Bound pesticide residues in soils: a review B. Gevao*, K.T. Semple, K.C. Jones Department of Environmental Science, Institute of Environmental and Natural Science, Lancaster University, Lancaster LA1 4YQ, UK Received 4 January 1999; accepted 12 July 1999 ``Capsule'': The current state of knowledge regarding the formation and biological/environmental signi®cance of bound pesticide residues in soils is reviewed. Abstract This article is a review of the current state of knowledge regarding the formation and biological/environmental signi®cance of bound pesticide residues in soils. We begin by de®ning various terms used in our discussions and identifying the types/classes of pesticides which may be added to soil and interact with it. We then consider various soil properties and aspects of land management which will in¯uence the nature and degree of the soil±pesticide association and discuss the possible physical and chemical binding mechanisms. We then move on to consider the role of microorganisms and other forms of soil biota in bound residue formation and the bioavailability of soil-borne pesticide residues. The review ends with a consideration of the signi®cance of bound pesticide residues. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Bound pesticide residues; Soils 1. De®nitions 1.1. `Bound' residues The terms `free' and `bound' residues were coined to indicate that the former can be readily extracted from soil without altering their chemical structures, whereas the latter are resistant to such extraction. However, the distinction between these two fractions is not always clear, because while they are in soil, even the free or extractable residues are not entirely free from any form of binding. On the contrary, at any time after the che- mical enters the soil, the extractable fraction may be sorbed to the soil solid phases and, therefore, show reduced bioavailability and degradation (Alexander, 1994). In other words, free residues may exhibit, to some extent, the properties that traditionally are attrib- uted to bound xenobiotics. It is, therefore, important to bear in mind that `extractability' of a compound will be operationally de®ned by the nature of the extractant and the experimental conditions under which an extraction is carried out. The decision to discontinue a given extraction may be taken arbitrarily and additional amounts of the bound chemicals can frequently be recovered by increasing the time or intensity of extrac- tion (Alexander, 1995). Dierent types of extraction procedures, such as supercritical ¯uid extraction (Capriel et al., 1986; Robertson and Lester, 1994; Koskinen et al., 1995), high temperature distillation techniques (Khan and Hamilton, 1980; Khan, 1982b; Worobey and Web- ster, 1982; You and Bartha, 1982), microwave extraction (Nicollier and Donzel, 1994) and silylisation prior to extraction (Drozd, 1975; Haider et al., 1992, 1993; Dec et al., 1997b) can perform dierently than conventional Soxhlet procedures. Another complexity is added to the issue of bound residues by the introduction of spectro- scopic techniques [e.g. nuclear magnetic resonance (NMR), infra-red (IR)] for which release from the soil matrix is not a precondition for identi®cation and char- acterisation (Dec and Bollag, 1997; Dec et al., 1997b). The de®nition of bound residues proposed by Roberts (1984), adopted by the International Union of Pure and Applied Chemistry (IUPAC) and generally accepted in the literature is ``chemical species originating from pes- ticides, used according to good agricultural practice, that are unextracted by methods which do not sig- ni®cantly change the chemical nature of these residues''. It seems reasonable to assume that there will always be a fraction of bound molecules that cannot be recov- ered from soil by exhaustive extraction. The partial 0269-7491/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0269-7491(99)00197-9 Environmental Pollution 108 (2000) 3±14 www.elsevier.com/locate/envpol * Corresponding author. Tel.: +44-1524-593972; fax: +44-1524- 593985 E-mail address: b.gevao@lancaster.ac.uk (B. Gevao).