| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Dipartimento di Scienze del Suolo, della Pianta e dell'Ambiente, Università di Napoli Federico II, Via Università 100, 80055 Portici (Napoli), Italy
* Corresponding author (violante{at}unina.it)
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Abbreviations: Ald, Al extracted by Na-dithionite-citrate • Alo, Al extracted by NH4-oxalate • EGME, ethyleneglycol monoethylether • Fed, Fe extracted by Na-dithionite-citrate • Feo, Fe extracted by NH4-oxalate • IEP, isoelectric point • IMt-2, Montana illite • KGa-1, Georgia kaolinite • OOMWW, olive oil mill waste water • PZC, point of zero charge • PZSE, point of zero salt effect • rf, molar ratio • SWy-1,Wyoming montmorillonite • TEM, transmission electron microscopy • XRD, x-ray diffraction
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
It has been established that the chemical behavior of AsO4 is similar to that of PO4 in soils. Both of these oxyanions are specifically adsorbed on soil minerals, mainly on variable charge minerals (Al, Fe, and Mn oxides; allophanes, imogolite), forming inner-sphere complexes. These oxyanions may form three different surface complexes on inorganic soil components: a monodentate complex, a bidentate-binuclear complex, and a bidentate-mononuclear complex in different proportions, depending on surface coverages (Lumsdon et al., 1984; Hsia et al., 1994; Sun and Doner, 1996; Fendorf et al., 1997; Smith et al., 1998; O'Reilly et al., 2001; Liu et al., 2001). Phosphate has been reported to suppress the sorption of AsO4 and to displace sorbed AsO4 from soils (Sadiq, 1997; Smith et al., 1998; Liu et al., 2001). Application of PO4 fertilizers is a management practice that can have a direct effect on the concentration of As in soil solution and may enhance arsenic's phytoavailability, but the role of soil components on AsO4 mobility is still obscure (Peryea, 1991; Melamed et al. 1995).
Fordham and Norrish (1979) found that when radioactive AsO4 was added to acid soils, it was retained mainly by Fe-oxides and to a much lesser extent by Ti oxides. Gibbsite was much less effective in sorbing AsO4 than Fe and Ti oxides. According to these authors, other components of the soil clay fractions (kaolinite, illite, vermiculite, feldspars) contributed to the total uptake of AsO4 but to a lesser degree than Fe oxides. Many studies (Livesey and Huang, 1981; Goldberg, 1986; Smith et al., 1998; Liu et al., 2001) have demonstrated that the amount of sorbed AsO4 in soils is significantly correlated with oxalate-extractable Al and Fe and with clay. Competition in sorption of PO4 and AsO4 may vary greatly on different soil minerals and on soils characterized by different mineralogy and chemical properties (e.g., Woolson et al., 1973; Roy et al., 1986; Peryea, 1991; Melamed et al., 1995). Roy et al. (1986) found that the competitive effects of PO4 on AsO4 adsorption on three soil samples were greater than those of AsO4 on PO4 adsorption. The opposite was found by Liu et al. (2001) by using goethite as sorbent. Furthermore, Woolson et al. (1973) found that heavy additions of PO4 to AsO4–polluted soils displaced large amounts of As, approximately 80% of the total As in the soils. In contrast, Peryea (1991) reported that large additions of PO4 to volcanic soils, rich in allophanic minerals, only partially displaced AsO4. Unfortunately, until now detailed information on the competitive sorption of PO4 and AsO4 on various minerals and soils with different mineralogical and chemical properties is not available. Sorption of AsO4 on different soil components may have a dominant role in regulating As mobility in soils.
