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Journal of Environmental Quality 32:966-976 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Organic Compounds in the Environment

Significance of Anion Exchange in Pentachlorophenol Sorption by Variable-Charge Soils

Seunghun Hyuna, Linda S. Lee*,a and P. Suresh C. Raoa,b

a Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907-2054
b School of Civil Engineering, Purdue Univ., West Lafayette, IN 47907-2051

* Corresponding author (lslee{at}purdue.edu)

Received for publication May 14, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Sorption data and subsequent predictive models for evaluating acidic pesticide behavior on variable-charge soils are needed to improve pesticide management and environmental stewardship. Previous work demonstrated that sorption of pentachlorophenol (PCP), a model organic acid, was adequately modeled by accounting for pH- and pKa–dependent chemical speciation and using two organic carbon–normalized sorption coefficients; one each for the neutral and anionic species. Such models do not account for organic anion interaction to positively charged surface sites, which can be significant for variable-charge minerals present in weathered soils typical of tropical and subtropical regions. The role of anion exchange in sorption of ionizable chemicals by variable-charge soils was assessed by measuring sorption of PCP by several variable-charge soils from aqueous solutions of CaCl2, CaSO4, Ca(H2PO4)2 as a function of pH. Differences in sorption from phosphate and chloride electrolyte solutions were attributed to pentachlorophenolate interactions with anion exchange sites. Suppression of PCP sorption by phosphate ranged from negligible in a soil with essentially no positively charge sites, as measured by negligible anion exchange capacity, to as much as 69% for variable-charge soils. Pentachlorophenolate exchange correlated well with the ratio of pH-dependent anion exchange capacity to net surface charge. Sorption reversibility of PCP by both CaCl2 and Ca(H2PO4)2 solutions was also demonstrated. Results for PCP clearly demonstrate that sorption to anion exchange sites in variable-charge soils should be considered in assessing pesticide mobility and that phosphate fertilizer application may increase the mobility of acidic pesticides.

Abbreviations: AEC, anion exchange capacity • CEC, cation exchange capacity • PCP, pentachlorophenol • PZNC, point of zero net charge


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SORPTION BY SOILS and sediments greatly moderates the mobility, degradation, and ultimate fate of pesticides and other organic contaminants in aquatic and terrestrial environments. Sorption of nonpolar organic chemicals by soils is primarily to organic matter; thus, quantitative relationships using sorption coefficients (Kd) normalized to the organic carbon (OC) content of soils (Koc) has worked well for obtaining reasonable estimates of their sorption behavior (Lyman et al., 1990). Development of similar models for ionizable organic chemicals has been hindered because of the need to consider simultaneously the physical and chemical reactions of the ionized and neutral species as well as changes in the electrostatic properties of the soil surface with changes in parameters such as pH, ionic strength, and ionic composition. For organic acids, the use of Koc in conjunction with speciation of the organic acid as a function of pH and the acid dissociation constant (pKa) has been shown to adequately describe sorption by several soils and sediments (Jafvert, 1990; Lee et al., 1990). In these models, sorption is described assuming hydrophobic partitioning onto soil organic matter with the neutral species having a much higher affinity for the soil than the anion. Such models, however, do not include interaction of the organic anion to positively charged sites on the soil surface, which can be significant for variable-charge minerals.

Weathered soils with variable-charge minerals constitute an important group of soils in tropical and subtropical agricultural regions such as South and Central America, Australia, South and East Asia, Central Africa, the southeastern and some northwestern regions of the United States and the Pacific Carribean Islands. Soil types include Ultisols, Andisols, and Oxisols (U.S. soil taxonomy), and contain substantial amounts of amorphous or crystalline oxides and hydrous oxides, which have pH-dependent charge characteristics; thus, the term variable-charge soils. Even for sandy soils in Florida that have less than 3% clay content, a majority of the clay-size particles are oxide minerals. At pH values lower than the point of zero net charge (PZNC), the soil surface can be protonated, yielding a net positive charge. The PZNC of a soil is the pH value where the sum of the negative and positive charges on the soil surfaces equal zero, that is, no net charge (Sposito, 1989). Iron and aluminum oxides have pH-dependent charge characteristics that enable the generation of both negatively and positively charged sites, and are the principal soil minerals responsible for positive surface sites in variable-charge soils (Evangelou, 1998). Therefore, sorption of organic acids by variable-charge soils is attenuated by the pH- and pKa–dependent speciation of both the organic acid and the oxide surface. Many variable-charge soils exhibit a positive charge under natural environmental conditions, especially for soils that are acidic to slightly acidic (pH < 7). Likewise, many organic acids will exist as anions under the same conditions; the lower a compound's pKa, the greater the likelihood that it will exist as an anion, thus increasing the potential for interactions with positively charged surfaces (Kung and McBride, 1991).

