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

Abstract

Reactions involved in the sorption of heavy metal cations and an-ions 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.

Assessment of the Binding of Protons, Al and Fe to Biochar at Different pH Values and Soluble Metal Concentrations

“…But, the biochar-metal binding behaviour in this study did not show a strong pH dependence. This was unexpected based on a prior assumption of a lack of negatively charged binding sites found on humic acid at pH 4 [18,19]. However, the reverse was true; more Al and Fe were bound at pH 4 than at pH 7, with higher soluble metal concentrations present in added solution at the lower pH.…”

“…Our results suggest the models developed for metal binding to natural organic matter in water and soils are not readily applicable to biochar. Other potential binding mechanisms for metals include ion exchange, surface complexation, electrostatic attraction, or physical adsorption [18,41]. It is clearly an advantage, however, if biochar can bind metals under acidic conditions, as these are the conditions in the drainage water from acid sulfate soils and mine sites.…”

“…Metal retention on organic materials is strongly pH dependent [18,19]. As pH increases, there is less competition for cation binding sites with protons [18]. Additionally, dissolved metals may precipitate as pH increases (e.g., [20]).…”

“…Metal adsorption efficiency varied, however, and is affected by factors, such as functional groups, surface area, and environmental conditions [10,16,17]. Metal retention on organic materials is strongly pH dependent [18,19]. As pH increases, there is less competition for cation binding sites with protons [18].…”

Sorption of Arsenic from Desalination Concentrate onto Drinking Water Treatment Solids: Operating Conditions and Kinetics

“…The significant decrease of arsenic sorption onto DWTS with increasing pH was a result of the deprotonation of the sorbent surface, resulting in an increasingly negative surface of DWTS. The increase of soluble organic matter from DWTS at high pH could also attribute to the decrease of arsenic uptake because of the formation of arsenic-NOM complexes [59].…”