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Soil Science Society of America Journal 64:1616-1622 (2000)
© 2000 Soil Science Society of America

DIVISION S-2-SOIL CHEMISTRY

Effectiveness of Phosphate and Hydroxide for Desorption of Arsenic and Selenium Species from Iron Oxides

Brian P. Jacksona and W.P. Millerb

a Advanced Analytical Center for Environ. Sci., Savannah River Ecology Lab., Univ. of Georgia, Aiken, SC 29803 USA
b Dep. of Crop and Soil Sci., Univ. of Georgia, Athens, GA 30602 USA

jackson{at}srel.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Phosphate and OH- are often used for the extraction of As and Se from soils, either as single extractants or as part of a sequential extraction scheme. However, the recovery of As and Se species and the integrity of the resulting solution speciation merit investigation. In this study the relative effectiveness of PO4 at 0.1 and 0.5 M and pH values of 3 and 6.7 and 0.1 M OH- to extract As(III), As(V), dimethylarsinic acid (DMA), monomethylarsonic acid (MMA), p-arsanilic acid (p-ASA), roxarsone (ROX), Se(IV) and Se(VI) sorbed to goethite and an amorphous Fe oxide were compared, and the speciation in the resulting extract was determined. The extent to which 0.1 M PO4 added to 0.25 M NH2OH·HCl or 0.175 M Na oxalate/0.1 M oxalic acid prevents readsorption of As(V) or Se(IV) to goethite during the dissolution of an amorphous Fe oxide was also assessed. Hydroxide was the most effective extractant for desorption of all species except As(III) from both oxide surfaces. Arsenite was extracted most efficiently by 0.5 M PO4 at low pH; however, amorphous Fe oxide exhibited a strong affinity for As(III) with a maximum of 18% of As(III) extracted by 0.5 M PO4 at pH 2.8. Partial oxidation of As(III) to As(V) occurred in all extractions where an Fe oxide solid phase was present, but only in the hydroxide extract in the absence of a Fe solid phase. Addition of 0.1 M PO4 to extractants used for the dissolution of the amorphous Fe oxide prevented the readsorption of As(V) and Se(IV) to goethite.

Abbreviations: DMA, dimethylarsinic acid • EXAFS, extended x-ray absorption fine structure • FTIR, Fourier transform infared • IC, ion chromatography • ICP–MS, inductively coupled plasma mass spectrometry • MMA, monomethylarsonic acid • p-ASA, p-arsanilic acid • ROX, roxarsone


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
ARSENIC AND SELENIUM are potentially toxic trace elements that are present in all soils at trace levels but can reach hazardous concentrations in soils contaminated by either anthropogenic or natural processes. The mobility and availability of As and Se are determined by the prevailing speciation of these elements and the physical, chemical, and mineralogical properties of the soil. Hence, an assessment of the fate of As and Se in soil systems requires a knowledge of both speciation and the partitioning of these species.

Arsenic and Se exhibit varying oxidation states in natural systems; in oxidized systems As may occur as As(V) arsenate and As(III) arsenite, as well as the methylated species monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) (O'Neill, 1995), and Se as Se(IV) selenite, Se(VI) selenate, and a number of organo–selenium (-II) compounds. The aryl-organoarsenical compounds p-arsanilic acid (4-aminophenylarsonic acid, p-ASA) and roxarsone (3-nitro-hydroxyphenylarsonic acid, ROX) are approved by the USFDA for use as animal feed additives and are commonly used in both the swine and poultry industries. It has been reported that these organoarsenicals are rapidly excreted with little breakdown to the inorganic species (Andreae, 1986), and, given that both swine and poultry waste are frequently land-applied, it is be expected that these compounds should be present in such waste-impacted soils (Morrison, 1969; Woolson, 1975).