To have useful information on the factors which may influence the mobility and potential toxicity of AsO4 in natural systems, we carried out an investigation on the competition in sorption of AsO4 and PO4 on selected inorganic soil components (phyllosilicates, crystalline, and short-range ordered Fe, Al, Ti, and Mn oxides, mixed Fe-Al oxides, allophanes), synthetic organo-mineral complexes and soil samples (Andisols, an Ultisol, an Oxisol, and Rhodoxeralfs) as affected by pH, oxyanion concentrations, surface coverage of both the oxyanions on selected samples, and the time of contact of the oxyanions with the sorbents.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
A noncrystalline Al hydroxide, a ferrihydrite, and mixed Fe-Al gels were prepared according to the method described in a previous work (Colombo and Violante, 1996). Stock solutions of 0.1 M Al(NO3)3 and 0.1 M Fe(NO3)3 were mixed in different proportions to synthesize samples having initial Fe/Al molar ratios (R) of 0 (no Fe present), 1, 2, 4, 10, and 
(no Al present). The solutions (henceforth referred as R0, R1, R2, R4, R10, and R) were potentiometrically titrated to pH 5.5 (except for R0, which was titrated to pH 7.0) by adding 0.5 mol L-1 NaOH at a feed rate of 0.5 mL min-1. The final volume of all samples was adjusted to 1 L. After 7 d of aging at room temperature (1 d of aging for sample R0), the suspensions were dialyzed. The R0 sample appeared a noncrystalline Al precipitation product to x-ray diffraction (XRD) analysis, whereas the R1-R oxides showed through XRD analysis broad peaks at about 0.255, 0.225, 0.220, 0.195, 0.170, and 0.150 nm characteristic of six-lines ferrihydrite (Cornell and Schwertmann, 1996). Table 1 shows the chemical composition, the surface area, and the mineralogy of these oxides. The nature, chemical composition, stability, and mineralogy of mixed Al and Fe(III) oxides as well as of noncrystalline Al precipitation products have been extensively described elsewhere (Colombo and Violante, 1996, 1997; Violante et al., 1998; Violante and Huang, 1992).
|
Two synthetic allophanes (AL-SI and AL-SI-FE) were generously provided by Dr. M. Mora (Mora and Canales, 1995). The AL-SI sample, synthesized by mixing 80 mL of 1.1 mol L-1 AlCl3 and 104 mL of 1.3 mol L-1 potassium silicate which were neutralized up to pH 5.0 by 0.1 mol L-1 NaOH, had a surface area of 717 m2 g-1, a Si/Al molar ratio of 0.21, and an isoelectric point (IEP) of 5.5. The AL-SI-FE allophane was obtained precipitating a suitable amount of Fe oxide on the AL-SI surface, as described elsewhere (Mora and Canales, 1995). The AL-SI-FE allophane had a surface area of 540 m2 g-1, an IEP of 8.6 and a SiO2, Al2O3 and Fe2O3 content, respectively of 42.0, 29.5, and 6.6, respectively. As described by Mora and Canales (1995), the specific surface area was determined by EGME method. The IEP value was determined by microelectrophoresis in a zeta meter (ZM-77) apparatus. The chemical composition of the allophanic samples was determined by dissolving them in teflon bombs and the Al, Si, and Fe content were analyzed by atomic absorption spectrophotometry.
Two organo mineral complexes containing Al were prepared by titrating 500 mL of stirred solutions containing 0.1 mol L-1 AlCl3 and 0.01 mol L-1 tannic or tartaric acid [Al(OH)x-TAN or Al(OH)x-TAR; tannic (or tartaric) acid/Al molar ratio of 0.1] with 0.1 mol L-1 NaOH at a rate of 2 mL min-1 to pH 8.0. Al(OH)x-TAN and Al(OH)x-TAR complexes had surface areas (EGME) of 350 and 430 m2 g-1 and PZSE of 5.4 and 6.5 respectively (Violante and Huang, 1989, 1992). These organo mineral complexes were noncrystalline materials. The chemical composition and surface properties, including XRD and thermal analyses and electron microscope observations of these samples are described elsewhere (Violante and Huang, 1989; 1992). A ferrous organo-mineral complex [Fe(OH)x-POL] was prepared by titrating 500 mL of a stirred solution of 0.1 mol L-1 FeCl3, containing 2 g of humic acid-like polymers, recovered from olive oil mill waste water (OOMWW) (relative average molecular mass of 150000 kDa; Arienzo and Capasso, 2000) with 0.1 mol L-1 NaOH at a rate of 2 mL min-1 to pH 6.0. Arienzo and Capasso (2000) demonstrated that the organic fraction of the OOMWW, contained polymeric polyphenols, polysaccharides, and proteins. The suspensions were aged for 24 h, centrifuged at 10000 x g, washed twice with deionized water, dialyzed until Cl- free, and freeze-dried. The sample appeared noncrystalline to XRD analysis and TEM observation.
