insight into the role of dissolved organic matter in sorption of sulfapyridine by semiarid soils

Insight into the Role of Dissolved Organic Matter in Sorption ofSulfapyridine by Semiarid SoilsHai Haham, Adi Oren, and Benny Chefetz*

Department of Soil and Water Sciences, Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, P.O.Box 12, Rehovot 76100, Israel

ABSTRACT: Sorption−desorption behavior of sulfapyridine was studied withthree distinct soil types low in organic carbon with or without the introduction ofexogenous dissolved organic matter (DOM). Experiments with bulk soils yieldedsorption coefficients equivalent to those obtained with soils richer in organicmatter, indicating an important sorptive role for soil mineral matrices.Cointroduction of sulfapyridine with DOM significantly reduced sulfapyridinesorption. However, decreasing solution pH from ∼9 to ∼6 limited the effect ofDOM and revealed the effect of ionic speciation of sulfapyridine on the sorptionpotential. Sulfapyridine sorption to soils precoated with DOM exhibitedcontrasting trends. Two of the coated soils exhibited decreased sorption ofsulfapyridine probably due to blockage of sorption sites by DOM. Conversely, thethird soil demonstrated cumulative adsorption of sulfapyridine. Precoating alsoenhanced sulfapyridine desorption, suggesting that sorption of sulfapyridine tomineral surfaces involves stronger chemisorptive binding as compared withinteractions with sorbed DOM. The capacity of soil to sorb DOM as well as the chemical fractionation of DOM during sorptionwere found to significantly affect binding of sulfapyridine. Competition between preferentially sorbed DOM moieties (e.g.,carboxyl, phenol) and sulfapyridine for sorption sites is proposed. This study suggests that the chemical nature of DOM cansignificantly affect the fate of sulfonamide compounds in soils.

■ INTRODUCTIONThe occurrence and fate of human and veterinary antibiotics inthe environment is of growing concern because of the potentialhealth and ecological effects associated with exposure to thesecontaminants.1 Limited removal efficiency in conventionalwastewater treatment and subsequent agricultural utilization ofreclaimed wastewater and biosolids can result in the exposureof agro-ecosystems to measurable amounts of antibiotics andtheir active metabolites.2 In addition to being sources ofpollutants, biosolids, composted manure, and reclaimedwastewater contribute significant amounts of dissolved organicmatter (DOM) to the soil, which may alter the sorptivebehavior of xenobiotics.3 DOM might enhance the retardationof organic pollutants through cumulative sorption 4 orcosorption.3 In contrast, processes such as competition orcotransport may facilitate the mobility of antibiotic compoundsin the soil system.5 The available data on effects of DOM onthe behavior of xenobiotics refer mainly to relatively hydro-phobic solutes,6−9 while the influence of DOM on the behaviorof polar contaminants (e.g., sulfonamides) has not receivedadequate consideration.Sulfonamide is a generic name for derivatives of p-

aminobenzenesulfonamide (sulfanilamide) that vary in theamide substitution. Sulfonamides are polar compounds whoseionic speciation depends on solution pH.10 Factors affecting thesorptive behavior of sulfonamide compounds in soils have beengenerally outlined in recent investigations, with soil organicmatter (SOM) being a major enhancing factor.11−13 The

chemical properties of the soil solution (e.g., pH and ioniccomposition) may also be important,14 particularly foramphoteric molecules.10,15 In general, sulfonamides exhibitrelatively low sorption affinity to solid phases in soils11 and aretherefore considered to be highly mobile compounds.16,17 Thismay be especially true in alkaline ecosystems where thetransport of sulfonamide compounds through soil columns hasbeen shown to approximate the mobility of conservativetracers.18

Sulfapyridine was chosen as the primary sorbate in this studysince it is one of the most frequently detected sulfonamides invarious environmental systems such as wastewater effluents andreceiving water bodies.19 In addition, sulfapyridine exhibits arelatively high pKa value that may help elucidate the sorptivebehavior of polar nonionized organic pollutants in calcareoussoils. Since it is difficult to evaluate the effects of exogenousDOM in SOM-rich soils (due to release of native DOM), weintroduced DOM to soils low in SOM in which DOM sorptionbehavior had been previously investigated.20,21

