WADE

Following the changeover of political power in Senegal in 2000, public transport reform was introduced in the capital. 1 Reform development began in the 1990s with the support of the World Bank, which defined the overall direction and financing mechanisms. At the time, the city of Dakar was heralded as a "pilot city" on the African continent. 2 The reform measures gave priority to "private" management of urban transport over the "public" management system in place until then. Yet an analysis solely in terms of "privatising" public action would not give us the full picture of the turmoil the reform generated in the sector. In Senegal, transport is dominated by so-called "informal" companies with close ties to the State, a situation that rules out any reform approach radically opposing the State to the market. In this context, liberal reform does not always imply a withdrawal of the State. On the contrary, it may give the State an opportunity to reaffirm or renegotiate the framework in which it intervenes. Contrary to the notion of privatisation, the notion of "discharge" 3 allows for mutual dependence between private and public actors. It points to the growing importance of private actors in state intervention, but suggests that these changes should be analysed as a restructuring of public action rather than simply as the retreat of the State. Hence, this article does not intend to examine the shift in the boundary line between public and private, but rather the nature of the ongoing negotiations between the two sectors in a context of reform.

The bus stations in the Dakar region were the main sites of government restructuring in the area of transport. The relationship between the transport sector and the State originally grew out of management issues in these stations, so it was logical for the reforms to take place there. This relationship was forged primarily within and in reference to "unions" that grouped together all the drivers working out of the same station, which had long been responsible for station management in place of the municipal government. Implementing the reform in the stations meant "privatising" the management of these spaces, and thereby calling into question the role of the unions. In addition, the stations were where the new vehicles required by the reform were installed. We will therefore base our analysis on the data collected in two of the so-called "urban" bus stations affected by the reform: the Petersen station and the Colobane station. The first is close to the centre of the capital and serves the suburb of Dakar. Opened in 1996, it was formed by combining three separate stations previously disseminated in the capital. It was the first bus station in the capital to have its management privatised. Furthermore, the first fleet renovation took place on the bus lines served by this station. The Colobane station also serves the Dakar suburb. It is located at the entrance to the city, along the National 1 highway. The privatisation of its management was announced later on, in 2008. Part of the fleet operating out of the Colobane station also underwent vehicle renovation. For purposes of comparison, we will include observations carried out at the Pompiers station, one of the main stations in Senegal and no doubt the oldest. It takes its name from the site where it was set up in 1962, near a fire station [in French: caserne de pompiers]. The Pompiers station serves the other cities and the lower region of Senegal. It is described as an "intercity" bus station. It has not yet been affected by the reforms, which so far have concerned only "urban" traffic.

The data used in writing this article were gathered during a twomonth study conducted between March and April 2010. 4 Analysis of the interviews, observations and documents produced by the various entities managing the bus stations reveals the logic that helped to shape the successive representatives of the State in the transport sector. After the changeover of political power, state intervention through the reform promoted by the country's donors has merely recycled the logic inaugurated during the socialist party era. Accordingly, the affirmation of the State's authority has not been the result of its pro-active intervention alone, but rather of a series of arrangements that suited its interests and those of the various actors in the transport world. Transport reform has not questioned the role of the State but rather that of its representatives in the sector, in this case the drivers unions.

The successive representatives of the State in the transport sector

Unlike peasant organisations during the liberalisation of Senegal's agricultural sector, the trade unions formed first by hauliers and then by drivers succeeded in becoming representatives of the State in the transport sector. 5 The reform of the transport sector ushered in by the World Bank in the early 2000s was merely an updated version of previous reforms carried out when the socialists were in power during the 1970s. Transport sector reform was one of the areas in which a socialist "hegemonic bloc" took hold in the first decade after decolonisation. Initially, it was set up for the benefit of hauliers, and subsequently for drivers, after the peaceful handover of political power from Senghor to Diouf. The reforms and the converging interests of the government and major hauliers and later drivers resulted in the creation of the unions. This is the structure inherited from the socialist era that post-changeover reform called into question.

Major hauliers

During the first decades following decolonisation, Senegalese hauliers gradually ceased to be marginalised by transport initiatives promoted by Europeans or the equivalent. They continued to be marginalised by the State, however, until economic "Senegalisation" began in the 1970s, when hauliers obtained government recognition.

Under colonial rule, intercity transport of goods and passengers was largely dominated by hauliers of Lebanese extraction. The Régie du transport du gouvernement general [General Government Transport Authority], which became the Régie des transports du Mali [Mali Transport Authority] in 1959, and then the Régie des transports du Sénégal [Senegal Transport Authority] in 1961, controlled urban transport. This public company was at first responsible for the transport of civil servants, but eventually widened its scope of action to serve the entire population. In addition to public transport, there were also private "rapid coaches", first in Saint-Louis and then in Dakar, belonging to Senegalese operators, each of whom owned a fleet of a dozen vehicles. Although the novelty of the new coaches and the obsolescence of horse-drawn carriages worked to the advantage of such initiatives,6 the economic policy pursued during the 1960s favoured the public transport network in the capital, whereas Lebanese hauliers maintained their dominant position in intercity transport throughout the country.

At the end of the 1960s, the regime of Léopold Sédar Senghor drew widespread criticism for its economic strategy of continuity with the colonial period. The opposition came, in particular, from Senegalese businessmen, among them the hauliers belonging to the Union des groupements économiques sénégalais [Union of Senegalese Economic Groupings] or UNIGES. They attacked Senghor for continuing its preferential treatment of French economic interests over those of Senegalese entrepreneurs. After promoting the creation of a more moderate hauliers trade union, the State finally managed to meet their demands. 7 The "Senegalisation" of the economy thus began in the early 1970s.

The State generated a new clientele for itself through financial institutions empowered to make loanswhich were seldom repaidto revitalise Senegalese business activities. Financing was made available in all sectors of the economy, including transport. Loans were granted mainly on the basis of the entrepreneurs' degree of proximity to the ruling power. 8 The new policy led to the gradual reintroduction of rapid coaches and their owners, who received de facto recognition from an inter-ministerial advisory board in 1973. These hauliers were incorporated into institutions such as the Transport Management Committee, responsible for jointly defining transport policy. 9 In 1976, they benefitted from a plan to renovate their fleets of vehicles. In 1980, a "masters' operation" offered young graduates jobs in the transport field and provided an initial opportunity to streamline the sector. 10 Some of the major hauliers combined their dominant economic positions with advantageous positions in religious and political spheres. The success of these businessmen earned them considerable status in the Mouride Brotherhood, even when they had no family ties with the marabouts.11 On the political side, certain large hauliers such as Bamba Sourang and Lobatt Fall won seats in parliament. They became members of state bodies such as the Economic and Social Council. They were also awarded numerous public procurement contracts to transport civil servants.

The community of drivers

The transport world was gradually integrated into government networks through individuals belonging to the many "unions" and the community of drivers scattered throughout Senegalese territory. In 1985, the Fédération nationale des groupements de transporteurs du Sénégal [National Federation of Senegalese Haulier Unions], the longstanding trade union of hauliers set up in 1963, was weakened by a split resulting in the creation of a second federation, the Fédération nationale des transporteurs du Sénégal [National Federation of Senegalese Hauliers]. In 1991, the hauliers were divided by a further split that produced a third trade union, the Syndicat national des transporteurs et chauffeurs du Sénégal [National Hauliers and Drivers Union of Senegal]. 12 During the same period, the drivers were increasingly challenging the legitimacy and ability of major hauliers to represent them. They decided to organise on their own within the country's various bus stations. President Abdou Diouf used these internal divisions to achieve his own political ends: the drivers unions joined the urban network of the socialist party.

At first, the splits between major organisations within the haulier confederation arose from the initial measures for sector liberalisation: the debate was focused on how much tax they had to pay. But the large hauliers were also competing within the socialist party for seats in parliament or various administrative bodies. Cracks gradually 6 appeared in trade union representation of the hauliers, weakening the overall ties between trade unionism and the ruling party.

The first versions of the structural adjustment plans in the transport sector modified the relationships between hauliers and drivers, thereby helping to generate opposition on the part of the drivers. In 1985, the longstanding practice of delivering licences according to itinerary was eliminated. In 1986, vehicle imports were liberalised. The number of hauliers grew and, with them, the number of drivers. The profiles of investors in the transport world diversified. During the 1990s, a growing percentage of hauliers had no experience as chauffeurs, mechanics or apprentices. This "de-professionalisation" of the sector created turmoil in labour relations between drivers and hauliers. Drivers were no longer assured of fixed wages and compensation for travel expenses; instead they were regularly required to pay a flat fee to the haulier for the use of its vehicle. 14 In this context, the drivers also accused the hauliers who represented them of thinking only about their own personal advantage and not "intervening" sufficiently on their behalf.