Both As(III) and As(V) are adsorbed to Fe oxide surfaces through inner-sphere mechanisms (Pierce and Moore, 1982; Jain et al.,1999), however, As(V) is reported to be more strongly sorbed to soils and pure mineral phases than As(III) (Bowell,1994). In contrast, Oscarson et al. (1983) found that amorphous Fe oxides adsorbed greater amounts of As(III) than As(V). In any case, it is apparent that amorphous Fe oxides have a great affinity for adsorption of As(III) and As(V) and adsorption maxima of 0.6 and 0.25 molAs mol-1Fe, respectively, have been reported (Raven et al., 1998). Although obviously dependent on the mineralogy of the soil, maximum As(III) adsorption generally occurs at pH > 8, while maximum As(V) adsorption occurs at pH < 7 (Manning and Goldberg, 1997a; Manning and Goldberg, 1997b, Raven et al., 1998). Arsenic solubility was initially increased under reducing conditions that promoted reductive dissolution of Fe oxides (Masscheleyn et al., 1991). However, the solubility of As(III) and As(V) added to a Pickney sand decreased during 68 d under both saturated and unsaturated conditions (Onken and Adriano, 1997). The methylated As species have been reported to be less strongly sorbed to oxide surfaces than the inorganic forms (Xu, 1991; Bowell, 1994). Extended x-ray absorption fine structure (EXAFS) studies have provided evidence that As(V) adsorbs by an inner-sphere mechanism to goethite (Fendorf et al., 1997) and ferrihydrite (Waychunas et al., 1993), and Fourier transform infared (FTIR) results have suggested inner-sphere adsorption of As(III) to goethite (Sun and Doner, 1996). Selenite is believed to adsorb to oxide surfaces by inner-sphere mechanisms, while Se (VI) is sorbed by outer-sphere mechanisms and is therefore more soluble (Balistrieri and Chao, 1987; Neal et al., 1987). However, EXAFS spectroscopy has provided contradictory evidence of Se(VI) adsorbtion on goethite with different studies indicating both inner-sphere (Manceau and Charlet, 1994) and outer-sphere (Haynes et al., 1987) adsorption.

Variably charged surfaces in general, and the oxyhydroxides of Fe, Al, and Mn in particular, are the primary solid phases that control As and Se solubility in soils (Livesey and Huang, 1981). One approach to assessing the labile fraction of As and Se in soils is to react the solid phase with a specifically adsorbing ligand in order to displace adsorbed As and Se. Both PO4 (Johnson and Barnard, 1979; Chao and Salazone, 1989) and OH- (Gustafsson and Jacks, 1995) have been used to effect this ligand displacement reaction; however, the extent to which either of these ligands can quantitatively desorb As and Se species from amorphous and crystalline phases and the integrity of the resulting speciation has not been widely studied.

Oxidation of As(III) by O2, or Fe oxyhydroxides is thermodynamically favorable, yet oxidation by O2 is kinetically very slow (Cherry et al., 1979), and As(III) stock solutions prepared in this laboratory have not undergone oxidation for periods >1 month. Nevertheless, more rapid oxidation of As(III) by O2 does occur in solutions with pH >9 (Manning and Goldberg, 1997a). Oxidation of As(III) by manganese oxides has been shown to occur by abiotic processes (Oscarson et al., 1980). Similarly, synthetic Fe oxyhydroxides prepared by slow oxidation of Fe(II) salts also rapidly oxidize As(III) (De Vitre et al., 1991).

Another approach to trace element partitioning has been the selected dissolution of a particular solid phase and the apportioning of trace elements in the resulting supernatant to prior sorption by the dissolved phase (e.g., Tessier et al., 1979). Amorphous Fe oxides are of particular importance in this regard, as these solid phases have large surface areas and a high affinity for trace element adsorption. Certain chemical extractants can selectively dissolve poorly ordered Fe oxides in the presence of crystalline oxide phases (Chao and Zhou, 1983). These dissolutions are conducted under conditions of low pH, where readsorption of oxyanions is favored by the positive surface charge of crystalline oxides. Indeed, As(V) and Se(IV) originally adsorbed to an amorphous Fe oxide were 100% readsorbed when the dissolution was conducted in the presence of goethite (Gruebel et al., 1988).

