Edith Archambault

Non-governmental organizations and foundations

Keywords: foundations, government partnership, public services, INGOs

Defining non governmental organizations (NGOs) and foundations, the entry outlines a global picture of the non-profit sector, emphasising the activities privileged in each major region through the analysis of the actions of NGOs and foundations. Then it examines various experiences and trajectories of these organizations' development at the local and national level. Finally it critically reviews actions of international NGOs and foundations that operate at inter-country or global level. It concludes on the importance of a real partnership between public authorities, NGOs and foundations to cope with the 21th century's main societal and global risks.

INTRODUCTION

In many developed and developing countries, non-governmental organizations (NGOs) and foundations interact with national and local governments to provide public services or to incite public authorities to support their causes in order to improve the capacity of both governments to respond to societal challenges. Some of them deliver services or advocate cross-country or at the international level.

The term 'non-governmental organization' has several meanings. It is often used as an equivalent of an association (see Entry "Associations"). The United Nations (UN) proposes a narrower understanding of this concept; according to its 1945 Charter, article 71, an NGO must have the following characteristics as conditions of recognition by the UN:

• Not profit-making or not distributing its eventual surplus • Independent from the direct control of government at any level, though often receiving public funding • Oriented to public good, not member interest In addition, in most countries NGOs have democratic governance based on the 'one member, one vote' principle. They are not constituted as political parties and their action is nonviolent; they are not criminal groups such as mafias (Willets 2006). The narrowest understanding of the term NGO is limited to organizations operating in several countries. In the following, the intermediary term will be used while international NGOs will be called INGOs.

A foundation (or a charitable trust in Anglo-Saxon countries) is an organization that devotes private funds to a public purpose of its choice. It is a non-membership-based organization, Therefore its governance is either the founder itself or a co-opted board rather than the democratic governance of NGOs. The other criteria are shared with NGOs: foundations are nonprofit, private, independent, and public good-oriented entities. Sometimes foundations are included in the broadest understanding of NGOs (Anheier 2001).

NGOs and foundations are part of the social economy because they undertake an economic activity even when run solely by volunteers, which is the situation in most developing countries and in many developed countries' nonprofit sectors. Most of them deliver welfare services, while advocacy NGOs provide information services for their members or the public at large. Operating foundations are producers of welfare services, while grant-making foundations change income or wealth distribution (Archambault, 1986). This entry outlines a global picture of the nonprofit sector, highlighting the privileged activities in each important region through the analysis of NGOs and foundations. It also examines the different experiences of these organizations at local and national level. The entry critically reviews the activities of international NGOs and foundations operating at inter-country or global level with a focus on the challenges, limitations and opportunities. It concludes with the importance of a real partnership between authorities, NGOs and foundations to address the key societal and global problems of the twenty-first century.

PANORAMA OF ASSOCIATIONS AND FOUNDATIONS WORLDWIDE

As said above, NGOs have several definitions which is why empirical data, if any, deal with associations, the legal status of most NGOs. Statistics for foundations are better in the USA and European countries. The most comprehensive data on the nonprofit sectors of 41 countries, the population of which is more than half of the world's population, are the following :

• On average, the full-time paid or volunteer employment in the nonprofit sector is 5.7% of the active population of these countries -the same percentage as the construction industry. The volunteer share of this workforce is 41%. For example, there are 3,456,000 regular voluntary workers in charities in England and Wales; a further 944,000 trustees can be added to this estimate. NGOs and foundations exist everywhere in the world. Their density is often seen as an indicator of democracy. However, even in dictatorial or authoritarian countries, NGOs are tolerated provided they do not engage in anti-establishment politics. In these countries, the public authorities closely control the creation and decisions of many NGOs which therefore cease to be independent. These organizations are often called Governmental NGOs (GNGOs) and they are often instruments of the social control of citizens, such as in China.

In many countries, NGOs and foundations deliver public services in addition to central or local governments. These various experiences and trajectories are discussed in the following section.

NGOs AND FOUNDATIONS PROVIDING PUBLIC SERVICES AT THE LOCAL AND NATIONAL LEVEL

Historically, NGOs have been pioneers of public services and they continue to play this role in many developing countries today. In these countries, local, national, or foreign NGOs provide education, health, and social services as well as utilities. For example, in South Morocco, development NGOs have installed the electricity connection of many villages and modernized the traditional irrigation network. Service-delivery NGOs provide public goods and services which governments of developing countries are unable to provide due to a lack of resources. They may be contractors or collaborate with government agencies to reduce the cost of public goods. In Continental Europe, services towards people with disabilities were often created by NGOs, and governments developed partnerships with these NGOs after both world wars. According to this partnership, NGOs deliver residential or day-care services or work integration to people with physical or mental disabilities, while the local or central governments pay the bulk of their expense through grants or contracts. The relative penetration of the nonprofit sector into education and health systems follows from the historical relations between the government and the Catholic Church, which has been a pioneer in these domains since the Middle Ages. Consequently, there exists a high penetration of NGOs in Belgium and Netherlands where these relations were peaceful and a lower one in France where government/Church relations have been crossed by recurring conflicts especially in education; however, these conflicts are now appeased.

Currently the partnership between NGOs, operating foundations, and government is not confined to continental Europe; it is beginning in many developing countries as well. Even in the United States, the country that champions liberalism, private non-profit hospitals are publicly funded partly through the Medicaid and Medicare programs and the Ivy League universities, most of which are foundations, and partly from public research contracts. Where the welfare partnership is ancient and equal between both partners, a co-construction of social policies is possible where NGOs bring their knowledge of the population concerned and of the best actions to privilege, while the government brings its ability to obtain money through taxes and its attention to the continuity of the public service and an equal access of every concerned citizen (Brandsen et al. 2018). The advantages and disadvantages of sharing welfare services between NGOS and public authorities are different locally and nationally.

Nonprofit organizations delivering welfare services at the local level

Locally, a good relationship between NGOs and public authorities is more likely because each partner has a good knowledge of the other, namely the results of its action, its efficiency, and financial situation. That is why decentralized countries are more likely to develop local welfare partnerships. However, an example of the advantages of such a partnership is given in a centralized country in Box 1.

Box 1: Chartreuse de Neuville, a win-win partnership in France Chartreuse de Neuville is an association devoted to heritage preservation which is currently restoring a fourteenth-century deserted and dilapidated monastery. It partnered with the regional and local authorities to rebuild these beautiful ruins. The association created a Work Integration Social Enterprise (WISE) to train young unemployed people with no skills in heritage restoration, allowing it to receive public funding. Artists have residencies in the rebuilt parts of the monastery and shows are regularly put on for the public. In addition, some parts of the monastery are rented for professional and family events. Chartreuse de Neuville is a real attraction for tourism in the Northern area, a disadvantaged French region. Source: (La Chartreuse de Neuville Various years; Le Labo de l'économie sociale et solidaire n.d.; Interview with the CEO for EU-Russia Report 2020 in 2020)

Disadvantages of a local partnership also exist: proximity can induce conflicts of interests in public procurement and also between NGOs or operating foundations and their suppliers.

Local governments can seek votes of the employees, volunteers, and beneficiaries of these organizations for its re-election; it could also condition its financing by control of the organizations' boards by its representatives. When the political colour of the local government changes, an abrupt cessation of public funding may occur, but this is more likely for advocacy NGOs than for those who manage a public service by delegation.

Nonprofit organizations delivering welfare services at the national level

NGOs and operating foundations provide educational, health, and social services nationally as well as locally. The sharing of these welfare services provisions between central and local governments and civil society organizations obviously depends on functions that have been devolved to local governments as well as on the level of externalization or subcontracting which authorities consider to be relevant. This sharing is variable across countries,and depends on the advantages and disadvantages for central governments to outsource to civil society organizations (CSOs). The benefits are the reduction of the cost of public services (as salaries are lower in NGOs, or nonexistent for volunteers), and the reduction of public employment, considered too high in Nordic countries and France. CSOs also have a better knowledge of minority populations. Lastly, the involvement of a large part of the population is required in a crisis situation such as the Covid-19 pandemic. Another example of civil society involvement is given in Box 2.

Box 2: The German partnership for 2015 migrants inclusion

In the years 2015-2018, Germany welcomed about one million Syrian and Afghan refugees. 'Wir schaffen das', said Angela Merkel, and five years later Germany did it: half of the migrants had jobs and most of them spoke German well or just about well. The distribution of refugees among the Länder was as a proportion of their population but also of their per capita income, with the richest Länder receiving more refugees than the poorest. Civil society's involvement was strong. In addition to the Catholic and Protestant parishes and existing NGOs providing social services, 50,000 new NGOs were created by volunteers to shelter refugees, distribute clothes, introduce the German language, or help with administrative procedures. In the Fall 2021 election, the migrant issue was no longer a major one in the debates among the leaders of the diverse political parties. Source: (L'Express Various years; Le Point. Various years; Interviews with anonymous German interviewees)

But the drawbacks of delegating public services to NGOs or foundations are also obvious: amateurism and particularism are common critiques of NGOs; operating foundations may be accused of deepening social inequalities, as is the case in education for expensive American universities or private schools in many developing and developed countries; foundations running hospitals may provide better quality and quicker healthcare for the richest while public hospitals welcome the rest of the population. Of course social services for the most disadvantaged are not the subject of this criticism, but they are accused of keeping the poor in poverty. This ambiguity of NGOs and foundations seems to be reinforced when they operate internationally.

The ambiguous role of NGOs and foundations at the global level

International NGOs (INGOs) or foundations have the source of their funding and their programs in different countries. Foundations and INGOs have a positive impact on developing countries and the alleviation of extreme poverty in the world, but they are also the subject of significant criticism. These praises and criticisms can be illustrated by the Bill and Melinda Gates Foundation, which alone represents half of private philanthropy. In the field of health, the BMGF, in cooperation with the WHO and the GAVI Alliance, nearly eradicated poliomyelitis, cured VIH holders, and gave access to vaccines to 55 million children and family planning in many African and Asian countries. In the field of agriculture, this foundation was an efficient partner of the 'green revolutions' in India and some African countries. However, critics outline that the assets of the BMGF ($50 billion) are higher than the GDP of many poor countries, and that its endowment is frequently invested in companies whose impact is inconsistent with its objectives, because they are harmful to health and the environment. Conflicts of interests exist also with vaccine producers and Monsanto in Indian and African green revolutions.

