S Awad

M Paraschiv

V Edwin Geo

M Tazerout

Mr S Awad

EFFECT OF FREE FATTY ACIDS AND SHORT CHAIN ALCOHOLS ON CONVERSION OF WASTE COOKING OIL TO BIODIESEL

Keywords: Biodiesel, Waste cooking oil, Transesterification, Methyl ester, Ethyl ester, Free fatty acids

In this article, the transesterification of three types of waste cooking oil (WCO) with methanol and ethanol was studied using alkali catalyzed process. The catalyst used in this study was sodium hydroxide. The effects of temperature, catalyst amount, alcohol to oil ratio, and the time of reaction on the yield were studied. The temperature and the catalyst amount were the most important factors affecting the yield of biodiesel. Also the process exhibited some sensitivity to the level of free fatty acids (FFA) in the WCO and to the type of alcohol. The yields of methyl esters varied from 97% with the lowest acidity (0.4% FFA WCO) to 76% with the highest acidity (3.25% FFA WCO). The ethyl esters yields were lower and the difference increased with the level of FFA in the oil, the maximum yield was 95% and 73% with the lowest and the medium acidities respectively and no reaction was registered with the highest one. The chromatographic analysis of the produced biodiesel showed high contents of fatty acid methyl esters varying from 96.5% to 98%. The physical-chemical characteristics of produced biodiesel were studied and compared to the European norm, EN 14214.

INTRODUCTION

The world population has increased from 2.5 to 6 billions between 1950 and 2000, and it is expected to reach 9 billions in the four coming decades. This increase of population and the change in their lifestyle demand more energy and raw materials. On the other hand, it generates more wastes that are becoming unbearable by the ecosystem which is dangerously suffering. In Europe, for example, a citizen produces about 600 kg of wastes per year. European parliament has set a law in 2008 for the wastes management which can be resumed by: reusing, recycling, and valorization. The waste cooking oils (WCO) represent a very important share of these wastes, and with the new life style of people and the increasing of fast food share in their nutritional habits, the waste edible oil quantities produced around the world and especially in the big cities are increasing. These wastes are considered as non-dangerous wastes. These wastes should not be landfill or burned into the open air, neither poured into the waste water-evacuation networks, but they can be used as raw material in biodiesel-production processes.

The majority of energy consumption for transportation is based on the rapidly depleting fossil fuels resources. So, the studies are focused on new energy sources that can be an alternative to petroleum fuels and that can meet stringent emissions norms. During the year 1990, biofuel production has started in several European countries and flourished in a meaningful way. In 2003, the European Union (EU) has set a target of the contribution of biofuels to 5.75% of its energy consumption by 2010 and 20% in 2020. France has adopted the EU target and has set more ambitious goals, in 2008 biofuels have represented 5.75% of its energy consumption and it tries to reach 10% in 2015, making an advance of 2 years on European targets [START_REF] Bozbas | Biodiesel as an alternative motor fuel: Production and policies in the European Union[END_REF]. Biodiesel is one of the most available sources of renewable energy, it is non-toxic, biodegradable, its heating value is high, it has zero sulfur content and its use in diesel engines reduces the emissions [START_REF] Dalvi | Preparation of methyl esters from Thespesia Populnea l. Oil and its engine exhausts studies[END_REF][START_REF] Ayhan | Recent developments in biodiesel fuels[END_REF][START_REF] Capareda | Engine Performance and Exhaust Emissions of Cotton Seed Oil Biodiesel[END_REF][START_REF] Kumar | A comparison of the different methods of using jatropha oil as fuel in a compression ignition Engine[END_REF]. It can be used directly in diesel engine without introducing any changes. However, there are technical problems related to its flow properties at lower temperatures, the emissions of NOx and stability during storage [START_REF] Bandel | Vegetable oil derived fuels and problems related to their use in diesel engines[END_REF][START_REF] Kalam | Emissions and deposit characteristics of a small diesel engine when operated on preheated crude palm oil[END_REF][START_REF] Vedaraman | Preparation of palm oil biodiesel and effect of various additives on NOx emission reduction in B20: An experimental study[END_REF]. The most important problem for the moment is high production cost of which raw materials accounts 60-85% of the total price of production. Using waste fats instead of edible oils as raw material for biodiesel production may resolve that problem [START_REF] Meng | Biodiesel production from waste cooking oil via alkali catalyst and its engine test[END_REF][START_REF] Bai | Dehydration of waste edible oil by hydrocyclones[END_REF].

In this respect, fermentation, transesterification, and pyrolysis of biomass, industrial, and domestic wastes have been proposed during the last years as an alternative solutions for biofuel production [START_REF] Luo | Research on biomass fast pyrolysis for liquid fuel[END_REF][START_REF] Leung | A review on biodiesel production using catalyzed transesterification[END_REF]. A number of methods are currently available and have been adopted for the biodiesel production, among which there are three primary methods are described in the literature: alkali-catalyzed, acid-catalyzed, and enzyme-catalyzed transesterification. Due to the process cost, alkalicatalyzed transesterification seems to be the most commonly adopted method [START_REF] Antolín | Optimisation of biodiesel production by sunflower oil transesterification[END_REF]Berchmans and Hirata 2008;[START_REF] Leduc | Optimizing biodiesel production in India[END_REF]).

On the whole, during these processes several variables (i.e., reaction temperature and reaction time) affect the ability of triglycerides to react and the yield of biodiesel production. The highest yield of biodiesel can be obtained under the optimal conditions, which are sensible, different in function of the raw material nature and chemical structure. As a consequence, the work presented in this article focused on the identification of these optimal conditions in the case of using three types of waste oils as raw materials for biodiesel production by alkali catalyzed transesterification process.

EXPERIMENTAL SETUP Basic Chemical aspects of Transesterification Process

Transesterification is a chemical reaction that consists on substituting alcohol groups of an ester by another type of alcohol which is given in Eq. (1). This process is widely used to reduce the viscosity of triglycerides, which limits their direct use in diesel engines. Transesterification is a reversible reaction, which consists of mixing the reagents. The presence of a catalyst greatly promotes this reaction and reduces considerably the reaction time.

R 1 COOCH 2 HOCH 2 R 1 COOCH 3 || R 2 COOCH + CH 3 OH NaOH -------⇀ ↽ -------HOCH + R 2 COOCH 3 || R 3 COOCH 2 HOCH 2 R 3 COOCH 3 Triglyceride Alcohol Glycerol Fatty Acid Esters (Biodiesel) (1)
Several processes have been developed for biodiesel production, either by catalysis: acidic, basic, and enzymatic, in homogeneous or heterogeneous systems, either by methods that operate without a catalyst at very high temperatures and pressures. The reaction following an alkali catalyst is the most widespread in the industrial field, for its low output price and triglyceride conversion rate. The problem of this method is related to the sensitivity of the response to moisture and the free fatty acids (FFAs) levels, which form soaps in the presence of a strong base [START_REF] Jeong | Optimization of transesterification of animal fat ester using response surface methodology[END_REF].

Raw Materials

In the present study, the WCO was collected from the restaurant of Ecole des Mines de Nantes. Based on their acid values, three types have been identified: WCO1, WCO2, and low acidity waste cooking oil (WCOLA). Its composition was described on the products wrapping. And it was as follows: 45% palm oil, 45% sunflower oil of which 35% are enriched with oleic acid, and 10% of rapeseed oil. WCO1 and WCOLA were used to make French fries and WCO2 which was used to cook meat and fishes. The physical-chemical characteristics of these WCOs are describes in Table 1.

Transesterification Process of WCOs

Initially, NaOH was mixed with the alcohol (methanol-CH 3 OH or ethanol-C 2 H 5 OH). Then, the mixture was stirred by means of a magnetic stirrer under constant temperature (30 • C) for 5-10 min, until any traces of solid NaOH are not observed, which means that the reaction is accomplished. The purpose of premixing alcohol with NaOH is to produce alkoxide, which reacts more effectively with the triglycerides. Then the WCO and alkoxide were mixed together under continuous and vigorous stirring at different temperatures and for different times that will be specified later in this article. At the end of reaction, the products were poured on a separating funnel in order to separate the glycerol phase from biodiesel.

