Mechanochemical Remediation Technology Of Heavy Metal-POPs Composite Contaminated Soil

With the continuous development of science and technology,the replacement speed of electronic products is accelerating.As a result,the content of electronic waste continues to reach new highs.Due to the existence of a large number of directly recyclable components and other high-value resources in electronic waste produces,has a huge economic value and accelerates the rise of the electronic waste dismantling industry.In the early days of my country,most of them were mainly dismantling operations in a small family workshops,and did not form a standardized industrial chain.The long-term irregular stacking and dismantling of electronic waste has resulted in toxic and harmful substances left in the topsoil.The heavy metals and organic substances in the surface soil affect the surrounding ecological environment through surface runoff and underground infiltration,and cause serious harm to surface animals and plants.The soil has complex structure and composition,which has strong adsorption and retention of heavy metals and organic matter,which makes it difficult to remediate the soil once it is contaminated.Therefore,the remediation of heavy metal-persistent organic pollutant（POPs）complex contaminated soils has always attracted much attention.The contaminated soil in the dismantling area of electronic waste has the double pollution characteristice of heavy metals and POPs.Based on the feasibility of the mechanical pure ball milling experiments,this study sets up mechanical chemical ball milling experiments with exogenous addition of remediation chemicals to enhance the remediation effect of contaminated soil,and systematically studied the changes of physical and chemical properties and kinetic effects of mechanical chemical ball milling remediation of e-waste contaminated soil through experiments.The analysis of the key influencing factors of the mechanical pure ball milling experiment showed that the ball milling speed had the greatest influence on the remediation of the composite contaminated soil.After the ball milling speed was increased from 250 rpm to650 rpm,the solidification rates of Cu,Pb and Cd increased from 34.69%,4.9%and 9.01%to90.05%,96.07%and 55.97%,respectively.The degradation rates of Trichlorobiphenyl,Tetrachlorobiphenyl,Pentachlorobiphenyl and Decabromobiphenyl ether increased from49.88%,50.13%,10.98%and 49.37%to 66.87%,76.46%,68.24%and 87.7%,respectively.Through the analysis of the heavy metal species of the raw samples and the samples treated at different ball milling speeds,it is found that the proportion of heavy metals in the oxidizable state and the residue state can be effectively increased after the mechanical pure ball milling treatment,and the proportion will increase with the increase of ball milling speed.Under the condition of mechanical ball milling at 650 rpm as it is,the weak acid extractable state of Cu is reduced from 79.02%to 16.32%,the proportion of reducible state is reduced from 11.93%to 6.14%,and the proportion of oxidizable state is increased from 3.33%to 30.98%,and the proportion of residue state is increased from 5.72%to 46.56%.It can be seen that the mechanical ball milling technology can fix the heavy metals in the soil and reduce the degree of environmental hazards.However,it is difficult to reduce the free state content of composite polluted soil to the national standard only based on the pure ball milling technology.Therefore,it is necessary to further optimize the mechanical ball milling technology.Through chemical screening experiments,it was found that Fe-CaO-Ca₃（PO₄）₂ had the best remediation effect on heavy metal-POPs composite contaminated soil.Fe-CaO-Ca₃（PO₄）₂ combined with mechanical ball milling treatment to analyze the key influencing factors of composite contaminated soil remediation process,and then determine the optimal reaction conditions and mechanism of action.The experimental results show that under the conditions of ball milling speed of 550 rpm,ball milling time of 4 h,and ball-to-material ratio of 14:1,the toxic leaching concentrations of heavy metals Cu,Pb and Cd in the composite contaminated soil are reduced from 5497.48 ppm,4104.14 ppm and 80.52ppm to 97.74 ppm,3.29 ppm and 0.93 ppm,respectively.The solidification rates reached98.22%,99.92%and 98.85%,respectively.The extraction concentrations of persistent organic pollutants trichlorobiphenyls,tetrachlorobiphenyls,pentachlorobiphenyls and decabromobiphenyl ethers in composite contaminated soil were reduced from 135.20 ppm,153.85 ppm,16.85 ppm and 116.36 ppm to 3.56ppm,4.24ppm,3.34pm and 0.8ppm,respectively,and their degradation rates reached 97.37%,97.22%,80.18%and 99.31%,respectively.By fitting the experimental results with the macro-structural dynamics model,it is found that the repair process of mechanical chemical ball milling conforms to the macro-structural dynamics model and can predict the reaction results well.Through scanning electron microscope（SEM）and laser particle size analyzer analysis,it was found that the ball-milled soil samples formed an agglomerated structure,the soil particle size was also significantly reduced and the specific surface area of the samples increased.The X-ray diffractometer（XRD）analysis showed that the crystal structure of the ball-milled samples changed significantly,with the process of mechanical ball milling,the crystal structure of the samples gradually collapsed and changed to amorphous structure.Raman spectroscopy was used to analyze the production of inorganic carbon in the sample,and the formation of G-peak was analyzed,which can be inferred that the benzene ring in the sample will polymerize or break the bond to form the structure of inorganic carbon after ball milling.The samples were experimentally analyzed by X-ray photoelectron spectroscopy（XPS）,and the results showed that the binding energy of all elements in the samples changed,indicating that the chemical structure of the substances changed and the pollutants were more easily destroyed,thus achieving a reduction in the concentration of pollutants.Gas chromatography-mass spectrometry（GC-MS）was used to analyze the organic species in the samples before and after ball milling treatment,and it was found that the concentration of organic pollutants in the soil was reduced by ball milling treatment,and new organic substances were generated by bond breaking and ring opening.