Abdou Diouf sought to break up the partisan blocs established during the Senghor's regime and used the drivers' demands for this purpose. In exchange, the drivers seized upon the president's ideas to create their own trade union representation. The alliancesreligious, trade union and partisanformed with the major hauliers on a national scale developed into local alliances within the bus stations.

Originally, the drivers unions were dahiras, an urban version of daaras, rural associations of Mourides. Drivers operating from the same station contributed to a dahira contingency fund. Even today, the unions are presented as social organisations. They serve as channels to communicate instructions from marabouts (N'diguel). 15 The unions therefore help to perpetuate the "Senegalese social contract". 16 They also oversee money collection for the marabouts. Indeed, each dahira is required to make an annual offering to its marabout. The transport sector appears to have become a haven for the Mouride Brotherhood weakened by the agricultural crisis in the mid-1970s.

When the unions were first formed in the mid-1980s, they also functioned as "works councils". They relayed calls to action from the 7 socialist party and kept the government informed about various social tensions. They were therefore fully involved in carrying out Abdou Diouf's strategy to erode the CNTS. It was not until the 1990s that the unions finally returned to the CNTS.

In their dealings with the Dakar municipality, the "works councils" or dahiras were called "management committees". Despite early decentralising measures, the municipality remained attached to the central government. [START_REF] Diop | Enjeux et contraintes politiques de la gestion municipale au Sénégal[END_REF] Relations with city authorities took the form of direct, personal relationships between the mayor and the most influential members of the unions. Thus the Pompiers union obtained new lighting as well as a paved boarding area from the mayor.

The unions were therefore neither an idea previously overlooked by the State and the municipality nor a sign of their "failure" to manage public space. On the contrary, the unions emerged "from a functional collaboration that ensured the link between the State's role in controlling space and social groups using potential political ploys to achieve their ends". [START_REF] Salem | Crise urbaine et contrôle social à Pikine. Bornes-fontaines et clientélisme[END_REF] In the transport sector, this special arrangement took concrete form in the unions. These are the structures that have been undermined by the recent transport reform.

Bus stations during the socialist party era

Drivers unions were "transparent/opaque institutions". 19 Though private, they exercised state authority in the stations. They were incorporated into the municipal and national administrative network mainly through the practice of "brokering". [START_REF] Blundo | Dealing with the Local State: The Informal Privatization of Street-Level Bureaucracies in Senegal[END_REF] The reform has called into question all of these daily interactions. . To describe day-to-day station management by driver unions, we have primarily used the example of the Pompiers bus station today. As an "intercity" station, it has not yet been affected by the reforms described above and thus reflects the "discharge" methods adopted at the end of the 1980s.21

The board and its "base"

The unions have a pyramid organisation with ordinary drivers at the bottom and a "board" at the top. Board members are the only ones with direct links to the marabouts, the trade union confederation and political parties. The method of union affiliation to the CNTS clearly illustrates these relationships. Only a handful of board members have a card proving their affiliation with the CNTS. In the case of a call for strike, for example, the heads of the confederation telephone their representatives on the board, who in turn relay the instructions within the station. Each week, the union as a whole pays the CNTS a fee drawn from station operating revenue. The amount of the fee varies according to the number of drivers affiliated by the union with the CNTS.

The board president is elected at a general assembly. The other board members are appointed by the president or recommended by his circle. These positions are coveted because they carry both prestige and fixed wages. Appointments to the posts are made in keeping with "factionalist"22 logic. The board members justify their positions on the basis of know-how, seniority or personal success, but this rhetoric partly conceals family or regional ties. At the Pompiers station, the secretary general of the union explains his position by his level of education. However, he is also the brother of a union member and had never worked in transport prior to occupying this post. 23 The distribution of posts and aid thus leaves some station players on the sidelines. The latter view the selective distribution system as a form of graft.

And indeed the board members have resources at their disposal, which they distribute in the form of posts or material advantages to various groups that use the site. Bus stations comprise several sectors of activity. There are, of course, boarding platforms, divided up according to the types of vehicles present in the market, and areas set aside for mechanics, hawkers and cheap restaurants. Others are reserved for vendors with fixed locations. The clientele of the union and its board is made up of all these sectors, each one operating in a well-defined territory of the station.

The board takes part in appointing the personnel in charge of passenger boarding. The station areas assigned to boarding are divided into "lines". Each line corresponds to a destination and a type of vehicle. There is a "line chief" at the head of each line, along with his team of "coaxers" responsible for drumming up customers. 24 The line chiefs and the coaxers are usually former drivers who previously worked on the destination they head up today. Coaxers are hired at the discretion of the line chief or of board members, depending on the charisma of the line chief. The line chiefs, on the other hand, are elected. Only the coaxers and drivers working on the line are allowed to vote. Elections are widely overseen by the union and its secretary general.

The personnel in charge of boarding pay a percentage of their daily income to the union. The customer pays the coaxer the price of the ride. The line chief centralises the money before the bus leaves the station and pays out a specified portion to the driver and the other coaxers. The amounts vary from one line to the next. By the end of the day, each line has taken in a considerable amount of revenue, a percentage of which goes directly to the union. The union members present the contribution process as a system of sharing the risks assumed by the drivers and coaxers. The payment must be sufficient to cover expenses arising from unforeseeable events such as death, illness or birth. This "withholding" tax is the union's chief source of revenue, which accounts for their fear of internal dissension or "tendencies". Allowing the lines and their personnel too much autonomy would limit union revenue. Union rules therefore stipulate: "Any member who seeks to divide the order and unity of the union shall be excluded". 25 In addition to posts, the board assigns spaces inside the station. Though the situation is seldom acknowledged, it appears to be impossible to engage peacefully in commercial activity in the bus station without union authorisation. Any business activity undertaken without prior approval from the union runs the risk of having its storefront obstructed by vehicles. This applies not only to mechanics and fixed vendors and hawkers, but also to business activities without any connection to transport that simply have their premises located inside the station. To obtain a parking place at the Pompiers station, a manager of an insurance agency had to pay a tax to the union as well as to the coaxers working in the vicinity of the parking place. In 10 addition, the agency occasionally lends its more spacious premises to the union for its meetings.

The board and the administration

Everyday corruption, informal practices and privatised public services are different types of "arrangement" worked out between the board and the authorities. The daily management of the bus stations is based on such "arrangements". These interactions concern exclusively the board members, state representatives and municipal authorities.

The union and its board appear first of all to act as an interface between the police, the gendarmes and site users. At Pompiers, the police occupy the same premises as the board members, where they share the telephone and their archives. The location and sharing of offices belonging to the union is the corollary of the inadequate resources available to civil servants in the police force present on the site and corresponds to an evergetist practice. It enables the police to have premises inside the bus station and is a corruptive investment for board members. The union premises are divided into two parts separated by a half-closed curtain, which shows the police agent's desk but hides the bench on which drivers with violations are kept waiting. 26 Police officers and board members spend their time in the same space and hence develop privileged relationships. They eat at the same place and joke together. The same policemen seem to work at the station for years. The proximity and longevity of these relationships lead to a sort of symbiosis between police and board members. Behind this daily routine and harmonywhether sincere or feignedadministrative relationships become personalised and a series of corruptive investments are made, resulting in the informal privatisation of public services. 27 This proximity takes concrete form particularly in the repression of traffic code violations.