As both phases contain alcohol and catalyst, after the separation step, the biodiesel was washed several times with water, until the aqueous phase became clear and neutral. Product drying is ensured by using anhydrous calcium chloride, and after that the biodiesel is filtered and weighted. The flow chart of transesterification process is given in Figure 1. Considering Eq. ( 1), the yield of biodiesel was calculated as the percentage of the weighed product from the expected weight, calculated as theoretical conversion. At the end of these operations, the physical-chemical characteristics of the produced biodiesel were studied.

The main factors affecting the alkali-catalyzed transesterification are:

r Alcohol quantity r Reaction temperature r Reaction time r Catalyst concentration
The aim of this study is to obtain the maximum yield of biodiesel by searching the optimal combination of the listed parameters. In order to study a higher number of parameters with a larger range of values, the effects of interaction were not taken into consideration. The optimization was done by optimizing each parameter and then the optimal condition was determined which is shown in Figure 2.

RESULTS AND DISCUSSION

Transesterification of WCO1

Transesterification with Methanol (CH 3 OH). The most common transesterification process uses methanol to produce methyl esters (commonly referred to as fatty acid methyl esters-FAME) as it is the cheapest alcohol available. Initially, transesterification process was realized at different reaction temperatures: 30, 40, 45, 50, and 60 • C with three different molar ratios of 1:6, 1:9, and 1:12 and with three reaction times of 30, 45, and 60 min. The effect of catalyst quantity was identified by using five concentrations: 0.5%, 0.6%, 0.8%, 1%, and 1.25% which is calculated from total oil weight of WCO.

Based on literature information [START_REF] May | Transesterification of palm oil: Effect of reaction parameters[END_REF][START_REF] Phan | Biodiesel production from waste cooking oils[END_REF][START_REF] Jeong | Optimization of transesterification of animal fat ester using response surface methodology[END_REF], the study started with the following combination of process parameters: reaction temperature: 60 • C, oil to alcohol ratio: 1:6, catalyst amount: 1%, and reaction time: 60 min; at 60 • C (the temperature recommended by literature because it is near to the boiling point of methanol). At temperatures higher or equal to 50 • C, gel and soap formation occurred without any phase separation. At 45 • C and 40 • C the conversion reached 75% and 79.2%, respectively, while, at 30 • C, no phase separation was detected. As a consequence, the temperature of 40 • C was adopted as optimal value for this type of WCO. Table 2 illustrates the optimization scheme followed for WCO1 with methanol.

Transesterification with Ethanol (C 2 H 5 OH). Ethanol can be used to produce an ethyl ester, commonly referred to as fatty acid ethyl ester (FAEE). The same approach that was used with methanol is used with the substituting the methanol with ethanol. In the case of using WCO1, the optimal values of temperature and catalyst amount have remained the same, but, the oil to alcohol ratio has been changed. For a ratio of 1:6 the reaction did not occur, for a ratio of 1:9 the yield reached 76.3%, and with 1:12 the yield decreased to 75.9%. And the reaction time required was 60 min. Table 3 illustrates the optimization scheme followed for WCO1 with ethanol.

no separation phase was formed. Considering these effects, there is a reduction of biodiesel yield while the acid value of WCO increases. At the same time, comparing with results obtained during the transesterification with methanol, ethanol shows a clear tendency to produce lower amounts of biodiesel with WCO1 and WCOLA, and the formation of gel and soaps in reaction with WCO2.

Effect of reaction time on the transesterification of WCOs. In transesterification, conversion rate increases with reaction time until the optimum value. In the applied alkali-catalyzed transesterification, the process begins with the mixing and dispersion of sodium alkoxide (methoxide and ethoxyde in the present study) molecules into the oil. The conversion rate is relatively low for this step of the process and due to oil-alcohol immiscibility the mixture has a milky aspect. After 10-15 min, the mixture becomes clear and the reaction proceeds relatively fast. An excessively reaction time led to a reduction in biodiesel yield as a consequence of backward reaction resulting in more FFAs, more susceptible under the applied conditions to form soap than organic esters.

Effect of reaction temperature on the transesterification of WCOs. The present study shows that the temperature has an essential effect on the reaction rate and the biodiesel yield. Increasing temperature accelerates the reaction and increases the yield of biodiesel [START_REF] May | Transesterification of palm oil: Effect of reaction parameters[END_REF][START_REF] Meher | Technical aspects of biodiesel production by transesterification a review[END_REF][START_REF] Phan | Biodiesel production from waste cooking oils[END_REF][START_REF] Jeong | Optimization of transesterification of animal fat ester using response surface methodology[END_REF], which is quit logical, as a higher reaction temperature results in a decreasing oil viscosity so that a more effective contact between triglycerides and alkoxide molecules. But on the other hand, it promotes parallel reactions, especially saponification, which could play an inhibitor role for transesterification [START_REF] Meng | Biodiesel production from waste cooking oil via alkali catalyst and its engine test[END_REF][START_REF] Phan | Biodiesel production from waste cooking oils[END_REF][START_REF] Jeong | Optimization of transesterification of animal fat ester using response surface methodology[END_REF]. [START_REF] Van Gerpen | Biodiesel production technology[END_REF] suggests that the temperature of 40 • C in order to avoid saponification.

In this study, the results were congruent more with Dreger's [START_REF] Dreger | Method of treating fatty glycerides[END_REF]) observations. That comes probably from the fact that the WCO has a higher acidity than pure oils, contains impurities and it was subject of thermal treatment which may be affected its chemical structure.

Effect of catalyst concentration on the transesterification of WCOs. It was established that the catalyst amount has an important influence on both reaction conversion and biodiesel yield. Without catalyst, the reaction does not occur unless using the supercritical alcohol method which requires high temperatures and pressures. In the case of using an alkali-catalyst, increasing the catalyst's amount accelerates the reaction but beyond a certain value the NaOH will react with fatty acids and the saponification will be favored [START_REF] Ma | Biodiesel production: A review[END_REF][START_REF] Lin | Biodiesel production from crude rice bran oil and properties as fuel[END_REF].

Comparison of Physical-Chemical Characteristics of Biodiesel Derived from WCOs

The produced biodiesel was subjected to several tests to determine its physical and chemical characteristics. The results are presented in Table 6 and the variation of kinematics viscosity with temperature is plotted in Figure 3.

The viscosity was measured by using a vibro-viscosimeter SV-10 made by A&D Company. Dynamic viscosities of ethyl and methyl esters were measured between 25 • C and 60 • C. The acid values were measured by the volumetric titrimetry method. For all biodiesel samples, the identified values were lower than 0.2 mg KOH/g. Water content was determined by using xylene vapors method and there is no water is present in the biodiesel. "1" shows that the ester is prepared by WCO1 transesterification, "2" is corresponding to WCO2 and "LA" to WCOLA).

The elemental analysis was conducted using a CHNS-O Analyzer FLASH 1112 series E.A and the biodiesel has the following composition: 76.59% C, 12.91% H, 0% S, 0% N, and 10.5% O. The molar composition is derived from the formula by dividing the mass composition of each element by its molar mass, thus 100 g of biofuel contains: 6.3825 mol of C, 12.91 mol of H, and 0.65625 mol of O. Finally, dividing the molar composition by 0.65625 (the molar contenance of Oxygen). It is concluded that the empirical formula of the biodiesel is C 9.72 H 19.66 O. This empirical formula will be used in the study of combustion of this biofuel in internal combustion engines. The higher heating value was measured with an isoperibolic PARR 6200CLEF calorimeter. The flash point was measured with a PENSKY MARTENS NPM440 apparatus. The amounts of FAME/FAEE, glycerol, and alcohol were measured by using the gas chromatography analysis. In this purpose, a Perkin Elmer "Clarus 500 Gas Chromatograph," equipped with a flame ionization detector and a capillary column (VARIAN WCOT Fused Silica, 30 m × 39 µm × 25 µm) was used. The mentioned compounds were identified by comparing their retention time to the retention time of standard compounds. The results of the Chromatographic Analysis are listed in Table 7. The properties of biodiesel compared with diesel and European norms are given in Table 8.

