ElEctrokinEtic rEmoval of HErbicidEs from soils

Alexandra B. Ribeiro and Eduardo P. Mateus

12

12.1 introduction

Agriculture depends on soil for water and nutrients supply, as well as for root fixa-tion. Soil is a vital natural resource, nonrenewable at a human scale, that performs a number of key environmental, social, and economic functions. Soil is an interface reactor, performing storage, filtering, buffering, and transformation functions, thus playing a central role in water protection and in the exchange of gases with the atmosphere. It is also a habitat and gene pool, an element of the landscape and cultural heritage, as well as a provider of raw materials. In order to perform its main functions, it is essential to maintain soil quality, which is crucial for sustainability and for sustaining agricultural productivity (Blum, 2005; Silva et al., 2006). However, soil is under increasing threat from a wide range of human activities, which degrade it and are undermine its long-term availability and viability [Blum, 2005; Santos, 2008; Commission of the European Communities (2002); Commission of the European Communities (2006)].

Environmental contamination with pesticides due mainly to agricultural prac-tices and accidental spills leads to soil, surface, and groundwater quality degrada-tion, which may have negative impacts on public health and biodiversity (Correia et al., 2006; Santos, 2008). This issue of increasing concern has lead to many studies describing and quantifying contamination with pesticides (Thorstensen et al., 2001; Boivin, Cherrier, and Schiavon, 2005; Silva et al., 2006; Abreu, 2008). Pesticides are toxic compounds deliberately released into the environment to fight plant pests and diseases. Their introduction may result in the damage or loss of some or several functions of soil and in the possible cross-contamination of water. They can accu-mulate in the soil, leach to the groundwater, and evaporate into the air, from which

1

Electrochemical Remediation Technologies for Polluted Soils, Sediments, and Groundwater, Edited by Krishna R. Reddy and Claudio CameselleCopyright © 2009 John Wiley & Sons, Inc.

249

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250 ELECTROKINETIC REMOVAL OF HERBICIDES FROM SOILS

further deposition onto soil can take place. They may also affect biodiversity and enter the food chain, entailing multiple negative consequences for human health, ecosystems, and other natural resources [Commission of the European Communities (2002); Commission of the European Communities (2006)].

Rice is one of the major crops cultivated worldwide (Chang, 2000). Among the major herbicides used to control plagues in rice fields are atrazine, molinate, and bentazone (Castro et al., 2005; Correia et al., 2006).

The food demand of an increasing world population challenges the scientific community to pay attention to how food production leads to environmental pres-sures and to accelerate efforts to develop methodologies to minimize their impacts on human health and the environment (Santos, 2008).

The electrokinetic process is a remediation technique for the removal of con-taminants from polluted sites. It uses a low-level direct current (DC), as the cleaning agent, to transport contaminants out of the soil toward one of the electrode com-partments, from where they can be removed. Several authors have critically reviewed its state of knowledge (Pamukcu and Wittle, 1992; Acar et al., 1995; Ottosen, 1995; Yeung and Datla, 1995; Page and Page, 2002; Virkutyte, Sillanpaa, and Latostenmaa, 2002; Ribeiro and Rodríguez-Maroto, 2006).

This chapter reports on the application of the electrokinetic process to remove herbicides such as atrazine, molinate, and bentazone from soils. The discussion is focused on (a) the assessment of the behavior of atrazine, molinate, and bentazone in soils when submitted to an electric field, and (b) the evaluation of the applicability of the technique to remove atrazine, molinate, and bentazone from soils.

12.2 HErbicidEs

12.2.1 atrazine

Atrazine (2-chloro-4-ethyl-amino-6-isopropylamino-s-triazine) is a selective tri-azine herbicide used pre- and postemergence with restricted permitted uses to control broad-leaved and grassy weeds in agriculture and in conifer reforestation plantings (FOOTPRINT Pesticide Properties Database). Atrazine is highly persis-tent in soil (its main physicochemical properties are presented in Table 12.1). Chemical hydrolysis, followed by degradation by soil microorganisms, accounts for most of the breakdown of atrazine. Hydrolysis is rapid in acidic or basic environ-ments but is slower at neutral pH levels. Addition of organic material increases the rate of hydrolysis. Atrazine can persist for longer than 1 year under dry or cold conditions. Atrazine is moderately to highly mobile in soils with low clay or organic matter content. Because it does not adsorb strongly to soil particles and has a lengthy half-life (60 to >100 days), it has a high potential for groundwater contami-nation despite its moderate solubility in water (EXTOXNET, 1996). As a conse-quence, atrazine is one of the most frequently detected contaminants in the waterbodies in Europe (Gascón, Dubina, and Barceló, 1997; Carabias-Martínez et al., 2002).

