220.127.116.11 Adsorption and Ion Exchange
The calcium phosphate minerals apatite and hydroxyapatite have been demonstrated to immobilise metals including Zn, Cd, Pb and U, commonly via microbi-ally mediated SO4 reduction and precipitation (Fuller et al. 2002; Conca and Wright 2006; Martin et al. 2008). Organic carbon media function either via microbially mediated sulfate reduction and the precipitation of sparingly soluble metal sulfides (Benner et al. 1999), or with exchange of both metal ions and organic compounds (Shukla et al. 2002; Bulut and Tez 2007). Granular activated carbon is a particularly effective form of organic carbon material for absorption of both organics and metal ions (Ferro-García et al. 1988). Activated carbon can be made from a range of materials (Johns et al. 1998), but wood, coconut shell and coal precursors are amongst the more common forms. Activated alumina (Tripathi et al. 2004) and crushed rock, particularly limestone (Baker et al. 1998; Cravotta and Watzlaf 2002; Komnitsas et al. 2004; Turner et al. 2008), have also shown promise for the removal of inorganics including a range of metals, the former medium by adsorption and the latter by adsorption and the manipulation of aquifer pH, with resultant precipitation of metal salts. Perhaps the most widely used natural material used for the adsorbtion and ion exchange of contaminants is the zeolite family of minerals, particularly clinoptilolite (e.g. Ouki and Kavannagh 1997; Park et al. 2002). Zeolites may be modified, for example with coatings of surfactants such as hexadecyltrimethylammonium (Bowman 2003), to help target organic compounds such as those found in petroleum hydrocarbons, and anions such as arsenate and chromate (Haggerty and Bowman 1994; Ranck et al. 2005). The use of other promising types of sorbents has been reviewed by Bailey et al. (1999).
Organic contaminants can be degraded by oxidation. There is a range of potential reactive media that can be used in PRB, but most work by releasing oxygen which then mineralizes petroleum hydrocarbon contaminants. A PRB using this strategy has been installed in the Arctic (Lindsay and Coulter 2003).
Reduction of dissolved species typically decreases their solubility, enhancing the possibility of saturated water chemistry and thus precipitation of sparingly soluble minerals and amorphous compounds containing the target contaminants. Reduction is also able to degrade some organic compounds including chlorinated solvents
(Tratnyek et al. 1997). The most common PRB media to achieve reduction is granular zero valent iron, which has been used for over a decade in North America to dissociate organic compounds (Johnson et al. 1996; Cheng and Wu 2000; Mu et al. 2004) and induce precipitation of metals (Puls et al. 1999). Organic materials have also been used to promote sulfate reduction and metal precipitation, both within PRB (Benner et al. 1997, 1999) and using naturally occurring carbon in aquifer plumes (Rectanus et al. 2007).
Microbial activity is important for the removal of some inorganic and organic contaminants. Bacterially mediated reactions can assist in the removal of metals from groundwater via processes including sorption/ion exchange, precipitation onto live cells, attachment to dead microbial biomass and enzymatically driven redox or other chemical reactions (White et al. 1997; Benner et al. 1999; England 2006). The rate of biodegradation of organic compounds depends on their form, with aliphatic compounds generally being more readily degraded than aromatics. The addition of nutrients may be necessary to stimulate microbial growth (Walworth et al. 1997, 2007).
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