Roadside interaction between drivers and the police or gendarmes can lead to one of two types of monetary penalty. The police agent may impose a direct monetary penalty on the driver by making him pay a fine immediately. On the other hand, the police agent may impose an indirect monetary penalty on the driver by confiscating his driving permit, which he can recover only by paying a fine at the police premises. The transactions linked to direct monetary penalties take place on the road and often allow some leeway for traditional bribery practices. They do not require the intervention of the union. In the case of indirect penalties, the board, like its counterparts in Benin or Nigeria, plays a central role. 28 Only one or two board membersthe representatives of the CNTS within the unionare authorised to deal with the police. Their membership in the confederation allows them to brandish the threat of a strike or the support of high-placed connections, putting them on equal footing in their dealings. The trade union representatives recover all the driving permits seized by the police from drivers working out of the bus station. The drivers involved then file past the desk of the union secretary general to recover their permits in exchange for financial compensation, which is intended to remunerate both the union's brokering and the police officers. This auxiliary function comes into play only on such occasions. It may also pertain to other types of attestations such as "off-road" permits required to transports passengers on unpaved roads or extensions of certification in the event of temporary licence withdrawal. The unions are thus seamlessly inserted in "the chain of transport corruption". 29 The partnership goes so far as to designate targets and point out violations to the policeman on duty in the station. The garage manager can "take disciplinary action against coaxers and their table representatives 30 who owe him obedience"; 31 he must also "ensure the smooth operation of the union". 32 Within the scope of his attributions, he therefore presents coaxers lacking union accreditation to the police for punishment. The secretary general of the union and the policeman assigned to station surveillance conduct joint rounds. Together, they settle parking and traffic problems around the station and combat "poaching", defined as boarding or disembarking customers outside the station walls. Poaching represents not only a financial loss for the union but it is interpreted first and foremost as an act of secession. The fight against poaching is therefore a union priority. The board assigns one of its members to work on this task full time. "Customer poaching" is subject to a fine that can vary from 6,000 CFA francs to 12,000 CFA francs for automobiles and 24 000 CFA francs to 50,000 28 N. Bako-Arifari, " 'Ce n''est pas les papiers qu'on mange!' La corruption dans les transports, la douane et les corps de contrôle", in J.-P. Olivier de Sardan and G. Blundo (eds.), État et corruption en Afrique. Une anthropologie comparative des relations entre fonctionnaires et usagers (Bénin, Niger, Sénégal), Paris, Apad/Karthala, 2007, pp. 179-225 29 Ibid., p. 195. 30 A "table" specifies the type of vehicle for each destination of a departing line. Ce que je veux dire c'est qu'un" tableau" est l'équivalent d'une "ligne". 31 Internal Union Rules, Dakar, 2010.

12 CFA francs for buses. 33 The amount of the fine depends on the driver's attitude towards the union in the past and at the time he is commits his offence. A portion of the fine is systematically given to the police officer. On the other hand, the board member in charge of combating poaching may sometimes keep some of the amount for himself. In the eyes of some bus station workers, the police are so completely subordinated to the union that, although civil servants, they are perceived as full-fledged employees of the union.

Taxation and rubbish disposal in the stations

Collecting taxes and site cleaning offer further opportunities for the union to "make deals"not with the police but with various municipal departments.

The collection of municipal taxes on entering vehicles seems to depend largely on the goodwill of the union, to such an extent that when both entities have tax collection systems on the same site, the union system takes precedence. Queries made to the municipal tax collector concerning station management are first referred to the union board, by the way. In the municipal tax collector's view, the union is the real manager of the station. Municipal tax collectors are located at the entrance to the station. Each vehicle pays 100 CFA francs (about 0.15 Euro) to the municipality in exchange for a ticket. The municipal tax collector's post is not easily identifiable as such: it takes the form of a curtain hung at the bus station exit. The person collecting the tax is usually surrounded by two or three other people. The "auxiliaries" 34 are there to ensure the safety of the tax collector, who is at the mercy of all sorts of aggressors. The outward appearance of the municipal tax collection post contrasts sharply with that of the union office, a permanent construction in the centre of the station where the union collects its own taxes. Upon arrival, each driver has to go to the union office and pay 50 CFA francs (about 0.07 Euro) to the secretary general. Contrary to cities like Abidjan, Ibadan, Lagos or Accra, in Dakar municipal tax collection is not entirely delegated to the union. The two collection systems operate side by side on the same site.

In addition to the authorisation to collect taxes on bus station premises in the strict sense, taxes may also be collected outside the station walls. Many unofficial bus stations have sprung up throughout Dakar. These stations are set up by drivers and hauliers who object to 33 6,000 CFA francs correspond to slightly more than 9 Euros; 50,000 CFA francs to more than 75 Euros. 34 J.-P. Olivier de Sardan and G. Blundo, "La corruption quotidienne……", art. cit., p. 22 the organisation of departures at the station and seek to circumvent the unions' management systems. Buses are indeed forced to await their turnsometimes for several daysbefore they are allowed to leave. To avoid waiting, drivers take passengers on board at other sites. These stations are not recognised as such by the State, but their activities are nevertheless taxed by the union. Board members travel around Dakar and take a percentage of the revenue generated by these initiatives. The union even has a vehicle for this purpose. To enforce the tax, board members sometimes call upon wrestlers.

The union is also an indispensable contact in station cleaning operations. Although municipal authorities, particularly the Urban Development Department, send contract employees to collect rubbish, any large-scale action apparently involves the union. Station sites are regularly criticised for their squalor. Lack of space and cluttered conditions might lead drivers to avoid certain bus stations, resulting in revenue losses for the union. The union therefore negotiates with the municipality. It prevents municipal agents from collecting taxes if rubbish has not been collected, thereby depriving the municipal authorities of a steady source of income. To continue collecting taxes, the municipality is thus forced to undertake cleaning. The work is carried out by intermediaries of the union, which is paid a certain sum of money by the municipality. The union also has its own cleaning staff, who sweep the litter to the exit, where it is picked up by the rubbish collection company. When the rubbish concerns retail spaces inside the station, the delegation of authority may take yet another form, in which the union once again plays a central role. The union members remunerate cheap restaurants for cleaning own their sites; the restaurants may in turn delegate the task to a young talibé [child beggar].

The union is an indispensable entity in the areas of security, rubbish disposal and tax collection. Through its board, it operates as a real brokers' association. By introducing new institutions, transport reform and its first visible effects called into question the role of institutional creation during the socialist era. What the reform questioned was more than a mere institution; it was a whole set of daily interactions that have helped to shape the standards for public action in the transport sector.

Bus station reforms during the PDS era

At the turn of the 21 st century, the reform of transport in the Senegalese capital was more intense than any other reform implemented under sector adjustment programmes launched by Abdou Diouf starting in the early 1990s. President Aboulaye Wade clearly took possession of transport reform, making it the symbol of his policy of change. He explicitly declared his intention on the front page of the pro-government newspaper Le Soleil: "I want to revolutionise urban transport". 35 The new president announced: "We are going to buy back rapid coaches to house fish and shrimp". 36 Though the measures were designed and financed by the World Blank, they were taken over by the Senegalese State and various economic actors in the transport sector. The reform was introduced at the expense of the unions, despite their support for the Parti démocratique sénégalais or PDS [Senegalese Democratic Party]. The contracts awarded at the time of bus station privatisation and vehicle fleet renovation gave the post-changeover government an opportunity to build itself a new network in the transport sector. The reform allows a kind of "spoil system37 " or in other word the change of the "discharge" contractor. Pro-active intervention by the State was possible only because it coincided with "circumstantial opportunities"38 for a series of economic players. The reform served the interests of a composite group that had been previously kept out of the management of passenger boarding sites. In the various bus stations studied that underwent reform, union opposition to a change in "discharge" contractors led to the coexistence of two modes of bus station management: a new mode, resulting from liberalisation, headed by hauliers through EIGs (economic interest groups) and the old mode, an outgrowth of the socialist era, embodied by union management of driver representatives.

Bus station privatisation

Bus station privatisation was carried out independently of a new traffic plan for Dakar backed by the World Bank. The Ministry of Finance decided to award car park management to private operators. Reassigning the sites was seen as a possible solution to traffic congestion in the centre of the capital in the late 1990s. Through an opaque bidding process, newcomers succeeded in rising to positions of bus station leadership. Those who played the role of experts in preparing the call for tenders won the contracts. 39 The municipality of Dakar issued the call for tenders. The high rates charged for parking in Dakar-Plateau went hand in hand with the installation of a parking area for vehicles at the entrance to the city and a shuttle service to the city centre. A private company known as Société africaine de gestion des espaces de stationnement or SAGES [African parking space management company], which already managed parking in Dakar-Plateau, took charge of the Petersen bus station. At the time, the company was made up of former members of the inner circle of the ruling socialist party, who had remained in government after the changeover. Some of them had also worked as consultants in preparing the call for tenders.

The Petersen station established a precedent for extending privatisation to other stations like Colobane. In the latter case, management was awarded to a private operator through a far less sophisticated false public procurement process. A simple "mutual agreement" between the municipality and the beneficiary allowed the latter to claim the authority to manage the bus station. The governor, as the direct representative of the government, played the role of arbiter in settling conflicts between various claimants. He decided against the union.

