SOLUBILITY OF CDS (GREENOCKITE) IN SULFIDIC WATERS AT 25 DEGREES-C

The solubility of synthetic greenockite (hexagonal CdS) at 25-degrees-C and 1 atm is reported. Important features of the experimental procedure include the following: long equilibration times (>100 days) in sealed glass ampules, avoidance of supersaturation to prevent colloid formation, and clean room analytical procedures to minimize contamination. For CdS(s) + H+ reversible Cd2+ + HS-, log K(sp) equals -14.36 +/- 0.26, as much as 2 orders of magnitude below published values for fresh CdS precipitates. Based on surface energy, only about one-third of the difference is ascribable to the small particle sizes in precipitates; the rest must be due to stacking irregularities, to isomorphous replacement of S2- by O2-, OH-, or HS-, etc. Solubility measurements in the pH range 5-9 with SIGMAS(-II) > 10(-4) M are explained by the following three species: Cd(HS)3-, Cd(HS)42-, and CdOHS-, the latter being equivalent to Cd(OH)2(HS)-. Respectively, the equilibrium constants for these species are log K2,1 = 2.08, log K3,1 = 3.53, and log K0-1 = -16.83, where log K(m,n) is for the general reaction, CdS(s) + mHS- + nH+ reversible Cd(S)1+m(H)n+mn-m. Minimum solubilities occur in mildly acidic solutions With SIGMAS(-II) between 10(-4) and 10(-3) M. Under such conditions, the potential is maximized for Cd removal from wastewaters by sulfide treatment. Framvaren Fjord and the Black Sea are substantially undersaturated with respect to pur CdS.