Chapter 1: General introduction & background 1 General introduction & background 1.1 GENERAL INTRODUCTION Industrialisation since the 1940’s resulted in a massive explosion of urban populations, which in turn led to widespread economic expansion fuelled largely by demand for energy resources. The post war years relied heavily upon the manufacturing and utilisation of various chemicals, designed to meet the growing standards of living. Several of these chemicals were tainted with heavy metal(loid)s and inappropriately used and disposed of in urban and agricultural surroundings. The result of industrialisation over the past 60 years has resulted in widespread heavy metal(loid) contamination of soils and groundwater across the globe. Today their legacy poses one of the biggest environmental challenges of the 21 st century: to effectively and economically restore and remediate heavy metal(loid)s contaminated sites (Bhandari 2007). Heavy metal(loid)s may be used to describe both biologically essential (copper; Cu, manganese; Mn and zinc; Zn), often referred to as ‘trace elements’ or ‘micro elements’, and nonessential (cadmium; Cd, lead; Pb, mercury; Hg, arsenic; As) elements. The nonessential elements are of greatest importance as they pose a serious threat to ecosystem, human and animal health. (Raskin et al. 1997). For example, heavy metal(loid) uptake (such as Cd) in agricultural and/or horticultural production systems can potentially threaten food quality, safety and marketability, and may have deleterious effects on plant growth through phytotoxicities (McLaughlin et al. 2000a; Kachenko and Singh 2006). Once in the environment, heavy metal(loid)s express contrasting behaviours due to different thermodynamic and phisico-chemical properties (Mukherjee 2001). For example, Cr, Hg and Pb are very strongly retained by the solid phase of most soils, hence accumulation of these elements poses negligible risk to plant and soil organisms. Conversely, Cd is readily accumulated by plants at levels that are well below those where phytotoxicity is expressed, potentially threatening animal and human consumers (McLaughlin et al. 2000b). 1 A. G. Kachenko In recent times, there has been growing awareness regarding the development and implementation of cost-effective strategies to alleviate the threat of heavy metal(loid) contaminates in the soil environment (Mulligan et al. 2001). There are thousands of sites throughout the world where corrective action is required to satisfy the growing need for sustainable management of the environment. In Australia alone, there are an estimated 80,000 contaminated sites, of which many are related to former mining activities (Naidu et al. 2002). For many of these sites, there has been a trend toward enhanced environmental management practices particularly the implementation of green environmentally neutral technologies to rectify the ailing state of the environment (Swindoll and Firth 1998). The possibility of utilising plants to remediate and potentially decontaminate the environment began during the later 19 th century when Baumann (1885) observed up to 1.7% Zn in violet (Viola caliminara) and mustard (Thalaspi calaminara) plants growing on Zn rich soils. However, almost a century passed before these unique plant species were first considered as a possible green solution to remediate heavy metal(loid)s contaminated soils. It was in 1977 that Brooks and others first described this phenomenon as hyperaccumulation (defined in section 1.3.3) and today, this term has since been associated with > 450 species world wide (Prasad and Freitas 2006). More recently, studies have focused on the ecophysiology of hyperaccumulation (e.g. Fernando et al. 2000a; Whiting et al. 2003; Bhatia et al. 2005a), and many have attempted to identify additional hyperaccumulating species (e.g. Meharg 2003; Wang et al. 2007). In this introductory chapter, an overview is presented on the mechanisms of heavy metal(loid) tolerance and the occurrence of hyperaccumulator species. The chapter begins with a brief background on heavy metal(loid)s in the soil environment and the process of heavy metal(loid) uptake and (hyper)accumulation. Further, this chapter provides an overview on the tolerance mechanisms employed by hyperaccumulator species in order to neutralise potentially inimical concentrations of accumulated heavy metal(loid)s, and the potential use of these species in the remediation of polluted soils. The chapter concludes with a discussion on heavy metal(loid)s in ferns followed by the rationale, aims and objectives of this thesis, including a thesis outline. 