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a Dep. of Plant and Soil Science, College of Agriculture and Natural Resources, Univ. of Delaware, Newark, DE 19717
b Dep. of Crop and Soil Environmental Sciences, College of Agriculture and Life Sciences, Virginia Tech., Blacksburg, VA 24061
c Dep. of Plants, Soils, and Biometeorology, College of Agric., Utah State Univ., Logan, UT 84322
* Corresponding author (eick{at}vt.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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-FeOOH) was investigated in the presence of either a peat humic acid (Hap), a Suwannee River Fulvic Acid (FA) (International Humic Substances Society, St. Paul, MN), or citric acid (CA). Adsorption edges and kinetic experiments were used to examine the effects of equimolar concentrations of organic adsorbates on As adsorption. Adsorption edges were conducted across a pH range of 3 to 11, while the kinetic studies were conducted at pH 6.5 for As (V) and pH 5.0 for As (III). Both Hap and FA decreased As (V) adsorption, while CA had no effect. Humic acid reduced As (V) between pH 6 and 9 by 
27%. Fulvic acid inhibited As (V) adsorption between pH 3 and 8 by a maximum of 17%. Arsenite adsorption was decreased by all three organic acids between pH 3 and 8 in the order of CA > FA Hap. The different pH regions in which Hap and FA decreased As (V) adsorption suggest that more than one functional group on these complex organic polymers may be responsible for binding to the -FeOOH surface. Similarly, the relative surface affinity of the As(III or V) species and that of the competing organic ligand as a function of pH may play a major role in the outcome of As adsorption on -FeOOH. The results of these experiments suggest that DOC substances are capable of increasing the bioavailability of As in soil and water systems in which the dominant solid phase is a crystalline iron oxide.
Abbreviations: -FeOOH, goethite • As, arsenic • As (III), arsenite • As (V), arsenate • CA, citric acid • DOC, dissolved organic C • FA, fulvic acid • Hap, peat humic acid • TOC, total organic C
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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The two most commonly occurring forms of As in the environment are As +5 and As +3, present as the oxyanions arsenate (AsO3-4) and arsenite (AsO3-3), respectively. Arsenite has been found to be the more mobile and toxic species in soil environments (Tamaki and Frankenberger, 1992). Arsenic bioavailability is a function of pH, redox potential, the presence and type of adsorbing surfaces, and microbial populations, which potentially could reduce As to volatile methylated As species (Massecheleyn et al., 1991; McGeehan and Naylor, 1994; Onken and Hossner, 1996). The adsorption mechanisms of As (III) and As (V) have been well documented (Fendorf, 1997; Grossl et al., 1997; Sun and Doner, 1996). Grossl et al. (1997) proposed that the adsorption of arsenate to -FeOOH involved a two-step ligand exchange reaction by which an inner-sphere bidentate surface complex is formed. Sun and Doner (1996) established that As (V) and As (III) replace two singly coordinated surface OH groups on -FeOOH to form binuclear bridging complexes.
The effects of pH and redox potential on the solubility and speciation of As was studied by Masscheleyn et al. (1991) for an As-contaminated aeric ochraqualf. High soil redox potentials (200–500 mV) decreased As solubility, and most of the As was in the form of As (V) (65–98%). Under alkaline or reduced conditions (0–100 mV), the solubility of As increased substantially due to the dissolution of iron oxyhydroxides and release of sorbed and coprecipitated As.
Competitive adsorption between As (V) and other oxyanions on kaolinite, montmorillonite, and illite and oxide minerals has been documented by Manning and Goldberg (1996a)(1996b). They found that phosphate 
adsorption was slightly greater at equal concentrations of PO3-4 and As (V), while As (V) adsorption was greatly reduced when PO3-4 was present at 10 times the concentration of As (V). However, molybdate 
inhibited As (V) adsorption only at a pH value <4, illustrating the importance of pH and oxyanion speciation for specific adsorption.
