The potential of granulated schwertmannite adsorbents to remove oxyanions (SeO32−, SeO42−, MoO42−, PO43−, Sb(OH)6−) from contaminated water

Abstract The mineral schwertmannite forms in acidic sulfate-rich environments (pH 2.9–4) and has a large sorption potential for oxyanions. In this study we tested the properties of granulated schwertmannite produced from material obtained from a mine water treatment plant to remove SeO32−, SeO42−, MoO42−, PO43−, and Sb(OH)6−. We studied the kinetics of sulfate release from the granulated schwertmannite in suspensions containing various concentrations of HCO3− and its effect on the sorption kinetics of the oxyanions in synthetic solutions and industrial wastewater through batch experiment at acidic and neutral pH. We further tested the suitability of the granulated schwertmannite adsorbents as filter bed materials to remove SeO32−, MoO42−, and PO43− from synthetic solution via column experiments. Using a two-step pseudo-first order model, we were able to distinguish between an initial fast sorption process and a second slow reaction with characteristic reaction times (1/k) of ~10 min and ~ 10 h, respectively. Ligand exchange with surface bound sulfate is proposed to be responsible for the fast reaction while intraparticle diffusion and exchange with structural sulfate control the slow process. At neutral pH values, sulfate release is complete within one week, suggesting partial transformation of schwertmannite into a new ferric oxyhydroxide phase. The sorption kinetics were reflected in the results from the column experiments where only the fast sorbing fraction of surface sites participated in the retention of oxyanions at a mean residence times τ of 8 min with retention capacities of 149 μmol ∙ (g SHM)−1 for SeO32− and 242 μmol ∙ (g SHM)−1 for MoO42−. Hydraulic residence time increased the retention of PO43− with 213 μmol ∙ (g SHM)−1 at τ = 4.75 min and 508 μmol ∙ (g SHM)−1 at τ = 47.5 min. In summary, granulated schwertmannite turned out to be a promising adsorbent to remove the tested oxyanions from contaminated waters. Our results demonstrated that its application in filter bed systems needs to consider the trade-off between residence time and sorption kinetics. An increase in residence time clearly increases the retention capacity.