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Published online 20 February 2008
Published in J Environ Qual 37:631-638 (2008)
DOI: 10.2134/jeq2007.0332
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Bioremediation and Biodegradation

Use of a Low-Cost Biosorbent to Remove Pesticides from Wastewater

Stéphanie Boudesocquea,*, Emmanuel Guillona, Michel Aplincourta, Frédéric Martelb and Sandrine Noëlb

a GRECI (Groupe de Recherche en Chimie Inorganique), Université de Reims Champagne-Ardenne, BP 1039, 51687 Reims Cedex 2, France
b ARD Société Agro Industrie Recherches et Développements, route de Bazancourt, 51110 Pomacle, France

* Corresponding author (stephanie.boudesocque{at}univ-reims.fr).

Received for publication June 21, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
A lignocellulosic substrate (LS) obtained from our local agroindustry was used as a low-cost and effective adsorbent for the removal of pesticides from wastewaters. The studied pesticides were terbumeton (N-(1,1-dimethyl)-Nethyl-6-methoxy-1,3,5-triazine-2,4-diamine), desethyl terbumeton (N-(1,1-dimethylethyl)-6-methoxy-1,3,5-triazine-2,4-diamine), dimetomorph (4-[3-(4-chlorophenyl)-3-(3,4-dimethoxyphenyl)acryloyl]morpholine), and isoproturon (3-(4-isopropylphenyl)-1,1-dimethylurea). Batch and column experiments were conducted as a function of pH and pesticide concentration under laboratory and industrial conditions. The concentration range studied for the pesticides varied from 2 x 10–7 to 3 x 10–4 mol L–1. The influence of organic and inorganic pollutants was assessed by studying the retention of pesticide in the presence of copper(II) and a surfactant. These experiments indicated that LS is an efficient adsorbent toward the investigated pesticides and has little influence of the other pollutants. The kinetic adsorptions are fast, and the amounts of adsorbed pesticide varied from 1 to 8 g kg–1 of LS. These retention capacities show that LS can provide a simple, effective, and cheap method for removing pesticides from contaminated waters. Thus, this biomaterial may be useful for cleaning up polluted waters.

Abbreviations: BV, bed volume • DET, desethyl terbumeton • DIM, dimetomorph • HPLC, high-performance liquid chromatography • ISO, isoproturon • LS, lignocellulosic substrate • TER, terbumeton


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
IN the last few years, the intensive use of pesticides has generated discussions about important environmental issues, including the pollution of surface water and ground water. This pollution may arise from run-off, leaching, or accidental application. Environmental regulations on the composition of drinking water have become very strict, especially regarding pesticide compounds. The European Union requires water to be treated before being supplied to customers as drinking water. Pesticide may not be present at concentrations exceeding 0.1 µg L–1 for a single pesticide and 0.5 µg L–1 for the sum of all pesticides contained in a sample (European guideline 98/83/CE). A range of conventional methods can be used to remove pesticides from wastewaters, and patents protect a number of them. These methods include UV treatment, ozonation, and disinfection (Hwang et al., 2001; Mason et al., 1990). Pesticide adsorption on different solid materials, such as clays (Sannino et al., 1997), modified clays (Cruz-Guzman et al., 2005), and activated carbon (Bacaoui et al., 2002), has also been investigated to eliminate pesticides from water. Unfortunately, these materials are expensive. This called for the development of low-cost adsorbents (Gupta and Ali, 2006) that are widely available. Pollard et al. (1992) suggested the use of adsorbents prepared from carbonaceous industrial wastes, agricultural byproducts, and mineral-derived sources. Sudhakar and Dikshit (1999) proposed low-cost adsorbents like wood charcoal, kimberlite tailings, silica, and the macro fungus Sojar caju. Gupta and Ali (2001; 2002) suggested the use of waste from the sugar industry to remove pesticides from wastewaters.

