SSSAJ Grow Your Career with SSSA
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (31)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Reynolds, J.G.
Right arrow Articles by Fendorf, S.E.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Reynolds, J.G.
Right arrow Articles by Fendorf, S.E.
Agricola
Right arrow Articles by Reynolds, J.G.
Right arrow Articles by Fendorf, S.E.
Soil Science Society of America Journal 63:1149-1156 (1999)
© 1999 Soil Science Society of America

DIVISION S-2-SOIL CHEMISTRY

Arsenic Sorption in Phosphate-Amended Soils during Flooding and Subsequent Aeration

J.G. Reynoldsa, D.V. Naylorb and S.E. Fendorfc

a Dep. of Agronomy, Pennsylvania State Univ., University Park, PA 16802 USA
b Division of Soil Science, Univ. of Idaho, Moscow, ID 83844-2339 USA
c Geological and Environmental Sciences, Stanford Univ., Palo Alto, CA 94305 USA

jgr6{at}psu.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
Phosphate enhances the mobility of As in well-aerated soils by competing for adsorption sites. Phosphate and As may also coexist in large concentrations in hydric soils, and the influence of P on As in anaerobic systems is largely unknown. To determine the effects of P on As dynamics during a soil flooding and aeration cycle, samples of two soils were amended with Na2HAsO4 and Na2HPO4 and incubated under a N2 atmosphere for 41 d, and then reaerated for 7 d. Subsamples were collected intermittently and dissolved As, Fe, Mn, Ca, S, P, and H3AsO3 concentrations were determined. Arsenic speciation in the soil solids was determined after 14 and 41 d of flooding and then after 13 h of aeration by X-ray absorption near edge structure (XANES) spectroscopy. Arsenic sorption was small under anaerobic conditions, and H2PO-4 additions enhanced As(V) reduction rate in both soils and slightly suppressed As sorption in one soil. Arsenopyrite (FeAsS) was identified in the soil solids. Rapid and simultaneous As sorption and Fe precipitation occurred during the first 0.25 d of aeration, suggesting that As was retained on freshly precipitated Fe (hydr)oxides. Manganese precipitation and concomitant As sorption occurred after 1 d of aeration. Arsenopyrite was largely destroyed upon aeration but As(III) persisted. Thus, As is partitioned into the solid phase under both aerobic and anaerobic conditions, although more appreciably under the aerobic conditions of this study, and P has little influence on dissolved As during soil flooding–aeration cycles.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Discussion
 REFERENCES
 
ARSENATE , arsenite (H3AsO3), and phosphate are oxyanions that behave similarly in well aerated soils (Wauchope, 1975). Arsenate is in the +5 oxidation state and H3AsO3 is in the +3 oxidation state. Arsenic is toxic to biota (Abernathy, 1993; Marin et al., 1992) and P has been found to enhance its mobility in well aerated soils by competing for sorption sites (Woolson et al., 1973). Many soils with elevated levels of both As and P undergo periodic flooding and draining cycles. Flooding a soil limits O2 availability, which can result in soil redox potential changes (Ponnamperuma, 1972). Soil redox changes greatly alter the form and solubility of As and P in soil (Masscheleyn et al., 1991; Holford and Patrick, 1979). Unfortunately, few empirical studies have investigated how H2PO-4 influences As sorption in hydric soils.

Arsenic sorption is controlled largely by Fe (hydr)oxides, and to a lesser extent Mn (hydr)oxides in aerated soils (Mok and Wai, 1994). When soil O2 and NO-3 are depleted, Mn(IV) and Fe(III) (hydr)oxides are reduced to soluble forms and As sorbed on them is released into solution (Deuel and Swoboda, 1972; Hess and Blanchar, 1977; Masscheleyn et al., 1991; McGeehan and Naylor, 1994). At longer flooding times, however, McGeehan and Naylor (1994) and Onken and Hossner (1995) observed a decrease in dissolved As. McGeehan (1996) and Onken and Adriano (1997) showed that the As that is eventually lost from solution is resorbed to the solid phase. The mechanism responsible for the As sorption at long flooding times in those studies has not been determined.

