The soil solution speciation and solid-phase fractionation of copper (Cu) and zinc (Zn) in 11 typical uncontaminated soils of South Australia were assessed in relation to heavy metal phytoavailability. The soils were analyzed for pH (4.9-8.4), soil organic matter content (3.5 to 23.8 g of C kg(-1)), total soil solution metal concentrations, Cu8 (49-358 microg kg(-1)) and Zn8 (121-582 microg kg(-1)), and dissolved organic matter (DOM) (69-827 mg of C L(-1)). The solid-liquid partition coefficient (Kd) ranged from between 13.9 and 152.4 L kg(-1) for Cu and 22.6 to 266.3 L kg(-1) for Zn. The phytoavailability of Cu and Zn could be predicted significantly using an empirical model with the solid-phase fractions of Cu and Zn, as obtained from selective sequential extraction scheme, as components. Phytoavailable Cu and Zn were found to significantly correlate with fulvic complex Cu (r= 0.944, P < 0.0001) and exchangeable Zn (r = 0.832, P = 0.002), respectively. The fulvic complex Cu was found to explain 89.2% of the variation in phytoavailable Cu, where as, the exchangeable Zn together with fulvic complex Zn could explain 78.9% of the variation in phytoavailable Zn. The data presented demonstrate the role of solid-phase metal fractions in understanding the heavy metal phytoavailability. The assessment of the role of solid-phase fractions in heavy metal phytoavailability is a neglected area of study and deserves close attention.

The low‐molecular‐weight organic acids (LMWOAs) secreted by plant roots modify the mobility of Cd through formation of soluble complexes in the soil rhizosphere; the kinetics of Cd release as influenced by organic acids and its impact on Cd bioaccumulation remains to be established. The influence of LMWOAs (10−3 and 10−2 M), viz., acetic, citric, oxalic, fumaric, and succinic acids, which are present in the soil rhizosphere, on the kinetics of Cd release from selected soils (Luseland, Waitville, and Jedbergh) of Saskatchewan, Canada, was investigated at 25°C and at an ionic strength of 0.1 M NaNO3. It was observed that the Cd release from the soils increased initially up to a reaction period of about 2 h and then slowly decreased with time. With time, the LMWOAs were apparently microbially degraded and the soil solution Cd2+, which was transformed from the Cd‐LMWOA complexes by microbial degradation, was adsorbed onto the negatively charged soil particles. Increasing amounts of Cd were released from the soils to the soil solution with the renewal of LMWOAs after every 2 h. The short‐term (reaction period of 0.25 to 1 h) kinetics of Cd release from the soils was described by a parabolic diffusion kinetic model. The overall diffusion coefficient of the Cd release from the soil and the Cd released by the renewal of LMWOAs followed the same trend as the Cd availability index of the soils: Luseland >> Waitville > Jedbergh. Thus, the results presented in this investigation indicate the importance of the kinetics of the Cd release from the soils by the LMWOAs excreted by root exudates in understanding the availability of soil Cd.

Unlike hydrophobic organic pollutants, the potential of organoclays to adsorb inorganic ionic contaminants is relatively underexplored. The present study attempts to characterise bentonite (QB) based organoclays synthesised from a commercially available, low-cost alkyl ammonium surfactant Arquad® 2HT-75 (Aq) and test their ability to adsorb hexavalent chromium (Cr (VI)) in aqueous solution. XRD, FTIR and TGA characterisation techniques prove successful modification of the bentonite structure and reveal that higher surfactant loadings gives rise to more ordered surfactant conformation in the organoclays. The zeta potential values indicate that higher surfactant loadings also create positive charges on the organoclay surfaces. Detailed isothermal and kinetic studies show that the organoclays effectively remove hexavalent chromium (Cr (VI)) from aqueous solution by both physical and chemical adsorption processes. Higher surfactant loadings provide better adsorption efficiency. The adsorption performance is reasonably efficient under the levels of pH, temperature, electrolyte concentration and natural organic matter concentration that generally prevail in contaminated soil and water. This study shows that organoclay sorbents offer good potential for remediating Cr (VI) under real environmental conditions.

A modified sequential chemical extraction procedure was developed for partitioning particulate Cd into eight fractions: exchangeable, carbonate-bound, metal-organic complex-bound, easily reducible metal oxide-bound, organic-bound, amorphous mineral colloid-bound, crystalline Fe oxide-bound, and residual. Results of experimental data on 16 surface soils of Saskatchewan, widely varying in physico-chemical properties, indicate the presence of little exchangeable Cd. Cadmium in these soils was predominantly in the form metal-organic complex-bound, accounting for 31-55%, with an average of 40%, of the total Cd present in the soils. The average relative abundance of the different forms of Cd present in these soils is in the order: metal-organic complex-bound (0.107 mg kg-1) > carbonate-bound (0.052 mg kg-1) > residual (0.042 mg kg-1) > organic-bound (0.035 mg kg-1) > crystalline Fe oxide-bound (0.016 mg kg-1) > easily reducible metal oxide-bound (0.010 mg kg-1) > amorphous mineral colloid-bound (0.002 mg kg-1). Statistical treatment of the Cd availability index, measured as ammonium hydrogencarbonatdiethylenetriaminepentaacetic acid (ABDTPA)-extractable Cd, with different particulate-bound Cd species showed high correlation (r = 0 . 9 1 6 , ~ = 6 X 10-7) of the Cd availability index with the metal-organic complex-bound Cd. The beta coefficients obtained from the multiple regression analyses have given an insight into the importance of Al-organic complex-bound Cd species in estimating the bioavailability of Cd in these soils. The relationship of the metal-organic complex-bound Cd and the mobility and bioavailability of soil Cd merits in-depth research in explaining the toxicity and food chain contamination of Cd in the environment.

Sorption and desorption processes control the mobility, toxicity, and availability of As in natural environments. Surface coverage and residence time may affect the kinetics of As sorption–desorption from soil components and the transformation of As from desorbable into resistant or undesorbable forms. We performed kinetic studies on the sorption of As(V) onto crystalline or poorly crystalline metal oxides (noncrystalline Al(OH)x, gibbsite, ferrihydrite, and goethite) and its desorption by PO4 at pH 6.0 as affected by the residence time and the surface coverage (50 or 100%) of As(V). Significant amounts of As(V) were sorbed during the initial period of 0.167 h, ranging from 37.9 to 71.8% when the surface coverage was about 100%. The kinetic data, explained best by the Elovich kinetic model, indicated the following order in As(V) sorption: gibbsite < Al(OH)x < goethite < ferrihydrite. By adding PO4 immediately after complete sorption of As(V) onto the oxides (50% surface coverage; PO4 added/As(V) sorbed molar ratio of 4), a much higher proportion of As(V) was desorbed after 24 h of reaction from Al oxides (48–56%) than from Fe oxides (18–23%). The amount of As(V) desorbed decreased with increasing residence time. The kinetics of As(V) desorption by PO4 as a function of residence time was explained best by the Elovich kinetic model. The kinetics described the rate of rearrangement of As(V) from desorbable into resistant or undesorbable forms, which occurred more rapidly in Al than Fe oxides. After a residence time of 360 h, the percentage of As(V) desorbed from the oxides was reduced significantly (<13%).