Marthe Fatin

Rouge Stefanini

Keywords:

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Introduction

Referendums are often portrayed as instruments that are dangerous to rights and freedoms, in particular in the cases of minorities [START_REF] Derrick | The Referendum : Democracy's Barrier to Racial Equality[END_REF][START_REF] Priscilla | Initiatives and Referendums : Direct Democracy and Minority Interests[END_REF][START_REF] Gamble | Putting Civil Rights to a Popular Vote[END_REF][START_REF] Donald | Lose, Win, or Draw ? A Reexamination of Direct Democracy and Minority Rights[END_REF]. This danger would come from a natural tendency of the majority to be tyrannical and discriminatory towards minorities. This fear was in fact an argument put forward by the Founding Fathers of the United States to outlaw referendums at the federal level (Magleby, 1995, 19-20), and that country is the source of the greatest number of studies on the impact of referendums on minorities. Those studies show that, statistically, the risks of infringement of the rights of minorities are greater in states where direct democracy procedures are employed [START_REF] Daniel | Direct Democracy and Minority Rights : Same Sexe Mariage Bans in the United States[END_REF]). Yet, the supposed hostility of referendum procedures to rights and freedoms has not always been taken for granted. For example, from the time of Jean-Jacques Rousseau's Social Contract to still recently in France, the sovereign people was perceived as best able to protect rights and freedoms, including by means of referendums (Fatin-Rouge Stefanini, 2003, 74). However, and more generally, the risk that referendums could kill freedom is a recurring criticism, and this criticism has grown louder with the crisis of representative democracy and rise of populism. In contrast, decisions adopted by representatives are reputed to be better able to protect minority rights because they are taken after deliberations in which various elements of the debate can be weighed one against the other [START_REF] Julian | Judicial Review of Direct Democracy[END_REF](Eule , 1525)). Decisions made by representatives are thus the results of discussions and compromises that take into account the different interests at stake, and these include the protection of rights and freedoms, including those of minorities (Moeckli, 2011, 777). In contrast, referendums allegedly do not favour such balancing because they do not leave room for debate (see, for example, Lewis, 2011, 366). This is not always accurate since, among the broad range of referendum formats, some take place after debates in assemblies, which may make recommendations (for example, the messages from federal authorities in Switzerland concerning popular initiatives), formulate the question (for example, in the case of the Portuguese referendum on article 115 of the constitution), or decide on the content of a text (for example, the French referendum on article 89 of the constitution). However, it is true that no debate can really be held with citizens, or among citizens. They will just have the choice of adopting or rejecting a text, of answering "yes" or "no" to a question, of choosing among several possible responses, but they will not be able to negotiate the content of the proposals. The supposed opposition between referendums and freedoms is, however, revealing of evolution of the very notion of democracy. This debate contrasts the traditional conception of democracy, that is, "government of the people, by the people, for the people", with the modern conception of democracy, which is sometimes called "constitutional democracy". This conception integrates the rule of law and the respect for fundamental rights and freedoms proclaimed in constitutional and international texts. The classical conception leads to enshrining political choices directly determined by the majority of citizens; the modern conception is intended to submit expression of the majority will, even when expressed through a referendum, to compliance with the law. The theory of the sovereignty of the people amplifies the differences between these approaches because the sovereign people, original holder of power, is by nature unlimited and is even less able to impose limits on itself when it expresses itself directly through a referendum. Moeckli points out: "Simply stated, this debate opposes, on the one hand, who depict the people as the absolute sovereign on whose will, finding its expression on direct democratic processes, no limits can be imposed with, on the other hand, those who argue that in a state based on the rule of law, even the people must comply with certain fundamental rules, including respect for human rights" (2011,775). Present-day respect for democratic values would therefore suppose that it is not possible to infringe on fundamental rights and freedoms, including by referendum. This theme is especially relevant in a period of return to grace of referendum procedures, in particular in the name of the need to allow the people to express itself directly on the issues that are the most important for the social, societal, political and economic future of states. Citizens themselves want to intervene more frequently in public debates and political decision-making ; such participation is seen as a means of overcoming the real or supposed failure of their representatives to respond to their concerns in an appropriate manner. The increase of popular initiatives throughout the world in recent years can be analyzed as a consequence of this claim.

However, this development is not without danger to rights and freedoms, since such initiatives can be powerful tools for challenging representatives' political choices, no matter what they are. While the risk that referendums could undermine freedom is real, especially for unpopular minorities, safeguards have nonetheless been established by some states to try to reconcile citizens' aims with the values to which modern democracies aspire.

1) Earlier research and key literature on the problem

A great many studies have been done on the dangers of referendums to minorities in the United States and, more recently and to a more limited extent, in Switzerland. Beyond these cases, recent research provides a spectrum of analysis that encompasses a few European cases, such as those of Hungary, Ireland and Lichtenstein (Marxer, 2012) as well as, more broadly member states of the Council of Europe, Latin America, Australia and New Zealand (Bochsler and Hug, 2009). Yet, given the few referendums outside of the United States and Switzerland, studies have often been very specific (focussing on one referendum in particular), or very general and theoretical. Indeed, aside from in those two states, referendums are so infrequent that a scientifically relevant comparative study on fundamental rights is difficult to conduct, 1 especially since the national context of each referendum and the political context at the time when it was held must be taken into consideration.

There are various classifications proposed by doctrine on referendums, but for this study we have chosen a distinction based on the conditions for triggering referendum procedures suggested by Francis Hamon (2012, 17). Hamon breaks the various types of referendums into two groups: "top-down" referendums instituted by representatives (the executive, legislature, parliamentary minorities, deliberative bodies of territorial communities…) and "grass roots" referendums triggered by citizens themselves through popular initiatives. This classification has the advantage of encompassing under the term "referendum" a complete series of procedures: compulsory or optional referendums, propositional referendums initiated at the discretion of institutional bodies or by citizens, abrogating referendums (concerning old laws) and veto referendums (on new laws) instituted by citizen or parliamentary minorities. Popular initiatives can therefore be considered as ways of triggering referendums when they are "direct", in other words, when they necessarily lead to the holding of a vote if the conditions of admissibility are met. When an initiative results only in the proposal being studied by parliament, it is described as indirect 2 (Eule, 1990, 1511) and we will not study it here. The states in which the number of referendums is the greatest are those where there are popular initiatives and, more generally, those where citizens can initiate referendums directly, whether the process is propositional, abrogating or designed to challenge a law that has just been adopted (veto referendum). The United States and Switzerland are the countries where the largest number of referendums are organized because citizens have the power to trigger referendums through popular initiatives at the level of the federal states in the USA, and at the level of communes, cantons and the federal government in Switzerland.

Such "grass roots" referendums are suspected of generating greater risks to freedom than "topdown" referendums. However, several remarks have to be made before presenting the content.

To begin with, the studies that have been done focus mainly on the discriminatory nature of some referendums, more than on how referendums in general place freedoms at risk. Yet, this distinction is important and merits that we ask questions about each of these terms. The rights and freedoms that will be taken into consideration are those qualified as "fundamental".

According to Louis Favoreu's definition, the adjective "fundamental" applies to "the set of rights and freedoms ascribed to physical persons as moral persons (under both private and public law) in virtue of the constitution, but also in virtue of international texts, and protected from both executive and legislative power by constitutional and international judges" (1990, 581 [our translation]). The fundamental rights and freedoms taken into consideration are thus those that are provided for in state constitutions and the international agreements with which states have made a commitment to comply.

Infringement on a freedom can be to different degrees, from the smallest limitation to suppression. Simple adjustment of a freedom in its implementation cannot be considered as killing freedom so long as there is no decline in exercise of the freedom or limitation that is disproportionate to the objective. In some cases, the decline can affect a minority, but it can also affect all citizens, notably when there are grounds of general interest or public order that can be decided by referendum (e.g., a prohibition against smoking in public spaces).

Moreover, it is important to point out that the scope of fundamental rights and freedoms is relative since it depends on states' constitutional and conventional choices. Such relativism in terms of the recognition and protection of so-called fundamental rights and freedoms is especially an issue when it comes to sensitive topics that are considered societal choices, such as abortion, the death penalty, medically assisted suicide, marriage and adoption by couples of the same sex, etc. On these issues where the boundary between the status quo, the permissible restriction and the infringement on the fundamental right is tenuous, referendums are numerous.

Discrimination is an infringement of the right to equality in that it supposes that two situations that could be treated equally, are not. There are many forms of discrimination. The discrimination dealt with in international 3 and constitutional 4 texts is negative and generally prohibited by those texts. Some policies designed to correct inequalities have, in contrast, been described as positive discrimination in that they establish preferential treatment in order to foster de facto equality (affirmative action policies). Here, the forms of discrimination to which I will be referring are mainly negative forms suffered by minorities. However, positive discrimination is also interesting for this study because some minorities that used to be covered by positive discrimination policies, such as the Black minorities in the United States, women and homosexuals, have seen those policies challenged and submitted to popular vote in some member states of the USA. 5 Finally clarifications must concern the notion of minority. There is no unanimity on the definition of that notion because, depending on the text and study, a group may be a minority in numerical, social or political terms. Thus, ethnic, religious and sexual minorities are numerical, whereas women were long a social and political minority, even though numerically they were sometimes in the majority. For this study, we will use Marxer's broad definition, which encompasses "not only ethnic, national, linguistic and religious minorities, but also social and political marginal groups and people with specific identifying characteristicssuch as nondominant sexual orientation or disabilities" (2012,8). People who have a criminal conviction can also be considered as a marginal social group, and thus as a minority.

Various studies take certain minorities into accountfor example, ethnic and racial minorities [START_REF] Derrick | The Referendum : Democracy's Barrier to Racial Equality[END_REF]Hajnal, Gerber, Lounch, 2002), religious minorities [START_REF] Christmann | Direct Democracy and Minority Rights : Direct and Indirect Effects on Religious Minorities in Switzerland[END_REF], sexual minorities [START_REF] Todd | Direct Democracy and Minority Rights : An Extension[END_REF][START_REF] Donald | Lose, Win, or Draw ? A Reexamination of Direct Democracy and Minority Rights[END_REF][START_REF] Daniel | Direct Democracy and Minority Rights : Same Sexe Mariage Bans in the United States[END_REF], foreigners [START_REF] Hainmueller | Does Direct Democracy Hurt Immigrant Minorities ? Evidence from Naturalization Decisions in Switzerland[END_REF] as well as, more broadly, a number of groups that are social or political minorities (for example: [START_REF] Gamble | Putting Civil Rights to a Popular Vote[END_REF][START_REF] Vargas | Judicial Review of Initiatives and Referendums in Which Majorities Votes on Minorities' Democratic Citizenship[END_REF], Buchsler and Hug, 2009;Vatter and Danaci, 2010).

