Use of stable isotopes to assess phytoremediation of soils contaminated with cadmium and zinc

Land contaminated with heavy metals such as Cd and Zn can be remediated using a number of different approaches. Many of these strategies are very expensive (e. g. removal and disposal of the soil, covering the soil with uncontaminated soil, extraction with acids or chelates) whilst others provide only short term solutions (e. g. reduction of metal bioavailability by liming or adding organic matter). An alternative approach is the use of hyperaccumulator plants to remove the heavy metals. Phytoremediation, as this process is known, is an attractive method for remediation of contaminated land since it is relatively inexpensive and has the potential through the appropriate selection of plant species to be effective. However, there are many factors that need to be considered for phytoremediation, perhaps the most important is the bioavailability of metals from different component or functional pools within the soil. The bioavailability of heavy metals has been assessed through a variety of approaches, with extraction using chemicals being the most common. Another approach is the isotopic exchange method which has been regularly used for estimating the bioavailable P in soil. This technique relies on the assumption that isotopically exchangeable P is available to a growing plant and yields the so called E-value for a soil. In past studies, particularly in highly P fixing soils, the E-value has been shown to overestimate the soil available P. To overcome this a second procedure based on the measurement of isotope uptake by a plant growing on a spiked soil has been proposed and yields the so called L-value. As well as P, this method has been applied to Ni and only very recently to Cd and Zn using radioisotopes. In the present work, stable isotopes are used for the first time to measure the Cd and Zn available for plant uptake using isotopic exchange principles based on the E and L-values using a contaminated (Great Billings) and a non-contaminated (Countesswells) soil. These values were compared to Cd and Zn bioavailable pool determined using more traditional chemical extraction methods (5 extractants). Of the chemical extractants O. 1M NaNO3,0.01M CaC12,0.5M NaOH, 0.43M CH3COOH and 0.05M EDTA (pH=7.0), the highest amounts of Cd and Zn were extracted by CH3COOH and EDTA. In addition, these two extractants extracted the Cd and Zn amounts close to the Lvalues determined using the isotopic exchange method. An isotope dilution thermal ionisation mass spectrometric (ID-TIMS) method for an accurate and precise determination of Cd and Zn in soils and plants via isotope ratio measurements was developed. This was then validated by quantifying Cd and Zn in soils, plant and animal Certified Reference Materials digested by three different methods. In addition, there was no significant difference between open tube aqua regia, closed microwave bombs and HF digestion procedures which was related to the accurate definitive method of ID. Also, an approach for the sequential isotopic analysis of Cd and Zn on the same filament within the TIMS was developed, allowing more rapid analysis and reducing running time on the instrument. Cd and Zn dual analysis on the same filament proved to be successful and produced results comparable to single analysis. Isotopic ratios of Cd and Zn were used to determine both E and L-values in two soils. Although the E-value determined (in soil solution system) was higher than the L-value (using plants) for the 2 soils tested, the 2 values were generally within agreement. L-values were determined using 3 different plant species, the hyperaccumulator alpine penny cress (T. caerulescens), dandelion (T. officinale) and spring barley (H. vulgare). The available pool of Cd and Zn for plant uptake was determined to be the same for the 3 different plant species examined although plant species had markedly different abilities to uptake metals. In addition to using the plant species to estimate the bioavailable metals, the potential of these plant species for phytoremediation of Cd and Zn was also evaluated. Among the plants tested, only T caerulescens proved to be a Cd hyperaccumulator. However, T. officinale removed an equivalent amount of Cd and more of Zn than the other two plant species; although its Cd and Zn content was not sufficiently high to be considered as a hyperaccumulator. Based on this, dandelion has a good potential for phytoremediation, which lead to an attempt to study Cd and Zn uptake in the rhizosphere of dandelion roots. However, the experiment which used soil between glass plates did not show a clear depletion in Cd and Zn concentrations towards the root surface or if metal concentration was related to the amount of root present in any soil section.