The main objective of the current study was to elucidate theeffect of DOM on the sorptive behavior of sulfapyridine insemiarid soils. The specific objectives were to characterizesorption and desorption of sulfapyridine with bulk soils; to

Received: August 7, 2012Revised: September 27, 2012Accepted: September 28, 2012Published: September 28, 2012

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© 2012 American Chemical Society 11870 dx.doi.org/10.1021/es303189f | Environ. Sci. Technol. 2012, 46, 11870−11877

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study the effects of DOM coating and DOM competition onsulfapyridine sorptive behavior; and to evaluate sulfapyridinesorptive behavior in a bisolute system. Our hypothesis was thatthe amount of DOM, its composition, and its mode ofapplication to the soil (i.e., precoating versus cointroduction)will all influence sulfapyridine sorption to soil.

■ EXPERIMENTAL SECTION

Reagents, Soils, and DOM. Sulfapyridine (4-amino-N-(pyridin-2-yl)benzene-1-sulfonamide, 99% w/w) and sulfanila-mide (4-aminobenzenesulfonamide, 99% w/w) were purchasedfrom Sigma-Aldrich (Rehovot, Israel). Selected physicochem-ical properties of these compounds are presented in Table 1.Soil cores (90−120 cm) were obtained from three agriculturalsoils in Israel. The soils were collected from Akko (33°127′ N,35°738′ E), Basra (32°433′ N, 35°549′ E), and Nir-Oz(31°457′ N, 35°041′, E). The soil samples were air-dried andsieved (<2 mm). Chemical and physical properties of the soilsare presented in Table 2.DOM was extracted from mature composted biosolids with

distilled water (1:10 w/w) as described by Oren and Chefetz.20

The isolated DOM (<0.45 μm; i.e., bulk DOM) was freeze-dried and redissolved in distilled water upon use. The following

DOM samples were used: (i) bulk DOM; (ii) desalted DOM;(iii) DOM-free fraction (i.e., ash); and (iv) nonbound DOM.To obtain the desalted DOM, the bulk DOM was placed indialysis tubes (cutoff of 100−500 Da, spectra/por, celluloseester dialysis membranes) and dialyzed against distilled water.To obtain the DOM-free fraction (i.e., residual ash), the bulkDOM was ignited (400 °C, 8 h). The nonbound DOM fractionwas the residual DOM remaining in the supernatant followingsorption of bulk DOM to the studied soils.20,21 The nonboundDOM was further filtered (0.45 μm) and dialyzed for saltremoval.

Sorption and Desorption Experiments. Sorption anddesorption of sulfapyridine were studied by a batch-equilibriumtechnique at 25 °C in 40 mL polypropylene centrifuge tubeswith a solid-to-solution ratio of 1:10 (w/w). Sulfapyridine wasdissolved in a background solution containing 3.7 mM CaCl2and 1.5 mM NaN3. These conditions are similar to theexperimental conditions used to study the DOM−soilinteractions.20 Nine sulfapyridine initial concentrations in therange of 0.04−11 mg/L were used. The tubes were horizontallyagitated in the dark for 4 days to reach sorption equilibrium(based on preliminary kinetics experiments and the timeneeded for sorption of DOM with the studied soils). Inaddition, a set of blanks (sulfapyridine without soil or soilwithout sulfapyridine) were incubated under the sameconditions to determine initial sulfapyridine and desorbedDOM concentrations, respectively. At equilibrium, the tubeswere centrifuged (12 000g, 6 min) and the supernatants filtered(0.45 μm). For desorption, 50% of the supernatant volume wasreplaced with a background solution free of the analyte. Twosequential desorption steps were performed, each for 4 days. Allexperiments were performed in triplicate. Similar sorptionprocedures were performed with sulfanilamide. In addition,bisolute sorption experiments were performed with sulfapyr-idine and sulfanilamide, testing for possible competitionbetween the two sulfonamides for sorption to soil. Sulfapyr-idine was applied as described above, and sulfanilamide wasadded at a constant concentration of 7.74 mg/L. Theindependent sorption of sulfanilamide and sulfapyridine tothe bulk soils served as control treatments to these competitionexperiments.To examine DOM effects on the sorption behavior of

sulfapyridine, DOM was applied in the following ways: (i) soilwas precoated with DOM: the soil samples were pre-equilibrated with 70 mg OC/L bulk DOM for 4 days accordingto ref 21, and after centrifugation the supernatant (containing

