Selective Ion Exchange for Removal of Inorganic Trace Contaminants from Water

Various natural and anthropogenic reasons lead to the presence of heavy metals in natural waters: (i) Mobilisation from soils or rocks during the natural water circulation, (ii) discharge of effluents of actual or former mining, (iii) discharge of waste effluents e.g. from the mineral processing and metal finishing industries, (iv) unlawful discharge of waste water into the underground by individuals, v) leaching of solid waste from landfill deposits or unlawful waste deposition, and (vi) oxidation of sulphides by nitrate input after applying fertilizers in agriculture. Relevant metal contaminants are mercury, cadmium, arsenic, nickel, uranium, and chromium because of their toxicity. Corresponding with the local situation further metals causing undesirable effects (copper, cobalt, molybdenum, barium, selenium, antimony) may be encountered in concentrations of up to 100 μg/L, in exceptional cases even 2000 μg/L. Because of the well-known harmful effects of heavy metals on human health the concentrations of the inorganic species in drinking water should not exceed standards established by WHO or by individual countries (1). The speciation of most of the metals strongly depends on the composition of the aqueous phase and may be comprised of free cations, inorganic complexes, complexes with natural organic substances or complexes with strong complexing agents. Among the metals eliminated in the respective studies, mercury can be present in a number of forms and, through interactions with the components of the water matrix, can be transformed into a variety of structures. The most commonly known forms include elemental mercury, divalent mercury and methyl mercury. The speciation of divalent transition metals includes cations Me but also complexes with different ligands, depending on the background composition of the water. Chromium is usually present as monovalent oxide anions HCrO4. Uranium forms stable complexes with carbonate ions and is present as e.g. UO2(CO3)2 carbonato complex species (2).