Controls on the stability of sulfide sols: Colloidal covellite as an example

The conditions under which dilute CuS sols might be stable in natural waters have been explored. In single electrolyte solutions at room temperature, coagulation data adhere approximately to the classic theory for electrostatic stabilization. From this theory, the Hamaker constant is estimated to be 4 × 10^?19J. In a simple, four-component, synthetic seawater, coagulation occurs at 0.14 g/kg salinity. The critical coagulation concentration for CaCl₂ drops by a factor of six between 25 and 175°C. These measurements, which appear to be the first over so large a temperature range, confirm previous suppositions about the effect of temperature. The results suggest that electrostatic stabilization will be ineffective in nature for substances like CuS with large Hamaker constants, because divalent cation concentrations in most aquatic environments are sufficient for coagulation. Three alternate mechanisms by which organic compounds can stabilize sulfide sols are reported: chelation of counterions. surface hydrophillization through adsorption, and steric stabilization by polymers. Humic acid, which contributes stability when present at less than 1 mg C/1, may stabilize by all three mechanisms. However, theory indicates that even in the absence of chemical stabilization, extremely dilute sols such as would be encountered in anoxic marine basins might require many years to coagulate, thus appearing stable even if not.