Heavy metals removal from effluents by adsorption on activated silica sols

Effluents are neutralized with lime forming a voluminous waste sludge which may slowly redissolve in groundwater. This sludge may also be a significant secondary metal source. The present research aims to develop a concept for metals removal and recovery using stabilized colloidal suspensions of activated silica sol, which acts as a neutralizing agent and an adsorption medium.; Silica sol was prepared by firstly acidifying a sodium silicate solution to initiate polymerization, which would ultimately yield a gel. Polymerization was then arrested before gelation by water dilution giving a stabilized, negatively charged sol. Adsorption and precipitation were distinguished by comparing the coloured products from adding silica sol, lime or sodium hydroxide to cobaltous sulphate solution.; Studies of dissolved copper/silica sol interactions showed copper adsorption/desorption to be a reversible pH-controlled process, which was monitored ‘in-situ’ using a cupric specific ion electrode. Dissolved copper values determined by the electrode were identical to those measured by atomic absorption spectrometry (AAS) for solutions at below pH 7. At above pH 7, the electrode showed zero dissolved cupric, but AAS reported some dissolved copper, attributed to desorbed colloidal cupric hydroxide. A copper/sol underflow sharply divided from a clear overflow were the adsorption products. Addition of concentrated sulphuric acid to centrifuged underflow produced a concentrated copper solution and regenerated sol. The separation of two metals (cupric and ferric) using pH control was also demonstrated.; Adsorption/desorption cycles were used to remove iron from synthetic effluents producing concentrated iron products. Three consecutive cycles were required to produce 30 g/L dissolved iron, suitable for sewage treatment, from 0.5 g/L iron effluent. Final pH control with lime precipitated desorbed colloidal hydroxides. Good settling characteristics following adsorption/precipitation from synthetic and actual mine effluents were observed. Problems included gypsum build-up, while excess silica addition gave dispersion, high silica losses and no phase separation.; Nickel was selectively recovered over iron and magnesium from a tailings pond sludge by acid leaching then solution treatment with activated silica/lime. One adsorption/desorption cycle produced up to a 17 g/L Ni product from 2 g/L Ni leach solution.