Most of the studies prior to the 2000s focussed on the United States and many of them denounced the discriminatory, conservative nature of decisions adopted by referendum, especially those that resulted from citizens' initiatives at the state level. The end of the nineteenth century and the twentieth century were marked by a number of racist or xenophobic popular initiatives, including those designed to exclude Black settlers (Johnson, 1995, 632), impose a certain level of education to exercise the right to vote (Oklahoma, 1910, the "Grand-Father Clause") or prevent people of Japanese origin from becoming landowners (California, 1920, the Alien Land Law). Beginning in the 1960s, desegregation policies were targeted by referendums opposing fair distribution of housing between Blacks and Whites (anti-fair housing initiatives) and more equitable distribution of Black and White people in schools (antibusing initiatives). Those ballots were almost always unfavourable to minorities until they were challenged, either by a federal law (the Fair Housing Act of 1968), or the courts (Lazos- Vargas, 1999, 426). Then, beginning in the 1980s, referendums were held that were unfavourable to homosexuals, foreigners ("English only" initiatives and initiatives opposing bilingual education), immigrants in irregular situations (especially in southern states), people suffering from AIDS, and positive discrimination policies. There were also many proposals designed to make abortion laws stricter, in particular to require parents' consent in the case of minors. Lastly, in recent years, there has been a series of referendums against same-sex marriage.

With regard to all of these votes, and their often positive outcomes, which were therefore unfavourable to minorities, the United States seems to be the country in which there is the greatest discrimination and restriction of freedom by means of referendums. Swiss citizens, although notoriously conservative, 6 seem more reasonable, at least at the federal level, since, until 2004, they rejected virtually all of the referendums that could have infringed on rights and freedoms, including on those of minorities. Prior to 2004, only the initiative adopted on August 20, 1893, prohibiting "the slaughtering of livestock without having stunned it first" [our translation] was openly discriminatory against Jews and their religious traditions (Moeckli, 2011, 779). Jews were the unpopular minority at the time. In contrast, other later initiatives targeting foreigners in particular either did not receive enough signatures, and so were declared void (1995), or they were rejected. Studies carried out at the cantonal and local levels were, moreover, less alarming than in the United States (Frey and Goette, 1998). Switzerland attracted international attention only after the 2000s, following a series of popular initiatives that went against fundamental rights protected by certain international treaties and conventions.

Four referendums in particular were subject to criticism owing to their positive outcomes and especially because of problems of compatibility with international law. They were referendums organized on the basis of a popular initiative proposing an amendment to the constitution since, at the federal level, popular initiatives are permitted only to amend the constitution partly or completely. 7 The first initiative was adopted on February 8, 2004, and was entitled "Internement à vie pour les délinquants sexuels ou violents jugés très dangereux et non amendables" ["Life-long custody for non-curable, extremely dangerous sexual and violent criminals"]. The federal authorities had recommended that citizens reject that proposal, in particular because only a broad interpretation of its provisions, which did not correspond to the intentions of its authors, 8 could make it compatible with the European Convention on Human Rights (ECHR). Nonetheless, 56.2% of the votes were in favour of adopting it, and it was introduced as article 123 of the federal constitution.

The second popular initiative received very strong international reactions because it was intended to prohibit the construction of minarets. Despite the federal authorities' recommendation that it be rejected, 9 this proposal was adopted on November 29, 2009 with a majority of 57.5%. It entered into effect immediately after having been introduced in article 72, paragraph 3 of the constitution. In addition to the fact that this prohibition entered into contradiction with a number of provisions of the Swiss constitution that have the same legal weight as it, this prohibition is directly incompatible with article 9 of the ECHR, which guarantees freedom of religion, and article 14 of the ECHR, which prohibits any form of discrimination. This prohibition also contradicts articles 2, 18 and 27 of the UN Covenant on Civil and Political Rights.

The title of the third popular initiative that was approved was "Pour le renvoi des criminels étrangers" ["For the expulsion of foreign criminals"], and provided for the automatic deportation of foreigners who had committed certain offences or had received social benefits or assistance illegally. The proposal was adopted on November 28, 2010, with 52.9% of the votes. Like the preceding ones, the proposal was also incompatible with international law, in particular with the ECHR, the UN Covenant on Civil and Political Rights, the Convention on the Rights of the Child and the bilateral agreement between Switzerland and the European Union on free movement of persons. The federal authorities recommended rejecting the proposal and submitted a counter-proposal designed to respect the intentions of the authors of the initiative while ensuring compatibility with constitutional provisions and international law. 10 The counter-proposal was rejected by the people.

Lastly, on February 9, 2014, the Swiss people adopted the initiative entitled "contre l'immigration de masse" ["against mass immigration"], which was designed to allow Switzerland to set annual quotas on the number of foreigners admitted. It challenged the bilateral agreement on free movement of persons between Switzerland and the European Union, and the federal authorities recommended it be rejected. 11 It was adopted with 50.3% of voters in favour.

These differences in situation between the United States and Switzerland are also reflected in the results of studies conducted in the two countries. While some American authors showed that, in 78% of cases, referendum outcomes were unfavourable to minorities in the United States, (Gamble, 1997, 245;Lazos Vargas, 1999, 425), Swiss authors have shown, in contrast, that referendums, whether at the federal, cantonal or local level, have been, on the whole, favourable to rights and freedoms, with an overall rate of infringement of 30% (Frey andGoette, 1998, 1343). In both cases, the numbers advanced have been contested by other studies, which consider that the discriminatory impact on minorities is more limited or must be put into perspective since not all minorities are affected in the same way (regarding the United States: [START_REF] Todd | Direct Democracy and Minority Rights : An Extension[END_REF][START_REF] Hajnal Zoltan | Are There Winners and Losers ? Race, Ethnicity and California's Initiative Process[END_REF]regarding Switzerland: Vatter and Danaci, 2011;[START_REF] Christmann | Direct Democracy and Minority Rights : Direct and Indirect Effects on Religious Minorities in Switzerland[END_REF]. The methods of analysis are not the same from one article to the next (in particular regarding the situations and minorities studied, and the scope of the study), and, consequently, among the studies done in the United States in particular, few come to similar conclusions (Matsusaka, 2005, 168). Nevertheless most of the studies conclude that the direct procedures of democracy are not favorable to minorities [START_REF] Donald | Lose, Win, or Draw ? A Reexamination of Direct Democracy and Minority Rights[END_REF][START_REF] Daniel | Direct Democracy and Minority Rights : Same Sexe Mariage Bans in the United States[END_REF][START_REF] Todd | Direct Democracy and Campaigns Against Minorities[END_REF], or even to freedoms in general [START_REF] Daniel | How Minorities Fare under Referendums. A Cross-national Study[END_REF].

III -The realities of referendums' danger to minorities and freedoms

There are many examples of discriminatory or freedom-killing referendum propositions.

However, the picture is not completely dark. Shades and tones have to be pointed out because each referendum is unique, occurs in a very specific context, and is subject to campaigns that can have considerable influence on the way people vote. Indeed, while referendums have strong potential for discrimination, especially against unpopular minorities (1), the negative effects of referendums have to be placed in perspective in some cases (2).

-Proven danger for unpopular minorities

The many examples of discriminatory referendums mentioned in doctrine do not target all minorities in the same way, but certain minorities whose unpopularity is climbing. Such unpopular minorities may be those demanding greater rights, such as in the case of the homosexual movement in the United States and other countries, or they may be minorities with no special political or social demands, but whom people are talking about because, in a given context, they give rise to fearsoften unjustified or with no basis whatsoeveron the part of the majority. This is the case of Latin-American immigrants in the southern United States and foreignersin particular those of the Muslim religionin Switzerland, who are perceived as dangerous to the economy, culture and very identity of the host state.

Thus, a number of authors have proposed a distinction between "in-group minorities", which are not particularly affected by direct democracy procedures because they are considered as well integrated into the community, and "out-group minorities", which are perceived as little or not integrated, and therefore foreign to the community considered to be the majority (Vatter and Danaci, 2010, 205;Christmann and Danaci, 2012, 134). The less the minority group is considered as integrated, the more it will be likely to suffer discriminatory proposals in referendums. A number of elements can be added. To begin with, the acceptance of a minority varies from one state to the next. For example, Vatter and Danaci's study covering the federal and cantonal levels in Switzerland between 1960 and 2007 shows that direct democracy procedures did not affect the situation of homosexual people (2010, 157), while in the United States, beginning in the 1980s, they were targeted by a number of referendums. In contrast, in both states, foreigners were considered at different times as especially unpopular. Indeed, in Switzerland, this is the minority that has been most affected by direct democracy procedures (Vatter and Danaci, 2010, 212). However, within the "foreigner" category, all are not targeted in the same manner, as can be seen from local referendums on naturalization in Switzerland, which have caused problems owing to discrimination against applicants depending on their geographical origin (Hainmueller and Hangartner, 2015, 35).

Popular reaction to some minorities and some categories of people is also often related to a very specific context: a shocking current event (serial killing in the case of initiatives in favour of the death sentence or life imprisonment; massive immigration -sometimes escalated by war-in a state in loss of identity or suffering an economic crisis (Alvarez and Butterfield, 2000, 2); growth in power of a minority claiming rights and protection (homosexuals, Muslims in secular or Christian-majority states…)). Such initiatives may also be in reaction against political choices made by representatives or against court decisions perceived as activist in that they allegedly protect a minority against the majority will. Thus, Donavan says that "directdemocracy campaigns over questions of minority rights are not simply about a particular right and a particular minority group but may also reflect reaction to counter-majoritarian aspects of democracy that are facilitated by courts and representative government" (2013,1743). He calls this reaction "populist backlash" (2013, 1740). Thus, anti-fair housing and anti-busing initiatives in the United States occurred in reaction to the desegregation policy; proposition 17 concerning the death penalty in California was approved in 1972 to counter the Supreme Court decision of the same year declaring capital punishment to be unconstitutional; 12 in Ireland, one of the referendums proposed by the government in 1992 was on abortion and was directly intended to counter the Supreme Court decision of the same year that led abortion to be permitted in case of risk of suicide; 13 in the United States, initiatives were filed against affirmative action policies following decisions by the Supreme Court in 1989, in City of Richmond v. J.A. Croson, 14 and in 1995, in Adarand Constructors, Inc. v. Peña 15 (Lazos-Vargas, 1999, 455); proposals designed to prohibit same-sex marriage also followed a number of decisions favourable to homosexuals, in particular the Supreme Court of Hawaii decision in 1993, which considered that refusing to marry a homosexual couple was discrimination in violation of the state constitution, 16 and the Supreme Court of Massachusetts in 2003 authorizing same-sex marriage 17 [START_REF] Todd | Direct Democracy and Campaigns Against Minorities[END_REF](Donavan, , 1747)); in Slovenia, the 2004 referendum initiated by the conservative opposition on the question of the "Erased" directly targeted a Slovenian constitutional court decision in 2003 that challenged the legislation's strict conditions on regularization of such situations (Hassid, 2015); 18 etc. The unpopularity of a minority targeted by a referendum is also often amplified in the context of campaigns led by lobbies or political parties that stigmatize the minority by blaming it for society's various ills (Bell, 1978, 20) in an unsubtle and often deceitful manner. Indeed, some political parties are aware that they can take advantage of such stigmatization and, through wellorganized, generously funded campaigns, take advantage of direct democracy mechanisms to criticise the political choices of parties in power in a more general way. This is especially the case with respect to foreign minorities, or minorities considered to be foreign, which are directly targeted by political parties, often on the extreme right, wishing to intensify nationalist sentiments, as is the case in the United States, Switzerland and also Slovenia, with respect to the "Erased". As Sylvia Lazos-Vargas has pointed out regarding proposition 187 against immigrants in irregular situations in California: "Illegal immigrants, a highly unpopular group, are an easy target for politicians who perceive that they can gain an electoral advantage by positioning themselves as being 'against' such groups". 19 Likewise, in Switzerland, the initiatives against foreigners and Muslims supported by the Swiss People's Party have considerably aided the rise of that political party (Moeckli, 2011, 780).

