Arsenic removal from surface waters by hydrotalcite-like sulphate minerals: field evidences from an old mine in Sardinia, Italy

The presence of hydrotalcite-like sulphates such as glaucocerinite and zincowoodwardite [(Zn, Cu)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 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 from waters. An extremely variable composition and low degree of structural order make natural HTs difficult to be characterized. For this reason a combination of several analytical techniques (ICP-OES, XRD, CHNS, SEM-EDX, TEM-EDX, IR, XPS) was adopted to obtain a good characterization of investigated samples from Baccu Locci. Along with field investigations, laboratory experiments on synthetic phases showed that glaucocerinite and zincowoodwardite remove As from waters, mostly through exchange processes, due to the strong capability of (AsO4)3- in replacing (SO4)2- in the interlayer of HTs structure, where anionic groups are hosted. The advantage of HT-like sulphates, in comparison with Fe-oxides-hydroxides (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 in arsenic sorption and a greater arsenic mobility account for an increased arsenic hazard.