Modeling of microbial dynamics and geochemical changes in a metal bioprecipitation experiment

Abstract

A biogeochemical transport modeling study was carried out to analyze large-scale laboratory column experiments in which ethanol was used as an electron donor to create favorable conditions for the immobilization of selected trace metals (Zn and Cu) in groundwater. Microbial activity was explicitly simulated to capture the dynamic changes of the redox zonation within the column (i) in the early phase of the experiment (microbial lag) and (ii) in response to a significant decrease in the pH of the feed solution introduced after 188 days. The simulated redox dynamics agreed well with the observations after the pH-dependency of microbial growth was incorporated into the microbial model. The study showed that residual minerals may have buffered the pH for a period after the pH of the feed solution was decreased. Where the buffering capacity was exhausted, the pH decreased, leading to a successive downstream movement of the redox boundaries. The simulations reproduced the Zn immobilization within the sulfate-reducing zone as well as its partial remobilization after this zone moved further downstream. The immobilization of Cu within the denitrifying zone could also be well explained by incorporating malachite (Cu2(OH)2CO3) precipitation in the simulations.