For unpopular minorities, direct democracy procedures seem to increase discrimination in two ways. On one hand, a number of studies show that representatives, not only because they discuss decisions to be taken among themselves within assemblies, but also because they have to justify their choices (Hainmueller, Hangartner, 2015, 34), are likely to make decisions that are more favourable to minorities. Representatives take into account a complex set of data, to justify a decision that will, in principle, be reasonable. In contrast, a referendum vote is considered more emotional than reasonable, especially since the various arguments and elements of the debate are often extremely simplified in referendum campaigns. Voters do not have to answer for the positions they take; the vote is "anonymous" (Christmann and Danaci, 2012, 136). Moreover, some authors have shown that the negative effects of referendums on minorities are not always immediately visible at the time of the vote [START_REF] Daniel | Direct Democracy and Minority Rights : Same Sexe Mariage Bans in the United States[END_REF]. The very existence of direct democracy mechanisms can have indirect effects, beyond the vote itself, on representatives. For example, Lewis (2011, 367, 376), and[START_REF] Christmann | Direct Democracy and Minority Rights : Direct and Indirect Effects on Religious Minorities in Switzerland[END_REF]Danaci, (2012, 147, 153) show, regarding same-sex marriage in the United States and the policy applying to Muslims in Switzerland, that the parliaments of the states where there are direct democracy mechanisms are influenced by the very existence of those mechanisms and can, owing to this, be tempted to adopt stricter legislation targeting minorities out of fear of political reprisals (in the form of popular initiatives, recall or failure to be re-elected) by the majority of citizens. 20 Minorities are thus in a lose-lose situation in that they are affected both by the decisions of the majority of citizens, but also by those of the majority of representatives. States that have direct democracy mechanisms that can be set into motion by citizens' initiatives are allegedly more conservative than states that do not have such mechanisms (Matsusaka, 2007, 20;Lewis, 2011, 367, 376). Such conservatism apparently affects not only the rights of minorities, but also fundamental rights in general, as a study on 52 states tries to show concerning a number of rights and freedoms: women's social and economic rights, the rights of sexual minorities, abortion rights, freedom of expression, freedom of assembly and association (Boschler, Hug, 2009, 22). The results of studies are however interpreted in accordance with the themes addressed; the issue of abortion, for example, apparently reveals a more open-minded attitude on the part of voters [START_REF] John | Direct democracy and Social Issues, Working paper, USC Marshall School of Business[END_REF]Buchsler and Hug, 2009). In contrast, the conservative reflex is obvious with respect to the theme of the death penalty in the United States. In November 2016, three states decided in referendums to maintain the death penalty or to challenge a law abolishing the death penalty (California, 21 Oklahoma, 22 Nebraska 23 ).

-Discriminatory effects to be brought into perspective

Discriminatory referendums are highly mediatized, and the many debates and criticisms to which they give rise tend to strengthen the feeling that referendums are a major danger to rights and freedoms. While there is this danger, it must be seen in relative terms in a number of respects.

To begin with, in Switzerland, for example, the successes of recent years have to be viewed in relation to the very large number of potentially discriminatory popular initiatives that failed, either at the signature-gathering or at the voting stage, as can be seen on the website of the Federal Chancellery. 24 All of the popular initiatives targeting foreigners prior to 2010 failed, including the initiative designed to overturn the jurisprudence of the Federal Supreme Court, 25 which means that naturalization can no longer be decided by referendum at the local level. 26 In the United States, while a certain number of proposals targeting minorities have ended up being adopted, 27 most of them have been challenged at the federated or federal level, and when a fundamental right is in question, the provisions adopted have strong chances of being declared unconstitutional by federal courts (see IV below).

It is also important to look more closely at the content of the referendum before drawing general conclusions. Manifestly discriminatory or provocative referendums that lead to a step back in minority rights or to a clear decline in freedom are generallythough not alwaysrejected (Lazos- Vargas, 1999, 431, 432 concerning certain initiatives against homosexuals and people suffering from AIDS in the United States). In especially tense situations, discriminatory and regressive arguments concerning protection for fundamental rights can, however, be the recipe for success (Mockli, 2011, 790). In contrast, referendums on proposals to maintain the status quo by opposing legislation or court decisions considered as favourable to minorities or on proposals against giving new rights to minorities have greater chances of being adopted (Christmann and Danaci, 2012, 155;Marxer, 2012, 165). According to those analyses, the rights of minorities are affected mainly when what is in question is their progress, in other words, the expansion of their rights; in contrast, it is rare to see true steps back in the rights of minorities, such as with proposition 187 in California or the prohibition against minarets in Switzerland. This reading of referendums can explain why, in the case of same-sex marriage, most American and European states (Croatia 28 , Slovakia 29 , Slovenia 30 ) that held referendums on that theme refused to give that right to the homosexual minority: members of that minority were asking for equality with respect to access to marriage, which was synonymous to them with progress in recognition of their rights. This attitude is also revealing of the conservatism of which direct democracy measures are generally accused. However, some authors have shown that when minority rights are encompassed in broader reforms, in particular in constitutional revisions, progress can be made in fundamental rights, including for unpopular minorities (Christmann and Danaci, 2012, 154). Expanding the proposed reform to integrate into it less popular measures makes it possible to avoid polarizing attention on one minority in particular, and thereby to dilute the negative resentment that the majority might express towards that minority if it were to be questioned specifically on that minority's rights.

While it is true that states such as Switzerland have been slower to accept reforms related to societal issues (women's right to vote, abortion, same-sex marriage…), referendum procedures can also be shown to be more liberal. Indeed, the great majority of studies focus attention on the danger that popular initiatives and referendums present for minorities, and it is rare to find one that bucks the apparent general trend toward defeatism and highlights the fact that direct democracy mechanisms can also be a means of acquiring new rights. For example, in 1994, Oregon adopted a popular initiative permitting medically assisted suicide, and, despite opponents' efforts, this measure has not been challenged since (Purvis, 2012, 271). Oregon was even the precursor in this area; it was followed a few years later, in 2008, by the State of Washington, which held a referendum on medical assistance for suicide, in which the outcome was positive, with 58% of the votes. As of 2016, eight USA states had approved by referendum the legalization of marijuana or cannabis use for recreational or medical purposes. Similarly, Ireland, going against the current in most American states, was the first state to accept homosexual marriage by referendum. 31 A compulsory constitutional referendum also allowed the Irish to confirm, in 2001, the law that had prohibited the death penalty since 1990 and to provide in the constitution that the death penalty cannot be reinstated under any circumstances, even in time of war. 32 The danger to freedoms seems higher when direct democracy procedures are actively employed than when representative institutions have the power to make decisions without having their choices subject to being threatened by a referendum. However parliamentary process is no absolute guarantee of protection of freedoms and respect for the rights of minorities. This justifies establishing anti-majority filters in order to limit the omnipotence of the parliamentary majority; in particular, acts adopted by parliaments are subject to judicial review, which is a way of preserving, notably, fundamental values enshrined in the constitution. Similarly, a certain number of safeguards have been developed in recent years with respect to direct democracy and, even though they are sometimes challenged on the basis that they deconsecrate the outcomes of referendums, they reveal awareness of referendums' dangers to rights and freedoms.

IV -Guarantees to limit referendums' threat to freedoms and minority rights

Many states became aware, more or less long ago, of the dangers that direct democracy procedures can create for the rights of minorities and fundamental freedoms in general. There are ways to limit these risks. To begin with, the choice may be to have an exclusively representative regime that provides for no form of referendum intervention at all at the national, federal or local level. This radical solution is, however, difficult to defend in an era when many states are dealing with crises in representation, and when citizens are seeking to participate more fully in public affairs and to intervene directly in political decision making, even though those decisions may be complex. Therefore, even in long-standing democracies, exclusively representative by tradition, referendum mechanisms have been introduced, either by law on a case-by-case basis (Holland, the United Kingdom) or in the constitution at the local level (Belgium 33 ). Some states have chosen to prohibit referendums on the national or federal level (Germany, 34 the United States) while authorizing them at the local or federated level. Indeed, in Germany and the United States, direct democracy procedures have developed largely at the level of the Länder and the states. 35 This principle of organization has a major advantage: at the federated level, citizens can make proposals or express their opposition to political choices, norms or court decisions adopted at the federal level, but within the limits of respect for the federal constitution and, notably, the rights and freedoms that it guarantees. Thus, the principle of a hierarchy of norms and that of respect for the federal constitution make it that, through judicial review, constitutional courts are, in principle, able to set aside discriminatory referendum choices adopted at the level of federated states.