More generally, some critics consider INGOs and foundations as part of a neo-colonial system to maintain the domination of northern countries over the less-developed southern countries. They play the role that nineteenth-century missionaries played as Trojan horses of the predatory northern companies. Less radical critics outline that international nonprofit organizations have a tendency to impose their system of values on countries with other traditions. In another context, the American foundations arrived in the countries of Eastern Europe as soon as the fall of the Berlin Wall; these foundations influenced eastern countries to adopt health and education systems more similar to the American than the European ones.

The Trust for Civil Society in Central and Eastern Europe contributed also to the emergence of NGOs and foundations in post-Communist countries

CONCLUSION

The European Union, where the partnership between government and the nonprofit sector is a dominant pattern, is in the best cases able to combine the strengths of both partners. On the government side, we can note a high legitimacy to decide, due to periodic elections, the weight of resources raised by tax, the ability to create rights to benefit public services and to verify the qualifications of employees delivering these services, and finally the concern to maintain equality between individuals and territories. On the nonprofit side, the strengths of NGOs and foundations are their roles in the detection and response to new needs in the field of health, research, education, culture, and social services, because they are nearer to the beneficiaries and more attentive to the diversity of populations. They are therefore quicker than the government to respond to emergency issues such as earthquakes, floods, or other disasters. Having to please neither their shareholders as corporations nor voters as central or local governments, NGOs and foundations can experiment with new and unpopular fields, for example alternatives to jail. They can also personalize public services through volunteer accompaniment and avoid the effects of silos of public social policies.

At the global level, the partnership with northern NGOs or foundations is also possible when the government of the host country is reliable and not corrupt. These organizations consider more and more that they have to cooperate with the local NGOs to avoid the above noted criticisms of neo-colonialism and ignorance of the history, religions, and culture of the developing countries.

Finally, the fight against the great challenges of the twenty-first century, such as poverty of people and countries, unemployment, the marginalization of part of the youth, the ageing of society, climate change and environmental degradation, racism, xenophobia, and all other forms of discrimination, presupposes collaboration between the government and the specialized SSE organizations. Reducing social and cultural inequalities for more inclusive growth and coping with the migrations of large populations also most often requires effective partnership between public authorities and NGOs and foundations.

Bibliography

Box 3: La Main à la Pâte, a French foundation that is becoming international

  The most common ones link a developed country with one or several developing or underdeveloped countries; in this case they are tools of private help to development. The best example of such organizations is the Bill and Melinda Gates Foundation (BMGF), the largest foundation in the world devoted mainly to health and agricultural development. Some INGOs are committed to reinforcing links between two or more northern and southern countries to advocate for specific causes or interests. An example is given in Box 3. During the early 2000s in France, it was clearly apparent that science education in primary and middle schools was not up to what the twenty-first century demanded. Initiated by the Nobel laureate Georges Charpak, La main à la pâte recommends that teachers implement an inquiry-based method with pupils and provides training to teachers in this active pedagogy. Over the last decade, it has spread internationally. Today, La main à la pâte is cooperating with more than 40 countries and 3 regional networks (European Union, Southeast Asia, and Latin America) and its establishment in Africa has begun. It receives grants from interested governments and large European and American foundations, such as the Siemens foundation and the Smithsonian Institute. Source:[START_REF] Charpak | L'Enfant et La Science: L'aventure de La Main à La Pâte[END_REF]; Fondation La main à la pâte n.d.; interview with Pierre Léna, the main founder of EU-Russia Report in 2020) Some INGOs are confirmed by the UN for consultation, and the European Union (EU) has similarly confirmed NGOs. According to articles 70 and 71 of the UN 1945 Charter, 'specialized agencies, established by intergovernmental agreement', such as the Food and Agriculture Organization (FAO) or the World Health Organization (WHO), could 'participate without a vote in UN deliberations', while a confirmed NGO could have 'suitable arrangements for consultation'.(United Nations, 1945) Therefore, there is a hierarchy between these two types of organizations, because deliberation is nearer to ultimate decisions than is consultation Confirmed INGOs and some other organizations can be used by northern countries or international organizations as transfer channels of public aid to development when the governments of those countries are considered corrupt or are not internationally recognized, such as the Taliban government in Afghanistan. For example, the European Community Humanitarian Office program (ECHO) is mainly oriented towards INGOs. Conversely, some large foundations, mainly BMGF, can also fund international public organizations such as FAO, WHO, and UNICEF for specific projects. Of course public aid to development, $152 billion, is more than twenty times private aid to development.INGOs or foundations are either operating or campaigning. The operating ones affect smallscale change, achieved through development projects such as affording sanitary water to the population of a village or building and running a hospital for a larger area. Service-delivery INGOs provide public goods and services which governments of developing countries are unable to provide due to their lack of resources. Campaigning INGOs aim to enact broader change through influence on the political system. Once INGOs do decide to influence public policy, they organize, in broad coalitions, specifically for this purpose. These umbrellas, networks, or caucuses are numerous to advocate for development, humanitarian, or environmental issues. An example is the World Social Forum, a rival convention of the World Economic Forum, an annual meeting focused on capitalist enterprise interests held in Davos, Switzerland. Similarly, World Social Forums, gathering hundreds of INGOs, have parallel meetings every year to influence the Conferences of the Parties (COP) where representatives of the governments of most countries discuss climate change and other environmental challenges.

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E F Mohamed

C Andriantsiferana

A M Wilhelm

H Delmas

Competitive adsorption of phenolic compounds from aqueous solution using sludge-based activated carbon

Keywords: water treatment, sludge, activated carbons, multicomponent adsorption, substituted phenols

Preparation of activated carbon from sewage sludge is a promising approach to produce cheap and efficient adsorbent for pollutants removal as well as to dispose of sewage sludge. The first objective of this study was to investigate the physical and chemical properties (BET surface area, ash and elemental content, surface functional groups by Boehm titration and weight loss by thermogravimetric analysis) of the sludge-based activated carbon (SBAC) so as to give a basic understanding of its structure and to compare to those of two commercial activated carbons, PICA S23 and F22. The second and main objective was to evaluate the performance of SBAC for single and competitive adsorption of four substituted phenols ( p -nitrophenol, p -chlorophenol, p -hydroxy benzoic acid and phenol) from their aqueous solutions. The results indicated that, despite moderate micropore and mesopore surface areas, SBAC had remarkable adsorption capacity for phenols, though less than PICA carbons. Uptake of the phenolic compound was found to be dependent on both the porosity and surface chemistry of the carbons. Furthermore, the electronegativity and the hydrophobicity of the adsorbate have significant influence on the adsorption capacity. The Langmuir and Freundlich models were used for the mathematical description of the adsorption equilibrium for single-solute isotherms. Moreover, the Langmuir-Freundlich model gave satisfactory results for describing multicomponent system isotherms. The capacity of the studied activated carbons to adsorb phenols from a multi-solute system was in the following order: p -nitrophenol > p -chlorophenol > PHBA > phenol.

Introduction

Sewage sludge is being generated in ever increasing amounts due to the rapid urbanization and higher effluent criteria implemented in recent decades. Without proper treatment and disposal, it causes a secondary pollution problem in the environment.

Sewage sludge is carbonaceous in nature and rich in organic materials. Hence, it has the potential to be converted into different kinds of activated carbon if pyrolysed under controlled conditions or with some chemical treatment. This conversion could offer the combined benefits of reducing the volume of sludge and producing a valuable adsorbent with lower cost than commercial activated carbons. Several investigations have shown the feasibility of the conversion [START_REF] Rio | Structure characterization and adsorption properties of pyrolysed sewage sludge[END_REF][START_REF] Rio | Experimental design methodology for the preparation of carbonaceous sorbents from sewage sludge by chemical activation: application to air and water treatments[END_REF][START_REF] Rio | Production of porous carbonaceous adsorbent from physical activation of sewage sludge: application to wastewater treatment[END_REF][START_REF] Liu | Characterization of mesoporous activated carbons prepared by pyrolysis of sewage sludge with pyrolusite[END_REF][START_REF] Gutiérrez-Segura | Comparison of Cd-Pb adsorption on commercial activated carbon and carbonaceous material from pyrolysed sewage sludge in column system[END_REF]. Activated carbons have been widely used as adsorbents in many water treatments, especially for drinking water, due to their outstanding adsorption capacity derived from their high surface area, tailor-made pore structure and the chemical character of their surface [START_REF] Hamdaoui | Modeling of adsorption isotherms of phenol and chlorophenols onto granular activated carbon Part II. Models with more than two parameters[END_REF][START_REF] Lu | The effect of functional groups on oligomerization of phenolics on activated carbon[END_REF][START_REF] Cougnaud | Removal of pesticides from aqueous solution: quantitative relationship between activated carbon characteristics and adsorption properties[END_REF].

The adsorption capacity of an activated carbon depends on [START_REF] Radovic | An experimental and theoretical study of the adsorption of aromatics possessing electron-withdrawing and electron-donating functional groups by chemically modified activated carbons[END_REF]:

• the nature of the adsorbent (e.g. functional groups present, surface area and pore size distribution, ash content); • the nature of the adsorbate (e.g. surface functional groups, polarity, hydrophobicity, molecular weight and size, solubility and pKa); and • the solution conditions (e.g. pH, temperature and adsorbate concentration, presence of competitive solutes, polarity of solvent).

Not surprisingly, apart from some well-established facts, there are many conflicting results in the literature. The treatment of drinking water and wastewater by activated carbon sometimes involves a multitude of adsorbates competing for adsorption sites on the carbon surface. Thus, a thorough understanding of the competitive effect of various organic compounds on carbon adsorption deserves more work [START_REF] Srivastava | Competitive adsorption of substituted phenols by activated carbon developed from the fertilizer waste slurry[END_REF][START_REF] Ha | Competitive removal of phenol and 2,4-dichlorophenol in biological activated carbon system[END_REF].

Phenolic compounds occur frequently in the aquatic environment through [START_REF] Mastral | Study of the adsorption of polyaromatic hydrocarbon binary mixtures on carbon materials by gas-phase fluorescence detection[END_REF][START_REF] Gallardo-Moreno | Arrangement of SDS adsorbed layer on carbonaceous particles by zeta potential determinations[END_REF][START_REF] Purkait | Adsorption of eosin dye on activated carbon and its surfactant based desorption[END_REF][START_REF] Canizares | Adsorption equilibrium of phenol onto chemically modified activated carbon F400[END_REF][START_REF] Mall | Characterization and utilization of mesoporous fertilizer plant waste carbon for adsorptive removal of dyes from aqueous solution[END_REF]:

• direct discharge of industrial wastewaters containing dyes, herbicides, surfactants, etc.; *Corresponding author. Email: henri.delmas@ensiacet.fr

• effluents from oil refineries and olive mills;

• effluents from the plastics, leather, paper, paint, pharmaceutical and steel industries.

