Sorption of Heavy Metals by Inorganic and Organic Components of Solid Wastes: Significance to Use of Wastes as Low-Cost Adsorbents and Immobilizing Agents

Reactions involved in the sorption of heavy metal cations and anions to the surfaces of inorganic and organic components of solid wastes are examined. The properties of various waste materials (e.g., fly ash, slag, red mud, water treatment sludge, fungal and bacterial biomass, tree bark, sawdust, paper mill sludge, seafood processing waste, and composted organics) and the use of these materials as heavy metal sorbents, in situ immobilizing agents, and stabilization-solidification agents are then reviewed and discussed. Specific adsorption/surface precipitation onto various mineral phases present on composite inorganic waste materials explains the capacity of such materials to sorb metals. Similarly, reactions of various functional groups (e.g., carboxyl, amine, hydroxyl, sulphydryl) on organic molecules (e.g., lignin, chitin, humic substances) with heavy metals explain the sorption capacities of biosorbents. Differences in operational parameters between studies make a comparison of sorption capacities between materials difficult. Ease of desorption is also an important consideration, because in the treatment of wastewaters, materials are used primarily as ion exchangers, while for in situ immobilization, the metals need to be irreversibly bound to the added adsorbent. In the future, there is a need to develop low-cost sorbents with a wide range of metal affinities (through the combination of several waste sorbent materials) that can remove a variety of metal ions from solution from multielement-contaminated waters or soils.