The bioavailability of As in the environment may also be affected by naturally occurring organic molecules, which may compete with As for sorption to surface sites. While oxyanions such as PO3-4, SO2-4, MoO2-4 have been shown to compete with As (V) for mineral surface sites, there is relatively little information examining the role of DOC on the adsorption behavior of As species (Xu et al., 1988). Xu et al. (1988) observed a reduction in As (V) adsorption on alumina in the presence of FA under varying FA to adsorbent concentration ratios.
Research has demonstrated that adsorption of DOC occurs via multiple mechanisms, including ligand exchange reactions of COOH and phenol/catechol OH functional groups (Gu et al., 1994). While there is very little information available on the effects of DOC on As adsorption, previous research has shown that PO3-4 adsorption onto soil constituent surfaces is decreased in the presence of Haps and FAs, as well as simple aliphatic acids like citrate (Geelhoed et al., 1998; Sibanda and Young, 1986). Sibanda and Young (1986) demonstrated that FAs and Haps strongly compete for adsorption sites with phosphate on -FeOOH and gibbsite, and two tropical soils at pH 4 and pH 7. Fontes et al. (1992) studied Oxisols from Brazil and determined that organic C compounds in the soil had COOH groups in their dissociated form that would sorb to -FeOOH surfaces. Furthermore, these researchers observed a reduction in PO3-4 adsorption on -FeOOH in the presence of Hap. In a similar study, Geelhoed et al. (1998) showed that CA will lower PO3-4 adsorption on -FeOOH at low pH values.
Accordingly, the objective of this research was to determine the effect of Hap, FA, and CA on the adsorption of As (V) and As (III) on -FeOOH surfaces. The DOC compounds and Fe-oxide were chosen because they are commonly found in natural soil and aquatic environments, and specific organic acids were chosen on the basis of their differential size and functional group content.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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-FeOOH used in all experiments was synthesized from reagent-grade Fe(NO3)3 using the method described in Schwertmann and Cornell (1991). We altered the -FeOOH preparation procedure only in one aspect, namely by adding 4 M NaOH in a very slow, drop-wise fashion to achieve a higher specific surface area -FeOOH. Excess salts were removed by electrodialysis until the conductivity of the wash solution was nearly equal to that of distilled, doubly deionized water. All solutions were prepared with distilled, doubly deionized water and contact with glassware was avoided to prevent silica contamination. The clean -FeOOH precipitate was subsequently washed for 1 h with 0.40 M HNO3 to remove any remaining amorphous phases, redialyzed and freeze-dried. x-ray diffraction and thermo-gravimetric analysis were used to verify the identity of the -FeOOH. X-ray diffraction and thermo-gravimetric analyses were diagnostic for -FeOOH and consistent with those present in Schwertmann and Cornell (1991). The specific surface area was 103 m2 g-1, as determined by a five-point Brunauer-Emmett-Teller N2 gas adsorption isotherm method.
Adsorption Edge Studies
Adsorption edge studies were conducted to examine the influence of pH on As and DOC sorption on -FeOOH. The adsorption of As (V) or As (III) and DOCs was examined as a function of pH (3–11) in a background electrolyte solution of 0.01 M NaNO3 at a constant adsorptive (1.0 mM) and adsorbent concentration (2.5g -FeOOH L-1). An appropriate quantity of N2 purged -FeOOH suspended in 0.01 M NaNO3 was added to a 500 mL Teflon-lined, flat-bottomed, water-jacketed reaction vessel (500 mL). The reaction vessel was covered with a removable glass lid containing entry ports for a mechanical stirrer, pH electrode, N2 gas, burette tip, and pipette, and allowed to fully hydrate overnight. The pH was adjusted to 11.00 using a Brinkmann Metrohm 718 Stat Titrino and the dropwise addition of 0.10 M NaOH. Each adsorption edge was kept well-stirred with the aid of a mechanical stirrer (model RZR-2000, Caframo, Wiarton, ON, Canada) spinning at 300 rev. min-1. All experiments were conducted at 298°K ± 0.1°K and 0.101 MPa pressure under a N2 environment to eliminate CO2 influences.