Arsenic sulfides are sparingly soluble (Webster, 1990), and their precipitation has been predicted on thermodynamic grounds in sulfidic sediments (Moore et al., 1988). Although As may retard SO2-4 reduction (Dowdle et al., 1996), it is evident that As is capable of forming authigenic sulfides in anaerobic soils or sediments. For instance, an authigenic As-sulfide resembling arsenopyrite has been identified by electron microscopy in sediments from the Clark Fork River, Montana (Rittle et al., 1995). In addition, As has been found in pyrite extracts from marine sediments (Belzile and Lebel, 1986). Sulfur additions were found to decrease solution As added as calcium arsenate to paddy soils (Epps and Sturgis, 1939).

Phosphate does not precipitate as a sulfide in soils, but can experience increased sorption under anaerobic conditions relative to aerobic conditions (Khalid et al., 1977). Anaerobic soils tend to buffer the solution concentration of P (Khalid et al., 1977; Shahandeh et al., 1994). The enhanced sorption of P in anaerobic soils suggests the existence of a mechanism besides sulfide precipitation for oxyanion retention. Phosphate is believed to be sorbed by mixed Fe(II)–Fe(III) hydroxides at higher pH and precipitate as ferrous phosphate at low pH (Holford and Patrick, 1979). The relationship between As and P competitive sorption and solution P buffering capacity of anaerobic soils has gone largely unstudied.

Reaeration of flooded soils changes the oxidation state of As and extent of As sorption. Flooding and drying soils enhances the extent of As sorption (Amrhein et al., 1993; McGeehan et al., 1998). Brannon and Patrick (1987) waterlogged 10 As-contaminated sediments for 45 d, reaerated them, then measured the release of As species over six months. They found that the sediments which underwent a pH increase in the first 30 min of aeration also released H3AsO3 into solution during that period. Arsenite was the dominant form of As in solution in all of the sediments after 30 min of aeration (Brannon and Patrick, 1987), but H2AsO-4 was the major species after one month and longer times of aeration. Takamatsu et al. (1982) established that acid-extractable H3AsO3 in flooded rice soils decreased but still endured for at least 1 mo after drainage.

The purpose of this study was to: (i) determine the impact of H2PO-4 on As sorption during flooding and aeration cycles, (ii) probe for As-sulfide precipitation, and (iii) obtain further empirical data on As dynamics during anaerobic soil aeration (Amrhein et al., 1993; Brannon and Patrick, 1987; Hess and Blanchar; 1977).


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Discussion
 REFERENCES
 
Solution Phase Analysis
Crushed (<2 mm), air-dried samples of A horizons of Palouse (a fine-silty, mixed, mesic Pachic Ultic Haploxeroll) and Santa (a coarse-silty, mixed, frigid Ochreptic Fragixeralf) soils were used in this study (Table 1) . Triplicate, 50-g samples were placed in 300-mL glass bottles with 250 mL of an H2AsO-4 -nutrient-electrolyte solution. The solutions contained 50 mg L-1 As added as Na2HAsO4 7H2O, 0.01 M CaCl2, 0.50 g L-1 (NH4)2SO4, and 1.5 g L-1 D-glucose. Separate samples (in triplicate) of the soils were prepared in the same manner except the solution also contained 25 mg L-1 P added as Na2HPO4.


View this table:
[in this window]
[in a new window]
  		Table 1 Selected chemical properties of the soils used in this study

 
Head spaces were purged with N2, and the samples were capped and shaken for 41 d on a reciprocal shaker. The pH of each suspension was measured with a Corning pH electrode, and 10-mL subsamples were taken inside a N2 filled glove box at 1, 3, 6, 10, 16, 24, 31, and 41 d of flooding. The subsamples were centrifuged, filtered with 0.20-µm filters, diluted 1:5 with 0.1% (v/v) ascorbic acid for H3AsO3 preservation (Rochette et al., 1998), and then refrigerated. Total dissolved As, Fe, Mn, S, and P were determined by inductively coupled plasma (ICP) atomic emission spectroscopy (Thermal Jerral Ash, IRIS, ICP Spectrophotometer, Thermo Jarrell Ash Corporation, Franklin, MA). Arsenite was determined in the Santa soil filtrates by hydride generation, where H3AsO3 was selectively reduced by 0.35% NaBOH4 to volatile AsH3 at pH 4.8 in acetate buffer (Hinners, 1980). The AsH3 gas was pumped through a gas liquid separator and then into the ICP for As analysis.

