Removal of heavy metals and arsenic by microbial sulfate reduction and the characterization and stability of the immobilized solid-phase products
Creators: name: Jong, Tony
Publisher: name: Charles Darwin University
Year: 2004

description: The immobilization of heavy metals and metalloids by microbial sulfate reduction and the various chemical forms of metalloids and heavy metals precipitated, and the fate of these species within such systems over time were investigated. A bench-scale upflow anaerobic packed bed (UAPB) bioreactor was inoculated with a mixed population of sulfate reducing bacteria (SRB), isolated from a wetland, and operated under progressively acidic conditions. The reactor was operated under continuous flow conditions and was shown to be capable of sulfate reduction at pH 6.0, 5.5, 5.0, 4.5, 4.0 and 3.5 in a medium containing 53.5 mM sodium lactate. This contrasted previously published work that showed biological sulfate reduction was completely inhibited in media containing greater than 5 mM organic acids at pH 3.8. The lowest pH at which the reactor could support significant sulfate reduction was pH 4.0. At pH 3.5, sulfide production was substantially lowered, with sulfate removal decreasing to &lt;7.6%. The use of conductivity measurements, compared to pH and redox potential, seems to be a more sensitive tool for monitoring the activity of SRB, and offers the potential for providing continuous and rapid monitoring information with respect to SRB activity. Mildly acidic metal (Cu, Zn, Ni, Fe, Al and Mg), arsenic and sulfate contaminated waters were treated, over a 14 day period at 25 °C, in a bench-scale upflow anaerobic packed bed reactor filled with silica sand, employing a mixed population of sulfate-reducing bacteria. The activity of SRB increased the water pH from ~4.5 to 7.0, and enhanced the removal of sulfate and metals in comparison to controls not inoculated with SRB. Addition of organic substrate and sulfate at loading rates of 7.43 and 3.71 kg d-1 m-3, respectively, resulted in &gt;82% reduction in sulfate concentration. The reactor removed more than 97.5% of the initial concentrations of Cu, Zn and Ni, while only &gt;77.5% and &gt;82% of As and Fe were removed, respectively. In contrast, Mg and Al levels remained unchanged during the whole treatment process. The removal patterns for Cu, Zn, Ni and Fe reflected the trend in their solubility for their respective metal sulfides, while As removal appeared to coincide with decreasing Cu, Zn, Ni and Fe concentrations, which suggests adsorption or concomitant precipitation with the other metal sulfides. It was found that bacterially produced metal sulfide (BPMS) precipitates had adsorptive properties comparable with other adsorbents with respect to the specific uptake of a range of metals and, the levels to which dissolved metal concentrations in solution can be reduced. The percentage of adsorption increased with increasing pH and adsorbent dose, but decreased with increasing initial dissolved metal concentration. The pH of the solution was the most important parameter controlling adsorption of Cd(II), Cu(II), Fe(II), Ni(II), Pb(II), Zn(H) and As(V) by BPMS. The adsorption data was successfully modelled using the Langmuir adsorption isotherm. Desorption experiments showed that the reversibility of adsorption was low, suggesting a high-affinity type adsorption governed by chemisorption. The mechanism of adsorption for the divalent metals was thought to occur via the formation of strong, inner-sphere complexes, involving surface hydroxyl groups. However, the mechanism involved for the adsorption of As(V) by BPMS appears to be distinct from those of surface hydroxyl exchange.