Effect of Competing Anions on the Adsorption of Arsenate and Arsenite by Ferrihydrite

Abstract

The competitive adsorption of arsenate and arsenite and the effect of phosphate and sulfate on adsorption of arsenate and arsenite by ferrihydrite were investigated in the pH range of 3 to 10 and at varying initial ligand concentrations. In dual anion systems, arsenate retention was greater at low pH compared with greater arsenite retention at high pH. In systems with arsenate and arsenite concentrations ≤2.08 molAs kg−1fer each, the effect of arsenate on arsenite sorption was more pronounced than vice versa. On the contrary, at arsenate and arsenite concentrations of 3.47 molAs kg−1fer each, arsenate did not influence arsenite sorption but arsenite substantially reduced arsenate adsorption. The different sorption behavior of arsenite at low and high arsenite concentrations might be due to surface polymerization of adsorbed arsenite at high concentrations. The presence of phosphate resulted in a significant reduction in arsenate and arsenite adsorption by ferrihydrite, with strong dependence on pH and phosphate concentration. The effect of phosphate on arsenate adsorption was greater at high pH than at low pH, whereas the opposite trend was observed for arsenite. Results indicated that arsenate and phosphate compete for the same surface sites, with a moderate preference for arsenate adsorption. There was evidence of the presence of some surface sites that exhibited much higher affinity for arsenite than phosphate. The presence of sulfate did not influence arsenate adsorption but resulted in a considerable reduction in arsenite adsorption below pH 7.0, with the largest reduction at the lowest pH.

A Gel Probe Equilibrium Sampler for Measuring Arsenic Porewater Profiles and Sorption Gradients in Sediments: I. Laboratory Development

“…Arsenic(V) and phosphate directly compete for surface sites, especially at neutral and alkaline pH (29,30), which is consistent with the similar maximum site densities observed for As(V) and phosphate in this study.…”

“…It is well-known that phosphate inhibits the sorption of both As(V) and As(III) on Fe (oxyhydr)oxides (24,(27)(28)(29)(30)(31)(32)(33), but it has a stronger effect on As(V) (Figure 3). At high phosphate concentrations, only 7% of total As(V) and 16% of total As(III) is adsorbed, compared to 45% adsorbed As(III) and As(V) in the absence of phosphate.…”

Mitigating heavy metal accumulation into rice (Oryza sativa L.) using biochar amendment — a field experiment in Hunan, China

“…4). In addition to an increase in soil pH, P entering into soil with biochar application (especially RBC) may also have been a contributing factor in immobilizing Cd, Zn, and Pb via the formation of phosphate precipitates (McGowen et al 2001;Tang et al 2004) and activating As via competing adsorption onto soil surface with arsenate (Jain and Loeppert 2000). Arsenate retained in soil particles can be desorbed by phosphate and released into soil solution.…”

A Gel Probe Equilibrium Sampler for Measuring Arsenic Porewater Profiles and Sorption Gradients in Sediments: II. Field Application to Haiwee Reservoir Sediment

“…Adsorption of As can be suppressed in the presence of competitively sorbing ions such as phosphate, inorganic carbon, organic carbon, and other compounds through electrostatic, steric, or competitive effects (2)(3)(4)(5). Phosphate competes directly with both As(III) and As(V) for surface sites on Fe oxides (2,3,(5)(6)(7)(8)(9)(10), but has a stronger inhibitory effect on As(V) (see Part I,11).…”

“…Phosphate competes directly with both As(III) and As(V) for surface sites on Fe oxides (2,3,(5)(6)(7)(8)(9)(10), but has a stronger inhibitory effect on As(V) (see Part I,11). Inorganic carbon may also suppress As adsorption when concentrations are elevated in groundwater because of microbial respiration of organic carbon and/or carbonate mineral dissolution (12)(13)(14)(15).…”