Phenolic compounds cause an unpleasant taste and odour (even at low concentrations) in drinking water, and can exert negative effects on different biological processes [START_REF] Dabrowski | Adsorption of phenolic compounds by activated carbon -a critical review[END_REF]. Most of these compounds are recognized as toxic carcinogens to humans and aquatic life and need to be eliminated for safe discharge [START_REF] Lu | The effect of functional groups on oligomerization of phenolics on activated carbon[END_REF]. The European Union and US Environmental Protection Agency (USEPA) have placed phenol and its derivatives (e.g. nitrophenols and chlorophenols) on their priority pollutants lists [START_REF] Hamdaoui | Modeling of adsorption isotherms of phenol and chlorophenols onto granular activated carbon Part II. Models with more than two parameters[END_REF][START_REF] Álvarez | Ozonation of activated carbons: effect on the adsorption of selected phenolic compounds from aqueous solutions[END_REF].

Adsorption appears as the best technique to remove phenols at low concentration. Especially phenol is widely taken as a reference compound for adsorption capacity determination [START_REF] Pan | Adsorptive removal of phenol from aqueous phase by using a porous acrylic ester polymer[END_REF]. The main drawback of the adsorption process is that, after saturation, the activated carbon is regenerated by ex situ high temperature processes which require long transportation high energy cost and moreover results in about 10-15% activated carbon losses.

This high adsorbent cost has limited adsorption applications to final water treatment, most often to produce drinkable water, when pollutant concentration has been very much reduced by several major processes such as biological and physicochemical treatments. In this perspective, it would be of great economic importance to develop either cheap in situ activated carbon regeneration processes or a cheap adsorbent to replace usual activated carbon. Our team has investigated both new in situ oxidative regeneration techniques [START_REF] Delmas | AD-OX: a sequential oxidative process for water treatment-adsorption and batch CWAO regeneration of activated carbon[END_REF][START_REF] Muranaka | Regeneration of activated carbon by (photo)Fenton oxidation for the treatment of phenol wastewater[END_REF] and the performance of new carbonaceous materials produced from sludge [START_REF] Julcour Lebigue | Application of sludge-based carbonaceous materials in a hybrid water treatment process based on adsorption and catalytic wet air oxidation[END_REF]. These was mainly achieved through the European project, REMOVALS (FP6-018525) with Imperial College London and GEPEA (Nantes) for the production of sludge-based activated carbons and their use in water treatment process involving both adsorption and catalytic wet air oxidation (CWAO). A previous common paper [START_REF] Julcour Lebigue | Application of sludge-based carbonaceous materials in a hybrid water treatment process based on adsorption and catalytic wet air oxidation[END_REF] presented the first results comparing six sludgebased activated carbons when used as adsorbents and as wet air oxidation catalysts. Both adsorption and CWAO were achieved with phenol as a model pollutant.

The present study focuses on the performance of the best SBAC as compared with two commercial activated carbons (S23 and F22) in the case of multicomponent adsorption. Four phenolic compounds ( p -nitrophenol, PNP; p -chlorophenol, PCP; p -hydroxy benzoic acid, PHBA; phenol, P) were used either in single or multicomponent aqueous solutions. This comparison also includes the physico-chemical properties of these three adsorbents so as to provide fundamental knowledge and basic understanding of the SBAC structure.

Materials and methods

Reagents

Four phenolic compounds (Aldrich Chemical) with different functional groups: phenol (-OH), p -chlorophenol (-Cl), p -hydroxybenzoic acid (-COOH) and p -nitrophenol (-NO 2 ) were used. Table 1 provides physico-chemical properties of phenol and its derivatives. All other chemicals used were analytical grade reagents.

Preparation of sludge-based activated carbon (SBAC)

According to the comparison of six SBACs in our preliminary work [START_REF] Julcour Lebigue | Application of sludge-based carbonaceous materials in a hybrid water treatment process based on adsorption and catalytic wet air oxidation[END_REF], steam-activated dewatered raw sludge prepared by Imperial College London was selected for this detailed investigation on multicomponent adsorption. Sewage sludges have been used as feedstock for the production of the carbon; municipal wastewater DRAW (dewatered, raw) originated from the UK (Little Marlow). The raw sludge used consists of 65.9% volatile contents, 41% carbon and 20.4% ash [START_REF] Pullket | Influence of source and treatment method on the properties of activated carbons produced from sewage sludge[END_REF]. Prior to carbonization, the sludges were sterilized in steam at 122 ° C for 15 minutes in an autoclave, then After the desired temperature (250-1000 ° C) was reached, the furnace was cooled down automatically to produce carbonized samples. When steam activation was applied, once the target temperature was reached, the equivalent of 0.7 g/min of steam was mixed with the nitrogen flow (acting as carrier) and the temperature was maintained for the whole duration of the prescribed activation period. The flow of the activation reagent was arrested and the furnace was left to cool at the end of the activation period. The steam activation procedure was optimized using the response surface methodology [START_REF] Karacan | Optimization of manufacturing conditions for activated carbon from Turkish lignite by chemical activation using response surface methodology[END_REF] to establish the relationship between the textural properties of the resulting carbonaceous materials and the applied conditions. From the models, it was calculated that the maximum BET surface area was attained when the activation temperature was 838 ° C and dwell time was 80 min [START_REF] Pullket | Influence of source and treatment method on the properties of activated carbons produced from sewage sludge[END_REF].

The F22 and S23 commercial activated carbons used as a reference were provided by PICA (Veolia Group) and were made from coal and oil, respectively.

Activated carbon characterization 2.3.1. Specific surface area and pore volume

Surface area was determined using nitrogen as the sorbate at 77K in a static volumetric apparatus (Micromeritics ASAP 2010 sorptometer). Specific total surface areas were calculated using the BET equation [START_REF] Brunauer | Adsorption of gases in multimolecular layers[END_REF].

Boehm titration

The acid/base properties of activated carbons were determined by the classical Boehm titration method [START_REF] Boehm | Some aspects of the surface chemistry of carbon blacks and other carbons[END_REF].

Thermal analysis

Thermogravimetric analysis (TGA) was performed under nitrogen flow from room temperature to 700 ° C with a heating rate of 10 ° C/min (Q600 SDT, TA Instrument).

Elemental analysis and ash content

Elemental analysis of the activated carbons was carried out using a Thermo Finnigan Flash EA111 for CHNSO determination and acid lixiviation (HCl: HNO 3 , 3:1 v/v) followed by inductively coupled plasma atomic emission spectroscopy (Jobin Yvon -Ultima 2R) for metals.

Metals whose oxides could act as oxidation catalysts were investigated.

The ash content was determined by calcination at 650 ° C until constant weight according to ASTM D2866 [START_REF] Astm D | Standard Test Method for Total Ash Content of Activated Carbon[END_REF].

Adsorption equilibrium studies 2.4.1. Single component isotherms

Isotherms were performed at 25 ° C using the batch bottle-point technique. A carbon sample of 0.5 g was placed into each bottle and 100 mL of single adsorbate aqueous solution with initial concentrations of 0.005-0.05 mol L -1 for P, 0.005-0.025 mol L -1 for PNP, 0.005-0.05 mol L -1 for PCP and 0.005-0.025 mol L -1 for PHBA were added. The initial pH was 5.6 for P, 3.8 for PHBA, 4.8 for PNP and 5.3 for PCP. Solutions were not buffered to avoid adsorption competition between organic and buffer. Bottles were shaken for three days before sampling.

Preliminary kinetic experiments indicated that adsorption equilibrium was reached in less than three days for all the activated carbons. All the samples were filtered using a 0.25 µ m filter syringe and analysed by high-performance liquid chromatography (HPLC) with an UV detector set at 254 nm for phenol and PHBA, 285 nm for PCP, and 315 nm for PNP; a ProntoSIL 120-5 AQ column (C-18, 250 mm × 4 mm, 5 µ m) was used, the mobile phase being acidified distilled water (pH 2.2 using H 3 PO 4 ):acetonitrile (60:40, v/v). The flow rate of the mobile phase was maintained at 1 mL min -1 . The amount of adsorbed phenolic compounds was then determined.

Multicomponent isotherms

The procedure for competition isotherms was the same as for the single component isotherms, with the initial solutions being composed of equimolar concentrations of the four phenolic compounds (0.007-0.025 mol L -1 ). Bottles were shaken for seven days before sampling. From a previous adsorption kinetic study, this period had proved sufficient to ensure that adsorption equilibrium was reached. The initial pH of the mixture solution was between 3.5 and 5.

Adsorption modelling 2.5.1. Single component model

Among the wide variety of available theoretical models and empirical fitting functions used in this field, the curves for single component isotherms were evaluated according to the two parameter models of Langmuir and Freundlich. The most extensively used isotherm equation for modelling the adsorption data is the Langmuir equation, which is suitable for monolayer adsorption onto a surface with a fixed number of identical sites and is given by the following equation:

where q e is the amount of adsorbate fixed on adsorbent at equilibrium (mol g -1 ), C e is the equilibrium concentration of the adsorbate in aqueous solution (mol L -1 ), and q max and K L are the Langmuir constants, representing the maximum adsorption capacity for the solid phase loading and the energy constant related to the heat of adsorption, respectively. A high K L value reveals a strong interaction between the pollutant and the surface of the adsorbent. The Langmuir parameters were evaluated by linearization of C e / q e versus C e .

The Freundlich model is an empirical equation based on adsorption on a heterogeneous surface [START_REF] Freundlich | Kapillarchemie[END_REF]. It is given by the following equation:

where K F is the Freundlich constant for a heterogeneous adsorbent, and n is related to the magnitude of the adsorption driving force and to the adsorbent site energy distribution. The value of n indicates favourable adsorption when 1 < n < 10; the more favourable, the lower its value within this range [START_REF] Rozada | Activated carbons from sewage sludge and discarded tyres: production and optimization[END_REF]. The Freundlich parameters were determined by linearization of log q e versus log C e . The adsorption models constants, the values of which express the surface properties and affinity of the adsorbent, can be used to compare the adsorption capacity of adsorbents for different adsorbates.

