Immobilization Of Heavy Metals In Contaminated Soils

The overall situation of soil environment in China is not optimistic.Human activities have caused the accumulation of heavy metals in certain areas in Beijing,Tianjin,South China,East China,Southwest China,and Henan Provinces,elevated levels of heavy metals in edible parts of crops have also been found in these areas,this may affect the health of local residents.The main purpose of this study was to investigate the immobilization of heavy metals in farmland soils near lead smelters in Jiyuan,Henan Province,and to reduce the risk of heavy metals in the crop grains.The results are as follows,(1)The acid Chunan soil(with elevated levels of Pb,Cd,Cu and Zn)and calcareous Jiyuan soil(with elevated levels of Pb and Cd)were amended with raw,or sulfuric acid-,or hydrochloric acid-washed HBCR at 4%,and cultivated for 60 d.Analysis of the soil indicated that,all three honeycomb briquette combustion residues(HBCR)increased soil pH and EC more in the Chunan soil than in the Jiyuan soil.The HBCR amendment significantly reduced the concentrations of DTPA-extractable Pb,Cd,Cu and Zn in the Chunan soil(p<0.05),while it had a less pronounced affect in the Jiyuan soil.In the Jiyuan soil,the amendments with the sulfuric acid-and hydrochloric acid-washed HBCR resulted in 20.9%and 33.1%decrease in the concentrations of DTPA-extractable Pb,respectively.In the Chunan soil,the amendments with all three HBCRs decreased DTPA-extractable Pb concentrations for 26.8%-41.8%;when applied alone or with phosphate,the sulfuric acid-washed HBCR reduced DTPA-extractable Cu and Zn more than the hydrochloric acid-washed HBCR.The results suggest that the HBCR has better immobilization effect on heavy metals in acid soils than in calcareous soils,the sulfuric acid-washed HBCR differs with hydrochloric acid washed-HBCR in the immobilization of heavy metals in soil.(2)A field experiment was conducted to investigate the effect of amendment with phosphate and foliar application of Zn on the accumulation of Pb and Cd in wheat grains grown in a lead-smelting polluted area in Jiyuan in northern China.The soil was amended with superphosphate with the P:Pb ratios(mol:mol)of 1.90 or 2.57 at wheat(Triticum aestivum L.)planting or 60%of the phosphate was applied at the planting,while the rest 40%was applied at jointing stage,Zn was sprayed on the canopy of the wheat plants at the jointing stage.The phosphate amendment increased soil P availability,it also resulted in lower DTPA(Diethylene triamine pentaacetic acid)-extractable Pb(1.39-10.7%lower than the control)and Cd(0.040-7.12%lower than the control)in the soil.No significant effect of split application of phosphate was found on Pb and Cd availability in soil,higher rates of P resulted in lower Pb and Cd availabilities in the soil.Grain Pb(5.41-21.5%lower than the control),Cd(3.62-6.76%)and Zn(4.29-9.02%)concentrations were negatively affected by the phosphate application,the higher rates of phosphate resulted in lower grain heavy metal concentrations.Foliar application of Zn during the wheat growth increased the grain Zn concentrations significantly(ca.10%)(p<0.05),while it had no statistically significant effect on Pb and Cd concentrations in the grain(p>0.05).Although the Pb and Cd concentrations in wheat grains were reduced by the phosphate application,their concentrations were still much higher than the maximum permissible concentrations of wheat grain in the national standards of China.The results suggest that it is feasible to reduce the wheat grain concentrations of Pb and Cd in Pb smelting polluted areas in northern China by soil application of superphosphate,however,the split application of the phosphate and the foliar application of Zn compounds do not have substantial effect on the accumulation of Pb and Cd in the wheat grains.(3)The effect of adding calcium and soil in soil-water system on the immobilization of lead with phosphate was investigated.Four soil rates(0,0.1,0.15 and 0.2 g)were used,for each soil rate,calcium nitrate was added at 10 rates.After the reaction,the amount of phosphorus,calcium,chlorine,and lead remaining in the solution and the composition of the solid product were determined.The results showed that the final concentrations of phosphorus,lead and calcium in the solution were significantly affected by the amount of calcium nitrate added.As the calcium level increased from 0 to 100 mmol,the lead concentration in the solution increased from approximately 204 mg/L to 662 mg/L,but the lead concentration tends to decrease as the amount of Ca increases at 0-20 mmol,and the phosphorus concentration increases from 29.3 mg/L to approximately 78.2 mg/L.The amount of soil influences the residual concentration of lead,calcium,and phosphorus in the solution.With the increase