The drivers unions at both sites systematically opposed privatising the bus stations; they mobilised station workers, claiming to act on their behalf, and succeeded in forcing the takeover entities to withdraw. In the case of the Petersen station, the entity was SAGES, whereas in the case of Colobane, the prospective buyer was an individual entrepreneur. In the Petersen case, the union circumvented SAGES authority. Tensions between the union and the new manager surfaced in everyday situations such as the payment of rent or collecting taxes on vehicles that frequented the station. The union refused to allow a SAGES employee to collect taxes directly from drivers, so a union employee performed the task. The SAGES employee in charge of collecting taxes from drivers arriving and leaving the station could only do so indirectly through the union. This system allowed the union to combine the tax collected in the name of SAGES with several other taxes intended for the CNTS, to which it was linked, and for itself. The union collected the taxes and distributed locations to retail outlets operating outside the buildings that followed station renovation. A henchman was mandated by the union to collect a tax from hawkers. To sell wares inside the station, the vendors had to buy a ticket with the words "Bus Station Management Committee" printed on it. The presence of these words created confusion in the minds of many vendors who thought they were paying a tax to the municipality. SAGES was thus caught between the union and city hall. Indeed the mayor, too, challenged the legitimacy of the new management company, accusing it of failing to meet the specifications of the lease contract, notably with regard to site upkeep. The municipal employees normally in charge of upkeep sometimes seemed to be adrift in the station. Some were reduced to begging from the shopkeepers, while others combined their official job with informal hawking inside the station. SAGES appeared to be focused on its most remunerative activitymanaging parking metres in the city centreinstead of investing in the bus station. Consequently, the company and its employees lacked the necessary legitimacy in the eyes of the municipality and users to collect taxes inside the station without going through the union.

In the case of the Colobane station, the union president relied on his networks in the administration and the Mouride Brotherhood to cancel the public procurement procedure, which took place in 2008. After losing an initial call for tenders, the union was ejected from the installations built during the station renovation. The union members therefore built their own installations next to the new ones, which were kept under lock and key. Subsequently, the president of the union denounced the terms of the call for tenders to the public contracts commission. Faced with what he saw as a "political plot", the union president appealed in particular to a former socialist party minister of transport who was a member of the commission. He also called upon a marabout in the Mouride Brotherhood to advocate on his behalf. A second call for tenders was issued and this time union won, and thus became the official manager of the bus station. Like the transport trade unions in Nigeria, the Colobane union played on its political and party relationships to maintain its control over the bus station. 40

Renewal of the vehicle fleet

Fleet renewal fostered the arrival of new actors; some had already been involved in the transport sector in the mid-1980s when the unions took control of the bus stations, others had worked in other sectors until then. The renewal came in response to one of the main criticisms of transport in the capital: the obsolescence of the old Mercedes buses that in some cases had been in use for more than thirty years. The State did not immediately find a partner to finance the reform in 1999, but in 2002, the World Bank approved a loan of 11 billion CFA francs (about 16.7 million Euros) to renovate the automobile fleet. 41 The financing was coupled with a requirement to "professionalise" 42 the sector. Indeed, the authorities intended to support the renewal by "formalising" the operation of transport vehicles

The selection of the first hauliers to benefit from fleet renewal played a decisive role in the exclusion of the unions. To benefit from the hire-purchase financing mechanism and receive new vehicles, the hauliers had to group together. After rejecting the idea of a single federation of hauliers, they agreed to join together in a financing association, which was supposed to manage the drafts. The Association de financement des transports urbains (AFTU) [Urban transport financing association] acted as an intermediary between the owners and the State in the loan repayment process. The Association was a federation of fourteen EIGs comprising the various beneficiaries of the renewal. The AFTU president was once a haulier himself and a Lebou like the Minister of Transport at the time. 43 This shared experience made him a privileged contact person. One of the main managers of reform implementation was the neighbour of the president of one of the EIGs in the AFTU, also a former haulier. They played football together on weekends. More surprisingly, some members of the EIGs included in the AFTU were in the professions or civil servants. In these cases, fleet renewal was involved: the operation enabled individuals outside the transport world to invest in it. One of the first beneficiaries was the trade union leader of the CNTS-FC (Confédération nationale des travailleurs sénégalais-Force changement) [National confederation of Senegalese workers -Force 41 "Renouvellement du parc des cars rapides, un crédit de 11 milliards de FCFA de la Banque mondiale", L''Info, 11 March 2002. 42 "CETUD rationalise l''exploitation des cars rapides", L''Info, 22 January 2002. 43 The Lebou people are historically the largest ethnic group in the Dakar region.

for change]. This confederation was the principal opponent of the CNTS, the longstanding ally of the socialist party prior to the changeover of political power. The creation of the CNTS-FC was openly supported by the presidential party. 44 Its creation in 2002 led to violent clashes with the unions, which had generally remained loyal to the CNTS. These confrontations took place in and around the bus stations and at the labour exchange where the two opposing trade unions were forced to cohabit. 45 The cause of the clashes was not directly political, since after the changeover the CNTS-affiliated unions also gave their support to the new ruling party. Their primary concern was control over the bus stations and the money generated by their management. 46 The changes brought about by the arrival of the new vehicle models allowed loan beneficiaries to impose their own personnel in the stations, at the expense of drivers union workers. Fearing violence would break out when the new buses arrived, the authorities reserved a special area for this personnel in the centre of the stations. The buses departed at fixed times instead of waiting until they were full. They no longer called upon coaxers to determine the order of arrivals and departures. Most coaxers were former drivers who were either retired or did not have a vehicle in proper operating condition. They saw themselves replaced by a "track chief" hired by the leaders of the EIGs. Changes in the structure of the new buses also affected the boarding process. For example, new vehicles no longer had a running board. Apprentices, who usually belonged to the driver's entourage, would perch on the running boards of the old vehicles. In the "modernised" transport operations, drivers were required to comply with stops and therefore no longer needed apprentices to harangue potential passengers along the way or signal to the driver when he should stop. The apprentices were thus replaced by "ticket agents", recruited from outside the world of transport, or employees of the EIGs. The ticket agents sold tickets to passengers when they boarded the bus and henceforth operated aboard the vehicle, where a special seat was reserved for them.

The presence of these various employees at the bus station enabled certain EIGs to set up operations at the boarding sites. Their employees took breaks in EIG premises. The EIG issued its own operating rules as well as those governing the bus station. Drivers, the rank and file of the unions and their main source of legitimacy, were no longer allowed to join trade unions made up of union members with links to the CNTS if they were employed by one of the EIGs of AFTU. The latter required their drivers to pay a tax at each departure and "intervene" on their behalf with the police in the event of a problem related to "brokering" practices. The changes applied only in the areas reserved for new vehicles; the unions continued to control the rest of the stations.

Unions: opposition to attempts at integration

The unions denounced "unfair operating conditions and discrimination, which they claim have accompanied the project from the start". 47 After initially opposing the creation of the AFTU, the unions tried to join the financing association: they, too, wanted to benefit from fleet renewal. They ran into opposition from the original members who had de facto control over the AFTU and its various EIGs. The reversal of the union's position can be explained by the high profits to be derived from the project and the State's involvement in it.

The "new" vehicles of the "new" hauliers were far more profitable because they controlled every aspect of the operation. The introduction of tickets made it possible to check the exact amount of revenue generated by the vehicles. Each haulier had to go in person to the AFTU offices to buy his tickets and each batch of tickets was numbered. The haulier therefore knew the number of tickets sold and consequently the amount due from the receiver. A different system was used in operating the old Mercedes vehicles. The apprentice controlled receipts during the journey and counted them up at the end together with the driver. The driver had to pay an average of 15,000 CFA francs (about 23 Euros) per day to the haulier, which a vehicle operating in the greater Dakar area could make in one return trip. The rest went to the driver and the apprentice. The new contractual relationship between drivers and hauliers, as well as between ticket sellers and hauliers, was equivalent to fixed wages. Haulier employees could no longer benefit from extra revenues. Driver and receivers no longer had anything to gain by overloading the vehicle or making erratic stops along the way to pick up every last passenger.

Mechanical issues, sometimes a point of contention between drivers and hauliers, were no longer a problem either, as Tata vehicles were repaired at a directly affiliated garage (Senebus, the company that assembles buses in Senegal). Drivers and mechanics could no longer work out a deal to siphon off a margin on the repairs to haulier vehicles.

The most influential hauliers in the AFTU also promoted parallel initiatives. They founded a new mutual fund and a new savings institution. Every AFTU employee was required to contribute to these initiatives, which made it a profitable business. As a result, some AFTU hauliers were able to join with members of the government to invest in a taxi service. Given the multiplication of yellow taxis bearing the name of their company, it is safe to assume that the initiative has been highly profitable.