Table 1. Sulfapyridine and Sulfanilamide’s Selected Properties13

Table 2. Major Soil Characteristicsa

Akko Nir-Oz Basra

clay (%) 67.5 ± 1.4 17.5 ± 0.7 20.0 ± 0.1silt (%) 15.0 ± 3 7.5 ± 0. 2 5.0 ± 0.1sand (%) 17.5 ± 3 75.0 ± 1.1 75.0 ± 0.9texture clay sandy loam sandy loamOCb (%) 0.145 ± 0.02 0.078 ± 0.01 0.081 ± 0.01SSAc (m2/g) 327 ± 2 135 ± 1 61 ± 1CECd (meq/100 g) 43.56 ± 1.61 16.81 ± 0.45 15.21 ± 1.49Na (meq/100 g) 0.98 ± 0.09 0.24 ± 0.00 0.56 ± 0.03Ca (meq/100 g) 32.95 ± 1.38 20.91 ± 0.51 4.69 ± 1.65CaCO3 (g/kg) 35 ± 2 84 ± 3 0.0 ± 0.0Fee (g/kg) 6.54 ± 0.23 1.9 ± 0.19 4.46 ± 0.19pHf 7.7 ± 0.1 8.0 ± 0.1 7.2 ± 0.1

aMean values ± standard errors are presented. bOrganic carbon;obtained by wet oxidation. cSpecific surface area; obtained by ethyleneglycol monoethylether method. dCation exchange capacity; deter-mined by ammonium acetate saturation method. eExtracted withdithionite−citrate−bicarbonate reagent. fObtained in soil aqueousextract (1:10 with distilled water).

Environmental Science & Technology Article

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nonbound DOM) was decanted and sulfapyridine solution wasintroduced to the coated soil; (ii) bulk DOM (60−70 mg OC/L) was cointroduced to soil with sulfapyridine; (iii) DOM-freefraction (i.e., ash) corresponding to a bulk DOM solution of 65mg OC/L was cointroduced to the soil with sulfapyridine; (iv)pH variation: treatments (ii) and (iii) were performed underbuffered conditions (pH 6); (v) varying DOM concentrationand composition: desalted DOM (0−70 mg OC/L) samples,bulk DOM, and nonbound DOM were cointroduced to soilwith sulfapyridine (5 mg/L); and (vi) control (no DOMaddition): bulk soils were used as sorbents, and sulfapyridinewas dissolved in a background solution free of DOM.Analytes were quantified by reversed-phase HPLC analysis

with a Merck-Hitachi LaChrom D-7000 system equipped witha LiChrospher 100 RP-18 column (25 cm × 4.6 mm, 5 μm)and photodiode array (L-7455, Merck-Hitachi) and fluores-cence (L-7458, Merck-Hitachi) detectors for sulfapyridine andsulfanilamide, respectively. The compounds were eluted fromthe column at a constant flow rate of 1 mL/min. When onlyone analyte was present (sulfapyridine or sulfanilamide), anisocratic program of 20% acetonitrile and 80% of 0.1% formicacid was employed for 9 min. When both sulfapyridine andsulfanilamide were present, a gradient program was appliedwith the level of acetonitrile increasing from 10 to 20% for 11min. Sulfapyridine was quantified at 264 nm and sulfanilamideat 276/342 nm (excitation/emission). Limit of quantification(LOQ) values for sulfapyridine and sulfanilamide were 15 and10 μg/L, respectively.Data Analysis. Mass-balance analyses ensured negligible

(<1%) sorption of the target analytes to the tubes or loss due tovolatilization, hydrolysis, or degradation; therefore, sorptionwas calculated by mass difference. Sorption isotherms werefitted to the Freundlich equation (q = KF × Ce

N) usingSigmaPlot 12 software (Systat Software Inc.), where q is thesorbed concentration (mg/kg), Ce is the equilibrium concen-tration (mg/L), KF is a Freundlich coefficient (mg1−N/kg LN),and N is a dimensionless linearity parameter. Distributioncoefficients (Kd) were calculated for a Ce of 0.05, 0.5, and 5mg/L using the equation Kd = KF × Ce

N−1 (Table 3).

