Removal of As(III) from acidic waters using schwertmannite: Surface speciation and effect of synthesis pathway

The removal of arsenic(III) from aqueous solution by three different schwertmannite specimens that were synthesized by i) biogenic precipitation (SHM_MS), ii) slow abiotic dialysis (SHM_DS) and iii) fast abiotic oxidation of Fe(II) (SHM_FS) was studied through sorption experiments at pH 3.0. Marked differences in dissolution rates, Fe:S ratios (4.7–6.4) and structural properties were found among the specimens. The sorption mechanisms are governed by multilayer processes as indicated by highly non-linear Freundlich adsorption isotherms. The affinities increased in the sequence SHM_FS (KF = 0.02 L0.57/(mol− 0.43 g)) < SHM_MS (KF = 0.04 L0.64/(mol− 0.36 g)) < SHM_DS (KF = 0.10 L0.66/(mol− 0.34 g)) possibly owing to the corresponding increase in specific surface area (5.3, 14.7, and 210 m2/g, respectively). XANES results show that no detectable oxidation of As(III) occurred on any of the schwertmannite surfaces within 5 days equilibration time even though oxidation both by O2 and Fe(III) was favorable thermodynamically. A relationship between As(III) uptake and sulfate release was observed at high initial As(III) concentrations only suggesting that As(III) retention through ligand exchange is of minor relevance under the experimental conditions examined in this study. Higher As(III) loading caused morphological degradation yielding angular particles with porous centers and the extent of degradation varied accordingly with As(III) partitioning. Appearance of new infrared absorption bands after As(III) sorption and shifting in diffractogram peaks with increasing As(III) concentrations suggest the formation of a ferric arsenite surface precipitate.

Research Highlights
► As(III) removal using various schwertmannite types is done here for the first time.
► Sorbed As(III) did not undergo surface oxidation by schwertmannite is a novel finding.
► Morphological degradation by As(III) retention is an important observation.
► SO42–↔As(III) exchange and surface precipitation control the overall As(III) uptake.