Organic-matter composition and microbial communities as key indicators for arsenic mobility in groundwater aquifers: Evidence from PLFA and 3D fluorescence

Abstract

The phospholipid fatty acids (PLFA) and dissolved organic matter (DOM) 3D fluorescent signatures were appointed to elaborate arsenic (As) mobilization mechanism in groundwater from the Datong Basin. Groundwater samples were collected along the groundwater flow path from recharge zones to discharge zones according to redox sensitive parameters. Variations of Eh, NO3− and SO42− manifested the redox condition shifted from a weakly oxidative environment to highly reductive environment. In recharge zones, some aerobic bacteria may serve as the dominant species, such as Thiothrix (S-oxidizing bacteria: SOB), Gallionella (Fe-oxidizing bacteria: FeOB) and Fluviicola, characterized by 16:1ω7, 16:0 and 18:1ω7 PLFAs. The biogeochemical processes were mainly governed by aerobic bacteria exploiting protein-like DOM as electron donors to maintain metabolism together with depleting of O2 and NO3− (electron acceptors), which restricted the reduction of As. While in discharge zones, the anaerobic microbes played a predominant role, such as Desulfosporosinus (Sulfate reducing bacteria: SRB) and Clostridia (Fe reducing bacteria: FeRB), indicated by cy17:0, cy19:0 and 18:1ω9 PLFAs. SRB and FeRB mainly utilized protein-like DOM as energy sources for respiratory action. Simultaneously, biogenic reductive dissolution of Fe(III) (hydr)oxides and reduction of As(V), SO42− resulted in geogenic immobilized As reductive desorption into aquifers. The positive linear correlations between humic substances and Fe, As suggested that As-Fe-DOM complexation can enhance the transport of As and Fe in aqueous environments. The negative relationships between SRB/FeRB and As/Fe concentrations demonstrated that the HS− could react with Fe(II) and As(III) to form secondary Fe(II) sulfides or As-bearing sulfides, which subsequently sequestered As from groundwater via sorption or coprecipitation. In conclusion, the utilization of biodegradable protein-like DOM by SRB/FeRB being associated with reductive dissolution of As-bearing Fe (hydr)oxides minerals and As-Fe-DOM complexation promoted As mobility in groundwater aquifers.