In this study, we focused on the retention capacities of a lignocellulosic substrate (LS), which is a byproduct of the local industry. Our sample is produced by the Agro Industrie Recherches et Développements company (ARD, Pomacle, France). The LS is generated by an industrial process from wheat bran leading to a compound essentially composed of lignin and cellulose. Lignin, which is one of the main components of soil organic matter, plays a key role in the fate of organic pollutants in the environment due to its high affinity toward pesticides (Barak et al., 1983). Using batch and column experiments, we have studied the retention of four pesticides on LS. The studied pesticides are widely used in the Champagne-Ardenne region and other vineyard regions and include terbumeton (N-(1,1-dimethyl)-N'ethyl-6-methoxy-1,3,5-triazine-2,4-diamine), a selective herbicide used for weed control in vineyards; desethyl terbumeton (N-(1,1-dimethylethyl)-6-methoxy-1,3,5-triazine-2,4-diamine), its main metabolite; isoproturon (3-(4-isopropylphenyl)-1,1-dimethylurea), a herbicide used in field crops; and dimetomorph (4-[3-(4-chlrophenyl)-3-(3,4-dimethoxyphenyl)acryloyl]morpholine, a fungicide used in vine growing.

The kinetics and extent of pesticide adsorption on LS were determined under laboratory and industrial conditions as a function of pH and pesticide concentration. The influence of copper(II) [Cu(II)] on pesticide adsorption has also been studied due to its known affinity toward pesticides (Pier et al., 1997; Sheals et al., 2001). Copper(II) was applied in great quantities for over a century as Bordeaux mixture to protect grapevines against fungal diseases. Thus, Cu(II) has become a ubiquitous metallic cation of vineyard soils and has been detected in the surface waters and ground waters of the Champagne Ardenne region. The influence of a surfactant on pesticide adsorption was also studied. Pesticide formulations are mixtures of active (pesticidal) and inert components, including surfactants that may aid in pesticide application or uptake. The results of the present work may aid in the development of an alternative way of removing pesticides from wastewaters.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Materials and Lignocellulosic Substrate
The purest commercially available grade of HNO3 and KOH were obtained from Fluka (Saint Quentin, Fallavier, France) (puriss grade). Copper(II) stock solution was prepared from Cu(NO3)2 purchased from Fluka. Terbumeton (TER), desethyl terbumeton (DET), isoproturon (ISO), and dimetomorph (DIM) certified standards (minimum purity 99%) were purchased from CIL Cluzeau (Sainte Foy La Grande, France). Their chemical structure, molecular weight, and solubility in water are given in Table 1 . The surfactant POEA (polyethoxylated tallow amine), used in some formulations of glyphosate, was purchased from Cognis (Monheim, Germany). All chemicals from commercial sources were of the highest available purity and used without purification.


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  		Table 1. Studied pesticides.

 
The LS (provided by ARD, Pomacle, France) was obtained from wheat bran after sequential acido-basic treatments: (i) an acidic hydrolysis (H2SO4 96%) to remove starch, hemicellulose, proteins, and polysaccharides; (ii) an alkali treatment (KOH) to dissolve lignins of low molecular weight; and (iii) an acid treatment with HNO3 for 3 d to protonate the surface sites. These treatments were necessary to obtain an insoluble substrate in the pH range 2 to 11 (DOC values <<1%). The LS composition and structure have been previously reported (Bouanda et al., 2002). Briefly, LS was analyzed by various techniques and is composed of 36% lignin and 64% cellulose. The amounts of acid groups, mainly involved in complexation, were 0.55 mol kg–1 carboxylic and 0.45 mol kg–1 phenolic (Merdy et al., 2002). The specific surface area (185 m2 g–1) was determined using the BET method (Brunauer et al., 1938) from aqueous vapor adsorption isotherms to approximate wet solid conditions, which corresponds to more realistic conditions.

Batch Experiments under Laboratory Conditions
The adsorption of pesticides onto LS was determined as a function of time (kinetic studies), the solution pH, and the introduced pesticide concentration. All studies were conducted in triplicate at room temperature (293 K).

Adsorption experiments conducted as a function of pH were performed by suspending 50 mg of LS in 15 mL of ultra-pure water (ALPHA Q, 18 M{Omega} cm) for 24 h to ensure the hydration of the solid. After this pre-equilibration step, the pesticide and/or copper solution was added to obtain final concentrations of 10–5 and 2 x 10–4 mol L–1, respectively. The pH was incrementally adjusted to a fixed value (ranging from 6 to 10) by the addition of 0.1 mol L–1 HCl or KOH. The final volume was adjusted to 25 mL, which led to a final LS concentration of 2 g L–1. The flasks were shaken with an automatic shaker for the time necessary to ensure a complete adsorption, and the pH value was recorded. The adsorption equilibrium time was predetermined by kinetic studies in which different contact times were used.

