Competitive Metal Adsorption Phenomena in Acid Mine Drainage Treatment using Blast Furnace Slag as Functional Adsorbent

This study has investigated the use of blast furnace slag as functional adsorbent for metal ions in acid mine drainage treatment. This opportunity has not yet been developed into a practical process partly because of limited understanding of rates and mechanisms of adsorption process and inconsistency in interpretations of adsorption data. The overall objectives were to determine the efficiency and mechanisms of sorption process of metal ions in a multiadsorbate system. We employed standard materials characterisation techniques to analyse composition, structure and surface morphology of the slag. Batch adsorption experiments were then conducted to study the mechanisms, kinetics and efficiency of metal sorption as a function of pH, phase ratio, mixing time, mass and particle sizes of slag. Both XRF and adsorption data suggested the slag to possess acid neutralising capacity. This slag is non-crystalline based on XRD pattern, has adequate porosity and pore volume comparable to similar commercial adsorbents but it has inferior pore structure, pore size distribution and BET surface area to be effective under wide applications. Metal sorption increased with decrease in slag particle diameter, decrease in initial metal concentration and increase in the mass of slag. Although slags have demonstrated to efficiently treat low concentrations of single and multiadsorbate effluents using fine particle sizes, they have limited capacity to treat multiadsorbate systems for some metal ions. This is probably due to low affinity, competitive adsorption or limitation in material?s adsorptive properties. Sorption mechanisms probably involve precipitation and ion exchange on the basis of alkaline pH and exchangeable slag surface ions. Sorption preference is proposed as Fe2+>Cu2+>>Cd2+>Co2+>Mn2+. Despite these limitations, deeper understanding of sorption behaviour of metal ions by slags for application in dilute and large volumes of effluents is necessary where precipitation and other standard techniques are either inefficient or uneconomic. Thus, this study has established that sorption capacity of slag to remove metal ions from acidic effluents is selective, limited to certain metal ions and for other ions only at low concentrations. Further adsorption studies in multiple solutes under various experimental conditions or those that can propose an economic slag activation process are needed that improves or widens adsorption window.