To assess the mobilities of Pb, Cd, and Zn from a contaminated soil, the effects of redox potential and pH value on metal solubilities were investigated. Both redox potential and pH were found to greatly affect heavy metal solubility in the soil. Results showed that the soil suspension under continuous oxygen aeration for 21 days resulted in increases of redox potential from 290 to 440 mV and pH value from 6.9 to 7.0, respectively. Soluble concentrations of Pb, Cd, and Zn varied with time, and were all lower than 1 mg kg−1. When the soil suspension was aerated with nitrogen, final redox potential was −140 mV and pH value of 7.1. The soluble metal concentrations were slightly higher than those aerated with oxygen. The equilibrium solubility experiments were conducted under three different pH values (3.3, 5.0, 8.0) and three redox potential (325, 0, −100 mV). Results showed that metals were sparingly soluble under alkaline conditions (pH = 8.0). Metal solubilities were higher when under slightly acidic conditions (pH = 5.0), and increased drastically when pH was kept at 3.3. When solubilities were compared under same pH values, it was observed that metal solubilities increased as redox potential decreased. Generally speaking, acidic and reducing conditions were most favorable for metal solubilization, and the effect of pH was more significant than that of redox potential. It was proposed that heavy metals were mostly adsorbed onto Fe-Mn oxyhydroxides. The pH-dependent metal adsorption reaction and the dissolution of Fe-Mn oxyhydroxides under reducing conditions was the mechanism controlling the release of heavy metals from soils.

Solubility of heavy metals in a contaminated soil: Effects of redox potential and pH

Dissimilatory metal reduction has the potential to be a helpful mechanism for both intrinsic and engineered bioremediation of contaminated environments. Dissimilatory Fe(III) reduction is an important intrinsic process for removing organic contaminants from aquifers contaminated with petroleum or landfill leachate. Stimulation of microbial Fe(III) reduction can enhance the degradation of organic contaminants in ground water. Dissimilatory reduction of uranium, selenium, chromium, technetium, and possibly other metals, can convert soluble metal species to insoluble forms that can readily be removed from contaminated waters or waste streams. Reduction of mercury can volatilize mercury from waters and soils. Despite its potential, there has as yet been limited applied research into the use of dissimilatory metal reduction as a bioremediation tool.

Bioremediation of organic and metal contaminants with dissimilatory metal reduction

Selenium (Se) is an essential element for humans and animals, which occurs ubiquitously in the environment. It is present in trace amounts in both organic and inorganic forms in marine and freshwater systems, soils, biomass and in the atmosphere. Low Se levels in certain terrestrial environments have resulted in Se deficiency in humans, while elevated Se levels in waters and soils can be toxic and result in the death of aquatic wildlife and other animals. Human dietary Se intake is largely governed by Se concentrations in plants, which are controlled by root uptake of Se as a function of soil Se concentrations, speciation and bioavailability. In addition, plants and microorganisms can biomethylate Se, which can result in a loss of Se to the atmosphere. The mobilization of Se across soil-plant-atmosphere interfaces is thus of crucial importance for human Se status. This review gives an overview of current knowledge on Se cycling with a specific focus on soil-plant-atmosphere interfaces. Sources, speciation and mobility of Se in soils and plants will be discussed as well as Se hyperaccumulation by plants, biofortification and biomethylation. Future research on Se cycling in the environment is essential to minimize the adverse health effects associated with unsafe environmental Se levels.

