Geochemical behaviour of heavy metals in sludge effluents and solid deposits on the Zambian Copperbelt: Implication for effluent treatment and sludge reuse.

Sludge effluents and solid deposits generated from the conventional lime treatment processes on the Zambian Copperbelt have led to reports of copper (Cu) and cobalt (Co) contamination into the nearby water bodies. To better understand the behaviour of the metals; partitioning, adsorption and their specific binding forms were studied through sequential extraction, batch adsorption experiments and surface complexation modeling (SCM). Results of mineral composition analyses indicated that micas, kaolinite, quartz and feldspar are abundant with hydrous ferric oxide (HFO) precipitates that formed as a result of the weathering of biotite grains existing as grain surface coating. Sequential extractionrevealed that Cu and Co metals are partitioned in the order of: exchangeable (F1: 600-1500 mg/kg Cu; 100-200 mg/kg Co), acid-soluble (F2: 2200-5500 mg/kg Cu; 190-220 mg/kg Co) and reducible fraction (F3: 2200-5500 mg/kg Cu; 260-300 mg/kg Co). Metals in F1 are hosted by kaolinite, F2 by both kaolinite and HFO whereas in F3 by dominantly HFO. Equal Cu concentration between F2 and F3 is due to both the limited amount of HFO (i.e. 5-10 g/kg) and desorption of loosely adsorbed Cu and Co metals to HFO surfaces. Batch adsorption experiments revealed adsorption as the dominant metal retention mechanism. According to modeling predictions, HFO sites are the dominant metal adsorption sites. At HFO site; >(s)FeOCo+, Co showed adsorption decrease from 40% in single system to 25% in binary system between pH 7 - 7.5 due to metal competition for adsorption sites. The high Cu concentration (i.e. 0.5-1.1% Cu) displaced low Co (i.e. 0.03-0.07% Co) concentration from the adsorption sites present in sludge, thus rendering Co mobile into the environment. To keep the adsorbed metals stable from release, optimal pH of 7.5 is suggested during treatment with lime. At this optimal pH, metals are decreased to below the regulation standard values and with less generation of voluminous sludge. Adsorbed Cu and Co can be recoverable from sludge through acid treatment at pH <3 based on sequential extraction results. The resultant metal-free sludge material has potential of been used as aggregate in construction.