Given the widespread use of ligands such as PO4 or OH- as either single extractants (e.g., Martens and Suarez, 1997) or as part of sequential extraction schemes (e.g., Chao and Sanzolone, 1989) for As and Se extraction from soils and sediments, the extent to which either of these ligands can quantitatively desorb As and Se merits investigation. This study examines the effectiveness of PO4 and OH- for the desorption of As(III), As(V), DMA, MMA, p-ASA, ROX, Se(IV), and Se(VI) sorbed to amorphous Fe oxide and goethite. The feasibility of using these extractants for speciation analysis of desorbed As and Se is examined by assessing the recovery of the particular As or Se species by ion chromatography–inductively coupled plasma mass spectrometry (IC–ICP–MS). The use of PO4 as a competing ligand to prevent readsorption of As(V) and Se(IV) during dissolution of amorphous Fe oxide in the presence of goethite is also examined.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Synthesis of Solid Phases
Amorphous Fe oxide was prepared by the rapid precipitation of a solution of 40 g Fe(NO3)3 salt at pH 7 with 1 M NaOH. The suspension was then poured into 250-mL centrifuge tubes and centrifuged at 2000 rpm for 10 min. The supernatant was decanted, and the amorphous Fe phase was resuspended in deionized water. After 10 min of shaking the suspension was centrifuged, the supernatant decanted, and this washing procedure repeated three times. The conductivity of the supernatant was checked to ensure removal of entrained salts (<10 µS cm-1), after which the Fe phase was kept in suspension. Gravimetric analysis was used to calculate the concentration of Fe oxide in the suspension. Total Fe was determined by dissolving 1 mL of the suspension in 10 mL of concentrated HNO3 and, after appropriate dilution, analyzing by flame atomic absorption spectroscopy. Because amorphous Fe oxide has been shown to increase in crystallinity upon aging (Schewertmann and Taylor,1989) a freshly precipitated batch was used for each experiment. Goethite was prepared according to the method of Schwertmann and Cornell (1991), dialysed, and stored as a suspension. X-ray diffraction was used to confirm the lack of crystallinity of the amorphous sample as well as the identity of goethite. Surface areas of the two Fe oxides were determined by N2 adsorption using a multi-point BET (Brunauer-Emmett-Teller adsorption isotherm). Surface areas were 213 and 41 m2 g-1 for the amorphous Fe oxide and goethite, respectively.

Arsenic and Selenium Solutions
Sodium arsenite (Fisher Scientific, Fair Lawn, NJ), sodium arsenate heptahydrate (Sigma, St. Louis, MO), sodium cacodylate (Fisher Scientific), p-arsanilic acid (Sigma), roxarsone (Aldrich, Milwaukee), calcium selenate (Cerac, Milwaukee), and selenous acid (Fisher Scientific) were used to prepare 1000 mg L-1 stock solutions (as As or Se) of arsenite, arsenate, DMA, p-ASA, ROX, selenate, and selenite, respectively. Monomethylarsonic acid (Crescent Chemicals, Hauppauge, NY) was obtained as 100 mg L-1 solution in methanol. All solutions were prepared in 18.3 {Omega}M deionized water.

Ligand Displacement Experiments
A 5 g L-1 suspension of amorphous Fe oxide or goethite was prepared in 0.02 M KNO3. The pH of this suspension was adjusted to pH 6, as a survey of the literature indicated that a pH range of 6 to 7 was a reasonable compromise to allow for maximum adsorption of the eight As and Se species. Ten-milliliter aliquots of the suspension were withdrawn by syringe and placed in 15-mL centrifuge tubes. The appropriate As or Se species (0.05 mL of a 50 mg L-1 As or Se stock solution; except 100 mg L-1 as MMA) was added to the suspension, resulting in 50 µg (As or Se species) g-1 Fe oxide. The tubes were shaken for 16 h on a reciprocating shaker, centrifuged at 2000 rpm for 10 min, and the supernatant was decanted, filtered (0.22 µm), and retained for ICP–MS analysis of non-adsorbed As and Se species. The pH of the supernatants was also measured at this time and was found to be unchanged from the original pH of 6. Although specific adsorption of oxyanions can lead to pH increases through OH- exchange at the solid surface, the low concentrations of As and Se species used in this experiment meant that this pH increase was not detected. The volume of entrained solution after decanting the supernatant was estimated by the difference in tube weight compared to the initial tube weight (accounting for the mass of the solid phase), and subsequent extraction results were corrected for this dilution.

