Slow Adsorption Reaction between Arsenic Species and Goethite (α-FeOOH):  Diffusion or Heterogeneous Surface Reaction Control

Abstract

The slow stage of phosphate or arsenate adsorption on hydrous metal oxides frequently follows an Elovich equation. The equation can be derived by assuming kinetic control by either a diffusion process (either interparticle or intraparticle) or a heterogeneous surface reaction. The aim of this study is to determine whether the slow stage of arsenic adsorption on goethite is more consistent with diffusion or heterogeneous surface reaction control. Adsorption kinetics of arsenate and dimethylarsinate (DMA) on goethite (alpha-FeOOH) were investigated at different pH values and inert electrolyte concentrations. Their adsorption kinetics was described and compared using Elovich (Gamma vs ln time) plots. Desorption of arsenate and DMA was studied by increasing the pH of the suspension from pH 4.0 to pH 10.0 or 12.0. The effective particle sizes and zeta-potential of goethite were also determined. Effective particle size increased rapidly as the pH approached pH(IEP), both in the absence and presence of arsenic. Inert electrolyte concentrations and pH had no effect on the slow stage of arsenate adsorption on goethite, while the kinetics of DMA adsorption on goethite was influenced by both parameters. The slow stage of arsenate adsorption on goethite follows an Elovich equation. Since effective particle size changes with both pH and inert electrolyte concentrations, and effective particle size influences interparticle diffusion, the arsenate adsorption kinetics indicate that the slow adsorption step is not due to interparticle diffusion. DMA also has complex adsorption kinetics with a slow adsorption stage. DMA desorbed completely and rapidly when the pH was raised, in contrast to the slow adsorption kinetics, indicating that the slow adsorption step is not due to intraparticle diffusion. The slow adsorption is not the result of diffusion, but rather is due either to the heterogeneity of the surface site bonding energy or to other reactions controlling arsenic removal from solution.

Arsenate and cadmium co-adsorption and co-precipitation on goethite

“…The pH edge of As(V) adsorption shows similar trend to the ones reported by several previous studies [25][26][27][28]. However, the surface loading concentration decreases from 0.85 to 0.58 mol/m 2 for 0.1 mM As(V), from 1.74 to 1.12 mol/m 2 for 0.5 mM As(V) in the pH interval of 3-10, less than the changes of the previously reported results [25][26][27][28]. It is probably due to the systemic variation in goethite surface area measurement and differences in electrolyte.…”

Adsorption behavior of arsenic onto protonated titanate nanotubes prepared via hydrothermal method

“…Natural materials such as zeolite [11], natural iron ores [7,12,13], siderite [14] and red mud [15] have been examined intensively for arsenic removal. Although these materials are regarded as cheap and effective adsorbents, there are several problems (their impurities, unknown stability and regeneration, low adsorption capacity and slow kinetics) associated with their use [4,13,16]. An ideal adsorbent should have suitable particle size or uniformly accessible pores, high surface area, and physical and/or chemical stability [4].…”

Speciation dynamics of oxyanion contaminants (As, Sb, Cr) in argillaceous suspensions during oxic-anoxic cycles

“…Markelova et al Applied Geochemistry 91 (2018) 75-88 particle diffusion (Zhang and Stanforth, 2005) or the formation of surface precipitates (Gallegos et al, 2007), such as Ca-arsenate phases (Bothe and Brown, 1999;Raposo et al, 2004). Based on the initial [As] concentration in Exp.…”