of soil dosage from 0 g to 0.2 g,the residual lead concentration in solution increases from about 204 mg/L to 210 mg/L.The remaining concentration of phosphorus decreased from 29.3 mg/L to approximately 24.3 mg/L.There is no significant effect of calcium on lead content,and the effect on phosphorus content isdifferent from that of calcium.According to the XRD analysis.no matter whether calcium nitrate was added to the reaction system or whether calcareous soil was added,the predominant product in the precipitate was still lead chlorophosphate.The above results show that in the soil-water system,a certain proportion of soluble calcium can promote the immobilization of lead with phosphate,and the soil has a certain adsorption effect on lead;the addition of calcium does not affect the solid product composition.The main product of immobilization of lead w-tih is lead chlorophosphate.(4)To understand different pollution level of straw to farmland soil pollution level of different weight is active,the influence of zhengzhou and soil heavy metal pollution in clean jiyuan soil with 4%of the amount of added clean straw or contaminated straw,training 50 d under the water content of 20%,and the dynamic observation test of heavy metals in the process of soil effective state and soil enzyme activity.It was found that the use of two kinds of straw in soil can be significantly increased soil organic matter content(p<0.05),and the use of contaminated straw than the use of clean straw increase soil organic matter content of more.Test early straw application significantly increased the effective state content of heavy metal(Pb and Cd)has high heavy metal contaminated straw themselves,compared with control treatment,effective state content of heavy metals pollution to add straw increase is relatively clear,test late straw application effect on the activity of soil heavy metals are becoming is not obvious.At the same time,the effect of straw on soil heavy metals is different from that of soil heavy metal pollution,straw type and application time.The application of straw can significantly increase the activity of catalase and sucrose enzyme in soil,in which the cleaning straw is more active than the contaminated straw,and can maintain a high enzyme activity for a long time.Soil organic matter and soil effective state of lead,cadmium and zinc,and soil heavy metal total amount was significantly positively related to soil organic matter and soil catalase and sucrase were significantly positive correlation between,and was also positively correlated between two enzymes.The results showed that it was effective to reduce the heavy metal content in soil by applying straw in different levels of soil pollution,but the pollution level of straw should be controlled.(5)Influence of different stabilization measure on that effectiveness of soil heavy metal and the accumulation of heavy metals in wheat(Triticum aestivum L.)were explore by field tests in the farmland near a large smelting enterprise in Jiyuan city.The results of the study,in contrast to individual phosphates,the use of a single addition of lime or phosphite and the addition of the soil DTPA to reduce the levels of lead,cadmium,copper,and zinc in the soil,and the reduced effect of the lime in isolation is significant(p<0.05).Compared to different stability measures in order to add lime alone the best effect,significantly reduce soil DTPA extraction of lead,cadmium,copper,and zinc content(p<0.05),was reduced by 12.2%,15.14%,29.4%and 12.2%respectively;Single use of phosphate and phosphate combined with bentonite or phosphate combined with lime significantly reduced soil DTPA extraction condition of lead and cadmium content,three kinds of stabilisation measures on soil DTPA extraction state reduction of lead and cadmium content of 5.21%12.72%and 5.71%12.8%respectively.For booting stage of wheat,different treatment on wheat(Pingan 9)no significant change in lead,separate application of phosphate,adding lime and phosphate add lime processing wheat(Zhengmai 103)levels decreased significantly(p<0.05),a decline between 22.1%and 30.1%,Phosphates added bentonite processing wheat(Pingan 9)cadmium content decreased significantly(22.3%lower than in control),single use of phosphate treatment(Zhengmai 103)significantly reduce the cadmium content of wheat(17.7%lower than in control),All treatments did not significantly reduce the content of copper and zinc in the two wheat varieties compared with the control(p>0.05).The above results show that different stability measures can effectively reduce the effectiveness of soil heavy metals,including the effect of adding lime alone the best,at the same time had significant effect on the content of lead and cadmium accumulation in wheat,and the differences between the two varieties of wheat.