Both inorganic and organic anions can associate with positively charged surfaces by both specific and nonspecific adsorption mechanisms, thus reducing their mobility. Nonspecific anion adsorption includes outer-sphere surface complexation and weak electrostatic interactions between an anion in the diffuse-ion swarm and a surface site, both of which are commonly referred to as anion exchange (Sposito, 1989). Specific anion adsorption includes ligand exchange, which may involve a combination of ionic and covalent bonding and often specifically involves exchange with a surficial hydroxyl (OH) or water group. Regitano et al. (1997) showed that for highly weathered soils containing significant amounts of both iron and aluminum oxides, sorption of the acidic pesticide imazaquin was greater than could be predicted assuming only hydrophobic partitioning, and that on addition of phosphate, sorption decreased. Based on results from stirred-flow sorption–desorption studies, Vasudevan et al. (2002) suggested that 2,4-D (pKa = 2.8) sorption by iron oxide–rich variable-charge soils appeared to be driven by nonspecific electrostatic interactions. Similar inferences were made previously by Watson et al. (1973) in their 2,4-D adsorption–desorption studies with synthesized goethite. Recent work quantifying NO-3 behavior in columns packed with variable-charge soils by Qafoku et al. (2000) demonstrated a strong positive correlation between retardation of NO-3 and the soil's anion exchange capacity. However, when the net negative surface charge is sufficiently large, NO-3 can be repelled from the soil surface, thus enhancing anion mobility (Black and Waring, 1979).

Sorption of anions onto positively charged sites in soils is affected by the presence of other anions and their relative affinity or selectivity for positively charged sites. An approximate order of selectivity for inorganic anions is as follows: phosphate > silicate > sulfate >> nitrate > chloride. For example, the sorption of sulfate on iron oxide was completely eliminated in solutions containing equal molar amounts of phosphate as H2PO-4 or HPO2-4 (Ryden et al., 1987) and silicate strongly competed with phosphate for sorption sites (Smyth and Sanchez, 1980). Several studies have demonstrated competition between organic and inorganic anions for sorption sites on variable-charge surfaces (Ali and Dzombak, 1996; Regitano et al., 1997). In addition, as the concentration of a given anion increases, its ability to displace another anion from the soil surface may increase, particularly for anions with a low relative affinity. Katou et al. (1996) modeled inorganic anion transport involving competitive sorption through variable-charge soil and estimated the relative mobility in terms of an exchange selectivity coefficient. Anions that can undergo specific adsorption such as phosphate may induce changes in the charge of soil minerals, thus the soil's PZNC, and are often referred to as potential determining ions (Anderson and Malotky, 1979; Ryden et al., 1977; Pyman et al., 1979).

Both the chemical and the oxide surface speciation in weathered soils will be pH-dependent. Therefore, especially at pH ranges where organic anions coexist with positively charged surfaces, anion sorption to these sites must be considered in predicting sorption and transport of acidic pesticides. Highly weathered soils and/or variable-charge soils are usually acidic and often have low fertility. Application of fertilizers in the form of inorganic salts such as sulfate, phosphate, or silicates usually increases soil pH while providing nutrients to crops. In such cases, shifts in speciation, competition by inorganic anions for positively charged sites, and alteration of the charge on the oxide surface could result in the enhanced mobility of acidic pesticides. Therefore, data for evaluating and predicting sorption of acidic pesticides by variable-charge soils are needed to provide knowledge and approaches necessary to improve pesticide management and minimize contamination of our soil and water resources. The primary objective of this experimental study is to quantify the contribution of anion exchange sites relative to hydrophobic interactions for sorption of pentachlorophenol (PCP) by variable-charge soils as a function of pH. Also included are results of preliminary investigations on the relative effects of different inorganic ions (calcium, potassium, phosphate, sulfate, and chloride) on PCP sorption and the reversibility of pentachlorophenolate sorption.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soils
Four highly weathered variable-charge soils collected from Brazil at a depth of 80 to 100 cm, three volcanic ash soils from South Korea, one weathered surface soil from Costa Rica, and one surface soil from Indiana (USA) were used (Table 1). All soils were acidic and represented a wide range in Al and Fe oxide content with the Indiana soil having the lowest oxide content. All soil samples were air-dried, sieved (<2 mm), and stored at laboratory conditions. Soil-solution pH was determined in water and 0.1 M KCl at soil mass (g) to solution volume (mL) ratio of 1:10, which was the ratio used in the sorption studies. Particle size analysis was performed using a pipette method (Gee and Bauder, 1986). Iron oxide and aluminum oxide content were determined using a citrate–bicarbonate–dithionite (buffered at pH 7) extraction method (Loeppert and Inskeep, 1996). The PZNC was determined using a KCl saturation method (Zelazny et al., 1996). Surface charge estimation by the ion sorption method has been reported to be more reliable than potentiometric titration, which can overestimate variable charge resulting from hydroxide dissolution, especially at pH > 7 and pH < 4 (Schulthess and Sparks, 1987; Marcano-Martinez and McBride, 1989).


