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Journal of Hazardous Materials 235– 236 (2012) 218– 223

Contents lists available at SciVerse ScienceDirect

Journal of Hazardous Materials

j our na l ho me p age: www.elsev ier .com/ locate / jhazmat

dsorption of paraquat on soil organic matter: Effect of exchangeable cations andissolved organic carbon

ora Gondara, Rocío Lópeza, Juan Antelob, Sarah Fiola, Florencio Arcea,∗

Departamento de Química Física, Facultad de Química, Universidad de Santiago de Compostela, E-15782 Santiago de Compostela, SpainDepartamento de Edafología y Química Agrícola, Facultad de Biología, Universidad de Santiago de Compostela, E-15782 Santiago de Compostela, Spain

i g h l i g h t s

At pH > 5.5, the amount of PQretained by the soil decreases due tothe presence of DOM.The pH and ionic strength effecton PQ adsorption is of electrostaticnature.An electrostatic model is used tointerpret the effect of the surfacecharge on PQ adsorption.The binding sites available for PQare reduced by the presence ofexchangeable cations.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 1 February 2012eceived in revised form 22 June 2012ccepted 23 July 2012vailable online 28 July 2012

eywords:araquatoil organic matter

a b s t r a c t

Herbicides that interact with soil organic matter do so with both the solid and the dissolved fractions, sothat the distribution of herbicide between the soil solution and solid phases is determined by compet-itive effects. In the present study, adsorption experiments were carried out with the cationic herbicideparaquat and untreated and acid-washed samples of a peat soil, at different values of pH and ionicstrength. Less herbicide was adsorbed onto the untreated peat than onto the acid-washed peat; thedifference was due to the presence of exchangeable cations, as demonstrated in experiments carriedout by adding Ca2+ to suspensions of acid-washed peat. The results were interpreted by an electrostaticmodel and the fitting parameters indicated that the adsorption constants were the same for both sam-

issolved organic matterxchangeable cationsH effectdsorption

ples of peat, although the number of binding sites available was different. Simultaneous resolution ofthe adsorption equilibrium of paraquat for the soil organic matter (SOM) and of the binding equilibriumbetween paraquat and dissolved organic matter (DOM) enabled the distribution of paraquat betweenthe solid and solution phases to be determined. The increased solubility of the SOM with increasing pHled to a decrease in the fraction of paraquat retained on the peat surface above pH 5.5, which favors themobility of the herbicide in the soil.

© 2012 Elsevier B.V. All rights reserved.

. Introduction

Pesticides are a class of persistent organic pollutants that play anmportant role in crop production and protection. Some pesticidesave a tendency to leach through the soil profile and contaminate

∗ Corresponding author. Tel.: +34 881816042; fax: +34 981545079.E-mail address: florencio.arce@usc.es (F. Arce).

304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2012.07.044

the groundwater as well as surface waters and may have adverseeffects on human health. Sorption is found to be one of the bestmethods for the removal of organic pollutants, which favors thedevelopment of new synthetic and economically good adsorbentsfor the removal of organic pollutants [1,2].

Paraquat is a cationic non-systemic contact herbicide. Since itfirst appeared for commercial purposes in 1962, it has been used inover 130 countries [3]. However, paraquat is known to be extremelytoxic if accidentally ingested [4], and is included in a priority list of

ous Materials 235– 236 (2012) 218– 223 219

hctlbi

dasslcspsardfbpis

slmamspqk

htpatD

ttsatemtttp

eoosiapo

tob

Table 1Peat soil properties.

pH (H2O) 4.0%C 48.6%N 2.4

1 M NH4Cl exchangeable cations(cmol+ kg−1)

Proton binding parameters

b 0.1Ca 22.2 KH,1 3.62Mg 7.8 Qmax,1 0.97Na 0.5 mH,1 0.39K 0.9 KH,2 7.49Fe 0.3 Q 1.02

