Physico-chemical characteristics of some soils from Mali and their potential in heavy metal removal

Abstract: Rivers, streams and groundwater contaminated by heavy metals is a growing concern world wide as the pressure on existing water resources is increasing. Unfortunately, low income countries are afflicted the most. There is a need for cost efficient and simple methods and techniques to solve the problem. Previous studies have indicated great potential for clay minerals in pollution control. However, the studies have been carried out on synthesized clays and through modeling predictions. The objective for this study was to investigate natural mixed clays from Mali and their potential in heavy metal removal. Experiments were carried out on three natural clay samples from Mali. Using X-ray diffraction and electron microscope the mineral composition and its morphology were investigated. The physico-chemical properties of the mixed clays were investigated by acid-base titrations preceded by a purification procedure. Finally, adsorption experiments were carried out with Pb2+, Cd2+, Cu2+, Zn2+ and Cr3+ ions in aqueous suspensions. The mineralogical analyses showed that all three clay samples mainly consisted of crystalline quartz. According to the literature and previous modeling predictions carried out on quartz, the pKa1 value is negative. Basically, it means that the hydrated quartz surface has no base properties, and that the only surface sites are ≡Si-OH and ≡Si-O-. It can therefore be assumed that rather than having a specific pHpzc the surface is neutral over a wide pH range. pKa2 was determined through alkalimetric titrations to 9.7 which is a value that corresponds relatively well with the literature. All clay samples proved to completely remove Pb2+, Cd2+, Cu2+, Zn2+ and Cr3+ from aqueous solution at different pH ranges. At pH >7, all the studied metal ions are adsorbed to the mineral surface. Since the surface is neutral at this pH it can be assumed that all metals are adsorbed to the surface as inner-sphere complexes. Using this clay mineral for heavy metal removal requires fairly high pH values for the water being treated. The pH has to be well above 7 to be certain that all metals are adsorbed. For a more wide spread application of clay minerals as heavy metal adsorbents further studies in clay mineral characterization are necessary. Every country has their unique soils with different properties and capacities that need to be mapped.