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  		Table 1. Selected physical and chemical properties of soils.

 
Chemicals
Pentachlorophenol (PCP) was selected as a model organic acid because it has been well characterized in the literature (Lee et al., 1990; Arcand et al., 1995; Fein, 1996), and has a pKa value that allows it to exist primarily as an anion in an environmentally relevant pH range. Pentachlorophenol has a molecular weight of 266.35 g/mol and a pKa in pure water of 4.75 (Howard, 1991). Pentachlorophenol's aqueous solubility (20°C) is 3.54 g/L at pH = 4.1 and 7.0 mg/L at pH = 7 (Arcand et al., 1995). The log Koc (organic carbon–normalized partition coefficient, L/kg) value in 0.005 M CaCl2 systems for neutral PCP is 4.3 (pH < 4) and for anionic PCP is 2.6 (pH > 8) (Lee et al., 1990). Analytical-grade PCP was purchased from Aldrich Chemical Co. (Milwaukee, WI) at a chemical purity of >98%. Calcium chloride dihydrate (CaCl2·2H2O) and sodium sulfate anhydrous (Na2SO4) were purchased from Fisher Scientific (Pittsburgh, PA). Calcium bis(dihydrogenphosphate) monobasic, anhydrous calcium sulfate, and ammonium fluoride were purchased from Fluka (Buchs, Switzerland). Toluene and acetonitrile at greater than 99% purity were purchased from Mallinckrodt Laboratory Chemicals (Phillipsburg, NJ).

Anion Exchange Capacity and Cation Exchange Capacity Estimation
The PZNC, cation exchange capacity (CEC), and anion exchange capacity (AEC) were determined at the natural soil pH for all soils and over a pH range (approximately 3 to 8) for a subset of soils using KCl as a saturating solution and NaNO3 as a replacing solution (Zelazny et al., 1996). The K+ and Cl- extracted by NaNO3 were used for CEC and AEC calculations. Concentration of K+ was measured by atomic absorption (AA) (Spectra AA; Varian, Palo Alto, CA), and Cl- was measured by ultraviolet absorbance (Lambda 19 spectrophotometer; PerkinElmer, Wellesley, MA) at 460 nm using the mercury(II) thiocyanate [Hg(SCN)2] method (Frankenberger et al., 1996). The pH value at which CEC is equal to AEC was accepted as the PZNC. In cases where CEC and AEC were not measured at a pH where they were equal (e.g., for high CEC soils), PZNC values were estimated by extrapolation.

Mineral Characterization
Clay fractions (<2 µm) were collected using a sedimentation and sampling method. X-ray diffraction of slides prepared with untreated, K+–saturated, or Mg2+–glycerine-treated clay fractions were performed on clay fractions collected from each soil. Samples were scanned from 2 to 30°2{theta} using a step size of 0.05°2{theta} and scanning for 10 s at each step. The X-ray patterns were collected using CuK{alpha} radiation from a Simens (Munich, Germany) Kristalloflex 4 X-ray generator and a Simens Type F vertical goniometer equipped with a 1° divergence slit, a 1/4° receiving slit, a graphite, diffracted-beam monochromator, and a scintillation counter. Digital diffraction patterns were collected using Databox (Materials Data, Livermore, CA) and manipulated with JADE 3.1 software (Materials Data). The K+–saturated clay slides were reanalyzed after heating for 2 h at 100, 300, and 550°C to identify the expandable and partly chlorinated clays (e.g., hydroxy-interlayered vermiculite) and kaolinite (Whittig and Allardice, 1986).

