Cadmium concentrations extracted from soils

Mean cadmium concentrations extracted from soils (mg Cd/kg soil) using different extractants and their regression coefficients (r ) with the phy toavailable cadmium content in durum wheat... 

Roberts et al. [59] have discussed the simultaneous extraction and concentration of cadmium and zinc from soil extracts. Extractions were conducted with calcium chloride adjusted to various pH values between 3 and 11. The simultaneous recovery of cadmium and zinc was essentially quantitative over the pH range 4-7, with values ranging from 92 to 102%. An extraction at pH 4.5 was adopted. Adequate recoveries were obtained when the procedure was applied to spiked soils. 

Manganese oxides have been used to remediate contaminated soils as well. Hydrous manganese oxides decreased Cd and Pb in rye grass (Lolium spp.) and tobacco (Nicotiana tabacum L.) from soils with pH 7.4-7.8 (Mench et al., 1994). Cadmium concentrations decreased in water and in 0.1 M Ca(N03)2 and acetic acid extracts of treated soils (Mench et al., 1994). 

What is essential in establishing traceability is that the measurand is specified unambiguously. This may be, e.g. in terms of extractable cadmium from soil by using a named acid mix or the concentration of a metal in a particular oxidation state, e.g. Fe(n) or Fe(m). The units used to report the result should also be known and acceptable SI units are preferred. The method used will be validated and if used in accordance with the written procedures should produce results that are fit for purpose . The class of glassware to be used will be specified in the method procedure, e.g. Class A pipettes and volumetric flasks, as these are manufactured to a specified tolerance. Instruments will be regularly calibrated and their performance verified daily. In terms of the chemicals used, these will... 

Tecator [20] has described a flow injection system for the determination of nitrate and nitrite in 2 mol/1 potassium chloride extracts of soil samples. Nitrate is reduced to nitrite with a copperised cadmium reductor and this nitrite is determined by a standard spectrophotometric procedure in which the soil sample extract containing nitrate is injected into a carrier stream. Upon the addition of acidic sulfanilamide a diazo compound is formed which then reacts with N-(l-naphthyl)ethylcncdiamine dihydrochloride provided from a second merging stream. A purple azo dye is formed, the intensity of which is proportional to the sum of the nitrate and the nitrite concentration. Nitrite in the original sample is determined by direct spectrophotometry of the soil extract without cadmium reduction. 

The second type of extraction involves removing metals from contaminated soil such that their concentration is below some defined level. Sometimes, this is called the actionable level. Above this level, some action needs to be taken to remove the contaminating metal. Below this level, no action need be taken. This type of extraction is limited to contaminated soil such as would be found at Superfund sites. Metals commonly of concern are chromium and cadmium other metals extracted are given in Table 11.2. At high levels, plant essential metals, mentioned earlier, are toxic and so may also be of concern under these conditions. 

Only limited work has been done on the bioaccessibility of metals in windborne mine waste and tailings material, and so much must be inferred. Mullins and Norman (1994) analyzed the size distribution, metal content, and metal extraction by simulated biofluids (lung, gastric, intestinal) of surface materials (soils) collected from several mine waste piles in the Butte, Montana, district. They found that the concentrations of arsenic, cadmium, copper, manganese, and lead were commonly greatest in the smallest size fractions (<4.7 pm) of the waste dump material. The percentage of metals leached from the hne fraction was quite variable, but not in any consistent way, between different metals, different dumps, and different extraction fluids. 

Fig. 2. Cadmium uptake by com shoots against molar concentration of CdCC and Cd2 in saturation extracts of the soils (redrawn from Sposito and Bingham, 1981).

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