D. Gondar et al. / Journal of Hazard

erbicides of potential concern established for the Mediterraneanountries by the European Union, due to its widespread usage inhis area [5]. Its strong adsorption to all types of soils [6] limitseaching and also makes the herbicide less available for biologicalreakdown in the soil solution, and therefore enables it to persist

n soil.The binding of paraquat to soil surface components is mainly

ue to the electrostatic interaction between the herbicide cationsnd the negative charge on clay minerals and/or organic matter,o that adsorption depends on variables such as pH and ionictrength. Although paraquat is strongly bound by expanding clayattices [7], soil organic matter can also adsorb large amounts ofationic herbicide [8]. The application of organic amendments tooil has beneficial effects, mainly because such amendments sup-ly organic matter and other nutritive elements to the soil–plantystem, but also because they contribute to controlling leachingnd sorption of herbicides. However, the role of organic matter ineducing the mobility of paraquat is somewhat limited, since theissolved organic matter may interact with the herbicide and thusavor leaching. Although the concentration of dissolved organic car-on (DOC) is usually low in soils, DOC has a high capacity to bindaraquat [9,10], and the herbicide may reach high concentrations

n drainage waters, thus influencing its transport through the soilystem [11].

The pH determines the distribution of paraquat between theolid and soil solution fractions. On one hand, an increase in pHeads to greater ionization of the acid groups of the natural organic

atter (NOM), which favors interaction with the cationic herbicide,nd on the other hand it increases the solubility of the solid organicatter [12], so that the concentration of DOC increases. In order to

tudy the effect of pH on the distribution of paraquat, the effect ofH on the solubility of SOM and the values of the parameters thatuantify the herbicide/SOM and herbicide/DOM equilibria must benown.

We have previously studied the binding of paraquat to a soilumic acid (HA) in solution [10] and used an electrostatic modelo interpret the experimental behavior, so that the values of thearameters that describe the paraquat/HA equilibrium binding arelready available. If HA is used to model the behavior of the DOM,he concentration of paraquat present in the soil solution bound toOM can be calculated for any pH value.

Although the role of the organic fraction of the soil surface inhe adsorption of cationic herbicides has been investigated [13,14],he effects of variables such as pH and ionic strength have not beentudied, and electrostatic models have not been used to describe thedsorption of paraquat on SOM. In the present study, we obtainedhe adsorption isotherms for paraquat and the peat soil at differ-nt values of pH and ionic strength, and interpreted the results byeans of an electrostatic model. As a result, and taking into account

hat in a peat soil (more than 90% organic matter) the adsorp-ion is exclusively due to the interaction with the organic matter,his provides the values of the parameters required to describe thearaquat–SOM adsorption equilibrium.

The adsorption of paraquat is influenced by the competitiveffects of the inorganic cations present in the peat soil. Therefore, inrder to analyze the effect of exchangeable cations, the adsorptionf paraquat on a peat soil was compared with the adsorption on aample of peat that was first washed in acid to remove inorganicons. In addition, as Ca2+ is the most abundant of the exchange-ble cations [15], adsorption experiments were carried out witharaquat and acid-washed peat to which different concentrationsf Ca2+ were added.

Taking into account that the solubility of peat (and thereforehe concentration of DOC) increases with pH, the distributionf the total concentration of paraquat between both phases cane simulated by simultaneously resolving paraquat–DOM and

max,2

Al 1.1 mH,2 0.43

Data from [15].

paraquat–SOM equilibria for different pH values. This hypothesiswas tested by comparison of the simulated distribution with theexperimental adsorption measurements at different pH values.

2. Materials and methods

2.1. Reagents

Paraquat (1,1′-dimethyl-4,4′-bipyridinium ion) was purchasedas the dichloride salt, from Aldrich. All reagents used were analyt-ical (p.a.) grade.