The kinetics of pesticide adsorption were studied at an initial pesticide concentration of 10–5 mol L–1 at the LS natural pH (pH of the LS solution at 2 g L–1). The contact time varied from 5 min to 48 h. After filtration through a 0.20-µm cellulose acetate membrane, the unsorbed pesticide concentration was measured by high-performance liquid chromatography (HPLC). The amount of pesticide adsorbed was deduced from the initial concentration.

Adsorption was determined as a function of pesticide concentration at the LS natural pH to study the effect of the introduced pesticide concentration on the adsorption phenomenon and to determine the maximum adsorption capacity of the LS. The pesticide concentration varied from 0.2 to 50 µmol L–1 in the case of TER and DET and from 0.2 to 300 µmol L–1 in the case of ISO and DIM. Experiments were performed as described previously.

Batch Experiments in Industrial Conditions
To determine pesticide adsorption under industrial conditions, experiments were conducted as described in the previous section but using 500 g of LS suspended in 100 L of purified water or an effluent solution (LS concentration of 5 g L–1). The experiments were conducted using ultra-pure water (ALPHA Q, 18 M{Omega} cm) or an effluent solution from a decantation basin located at the village of Reuil (Marne, France), which collects the run-off waters of a vineyard bank basin. None of the studied pesticides were detected in the natural effluent before enrichment. For our studies, we enriched this effluent with pesticides and/or surfactant at concentrations of 10–6 and 2 x 10–7 mol L–1, respectively. Then, 10 mL of supernatant were removed at times varying from 5 min to 4 d. The supernatant was filtered through a 0.20-µm cellulose acetate membrane, and the unsorbed pesticide concentration was measured by HPLC. The unsorbed surfactant amount was determined by surface tension measurements.

Column Experiments
The bottom of a glass column was covered with fine gravel to retain the fine particles of LS. The LS was suspended in water solution to allow a better introduction to the column (bed volume [BV] = 21 mL; 22 g of LS, height = 2.5 cm, diameter = 3 cm) and to increase packing uniformity. After column preparation, pesticide solution at a concentration of 10–5 mol L–1 was pumped continuously to the top of the LS column using the peristaltic pump. The flow rate of the pump was 0.9 mL min–1, which corresponds to 2.5 BV h–1. The pesticide solution passed down the LS column and was collected as 25-mL fractions at the outlet. Each fraction was filtered through a 0.20-µm cellulose acetate membrane, and the pesticide concentration was measured by HPLC analysis.

Pesticide Analysis
The unsorbed pesticide concentration was quantified using an HPLC system (Varian ProStar Chromatograph; Varian, Palo Alto, CA) equipped with a photodiode array detector (Varian). A reversed-phase Kromasil C18 column (5 µm x 250 mm x 3.0 mm) was used. Samples were eluted isocratically and analyzed using the following mobile phases: 70/30 (v/v) CH3CN/H2O for TER, DIM, and ISO; 50/50 (v/v) CH3CN/H2O for DET. The flow rate was 0.7 mL min–1 in the case of TER and DET, 0.5 mL min–1 in the case of DIM, and 1 mL min–1 in the case of ISO. The injected volume was 20 µL in all cases. The analytes were quantitatively determined by UV detection at 219 nm for TER, DET, and DIM or 242 nm for ISO. The detection limit for the pesticides was 2 µg L–1, which corresponds to 8.9 x 10–9 mol L–1 for the TER, 1 x 10–8 mol L–1 for the DET, 9.7 x 10–9 mol L–1 for the ISO, and 5.2. x 10–9 mol L–1 for the DIM. The amounts of pesticide and surfactant adsorbed onto LS were calculated as the difference between the initial solution concentration and the solution concentration after equilibration.

Surfactant Analysis
Tension surface measurements were performed, at room temperature (293 K), using a KRUSS Processor Tensiometer K100 equipped with a platinum strip.

Adsorption Isotherms
The Langmuir and Freundlich isotherm equations were used to describe the adsorption of pesticides from aqueous solution onto LS.

The Langmuir isotherm is expressed by

Formula 1[1]
where KD is the distribution coefficient that characterizes the affinity of the pesticide for the sorbent, CSmax is the maximum adsorption capacity of the solid, Ce is the measured concentration in the equilibrium solution (µmol L–1), CS is the measured adsorption per unit weight of solid (µmol g–1), and b represents the Langmuir bonding term related to the adsorption energy.