After 41 d of flooding, the bottles were uncapped and the suspensions were stirred on a stir plate so that air was allowed to enter. Subsamples (10 mL) were collected at 0.02, 0.25, 1, 3, 5, and 7 d, filtered, preserved, and analyzed as described above. The 0.02-d (30-min) samples were not centrifuged prior to filtration in order to minimize sampling time.

Mean total As data for both treatments were compared by the least significant difference test (SAS Institute, Inc., 1985).

Solid-Phase Arsenic Speciation
X-ray absorption near edge structure (XANES) spectroscopy can be used to monitor the electron density around trace elements in soils (Fendorf and Sparks, 1996), and has been used to speciate As in soil solids (McGeehan, 1996; Rochette et al., 1998). Crushed (<2-mm), air-dried samples of the A horizon of the Palouse and Santa soils were used in this study. Soil samples (5 g) were suspended in 25 mL of the H2AsO-4-nutrient-electrolyte solution (50 mg L-1 As added as Na2HAsO4 · 7H2O, 0.01 M CaCl2, 0.50 g L-1 (NH4)2SO4, and 1.5 g L-1 D-glucose) in 50-mL plastic centrifuge tubes, with some solutions containing 25 mg L-1 P added as Na2HPO4. One sample of each soil and soil treatment (±P) was purged with N2 and placed in an N2 atmosphere glove box at room temperature for 14 and 41 d. At the end of the incubation period, the solution was decanted and the soils were mounted as wet pastes in a 3- by 5- by 40-mm slot cut in an acrylic plate in an N2 (g) environment. The sample cell was sealed with a Kapton polymide film to prevent oxidation and moisture loss while minimizing X-ray absorption. The samples were refrigerated under N2 gas for 2 d prior to As speciation by XANES spectroscopy. After XANES analysis of anaerobic samples, the polymide film was removed so that air could reenter. The low water content of the aerated XANES samples should have allowed O2 to infiltrate more rapidly than in the soil suspensions. Selected samples were analyzed after 13 h (Santa soil) or 5 and 13 h (Palouse soil) of aeration.

To speciate As in soil solids, XANES spectroscopy was performed on beamline 4-3 (an 8-pole wiggler) at the Stanford Synchrotron Radiation Laboratory (SSRL). The ring operated at 3 GeV with a current ranging from Å 100 mA to Å 50 mA. Energy selection was accomplished with a Si (220) monochromator with an unfocused beam. XANES spectra were recorded by fluorescent X-ray production using a wide-angle ionization chamber for model compounds (Stern and Heald, 1979) and a 13-element Ge detector for soil samples (Cramer et al., 1988). A Ge filter was used with the ionization chamber to limit counting of scattered primary radiation. Incident and transmitted intensities were measured with in-line ionization chambers. XANES spectra were recorded over the energy range of -200 to +500 eV about the K-edge of As (11 867 eV). Each scan was calibrated by placing an elemental foil of As between second and third in-line ionization chambers, with the inflection point of As(0) set to 11 867 eV. Between three and six individual spectra were averaged for each sample.

Spectral backgrounds were subtracted from the averaged spectra by a low-order polynomial function fit to the pre-edge region (approximately -200 to -50 eV relative to the As edge). Spectra were normalized by setting the total atomic cross-sectional absorption to unity. Following the data reduction, first-derivative curves were obtained using a Savitzky-Golay function with smoothing between 3 and 5%. Qualitative identification of As species was accomplished by noting peak positions in the first-derivative curves.

A two-step approach was used for quantitative XANES analysis. First-derivative As-XANES spectra were first de-convoluted by a series of Gaussian-Lorenzian peaks, which described the mixing of true and instrumental line shapes (Frank et al., 1994). Arsenic species were quantified using peak areas of the Gaussian-Lorenzian components and compared with those of the standard compounds, which include As-sulfides (arsenopyrite FeAsS, realgar AsS, and orpiment As2S3), NaAsO2, and Na2HAsO4. Values obtained by the de-convolution method were then refined by combining a linear set of the standard spectra to the unknown using a least-squares routine to minimize the error between the unknown and reconstructed spectrum using the WinXAS code (Ressler, 1998). Because of slight errors in calibration, a constrained 1-eV shift in standard spectra was allowed during fitting. Our approach is similar to that used by others to quantify As, S, Se, and Mn species in complex media using XANES spectroscopy (Fendorf et al., 1999; Foster et al., 1998; Pickering et al., 1995; Vairavamurthy et al., 1994; Waldo et al., 1991).