Birnessite was synthesized by adding hydrochloric acid to a hot solution of KMnO4, as described by McKenzie (1983). The surface area was of 40 m2 g-1. Pyrolusite (MnO2) and anatase (TiO2) used in our study were obtained from Aldrich Chemical Company (Milwaukee, WI). The point of zero charge (PZC) of these samples were not determined, but PZC of Mn oxides are usually low (<4.6; 2.0–2.5 for birnessite), whereas that of TiO2 ranges from 4.0 to 6.0 (McKenzie, 1981; 1983).
Phyllosilicates
Five clay minerals (Wyoming montmorillonite [SWy-1], Georgia kaolinite [KGa-1], nontronite, Montana illite [IMt-2], and Washington ferruginous smectite), obtained from the Clay Minerals Society Source Clay Minerals Repository (Columbia, MO), were lightly ground to pass a 0.315-mm sieve, and were used without further preparation. Manning and Goldberg (1996b), who used some of these clay minerals (SWy-1, KGa-1, and IMt-2) measured a surface area (Brunauer–Emmett–Teller [BET] N2 adsorption) of 9.1 for Kga-1, 18.6 for Swy-1, and 24.2 m2 g-1 for IMt-2. These authors found that kaolinite contained trace impurities of feldspar and vermiculite, montmorillonite showed a trace impurity of mica, whereas illite was x-ray pure. Nontronite and ferrouginous smectite contained trace of quartz and calcite.
Soil Samples
The <2-mm fraction of three Andisols (a A1 horizon of an Eutric Fulvudand, a 3Bw horizon of an Eutric Pachic Fulvudand, and a 2C horizon of a Typic Hapludand) from Roccamonfina Volcano (Italy), an Ultisol and an Oxisol from China and the <2-mm fraction of two red Mediterranean soils (Rodoxeralfs) from Apulia (Italy) were used in this work. The general descriptions of these soils are given by Vacca et al. (2002)(Andisols), He et al. (1998)(Oxisol and Ultisol), and Colombo et al. (1996)(Rodoxeralfs). Selected chemical and physicochemical properties of the soil samples and their mineralogical composition are reported in Table 2. Organic C, pH, allophane content, clay content, and Fe and Al extracted by Na-dithionite-citrate (Fed, Ald) and by NH4-oxalate (Feo, Alo) used, respectively, for the complete dissolution of Fe oxides (Fed) and for the extraction of short-range order Fe oxides (Feo) and Al precipitation products (Alo), were determined as reported by He et al. (1998) and Vacca et al. (2002).
|
radiation generated at 40 kV and 30 mA (Rigaku Co., Tokyo, Japan). For TEM examination, one drop of sample suspension was deposited onto a carbon-coated Forvar film Cu grid. The TEM micrographs were taken with a Philips CM 120 microscope (Philips Ind. S.A. Wavre, Belgium).
Phosphate and Arsenate Sorption Isotherms
Suitable amounts (50–300 mg) of the clay minerals, organo-mineral complexes, and soil samples were equilibrated at 293 K with 43 mL of 0.05 mol L-1 KCl at pH 5.0 and 7.0. Suitable amounts of 0.01 mol L-1 solutions containing PO4 or AsO4 were then added to have initial PO4 or AsO4 concentration in the range 5 x 10-4 to 10-2 mol L-1. The pH of each suspension was kept at the initial value by adding 0.1 or 0.01 mol L-1 HCl or KOH. The suspensions were shaken for 24 h. The final suspensions (45 mL) were centrifuged at 10000 x g for 20 min. and filtered through a 0.22-µm membrane filter.
