Biogeochemical transformation processes of iron, manganese, ammonium under coexisting conditions in groundwater based on experimental data

Abstract A dynamic column experiment using aquifer medium collected from a riverbank filtration (RBF) site in Northeast China was conducted to investigate the biogeochemical transformation processes of iron (Fe2+), manganese (Mn2+), ammonium (NH4+) and their interaction under coexisting conditions in real groundwater. Water parameters such as contaminant concentrations, pH and dissolved oxygen (DO) were determined while microbial analysis for the aquifer medium was performed through 16S rRNA sequencing. The results revealed that biochemical oxidation, reductive dissolution, adsorption were among the critical biogeochemical processes involved in the transformation of Fe2+, Mn2+ and NH4+, in which biological processes were the core process. The transformation rate of Fe2+, Mn2+ and NH4+ jointly revealed that the competition for DO existed throughout their co-oxidation process, and determined the spatial characteristics of their transformation processes to a certain extent. Fe2+ oxidation and nitrification primarily occurred in the upper oxidation zone (100 cm–70 cm), Mn2+ transformation and anaerobic ammonium oxidation (ANAMMOX) occurred mainly beneath the oxidation zone. The predominant area of microbial growth and metabolism was another key factor related to the spatial characteristics of biotransformation. The microbial analysis illustrated that Pseudomonas, Arthrobacter, Gallionella, Acinetobacter, Bacillus were among the dominant genera related to iron-oxidizing bacteria (IOB) and manganese-oxidizing bacteria (MnOB). The IOB-related genera were distributed mainly at 80 cm–60 cm with a relative abundance of 49%, MnOB-related genera were the most abundant (35%) bacteria at 60 cm–40 cm, and nitrifying bacteria (Nitrospira and Nitrosomonas) mainly existed in the upper part of the 100 cm–70 cm where nitrification occurred. The influence of microbial activity by changing chemical environment was manifested through the variation of transformation intensity during the experiment. The iron and manganese oxides generated by the bio-oxidation of Fe2+ and Mn2+ contributed significantly to the transformation of all three contaminants. The results may be valuable for understanding the evolution mechanism of groundwater when contaminated simultaneously with Fe2+, Mn2+ and NH4+.