Additionally, atrazine has been reported as a potential endocrine disruptor (Renner, 2000, 2002).

2

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HERBICIDES 251

tablE 12.1. Physicochemical Properties of the Herbicides atrazine, molinate, and bentazone

Parameter Atrazine Molinate Bentazone

Structure diagram Cl

N

NN

CH3CH2N NHCH(CH3)2

N

SO CH3

N

N

H

O

SO2

CH(CH3)2

CAS 1912-24-9 2212-67-1 25057-89-0EC number 217-617-8 218-661-0 246-585-8Chemical group Triazine Thiocarbamate BenzothiazinoneChemical formula C8H14ClN5 C9H17NOS C10H12N2O3SMolecular mass (g/mol) 215.68 187.3 240.3Physical state White crystals Clear liquid Dirty white crystalsBulk density (g/ml)/

specific gravity1.23 1.06 1.41

Solubility in water at 20 °C (mg/l)

35 1100 570

Octanol–water partition coefficient (LogP)

2.7 2.86 −0.46

Dissociation constant (pKa at 25 °C)

1.7 Not applicable (neutral)

3.28

Vapor pressure at 25 °C (mPa)

0.04 500 0.17

Henry’s constant at 25 °C (Pa m3 mol−1)

1.50E-04 6.87E-01 7.20E-05

Soil degradation DT50 (days)

75 28 13

GUS leaching potential index

3.75 2.49 2.55

Organic carbon sorption constant Koc (ml/g)

100 190 51

Source: FOOTPRINT Pesticide Properties Database.

50

12.2.2 molinate

Molinate is a selective herbicide that belongs to the thiocarbamate chemical class (EXTOXNET, 1996). It is used worldwide to control germinating broad-leaved and grassy weeds in rice paddies (Barreiros et al. 2003; WHO, 2003). Molinate is rapidly taken up by plant roots and transported to the leaves. In the leaves, molinate inhib-its leaf growth and development. It is rapidly metabolized in nonsusceptable plants (EXTOXNET, 1996). Several studies reported molinate to be found in groundwater (Castro et al., 2005), and toxicological studies showed that molinate could lead to adverse effects such as reproductive toxicity in mammals (Cochran et al., 1997; APVMA, 2003). Molinate is stable under normal temperatures and pressures, but thermal decomposition may release toxic oxides of nitrogen and sulfur (EXTOXNET,

c12.indd 251 5/21/2009 6:39:20 PM

252 ELECTROKINETIC REMOVAL OF HERBICIDES FROM SOILS

1996). The main physicochemical properties are presented in Table 12.1. Molinate presents high solubility in water (1100 mg/l), is highly volatile and moderately mobile, bioaccumulates moderately (LogP = 2.86) and presents low persistence in the soil.

12.2.3 bentazone

Bentazone [3-(1-methyltethyl)-1H-2, 1, 3-benzothiadiazin-4(3H)-one 2, 2-dioxide] is a postemergence herbicide used for selective control of broad-leaved weeds and sedges (a weed) in beans, rice, corn, peanuts, mint, and others. Bentazone is a contact herbicide, which means that it causes injury only to the parts of the plant to which it is applied. It interferes with the ability of susceptible plants to use sun-light for photosynthesis. Visible injury to the treated leaf surface usually occurs within 4–8 h, followed by plant death (EXTOXNET, 1996). The sorption and desorption on organic matter is strongly pH controlled for bentazone due to the fact that it shows an acid behavior (Boivin, Cherrier, and Schiavon, 2005). The sorption is stronger for pH 2–3 (Poll and Vink, 1996; Aguilar et al., 1999; Santos, Rocha and Barceló, 2000; Boivin et al., 2004; Fontanals, Marcé, and Borrull, 2004; Boivin, Cherrier, and Schiavon, 2005). The dissociation constant of bentazone is pKa = 3.28, the solubility in neutral water is 570 mg/l, and its log KOW under neutral conditions is −0.318 (FOOTPRINT Pesticide Properties Database). These proper-ties (several others are presented in Table 12.1) show the polar character of ben-tazone (Aguilar et al., 1999; Fontanals, Marcé, and Borrull, 2004).