Five extracting solutions were prepared: (i) 0.1 M PO4 at pH 3 was prepared from a 9:1 mixture of 0.1 M NaH2PO4·H2O and 0.1 M H3PO4, respectively; (ii) 0.1 M PO4 at pH 6.7 from a 1:1 mixture of 0.1 M NaH2PO4·H2O and 0.1 M Na2HPO4; (iii) 0.5 M PO4 at pH 2.8 from a 9:1 mixture of 0.5 M NaH2PO4·H2O and 0.5 M H3PO4, respectively; (iv) 0.5 M PO4 at pH 6.7 from a 1:1 mixture of 0.5 M NaH2PO4·H2O and 0.5 M Na2HPO4 and (v) 0.1 M NaOH. Ten milliliters of the appropriate extracting solution were added to the solid phase, which was then resuspended by vortexing, and the suspension was shaken for 4 h and then centrifuged at 2000 rpm for 20 min. A 5-mL aliquot of extracted solution was filtered (0.22 µm) and analyzed by ICP–MS for extracted As and Se and for dissolved Fe. An unfiltered aliquot was also preserved for ICP–MS analysis to assess the efficiency of the centrifugation process by comparing filtered and unfiltered Fe concentrations. The Fe solids were well flocculated in the extractant solutions and there was little difference between filtered and unfiltered Fe concentrations, which were <1% total added Fe in all solutions. Speciation of As and Se in the resultant extracts was examined using ion chromatography coupled to ICP–MS (IC–ICP–MS); details of the methodologies have been published elsewhere (Jackson and Miller, 1998; Jackson and Miller, 1999). Extraction solutions without an Fe solid phase were also spiked with As(III) and treated in an equivalent manner to the solid phase extractions prior to IC–ICP–MS analysis to assess homogenous oxidation of As(III) in these solutions.

Readsorption of As(V) and Se(IV) during Dissolution of an Amorphous Fe Phase
In this experiment 0.1 M PO4 was added to two reagents commonly used for the dissolution of amorphous Fe oxides to test its effect on preventing re-adsorption of As(V) and Se(IV) to a crystalline Fe solid phase (goethite). One liter (0.01 M NaNO3) of amorphous Fe suspension containing 2.9 g Fe oxide was prepared. A 3 g L-1 goethite suspension was similarly prepared. The suspensions were adjusted to pH 7 ± 0.2, and 2-mL 100 mg L-1 As(V) and Se(IV) solutions were added to each suspension under rapid stirring. Thirty-milliliter aliquots were withdrawn by syringe and placed in 50-mL centrifuge tubes that were then further shaken for 4 h, then the suspensions were centrifuged at 2000 rpm, after which 20 mL of the supernatant was withdrawn by syringe, filtered, and retained for ICP–MS analysis of As, Se and Fe.

