Immobilization of arsenic as scorodite by a thermoacidophilic mixed culture via As(III)-catalyzed oxidation with activated carbon
Vega-Hernandez, Silvia ; Weijma, Jan ; Buisman, Cees J.N. - \ 2019
Journal of Hazardous Materials 368 (2019). - ISSN 0304-3894 - p. 221 - 227.
arsenic removal - arsenite oxidation - biogenic scorodite - ferrous iron oxidation - granular activated carbon - thermoacidophilic mixed culture
In this study we describe the immobilization of arsenic as scorodite (FeAsO4.2H2O) by a thermophilic iron-oxidizing mixed culture from an acidic sulfate medium containing 500 mg L-1 of Fe(II), 500 mg L-1 As(III) and granular activated carbon (GAC) as the main arsenite oxidant. This study shows that crystalline scorodite can only be precipitated in the presence of the ferrous iron-oxidizing mixed culture (pH 1.3 and 70 °C). The efficiency of arsenite oxidation was over 99% with a maximum specific oxidation rate of 280 mgAs(III) gGAC-1 day-1. Ferrous iron and arsenite were also oxidized in the absence of the mixed culture, however, no scorodite precipitated under these conditions; consequently, scorodite precipitation was biologically induced. The precipitated scorodite particles had a size between 0.5 and 10 μm with an average of 5 μm, resulting in low settling rates. Ion activity product calculations and observations by Scanning Electron Microscopy (SEM) indicated that microbial cells served as surface for heterogeneous nucleation. The potential of the thermophilic mixed culture for the scorodite formation explored in this study provides the basis of a new approach for the treatment of As(III) polluted streams.
Iron oxidation kinetics and phosphate immobilization along the flow-path from groundwater into surface water
Grift, B. van der; Rozemeijer, J.C. ; Griffioen, J. ; Velde, Y. van der - \ 2014
Hydrology and Earth System Sciences 18 (2014)11. - ISSN 1027-5606 - p. 4687 - 4702.
suspended sediment - ferrous iron - fresh-water - phosphorus limitation - nutrient dynamics - fe(ii) oxidation - arsenic removal - natural-waters - riparian zone - river
The retention of phosphorus in surface waters through co-precipitation of phosphate with Fe-oxyhydroxides during exfiltration of anaerobic Fe(II) rich groundwater is not well understood. We developed an experimental field set-up to study Fe(II) oxidation and P immobilization along the flow-path from groundwater into surface water in an agricultural experimental catchment of a small lowland river. We physically separated tube drain effluent from groundwater discharge before it entered a ditch in an agricultural field. Through continuous discharge measurements and weekly water quality sampling of groundwater, tube drain water, exfiltrated groundwater, and surface water, we investigated Fe(II) oxidation kinetics and P immobilization processes. The oxidation rate inferred from our field measurements closely agreed with the general rate law for abiotic oxidation of Fe(II) by O-2. Seasonal changes in climatic conditions affected the Fe(II) oxidation process. Lower pH and lower temperatures in winter (compared to summer) resulted in low Fe oxidation rates. After exfiltration to the surface water, it took a couple of days to more than a week before complete oxidation of Fe(II) is reached. In summer time, Fe oxidation rates were much higher. The Fe concentrations in the exfiltrated groundwater were low, indicating that dissolved Fe(II) is completely oxidized prior to inflow into a ditch. While the Fe oxidation rates reduce drastically from summer to winter, P concentrations remained high in the groundwater and an order of magnitude lower in the surface water throughout the year. This study shows very fast immobilization of dissolved P during the initial stage of the Fe(II) oxidation process which results in P-depleted water before Fe(II) is completely depleted. This cannot be explained by surface complexation of phosphate to freshly formed Fe-oxyhydroxides but indicates the formation of Fe(III)-phosphate precipitates. The formation of Fe(III)-phosphates at redox gradients seems an important geochemical mechanism in the transformation of dissolved phosphate to structural phosphate and, therefore, a major control on the P retention in natural waters that drain anaerobic aquifers.