Removal of Arsenic in a passive treatment system for mine drainage

Mine drainage from abandoned mines is a major source of Arsenic (As); a ubiquitous, toxic and carcinogenic metalloid affecting over 200 million people worldwide. Recently, we observed extensive (up to 90%) co-removal of As in a vertical flow pond (VFP) passive treatment system that was designed to remove zinc from mine water drainage by precipitating ZnS following microbial sulphate reduction. However, the mechanism of As removal in the passive treatment system was unclear, even as microbial sulphate reduction is an emerging and cost-effective innovation for treating As contamination yet has received limited attention. Hence, the aim of this research was to investigate the main mechanism of As removal in the passive treatment system.

To understand the complex biogeochemical interactions of As with redox sensitive elements (Fe, S) and dissolved organic carbon (DOC), we conducted monthly field sampling over one year at the passive treatment system at the Force Crag abandoned mine site, Cumbria, UK. Aqueous sample and porewater of three depth profiles including overlying water in the VFP were collected and analysed for total element concentration, speciation (As, Fe) and DOC. Elemental composition was determined with ICP-MS. Speciation of As and Fe in aqueous phase were determined using solid phase extraction cartridges and phenanthroline method respectively, where DOC was determined with TOC Analyser.

The concentration of As (total, dissolved and colloidal) were consistently positively correlated with total, dissolved and colloidal Fe at the influent and four effluents, with concomitant decrease of both elements at the four effluents indicating potential influence of Fe on As mobility. Highest concentration of dissolved As and Fe were recorded in the porewater, which increased with depths, possibly due to vertical transportation and accumulation through the VFP, although highest level of DOC and sulphate in porewater may have caused competitive adsorption with As, resulting to weak retention of As on the binding sites. As(III) and Fe(II) were predominant in all aqueous samples, including the porewater, suggesting, to our surprise, the absence of redox transformations of As and Fe in the  VFP. Decreased As concentrations at the four effluents coincided with decreased redox potentials (anaerobic), decreased sulphate and increased DOC, indicating that organic substrates were available as electron donor and may have fuelled microbial sulphate reduction, and subsequently generating sulphide. Combined with geochemical modelling of mineral saturation indices, our results point to the precipitation of As sulphides and/or co-precipitation with Fe sulphides as the likely mechanism(s) through which As was scavenged in the treatment system. We suggest that this passive treatment system relying on microbial sulphate reduction could be further developed for treatment of As contamination in mine water effluents.