Another way to limit the risk of discrimination or elimination of freedoms is to leave the national or federal referendum initiation procedure in the sole control of those in power. Most states do not allow a minority of citizens or a political minority (determined by a minimum number of parliamentarians) to trigger referendum procedures (for example, Austria, 36 Spain, 37 Greece, 38 Ireland 39 ), or they make the conditions to be met so difficult that it is practically impossible for a referendum to be held (France, 40 Russia 41 ). Thus, referendums remain a form of recourse that is exceptional and controllable by government or parliament as to avoid any abuses. However, this does not prevent those in power from proposing discriminatory referendums themselves, as can be seen, for example, from the 1992 referendum on abortion in Ireland, on a question designed to challenge the Supreme Court decision, 42 and the referendum in Hungary on October 2, 2016, targeting the European Union's policy on migration quotas, and thus, indirectly, migrants. 43 Some states have also decided to shelter fundamental rights and freedoms from popular votes by adopting provisions that limit the areas to which referendums can apply. This limit can be general, providing that some referendums initiated by citizens cannot concern constitutional questions, and thus cannot amend the constitution. For example, in Italy, article 75, paragraph 1, which provides for referendums on repealing legislation, states: "A popular referendum shall be held to abrogate, totally or partially, a law or an act having the force of law…" [our emphasis]. This excludes provisions in the constitution, and therefore a provision on fundamental rights and freedoms, from being the targets of abrogating referendums. (This was the case, for example, of a radical proposal to repeal the law on abortion in 1981. 44 ) The situation is the same in Portugal, where article 115, paragraph 4 expressly removes constitutional amendments from the field of referendums. That limit can also be more specific and directly target the category of fundamental rights and freedoms. This is the case of referendums in Slovakia (article 93, paragraph 3 of the constitution) and with respect to popular initiatives in Mississippi (article XV, section 273, part 5). 45 The limitation can also target, as in Massachusetts, only certain freedoms (article XLVIII, part 2 of the constitution 46 ). However, these limits can be respected only at the cost of efficient control of compliance with provisions on referendums.

The efficiency of control over referendums will depend on the authority that performs it, the jurisdiction and margin of manoeuvre that authority has, the impacts of its decisions and, more generally, the system of which it is a part. It is, however, important that the authority responsible for making the decision be independent, which entails a priori setting aside control by political authorities. Thus, in Europe, after World War II and the fall of the Berlin Wall, a number of new democracies that had been victims of authoritarian regimes introduced or reintroduced into their constitutions direct democracy procedures that could be initiated by citizens. These procedures were often enshrined at the price of strict control over them out of fear of antidemocratic hijacking of referendum tools and destabilization of institutions that were sometimes still fragile. The referendums were therefore accompanied by procedures to ensure prior control over referendum proposals, and that control was generally placed in the hands of constitutional courts (Albania, 47 Croatia, 48 Italy, 49 Portugal, 50 Romania, 51 Russia, 52 and Slovakia: 53 Fatin-Rouge [START_REF] Stefanini | Le contrôle de la recevabilité des référendums en droit comparé ou la recherche d'un équilibre entre différentes exigences démocratiques[END_REF].

Such control prior to the holding of a referendum flows from a concern to "rationalize" the direct expression of the general will (Taillon, 2012, 479) so as to preserve the constitution and the various interests it encompasses.

Control performed before the vote so as to prevent referendums from being held that could infringe on fundamental rights and freedoms has a major advantage, which is that this avoids tensions created by the campaigns themselves. However, such preventive control is not always possible or desirable. In the United States, for example, the courts rarely agree to verify the legality and constitutionality of the merits (the substantive grounds) of referendum proposals, especially when they are popular initiatives, before the vote is held. There are a number of reasons for this. To begin with, when a popular initiative is provided for under the constitution, it is considered to be a citizens' right that is constitutionally protected at the state level (Radcliffe, 1994, 425). Preventive control is thus perceived by some courts as infringement on citizens' rights. 54 Moreover, technical reasons can be advanced by the courts in the American justice system to justify the absence of control over the content of proposals submitted to referendums: the prohibition against advisory opinions, the need for cases to be in a state amenable to judgment and thus on concrete issues, and the fact that court intervention must be on a strictly necessary basis … (Gordon and Magleby, 1989, 304). Thus, those affected by a referendum decision must wait until the disputed provision comes into effect before initiating legal proceedings and, in most cases, such proceedings occur after the vote, if the outcome is positive. Some so-called activist courts are more open to exercising control before the vote, or interpret more broadly their formal control over the manner in which the question, proposal or title of the proposal is written. For example, the Arkansas Supreme Court struck a number of referendum proposals from the ballot a few days before the vote was to be held for reasons related to the vagueness of the proposals as formulated in the titles or in the texts. 55 However, a posteriori controls, although respecting the freedom of expression of voters and a form of sovereignty of the people in the Federated States, raise some political difficulties in the United States. Indeed, at the federal level, the judges in the various state courts are often elected.

And it happened on several occasions that their position on certain cases, which had led them to censure the referendum decision, caused them to be recalled or not being reelected [START_REF] Julian | Judicial Review of Direct Democracy[END_REF](Eule, ,1582;;[START_REF] Todd | Direct Democracy and Campaigns Against Minorities[END_REF]Donavan, , 1741)). As a result, some judges express their discomfort at the decisions adopted by popular vote and tend to show some form of deference towards them.

However, in the United States, many proposals -even constitutional amendments -adopted or amended at the state level are being challenged in court for non-compliance with the federal Constitution. The Federal Courts and ultimately the Supreme Court of the United States have thus invalidated several provisions considered to be discriminatory or contrary to rights and freedoms. Thus, for example, the Romer v. Evans' decision, according to which Amendment No. 2 to the Constitution of Colorado was discriminatory for gays and lesbians, or the Obergefell v. Hodges' decision, which invalidated -on the basis of the XIV Amendment -all state provisions, whether adopted or not by referendum, prohibiting marriage between persons of the same sex. It should be noted, however, that the constitutional recognition of a right even at the federal level does not prevent new initiatives from being filed in order to circumvent either the constitutional provision or the court decision recognizing that right, as has been the case for example with abortion issues following the Roe v. Wade's decision by the Supreme Court in 1973(Carter, 2011, 305). In Europe, even in the case of constitutional provisions that discriminate or threaten individual freedoms adopted by referendum, a final appeal can be lodged with the European Court of Human Rights, which may declare the decision incompatible with the Convention. This has been the case, for example, with the provisions of the Irish Constitution on abortion, introduced by referendum in 1983, which prohibited associations from publishing information on the possibility for Irish women to have abortion abroad (CEDH, 29 October 1992, Open Door and Dublin Well Woman v. Irlande).

Judicial intervention, whether before or after voting, has the further virtue of shifting the debate on the legal level and thus offering answers based on the right to political questions with a high emotional content. Such answers usually help alleviate this tension since fundamental democratic values are recalled.

In Switzerland, the problem raised by referendums at the federal level is still different. Indeed, popular initiatives that have aroused controversy have a constitutional status since it is allowed at federal level to propose a constitutional revision by means of a popular initiative. However, because of their place in the hierarchy of norms and the great respect traditionally accorded to popular rights and the decisions adopted directly by citizens in this country, popular initiatives enjoy full immunity. An a priori control of the admissibility of popular initiatives is actually provided for in the Constitution 56 , but not only is it exercised by a political authority -the Federal Parliament -but it is also rather succinct. Indeed, until 1994, the control of admissibility, apart from the requirements for the collection of signatures, was limited to respect for the unity of form, which concerns the way in which the initiative is drafted, and the unity of matter, which means that the popular initiative must not deal with separate subjects, without relations between them, on which citizens might want to express different opinions. Since 1955, the Federal Parliament also controls the executability of the popular initiative 57 . In the mid-1990s, the Federal Assembly was faced with an initiative entitled "For a reasonable policy of asylum" which violated several international conventions (including the Geneva Convention of 8 July 1951, the European Convention on Human Rights, the United Nations Convention against Torture and the International Covenant on Civil and Political Rights), by prohibiting in particular the right to appeal for asylum seekers. This initiative was declared void in the name of respect for the rules of international law 58 -jus cogens -and this new limit was enshrined in the Constitution during the 1999 total revision. However, this limit has not prevented the holding of other popular votes leading to the adoption of discriminatory provisions since 2004, since even if these provisions are contrary to international law, the rules of jus cogens are not at issue. These initiatives raise problems of compatibility between national constitutional law and international law, without the question of the primacy of one or the other having been finally settled. Consultations were held on the possibility of introducing new limits taking into account, for example, the protection of the "essence of fundamental rights"; however, faced with the difficulty of agreeing on the content of these limits, no reform has yet been undertaken (Moeckli, 2011, 786).

However, the Swiss authorities, hampered by the stigmatization of these initiatives at diplomatic and international levels, have tried, where possible, to find compromise solutions in the implementation of these new provisions in order to ensure respect for the law international.

With respect to the 2004 initiative on life internment for sexually or violent offenders found to be very dangerous and non-amendable, the federal Parliament, using its power to interpret adopted proposals in their implementation phase, has adopted in 2007 an implementing law enabling its conformity with international law (Article 64 (1) (a) of the Criminal Code) 59 .

Similarly, with respect to the 2010 initiative on "Removal of Foreign Criminals", the federal Parliament relaxed the rigor of the rule adopted by referendum in its enforcement measures by providing, inter alia, a degree of discretion left to the judge to decide expulsion, in spite of the fact that this room for manoeuver (which had been proposed in a counter-project voted together with the initiative) had been expressly rejected by the voters. In response to these implementing measures, however, new popular initiatives have been introduced 60 , which shows that the solutions of compatibility found by the Parliament for certain initiatives are fragile, all the more since international law and the law of the ECHR, particularly in Europe, is increasingly contested by States, with the result that the guarantee of fundamental rights and, beyond, the respect for democratic values, appears increasingly challenged.

V -Conclusion :

It is difficult to draw general conclusions about the effects of referendums on fundamental rights and freedoms. The procedure for calling the referendum, the questions that can be put to the vote, the rank of the rules adopted in the hierarchy of norms, the context of the referendum, the subject matter, how the question is formulated, how the campaign is organized and funded, are all factors that must be taken into account in order to understand why certain referendums may infringe rights and freedoms, in particular minorities, in certain States, and why this is a limited or non-existent risk in others. The case of Switzerland is topical and has attracted much attention in so far as, faithful to a traditional conception of democracy and confident in a system that has worked relatively well for a century -thanks in particular to the sometimes very motivated messages of the Federal assemblies and to the technique of the counter-projectrights and freedoms, including those of minorities, seemed preserved. The wisdom of the Swiss population seemed to be able to protect against the risk of violating the freedoms. However, in a troubled period, the balances on which the Swiss democratic system rests are found to be fragile, which makes it necessary to provide appropriate mechanisms to ensure respect for the fundamental rights of minorities. Such mechanisms exist in many countries which have become aware of the importance of allowing citizens to participate more actively in political life but also of the need to frame the popular will in the name of respect for the different components of democracy. These counter-majoritarian mechanisms make it possible to deal with the brutality of certain proposals submitted to popular vote, while respecting democratic values, either by preventing referendums on issues that are particularly sensitive to rights and freedoms or by allowing the debate to continue in front of the courts, and may even lead to a revision of the Constitution or to the renunciation of a treaty (which has not yet occurred), but after a genuine democratic reflection on the choices to be made.

