Organically modified clays for pollutant uptake and environmental protection

Abstract

Because of their natural abundance and widespread occurrence, clays have been used 'in the service of man' (Konta 1995) since antiquity, notably as the raw materials in pottery and ceramics. Similarly, the use of fuller's earth (a calcium-rich montmorillonite) as a scouring and cleaning agent of raw wool dates back to before 2000 BC (Robertson 1986). Clays have also long served as medicinal and therapeutic agents, among which Bolus Armenus (a red clay from Cappadocia) and terra sigillata (a kaolinite-rich material from the island of Lemnos) are well known for their efficacy in curing festering wounds, skin afflictions, and snake bites (Robertson 1986, Carretero et al. 2006, Droy-Lefaix and Tateo 2006). Likwise, Sudanese villagers along the Nile have traditionally used a local bentonite clay to rid river water of viruses and bacteria (Lund and Nissen 1986, Madsen and Schlundt 1989). For a description of the modern-day uses of clays and clay materials in various industries the reader is referred to the reviews by Murray (2003) and Harvey and Lagaly (2006). Clay therapy and many practical applications of clays rely on the ability of these minerals to sorb and retain harmful and undesirable substances from their immediate environment. The sorptive capacity of clays is related to their small particle size, extensive surface area, layer structure, and charge characteristics. The vast literature on the interactions of clay minerals, especially smectites, with small and polymeric organic molecules has periodically been reviewed (Mortland 1970, Theng 1974, 1979, Huang and Schnitzer 1986, Yariv and Cross 2002, Lagaly et al. 2006). Although the reactivity of these minerals might be expected to extend to anthropogenic and industrial pollutants, the use of smectites for environmental protection is a relatively recent development. Its emergence is prompted by a growing awareness that industrial pollutants pose a threat to environmental and human health, and the need to find inexpensive and environmentally friendly materials for pollution control (Kowalska et al. 1994, Xu et al. 1997, Prost and Yaron 2001). Here we assess the literature that has accumulated over the past two decades on the use of smectites and their organically modified forms as sorbents of non-ionic organic compounds and pollutants. As far as is possible, we will refer to key papers and reviews, rather than cite individual authors.