Sorption Experiments
Pentachlorophenol sorption by soils was measured from aqueous solutions consisting of 0.005 M CaCl2, CaSO4, or Ca(H2PO4)2 using standard batch techniques (Rao et al., 1990). Pentachlorophenol dissolved in ethanol was spiked into preweighed 35 mL-glass centrifuge tubes followed by evaporation of ethanol. Soil (2 g) and pH-adjusted electrolyte solution (20 mL) were added and rotated end-over-end (30 rpm) for 48 h at 23 ± 2°C. All samples were performed in duplicate. In a preliminary time study, 48 h was shown to be sufficient to attain equilibrium. For each target pH, the amount of dilute HCl or NaOH needed to achieve a similar pH in each of the different inorganic matrices was predetermined for each soil. A 5-mL aliquot of the supernatant after centrifugation was transferred to a new tube and acidified to pH < 2 for liquid–liquid extraction with toluene (1:1). Mixtures were equilibrated overnight in 15-mL Teflon-lined screw-capped tubes using an orbit shaker. Toluene extracts were passed through an anhydrous Na2SO4 tube to remove residual water and then analyzed for PCP by gas chromatography–electron capture detector (GC–ECD).

For GC analysis, an automated Model 17A gas chromatograph (Shimadzu, Kyoto, Japan) was used with a 30-m x 0.32-µm SPB-5 capillary column (Supelco, Bellefonte, PA). The injector and detector temperatures were 300°C, and the temperature program was as follows: ramped from 150 to 210°C at 10°C/min, then ramped to 285°C at 25°C/min and held for 2 min. Injections were 1 µL at a split ratio of 1:5. The pH of the remaining supernatant was measured using a glass pH electrode. Soils were extracted with 10 mL of 4:1 (v/v) acetonitrile–0.25 M NH4F solution for 24 h. Samples were centrifuged and supernatants analyzed using a Shimadzu high performance liquid chromatography (HPLC) system equipped with an ultraviolet detector ({lambda} = 225 nm) and a Supelco LC-ABZ reversed-phase column (15-cm length x 4.6-mm ID, 5-µm film thickness). The mobile phase was a methanol–phosphate buffer (pH 2.2), 90:10 (v/v) solution at a 1.5 mL/min flow rate.

Solute mass extracted with the acetonitrile–NH4F solution is assumed to be the sorbed concentration. No significant PCP was detected in a second sequential extraction except for carryover from the first extraction step. Sorption isotherms were constructed using measured sorbed- and solution-phase concentrations. Overall PCP mass balance calculated by summing the PCP mass measured in the aqueous supernatants at equilibrium and extracted from the soil was 90 ± 10%.

The A1 soil was chosen to investigate the potential for the formation and sorption of positively charged ion pairs. Soil was saturated with either K+ or Ca2+ cation by gradually suspending soil in 0.1, 0.05, and 0.01 M KCl or 0.05, 0.025, and 0.005 M CaCl2. After saturation, excess salts were removed by washing with a 95% ethanol solution. Soil samples were freeze-dried and stored in capped bottles for further use.

Desorption of Pentachlorophenol
The sorption reversibility of pentachlorophenolate was assessed on the A1 Oxisol. After 2-g samples were equilibrated with PCP in 0.005 M CaCl2 resulting in a sorbed concentration of approximately 60 µmol/kg, desorption was performed by replacing supernatant with PCP-free 0.005 M CaCl2 solution and reequilibrating for 48 h. This desorption procedure was repeated two additional times before a final desorption step in 0.005 M Ca(H2PO4)2 solution. After each step, the supernatants were analyzed for PCP. After the final desorption step, residual PCP in the soil was also measured. Extraction and analysis of PCP in the supernatant and soils were performed as previously described.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Properties
Selected soil properties are summarized in Table 1. X-ray diffraction patterns showed that all soils except Toronto contained one or more variable-charge minerals such as gibbsite, goethite, hematite, and kaolinite. The Korean soils, which have been characterized as allophonic Andisols, showed strong peaks at the kaolinite position (0.72 nm) and weak broad bands at lower wavelengths (0.25 nm), indicative of allophane's noncrystalline properties (Harsh et al., 2002). The measured net surface charge of the Brazilian soils at their natural acidic soil pH (pH < 6.1) was negative with CEC values between 1 and 7 cmol/kg. These CEC sites probably originate from kaolinite and highly acidic functional groups in soil organic matter such as humic and fulvic acids. The Brazilian samples also posses a significant amount of AEC at their natural soil pH, originating primarily from the hydroxide minerals. Korean soil AEC values are similar or higher than the Brazilian soils; however, the Korean soils have higher CEC values originating from the high organic matter contents in these soils. The net effect is that net surface charge is more negative than that observed for the Brazilian soils. Toronto soil is primarily smectitic and has a low AEC. The PZNC values ranged from <1 to 5.1, with the Brazilian soils generally having the higher values.

