Metal bioavailability in soils

Diethylenetriaminepentaacetic acid (DTPA) possesses amine and carboxylic acid functional groups and could form octadentate complexes with heavy metals. It is used as a chelator to determine metal bioavailability in soil to plants and mobility to the aquatic environment. 

Sloan, J. J., Dowdy, R. H., Dolan, M. S., Linden, D. R. (1997). Long-term effects of biosolids applications on heavy metal bioavailability in agricultural soils. J. Environ. Qual. 26,966-974. 

The Heavy Metal Binding Capacity (HMBC) test is a bioassay that helps to quickly determine metal bioavailability in aquatic environments. HMBC can also be applied to soils and to root exudates from aquatic and terrestrial plants. The HMBC test is based on MetPLATE, a bacterial toxicity test that selectively detects metal toxicity. 

The significance of this segregating mechanism in controlling metal solubility and bioavailability in soils can be easily imagined, as it provides a means of literally burying toxic or essential metals in a form inaccessible to desorption. Furthermore, the solubility of a toxic or essential metal could be lowered well below that predicted from the solubility of the pure solid phase. That is, if the trace metal of interest is symbolized by B, the solubility of B in equilibrium with the solid solution would be lower than that in equilibrium with pure BY. This fact is demonstrated by combining equations 4.78 and 4.83 to give... 

In fact, for metal cations in soils, the following general rules relate bioavailability to speciation ... 

The need to assess the speciation of trace metals and to estimate their bioavailability in the plant root environment is crucial in order to establish the extent of trace metal contamination in soils and to address their potential adverse effects. Although the bioavailability of metals is best predicted by their liquid phase speciation, the assessment of the trace metal fractionation in the solid phase is still essential because the replenishment rate of metals in the soil solution is controlled by solid phase metals (Minnich et al., 1987 Zhang et al., 2001 Krishnamurti et al, 2002). For instance, Zhang et al (2001) showed that Cu concentrations in plant parts correlated best with the effective Cu concentration, which was defined as the Cu concentration in the soil solution combined with a... 

To understand the changes in metal fractionation within the rhizosphere, we need more insight into the key factors influencing metal fractionation. It has been assumed that the factors affecting metal fractionation and bioavailability in soil include root-induced pH changes, metal binding by root exudates, root-induced... 

These Fe deficiency-induced processes are likely to operate ubiquitously in soils because of limited bioavailability of Fe, especially in neutral to alkaline soils. Iron deficiency in crops may thus affect metal speciation in soil, and ultimately enhance the uptake of metals by plants. In the present study, we examined this hypothesis in the case of copper (Cu) and zinc (Zn), which are micronutrients and also potential metal contaminants. 

Accurate assessment of human health risks associated with oral exposure to metals requires knowledge of the fraction of the dose absorbed into the blood. This information is important for As-contaminated environmental media, such as soil and mine waste, because metal contaminants exist in a variety of soluble and insoluble forms and may be contained within particles of inert matrix, such as rock or slag. Physicochemical properties such as these influence the enteric absorption fraction (bioavailability) of ingested metals. Therefore, site-speciflc data on metal bioavailability in the environmental media of concern will increase the accuracy and decrease the uncertainty in human health risk estimates. 

Heavy metals occur in soils and sediments in various chemical forms, and the variety of heavy metals speciation significantly influences their environmental mobility and bioavailability, which finally determines their potential for environmental contamination (McBride, 1994). Representative speciations of heavy metals in the geoenvironment can be summarized as follows ... 

Since the early 1970s, the Institute of Soil Science and Agricultural Environment Protection of the Agricultural University of Wroclaw has carried out monitoring of the heavy metal content in soils, in order to estimate environmental risks associated with soil contamination (Kowalihski et al. 1972, 1979 Roszyk and Szerszeh 1988ab Szerszeh et al. 1993 Borkowski et al. 1991). Some results and conclusions of this work have been briefly summarised and reported in this chapter. However, the main theses of Chapter 6 refers to the mobility and bioavailability of metals in contaminated soils, as the main aspects of the... 

Other studies use soil or sediment samples for a more accurate indication of microbial activity in natural environments. In these samples, organic matter and clay particles play a role in metal toxicity.76112113 Both organic material and clay particles in soil can bind metals and reduce their bioavailability. For example, Pardue et al.87 demonstrated that much less solution-phase cadmium was required to inhibit trichloroaniline (TCA) dechlorination in a mineral-based soil than in a soil containing a higher concentration of organic matter. Other studies have shown that adding clay minerals to a medium mitigates toxicity. Clay minerals, such as kaolinite, montmorillonite, bentonite, and vermiculite, can bind to metals to decrease the amount that is bioavailable.112 115... 

Pierzynski GM, Schwab AP. 1993. Bioavailability of zinc, cadmium, and lead in a metal contaminated alluvial soil. J Environ Qual 22 247-254. 

Zhao K, Liu X, Zhang W, Xu J, Wang F (2011) Spatial dependence and bioavailability of metal fractions in paddy fields on metal concentrations in rice grain at a regional scale. J Soils Sediments 11(7) 1165—1177. doi 10.1007/s 11368-011-0408-6... 

In addition, dissolved organic carbon (DOC) is also an important soil solution solute affecting speciation and bioavailability of many trace elements in soil solution. Many trace elements and heavy metals complex with dissolved organic carbon. This is especially important in arid and semi-arid environments since high soil pH increases the solubility of organic molecules and accordingly increases concentrations of dissolved organic carbon in soil solution. 

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