2.2. Samples

The soil sample, which was collected from an ombrotrophic peatbog located in Galicia (NW Spain), was sieved to remove stones androots, placed in polythene bags, and stored in darkness at +4 ◦C.The sample was freeze-dried before beginning the experiments. Toremove the inorganic ions, a sub-sample was acid-washed withconcentrated HNO3 at pH 1.0, following a method similar to thatproposed by Smith et al. [15,16]. The site characteristics and chem-ical properties of the peat soil are described elsewhere [15], and themost relevant are summarized in Table 1.

2.3. Adsorption experiments

Adsorption isotherms for paraquat on acid-washed peat wereobtained at pH 4.0 and 5.0, and ionic strengths 0.02 and 0.1 Mwith KCl as inert electrolyte. Since one of the objectives of thisstudy was to compare the peat samples, the adsorption on theuntreated peat was studied for some of the previous conditions:isotherms were obtained at pH 4.0 for both ionic strengths, andanother isotherm was also obtained at ionic strength 0.02 M andpH 5.0. Batch adsorption experiments were carried out with 10 mLof peat suspension. To obtain isotherms covering similar ranges ofparaquat concentration for all experimental conditions, differentconcentrations of peat suspension were used: 5 g L−1 at pH 5.0 andionic strength 0.02 M, and 10 g L−1 for the other conditions. The con-centration of paraquat initially added ranged between 4.0 × 10−5 Mand 1.6 × 10−3 M. The suspensions were shaken for 24 h, which issufficient time to reach the adsorption equilibrium, as shown inprevious kinetic assays (Fig. S1 in Supplementary data). To verifythe reproducibility of the experiments, each isotherm comprisedadsorption data obtained in at least two different series of experi-ments.

The concentration of paraquat in the supernatant was measured,

and the amount of paraquat adsorbed on the peat was calculatedas the difference between the initial concentration added and theconcentration remaining in solution. The equilibrium concentra-tion of paraquat in solution was determined by HPLC, with a Waters

2 ous Materials 235– 236 (2012) 218– 223

2d(wwom2

abotbo71twtd

2

ateotK

eawa

3

3

tctaeTsbmnwaKpa

K

wtpt

t

Table 2Paraquat binding parameters.

Acid-washed peat Untreated peat Dissolved HAa

Log Kint −0.08 ± 0.03 −0.08 (fixed value) −0.10Mint (mol kg−1) 0.27 ± 0.01 0.17 ± 0.02 0.84m 0.57 ± 0.01 0.58 ± 0.06 0.66

r2 0.99 0.98RSS 1.4 × 10−4 1.4 × 10−3

20 D. Gondar et al. / Journal of Hazard

695 separation module and a Waters 2996 PDA (photodiode array)etector. A Simmetry® C18 column packed with 5 �m particles150 mm × 3.9 mm i.d.) was used, at 25 ◦C. The isocratic separationas carried out at a flow rate of 1 mL min−1. The injection volumeas 40 �L. The mobile phase was an aqueous solution of 25 mM 1-

ctanesulfonic acid sodium salt monohydrate, pH 3.0, mixed withethanol at a ratio of 40:60 (v/v). The paraquat was analyzed at

57 nm, which is the wavelength of maximum absorption.Adsorption of the herbicide was measured in suspensions of

cid-washed peat to which different concentrations of Ca2+ hadeen added. Two series of adsorption experiments were carriedut: (i) addition of a fixed concentration of paraquat (1.2 × 10−3 M)o samples of peat to which different concentrations of Ca2+ hadeen added (between 5.0 × 10−4 and 6.7 × 10−3 M), and (ii) additionf different concentrations of paraquat (1.90 × 10−4, 3.90 × 10−4,.80 × 10−4, and 1.17 × 10−3 M) and a fixed concentration of Ca2+

.5 × 10−3 M (30 cmol+ kg−1), which corresponds to the charge ofhe exchangeable cations in the peat soil (Table 1). After the Ca2+

as added the samples were shaken for 4 h before the addition ofhe paraquat. The adsorption experiments were then carried out asescribed in the previous paragraph.