■ RESULTS AND DISCUSSION

Sorption and Desorption of Sulfapyridine in DifferentSoil Types. Sorption−desorption isotherms for sulfapyridinewith the Akko and Nir-Oz bulk soils are presented in Figure 1(left). The Basra soil exhibited relatively low sorption ofsulfapyridine, thus the isotherm is presented separately inFigure 2. The low sorption affinity for sulfapyridine obtainedwith the Basra soil was similar to the affinity obtained foranother sulfonamide, sulfadiazine, by a soil with similarcharacteristics (23% clay, 0.06% OC, pH 8.0).22 In this studywe used the Freundlich model to evaluate sorption parameterssince this model has been widely used in the study ofsulfonamides’ sorption to soils.12,13,23−25 Since KF values aredependent on N values, the obtained parameters cannot bedirectly compared. Hence, we used Kd values to compare thesorbents (Table 3). The calculated Kd values for the Akko andNir-Oz soils were similar or even higher than the Kd value (3.5L/kg) that was obtained for sulfapyridine with a Chernozemsoil (1.6% OC; pH 7.0) using linear regression of the sorptionisotherm.13 This similarity is unexpected considering thesignificantly lower OC content in our studied soils.Sorption of sulfonamides is considered to be largely

governed by SOM content.11,12,26,27 Normalizing the calculatedKd values to OC content yielded KOC values in the range of2200−4300 and 4000−12 700 L/kg of OC with Akko and Nir-Oz soils, respectively. These values are 1 order of magnitudehigher than those reported for the sorption of sulfonamides topure organic sorbents such as humic acid.28 Thus, we suggestthat sorption of sulfapyridine to the currently studied soils,which were very low in SOM (<0.15%), occurs mainly viainteractions with sites on the mineral matrix which may be aseffective as SOM binding sites. Nowara et al.29 reported thatsoil clay minerals act as important sorbents for antimicrobialagents from classes other than sulfonamides. Several relevantmechanisms have been proposed for the sorption of polarcompounds to soil minerals, among them cation-exchange,cation-bridging at clay surfaces, surface complexation, andhydrogen bonding.30 In the neutral-to-alkaline pH range of thecurrently investigated soils, cation exchange is not a relevantmechanism. Since the nonionized species of sulfapyridineprevails in the studied systems, binding through charge-transfercomplexes with clay mineral surfaces involving the stronglypolar amino groups of sulfapyridine can be highly relevant.13

Despite the particularly low SOM content in the currentlyinvestigated soils, some involvement of organic matter in thesorption of sulfapyridine might still subsist, possibly throughhydrogen bonding31 or covalent bonding of the aromatic aminemoiety of sulfapyridine.32

While the contribution of mineral phases to the sorption ofsulfonamides is commonly considered less important than thatof organic materials, sorption of sulfapyridine in its neutral form(pH 7) to clay minerals can reach Kd values of up to 5 L/kg.33

In soils, minerals are mostly present in association with organicmatter rather than as discrete mineral phases,34 and mineralsurfaces are mostly covered with organic coatings. Therefore,sorption of sulfonamides is mostly associated with SOM ratherthan with soil minerals. In contrast, Sukul et al.12 found thatamong five widely variable soils the second strongest sorbentfor sulfadiazine was a soil with low SOM content (0.6% OC)but a high clay content (42%). Interestingly, the sorptionisotherms of sulfadiazine with all five soils12 were S-shaped, assimilarly observed for the sorption of sulfapyridine to the Akko