Sorption of Arsenate and Phosphate as a Function of pH
Forty-three milliliters of 0.05 mol L-1 KCl were added to suitable amounts (50–300 mg) of the clay minerals, organo-mineral complexes, or soil samples in reaction flasks. Predetermined quantities of 0.01 mol L-1 solutions containing AsO4, PO4, or a mixture of both oxyanions, whose pH values were previously adjusted (pH 4.0–8.0), were pipetted into the flasks to yield nearly the maximum adsorption of each oxyanion per kilogram of sample, as previously determined by adsorption isotherms (not shown). Experiments were also carried out to yield a lower surface coverage of both oxyanions (about 35–70% of surface coverage). Competitive sorption of AsO4 and PO4, added as a mixture, was also carried out by adding suitable amounts of AsO4 (or PO4) in the presence of increasing quantities of PO4 (or AsO4) to achieve initial PO4/AsO4 (or AsO4/PO4) molar ratios ranging from 0 to 2.0. The pH of each suspension was usually kept constant for 24 h (only for some experiments for 0.02–1440 h) by adding 0.1 or 0.01 mol L-1 HCl or KOH. The final suspensions (45 mL) were centrifuged at 10000 x g for 20 min, and filtered through a 0.22-µm filter. Arsenate and PO4 were determined in the supernatant as described below. When necessary, samples were collected and filtered through a 0.22-µm membrane filter.
Arsenate, Phosphate, Aluminum, and Iron Determination
Arsenate and PO4 were determined by ion chromatography, using a Dionex DX-300 Ion Chromatograph (Dionex Co, Sunnyvale, CA), an IonPac AS11 column (4.0 mm), an eluent of 0.05 mol L-1 NaOH at a flow rate of 1 mL min-1, and a CD20 Conductivity Detector combined with autosuppression. Average PO4 and AsO4 retention times were 3.0 and 4.2 min, respectively. The standard concentrations were 0.2 to 2 mmol L-1 for AsO4 and PO4. The amount of ligands adsorbed was determined by the difference between the initial and final concentrations. The data are the mean of two or three determinations. Coefficients of variation ranged from 1.5 to 5%.
Aluminum and Fe in the final solutions were determined by atomic absorption spectrophotometry. No or negligible quantities of soluble Al or Fe were found in the final solutions. Most of the systems, particularly those at pH > 5.0, were undersaturated in respect to the sparingly soluble AsO4 and PO4 compounds that may form in soils.
|
RESULTS AND DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
|
|
|
|
Competitive Sorption of Arsenate and Phosphate
Competition in sorption between AsO4 and PO4 on phyllosilicates, metal oxides, organo-mineral complexes, and soils was studied at pH 4.0 to 8.0 (Tables 3, 4, and 5). The oxyanions were added as a mixture at an initial PO4/AsO4 molar ratio of 1 and near their maximum surface coverage. Phosphate and AsO4 strongly competed for the surface sites of the sorbents, but a large variation in competitiveness between these ligands for the sorbents used was observed.
The sorbed AsO4/sorbed PO4 molar ratio (rf) indicated the selectivity of each sample to adsorb preferentially one of the two ligands. The rf values for kaolinite, gibbsite, boehmite, AL-SI allophane (Table 3), and noncrystalline Al hydroxide (R0; Table 5) were usually <0.6, indicating a much greater affinity of PO4 than AsO4 for the surfaces of these minerals. The opposite was true for goethite, pyrolusite, birnessite, nontronite, ferruginous smectite (Table 3), and ferrihydrite (R, Table 5); in fact, the rf values for these samples were usually >1. For montmorillonite, vermiculite, and illite, the rf values ranged from 0.80 to 0.97, indicating that the affinity of PO4 for these minerals is only slighter greater than that of AsO4.
