Arsenate aqueous solution – anglesite PbSO4 interface: dissolution, precipitation, and arsenic sequestration

The circulation of toxic elements through soils, sediments and aquatic environments remains a significant environmental problem, which implies several, often unrecognized health risks. Dissolution and precipitation reactions that result in formation of sparingly soluble crystalline compounds containing toxic elements e.g. As, appear to be a promising strategy for reducing their chemical mobility and bioavailability (Magalhães, 2002; Wang et. al., 2013).

In this study, the processes occurring at the interface of anglesite and solutions containing AsO₄^3- ions were investigated. The results provide insight into the mechanism of As immobilization on the mineral surface through transformation of labile form into less reactive and more thermodynamically stable phases. Synthetic anglesite powder and fragments of natural anglesite crystals (Tsumeb, Namibia) were reacted for up to 3 months with solutions containing AsO₄^3- (50 mg As/l) in the presence of Cl^- ions at pH range 2 - 8. The experiments were conducted at room temperature (20 ^oC) or in the autoclave (120 ^oC).

Rapid sequestration of As from the solution was associated with precipitation of mimetite Pb₅(AsO₄)₃Cl: over 66% of As was removed within 24 hours. After 7 days of the reaction, the concentration of As decreased from 50 to less than 11 mg As/L. The concentration of Pb²⁺ ranged from 0.6 mg Pb/L at pH 8 to 20 mg Pb/L at pH 2. Both, homogeneous and heterogeneous precipitation of mimetite was observed, and some anglesite crystals were covered by incrustations. At pH=2, mimetite was associated with schultenite PbHAsO₄.

Scanning Electron Microscopy observations of the natural crystals surface, indicated that the reaction of As-containing solutions with anglesite at pH 4-8, involves dissolution of PbSO₄, resulting in formation of etch pits, followed by precipitation of randomly intergrown mimetite as a loosely bound crust. The crust was formed in the vicinity but is separated from anglesite surface. In contrary, at very acidic conditions (pH=2), anglesite is replaced by a secondary phase as a result of a coupled dissolution and precipitation, leading to formation of lead arsenate pseudomorph. Surprisingly, temperature had no significant effect on the reaction, while the pH control is of great importance and determines the mechanism.

References:

Magalhães, M. C. F. (2002). Arsenic. An environmental problem limited by solubility. Pure and Applied Chemistry, 74(10), 1843–1850.

Wang, L., Putnis, C. V., Ruiz-Agudo, E., King, H. E., & Putnis, A. (2013). Coupled dissolution and precipitation at the cerussite-phosphate solution interface: Implications for immobilization of lead in soils. Environmental science & technology, 47(23), 13502-13510.

This research was funded by AGH University of Kraków project No 16.16.140.315.