Table 3. Parameters for Sulfapyridine Sorption to Bulk andDOM-Coated Soilsa

Bulk Soils

Kdc

KFb N r2 0.05 0.5 5

Akko 4.02 ± 0.43 0.85 ± 0.06 0.92 6.36 4.47 3.14Nir-Oz 4.73 ± 0.26 0.75 ± 0.03 0.98 9.97 5.62 3.17Basra 0.35 ± 0.03 0.66 ± 0.06 0.89 0.95 0.44 0.20

DOM-Coated Soils

Kdc

KFb N r2 0.05 0.5 5

Akko 2.20 ± 0.20 1.11 ± 0.05 0.98 1.15 2.03 2.66Nir-Oz 2.59 ± 0.27 0.90 ± 0.05 0.96 3.47 2.77 2.22Basra 1.02 ± 0.44 0.79 ± 0.03 0.98 1.93 1.18 0.73

aMean values ± standard errors and correlation coefficients (r2) arepresented for Freundlich model parameters; distribution coefficients(Kd) are presented for equilibrium concentrations of 0.05, 0.5, and 5mg/L. bmg1−N/kg × LN. cL/kg.



and Nir-Oz bulk soils (Figure 1). We assume that competitionwith dissolved components of native SOM (i.e., native DOM)is responsible for the S-shaped isotherms. The fact that the Sshape was more pronounced with Akko than with Nir-Oz soilscoincides with the significantly larger amount of SOM thatturns into DOM in the Akko soil system compared with theNir-Oz soil during the sorption experiments.21 This suggestscompetition between sulfapyridine and DOM for sorption sites.This issue is discussed in detail further on.

Within the soil mineral fraction, the contribution of ironoxides to sulfapyridine sorption has been suggested as well.13

However, the low sorption of sulfapyridine to the Basra soil,which contains more than twice the amount of iron oxides inthe Nir-Oz soil, implies that other properties are probably moreinfluential than iron-oxide content for the sorption ofsulfapyridine. Yet, the similarity between Akko and Nir-Ozsoils in the sorption affinity for sulfapyridine cannot beexplained by the effect of surface area (Akko > Nir-Oz).Thus, we assume that the composition of exchangeable cationsand their relative hydration radii might influence sulfapyridinesorption. The composition of exchangeable cations has beenshown to affect the accessibility of polar aromatic compoundsto binding sites.35,36 The results of Gao and Pedersen 33

support our assumptionssaturation of the exchange complexof clays by Ca2+ cations resulted in greater sorption ofsulfamethazine (mostly in nonionized form) relative tosaturation with Na cations. The proposed mechanisms werewater bridging with the aniline group (i.e., outer-spherecomplexation) or alternatively complexation of exchangeablecations with pyridine N or −SO2 groups. In our soils, bothAkko and Nir-Oz were rich in exchangeable Ca2+ (Table 2),thus facilitating similar interactions with sulfapyridine. The ratiobetween exchangeable Ca2+ to Na+ was 33 and 87 for Akko andNir-Oz soils and only 8 in Basra soil.Desorption of sulfapyridine from the Akko and Nir-Oz bulk

soils was greatly restricted (Figure 1, left). Most of the sorbedsulfapyridine (60−100%) remained in the sorbed phase of theAkko soil at the end of the second desorption step. In Nir-Oz

Figure 1. Sorption−desorption isotherms for sulfapyridine with bulk and DOM-precoated soils. Filled and open symbols designate sorption anddesorption, respectively.

