【作者】 杜娟；

【导师】 张增强；

【作者基本信息】 西北农林科技大学 ， 环境工程， 2020， 博士

【摘要】 土壤植物修复法因其环境友好、效果永久,并可实现大范围原位修复等优势而被广泛应用于重金属污染土壤的治理。然而,土壤修复后产生的大量重金属富集植物若得不到合理处置,势必会对土壤环境、水环境等产生二次污染,进而对农产品安全及人类健康等造成危害。因此,如何安全并高效地处理重金属富集植物,且尽可能地对其加以回收利用是解决此类问题的关键。本研究以陕西省宝鸡市凤县典型铅锌污染场地内采集的聚合草（Symphytum officinale L.）、串叶松香草（Silphium perfoliatum L.）及金盏银盘（Bidens biternata（Lour.）Merr.et Sherff）为试验原料,采用热处理技术及固化/稳定化技术作为重金属富集植物的后处理方法,分别研究了热解温度和添加剂（粘土矿物和磷酸盐）对热解产物生物炭中重金属（Cd、Pb和Zn）稳定性的影响以及重金属的稳定化机理;同时,研究了地聚物固化法对生物质中重金属的固化效果及固化机理,旨在为重金属富集植物的安全、高效处理提供新思路与理论参考。本研究获得的主要结果如下:（1）不同重金属污染程度生物质的热反应动力学分析表明,重金属（Cd/Zn）的存在并不会对生物质的热降解机制产生明显的影响;重金属（比如Zn）可在一定程度上促进热解过程中H₂的产生。（2）生物质中的重金属在热解过程中基本呈现出在生物炭中富集的趋势。其中,Cd在低温（<350℃）时富集,而Pb和Zn在中低温（<550℃）时富集。根据重金属的化学形态分析结果得出,高温热解使得重金属逐步由不稳定性或高毒性向稳定性或低毒性的形态转化。重金属的浸提试验结果表明,热解处理降低了生物炭中重金属的浸出毒性、生物有效性和潜在释放性。同时,风险指数评价结果显示,随着热解温度的升高,生物炭的风险性由高风险转变为了低风险。（3）生物炭中重金属（Cd、Pb和Zn）的长期浸出试验表明,同一热解温度生物炭中重金属的浸出量及浸出率随着浸提时间的增加而增大。其中,350℃生物炭的重金属浸出量最大,而750℃生物炭的重金属浸出量在浸提周期内增量不明显,这表明了高温炭中重金属的强稳定性。另外,高温炭在H₂O₂氧化条件下的抗氧化性要强于低温炭,且未有重金属释放,即高温热解增强了生物炭的抗氧化性并有利于抑制重金属的释放。（4）土壤培养条件下生物炭中重金属的稳定性分析试验表明,750℃生物炭中的有效态重金属含量除Pb在培养期内有小幅增加外,其余元素无显著变化。本试验中生物炭对水溶液中Cd²⁺的最大吸附量为25.17 mg g^-1,具有作为重金属吸附剂的潜力。（5）含粘土矿物（高岭土、钙基膨润土和伊利石粉）和磷酸盐（磷酸二氢钙）添加剂的生物质热重-质谱分析结果表明,钙基膨润土可有效促进生物质热解过程中可燃气组分（如H₂、CO、CH₄和其它短链烃）的生成。粘土矿物或磷酸盐添加剂提高了热解过程中重金属的回收率。高温热解,特别是与矿物添加剂的共热解使得重金属的化学形态由不稳定态向稳定态转化;同时,添加剂的存在降低了生物炭中重金属的浸出毒性。（6）重金属富集植物制备地聚物保温材料以固化/稳定化重金属的试验结果表明,生物质的最佳添加比例为3%,此时地聚物材料的抗压强度达到最大值32.6MPa,且导热系数为0.11W/（m·K）,满足保温材料的要求。地聚物试样的浸出试验结果表明,地聚物对生物质中重金属的固化率可达98%以上,且重金属的浸出浓度满足相关标准限值要求。综上所述,高温热解增强了生物质中重金属的稳定性,且粘土矿物和磷酸盐有利于强化重金属的稳定化;另外,地聚物固化法可同时实现重金属的有效固化和制备保温材料。本研究可为重金属富集植物的后处理和回收利用提供有益思路、理论支撑和技术支持。更多还原

