Oxidative dissolution of metacinnabar (beta-HgS) by dissolved oxygen

The oxidative dissolution rate of metacinnabar by dissolved O-2 was measure d at pH similar to5 in batch and column reactors. In the batch reactors, th e dissolution rate varied from 3.15 (+/-0.40) to 5.87 (+/-0.39) x 10(-2) mu mol/m(2)/day (I = 0.01 M, 23 degreesC) and increased with stirring speed, a characteristic normally associated with a transport-controlled reaction. However, theoretical calculations, a measured activation energy of 77 (+/-8 ) kJ/mol (I = 0.01 M), and the mineral dissolution literature indicate reac tion rates this slow are unlikely to be transport controlled. This phenomen on was attributed to the tendency of the hydrophobic source powder to aggre gate and minimize the effective outer surface area. However, in a column ex periment, the steady-state dissolution rate ranged from 1.34 (+/-0.11) to 2 .27 (+/-0.11) x 10(-2) mu mol/m(2)/day (I = 0.01 M, 23 degreesC) and was al so influenced by flow rate, suggesting hydrodynamic conditions may influenc e weathering rates observed in the field. The rate of Hg release to solutio n, under a range of hydrogeochemical conditions that more closely approxima ted those in the subsurface, was I to 3 orders of magnitude lower than the dissolution rate due to the adsorption of released Hg(II) to the metacinnab ar surface. The measured dissolution rates under all conditions were slow c ompared to the dissolution rates of minerals typically considered stable in the environment, and the adsorption of Hg(II) to the metacinnabar surface further lowered the Hg release rate. (C) 2001 Elsevier Science Ltd. All rig hts reserved.