Field evidences on release/removal of arsenic by hydrotalcite-like compounds in mining waters. The example of Baccu Locci mine (Sardinia, Italy)

Minerals belonging to the hydrotalcite group are assuming an increasing interest in environmental mineralogy. In fact, thanks to a high anionic exchange capacity, they can find applications in site-remediation as contaminant removers from polluted waters. The presence of hydrotalcite-like sulphates such as glaucocerinite, zincowoodwardite, hydrowoodwardite and woodwardite [Me2+ 1-xAlx(SO4)x/2(OH)2·mH2O] has been documented in a number of mining areas. They usually occur as fine-grained precipitates from mining waters or as porous crusts. While their role on removing cations such as Zn, Cu and Al from waters is immediately evident, the role they have in incorporating undesirable anions, other than (SO4)2-, is often disregarded. However, field evidences from a mine area in Sardinia (Italy) showed that these phases have a great capability to remove arsenic, probably as (AsO4)3-, from waters. The advantage of these hydrotalcites-like sulphates, in comparison with Fe-oxideshydroxides (generally considered the most efficient arsenic removers, naturally occurring in mining areas), is to act as removers in waters at high pH values, when a reduced efficiency of Fe-oxides-hydroxides for arsenic sorption and a greater arsenic mobility account for an increased arsenic hazard.