| Following rapid social and economic development over the past several decades,heavy metal pollution in soil caused by anthropogenic activities has been both serious and widespread in China.Excessive heavy metals in soils may pose risks and hazards to humans and the ecosystem through their presence in the food chain.The latest survey report had shown that soil contaminants point in China exceeding the rate of 16.1%,approximately 7.0% of soil Cd exceeded the national standard and the pollution degree was mainly light and mild.In situ immobilization had been regarded as the most effective and practical strategy for the remediation of heavy metalcontaminated farmlands.According to the investigation of heavy metal pollution in farmland soil around the main metal mining areas of Huangshi city.We selected a Cd contaminated farmland(Cd 2.84 mg/kg)near the Tongshankou mining area to carry out a three-year in-situ passivation remediation field experiment.Rice straw,biochar,calcium carbonate and bacteria P.putida X4/p IME were selected and tested for their ability to remediate heavy metal contaminated soil.Single amendment and combined amendment were added to the soil with one or more times.During the experiments,the plant growth and cadmium absorption of lettuce were measured.In addition,the response of soil organic matter,iron oxide,enzyme activity to different treatments and the relationship between soil properties and cadmium forms were discussed,and then the effects and stability of several immobilization approaches were evaluated.The main research contents and results were as follows:(1)The effects of several stabilizing agents added once and year on the absorption of Cd content in lettuce were investigated,and the best way to reduce Cd bioavailability were explored.For treatments that the amendments were added to soil only in the first year,the edible part Cd concentration of lettuce for C,L,RS,RSL and RSLB treatments decreased from 0.87 mg/kg to 0.39 ～ 0.67 mg/kg and then remained stable over three years,while the cadmium content of lettuce treated by the CL and RSB agents reduced to 0.42 mg/kg and 0.46 mg/kg and then gradually increased from the second year.In addition to the application of RS alone,the effects of adding agents multiple times to reduce the absorption of Cd in lettuce were better than that of adding agents only once.After continuous application of immobilization agents for three years,the Cd content in lettuce leaves were 0.23 ～ 0.42 mg/kg for C,L,RS,RSL and RSB treatments.At the same time,the Cd content of lettuce leaves decreased to 0.18 mg/kg and 0.20 mg/kg in RSLB and CL treatments,and this levels were lower than the maximum permissible concentration for Cd in leafy vegetables(0.2 mg/kg)according to the National Food Hygiene Standard of China.Among all agents,RSLB had the best effect in reducing the absorption of Cd in lettuce and maintains stability.(2)The influence of different immobilization agents and application methods on soil Cd fractionation and activity was revealed.After the addition of immobilization agents at first year,soil labile species Cd(sum of water soluble,exchanged and carbonate-bound Cd)of all treatments decreased by 0.8% ～ 26.7%,the order of reduction was L ≈ CL > RSL ≈ RSLB ≈ RSB > C ≈ RS.At the same time,the humic acid-bound Cd content of L,RS,CL treatments increased by 41.0% ～ 92.7%,and the increase of L treatment was larger than that of CL and RS treatments.The residual Cd content of L,CL,RSL,RSB,and RSLB treatments increased by 48.9% ～ 98.6%,the order of increase was RSL ≈ RSB ≈ RSLB > L ≈ CL.The content of labile species Cd increased gradually from second year for all treatments that agents was added for only once.Moreover,the active forms Cd content of L and CL treatments increased faster than other treatments.It shows that the stability of L and CL treatments were relatively weak.With the continuous addition of C,RS,RSB agents,the change trend of soil Cd distribution did not change again.However,the soil active forms Cd content of L,CL,RSL and RSLB treatments consistently lower than control group with the continuous addition of agents.After the application of immobilization agents for three years,the content of labile species Cd of L,CL,RSL and RSLB treatments decreased by 17.7%～ 21.8%.Among several immobilization agents,L,CL,RSL and RSLB were more effective in inactivating soil labile species Cd,and the effect of RSL and RSLB can be maintained for a longer time.(3)The effects of different treatments on enzyme activities in contaminated soil were explored,while the relationship between soil enzyme activities and the bioavailability and chemical activity of soil Cd were investigated.After the addition of immobilization agents at first year,the activities of invertase and catalase of all treatments increased by 35.9% ～ 233.0% and 21.9% ～ 139.3%.The increase of invertase activities followed the order of RSL > RSB > RSLB > CL > RS > C and L,while the increasement of catalase activities followed the order of RSLB > L ≈ RSL >RS ≈ CL > RSB > C.The urease activities of L,RSB and RSLB treatments increased by 22.5% ～ 83.6%,and the enzyme activity of RSB treatment was higher than that of L and RSLB treatments.The phosphatase activities of L,CL,RSL,RSB and RSLB increased by 4.9% ～ 13.0%,the increasement of catalase activities followed the order of CL > RSB ≈ RSLB > L ≈ RSL.Multiple additions of immobilization agents further enhanced soil enzyme activities to varying degrees.With the addition of stabilizing agents for three times,the activities of invertase and catalase of all treatments increased by 49.8% ～ 184.2% and 65.5% ～ 239.4% respectively,the urease activities of CL and RSLB treatments increased by 28.1% ～ 38.6%,the phosphatase activities of C,L and RSL treatments increased by 15.4% ～ 31.1%.Among the four soil enzymes,catalase activity was correlated with soil labile species Cd content(r =-0.456,P < 0.01),and positively correlated with iron-manganese oxide bound Cd content(r = 0.321,P < 0.05),sucrase(r =-0.545,P < 0.01)and catalase activity(r =-0.429,P < 0.01)were negatively correlated with the Cd content of lettuce leaves.This indicated that catalase activity can reflect the bioavailability and chemical activity of Cd in the soil,while invertase activity could only reflect the bioavailability of soil Cd.(4)The relationship between important soil properties and the Cd content of lettuce,soil Cd form and soil enzyme activity were analyzed.Based on these results,the key factors that affecting the effect and stability of soil Cd immobilization were determined.Multiple linear regression analyses showed that soil p H was the main factor affecting soil enzyme activity and the Cd content in lettuce.In addition,the active form of soil Cd was mainly affected by soil p H,while the stable form of soil Cd was mainly affected by iron oxide.Calcium carbonate and the combination of calcium carbonate with other materials could greatly increase the p H of the soil.The combination of rice straw with calcium carbonate and bacteria would not only increase the p H of the soil,but also had a greater impact on the morphological composition of soil iron oxides.Among several agents,RSLB had the best effect and stability in remediation of soil Cd pollution.The main materials such as rice straw and calcium carbonate in this agent had wide sources and low cost,it had a good application prospect in the treatment of Cd-contaminated farmland soil.In addition to the immobilization of heavy metals,this agent significantly improved soil quality and soil fertility.It could be combined with organic fertilizer to produce biological organic fertilizer products with both heavy metal passivation and soil fertilizer cultivation,which would further enhance the application prospects of this agent in the remediation of heavy metal polluted farmland soils. |
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