12.3 casE study

The laboratory study procedure and results are presented in this section to demon-strate the electrokinetic removal of herbicides from soils.

12.3.1 soils

Four types of soil were used. The first, sampled at Valadares (Vale de Milhaço, Portugal), corresponds to a Eutric Regossol (FAO/UNESCO soil classification). An average sample was collected at 0- to 15-cm depth; it has a sandy texture, and its characteristics are shown in Table 12.2 (Soil 0). More details related to its physical, chemical, and mineralogical properties can be found in Ribeiro (1992).

The second soil, collected at a depth of 0–20 cm from the rice crop fields of the Ebro Delta area (Tarragona, Spain), where the pesticides are currently applied, was “naturally” contaminated with aged residues corresponding to the last atrazine application carried out 40 days before its sampling. Characteristics of this soil are shown in Table 12.2 (Soil 1), and more details are available in Durand, Forteza, and Barceló (1989).

Soil 2 and soil 3 were sampled at Bico da Barca, an experimental rice site, located at the Rio Mondego valley, Portugal. Several physical and chemical characteristics of these soils (soil 2 and soil 3) are presented in Table 12.2. Soil 2 and soil 3 have different histories of pesticide application: Soil 2 comes from a rice field with no tradition of specific type of pesticide application, which has been used for biological

3

4

5

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CASE STUDY 253

tablE 12.2. Physical and chemical characteristics of the soils used

Parameter Soil 0 Soil 1 Soil 2 Soil 3

Soil texture (%) Coarse sand (200 < Ø < 2000 µm) 69.8 1.8 1.7 2.1 Fine sand (20 < Ø < 200 µm) 24.2 42.3 26.5 26.1 Silt (2 < Ø < 20 µm) 3.5 24.1 51.1 49.7 Clay (Ø < 2 µm) 2.5 31.8 20.7 22.1 Textural classification Sandy Clay loam Silt loam Silt loampH (H2O) 6.0 6.5 6.46 5.83Organic carbon (g/kg) — — 21.1 26.8Organic matter (%) 0.41 1.64 3.64 4.61Total nitrogen (g/kg) — — 1.55 1.72Exchangeable cations (cmol(c)/kg) Ca2+ 0.35 — 6.44 4.46 Mg2+ 0.07 — 1.34 1.32 K+ 0.07 — 0.59 0.61 Na+ 0.07 — 0.89 0.23Sum of exchangeable cations (cmol(c)/kg) 0.54 — 9.26 6.62Cation exchange capacity (cmol(c)/kg) 1.47 12.36 9.26 10.0Base saturation (%) 36.7 — 100.0 66.2

agriculture, and soil 3 comes from a rice field with a history of traditional herbicide application (bentazone, propanil, and MCPA). However, both soils (soil 2 and soil 3), together with soil 0, were initially free of herbicide residues.

The soils were then spiked with herbicides without residue aging: soil 0 with atrazine, soil 2 with molinate, and soil 3 with molinate, or with bentazone (according to Table 12.3).

12.3.2 reagents and chemicals

The enzyme-linked immunosorbent assay (ELISA) kits, Atrazine RaPID Assay® (A00071) and Magnetic Separation Rack (A00004), used were obtained from Strategic Diagnostics Inc. (USA). All organic solvents used were high-performance liquid chromatography (HPLC) grade and purchased from Merck (Darmstadt, Germany), Sigma-Aldrich (Steinheim, Germany), or Panreac (Barcelona, Spain). The acetonitrite, methanol, and acetone were gradient grade, the methylene chlo-ride was Pestanal grade, and the diethyl ether and pentane were PA grade. The Supelclean ENVITM 18 Disks, 47 mm in diameter, for solid-phase extraction (SPE), were purchased from Supelco (Bellefonte). All triazine standards, atrazine, sima-zine, and propazine, as well as molinate and bentazone standards, were Pestanal grade, purchased from Riedel-de-Haën (Seelze, Germany).