After a minimum hydration period of 12 h, appropriate volumes of adsorptives were added to the -FeOOH suspension. Adsorptives for the adsorption edges were obtained from stock solutions [0.10 M As (V) and 0.10 M As (III)] prepared from sodium salts. All DOC stock solutions were prepared using a background electrolyte of 0.01 M NaNO3. A Hap solution of 0.01 M C was prepared by placing required amounts of freeze-dried Hap in the background electrolyte at pH 8. The background electrolyte was previously purged with N gas for 20 min before the addition of the Hap. While dissolving, the pH was maintained at 8.00, and the system was continuously purged with N gas. The total acidity of the Hap is estimated according to reports from Stevenson (1994) and Navarro et al. (1993) to be 7mmolc g-1 of Hap. On a weight to weight basis, Hap (International Humic Substances Society, St. Paul, MN) contains 56.37% C. Suwannee River Fulvic Acid stock solution was prepared (0.01 M C) in a similar manner at pH 7. The total acidity of FA has been reported to be 13.9 and 14.2 mmolc g-1 (Yates and von Wandruszka, 1999). On a weight to weight basis, FA (International Humic Substances Society, St. Paul, MN) contains 52.31% C. A CA stock solution (0.01 M C) was prepared by dissolving an appropriate quantity of sodium citrate salt in 0.01 M NaNO3 at pH 7. The total acidity of CA can be calculated from its formula weight and the assumption that only COOH are active in the pH range of 3 to 11. The total acidity is then 15.62 mmolc g-1.
Adsorption edges were conducted using three different addition orders for the adsorptives. In the first case, we added the DOC species before the As species, in the second case, we added the As species before the DOC species, and in the last case, we simultaneous added both adsorptives. After the addition of each adsorptive, the pH was titrated to 11.00 and allowed to react for a minimum period of 2 h. A start pH of 11 was chosen to fully account for the reactivity range of all adsorbates used in the experiments. Arsenite has a pKa1 value of 9.29, and Hap and FA have phenol and catechol functional groups with pKa value of 10 (Rubinson and Rubinson, 1998).
The suspension pH was lowered either in half or full pH units from pH 11.00 to 3.00 using the aforementioned pH-stat unit and 0.10 M HNO3. After a 2 h minimum reaction time at each pH, a sample was removed from the reaction vessel. For As (V), As (III), and DOC edges, 12 mL aliquots were removed using a Rainin automated digital pipet and filtered through a 0.10 µm Gelman metrical membrane into previously acid-washed polypropylene test tubes. The 12 mL aliquot was divided into equal 6 mL samples for the analysis of As and total organic C (TOC). For the competitive adsorption edges, aliquots of 18 mL were taken for each adsorptive. The 18 mL aliquots were divided again into equal volumes of 6 mL for As analysis, As (V) speciation, and TOC analysis. Arsenic was measured using a Spectro inductively coupled plasma atomic emission spectrometer (ICP-AES). Humic acid samples were analyzed using a Phoenix 8000 C Analyzer (Tekmar-Dohrmann, Cleveland, OH). All adsorption edges were run in duplicate.
Arsenic Speciation
Arsenate species were measured using a colorimetric assay by Cummings et al. (1999). Samples were mixed with appropriate amounts of 25 mM HCl and reagent mix to give a final volume of 3 mL in a 4-mL plastic cuvette. The reagent mix consists of four equal amounts of potassium antimony tartrate (0.544 g L-1), ammonium molybdate (24 g L-1), sulfuric acid (269.2 mL conc. sulfuric acid L-1), and ascorbic acid (43.2 g L-1). The reagent mix was prepared fresh within 2 h prior to the assay. Arsenate standards of 0, 250, 500, 750, and 1000 µM were prepared from stock solutions of known concentrations. Standards and samples were placed into a water bath at 351 ±1°K for exactly 10 min, immediately removed, and placed in an ice bath for 5 min. Arsenate was speciated using a Beckmann Coulter DU-640 spectrophotometer using a wavelength of 640 nm. Any observed difference between As content established by ICP-AES and the colorimetric technique was considered a reduction of As (V) to As (III). No such observation was made. To determine potential interference of the organic acids, we prepared samples of known As (V) concentration in a background of the respective organic acid, and followed the procedure outlined above. No significant interference could be noted from the presence of Hap, FA, or CA.
