Chemical equilibria in wastewaters during toxic metal ion removal by agricultural biomass

After an overview of environmental pollution and the environmental threat posed by metals, this paper focuses on a literature survey of the different analytical methodologies and techniques used to investigate the functional groups involved in metal sorption onto biomass derived from agricultural waste. The recent literature has mainly focused on identifying the binding groups on the biomass surface through the use of different analytical techniques, from the traditional potentiometry to the most innovative and sophisticated solid surface analysis. In most of the biosorption studies, carboxylic group is the main origin for metal uptake, followed by hydroxyl group, aromatic ring and amine. The extent of the use of spectroscopic methods compared to traditional chemical analysis is being questioned. We stress that the study of biosorption mechanisms requires a multidisciplinary approach, and the efforts of analytical chemists and solution equilibrium experts really do contribute to the knowledge and to the optimization of these processes. This is the context in which we present a speciation study of the binding of various toxic metal ions by some coordinating groups on the biomass surface, which allows the classification of sorption mechanisms as a function of the pH dependence of the binding phenomena.