All the materials were washed to prevent contamination, being immersed over-night in water with Sodosil from Riedel-de-Haën. After cleaning, the materials were rinsed with water in order to remove detergent residues, followed by rinsing with a 50/50 acetone/distilled water (v/v), and then with distilled water. At the end, the materials were dried in a furnace at 40 °C.

6

7

8

9

c12.indd 253 5/21/2009 6:39:20 PM

ta

bl

E 1

2.3.

Exp

erim

enta

l con

diti

ons

for

the

Ele

ctro

kine

tic

rem

edia

tion

Exp

erim

ents

Exp

erim

ent

Soil

(Spi

ked

Wit

h)T

ime

(day

s)C

urre

nt

Inte

nsit

y (m

A)

Cen

tral

Sl

ice

(g)

Tot

al

Soil

Mas

s (g

)

Mas

s of

H

erbi

cide

in

Soil

(mg)

pH A

djus

tmen

tF

low

R

ate

(ml/m

in)

ASo

il 0

(atr

azin

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10N

o34

26.

5C

atho

lyte

pH

≈ 3

wit

h H

NO

31.

4B

Soil

0 (a

traz

ine)

910

9834

24.

4C

atho

lyte

pH

≈ 3

wit

h H

NO

31.

4C

Soil

0 (a

traz

ine)

910

103

342

4.6

Cat

holy

te p

H ≈

3 w

ith

HN

O3

1.4

DSo

il 1

wit

h at

razi

ne

aged

res

idue

s9

10N

o21

40.

033

Cat

holy

te p

H ≈

3 w

ith

HN

O3

1.4

ESo

il 2

(mol

inat

e)13

1010

237

920

.1N

o3.

0F

Soil

2 (m

olin

ate)

710

121

379

50.1

No

2.5

GSo

il 3

(mol

inat

e)10

1082

299

52.4

Ano

lyte

pH

≈ 8

wit

h 6

M N

aOH

5.0

HSo

il 3

(ben

tazo

ne)

710

9230

125

.1N

o5.

0I

Soil

3 (b

enta

zone

)22

2011

029

614

.6N

o5.

0J

Soil

3 (b

enta

zone

)6

2011

126

715

.2A

noly

te p

H ≈

8 w

ith

6 M

NaO

H5.

0K

Soil

3 (b

enta

zone

)10

1078

337

24.1

Ano

lyte

pH

≈ 8

wit

h 6

M N

aOH

5.0

51254

c12.indd 254 5/21/2009 6:39:20 PM

CASE STUDY 255

12.3.3 Electrokinetic laboratory cell

Eleven electrokinetic experiments were carried out in a laboratory cell schemati-cally presented in Figure 12.1. The cell is divided into three compartments, with an internal diameter of 8 cm, consisting of two electrode compartments (L = 7.46 cm) and a central one (L = 3 cm), in which the soil, saturated with deionized water according to Rhoades (1982), is placed (Fig. 12.1). A set of five cellulose filters, previously tested and known to work as passive membranes (Ribeiro, 1998), assured the separation between the central compartment and the electrode ones. A power supply (Hewlett Packard E3612A) maintained at a constant DC and a multimeter (Kiotto KT 1000H) monitored the voltage drop. The electrodes (diameter = 3 mm, L = 5 cm) were platinized titanium bars (Bergsøe Anti Corrosion A/S, Denmark). In all experiments, the electrolyte was a 10−2 M NaNO3 solution (with a pH value according to Table 12.3), circulating due to a peristaltic pump (Watson-Marlow 503 U/R) with one head and three extensions.

12.3.4 Electrokinetic Experimental conditions

The detection and quantification of the initial atrazine in soil 1 and the proof of the nonexistence of atrazine, molinate, and bentazone in soil 0, soil 2, and soil 3 were carried out before all experiments. The polluted soils (spiked and natural) were submitted to the electrokinetic process under different experimental conditions presented in Table 12.3.

The spiking solutions were prepared by dissolving the masses of the herbicide standards, referred to in Table 12.3, in 25 ml of diethyl ether to facilitate the homo-geneousness and dispersion of the active principle throughout the soil. The spiking solutions were added to the saturated soil.

Electrolyte samples (catholyte and anolyte) were collected during the experi-ments for further quantification of the herbicides (further on explained), and the pH and respective volume were registered.

