Biochelators as an alternative to EDTA and other synthetic chelators for the phytoextraction of heavy metals (Cu, Cd, Pb) from soil

摘要

Heavy metal pollution of soil is a global problem. Diffuse contamination of large areas causes particular difficulties, since most classical engineering technology directed at soil decontamination is associated with a loss of land, and is usually costly. For arable land, in which low to medium level contamination threatens soil fertility and human health in the long term, gentle remediation techniques are required which not only reduce toxicological risks, but also restore soil fertility. In order to overcome these problems, the use of plants to extract metals from soil (Phytoextraction) has repeatedly been suggested as a novel clean-up technology. Phytoextraction using hyperaccumulator plants is usually limited by low biomass, whereas metal uptake by high biomass plants usually suffers from low phytoavailability of the metal. One strategy to overcome these limitations is to enhance metal phytoavailability by the application of chelates. EDTA has been the most commonly used chelating agent for phytoextraction. Its disadvantage, though, is its persistence in the environment. This leads to a high risk of metal leaching into the groundwater. Furthermore, there have been signs of high toxicity towards plants and microorganisms. Various naturally occurring chelating agents, such as natural low molecular weight organic acids (NLMWOA), cyclodextrins (CyD), (S,S)-N,N’-ethylenediamine disuccinic acid (EDDS) and wool in hydrolysed form were selected for use. All these chelating agents have a common denominator, they are synthesised by natural processes, such as microorganisms, plants or animals and are thus biodegradable. The ability of the above mentioned chelating agents to mobilise heavy metals was assessed in slurry and column experiments. Phytotoxicity experiments evaluating the toxic effect of the chelates on plants were succeeded by phytoextraction pot experiments in the greenhouse. The degree of biodegradability of the chelates was evaluated with oxitop and pot experiments. Attention was given on the influence of the chelating agents on the bioavailability of heavy metals during the degradation process. Microorganisms which degrade the applied chelating agents were characterised in the case of NLMWOA. The studied NLMWOA, citric acid, oxalic acid and tartaric acid, displayed low toxicity to plants and high Cu mobilisation effectiveness. The high effectiveness was not confirmed in the phytoextraction experiment, where EDTA, considering the amounts applied was more effective. The reason for this lies in the high degree of biodegradability of the NLMWOA. All three investigated NLMWOA were degraded by the same microorganisms and displayed a significant pH increase in the soil during the degradation. This pH increase was accompanied by a decrease in the bioavailability of the heavy metals. The CyD revealed a high toxicity towards tobacco and a very low capacity mobilising heavy metals. Additionally a high degree of biodegradability was observed. EDDS, compared to EDTA, displayed a higher toxicity to tobacco. Both showed the same effectiveness in mobilising Cd and Cu in a column experiment. In the phytoextraction experiment EDDS increased the uptake of Cu but not of Cd, and both chelates failed to increase the root to shoot translocation rate. EDDS displayed a degree of biodegradability lower than often currently stated. A replanting of the pots revealed a prolonged toxicity effect of the remaining EDDS in the soil on the seedlings revealing new challenges for the field of chelate assisted phytoextraction. Wool hydrolysate displayed a high efficiency mobilising Cu in soil. Depending on the degree of hydrolysation for the applied wool, it increased the uptake by up to 850%. It revealed a very high degree of biodegradability, which, moreover was not accompanied by side effects, such as the increase of soil pH, and the accompanied decrease of the bioavailability of the heavy metals. The results from this study demonstrate that hyrolysed wool may be applied as a perfect substitute of the commonly used EDTA for enhancement of phytoextraction. The other tested biochelators, however, are not suitable to replace EDTA.