Revegetation of lead/zinc mine tailings at shaoguan, guandong province, China: phytotoxicity of the tailings

The Shaoguan lead/zinc mine is located in Guangdong Province, south of China. The residual tailing materials from the extraction of lead and zinc ores have been retained in a tailing pond since 1986 as a slurry. The operation ceased in 1978 and the tailings became arid in 1980 with a pH of 10. The pH of the tailings was further reduced to 7.8 in 1993 with a limited number of plant species and a sparse coverage. This article is an attempt to study the major constraints for the natural colonization of plants on the tailings, evaluated by the phytotoxicity. There are only 12 plant species thriving in the tailings, Neyrudia reyaudinana, Imperta cylindraca, Rhus chinensis, and Pteridium aquilium being the most common species, with a relatively higher coverage. The adverse edaphic factors seemed to govern plant establishment and colonization on the tailings. These included the high concentrations of heavy metals (Pb 34300, Zn 36500, Cu 251 and Cd 83 mg/kg, total concentrations), lack of organic matter and nitrogen. A pot trial was conducted by growing a grass species Stylosanthes guianensis cv Graham on tailings mixed with sand at the ratio of 0, 25, 50, 75 and 100% (w/w). There were evident chlorosis and stunting symptoms at 25% tailings. Plant growth was inhibited at 50% tailings upward. A subsequent toxicity test was conducted using seeds of Brassica chinensis growing in a series of 0.1 M HCl extracts of tailings (Tailings: HCl=1:5 w/v; and diluted 1-, 5-, 10-, and 20-fold), with adjustment of pH to 7. The effects of tailing extracts on root elongation of B. chinensis matched the results of the pot trial, which indicated the possibility of using the root elongation test as a fast screening method for assessing soil toxicity. A column leaching experiment was also conducted to investigate the effects of acid precipitation on the mobility of heavy metals (Pb, Zn, Cu, Cd). The results indicated that acidic precipitation gave rise to the release of a higher concentration of heavy metals, which might cause soil and groundwater contamination.