Vatter Adrian, Danaci Deniz, (2010), Mehrheitstyrannei durch Volksentscheide? : zum Spannungsverhältnis zwischen direkter Demokratie und Minderheitenschutz, Politische Vierteljahresschrift, 51, 205. 1 See, however, Boschler and Hug (2009) for a study of the influence of the existence of direct democracy mechanisms on Parliament decisions involving minority rights and civil rights. 2 See, for example, the indirect popular initiative provided for in article 71 of the Italian Constitution. 3 Article 6 of the International Covenant on Civil and Political Rights. 4 See, for example, Article 3 of the German Basic Law, Article 11 of the Belgian Constitution or Article 14 of the Spanish Constitution. Most often, the constitutions mention prohibited "distinctions": for example, the Constitution guarantees "equality before the law of all citizens without distinction of origin, race or religion" (Article 3). 5 See, in particular, regarding positive discrimination in favour of homosexuals, Amendment No. 2 in Colorado in 1992, Proposal 209 in California in 1996, and Initiative 200 in Washington State in 1998. 6 E.g. about the right to vote. Women did not acquire this right at federal level until 1971 and in some cantons it was granted only in 1990. 7 Art. 138 and 139 of the Constitution, an amendment which will be submitted to the referendum (article 140 of the Constitution). 8 Message from the Federal Council, Federal Law (FF) 2001 3265. For the Parliament: FF 2003 3979. 9 FF 2008 6923 et FF 2009 3903. 10 FF 2009 4571 et FF 2010 3853. 11 FF 2013 279 et FF 2013 6575. 12 Furman v. Georgia, 480 US 238, and, at the level of California : People v. Anderson, 493 P.2d 880, 6 Cal. 3d 628 (Cal. 1972). 13 Attorney General v. X, [1992] IESC 1; [1992] 1 IR 1. 14 488 U.S. 469 (1989). 15 515 U. S. 200 (1995). 16 Baehr v. Lewin, 852 P. 2d 44 (Haw.1993). 17 Goodridge v. Dep't of Pub. Health, 798 N.E.2d 941 (Mass. 2003). 18 The referendum was held on April 24, 2004. 31.54% of the voters participated in the vote and rejected the law applying this decision to 94.59%. 19 1999, 453. 20 The same observations can be made for other minorities. For migrants, see, for example, Lazos Vargas, 1999, 473, 474. 21 California, Proposal 62, Repeal of the Death Penalty. The purpose of this proposal was to abolish the death penalty that was reintroduced by referendum in 1978, after the procedure was declared unconstitutional by the Supreme Court of California in 1972 (People v. Anderson). 22 Oklahoma Death Penalty Amendment, State Question 776. 23 Nebraska, Referendum 426, Death Penalty Repeal. This veto-referendum is against the law LB 268, passed in May 2015, prohibiting the death penalty in Nebraska. It was adopted with 60.64% of the cast votes (source: ballotpedia). 24 https://www.admin.ch/ch/f/pore/vi/vis_2_2_5_1.html?lang=fr 25 Decisions of the Swiss Federal Court of 9 July 2003: ATF 129 I 217 and ATF 129 I 232. 26 Popular initiative "For democratic naturalisations" refused on 1 June 2008. 27 See ballotpedia.org, which lists by subject, state and year all proposals submitted for voting (https://ballotpedia.org/List_of_ballot_measures_by_topic). 28 Referendum of 1 December 2013 deciding to write in the Constitution that marriage can only be celebrated between a man and a woman. 29 Although the Constitution only permits marriage between a man and a woman, a minority initiative consultation was held on 7 February 2015, asking three questions, one to go further in the prohibition of samesex marriage and another one to deny adoption by homosexual couples. Although the results of this consultation were largely positive, they were invalidated because of the quorum of participation not met. 30 Referendum of 20 December 2015 rejecting the law authorizing marriage between persons of the same sex. 31 Referendum of 22 May 2015. 32 This inscription was probably influenced by Ireland's desire to ratify the Additional Protocol 13 to the ECHR on the abrogation of the death penalty in all circumstances.


Elina Manova

Daniela Paneva

Boris Kunev

Claude Estournès

Éric Rivière

Krassimir Tenchev

Anne Léaustic

Ivan Mitov

E Rivière

Mechanochemical synthesis and characterization of nanodimensional iron-cobalt spinel oxides

Keywords:

Introduction

Nowadays the synthesis of spinel ferrite nanoparticles has been intensively studied, because of their remarkable electrical and magnetic properties and wide practical application to information storage system, ferrofluid technology, magnetocaloric refrigeration, catalysis, and medical diagnostics. The principal role of the preparation conditions on the morphological and structural features of the ferrites have been discussed in several papers [1][2][3][4][5][6]. High-energy milling as a solid-state method of synthesis of nanodimensional materials has been the subject of considerable interest in recent years [START_REF]Proceedings of International Symposium on Metastable, Mechanically Alloyed and Nanocrystalline Materials[END_REF][START_REF] Boldyrev | [END_REF][START_REF] Suryanarayana | Mechanical Alloying and Milling[END_REF][START_REF] Suryanarayana | Mechanical alloying for advanced materials[END_REF]. The highly non-equilibrium nature of the milling process creates the opportunities to prepare solids of improved and/or novel physical and chemical properties. Mechanical milling is a technique with an advantage that it can easily be operated and produces large amounts of nanostructured powders for a short period of time [START_REF] Fecht | [END_REF]. A mechanochemical route for the preparation of ferrites has been reported [12][13][14][15], however, synthesis was generally performed starting from a mixture of iron and other metal oxides. In our previous studies, we reported ferrites formation after a mechanical milling of the corresponding hydroxide carbonates [16,17]. Among spinel ferrites, cobalt ferrite CoFe 2 O 4 is especially interesting because of the high cubic magnetocrystalline anisotropy, high coercivity, and moderate saturation magnetization. The properties of these ferrites are highly sensitive to the concentration of divalent metal ions, to substituting other metallic ions for the divalent ions and to the crystallite size [18][19][20][21]. Many synthesis strategies for preparing nanosized cobalt ferrite have been reported [21][22][23][24][25].

In this paper, for the first time we describe the mechanochemical synthesis of Co 2 FeO 4 . We also present the mechanochemical synthesis of CoFe 2 O 4 under the same conditions as for Co 2 FeO 4 and the characterization of the obtained materials using powder X-ray diffraction, Mössbauer spectroscopy, infrared spectroscopy, temperature-programmed reduction, and magnetic measurements of samples prepared at different milling times.

Experimental

The synthesis was performed by two steps: co-precipitation and mechanical milling of the co-precipitation precursors. The starting materials used were Fe(NO3)3•9H2O powder (purity 99%), Co(NO3)2•6H2O powder (purity 96%), and Na2CO3. In the co-precipitation processing route, a 0.5 M solution of metal salts containing Co and Fe were taken in a desired molar ratio: Fe/Co = 2 and 0.5. Mixtures of cobalt and iron hydroxide carbonates precursors were formed when a 1 M sodium carbonate solution was added at pH 9. The precipitates were washed and dried at 348 K for 3 h. The as-obtained precursors (named as CoFe2HC and Co2FeHC) were milled using a Fritsch Planetary mill in a hardened steel vial together with 15 grinding balls having different diameters (from 3 to 10 mm). The ball-to-powder mass charge ratio was 10:1. The powders were milled for 1 and 3 h (samples denoted as CoFe2MS1, CoFe2MS3, Co2FeMS1, and Co2FeMS3, where MS indicates mechanochemical synthesis). Thermal synthesis was performed in two steps: co-precipitation (starting materials and procedure are described above) and subsequent annealing of the coprecipitation precursor at 773 and 573 K for CoFe2O4 (denoted as CoFe2TS, where TS indicates thermal synthesis) and Co2FeO4 (denoted as Co2FeTS), respectively. According to our previous results [26] the thermal treatment of Co2Fe-hydroxide carbonate at temperatures above 573 K leads to the formation of two spinels of different Co/Fe ratio, while for CoFe2O4 this temperature is too low for spinel formation and for this reason the CoFe2-hydroxide carbonate is annealed at 773 K.

The structure was determined by X-ray diffraction (XRD) using TUR M62 diffractometer with Co K␣ radiation. Data analysis was carried out using JCPDS database. Transmission Mössbauer spectra were obtained at room temperature (RT) with a Wissel electromechanical Mössbauer spectrometer (Wissenschaftliche Elektronik GmbH, Germany) working at a constant acceleration mode. A 57 Co/Cr (activity ∼ = 10 mCi) source and an ␣-Fe standard were used. Experimentally obtained spectra were treated using the least squares method. The parameters of hyperfine interaction such as isomer shift (IS), quadrupole splitting (QS), and effective internal magnetic field (H eff ) as well as line widths (FWHM) and relative spectral area (G) of the partial components of the spectra were determined. The infrared (IR) spectra were taken on a Spectrum 1000 (PerkinElmer) FTIR spectrometer in KBr pellets. Isothermal magnetizations at RT were obtained with a Princeton Applied Research vibrating sample magnetometer Model 155 (VSM -maximum static field of ±1.8 T). Temperature and field dependences of the magnetization of the cobalt ferrites were measured on a Quantum Design SQUID Magnetometer. Zero-field cooled (ZFC) magnetization of the sample was measured by cooling down the sample to 5 K in zero-field and monitoring the magnetization of the sample from 5 to 400 K in a field of 20 Oe. The field-cooled (FC) magnetization was measured by cooling the sample down to 5 K in the same field. In order to avoid sample rotation in the applied field the nanoparticles were embedded in a polymer matrix, PMMA (poly methyl methacrylate). Transmission electron microscopy (TEM) investigations were made using a Topcon 002B electron microscope operating at 200 kV with a point-topoint resolution r = 1.8 Å. The samples were sonicated in ethanol and deposited on the copper grid precovered by polymer. Temperature-programmed reduction (TPR) of the samples was carried out in the measurement cell of a differential scanning calorimeter (DSC-111, SETARAM) directly connected to a gas chromatograph (GC). Measurements were carried out in the 300-973 K range at a 10 K/min heating rate in flow of Ar:H2 = 9:1, the total flow rate being 20 ml/min. A cooling trap between DSC and GC removes the water obtained during the reduction.

