Microbial transformation of biogenic and abiogenic Fe minerals followed by in-situ incubations in an As-contaminated vs. non-contaminated aquifer

Abstract

Fe(III) minerals play a crucial role for arsenic (As) mobility in aquifers as they usually represent the main As-bearing phases. Microbial reductive dissolution of As-bearing Fe(III) minerals is responsible for the release of As and the resulting groundwater contamination in many sites worldwide. So far, in most studies mainly abiogenic iron minerals have been considered. Yet, biogenic minerals that possess different properties to their abiogenic counterparts are also present in the environment. In some environments they dominate the iron mineral inventory but so far, it is unclear what this means for the As mobility. We, therefore, performed an in-situ aquifer Fe(III) minerals exposure experiment i) to evaluate how different biogenic and abiogenic Fe(III) minerals are transformed in a strongly reducing, As-contaminated aquifer (25 m) compared to As-free moderately reducing aquifer (32 m) and ii) to assess which microbial taxa are involved in these Fe(III) minerals transformations. We found that higher numbers of bacteria and archaea were associated with the minerals incubated in the As-contaminated compared to the non-contaminated aquifer and that all Fe(III) minerals were mainly colonized by Fe(III)-reducing bacteria, with Geobacter being the most abundant taxon. Additionally, fermenting microorganisms were abundant on minerals incubated in the As-contaminated aquifer, while methanotrophs were identified on the minerals incubated in the As-free moderately reducing aquifer, implying involvement of these microorganisms in Fe(III) reduction. We observed that biogenic Fe(III) minerals generally tend to become more reduced and when incubated in the As-contaminated aquifer sorbed more As than the abiogenic ones. Most of abiogenic and biogenic Fe(III) minerals were transformed into magnetite while biogenic more crystalline mixed phases were not subjected to visible transformation. This in-situ Fe(III) minerals incubation approach shows that biogenic minerals are more prone to be colonized by (Fe(III)-reducing) microorganisms and bind more As, although ultimately produce similar minerals during Fe(III) reduction.