Effect of ph on the methylation of mercury and arsenic by sediment microorganisms

Abstract The effect of pH on the methylation of mercury and arsenic in lake sediment amended with nutrients was examined. The formation of methyl mercury from inorganic mercuric chloride occurred only in the pH range of 5.5–6.5. Dimethyl mercury was not produced. Mercury methylation in this narrow pH range was probably the result of a microbial population which had adapted to the in situ pH of 5.8. The levels of methyl mercury were reduced in the presence of sulfuric acid, probably due to sulfide formation. In the arsenic methylation experiments, volatile arsine or methyl arsines were rarely detected. Analysis of the culture fluid revealed that methyl arsonic acid and dimethyl arsinlcacid were formed over the pH range of 3.5 to 7.5. In contrast to the mercury methylation experiments, the arsenic methylating microorganisms were not as sensitive to changes in pH and different microorganisms may have been responsible for methylating arsenic over this pH range.