Kinetic Studies
Kinetic studies were conducted to examine the influence of time on the competitive adsorption between As and DOC. These studies were conducted at a pH of 6.5 and 5.0 for As (V) and As (III), respectively. We chose pH 6.5 for the As (V) study because at this pH we observed the greatest effects of Hap on As (V) adsorption, while a pH of 5.0 was chosen because, at this pH, we observed strong effects by CA and lesser effects of Hap and FA on As (III) adsorption. We avoided pH values lower than 5.0 to prevent Hap precipitation from solution (Schulthess and Huang, 1991). Equipment conditions and -FeOOH suspension equilibration times were the same as those of the adsorption edge experiments. The DOC was added 2 h prior to the addition of either As (V) or As (III). Sampling commenced 4 min after the addition of the As species, and continued every 4 min during the first 0.5 h, and every 8 min during the second 0.5h. After the first hour, sampling proceeded every 0.5 h. Samples were analyzed in the same manner as samples of adsorption edge experiments.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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-FeOOH are presented (Fig. 1)
as a reference for the relative positions of the individual adsorption edges to each other.
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(Rubinson and Rubinson, 1998). Similar adsorption edges have been observed by Manning and Goldberg (1996b) on -FeOOH and gibbsite. Ligand exchange reactions with surface functional groups result in different surface complexes (e.g., monodentate vs. bidentate), which are also dependent on sorbate surface coverage (Fendorf, 1997). Arsenite adsorbs to -FeOOH surfaces via ligand exchange reactions as well as forming monodentate and bidentate complexes with mostly A-type hydroxyls and H-bonding with C-type (Sun and Doner, 1996).
The adsorption of organic acids is not as well defined, but is assumed to be an intricate interaction involving several mechanisms including ligand exchange reactions, H-bonding, and electrostatic interactions. Humic acid (peat) adsorption was complete by pH 9 and independent of pH below pH 9. Suwannee River Fulvic Acid adsorbed in a similar manner, while CA showed an adsorption maxima around pH 5. The citrate and Hap adsorption edges compare well with adsorption edges obtained by others (Geelhoed et al., 1998; Schulthess and Huang, 1991). The adsorption of FA differed somewhat from that reported in the literature, showing an adsorption maxima at a higher pH value (e.g., pH 9). This may be attributed to differences in initial adsorptive concentrations. Filius et al. (2000) found that FA adsorption maxima on -FeOOH are dependent on initial DOC concentration. At low initial coverage (75 mg FA L-1), the adsorption maximum is reached at pH 9, however with increasing FA concentration, the adsorption maxima are at successively lower pH values.
Arsenate(V)–Dissolved Organic Carbon Adsorption Systems
The variation of the order of ligand addition to the -FeOOH suspension did not result in significant differences of As sorbed. Hence collected values were averaged and their standard deviations were calculated on the basis of an average of five degrees of freedom. Averages and corresponding standard deviations were then plotted in the subsequent graphs as error bars for As (V) and As (III) adsorption edges in the presence of the organic acids (Figs. 2–4)
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-FeOOH by 27% between pH 6 and 9. Fulvic acid increasingly reduced As (V) adsorption with decreasing pH from 8 to 3 with a maximum effect of 17%. Similar results were obtained by Xu et al. (1988) who examined As (V) adsorption on alumina in the presence of FA.
