| At present,the heavy metal contamination of agricultural soil in China is a very serious problem,and the exceeding stardard rate of heavy metal is 19.4% according to the report in the Soil Pollution Condition Investigation Gazette in 2014,which pose a high risk to the safty of agriculture products and human's health.Biochar has a huge potential for soil application such as improving soil physical and chemical properties,increasing soil fertility,promoting plant growth and remedying contaminated soil effectively,due to its excellent charactersistics.Thus,in this study,biochars derived from municipal sewage sludge was prepared and the samples were characterized via analysis for Elemental analysis,Specific surface area,Scanning electro microscopy,Fourier transform infrared spectroscopy,X-ray diffraction and Thermogravimetric analysis.The effect of biochar characteristics on adsorption was explored through the adsorption experiment.A 12 months incubation experiment and a column leaching experiment were carried out to evaluate the effect of biochar on speciation,mobility of heavy metals in soil,and a pot experiment was conducted to analysis the effect of biochar on heavy metal bioavailability,which providing theoretical basic for the feasibility of sewage sludge on heavy metal polluted agricultural soil remediation.Main conclusions are as follows:(1)The structure characteristics of sludge biochar was mainly influenced by the pyrolysis temperature,the biochar p H enhanced and CHON element content decreased because of the losing of volatile substance and the formation of alkaline materials with increasing pyrolysis temperature,while the specific surface area,pore volume,number of oxygen containing functional groups and negatively charged quantity gradually increased under the low temperature condition(300~400?),and decreased under the high temperature condition(500~800?),there exist a critical point between 400 and 500?.-OH,-COOH and –C=O-are main characteristic organic functional groups on the surface of the sludge biochar.(2)The adsorption behavior of sludge biochar fitted the isothermal model of Langmuir and pseudo-second order kinetics model,so the adsorption capacity is proportional to the number of active sites of biochar.The maximum adsorption capacity of Cr,Mn and Cu on BC400 were 5.727,6.492 and 6.56 mg·g-1,respectively,and the maximum adsorption capacity of Zn was 6.124 mg·g-1 on BC300.The enhance of pyrolysis temperature adverse to adsoption ability of sludge biochar.Different from Cr,Mn and Cu,Cr mainly in the form of anions,so the adsorption efficiency of Cr on biochar reduced with solution p H increased,with the max efficiency of 91.13% at p H3.The adsorption efficiency of Cr on biochar gradually decreased with the increase of heavy metal concentration in mono-metal solution,while there showed an opposite trend in multi-metal solution,which might contributed to the competition for adsorption sites between heavy metals in the solution.The adsorption capacity of BC400 depends on three mechanisms: the physical surface adsorption process,the electrostatic attraction between-OH,-COOH and –C=O-on the surface of BC400 and heavy metal ions,and the complexation between inorganic mineral such as kaolinite on the surface of BC400 and heavy metal ions.(3)The results of 12 months incubation experiment showed that BC400 can decrease the soil exchangeable acid significantly,and enhance the soil organic matter,which improve soil physical and chemical properties and soil ferility.Compared with control,BC400 reduced the acid-soluble fraction of Cr,Mn,Cu and Zn both in single and compound heavy metals polluted soil,with the decreasement of 99.62% of Cr,40.37% of Mn,5.84% of Cu and 37.44% of Zn,respectively,in single heavy metal polluted soil,and 77.2% Cr,70.38% of Mn,50.43% of Cu and 29.78% of Zn,respectively,in compound heavy metals polluted soil.Leaching test showed that the lower p H of the simulated acid rain,the higher risk of heavy metals leaching into the environment.In contrast with the control,the acid buffering capacity of the soil treated with BC400 improved,and the heavy metals Cr,Mn,Cu,and Zn accumulated in leachate from the BC400 treated soil significantly lower than that from untreated soil after the simulated acid rain leaching.The content of Cr,Mn,Cu,and Zn in leachate were progressively dropped about 0.113,0.023,0.01 and 0.332 mg·L-1 with the p H3.5 simulated acid rain leaching.(4)The 3 months pot experiment results showed that BC400 could significantly reduce the accumulation of heavy metals in edible plants.What's more,the effect was more obvious with a higher dosage,the uptake of Cr,Cd,Cu and Zn by shoots(the edible part)reduced 70.07%,81.81%,85.07% and 85.82%,respectively,and by roots reduced 63.63%,84.2%,53.16% and 58.46%,respectively,with 10% adding dosage.The effect of BC400 on soil restoration was much better than the sewage sludge(SS)and rice husk ash(RHA),indicating a good application prospect.BC400 effectively reduced the available fraction of heavy metal(acid-soluble fraction)according to the speciation analysis,thereby reduced the uptake of plants.Compared with CK,the addition of BC400 reduced 29.12%,30.38%,28.67% and 37.71% with 10% adding dosage.(5)The effect of BC400 in heavy metal contaminated soil restoration was prior to SS and RHA,which showed a good application prospect.Though the higher p H and specific surface area of RHB contribute to its higher remedy effency than BC400,the ultization of sludge as a resource has a significant value.In addition,the modification or pretreatment of BC400 will be valid before its large scale application in order to achieve a better soil improvement and heavy metal immobilized effects. |
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