CONCLUSIONS

In this work, biodiesel was derived from various WCOs by alkali-catalyzed transesterification reaction, using two types of alcohol, methanol, and ethanol. Also the optimal conditions for these reactions were determined, depending on acidity of WCO and alcohol types. The effects of the following process variables were investigated: reaction temperature, catalyst amount, type of alcohol, reaction time, and WCO acidity. Based on the present work, the following conclusions are drawn: r The high-quality biodiesel was successfully derived from WCOs by alkali-catalyzed transesterification.

r The measured physical and chemical properties of produced biodiesel were almost the same, and independent of raw materials acidity. Moreover, slight differences between ethyl esters and methyl esters have been obtained.

r The lower heating value of the biodiesel is a little lower than that of diesel oil, which will increase the fuel consumption of the engine for the same power output.

r It is concluded that the FFA content has a negative effect on the whole process.

The FFAs form soap with the catalyst, which reduce considerably its efficiency for transesterification. Thus higher amounts of catalyst are required. The formation of soap during alkali-catalyzed reaction creates proper conditions for emulsions appearance; so that, the esters are captured into a stable emulsion with glycerol and excess alcohol and at the end of process the phase's separation will not be possible.

Based on this work, it is recommended to use methanol in alkali-catalyzed transesterification of waste oils with relatively high FFA contents. To avoid their negative effects, FFAs in the base oil are either converted to soap, and removed from the process, or they may be esterified (yielding more biodiesel) using an acidic catalyst.

Figure 2

 2 Figure 2 Flow Chart for Optimisation of Reactions.

Figure 3

 3 Figure 3Evolutions of Kinematic viscosities of biodiesel wit temperature (ME -methyl ester, EE -ethyl ester. "1" shows that the ester is prepared by WCO1 transesterification, "2" is corresponding to WCO2 and "LA" to WCOLA).

Table 1

 1 Physical-Chemical Characteristics of WCOs

	WCO	Acid value	Moisture	Viscosities at 40 • C	High heating	Melting	Density
	type	(mg KOH /g)	(%)	Dynamic (mPa•s)	Kinematic (mm 2 /s)	value, (HHV) (MJ/kg)	point ( • C)	(g/cm 3 )
	WCOLA	0.8	<0.05	32.9	37.8	40	30	0.87
	WCO1	1.4	<0.05	33.1	38.0	40	30	0.87
	WCO2	6.5	<0.05	33.0	37.9	40	30	0.87

Figure 1 Flow Chart of Transesterification Process.

Table 6

 6 Physical-Chemical Characteristics of Produced Biodiesels

	Characteristic	Units	EELA	MELA	ME1	EE1	ME2
	Density	g/cm 3	0.868	0.869	0.870	0.868	0.870
	Carbon content	w/w	76.57	76.60	75.96	77.23	76.7
	Hydrogen content	w/w	12.96	12.85	13.04	12.8	12.7
	Oxygen content	w/w	10.47	10.55	11	9.97	10.6
	Higher heating value	MJ/kg	40.13	40.05	39.92	40.24	39.98
	Lower heating value	MJ/kg	37.51	37.43	37.29	37.61	37.17
	Kinematic viscosity @ 40 • Cm m 2 /s	3.88	3.73	3.61	3.95	3.7
	Flash point	• C	158	160	162	160	158
	Water content	mg/k g-----
	Acid value	mg KOH /g	-	0.08	0.07	0.12	0.1
	Sulfur content	mg/k g-----
	Abbreviations: EELA, ethyl ester of low acid value waste cooking oil;
	MELA, methyl ester of low acid value waste cooking oil;
	ME1, methyl ester of waste cooking oil 1;
	EE1, ethyl ester of waste cooking oil 1;
	ME2, methyl ester of waste cooking oil 2.

Table 7

 7 Methyl Esters Compositions of Biodiesel Produced from the Different WCO

		WCOLA	WCO1	WCO2
	Myristic acid methyl ester	0,90	0,94	0,99
	Palmitoleic acid methyl ester	0,25	0,15	0,82
	palmitic acid methyl ester	41,00	39,00	33,88
	Stearic acid methyl ester	12,20	13,10	16,85
	Oleic acid methyl ester	42,70	43,10	40,58
	Linoleic acid methyl ester	2,15	2,33	2,99
	Methanol	Traces	Traces	Traces
	Glycerol	Traces	Traces	Traces
	Total	99,20	98,60	96,13

Table 8

 8 Characteristics of Biodiesel Compared to European Norm EN 14214

	Characteristics	Units	EN 14214 limits	Produced biodiesel
			Min	Max
	Density	g/cm 3	0.86	0.90	0.87
	Kinematic viscosity	mm 2 /s	3.50	5.00	3.61-3.95
	Flash point	• C	110	-	158-162
	Water content	mg/kg	-	500	0
	Acid value	mg KOH /g	-	0.50	0.08-0.12
	Sulfur content	mg/kg	-	10	0
	Esters	%	96.5	-	98
	Methanol	%	-	0.2	Traces (ppm)
	Glycerol	%	-	0.02	Traces (ppm)

6

Transesterification of WCO2

Transesterification with Methanol (CH 3 OH). In order to establish the optimal conditions for WCO2, the same approach as for WCO1 has been used. With the same optimal conditions as that of WCO1, the transesterification of WCO2 has not occurred. It is known that the high FFA content will react with catalyst and neutralize it. By increasing the amount of catalyst, this problem could be overcome. Several amounts of catalyst were tried: 1.25%, 1.5%, and 2%. The optimum yield occurred between 1.25% and 1.5% of catalyst amount. Using 2% NaOH, the soap formed during the reaction resulted on a gel and froze the biodiesel production. Thus, the value of 1.25 was fixed as an optimal catalyst amount. In order to optimize the oil to alcohol molar ratio, the following values were used: 1:6, 1:7.5, and 1:9. The maximum biodiesel yield was obtained for a molar ratio of 1:7.5. For superior oil to methanol ratios, lower yields were obtained because of the tendency of FAMEs to make emulsions with glycerol in the presence of higher amounts of alcohol, and thus a very difficult phase separation occurs at the end of process. The study on the effect of reaction temperature and time variation indicates that the best biodiesel yield is obtained at 45 • C and 45 min. The detailed optimization process is shown in Table 4.

Transesterification with Ethanol (C 2 H 5 OH). The WCO2 transesterification using ethanol failed after several experiments, in all tests gels were obtained and any phase separation was not observed. So, it is concluded that the production of ethyl esters is more sensitive to oil acidity than that of methyl esters.

Transesterification of WCOLA

The WCOLA samples were obtained in a later period of this study, after the optimization of the reaction with the WCO1 and WCO2. In order to evaluate the effect of increasing or decreasing the acid value of the raw material, the optimization of the reaction with the latest sample was planed. Since its acid value was nearest to WCO1, the optimization started with the optimal values obtained in Tables 2 and3, the yield reached 97% and 95% with methanol and ethanol, respectively. So the need of optimization was not justified with these reaction efficiencies, and the optimal parameters were set like those of WCO1. The WCOLA can be considered as a reference raw material for these experiences, because its acid value is very close to those of pure vegetable oil. The established optimum conditions for the applied process are listed in Table 5.

Effects of Different Reaction Parameters on the Transesterification Process

Effect of alcohol quantity on the transesterification of WCOs. Theoretically, as the reaction stoichiometry shows, transesterification reaction requires 3 mol of alcohol for 1 mol of triglycerides to produce 3 mol of fatty acid ester and 1 mol of glycerol [refer Eq. ( 1)]. As a reversible reaction, an excess of alcohol is needed to shift the equilibrium toward the biodiesel production to ensure that triglycerides will be completely converted to esters. Although its positive effect on the transesterification reaction, increasing the excess of alcohol, enhances the formation of emulsions of biodiesel in glycerol phase which decreases the recovered fuel. In this work, this problem appeared more likely when ethanol is used, and that may explain the lower yields of ethyl esters and, on the other hand, the loss in recovered biodiesel.

The catalyst will form soap with FFAs and, in this environment, the produced esters seem to exhibit more tendencies to form gels. Moreover, it is known that soaps are good emulsifiers and they create proper conditions for the reaction mixture to become a stable emulsion. In this case, the esters remain captured in the emulsion and at the end of process


Jonatan Torres-Perez

Claire Gerente

Yves Andres

Andres

Conversion of agricultural residues into activated carbons for water purification: Application to arsenate removal

Keywords: Activated carbon, adsorption, arsenate, iron impregnation, residue valorization, water treatment

Introduction

Activated carbon (AC) is currently one of the most used adsorbents for water and air purification on an industrial scale. The production process is commonly divided in 2 steps: pyrolysis of the precursor (coal, coconut, wood, etc.) followed by its activation to create porosity. The first step is often conducted where the precursor is locally produced while the second occurs in another place from where the ACs are distributed all around the world.

