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Content archived on 2024-04-16

A unifying concept for the assessment of the bio-availability and leachability of cadmium and zinc


The objective is to develop a laboratory technique in which the plant uptake response to concentrations and activities of cadmium and zinc (cadmium(2+) ions and zinc(2+) ions) in soil solution can be measured and to determine whether plants respond closely to these under different soil conditions (pH, metal load) and what is the influence of intensity and capacity.
A project has been set up to investigate plant uptake response to concentrations and activities of cadmium and zinc ions in soil solution. In particular the objectives were:
to establish a method of sampling soil solution and solve remaining problems regarding speciation of small samples of soil solution;
to monitor changes over 1 year in cadmium and zinc solubility after spiking to 10 levels of cadmium or zinc of two soils of different pH;
to assess a new method of linking nutrient additions to plant transpiration;
to further test the hypothesis that plant uptake of cadmium and zinc is determined by solution ion activity and mass flow. 8 major experiments were conducted:
experiment A: a method of cadmium and zinc speciation was developed for 50 ml samples of soil solution down to 0.1 ppb cadmium and 1 ppb zinc concentrations;
experiment B: the ion exchange speciation method for cadmium or zinc was compared to a fractionation method by dialysis;
experiment C: a new pressure method of obtaining leachate was compared to the water displacement method for planted and unplanted pots. Organic carbon and potassium in solution were not increased in the pressure solutions;
experiment D: the exact point of breakthrough of added water in the displacement method was determined using chromatographical analysis. More solution could be obtained from unplanted pots before breakthrough;
experiment E: changes over time of cadmium and zinc uptake by radish were assessed by sampling 10 times during plant growth. Uptake of cadmium and zinc seemed to be related to ion activity in soil solution;
experiment F: the effect of variations in cadmium and zinc concentration and activity in soil solution was studied in a plant experiment using soils varying in metal load or pH. Increases in cadmium and zinc in soil solution were positively correlated to plant uptake of cadmium or zinc from polluted but not from uncontaminated soils;
experiment G: it was found that mass flow explains some of the variation in cadmium and zinc uptake between species;
experiment H: 2 uncontaminated soils of different pH were spiked to 10 levels of either cadmium or zinc and incubated. It was found that no precipitation had occurred after 7 months of incubation.
Phase 1 involves the development of methods and testing of concepts. The hypothesis to be tested is that plant uptake response can be related to either the concentration or activity of cadmium and zinc present in the soil solution. A conceptual model will be built up from the results of pot experiments in soil using plant species that are widely grown in Europe (eg a graminaceous species, a dicotyledon). Factors to be tested in Phase 1 are those that influence both soil solution metal content and the chemical species present:
pH, 3 levels (2 contrasting soil types, with 3 different long established pHs of each, are available at Rothamsted);
metal loading, 4 or 5 levels (more in the initial experiments to establish response curves);
and metal addition, cadmium or zinc as sulphate salts.

Phase 2 involves the use of the developed concepts on different soils. About 15 soils will be collected from different countries to give a range of soil types and properties (range of pH, organic matter and textures representing the variety of European soils) and also different forms of pollution. These should include natural mineralization, mining waste, industrial effluents, disposal of sewage sludge, disposal of dredged materials, fallout from smelters, and normal agricultural soils with little pollution present. The choice of soils will be coordinated with the Community Bureau of Reference who are coordinating trials of methods for chemical speciation of metals in soil and sediment.These soils will be used in experiments like those above and both uptake of metals and full characterization of the metals in soil solution will be done.

Phase 3 involves the development of a simple testing procedure. From the concepts derived in Phase 1 and 2 a procedure will be developed that will be more simple but still capable of providing the information necessary to indicate the relative risk of the metal contamination present in any soil. Extraction with neutral salts seems to be promising because essentially those mimic soil solution. Present methods using calcium chloride, however, are unable to account for the effect that differences in soil solution calcium concentration have on uptake by plants. Therefore salts such as strontium chloride will be tried out, as strontium is chemically similar to calcium but will permit measurement of the calcium concentration in the extract.


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West Common
AL5 2JQ Harpenden, Herts
United Kingdom

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