Competitiveness between the anions also changed at different pH values. The rf values usually decreased by increasing the pH of the systems, indicating that PO4 inhibits AsO4 sorption on the surfaces of most of the samples used in this study more in neutral and alkaline systems than in acidic systems (Tables 3, 4, and 5). Only for birnessite the rf values increased (from 1.73 to 2.10) by increasing pH. It is particularly interesting to note that the rf values for the AL-SI allophanic sample ranged from 0.48 (pH 4.0) to 0.05 (pH 7.0; Table 3), whereas those for the AL-SI-FE allophanic sample, containing only 6.6% Fe, the rf values were greater and ranged from 0.79 at pH 4.0 to 0.40 and 0.38 at pH 5.0 and 7.0 (data not shown). Finally, some experiments with anatase (TiO2) carried out at pH 
4.0 showed that this oxide-sorbed AsO4 more selectively than PO4 (rf values >1.5 at pH 4.0; data not shown). Fordham and Norrish (1979)(1983) found that in several acidic soils, AsO4 was retained mainly by Fe and Ti oxides. Titanium oxides competed with Fe oxides for AsO4 and were able to dominate uptake when Fe oxides were removed chemically.
The rf values for the clay minerals usually increased according to the following sequence:
AL-SI allophane < gibbsite 
noncrystalline Al precipitation product (R0) kaolinite < boehmite < illite 
montmorillonite < AL-SI-FE allophane < nontronite < ferruginous smectite goethite ferrihydrite (R) < pyrolusite < anatase birnessite.
Our findings clearly showed that Fe, Mn, and Ti oxides and phyllosilicates particularly rich in Fe (nontronite, ferruginous smectites vs. montmorillonite, illite, vermiculite, and kaolinite) were more effective in sorbing AsO4 than PO4. In contrast, minerals richer in Al (gibbsite, boehmite, noncrystalline Al precipitation products, allophane, kaolinite) were much more effective in retaining PO4 than AsO4. The organo-mineral complexes also showed a similar behavior. In fact, the rf values for the organo-mineral complexes containing Al [Al(OH) x-TAN and Al(OH)x-TAR] ranged from 0.22 to 0.53, whereas those of the Fe(OH)x-POL complex were greater (0.83–0.86; Table 3).
The rf values for soil samples, were always <1.0, ranging from 0.79 for Rhodoxeralf-1 to 0.37 for Andisol-8 at pH 4.0 (Table 4). The soils rich in kaolinite, halloysite, allophane, and organic C (Andisols, Ultisol, and Rhodoxeralf-3) usually showed rf values particularly low (say <0.50). However, rf values were, particularly at pH 
5.0, greater than those determined for kaolinite, synthetic allophane, or gibbsite (Table 3), probably because of the presence in soil samples of illite, vermiculite, and oxalate- or dithionite-soluble Fe (Table 2). Manning and Goldberg (1997) found that the soils containing relatively high citrate-dithionite extractable Fe and clay content had the greatest affinity for AsO4. Smith et al. (1998) showed the same behavior by four Australian soils.
The low rf values (0.37–0.45) ascertained for the Andisol-1, Andisol-8, Ultisol, and Oxisol, characterized by relatively high amounts of Feo and Fed, could be attributed, at least in part, to the presence of organic C. Fordham and Norrish (1983) demonstrated that in soil, Fe-oxide particles associated with organic matter (Fe-organic complexes) sorbed lower quantities of AsO4 and were more reactive after H2O2 treatment. Recently, Grafe et al (2001) demonstrated that dissolved organic C substances may be capable of increasing the mobility of AsO4 in soils containing Fe oxides.
Because many factors (e.g., pH, clay content, nature of clay minerals, organic matter, and chemical composition) may affect the sorption and desorption of AsO4 and PO4 on or from natural soils the competition in sorption of these oxyanions on natural soils deserve closer attention.