In 2007, Ioannidou and Zabaniotou [START_REF] Ioannidou | Agricultural residues as precursors for activated carbon production -a review[END_REF] published a review documenting more than 100 references about activated carbon production from agricultural residues on a laboratory scale. They concluded that steam or CO 2 activation is easier to handle, cleaner and cheaper than chemical activation. Moreover, if this treatment is conducted in a one-step process (direct activation), there is also an economic benefit. However, depending on the nature of the precursors, these ACs exhibit less satisfactory characteristics for further use as adsorbents or filters. [START_REF] Ioannidou | Agricultural residues as precursors for activated carbon production -a review[END_REF] In terms of residue valorization, the main idea of this work is to prepare sustainable ACs Address correspondence to Claire Gerente, LUNAM Université, Ecole des Mines de Nante, CNRS, GEPEA, UMR 6144, 4 rue Alfred Kastler, BP 20722, 44307, Nantes cedex 03, France; Email: claire.gerente@mines-nantes.fr from local resources by direct and physical activation using steam.

Two agricultural residues were selected: sugar beet pulp, a typical French waste accounting for 33 million tons per year, and peanut hulls, a Mexican waste with an annual production of 70,000 tons. As for arsenic contamination of surface and ground water, this is a worldwide problem due to its established toxicity and its presence in overcrowded areas. [START_REF] Jain | Arsenic: occurrence, toxicity and speciation techniques[END_REF] In 1993, the guideline concentration limit in drinking water recommended by the WHO was fixed at 10 µg. L -1 . The European Union has accepted this value in their regulatory systems but in Mexico, for example, it is still 25 µg.L -1 . [START_REF] Nom | Salud ambiental[END_REF] Two predominant species found in natural waters are inorganic forms of arsenic, namely arsenate, As (V), and arsenite, As (III), and their presence depends on the pH and redox conditions. As (V) is the thermodynamically stable form, found in oxic surface waters, rivers and lakes, and presents three pKa: 2.2, 7 and 11.6. This means that in most natural waters, arsenic (V) is mainly in the H 2 AsO 4

-form (at 3 < pH < 6) or associated with the HAsO 4 2-form (at 6.5 < pH < 7.5). Above pH 8, HAsO 4 2-is the predominant species. [START_REF] Sengupta | Arsenic in subsurface water: Its chemistry and removal by engineered processes[END_REF] Although various technologies are well known for the removal of arsenic from waters, adsorption is one of the best available to achieve the lowest concentration in the effluent after treatment. [START_REF] Mohan | Arsenic removal from water/wastewater using adsorbents -a critical review[END_REF] The most widely studied media for adsorption processes include iron hydroxide and oxide, such as amorphous Table 1. Selection of papers reporting the adsorption of arsenic onto activated carbons.

Steam-activated carbons from agricultural residues

Precursor

Activation conditions q m of As (III) µgg -1 q m of As (V) µgg -1 Ref.

Oat hulls H 2 O, Fast pyrolysis (500 [20] Commercial (NC100) 0.05 M FeCl 3 and 0.5 M HCl solution.

1.3-1.5 15-20 [20] hydrous ferric oxide, ferrihydrite and goethite, activated alumina, zeolites and chitosan. [START_REF] Raven | Arsenite and arsenate adsorption on ferrihydrite: kinetics, equilibrium, and adsorption envelope[END_REF][START_REF] Driehaus | Granular ferric hydroxide -a new adsorbent for the removal of arsenic from natural water[END_REF][START_REF] Appelo | Surface complexation of ferrous iron and carbonate on ferrihydrite and the mobilization of arsenic[END_REF][START_REF] Singh | Equilibrium, kinetic and thermodynamic studies for adsorption of As(III) on activated alumina[END_REF][START_REF] Jimenez-Cedillo | Adsorption kinetic of arsenates as water pollutant on iron, manganese and iron-manganese-modified clinoptilolite-rich tuffs[END_REF][START_REF] Gerente | Interactions of natural aminated polymers with different species of arsenic at low concentrations: Application in water treatment adsorption[END_REF] As far as activated carbons are concerned, only a few recent publications have shown that physical activation of agricultural residues by steam leads to carbonaceous materials which are effective for As removal. [START_REF] Chuang | Adsorption of arsenic(V) by activated carbon prepared from oat hulls[END_REF][START_REF] Budinova | Removal of arsenic(III) from aqueous solution by activated carbons prepared from solvent extracted olive pulp and olive stones[END_REF][START_REF] Budinova | Biomass waste-derived activated carbon from the removal of arsenic and manganese ions from aqueous solutions[END_REF] On the other hand, in the last decade more than 10 papers have reported that arsenic sorption capacities are enhanced when ACs are impregnated with iron salt solutions, with or without a preliminary oxidation step. [START_REF] Fierro | Arsenic removal by iron-doped activated carbons prepared by ferric chloride forced hydrolysis[END_REF][START_REF] Reed | As(III), As(V), Hg and Pb removal by Fe-oxide impregnated activated carbon[END_REF][START_REF] Chen | Arsenic removal by iron-modified activated carbon[END_REF][START_REF] Jang | Preloading hydrous ferric oxide into granular activated carbon for arsenic removal[END_REF][START_REF] Mondal | Effects of adsorbent dose, its particle size and initial arsenic concentration on the removal of arsenic, iron and manganese from simulated ground water by Fe 3+ impregnated activated carbon[END_REF][START_REF] Mu Ñiz | Synthesis, characterization and performance in arsenic removal of iron-doped activated carbons prepared by impregnation with Fe(III) and Fe(II)[END_REF] The combination of activated carbon and iron loading is attractive: AC is as an ideal support medium for iron preloading due to its high BET surface area while iron has a high affinity for arsenate and arsenite. However, all these modifications have only been conducted on commercial materials. Table 1 reports a selection of these papers and focuses on (i) physically activated carbons prepared from agricultural residues and (ii) commercial activated carbons modified with iron for arsenic removal. Regarding arsenate removal, only one study [START_REF] Chuang | Adsorption of arsenic(V) by activated carbon prepared from oat hulls[END_REF] describes activated carbons obtained from steam activation of oat hulls with a maximum capacity, deduced from the Langmuir model, of 3100 µg.g -1 . As far as commercial activated carbons are concerned, different impregnations are possible leading to an iron content ranging between 2.2 and 11.7% and q m values for arsenate are very scattered from 0.008 to 51.3 µg.g -1 . [15 , 17] This work thus has two main objectives. First, the conversion of cheap and renewable agricultural residues into sustainable ACs is studied in a one-step process with steam activation. The second aim is to show the value of these ACs for As (V) removal, including the improved and tailored AC after iron impregnation. After a physical and chemical characterization of the porous materials, adsorption experiments are performed in terms of contact time experiments, adsorption isotherms in deionized and natural waters and the effect of a preliminary oxidation. Different models are presented: intraparticle diffusion and pseudo-second-order equations for kinetic data, and Langmuir and Freundlich models for equilibrium data.

Materials and methods

All chemicals were of reagent grade from Sigma. Arsenic solutions were made from Na 2 HAsO 4 -7H 2 O. Sugar beet pulp was commercially supplied by Lyven (Cagny, France) and peanut hulls were obtained from a local market. In a previous study, a complete characterization of the raw sugar beet pulp revealed that polysaccharides accounted for 72.5% of the dry matter, ash for 3.9% and calcium for 1.1%. [START_REF] Reddad | Ni(II) and Cu(II) binding properties of native and modified sugar beet pulp[END_REF] Raw peanut hulls had an ash content of 1.7%. In addition, a commercial granular activated carbon (GAC), also produced from a biomass (coconut) and physically activated with steam, was used as a reference for the physicochemical characterization and for comparison of sorption performances.

Activated carbon production

The sugar beet pulp (BP) and peanut hulls (PH) were first dried at 110 • C for 24 hours before being heated at 10 • C min -1 up to 850 • C under a nitrogen flow rate of 0.5 L min -1 . At 850 • C, steam was also introduced into the furnace at 0.7 mL min -1 for 80 min. Then, heating was stopped and the fall in temperature occurred while still under the nitrogen atmosphere. These conditions were optimized from a previous study, [START_REF] Rio | Preparation of adsorbents from sewage sludge by steam activation for industrial emission treatment[END_REF] and they are resolutely placed in the field of sustainable production because (i) only local resources are investigated, (ii) direct activation is used and (iii) no chemicals are employed as activating agents.