【Abstract】 Phytoremediation is widely used to restore heavy metal contaminated soil due to its environmental friendliness,permanent effect,and application in large-scale field for in-situ remediation.However,plant waste from phytoremediation will inevitably cause secondary soil or water pollution,and then endanger the safety of agricultural products and human health if they are not properly disposed.To solve this problem,finding the proper ways for safely disposing the phytoremediation plant waste and recycling the resources and energy simultaneously is the key.In this study,Symphytum officinale L.,Silphium perfoliatum L.,and Bidens biternata（Lour.）Merr.et Sherff harvested from a typical lead-zinc contaminated sites in Fengxian County,Baoji City,Shaanxi province were used as experimental materials.Thermal treatment and solidification/stabilization techniques were applied for the post-treatment of plant waste accumulated with heavy metals.The effects of pyrolysis temperature and additives（clay minerals and phosphates）on the stability of heavy metals（Cd,Pb and Zn）in biochar and the stabilization mechanism of heavy metals were studied.Additionally,the solidification/stabilization effect and mechanism of geopolymers on heavy metals in plant waste were studied.This study can provide new perspective and theoretical reference for the safe and efficient treatment of plant waste accumulated with heavy metals.The main results are as follows:（1）The thermokinetics analysis of biomass with different heavy metal content demonstrated that the existence of metals（Cd/Zn）had little effect on the thermal degradation mechanism of feedstocks,and the existence of Zn could catalyze the H₂ evolution during the biomass pyrolysis.（2）Heavy metals mostly concentrated on the biochar derived from the phytoremediation plant in pyrolysis.Cd concentrated at low temperature（<350℃）,Pb and Zn concentrated at medium and low temperature（<550℃）.The speciation analysis of HMs indicated that HMs in the biochar could transform into more stable and less toxic forms by high-temperature pyrolysis.HMs leaching results indicated that the thermal conversion could reduce the leaching toxicity,bioavailability and potential release of HMs in the biochar.The risk index evaluation showed that the high-risk biochar changed into low risk ones as the pyrolysis temperature increasing.（3）The long-term leaching behavior of HMs in the biochar indicated that the leaching amount and rate of HMs increased with the increasing leaching time for a certain biochar.Heavy metals in the 350℃biochar was sensitive to the leaching while that in the 750℃biochar was not affected,which suggested that high-temperature pyrolysis could significantly increase the stability of HMs in the biochar.High-temperature biochar showed greater oxidation resistance with little HMs releasing than lower temperature biochar.High-temperature pyrolysis enhanced the oxidation resistance of biochar and prevented the HMs from releasing.（4）The analysis of HMs stability in the biochar in soil incubation showed that the available HMs conent in the 750℃biochar did not change significantly except that Pb increased slightly.The Cd²⁺adsorption capacity of biochar(the maximum is 25.17 mg g^-1)is comparable with the straw biochar,which suggested the potential use of biochar as heavy metals adsorbent.（5）The thermogravimetry-mass spectrometry（TG-MS）analysis of biomass with clay minerals（kaolin,calcium bentonite and illite）or phosphates（calcium dihydrogen phosphate）additives indicated that the addition of calcium bentonite could effectively promote the evolution of combustible gas,such as H₂,CO,CH₄,and other short-chain hydrocarbons during pyrolysis.The addition of clay minerals or phosphate could improve the recovery rate of HMs during pyrolysis.The speciation analysis showed that HMs were transformed from unstable forms to more stable forms in the co-pyrolysis of mineral additives and biomass at high temperature.The additives reduced the leaching toxicity of HMs in the biochar.（6）The optimum proportion of biomass in the geopolymer insulation material for HMs solidification/stabilization was 3%,of which the compressive strength reached a maximum of32.6 MPa and the thermal conductivity was 0.11 W/（m·K）meeting the requirements of insulation material.The leaching results indicated that the solidification rate of HMs in the geopolymer materials was higher than 98%,and the leaching concentration of HMs were lower than the standard limits.Overall,high-temperature pyrolysis enhances the stability of HMs in the plant waste,and the addition of caly minerals and phosphate is benefical for HMs stabilization.Moreover,geopolymer technique can stabilize the heavy metals and prepare insulation materials simultaneously.Hence,this research could provide a helpful insight and important theoretical support for the post-treatment and the reutilization of plants waste accumulated with heavy metals.更多还原