A further 5 mL of supernatant was withdrawn from half of the amorphous Fe replicates, and 5 mL of a 20 g L-1 goethite solution (with no sorbed As or Se species) was added. The reason for adding goethite to half the amorphous suspensions was to assess the extent of readsorption of As(V) and Se(IV) to this crystalline phase upon the dissolution of the amorphous phase. This resulted in three treatments: amorphous Fe with adsorbed As(V) and Se(IV); goethite with adsorbed As(V) and Se(IV); and amorphous Fe with adsorbed As(V), Se(IV), and 100mg of added goethite. For each of these treatments two methods commonly used for the dissolution of amorphous Fe oxides were performed; namely, the reductive dissolution using hydroxylamine hydrochloride at 50°C for 30 min (Chao and Zhou, 1983) and oxalate–oxalic acid in the dark for 2 h (Tamm's reagent). One liter of 0.25 M NH2OH·HCl in 0.25 M HCl was prepared, and to 500 mL of this solution 6.05g NaH2PO4 was added to give a solution that was also 0.1 M PO4. One liter of 0.1 M oxalic acid/0.175 M Na oxalate was also prepared, and 6.05g NaH2PO4 was added to 500 mL of this solution to give a background concentration of 0.1 M PO4. Dissolution of the amorphous Fe oxide phase was carried out both with and without 0.1 M PO4.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Desorption of Adsorbed Arsenic and Selenium Species
A change in pH of extractant affects the surface charge of the Fe oxide and the extent of dissociation of PO4 and As and Se oxyanions. The point of zero charge for amorphous Fe oxide and goethite have been reported as approximately pH 8 (Dzombak and Morel, 1990; Sposito, 1984), and the dissociation constants of the As, Se species (except ROX), and PO4 used in this study are given in Table 1 . All the As and Se oxyanions species except As(III) are dissociated to some extent at the pH of adsorption (pH 6). At the pH 3 extraction, the main phosphate species is H2PO-4, and at pH 6.7 H2PO-4 and HPO2-4 are present at almost equivalent concentrations. The surface site densities of the Fe oxides used in this experiment have been estimated at 2.25 x 10-3 mol sites g-1 for a freshly precipitated amorphous Fe oxide (Dzombak and Morell, 1990) and 9.1 x 10-4 mol sites g-1 for goethite (Greubel et al., 1988), the greater site density of the amorphous phase being a consequence of the greater surface area per gram of the amorphous Fe oxide compared to crystalline goethite. Arsenite, As(V), Se(IV), Se(VI), MMA, DMA, p-ASA, and ROX were added at concentrations of {approx}6.5 x 10-7 M (As or Se) g-1 solid phase (2.5 µg as As or Se species added to 50 mg solid), which we consider a relatively low surface loading. No As or Se was detected in the supernatant after equilibration of the As and Se species with the amorphous Fe oxide. For goethite both DMA (0.7% of total) and Se(VI) (27% of total) were detected in the supernatant, and the results on extraction efficiency below are based on the sorbed fraction of these species.


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  		Table 1 Dissociation constants of P, As, and Se species.{dagger}

 
Extraction efficiencies for desorption of As and Se species from amorphous Fe oxide and goethite are shown in Fig. 1A and 1B , respectively. Although the total recovery of any one As or Se species differed between the two oxide surfaces, the relative effectiveness of any one of the five extracting solutions for desorption of a particular As or Se species from either oxide was very similar. Selenite was a minor exception to this observation in that 0.5 M PO4 at either pH 3 or pH 6.5 extracted {approx}70% of Se(IV) from goethite, whereas PO4 at pH 6.7 was much more effective than pH 3 at either molarity in extracting Se(IV) from the amorphous Fe oxide.



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  		Fig. 1 Effectiveness of various extraction solutions for desorption of As and Se species from (A) amorphous Fe oxides and (B) geothite. Error bars are standard deviations;

 
With the exception of As(III), 0.1 M OH- was generally the most effective extractant with >70% extraction efficiency for As(V), DMA, p-ASA, ROX, Se(IV), and Se(VI) from the amorphous Fe oxide and >70% extraction efficiency for DMA, MMA, p-ASA, ROX, Se(IV), and Se(VI) from goethite. The effectiveness of this extractant results from ligand exchange by OH-, with readsorption being disfavored by the resultant negative surface charge at the oxide surface. Adsorption studies of As(III) and As(V) on an amorphous Fe oxide have also shown that adsorption of As(V) rapidly decreases at pH > 9, while As(III) was strongly adsorbed through pH 11 (Raven et al.,1998). None of the extractants tested was applicable for all the As and Se species examined, and only OH- approached 80 to 100% recovery for most of these species.

Arsenite was preferentially extracted at low pH with 0.5 M PO4 being the most effective extractant of As(III) from both oxides. Arsenite, and to a lesser extent MMA, were poorly extracted from the amorphous Fe oxide with a maximum of 18% of As(III) and 59% of MMA extracted by 0.5 M PO4 (Fig. 1A). High affinities of As(III) for amorphous Fe oxides have been reported for pure solid phases (Raven et al., 1998) and soils (Manning and Goldberg, 1997b). This high affinity of As(III) for the amorphous Fe oxide surface may be indicative of high energy sites with some degree of specificity for As(III).