Figure 2. Sorption isotherm for sulfapyridine with the bulk (●) andthe DOM-precoated (○) Basra soils.



soil, the nondesorbed fraction of sulfapyridine was close to100% at all sulfapyridine concentrations. Similarly, desorptionof sulfadiazine from variable soil types did not exceed 14% ofthe sorbed amount after an equilibration time similar to thatused in the current study.12 As the OC content of the soils usedin that study ranged from 0.5 to 2.9%, it appears that bothmineral and organic constituents can contribute to the firmretention of sulfadiazine in soil. Similarly, both a humic acid37

and an annealed soil matrix22 exhibited greatly nonreversiblesorption behavior of sulfonamide compounds. While themechanisms involved in the irreversible binding of sulfona-mides are not clear, we suggest that strong H-bonding betweenthe amine functionalities and H-acceptor moieties in both themineral matrix and SOM may be accountable. This assumptionis based on the finding of Teixido et al.31 that under alkalineconditions sorption of sulfamethazine to biochar involvesexceptionally strong H-bonding.Sulfapyridine Sorption to DOM-Coated Soil. Presorp-

tion of DOM resulted in a significant decrease in sulfapyridinesorption to both the Akko and Nir-Oz soils (Figure 1 right;Table 3). Moreover, the shapes of both soils’ isotherms werealtered (the corresponding N values increased; Table 3) as aresult of surface modification caused by precoating with DOM.While the S shape observed with the bulk Akko soil became lessexplicit, the opposite was observed with the Nir-Oz soil. Thedifferent effects of coating on the two sorbents were mostobvious at low concentration, where sulfapyridine sorption wassignificantly reduced in the DOM-coated Nir-Oz soil comparedwith the corresponding no-DOM treatment. It is important tonote that the equilibrium pH values following DOM coatingwere not different from the pH values recorded in theequilibrium solutions of the respective bulk soils (Table 2).Spectroscopic analyses of sorbed DOM demonstrated the

preferential sorption of carboxylic and aromatic (particularlyphenolic) groups to these soils’ surfaces,20 with a strongerpreference in Akko than Nir-Oz soils. The sorption affinity ofsulfapyridine to SOM components such as carboxylic, alkyl-aromatics, lignin constituents, and phenols can be high.26,38

The limited amount of such constituents in the sorbed phase ofthe Nir-Oz soil probably could not provide effective sorptionsites for sulfapyridine. Conversely, coating of the soil mighthave occupied potential binding sites for sulfapyridine at themineral matrix.Data obtained for the Basra soil revealed a significant

increase in sulfapyridine sorption (by up to 250%; Table 3) as aresult of precoating with DOM (Figure 2). While this findingcontrasts with the observations from the Akko and Nir-Oz soils,it supports the notion of coating-induced supplementation withfavorable sorption sites. The Basra soil has a particularly lowsurface area but relatively large surface coverage by iron oxides,which have a high affinity for DOM. As a consequence, sorbedDOM in the Basra soil accounted for 2.7 μg OC/m2, comparedwith only 0.76 and 0.96 μg OC/m2 in the Akko and Nir-Ozsoils, respectively. The DOM coating in the Basra soil couldincrease the effective surface area and provide constructivesorption sites for sulfapyridine.21 Sulfamethazine sorption tohumic acid−smectite complexes was similarly found to exceedthat to the corresponding smectites, with the effect of humicacid increasing with loading.38

Desorption of sulfapyridine from the precoated soils (Figure1, right) was less restricted as compared with desorption fromthe respective untreated soils (Figure 1, left). It seems thatsulfapyridine sorption to the mineral soil matrix involves strong

chemisorptive binding, whereas sorption to sorbed DOM isweaker and involves relatively more physical forces (e.g., vander Waals interactions). Detachment of cumulatively sorbedsulfapyridine−DOM complexes is also a possible contributor tothe increased desorption from coated soils. The observed datacontradict the trends observed for carbamazepine, whichexhibited increased retardation in soils precoated withDOM.39 There it was suggested that DOM surfacemodification decreases the release potential of carbamazepine.It seems that the more polar and partially ionized compoundsused in the current study interact differently with sorbed DOM.