The Freundlich isotherm is expressed by

Formula 2[2]
where KF is the Freundlich distribution coefficient related to the total adsorption capacity of the solid, and n is a nonlinear constant.

Kinetic Model
The Lagergren expression (Lagergren, 1898) was used to analyze the kinetic adsorption of solutes from a liquid solution. This first-order rate equation is

Formula 3[3]
where C and Ce are the grams of solute sorbed per gram of sorbent at any time and at equilibrium, respectively, and k1 is the rate constant of first-order adsorption.

The rate law for the second-order reaction is expressed as

Formula 4[4]
where k2 is the second-order rate constant of adsorption. Equation [4] can be rearranged to obtain the following linear form:

Formula 5[5]
The constant can be determined by plotting t/C versus time.


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
Pesticide Sorption Isotherms
Before adsorption studies, to obtain kinetic information about the pesticide adsorption processes, kinetic experiments were performed using the batch equilibrium method. These experiments were conducted under laboratory and industrial conditions at pH 4.8. The results are reported in Fig. 1 and 2 , respectively. Under laboratory conditions, kinetic studies show that the adsorption process can be divided into two steps for TER and DIM: a rather fast first step (t < 1 h) followed by a second step (1 h < t < 4 h) corresponding to a much slower adsorption process. Sorption was more than 80% complete within 1 h and complete after about 4 h. Desethyl TER and ISO were sorbed within 15 min. These preliminary results showing rapid pesticide adsorption are encouraging for future application of this LS in the removal of pesticides from contaminated waters. The kinetic study was also performed under industrial conditions with TER in distilled water and in a natural effluent to estimate the influence of the presence of competitive molecules (other pesticides, metallic cations, dissolved organic matter, etc.). In distilled water, rapid adsorption of the pesticide occurs, and sorption equilibrium is reached after 4 h (Fig. 2). In the effluent solution, TER adsorption is slower, and equilibrium is reached in about 50 h. This difference suggests that competitive adsorption occurs on the LS surface between TER and components present in the effluent. Nevertheless, more than 60% of the potentially adsorbable amount of TER is adsorbed within an hour. The two curves reach the same final adsorption percentage (around 70%) (Fig. 2). This indicates that even if there were a competitive effect, the presence of competitive compounds present in this effluent had no influence on the adsorption capacity of the LS.


Figure 1
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  		Fig. 1. Adsorption kinetics of terbumeton (circles), desethyl terbumeton (triangles), dimetomorph (squares), and isoproturon (X) on lignocellulosic substrate (2 g L–1) under laboratory conditions at pH 4.8, [pesticide] = 10–5 mol L–1, V = 25 mL, distilled water.

 

Figure 2
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  		Fig. 2. Adsorption kinetics of terbumeton on lignocellulosic substrate (5 g L–1) in industrial conditions at pH 4.8 (squares, effluent solution; circles, distilled water), [TER] = 10–6 mol L–1, V = 100 L.

 
Pesticide adsorption on the LS under laboratory conditions followed a second-order kinetic model (data not shown). The values of the rate constant, k2, and the amount of pesticide sorbed at equilibrium, Ce, are summarized in Table 2 . In the case of DIM and TER, the Ce values are greater than those obtained with the two other products. The correlation coefficients for the linear plots of t/C against time (from the second-order kinetic equation) are >0.994 for all systems. This suggests that this adsorption system is not a first-order reaction (r2 between 0.553 and 0.907) (Table 2) and that the second-order model provided the better description of the data. In these systems, the rate-limiting step may be chemisorption involving valency forces through sharing or exchange of electron between sorbent and sorbate (Ho and Mc Kay, 1999).


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  		Table 2. Kinetic parameters derived from the second-order equation of Lagergren (laboratory conditions).

 
The kinetics of TER adsorption under industrial conditions were also analyzed with the Lagergren equation. The values of the rate constant k are 0.68 x 10–2 g mg–1 h–1 and 0.34 x 10–2 g mg–1 h–1 in distilled water and in effluent solution, respectively. These values are greater than the one obtained under laboratory conditions (0.11 x 10–2 g mg–1 h–1). This is in accordance with the greater solid/volume ratio for the industrial conditions (5 g L–1 compared with 2 g L–1), which leads to a faster adsorption process. Equilibrium is reached after 4 h in distilled water and in 50 h in the effluent (Fig. 2).

