Heavy Metal Contamination Of Soils And Bioaccumulation By Dominant Plants In Three Manganese Minelands In Guangxi

An extensive ecological survey and sampling of soils and dominant plants were conducted in three typical manganese minelands (Quanzhou, Bansu and Xialei) in Guangxi. The soil nutrients and heavy metal concentrations were determined to assess nutrient status and heavy metal contamination of the mine soils; Accumulation of heavy metals by dominant plants was investigated to screen the metal-tolerant plants and potential hyperaccumulators potentially used for phytoremediation. Also, heavy metal speciation in soils was determined to further reveal the bioavailability and toxicity to plants. The main research findings were as follows:1 The soil pH indicated an acid nature. The mean EC is 12.02~542.23 ds·m-1, which existed significant variation in three minelands. TN in restored Quanzhou mine area was in a higher level, but on the low side for other two minelands. The highest TP was found in Quanzhou, and the lowest in Bansu, and TP levels in the three tailings dams were significantly higher than those in restored areas. TOM of three minelands were basically low and may become one of factors limiting plant growth.2 The total concentrations of Mn, Pb, Zn, Cr, Cu and Cd in the minesoils all exceeded the relevant soil background values for Guangxi. According to China Environmental Quality Standard for Soils (GB 15618-1995 pH<6.5), Cd was beyond the Class III and soil Mn level surpassed the upper limit of the moderate range. The highest concentrations of Cu and Cd were 86.29 and 7.41 mg·kg-1 in Bansu mineland, respectively. The highest concentrations of Cr were 171.94 mg·kg-1 in Quanzhou. For peak Zn and Mn levels existed in Quanzhou mineland, but no significant difference was found among the three minelands.3 Simple pollution index, modifed Nemerow index and Fuzzy comprehensive assessment method were used to assess heavy metal contamination of minesoils. Pi indicated that minesoils were not polluted by Pb, Zn and Cu, but slightly polluted by Cr and seriously polluted by Cd. On the whole, the pollution pattern of the studied heavy metals were consistent assessed with the three methods, but variations existed. However, the comprehensive pollution level of the minesoils reached heavy grade. The combination of three methods in pollution assessment may give a more objective result.4 According the sequential extraction, Cu was dominated by iron-manganese oxides and residual fraction; Cd was dominated by iron-manganese oxides and residual fraction in tailing dams and in restored eucalyptus forest soil, while was dominated by exchangeable fraction in unexploitated areas and litchi forest soil; Pb was dominated by iron-manganese oxides and residual; Zn and Cr were dominated by residual; Mn was dominated by iron-manganese oxides fraction.5 From the bioavailability of different forms of heavy metals, bioavailable and potentially bioavailable parts were the main forms of Cu, Mn and Cd; Bioavailable forms of Cd accounted for 68.0%, while potentially bioavailable parts reached 21.5%. For Pb, bioavailable and potentially bioavailable parts were almost half percent (49.2%); For Zn and Cr, they were dominated by inert fraction, highly unavailable to plants.6 In minesoils, exchangeable and iron-manganese oxide Cu was positively correlated with total Cu; A significantly positive correlation existed between the total Cd and its iron-manganese oxide, between the total Mn and its iron-manganese oxide, and between the total Cr and its residual fraction. There was also a great significant correlation between exchangeable Cd and organic matter, and C/N ratio; Also, a significant positive correlation between residual Pb and organic matter, and C/N ratio. Therefore, organic matter in soils had effect on the elemental speciation of heavy metals.7 The heavy metal contents in dominant plants showed the order of Mn> Zn> Cr > Cu >Pb>Cd. Miscanthus floridulus and Erigeron acer can be used as good pioneer species in the ecological restoration on the mine wastedlands. Phragmitas communis was a good stabilizer of tailing dams and slagheaps owing to its high accumulation of Mn and Cr. Blechnum orientale, Paspalum orbiculare, Arundinella nepalensis and Urena lobata employed metal-avoidance strategy to adapt to the mining environment. Yet Schima superba, Camellia oleifera, Phragmitas communis and Phytolacca acinosa evolved a typical tolerance measure. All of these species can be considered as a good choice for the ecological restoration of minelands. 8 In the three minelands, the highest Cu, Cr and Mn concentrations in the edible crops were in the tuber of Colocasia esculenta; for Pb, the highest level was found in Cucurbita moschata, for Cd, that was in Sonchus oleraceus, and for Zn, in cucumber. The six metal contents in Ipomoea batatas were low and within normal range. From the view of food categories, vegetables contained highest heavy metal concentrations in the three Mn minelands. Composite pollution index (Nemerow index, PN) assessment against China Food Safety Standards indicated the pollution rates of agronomic crops reached 94%, in which heavy pollution grade represented more than 88.9%. For individual metal, no Zn and Cu pollution was found among the edible crops, but heavy pollution existed for Pb, Cd and Cr in crops. The pollution rate of Pb, Cr and Cd in agronomic crops was 83.3%, 77.8% and 50.0%, respectively.