Sorption of Arsenate (Phosphate) in the Presence of Increasing Concentrations of Phosphate (Arsenate)
Experiments were carried out on the sorption of AsO4 and PO4 onto goethite and gibbsite in the presence of increasing concentrations of PO4 or AsO4, respectively (Fig. 2)
. The amounts of PO4 and AsO4 sorbed are reported in percentages and are referred to the quantities of AsO4 and PO4 sorbed when added alone (AsO4/PO4 or PO4/AsO4 = 0). The amounts of AsO4 (Fig. 2A) and PO4 (Fig. 2B) added to the sorbents were near at 50% of their maximum surface coverage. The sorption of AsO4 (or PO4) on goethite and gibbsite usually decreased by increasing the initial PO4/AsO4 (or AsO4/PO4) molar ratio, but whereas PO4 inhibited AsO4 sorption more on gibbsite than on goethite (Fig. 2A), conversely, AsO4 prevented PO4 sorption more on goethite than on gibbsite. In fact, at an initial PO4/AsO4 molar ratio of 1.5, 72% of AsO4 was sorbed on goethite and only 54% on gibbsite; on the contrary, at the same AsO4/PO4 molar ratio, 83% of PO4 was sorbed on gibbsite and only 64% on goethite.
|
Competitive Sorption of Arsenate and Phosphate on Mixed Iron-Aluminum Oxides: Effect of Time
The competitive sorption of AsO4 and PO4 was also studied by using synthetic mixed Fe-Al gels, formed at pH 5.5 by coprecipitating Fe and Al at Fe/Al molar ratios of 1, 2, 4, and 10 (referred as R1, R2, R4, R10) and characterized by similar surface area (260–285 m2 g-1) and mineralogy (6-lines ferrihydrites with different percentages of Al isomorphic substitution) but different chemical composition (Al and Fe content; Table 1). Two other oxides, R0 (Fe/Al molar ratio of 0) and R (Fe/Al molar ratio of ), showed the presence of a noncrystalline Al precipitation product and ferrihydrite, respectively.
A variation in competitiveness between PO4 and AsO4 on these samples was determined at pH 5.0, 6.5 (data not shown), and 8.0 (Table 5), in spite of the fact that mixed Fe-Al oxides had similar surface area and mineralogy. The rf values increased with increasing amounts of Fe present in the mixed Fe-Al gels. For example, at pH 8.0 after 24 h of reaction time, rf was 0.38 for R0, 0.47 for R1, 0.69 for R4, and 1.05 for R. The rf value for each sample was usually lower at pH 8.0 than at pH 5.0 or 6.5 (data not shown), strengthening the observation that AsO4 competes with PO4 more in acidic than neutral (Tables 3 and 4) and alkaline systems (Table 5).
Sorption studies conducted from 5 to 96 h showed that AsO4 and PO4 sorption increased with time, but the residence time influenced the rf values, which generally increased with time. Figure 3A reports the amounts of PO4 and AsO4 sorbed at pH 4.0 on the Andisol 16 after 0.02 to 3 h when the anions were added alone or as a mixture. The amounts of PO4 and AsO4 sorbed after 3 h, when added alone, were 2.54 and 3.78 times greater, respectively, than those sorbed after 0.02 h. Arsenate and PO4 sorption continued to increase up to 24 h (data not shown). When added as a mixture, the amounts of PO4 and AsO4 sorbed were 2.18 and 3.68 times greater, respectively, than those sorbed after 0.02 h. In the latter system, the rf values continuously increased with time from 0.25 after 0.02 h to 0.42 after 3 h (Fig. 3B) and then to 0.51 after 24 h (data not shown).
|
Effect of Surface Coverage of the Sorbents on the Competitive Sorption of Arsenate and Phosphate
The surface coverage of the sorbents had an influence on the effectiveness of PO4 to inhibit the sorption of AsO4 and vice versa. Table 6 reports the inhibition of AsO4 sorption on the Andisol-16 at pH 5 in the presence of increasing concentrations of PO4, when the amounts of AsO4 added to the soil sample were respectively at about 35, 70, and 100 surface coverage.
|
 |
At about 35% of surface coverage, AsO4 sorption was no or very poorly inhibited by the presence of increasing concentrations of PO4. Only at an initial AsO4/PO4 molar ratio of 0.5 was AsO4 sorption reduced by 13%. This behavior was attributed to the many sites still being available on the surfaces of the allophanic clays of the Andisol.