.4. pH and solubility effects

Adsorption experiments were also carried out to assess themount of paraquat adsorbed onto acid-washed peat as a func-ion of pH, between 3.5 and 9.0, at an ionic strength of 0.02 M. Thexperiments were carried out as previously described, with 10 mLf a 10 g L−1 peat suspension, and 9.7 × 10−4 M paraquat. The pH ofhe suspension was adjusted by adding minimum volumes of 0.1 MOH.

To measure the effect of pH on the solubility of the peat, similarxperiments to those described above were carried out withoutddition of paraquat to the initial suspension. After the suspensionsere centrifuged, the DOC was measured in the supernatant with

Shimadzu TOC-5000 analyser.

. Modeling

.1. Paraquat-peat soil binding

As paraquat is present in solution as the PQ2+ cation, the elec-rostatic interaction is expected to determine the binding of theationic herbicide to the negatively charged peat soil and, in par-icular, the effect that pH and ionic strength have on herbicidedsorption. Therefore, to interpret the experimental results, anlectrostatic model based on the Donnan approach [17] was used.his model has been used to interpret the effect of pH and ionictrength on paraquat-dissolved HA binding [10]. The interactionetween the PQ2+ and the negatively charged surface of the organicatter can be separated into the following contributions: (i) an

on-specific electrostatic contribution, Kelect, the magnitude ofhich depends on the electrostatic potential generated by the neg-

tive surface charge of the peat, and (ii) a specific contribution,int, which depends on the affinity of the peat functional groups foraraquat. Thus, the conditional binding constant can be expresseds follows:

cond = K int × Kelect = K int exp(−zF�D

RT

)(1)

here z is the charge on the paraquat cation (2+), � D is the elec-rostatic potential created by the negative surface charge of the

eat, F is the Faraday constant, R is the gas constant, and T is theemperature.

In order to calculate the electrostatic contribution and to modelhe adsorption of paraquat on the peat, we proceeded as follows:

RSS, residual sum of squares; r2, coefficient of determination.a Data from [10].

1. The values of the NICA-Donnan model parameters that describethe distribution of the acid groups in the peat were reported ina previous study [15]. As the negative charge of the peat is dueto ionization of the acid functional groups, these parameters canbe used to calculate the charge at any pH and ionic strength.

2. Once the value of the charge was determined, the volume of theDonnan phase of the peat, VD [15], was used to calculate theelectrostatic potential, � D, created by the charge on the peat.

3. The electrostatic potential relates the concentration of theparaquat in solution, [PQ2+], to the concentration in the vicinityof the peat surface, [PQ2+]D:

[PQ2+]D = [PQ2+] exp(−zF�D

RT

)(2)

Once the electrostatic interaction term was included, the adsorp-tion isotherms corresponding to the different conditions of pHand ionic strength coincided in a single binding curve, in whichthe concentration of paraquat adsorbed, [PQ2+]ads, was plottedagainst [PQ2+]D.

4. Analysis of the “unique binding curve” was carried out by meansof a Langmuir–Freundlich isotherm,

[PQ2+]ads = Mint(K int[PQ2+]D)m

1 + (K int[PQ2+]D)m (3)

This equation enabled calculation of the parameters Mint, Kint andm, which do not depend on the experimental conditions. Takinginto account that the adsorption isotherms were obtained at pH4.0 and 5.0, it was assumed that the charge on the peat was onlydue to ionization of the carboxylic groups and that ionization of thephenolic groups was not significant.

Finally, the value of the conditional adsorption constant, Kcond,which depends on the experimental conditions (pH and ionicstrength) was obtained from Eq. (1). The values of Kcond, Mint and mcan be used to simulate the paraquat adsorption isotherm for theexperimental conditions.

The calculations were carried out using a procedure based onKinniburgh’s FIT software [18].

3.2. Distribution of paraquat between SOM and DOM

In order to quantify the distribution of the cationic herbicidebetween the solid and liquid phases, competition between bothphases must be taken into account and the paraquat/SOM andparaquat/DOM binding equilibria must be resolved simultaneously.

