Prevention of Acid Mine Drainage Through Complexation of Ferric Iron by Soluble Microbial Growth Products

Acid mine drainage (AMD) is a widespread environmental problem with deleterious impacts on water quality in streams and watersheds. AMD is generated largely by the oxidation of metal sulfides (i.e. pyrite) by ferric iron. This abiotic reaction is catalyzed by conversion of ferrous to ferric iron by iron and sulfur oxidizing microorganisms. Biostimulation is currently being investigated as an attempt to inhibit the oxidation of pyrite and growth of iron oxidizing bacteria through addition of organic carbon. This may stimulate growth of indigenous communities of acidophilic heterotrophic bacteria to compete for oxygen. The goal of this research is to investigate a secondary mechanism associated with carbon addition: complexation of free Fe(III) by soluble microbial growth products (SMPs) produced by microorganisms growing in waste rock. Exploratory research at the laboratory scale examined the effect of soluble microbial products (SMPs) on the kinetics of oxidation of pure pyrite during shaker flask experiments. The results confirmed a decrease in the rate of pyrite oxidation that was dependent upon the concentration of SMPs in solution. We are using these data to verify results from a pyrite oxidation model that accounts for SMPs. This reactor model involves differential-algebraic equations incorporating total component mass balances and mass action laws for equilibrium reactions. Species concentrations determined in each time step are applied to abiotic pyrite oxidation rate expressions from the literature to determine the evolution of total component concentrations. The model was embedded in a parameter estimation algorithm to determine the reactive surface area of pyrite in an abiotic control experiment, yielding an optimized value of 0.0037 m². The optimized model exhibited similar behavior to the experiment for this case; the root mean squared of residuals for Fe(III) was calculated to be 7.58 x 10^-4 M, which is several orders of magnitude less than the actual Fe(III) concentrations. The model was refined to include Fe(III)-SMP complexes, but these are not well documented and vary depending upon the nature and origin of the growth products. Well known chelating agents form predictable complexes with Fe(III) iron through documented complexation reactions. If chelation of soluble Fe(III) by SMPs is similar to such a chelator, the latter may be used as a basis to parameterize inhibition of pyrite oxidation due to complexation of Fe(III) by SMPs. Fe(III) complexation by known ligands or SMPs may adequately be represented by a bulk complex whose stability constant reflects the extent to which free Fe(III) is diminished. The stability constant may differ among the different SMPs experiments depending upon their origin but can be optimized for each case using inverse modeling techniques. We present results from these inverse modeling exercises to demonstrate the validity of using bulk Fe(III)-SMP complexes to explain inhibition of pyrite oxidation in the presence of SMPs. Our results will facilitate the design of in-situ carbon addition strategies by determining organic carbon dose intensity and application frequency required to effectively mitigate impacts on receiving water quality.