Does iron cycling trigger generation of acidity in groundwaters of Western Australia?

In large areas of Western Australia, acidic groundwaters occur with pH values distinctly lower than 3, generation of which has been attributed to the oxidation of Fe(II). Incubation experiments performed with sediments from playas receiving acid groundwater demonstrated occurrence of reductive dissolution of ferric iron minerals at rates [670 nmol (g reactive iron)(-1) h(-1)] similar to those observed in sediments of acidic mining lakes (AML), indicating thatthe pH was established through an acidity-driven iron cycle in analogy to processes occurring in AML systems. The low pH values observed in acidic groundwaters and AML, however, can only be achieved if the anion corresponding to Fe(III) is that of a strong acid. In AML, sulfate is derived from pyrite oxidation. Because this process is reported not to occur in the acidic groundwater systems of Western Australia, we have derived a conceptual model according to which sulfate is generated upon reaction of weathering-derived alkalinity with gypsum to form calcite, which is abundant in these areas. The model proposes that part of the alkalinity generated during weathering is stared as calcite in the landscape, which leads to spatial separation of acidity and alkalinity.

Quantification of abiotic reaction rates in mine tailings: evaluation of treatment methods for eliminating iron- and sulfur-oxidizing bacteria.

Effective treatment techniques for eliminating iron-oxidizing (IOB) and sulfur-oxidizing bacteria (SOB) are required for the comparison of abiotic and microbial sulfide oxidation rates and mechanisms in mine tailings. This study evaluates the effect of autoclaving, repeated heating, ethanol treatment, antibiotic treatment, gamma-radiation, and washing with deionized water on tailings characteristics and concentrations of IOB and SOB. Most probable number enumeration indicates that IOB and SOB were present at very low concentrations or below detection limits following treatment with all methods except rinsing and antibiotics treatment, where higher concentrations of IOB and SOB were present. The physical, chemical, and mineralogical characterization of the tailings indicated no changes in bulk mineralogy or bulk chemical composition as a result of treatment. However, an increase in oxidized sulfur species at the tailings surface, as determined by X-ray photoelectron spectroscopy, was observed for the heating, autoclaving, and antibiotics treatments. Batch weathering experiments, used to evaluate the effect of treatment on element release rates, indicated that the final element release rates (after >30 d) were similar between treated and untreated control samples. On the basis of the results of this study, experiments over relatively long periods (>30 d) are to be recommended forthe establishment of microbial and abiotic weathering rates in mill tailings samples. For the determination of abiotic reaction rates, treatment by gamma-radiation is suggested to be the most appropriate method for sulfide-rich tailings.