Competitive adsorption models

Because of the limited success of the generalized Langmuir model in predicting multicomponent adsorption equilibria, several authors have modified the equations by the introduction of a power law expression of Freundlich form [START_REF] Sips | On the structure of a catalyst surface[END_REF]. In this work, the competitive isotherms were evaluated according to the Langmuir-Freundlich model as shown in the following equation [START_REF] Ruthven | Principles of Adsorption and Desorption Processes[END_REF]:

where q e, i is the amount of component i adsorbed per unit weight of adsorbent, C e, i the equilibrium concentration of the component i and C e, j the equilibrium concentration of the other component j , q max, n and K L are parameters obtained from the Langmuir model and the Freundlich model for each single component.

The values of the Optimized Extend Langmuir-Freundlich model parameters were obtained from nonlinear regression. Identification was made by using the generalized reduced gradient method (Solver Microsoft Excel) to minimize the residual sum of squares (RSS), as shown in Equation ( 4):

where q exp,i are the elements of the vector q exp,i containing the given experimental adsorbed phase concentrations, n is the data points and q t,i are the corresponding theoretical values calculated by model being studied.

Based on the objective function values (F OBJ ), relative average deviation (RAD) and a correlation coefficient (R 2 ) between the measured and the modelpredicted values, statistical analysis was carried through to evaluate the model's behaviour. The objective function (F OBJ ) and the relative average deviation (RAD) were calculated using the equations: where q exp, i , and q t,i are the experimental and calculated values for each data point, respectively, and n is the number of experimental data.

Results and discussion

Characterization of the studied carbons

Table 2 shows that the total mesopore volume and micropore volume of the SBAC are 0.17 and 0.11 cm 3 g -1 , respectively; the two commercial activated carbons exhibited much more microporosity. The BET surface area of the sludge-based carbon was about 265 m 2 g -1 which was nearly four times lower than that of the commercial activated carbons. The lower specific surface area of the SBAC is related to its lower microporosity, which is primarily attributed to the properties of the precursor.

Table 3 gives the contents of oxygen-containing surface functional groups of the activated carbons. The total acidic groups content of the SBAC (1.1 mmol g -1 ) was found to be higher than those of commercial activated carbons, 0.2 and 0.65 mmol g -1 for S23 and F22,

q q e L e L e = ⋅ ⋅ + ⋅ max , ( ) K C K C 1 1 q e F e n = ⋅ K C 1 2 / , ( )
q , j e L e L e = ⋅ ⋅ + ⋅ ∑ q K C K C j i ni j j nj j max , , , , ( ) ( ) , ( ) 1 3

RSS q q

i i i l n = - = ∑ ( ) , ( ) exp, , t 2 4 F q q q i i i i j n OBJ exp, t, exp, = -       = ∑ 2 5 , ( )
RAD q q q i i i i j n = -       = ∑ 100 6 2 N exp, t , exp, , ( )
respectively, while S23 has the greatest content of basic groups (0.98 mmol g -1 ). However, it is important to note that Boehm titration is not convenient for the SBAC due to the release of either carbonates in basic solutions or phosphates and ferric ions in acidic solutions. The presence of basic groups enhances the adsorption of aromatic compounds through the formation of an electron acceptor-donor complex [START_REF] Haydara | Adsorption of p-nitrophenol on an activated carbon with different oxidations[END_REF]. Thermogravimetric analysis of the three activated carbons reveals the same global behaviour (Figure 1). The higher difference between the three activated carbons is observed during the first mass loss (<273°C) and mainly represents the degree of humidity and small adsorbed molecules. During the thermal decomposition of the oxygenated surface groups of the activated carbons studied, the loss of mass at temperatures ranging from 200 to 500°C is essentially attributed to the decomposition of carboxylic acid groups to CO 2 , while the loss of mass observed at higher temperatures, from 500 to 700°C, is mainly due to the decomposition of the lactonic and phenolic groups [START_REF] Figueiredo | Modification of the surface chemistry of activated carbons[END_REF]. Table 4 shows that SBAC has a higher ash content compared with the commercial activated carbons. This is in accordance with Smith et al. [START_REF] Smith | Sewage sludge-based adsorbents: a review of their production, properties and use in water treatment applications[END_REF] for the production of SBAC by carbonization and steam activation where the lower BET surface area was related to the lower carbon content of the parent sludge. It was also characterized by the presence of a large amount of iron, about 100 times more than that found in the commercial activated carbons.

Moreover, SBAC contained a rather high percentage of oxygen (Table 5) suggesting the presence of acidic surface functional groups and even more of inorganic salts and oxides. From Tables 4 and5, it was found that the ash content and the elemental analysis cannot be merged in a consistent mass balance due to other elements not analysed by these two methods.

Single component adsorption 3.2.1. Effect of carbon surface chemistry

Figure 2(a), (b) and (c) shows the single component adsorption isotherms for the four substituted phenols in aqueous solution onto SBAC, S23 and F22, respectively. The results show that the phenols adsorption capacity of the sludge-based activated carbon (SBAC) is lower than that obtained by the two commercial activated carbons (S23 and F22) investigated for comparison (Table 6). This lower performance could be due to the much lower BET surface area (265 m 2 g - 1 ) and the less developed microporous structure of SBAC (Table 2). Phenol molecules, due to their molecular size, are mainly adsorbed in pores with diameter greater than 1 nm, i.e. within the microporous range. It can be noted that the maximum adsorption capacity of SBAC for phenol as a reference compound is half that of F22 and S23, while the ratio of their surface areas is nearly 1:4 for F22 and about 1:5 for S23. The adsorption capacity of SBAC is therefore higher than expected from the BET surface area. This would suggest an economically attractive utilisation of SBAC as a preliminary adsorbent in water treatment.

Due to competition between water and phenol compounds, the acidic nature of SBAC also plays an unfavourable role in its adsorption capacity. Water molecules adsorbed on oxygen groups become secondary adsorption centres, retaining other water molecules by means of hydrogen bonds. The adsorbed water molecules block the entry of organic molecules to significant parts of the surface; this effect is generally called 'solvent effect' [START_REF] Franz | Effect of chemical surface heterogeneity on the adsorption mechanism of dissolved aromatics on activated carbon[END_REF]. Moreover, the acidic surface oxygen groups, located at the edge of the graphite Table 6. Parameters of Langmuir and Freundlich models for the adsorption of single solute of P, PCP, PNP and PHBA on three types of activated carbons at 25°C.

Langmuir model

Freundlich model layers, remove electrons from the π-electron system, so creating positive holes in the conducting π-band of the graphite layer, and weakening interaction between the π-electron of the phenol aromatic ring and π-electron of the graphite layer leading to reduce the phenols uptake [START_REF] Coughlin | Role of surface acidity in the adsorption of organic pollutants on the surface of carbon[END_REF].

ACs Phenol q max mol g -1 K L L mol -1 R 2 n K F (mol g -1 (L mol -1 )
The sludge-made activated carbon also displayed a distinctive chemical composition; its carbon content was much lower whereas its ash, oxygen and nitrogen contents were higher than in commercial activated carbons.

While S23 and F22 have nearly the same surface area, S23 exhibits the greatest adsorption capacities. This seems to be related to the basic nature of this activated carbon where the basic functional group present on carbon surface acts as an electron-donor and interacts with the aromatic ring of phenols as electron-acceptor in a complex donor-acceptor mechanism [START_REF] Haydara | Adsorption of p-nitrophenol on an activated carbon with different oxidations[END_REF].

Effect of the adsorbate nature

Prior to analysis of the competitive effect on the equilibrium adsorption of different substituted phenols, it was necessary to obtain the single adsorption isotherm for each compound in aqueous solution.

The single component isotherms revealed the strongest adsorption capacity for PCP whichever activated carbon was used, followed by PNP, P and PHBA (Figure 2). Different behaviour was observed in the case of SBAC at higher concentrations; the adsorption capacities were increased strongly for PNP and PHBA, and the isotherms of these two compounds were represented by type II isotherm, whereas type I is convenient for all the other adsorption isotherms [START_REF] Sun | Adsorption. Aspects théoriques[END_REF]. A type II isotherm indicates a multilayer adsorption consecutive to additional interactions between the molecules. The possibility of the formation of these adsorbates is due to the mesoporous nature of SBAC (Table 2); it is not observed for the other microporous activated carbons.

The activated carbons are mainly hydrophobic and display a strong affinity for organic molecules with a limited solubility in water. Hydrophobic compounds tend to be pushed to the adsorbent surface and hence they are more adsorbed than hydrophilic compounds [START_REF] Lu | The effect of functional groups on oligomerization of phenolics on activated carbon[END_REF]. The higher capacity for PCP may be essentially explained according to the hydrophobicity of the adsorbed molecules. Among these four phenols, PCP is more hydrophobic than PNP, P and PHBA, as expected from the magnitude of the octanol-water partition coefficient; log K ow,PCP < log K ow,PNP < log K ow,P < log K ow,PHBA (Table 1).

On the other hand, the adsorption capacities are known to depend on the solubility of the phenols compounds in water. Moreno-Castilla et al. [START_REF] Moreno-Castilia | Adsorption of some substituted phenols on activated carbons from a bituminous coal[END_REF] reported that the adsorption capacity increased with decreased water solubility of the phenolic compounds. In the present study, this parameter seems to be secondary since PHBA is the less adsorbed although it is the less soluble compound (Table 1).

Another parameter generally taken into account is the dispersive interactions between the activated carbon (electron donating) and the aromatic molecules (electron accepting). The functional groups present at the surface of the activated carbon can decrease or increase the electronic density of the graphitic planes [START_REF] Michailof | Enhanced adsorption of phenolic compounds, commonly encountered in olive mill wastewaters, on olive husk derived activated carbons[END_REF]. At the same time, the chemical structure of the molecule can also play an important role in the adsorption, where the nature of substituents could increase (electron donating effect) or decrease (electron withdrawing effect) the electron density of the aromatic ring and influence the interactions between the surface groups and the aromatic ring [START_REF] Haydara | Adsorption of p-nitrophenol on an activated carbon with different oxidations[END_REF][START_REF] Moreno-Castilia | Adsorption of some substituted phenols on activated carbons from a bituminous coal[END_REF].

At a value pH < pKa, the phenol compounds are in the non-ionized forms and the electrostatic interactions do not play an important role in the adsorption phenomenon. The interaction between the carbon surface and the phenols in this case can be attributed to dispersion effects. The enhanced interaction in the case of PNP and PCP is due to the electron-withdrawal effect of their -NO 2 and the -Cl substituents, respectively, as they reduce the overall electron density of the aromatic ring, increasing its ability to accept the electron of the graphite layers of the activated carbon [START_REF] Dabrowski | Adsorption of phenolic compounds by activated carbon -a critical review[END_REF][START_REF] Moreno-Castilia | Adsorption of some substituted phenols on activated carbons from a bituminous coal[END_REF][START_REF] Streat | Sorption of phenol and para-chlorophenol from water using conventional and novel activated carbons[END_REF].

