Experimental model of cerussite PbCO3 replacement by mimetite Pb5(AsO4)3Cl at pH 2 – 8

The mobility of arsenic in aquatic environments is controlled by oxidation states of arsenic, stability of solid phases, and chemical composition of water (Meng et al., 2002). Binding of arsenic in the environment may occur through precipitation of low-solubility salts (Magalhães, 2002), like mimetite Pb₅(AsO₄)₃Cl. The aim of this study is to experimentally investigate reactions between cerussite (PbCO₃) and solutions containing AsO₄^3- at various conditions favouring mimetite formation. These observations may provide a new recognition for As immobilization, which might be relevant in remediation of contaminated natural waters.

The mechanism of cerussite reaction with arsenate solutions (50 mg As/L) was studied at pH 2 – 8 using synthetic cerussite powder and fragments of natural cerussite crystals (Mibladen, Morocco). The reaction was carried out by direct contact of 500 ml of As-containing solution with PbCO₃, in presence of Cl^- ions.Cerussite was reacted for up to 4 weeks at in situ and ex situ setups. X–Ray Diffraction (XRD), Scanning Electron Microscopy with Energy Dispersive Spectrometry (SEM-EDS) and Electron Microprobe Analysis (EMPA) were used for analysis of the solid products of the experiments. The solutions were tested for Pb with Atomic Absorption Spectroscopy (AAS) and for AsO₄^3-  using colorimetry.

Precipitation of mimetite on cerussite powder crystals is observed already after 1 day of the reaction with arsenate solutions, at the whole range of pH. Mimetite forms hexagonal rods or needles less than 1 µm in size precipitating in the form of incrustations on PbCO₃ crystals. Their size depends on the pH: a fine-grained precipitate forms at higher pH. Observations of natural crystals show replacement of cerussite by polycrystalline mimetite crust. The crust made of columnar and needle crystals is porous allowing for solution penetration and progress of the reaction. The replacement features indicate similarity to pseudomorphic reactions, and the mechanism elucidated as interface coupled dissolution - precipitation. Overall, cerussite replacement by mimetite reduces AsO₄^3- concentration from 50 ppm to below 1 ppm. It also depends on the pH.

This research was funded by AGH University of Science and Technology project No 16.16.140.315.

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

Meng, X., Jing, C., & Korfiatis, G. P. (2002). A Review of Redox Transformation of Arsenic in Aquatic Environments. Biogeochemistry of Environmentally Important Trace Elements, 70–83.