Immobilization of mercury and arsenic and microbial response in contaminated soil treated with coal-derived iron-modified carbon foams

Industrial sites contaminated by metallurgical activity pose a significant risk to human health and the environment. In this study, novel carbon foam materials, either by themselves (CF) or combined with goethite nanocomposites (CFGo), were tested for metal(loid) remediation in a brownfield soil (El Terronal, Spain) highly contaminated with arsenic and mercury. Their use requires preserving the soil's physicochemical and biological properties, in which microorganisms play a crucial role. We analysed changes in soil chemistry, potential contaminant interactions with the CFs, and dynamics of microbial populations over 17 days of treatment. Incubation with foams decreased As availability by 42 % (CF) and 6 % (CFGo), while Hg availability was reduced by 79 % (CF) and by more than 80 % (CFGo). However, under CF treatment, Hg availability was initially increased, promoting selection of bacterial populations with the genetic capacity to attenuate its toxicity. Overall reduction in bacterial diversity was observed through molecular analysis with ARISA and Illumina 16S rRNA gene sequencing, while direct tests with CF and CFGo on isolated bacteria showed no harmful effects. Bacteria from the phylum Patescibacteria, possibly parasitic and not cultured in the laboratory, were favoured in the presence of CF, while CFGo treatment promoted, from the outset, development of bacterial community distinct from that of the original and control soils, dominated by the genera Pseudomonas, Cupriavidus, and Pseudarthrobacter from the phylum Proteobacteria. Therefore, those new amendments were found suitable for field-scale use in the El Terronal soil, opening new lines of research on bacterial interactions with contaminants and CF.