By increasing the surface coverage to about 70 to 100%, the effect of PO4 in preventing AsO4 sorption strongly increased because of the greater competition of the ligands for the sorption sites available. At an initial AsO4/PO4 molar ratio of 0.5, the inhibition of PO4 on AsO4 sorption increased to 47 and 67% respectively at surface coverages of about 70 and 100%. As a consequence the rf values decreased by increasing the surface coverages of the oxyanions on the sorbents.
It is well known that there is a continuum between surface complexation (adsorption) and surface precipitation. At low surface coverages surface complexation tends to dominate, but as surface coverage increases nucleation occurs and results in the formation of distinct entities. As surface coverage increases further, surface precipitation become the dominant mechanism (Scheidegger and Sparks, 1996; Sparks, 1999). However, Waychunas et al. (1993) who studied the kinetics and mechanisms of As(V) sorption on ferrihydrite demonstrated that, despite the high concentration of AsO4 in their precipitates, EXAFS spectroscopy showed that neither ferric AsO4 nor any other As-bearing surface precipitate or solid solution was formed.
The findings reported in this work clearly showed that AsO4 has an affinity greater than PO4 for phyllosilicates, metal oxides, and organomineral complexes containing Fe and Mn, the opposite is true for metal oxides containing Al (Tables 3, 4, 5, and 6; Fig. 2 and 3). A complete understanding of the mechanisms of AsO4 and PO4 sorption and desorption on or from soil components and soils is particularly difficult because different processes may concur in these reactions, as (i) the kind of surface complexes formed by the oxyanions when added as a mixture in different amounts, (ii) the change in the surface charge after oxyanions sorption and (iii) the effect of time on competition. In spite of the fact that there are some information on the molecular structure of AsO4 or PO4 adsorbed, when added alone, on metal oxides, soil clays, and soils as affected by pH, nature of sorbent, and surface coverage of oxyanions (Hsia et al., 1994; Lumsdon et al., 1984; Manning and Goldgerg, 1996a,b; Scheidegger and Sparks, 1996; Sun and Doner, 1996; Fendorf et al., 1997; Liu et al., 2001) however, studies which compare the nature of the complexes formed by both these oxyanions when added alone or as a mixture on the same sorbents and at the same pH values and surface coverages are not available.
Recent studies by x-ray adsorption fine structure (EXAFS) and transmission-Fourier transform infrared (T-FTIR)-attenuated total reflectance-FTIR (ATR-FTIR) (Waychunas et al., 1993; Sun and Doner, 1996; Grossl et al., 1997) showed that the strong retention of AsO4 on goethite and ferrihydrite is most likely caused by the formation of binuclear (mainly) and trinuclear complexes with the Fe oxides. Furthermore, Lumsdon et al. (1984) claimed that AsO4 may sorb more strongly on goethite than PO4 because the AsO4 ion is larger in size and interacts more strongly with some of the OH groups that remain on the surface. Our study seems to strengthen these findings that on some metal oxides and soil components, mainly those containing Fe and Mn, AsO4 may form surface complexes relatively stronger than those formed by PO4 (Tables 3, 4, and 5 and Fig. 2).
We have also demonstrated that the extent of competition between oxyanions was also related to sorption kinetics. On different variable charge sorbents, both the oxyanions were capable of slow sorption, but PO4 sorption was initially faster than AsO4 sorption (Table 5 and Fig. 3). However, with time AsO4 sorption increased and consequently its competitiveness with PO4 also increased with time. Probably, an initial reduction in surface charge because of the initial sorption of PO4 and AsO4 may differently reduce the rate of oxyanions sorption which may be responsible for the observed residence time effect. Hypotheses for the slow sorption mechanisms include different site reactivity, diffusion, or surface precipitation (Scheidegger and Sparks, 1996).
