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

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Abstract
The effect of pH on the methylation of Hg and As in lake sediment amended with nutrients was examined. The formation of methyl mercury from inorganic HgCl2 occurred only in the pH range 5.5-6.5. Dimethyl mercury was not produced. Hg 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 H2SO4, probably due to sulfide formation. In the As methylation experiments, volatile arsine or methyl arsines were rarely detected. Analysis of the culture fluid revealed that methyl arsonic acid and dimethyl arsinic acid were formed over the pH range 3.5-7.5. In contrast to the Hg methylation experiments, the As methylating microorganisms were not as sensitive to changes in pH, and different microorganisms may have been responsible for methylating As over this pH range.