Uptake of dissolved lead by anhydrite surfaces

Abstract

The fate of harmful metals in the Earth crust is importantly affected by sorption processes on mineral surfaces. Here, a study of the ability of anhydrite surfaces to uptake dissolved Pb is presented. Experiments were conducted at room temperature using initial Pb concentration ([Pbaq]0) ranging between 10 and 1000 mg/L and a batch type set-up. Inductively coupled plasma optical emission spectrometry analyses showed that [Pbaq] progressively decreased as the time of interaction increased, to reach a final steady state value of ~3.0 mg/L, irrespectively of [Pbaq]0. However, the time elapsed before the steady state value was reached strongly depended on [Pbaq]0, with the drop to this final value occurring in less than 1 day interaction when [Pbaq]0 ≥ 50 mg/L and after 20 days when [Pbaq]0 < 50 mg/L. Scanning Electron Microscopy and X-ray diffraction analyses confirmed the epitactic growth of anglesite (PbSO4) crystals on anhydrite surfaces when [Pbaq]0 ≥ 50 mg/L. X-ray Absorption Near Edge Structure spectroscopy points to a different sorption mechanisms when [Pbaq]0 < 50 mg/L. The results show that the epitactic growth of anglesite on anhydrite has no significant impact on the ability of anhydrite surfaces to remove Pbaq, which show equal effectiveness as that of gypsum surfaces. The high reactivity of anhydrite surfaces renders this phase potentially important in the control of the fate of dissolved metals in nature.