Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2006
Publication Date: 5/16/2007
Citation: Herbel, M.J., Suarez, D.L., Goldberg, S.R., Gao, S. 2007. Evaluation of chemical amendments for ph and redox stabiliztion in aqueous suspensions of three california soils. Soil Science Society of America Journal. Vol 71(3): 927-939 Interpretive Summary: Discharge of selenium in agricultural drainage waters is of environmental concern and subject to increasing regulation. Development of management practices to reduce concentrations in discharge water requires that we understand the processes that control transport. Selenium mobility is highly dependent on oxidation state. In this study we evaluate the use of chemical amendments to stabilize pH and redox in order to study the rates of selenium reduction under different anaerobic conditions (redox). Various amendments were successful in stabilizing pH and Eh however they resulted in differing distributions of redox species for several elements, indicating that Eh characterization is not sufficient to represent oxidation states of individual elements. These results indicate that we will have to measure a variety of redox-sensitive species when characterizing the anaerobic states of a soil. This complexity suggests that we utilize an empirical approach to characterizing selenium reduction rates.
Technical Abstract: Many chemically and biologically important trace element, heavy metal, and organic contaminant reactions in soils are constrained by pH and redox conditions and changes in these conditions can significantly affect reaction rates. Although closed-system, batch methods have been used for many years to study contaminant reaction kinetics; redox conditions in such suspensions over extended durations have not been well evaluated. Here we tested a suite of readily available chemicals for their ability to buffer pH and redox potential (Eh) of anaerobic soil-water (1:4 or 1:5) suspensions at specific levels under closed conditions. After initial titration, Good buffers (20 mM MES, PIPES, HEPES, TAPS and CHES) were able to stabilize the pH of the soil suspensions within plus or minus 0.3 units of the target pH (5.8 to 8.9) for a period of at least 8 d. The ability of redox active chemicals, such as NaNO3, Fe(III)NTA, Ti(III)NTA, Fe and Mn oxyhydroxides, cysteine-HCI, dithiothreitol and ascorbic acid, to stabilize Eh at specific levels (+400 to -300 mV) depended heavily on the initial concentration of the chemical as well as the organic carbon and nitrogen status of the soil. Redox stabilization in most soils was typically achieved within a period of 3-4 weeks. Fe(II):Fe(III) ratios in 1 M HCI soil extracts and S(¬II):SO4(2-) ratios in filtrates generally reflect the redox condition as measured by the platinum electrode, although ascorbic acid and cysteine-HCI tended to enhance reduced products over that of dithiothreitol and Ti(III)NTA, which yielded similarly low Eh readings. This investigation demonstrates that the pH and Eh of enclosed soil-water suspensions can be buffered for long periods over a wide range of conditions using soluble chemicals alone.