We explored the influence of pH on the adsorption of pesticides onto LS. Many studies (e.g., Wang et al., 1992; Gao et al., 1998; Hsieh and Kao, 1998) have shown that the adsorption of pesticides onto soil organic matter strongly depends on the pH. In our study, the adsorption of the pesticides was examined over the pH range 6 to 10 (Fig. 3 ), which corresponds to the range most frequently encountered in the environment. It seems that the retention capacity of the LS is practically independent of the pH (Fig. 3). This result is an advantage compared with other solids used to remove pesticides from water, such as activated carbon or resins (Yang et al., 2004; Kyriakopoulos et al., 2003). The retention of pesticides on resins is a selective process that depends on the solution pH and the characteristics of the resin. In our case, the pesticide removal by LS was independent of the ionizable character of the pesticides.


Figure 3
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  		Fig. 3. Adsorption of terbumeton (circles), desethyl terbumeton (triangles), isoproturon (x), and dimetomorph (squares) as a function of pH on the lignocellulosic substrate (2 g L–1) under laboratory conditions, [pesticide] = 10–5 mol L–1, V = 25 mL, distilled water.

 
In the pH range 6 to 10, TER (pKa = 4.68) and DET (pKa = 4.39) are neutral (Abiven et al., 2006). Dimetomorph and ISO do not have ionizable protons and are neutral. Moreover, at pH values below 6 (data not shown), the adsorption capacity of LS was similar to the one obtained at higher pH values, which confirmed the independence of the ionizable character of the pesticides. The pHpzc of LS is equal to 4.1 (Sayen et al., 2006). Above this pH value, the surface charge becomes negative. In the pH range studied (6–10), the charge of the LS surface and pesticides is constant, which can explain the horizontality of the adsorption curves (Fig. 3). Moreover, the kinetic experiments, which were conducted at pH 4.8 (Fig. 1a), showed a greater amount (at equilibrium) of TER and DET sorbed onto the LS surface compared with the sorption experiment conducted as a function of pH (between 6 and 10). The sorbed TER amount decreases from 60 to 25%, and the DET decreases from 20 to 10%. This can be explained by the fact that at pH 4.8 the two pesticides are partially positively charged (pKa values around 4.5), which increases their affinity toward the negatively charged LS surface. The amount of DIM and ISO sorbed was constant (20% for ISO and 50% for DIM) for these two experiments because these pesticides are neutral at all pH values. For further applications of LS, it is important that pH has no influence on its adsorption capacities, which enables the treatment of water from different origins.

To determine the maximum adsorption capacity of LS, adsorption was determined at the natural pH of the LS as a function of the introduced pesticide concentration. The maximum concentrations of pesticide introduced were 0.05 mol kg–1 of LS for TER and DET (11.2 g kg–1 and 9.8 g kg–1, respectively) and 0.3 mol kg–1 for ISO and DIM (61.8 g kg–1 and 107.3 g kg–1, respectively). A sorption plateau was reached only in the case of DIM (approximately 10 g kg–1). For the other pesticides, the saturation of the solid surface was not obtained (Fig. 4 ) in our experimental conditions, although maximum concentrations were near or in excess of the solubility, much higher than those commonly observed in polluted waters. To compare the adsorption capacities of LS toward the different pesticides, the amount of adsorbed pesticide was calculated for an equally introduced pesticide quantity (0.05 mol kg–1). Thus, 1 kg of LS sorbed 1 g of ISO, 2 g of DET, 5 g of TER, and 8 g of DIM. The amounts adsorbed are similar to those obtained in the case of the retention of paraquat (Hamadi et al., 2004) and diuron (Yang and Shen, 2003) by activated carbon. For the all studied pesticides, the amounts adsorbed on LS are higher than those detected in the polluted waters.


Figure 4
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  		Fig. 4. Adsorption isotherms of terbumeton (circles), desethyl terbumeton (triangles), isoproturon (X), and dimetomorph (squares) on the lignocellulosic substrate (2 g L–1) under laboratory conditions at pH = 4.8, V = 25 mL, distilled water (upper panels). Ce, equilibrium concentration; Cs, concentration of adsorbed pesticide. (a) Freundlich and (b) Langmuir modeling of pesticide adsorption on lignocellulosic substrate derived from data of the adsorption isotherms (lower panels).