At the end of each experiment, the total soil in the central cell compartment was sectioned in slices, one nearer the anode (AN soil slice), another one nearer the cathode (CAT soil slice), and a central soil slice (central soil slice). Their respective masses were determined, and subsamples were collected to humidity and pH in H2O (1 : 2 . 5) measurements. The rest of the known mass of each “slice” was submitted

10111213

14

15

16

figure 12.1. Schematic representation of the laboratory cell.

DC power

+ –Cathode

Contaminatedsoil

Passive membrane Passive membrane

Anode

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256 ELECTROKINETIC REMOVAL OF HERBICIDES FROM SOILS

to extraction three times by sonication with a Bandelin Sonarex Super RK102 H for 10 min using 50 ml of fresh methanol in experiments A–F, and using 50 ml of acetone in experiments G and H–K. The soil and passive membrane extract samples were concentrated on a rotavapor until 5–10 ml and refrigerated until analysis.

For the determination of the herbicide adsorption to the passive membranes, those were put in 100 ml of methanol (in experiments A–F) or acetone (in experi-ments G and H–K) and submitted to sonication with a Bandelin Sonarex Super RK102 H for 10 min. The obtained extracts were concentrated to 10 ml and filtered by Acrodisc Gelman filters (0.45 µm in diameter) before being submitted to analysis by ELISA, in the case of atrazine (experiments A–D), or to gas chromatography with flame ionization detector (GC-FID) in the case of molinate (experiments E and F), or to HPLC with ultraviolet detection (HPLC-UV) in the case of molinate and bentazone (experiments G and H–K).

12.3.5 Herbicide analysis

Atrazine analysis were performed by ELISA, in soils and electrolyte solutions, according to the procedures stated on the Technical Notes 0003 and A00071 from Strategic Diagnostics, Inc. The absorbance measurements were performed on a spectrophotometer UNICAM Helios α v2.03 at 450 nm. The detection limit achieved for atrazine was 0.05 µg/l (Ribeiro et al., 2005). Confirmation of atrazine detection (positive ELISA) was conducted using a Merck-Hitachi HPLC system with a UV detector in 10% of the samples. The analytical separation was performed on a Lichrospher RP-18 (125 mm × 4.6 mm, 5 µm) from Merck. The UV wavelength was setup to 220 nm. The analysis was performed in isocratic mode using water– methanol (65 : 35), with a flow rate of 1.0 ml/min (Ribeiro et al., 2005). When the atrazine content in an electrolyte sample was below the detection limit of the ELISA method, a SPE step was used to concentrate the samples based on Technical Note T00020 from Strategic Diagnostics, Inc and Supelco application note 59 (Ribeiro et al., 2005). Quantitative analysis of atrazine in soils for each experiment was carried out by extracting twice 5 g of soil with 20 ml of methanol by sonication using a Bandelin Sonarex Super RK102 H for 10 min. Both extracts were collected in conjunction and concentrated to 10 ml under a nitrogen flux. When the atrazine content was smaller than the detection limit of the ELISA method, the extracts were concentrated to 1 ml.

Molinate and bentazone analysis were performed by chromatography. GC was used for molinate determination in experiments E and F electrolyte solutions. HPLC was used for bentazone determination in soils and electrolyte solutions, and for molinate determination in soils and experiment K electrolyte solution.

The GC analysis was performed on a Carbon Erba HRGC 5300 Mega series gas chromatograph equipped with an FID. The capillary column used was a ZB-5 (5% phenyl, 95% dimethylpolysiloxane), 30 m × 0.25 mm in i.d. and 0.25 µm in film thick-ness, from Phenomenex (Torrance). The sample injection was made on a slit-splitless injector, with the slit ratio set to 1.4. The oven temperature was programmed to start at 100 °C for 5 min, after being increased to 120 °C at a rate of 6 °C/min and then increased at 10 °C/min to 290 °C, where it held for 15 min. The temperatures of the injector and detector were 250 and 300 °C, respectively. The carrier gas was

17

18

19

2021

22

23

2425

2627

c12.indd 256 5/21/2009 6:39:20 PM

CASE STUDY 257

hydrogen set at 1.0 ml/min. Data were registered using the Merck-Hitachi integrator D-200.

