Field Determination of Fe2+ Oxidation Rates in Acid Mine Drainage

Using a Continuously-Stirred Tank Reactor

 

Carl S. Kirby

Jennifer A. Elder Brady

 

Department of Geology, Bucknell University, Lewisburg PA 17837, kirby@bucknell.edu, 570-524-1385

 

1998, Applied Geochemistry, 13, 4, 509-520

ABSTRACT

A gravity-fed, battery-powered, portable continuously-stirred tank reactor has been developed to directly measure aqueous reactions rates in the field. Dye and tracer experiments indicate the reactor is well-mixed. Rates of ferrous iron oxidation at untreated and passively treated coal mine drainage sites in Pennsylvania were measured under ambient conditions and with the addition of either O2 gas or NaOH solutions. Rates at five sites ranged from below the detection limit for this technique (approximately 10-9 mol L-1 s-1) to 3.27±0.01x10-6 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 ferrous iron, dissolved oxygen, and pH (Thiobacillus populations were not measured), although data for two more acidic sites were fit well by the model log rate = log k + a log [Fe2+] + b log [OH-] + c log [O2]. Field rates of iron oxidation from this and other studies vary by four orders of magnitude. A model using the ambient field rate of iron oxidation from this study successfully reproduced independently-measured Fe2+ concentrations observed in a passive wetland treatment facility.