The activated carbonaceous materials thus obtained from beet pulp and peanut hulls were called BP-H 2 Oand PH-H 2 O, respectively. Next, they were crushed in a hammer mill and the particles were sieved so that only particle sizes of 0.50-1.00 mm were used for the arsenic adsorption experiments. This fraction was washed with deionized water(10gL -1 ) until the pH remained constant, filtered and dried at 110 • C. Mass yields were 16 and 24% for BP-H 2 O and PH-H 2 O, respectively. To obtain iron-modified ACs, 1gofBP-H 2 O and PH-H 2 O were also placed in a FeCl 3 -6H 2 Osolution(0.1MandpH= 1.7) and the mixture was shaken for 24 hours at room temperature.

Once again, this modification is in agreement with sustainable development since no toxic chemicals are used (only FeCl 3 -6H 2 O), with no added oxidant and no additional heating. [START_REF] Fierro | Arsenic removal by iron-doped activated carbons prepared by ferric chloride forced hydrolysis[END_REF][START_REF] Reed | As(III), As(V), Hg and Pb removal by Fe-oxide impregnated activated carbon[END_REF][START_REF] Chen | Arsenic removal by iron-modified activated carbon[END_REF][START_REF] Jang | Preloading hydrous ferric oxide into granular activated carbon for arsenic removal[END_REF][START_REF] Mondal | Effects of adsorbent dose, its particle size and initial arsenic concentration on the removal of arsenic, iron and manganese from simulated ground water by Fe 3+ impregnated activated carbon[END_REF][START_REF] Mu Ñiz | Synthesis, characterization and performance in arsenic removal of iron-doped activated carbons prepared by impregnation with Fe(III) and Fe(II)[END_REF] Iron-impregnated beet pulp activated carbon (BP-H 2 O-Fe) and iron-impregnated peanut hull carbon (PH-H 2 O-Fe) were then washed several times with deionized water until water was free of iron. The iron content of BP-H 2 O-Fe and PH-H 2 O-Fe was measured after an acid digestion (concentrated HCl), by an atomic absorption spectrophotometer with flame atomization (AAS Analyst 400, Perkin Elmer). Finally, the adsorbents were dried at 105 • C.

Activated carbon characterization

Physical characterization -BET surface area analysis

Textural parameters of the samples, like surface area and porosity, were obtained from nitrogen adsorption isotherms. In order to eliminate any adsorbed humidity and/or gases, between 0.8-1.2 g of the carbon was outgassed at 350 • C for 24 hours. Then, N 2 isotherms were recorded at 77 K using automatic equipment (ASAP 2010, Micromeritics). The total pore volume (V total ) and specific surface area (S BET ) of the adsorbents were obtained from the Brunauer-Emmet-Teller (BET) isotherm model.

In addition, Horvath-Kawazoe (HK) and Barret, Joyner and Halenda (BJH) models were used to evaluate the micropore (V micro ) and mesopore (V meso ) volumes. The micropores have diameters of less than 2 nm while the mesopores have diameters between 2 and 50 nm. All the results were duplicated but the raw materials (sugar beet pulp and peanut hulls) were not characterized due to their poor specific surface areas and absence of porosity. The en-tire study was carried out with the same precursors and no adsorption of As (V) onto the raw materials was observed.

Elemental analysis and total ash content

Each sample was ground into a fine powder for the determination of elemental content (Flash EA 1112, Thermofinnigan). This analysis was repeated three times for each sample to provide an average reading. Results were indicated as percentages of carbon, hydrogen, nitrogen and oxygen. No sulfur content was detected. The total ash content in percentage was calculated by the standard test method for activated carbon. [START_REF] Standards | Standard Test Method for Total Ash content of Activated Carbon[END_REF] pH PZC determination This was carried out as follows: 100 mL of 0.1 M NaCl solution was placed in a closed Erlenmeyer flask and the pH was adjusted to between 2 and 12 by adding solutions of HCl or NaOH (0.1 M). Then, 0.05 g of each sample was added and the final pH measured after 5 days under stirring at room temperature. The pH PZC is the point where the curve pH final vs. pH initial crosses the line pH final = pH initial . [START_REF] Faria | Adsorption of anionic and cationic dyes on activated carbons with different surface chemistries[END_REF]

EDX and SEM

The porous structure of the carbonaceous materials and commercial activated carbons was observed using a JEOL 6400F Scanning Electron Microscope. A magnification ranging from 100X to 500X was used for visualization of the images. Imaging is typically achieved using secondary electrons to obtain the best resolution of the fine surface typographical features. An EDX analysis was carried out to verify the presence of iron on the surface of the modified carbons.

Arsenic removal with AC

Sorption kinetic experiments

Batch contact time experiments were conducted at 20 • Cby stirring 0.05 g of adsorbent with 100 mL of As (V) solution (1000 µgL -1 ), at 200 rpm. Initial pH values were close to 6 and were recorded at the end of the experiment. For each adsorbent, the equilibrium time between the solid and the solution was determined by plotting C t versus time.

To investigate the mechanism of sorption, two kinetic models were tested. The intraparticle diffusion-controlled adsorption model is generally applied to porous powdered materials and at the beginning of the kinetic decay curve in order to determine the intraparticle diffusion rate constant. [START_REF] Morris | Removal of biologically-resistant pollutants from waste waters by adsorption -Advances in Water Pollutions Research[END_REF] Eq. 1 expresses the adsorption capacity as a function of time, where k i is the intraparticle diffusion rate constant (mg g -1 min -0.5 ):

q t = k i t 0.5 (1)
The adsorbents were thus ground into powder and new kinetic curves were carried out during the first 2 hours of the decay, with vigorous stirring (300 rpm) at 20 • C. The k i values for each material were deduced from the slope of the line q t versus t 0.5 .

The second model is the pseudo-second-order sorption model proposed by Ho and McKay [START_REF] Ho | Kinetic models for the sorption of dye from aqueous solution by wood[END_REF] and used to describe the whole kinetic decay curve. Its equation is the following (Eq. 2)

dq t /dt = k 2 (q e -q t ) 2 (2)
where q t (µgg -1 ) is the amount of arsenate in the material at any time t (min), q e (µgg -1 ) is the amount of arsenate sorbed at equilibrium and k 2 (g µg -1 min -1 )i st h er a t e constant of the pseudo-second-order equation. Taking into account the boundary condition q t = 0a tt= 0, and the initial sorption rate h (µgg -1 min -1 ),

h = k 2 q 2 e (3)
Eq. 2 can be rearranged to obtain:

t/q t = 1/ h + 1/q e t (4)
By plotting t/q t versus t, the values of h, k 2 ,q e calculated were determined for all the materials.

Adsorption isotherms

Batch adsorption isotherms were conducted with 100 mL of synthetic solutions from 100 to 5000 µgL -1 of arsenate and 0.05 g of activated carbons. The bottles were sealed and put on a shaker for 5 days at 20 • C. After shaking, the equilibrium pH was measured and recorded, the samples were filtered through a 0.45-µm regenerated cellulose filter and the concentration of arsenic in the filtrate was determined by atomic absorption spectroscopy with a graphite tube (AAS Analyst 600, Perkin Elmer). Analytical measurements were obtained with a quantification limit of 4 µg L -1 and a detection limit of 2 µgL -1 . It was verified when necessary that no adsorption occurred on the walls of the plastic bottles or on the filters. Langmuir and Freundlich models were applied to describe the experimental results. The Langmuir equation [START_REF] Langmuir | The constitution and fundamental properties of solids and liquids[END_REF] is the following (Equation 5):

q e = bq m C e 1 + bC e ( 5
)
where q e (µgg -1 )andC e (µgL -1 ) are the amount of arsenic at equilibrium in the solid and liquid phase, respectively. q m and b are the parameters representing the maximum sorption capacity (µgg -1 ) and the sorption constant (L µg -1 ), respectively. According to the Langmuir model assumptions, adsorption occurs in a monolayer coverage characterized by the q m value, on homogeneous adsorption sites of constant energy, and adsorbed species do not interact with each other. The empirical Freundlich model [START_REF] Freundlich | Over the adsorption in solution[END_REF] is formulated as:

q e = K f C n e ( 6
)
K f (µg 1-1/n L 1/n g -1 )
and n are the Freundlich constants that indicate the adsorption capacity and the heterogeneity factor, respectively. The assumptions are (i) the adsorption sites of different energy are heterogeneous; (ii) the adsorbed species could interact with each other; and, (iii) there is no upper limit to adsorption, which restricts its use to diluted media.