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Discussion
 REFERENCES
 
The pH of the soil suspensions decreased during the first 3 d of incubation (Fig. 1a) , reaching mean minimums values of 5.0 in the Palouse soil and 4.9 in the Santa soil. The decrease in pH during the early flooding period in soils was likely the result of the biotic transformation of easily degradable organic material to organic acids and CO2 (Motomura, 1962). The pH increased at longer flooding times, and reached a mean maximum of 5.9 after 6 d in the Palouse soil and after 41 d in the Santa soil (Fig. 1a). The pH increase observed in anaerobic soils was likely the result of Mn and Fe oxide reduction (Ponnamperuma, 1972). A large pH increase was observed in the first 30 min of aeration in the Santa suspensions (Fig. 1b). A decrease in pH was then observed for the rest of the first day of aeration for both soils. The pH increased for the remainder of the aeration period and approached neutrality for both soils (Fig. 1b).



View larger version (18K):
[in this window]
[in a new window]
  		Fig. 1 Change in pH as a function of (a) flooding or (b) aeration time. Error bars denote one standard deviation. Santa, no P—{circ}; Santa, with P—{square}; Palouse, no P—{triangleup}; Palouse, with P—{triangledown}

 
Soluble Fe in anaerobic soils is produced from the reduction of Fe oxides and oxyhydroxides (Gotoh and Patrick, 1974). Dissolved Fe concentrations remained relatively small during the first day of flooding in both soils (Fig. 2a) , indicating that Fe reduction was not occurring during this time period. Dissolved Fe levels increased steadily until reaching plateaus after 15 d in the Palouse and 24 d in the Santa soils. The Santa soil maintained the larger concentration of dissolved Fe (228 mg L-1 after 24 d) during flooding. Dissolved Fe concentration rapidly decreased when the soils were aerated, approaching our detection limits by the end of the first day of aeration (Fig. 2b). This rate of Fe loss from solution was similar to what other researchers have observed during the oxidation of soil (Ahmad and Nye, 1990) and natural waters (Davidson and Seed, 1983). Oxidation of Fe(II) to Fe(III) is the most important sink for incoming O2, and Fe(III) is sparingly soluble in the pH range of these soils, so the loss of Fe is likely from precipitation of hydrous oxides (Schwertmann and Taylor, 1989). Large amounts of dissolved Fe(III)-organic complexes have been observed in anaerobic soils only in rare occurrences (Motomura, 1962). The rapid and immediate loss of dissolved Fe in this study is strong evidence that the dissolved Fe was Fe(II) rather than Fe(III) because Fe(III)-organic matter complexes should not dissociate simply because O2 was added to the system.



View larger version (28K):
[in this window]
[in a new window]
  		Fig. 2 Change in dissolved Fe as a function of (a) flooding time and (b) aeration time, as well as dissolved Mn as a function of (c) flooding time and (d) aeration time. Error bars denote one standard deviation. Santa, no P—{circ}; Santa, with P—{square}; Palouse, no P—{triangleup}; Palouse, with P—{triangledown}

 
Solution Mn concentration in the soil suspensions increased during the first 6 to 24 d of flooding and subsequently reached steady-state values (Fig. 2c). Dissolved Mn was assumed to be the free ion or soluble complexes of Mn(II) produced from the reductive dissolution of Mn (hydr)oxides. A decrease in dissolved Mn commenced after 1 d of aeration (Fig. 2d), corresponding to the time after which most of the Fe had precipitated. Loss of aqueous Mn was slow after a rapid drop between 1 and 3 d of aeration (Fig. 2d). Although Mn(II) is soluble in soils, Mn(II) can be oxidized to Mn(III) or Mn(IV) which are sparingly soluble at the neutral pH of these suspensions (McKenzie, 1989); therefore, the loss of Mn from the oxidized solutions was possibly because of the oxidation of Mn(II) and subsequent precipitation.

