Kinetics of Molybdenum Adsorption and Desorption in Soils

Abstract

Much uncertainty exists in mechanisms and kinetics controlling the adsorption and desorption of molybdenum (Mo) in the soil environment. To investigate the characteristics of Mo adsorption and desorption and predict Mo behavior in the vadose zone, kinetic batch experiments were performed using three soils: Webster loam, Windsor sand and Mahan sand. Adsorption isotherms for Mo were strongly nonlinear for all three soils. Strong kinetic adsorption of Mo by all soils was also observed, where the rate of retention was rapid initially and was followed by slow retention behavior with time. The time-dependent Mo sorption rate was not influenced when constant pH was maintained. Desorption or release results indicated that there were significant fractions of Mo that appeared to be irreversible or slowly reversibly sorbed by Windsor and Mahan. X-ray absorption near edge structure (XANES) analysis for Windsor and Mahan soils indicated that most of Mo had been bound to kaolinite, whereas Webster had similar XANES features to those of Mo sorbed to montmorillonite. A sequential extraction procedure provided evidence that a significant amount of Mo was irreversibly sorbed. A multireaction model (MRM) with nonlinear equilibrium and kinetic sorption parameters was used to describe the adsorption-desorption kinetics of Mo on soils. Our results demonstrated that a formulation of MRM with two sorption sites (equilibrium and reversible) successfully described Mo adsorption-desorption data for Webster loam, and an additional irreversible reaction phase was recommended to describe Mo desorption or release with time for Windsor and Mahan soils.

Kinetic Modeling of pH‐Dependent Molybdenum(VI) Adsorption and Desorption on Iron Oxide‐Coated Sand

“…Moreover, K f values decreased from 294.9 L kg −1 at pH 4.0 to 8.7 L kg −1 at pH 7.5, resulting from the decreased amount of adsorption. This is consistent with our previous study that there was a significantly negative relation between solution pH and K f (Sun and Selim, 2018). The Langmuir equation S / S max = ω C /(1+ω C ) is another commonly used equilibrium sorption model, where S max (mg kg −1 ) represented maximum sorption capacity and ω (L mg −1 ) is a Langmuir coefficient related to the binding strength.…”

“…They suggested that Mo adsorption on oxides increased from pH 2 to 4, exhibited a peak near pH 4, and decreased with increasing pH above 4. Recently, Sun and Selim (2018) found that Mo(VI) sorption capacity and kinetic rate on an acidic soil was much higher than that of a neutral soil. It is commonly accepted that the maximum in the adsorption of anions occurs at pH values close to their dissociation constants (p K ).…”

“…Compared with equilibrium studies, only a limited number of studies focused on the kinetics of Mo(VI) adsorption/desorption on minerals and soils (Zhang and Sparks, 1989; Lang et al, 2000; Vistoso et al, 2012; Kim and Jang, 2010; Sun and Selim, 2018). It was reported that the sorption rate of Mo(VI) was initially rapid within few hours, and was followed by a slow process that extends for several days to weeks.…”

“…To our knowledge, batch experiments examining kinetics of Mo(VI) adsorption/desorption on IOCS have not yet been reported. A multi‐reaction model which assumes that a fraction of the total sorption sites is rate limited (kinetic site) whereas the remaining fractions interact rapidly or instantaneously with solute (equilibrium site) has been developed to describe the kinetic adsorption and desorption behavior of Mo in soils (Sun and Selim, 2017, 2018). However, this multi‐reaction modeling only dealt with generalized kinetic sites, and lacks a unified modeling framework that account for the change of individual kinetic sites (e.g., monodentate, bidentate) over time (Zhang and Selim, 2011).…”

The Influence of Phosphate on the Adsorption–Desorption Kinetics of Vanadium in an Acidic Soil

“…It is well established that both V and phosphate are specifically retained on mineral via similar inner‐sphere surface complexes (Arai and Sparks, 2001; Rahnemaie et al, 2007; Larsson et al, 2017a). Moreover, Freundlich N is a measure of the extent of heterogeneity of the sorption sites having different affinities for solute retention by matrix surfaces (Kinniburgh, 1986; Ho et al, 2002; Sun and Selim, 2018). In this study, although the N values increased with increasing applied phosphate concentrations, they did not vary significantly among all three applied phosphate concentrations ( p > 0.05).…”