Cointroduction of DOM and Sulfapyridine to Soil.Cointroduction of sulfapyridine and bulk DOM resulted innearly complete elimination of sulfapyridine sorption to boththe Akko and Nir-Oz soils (Figure 3, bottom; filled symbols) as

compared with the corresponding no-DOM treatments (Figure1). Since the ionic speciation and charge of sulfapyridine is pH-dependent,10 the decrease in sorption seems to be closelyrelated to the strong shift in pH caused by the bulk DOMaddition. The reaction pH diverged from <8 in the native soils’solutions to ∼9 following the bulk DOM’s application, crossingthe pKa (8.4) and bringing sulfapyridine to ∼80% ionization.Strong reduction in the sorption of sulfonamide compounds tosoils11,27 and clay minerals33 with increasing pH is well

Figure 3. Sorption isotherms for sulfapyridine at pH 6 (top) and 9(bottom) for Akko (●, ○) and Nir-Oz (▼, Δ) soils. Filled and opensymbols designate cointroduction experiments with bulk DOM andDOM-free fraction (i.e., ash), respectively.



documented.40 In contrast, sorption of sulfonamides (includingsulfapyridine) to soil organic sorbents (e.g., humic acid) doesnot always follow a clear pH dependence, possibly because ofthe greater contribution of van der Waals interactions.28 Theeffect of pH in the current investigation was confirmed byperforming parallel sorption experiments in a bufferedenvironment (pH 6) where sulfapyridine exists entirely in itsneutral form (Figure 3; top). The effect of bulk DOM inreducing sulfapyridine sorption diminished at pH 6 ascompared with noncontrolled pH (i.e., ∼9). Nevertheless,reductions in the sorption of sulfapyridine were also evident atpH 6, implying that DOM may also affect sulfapyridinesorption independent of pH modifications.To differentiate between the effects of pH and solution

composition (i.e., DOM and salts), cointroduction ofsulfapyridine with the DOM-free fraction (i.e., ash) was alsoexamined at both pH values (Figure 3; open symbols). For Nir-Oz soil, similar sorption-reducing effects were demonstrated bythe bulk DOM and the DOM-free fraction (under both pHconditions). In contrast, the effect of the DOM-free fraction onsulfapyridine sorption to the Akko soil at pH 6 was enhancive(compared with sorption to bulk soil). This provides evidencefor an effect of DOM molecules, rather than pH or ioniccomposition, on reducing sulfapyridine sorption to this soil.The different effects obtained with the cointroduced DOM onsulfapyridine sorption to the two soils can result from differentsurface interactions of DOM. The Akko soil has been shown tobe a significantly stronger sorbent for DOM than the Nir-Ozsoil 21 due to higher clay and iron-oxide contents in the former.Thus, competition for sorption sites between DOM (higheraffinity in Akko) and sulfapyridine may limit sulfapyridinesorption to the Akko soil.Effect of DOM Composition on Sulfapyridine Sorp-

tion. To elucidate the effect of DOM on sulfapyridine sorption,experiments were performed with desalted DOM at varyingDOM-to-sulfapyridine ratios (cointroduction). This allowedthe detection of effects that could be solely attributed to DOMreactivity. As noted in Figure 4, the Kd values for sulfapyridinesorption decreased by ∼95% at the highest DOM concen-tration (70 mg OC/L). However, maximum reduction insulfapyridine sorption was already attained at a DOM-to-sulfapyridine ratio of ∼1.5 and persisted with increasing DOMconcentration. Similar to our results, sulfapyridine sorption to apig slurry−soil mixture decreased relative to sorption to bulksoil.41 Likewise, sorption of sulfapyridine to graphite andcarbon nanotubes decreased with increasing concentrations ofdissolved humic acid.42 These results support our hypothesisthat the effect of DOM on sulfapyridine sorption isconcentration dependent.DOM can affect sorption of organic compounds by

competition and/or direct interactions (complexation). DirectDOM−sulfapyridine interactions were studied using dialysistube experiments and IR analyses (data not presented). Neitherapproach provided evidence for direct interactions betweenDOM and sulfapyridine in the solution. Therefore, our datapresented in Figure 4 may support competitive sorption ofsulfapyridine and DOM.The effect of DOM on sulfapyridine sorption to the soils was

found to be not only concentration-dependent but also, moreimportantly, composition-dependent. For similar DOM-to-sulfapyridine ratios, the effects of bulk DOM (●) were greaterthan those of nonbound DOM (○; Figure 4). pH as a possiblecause for these differences may be ruled out considering that

the pH values of the nonbound DOM (7.9) and the bulk DOM(8.0) solutions were similar. DOM sorption to the currentlyinvestigated soils has been shown to be selective, withpreference for certain organic moieties rich in phenol andcarboxyl functionalities.20 Decreasing the fraction of thesefunctional groups (i.e., nonbound DOM fraction) evidentlymodified the effect of the DOM. We therefore assume that lessintense competition was exerted on sulfapyridine in thepresence of nonbound DOM as compared with the DOMprior to soil interaction.