For a comparison of single solute isotherm results in a quantitative manner, the isotherms were described by the Langmuir and Freundlich models; the constants obtained according to these two models are listed in Table 6. The correlation coefficients square (R 2 ) for both models at 25°C suggested that the empirical Langmuir equation was more convenient than Freundlich equation in describing the behaviour of PCP, PNP, P and PHBA adsorption onto the two commercial activated carbons (S23 and F22). However, for SBAC, the Freundlich model was found to better represent the adsorption data for PNP, P and PHBA, while the Langmuir model described well the data of PCP.

The convenience of the Langmuir equation for the two commercial activated carbons clearly indicates that S23 provides the highest maximum adsorption capacity (q max ) for the four phenols, and the best adsorption equilibrium constant (K L ) for P and PCP. The higher value for PCP proves its high affinity with this carbon surface compared with the other phenols studied in this work. For F22, the values of K L are nearly the same for PCP, PNP and PHBA, and the higher value of q max again indicates more affinity of PCP with the carbon surface.

Competitive adsorption isotherms

The studies of such competitive systems, especially those containing a large number of solutes, are very complex and rarely detailed. Many factors influence adsorption equilibria such as the structural and energetic heterogeneity of the adsorbent and the differences in physicochemical properties of adsorbates, which makes it a very difficult problem [START_REF] Lu | The effect of functional groups on oligomerization of phenolics on activated carbon[END_REF].

The results of competitive adsorption of an equimolar mixture of four phenols onto SBAC, S23 and F22 are shown in Figures 3(a Whichever the activated carbon, the adsorption isotherms of these phenols range in the following order: PNP > PCP > PHAB or P (Table 1). For these two last phenols, the order depends on the activated carbon and the phenols' concentrations. At high concentrations, an inversion of selectivity between PHBA and P onto SBAC and S23 was observed. This result was in agreement with that obtained by Monneyron et al. [START_REF] Monneyron | Competitive adsorption of organic micropollutants in the aqueous phase onto activated carbon cloth: comparison of IAS model and neural networks in modeling data[END_REF] who reported that, as the concentration increases, solubility is considered the critical factor for the adsorption of phenols. Different authors have reported that the order of adsorption affinity may be interpreted on the basis of the chemical and physical characteristics of molecules and the nature of the different groups located on the aromatic ring [START_REF] Baudu | Pollutant adsorption onto activated carbon membranes[END_REF][START_REF] Juang | Liquid-phase adsorption of phenol and its derivatives on activated carbon fibers[END_REF][START_REF] Deng | Preparation and characterization of active carbon adsorbents for wastewater treatment from elutrilithe[END_REF][START_REF] Zhou | Adsorption isotherm comparison and modelling in liquid phase onto activated carbon[END_REF][START_REF] Mihalache | Competitive adsorption models of organic pollutants from bi-and tri-solute systems on activated carbon[END_REF][START_REF] Chern | Competitive adsorption of benzoic acid and p-nitrophenol onto activated carbon: isotherm and breakthrough curves[END_REF]. Among the four phenols investigated here, the order of decreasing solubility is PHBA < PNP < PCP < P which does not at all correspond to the order of single adsorption. However, the hydrophobic nature of the adsorbate could be taken into account; PNP and PCP, which have hydrophobic characteristics, are adsorbed to a greater extent than P and PHBA, which have hydrophilic groups [START_REF] Dabrowski | Adsorption of phenolic compounds by activated carbon -a critical review[END_REF]. Finally, it appears that various chemical characteristics of the phenols are involved in the competition.

As shown previously among the four substituted phenols used in this work, PNP is the most strongly adsorbed followed by PCP, whereas phenol is the least adsorbable. This is in accordance with Srivastava and Tyagi [START_REF] Srivastava | Competitive adsorption of substituted phenols by activated carbon developed from the fertilizer waste slurry[END_REF] who reported that PNP is preferentially adsorbed in PNP-PCP mixtures. It is in agreement with the well-known theory of the formation of π-π bonds, where activated carbon acts as an electron donor and the solute benzene ring has an electronwithdrawing character [START_REF] Bansal | Activated Carbon Adsorption[END_REF]. The nitro-group of nitrophenol is electron withdrawing, thereby causing a decrease in electron density of the π-electrons of the benzene ring. Therefore, dispersion forces between the carbon surface and the phenolic compound are expected to be the strongest for PNP. In the order of preferential adsorption, PCP is the second absorbable compound owing to the withdrawing effect of its -Cl group [START_REF] Ayranci | Adsorption behaviors of some phenolic compounds onto high specific area activated carbon cloth[END_REF].

In the field of competitive adsorption, several complex models have been formulated in the literature. In this study, the modified three parameters Langmuir-Freundlich model was applied. The simple formula makes this model very attractive. Although not thermodynamically consistent, this expression (Equation ( 3)) has been shown to provide a reasonably good empirical correlation of multicomponent equilibrium data on the adsorbent. Moreover, the parameter n characterizes the heterogeneity of the adsorbent surface with respect to the adsorbing molecules [START_REF] Dabrowski | Adsorption of phenolic compounds by activated carbon -a critical review[END_REF]. The Langmuir-Freundlich model was initialized with the parameters obtained from the Langmuir and Freundlich models of single solute isotherms. The values of the Optimized Extend Langmuir-Freundlich model parameters were then calculated minimizing the residual sum of squares (RSS) expressed by Equation [START_REF] Liu | Characterization of mesoporous activated carbons prepared by pyrolysis of sewage sludge with pyrolusite[END_REF].

Statistical analysis of the multicomponent isotherms data obtained at 25°C shows that the equilibrium data for all the pollutants fitted well to the Langmuir-Freundlich model (Table 7). For the three activated carbons, the higher values of correlation coefficient (R 2 ) and the lower values of residual sum of squares (RSS), the objective function values (F OBJ ) and the relative average deviation (RAD) obtained with this model confirm the good agreement between the experimental and calculated data for the fourcomponent system.

Conclusions

Despite the rather low surface area, it may be concluded that the dried activated carbon resulting from sludge treatment (SBAC) shows an adsorption capacity of about half those of high-performance commercial activated carbons. As it may still be improved, it should be considered as a convenient low-cost material for the removal of phenol and substituted phenols from polluted water. Production of SBAC from sludge thus constitutes a possible way of valorisation of this abundant waste.

Concerning adsorption features of four phenols on various activated carbons, it may be concluded that for single-solute adsorption, hydrophobicity is the main factor to explain the higher capacity of adsorption of PCP.

Studies on competitive adsorption have been performed to give an insight into some of the factors affecting selective uptake of various phenols on activated carbon from multi-solute aqueous solutions. The preferential adsorption of PNP followed by PCP could be explained by the electron-withdrawing effect of the -NO 2 group and -Cl group. The simple Langmuir-Freundlich model gives satisfactory results for describing the four component experimental data.

Figure 1 .

 1 Figure 1. Thermogravimetric analysis (TGA) of commercial activated carbons and SBAC.

Figure 1 .

 1 Figure 1. Thermogravimetric analysis (TGA) of commercial activated carbons and SBAC.

Figure 2 .

 2 Figure 2. Adsorption isotherms at 25 °C of single component of phenol, PHBA, p-nitrophenol and p-chlorophenol onto activated carbons (a) SBAC, (b) S23 and (c) F22.

Figure 2 .

 2 Figure 2. Adsorption isotherms at 25°C of single component of phenol, PHBA, p-nitrophenol and p-chlorophenol onto activated carbons (a) SBAC, (b) S23 and (c) F22.

  ), (b) and (c), respectively.

Figure 3 .

 3 Figure 3. Comparison of experimental data of adsorption isotherms at 25 °C with the calculated data by the Langmuir-Freundlich model for an equimolar mixture of phenol, PHBA, p-nitrophenol and p-chlorophenol onto (a) SBAC, (b) S23 and (c) F22.

Figure 3 .

 3 Figure 3. Comparison of experimental data of adsorption isotherms at 25°C with the calculated data by the Langmuir-Freundlich model for an equimolar mixture of phenol, PHBA, p-nitrophenol and p-chlorophenol onto (a) SBAC, (b) S23 and (c) F22.

Table 1 .

 1 Basic information concerning phenol and its derivatives[START_REF] Vidic | Oxidative coupling of phenols on activated carbon: impact on adsorption equilibrium[END_REF] and comparison of its adsorption capacities in single-and multi-solute systems.

	Properties	Phenol	p -Chlorophenol	p -Nitrophenol	p -Hydroxy benzoic acid
	Symbol	P	PCP	PNP	PHBA
	Molecular formula	C 6 H 6 O	C 6 H 5 OCl	C 6 H 5 O 3 N	C 7 H 6 O 3
	Chemical structure
	pKa at 25 ° C Solubility at 25 ° C (g L -1 )	9.89 93	9.37 27	7.15 17	4.58/9.23 5
	Log K ow	1.5	2.39	1.91	1.37

Table 2 .

 2 Characterization of activated carbons.

	Activated carbon	BET surface (m 2 g -1 )	Mesoporous (cm 3 g -1 )	Microporous volume (cm 3 g -1 )	Pore diameter (Å)	Apparent density (kg m -3 )
	S23	1230	0.04	0.49	17	1013
	F22	985	0.12	0.40	20	993
	SBAC	265	0.17	0.11	35	1471

Table 3 .

 3 Results of Boehm titration.

	Activated carbon	Carboxylic group (mmol g -1 )	Lactonic group (mmol g -1 )	Phenolic group (mmol g -1 )	Total acidic sites (mmol g -1 )	Total basic sites (mmol g -1 )	pH PZC
	S23	0.00	0.09	0.21	0.30	0.98	9.8
	F22	0.05	0.01	0.18	0.24	0.51	9
	SBAC	0.15	0.15	0.80	1.10	0.64	6.3

Table 5 .

 5 Elemental analysis of activated carbons.

	1/n	R 2

Table 4 .

 4 Metal composition analysis of the activated carbons.

		Zn Ni Co Mn	Fe	Cu
	Carbon				(ppm)			Ash
								(wt%)
	PICA S23	13	3	0	5	116	74	3.2
	PICA F22	13 24	0	6	578	28 12.0
	SBAC	613 28	1	149 4.79•10 4 672 65.0

Table 7 .

 7 Correlation coefficient (R 2 ) residual sum of squares (RSS) objective function values (F OBJ ) and the relative average deviation (RAD) for multicomponent system using the Langmuir-Freundlich model.