Contribution of Anion Exchange Sites to Pentachlorophenol Sorption at the Natural Soil pH
To quantify the contribution of adsorption to positively charged sites relative to hydrophobic interactions, sorption of PCP was measured from 0.005 M CaCl2 and Ca(H2PO4)2 solutions. Chloride is weakly sorbed to anion exchange sites and will not be a strong competitor for anion exchange sites relative to pentachlorophenolate, even at concentrations much higher than PCP. However, since phosphate has a high affinity for exchange sites, we assumed that phosphate would essentially inhibit pentachlorophenolate sorption to anion exchange sites. Specific adsorption of phosphate (e.g., ligand exchange) may also modify charge arising from hydrous oxides to a neutral or negative charge, resulting in a shift of the PZNC to a lower pH (Ryden et al., 1977; Anderson and Malotky, 1979; Sposito, 1989; Bolan et al., 1999), which will contribute to a further reduction in pentachlorophenolate sorption. The equilibrium pH of soils suspended in 0.005 M Ca(H2PO4)2 was 0.2 to 0.8 pH units higher than that in 0.005 M CaCl2, indicating that hydroxyl groups (OH) on hydrous oxides were displaced by phosphate and released to the bulk solution. Therefore, hydroxyl group–induced anion exchange sites were reduced on the soil surface (Parfitt, 1977; Ryden et al., 1977; Yu, 1997), implying a potential-determining sorption mechanism. Differentiation between phosphate-induced surface potential changes from specific adsorption and competition for anion exchange sites was not attempted in this study. In the phosphate system, we assumed sorption of PCP to be primarily from hydrophobic interactions given the high selectivity of phosphate and that the phosphate concentration applied was at least 20 times greater than the measured AEC. In the chloride system we assumed that both hydrophobic interactions and sorption to positively charged sites are occurring. Therefore, the difference in sorption between the phosphate and chloride systems is assumed to demonstrate the contribution of positively charged sites to pentachlorophenolate sorption relative to hydrophobic interactions.

Representative PCP sorption isotherms in 0.005 M CaCl2 and Ca(H2PO4)2 are shown in Fig. 1 for six of the soils. To compare PCP sorption from 0.005 M Ca(H2PO4)2 and 0.005 M CaCl2 without having to account for pH shifts, small acid–base adjustments were made to achieve similar equilibrium pH values in both electrolyte systems near the natural soil pH. Sorption data were fit with linear and Freundlich sorption models: Cs = KdCw and Cs = KfCNw, respectively. The terms Cs (µmol/kg) and Cw (µmol/L) are equilibrium sorbed and solution concentrations, respectively, Kd is the linear sorption coefficient (L/kg), Kf is the Freundlich sorption coefficient (µmol1-N LN kg-1), and N (unitless) is a measure of isotherm linearity. The Freundlich Kf value is equal to the linear distribution coefficient Kd (L/kg) when an isotherm is linear (N = 1) or at Cw = 1 µmol/L. Coefficients for both linear and Freundlich model fits are summarized in Table 2. The magnitude of PCP sorption by these soils ranged over an order of magnitude. Sorption nonlinearity ranged from 0.66 to 0.86 and was similar for a given soil in the two electrolytes except for soil DRC. In most cases, Kd and Kf values were within 10% of each other, with a few exceptions (soil A2 in both electrolytes and soil DRC in CaCl2) where differences were between 20 and 30%, suggesting that linear approximation may suffice.



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  		Fig. 1. Pentachlorophenol (PCP) sorption isotherms measured in soils suspended in 0.005 M CaCl2 or Ca(H2PO4)2 near the natural soil pH. The term fa is the fraction of PCP existing as an anion at the noted pH.

 

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  		Table 2. Coefficients from linear and Freundlich model fits to pentachlorophenol (PCP) isotherms measured on several soils in 0.005 M CaCl2 and Ca(H2PO4)2. The goodness of fit (r2 values) for the linear isotherm and Freundlich isotherm models was >=0.93 and >=0.97, respectively, with r2 values of 0.99 for a majority of the fits.{dagger}

 


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  		Fig. 3. Anion exchange capacity (AEC) and cation exchange capacity (CEC) as a function of pH on selected soils. The value in the x axis represents the absolute values of AEC and CEC.