In adsorption processes, it is well known that the presence of other ions in solution can greatly modify the sorption capacities. To determine this effect, sorption isotherms were also conducted in two natural spring waters, called HMC and LMC for high mineral content water and low mineral content water respectively, whose compositions are shown in Table 2. The main differences are in their anion concentrations, especially SO 4 2-, HCO 3

-and Cl -, which can potentially lead to competition with arsenate species and, to a lesser extent, the cation content like Ca 2+ ,Mg 2+ and Na + . These spring waters were spiked with arsenic from 100 to 5000 µgL -1 and the same protocol was followed for the adsorption isotherms.

Results and discussion

Characterization of the activated carbons

Before the sorption experiments, the four activated carbons produced at lab scale from sugar beet pulp (BP-H 2 O and BP-H 2 O-Fe) and peanut hulls (PH-H 2 O and PH-H 2 O-Fe) were characterized in terms of their textural properties (BET surface area, porous volumes, micro/mesoporosity percentage), chemical composition (elemental analysis, ash content, pH PZC ) and SEM pictures. GAC was used as a reference.

Physical characterization -BET surface area analysis

The results provided by nitrogen adsorption isotherms are presented in Table 3. The BET surface areas are 821 and 762 m 2 g -1 for BP-H 2 O and BP-H 2 O-Fe, and 829 and 718 m 2 g -1 for PH-H 2 O and PH-H 2 O-Fe, respectively. Previously, determinations of BET surface area were done on a char (without activation) from sugar beet pulp and on the raw precursor (data not shown) but the results revealed very low values of 6.6 m 2 g -1 and 0.2 m 2 g -1 , respectively. [START_REF] Reddad | Ni(II) and Cu(II) binding properties of native and modified sugar beet pulp[END_REF] These values confirm that activation is necessary to create porosity in a carbonaceous material.

Another point to note is that the iron impregnation on BP-H 2 O and PH-H 2 O only causes a slight reduction in the BET surface area, leading to 762 m 2 g -1 for BP-H 2 O-Fe and 718 m 2 g -1 for PH-H 2 O-Fe. Although the precipitation of iron is difficult to control because of the complexity and black color of the mixture, it can be deduced that iron impregnation is probably not a homogeneous coating that closes the porosity but rather an amount of iron oxide probably fixed in specific sites. The SEM pictures and EDX spectra confirm this assumption (Fig. 1). The BET surface areas developed by BP-H 2 O and PH-H 2 O are 25% lower than for GAC.

Taking into account that our first goal was not the optimization of the AC production, in terms of activation temperature, nature of activating gas, temperature ramp and dwell time, these values are considered acceptable for activated carbons produced at laboratory scale. To demonstrate this, a selection of papers is reported in Table 4 based on agricultural residues, the conditions of physical activation, BET surface areas and total porous volumes of the obtained ACs. [START_REF] El-Hendawy | Adsorption characteristics of activated carbons obtained from corncobs[END_REF][START_REF] Malik | Use of activated carbons prepared from sawdust and rice-husk for adsorption of acid dyes: a case study of Acid Yellow 36[END_REF][START_REF] Juang | Characterization and use of activated carbons prepared from bagasses for liquid-phase adsorption[END_REF][START_REF] Yang | Characteristics of activated carbons prepared from pistachio-nut shells by physical activation[END_REF][START_REF] Wang | Structural and surface property changes of macadamia nut-shell char upon activation and high temperature treatment[END_REF][START_REF] Valix | Preparation of activated carbon using low temperature carbonization and physical activation of high ash raw bagasse for acid dye adsorption[END_REF] It is clear that the BET surface area depends on the precursor, the nature of the activating gas and the activation temperature.

In a study by El-Hendawy et al., [START_REF] El-Hendawy | Adsorption characteristics of activated carbons obtained from corncobs[END_REF] the corncob was first carbonized at 500 • C for 2 hours and a portion of this char was steam-activated at different temperatures (from 600 • C to 850 • C) and times (1 to 2 hours). The resulting BET surface areas ranged between 607 and 786 m 2 g -1 .

In another case, an increase of 90 • C in activation temperature induced an enhancement of BET surface area from 446 to 607 m 2 g -1 on an AC produced from bagasse. [START_REF] Juang | Characterization and use of activated carbons prepared from bagasses for liquid-phase adsorption[END_REF] On the other hand, CO 2 activation led to the highest BET surface areas in the same range of temperatures. [START_REF] Yang | Characteristics of activated carbons prepared from pistachio-nut shells by physical activation[END_REF] Nevertheless, in all cases, the BET surface areas determined in this work, from sugar beet pulp and peanut hulls, are quite high in comparison with the data from this literature review.

Considering the experimental total porous volumes, the value obtained with sugar beet pulp (BP-H 2 O) is similar to those of GAC: 0.643 cm 3 g -1 compared to 0.623 cm 3 g -1 , respectively, while PH-H 2 O presents a lower volume of 0.403 cm 3 g -1 . Once again, iron impregnation causes little effect on the porous volumes (-10% in BP-H 2 Oand-14% in PH-H 2 O). They are also greatly improved in comparison with the values shown in Table 3. Looking at the balance between microporous and mesoporous volumes, PH-H 2 O is the highest microporous carbon (84%), followed by GAC (73%) and then BP-H 2 O (50%).

However, BP-H 2 O and PH-H 2 O have similar microporous volumes (0.35 cm 3 g -1 ). As far as iron-modified activated carbons are concerned, impregnation does not modify the percentage in terms of micro-and mesoporosity. In the composition of raw sugar beet pulp, a high percentage of ash was observed (4%) corresponding to the natural presence of calcium ions. [START_REF] Reddad | Ni(II) and Cu(II) binding properties of native and modified sugar beet pulp[END_REF] Consequently, the ash content of the carbon obtained is also high at 14% (Table 3). More generally, high ash content in the precursor tends to decrease the microporous volume balance. This observation has already been made in a previous work where activated carbons produced from raw bagasse with a high ash content (15%) by physical activation had some inorganic oxides filling or blocking a part of the microporous volume. [START_REF] Valix | Preparation of activated carbon using low temperature carbonization and physical activation of high ash raw bagasse for acid dye adsorption[END_REF] Another study using activated sludge with more than 20% ash as the precursor led to the same result. [START_REF] Julcour Lebigue | Application of sludge-based carbonaceous materials in an hybrid water treatment process based on adsorption and catalytic wet air oxidation[END_REF]

Chemical characterization: Elemental analysis and pH PZC

The major element contents, C, H, N and O, are given in Table 3. The carbon percentage is 78% and 68% for BP-H 2 O and BP-H 2 O-Fe and 91% and 84% for PH-H 2 O and PH-H 2 O-Fe, respectively. These values depend on the precursor but are still comparable to the carbon content of a commercial GAC (90%). In that sense, the adsorbents produced from sugar beet pulp and peanut hulls are really activated carbons, with a high carbon content. The second major element is oxygen. Although the determination by elemental analysis is sometimes tricky, it gives an order of magnitude confirmed by triplicates. BP-H 2 O, PH-H 2 O and GAC present similar percentages (7%, 6% and 9%, respectively) and these increase in both BP-H 2 O-Fe and PH-H 2 O-Fe after iron impregnation (16% and 14%, respectively). It should be noted that the FeCl 3 solution has an acidic pH of 1.7, which probably favored some surface oxidation of the sorbents and consequently the oxygen content. In BP-H 2 O-Fe and PH-H 2 O-Fe, the iron concentration was determined after an acid digestion of the sorbents and measured by atomic absorption spectrometry. The iron content deduced from this methodology is 4.8 and 0.5% for BP-H 2 O-Fe and PH-H 2 O-Fe, respectively.

The efficiency of this impregnation method depends on the nature of the precursor but leads to an increase in iron content compared with BP-H 2 O (0.1%) and PH-H 2 O (not significant). From the state of the art process (Table 1), modified commercial ACs can exhibit higher iron concentrations because the methodologies employed are often either preceded by an oxidation step or followed by solution evaporation or thermal treatment.