We observed no effect of CA on As (V) adsorption on -FeOOH. This is in contrast to Geelhoed et al. (1998), who observed a reduction in phosphate adsorption on -FeOOH in the presence of CA. These results may be related to differences in adsorbent and adsorptive concentrations used in this study. For example, Geelhoed et al. (1998) showed reduced PO3-4 adsorption at equimolar PO3-4: citrate ratios and a citrate to -FeOOH suspension concentration ratio of 0.0002, while our citrate to -FeOOH concentration ratio was 0.0001, hence lower citrate coverage. Another difference is that we carried out all CA related experiments in the dark to avoid effects of CA induced photo-reduction of -FeOOH, while Geelhoed et al. (1998) did not check iron dissolution as a function of citrate concentration in solution.
The corresponding data for Hap, FA, and CA adsorption on -FeOOH in the presence of As (V) (Fig. 4A–C) showed a strong reduction in the amount of all three organic acids adsorbed on the -FeOOH surface. This may be attributable to weaker, more physical adsorption mechanisms as well as steric effects. Humic acid (peat) adsorption was reduced by 30% between pH 6 and 11. This reduction in adsorption was consistent with the reduction in As (V) adsorption in the presence of Hap (27%). Below pH 6, a marked increase in Hap adsorption was observed. This increase may be attributable to interactions between Hap COOH functional groups and the -FeOOH surface and/or it may be precipitation reactions of the Hap in the increasingly acidic environment. Schulthess and Huang (1991) observed precipitation of Hap in the presence of silica and aluminum oxides at pH values as high as 6, and this precipitation increased with decrease in pH.
Adsorption of FA was inhibited throughout the entire pH range by As (V). A slight increase in the adsorption of FA at pH values <6 may be attributed to increased surface interactions between COOH and the -FeOOH surface. Several researchers have pointed out the importance of COOH functional groups in specific and nonspecific adsorption processes of natural organic matter on variably charged surfaces (hematite, -FeOOH, gibbsite) and the associated increase in adsorption at low pH (Evanko and Dzombak, 1999; Filius et al., 2000; Gu et al., 1994; Kaiser et al., 1997; Varadachari et al., 1997; Parfitt et al., 1977). Citric acid adsorption was strongly inhibited by As (V) between pH 9 and 3, which is consistent with the corresponding As (V) adsorption data. Similar results were observed by Geelhoed et al. (1998) for citrate adsorption on -FeOOH in the presence of PO3-4. In the kinetic experiments at pH 6.5 (Fig. 5A)
, the rate of the initial reaction (first 124 min) for As (V) adsorption on -FeOOH may be controlled by the presence of DOC molecules on the surface, because the initial reaction rate followed the order of As (V) > As (V)FA > As (V)CA > As (V)Hap (Fig. 5A). However, these small differences may also be due to experimental errors. After 124 min, substantially less As (V) was sorbed to the surface in the presence of Hap than in the presence of either FA or CA. Humic acid (peat) reduced As (V) adsorption by 27% (time = 124 min), which is almost identical to the results we observed in adsorption edges.
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-FeOOH suspension is capable of reducing the amount of As (V) adsorbing on the -FeOOH surface. The adsorption edge data of As (V) on -FeOOH in the presence of Hap or FA in the pH range studied indicates that two or more types of organic functional groups may be involved in competitive sorption reactions, or similarly, that the relative acidity of the involved organic functional groups varies considerably. Gu et al. (1994) examined the adsorption of Hap on hematite using Fourier Transformed Infrared Spectroscopy and heat of adsorption. The results indicated that both COOH and phenolic/catecholic OH groups were involved in ligand exchange reactions with the surface. Sibanda and Young (1986) observed a competitive effect of Hap and FA on phosphate sorption to -FeOOH at both pH 7 and pH 4, while the effect at lower pH was greater for both FA and Hap. These researchers attributed the ability of organic acids to increase solution PO3-4 concentrations in part to an unfavorable electrostatic field around the adsorbed Hap or FA molecules, as well as a pH-dependent effect associated with the relative pH regions of the maximum buffering capacity of the organic acids and PO3-4. Xu et al. (1988) argued similarly that FA reduced As (V) adsorption on alumina as result of a predominantly negatively charged surface created by the adsorption of the FA. Fontes et al. (1992) examined the association of microcrystalline -FeOOH and Hap in Brazilian Oxisols and observed that Hap can effectively compete with PO3-4 for adsorption sites. Infrared spectra in this study showed that COOHs were deprotonated and bonding through these groups to the -FeOOH surface was suggested.
