ABIOTIC REDUCTION OF MERCURY BY HUMIC SUBSTANCES IN AQUATIC SYSTEM - AN IMPORTANT PROCESS FOR THE MERCURY CYCLE

The potential ability of humic substances to reduce Hg(II) to Hg(0) in aqueous systems and, consequently, strongly influence Hg speciation and mobility in the environment is known but has not been studied in detail. A demonstration of the redox behavior of Hg in the presence of humic substances is made in the present work. Calculations show that the reduction is thermodynamically possible. The effects of some chemical parameters (pH, aerobic/anaerobic conditions, presence of chloride) on the process were studied experimentally. Hg(0) production was highest in O2-free systems in the absence of chloride at pH ca 4.5, when ca 25% of initially 2 x 10(-6) M Hg(II) was reduced to Hg(0) in 50 hr. The presence of a competing ion (10(-4) M Eu) in the system as well as methylation of the carboxyl groups in the humic substance considerably reduced the Hg(0) production. The practical importance of the abiotic reduction of Hg in the environment is pointed out.