The HPLC analysis was performed on an Agilent 1100 series LC, equipped with a quaternary pump (G1311A) and a UV multiwave length detector (G1365B), set at 220 and 280 nm. The injector was a Rheodyne manual injector valve, model 7725i, equipped with a 20-l sample loop. The column temperature was controlled using a ThermaSphere TS-130 oven from Phenomenex set at 40 °C.

Herbicide separations were carried out using a Chromolith Performance RP-18e column with 100 mm × 4.6 mm from Merck, protected using Onyx SecurityGuard cartridges (4 × 3.0 mm) from Phenomenex. All sample analysis were performed with a flow rate of 1 ml/min at gradient mode. The eluents were acetonitrile/water solu-tions (solution A: 35/65; solution B: 90/10), adjusted to pH = 2.8 using a formic acid solution (Panreac, 98%).

The gradient run was set at 100% A from 0 to 8 min, then to 100% B until 14.5 min, where it held until minute 17. The system reequilibration was performed by changing to 100% A from minute 17 to 18 and remaining with 100% A until minute 20. All operations and data analysis were processed by the Chemstation software v.8.03 (Agilent Technologies).

Before analysis, the herbicides were extracted from the electrolyte solutions by means of SPE and by solvent extraction, using sonication, from the soil and passive membranes. The pH values of the collected electrolyte solution was between 4 and 7 for experiments E and F, and below 4 for experiment G. For experiments H–K, the collected electrolyte solutions were adjusted to pH 2 by adding NaOH or HNO3.

For experiments E, F, and G, the SPE has been performed using Strata C18-T cartridges (Phenomenex). The SPE sorbent amount has been selected according to the sample volume of 500 mg for volumes lower than 300 ml, and 1000 mg when the sample volume was higher than 300 ml. For experiments H–K, the SPE has been performed using 200-mg Strata-X cartridges (Phenomenex). All analytes were eluted after extraction with methylene chloride.

After SPE extraction, the extracts were concentrated under a gentle stream of nitrogen, until 1.0 ml in experiments E and F. In experiments G and H–K, the extracts were evaporated to dryness and resuspended to 2.0 ml with methanol/ distilled water (1 : 1).

After concentration, the samples for GC analysis were transferred to a 2.0-ml vial and kept at −20 °C until analysis. The samples for HPLC analysis, in methanol/water, were kept at 5 °C and analyzed in 24 h.

Molinate and bentazone analysis were performed in soil, passive membranes, and electrolyte sample solutions according to the method developed and validated by Buchholz (2007).

12.3.6 results and discussion

The experiments A–H and K, all carried out at 10 mA, presented similar values of voltage and current (results not shown). In the experiments with no pH control in the electrolytes, the catholytes presented predominantly an alkaline pH, and the anolytes an presented acid pH that varied between 3 and 4. The accumulated volumes of the catholyte and anolyte of the experiments presented close values

2829

30

31

32

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258 ELECTROKINETIC REMOVAL OF HERBICIDES FROM SOILS

because the electroosmotic flow was low, but the anolyte cumulative volumes were higher than the catholyte ones, particularly in experiments A–D, in which the catholyte pH was adjusted to approximately 3. Experiment B was the one that registered a higher difference between the anolyte and catholyte volumes, and clearly indicates an electroosmotic flow toward the anode (results not shown) at an acidic soil pH.

Figure 12.2 illustrates the cumulative mass of herbicide removed in some experi-ments: (a) atrazine in experiment C; (b) molinate in experiment E; and (c) ben-tazone in experiment I. Figure 12.3 presents the quantities of herbicide remaining at the end of all the experiments: in the passive membranes, nearer the cathode (CAT membrane) and nearer the anode (AN membrane); in the soil, in the soil slice near to the cathode (CAT soil slice), in the central soil slide and in the soil slice nearer to the anode (AN soil slice); as well as an estimate of the removed herbicide percentages obtained.