Under flooded conditions, dissolved P concentrations decreased rapidly in the P amended suspensions before reaching a steady state level (Fig. 3a) . Addition of 25 mg L-1 of P to the flooded soils resulted in 2 mg L-1 increase in dissolved P in the Palouse soil suspensions and in 1 mg L-1 increase in the Santa soil suspensions (Fig. 3a), reflecting the solution P buffering capacity of flooded soils observed by many researchers (Khalid et al., 1977; Shahandeh et al., 1994). Dissolved P decreased during the first 30 min of aeration (Fig. 3b), corresponding to the rapid removal of Fe from solution. Flooding and draining soils can enhance P sorption because P is sorbed on freshly precipitated Fe oxyhydroxides (Mandal, 1979).



View larger version (32K):
[in this window]
[in a new window]
  		Fig. 3 Change in dissolved P as a function of (a) flooding time and (b) aeration time, as well as dissolved S as a function of (c) flooding time and (d) aeration time. Error bars denote one standard deviation. Santa, no P—{circ}; Santa, with P—{square}; Palouse, no P—{triangleup}; Palouse, with P—{triangledown}

 
Dissolved S initially increased but then tended to decrease with flooding time in the soils (Fig. 3c). Dissolved S concentration decreased in the first 30 min of soil aeration, but increased after longer aeration times in the Santa soil (Fig. 3d).

Dissolved As reached an approximate steady state in the Santa soil within 10 d of flooding (Fig. 4a) , consistent with data reported by McGeehan (1996) for this soil. Mean total As concentrations of the P-amended samples were usually larger than the nonamended samples (Fig. 4b) in the Palouse soil. The differences in the means were only statistically significant (least significant difference test) at the P = 0.05 level for the Day 1 measurement. On the basis of the least significant difference test, total dissolved As and H3AsO3 concentrations were not significantly different between treatments in the Santa soil, and the means were often lower in P-amended than in nonamended suspensions. Large deviations occurred among replicates in both soils. Dissolved As concentrations increased during the same time periods that Fe dissolution was occurring (Fig. 2a and 4a,b). This was consistent with the hypothesis that As desorption occurs as a result of ferric hydroxide reduction (Deuel and Swoboda, 1972; Hess and Blanchar, 1977; Masscheleyn et al., 1991; McGeehan and Naylor, 1994).



View larger version (34K):
[in this window]
[in a new window]
  		Fig. 4 Change in total dissolved As (both soils) and H3AsO3 (Santa soil) in the (a) flooded Santa soil, (b) flooded Palouse soil, (c) re-aerated Santa soil, and (d) re-aerated Palouse soil. Error bars denote one standard deviation. Total As, no P—{circ}; Total As, with P—{square}; H3AsO3, no P—{triangleup}; H3AsO3, with P—{triangledown}

 
The Santa soil solution contained less than 2 mg L-1 H3AsO3 at Day 1; however, H3AsO3 increased to 10 mg L-1 by Day 3 of flooding (Fig. 4b). Arsenite accounted for the majority of As in solution by Day 3 of flooding. Assuming that H2AsO-4 was equal to total As minus H3AsO3, some H2AsO-4 persisted throughout the flooding period.

Concentrations of dissolved H3AsO3 and total As decreased rapidly during the first 0.25 d (6 h) of aeration (Fig. 4c, d), concurrent with the rapid loss of Fe from solution. Arsenic is sorbed onto or coprecipitated with Fe oxyhydroxides when present during Fe(II) oxidation and hydrolysis (Fuller et al., 1993; Mok and Wai, 1994). A second period of relatively rapid As and H3AsO3 sorption occurred between 1 and 3 d of soil aeration (Fig. 4c, d), concurrent with the loss of Mn from solution. Phosphate-amended aerated Palouse suspensions had larger mean dissolved As concentration than did the nonamended samples (Fig. 4d). In contrast, P-amended Santa soil suspensions did not maintain larger dissolved As concentrations than did the nonamended suspensions.

