Figure 4: Relationship between the formation rate of CH₃Hg⁺ and the product of Hg²⁺ and CH₃I concentrations in a pond water.

To calculate the relationship between the formation rate of CH₃Hg⁺ (r) and concentrations of Hg²⁺ and CH₃I, various concentration of CH₃I (0, 0.1, 0.2, 0.5, 1.0 and 2.0 mmol l⁻¹) and ¹⁹⁹HgCl₂ (0, 1.0, 2.0, 3.0, 4.0 and 5.0 nmol l⁻¹) were prepared using pond water containing 1 ng l⁻¹ (5.0 pmol l⁻¹) . The formation rate was observed to be positively related to the product of Hg²⁺ and CH₃I concentrations. Photomethylation rate constant of Hg²⁺ by CH₃I (k_M, see equation (13) in main text) was calculated to be (1.51±0.19) × 10⁻² l mol⁻¹ E⁻¹ m². All the tests were conducted in triplicate, and the error bars indicate the s.d. of three repeated measurements.