Plant metal concentrations

Although many of the procedures listed in Table 11.9 are used for contaminated soils, they will not necessarily measure plant-available contents but rather, the labile or mobile species contents. They can best be indicated as potentially plant-available contents rather than actual contents. The most commonly used extractants are diethylenetriaminepentaacetic acid-triethanolamine (DTPA-TEA), buffered at pH 7.3 (Lindsay and Norvell, 1978) and 0.05 M CaCh (Sauerbeck and Styperek, 1984). Some authors also reported no relationship between extractable metals and test plant metal concentrations (e.g., Haq and Miller, 1972 Rappaport et al., 1988). O Connor (1988) has subsequently identified an entire series of misuses of the DTPA test, which probably account for failure of the test,. Two major constraints with the DTPA extractant include the high pH (that may not typify soil pH) and chelation effect of the ligand ion. The chelate-based extractants tend to extract significantly higher amounts of trace elements and thus may not necessarily reflect the plant-available content in soils. To compensate for the high pH of the extractant, O Connor (1988) suggested inclusion of pH as one of the variables in the correlation studies. 

Although much effort has been spent testing different soil extraction techniques for characterizing metal phytoavailability, Sharma and Shupe (1977) found a surprisingly good relation (r = 0.883 for Cd) between total metal concentrations in the soil and total metal concentrations in plants. The statistical relationship between total soil and plant metal concentration may be fortuitous, since the total amount of Cd in soil is seldom indicative of its effect on Cd accumulation in plants (Cottenie et al., 1983). More attention should be paid to Cd speciation in relation to Cd bioavailabihty, especially in the rhizosphere. 

Many extractants were recommended for use as bioavailable indices based on significant correlation between quantities of metal extracted from the soils by the extractants and metal uptake by plants. The most commonly used extractants were ammonium bicarbonate-diethyl triamine penta acetic acid (AB-DTPA) (Soltanpour and Schwab, 1977 Norwell, 1984) and ammonium acetate acetic acid-ethylene diamine tetra acetic acid (AAAc-EDTA) (Lakanen and Ervio, 1971 Sillanpaa and Jansson, 1992). AB-DTPA was used successfully as an extractant for characterizing the bioavailability of both native soil metals as well as metals added to soils in sewage sludges. Some authors also reported insignificant relationships between the AB-DTPA-extracted metals and test plant metal concentrations (e.g. Haq and Miller, 1972 Rappaport et al, 1988). O Connor... 

Flow Sheets. AH minerals processing operations function on the basis of a flow sheet depicting the flow of soHds and Hquids in the entire plant (6,13,14). The complexity of a flow sheet depends on the nature of the ore treated and the specifications for the final product. The basic operations in a flow sheet are size reduction (qv) (comminution) and/or size separation (see Separation, size), minerals separation, soHd—Hquid separation, and materials handling. The overaH flow sheet depends on whether the specification for the final mineral product is size, chemical composition, ie, grade, or both. Products from a quarry, for example, may have a size specification only, whereas metal concentrates have a grade specification. 

Standards imposed to the industrial waste streams charged in heavy metals are more and more drastic in accordance with the updated knowledges of the toxicity of mercury, cadmium, lead, chromium... when they enter the human food chain after accumulating in plants and animals (Forster Wittmann, 1983). Nowadays, the use of biosorbents (Volesky, 1990) is more and more considered to complete conventional (physical and chemical) methods of removal that have shown their limits and/or are prohibitively expensive for metal concentrations typically below 100 mg.l-i. 

Phytoextraction has several advantages. The contaminants are permanently removed from the soil and the quantity of the waste material produced is substantially decreased. In some cases, the contaminant can be recycled from the contaminated biomass. However, the use of hyperaccumul-ating plants is limited by their slow growth, shallow root systems, and small biomass production. In order for this remediation scheme to be feasible, plants must tolerate high metal concentrations, extract large concentrations of heavy metals into their roots, translocate them into the surface biomass, and produce a large quantity of plant biomass. 

Ecosystem functioning Free metal ion concentration in soil solution re-calculated to total dissolved metal concentration in soil drainage water (in view of effects on soil microorganisms, plants and invertebrates) 1.7-20.4 1.3-3.2 ... 