Results and discussion

XRD patterns of the precursors and those of the samples obtained after different milling times as well as after thermal treatment are shown in Fig. 1. The patterns of the precursors are characteristic of layered double hydroxides (LDH) as found for pyroaurite (PDF 25-0521) and hydrotalcite (PDF 41-1428). After 1 h of milling the intensive lines of LDH disappeared and broad peaks of a new phase were registered. Their positions and intensities suggest the formation of a spinel phase with cubic structure. After 3 h of milling, the diffraction lines of the spinel phase (CoFe 2 O 4 or Co 2 FeO 4 ) are well defined. The average crystallites size (D), the degree of microstrain (e) and the lattice parameter (a) of the studied cobalt ferrites were determined from the experimental XRD profiles (Table 1) by using the Williamson-Hall equation [27]:

ˇcos  = 0.9 D + 4ε sin  (1)
where ˇis the full width at half maximum (FWHM) of the XRD peaks,  is the Bragg angle, is the X-ray wavelength, D is the crystallite size, and ε is the value of internal strain. By plotting the value of ˇcos  as a function of 4sin  the microstrain ε may be estimated from the slope of the curve, whereas the crystallite size D can be obtained from the intersection with the vertical axis. A well-defined effect of the crystal size increase and decrease in the crystal defects and lattice parameter with milling time is observed with the mechanochemically prepared materials.

Typical morphologies of the synthesized Co x Fe 3-x O 4 (x = 1, 2) particles visualized by TEM show that in all cases the prepared particles are nanosized, nearly spherical in shape and tend to agglomerate. As an example transmission electron micrographs of Co2FeTS and CoFe2MS3 are presented in Fig. 2. The average values of particles diameter estimated from the TEM images are in good agreement with those calculated from XRD results.

The bands characteristic of carbonate groups vibrations (1480, 1345, 1100, 840 cm -1 ) and those due to M-O stretching mode (350 cm -1 ) and M-OH vibrations (475, 710 cm -1 ) appear in the IR spectra (not shown) of the hydroxide carbonate precursors. In the IR spectra of the samples obtained after 1 h of mechanical treatment the bands of the carbonate vibrations are less intense and two bands characteristic of spinel phases appear. The IR spectra of samples mechanically treated for 3 h and the thermally obtained samples present only bands typical of a spinel phase (Fig. 3). The higher frequency band l is due to AOB 3 , and that of the lower frequency ( 2 ) arises from the BOB 2 vibrations in the spinel lattice, where A and B denote cations in tetrahedral and octahedral sites, respectively, of the spinel structure [28]. For Co 2 FeO 4 the TPR profiles of mechanochemically and thermally obtained materials are presented in Fig. 4 and show mainly one reduction peak with shape and maximum position depending on the iron/cobalt ratio. A broadening is observed for the iron-rich samples and a shift of the peak maxima towards higher temperatures compared to the cobalt-rich ones. For the studied iron-cobalt mixed oxides the elementary steps of Fe 3+ , Co 3+ , and Co 2+ reduction cannot be distinguished from the TPR curves, thus leading to the suggestion that the more easily reducible cations (in this case Co 3+ and Co 2+ ) promote the reduction of iron cations. CoFe2MS3 is reduced in the range of 400-750 K with peak maximum at 650 K and a shoulder at ca. 700 K. It is clearly seen that these two reduction peaks are shifted to a higher temperature with the thermally pre- pared sample. This more difficult reduction can be due to increased crystallinity of the sample obtained by thermal treatment in comparison with the mechanochemically synthesised one. In the case of Co 2 FeO 4 , mainly one reduction peak is observed that can be due to a larger amount of cobalt present in the samples. As for CoFe 2 O 4 , the peak of the thermally synthesised sample is shifted to higher temperatures and the reason is probably a higher crystallinity.

Mössbauer spectroscopy was applied to gain information about the cationic occupations and/or different state distribution of iron ions in the studied ferrite materials. Fig. 5 shows RT Mössbauer spectra of samples taken from different steps of the processing route. The corresponding parameters determined from simulations of the spectra are listed in Table 2. The co-precipitated precursors exhibit a quadrupole doublet with IS = 0.34 mm/s, QS = 0.65-0.70 mm/s, indicating that the hydroxide carbonates are paramagnetic. As shown in Fig. 5b the spectra of Co 2 FeO 4 appear always as doublets. The spectra of CoFe2MS1 and CoFe2MS3 are doublets, while the spectrum of CoFe2TS contains only sextet components (Fig. 5a). It should be noted that reasonable data fitting of the Mössbauer spectrum of CoFe2TS exhibiting magnetic splitting at RT could be obtained only when the B-site pattern is assumed to be a superposition of more than one sextet. In our case the hyperfine interaction of the B site could be fitted up to the four overlapping six-line pattern (belonging to high spin Fe 3+ ions with 0-3 Co nearest neighbours), which is in agreement with observations of other authors for ferrite samples [12,32]. The experimentally calculated intensity ratio of the B-site peaks is different from that calculated using a binomial distribution, proposed by Sawatzky et al. [33] and de Bakker et al. [34]. The reason for the observed difference is the method of preparation, which does not ensure a statistical equilibrium distribution of the metal ions in the ferrite sample. Additionally the size effect of small particles could increase the relative spectral area of the components having smaller H eff . The results obtained after simulation of the Mössbauer spectrum indicate that the spinel structure of the thermally obtained compound is not completely inverse (of the order of 10% Co on A sites).

Taking into account the XRD data we suppose that the doublets observed in all milled samples and Co2FeTS arise from Fe(III) ions in ultrafine ferrite particles exhibiting superparamagnetic behaviour [35]. To confirm the superparamagnetic behaviour magnetic measurements were carried out.

Room temperature isothermal magnetizations of the mechanochemically and thermally prepared phases are shown in Figs. 6 and7, respectively. The magnetizations of all samples obtained after mechanochemical treatment are weak (a few emu/g). The small value of the measured magnetization could be related to strong magnetic anisotropy and possible local canting of magnetic ions due to the imperfect structure, and mainly to the surface effect of nanosize particles of large surface area. The absence of saturation in the magnetic field range explored, the "S" shape of the curves together with the lack of coercivity indicate the presence of small magnetic particles exhibiting superparamagnetic behaviour [36]. This particle size effect is in good agreement with the room temperature Mössbauer spectrometry where only doublet components are observed. The RT magnetization of CoFe2TS is 63.2 emu/g and the coercive field -1.5 kOe. Cobalt ferrite bulk material (CoFe 2 O 4 ) is known to be a ferrimagnetic material with very high cubic magnetocrystalline anisotropy leading to high theoretical coercivity: 25.2 and 5.4 kOe at 5 and 300 K, respectively, and a saturation magnetization of 93.9 and 80.8 emu/g at 5 and 300 K, accordingly [37,38]. However, the values of the magnetic properties of CoFe2TS are lower than the values of pure crystalline cobalt ferrite indicating that either the objects formed are core/shell particles with spin-glass-like surface layer [39] or that some of the ultrafine particles with superparamagnetic behaviour remain intact. Our values are in accordance with results obtained with nanocrystalline CoFe 2 O 4 of similar grain size [40]. Concerning Co 2 FeO 4 , it is important to note that saturation of the magnetization is never reached (in the magnetic field range used (5 T)). The absence of coercivity, remanence, and saturation at 295 K (Figs. 6b and7b) can be explained by size effects and suggests that this material behaves as a superparamagnetic at room temperature. Magnetization measurements as a function of applied temperature have been performed and as an example, the FC-ZFC curves of CoFe2MS3 and Co2FeMS1 are presented in Fig. 8. The blocking temperature (TB) associated with the maximum in the zero field cooling magnetization curve increases with the milling time and the iron content (170 and 210 K for CoFe2MS1 and CoFe2MS3; 75 and 110 K for Co2FeMS1 and Co2FeMS3, respectively). TB can be associated with an average size of the particles. The increase of the particle size is confirmed by the increase of the blocking temperature with mechanochemical treatment time and is in accordance with the XRD, TEM, and Mössbauer spectroscopy results. The point at which ZFC-FC starts to diverge is usually associated with the blocking temperature of the larger particles. The difference between these two temperatures reflects the distribution in the particle size. Thus, the cobalt rich sample exhibits a wide particles size distribution with the larger particles magnetically blocked at room temperature.

Conclusions

High-energy ball milling of layered cobalt-iron hydroxide carbonates results in the formation of nanocrystalline cobalt ferrites, where the particle size is below 10 nm and can be controlled by the treatment time. The measurement techniques employed indicate that the crystal size increases while the number of defects decreases with treatment time. The magnetic and reduction properties of the obtained materials are affected by the synthesis conditions, the iron/cobalt ratio, and the particles size. This work shows that nanosized CoFe 2 O 4 and Co 2 FeO 4 particles can be synthesized by combining the co-precipitation method with subsequent highenergy ball milling. This is a promising technique for a relatively large-scale preparation of cobalt ferrite nanoparticles with tailored properties.

Fig. 1 .

 1 Fig. 1. Powder X-ray diffraction patterns of CoFe2O4 (a) and Co2FeO4 (b) precursors after different milling times and thermal treatment.

Fig. 2 .

 2 Fig. 2. Transmission electron micrograph of Co2FeTS (A-C) and CoFe2MS3 (D).

Fig. 3 .

 3 Fig. 3. IR spectra of mechanochemically and thermally obtained cobalt ferrites.

Fig. 4 .

 4 Fig. 4. TPR of mechanochemically and thermally obtained cobalt ferrites.

Fig. 5 .

 5 Fig. 5. Mössbauer spectra of CoFe2O4 (a) and Co2FeO4 (b) precursors after different milling times.

Fig. 6 .

 6 Fig. 6. Isothermal magnetizations of CoFe2O4 (a) and Co2FeO4 (b) after different milling times.

Fig. 7 .

 7 Fig. 7. Isothermal magnetizations of thermally synthesized CoFe2O4 (a) and Co2FeO4 (b).

Fig. 8 .