Bisolute Sorptive Behavior. As expected, the considerablylower solubility of sulfapyridine resulted in greater sorptioncompared with sulfanilamide (Figure 5). Kd values obtained forsulfanilamide with the Akko and Nir-Oz soils were 0.48 and0.82 L/kg, respectively, in comparison to 1.7 L/kg obtained forthe sorption of sulfanilamide to soil with 1.6% OC at pH 7.0.13

A lower affinity of this compound to soil minerals as comparedwith SOM may be inferred, possibly due to the relatively lowelectronegativity of the functional group attached to thesulfonamide core (i.e., absence of a pyridine group). In thecurrent study, the shapes of the sorption isotherms obtainedwith sulfanilamide were similar to those obtained withsulfapyridine. Sorption to the Akko soil produced pronouncedS-shaped isotherms for both compounds, possibly reflecting thepresence of competing DOM moieties.

Figure 4. Effect of concentration and composition of DOM onsorption affinity of sulfapyridine. Sulfapyridine sorption affinity undervarying DOM concentrations (Kdi) is presented relative to maximumsorption affinity (i.e., without DOM; Kd (no DOM)) as a function of OC-normalized DOM:sulfapyridine ratio.



The cointroduction of sulfanilamide (45 μmol/L) withsulfapyridine resulted in an even more pronounced S-shapedisotherm for the sorption of sulfapyridine to the Akko soil(Figure 5, ○). In the low sulfapyridine concentration range(Figure 5, enlarged area), sulfapyridine sorption was noticeablysuppressed. This was also the case with the Nir-Oz soil, whosesulfapyridine sorption isotherm became slightly S-shaped aswell. These results suggest that sulfanilamide effectivelycompetes with sulfapyridine for sorption sites but only at ahigh sulfanilamide-to-sulfapyridine molar ratio (i.e., >15). Thisis in agreement with Thiele-Bruhn et al.13 who reportednegligible competition among sulfonamides for sorption to soil.The effective competition of sulfanilamide with sulfapyridine atlow concentrations further supports our assumption that DOMin our experimental setup competes with sulfapyridine andpossibly with other polar antibiotic agents for binding sites.Environmental Implications. The current investigation

highlights the important contribution of the mineral soil matrixto the sorption of sulfonamide compounds. The way in whichexogenous DOM affects the chemical reactivity of sulfonamideantibiotics in SOM-poor soils strongly depends on soil surfaceproperties as well as solution conditions. The DOM itselfundergoes molecular modifications in the soil environment. Itcan interact with soil constituents and become integrated in thesolid matrix, thus modifying surface properties. Alternatively,DOM moieties with low surface reactivity (nonbound DOM)

can alter the properties of the soil solution. In the current study,DOM coating on SOM-poor soils acted as an efficientcompetitor for sulfapyridine. Furthermore, when sorbed tosoil, DOM facilitated sulfapyridine desorption. The competitiveeffect of DOM proved to be concentration- as well ascomposition-dependent. In contrast, when very low surfacearea matrices are considered, DOM sorption might enhancesulfapyridine binding through cumulative sorption.

■ AUTHOR INFORMATION

Corresponding Author*Tel.: 972-8-9489384. Fax: 972-8-9475181. E-mail: [email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The research was supported by the Canada-Israel BARDprogram (CA 9114-09).

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Figure 5. Sorption isotherms of sulfapyridine (●), sulfanilamide (▼),and sulfapyridine in the presence of 45 μmol/L of sulfanilamide (○).



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insight into the role of dissolved organic matter in sorption of sulfapyridine by semiarid soils

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