Mass fractions of As(III), As(V), and FeAsS in the soil solids are given in Table 2 , the values of which are the result of three to six replicate analysis of one sample for each time and treatment. Realgar (AsS) and orpiment (As2S3) were not observed in the XANES spectra. Arsenopyrite was observed by 14 d of flooding in both soils yet As(V) was present in all samples. Less As(V) was found in the P-amended treatments than nonamended treatments for both soils at all times (Table 2), indicating that P amendments enhanced As(V) reduction. Because of the lower water content, O2 could infiltrate more quickly into the samples prepared for XANES analysis than the suspensions during aeration, and dissolved species were lost. Arsenopyrite was destroyed during the aeration of the Santa XANES samples and largely disappeared in the Palouse XANES samples (Table 2). Arsenite was still present after 13 h of aeration, and constituted about 0.33 of the As in the Palouse and Santa soil solids.


View this table:
[in this window]
[in a new window]
  		Table 2 Mass fraction of H3AsO-4, H3AsO3, and FeAsS in the soil solids determined by XANES spectroscopy. These results are from a single replicate at each time and treatment

 

   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
REFERENCES
 
Flooding Period
In the early stages of flooding when the soil is still aerobic, As sorption is largely controlled by amorphous and crystalline Fe and Al hydroxides (Mok and Wai, 1994). When the soil becomes more reduced, the H2AsO-4 sorbed on Fe (hydr)oxides is released into solution and reduced to H3AsO3 (Deuel and Swaboda, 1972; McGeehan and Naylor, 1994; Rochette et al., 1998). Arsenite may be removed from solution as observed by McGeehan (1996) and McGeehan and Naylor (1994), or remain partially soluble as observed in this study (Fig. 4). McGeehan (1996) and McGeehan and Naylor (1994) observed the disappearance of H3AsO3 and H2AsO-4 from the Santa soil solution within 10 d of flooding. One difference between these studies is that we added less glucose as a microbial stimulant (0.375 g kg-1 versus 1.0 g kg-1) and we added (NH4)2SO4. The suspensions prepared by McGeehan and Naylor (1994) had a lower pH and redox potential than the suspensions generated here (data not shown).

Like As, H2PO-4 sorbed on Fe (hydr)oxides is released into solution when Fe is reduced, and then can resorb onto the solid phase after prolonged flooding times (Quang and Duffey, 1995). Unlike As, P does not form a sulfide phase in soils. Reduced soils are, however, able to buffer the dissolved H2PO-4 concentration, and have a larger P sorption maximum than their oxidized counterparts in some cases (Khalid et al., 1977; Shahandeh et al., 1994). Our H2PO-4 amendments only slightly increased the dissolved P concentration once Fe reduction commenced (Fig. 2a and 3a). This was consistent with the observation that P amendments resulted in a minimal, at most, increase in the dissolved As concentration in the flooded soils (Fig. 4a, b). Nevertheless, a larger P amendment may have a greater influence on dissolved As concentrations.

XANES analysis showed that P amendments decreased the mass fraction of As(V) in the Palouse and Santa soil solids (Table 2), indicating an enhanced As(V) reduction rate. In a study by Dowdle et al. (1996), solution data from anoxic salt marsh sediments showed only limited enhancement of As(V) reduction from P amendments. Our solution data (Fig. 4a) also does not indicate enhanced As(V) reduction rate from P amendments (Fig. 4a); only through our solid-phase data is it apparent that the reduction rate increased (Table 2). These results suggest that solution-phase data alone are not sufficient to describe As redox reactions in soils, because the As may be removed from solution upon reduction. Phosphate can desorb H2AsO-4 (Dean and Rubins, 1947; Peryea, 1991) and the rate of As redox transformations are controlled by sorption–desorption kinetics (Holm et al., 1979).

