| The mining tailings, powder wastes produced in the flocculate flotation of copper ore, are generally piled up in the tailings impoundment leading to the formation of copper mine wasteland. Acidification of mine wasteland is caused by the oxidization of metal sulfides, resulting in serious pollution to the surrounding environment. Phytoremediation is widely used in ecological restoration of mine wastelands. In this study, the forms and contents of iron and sulfur, the net acid generation and the oxidation rate of mine tailings were analyzed in Yangshanchong and Shuimuchong mine wastelands differing in the contents of metal sulfides, in order to investigate the biological and chemical oxidation processes of metal sulfides in mine wastelands under natural and artificial plant communities, the effects of plant residues and roots on the oxidation processes of sulfides, and the development of plant communities with different amendments and the consequent changes of oxidation of metal sulfides. The main results were as follows:1. The wastelands of copper mine tailings in the profile (0-60cm) showed increasing pH and acid neutralization capacity, and decreasing net acid generation and oxidation rate of metal sulfides. The top30cm showed obvious acidification in bare wastelands, while only the top10cm of wasteland under plant communities was significantly oxidized in Shuimuchong mine wasteland. The oxidation processes were dominated by chemical oxidation. The extreme acidic condition of top tailings favored the oxidation of sulfides by acidophilic bacteria, resulting in the high biological oxidation rate. The establishment of plant communities increased the water content and porosity of mine wasteland, and thus promoted the biological and chemical oxidation of sulfides in mine tailings. Bare wasteland showed higher net acid generation and oxidation rate than wastelands under plant communities. The reducing of light, temperature and oxygen content on the surface of mine wasteland resulted from phytoremediation was likely to be the main reason for the weakness of sulfide oxidation in the tailings. 2. The mine wasteland under Rhus chinensis showed high net acid generation and oxidation rate than that under I. cylindrica and V. zizanioides. The principal component analysis (PCA) showed that the layers of40to60cm were not significantly affected by plant communities. The development of plant communities was beneficial to slow the acidification of mine tailings, and effectively inhibit the migration of oxidation front. Shuimuchong wasteland displayed significantly higher net acid generation and oxidation rate than Yangshanchong wasteland with low sulfide content, and the effect of phytoremediation on the acidification of Yangshanchong wasteland was not significant. The content of metal sulfides in the copper tailings was the key factor leading to the acidification.3. Compared to nonrhizospheric tailings from I. cylindrical, V. zizanioides, Cynodon dactylon and Miscanthus sinensis in Shuimuchong wasteland, the rhizospheric tailings showed low pH and NAG-pH, and high net acid generation, respectively. The pH and NAG-pH of rhizospheric tailings increased in the following order:Cynodon dactylon<M. sinensis<I. cylindrical<V. zizanioides. The rhizospheric and nonrhizospheric tailings with circumneutral pH (6.13～7.38) dominated by chemical oxidation of sulfides. The high contents of organic matter and nutrients in rhizosphere contributed to the biological and chemical oxygen consumption, and the rhizospheric tailings of C. dactylon showed the highest biological and chemical oxidation rate among four dominant plants. The changes of oxygen content and oxidizing bacteria activities caused by plant roots slowed the oxidation rate of metal sulfides in rhizosphere to a certain extent, which was conducive to suppress a decrease in pH of the rhizosphere.4. Cluster analysis showed that the oxidation characteristics of rhizospheric and nonrhizospheric tailings from the same plant displayed a high correlation, and a low correlation was presented between C. dactylon and the other three dominant plants. Distribution of plant roots in the tailings exerted an important impact on the oxidation of rhizospheric tailings. C. dactylon with shallow roots showed the greatest acidification, while the rhizospheric tailings from V. zizanioides with well-developed root presented weak acidification, which may be related to the low oxygen content in deep rhizospheric tailings.5. The contents of oxygen and water in the mine tailings with I. cylindrical and V. zizanioides residues covered (5cm, to10cm and20cm thick layer) significantly decreased and increased, respectively. With the coverage of plant residues, the circumneutral unoxidized tailings showed decreasing pH compared with controls, and the increasing thick of plant restudies enhanced the pH decline. The top layer tailings (0-10cm) were in the greatest acidification, and dominated by chemical oxidation. However, the acidic oxidized tailings showed increasing pH with residues covered, and the pH amplification increased with the thick of plant restudies; among the three tailings layers,10-20cm tailings displayed the lowest pH, and the highest ion concentration; the most acidified layer of controls was0-10cm tailings with the formation of the hardpan, and biological oxidation was the main oxidizing process.6. The coverage of I. cylindrica and V. zizanioides residues restrained the chemical oxidation of metal sulfides in mine tailings. Meanwhile, the residues caused a decline of biological oxidation of circumneutral tailings, and an improvement of oxidizing bacteria activity in oxidized tailings. As a whole, the coverage of residues was beneficial to slow the oxidation rate of mine tailings, and no significan difference was shown due to the types of residues. Multiple regression analysis showed that the amounts of excessive cations added as plant residues and the initial pH of tailings were main factors affecting the extent of tailing pH changes.7. The copper tailings covered with soil layer (30～40cm) in Shuimuchong mine wasteland showed low content of organic matter, available nitrogen and phosphorus; the free iron oxides content, net acid generation, and biological and chemical oxidation rate of the tailings was significantly lower than that of tailings without amendments. The silty loam cover layer can effectively reduce the mineral weathering of tailings, and low contents of oxygen and nutrient restrained the chemical and biological oxidation of metal sulfides in the copper mine tailings.8. The available nitrogen content of Shuimuchong mine wasteland increased significantly with the application of urea; the wastelands (pH4.44～5.25) under plant communities of I. cylindrica and V. zizanioides in young phase displayed a great improvement in free iron oxide content and biological and chemical oxidation rate, and a significant decrease in net acid generation; meanwhile, the oxidation of metal sulfides in wastelands (pH6.9～7.1) under plant communities of I. cylindrica and V. zizanioides in mature phase was not significantly affected by the application of urea, but net acid generation greatly increased. The pH change of mine wastelands depended on the transformation of urea and the acidity of tailings.9. The inoculation of phosphate-solubilizing fungi Aureobasidium pullulans in mine wasteland promoted the development of plant communities, the contents of free iron oxides, available nutrients (nitrogen and phosphorus) and organic matter, and biological and chemical oxidation rate of sulfides, but the effect was not significant, which was likely related to the inhibition of hostile condition of mine wasteland on A. pullulans growth and metabolism.In conclusion, the establishment of vegetation is effective in suppressing the acidification of mine wastelands; the effect of plant residues and root on sulfide oxidation in the tailings differs in tailings and plant species, and the tolerant plant with high surface coverage and deep roots (such as V. zizanioides) can be effectively applied to the restoration of mine wastelands. The effect of assisted remediation (fertilization or soil covers) on wasteland acidification is close related to the properties of mine tailings. |
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