The statistical estimation of heavy metal concentrations in the Spruce Forest ecosystems of the Boreal climatic zone is the subject of wide variation, with coefficient of variation from 36 to 330%. However, we can note the clear trend in biogeochemical peculiarities and relevant exposure to heavy metal uptakes by dominant plant species. 

In many mountain-industrial areas there are 3 1 landscape-functional zones with different extents of the anthropogenic transformation of natural environments. As a rule, the first zone is the spatial complex joining mines, pits and tails site area with almost whole degradation of soil and vegetation cover and high metal concentrations in dust, technogenic depositions, waters and plants. 

Mnshrooms are important in the ecosystem because they are able to biodegrade the substrate and therefore use the wastes of agricultural production [1]. Mushrooms have also been reported as therapeutic foods, useful in preventing diseases such as hypertension, hypercholesterolemia, and cancer. These functional characteristics are mainly due to their chemical composition [2]. Heavy metal concentrations in mushroom are considerably higher than those in agricultural crop plants, vegetables and fmit. This suggests that mushrooms possess a very effective mechanism that enables them readily to take up some heavy metals from the ecosystem. 

Much of the optimization of the solvent extraction plant can be achieved in the pilot plant testing. As noted earlier on the subjeet of proeess design, one must investigate the dependence of the dispersion and eoaleseence char-aeteristies and their effect on extraction and phase separation. Also, such variables as metal concentration, equilibrium pH (or free aeidity or free basieity), salt concentration, solvent concentration (extraetant, diluent, and modifier), and temperature have to be studied to determine their effect on mass transfer. Although many of the variables can be tested in the pilot plant, many circuits are not optimized until the full-scale plant is in operation. 

Karam, N. S., Ereifej, K. L, Shibli, R. A., AbuKudais, H., Alkofahi, A., Malkawi, Y. (1998). Metal concentrations, growth, and yield of potato produced from in vitro plantlets or microtubers and grown in municipal solid-waste-amended substrates. J. Plant Nutr, 21, 725-739. 

Wetland remediation involves a combination of interactions including microbial adsorption of metals, metal bioaccumulation, bacterial oxidation of metals, and sulfate reduction (Fennessy Mitsch, 1989 Kleinmann Hedin, 1989). Sulfate reduction produces sulfides which in turn precipitate metals and reduce aqueous metal concentrations. The high organic matter content in wetland sediments provides the ideal environment for sulfate-reducing populations and for the precipitation of metal complexes. Some metal precipitation may also occur in response to the formation of carbonate minerals (Kleinmann Hedin, 1989). In addition to the aforementioned microbial activities, plants, including cattails, grasses, and mosses, serve as biofilters for metals (Brierley, Brierley Davidson, 1989). 

Microbial mat formation may also stimulate metal removal through sulfate reduction. Barnes, Scheeren Buisman (1994) have developed a process that specifically uses sulfate-reducing bacteria to treat metal-contaminated groundwater. In this process, as groundwater is pumped through the water treatment plant, sulfide produced by sulfate-reducing bacteria precipitates the metals in the water. Metal concentrations in the treated water were reportedly reduced to fig/l quantities and the water was suitable for release into the environment. 

Heavy metal toxicity in plants is infrequent (143). In many cases, metal concentrations in plant parts show poor correlation with soil concentrations of the element (147). Plants tend to exclude certain elements and readily accept or concentrate others. Lisk (148) reported natural plant soil concentration ratios of 0.05 or less for As, Be, Cr, Ga, Hg, Ni, and V. Cadmium appears to be actively concentrated and selenium appears to be easily exchangeable. Indicator plants are capable of markedly concentrating specific elements, e.g., Astragalus spp. for selenium (138) and Hybanthus floribundus for nickel (149). Plants growing on mine wastes have been shown to evolve populations which exhibit metal-specific tolerances (150). 

St-Cyr, L. and Campbell, P.G.C. (1996) Metals (Fe, Mn, Zn) in the root plaque of submerged aquatic plants collected in situ relations with metal concentrations in the adjacent sediments and in the root tissue, Biogeochemistry 33, 45-76. 

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