	ACs	Phenol	R 2	RSS	F OBJ	RAD%
	S23	P	0.9945 2.88E-07	0.7765	6.4708
		PCP	0.9623
		PNP	0.9923
		PHBA	0.7897
	F22	P	0.9730 2.39E-08	0.0676	0.6756
		PCP	0.9949
		PNP	0.9981
		PHBA	0.9902
	SBAC P	0.9782 6.92E-08	0.2340	2.6002
		PCP	0.9606
		PNP	0.9846
		PHBA	0.9335

Acknowledgements

The authors wish to express their appreciation to PICA (Veolia Group) for supplying the activated carbons S23 and F22, Imperial College of London for supplying AC SBAC, and Martine Auriol and Christine Rouch (SAP, LGC Toulouse) for characterization of the activated carbons. This work has been partly supported by the EC FP-6 programme 'Removals'.


M Asif Inam
email: asifinam1976@yahoo.com

Soualo Ouattara

Christine Frances

Effects of concentration of dispersions on particle sizing during production of fine particles in wet grinding process

Stirred media milling is a prospective technology for producing colloidal dispersions by means of wet grinding process. In the past, many researchers have studied the effects of different operating parameters such as size, shape, nature and quantity of grinding medium, the speed of agitator in grinding chamber, the feed rate of dispersions, etc. in stirred media mills. However, it is still less known how particle sizing which generates valuable information of particle size of the product to interpret, control and optimize the grinding process, is influenced by the concentration of the dispersion during stirred media milling where particles change their size from micron to colloidal range rapidly. One of the reasons of this lack had been our incapability in the past to study the particle size distribution of dispersions without dilution. The recent advent of acoustic attenuation spectroscopy is known to be capable of studying dispersions without dilution, under real process conditions and on line. The study employs acoustic attenuation spectroscopy to investigate the effects of concentration of dispersions of CaCO 3 on its particle sizing during size reduction process in a stirred media mill (LabStar manufactured by NETZSCH). The dispersions of CaCO 3 at 5%, 10%, 20% and 30% (m/m) were studied about six hours under a selected set of operating conditions. Contrary to the existing knowledge obtained through other techniques of particle sizing that are based on the principle of dilution, acoustic attenuation spectroscopy shows that, under certain grinding time at given operating conditions, increase in concentration of dispersion results in better grinding results yielding smaller particles. The causes behind the differences in results of acoustic attenuation spectroscopy and dynamic light scattering have been thoroughly investigated. We find certain limitations of acoustic attenuation spectroscopy in particle sizing. A typical phenomenon which causes misleading trends in particle sizing is multiple scattering in acoustic measurements. Multiple scattering, particularly, influences acoustic results when particles approach to fine size range during size reduction process.

Introduction

Many industries such as of paints, dyes, cosmetics, pharmaceuticals, ceramics, micro-electronics, etc. employ colloidal dispersions in fabrication of many products. The demand of such industries for stable colloidal dispersions is on increase because use of fine particles may improve homogeneity, solubility, strength, reactivity etc. In general, there are two approaches for production of fine particles: (i) Bottomup (in which fine particles may be produced by a reaction, condensation or aggregation of molecules, etc.). (ii) Top-down (in which coarse particles are ground to produce fine particles). Wet grinding is often employed for production of fine and ultrafine particles suspended in a liquid. The stirred media milling is a prospective technology in domain of wet grinding because of many advantages such as relative low agglomeration tendency, low material losses, less possibility of oxidation, easy handling of toxic materials, elimination of need of employing a device for air cleaning, elimination of the problem of dust explosion, better heat transfer options, etc. In the past, many researchers have studied various aspects of wet grinding process. Today, it is known to a considerable extent how different operating parameters such as size, shape, nature and quantity of grinding medium, the speed of agitator in grinding chamber, and the feed rate of dispersions may affect size reduction process in stirred mills under certain conditions. However, it is still less known how particle sizing which generates valuable information of particle size of the product to interpret, control and optimize the grinding process, is influenced by the concentration of a dispersion during stirred media milling where particles enter from micron to colloidal size rapidly. One of the reasons of this lack had been our incapability in the past to study the particle size distribution of dispersions without dilution. The techniques of particle sizing based on principles of laser diffraction, dynamic light scattering, etc., may measure the particle size of dispersion but they require the sample to be diluted before measurement. The dilution step may result in destabilization of dispersion, formation or deformation of flocs and/or aggregates etc. All the techniques based on the dilution step have a risk of not showing the right information of particle size which is quite important in understanding, controlling, designing and optimizing the grinding processes.

Acoustic attenuation spectroscopy, which has appeared, relatively, in the recent past, is considered a solution for such problems. Different reviews on the technique explain that it has advantages over many existing techniques because of a wide range of particle sizing (10 nm to 1000 µm), because of being non-destructive, non-invasive and capable of measuring dispersions on line, at real process conditions and without dilution till a concentration of 50% (m/m) under certain conditions [START_REF] Alba | Acoustic spectroscopy as a technique for particle sizing of high concentration colloids, emulsions and suspensions[END_REF][START_REF] Julian | Ultrasonic measurements in particle size analysis[END_REF]. The technique has been employed in many studies for different applications. For example, Richter et al. [START_REF] Richter | Polydisperse particle size characterization by ultrasonic attenuation spectroscopy in the micrometer range[END_REF] employ it for characterization of polydisperse particles in micrometer range, Stolojanu and Prakash [START_REF] Stolojanu | Characterization of slurry systems by ultrasonic techniques[END_REF] consider it for characterization of slurry systems, Tourbin and Frances [START_REF] Tourbin | Monitoring of the aggregation process of dense colloidal silica suspensions in a stirred tank by acoustic spectroscopy[END_REF] monitor aggregation process of dense colloidal silica suspensions with this technique. Takeda and Goetz [START_REF] Takeda | Dispersed/flocculated characterization of alumina particles in highly concentrated slurries by ultrasonic attenuation spectroscopy[END_REF] and other researchers like Hibberd et al. [START_REF] Davidj | Ultrasonic characterization of colloidal dispersions: detection of flocculation and adsorbed layers, Colloids and Surfaces[END_REF] use the technique for characterization of flocculation in different applications. The applications of the technique in domain of crystallization may be consulted in the work of Hipp et al. [START_REF] Alexanderk | In-situ monitoring of batch crystallization by ultrasound spectroscopy[END_REF], Mougin et al. [START_REF] Mougin | In situ ultrasonic attenuation spectroscopy study of the dynamic evolution of particle size during solutionphase crystallization of urea[END_REF], Li et al. [START_REF] Li | On-line crystallization process parameter measurements using ultrasonic attenuation spectroscopy[END_REF], etc. However, sufficient work has not been performed yet in application of the technique in fine wet grinding processes which are typical in the sense: (i) The change in average particle size of the product is rapid in the grinding mill due to breakage of particles. (ii) The degree of polydispersity changes with time in grinding process.

(iii) The nature of interaction between particles is non-colloidal before grinding operation which changes to colloidal with increasing grinding time. This study is an attempt to explore such complexities. In particular, we investigate how different concentrations of a dispersion influence particle sizing during production of fine particles in wet grinding process carried out by means of a stirred media mill.

Fundamentals of the techniques of particle sizing

Acoustic attenuation spectroscopy

When a sound wave of certain frequency is passed through a particulate system, it interacts with the particles (transmission, reflection, absorption, scattering etc.) and as a result it undergoes attenuation (decrease in amplitude) that can be measured in terms of attenuation coefficient. It is defined as a decrease in the amplitude of the sound wave per unit distance travelled and may be expressed as [START_REF] Julian | Ultrasonic measurements in particle size analysis[END_REF]:

α = - 1 x ln Α x Α 0 ð1Þ
where A 0 is the initial amplitude of the sound wave (x = 0) and A x is the amplitude after it has travelled a distance x as illustrated in Fig. 1. If a band of sound waves of different frequencies is passed through dispersion from a certain distance (x), the attenuations in each sound wave of certain frequency may be calculated using Eq. ( 1). An acoustic attenuation spectrum describes acoustic attenuation constants expressed as a function of frequencies of sound waves. There are two major steps in particle sizing by acoustic attenuation spectroscopy: (i) development of an acoustic attenuation spectrum. (ii) Interpretation of the resulting acoustic spectrum in terms of particle size distribution using an appropriate theory. A commercial acoustic attenuation spectrometer has appropriate arrangements for measurement of attenuation spectrum. Moreover, it has an appropriate software program to interpret acoustic attenuation spectrum in terms of particle size distribution based on some appropriate theory such as ECAH. This theory is based on differential wave equations that have been derived from fundamental laws of conservation of mass, energy, momentum, thermodynamic equations of state and stress-strain relations for isotropic elastic solids (or viscous fluids). The details of ECAH theory may be studied in papers of Epstein and Carhart [START_REF] Epstein | The absorption of sound in suspensions and emulsions[END_REF] and Allegra and Hawley [START_REF] Allegra | Attenuation of sound in suspensions and emulsions: theory and experiments[END_REF] who contributed to the development of this theory.

Dynamic light scattering

Dynamic light scattering is also known as photon correlation spectroscopy. The basic principle of this technique is when particles are illuminated with laser; the intensity of the scattered light fluctuates at a rate that is dependent upon the size of the particles. Smaller particles are pushed further by the solvent molecules and move more quickly. Analysis of these intensity fluctuations gives the velocity of the Brownian motion and thus the particle size using the Stokes-Einstein relationship. Dynamic light scattering measures hydrodynamic diameter which refers how a particle diffuses within a fluid. Hydrodynamic diameter is the diameter of a sphere that has the same translational diffusion coefficient as the particle being measured.

Materials

The aqueous dispersions of calcium carbonate were used in grinding, in this study. Availability of good literature on dispersing behavior of CaCO 3 , its non-corrosive nature, low hazards and reasonable price make it, relatively, a good choice as a material of grinding in our study.

CaCO 3

It was procured through Merck KGaA Germany. It had purity N99%, density = 2.656 g/cm 3 at 20 °C, thermal decomposition N825 °C, specific surface area (BET) = 0.5 m 2 g -1 and hardness in the Mohs scale = 3. Its mean particle size (in dry form) was found to be 30 µm using technique of laser diffraction (Malvern Mastersizer). Its particle size distribution, as shown in Fig. 1, explains that most of the particles had a size between a few microns and one hundred microns. To study the shape of CaCO 3 , scanning electron microscopy (SEM) was employed. It was observed that the product is constituted by compact aggregates made of rhombohedral particles of about 30 µm in size, as shown in Fig. 2. A reasonable agreement was found in the results of both techniques.