2 Chapter 1: General introduction & background 1.2 HEAVY METAL(LOID)S IN THE SOIL ENVIRONMENT 1.2.1 Definition The term heavy metals is a widely used term in literature to describe a disparate group of potentially toxic elements, however, its definition is arbitrary and somewhat imprecise (Hodson 2004). In an environmental context, it is often used to describe a group of elements associated with pollution and potential toxicity (Hodson 2004). Chemically, heavy metals are defined as a group of elements with an atomic density greater than 6 g cm 3 (Phipps 1981). Although this definition appears precise, great debate has evolved over the correct interpretation and scientific merit of the term heavy metals. A concise summary of current definitions based on six parameters; density (specific gravity), atomic weight (relative atomic mass), atomic number, chemical properties, toxicity or criteria used to define heavy metals before 1936 has been recently reviewed (Duffus 2002). Phipps (1981) discussed a biologically significant classification of metals based on the four (s-, p-, d- and f-) blocks of the periodic table. The d-block elements are particularly important as their redox behaviours and complex formation properties underlie their catalytic role in enzymes, whilst higher atomic number (p-block) elements bind strongly to sulphur, often resulting in a toxic effect. A weakness in Phipps (1981) proposal is that there is no differentiation between the metal ions in each of the different blocks, as the scheme is based on reactivity (Duffus 2002). An alternative theory suggested by Neiboer and Richardson (1980) classified metal ions in terms of differential Lewis acidity based on their relative affinity for N-, O- and S- containing ligands. This theory, based on chemical properties alone, is not absolute. It is largely empirically based and ion specific, however is relevant in a biological, toxicological and environmental contexts (Duffus 2002). A summary of the Lewis acidity based classification of heavy metals is presented in Table 1.1. 3 A. G. Kachenko Table 1.1 Classification of heavy metals based on Lewis acidity adapted from Neiboer and Richardson (1980). Class A (Hard) metals Lewis acids (electron acceptors) of small size, low polarizability (hardness), are easily displaced and mobile, generally forming ionic bonds. Examples : Li +, Fe 3+, Rb +, Sr 2+, Zr 4+ , Cs +, Hf 4+ , Ti4+ , Co 3+ Class B (Soft) metals Lewis acids (electron acceptors) of large size, high polarizability (softness), show strong affinity to soft ligands such as sulphide or sulphur donors, often forming covalent bonds. Examples : Cu +, Pd 2+ , Ag +, Cd 2+ , Hg +, Pt2+ Borderline (Intermediate) metals Size and polarizability between class A and class B metals, form relatively stable complexes with both hard and soft donor ligands. Examples : Pb 4+ , Cu 2+ , Fe 2+ , Co 2+ , Ni 2+ , Zn 2+ In this thesis, class B and borderline metals will be collectively referred to as heavy metals. Although imprecise, this term is still frequently used to describe metals of environmental significance with no unambiguous alternative available (McGrath et al. 1997; Citterio et al. 2003). Further, in this thesis the metalloid As will be referred to as a heavy metalloid when discussed. 1.2.2 Sources The soil environment acts as a large repository of heavy metal(loid)s that enter from numerous sources classified as either natural or anthropogenic. Irrespective of their origin, their persistent nature in soils pose significant environmental hazards to plant, animal and most importantly human health (McIntyre 2003; Krämer 2005). Human exposure to As my result in short term diseases such as hypertension or cardiovascular disease or long term illness such as skin, lung or bladder cancer. The clinical manifestations of As exposure has resulted in United States Environment Protection listing As as the number one toxin of prioritised pollutants (Ng et al. 2003). Heavy metal(loid)s arising from natural sources result 4 Chapter 1: General introduction & background from weathering of parent material, wind borne particles, volcanoes, forest fires and biogenic processes (Ernst 1998). For example, geogenic As is commonly associated with potable groundwater supplies throughout South East Asia, placing millions of people at direct risk of As exposure (Juhasz et al. 2003). Anthropogenic sources of heavy metal(loid)s include mining, smelting and refining of metalliferous ore including by-products such as slag, emissions from industrial manufacturing processes including electroplating, energy and fuel production, and agricultural inputs such as the application of fertilizers, pesticides, fungicides and municipal sludges to land (Singh 2001). For example, at the abandoned Mount Perry Cu mine, South Eastern Queensland, a recent study reported mining-influenced stream sediment contained Cu and As levels that were typically higher than international