 
The Langmuir and Freundlich models were fitted (Fig. 4a, 4b) to adsorption isotherms in the case of DIM and in the case of TER, DET, and ISO, respectively. In the Langmuir model, a monolayer is considered, and the Freundlich model is based on a multilayer mode. In the case of DIM, for which a plateau in the isotherm is reached (Fig. 4), a Langmuir model (monolayer) is in accordance with its larger steric hindrance. The Kf values are equal to 340 L kg–1, 620 L kg–1, and 1090 L kg–1 for DET, ISO, and TER, respectively. These values show that TER has a greater affinity for the LS than the two other pesticides. These values are higher than those obtained by Van Beinum et al. (2006) in the case of the sorption of ISO on lignin. The Freundlich nonlinearity constant, n, ranged from 0.59 to 0.88 (n < 1), indicating that the proportion of pesticide adsorbed decreases with increasing initial concentration of pesticide. This may be explained by an increased difficulty in accessing adsorption sites when the pesticide concentration increases.

Influence of Other Pollutants on the Pesticide Adsorption
Because surfactants are commonly used in pesticide formulations and are frequently detected in surface waters (Wang et al., 2005), competitive experiments were conducted. The influence of Cu(II) was also studied due to the great quantity found in Champagne-Ardenne waters (European Project LIFE).

Figure 5 shows the adsorption of TER on LS in the presence and absence of Cu(II) as a function of the pH in laboratory conditions. This figure shows that the amount of TER adsorbed is practically the same with or without Cu(II) ion only when the pH is in the range of 8 to 9. At lower pH, the percentage of TER adsorbed without Cu(II) ions is 30%, whereas in presence of Cu(II) ions the percentage decreases to 20%, probably due to a competitive effect. This result differs from the results of Flogeac et al. (2005), where the amount of amitrole adsorbed increased in the presence of Cu(II). Sheals et al. (2003) showed similar results with the glyphosate.


Figure 5
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  		Fig. 5. Adsorption of terbumeton alone (circles) and in presence of copper(II) (triangles) on lignocellulosic substrate (2 g L–1) as a function of pH in laboratory conditions, [TER] = 10–5 mol L–1, [Cu(II)] = 2 x 10–4 mol L–1, V = 25 mL, distilled water.

 
The influence of a surfactant was studied, and the results are presented in Fig. 6 . The adsorption of TER was studied in the presence and absence of POEA (polyethoxylated tallow amine), which is a non-ionic surfactant used in glyphosate formulations. Some studies (Iglesias-Jimenez et al., 1996; Beigel et al., 1998; Dai et al., 2001) have shown that the presence of surfactants in the soil–water system modified the adsorption of pesticides depending on the nature (cationic, anionic or non-ionic) of the surfactant (Esumi et al., 1998). In our case, the retention of TER was not modified in the presence of POEA. The amount of adsorbed pesticide and the sorption equilibrium time were the same in the presence or absence of POEA (Fig. 6).


Figure 6
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  		Fig. 6. Adsorption of terbumeton alone (circles) and in presence of surfactant (X) on the lignocellulosic substrate (5 g L–1) as a function of time, in industrial conditions, [TER] = 10–6 mol L–1, [surfactant] = 2 x 10–7 mol L–1, V = 100 L, distilled water.

 
We measured the surface tension as a function of time of different mixtures (LS + distilled water, TER + distilled water, POEA + distilled water, and LS + POEA + TER) to obtain better information about the adsorption processes. The initial surface tension of the mixture LS + POEA + TER was 40 mN m–1, the same as that of POEA + distilled water, which indicates that the surface tension is strongly dependent on POEA. Two hours after the solution was placed in contact with LS, the surface tension increased to 55 mN m–1, which is an intermediate value between the mixture TER + distilled water and the mixture LS + distilled water. This result indicates that POEA is adsorbed onto LS before the TER. After 8 h, the surface tension had decreased to 35 mN m–1. We cannot explain this drop. Thirty hours after addition to LS, the surface tension was 50 mN m–1, which is the same as in the mixture LS + distilled water, in accordance with the simultaneous sorption of POEA and TER. These competitive adsorption studies indicate that the presence of other chemicals (inorganic or organic compounds) may have little or no influence on the retention properties of LS, which will be useful in its application to remove pesticides from contaminated waters.

