Chapter 11.1 Clays and Clay Minerals for Pollution Control
Section snippets
Control of Heavy Metal Cations and Simple Cations
As cation exchangers, clays are effective for the control of cations in solution, although, where only ion exchange is involved, their effectiveness is dictated by the imperative of attaining equilibrium between ions in solution and those on exchange sites. However, clays are most useful for the control of cationic pollutants when the mechanism of uptake extends beyond that of just cation exchange. This is particularly the case for heavy metal ions, on which the following discussion is
Control of Organic and Biological Cations
Since the majority of clay minerals are negatively charged they would have a strong affinity for organic cations. Although the number of organic species that can acquire a positive charge or act as a base may be limited (Theng, 1974), some of these are important. For example, the pesticides paraquat and diquat present problems as pollutants whereas amines, especially alkylammonium cations, are useful for modifying clay properties, and amino acids, peptides and proteins are biologically
Control of Non-Ionic Organic Compounds
Because of their charge characteristics, clays are naturally hydrophilic. Nevertheless, their high-surface areas and volume of fine pores enable them to adsorb significant amounts of non-ionic substances. There are records of the use of clays for ‘fulling’, i.e., cleaning grease from wool, that date back before 2000 BC, hence the term ‘fuller's earth’ (Robertson, 1986). Fuller's earth generally denotes calcium montmorillonite, although it is sometimes used to refer to palygorskite
Uptake of Anions by Unmodified Clays
As clay minerals are predominantly negatively charged, they have only a small capacity for taking-up anions. Anion exchange generally occurs on the edges of the aluminosilicate layers and is pH-dependent. The anion exchange capacity (AEC) of clays increases with decreasing pH but its magnitude is never high, being <5 cmol/kg for smectites (Borchardt, 1989) and apparently not more than 2 cmol/kg for kaolinites (Dixon, 1989). Some anions, notably phosphates, may be adsorbed, at least partially
Control of Turbidity in Water Treatment
The addition of clays, and particularly bentonite (i.e., smectite, as mined), as an aid to flocculation/coagulation during the treatment of water or wastewater is a well-established technology. Its historical use in the Sudan to clean pathogenic organisms from river water has already been observed. Bentonite was included, along with the soluble cationic polymeric flocculant, ferric chloride, and aluminium sulphate (alum) for use in a water filtration plant planned for Los Angeles (McBride et
Concluding Remarks and Future Prospects
The use, and enhancement of the utility, of clays, for the control of each of the different classes of pollutants has reached its own particular stage of maturity or development. This point will be discussed, along with research which shows promise for new targets for the environmental use of clays and also for overcoming some of the problems raised in their environmental applications to date.
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