Ferrihydrite adsorbed arsenate

This band is at a lower frequenqf than that of the Si-O stretch in pure Si oxide gel (1080 cm ) probably because the Si-O stretch is influenced by neighbouring Fe atoms. For adsorbed arsenate and selenate on ferrihydrite, Harrison Berkheiser (1982) reported a poorly resolved doublet at 805 and 875 cm , and weak bands at 700 cm and at 820, 880 and 910 cm respectively. These were attributed to splitting of the fundamental v vibration. 

Waychunas, G.A. et al., Surface chemistry of ferrihydrite Part 1. EXAFS studies of the geometry of coprecipitated and adsorbed arsenate, Geochim. Cosmochim. Acta, 57, 2251, 1993. 

Warburton WK (1986) Filtered energy dispersive detector (EDD) arrays superior detectors of EXAFS from very diluted solutions. Nucl Inst Methods A246 541-546 Waychunas GA, Brown GE (1994) Fluoresence yield XANES and EXAFS experiments Applications to highly dilute and surface samples. Adv X-ray Anal 37 607-617 Waychunas GA, Rea BA, Fuller CC, Davis JA (1993) Surface chemistry of ferrihydrite. Part I. EXAFS studies of the geometry of coprecipitated and adsorbed arsenate. Geochim Cosmocliim Acta 57 2251-2269... 

The oxalate method also provides information about the capacity of soils to adsorb certain compounds such as phosphate, arsenate etc. and to supply Fe to the plant root because both are influenced by their ferrihydrite content. In fact, a negative correlation was found between Fe, and the severity of chlorosis of sorghum in calcareous soils of Texas (Loeppert Hallmark, 1985) and FCo has been used for the sandy soils of the Netherlands to predict their capacity to adsorb phosphate and prevent P... 

Fuller et al. (1993) measured desorption rates for As(V) from ferrihydrite, aged for 24 h, as a result of increasing pH. Arsenate was first equilibrated with ferrihydrite for 144 h at pH 8.0 The molar ratio of As(V) adsorbed to Fe in ferrihydrite was 0.10 (Fig. 7). Desorption was initiated by rapidly increasing the pH to 9.0. Within a few hours, the molar ratio of As(V)/Fe had decreased to about 0.08. The rate of desorption then slowed as the rate became limited by diffusion of As(V) from pores within mineral aggregates. Within 96 h, the concentration of As(V) still adsorbed was only about 5% greater than the adsorbed concentration of As(V) determined in a separate adsorption experiment at pH 9.0. Similar desorption behavior for As(V) from ferrihydrite were measured by Fuller and Davis (1989). 

Fig. 4. (A) Adsorption of Cu and Zn at pFl 4.0 on a humic acid-like polymer (POL) added alone (T, Cu , Zn) or as a mixture (VCu adsorbed in the presence of Zn [Cu (-1- Zn)] , Zn adsorbed in the presence of Cu [Zn (+ Cu)]. Dashed curve indicates the adsorption of Cu on the Fe(OH) -POL complex. (B) Adsorption of arsenate (As) at pFl 4.0 onto ferrihydrite, Fe(OH) -POL complex and humic acid-like polymer (POL).

Finally, there is the question of sorption of both arsenic species to mineral phases. Arsenate adsorbs strongly to a number of common minerals, such as ferrihydrite, goethite, chlorite, and alumina, which constrains its mobility in aquifers, soils, and sediments. This is a complicated, pH-dependent phenomenon (36). Phosphate has chemical properties similar to arsenate and is a common anion... 

Figure 8 Schematic representation of three possible microbial mechanisms for mobilization of arsenic oxyanions adsorbed to ferrihydrite surfaces by respiratory reduction. Bottom left Shewanella alga reduces Fe(lll) to Fe(It), thereby releasing As(V) into solution (41). Lower right bacterially reduced electron shuttle molecules pass electrons to solid-phase As(V) and Fe(III) (48). Top Sulfiirospirillum bamesii directly reduces both As(V) and Fe(III) (43).

Arsenate adsorption on ferrihydrite consisted of a period of rapid uptake followed by slow adsorption for at least 8 days 43). The rate of the slow adsorption reaction is considered to be limited by diffusion into the ferrihydrite aggregates. Slow adsorption kinetics similar to those for phosphate are expected for arsenate because of the similar chemistry of these two anions. Arsenate adsorption data adhere to the Elovich kinetic model indicating a diffusion limited reaction. Arsenate desorption rates were much slower than arsenate adsorption rates, also consistent with a diffusion limited process. A model was developed that assumes that 63% of adsorbing sites are located at the exteriors of aggregates and reach arsenate equilibrium rapidly, while 37% of adsorbing sites are located in the interiors of aggregates with access being diffusion limited. 

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