 8 Fig. 8. ZFC-FC measurements of CoFe2MS3 (a) and Co2FeMS1 (b).

Table 1

 1 Crystallite size (D), microstrain (e) and lattice parameter (a) of investigated samples.

	Sample	D (nm)	e × 10 3 (a.u.)	a (Å)
	CoFe2MS1	3.4	8.97	8.42
	CoFe2MS3	8.6	7.83	8.40
	CoFe2TS	18.9	1.80	8.37
	Co2FeMS1	2.7	11.50	8.39
	Co2FeMS3	3.8	8.08	8.29
	Co2FeTS	10.5	7.20	8.26

Table 2

 2 Mössbauer parameters of samples after different milling times.

	Sample	Compounds	IS (mm/s)	QS (mm/s)	H eff (T)	FWHM (mm/s)	G (%)
	CoFe2MS1	Fe 3+ , CoFe2O4	0.34	0.67	-	0.48	100
	CoFe2MS3	Fe 3+ , CoFe2O4	0.34	0.65	-	0.57	100
	CoFe2TS	Sx 1 -Fe 3+ , CoFe2O4	0.28	0.00	49.0	0.41	44
		Sx 2 -Fe 3+ , CoFe2O4	0.37	0.00	52.4	0.42	18
		Sx 3 -Fe 3+ , CoFe2O4	0.37	0.00	50.7	0.42	16
		Sx 4 -Fe 3+ , CoFe2O4	0.37	0.00	46.7	0.42	15
		Sx 5 -Fe 3+ , CoFe2O4	0.37	0.00	43.2	0.42	8
	Co2FeMS1	Fe 3+ , Co2FeO4	0.34	0.70	-	0.49	100
	Co2FeMS3	Fe 3+ , Co2FeO4	0.33	0.66	-	0.47	100
	Co2FeTS	Fe 3+ , Co2FeO4	0.32	0.73	-	0.47	100

IS: isomer shift relative to metallic ␣-Fe at RT; QS: quadrupole splitting for doublets or quadrupole shift for sextets; H eff : effective magnetic field.

Acknowledgements

The authors thank the National Science Fund of Bulgaria for financial support through Projects X-1504/05 and Rila4-412 (DO 02-29/2008).


G Perchet

G Merlina

J.-C Revel

M Hafidi

C Richard

E Pinelli

Evaluation of a TiO 2 photocatalysis treatment on nitrophenols and nitramines contaminated plant wastewaters by solid-phase extraction coupled with ESI HPLC-MS

Keywords: Dinoseb Dinoterb Hexahydro-1, 3, 5-trinitro-1, 3, 5-triazine Octahydro-1, 3, 5, 7-tetranitro-1, 3

5,7tetrazocine UV/TiO2 a b s t r a c t Nitration reactions of aromatic compounds are commonly involved in different industrial processes for pharmaceutical, pesticide or military uses. For many years, most of the manufacturing sites used lagooning systems to treat their process effluents. In view of a photocatalytic degradation assay, the wastewater of a lagoon was investigated by using HPLC coupled with mass spectrometry. The wastewater was highly concentrated in RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) and two herbicides Dinoterb (2-tert-butyl-4,6-dinitrophenol) and Dinoseb (2-sec-butyl-4,6-dinitrophenol). First of all, an analytical method using solid-phase extraction (SPE) combined with HPLC ESI MS/MS was put in work for identification and titration of RDX, HMX and the two dinitrophenols in a complex natural matrix. Then, the UV/TiO 2 treatment was investigated for pollutants removal. Dinitrophenolic compounds were significantly degraded after a 8-h-exposition of the wastewater/TiO 2 suspension, whereas RDX and HMX were poorly affected.

Introduction

For several decades, a wide range of nitro-aromatic and nitramine molecules were processed on the same manufacturing site for pyrotechnic synthesis (e.g. RDX (hexahydro-1, 3,5-trinitro-1,3,5-triazine), HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), TNT (trinitrotoluene), picric acid or for pesticide synthesis (e.g. nitrophenols such as Dinoseb (2-sec-butyl-4,6-dinitrophenol), Dinoterb (2-tert-butyl-4,6-dinitrophenol)). Wastewater from these plants is often contaminated with complex mixtures of these different xenobiotics. Considering the persistence of nitro-aromatic compounds, this wastewater is today a source of contamination for soils, groundwater and surface water [1]. In addition, most of the derivatives are known to be toxic. RDX and its metabolites are currently reported to damage soil and water organisms [2][3][4]. Robidoux et al. have described significant effects of HMX on the growth and reproduction of the earthworm Abbreviations:

HMX, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine; RDX, hexahydro-1,3,5-trinitro-1,3,5-triazine; TNT, tri-nitro-toluene; ESI, electospray ionization; LOD, limit of detection; LOQ, limit of quantification; NL, noise level; SPE, solid-phase extraction; TIC, total ionic current.

* Corresponding author. Tel.: +33 562193945.

E-mail address: pinelli@ensat.fr (E. Pinelli).

(Eisenia andrei). The Dinoseb 96 h LC 50 value for Chinook salmon (Salmo salar) alevins has been established at 70.6 ppb and 87% mortality for Chinook salmon alevins was observed after a 96-hexposure to 100 ppb [5]. Dinoterb is reported to have hepatotoxic effects between 10 and 100 mol L -1 after 2 h of incubation on rat hepatocytes and becomes much more toxic for a 24-h exposure. Dinoterb was withdrawn from sale in April 1998 by the European Authorities (decision 98/269/CE) and nitrophenols are registered as priority pollutants (HR-3 grade) by the US EPA (US EPA, 1976).

To overcome these environmental problems, several biochemical techniques (ultrafiltration, coagulation, evaporation, etc.) have been tried [6,7] but owing to their high cost, few have ever been applied on an industrial scale. Biodegradation processes are reported to be successful on wastewater sludges [8,[START_REF] Ruggieri | Biodegradation of animal fats in a cocomposting process with wastewater sludge[END_REF]. Previous study showed that consumption of Dinoterb was obtained after 5 days in presence of Pseudomonas citronellolis and glucose [10]. But, the time necessary to degrade all nitrophenolic compounds contained in the sludge was of 10 weeks.

In recent years, several works have been focused on the destruction of these synthetic wastes using different oxidation technologies such as chemical oxidation [11], advanced oxidation processes (AOP) [12] catalytic wet air oxidation (CWAO) (Oliviero et al., 2003) or electrochemical oxidation [13]. Photocatalytic processes [START_REF]Photocatalytic Purification and Treatment of Water and Air[END_REF] may be an alternative method to allow improving of degradation duration. Heterogeneous photocatalysis, involving photoinduced redox reactions at the surface of semiconductor minerals, is a promising technique for the treatment of water contaminated by organic molecules [START_REF] Tariq | [END_REF][16][17]. Among the semiconductors available, titanium dioxide, or titania, is the most widely used in due to its outstanding stability [18]. Previous studies showed that dinitrobenzene (DNB) could be degraded with photocatalysis by UV/TiO 2 treatment [19]. In these conditions, pH 7 was shown to lead to the most efficient degradation of DNB. At concentration of 50 mg/L, 4 h were necessary to degrade the totality of DNB. Most of studies were realized on pure compounds, and few on mixed compounds coming from industrial wastewater.

The goal of the present study is to degrade nitrated compounds contained in wastewater by using a photocatalytic treatment. Firstly, an assessment of compounds present in the wastewater was realized at the start of the treatment. Thus, a simple and selective method let to analyze two nitramine compounds (RDX and HMX) and two dinitrophenolic herbicides (Dinoterb and Dinoseb) in wastewater contained in an old industrial lagoon. After SPE of the wastewater, the compounds were investigated by a multi-residue approach using ESI-LC/MS in the negative mode and taking into account the same molecular weight and the proximal physicochemical characteristics of Dinoseb and Dinoterb by the MS/MS mode. This approach comes in response to the need for sensitive and reliable analytical methods for the simultaneous assay of organic compounds from several families and the need of new treatments able to degrade hazardous nitrated compounds. Finally, this paper reports the photocatalytic degradation of nitrophenols in a multiresidue wastewater containing several herbicides and explosives.

Methods

Samples

Our contaminated material, wastewater from an old decantation lagoon which received effluent from a military manufacturing plant for about 100 years, contained at least four nitrified cyclic compounds (RDX, HMX, Dinoseb (2-sec-butyl-4,6-dinitrophenol)

and Dinoterb (2-tert-butyl-4,6-dinitrophenol). The samples were stored at 4 • C until extraction.

Chemicals

Standard Dinoseb and Dinoterb herbicides (Fig. 1A) were purchased from Cluzeau Info Labo (Paris, France). Standard explosives RDX and HMX (Fig. 1B) were obtained from Restek (Paris, France). Methanol, acetonitrile and HPLC grade water were obtained from Fisher Analytics (Illkirch, France). Helium was provided by air liquid (Labège, France). Nitrogen (purity > 99.9%) was provided by a nitrogen generator NM18LA (F-DBS, Courtaboeuf, France).

Extraction of explosives and herbicides

As the solubility of these organic compounds changes with pH, the wastewater pH was adjusted to 7.5. Solid-phase extraction (SPE) Porapak RDX cartridges (Waters, Massachusetts) were used to extract both explosive and herbicide compounds. SPE cartridges were conditioned prior to sample extraction with 5 mL acetonitrile. Acetonitrile was then flushed from the cartridges using 30 mL of reagent grade water. Care was taken to ensure that the cartridges were never allowed to dry after the initial cleaning. A 5-mL aliquot of sample was vacuum-siphoned at a flow rate of 10 mL/mn through the SPE cartridge. Studied compounds were then eluted from the SPE cartridge with 5 mL of acetonitrile since their solubility in this solvent is higher than in water. The eluate was diluted to 1/50 and transferred to an HPLC auto-sampler vial (Thermo Fisher) for analysis.

Analysis of explosives and herbicides

The concentrations of HMX, RDX, Dinoseb and Dinoterb were identified using a high-performance liquid chromatography-mass spectrometry (LC-MS) system (Surveyor Pump, Thermo; Surveyor Autosampler, Thermo). Ten microliters of the extract were injected into the HPLC system equipped with a reverse-phase column Mass spectrometry was performed with an ion trap spectrometer (LCQ Advantage Max, Thermo). The tune function was used to optimize essential parameters of the ESI-MS system. Other conditions, such as capillary temperature or nitrogen gas flow, were manually optimized. The final ESI-MS conditions are reported in Table 1. For all ESI-MS and ESI-MS/MS measurements, solutions of explosives were continuously infused by means of a syringe pump. Scan parameters were set from m/z 100 to 600 amu in full-scan investigations. A negative-ion ESI mass spectrum of Dinoseb and Dinoterb was acquired by flow injection into an eluent consisting of methanol:water (50:50), leading to a common molecular ion with an m/z value of 239 amu. The MS/MS mode was used to separate Dinoseb and Dinoterb. The collision energy for each ion transition was optimized to produce the highest intensity of the selected ion peak and common optimized relative collision energy of 42% was used for all MS/MS works.

The concentrations of potential explosives and herbicides were investigated using a high-performance liquid chromatography separation device coupled with detector-mass spectrometry (LC-MS) by comparing the areas under the curve obtained to those of the corresponding calibration sample. Concentrations of herbicides (Dinoseb and Dinoterb) and explosives (RDX and HMX) were quantified using the Excalibur data system from Thermo Fisher.