Barrow (1992) studied the effects of time on the competition between PO4 and selenite and demonstrated that competitive effects were smallest after brief periods of mixing and increased with time. According to this author because true adsorption would be expected to occur quickly, the competition for adsorption sites was not an important component in his study and was then largely through changes in the electric potential of the surface. Our findings on the effect of surface coverage on the competitive sorption of the oxyanions (Table 6) and the observation that AsO4 and PO4 compete even after a very brief period of reaction (0.02 h Fig. 3) demonstrates that competition for sorption sites was a very important mechanism in our experiments, but certainly reduction in the surface charge of the sorbents concur to explain our results.
A complete understanding of competitive sorption of AsO4 and PO4 (as well as other ligands) may be reached only combining sorption and desorption studies with spectroscopic techniques, surface complexation models and kinetic experiments (Scheidegger and Sparks, 1996).
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
The competitive sorption of AsO4 and PO4 on different soil minerals, organo-mineral complexes and soils may be influenced by the type and stability of surface complexes formed by the oxyanions and, the effect of time on the electrical potential of the surfaces, which may affect the kinetics of sorption of AsO4 and PO4.
The information reported in this work provide to better understand the role of soil components on the mobility and potential toxicity of AsO4 in natural environments.
|
ACKNOWLEDGMENTS |
|---|
Received for publication August 2, 2001.
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
-FeOOH). J. Soil Sci. 35:381–386.This article has been cited by other articles:
|
|
 |
|
  K. Szegedi, D. Vetterlein, H. Nietfeld, R. Jahn, and H.-U. Neue New Tool RhizoMath for Modeling Coupled Transport and Speciation in the Rhizosphere Vadose Zone J., May 27, 2008; 7(2): 712 - 720. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 D. Vetterlein, K. Szegedi, J. Ackermann, J. Mattusch, H.-U. Neue, H. Tanneberg, and R. Jahn Competitive Mobilization of Phosphate and Arsenate Associated with Goethite by Root Activity J. Environ. Qual., October 24, 2007; 36(6): 1811 - 1820. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. Zhang and H. M. Selim Modeling Competitive Arsenate-Phosphate Retention and Transport in Soils: A Multi-Component Multi-Reaction Approach Soil Sci. Soc. Am. J., June 29, 2007; 71(4): 1267 - 1277. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Pigna, G. S. R. Krishnamurti, and A. Violante Kinetics of Arsenate Sorption-Desorption from Metal Oxides: Effect of Residence Time Soil Sci. Soc. Am. J., October 27, 2006; 70(6): 2017 - 2027. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 I. R. Sneddon, H. Garelick, and E. Valsami-Jones An investigation into arsenic(V) removal from aqueous solutions by hydroxylapatite and bone-char Mineralogical Magazine, October 1, 2005; 69(5): 769 - 780. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. Alvarez-Benedi, S. Bolado, I. Cancillo, C. Calvo, and D. Garcia-Sinovas Adsorption-Desorption of Arsenate in Three Spanish Soils Vadose Zone J., May 12, 2005; 4(2): 282 - 290. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Quaghebeur, A. Rate, Z. Rengel, and C. Hinz Desorption Kinetics of Arsenate from Kaolinite as Influenced by pH J. Environ. Qual., March 1, 2005; 34(2): 479 - 486. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 null Feng Xiong Han, X. H. Feng, F. Liu, W. F. Tan, and X. W. Liu SYNTHESIS OF BIRNESSITE FROM THE OXIDATION OF Mn2+ BY O2 IN ALKALI MEDIUM: EFFECTS OF SYNTHESIS CONDITIONS Clays and Clay Minerals, April 1, 2004; 52(2): 240 - 250. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||
