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

This work describes the influence of temperature on two Hg(II)-cysteine modified clay minerals with different layer charge (i.e.,vermiculite and montmorillonite) by using chemical analyses,thermal analyses coupled with evolved gas mass spectrometry(TGA-MSEGA), synchrotron-based X-ray absorptionspectroscopy (XAS), and X-ray powder diffraction (XRPD)techniques. These two latter techniques were applied onsamples heated “in situ” up to 900°C. Hg binds cysteine by thethiol group with Hg–SH bond lengths similar in both clayminerals, thus suggesting their independence from layer charge.On the contrary, the thermal behaviour of the adsorbedorganometallic complexes is strictly layer charge dependent, asevidenced by decomposition temperatures, which are commonlylower in montmorillonite than in vermiculite. Results from Xrayabsorption near edge structure (XANES) spectroscopyevidence a nearly completely Hg removal in montmorillonite attemperature values ranging from 600 to 700°C, unlike invermiculite where Hg is still present at these temperatures.