State support and the involvement of certain government members also seem to have motivated project participants. "To have greater control, one should always be close to the manager. One should always be close to the President", says one of the beneficiaries of fleet renewal. This idea has been reinforced by the fact that the police check old vehicles more often than new ones. Similar reasoning has prompted the unions and their leaders to draw closer to the ruling party 48 . This article does not claim to explain every aspect of transport reform. We have not discussed, for example, the creation of the CETUD49 (Comité exécutif des transports urbains dakarois) [Executive committee of Dakar urban transport], which is also an important aspect of the reform in the region. Our aim has been first and foremost to illustrate the ongoing process through which relationships between the public and private sectors are formed, using the example of the implementation of two reform measures.

So far, bus station privatisation and vehicle fleet renewal has left out the drivers unions. The reform has called into question a whole series of daily interactions between the members of the "board", which exists for and through bus station management and the agents of the State and the municipality working at the boarding sites. It is not so much the State that has been called into question by the reform financed by the World Bank, but rather the role of union board members in the local and national administrative system. The Senegalese State has circumscribed their power by introducing new institutions.

An analysis in terms of "discharge" offers a more detailed explanation of the renegotiations at work when the reform was introduced, revealing the bridges between the public and private sectors. It also emphasises the reform's logic of "diverting" and "cornering" revenues. Both the Senegalese State and the transport actors have invested and used the measures demanded by donors in accordance with their own reasoning. The success of the reform and the State's ability to keep representatives in the transport sector is the result of a combination of state determination and the successive interests of certain sector players.

Sidy Cissokho

Université Paris 1-Panthéon-Sorbonne

40

  See I. O. Albert, "Between the State and Transporter Unions: NURTW and the Politics of Managing Public Motor Parks in Ibadan and Lagos, Nigeria", in L. Fourchard (ed.), Gouverner les villes d''Afrique. État, gouvernement local et acteurs privés Paris, Karthala, 2007, pp. 125-139

  Version avant impression de "Outsourcing the State? Public Transport reform in Dakar under President Abdoulaye Wade", B. Hibou, B. Samuel, L. Fourchard (dir.), The Spirits of Neoliberal Reforms and Everyday Politics of the State in Africa, Amalion Publishing, Dakar, 2016.

	SIDY CISSOKHO
	OUTSOURCING THE STATE? PUBLIC TRANSPORT
	REFORM IN DAKAR UNDER PRESIDENT ABDOULAYE

In

2000, Senegal experienced its first changeover of political power after a forty-year socialist reign. The change brought Abdoulaye Wade to power for a period of twelve years. 2 X. Godard, "D comme Dakar, ou le bilan mitigé d''une ville-pilote", in X. Godard (ed.), Les Transports et la ville en Afrique au sud du Sahara. Le temps de la débrouille et du désordre inventif Paris, Karthala/Inrets, 2002, pp. 57-73 3 See the special feature under the direction of Béatrice Hibou on "L''État en voie de privatisation", Politique africaine, no. 73, March 1999; B. Hibou (ed.), La Privatisation des États, Paris, Karthala/Ceri, 1999.

S. Cissokho, La Réforme d''un secteur informel: les transports à Dakar, Master 2 thesis in African Studies, Université Paris I Panthéon-Sorbonne, 2010.

C. Vandermotten and C. O. Ba, "Les organisations de producteurs dans les politiques agricoles", in T. Dahou (ed.), Libéralisation et politique agricole au Sénégal, Paris, Crepos/Karthala/Enda Graf Diapol, 2008, pp. 25-47.

For further information on the transport sector in the capital during this period, see M. Coulibaly, Les Transports urbains au Sénégal de 1945 à 1970: le cas de Dakar, Master's thesis in history, Université Cheikh

For further details on these reforms, see I. Diouf, "C comme Car rapide ou les tentatives d''intégration du transport artisanal", in X. Godard (ed.), Les Transports et la ville……, op. cit., pp. 45-57.

We might mention the example of hauliers such as Lobatt Fallu or Ndiaga Ndiaye whose name was given to the famous vehicles. Both men became secretaries of prestigious marabouts.

J. Lombard and O. Ninot, "Impasses et défis……", art. cit.

J. Lombard and O. Ninot, "Impasses et défis……", art. cit

"Instruction" in Wolof. Like the word "instruction" in English, the word n''diguel can refer to different realities. Here we are talking specifically about political n''diguel, in other words, instructions on which candidate(s) to vote for.

D. C. O''Brien, "Le contrat social sénégalais à l''épreuve", Politique africaine, no. 45, March 1992, pp. 9-21.

There is a project, which has been postponed for several years, to move the station to the suburbs of Dakar and privatise its management.

The notion of factionalism seems more appropriate than the notion of patronage to describe the internal operation of the unions. See T. Dahou, "Entre engagement et allégeance. Historicisation du politique au Sénégal", Cahier d''études africaines, no. 167, 2002, pp. 499-520.

The secretary general of the Pompiers union had previously worked at a market. In his view, "the same language" applies to markets and bus stations. This assertion and the ease with which the secretary general assumed his role suggest that a logic similar to the one existing in bus stations is at work in the organisation of markets. Source: interview with the general secretary of the Pompiers union, Dakar, March 2010

. S. M. Seck, "Transport et territoires: les ""coxeurs"" de Dakar, du bénévolat à la prestation de service", Bulletin de la Société géographique de Liège, no. 48, 2006, pp. 7-17.

Internal Union Rules, Dakar, 2010

These offices were razed in 2011. The board members and police forces now cohabit in another space organised in a more rudimentary fashion.

J.-P. Olivier de Sardan and G. Blundo, "La corruption quotidienne en Afrique de l''Ouest", Politique africaine, no. 83, October 2001, pp. 8-37.

I. K. Ndiaye, "Le Président Wade veut révolutionner le transport", Le Soleil, 6 September 2005.

Ibid.

F. Dreyfus, L'invention de la bureaucratie, Paris, La Découverte, 2000, p. 188-189 Au Etats Unis, dans la seconde moitié du 19 ème , pour s'assurer de la fidélité de leur administration, les nouveau gouvernements remplaçait systématiquement les membres de l'administration par des fidèles de leur parti. Ce mode de fonctionnement était appelé le « spoil system ».

B. Hibou, Anatomie politique de la domination, Paris, La Découverte, 2011, p. 207.

See S. Bredeloup, S. Bertoncello and J. Lombard (eds.), Abidjan, Dakar: des villes à vendre? La privatisation ""made in Africa"" des services urbains, Paris, L''Harmattan, 2008, p. 69

M. A. Yalli, "Congrès constitutif de la CNTS FC, le PDS dispose de sa centrale syndicale", L''Info, 14 January 2002.

O. Diouf, "Incident à la Bourse du Travail: le feu couvait depuis deux ans à la CNTS FC", Le Soleil, 22 March 2002

The clashes were more explicitly political in nature in the cities of Ibadan and Lagos. See L. Fourchard, "Lagos, Koolhaas and Partisan Politics in Nigeria", International Journal of Urban and Regional Research, vol. 35, no. 1, 2011, pp. 40-56

D. Mané, "Les nouveaux minibus en service dès lundi", L''Info, 25 November 2005

The transport branch of the CNTS, for example, was introduced under the PDS label at the time of local elections in 2009 in Djender, in the Thiès department.

The Cetud coordinates transport policy for the Dakar region. Set up at the end of the 1990s, it was first marginalised after the political changeover before becoming one of the core elements of transport policy in the Dakar region.


Nathan Martin

Vivien Lacour

Cécile Mong-Tu Perrault

Emmanuel Roy

Yamin Leprince-Wang
email: yamin.leprince@univ-eiffel.fr

High flowrate microreactors integrating in situ grown ZnO nanowires for photocatalytic degradation

A high-volume, low-pressure microfluidic photocatalytic microreactor was created and demonstrated here. This high-flowrate microreactor, based on an innovative design developed by the start-up Eden Tech, was produced by a simple thermal pressing process, using Eden Tech's easy-to-use and environment-friendly thermoplastic polymer, Flexdym™. ZnO nanowires (ZnO NWs) were then synthesized in situ, using an environment-friendly, low-cost, two-step hydrothermal method. Varying the synthesis parameters (growth solution flowrate, time and temperature) allowed us to get an insight of the optimal synthesis conditions for photocatalytic purposes. The in situ grown nanowire morphology was characterized under Scanning Electron Microscopy (SEM). The photocatalytic efficiency of the as-synthetized microreactors was tested by degrading continuous streams of water polluted with Acid Red 14 (AR14), a commonly used organic dye, under UV irradiation, followed by UV-visible spectrometry. We found that the best results are obtained with the following synthesis parameters: a 1h growth time, a growth flowrate of 200 µL/min and a temperature of 80°C, as after 4 passes in the reactor, which is roughly equivalent to 40 seconds of UV light irradiation, the dye was degraded at 98%. Increasing the photocatalysis flowrate from 200 µL/min to 500 µL/min showed a small decrease in efficiency, but is still encouraging in the aim of high-flowrate degradation.