A methodology adapted from Faria et al. [START_REF] Faria | Adsorption of anionic and cationic dyes on activated carbons with different surface chemistries[END_REF] and based on pH measurement was used to determine the pH of point of zero charge (pH PZC ). It depends on the chemical and electronic properties of the functional groups on the surface and so is a good indicator of these properties. After an optimized contact time of 5 days, if the final pH of the solution is the same as before the AC introduction, then this is the pH PZC .

Moreover, it is generally admitted that (i) at pH < pH PZC , the overall surface charge of the solid is mostly protonated thus positively charged, (ii) at pH = pH PZC , the surface presents an equal positive and negative charge, and (iii) at pH > pH PZC , the overall surface charge of the solid is mostly deprotonated thus negatively charged. [START_REF] Rivera-Utrilla | Activated carbon surface modification by adsorption of bacteria and their effect on aqueous lead adsorption[END_REF] BP-H 2 O, BP-H 2 O-Fe, PH-H 2 Oa n dG A Cr eveal strong basic behavior with pH PZC ranging from 8 to 9.8 (Table 3).

This means that when placed in solution, they will induce a basic pH, which is useful because it gives an idea of the pH variation during the sorption experiments. After iron impregnation, it has been shown that O content increases after a partial oxidation of the surface. However, it probably also leads to an increase in the oxygenated chemical moieties like carboxyl, lactone, lactol or hydroxyl groups. Thus, pH PZC decreases from 9.8 to 9 for BP-H 2 O and BP-H 2 O-Fe and from 9.8 to 6 for PH-H 2 O and PH-H 2 O-Fe. The latter acidic value is surprising and needs to be confirmed by further experiments on surface chemical moieties.

Scanning electron microscopy (SEM)

Surface images of the carbonaceous materials were analyzed by scanning electron microscopy as shown in Figure 1 (a, c, e andg). From these pictures, it is possible to appreciate the rough edges of the surface of the materials. Only Figure 1(c) (BP-H 2 O-Fe) shows clearly the presence of iron traces and just in specific spots on the surface. To complete these pictures, EDX spectra were carried out on BP-H 2 O, BP-H 2 O-Fe, PH-H 2 O and PH-H 2 O-Fe (Fig. 1(b, d, f andh)). These were only used for a qualitative analysis: carbon and oxygen are the major elements present in all samples, no iron is observed on BP-H 2 O and PH-H 2 O, and its content in BP-H 2 O-Fe is higher than in PH-H 2 O-Fe because of the size of the peak which was confirmed in the previous section.

Arsenic removal with carbonaceous materials and GAC

Sorption kinetics

Batch contact experiments were conducted on BP-H 2 O, BP-H 2 O-Fe, PH-H 2 O, PH-H 2 O-Fe and GAC and the kinetic decay curves are plotted in Figures 2a and2b. It has previously been verified that no adsorption occurs on raw beet pulp and the corresponding char (data not shown) so Table 5. Parameters of intraparticle diffusion and pseudo-second-order kinetic models.

Intraparticle diffusion

Pseudo-second-order Material k id (µgg -1 min -0.5 )R 2 h(µgg -1 min -1 )q e, calc. (µgg -1 )q e, exp (µgg -1 )k 2 (g µg -1 min -1 )R that activation is essential for arsenate adsorption. The optimal contact time deduced from the curves and common to all the materials is 120 h (5 days). The comparison of kinetic data obtained with BP-H 2 O, PH-H 2 Oa n dG A C (Fig. 2a) shows that PH-H 2 O and GAC have a similar behavior in terms of rate and uptake amounts. After iron impregnation (Fig. 2b), the capacities are undoubtedly enhanced and kinetic rates seem to be faster. As this chemical treatment slightly affects the porous structure, a chemisorption reaction is probably involved.

To investigate the mechanism of sorption and potential rate-controlling steps, kinetic models were used to describe the experimental data. The intraparticle diffusion-controlled adsorption model (Eq. 1) requires the adsorbent to be reduced to powder and focuses on the first hours of the kinetic curves. [START_REF] Morris | Removal of biologically-resistant pollutants from waste waters by adsorption -Advances in Water Pollutions Research[END_REF] If the rate-limiting step is intraparticle diffusion, a plot of the sorbed solute against the square root of contact time should yield a straight line passing through the origin. [START_REF] Gérente | Application of chitosan for the removal of metals from wastewaters by adsorption -Mechanisms and models review[END_REF] The model led to acceptable correlation coefficients (Table 5) suggesting that intraparti- cle diffusion is probably a limiting step of the adsorption reaction between activated carbons and arsenate ions. The results also show that BP-H 2 O-Fe and PH-H 2 O-Fe exhibit higher intraparticle diffusion constants. As diffusivity is a function of the solute affinity for the adsorbent material and the activated carbon site density, it can be confirmed that As (V) has a great affinity for iron-loaded sorbents. [START_REF] Axe | Intraparticle surface diffusion of metal contaminants and their attenuation in microporous amorphous Al, Fe, and Mn oxides[END_REF] The pseudo-second-order model [START_REF] Ho | Kinetic models for the sorption of dye from aqueous solution by wood[END_REF] (Equations 2-4) was also tested and it described the experimental data perfectly: as shown in Table 5, the R 2 values are close to unity and the calculated q e values are similar to the experimental ones, demonstrating that the pseudo-second-order model can be applied to the whole curve. This is also in agreement with a chemisorption mechanism being the rate-controlling step. Nevertheless, it is a pseudo-kinetic model, so a specific but different rate constant is obtained for each change in a system variable.

However, all the curves are obtained in the same experimental conditions and the constants are compared with each other. It is clear from Table 5 that iron impregnation increases the initial sorption rate (h) by a factor of 14 on BP-H 2 O-Fe (7.06 µgg -1 min -1 )a n db yaf a c t o ro f7o n PH-H 2 O-Fe (14.27 µgg -1 min -1 ). Moreover, the h values obtained with PH-H 2 O and GAC are similar, 1.91 and 1.42 µgg -1 min -1 respectively, and higher than BP-H 2 O (0.5 µgg -1 min -1 ). In another work, kinetic data on arsenic sorption from groundwater (initial concentration of 300 µgL -1 ) using commercial iron-doped activated carbons were also presented. [START_REF] Fierro | Arsenic removal by iron-doped activated carbons prepared by ferric chloride forced hydrolysis[END_REF] Initial sorption rates deduced from the pseudo-second-order model were of the same order of magnitude, ranging between 0.65 and 38.1 µgg -1 min -1 .

Adsorption isotherms

Adsorption isotherms were conducted with 120 h of contact time between the arsenate solution (initial concentrations ranged between 100 to 5000 µgL -1 and pH between 6 and 7) and ACs. Experimental points, as well as the curves modeled by Langmuir and Freundlich equations, are shown in Figures 3 and4. The constants of each model are reported in Table 6. Regarding the Freundlich model, K F values are found to be 5 and 30 µg 1-1/n L 1/n g -1 for steam-activated carbons (BP-H 2 O and PH-H 2 O), 205 and 640 µg 1-1/n L 1/n g -1 for iron-loaded materials and 300 µg 1-1/n L 1/n g -1 for GAC. The values for n are less than 1 in all cases, which indicates that adsorption is favorable. [START_REF] Levan | Adsorption and ion exchange[END_REF] The pH at the end of the isotherm depends on the AC. It is higher than initial values (6 to 7) for BP-H 2 O and BP-H 2 O-Fe (9 to 9.5 and 8.5 to 9, respectively), similar for PH-H 2 O and GAC (6.5 to 7) and lower for PH-H 2 O-Fe (4.5 to 6). This means that the main arsenate species present in solution is H 2 AsO 4 -and that this is also the main species adsorbed on PH-H 2 O-Fe, PH-H 2 O and GAC. On the contrary, HAsO 4 2-is probably the form sorbed on BP-H 2 O and BP-H 2 O-Fe. In these conditions, all the sorbents are rather protonated because all the final pH values are below the pH PZC .

