The Chemistry Behind the Use of Agricultural Biomass as Sorbent for Toxic Metal Ions: pH Influence, Binding Groups, and Complexation Equilibria

Waters, because of human activities, are often characterized by different kinds of contamination. In this chapter we will deal with contamination due to toxic metal ions. To purify wastewaters from these pollutants different treatment processes are applied, which include chemical precipitation, chemical oxidation or reduction, electrochemical treatment, membrane filtration, ion exchange, carbon sorption, and coprecipitation/sorption. A number of these processes are extremely expensive and some of them are ineffective at low concentrations. Alternative cost effective technologies based on low cost sorbents are nowadays of great concern in the applied research. These low cost sorbents must be abundant in nature, easily available, and above all they have to fit the worldwide request of recycling. Certain waste products from agricultural operations may become inexpensive sorbents and the potential of some of these wastes for the removal of a number of metal ions has been extensively investigated. The use of these wastes as sorbents fulfills two important scopes for the protection of environment: the reuse of waste materials and the detoxification of wastewaters. The biomass source depends on the agricultural production prevailing in the geographical areas where pollution and subsequent decontamination process take place. The real challenge in the field of biosorption is to identify the chemical mechanism that governs metal uptake by biosorbents. Vegetal biomaterials, constituted principally by lignin, cellulose and by a non-negligible portion of fatty acid as major constituents, can be regarded as natural ion-exchange materials. Furthermore, the functional groups on the biomaterial surface, such as hydroxyl, carbonyl, amino, sulphydryl and carboxylic groups, allow the sorption of metal ions by strong coordination. Therefore, identification of the functional groups can help in shedding light on the mechanism responsible for metal uptake. Also some factors affecting the sorption process such as particle size, pH, metal ion concentration, agitation time, and kinetics must be investigated. The results obtained contribute to the knowledge of the overall process that takes place.