Aeration Period
Soil properties changed rapidly when aerated. A pH increase was observed in the first 0.02 d (30 min) of aeration of the Santa soil, as was a rapid loss of dissolved Fe, As, P, and S in both soils (Fig. 1, 2, 3, and 4). The oxidation of Fe(II) is the most important sink for incoming O2 during the initial aeration of flooded soils (Reddy et al., 1980). Rapidly oxidized Fe usually precipitates as hydrous ferric oxide, which is an excellent scavenger of oxyanions such as H2AsO-4 and H3AsO3 (Schwertmann and Taylor, 1989). Furthermore, As sorption rates by ferrihydrite are enhanced when present during precipitation from Fe(II) solutions (Fuller et al., 1993). Arsenic and P are usually associated with amorphous Fe oxyhydroxides at the oxic/suboxic interface in sediments and soils (Belzile and Tessier, 1990; Kuo and Mikkelsen, 1979). In addition, As and P solubility is diminished in flooded–dried soils and is correlated with an increase in amorphous Fe oxyhydroxides (McGeehan et al., 1998; Mandal, 1979). Finally, As and Fe loss from solution proceeded simultaneously during the first 0.02 d of aeration. Therefore, we concluded that Fe oxyhydroxides were primarily responsible for the P and As sorbed during at least the first 0.02 d of aeration in these suspensions. Additional As sorption occurred during Fe precipitation between 0.02 and 0.25 d of aeration.

Differences in the concentration of dissolved Fe and As complicated comparison of As sorption and Fe precipitation between soils. The mols of As sorbed per mol of Fe precipitated in the first 0.02 d of aeration for each suspension was estimated from the equation:

(1)
where Asan and Fean are the mean molar dissolved As and Fe concentration at 41 d of flooding, and As0.02 and Fe0.02 are the mean molar dissolved As and Fe concentration at 0.02 d (30 min) of aeration.

Fuller et al. (1993) reported that As:Fe mol ratios of greater than 0.1 and as large as 0.7 coprecipitated during the oxidation and hydrolysis of HAsO2-4-containing alkaline Fe(II) and Fe(III) solutions. In this study, the mol ratio of As to Fe lost from solution was 0.07 for both treatments in the Santa soil, 0.11 for non–P amended and 0.15 for the P amended Palouse suspensions. Thus the quantities of As sorbed because of the oxidation of Fe containing solutions is much smaller in soils than in the simple systems studied by Fuller et al. (1993). The As:Fe mole ratio of the ferrihydrite precipitate reported by Fuller et al. (1993) depended on the pH and As:Fe mole ratio in solution. In soil systems, the quantity of As coprecipitation with Fe can also be expected to depend on the form of As, competing ions, and the form of Fe. Much of the Fe(II) in flooded soils is found in exchangeable forms (Gotoh and Patrick, 1974), or complexed by dissolved organic matter (Motomura, 1962; Olomu et al., 1973). Presumably the oxidation and hydrolysis of exchangeable Fe is responsible for sorbing some of the As in the reaerated soils. On the other hand, Fe complexed by organic matter would be expected to be less effective at sorbing As during aeration, as it is for P (Koenings and Hooper, 1976).

Between 1 and 3 d, As sorption occurred concomitant with the loss of Mn from solution (Fig. 4b, c). While a number of mechanisms could explain the loss of Mn and As from solution during this time period, one likely mechanism is that the Mn(II) was oxidized and precipitated as a (hydr)oxide phase which enhanced As sorption. Arsenic has been found to be associated with Mn hydrous oxides in lake sediments (Takamatsu et al., 1985). Arsenite can be adsorbed and oxidized by Mn (hydr)oxides, and cryptomelane ({alpha}-MnO2) as well as pyrolusite (ß-MnO2) can adsorb H2AsO-4 (Oscarson et al., 1983a).

Other researchers have found H3AsO3 to persist for days (Questel and Scholefield, 1953) to weeks (Takamatsu et al., 1982) in well-aerated soils. In this study, H3AsO3 persisted for at least 7 d in the aerated suspensions. This is a short time relative to the rate of H3AsO3 oxidation by Fe (hydr)oxides or O2 at the pH of these suspensions, but long relative to Mn (hydr)oxide catalyzed oxidation of arsenite (Oscarson et al., 1981; Sun and Doner, 1998; Tallman and Shaikh, 1980). The time difference between Fe and Mn precipitation in these suspensions gave Fe the first opportunity to sorb As when aeration commenced. In turn, Fe (hydr)oxides inhibit the oxidation of H3AsO3 by Mn (hydr)oxides (Oscarson et al., 1983b). Consequently, the sluggish kinetics of Mn(II) oxidation is a contributing factor to H3AsO3 persistence in reaerated soils.