Sodium polyacrylate

It was used as a dispersing agent. Foissy et al. have demonstrated its ability to disperse the particles of CaCO 3 in water [START_REF] Foissy | Stability measurement and dispersion process of CaCO 3 suspension in sodium polyacrylate aqueous solutions[END_REF]. It is based on anionic polyacrylate with a flexible chain able to form mono or bi coordinated complexes with the calcium ions of the solution. In ultrafine wet grinding for aqueous dispersions of CaCO 3 , Garcia et al. have shown its suitability as a dispersing agent [START_REF] Garcia | On fragmentation and agglomeration phenomena in an ultrafine wet grinding process: the role of polyelectrolyte additives[END_REF]. It was procured through Sigma-Aldrich Co. It had density of 0.55 g/cm 3 at 25 °C and average Mw ~5100 (determined by gel permeation chromatography (GPC)).

H 2 O

Water served as dispersion medium in the dispersions employed in the study. Ultrapure water was produced by the equipment "Purelab Ultra" of VWS (UK) Ltd. Its density was found to be 0.9952 g/ cm 3 at 25 °C where as its pH varied between 6.3-6.8 at 25 °C.

Experimental setup

Four dispersions, as mentioned in Table 1, were studied individually for about 6 h in wet grinding process carried out in a stirred media mill that is commercially known as LabStar and manufactured by NETZSCH.

In each of the four cases, the dispersions (suspensions) were prepared in conical flask. Sodium polyacrylate was first dissolved in water with the help of a magnetic stirrer followed by addition of CaCO 3 . All dispersions were agitated at 1100 rpm. for about 20 min at 25 °C before transferring them to a stirrer tank for starting comminution. Fig. 3 shows the experimental setup that provides a circuit mode comminution of the product.

In general, in each of the four cases, the dispersion was pumped through the stirrer tank to the inlet of the grinding chamber of the stirred media mill (LabStar) with the help of flexible-tube pump. After experiencing a certain comminution in grinding chamber, the dispersion left through a steel (Cr-Ni-steel) separating cartridge (filter) that was installed at out let of the grinding chamber. The dispersion was recycled to the stirred tank after being passed through the acoustic attenuation spectrometer for online measurement of particle size in real environment (i.e. without need of dilution, on line and at process conditions). We had access to an acoustic attenuation spectrometer called Malvern Ultrasizer to measure the acoustic attenuation spectra of the dispersions and determining particle size distributions. The full details of the equipment are mentioned in US patent No. 5,121,629 3 [START_REF] Alba | Acoustic spectroscopy as a technique for particle sizing of high concentration colloids, emulsions and suspensions[END_REF]. Table 2 shows the set of physical properties (matrix) that was used in determination of particle size distributions using the software of the acoustic attenuation spectrometer. In particle size determination by Malvern Ultrasizer, "spectral quality" and "residual" are two important quality parameters that must be ≤5 for acceptable results. All acoustic measurements in the study meet quality criteria.

From the outlet of the grinding mill after certain intervals of time, the samples of the product were taken manually to analyze with pH meter and Zetasizer Nano ZS. The latter is manufactured by Malvern Instruments, works on principle of dynamic light scattering and measures zeta potential and particle size distribution of dilute samples. A reasonable repeatability was found in all the measurements taken by Malvern Ultrasizer, Zetasizer Nano ZS and pH meter. In the grinding mill (LabStar), torque and number of revolutions were measured by a torque sensor that was installed at a stirrer shaft by the manufacturer of the mill. The problem of contamination of the product by the materials of the mill had been reduced in the grinding chamber by lining with Cr-Ni-stirred walls and equipping the agitator shaft with tungsten-carbide pegs. The core objective of the experiments was to study the influence of concentrations of dispersions (as mentioned in Table 1) on particle sizing of the product obtained after certain times under a set of fixed operating parameters as described in Table 3.

The residence times in case of 5%, 10%, 20% and 30% (m/m) aqueous dispersions of CaCO 3 were 52.3 min, 50.8 min, 47.6 min and 44.0 min respectively.

Results and discussion

General considerations in particle sizing

It has been discussed that particle sizing in fine wet grinding process is typical in the sense: (i) the change in average particle size of the product is rapid due to relatively fast breakage of particles. (ii) The degree of polydispersity in product changes with grinding time. (iii) The nature of interaction between particles is non-colloidal before grinding process; this interaction becomes colloidal after a certain grinding time. In our preliminary work (i.e. before experiments of stirred media milling), we have verified suitability of acoustic attenuation spectroscopy for measuring the changes discussed in (i) and (ii) by taking into consideration different dispersions prepared with different proportions of coarse CaCO 3 (30 µm) and relatively fine CaCO 3 (1.6 µm), as mentioned in Table 4.

Acoustic attenuation spectroscopy is capable of detecting the changes in dispersions due to increase in quantity of fine particles, and due to variation of proportion of relatively fine particles. Fig. 4(a) and (b) demonstrates this respectively.

The results have been further verified. We describe the method of verification with the help of an example that focuses on dispersion nos. ( 1), ( 6) and [START_REF] Epstein | The absorption of sound in suspensions and emulsions[END_REF] referred in Table 4. For these dispersions, acoustic attenuation spectra were determined with the help of acoustic attenuation spectrometer. M. Tourbin and C. Frances [START_REF] Tourbin | A survey of complementary methods for the characterization of dense colloidal silica[END_REF] have given the expression for the attenuation spectrum of water as a function of frequency (f) as:

α w ðf Þ = ð0:0047f 2 -0:0083f Þ: ð2Þ
From the acoustic attenuation spectra of each dispersion, the acoustic contribution of water was subtracted in order to get the acoustic attenuation spectra merely due to particles present in the dispersions. Fig. 4(c) represents resulting acoustic attenuation spectra corresponding to (I) 200 g of CaCO 3 (30 µm), (II) 150 g of CaCO 3 (1.6 µm) and (III) a mixture of 200 g of CaCO 3 (30 µm) and 150 g of CaCO 3 (1.6 µm). If the sum of the acoustic attenuation spectra of (I) and (II) is equal to acoustic attenuation spectrum of (III), then it may be considered that the technique is able to detect the presence of both fine and coarse particles. Fig. 4(d) compares the sum of acoustic attenuation spectra of (I) and (II) with acoustic attenuation spectrum of (III). The reasonable agreement in the results confirms the suitability of acoustic attenuation spectroscopy for measuring the changes discussed in (i) and (ii).

Effects of concentration of dispersions on particle sizing during wet grinding

Particle sizing by acoustic attenuation spectroscopy

It has been mentioned that grinding experiments were planned to study the effects of concentration of aqueous dispersions of CaCO 3 at varying proportions (i.e. 5%, 10%, 20%, and 30% (m/m)). The particle sizing of aqueous dispersion of CaCO 3 was carried out online, at real process conditions and without diluting the sample by means of acoustic attenuation spectroscopy. The results of average particle size of the dispersions obtained at different grinding times have been mentioned in Fig. 5.

In the beginning of the grinding process (i.e. first hour), the comparative study reveals that the higher the dilution of a dispersion, the smaller its average particle size (i.e. use of dilute dispersion favors the process of size reduction). With time, when particles undergo further size reduction, a change in tendency is observed. From the second to sixth hour of grinding, it is evident that dispersion of 5% (m/m) has bigger average particle size in comparison with all other dispersions. From the second to third hour of grinding, the dispersion of 10% (m/m) mostly shows the least average particle size as compared to all other dispersions. The change in trend becomes more evident from the 3rd to 5th hour of grinding where dispersion of 20% (m/m) shows mostly least average particle size in comparisons to other dispersions. The sixth hour of grinding shows a completely inverse trend as compared to the trend observed in beginning of the grinding process (i.e. the higher the concentration of a dispersion, the smaller its average particle size) or in other words, the use of concentrated dispersions favors the process of size reduction.

Particle sizing by dynamic light scattering

The technique of dynamic light scattering is different from acoustic attenuation spectroscopy in two major aspects: (i) it measures the particle size of dispersions in dilute state. (ii) The particle size measured by this technique is hydrodynamic and not volume-based as in acoustic attenuation spectroscopy. Fig. 6 demonstrates the results of average particle size of the dispersions obtained at different grinding times.

The results explain that size reduction process is favored by the use of dilute concentrations. The trend remains un-changed throughout the grinding process.

Causes of different trends

Techniques of acoustic attenuation spectroscopy and dynamic light scattering generate different results in terms of numerical values and trends as shown in Fig. 7.

In the absence of scientific explanation of such opposing trends, one may get misled during the process of controlling, designing and optimizing the wet grinding process for production of fine particles. It is, therefore, important to investigate the roots of the problem that may lie in the process of particle sizing. The following hypotheses were developed in order to investigate the problem. H1: Different trends are due to different types of diameters of particles measured by two different techniques of particle sizing. H2: Different trends are due to dilution of samples, in case of particle sizing by dynamic light scattering, which changes the particle size generating misleading information. H3: The two different trends are due to multiple scattering in acoustic measurements that generates misleading information.

Effects of different diameters on measurements

The diameter of particles measured by dynamic light scattering is hydrodynamic diameter. Based upon the theory behind the technique (Section 2.2), hydrodynamic diameter may be regarded as the diameter of the particle plus the thickness of the layer of ions surrounding the particle, at both ends of the diameter [START_REF] Tourbin | A survey of complementary methods for the characterization of dense colloidal silica[END_REF]. Approximately, D H = D + 2/k (where D H = hydrodynamic diameter, D = diameter of particle and 1/k = thickness of layers of ions around the particle, known as Debye length) [START_REF] Tourbin | A survey of complementary methods for the characterization of dense colloidal silica[END_REF]. The value of the thickness of the layer of ions (from both ends of diameters) may be subtracted from the hydrodynamic diameter in order to get diameter of the particle (D). The method proposed by Israelachivili for the calculation of Debye length was employed [START_REF] Israelachvili | Intermolecular and Surface Forces[END_REF]. For the electrolytes at 25 °C involving monovalent cations and anions (such as sodium polyacrylate), the following equation may be used for calculation of Debye length [START_REF] Israelachvili | Intermolecular and Surface Forces[END_REF]:

1 κ = 0:304 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Electrolytes ½ p ð3Þ
where 1/k = thickness in nanometers of layers of ions around particle known as Debye length, and [electrolytes] = concentration of the electrolyte in moles/litre. The magnitude of Debye length depends solely on the properties of the liquid and not on any property of the surface such as its potential or charge [START_REF] Israelachvili | Intermolecular and Surface Forces[END_REF]. Table 5 shows the results for diameter of particles (D) at 25 °C when particles of CaCO 3 (insoluble) are surrounded by sodium polyacrylate (electrolyte).

