Evaluation of silicate minerals for pH control during bioremediation: Application to chlorinated solvents

Accurate control of groundwater pH is of critical importance for in situ biological treatment of chlorinated solvents. This study evaluated a novel approach for buffering subsurface pH that relies on the use of silicate minerals as a long-term source of alkalinity. A screening methodology based on thermodynamic considerations and numerical simulations was developed to rank silicate minerals according to their buffering efficiency. A geochemical model including the main microbial processes driving groundwater acidification and silicate mineral dissolution was developed. Kinetic and thermodynamic data for silicate minerals dissolution were compiled. Results indicated that eight minerals (nepheline, fayalite, glaucophane, lizardite, grossular, almandine, cordierite and andradite) could potentially be used as buffering agents for the case considered. A sensitivity analysis was conducted to identify the dominant model parameters and processes. This showed that accurate characterization of mineral kinetic rate constants and solubility are crucial for reliable prediction of the acid-neutralizing capacity. In addition, the model can be used as a design tool to estimate the amount of mineral (total mass and specific surface area) required in field applications.