Effect of temperature on Hg-cysteine complexes in vermiculite and montmorillonite

Abstract

This study describes the influence of temperature on two Hg(II)-cysteine modified clay minerals with different layer charges ( i.e., montmorillonite and vermiculite) by using chemical and thermal analyses coupled with evolved gas mass spectrometry (TGA-MSEGA), synchrotron-based X-ray absorption spectroscopy (XAS), and X-ray powder diffraction (XRPD) techniques; these two latter techniques were applied on samples heated “ in situ” up to 900 °C. Hg bonds cysteine by the thiol group with Hg–SH bond lengths similar in both clay minerals, thus suggesting their independence from layer charge. On the contrary, the thermal behaviour of the adsorbed organometallic complexes was strictly layer charge dependent, as evidenced by decomposition temperatures, which were lower in montmorillonite than in vermiculite. Results from X-ray absorption near edge structure (XANES) spectroscopy evidenced nearly complete Hg desorption in montmorillonite between 600 and 700 °C, unlike in vermiculite where Hg was still present at these values.