The results demonstrate that the trend (i.e. use of dilute dispersions favors size reduction) as observed in Fig. 6 (in case of dynamic light scattering) remains the same even after subtracting the Debye lengths from the results of dynamic light scattering.

Effects of dilution

Acoustic attenuation spectroscopy and dynamic light scattering when employed for studying the effects of concentration on particle size in wet grinding, yielded two different trends disabling us to reach a conclusion. Particle sizing by dynamic light scattering requires the samples to be diluted before measuring the particle size. The step of dilution may change the particle size leading to different trends. In order to investigate, all the dispersions (5%, 10%, 20%, and 30% (m/m)) obtained after grinding experiments were diluted gradually. In principle, the zeta potential may show the stability of a dispersion indicating any change in particle size or otherwise; but the option could not be employed as measurement of zeta potential itself with Zetasizer Nano ZS require dilute of the sample. Therefore to study the change in particle size with dilution, pH values and acoustic spectra were measured at each step of dilution. The maximum changes in pH values were found to be 0.31, 0.41, 0.77 and 0.88 respectively in the process of dilution of the dispersions of 5%, 10%, 20%, and 30% (m/m) as shown in Fig. 8.

The changes in values of pH are not considerably high. Zeta potential that can provide information about stability of dispersions was equally measured (by diluting the samples), in all dispersions after 6 h of grinding, as shown in Fig. 9.

All the dispersions were found to be having values less than -26.6 mV indicating reasonable repulsive forces between the particles in the dispersions. Change in Zeta potential depends upon change in pH, concentration or quantities of additives, etc. Based on the results as shown in Fig. 8 it can be considered that pH has not affected the zeta potential. It is difficult to study the effects of concentration change (dilution) on zeta potential using dynamic light scattering technique (Zetasizer Nano ZS). Acoustic attenuation spectroscopy, therefore, was employed at each step of dilution to understand how this technique explains the effects of dilution on stability and particle size. In general, we observed an ascending trend in particle size in dispersions of 20% (m/m) and 30% (m/m) whereas a descending trend in particle size in dispersions of 5% (m/m) and 10% (m/m) as shown in Fig. 10.

Acoustic attenuation spectroscopy is unsuitable for analyzing very dilute dispersions (suspensions) particularly below 1% (m/m) [START_REF] Julian | Ultrasonic measurements in particle size analysis[END_REF]. In our experiments, the limit of un-suitability seems to start from 2.6% (m/m) as shown by the dotted line in Fig. 10. A reason for ignoring the acoustic results during dilution process below the value of 2.6% (m/m) is our pH study of the dispersions that indicated that pH values remained almost constant below that concentration. Fig. 11 compares the results of particle sizes obtained after 6 h of grinding with the particle sizes of the same dispersions when each one was diluted to 2.6% (m/m).

It is observed that the trend is reversed. When this trend was compared with the trend in particle sizing by dynamic light scattering, both were found to be in accordance as shown in Fig. 12.

Effects of multiple scattering

Multiple scattering is a phenomenon in which attenuated acoustic wave from one particle is scattered by other particles changing the acoustic signal and thus bringing to misleading results. Nonlinearity observed in acoustic curves (attenuation plotted as a function of concentration) may indicate the presence of multiple scattering [START_REF] Tourbin | A survey of complementary methods for the characterization of dense colloidal silica[END_REF][START_REF] Hipp | On multiple-particle effects in the acoustic characterization of colloidal dispersion[END_REF]. The method adopted by M. Tourbin and C. Frances [START_REF] Tourbin | A survey of complementary methods for the characterization of dense colloidal silica[END_REF] was followed to confirm the existence of multiple scattering. The method takes into consideration the excess of attenuation which may be described as [START_REF] Tourbin | A survey of complementary methods for the characterization of dense colloidal silica[END_REF]:

α e ðf Þ = α T -ð1-/Þð0:0047f 2 -0:0083f Þ ð4Þ
Where α e (f) = excess attenuation (db/in.) at a certain frequency f, α T = total attenuation (db/in.) of the dispersion, ø = %volume concentration of the dispersion, and f = frequency (MHz) of acoustic wave. A new series of experiments was performed to obtain data required in Eq. ( 4). Each of the four dispersions (5%, 10%, 20%, and 30% (m/m)), after the grinding experiment was diluted to a certain volume concentration; ultrasound waves of different frequencies were passed through the dispersion to measure the excess and total attenuation in the sample. The same dispersion was diluted further to several levels of volume concentrations and at each level of dilution the excess and total attenuation were measured. The same procedure was adopted in all four cases of dispersions (5%, 10%, 20%, and 30% (m/m)).

Our investigation brings out that particles obtained after 6 hours of grinding process, in each case of the dispersions (5%, 10%, 20%, and 30% (m/m), show multiple scattering. Fig. 13 demonstrates the excess of attenuation measured against concentration for the 20% (m/m) dispersion as an example. Non-linearity at different frequencies indicates presence of multiple scattering.

Conclusions

The concentration of a dispersion plays an important role in particle sizing in size reduction process. This role emerges very distinctively when particles enter from micron to fine particle size range during stirred media milling. Acoustic attenuation spectroscopy, which is generally considered to be capable of measuring average particle size of the dispersions at different values of concentrations, shows certain limitations in stirred media milling. The technique explains that the use of relatively concentrated dispersions may be favorable in achievement of finer particles during stirred media milling carried out under a set of fixed operating parameters. However, dynamic light scattering demonstrates that the use of relatively dilute dispersions favors in achievement of finer particles during stirred media milling carried out under a set of fixed operating parameters. Our investigations bring out that particle sizing by acoustic attenuation spectroscopy cannot be considered valid at higher concentration of samples of dispersions during stirred media milling because of multiple scattering that influence the results. If concentration of the sample of dispersions is reduced considerably in particle sizing by means of acoustic attenuation spectroscopy in stirred media milling, then anomalies in the results may be avoided. In such cases both the techniques shows reasonable agreement that use of relatively dilute dispersions favors in achievement of finer particles during stirred media milling carried out under a set of fixed operating parameters.

Fig. 1 .

 1 Fig. 1. Passage of a sound wave through dispersion.

Fig. 2 .

 2 Fig. 2. Particle size distribution and shape of CaCO 3 used.

Fig. 3 .

 3 Fig. 3. Experimental setup.

Fig. 4 .

 4 Fig. 4. (a): Changes in acoustic spectra due to increase in quantity of relatively fine particles. (b): Changes in acoustic spectra due to changes in proportion of relatively fine particles. (c): Acoustic spectra corresponding to different disperse media. (d): Comparison of acoustic attenuation spectra.

Fig. 5 .Fig. 6 .Fig. 7 .

 567 Fig. 5. Particle sizes measured by acoustic attenuation spectroscopy during size reduction process in stirred media mill.

Fig. 8 .Fig. 9 .Fig. 10 .

 8910 Fig. 8. Changes in pH during dilution of dispersion after grinding.

Fig. 11 .

 11 Fig. 11. Comparative study of particle sizes by acoustic attenuation spectroscopy.

Fig. 12 .Fig. 13 .

 1213 Fig.12. Comparative study of particle sizes by two different techniques after dilution.

Table 1

 1 Compositions of dispersions used.

		Disp. 1	Disp. 2	Disp. 3	Disp. 4
	% Concentration (m/m)	5.000	10.000	20.000	30.000
	% Concentration (v/v)	1.934	3.997	8.565	13.837
	Water (g)	1700	1700	1700	1700
	CaCO 3 (g)	89.47	188.89	425	728.57
	Sodium polyacrylate (g)	7.158	15.111	34.000	58.286
	(8% of mass of CaCO 3 )

Table 2

 2 Physical properties at 25 °C used in calculations for particle sizing.

	CaCO 3
	Density (g cm -3 )	2.6560
	Sound speed (cm s -1 )	5.5600 × 10 5
	Thermal dilation (°C -1 )	2.2130 × 10 -6
	Thermal conductivity (erg cm -1 s -1 °C-1 )	2.2594 × 10 5
	Heat capacity (erg g -1 °C-1 )	8.3680 × 10 6
	Shear rigidity (dynes cm -2 )	3.0800 × 10 11
	Water
	Density (g cm -3 )	9.9524 × 10 -1
	Sound speed (cm s -1 )	1.4970 × 10 5
	Thermal dilation (°C -1 )	2.5677 × 10 -4
	Thermal conductivity (erg cm -1 s -1 °C-1 )	5.9525 × 10 4
	Heat capacity (erg g -1 °C-1 )	4.1785 × 10 7
	Viscosity (poise)	9.0300 × 10 -3

Table 3

 3 Set of fixed operating parameters.

	Grinding media	Zirconium oxide beads stabilized by yttrium
	Size of grinding media	300-500 µm
	Density of grinding media	3.7850 g/cm 3
	Volume of grinding media	415 ± 2 cm 3
	Volume of grinding chamber	670 cm 3
	Flow rate of dispersion	4.82 ± 0.1 cm 3 /s
	Grinding system	Zeta
	Speed of stirrer in grinding chamber 1500 rpm
	Speed of agitator in stirred tank	600 rpm
	Separating cartridge (filter)	100 µm
	Temperature	25	°C

Table 4

 4 Aqueous dispersions of CaCO 3 mixed with different proportion of coarse and relatively fine particles.

	Dispersion	H 2 O	CaCO 3 (30 µm)	CaCO 3 (1.6 µm)
	no.	g	g	g
	(1)	700	200	0
	(2)	700	0	5
	(3)	700	0	25
	(4)	700	0	50
	(5)	700	0	100
	(6)	700	0	150
	(7)	700	200	5
	(8)	700	200	25
	(9)	700	200	50
	(10)	700	200	100
	(11)	700	200	150

Table 5

 5 Debye length and diameter of particle (D) at 25 °C in different dispersions.

	Dispersion%	Moles of	Concentration	1/k	D H	D
	(m/m)	electrolyte	(mol/L)	(nm)	(nm)	(nm)
	5	0.00140	0.000826	10.5800 155.9	134.7
	10	0.00296	0.001743	7.2818 165.1	150.5
	20	0.00667	0.003922	4.8545 187.3	177.6
	30	0.01143	0.006723	3.7077 217.7	210.3