To calculate the concentration of paraquat bound to the peatsurface, the parameters that describe the adsorption of herbicideto the acid-washed peat were used, and the procedure described inSection 3.1 was followed.

Iglesias et al. [10] studied the effect of pH and ionic strength on

the binding of paraquat to dissolved humic acid and described thebinding behavior with an electrostatic model. The intrinsic bindingparameters thus obtained (Table 2) enabled calculation of the equi-librium concentration of herbicide bound to HA in solution. These

D. Gondar et al. / Journal of Hazardous Materials 235– 236 (2012) 218– 223 221

0.00

0.05

0.10

0.15

0.20

0.25

0.0E+0 20 .0E-04 4.0E-0 64 .0E-04 8.0E-0 14 .0E-03 1.2E-0 3

[PQ]solution (mol L-1 )

[PQ

] ads

orbe

d(m

ol k

g-1)

0.00

0.05

0.10

0.15

0.20

0.25

0.0E+0 20 .0E-04 4.0E-0 64 .0E-04 8.0E-0 14 .0E-03 1.2E-0 3

[PQ]solution (mol L-1 )

[PQ

] ads

orbe

d(m

ol k

g-1)

0.00

0.05

0.10

0.15

0.20

0.25

0.0E+00 2.0E-04 4.0E-0 4 6.0E-04 8.0E-0 4

[PQ]solution (mol L )-1

[PQ

] ads

orbe

d(m

ol k

g-1)

(a)

(b)

Fig. 1. Adsorption isotherms of paraquat on (a) acid-washed peat and (b) untreatedp(

pbic

[

4

4

i(e

udsiimibbtiphi

-2.00

-1.75

-1.50

-1.25

-1.00

-0.75

-5.0 -4.5 -4.0 -3.5 -3.0 -2.5

log [PQ]solution (mol L-1 )

log

[PQ

] ads

(mol

kg

-1)

Fig. 2. Effect of the presence of Ca2+ on the adsorption of paraquat by the acid-

and m were optimized. This enabled reproduction of the adsorption

eat. pH = 5.0 (triangles); pH = 4.0 (squares). I = 0.02 M (filled symbols); I = 0.1 Mempty symbols). The lines correspond to the model fit, Eq. (3).

arameters can be used to calculate the concentration of paraquatound to the DOM derived from dissolution of the peat, in a sim-

lar way as the concentration of paraquat bound to the SOM wasalculated.

The calculations were carried out using the ECOSAT program19].

. Results and discussion

.1. Adsorption isotherms: effect of Ca2+

For both concentrations of paraquat used in the kinetic exper-ments the adsorption equilibrium was reached within 24 hFig. S1 in Supplementary data) as also reported by Pateiro-Mouret al. [11].

The adsorption isotherms for paraquat on acid-washed andntreated peat soil obtained under different experimental con-itions (Fig. 1) show that the qualitative effects of pH and ionictrength on the concentration of paraquat adsorbed were the samen both peat samples. Because of its cationic nature, paraquatnteracts with the acid groups in peat via a cationic exchange

echanism, so that the adsorption depends on the degree of ion-zation of the acid groups present in the SOM. Therefore, paraquatinding increases as the pH of the medium increases, which cane attributed to the greater electrostatic attraction derived fromhe presence of a more negative surface charge of the peat. This

s consistent with previous reports on the interaction betweenaraquat, as well as other bipyridinic herbicides and dissolvedumic substances [6,20]. Paraquat adsorption also decreases as the

onic strength increases, which is a common effect in interactions

washed peat. pH = 4.0; I = 0.1 M. (�) Acid-washed peat; (�) untreated peat; (∗) acid-washed peat; added [Ca2+] = 1.5 × 10−3 M.

between reactants of opposite charge and reflects how the inertelectrolyte shields the electrostatic interactions between adsorbentand adsorbate.