The relative effectiveness of the five different solutions for extracting p-ASA or ROX from a given solid phase was almost identical. These two organoarsenicals both contain an As(V) functional group bound to a benzene ring through a As–C bond, and they differ in that p-ASA has an NH2 functional group in the para- position, while ROX has a NO2 group in the meta- position and a OH group in the para- position. Given the similarities in the desorption of these two species it seems that the substitution of the benzene ring [other than the AsO(OH)2 functional group] does not affect their adsorption properties to Fe oxides.

None of the extractants dissolved either oxide to great extent, with the greatest dissolution occurring for 0.5 M PO4 extraction of goethite at pH 2.8, in which 1% of total Fe was solubilized. The 0.1 M PO4 extraction of the amorphous Fe phase lead to dispersion of the solid phase, and there was significantly more Fe in the unfiltered extract than the filtered extract; however, even in this case the suspended Fe was < 0.3 % of total Fe. Extractions of the amorphous Fe oxide with 0.1 M and 0.5 M PO4 at pH 3 and 2.8, respectively, raised the pH of the suspension to 5.33 and 3.3, respectively, whereas no such change in pH was observed for goethite. Hence, the similarity in extraction efficiencies for the two 0.1 M PO4 treatments of the amorphous Fe oxide is a result of the similarity in the final extraction pH values of the two treatments. The increase in pH arises from ligand exchange of PO4 for OH- at the oxide surface, and the increase in pH for the amorphous Fe oxide (but not for goethite) is a result of the greater surface area per gram for the amorphous phase.

The speciation of desorbed As in the ligand displacement extracts of As(III) sorbed to the oxide surfaces was determined by IC–ICP–MS; the results are presented in Table 2 . Arsenic (V) was detected in all the extractants where As(III) had been initially sorbed to the solid phase. By contrast, no oxidation of As(III) to As(V) was detected in the PO4 extractants when no solid phase was present. Homogenous oxidation of As(III) did occur in the 0.1 M OH- extractant in the absence of an Fe oxide solid phase, in agreement with previous reports (Manning and Goldberg, 1997a). However, with goethite present, a greater proportion of As(III) was oxidized, with the major species in the extracted solution being As(V). Oxidation of As(III) was also detected in all the amorphous Fe oxide treatments, but it should be noted that the total amount of As desorbed from the amorphous solid was much lower than for goethite. Hence, although 100% of As desorbed from the amorphous Fe solid by OH- was present as As(V), this corresponded to 0.1 µg As, while for goethite 60% of desorbed As was oxidized to As(V) in the OH- extraction, corresponding to 0.41 µg As . These results indicate the involvement of the Fe oxide surface in the oxidation of As(III). Another study has reported oxidation of As(III) sorbed to diagenetic Fe oxyhydroxides, but in this case As speciation was determined after dissolution of the Fe phase (De Vitre et al., 1991). Speciation of the other As and Se species was unchanged in the extracts, except for MMA, where a small unidentified peak occurred along with MMA in the OH- extract, and for p-ASA, where the peak was shifted to a longer retention time in the OH- extract.


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  		Table 2 Speciation of desorbed As in the extracts of As(III) expressed as a percentage of the total extractable As

 
Amorphous Fe Oxide Dissolution: Effect of PO4 on Readsorption of As and Se to Crystalline Phases
Amorphous Fe oxides have a high affinity for adsorption of trace elements because of their large surface area and the reactivity of surface functional groups. Changes in pH and pE can affect the solubility of these amorphous phases; and adsorbed trace elements may, therefore, also be solubilized upon dissolution. As a result of their reactivity, selective extractants have been developed to specifically extract these amorphous phases from soil and sediment systems. Because of the acidic nature of these selective extractants, the readsorption of liberated trace cations to existing crystalline phases within the system is not favored. However, adsorption of oxyanions such as As and Se occurs readily under acid conditions. Greubel et al. (1988) have shown that As(V) and Se(IV) desorbed from an amorphous Fe solid phase are completely readsorbed when the dissolution is conducted in the presence of goethite.

