Sorption kinetics of isotopically labelled divalent mercury (196Hg2+) in soil.

Understanding the sorption kinetics of Hg2+ is the key to predicting its reactivity in soils which is indispensable for environmental risk assessment. The temporal change in the solubility of 196Hg2+ spikes (6 mg kg-1) added to a range of soils with different properties was investigated and modelled. The sorption of 196Hg2+ displayed a biphasic pattern with a rapid initial (short-term) phase followed by a slower (time-dependent) one. The overall reaction rate constants ranged from 0.003 to 4.9 h-1 and were significantly correlated (r = 0.94) to soil organic carbon (SOC). Elovich and Spherical Diffusion expressions compellingly fitted the observed 196Hg2+ sorption kinetics highlighting their flexibility to describe reactions occurring over multiple phases and wide timeframes. A parameterized Elovich model from soil variables indicated that the short-term sorption is solely controlled by SOC while the time-dependent sorption appeared independent of SOC and decreased at higher pH values and Al(OH)3 and MnO2 concentrations. This is consistent with a rapid chemical reaction of Hg2+ with soil organic matter (SOM) which is followed by a noticeably slower phase likely occurring through physical pathways e.g. pore diffusion of Hg2+ into spherical soil aggregates and progressive incorporation of soluble organic-Hg into solid phase. The model lines predicted that in soils with >4% SOC, Hg2+ is removed from soil solution over seconds to minutes; however, in soils with <2% SOC and higher pH values, Hg2+ may remain soluble for months and beyond with a considerable associated risk of re-emission or migration to the surrounding environments.