Cyanid-Phytoremediation mit Eichhornia crassipes : eine alternative Methode zur Aufbereitung cyanid- und kupferhaltiger Abwässer aus dem Goldbergbau

摘要

Most of the highly toxic cyanide used in industrial mining is handled without observable devastating consequences, but in informal, small-scale mining, the use is poorly regulated and the waste treatment is insufficient. Cyanide in the effluents from the latter mines could possibly be removed by constructed wetlands with water hyacinths (Eichhornia crassipes) because of its high biomass production, wide distribution, and tolerance to cyanide and metals. The aim of this thesis was to evaluate the use of E. crassipes for the treatment of gold mining effluents containing high amounts of cyanide and copper. The phytotoxicity and the cyanide and copper removal-capacity were tested in lab-scale experiments. The cyanide degradation studies were performed applying 14C-labelled potassium cyanide (K14CN). The results were verified with E. crassipes in field scale in the hydraulically controlled treatment plant AMOVA. Toxicity to cyanide and copper was quantified by measuring the mean relative transpiration over 96 h. At 5 mg CN or Cu L–1, only a slight reduction in transpiration-rate was visible. The EC50 value was calculated by probit analysis to be 13 mg CN L–1 and 29.8 mg Cu L–1. If copper and cyanide were supplied simultaneously phytotoxicity decreased. This effect was due to the detained absorption of copper in complex form. E. crassipes removed approximately 98% of applied copper in 24 h. Copper absorption was enhanced in solutions with lower pH (pH 5.5). The copper content in the leaves did not increase obviously, compared to the control plants, whereas the content in roots was 500-fold higher (12 mg (g dry weight)–1). The copper-cyanide complex was not absorbed significantly (p > 0.05) by the plants in 72 h. Metabolism of K14CN was measured in batch systems with leaf and root cuttings. Similar first-order removal kinetics were observed in all systems with plant tissue. Leaf cuttings converted about 10% to 14CO2 and accumulated about 35% of the applied radioactivity in the tissues. The calculated KM of the leaf cuttings was 12 mg CN L–1, and the vmax was 33 mg CN (kg fresh weight)–1 h–1 (non linear regression). The radioactivity in the tissues was not attributed to 14C-labelled cyanide. The production of 14CO2 was probably due to metabolism of asparagine, the metabolite of cyanide in plants described in the literature. The formation of 14C-labelled asparagine and aspartic acid was verified after an extraction with ethanol and derivatisation with PITC. The fate of K14CN in entire plants was investigated using water hyacinths in hydroponic-systems, which were set in flow through systems connected to NaOH traps. The radioactivity in the traps was attributed to leaf-volatilisation. The decrease of radioactivity in these experiments followed zero order kinetics. This probably indicated a diffusion controlled removal mechanism. After preincubation with cyanide the kinetic changed to first order. This phenomenon may point to a physiological adaptation of the plants and/or the root-associated microflora to cyanide. If copper was added simultaneously the removal of radioactivity was inhibited. Approximately 50% of the applied radioactivity of each experiment was found in the NaOH-traps. In consequence this amount of radioactivity was released by the leaves as 14CO2 or H14CN. The amount of radioactivity in root and leaf extract was similar, which showed a high root to shoot translocation. In semi-field scale experiments in the AMOVA, E. crassipes showed a higher tolerance to cyanide. The cyanide-removal capacity increased over the experimental period. On contrary to the findings of the lab-scale experiments the plants were able to tolerate cyanide in concentrations of 14 mg CN L–1 (adding 30 L of a 470 mg CN L–1 solution) without visible toxicity symptoms. The system showed a maximum cyanide removal capacity of 50 g NaCN in 70 h. Additionally CuCN removal was observed short time periods of 24-48 h. To examine if an adaption to cyanide appeared during the experimental period, plants from the AMOVA were tested in hydroponic experiments. These plants showed an 8-fold higher cyanide removal in comparison to plants from the greenhouse. One year later cyanide was applied in the AMOVA again, but this time without an adaption-period. The results of the previous year could not be reproduced. A slower cyanide removal was observed and about 70% of the plants died during the treatment. In consequence, an adaption to cyanide in the previous year most likely occurred und must be considered if this technique is applied in the treatment of gold mining effluents, to avoid overloading and collapsing of the treatment plant during the first applications. The results indicate a high potential of E. crassipes in treating cyanide effluents from small-scale gold mining.