Our own observations indicate that similar direct competitive sorption as well as indirect electrostatic effects may be responsible for the observed effects on As (V) adsorption on -FeOOH in the presence of Hap and FA. Furthermore, some steric effects may be in place also, because Hap and FA were able to increase As (V) solution concentration, but CA did not. The minimal effect of FA and CA on the adsorption of As (V) on -FeOOH is unexpected, since researchers have observed competitive sorption effects between PO3-4 and similar organic anions. Moreover, this has been shown to be enhanced by a short chain length and an increase in the density of COOH functional groups (Geelhoed et al., 1998; Schulthess and Huang, 1991; Struthers and Sieling, 1950). Our results are in contrast to the findings of Geelhoed et al. (1998), Schulthess and Huang (1991) and Struthers and Sieling (1950), and may be attributable to differences in adsorptive to adsorbent loading ratios. Struthers and Sieling (1950) observed phosphate precipitation in the presence of various organic acids during the preparation of Fe and Al oxides, and hence differs significantly in its experimental approach from our study. The small effect that we do see in the presence of FA at low pH values demonstrates the importance of COOH functional groups. At low pH values, FA forms polydentate surface bonds, which may effectively compete with As (V) for surface sites (Kaiser et al., 1997). This is in part supported by our measurements, which show a 17% decrease in As (V) adsorption at pH 3 and increase in FA adsorption of 13% across the pH range from 6 to 3.
The effects of Hap and FA on As (V) adsorption on -FeOOH may also be linked to the dominant organic functional groups (COOH or phenolic/catecholic OH groups) and their relative acidity and is overall consistent with solubility characteristics of both Hap and FA. However, a definitive explanation for the reduction in As (V) adsorption in the presence of Hap and/or FA requires further investigation using techniques such as 13C-NMR or IR spectroscopy and actual surface charge measurements to elucidate the exact adsorption mechanism(s) of the organic acids.
As (III)–DOC Adsorption Systems
All three organic acids reduced As (III) adsorption on -FeOOH. Generally, As (III) adsorption was not reduced between pH 8 and 11, however with decreasing pH, the amount of As (III) adsorbing on the -FeOOH surface decreased. At pH 3, CA decreased As (III) adsorption more than FA and Hap (35%, and 11–15%, respectively). Arsenite inhibited the adsorption of Hap, FA, and CA in alkaline pH regions. In contrast to Hap and CA adsorption, FA adsorption on -FeOOH was significantly lower in the presence of As (III). This may again be attributable to the relative distribution of functional groups on the organic acids, or similarly, the relative acidity of their functional groups. Data from the As (V)–Hap and As (III)–Hap systems suggest that Hap may be a weaker acid than FA or CA. This is supported above by reported literature values of the total acidity of the DOC species. Since phenolic and catecholic OHs as well as As (III) have similar pKa values between 9 and 10, greater competition for surface sites would be expected from Hap than from FA or CA at neutral to alkaline pH values. The lack of an As (III) effect on CA adsorption may be more attributable to steric effects than functional group content.
In the kinetic experiments conducted at pH 5.0 (Fig. 5B), we observed an almost identical effect of CA on As (III) adsorption (30% reduction), however, the effects noted earlier by Hap and FA were not reproducible. Again, this may be because of differences in the experimental protocol. Interestingly, the adsorption rate in the presence of FA and Hap is faster than As (III) adsorption alone. However, these small differences may also be due to experimental errors.
