Arsenate and chromate incorporation in schwertmannite

Arsenate and chromate incorporation in schwertmannite Original Research Article Pages 1226-1239 Simona Regenspurg, Stefan Peiffer Close Close preview | Purchase PDF (390 K) | Related articles | Related reference work articles AbstractAbstract | Figures/TablesFigures/Tables | ReferencesReferences Abstract High concentrations of Cr (up to 812 ppm) and As (up to 6740 ppm) were detected in precipitates of the mineral schwertmannite in areas influenced by acid mine drainage. Schwertmannite may act as well as a natural filter for these elements in water as well as their source by releasing the previously bound elements during its dissolution or mineral-transformation. The mechanisms of uptake and potential release for the species arsenate and chromate were investigated by performing synthesis and stability experiments with schwertmannite. Schwertmannite, synthesized in solutions containing arsenate in addition to sulphate, was enriched by up to 10.3 wt% arsenate without detectable structural changes as demonstrated by powder X-ray diffraction (XRD). In contrast to arsenate, a total substitution of sulphate by chromate was possible in sulphate-free solutions. Thereby, the chromate content in schwertmannite could reach 15.3 wt%. To determine the release of oxyanions from schwertmannite over time, synthetic schwertmannite samples containing varying amounts of sulphate, chromate and arsenate were kept at a stable pH of either 2 or 4 over 1 year in suspension. At several time intervals Fe and the oxyanions were measured in solution and alterations of the solid part were observed by XRD and Fourier-Transform infrared (FT-IR) spectroscopy. At pH 2 schwertmannite partly dissolved and the total release of arsenate (24%) was low in contrast to chromate (35.4–57.5%) and sulphate (67–76%). Accordingly, the ionic activity product (log IAP) of arsenated schwertmannite was lowest (13.5), followed by the log IAP for chromated schwertmannite (16.2–18.5) and the log IAP for regular (=non-substituted) schwertmannite (18). At pH 4 schwertmannite transformed to goethite, an effect which occurred at the fastest rate for regular schwertmannite (=arsenate- and chromate-free), followed by chromate and arsenate containing schwertmannite. Both chromate and more evidently arsenate have a stabilizing effect on the schwertmannite structure, because they retarded the dissolution and transformation reactions. These kinetic investigations as well as crystallographic considerations demonstrated that the strength of the Fe(III) complexes with the anions controls the formation process and the stability of schwertmannite: with increasing affinity of the oxyanions to form complexes with Fe(III), the strength of the resulting binding and thus the stability and substitution preference increases. Article Outline 1. Introduction 1.1. Schwertmannite structure 1.2. Arsenate and chromate 2. Materials and methods 2.1. Analytical methods 2.2. Mineral synthesis 2.3. Stability experiment 2.4. Natural specimens 3. Results 3.1. Enrichment of arsenic and chromium in natural schwertmannite 3.2. Identification of schwertmannite by X-ray diffraction 3.3. Processes and products of schwertmannite synthesis 3.4. Alteration of schwertmannite suspensions at pH 2 and 4 over time 3.4.1. Release of iron, sulphate, arsenate and chromate 3.4.2. Variation in crystal structure 4. Discussion 4.1. Oxyanion incorporation by solid solution 4.2. Anion affinity to iron(III) as initial controlling factor for schwertmannite formation 4.3. Influence of oxyanions on schwertmannite stability Acknowledgements References