Research On Post-treatment Of Plant Waste Accumulated With Heavy Metals And Stabilization Of Heavy Metals

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.