 
The presence of phosphate suppressed PCP sorption as much as 69% relative to sorption from 0.005 M CaCl2 (Table 2). For all variable-charge soils except the Korean soils, decreases in sorption occurred at soil pH values above the PZNC (CEC > AEC), supporting the discrete behavior of coexisting positive and negative surface charge sites (Black and Waring, 1979; Yu, 1997), and the need to consider the role of AEC when predicting sorption of anions even at pH values above the soil PZNC. For Toronto soil from Indiana, the hydrous oxide content was too small for anion exchange to be significant. The largest decreases in PCP sorption with phosphate present occurred on A1 and DRC soils. For the Andisols (K1, K2, and K3), changes in sorption in the presence of phosphate were less than 3%, although all three soils have significant AEC, with K1 and K3 having the highest AEC values of all soils investigated. As exemplified by the Andisols, phosphate-suppressed PCP sorption is not directly proportional to the AEC measured at the isotherm pH values (a linear regression of decreases in sorption versus AEC yielded a poor correlation, r2 = 0.003). However, a good linear correlation is observed when AEC is normalized to CEC (r2 = 0.85, not shown) or the net surface charge (r2 = 0.86, Fig. 2) . The better correlation when normalized to CEC or the relative surface charge suggests that although sorption to anion exchange sites occurs even when the surface has a net negative charge, sorption of anions to positively charged surface sites is hindered by the net negative surface charge (Smyth and Sanchez, 1980; Yu, 1997).



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  		Fig. 2. Correlation between the decrease in the pentachlorophenol (PCP) Kd value measured in Ca(H2PO4)2 relative to that measured in CaCl2 versus the absolute value of the ratio of anion exchange capacity [AEC, cmol(+)/kg] to the absolute value of the net surface charge. The line is a linear regression fit with R2 = 0.86 for the correlation coefficient.

 
Pentachlorophenol Sorption as a Function of pH
Anion exchange capacity, CEC, and PCP speciation will change as a function of pH (Sposito, 1989; Lee et al., 1990; Zelazny et al., 1996; Yu, 1997). Anion exchange capacity and CEC were measured as a function of pH for all soils as shown in Fig. 3 for six soils. Anion exchange capacity increased and CEC decreased as pH decreased; thus, AEC to CEC ratios increased with decreasing pH. Citrate–bicarbonate–dithionite (CBD)-extractable Fe and Al (Table 1) are highly correlated with free metal oxide (Loeppert and Inskeep, 1996), a primary source of anion and ligand exchange sites. The Toronto soil had low CBD-extractable Fe and Al corresponding to very low AEC and PZNC values; thus, pH-induced changes in AEC were not significant (data not shown).

Pentachlorophenol sorption as a function of pH was measured in duplicate in CaCl2, Ca(H2PO4)2, and CaSO4 at a single initial PCP concentration for all soils except the Andisols, where phosphate had little effect on PCP sorption. The Kd values estimated from the single-point isotherms are plotted versus pH in Fig. 4 . Aqueous equilibrium concentrations (Cw) for the single-point estimates for each soil were approximately midrange of the Cw values observed in the whole isotherms for the same soil (Fig. 1). Also included in Fig. 4 are the Kd values estimated from the multiconcentration isotherms near the natural soil pH (Fig. 1; Table 2). In all three electrolytes, sorption decreased with increasing pH as the fraction of pentachlorophenolate increased and AEC decreased (Fig. 4). Hydrophobic sorption of neutral PCP is two orders of magnitude greater than pentachlorophenolate; therefore, at lower pH values hydrophobic sorption of PCP increases (Lee et al., 1990). At lower pH values, AEC increases, and sorption of pentachlorophenolate increases as well. Therefore, PCP sorption by both mechanisms is higher at lower pH values.



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  		Fig. 4. Linear sorption coefficient (Kd) as a function of pH for pentachlorophenol (PCP) sorption from 0.005 M CaCl2, Ca(H2PO4)2, and CaSO4. Linear distribution coefficients (Kd, L/kg) were estimated from either a zero-intercept linear regression or from a single-point concentration where Cw was between 1 and 5 µmol/L. Fitted curves are arbitrary lines to visualize the difference of Kd as a function from 0.005 M CaCl2, Ca(H2PO4)2, and CaSO4 matrices. The terms Cl, P, and S noted in the legend refer to 0.005 M CaCl2, Ca(H2PO4)2, and CaSO4 matrices, respectively.