Column Studies
Given the encouraging experimental results obtained in batch reactors, an adsorption under dynamic conditions was tested. The affinity of the pesticides for the LS was well exhibited by the results obtained from column experiments. Figure 7 displays the percentage of TER adsorbed and the quantity of TER introduced as a function of the eluted volume of a TER solution. At elution volumes up to 3 L, the LS column retained about 98% of the introduced pesticide, which naturally leads to a very small amount of unsorbed pesticide. At volumes >3 L, the percentage of adsorbed pesticide significantly decreased, indicating the beginning of column saturation. The amount of unsorbed pesticide detected in the collected samples increased and the percentage of adsorbed pesticide decreased as the elution volume increased, reaching 75% of the applied TER adsorbed at a volume of 10 L.


Figure 7
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  		Fig. 7. Percentage of adsorbed terbumeton (squares) and amount of introduced terbumeton (triagles) vs. the volume of the eluted solution of terbumeton (10–5 mol L–1) in the case of the column experiment. The black diamonds represent the amount of unsorbed pesticide.

 
We arbitrarily fixed the limit of the LS column efficiency at 90% retention of the applied pesticide. Under these experimental conditions, below this percentage, the concentrations of unsorbed pesticide are greater than those allowed by the European Union drinking water guideline. Our column was efficient until 7 L of pesticide solution was eluted, which corresponds to about 16.5 mg of TER introduced (i.e., 15 mg; 90% of 16.5 mg) of TER sorbed. Under these experimental conditions, the retention capacity of the LS was about 0.7 mg of TER per gram of LS under dynamic conditions, which is lower than the one obtained under batch experiments (5 mg g–1 of LS). This retention capacity can be compared with those of other adsorbents used for pesticide removal. Martin-Guillon and Font (2001) studied atrazine removal with activated carbon (granular or fiber) columns. They obtained very similar adsorption capacities (around 1 mg g–1). Bras et al. (1999) found similar retention capacities for pine bark, a wood industry by-product.

The dynamic behavior of TER adsorption to LS was represented through a breakthrough curve (Fig. 8 ). This representation shows that the amount of pesticide adsorbed at the breakpoint (BP, corresponding to 150 BV) is equal to 7 mg (corresponding to 0.3 mg g–1), whereas the amount adsorbed at the saturation point (SP, corresponding to 500 BV) is about 17 mg (corresponding to 0.77 mg g–1), in accordance with our arbitrary limit of the LS column efficiency fixed at 90%. These amounts (7 and 17 mg) are deduced from the curve corresponding to the amount of introduced TER (Fig. 7).


Figure 8
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  		Fig. 8. Breakthrough curve of terbumeton adsorption on the lignocellulosic substrate.

 

   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
In this paper, we presented a detailed experimental analysis of the adsorption of pesticides onto LS, a low-cost biomaterial, in batch reactors and under dynamic conditions. The study of various experimental conditions showed that LS is an efficient adsorbent toward TER, DET, DIM, and ISO. These adsorption studies showed that the retention process is fast (<4 h) and independent of the pH in the range 6 to 10. The amounts of adsorbed pesticide onto 1 kg of LS vary from 1 g, in the case of ISO, to 8 g in the case of DIM. In addition, the presence of other pollutants such as Cu(II) or a nonionic surfactant does not affect the adsorption process, with the exception of the TER for which a weak decrease of the adsorption was observed in presence of Cu(II).

Several practical applications can be envisaged (Guillon et al., 2006). Lignocellulosic substrate can be spread in the bottom of the decantation basin or it can be used inside cartridges. The LS can also be used in place of products like activated carbon or resins. The spent LS material could be regenerated by an acidic treatment or burned if the pollutants could not be desorbed.

Finally, the results presented in this study show that LS is a good means of remediating water polluted by pesticides. Moreover, although the pesticide adsorption capacities are, to some extent, lower than those of synthetic resins or similar to those of activated carbon, the main advantages are the substantially lower cost of the LS (around 200 {euro} per metric ton) and its economic feasibility. These preliminary results warrant further investigations on the use of LS to reduce pesticide concentrations in water.


   ACKNOWLEDGMENTS
 
We thank the Département de la Marne and the Ville de Reims for their financial support (programme Zérophyto/AQUAL). We also thank the Département de la Marne for the grant to S.B. and reviewers for their constructive remarks.


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J. Environ. Qual., February 25, 2009; 38(2): 719 - 728.
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