Heavy metals toxic concentrations

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). 

The initial species present, their relative concentrations, the induction of their enzymes, and their ability to acclimate once exposed to a chemical are likely to vary considerably, depending upon such environmental parameters as temperature, salinity, pH, oxygen concentration (aerobic or anaerobic), redox potential, concentration and nature of various substrates and nutrients, concentration of heavy metals (toxicity), and effects (synergistic and antagonistic) of associated microflora (Howard and Banerjee, 1984). Many of the parameters affect the biodegradation of chemicals in the environment as well as in biodegradation test systems used to simulate the environment. 

In an earlier section it was reported that sulfides were excellent for preventing heavy metal toxicity. McCarty et al. 21) reported however, that soluble sulfide is toxic if the concentration exceeds 200 mg/hter. The soluble sulfide concentration in a digester is a function of the incoming sources of sulfur, the pH, the rate of gas production, and the availability of heavy metals to act as precipitants. Excess sulfide can be removed by adding iron to precipitate FeS. 

Plants could maintain the necessary concentrations of essential metal ions in cells by homeostatic mechanisms. They are also involved in the reduction of damage induced by heavy metal toxicity. One of the major tolerance mechanisms is chelation of heavy metals by a family of peptide ligands, the phytochelatins (similar to other xenobiotics) [108], The molecular basis for the chelators and chaperone synthesis is well known and could be applied in the modification of tolerant plants. Tolerance to Cd and As is largely dependent on the phytochelatin pathway, but molecular biology of Cd hypertolerance in certain plant species, such as the... 

The acute phase response protein, metallothionein, MT, belongs to a family of low molecttlar weight cysteine-rich proteins that can bind nutritionally important bivalent cations such as zinc and copper, as well as the envirorunental toxicants, cadmittm and mer-ctuy. Two of the MT proteins (MT-1 and MT-2) are widely expressed in the body with highest concentrations in the liver, kidney, intestine and pancreas. The MT-3 isoform is predominantly foimd in brain while expression of the MT-4 isoform has been detected in skin and tongue squamous epithelium and maternal deciduttm. " Postulated functions of MT include protection against heavy metal toxicity, as a contributor to zinc homeostasis and metabolism, and as an antioxidant and flee radical scavenger. 

Heavy metals such as copper, cadmium, mercury, and lead are found in wastewaters from a number of industrial processes. Because of the toxicity of many heavy metals, their concentrations must be reduced to very low levels before release of the wastewater. Several approaches are used in heavy metals removal. 

In general, nonconventional protein foods must be competitive with conventional plant and animal protein sources on the bases of cost delivered to the consumer, nutritional value to humans or animals, functional value in foods, sensory quality, and social and cultural acceptability. Also, requirements of regulatory agencies in different countries for freedom from toxins or toxic residues in single-cell protein products, toxic glycosides in leaf protein products, pathogenic microorganisms, heavy metals and toxins in fish protein concentrates, or inhibitory or toxic peptide components in synthetic peptides must be met before new nonconventional food or feed protein products can be marketed. 

Industrial Wastewater Treatment. Industrial wastewaters require different treatments depending on their sources. Plating waste contains toxic metals that are precipitated and insolubiHzed with lime (see Electroplating). Iron and other heavy metals are also precipitated from waste-pidde Hquor, which requires acid neutralization. Akin to pickle Hquor is the concentrated sulfuric acid waste, high in iron, that accumulates in smokeless powder ordinance and chemical plants. Lime is also useful in clarifying wastes from textile dyeworks and paper pulp mills and a wide variety of other wastes. Effluents from active and abandoned coal mines also have a high sulfuric acid and iron oxide content because of the presence of pyrite in coal. 

Sulfur Polymer Cement. SPC has been proven effective in reducing leach rates of reactive heavy metals to the extent that some wastes can be managed solely as low level waste (LLW). When SPC is combined with mercury and lead oxides (both toxic metals), it interacts chemically to form mercury sulfide, HgS, and lead sulfide, PbS, both of which are insoluble in water. A dried sulfur residue from petroleum refining that contained 600-ppm vanadium (a carcinogen) was chemically modified using dicyclopentadiene and oligomer of cyclopentadiene and used to make SC (58). This material was examined by the California Department of Health Services (Cal EPA) and the leachable level of vanadium had been reduced to 8.3 ppm, well below the soluble threshold limit concentration of 24 ppm (59). 

The principal objectives of pretreatment are to remove heavy metals prior to subsequent treatment, to neutralize the wastewater to a suitable pH for discharge or subsequent treatment to remove high concentrations of suspended soflds, to eliminate or reduce toxicity, and to eliminate or reduce volatiles. The concentrations of various pollutants that make pretreatment desirable are summarized in Table 7. 

The behavior of elements (toxicity, bioavailability, and distribution) in the environment depends strongly on their chemical forms and type of binding and cannot be reliably predicted on the basis of the total concentration. In order to assess the mobility and reactivity of heavy metal (HM) species in solid samples (soils and sediments), batch sequential extraction procedures are used. HM are fractionated into operationally defined forms under the action of selective leaching reagents. 

HEAVY METALS A gi oup of metals which are sometimes toxic and can be dangerous in high concentrations. The main heavy metals covered by legislation are cadmium, lead, and mercury. Industrial activities such as smelting, rubbish burning, waste disposal and adding lead to petrol increase the amount of toxic heavy metals in the environment. 

Table 16.13 Toxic concentrations of heavy metals to rainbow trout...

---