 
The relative changes in PCP sorption with increasing pH are different in the three electrolyte solutions. In all cases except Toronto soil where sorption through anion exchange is insignificant, the electrolyte solution containing phosphate suppressed sorption until the pH value at which the AEC approached zero. Specific adsorption of phosphate by variable-charge soils is favored at acidic pH values (Parfitt, 1977); therefore, decreasing pH resulted in increasing suppression of PCP sorption by phosphate. Even at pH values where aqueous-phase PCP exists primarily as a neutral species (e.g., fa = 0.02 at pH 3), phosphate suppressed PCP sorption indicating that PCP sorption through ion exchange reactions is significant even when the pentachlorophenolate species is negligible in the aqueous-phase species distribution. Fabrega et al. (1998) reported similar results for sorption of aniline, an organic cation with a pKa of 4.63, where the combined magnitude of the exchange coefficient and sites for exchange was much greater than the hydrophobic sorption coefficients and the hydrophobic domain. At pH > 7, PCP sorption is not significantly different between the different electrolytes, even though most of the PCP in the aqueous phase is in the anionic form. Anion exchange capacity approaches zero in this pH region, and electrostatic repulsion from an increasing number of CEC sites (negative charges on the soil surface) is large. Also, with increasing pH more hydroxyl ions are present to outcompete other anions for any remaining positively charged sites.

For the A1, A2, and A3 Oxisols, the decrease in PCP sorption in the presence of sulfate and phosphate are similar, but not consistent. For the A3 Oxisol, the presence of sulfate had a greater effect on PCP sorption than phosphate, whereas for the A1 and A2 soils, phosphate suppressed PCP sorption a little more than sulfate. For the DRC soil, sulfate had no effect on PCP sorption relative to chloride systems. Phosphate sorption is generally considered to be through a chemically coordinated ligand exchange reaction (e.g., such as a binuclear bridge complex) to positively charged surface sites, for example, hydroxyl groups singly coordinated to Fe3+ on an iron oxide surface and exposed edge hydroxyl groups on aluminum oxide and kaolinite (Atkinson et al., 1974; Hingston et al., 1974; Parfitt et al., 1976; Parfitt, 1977; Stumm et al., 1980; Bolan et al., 1986). There is less agreement with regard to sulfate sorption mechanisms, but mechanisms include ligand exchange similar to phosphate (Rajan, 1978; Parfitt and Smart, 1978; Martin and Smart, 1987) as well as outer-sphere complexation (Zhang and Sparks, 1990; He et al., 1997). Common observations for sulfate and phosphate sorption include (i) release of hydroxide ions, (ii) sorption greater than what could be accounted for from anion exchange sites, and (iii) PZNC shifts to lower pH values (Rajan, 1978; Pyman et al., 1979; Marsh et al., 1987; Zhang et al., 1987; Inskeep, 1989; Curtin and Syers, 1990; Guadalix and Pardo, 1991). However, major differences in selectivity and reversibility between phosphate and sulfate have been reported. Phosphate has been shown to preferentially adsorb over sulfate (Ryden et al., 1987) and to be irreversibly sorbed (Hingston et al., 1974), whereas sulfate sorption tends to be reversible (Curtin and Syers, 1990; Guadalix and Pardo, 1991). Therefore, sorption mechanisms for phosphate and sulfate may or may not be similar depending on the specific sorption sites, which will be dependent on mineral type. Both mineral type and content vary across soils (Table 1), which probably accounts for the relative difference between sulfate and phosphate effects on PCP sorption. Both DRC and A3 soils are listed as having more kaolinite, and A1 and A2 soils more gibbsite. The A3 soil is the only one of the four Oxisols that has no gibbsite or goethite.

Pentachlorophenol Sorption Reversibility
Given the different sorption mechanisms applicable to anion sorption and the subsequent effect on sorption reversibility, sorption reversibility of pentachlorophenolate was assessed on the A1 Oxisol at a pH of 5.8. Data for sequential desorption with 0.005 M CaCl2 and the final desorption with Ca(H2PO4)2 solution are shown in Fig. 5 along with linear regression fits to the multiconcentration sorption data measured in 0.005 M CaCl2 and Ca(H2PO4)2 solutions (shown in Fig. 1). Overall mass balance calculated by summing the PCP mass in aqueous and sorbed phases versus spiked PCP mass was 101 ± 1%. Pentachlorophenol desorption by CaCl2 follows closely the sorption isotherm measured from CaCl2 solutions, which is supportive of anion exchange as defined by diffuse-swarm and outer-sphere complexation. Likewise, PCP data from desorption with Ca(H2PO4)2 falls close to the sorption isotherm measured from Ca(H2PO4)2 solutions, demonstrating phosphate's effectiveness for displacing pentachlorophenolate. Desorption with both CaCl2 and Ca(H2PO4)2 solutions at a constant ionic strength and pH reflects sorption reversibility of pentachlorophenolate. Hingston et al. (1974) suggested that desorption of specifically adsorbed anions, for example, phosphate, from oxide surfaces is irreversible when constant pH and ionic strength are maintained. The latter conditions apply to our PCP desorption experiment as well. Therefore, while ligand exchange of pentachlorophenolate may not be explicitly excluded, little to no hysteretic behavior in PCP desorption by both CaCl2 and Ca(H2PO4)2 solutions suggests that for at least the A1 Oxisol, pentachlorophenolate sorption is primarily through nonspecific anion exchange reactions.