Under all the conditions of pH and ionic strength studied, moreparaquat was adsorbed on acid-washed peat than on untreatedpeat (Fig. 1), which indicates that the presence of exchange-able cations in the untreated peat leads to a decrease in thenumber of sites available for binding the paraquat. This interpre-tation is supported by the results obtained in paraquat adsorptionexperiments on acid-washed peat in the presence of Ca2+. Thus,an increase in the concentration of Ca2+ (with a constant ionicstrength) caused a decrease in the concentration of paraquatadsorbed (Fig. S2 in Supplementary data). Moreover, the adsorptionof paraquat on acid-washed peat in the presence of a concentra-tion of Ca2+ equivalent to the total charge of the exchangeablecations was almost the same as the adsorption of paraquat onuntreated peat (Fig. 2). This reduction in adsorption is explainedby the fact that Ca2+ competes with paraquat for adsorption sites,but is not consistent with simple attenuation of long-range elec-trostatic interactions, a characteristic effect of the inert electrolyte,as suggested, for example, in the Gouy–Chapman theory [21].

4.2. Modeling performance

The electrostatic model provided a good description of theadsorption isotherms for paraquat on the acid-washed peat underall conditions studied (Fig. 1(a)). This shows that the effect of pHand ionic strength on the adsorption of the cationic herbicide isbasically electrostatic. This effect can be attributed to the fact thatchanges in these variables modify the negative surface charge of thepeat as a consequence of the different ionization of the acid groups(mainly carboxylic groups) present in the NOM. The value of theintrinsic binding constant, log Kint, was similar to that obtained forthe binding of paraquat to dissolved humic acid (Table 2).

As already mentioned, less paraquat is adsorbed onto untreatedpeat than onto acid-washed peat, an effect that can be attributed tothe lower number of binding sites available in the former. There-fore, in order to model the adsorption of paraquat on the untreatedpeat, the distribution of binding sites was defined by the value ofKint obtained for the acid-washed peat, and the parameters Mint

isotherm (Fig. 1(b)). The difference between the values of parame-ter Mint (Table 2) for both peat samples (20 cmol+ kg−1) was of thesame order of magnitude as the charge of the exchangeable cations(33 cmol+ kg−1).

222 D. Gondar et al. / Journal of Hazardous Materials 235– 236 (2012) 218– 223

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

6543 87 9

pH

DO

C (g

Cdi

ssol

ved

/ 100

g p

eat)

Fp

4

ictotp

pnts

uSp

rcsdbtipTcp

aastwsin5tDwat(e

70

75

80

85

90

95

543 87 9

PQ(%

)

100

6

pH

adso

rbed

(a)

0

5

10

15

20

25

30

876543 9

pH

PQ

boun

d(%

)

0

20

40

60

80

100

43 65 87 9pH

PQad

sorb

ed/b

ound

(%) (b)

Fig. 4. (a) Simulation of the effect of pH on the distribution of paraquat between theSOM (filled symbols) and DOM (empty symbols). I = 0.02 M; [PQ2+]total: 1 × 10−5 M(diamonds), 1 × 10−4 M (squares), 1 × 10−3 M (triangles). (b) Simulation of pesticideadsorption on SOM in absence of DOM (filled triangles), and binding by DOM inabsence of SOM (empty triangles). [PQ2+]total = 1 × 10−3 M; I = 0.02 M.

60

70

80

90

100

9876543

pH

PQad

sorb

ed(%

)

Fig. 5. Effect of pH on the amount of pesticide adsorbed on SOM for the acid-

ig. 3. Concentrations of dissolved organic carbon measured in the acid-washedeat suspension at different pH values. I = 0.02 M.

.3. Effect of DOM on the adsorption of paraquat

In accordance with the results described in Section 4.1, anncrease in the pH of the medium would cause an increase in theoncentration of paraquat retained by the peat (solid) organic mat-er. However, it must also be taken into account that the solubilityf the peat increases with increasing pH; therefore, as the concen-ration of dissolved organic carbon is higher, the concentration ofaraquat bound to the DOM will increase.

