A critical review of three methods used for the measurement of mercury (Hg2+)-dissolved organic matter stability constants

Three experimental techniques – ion exchange, liquid–liquid extraction with competitive ligand exchange, and solid-phase extraction with competitive ligand exchange (CLE–SPE) – were evaluated as methods for determining conditional stability constants (K) for the binding of mercury (Hg2+) to dissolved organic matter (DOM). To determine the utility of a given method to measure stability constants at environmentally relevant experimental conditions, experimental results should meet three criteria: (1) the data must be experimentally valid, in that they were acquired under conditions that meet all the requirements of the experimental method, (2) the Hg:DOM ratio should be determined and it should fall within levels that are consistent with environmental conditions, and (3) the stability constants must fall within the detection window of the method. The ion exchange method was found to be limited by its detection window, which constrains the method to stability constants with log K values less than about 14. The liquid–liquid extraction method was found to be complicated by the ability of Hg–DOM complexes to partition into the organic phase. The CLE–SPE method was found to be the most suitable of these methods for the measurement of Hg–DOM stability constants. Stability constants for DOM isolates measured using the CLE–SPE method at environmentally relevant Hg:DOM ratios were log K = 25–30 (M−1). These values are consistent with the strong Hg2+ binding expected for reduced S-containing binding sites.