Uptake and release of arsenic and antimony in alunite-jarosite and beudantite group minerals

Abstract

Arsenic and antimony are highly toxic to humans, animals and plants. Incorporation in alunite, jarosite and beudantite group minerals can immobilize these elements and restrict their bioavailability in acidic, oxidizing environments. This paper reviews research on the magnitude and mechanisms of incorporation of As and Sb in, and release from, alunite, jarosite and beudantite group minerals in mostly abiotic systems. Arsenate-for-sulfate substitution is observed for all three mineral groups, with the magnitude of incorporation being beudantite (3-8.5 wt. % As) > alunite (3.6 wt. % As) > natroalunite (2.8 wt. %) > jarosite (1.6 wt. % As) > natroalunite (1.5 wt. % As) > hydroniumalunite (0.034 wt. % As). Arsenate substitution is limited by the charge differences between sulfate (-2) and arsenate (-3), deficiencies in B-cations in octahedral sites and for hydroniumalunite, difficulty in substituting protonated H2O-for-OH- groups. Substitution of arsenate causes increases in the c-axis for alunite and natroalunite, and in the c- and a-axes for jarosite. The degree of uptake is dependent on, but limited by, the AsO4/TO4 ratio. Aerobic and abiotic As release from alunite and natroalunite is limited, especially between pH 5 and 8. Release of As is also very limited in As-bearing jarosite, natrojarosite and ammoniumjarosite at pH 8 due to formation of secondary maghemite, goethite, hematite and Fe arsenates that resorb the liberated As. Abiotic reductive dissolution of As-bearing jarosite at pH 4, 5.5 and 7 is likewise restricted by the formation of secondary green rust sulfate, goethite and lepidocrocite that take up the As. Similar processes have been observed for the aerobic dissolution of Pb-As-jarosite (beudantite analogue), with secondary Fe oxyhydroxides resorbing the released As at pH 8. Higher amounts of As are released, however, during microbial-driven jarosite dissolution. Natural jarosite has been found to contain up to 5.9 wt. % Sb5+ substituting for Fe3+ in the B-site of the mineral structure. Sb(V) is not released from jarosite at pH 4 during abiotic reductive dissolution, but at pH 5.5 and 7, up to 75% of the mobilized Sb can be structurally incorporated into secondary green rust sulfate, lepidocrocite or goethite. Further research is needed on the co-incorporation of As, Sb and other ions in, and the uptake and release of Sb from, alunite, jarosite and beudantite group minerals, the influence of microbes on these processes and the long-term (>1 year) stability of these minerals.