| Heavy metal pollution to soil caused by mineral resource development has become a major global environmental problem. When farmland soil is polluted by heavy metals, heavy metals in available fractions would be accumulated by crops and damage people’s heath through the food chain. In this paper, Daye, as a city of mining and metallurgy, was chosen as the research object to investigate the current status of the heavy metal pollution to farmland soil. Mineral materials were chosen to make an indoor passivation test and outdoor field test to launch a low-cost study to prevent and control heavy metal pollution to soil. The main conclusions are as follows:(1) Heavy metal contents in farmland soil of 10 districts in Daye were investigated. The results showed that average contents of Cu, Zn, Pb, Cd in farmland soil were much higher than the background value of Hubei province, and were respectively 4.14, 1.25, 2.63, 15.29 times the background values.Background values of soil in Hubei province were taken as evaluation standards, and the evaluation results of Nemerow pollution index method showed that all sampling points and districts were heavily polluted, Cd and Cu made the greatest contribution to heavy metal pollution in this district, followed by Pb and Zn. Evaluation results of potential ecological risk index method showed that most of sampling points and districts suffered great potential ecological risks, Cd caused the greatest potential ecological risks, followed by Cu. Among 10 sampling districts, districts facing greatest pollution and potential ecological risks were Tonglvshan, Kewan and Dajipu, respectively.(2) Heavy metal contents of rape and wheat as main crops in Daye were investigated. The results showed that Cu, Zn and Pb contents in crop seeds exceeded limitation by varying degrees. Crop seeds showed the greatest ability to accumulate Zn, followed by Cu and Pb. In comparison, rape seeds had a greater ability to accumulate Zn while wheat seeds showed a greater ability to absorb Cu. The BCR method is used for fractionation of Cu, Zn and Pb in soil. Except Pb in rape soil, other heavy metals were mainly in the residual fraction, and the acid-soluble fraction was the lowest, with the average proportion less than 15%. Cu contents in crop seeds had a significantly positive correlation with contents of Cu in the non-residual fraction, and had the most significant correlation with contents of Cu in the acid-soluble fraction.(3) Indoor immobilization experiments were conducted to evaluate the effectiveness of dolomite, limestone, wollastonite and phosphate rock in immobilizing Cu, Zn and Pb in contaminated soils. The four minerals could immobilize heavy metals in contaminated soils, while improving the soil p H, and immobilization effects of a great additive amount are better than a small additive amount. The decrease extent of acid-soluble fractions and the increase extent of residual fractions in p H 4.79 and p H 5.13 soil were greater than in p H 6.70 and p H 7.27 soil. In most cases, dolomite and limestone showed a better effect on immobilization of Cu, limestone and wollastonite had a better effect on immobilization of Zn, and phosphate rock and wollastonite produced a better effect on immobilization of Pb.(4) Outdoor field experiments were conducted to evaluate the actual effectiveness of dolomite and wollastonite in immobilizing heavy metals in contaminated soils. The addictive amounts of minirals were 1%, 2% and 4%(W/W), respectively. The activity of heavy metals in soils and the uptake of heavy metals by maize were reduced effectively by two minerals. Compared with the control, dolomite showed a better effect on Cu and Zn, which decreased by 37.54%~51.52% and 17.90%~25.58% in the acid-soluble fractions, respectively. Cu and Zn contents in maize seeds decreased by 29.86%~39.06% and 19.71%~24.05%, respectively. Wollastonite showed a better ability to reduce the mobility of Pb. Residual Pb in soils amended with wollastonite was 2.18 to 3.00 times that in the control, and the Pb content in maize seeds decreased by 30.25%~33.46%. In addition, the two minerals could promote the growth of maize. Along with the increase in the addictive amount of the two minerals, the maize plant heights and yields increased first and then decreased, and reached the peak when the addictive amount was 2%(W/W). Dolomite showed a better effect on improving the maize plant heights, while wollastonite had a better effect on increasing yields. |
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