Measurement of the solubility of the peat at different values ofH (Fig. 3) showed that at pH < 5.5, the amount of C dissolved wasot significant and remained almost constant, but that at pH > 5.5,he solubility increased considerably. This increase may becomeignificant as it reached 30 g C/kg peat at around pH 8.5.

Three total concentrations of herbicide were selected to sim-late the effect of pH on the distribution of paraquat between theOM and DOM: 10−5 M (at which the adsorption of paraquat on theeat at acid pH (4.0–6.0) was almost 100%), 10−4 M, and 10−3 M.

The effect of the presence of DOM on the fraction of paraquatetained by the SOM was only significant at pH > 5.5 (Fig. 4(a)), as aonsequence of the increase in the solubility of the peat. Only a verymall fraction of the herbicide was bound to the DOM at pH < 5.5,ue to the low solubility of the peat at this pH, but this fractionecame more important at higher pH (Fig. 4(a)). Thus, at pH > 8.0,he herbicide remaining bound to the DOM in the soil solution,s expected to range between 8%, for the lower concentration ofaraquat, up to almost 16% in the case of the highest concentration.his behavior was particularly notable in the case of the highestoncentration of the herbicide, as in absence of DOM the fraction ofaraquat adsorbed on the SOM reaches 100% at pH > 7 (Fig. 4(b)).

The decrease in the amount of pesticide adsorbed on SOM when significant amount of soil organic matter has been dissolved at rel-tively high pH appears to be due to the competition between theolid and dissolved organic matter to bind paraquat. The simula-ion of the effect of DOC on paraquat retention by SOM is consistentith the experimental results. The adsorption data (Fig. 5) demon-

trated an increase in herbicide retention up to pH 5.5, due to thencrease of the electrostatic interaction between cationic PQ2+ andegatively charged SOM surface. However, as pH increased above.5 and the DOC concentration became significant, the retention ofhe herbicide by the SOM decreased. Effective competition from theOM to bind paraquat led to a decrease in the adsorption on SOM,hich, in turn, promoted pesticide mobility. Modeling of paraquat

dsorption on soil organic matter, including both solid and solu-ion phases, provided a good reproduction of experimental resultsFig. 5). The results show that the DOC fraction significantly influ-nces the ability of SOM to retain cationic herbicides. Therefore, this

washed peat. I = 0.02 M. Symbols, experimental data; line, simulation using Eq. (3)and parameters from Table 2.

fraction must be taken into account to make adequate predictionsabout the fate of this contaminant in soil systems.

5. Conclusions

Less paraquat was adsorbed by untreated peat than by acid-

washed peat, which is attributed to the presence of exchangeablecations that led to a decrease in the number of binding sites avail-able in the former. This was confirmed by the fact that the addition

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D. Gondar et al. / Journal of Hazard

f Ca2+ led to a decrease in the adsorption of paraquat by the acid-ashed peat.

The pH and ionic strength affect the adsorption of the cationicerbicide, so that the experimental results were interpreted by aimple electrostatic model that accounts for the effect of both vari-bles on the surface charge of the peat. The model indicated thathe distribution functions of the paraquat adsorption constants arehe same for untreated peat and acid-washed peat, and that the dif-erent adsorption on both samples was explained by the differencen the number of binding sites available.

The distribution of paraquat between the soil and the soilolution is the result of competition between adsorption of theerbicide on the SOM and binding of the herbicide by the DOM.t pH > 5.5, the fraction of paraquat adsorbed on the peat sur-

ace decreases as the pH increases, thus increasing the mobilityf paraquat and its presence in the soil solution.

cknowledgment

This work was financed by the project PGIDIT 10PXIB209014PRXunta de Galicia).

ppendix A. Supplementary data

Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/.jhazmat.2012.07.044.

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[

[

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