Introduction

Due to the combined effects of growing human population, climate change and increasing industrial consumption, the pressure on water resources is steadily rising and is a hugely pressing issue. To ensure the sustainability of Humanity's water supplies, it is necessary to find efficient, low-cost and environmentally friendly ways to purify urban and industrial wastewater effluents.

One of the most studied solution for water purification is the photocatalytic process [1,2], a sustainable degradation process using a photocatalyst such as ZnO or TiO2. This process presents the advantage of using a renewable and abundant energy source: the sunlight. It also leads to the mineralization of the pollutants, preventing the risk of leaking harmful chemicals into the environment [3]. Finally, as it is a catalytic process, the product needed to perform the depollution can easily be retrieved and reused. ZnO and TiO2 are large-bandgap metallic oxides with similar photocatalytic properties [4,5], and TiO2 has already been widely studied [6,7]. However, ZnO presents the advantages of being a low-cost photocatalyst, whose raw materials are abundant in nature [1]. It is even supposed to be a better photocatalyst than TiO2 under visible light [4,8], and is also non-toxic and biocompatible [1,2,9,10]. Thus, in the aim of creating an environmentally friendly and efficient photocatalytic reactor, ZnO seems to be a better choice, which is why it is our photocatalyst of choice.

To increase the contact between the photocatalyst and the molecules to be degraded, nanostructures are often used, such as nanoparticles, nanorods or nanowires. Those nanostructures present high surface-to-volume ratios, which increases the contact opportunities between the material and the molecules to be degraded, and so, the photocatalytic efficiency of the samples [11].

Nanostructured photocatalysts can be used by direct dispersion into water, like a powder [12][13][14]. However, once the purification is done, the separation of the photocatalytic particles from the water is far from easy and often requires further filtration of the effluents, adding significant time and energy costs, both financial and environmental, reducing the effective yield of depolluted water, and making the scale-up from laboratory scale difficult [15]. Furthermore, considering the reduced size of the nanoparticles, leakage into the environment is difficult to avoid, with uncertain consequences [16].

To prevent these risks and the filtering step, the nanostructures can be immobilized on a substrate [15,17,18]. This, however, tends to reduce the general photocatalytic efficiency [15] due to the unfavourable surface-to-volume ratio: a large volume of water is in contact with a relatively small photocatalytic surface. On the other hand, in addition to the reduced leakage risks and the possibility to easily reuse the photocatalyst, this allows to create reactors using the ZnO nanostructures in order to try to purify bigger quantities of water. For example, several designs of photocatalytic macroreactors have already been studied [19][20].

Performing photocatalysis with immobilized nanostructures at high-efficiency is achievable by incorporating them into microreactors, as previously done by several synthesis method, such as chemical bath deposition [21], sol-gel [22], electrospinning [23], and hydrothermal growth [24][25][26][27].

Microreactors have several very interesting properties for photocatalytic water depollution [21,22,28,29]: the reduced size of the microchannels increases the contact between the pollutant and the photocatalyst, while reducing the absorption of light irradiation by the liquid. This is especially important in the degradation of coloured pollutants. The continuous stream and small size of the microchannels also create a situation where mass transfer is no longer limiting the kinetic of the reaction, improving the reaction rate. All of these advantages imply that the reaction kinetic of the degradation is quicker in microreactors than in bigger reactors.

A major reported limitation of microreactors is the very small liquid quantities that can be treated within microchannels (often in the range of less than 1 mL/min [30]), and the high pressure required to achieve high-flowrate, making the solution costly for large treatment volumes. However, it is possible to circumvent the flowrate problem by plugging microreactors to one another in chain. Sadly, this means the pressure needed to ensure the flow of liquid will be even higher [28,31].

The start-up Eden Tech has developed a new design of microreactor [32], in which the pressure needed to make the liquid flow inside of the reactor is minimal, allowing high flowrate without large pressure needs. This microreactor is more suitable for water purification applications which need to treat large water quantities.

We used a low-cost and environmentally friendly hydrothermal method to synthesize ZnO NWs into a microreactor composed of a Si-wafer substrate and a Flexdym™ chip, on which microchannels had been patterned by a simple thermal process. Flexdym™ is a thermoplastic polymer, developed and produced by Eden Tech, which presents very interesting properties for its use in microfluidics for the environment, such as biocompatibility, optical transparency, stable surface chemistry, low water evaporation and ease of use [33]. It has also been proven that Flexdym™ is ideally suited for the use in tough conditions [34] and does not require a clean room for its use.

The parameters of the synthesis have been investigated in order to find the optimal synthesis conditions for the photocatalytic efficiency. The microstructure of the synthesized NWs were characterized by SEM observation and their photocatalytic efficiency was studied by degrading a continuous stream of AR14-polluted water, an organic dye often found in urban, pharmaceutic and agro-food industry effluents, under UV-irradiation, followed by UVvisible spectrometry.

Experimental

Materials and Methods

The reactor is composed of two different kinds of microchannels. Long microchannels, which weave from the inlet to the outlet, with a length of 575.92 mm and a height of 100 µm, ensure the sustainability of the high flowrate. Short microchannels, with a length of 0.85 mm and a height of 10 µm, connect the long microchannels to each other, decreasing the pressure needed to pump liquids into the reactor, as well as allowing a better contact between the pollutants and the NWs, improving the photocatalytic efficiency. The total volume of the microreactor is of 0.2 mL. The design has a circular shape, to fit 4 inches wafers, ease the stacking and allow all the outlets to be at the same distance of the inlets. During preliminary experiments, our reactor proved to be able to sustain pressures up to 1500 mbar. As we used a syringe pump in our experimental set-ups, the pressure at the inlets of the reactor could not be measured. However, as the 1500 mbar pressure lead to a flowrate of 6.1 mL/min, we are confident in the ability of our reactors to sustain flowrates even higher than the ones presented here. This design was printed on 2 mm thick Flexdym™ chips by vacuum thermal pressing (Sublym100 TM , Eden Microfluidics, France) using an epoxy mold imprinted with the negative design. First, the polymer sheet and the mold were pre-heated at the pressing temperature, 165°C, in the Sublym100 TM press for 10 minutes. Then, vacuum was applied for 760 seconds, still at 165°C, before release. The molded Flexdym TM sheet was unfolded from the mold and stored between two Teflon sheets for later use.

The hydrothermal synthesis was performed following a method adapted from our previous works [Erreur ! Source du renvoi introuvable.]: first, 4-inch circular Si wafers were spin-coated (200 µL/cm 2 ) with a precursor solution composed of 10g of polyvinyl-alcohol (PVA) and 1g of Zinc Acetate dissolved into 100 mL of deionized (DI) water (R = 18.2 MΩ). The substrates were then calcined at 500°C during 3 hours, to remove the PVA. After the calcination, the Flexdym™ chip was bound into the substrate for 4 hours at 120°C in an oven, without external pressure. The obtained non-functionalized reactor is illustrated in Figure 1(a). The resulting growth solution was heated at the desired temperature (80°C, 85°C or 90°C) using a thermostated bath and injected into the microreactor with a syringe pump, at a constant flow rate (from 100 to 400 µL/min) for the desired growth time (30 min to 1h30). During the circulation of the growth solution, the microreactor was placed on a heating plate to ensure a constant temperature throughout the process. The experimental set-up is displayed in Figure 1(b). As Flexdym TM cannot sustain the high temperatures of the post-annealing (350°C) we usually perform, this step was not performed in these experiments. This could lead to a small decrease in efficiency, but the biggest effect of the impossibility to anneal the samples after each experiment will more likely be a decrease in efficiency for subsequent experiments.

The obtained NWs were characterized by SEM observation, by taking apart the microreactor and imaging the NWs directly. The photocatalytic experiments were conducted by degrading a solution containing 10 µmol/L of AR14, pumped into the microreactor at a constant flow rate of 200 µL/min with a syringe pump, under UV irradiation. The surface of the tested microreactors was maintained at 10 cm of the UV lamp (Hamamatsu LC8, Japan) corresponding to an UV intensity of 35 mW/cm². The photodegradation of the studied dye was monitored using UV-visible spectrometry (Perkin-Elmer, Lambda 35). Every time, the syringe was rinsed with DI water, and the solution was pumped back into it to resume the experiment.

