Adsorption of Antimonate by Gibbsite: Reversibility and the Competitive Effects of Phosphate and Sulfate

Abstract

Antimony (sb) is a potential environmental contaminant of emerging concern that occurs in soils in the sb(V) oxidation state as the antimonate species sb(OH) 6 -. In soils, metal oxyhydroxides play an important role in the immobilization of contaminants and in restricting bioaccessibility. One such mineral is gibbsite, which bears the reactive aluminol surface functional group. Both inner-and outer-sphere surface complexation mechanisms have been inferred from conflicting sb(V) adsorption findings involving aluminol-bearing minerals. The objectives of this research are to characterize sb(V) adsorption by gibbsite and to use the macroscopic findings to develop mechanistic adsorption models. Antimonate adsorption envelopes were developed using two equilibrium techniques: continuous pH titration and batch. The adsorption of sb(V) decreases with increasing pH and increasing ionic strength, suggesting that outer-sphere surface complexation is an important adsorption mechanism. However, sb(V) retention is irreversible at pH <6 but is reversible in pH >7 systems, suggesting that inner-sphere species may be significant in acidic environments. Adsorbed sb(V) also generates a downward shift in the gibbsite isoelectric point, further supporting the formation of inner-sphere sb(V) surface complexes. Both PO 4 and sO 4 decrease adsorbed sb(V) concentrations: PO 4 throughout the pH 3 to 10 range and sO 4 in pH <7 systems. The triple-layer surface complexation model successfully predicts sb(V) adsorption by using both outer-sphere and inner-sphere surface species. The triple-layer model also predicts ligand adsorption in the competitive systems without the need for reoptimization.Abbreviations: BET, Brunauer-Emmett-Teller; ICP-AES, inductively coupled argon plasma-atomic emission spectroscopy; IEP, isoelectric point; SCM, surface complexation model; TLM, triple layer model; XRD, x-ray diffraction.A ntimony is one of the least abundant elements in soils, having a median concentration in uncontaminated soils from around the world of 1 mg kg −1 (range, <0.2-10 mg kg −1 ) (Bowen, 1979;Filella et al., 2002;Helmke, 2000). However, anthropogenic activities can result in elevated soil Sb levels (Crecelius et al., 1975;Filella et al., 2002;Li and Thornton, 1993;Nriagu, 1989;Nriagu and Pacyna, 1988;Scheinost et al., 2006). Soils that are particularly affected by Sb are those at military and civilian shooting ranges. Antimony is a hardening agent in lead bullets, which contain ~5% Sb (Carlin, 2000;Johnson et al., 2005;Kilgour et al., 2008). As bullets and bullet fragments corrode, Pb and Sb are released to the soil environment (Clausen and Korte, 2009;Kilgour et al., 2008). The resulting Sb concentrations can far exceed native soil levels. Antimony concentrations in military and civilian shooting range soils have been reported to range from <517 to <17,500 mg kg −1 (Basunia and Landsberger, 2001;Johnson et al., 2005;Kilgour et al., 2008;Knechtenhofer et al., 2003;Mitsunobu et al., 2006;Okkenhaug et al., 2013;Spuller et al., 2007), alth...

Antimony (V) Adsorption at the Hematite–Water Interface: A Macroscopic and In Situ ATR-FTIR Study

“…The adsorption decreased steeply from pH 6.5 to 9 from a value of about 80% to below detectable levels. The behavior of the Sb (V)-hematite adsorption envelope followed a similar trend to other oxide minerals [10,33,40]. For example, Leuz et al (2006) [10] found that between pH 3 and 6, about 96% of Sb (V) was adsorbed on goethite at ionic strength values of 0.1 and 0.01 M KCl.…”

“…In general, chemical models that produce V Y values less than 20 are deemed acceptable, although the desired result is to identify a chemical model that generates the smallest value of V Y , and is supported by a mechanistic interpretation of the experimental ATR-FTIR data. Several surface complexation reactions have been proposed to describe Sb retention at variable-charge surface functional groups [39][40][41][42]. Commonly, these include inner-sphere monodentate or bidentate and outer-sphere surface complexes, alone or in combination.…”

“…In the literature, various surface complexation reactions have been proposed to describe Sb (V) retention at variable-charge surface functional groups [33,[39][40][41][42][43]. Here we fit the adsorption envelope data using model I (assumes inner-sphere [≡FeOSb(OH) 4 • ] and outer-sphere [≡FeOH 2 + -Sb(OH) 6 − ] complexes), model II (assumes inner-sphere [≡FeOSb(OH) 5 − ] and outer-sphere [≡FeOH 2 + -Sb(OH) 6 − ] complexes), and model III (assumes bidentate inner-sphere [(≡FeO) 2 Sb(OH) 3 • ] and outer-sphere [≡FeOH 2 + -Sb(OH) 6 − ] complexes) ( Table 1).…”

Selective converting surface states of hematite photoelectrodes to catalytic active sites

“…Importantly, formation of inner‐sphere complexation was highly thermodynamically stable that is responsible for the irreversible adsorption observed in the environment. [ 29 ] In such inner‐sphere complexation configuration, surface states are strongly occupied by the adsorbed ions and consequently they are no longer available for water oxidation. Due to this unique adsorption behavior, although arranging a condition with trace concentration (~μM) of Zn(II) would not elaborate any superior performance afterword, it would be an excellent opportunity to quantify the concentration of surface states, which is barely reported in any literature.…”

Novel electroblowing synthesis of submicron zirconium dioxide fibers: effect of fiber structure on antimony(v) adsorption

“…Several studies have suggested that the decrease of the adsorbent's zeta potential aer adsorption of anions is related to an inner-sphere complexation in which chemical bond is formed between Sb(OH) 6 À and adsorbent surface. 18,48,49 In contrast, an outer-sphere complexation reaction is based on physical interaction between the anion and positively charged adsorbent surface and does not change the surface charge of the adsorbent. Our results show that the zeta potential decreases drastically aer Sb(V) adsorption indicating an inner-sphere complexation reaction.…”