TiO 2 /UV treatment

The following protocol results from a previous study realized on olive mill wastewater photocatalytic successful treatment [20]. For this experiment, sludge was diluted to 1/100 and filtered through a 0.45-m glass fiber filter to avoid the presence of a particular phase and crystalline forms of the studied products. The photocatalyst was TiO 2 Degussa P25. It was added at a level of 1 g/L. The reactor had a working volume of 100 cm 3 . The reactor was composed of a water-cooled double-walled Pyrex glass tube. The reaction mixture was continuously homogenized by magnetic stirring. It was illuminated by a vertical UV tube TLAD Philips 415W/05 (300-450 nm with an emission maximum at 365 nm). During the reaction, the suspension was maintained under constant oxygen bubbling. The medium was mixed in the dark (TiO 2 only condition) during half an hour prior to irradiation. This time was sufficient to reveal chemical compounds as explosives or pesticides adsorption on the catalyst [21][22][23]. Samples were collected at t = 0 h before irradiation and at 0.5, 1, 2, 4, and 8 h during illumination. They were filtered using 0.45 m Millipore filters to eliminate the excess of TiO 2 prior to analysis.

Results and discussion

Assessment before the beginning of photochemical treatment

Solid-phase extraction carried out with a specially cleaned divinyl benzene-vinylpyrrolidone copolymer resin (Porapak RDX) was commonly used to detect nitramines and nitro-aromatic explosive compounds in water [24]. In this study, SPE was used to clean complex wastewater samples from the impurity and mineral salt load in contrast to the common use as a pre-concentration step for explosives [25]. The recovery percentages on extraction cartridges of a mix of Dinoseb, Dinoterb, RDX and HMX (0.5 g mL -1 per compound) were evaluated at 87 (±2%), 87 (±3%), 86 (±1%) and 90% (±3%), respectively.

Retention times, molecular weights, detected ions, limits of detection (LODs) and limits of quantification (LOQs) of standards using ESI-LC-MS are presented in Table 2. The use of a gradient elution method allowed us to distinguish both explosives from Dinoseb and Dinoterb. The criteria for estimating the LODs were the smallest amount injected that yielded a signal-to-noise ratio greater than 3 for the mass chromatogram based on the characteristic ions. LODs were measured for these compounds by a triplicate injection from a series of solutions in methanol. The concentrations used for LODs determination were 1, 2, 5, 10 and 50 g/L and those used for HPLC calibration were 1, 5, 10 and 25 g/L. The correlation coefficient varied in the range of 0.950-0.999 for all the investigated analytes.

As shown by Fig. 2, the total ion current (TIC) chromatogram obtained in the ESI negative mode (A) authorized the extraction of the mass spectra of each component of the explosive mix (B, C and D) showing the characteristic ions of each one. HMX is well identified by LC-MS in the negative mode since the [M+NO 2 -H] -cluster ion constitutes the most abundant ion (mass spectrum B). Another cluster ion is observed at m/z 355, and concerns the [M+NNO 2 -H] - structure. Like HMX, RDX forms the same couple of cluster ions [M+NO 2 -H] -and [M+NNO 2 -H] -(mass spectrum C). This case has been reported previously [26]. Fragmentation processes characterized by losses of CH 2 NNO 2 , NNO 2 , NO 2 and NNO are represented by the ions m/z = 226 and m/z = 197 under our experimental conditions for these molecules. This route predominates with TNT, characterized by a signal at 226 corresponding to the deprotonated parent ion [TNT-H] -, whereas m/z = 197 concerns the ion [TNT-NO] -resting form in accordance with other studies [26,27].

As shown in Fig. 3, a good separation between the herbicides was observed for Dinoseb and Dinoterb as seen by their common molecular ion m/z 239. In addition to their different retention times, MS/MS gives another proof of their respective identities (Fig. 4). The ESI-MS/MS spectrum showed two specific ions at m/z = 194 and m/z = 221 for Dinoseb, while Dinoterb had a single specific ion at m/z = 207. Studies of the MS/MS spectra of Dinoseb showed that the ion m/z = 194 in the MS/MS spectrum is formed through the cleavage in the neutral molecule CH 3 NO released from the side chain and the remaining nitrophenolate [28].

Table 2

Retention time, molecular weight (MW), detected ions, limits of detection (LODs) (g L -1 ) and method detection limits (LOQs) in mass spectrometry (g L -1 ) for the studied compounds.

Retention time

Analyte MW (g mol This study demonstrates that the use of an SPE method combined with HPLC ESI MS/MS is suited to the analysis of compounds presenting two different ring cycles (nitramine and nitrophenolic compounds) in a complex matrix leading to good separation. As shown in Fig. 5

Photocatalytic treatment

Several methods of treating TNT, or other nitrophenolic or nitramine compounds in contaminated water have been developed. It was shown that TNT photocatalytic degradation was more effective at neutral pH [21]. Another study showed that photocatalysis is an effective process for the degradation of TNT, RDX and HMX. These compounds could be completely degraded in 150 min with 1.0 g/L TiO 2 at pH 7 [29]. Thus, in our experimental conditions, pH was adjusted to 7 by hydrochloric acid addition before beginning the photocatalytic treatment. The initial pH was stable at 7.4 at start and during the dark period (30 min before illumination). It slightly decreased to 7.2 during the 8 h of UV illumination.

The development of the analytical method previously described in this study allowed us to monitor RDX, HMX, Dinoterb and Dinoseb concentrations during the photocatalytic treatment. Chromatograms of Dinoseb and Dinoterb during UV treatment are presented in Fig. 6. Dinoseb and Dinoterb are no longer detected after 8 h exposition. As shown by Fig. 7, after 8 h exposition, 98.1% of Dinoterb and 94.6% of Dinoseb were degraded. In these conditions, only 13.7% of RDX was degraded, whereas HMX was not degraded. In 8 h, nitramines were weakly affected by the photocatalytic treatment, showing that this treatment is less destructive for these compounds as for the phenolic ones. This could be associated with the absence of a marked pH variation corresponding to nitramines decomposition [22,30].

The irradiation of TiO 2 yields hydroxyl radicals which are very oxidant species able to react with a great variety of organic chemicals, alone or in mixture. Under this treatment, olive mill wastewater with UV/TiO 2 for 24 h led to the almost complete dis- appearance of the mixture of phenol compounds [20]. Since these hydroxyl radicals are produced at the surface of the photocatalyst, only chemicals approaching very close the photocatalyst surface can be degraded. The weak reactivity of RDX and the non-reactivity of HMX are due either to their chemical stability or to their incapacity to approach the catalyst surface under this experimental pH condition. The s-triazine derivative cyanuric acid which possesses one OH substituent on each of the C of the ring is reported to be resistant to photocatalytic treatment or other types of oxidation treatment involving hydroxyl radicals [31]. The structure of RDX and HMX is quite different from that of cyanuric acid because NO 2 substituents are beared by the cyclic N, and the ring is saturated, but they have in common a very symmetric structure. To improve their degradation, studies would be necessary to determine the adsorption capacities of RDX and HMX, alone or in the reactional mixture, in function of pH and presence of solvent as acetonitrile [30].

Conclusion

LC/MS/MS appears to be a powerful technique for the analysis of nitramine and nitro-aromatic compounds. This method enables the sensitive and selective assay of two nitramines (RDX and HMX) and two nitroaromatic compounds (Dinoseb and Dinoterb). Other nitro-aromatic compounds like TNT can also be detected even if they were not present in the wastewater studied. This method highlighted a successful photocatalysis TiO 2 treatment leading to 8 h degradation of nitrophenols, comparing to 78 days necessary in liquid anaerobic bioremediation assays. However, RDX and particularly HMX appeared more resistant to TiO 2 photocatalytic treatment.

Fig. 1 .

 1 Fig. 1. Molecular structure of the herbicides (Dinoseb and Dinoterb) and explosives (RDX and HMX) studied.

Fig. 2 .

 2 Fig. 2. Mass analysis of standard explosives. (A) Total ionic current for the explosive mix obtained in the ESI mode. (B) Mass spectrum for HMX. (C) Mass spectrum for RDX. (D) Mass spectrum for TNT.

  , Dinoseb and RDX have the same retention time. Quantification could be carried out by the specific ions of RDX (m/z = 267 and m/z = 281) and Dinoseb (m/z = 239), respectively in the total mass chromatogram. Under the chromatographic conditions described in our study the concentrations of HMX, RDX, Dinoseb and Dinoterb were 4.03, 25, 15.6 and 4.65 mg L -1 , respectively, in the wastewater sample at t = 0 (before the UV irradiation).

Fig. 3 .

 3 Fig. 3. Negative-ion ESI mass chromatograms obtained for the two dinitrophenols Dinoseb and Dinoterb.

Fig. 4 .

 4 Fig. 4. Negative ion ESI-MS/MS mass spectrum of dinitrophenols Dinoseb and Dinoterb.

Fig. 5 .

 5 Fig. 5. Negative ion ESI-MS total ionic current chromatogram (A) for the studied wastewater sample and specific ion chromatograms for Dinoseb and Dinoterb (B), for HMX (C) or RDX (D).

Fig. 6 .

 6 Fig. 6. Negative ion ESI-MS specific ion chromatograms for Dinoseb and Dinoterb during UV/TiO2 treatment.

Fig. 7 .

 7 Fig. 7. Time course HPLC analyses of sample filtrate wastewater samples after photochemical UV/TiO2 treatment.

Table 1

 1 Source parameters for LCQ ion trap mass spectrometer for electrospray ionization configuration.

	ESI source	Ion optics
	Mode: negative	Capillary temp ( • C): 280
	Ion spray voltage (kV): 4	Multiple 1 offset (V): 2.25
	Sheath gas flow rate (UA): 40	Tube lens offset (V): 0
	Aux/sweep gas flow rate (UA): 10	Multiple 2 offset (V): 5
	Capillary voltage (V): -6	Multiple RF Amp (Vp-p, sp): 400
	(Hypersil ODS C18, 3 , 4.6 mm I.D. × 150 mm, Thermo). The sol-
	vent systems used were a mixture of solvent A (methanol) and
	solvent B (HPLC grade water). Compounds were eluted at a flow
	rate of 0.8 mL min -1 under a gradient starting with 26% of solvent
	A, ending at 74% of solvent A in 18 mn.

Acknowledgements

This work was supported by grant no. 03-79-144 from the Direction Générale de l'Armement (DGA, France). We would like to thank Mr. Pechoux and Mr. Mericam-Bourdet from DGA. We also thank Mr. Spyckerelle (EURENCO, France) for his welcome advice on safety.