The atrazine quantities removed by the electrokinetic process for the cathode and the anode compartments of the cell showed that in experiments A–D, the highest amounts are in the anolyte, and that 30%–50% of its initial amount is removed from the soils within the first 24 h. In experiment A, the quantity of atra-zine in the catholyte is quite significant in relation to the ones in experiments B and C (results not shown). This is due to the fact that in experiment A, all the soil in contact with the membranes was spiked, allowing atrazine to pass into the cathode compartment by diffusion. At the end of the experiments, higher quantities of atrazine were obtained in the central soil slice and in the soil slice nearer to the anode, in experiments B and C (Fig. 12.3), corroborating the results previously obtained in the electrolytes, confirming that under the action of an electric field, atrazine is mainly mobilized toward the anode compartment due to reverse electroosmosis. In experiment A, the quantities of atrazine in the soil slice nearer the cathode and nearer the anode are similar (Fig. 12.3), probably due to the previ-ously pointed out passage of atrazine toward the cathode due to diffusion. In the electrokinetic experiments carried out with atrazine, the removal efficiencies in the soil solution where high, ≥89% (Fig. 12.3). No atrazine was found in the passive membranes.

Experiment G was the one that registered the highest difference between the catholyte and anolyte volumes (results not shown), giving a strong indication of an electroosmotic flow toward the cathode. In fact, there seems to be a relation between the electroosmotic flows observed in the three experiments carried out with molinate (experiments E–G) and the range of pH values obtained at their anolytes. It seems that the more acid pH the anolyte presented, the bigger the tendency to obtain a reversed electroosmosis. This is in accordance with the conditions that determine the reversion of the electroosmotic flow, namely the acidification of the soil pro-voked by the propagation of an acid front due to the generation of H+ at the anode reaction. It seems that, independent of the initial amount of molinate and of the flow rate of the electrolyte solution, most of the molinate (>80%) found in the electrolytes is removed in the first 4 days. Molinate seems to be mobilized prefer-entially to the cathode compartment (e.g. experiment E in Fig. 12.2b). The pH control of the incoming anolyte solution of experiment G (according to Table 12.3) may have contributed to the mobilization of molinate toward the cathode compart-ment as it counteracted the reversion of the electroosmosis, and therefore impeding

c12.indd 258 5/21/2009 6:39:20 PM

CASE STUDY 259

figure 12.2. Cumulative mass of herbicide removed for the electrolytes: (a) atrazine in experiment C; (b) molinate in experiment E; and (c) bentazone in experiment I.

0.0

0.5

1.0

1.5

2.0

0 50 100 150 200

0 50 100 150 200 250 300

0 50 100 150 200 250 300 400 450350 500

Time (h)

Cum

ulat

ive

mas

s (m

g)Catholyte

Anolyte

0123456789

10

Time (h)

Cum

ulat

ive

mas

s (m

g)

Catholyte

Anolyte

0

1

2

3

4

5

Time (h)

Cum

ulat

ive

mas

s (m

g)

Catholyte

Anolyte

(a)

(b)

(c)

the acidification of soil, according to Acar et al. (1996), Yeung, Hsu, and Menon (1996), and Saichek and Reddy (2003). After the electrokinetic experiments were completed, the mass balance conducted for the molinate in the system, compared with the molinate spiked in the soil, presented very high removal efficiencies, greater than 90% (Fig. 12.3). Most of the molinate was found in the soil (central

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260 ELECTROKINETIC REMOVAL OF HERBICIDES FROM SOILS

figure 12.3. Quantities of herbicide remaining at the end of all the experiments: in the passive membranes, nearer the cathode (CAT membrane) and nearer the anode (AN mem-brane); in the soil, in the soil slice near to the cathode (CAT soil slice), in the central soil slide and in the soil slice nearer to the anode (AN soil slice); as well as an estimate of the removed herbicide percentages obtained.

A0.0

20.0

Nor

mal

ized

val

ues

(% o

f her

bici

de)

40.0

60.0

80.0

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B C D E FExperiment

CAT membrane

AN membrane

CAT soil sliceCentral soil sliceAN soil slice

G H I J K

A98

ExperimentsRemoval rates (%) 98 98 98 99 92 98 99999389

B C D E F G H I J K

soil slide) and in the soil slice nearer to cathode (CAT soil slice, in Figure 12.3, for experiment E). These results corroborate the direction of the movement of molinate obtained toward the electrolyte solutions.