Results and discussion

The photocatalytic properties of our samples are given below. Each graphic shows the degradation rate versus the number of passes made for each reactor. Each pass represents 25 minutes of injection at 200 µL/min. Considering the volume of our microchannels and the flowrate, the residence time of the liquid is of 10 seconds under the UV light, meaning that 4 passes are roughly equivalent to 40 seconds of photocatalytic process in the reactor.

Study of flowrate on ZnO NWs growth

Flowrate is critical to the proper growth of the NWs. A flowrate too slow can prevent the full filling of the microchannels, whereas a too fast flowrate reduces the contact with the seed layer and prevents the growth of the NWs, as the precursor molecules cannot diffuse to the wall (seed layer). The ratio between convection and diffusion is represented by the Péclet number, calculated according to the following equation [Erreur ! Source du renvoi introuvable.]:

Pe = H 2 .v.D -1 .L -1
Where H stands for the height of the microchannels, v stands for the velocity of the liquid in the microchannels, L stands for the length of the microchannels, and D stands for the diffusion coefficient of the molecules. A Péclet number higher than 1 means that the diffusion time of the molecules in the liquid is slower than their advection time, thus that all the molecules don't have the time to reach the bottom of the microchannels before exiting the reactor. In our case, this could lead to a waste of precursors, as they might just pass through the reactor and not be consumed by the growth reaction. Furthermore, a too high pressure, induced by a too high flowrate, could lead to a leakage of the chip, damaging the reactor. To ensure the integrity of the reactor and improve the chances of contact between the suspended precursor molecules and the seed-layer, the chosen flowrates were fixed at values between 100 µL/min and 400 µL/min, leading to Péclet numbers between 0.55 and 2.21.

The ZnO NWs were grown for 1 hour at the temperature of 80°C. Those parameters were chosen to ensure that the NWs were long enough to perform the photocatalytic degradation of the AR14, and reduce the risk of precipitation or evaporation. The effect of the flow velocity of the precursor solution on photocatalytic efficiency are shown in Figure 2. As can be seen, the best results were obtained for NWs grown at a flowrate of 200 µL/min. This means the Péclet number is of 1.1, and so, that the diffusion transfer is almost equal to the advection transfer. The obtained nanostructures were then characterized by SEM observation, as seen in Figure 3.

As shown in the SEM images, we obtained a homogenous NWs layer on our samples surface at 100 and 200 µL/min, but at higher flowrates, the NWs seem to agglomerate and cover less surface. The difference in efficiency between the 100 µL/min and the 200 µL/min samples is due to the characteristics of the NWs layer: as it is visible on the SEM image, the surface of the 100 µL/min sample is not totally covered by NWs, whereas the surface of the 200 µL/min sample is fully covered. Furthermore, the mean diameter of the NWs varies from 41 ± 5 nm for the 100 µL/min one to 58 ± 5 nm for the 200 µL/min, meaning their surface area is lower than the surface area of the 200 µL/min sample. The smaller density and mean diameter of the NWs in the 100 µL/min samples advocate for a smaller reaction rate of the NWs growth. This is because, at 100 µL/min, the smaller velocity of the flow equals a lower regeneration rate of the precursors, leading to a reduced reaction rate. The surface coverage and the density increase of the NWs explain the difference in efficiency between the two samples. When the flowrate increases, the increase of the Péclet number indicates that the relation between the advection time and the diffusion time reverses. As the velocity of the flow increases, the size of the depletion layer, defined as the layer in the flow of growth solution right above the bottom of the microchannels, where there is no precursor, as it is consumed by the growth reaction, decreases. This leads to an increase in available precursors near the bottom of the microchannels, promoting the growth and agglomeration of the ZnO nanowires in clusters. This explains the loss in photocatalytic efficiency: as the nanostructures tends to agglomerate on big clusters, the catalyst surface available for the photocatalysis decreases due to the screen effect, and such, the photocatalysis efficiency.

Study of growth time on ZnO NWs growth

The second synthesis parameter we studied is the growth time. A too short growth time would lead to an incomplete NWs growth, resulting in a lack of surface area, and such, a reduced efficiency. However, a too long growth time could lead to NWs with an excessive length that may block the microchannels. The Figure 4 indicates that the best photocatalytic performance of as fabricated microreactor was obtained for a growth time of 1h. The SEM images of the different NWs layers are presented in Figure 5.

The SEM images give us access to the factors affecting the efficiency of our samples, which are summarized in Table 1.

Those two parameters help to understand the differences in efficiency measured between the samples. Starting with the mean density, more NWs means more contact area, and should lead to an improved photocatalytic efficiency. However, when the density reaches a certain threshold, the NWs start to cover each other, preventing the liquid and the light from reaching the nanostructures. This diminishes the NWs area available for the photocatalytic reaction, and so reduces the photocatalytic efficiency.

As for the diameter, a larger diameter results in an increase of the surface area available for the photocatalysis, thus a higher efficiency. However, the mean diameters have to be linked with the density to have an overall picture of the efficiency of the samples. The decrease in the mean diameter we can see for growth times higher than 1h is due to the apparition of very thin NWs (with diameters around 20 nm) between bigger NWs, which diameters are closer to the one measured for the 1h sample. Those small NWs crowd the sample even more, leading to the coverage we discussed earlier. We can also see that the mean diameter of the NWs increases to a maximum with a growth time of 1h, then decreases after 1h, due to the apparition of smaller NWs. This means that the NWs of the 30 min sample have a smaller diameter than the other, and thus, their surface area is smaller than the one of the NWs of the other sample; further explaining their lack of efficiency.

Finally, even if performing cross-sections on our samples is difficult, we managed to evaluate the length of the nanowires synthesized in the optimal conditions by observing a damaged reactor with SEM, and comparing the obtained results with theoretical results obtained from an UV-vis analysis [Erreur ! Source du renvoi introuvable.]. The theoretical length of the ZnO NWs is 650 ± 65 nm, while the estimated length is 710 ± 50 nm. This size is inferior to the one we obtain with our usual method, but this could be explained by the fact that the in situ growth was performed only for 1h, whereas the classic growth has a duration of 4h.

Study of temperature on ZnO NWs growth

Finally, the last parameter we studied was the growth temperature. This temperature should not be too low, to ensure the NWs growth is taking place [Erreur ! Source du renvoi introuvable.]. However, if it is too high, the growth solution might precipitate into the Teflon tube, or start to evaporate, creating gas bubbles into the microchannels and blocking them in both cases. Thus, the selected growth temperatures were 80°C, 85°C and 90°C.

Table 1. Morphological characteristics of the NWs according to their growth time.

The flowrate is still fixed at 200 µL/min, and the growth time at 1h, according to the previous studies. The results of the influence of the growth temperature on the photocatalytic efficiency are presented in Figure 6.

Fig. 6. Impact of the growth temperature on the photocatalytic efficiency.

The influence of the temperature is important and interesting. As the growth solution can precipitate in the Teflon tube, it can also precipitate in the microchannels, thus creating blockage in some microchannels, and even engulfing NWs, preventing the dye molecules to reach them, leading to a reduced photocatalytic efficiency.

Considering the small quantities of liquid involved, it is also possible that a part of the liquid started to evaporate at higher growth temperature, creating air bubbles into the reactor, and such, areas where the NWs growth does not take place, further reducing the photocatalytic efficiency.

The nanostructures obtained via the synthesis at different temperatures have also been observed under SEM. The images are given in Figure 7. When increasing the temperature further, from 85°C to 90°C, we obtained nanostructures that are similar to agglomerates of nanorods, while the NWs layer seems to have disappeared. There are still some empty spaces in the nanostructure layer, and those are bigger than the ones obtained at 85°C.

Those observations are in accordance with the hypothesis that the increase of the temperature of the solution leads to the apparition of air bubbles inside of the liquid flow. Those air bubbles then stop the growth of the NWs, by preventing the growth solution to bring reagents in contact with the seeds. As the temperature of the growth solution increases, the solubility of the dissolved gases decreases, leading to the apparition of gas bubbles. Considering the growth solution is heated before entering the reactor, and as the reactor design is conceived to allow an efficient evacuation of the air bubbles that could be formed insides the microchannels, the air bubbles should be evacuated before they have an effect on the growth. However, during the reaction pathway for the ZnO NWs growth, there is formation of NH318. Thus, the observed gas bubbles could be NH 3 bubbles. As the solubility of NH3 decreases when the temperature increases, the as-formed bubbles size increases, thus leading to the holes in the NWs layer.

