Potentially toxic metals soil contaminants

Table 9.1 Amounts of some potentially toxic metals (PTMs) measured in the soil matrix (mg- kg a) and soil solution (ug L a) obtained by various techniques from uncontaminated and contaminated soils... 

Table 9.4 Examples of potentially toxic metals (PTMs ) bioaccessibility values (% of soil total content in mg-kg ) in contaminated soils determined with different in vitro digestion models... 

Weis, 2004). If the rhizosphere plays an important role in the sequestration of metals in wetland soils, fluxes of potentially toxic metals out of the wetland to adjacent aquatic systems will, in theory, be reduced. However, plants may also increase metal mobilization through rhizosphere acidification and the oxidation of metal-sulfide complexes (Jacob and Otte, 2003). They may also represent only a temporary sink for metals if rhizosphere Fe plaque is reduced following plant senescence. Furthermore, metals can be exported from the ecosystem if contaminated plant parts are consumed by people or wildlife. The pathways and possible health effects of metal consumption have been especially well studied in Southeast Asia, where metal contamination (notably As) of rice crops is a serious public health issue (e.g., Meharg and Rahman, 2003 Meharg, 2004). 

This paper [353] also appears to be based on the assumption that as long as phytotoxic symptoms and reduction in yield in crops are avoided, it does not really matter how much metal-contaminated sludge is applied to agricultural soils. On the basis of this view and rather dubious assumptions about the relationship between zinc equivalent and phytotoxic potential, carte blanche has been given in England in recent years for the application of sludges to agricultural land, with little reference to their contents of potentially toxic metals other than zinc, copper and nickel. Since the maximum proposed application is 50 tons dry matter/acre/annum and this would be equivalent to... 

Some heavy metal-tolerant bacterial strains and their sorption capacities for Cu and Cd are listed in Table 1. These bacteria show great potential for remediating soils that are contaminated with toxic metals. Our pot culture experiments showed that the growth of tobacco plants in a Cd-polluted Yellow Brown Soil (Alfisol) was significantly promoted by inoculating the soil with P. Putida in comparison with the non-inoculated soil (Fig. 2). 

Metal reclamation of sediments uses many of the same approaches as for soils, except that sediment access is often more difficult. Once removed from the bottom of a lake or river, sediments can be treated and replaced, or landfilled in a hazardous waste containment site. The actual removal of sediments involves dredging. This can pose serious problems since dredging includes the excavation of sediments from benthic anaerobic conditions to more atmospheric oxidizing conditions. This can result in increased solubilization of metals, along with increased bioavailability (see Section 10.3) and potential toxicity, and increased risk of contaminant spreading (Moore, Ficklin Johns, 1988 Jorgensen, 1989 Moore, 1994). There are ongoing discussions as to whether it is more detrimental to remove sediments, whether for treatment or removal, or simply to leave them in place. 

The assessment of plant-available soil contents can frequently be achieved and validated by field experiments for nutritionally essential elements, and, for a few potentially toxic elements such as chromium, nickel and molybdenum, at the moderately elevated concentrations that can occur in agricultural situations. The validation of extraction methods, devised for agricultural and nutritional purposes, is much less easy to achieve when they are applied to heavy metals and other potentially toxic elements, especially at the higher concentrations obtained in industrially contaminated land. This is not surprising in view of the fact that for some heavy metals, for example lead, there is an effective root barrier, in many food crop plants, to their uptake and much of the metal enters plants not from the root but by deposition from the atmosphere on to leaves. In these circumstances little direct correlation would be expected between soil extractable contents and plant contents. For heavy metals and other potentially toxic elements, therefore, extraction methods are mainly of value for the assessment of the mobile and potentially mobile species rather than plant-available species. This assessment of mobile species contents may well, however, indicate the risk of plant availability in changing environmental conditions or changes in land use. 

A further area in which sequential extraction continues to be applied successfully is in assessment of the likelihood of mobilisation of metal contaminants from sediment-derived soil. When dredged sediment is used to reclaim land from the coastal margins or applied to arable soil to improve fertility, there is concern that potentially toxic elements accumulated under reducing conditions may be released on exposure to an oxygen-rich environment. Sequential extraction can be used to characterise the sediment prior to application, or to monitor changes in metal availability in the soil with time (e.g. Singh et al, 1998). 

In addition to soil contamination with explosives and MC-related compounds, DND sites often have elevated concentrations of metals [53,54], Consequently, risk assessment at such sites should consider not only the presence of MC but also the potential effects of their interactions with metal co-contaminants on the toxicity to ecological receptors [61], Robidoux et al. [53,54] reported that ecotoxicological effects (such as the effects on earthworm reproduction) in soil with contaminant mixtures could not be attributed entirely to the toxicity of MC when in the presence of elevated concentrations of metals. In contrast, reproduction toxicity in earthworms correlated with TNT concentrations in TNT-contaminated soils having low metal concentrations [62], Findings in these and other reports show that mixtures... 

It has been known for some time that tolerance towards high levels of both essential and toxic metals in a local soil environment is exhibited by species and clones of plants that colonize such sites. Tolerance is generally achieved by a combination of exclusion and poor uptake and translocation. Some species can accumulate large quantities of metals in their leaves and shoots at potentially toxic levels, but without any harmful effects. These metal-tolerant species have been used in attempts to reclaim and recolonize metal-contaminated wastelands. More recently such species have attracted the attention of inorganic chemists. There is abundant evidence that the high metal levels are associated with carboxylic acids, particularly with nickel-tolerant species such as Allysum bertolonii. The main carboxylic acids implicated are citric, mahc and malonic acids (see refs. 30 and 31 and literature cited therein). Complexation of zinc by malic and oxalic acids has been reported in the zinc-tolerant Agrostis tenuis and oxalic acid complexation of chromium in the chromium-accumulator species Leptospermum scoparium ... 

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