Leaching of trace metals

Leaching of trace metals from CCB materials is of potential concern for contamination of natural water supplies. For example, Groenewold etal. (1985) found elevated levels of As, Cr, Mo, Pb, and Se in groundwater below a lignite fly ash landfill in North Dakota, and Shende et al. (1994) found stockpiled coal bottom ash contributed leachable metals to adjoining rivers. However, it also has been observed that weathered CCBs have the ability to retain many metals (Janssen-Jurkovicova et al. 1994 Steenari et al. 1999). 

Top soils under forest stands show a strong enrichment of anthropogenic trace metals. However, a part of this enrichment is due to the leaching of trace metals (Lindberg and Harriss 1981). 

The possibility of leaching of trace metals from the ash into ground or surface waters is also a possibility. The difference in physical properties between slag and fly ash would suggest a different leaching behavior. In fact, leaching behavior may vary within different types of fly ash especially where there is a size dependence of the physical and chanical properties (Fisher and Natusch, 1979). 

Uses. The principal uses of NaBH are ia synthesis of pharmaceuticals (qv) and fine organic chemicals removal of trace impurities from bulk organic chemicals wood-pulp bleaching, clay leaching, and vat-dye reductions and removal and recovery of trace metals from plant effluents. 

Some trace metals are transported into the ocean as a component of hydrothermal fluids. This process is discussed further in Chapter 19- To briefly summarize, hydrothermal fluids are produced when seawater penetrates into cracks in the crust near tectonic spreading centers. The seawater is heated as it comes into contact with magma. The hot seawater leaches a number of trace metals from the magma. The resulting hydrothermal fluids are acidic and do not contain O2, so most of the metals are present in reduced form. Because of their high temperatures, the hydrothermal fluids have a lower density than cold seawater. Their increased buoyancy causes them to rise until they are emitted into the deep sea. Admixture with cold, oxic, alkaline seawater causes the hydrothermal metals to undergo various redox and precipitation reactions. 

The Clean Air Act of 1990 has made trace metal content in fuels and wastes the final ash-related compositional characteristic of significance. Considerable attention is paid (ca 1993) to emissions of such metals as arsenic, cadmium, chromium, lead, mercury, silver, and zinc. The concentration of these metals in both grate ash and flyash is of significance as a result of federal and state requirements of particular importance is the mobility of metals. This mobility, and the consequent toxicity of the ash product, is determined by the Toxic Characteristic Leaching Procedure (tclp) test. Tables 8—10 present trace metal contents for wood wastes and agricultural wastes, municipal waste, and refuse-derived fuel, respectively. In Table 8, the specific concentration of various components in the RDF governs the expected average concentration of trace metals. 

T1he adsorption of metal ions from aqueous solutions is a phenomenon of immediate interest to workers in many diverse disciplines. The incorporation of metals into geological sediments, removal of metal ions from industrial and civic effluent, interference of trace metal ions in analytical and electroanalytical chemistry, ore flotation, metallurgical leaching processes, and the stability of ceramic slips are all processes which are controlled to a large extent by interaction of metal ions with solid-liquid interfaces. 

Sahuquillo, A., Rigol, A. and Rauret, G. (2003) Overview of the use of leaching/extraction tests for risk assessment of trace metals in contaminated soils and sediments, Trends in Analytical Chemistry 22,... 

Chester, R., Lin, F.J. and Murphy, K.J.T. (1989) A three stage sequential leaching scheme for the characterisation of the sources and environmental mobility of trace metals in the marine aerosol. Environ. Technol. Letts, 10, 887-900. 

Tillekeratne, S., Hiraide, M. and Mizuike, A. (1984) Selective leaching of trace heavy metals associated with suspended matter in fresh waters. Mkrochim. Acta, 1984 III, 69. 

In the case of solid environmental materials such as soils, sediments or indeed suspended particulates the use of selective chemical leaching methods can provide some basic information regarding the nature of the radionuclide species bound to the solid matrix. A typical sequential leaching might follow a procedure similar to that suggested for the study of trace metal binding to sediments by Gupta and Chen (1975) as shown in Fig. 13.2. 

There are many review articles concerning application of USE in food technology [57] and for isolation of bioactive substances from herbs and other plant materials [58], as well as leaching of heavy metals from environmental and industrial samples [59]. Application of ultrasound during sequential extraction of trace elements significantly shortens the whole procedure however, for satisfactory efficiency it is necessary to increase the temperature and modify the matrix. 

Some leaching tests were undertaken to ascertain the extent of trace metals contamination by sampling devices when collecting surface/deep sea water and snow/ice cores (52). Go-flow bottles, polyethylene tanks corers (especially knives and head) as well as all plastic items and Teflon components of the Differential Pulse (DP) ASV instrumentation used for the analytical determinations had to be preliminarily cleaned according to very stringent procedures. All this resulted in a significant abatement of the contamination by Cd and Pb. 

A simplified version of the scheme of Tessier et al. has been developed during the past years (40, 41). The scheme is made of three sequential extractions called A, B, C, which can leach respectively the exchangeable and carbonatic fraction (A), the reducible Fe-Mn hydroxides phase (B) and the organic matter together with the sulphides (C). The residual fraction is normally disregarded because total dissolution of the same samples is always performed, so that the residual fraction can be caleulated as the difference. Moreover, the residual fraction is the less important one from the viewpoint of bioavailability of trace metals. 

Fedotov, P. S., Wermrich, R., Stark, H.-J., and Spivakov, B. Ya. (2005b). Continuous-flow fractionation of trace metals in environmental solids using rotating coiled columns some kinetic aspects and applicability of three-step BCR leaching schemes. J. Environ. Monit. 7, 22-28. 

Table I lists the EPA contaminants and the criteria that EPA has established to constitute a hazardous toxic waste. Table 2 shows the concentrations of the inorganic contaminants in the extract from the phosphogypsum samples. All of the organic compounds listed by EPA as hazardous toxic waste.s were tested by the standard EPA procedure none were detected. These included endrin, lindane, methoxychlor, toxaphene, 2,4-D silvex, and 2,4,5-TP silvex. All of the metals listed in Table I were found to be present in the extract at concentrations lower than allowed by EPA (as shown in Table 2). Therefore, by EPA definition phosphogypsum is not a hazardous toxic waste material. This confirms earlier research conclusions 71 that the leaching of trace elements from phosphogypsum is not significant in introducing hazardous toxic waste materials into the environment.

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