This experiment examined the use of 0.1 M PO4 to prevent readsorption of As(V) and Se(IV) to goethite during the dissolution of an amorphous Fe phase. Hence As and Se recovery from the amorphous Fe phase was compared for treatments with and without added goethite. The two most commonly used reagents for dissolution of amorphous Fe oxides, 0.1 M oxalic acid/0.175 M Na oxalate and 0.25 M NH2OH·HCl/0.25 M HCl, were used both with and without 0.1 M PO4. The percent recoveries of As and Se from each solid phase with each extractant are shown in Fig. 2 .



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  		Fig. 2 Recovery of As(V) and Se(IV) with (A) 0.25 M NH2OH·HCl/0.25 M HCl; (B) 0.25 M NH2OH·HCl/0.25 M HCl/0.1 M PO4; (C) 0.1 M oxalic acid/0.175 M Na oxalate; (D) 0.1 M oxalic acid/0.175 M Na oxalate/0.1 M PO4

 
Both extractants completely dissolved the amorphous Fe phase (confirmed by atomic absorption spectroscopy analysis of Fe in the extracts). Similarly, neither extractant dissolved goethite to an appreciable extent during the course of extraction. There was 100% recovery of As and Se from the amorphous Fe phase in the absence of goethite; however, in the presence of goethite, but without PO4 in the extracting solution, substantial readsorption took place. In the hydroxylamine extract (pH <1) readsorption of As and Se was {approx}100% and this observation is in agreement with the results obtained by Greubel et al. (1988). In the oxalate extraction Se was almost completely readsorbed, while 42% of As was recovered in the extract.

The results of the extracts performed in 0.1 M PO4 show the effectiveness of this ligand for preventing readsorption of As and Se at the crystalline oxide surface. One hundred percent of adsorbed As and 70% of adsorbed Se were recovered in the extract of the mixed oxide system. The high affinity of Se for the goethite surface at low pH was evident and resulted in 30% Se readsorption despite the presence of 0.1 M PO4. Obviously, PO4 is nonspecific and will displace adsorbed As and Se from other solid phases during this extraction step. This was evident in the results of the goethite extraction where 40% As, 50% Se, 67% As, and 38% Se were desorbed from the goethite surface with oxalate–PO4 and hydroxylamine–PO4, respectively. Hence, although PO4 can be used to prevent readsorption in the selective dissolution of amorphous Fe oxide, the nonspecificity of PO4 for a particular oxide surface means that As and Se in the resultant extract cannot be ascribed to the amorphous Fe phase.


   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
Even in our relatively simple system none of the extractants tested proved effective in extracting all the As and Se species from goethite and amorphous Fe oxide. Hydroxide was generally the most efficient extractant; however, it was particularly ineffective in extraction of As(III) from the amorphous Fe oxide and As(V) from goethite. In addition, substantial oxidation of As(III) occurred in the OH- extracts both with and without an Fe oxide solid phase present. Phosphate was most effective at 0.5 M when extractions were conducted at pH 3, yet poor recoveries of As(III) and Se(IV) occurred under these conditions. Addition of 0.1 M PO4 to reagents used for specific dissolution of amorphous Fe oxides in the presence of crystalline oxides was effective in preventing readsorption of As(V) and, to a lesser extent, Se(IV) to goethite. However, extractions conducted under these conditions also caused desorption of As(V) and Se(IV) originally sorbed to goethite; thus, in a real system, As and Se solubilized under these conditions could not be apportioned to prior sorption by the amorphous phase.Hayes Roe Brown Hodgson Leckie Parks 1987; Pergantis Heithmar Hinners 1997; Schwertmann Taylor 1989; Xu Allard Grimvall 1991


   ACKNOWLEDGMENTS
 
Manuscript preparation was partially supported by Financial Assistance Award Number DE-FC09-96SR18546 from the US Dep. of Energy to the Univ. of Georgia Research Foundation. The thoughtful comments of two anonymous reviewers are gratefully acknowledged.

Received for publication February 24, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 




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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