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SOIL CHEMISTRY

Evaluation of Chemical Amendments for pH and Redox Stabilization in Aqueous Suspensions of Three California Soils

^a Dep. of Environmental Sciences, University of California, Riverside, CA 92521
^b USDA-ARS, U.S. Salinity Lab., 450 W. Big Springs Rd., Riverside, CA 92507
^c USDA-ARS, SJVASC-Water Management Research, 9611 S. Riverbend Ave., Parlier, CA 93648

^* Corresponding author (mjherbel{at}yahoo.com).

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 for extended durations have not been well evaluated. We tested a suite of readily available chemicals for their ability to buffer pH and redox potential (E_H) of anaerobic soil–water (1:4 or 1:5) suspensions at specific levels under closed conditions. After initial titration, 20 mM Good buffers (e.g., PIPES) were used to stabilize the pH of the soil suspensions within ±0.3 units of the target pH (5.8–8.9) for a period of at least 8 d. The ability of redox active chemicals, such as NaNO₃, Fe(III)nitrilotriacetic acid (NTA), Ti(III)NTA, Fe and Mn oxyhydroxides, cysteine-HCl, dithiothreitol, and ascorbic acid, to stabilize E_H at specific levels (400 to –300 mV) depended heavily on the initial concentration of the chemical as well as the organic C and N status of the soil. Redox stabilization in most soils was typically achieved within a period of 3 to 4 wk. Iron(II)/Fe(III) ratios in 1 M HCl soil extracts and S(–II)/SO₄^2– ratios in filtrates generally reflected the redox condition as measured by the Pt electrode. This investigation demonstrates that the pH and E_H of enclosed soil–water suspensions can be buffered for long periods across a wide range of conditions using soluble chemicals alone.

Abbreviations: AIW, artificial irrigation water • DDW, distilled deionized water • Eh, redox potential