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  		Fig. 5. Pentachlorophenol (PCP) sorption–desorption isotherms from 0.005 M CaCl2 and 0.005 M Ca(H2PO4)2 matrices noted as Cl and P, respectively. Solid and dotted lines are linear regression fits to the sorption isotherms measured in Cl and P, respectively, reported in Fig. 1. Solid circles are the data from three sequential desorption steps with Cl and the open circles are data from the final desorption step with P.

 
Role of Cation in Pentachlorophenol Sorption
Enhanced sorption by variable-charge soils of inorganic anions in the presence of divalent inorganic cations has been reported (Ryden et al., 1977; Bolan et al., 1993). Bolan et al. (1993) observed higher sorption of SO2-4 by variable-charge soils in the presence of Ca2+ relative to K+. Mechanisms that have been suggested for cation-enhanced sorption of inorganic anions include: (i) formation of a surface complex between the anion and divalent cation, reducing the repulsive force between adjacent anions; (ii) precipitation reactions occurring at high pH; and (iii) divalent cation-induced increase in positive charge on the surface. To assess the role of cation-induced sorption of the organic anion pentachlorophenolate, PCP isotherms were measured on a A1 Oxisol equilibrated with 0.1 M KCl or 0.005 M CaCl2 at different pH values. Pentachlorophenol sorption as a function of pH for the CaCl2–A1 and KCl-A1 soils is shown in Fig. 6 along with PCP sorption from CaCl2 in the nonhomoionic system (same as data shown in Fig. 4). No cation-induced sorption of PCP was observed at any pH.



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  		Fig. 6. Dependence of pH on Kd for pentachlorophenol (PCP) sorption by A1 soil from KCl-saturated, CaCl2–saturated, and nonhomoionic 0.005 M CaCl2 systems.

 
A sorption maximum was observed near the pKa of PCP where the change in ionic species with respect to pH is the greatest, resulting in a sorption envelope. Pentachlorophenol sorption in the nonhomoionic CaCl2 systems is also slightly higher at the pH near PCP's pKa compared with lower pH values. Similar trends were not observed with the other soil investigated, and the A1 Oxisol is the only soil that has a PZNC close to the pKa of PCP. The appearance of a sorption envelope has been reported for sorption of inorganic and organic anions such as fluoride, silicate, and 2,4-D anion by pure hydrous oxides and water system (Hingston et al., 1972; Watson et al., 1973). Hingston et al. (1972) showed that the energy required to remove the proton from a weak acid is minimum at the chemical's pKa and hence maximum anion sorption will be observed at pKa. The much greater sorption maximum observed in the homoionic system is probably due to the fact that all anion exchanges sites are equilibrated with chloride, an indifferent and readily exchangeable anion. Also, sorption of PCP in the pH 4 to 5 range is much greater on both the KCl- and CaCl2–saturated soils relative to the nonhomoionic system. The systems where Hingston et al. (1972) and Watson et al. (1973) observed sorption maximum at or near the chemical's pKa were homoionic and pure mineral systems.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Results clearly demonstrate that organic acid sorption to anion exchange sites on variable-charge soils is significant and should be considered when predicting organic acid sorption by variable-charge soils. Since anion exchange is affected by the presence of other anions, electrolyte composition also needs to be considered to better predict sorption of acidic pesticides. Pentachlorophenolate was easily desorbed on addition of phosphate with no apparent hysteresis, suggesting that PCP sorption is primarily through nonspecific ion exchange reactions. Application of phosphate fertilizers, a common agricultural management strategy, serves to not only add nutrients for plant growth but also as an amendment to increase CEC of the soil. Both the increase in CEC and the highly selective and competitive sorption of phosphate by limited available sorption sites will result in a decreased sorption and increased mobility of soil-applied acidic pesticides. The relative effect of phosphate addition will be dependent on both the soil minerals present and the soil-solution pH. Theoretically, contribution of anion exchange on organic acid sorption by variable-charge soil will be most significant in the pH range where organic anion and AEC coexist (pKa < pH < PZNC). Therefore, as chemical acidity (pKa values) and soil AEC increase, the pH range where the electrolyte composition may affect sorption of acidic pesticides will probably be extended.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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