The bigger nanostructures and their agglomeration are most likely due to the increase in the liquid agitation with the increase in temperature, as well as the surface effects of the bubbles inside of the liquid. Those two effects lead to a strong difference between areas were the liquid is concentrated, thus more available for nanostructure growth, and zones where there is no liquid, and so, no growth. The creation of those aggregates leads to a diminution of the available surface area for the photocatalysis, thus reducing the photocatalytic efficiency.

Photocatalysis at high flowrates

The previous results have allowed us to determine the best synthesis conditions to have the best performing reactors. However, the photocatalysis flowrate have been maintained at 200 µL/min, which is still a low flowrate, as our final goal is to overcome the usual flowrate one can expect from microreactors. Thus, we conducted further photocatalytic experiments where the polluted solution flowrate was increased from 200 µL/min to 500 µL/min. The growth conditions in the reactor were the optimized conditions we established in the previous paragraphs. The results of those photocatalytic experiments are presented in Figure 8. As we can see, the degradation rate we obtained is lower than the one obtained at the smaller flowrate, but stays very good, at around 87% of the pollutant degraded in 5 passes, which is roughly equivalent to 20 seconds under the UV light. This is the same time as 2 passes for the previous flowrate, which gave a degradation rate of 93%, meaning the photocatalysis is less efficient at higher flowrates, but the loss in efficiency is still minimal. This can be explained by the fact that, as the pollutant spends less time in the reactor, it stays under the UV irradiation for a shorter time, and so the photocatalysis degrades a smaller quantity of pollutant, leading to a smaller efficiency. Those results, however, stay very encouraging, as they prove that increasing the flowrate does not lead to a drastic decrease of efficiency, meaning that our reactor should be able to ensure the photocatalytic degradation of pollutants at even higher flowrates. Influence of the microchannels dimensions on the nanowires characteristics The reactor design we used is divided in two kinds of microchannels: long microchannels, which ensure the reactor is able to handle high flowrates, and short microchannels, which ensure the good distribution of the liquid inside the microreactor. Besides the length difference, the different microchannels also present a height difference: the long microchannels are 100 µm high, whereas the short ones are 10 µm high. Thus, we observed the NWs separately, depending on their localization into the microreactor. The SEM image of a connection between a long and a short microchannel is presented in Figure 9(a).

As the image shows, there is a very clearly defined separation between the different microchannels. The entrance of the short microchannel shows an accumulation of nanostructures, which progressively disappears, as shown in Figure 9(b), giving the way to an homogenous layer of NWs. This accumulation is attributed to the recirculation of the precursor solution at the entrance of the microchannel, caused by the height difference between the short and the long microchannels.

To check if the different flowing conditions had an effect on the NWs morphology, we studied their morphological characteristics and their visible aspect according to their location. The measured characteristics are summarized in Table 2 shows that the morphologies of the NWs are very similar, even when their growth occurred in the different kinds of microchannels. The sharp tops of the NWs might be attributed to less available precursors in the long microchannels, which means that the NWs will have a very slow diameter increase with time, while the height increase stays higher, leading to sharp tops. In the short microchannels, however, the flat tops seem to indicate that the precursors are more easily available for the growth reaction, as the diameter increase seems not to be affected the same way as in the long microchannels.

This might be due to the difference in height of the microchannels: as the volume of precursors solution is bigger in the long microchannels, the precursors molecules which are consumed by the NWs growth are replaced less quickly than in the short microchannels, where the volume of precursors solution is smaller, meaning that the molecules are more easily available for the NWs growth occurring at the bottom of the reactor.

Conclusions

We successfully created a high-volume photocatalytic microreactor by synthesizing ZnO NWs inside a microfluidic reactor, using a simple and environment-friendly hydrothermal method adapted from our previous works. The synthesis conditions were studied, and we concluded that the optimal conditions were a flowrate of 200 µL/min, a growth temperature of 80°C and a growth time of 1h. Under these conditions, the reactor achieved a 98% degradation rate over 4 passes, the rough equivalent of 40 seconds under UV irradiation. We also demonstrated that even at 500 µL/min, our reactor keeps most of its efficiency, degrading 87% of the dye in 20 seconds, against 93% at 200 µL/min. The next steps will be to increase the degradation flowrates, test the efficiency of the microreactors on different kinds of dye, and try to improve the efficiency even further by considering to perform an in situ metal doping of the NWs, as it has been proved metal doping leads to a better degradation [Erreur ! Source du renvoi introuvable.].

Those results are really promising, as they suggest that it is possible to produce environment-friendly and very efficient microreactors for the photocatalytic depollution of water, with high flowrate throughput, by using a simple production methods, easy-to-use and biocompatible polymers, and a low cost synthesis method at low temperature and atmospheric pressure.

Fig. 1 .

 1 Fig. 1. Non-functionalized reactor (a) and in situ ZnO NWs growth set-up (b).

Fig. 2 .

 2 Fig. 2. Impact of the ZnO NW growth solution flowrate on the photocatalytic efficiency.

  Photolysis 100 µL/min 200 µL/min 300 µL/min

Fig. 3 .

 3 Fig. 3. SEM images of the synthesized nanostructures at the different flowrates: (a) 100 µL/min (b) 200 µL/min, (c) 300 µL/min, (d) 400 µL/min. For the 100 µ/min, some zones devoid of NWs are highlighted in red.

Fig. 4

 4 Fig. 4 Impact of the ZnO NW growth time on the photocatalytic efficiency.

  Photolysis 30 min 1h 1h15 1h30

Fig. 5 .

 5 Fig. 5. SEM images of the synthesized NWs with different growth time: 30 min (a), 1h (b), 1h15 (c), and 1h30 (d).

Fig. 7 .

 7 Photolysis 80°C 85°C 90°C

Fig. 8 .

 8 Fig. 8. Photodegradation of AR14 at a flowrate of 500 µL/min by the optimized reactor, synthesized at 200 µL/min and 80°C for 1h.

Fig. 9

 9 Fig. 9 SEM Images of the nanostructures grown (a) at the junction between the short and long microchannels (b) at the other end of the short microchannel and zoom on the NWs grown into the short (c) and long (b) microchannels.

  Figure 9(c) and 9(d) reveal that even if the NWs have nearly identical morphologies, the ones synthesized in the short microchannels present a flat top, whereas the ones synthesized in the long microchannels possess a sharp top. The growth kinetics of the ZnO NWs have already been studied and tends to show that the NWs will first follow a very fast growth along the vertical axis, which will start to decrease after some reaction time, due to the decrease of the precursors concentration in the growth solution [Erreur ! Source du renvoi introuvable.-Erreur ! Source du renvoi introuvable.]. The growth along the horizontal axis is slower than the one along the vertical axis, but follows the same pattern [Erreur ! Source du renvoi introuvable., Erreur ! Source du renvoi introuvable.].

Source du renvoi introuvable., Erreur ! Source du renvoi introuvable

  Reactions are supposed to occur faster in microfluidic reactors compared to their batch counterparts [Erreur ! Source du renvoi introuvable.]. It has already been demonstrated that reduced reaction times lead to ZnO NWs with lengths equals or even greater than ZnO NWs grown in batch conditions [Erreur !

	.]. Furthermore, the synthesis of
	2-µm-long ZnO NWs in few minutes by using a continuous flowrate in capillary tubes
	has been reported [Erreur ! Source du renvoi introuvable.]. Thus, we decided to
	investigate the reduction of the growth time, by using timeframes ranging from 30
	minutes to 1h30.
	We selected a temperature of 80°C and a flowrate at 200 µL/min, conditions which
	previously demonstrated the highest photocatalytic efficiency. The effects of growth
	time on photocatalytic efficiency are shown in Figure 4.

Table 2

 2

	, and the SEM

Table 2 .

 2 Morphological characteristics of the NWs grown into the short and long microchannels.

	Microchannel type	Mean NWs density	Mean diameter (µm)
		(NWs/µm 2 )
	Long microchannels	64 ± 5	58 ± 5
	Short microchannels	61 ± 5	63 ± 5

Acknowledgements

The authors want to thank their colleagues in ESYCOM laboratory and at Eden Tech Company, for the scientific discussions and their general help. Special thanks to Dr. Martine Capo-Chichi for her help with the chemical aspects of the experiments and Dr. Mazen Erfan for his numerous and wise advices concerning the practical aspects of the microfluidic experiments.

Conflicts of interest

There are no conflicts to declare.