OXIDATION CAPACITY OF AQUIFER SEDIMENTS

A laboratory extraction method (Ti3+-EDTA extraction) for the determination of the oxidation (electron accepting) capacity related to oxides and hydroxides of aquifer sediments was developed. At room temperature, titanous ions (0.008 M Ti3+) in a solution of 0.05 M ethylenediaminetetraacetic acid (EDTA) reduced oxidized aquifer species. This operationally defined oxidation capacity (OXC) was determined as mumoles of electrons accepted per gram of sediment sample. Well-described oxidized iron and manganese minerals were reduced (ferrihydrite 98%, akageneite 100%, goethite and hematite 93%, magnetite 9%, pyrolusite 99%) whereas organic matter in the sediments was not reduced significantly. The method was applied to reduced as well as oxidized aquifer sediments from a well-described Danish sandy aquifer. The OXC (23-31 muequiv/g) of the oxic sediments could be attributed to the content of extractable Fe(M). The OXC was significantly lower for the reduced sediments (4-10 muequiv/g), approaching the detection limit of the method. Drying of the sediments was shown to be unacceptable because of the oxidation of Fe(II) compounds, whereas storing of sediments inside an anaerobic chamber for up to 1 month was acceptable. The oxidation capacity of aquifer sediments may be a key parameter in predicting contaminant plume development and size and in understanding aquifer redox zones in general.