Cyanide phytoremediation by water hyacinths (Eichhornia crassipes)

Although cyanide is highly toxic, it is economically attractive for extracting gold from ore bodies containing only a few grams per 1000 kg. Most of the 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 the water hyacinth Eichhornia crassipes because of its high biomass production, wide distribution, and tolerance to cyanide (CN) and metals. We determined the sodium cyanide phytotoxicity and removal capacity of E. crassipes. Toxicity to 5–50 mg CN L−1 was quantified by measuring the mean relative transpiration over 96 h. At 5 mg CN L−1, only a slight reduction in transpiration but no morphological changes were observed. The EC50 value was calculated by probit analysis to be 13 mg CN L−1. Spectrophotometric analysis indicated that cyanide at 5.8 and 10 mg L−1 was completely eliminated after 23–32 h. Metabolism of K14CN was measured in batch systems with leaf and root cuttings. Leaf cuttings removed about 40% of the radioactivity from solution after 28 h and 10% was converted to 14CO2; root cuttings converted 25% into 14CO2 after 48 h but only absorbed 12% in their tissues. The calculated Km of the leaf cuttings was 12 mg CN L−1, and the Vmax was 35 mg CN (kg fresh weight)−1 h−1. Our results indicate that E. crassipes could be useful in treating cyanide effluents from small-scale gold mines.