Thus, positively-charged surfaces could react favorably with the negative species of arsenate. Iron impregnation involves a combination of two effects: first, it strongly increases the initial adsorption rates and uptake capacities of both BP-H 2 O-Fe and PH-H 2 O-Fe, confirming the great affinity between arsenate ions and iron-based sorbents. Secondly, and to a lesser extent, it decreases the pH PZC and subsequently the solution pH. This leads to the monovalent ion (H 2 AsO 4 -) being the major arsenate species present whose adsorption is the highest on PH-H 2 O-Fe exhibiting only 0.5% of iron.

Finally, the results of many studies (Table 1) have shown the advantages of commercial iron-impregnated AC but have also revealed large discrepancies between the q m values (from 20 µgg -1 to more than 50 000 µgg -1 ). Nevertheless, the latter seem to be correlated with the iron content. The results obtained in this work are correctly placed within this field if we consider activated carbons produced from agricultural wastes at laboratory scale thus trying to increase the iron content of the proposed sorbent should be an interesting perspective.

To show the potential effect of competing ions, adsorption isotherms were conducted in natural waters (Fig. 5a and5b) whose major components are shown in Table 2 and to which arsenic was added. PH-H 2 O and PH-H 2 O-Fe, the best activated carbons in terms of adsorption capacities, were selected for this experiment. From Figure 5b, it is clear that in high mineral content water, PH-H 2 O loses its adsorptive properties completely. On the contrary, isotherms carried out with PH-H 2 O-Fe on both natural waters (Fig. 5a) exhibit weakened but useful capacities compared to GAC. The isotherm curves clearly show some waves that prevented the Langmuir or Freundlich models from being tested. Iron impregnation seems to moderate the competitive effect of anion or cationic, monovalent or divalent ions and is necessary for the adsorbents to keep their capacities in natural waters.

Conclusions

The aim of this work was to prepare sustainable activated carbons from agricultural wastes, namely sugar beet pulp and peanut hulls, and to use them to remove arsenate from water. A direct physical activation by steam led to activated carbons, called BP-H 2 O and PH-H 2 O, with respective mass yields of 16% and 24%. A chemical treatment by iron impregnation was performed leading to 4.8% and 0.5% of iron in BP-H 2 O and PH-H 2 O. An investigation of the porous structure showed that the BET surface area is close to 800 m 2 g -1 for both adsorbents, total porous volume is higher in BP-H 2 O (0.64 cm 3 g -1 ) than in PH-H 2 O (0.40 cm 3 g -1 ) but microporous volumes are similar (0.35 cm 3 g -1 ). Moreover, BP-H 2 O-Fe and PH-H 2 O-Fe keep their porous properties after iron impregnation. Carbon contents are up to 70% and ash contents depend greatly on the precursor. pH PZC measurements reveal a clear basic behavior, except for PH-H 2 O-Fe. Based on these results, the production of activated carbons from agricultural residues is clearly successful at lab scale. In terms of arsenic removal, the carbonaceous sorbents need first to be steam-activated and then iron-impregnated to increase adsorption capacities. From kinetic studies, the equilibrium time is close to 120 hours and the rate-controlling steps are probably related to intraparticle diffusion and chemisorption.

Regarding equilibrium data, experimental isotherms are correctly described by the Langmuir and Freundlich models. The best fit is obtained with the Freundlich equation while the best q m values provided by Langmuir are 3280 µg g -1 for PH-H 2 O-Fe, 2930 µgg -1 for BP-H 2 O-Fe, 2820 µg g -1 for PH-H 2 O and 690 µgg -1 for BP-H 2 O. In natural waters, the results also confirm the good performances of iron-rich materials. Iron impregnation creates a double effect: first, it strongly increases the initial adsorption rates and uptake capacities for both BP-H 2 O-Fe and PH-H 2 O-Fe. Secondly, it decreases the pH PZC of the adsorbents and subsequently the solution pH leading to a majority of the adsorbable arsenate species H 2 AsO 4

-. This study displays that sustainable AC produced at laboratory scale, improved by a simple chemical treatment and successfully applied to arsenate removal, could be used in a continuous process.

Fig. 1 .

 1 Fig. 1. SEM micrographs and EDX spectra of BP-H 2 O (a-b), BP-H 2 O-Fe (c-d), PH-H 2 O (e-f), PH-H 2 O-Fe (g-h) (color figure available online).

Fig. 2 .

 2 Fig. 2. Kinetic decay curves for removal of As (V) on (a) BP-H 2 O, PH-H 2 O and GAC and (b) BP H 2 O-Fe, PH-H 2 O-Fe and GAC.

Fig. 3 .

 3 Fig. 3. Experimental adsorption isotherms of As (V) and modeled results using the Langmuir equation.

Fig. 4 .

 4 Fig. 4. Experimental adsorption isotherms of As (V) and modeled results using the Freundlich equation.

Fig. 5 .

 5 Fig. 5. Sorption isotherm of arsenic (V) onto: (a) PH-H 2 O-Fe and (b) PH-H 2 O in deionized water (DW) and spring water with a high (HMC) or low (LMC) mineral content.

Table 2 .

 2 Mineral content of spring waters used for As (V) adsorption.

	Water	Ca 2+	Mg 2+	Na +	K +	Cl -	NO 3	-	HCO 3	-	SO 4	2-	pH
	HMC (mg.L -1 ) *	230	66	40	8	58	< 1	280		620	7.2
	LMC (mg.L -1 )

* * 4.11 .72 .70 .90 .90 .82 5 .81 .17 .3 * Spring water with a high mineral content (HMC), * * Spring water with a low mineral content (LMC).

Table 3 .

 3 Characterization of activated carbons produced from sugar beet pulp, peanut hulls and commercial GAC.

		BP-H 2 O	BP-H 2 O-Fe	PH-H 2 O	PH-H 2 O-Fe	GAC
	S BET (m 2 g -1 )	821	762	829	718	1138
	V micro (cm 3 g -1 )	0.3478	0.3227	0.3551	0.3084	0.4852
	V meso (cm 3 g -1 )	0.3612	0.3227	0.0699	0.0569	0.1761
	Microporosity%	49	50	84	84	73
	Mesoporosity%	51	50	16	16	27
	Total porous volume (cm 3	0.6430	0.5778	0.4028	0.3446	0.6230
	g -1 )
	C%	77.9	67.8	91.2	83.9	90.1
	H%	0.9	1.0	0.8	0.6	0.2
	N%	0.6	0.7	0.2	0.4	ns
	O%	7	16	6	14	9
	Fe%	0.1	4.8	ns	0.5	ns
	Ash%	13.6	13.9	1.3	1.5	0.4
	pH PZC	9.8	9	9.8	6	8

ns: not significant.

Table 4 .

 4 Selection of papers dealing with the production of activated carbons from agricultural residues, in similar conditions to this work.

	Material	Activation conditions	S BET (m 2 g -1 )	Total porous volume (cm 3 g -1 )	Reference
	Corncob	Steam/850 • C (1 h)	607	0.296	[29]
		Steam/600 • C (2 h)	618	0.321
		Steam/700 • C (2 h)	786	0.430
	Sawdust	Steam/800 • C (1 h)	516	-	[30]
	Bagasse	Steam/750 • C (2 h)	446	0.287	[31]
		Steam/840 • C (2 h)	607	0.445
	Pistachio nut-shells	CO 2 /800 • C(1 h)	884	-	[32]
		CO 2 /800 • C(2.5 h)	964
	Macadamia nut-shells	CO 2 /500 • C(4 h)	750	-	[33]
	Bagasse	CO 2 /900 • C(1 h)	614	0.310	[34]
	Beet pulp	Steam/850 • C(1.3 h)	821	0.643	This work
	Peanut hulls	Steam/850 • C(1.3 h)	829	0.403	This work

Table 6 .

 6 Adsorption constants for Langmuir and Freundlich models.

			Langmuir			Freundlich
	Material	q m (µgg -1 )b L (L µg -1 )R 2	K F (µg 1-1/n L 1/n g -1 )n	R 2
	BP-H 2 O	690	0.00066	0.956	5	0.56	0.968
	BP-H 2 O-Fe	2930	0.00312	0.964	205	0.33	0.990
	PH-H 2 O	2820	0.00066	0.971	30	0.52	0.993
	PH-H 2 O-Fe	3280	0.00089	0.945	640	0.21	0.991
	GAC	1240	0.01454	0.966	300	0.18	0.968

Acknowledgments

We acknowledge financial support from the Consejo Nacional de Ciencia y Tecnología de México (CONACYT) by a scholarship grant for JTP.