Bentazone was mobilized from the soil when submitted to the action of an elec-tric field, toward both electrode compartments, but mostly toward the anolyte (e.g. Fig. 12.2c). The accumulated volumes of the catholyte and anolyte of experiments H–K presented slighter higher values in the catholytes because the electroosmotic flow was toward that compartment (results not shown). The electrokinetic process was able to remove bentazone in the soil solution, reaching a balance after 70 h of the process. At the end of the experiments, more than 90% of the bentazone initial quantities were removed (Fig. 12.3). For the bentazone mobilized toward the cathode compartment, it takes longer to reach the balance (approximately 150 h). It seems that an application of a lower current intensity (10 mA) is related to a higher mobilization of this herbicide toward the cathode compartment, whereas higher current intensities (20 mA) promoted a greater rate of bentazone mobiliza-tion also toward the anolyte (Fig. 12.3, for instance, for experiment J, where the soil slice nearer to anode—AN soil slice—presented the highest bentazone concentra-tion; or Fig. 12.2c, for experiment I).

12.3.7 conclusions from the case study

Rice culture contributes to environmental contamination with pesticides, leading to soil, surface, and groundwater quality degradation, which may have negative impacts on public health and biodiversity. Atrazine, molinate, and bentazone are examples of herbicides of concern. The electrokinetic process is able to remove these herbi-cides from soils, since they are all mobilized in the soil solution, under the action of an electric field. The case study presented showed the following:

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ACKNOWLEDGMENTS 261

• Atrazine was mainly mobilized toward the anode compartment due to reverse electroosmosis, and about 30%–50% of its initial quantity was removed during the first 24 h. This behavior occurred in all experiments (spiked and natural contaminated soils).

• Molinate removal efficiencies were high, above 90%. It seems that, indepen-dent of the initial amount of molinate and of the flow rate of the electrolyte solution, most of the molinate (>80%) found in the electrolytes is removed in the first 4 days. Molinate seems to mobilize preferentially to the cathode com-partment. The pH control basification at the anolyte may have contributed to the mobilization of molinate toward the cathode compartment since it should counteract the reverse electroosmosis.

• Bentazone was mobilized from the soil when submitted to the action of an electric field, toward both electrode compartments, but mostly toward the anolyte. At the end of the experiments, more than 90% of the bentazone initial quantities were removed. It seems that an application of a lower current inten-sity (10 mA) is related to a higher mobilization of this herbicide toward the cathode compartment, whereas higher current intensities (20 mA) promoted greater rate of bentazone mobilization also toward the anolyte.

12.4 summary

The atrazine, molinate, and bentazone behavior in soils when submitted to an electric field is presented as a case study. The applicability of the electrokinetic process in these herbicides’ soil remediation is evaluated. Four polluted soils were used, respectively with and without herbicides residues, with the last ones being spiked. Eleven electrokinetic experiments were carried out at a laboratory scale. Determination of the herbicide residues were performed by different methods: by ELISA for atrazine, by GC-FID for molinate, and by HPLC-UV for molinate and bentazone residues. GC hyphenated with mass spectrometry was used to confirm and identify molinate on the samples.

The results show that the electrokinetic process is able to mobilize atrazine, molinate, and bentazone from soils, and efficiently remove them from the soil solu-tion. Atrazine is mainly carried out toward the anode compartment, and estimations show that 30%–50% of its initial amount is removed from the soil within the first 24 h. Molinate seems to be mobilized preferentially to the cathode compartment, with removal efficiencies higher than 90%. Bentazone also shows a high removal from soil, reaching efficiencies above 92%. It is mainly carried out toward the anode compartment.

acknowlEdgmEnts

The authors thank the Environment Quality Group of the DCEA/FCT/UNL for allowing us to use the UV spectrophotometer for the immunoassays readings and Professor Marco Gomes da Silva from REQUIMTE of the DQ/FCT/UNL for sharing the gas chromatograph for the molinate determination; and Professor Damià Barceló of the Environmental Chemistry Department, CID-CSIC, Barcelona,

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262 ELECTROKINETIC REMOVAL OF HERBICIDES FROM SOILS

Spain, for supplying the contaminated soil with atrazine, and Professor Olga Nunes of the Chemistry Department, FEUP, Portugal, for supplying soils S2 and S3. Drs. Helena Gomes, Joana Serpa Santos, Carla Abreu, and Christoph Buchholz are thanked for all their experimental and analytical work carried out with atrazine, molinate, and bentazone. This work was partly funded by the Portuguese Project PPCDT/AMB/59836/2004, approved by the FCT.

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