Residence Time Effects on Arsenate Adsorption/Desorption Mechanisms on Goethite

and anatase, but it does not sorb significantly to pure clay minerals or soil organic matter (Fordham and NorIn order to make sound decisions regarding arsenate contamination rish, 1979, 1983; Jacobs et al., 1970). Arsenate sorption in soil and water environments, it is necessary to have a thorough understanding of the mechanisms of arsenate sorption and desorption on soils and soil components vs. pH increases until maxiover extended periods. The major objectives of this study were to mum sorption is reached and then sorption decreases determine the effects of aging or residence time on the kinetics of with further pH increase (Goldberg and Glaubig, 1988; arsenate sorption and desorption on goethite, and to combine spectroXu et al., 1988). For example, arsenate sorption on scopic x-ray absorption fine structure (XAFS) and macroscopic studies montmorillonite and kaolinite increased at low pH, disin order to determine sorption and desorption mechanisms over time played a peak near pH 5, and decreased at higher pH at pH 4 and 6. Sorption studies, conducted from 4 min to ≈12 mo, values (Goldberg and Glaubig, 1988). showed that arsenate sorption on goethite increased with time. SorpGoethite (a–FeOOH), the most common iron oxide tion was initially rapid, with over 93% arsenate being sorbed in a 24-h in soils, has double bands of FeO3(OH)3 octahedra period at pH 6. Similar arsenate adsorption behavior was observed which share edges and corners to form 2 by 1 octahedra at pH 4. Analysis of the samples with extended x-ray absorption fine structure (EXAFS) revealed that there exist two distinct atomic shells tunnels (only large enough to accommodate the passage surrounding the adsorbed As. The closest atomic shell was identified of protons) partially bonded by H bonds (Cornell and as an O atom, the next shell out was identified as an Fe atom. The Shwertmann, 1996; Schwertmann and Cornell, 1991; As–Fe bond distance of 3.30 Å, derived from XAFS data, is indicative Sparks, 1995). Goethite exhibits needle-shaped crystals of a bidentate binuclear bond forming between the arsenate atoms with grooves and edges (Sparks, 1995). and the goethite surface. This is in agreement with the findings of Researchers have shown that arsenate is specifically previous researchers. Analysis of the As EXAFS from samples incusorbed onto iron oxides such as goethite through an bated for various periods indicated that the molecular environment inner-sphere complex via a ligand exchange mechanism did not change over time. Complimentary desorption kinetic studies (Fuller et al. 1993; Fendorf et al., 1997; Grossl et al., showed that when aging was increased, there was no significant change 1997; Hsia et al., 1994; Lumsdon et al., 1984; Parfitt, in the amount of arsenate desorbed from goethite by PO32 4 . Initially, desorption was quite rapid with .35% of the total adsorbed As being 1978; Sun and Doner, 1996; Waychunas et al., 1993). desorbed within 24 h at pH 6. After the initial rapid desorption, only Sun and Doner (1996), using Transmission-Fourier a small amount of additional desorption occurred at longer times. A Transform Infrared (T-FTIR) and Attenuated Total significant amount of arsenate remained bound to the goethite after Reflectance-FTIR (ATR-FTIR) spectroscopy, found 5 mo of desorption even though the PO32 4 desorptive solution was that arsenate replaced two singly coordinated surface three times stronger than the initial arsenate sorptive solution. Sulfate OH groups to form binuclear bridging complexes. Lumswas much less effective at promoting arsenate desorption; at pH 6, don et al. (1984), using infrared spectroscopy, discovno more than 2.5% of the total sorbed arsenate desorbed over a 5-mo ered that the HAsO22 4 ion participated in ligand experiod. Desorption results at pH 4 were similar to the desorption change reactions displacing singly coordinated surface behavior at pH 6. The XAFS analyses of PO32 4 desorbed arsenate hydroxl groups to adsorb as a binuclear species. EXAFS samples showed that the molecular environment of the adsorbed arsenate did not change. studies by Fendorf et al. (1997), Waychunas et al. (1993), and Manceau (1995) found that bidentate binuclear complexation was the major bonding mechanism for A a result of natural and anthropogenic sources, arsenate adsorption on goethite. On the basis of a presthe distribution of As is virtually ubiquitous in the sure-jump relaxation study and confirmed by EXAFS, environment. The major oxidation states of As in the Grossl et al. (1997) and Fendorf et al. (1997) demonsoil environment are As(III) (arsenite) or As(V) (arsestrated that arsenate can form three types of surface nate). As(III) can be oxidized in soils by manganese complexes on goethite depending on the surface coveroxides to form As(V) which is the dominant species age level (Fig. 1 and Table 1). At their highest loading under nonreducing conditions in soils. The acid dissocialevel (G 5 mol As/mol Fe; log G 5 22.05), Fendorf et tion constants for H3AsO4 are: pK1 5 3.60, pK2 5 7.25, al. (1997) fit a shell at 3.24 Å dominated with the and pK3 5 12.52 (Whitten et al., 1992). The primary fits being improved significantly by the addition of a sorbent phases for arsenate are hydr(oxides) of Fe and shell at 2.85 and 3.59 Å. Their XAFS data indicated Al (Fordham and Norrish, 1974, 1983; Huang, 1975; that monodentate surface complexes (RAs–Fe 5 3.59 Å) Livesey and Huang, 1981; Pierce and Moore, 1982). dominated at low surface coverages, bidentate mononuArsenate can also sorb to micron-size particles of rutile clear complexes (RAs–Fe 5 2.85 Å) dominated at high surface coverages, and bidentate binuclear complexes Dep. of Geological Sciences, 4044 Derring Hall, Virginia Polytech. (RAs–Fe 5 3.24 Å) dominated at surface coverages near Inst. and State Univ., Blacksburg, VA 24061-0420; Dep. of Plant, Soil, monolayer capacity. Grossl et al. (1997) proposed that, and Entomological Sciences, Univ. of Idaho, Moscow, ID 83844-2339; at extremely low surface coverages, a ligand exchange Dep. of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717-1303. Received 12 Nov. 1999. *Corresponding author (soreilly@ Abbreviations: EXAFS, extended x-ray absorption fine structure; vt.edu). PZSE, point of zero salt effect; RSF, radial structure function; XAFS, x-ray absorption fine structure. Published in Soil Sci. Soc. Am. J. 65:67–77 (2001).