Accumulation Characteristics Of Heavy Metals And Chemical Controlling Study On Cd Pollution In Soils From Wuhan City

With the increase of agricultural pesticide, chemical fertilizer consumption and industrial and mining waste, contaminations containing heavy metals have entered into the soil through various sources which cause serious pollution, decreased agricultural product quality and yield, and produce adverse effects on human health through the biological chain. Cognizant of regional soil heavy metal pollution status and appropriate repair technology applications, has increasingly to ensure agricultural products and producing area conditions necessary for security. Chemical controlling method has the advantage of economy, convenience, to little effect on physical and chemical properties of soil and so on. Remedying the heavy metal contaminated soils with this method is a kind of effective method. This study adopts field sampling analysis and indoor simulation training method, evaluated metal accumulation characteristics of Wuhan city. On the basis of clear all kinds of control agent on cadmium soil influences of the leaching-out toxicity, I studied two typical control agent effect of cadmium form on the contaminated soil. Final, I discussed sepiolite and peat on Cd passivation effect mechanism from the adsorption mechanism aspects, then studied sepiolite and peat adsorption phosphorus (P) characteristics. The main results obtained as follows:(1) The average concentrations of Hg, As, Cd, Pb, Cr and Cu in topsoil from urban districts were all higher than those from suburbs. Hg, Pb, and Cd contents in urban topsoils accumulated significantly, especially in traffic areas, industrial districts and park districts, and their average contents exceeded corresponding standard levels 186.1%,83.2% and 76.6%, respectively. The contents of Hg, Cd, Pb, and Cu also had significant accumulation in Wuhan suburb soils. Among the suburb soils, the average content of Hg in woodland soils exceeded its standard level 266.8%. Besides As in woodland which exceeded background value 13.8%, the contents of As in vegetable field, dry land, and paddy soils, and Cr in all suburban soils were all lower than the background level of heavy metals in topsoils of Hubei Province.(2) The pollution indexs of Hg, As, Cd, Pb, Cr, and Cu in soils of Wuhan were all less than 0.7. Cd pollution in soils was most serious, followed by As, Hg, Cu, Cr, and Pb. Comprehensive evaluation showed that soil pollution index were all less than 0.7 from both various use type in suburbs and different function districts in urban, which indicated topsoils in Wuhan kept the clean non-polluting condition. (3) Add all kinds of control agent on Cd=3.0mg/kg and Cd=6.0mg/kg of acidic soil, the best passivation effects to moderate and severely contaminated acid soil were peat and calcium magnesium phosphate fertilizer, the highest rate of passivation to 19.2% and 38.4% respectively. Lime control effect was the worst, did not show passivation effects, but increases Cd of leaching quantity, the highest leaching rate could reach 99.6%. Passivation Cd pollution of alkaline soil for the best control agent was calcium magnesium phosphate fertilizer, it can reach the highest rate was 72.5%. Overall, on Cd pollution, whether acidic or alkaline soil, the two best passivation effects were calcium magnesium phosphate fertilizer and control agent peat, And for the same pollution degree and add doses of soil, the passivation rate of two control agent on the alkaline soil was higher than that of acidic soil.(4) Add calcium magnesium phosphate fertilizer or peat to Cd=3.0mg/kg of soil, the effective state Cd content in the soil with control agent were reduced in the acidic soil in 28 days training period. And in the seven days to the fourteen days appeared fluctuations, the content of Cd was increased. For Cd=3.0mg/kg of alkaline soil, dosing calcium magnesium phosphate fertilizer or peatmoss, from 0 days to 28 days,the content of effective state Cd decreased obviously. While for Cd=6.0mg/kg of alkaline soil, from 0 days to 7 days, the content of soil effective state Cd was significantly lower, from end of 7 days to the end of cultivate, the content of effective state Cd did not changed significantly. As peat dosing quantity increases, the percentage of effective state, inorganic combination state and residue state were trend to decrease, the percentage of organic combination was trend to decrease.With calcium magnesium phosphate fertilizer dosing quantity increased, the water soluble state and exchange state, inorganic combination state of Cd were significantly lower proportion trend, the organic combination state did not change significantly, but the proportion of residue state increasing significantly. The largest rise of residue state was add calcium magnesium phosphate fertilizer in soil of Cd=6.0mg/kg, the residue state up to 49.4%.(5) Sepiolite and peat to Cd2+ and P adsorption isotherm both conform to Langmuir and Freundlich equation. The saturation of Cd2+ contained adsorption quantity was 17.32mg/g and significantly greater than sepiolite saturated adsorption which was 5.61mg/g. Both sepiolite and peat adsorption quantity of Cd2+ all along with the increase of concentration Cd2+, the present adsorption rate trend to decrease. Sepiolite and peat amount of phosphorus adsorption increases with phosphorus ion concentration. Peat easier than sepiolite adsorpt P. Peat saturated the adsorption quantity was 1.33mg/g higher than sepiolite's 0.83mg/g.(6) PH was a main factor in influence sepiolite and peat adsorption Cd and P. Peat on Cd adsorption capacity were increase with pH value increase. While sepiolite first decrease then increase with pH value increase, adsorption with 4.06mg/g was lowest in pH=6. Peat on P adsorption capacity was slightly increase with pH value increase, by 0.29mg/g increase to 0.30mg/g. But sepiolite to P adsorption capacity decreases with pH value increase, by 0.28mg/g to 0.11mg/g.