Thiosubstituted Organophosphinic Acids as Selective Extractants for Mercury(II) from Acidic Chloride Solutions

This paper reports results on the extraction behavior of mercury (II) from dilute hydrochloric acid solutions using mono‐ and di‐sulphur analogs of bis(2,4,4‐trimethylpentyl)phosphinic acid, namely, bis(2,4,4‐trimethylpentyl)monothiophosphinic acid (CYANEX 302) and bis(2,4,4‐trimethylpentyl)dithiophosphinic acid (CYANEX 301). The effect of various parameters such as, concentrations of mineral acid, metal ion, extractant, and the nature of the diluent on the extraction of mercury (II) has been investigated. The results clearly demonstrate that CYANEX 302 and CYANEX 301 do not behave as cation‐exchange extractants but as solvating extractants, owing to the strong affinity of sulphur atom of the extractant for the soft metal ion mercury. The extraction equilibrium of mercury (II) can be represented as: where HX represents CYANEX 302 and CYANEX 301. The equilibrium constants of the extracted complexes have been calculated by non‐linear regression analysis with the aid of suitable chemical model. IR spectral studies of the extracted complex have been used to further clarify the nature of the extracted complex. The potential of CYANEX 301 for decontaminating mercury from the brine‐sludge of a Chlor‐Alkali plant has been assessed.