There is no published literature available to which we could compare our As (III) adsorption results in the presence of DOCs. Arsenite is a weak acid, whose pKa1 value is 9.29 (Rubinson and Rubinson, 1998), and whose surface activity on -FeOOH is maximized at alkaline pH values (see Fig. 1 or 3). Similarly, maximum adsorption for Hap and FA is also observed at alkaline pH; however, the interactions between the surface and the organic acid (whether ligand exchange, electrostatic attraction, or van der Waals forces) appear to be weaker than those of As (III) and the surface. This is supported by a reduction in Hap (pH 8 and 9), FA (pH 6 to 11), and CA (pH 7 and 8) adsorption in the presence of As (III) (Fig. 4A–4C). As the pH dropped, the surface affinity of the neutral As (III) oxy-acid decreased and the adsorption of Hap, FA, and CA increased (Fig. 3A–3C and 4A–4C). Consequently, As (III) adsorption decreased in the presence of DOCs in the order of acidic strength: CA > Hap FA. Factors of chain length and COOH functional group density (and their relative position to each other) may hence become relevant only when the oxy-acid they compete with for the mineral surface is a weaker acid or as observed, when the buffering capacities of the competing ions lie in significantly different pH regions (Sibanda and Young, 1986). Hence, we observed increased efficacy in reducing As (III) adsorption by CA, but not As (V) adsorption. The role of organic functional groups and their acidity in competitive adsorption processes needs to be further addressed if we are to achieve a thorough understanding of which processes govern a decrease in As (III) adsorption on mineral surfaces.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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-FeOOH was examined using pH adsorption edges and kinetic studies. Arsenate adsorption was reduced by Hap and FA, but not by CA. Arsenite adsorption was reduced by all three organic acids in the order of CA > Hap FA. Adsorption and kinetic data suggest that type, density, and acidity of functional groups on organic polymers and suspension pH may greatly influence the adsorption of As (III and V) on -FeOOH. A reduction of As (III and V) adsorption on -FeOOH, according to our data, occurred in pH regions in which As (III) or As (V) have a reduced surface affinity, while that of the competing organic ligand may be higher. As such, stronger oxy-acids, like As (V), may be out-competed by weaker organic acids like Hap, whose probable abundant phenolic/catecholic OH content may be able to compete for surface sites on -FeOOH. According to other literature, an electrostatically unfavorable field for As (V) around adsorbed organic molecules may be created as well (Sibanda and Young, 1986; Xu et al., 1988). Effects of chain length, COOH group density, and their relative position may not be sufficient to interfere significantly with As (V) adsorption on -FeOOH. The slight effects of FA on As (V) adsorption on -FeOOH may be ascribed, however, to the presence of an abundant carboxyl functional group content, which possibly forms polydentate bonds between the surface and FA. The affinity constant for COOH chemisorption of FA, however, is low, and inner sphere complexes are only formed at low pH values because of strong electrostatic attraction between the fulvate molecule and the positively charged functional groups on -FeOOH (Filius et al., 2000). Since adsorbed As (V) lowers the point of zero charge of variably charged surfaces, such strong electrostatic forces may have been absent, which may prevent most of the carboxylate groups on FA from undergoing ligand exchange reactions with the surface. For weaker acids [e.g., As (III)], increased acidity of the organic acid may increase its surface activity. Additionally, the organic acid's chain length, COOH functional group density, and their relative positioning to each other may enhance the organic acid's competativeness for surface sites with As (III) (Evanko and Dzombak, 1999; Struthers and Sieling, 1950). Our research suggests that naturally ubiquitous organic acids such as Hap, FA, and CA may influence the potential bioavailability of As in natural systems. The ability of DOCs to increase As bioavailability will likely depend on many factors, including the type of DOC species, the oxidation state of As, and the type of mineral surface present. Additional research is necessary to further elucidate reaction mechanisms and to evaluate the effect of DOC on As transport in whole soils. In highly weathered soils, such as the Oxisol and Ultisol orders, the retention of As (V) or As (III) may be reduced in the presence of DOC species.
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ACKNOWLEDGMENTS |
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Received for publication January 16, 2001.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Agronomy Journal | Crop Science | |||
| Journal of Natural Resources and Life Sciences Education |
Vadose Zone Journal | ||||
| Journal of Plant Registrations | Journal of Environmental Quality |
The Plant Genome | |||
