Field determination of Fe2+ oxidation rates in acid mine drainage using a continuously-stirred tank reactor

A gravity-fed, battery-powered, portable continuously-stirred tank reactor has been developed to directly measure aqueous reaction rates in the field. Dye and tracer experiments indicate the reactor is well-mixed. Rates of Fe2+ oxidation at untreated and passively treated coal mine drainage sites in Pennsylvania were measured under ambient conditions and with the addition of either O-2 gas or NaOH solutions. Rates at 5 sites ranged from below the detection limit for this technique (approximately 10(-9) mol L-1 s(-1)) to 3.27 +/- 0.01 x 10(-6) mol L-1 s(-1). Uncertainties in rates ranged from 70% near the lower limit of measurement to as little as 1% at higher rates of reaction. Multiple linear regressions showed no universal correlations of rates to Fe2+, dissolved O-2, and pH (Thiobacillus populations were not measured), although data for two more acidic sites were found to fit well for the model log rate = log K + a log [Fe2+] + log [OH-] + c log [O-2]. Field rates of Fe oxidation from this and other studies vary by 4 orders of magnitude. A model using the ambient field rate of Fe oxidation from this study successfully reproduced independently-measured Fe2+ concentrations observed in a passive wetland treatment facility. (C) 1998 Elsevier Science Ltd. All rights reserved.