The role of adsorption in adsorbing colloid flotation of anions

Adsorbing colloid flotation of mercury(II), as a function of chloride concentration and pH, and sulphate and phosphate as a function of pH was carried out in the total carry-over mode (Sanciolo, 1993) and found to be highly successful. A mixed hydrous oxide containing equal portions of aluminium(III) and iron(III) was used in the case of mercury and a mixed hydrous oxide containing a 2:1:1 ration of aluminium(III):iron(III):iron(II) was used in the case of sulphate and phosphate. A dual surfactant mixture of sodium dodecyl sulphate (SDS) and dodecanoic acid (DA) was used in all three cases. Flotation was found to be effective both for model, simulated waste samples and for industrial effluents from a chloro-alkaline factory (high in mercury(II)) and a dye-making factory (high in sulphate). Adsorption experiments were also carried out, for the simulated waste, under similar conditions to the flotation experiments, and were found to correlate remarkably well with each other. Precipitates of small size were found to occur at low to medium pH due to the relatively insolubility of dodecanoic acid at low pH. In the presence of mercury, these precipitate altered in appearance, particle size and number. Surfactant, specifically dodecanoic acid was then found to significantly change the adsorption profile of mercury, especially at high chloride concentrations. Sulphate and phosphate were found to adsorb, and to float, in a fashion expected of anionic materials. Specifically, their removal decreased as pH increased. Mercury, in the presence of high concentrations of chloride, exists predominantly in an anionic form (eg. HgCl42-) and might have been expected to mimic the behaviour of sulphate and phosphate. However, it did not. Mercury removal was enhanced at high pH values, similar to that observed with aqueous heavy metal cations (Crawford et al., 1993a), and chloride inhibited both adsorption and flotation.