Selenium Cycling Across Soil-Plant-Atmosphere Interfaces: A Critical Review

Microbial Reduction of Iron, Manganese, and other Metals

A critical review of thermodynamic data for selenium species at 25°C

This study was conducted to characterize batch adsorption of selenate (SeO²⁻₄), selenite (SeO²⁻₃), and selenomethionine (Se-CH₂CH₂CHNH₂COOH)₂ and transformations of Se in the presence of microbial activity in Punoche loam (fine-loamy, mixed, calcareous, thermic Typic Torriorthents). The effects of sterilization, enhanced microbial activity, and aerobic vs. anaerobic conditions on the transport of pulse-fed Se in soil columns were also investigated. Selenate was rapidly leached in the soil columns compared with selenomethionine and selenite both of which invaded the columns to only a limited extent. Adsorption was highest for selenite and least for selenate. Selenate was transformed to reduced and less mobile forms when soil was C-enriched, particularly with smaller water fluxes and under anaerobic conditions. Systems of partial differential equations (PDEs) involving equilibrium adsorption of Se are set up to simulate transport of selenate, selenite, and selenomethionine under sterilized conditions. The transport model is solved by an iterative procedure using a finite difference scheme. Experimentally determined adsorption parameters of selenite and selenomethionine did not produce a satisfactory fit when used in simulation of these Se species in column studies. The Se transport model adequately simulated Se concentrations in soil solution under sterilized conditions when the adsorption coefficients derived from batch experiments were adjusted. No transport modeling was considered for nonsterilized conditions. Contribution from Dep. of Land, Air, and Water Resour., Univ. of California, Davis. Funding was provided by the Univ. of California Salinity/Drainage Task Force.

Modeling selenium transport in steady-state, unsaturated soil columns

Transport of selenate (Se⁶⁺) in steady-state, water-saturated soil columns was evaluated under different pore water velocities, ionic compositions, and two soil bulk densities. The soil was Panoche loam (Typic Torriorthents) saturated with 0.005 M CaSO₄, 0.005 M CaCl₂, or 0.05 M CaCl₂. To examine the movement of selenate in soil relative to water and SO²⁻₄ (a biogeochemically similar anion), breakthrough curves of ⁷⁵SeO²⁻₄, along witH⁺³H₂O or ³⁵SO²⁻₄, were observed. Selenate appeared before elution of one pore volume and always ahead of the ³H₂O and SO²⁻₄ except in 0.05 M CaCl₂, indicating exclusion of selenate by the soil matrix. On the other hand, soil analysis of selenate following the leaching indicated that selenate was retained by the soil, suggesting simultaneous exclusion and adsorption of selenate. Increasing pore water velocity and soil bulk density did not shift the position of the breakthrough curves significantly. Assuming equilibrium linear adsorption-desorption of selenate by soil, the values of the apparent diffusion coefficient (D) and the retardation factor (R) were obtained by fitting the solution of the solute transport equation without sink and source terms to the measured breakthrough curves of selenate. The values of D showed an increasing trend with pore water velocity and the values of R were influenced by ionic composition of the soil solution. Contribution from the University of California, Davis. Funding was provided by the University of California Salinity/Drainage Task Force.

Selenate Transport in Steady‐State, Water‐Saturated Soil Columns

Selenium-contaminated sediments were collected from evaporation ponds at the Kesterson National Wildlife Refuse in California. Columns of undisturbed, freeze-dried, and air-dried sediments were each leached with deionized water, California aqueduct water, and saline water. Total Se and inorganic Se were measured in the effluent. Following leaching, the sediment cores were sectioned, extracts were prepared using deionized water, and analyzed for total Se, inorganic Se, and selenite using fluorimetric techniques. Selenium remained immobile in the undisturbed sediments, but freeze- and air-drying increased Se mobility. The maximum concentrations of total Se in the effluent from undisturbed, freeze-dried, and air-dried sediments were 32, 7600, and 232 650 nmol L⁻¹, respectively. Microbial activity played an important role in the reduction of Se to immobile and volatile forms. Se recovered in the effluent and from water extraction of the sediments following leaching was highest for deionized water and least for aqueduct water under nonsterilized conditions. The composition of solution did not significantly influence the leaching of Se under sterilized conditions. Introducing Se to previously leached sediments resulted in an increase in water-extractable Se, particularly near the surface. However, when the concentration of added Se was high (31.70 µmol L⁻¹ in this study), the increase in water-extractable Se also occurred at greater depths in the sediments. Contribution from Univ. of California, Davis. Funding was provided by the Univ. of California Salinity/Drainage Task Force.

M. H. Alemi

Papers

Modeling selenium transport in steady-state, unsaturated soil columns

Selenate Transport in Steady‐State, Water‐Saturated Soil Columns

Elution of Selenium from Contaminated Evaporation Pond Sediments