Risk Assessment Of Soil Polluted By The Wastewater From Galvanizing Plant And Research Of Phytoremediation Plant Incineration Technology

AIM: An investigation and risk assessment about the content of heavy metals in rice grain and soil of farmland around a galvanizing plant in suburb of Hangzhou were investigated to provide the scientific basis for risk management.Fluidized bed incineration technique was applied for the treating of Sedum plumbizincicola, a zinc and cadmium hyperaccumulator, to provide a safety disposing method for the phytoremediation plant.METHODS: In experiment of heavy metal contamination of soil and rice grain and its risk assessment around a galvanizing plant. 38 samples of 0-20 cm topsoil and 24 samples of rice grain were collected to determine the total and DTPA extractable concentration of heavy metals in soil, and analysis the concentration of heavy metal in brown rice.In experiments of conventional pollutants and heavy metal migration and transformation in the incineration process of Sedum plumbizincicola .The gas flow was 1 m³/h and temperatures were 650°C, 750°C, 850°C, 950°C, respectively; At 850°C, Al₂O₃ and CaO were added to control the concentration of heavy metal in gas, added CaO with Ca/S of 0, 2, 4, 6 respectively. For burning test, the concentration of conventional gas pollutant were determined, the concentrations of heavy metal in flue gas, fly ash and bottom ash were also determined.In experiments of polycyclic aromatic hydrocarbons (PAHs) migration and transformation in incineration process of Sedum plumbizincicola plant, the gas flow was 1 m³/h, the temperatures were 750°C, 850°C and 950°C, respectively, for burning test. 15 kinds of PAHs contents in gas, fly ash and bottom ash were determined. RESULTS: The results showed that Zn, Cu and Cd concentrations in soil and brown rice were very high in the investigated area. Bioavailability coefficient of heavy metal was Cd > Cu > Zn. The significantly positive relationships were found between the concentrations of DTPA extractable heavy metals and the total amount of heavy metals. DTPA extractable heavy metals and the total amount of heavy metals were decreased exponentially with the distance from the factory. The stipulated standards were not exceed by the concentrations of Cu and Zn in brown rice. The over standard rate of Cd in brown rice was 79.2%. RfD values calculated followed the USEPA methods indicated that rice grown on soils of this region would pose significant Cd exposure risk to food safety.The concentrations of CO and NOx in gas were higher than the limits of GB18484-2001. With the burning temperature increased, CO emissions decreased, while the NOx emissions increased. When incinerating plant,Increase trends of CO and NOx were found with the addition of CaO .The concentration of Zn in flue gas increased with increasing temperature, while the Cd concentration at 650°C and 850°C was lower than that of 750°C, when the burning temperature was higher than 750°C, the concentration of Cd in flue gas was higher than 0.1 mg/m3 specified in GB18484-2001; In addition, with the burning temperature increased, the recovery rates of Zn and Cd in the fly ash and bottom ash were also decreased, while that of Pb had the highest at 750°C; when the temperature was higher than 750°C, the recovery of Pb shows a downward trend.At 850°C, addition of CaO and Al2O3 were conducive to lower the concentrations of Cd and Zn in gas, and improved the total recovery rate of Cd and Pb, but added CaO was not conducive to the recovery rate of Zn; Compare with Al2O3, added CaO had higher efficient to decrease concentrations of Cd in gas and increase the recovery rate of Zn and Cd. With the addition of CaO increased, the recovery rate Cd and Pb showed an increasing trend, while that of Zn showed decline trend; the concentrations of Zn and Cd peaked while the [Ca]/[S ]was 2; when the Ca/S was > 2, the concentrations of Zn and Cd decreased whit the addition of CaO increased, and the concentration of Cd in gas was below the limit of GB18484-2001. 13 kinds of PAHs were detected from the gas, while 12 kinds in the fly ash and 6 in the bottom ash were also found. The total concentration of PAHs in the gas and bottom ash decreased with the increasing of temperature. It increased 37.60 % of the total concentration of PAHs in 850°C compared with the burning temperature of 750°C. In addition, with the burning temperature increased, toxic equivalent of PAHs in gas was increased, while toxic equivalent declined in the fly ash and the bottom ash.Benzo[a]pyrene (BaP) is the main carcinogenicity PAHs in gas during the thermal treatment of Sedum plumbizincicola. The concentration of BaP increased with increasing temperature and it was higher than 0.01μg/m3 stipulated in GB3095-96; Its toxicity equivalent at 750°C, 850°C and 950°C accounted for 62.29%, 59.46% and 67.95% of PAHs TEQ, respectively.CONCLUSION: Soil of the farmland around the galvanized factory had been seriously polluted and rice grown on the soils of this region would pose significant Cd exposure risk to food safety.The major pollutants in gas were Cd, CO, NOx and PAHs in the thermal treatment of the Sedum plumbizincicola by fluidized bed; Addition of Al2O3 and CaO to Sedum plumbizincicola can decrease the Cd concentration in gas to meet the discharge standard; When the amount of CaO were at Ca/S > 4, the concentration of Cd in gas can reach discharge standard. BaP is the main carcinogenic PAHs in gas; the recovery of heavy metal from fly ash and bottom ash is low, but it can be improved by addition CaO and Al2O3. PAHs toxic equivalent in fly ash are high, and they must be rationally controlled to prevent the secondary pollution.