The role of anaerobes and organic substances in the mobilisation of arsenic in groundwater

UoM administered thesis: Phd

  • Authors:
  • Babajide Macaulay

Abstract

The use of arsenic-laden groundwater for domestic use and crop irrigation is of global concern, particularly in South East Asia where the magnitude of contamination is causing the largest mass poisoning in human history, affecting the lives of hundreds of millions of people. Several studies showed that the geogenic release of arsenic into aquifers is largely driven by organic-microbe-mineral interactions in the subsurface. However, it remains unclear how microbes transfer electrons to insoluble arsenic-bearing iron minerals and what exactly the role organic matter (OM), particularly the dissolved OM (DOM), plays in this mobilisation. Most studies to date are focussed on lab-based microcosms and their findings may not be directly applicable in the field, indicating the need for in-situ studies to compliment these lab-based findings. To avoid the need to transport large volumes of water to the lab for further analyses, methods that pre-concentrate DOM and capture the relevant microbial community are required. Thus, the overall aims of this study are to develop and test novel in-situ methods of capturing a wide variety of Fe(III)/As(V)-metabolising microbes and DOM in the water column (groundwater) to further our understanding of how they solubilise Fe and/or As at the subsurface. Illumina 16S rRNA gene sequencing and organic geochemical analyses indicate that pumice and sand coated with Fe(III) and/or As(V) can successfully be used to capture a wide range of dissimilatory iron-reducing bacteria (DIRB), dissimilatory arsenic-respiring prokaryotes (DARPs) and low molecular weight DOM in-situ using “microbe baits” deployed in boreholes or used to filter groundwaters. Subsequently, microcosm experiments using the captured microbial biomass and OM, indicated the release of Fe(II) and As(III), confirming the capture/presence of Fe(III)/As(V)-metabolising microbes. In addition, organic geochemical analyses indicate that this release was coupled to the depletion of macromolecular organic compounds present and, in the case of As(V) reduction, was accompanied by the release of nitrogen-containing compounds likely secreted by the native microbes. A filtration technique for the capture of DOM in the field using a polymer-based Bond Elut-PPL cartridge, was tested and optimised. Water sample acidity and sampling flow rate were found to reduce the DOM extraction efficiency of this cartridge, limiting the usability in the field. The analyses of macromolecular OM using pyrolysis GCMS was also optimised, reducing pyrogram analyses times by ~60%. Although, the techniques developed still require further field trials, the contributions from this work will further our understanding of the subtle organic-microbe-mineral interactions driving groundwater arsenic mobility.

Details

Original languageEnglish
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Award date31 Dec 2019