From another perspective, considerable As(V) was produced in just 13 h in the solids of both soils during aeration of the XANES samples, with essentially all of the oxidation occurring within 5 h in the Palouse soil (Table 2). This oxidation rate is fast when compared with oxidation by O2 or ferric (hydr)oxide catalysis at neutral pH, and more consistent with catalysis by Mn (hydr)oxides (Oscarson et al., 1981; Sun and Doner, 1998; Tallman and Shaikh, 1980) or oxidation in the surface reaction layer of FeAsS (Nesbitt et al., 1995). The mechanism of As oxidation could not be deduced from our data in either the XANES samples or the soil suspensions. The soil solution was decanted from the XANES samples prior to aeration. Consequently, most of the soluble components were lost from the XANES samples and the aerated XANES samples were not equivalent to the aerated soil suspensions.

Because of the conversion of crystalline Fe to amorphous Fe, both P and As sorption is enhanced in well aerated soils if the soils are previously subjected to flooding and aeration cycles (Mandal, 1979; McGeehan et al., 1998). In this study, P slightly depressed As sorption in the reaerated Palouse samples even though little dissolved P was maintained (Fig. 3 and 4). The enhancement of As sorption through flooding and aeration cycles may be complicated by conversion of As(V) to As(III), which is increased by P amendments. The data presented here, and by others (Brannon and Patrick, 1987; Questel and Scholefield, 1953; Takamatsu et al., 1982), indicate that H3AsO3 can persist in well-aerated soils. Arsenite has a tendency to be less strongly sorbed to soil (Holm et al., 1979; Keaton and Kardos, 1940; McGeehan et al., 1992) and is more toxic than H2AsO-4 to both humans and plants (Abernathy, 1993; Marin et al., 1992). While temporary flooding enhances As sorption in well-aerated soils (McGeehan et al., 1998), consideration for the conversion of As(V) to As(III) needs to be considered before subjecting As-contaminated soils to flooding and aeration cycles. Reaerated soils may have an enhanced sorption capacity for As, but the transition from aerobic to anaerobic or anaerobic to aerobic may lead to pulses of elevated dissolved As concentrations because different retention mechanisms are operational at each end point.Quastel Scholefield 1953


   ACKNOWLEDGMENTS
 
We gratefully acknowledge the laboratory advice and assistance of Drs. Beth Rochette, Guang Chao Li, and Steve McGeehan. We gratefully acknowledge support of this research provided by the USDA-NRI competitive grants program (grant number 9501364).

Received for publication December 23, 1997.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 




This article has been cited by other articles:


Home page
 J. Environ. Qual.Home page
P. M. Fox and H. E. Doner
Accumulation, Release, and Solubility of Arsenic, Molybdenum, and Vanadium in Wetland Sediments
J. Environ. Qual., November 1, 2003; 32(6): 2428 - 2435.
[Abstract] [Full Text] [PDF]

Home page
 J. Environ. Qual.Home page
L. E. Williams, M. O. Barnett, T. A. Kramer, and J. G. Melville
Adsorption and Transport of Arsenic(V) in Experimental Subsurface Systems
J. Environ. Qual., May 1, 2003; 32(3): 841 - 850.
[Abstract] [Full Text] [PDF]

Home page
 J. Environ. Qual.Home page
P. M. Fox and H. E. Doner
Trace Element Retention and Release on Minerals and Soil in a Constructed Wetland
J. Environ. Qual., January 1, 2002; 31(1): 331 - 338.
[Abstract] [Full Text] [PDF]

Home page
 Soil Sci.Home page
S. Beauchemin, D. Hesterberg, and M. Beauchemin
Principal Component Analysis Approach for Modeling Sulfur K-XANES Spectra of Humic Acids
Soil Sci. Soc. Am. J., January 1, 2002; 66(1): 83 - 91.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (31)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Reynolds, J.G.
Right arrow Articles by Fendorf, S.E.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Reynolds, J.G.
Right arrow Articles by Fendorf, S.E.
Agricola
Right arrow Articles by Reynolds, J.G.
Right arrow Articles by Fendorf, S.E.

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