Sorption of Heavy Metal Ions from Aqueous Solutions: A Review

Abstract

Sorption techniques are widely used to remove heavy metal ions from large volumes of aqueous solutions. Herein, the natural and some artificial materials, such as clay minerals, biosorbents, carbon-nanomaterials, metal oxides, are reviewed as adsorbents in the removal of different heavy metal ions, such as Ni(II), Cu(II), Pb(II), Cd(II), Cs(I), Eu(III), Th(IV), Cr(VI) from large volumes of aqueous solutions. The sorption kinetics and thermodynamics, the influ- ence of environmental factors on the sorption, the possible sorption mechanism of heavy metal ions and the modification of adsorbents on the removal of heavy metal ions are discussed in detail. The sorption properties of the different adsorb- ents are also described. The different methods such as batch sorption techniques and spectroscopy techniques (such as XPS, FTIR, EXAFS, ATR-IR) in the determination of heavy metal ion sorption properties and sorption mechanisms are presented. Different models for the simulation of sorption isotherms and kinetic sorption data are given as a comparison to understand the sorption mechanism. Many amphibolous matters for further research and the necessity in the study of the selective adsorbents in multi-component sorption systems are also summarized. The transfer of substances from a mobile phase (liquid or gaseous) to a solid phase is a universal phenomenon among the mobility of substances in aqueous porous media and aquatic environments. Sorption properties of metal ions are crucial for the evaluation of metal ion behavior in the natural environment. First, various treatment techniques and proc- esses have been used to remove the pollutants from contami- nated water. Among all the approaches proposed, sorption is one of the most popular methods and is currently considered as an effective, efficient and economic method for wastewa- ter purification. Second, the relationship between catalysis and sorption is considered as the most important domain of surface science. The prerequisite of heterogeneous catalysis to occur is sorption (usually chemical one) of molecules of the reacting substances on the inner or outer surface of the adsorbent or of the catalyst; then molecular dissociation of at least one or two reacting components, usually preceded by surface diffusion. Besides, catalysts combined with adsorb- ents owning high sorption capacity are important to achieve high catalytic activity. For example, TiO2 supported adsorb- ent provides higher specific surface area and facilitates more effective sorption sites than bare TiO2 (1, 2). As a result, the degradation rates of organic pollutants are usually higher than that of bare TiO2. Bhattacharyya et al. (3) loaded TiO2 on three different kinds of porous adsorbents, mesoporous (MCM-41), microporous (� -zeolite) and pillared structure (montmorillonite) using sol-gel method, and the photocata- lytic efficiency of the supported catalysts was evaluated