Activation Of Red Mud-Biochar Composite Material On Persulfate For Degradation Of Diesel Pollution In Soil

Red mud is a by-product of the alumina industry.In this research,an environmental-friendly materials was prepared by co-pyrolysis of red mud and wood chips（RMBC）,which was applied for the activation of sodium persulfate to degrade diesel pollution in soil.The influences of raw material ratio（red mud:biochar）and co-pyrolysis temperature on the material properties were investigated,and the optimal preparation parameters were obtained.The materials were characterized by ESEM-EDS,XRD,RAMAN,BET,FTIR,XPS,etc.The effects of different environmental conditions on the properties of the materials to activate sodium persulfate for diesel degradation were investigated by batch experiments.The mechanism of the degradation process of the system was analyzed by free radical burst experiments,electron paramagnetic resonance spectroscopy（EPR）and electrochemical experiments（CV,LSV,I-T）.By comparing and analyzing the soil physicochemical properties,soil functional groups and mineral crystals before and after the reaction,as well as seed germination experiments,T.E.S.T toxicity prediction,the main substance conformation and energy cost calculation,the feasibility of this method in environmental,safety,and economic perspectives was evaluated.Research on the influences of the preparation conditions on RMBC revealed that the pyrolysis temperature obviously affected the metal morphology,the degree of material defects and the specific surface area,while the raw material ratio（RM:BC）mainly affected the metal element distribution on the material surface.The RMBC prepared at the RM:BC ratio of 1:3 and the pyrolysis temperature of 900°C performed best on activation of sodium persulfate for the degradation of diesel,and the highest degradation efficiency of 88.7%was achived.Under the above preparation conditions,red mud was successfully loaded both on the surfaces and inside of biochar pores.Typical functional groups（Metal-OH/OFGs）of the two raw materials occoured on RMBC.In addition,several characteristics,such as higher degree of surface defects（I_D/I_G=1.99）,larger specific surface area（72.392m²/g）,more micro-and mesopores,lower Fe:C（0.102）and O:C（0.856）,and better catalytic properties of the metal crystalline form（Fe₂O₃ converted to Fe₃O₄）,were obtained,thus providing more catalytically active sites and increasing the adsorption properties of the material.The properties of the composite material（RMBC₃-900）obtained under the above optimal preparation conditions were studied on activation of peroxynitrite（PDS）for degradation of diesel fuel in soil under different environmental conditions.It was found that,under the optimal environmental conditions of material dosage of4%,PDS concentration of 50 g/L and reaction temperature of 25°C,the highest removal effencicy of 88.7%was achieved.The material perpormed well catalysis capacities when the water-to-earth ratio washigher than 1:1 and p H was at the range of 2～11.The radical burst experiments and EPR analysis proved that the main active substances in the system were SO₄^—·,·OH.Surface-bound radicals also played an important role in the activation,with the contribution of hydroxyl radicals of 61.19%.Other radicals and non-radical pathways are also present.Electrochemical experiments demonstrated the existence of electron transfer activation pathways in the materials for the degradation of pollutants.The correlation between the logarithm of the reaction rate constant（Lg（K））and XPS characterization showed that the content of C=O,lattice oxygen（Lattice O）,surface adsorbed oxygen（Absorbed O）and Fe（II）/Fe（III）ratio in RMBC effected on catalytic properties obviously.The application effect of RMBC on activation of the PDS system to degrade diesel pollution in soil was evaluated further.The results were as follows:the soil p H changed barely after the reaction,indicating that the material has a certain buffering capacity for the acidic environment caused by the PDS reaction.After the reaction,the soil agglomerates reducesd,but the soil composition did not change significantly.In the seed germination test,the germination rate,root length and average stem and leaf length（86%,2.62 cm and 3.04 cm,respectively）of wheat in the remediated soil were better than those of the contaminated soil（73%,1.52 cm and 2.74 cm,respectively）,indicating that the system significantly reduced the toxicity of diesel pollution to plants after remediation.Toxicity prediction(LC₅₀ of black fathead minnow,LC₅₀ of Daphnia Magna,IGC₅₀ of pear-shaped tetrahymena,oral LD₅₀ of rats,growth and development toxicity,and bioconcentration factor)was performed for four main components of diesel fuel（N-alkanes,Isoalkanes,Olefins and Aromatic compounds,among which aromatic substances are the most difficult to degrade）.The toxicity ranking was:Aromatic compounds>N-alkanes>Isoalkanes>Olefins.After the reaction,the concentration levels of all components except aromatic substances were greatly reduced,which proved that the toxicity of soil contaminants was significantly weakened after the remediation of this system.Based on the analysis of material preparation and consumption during the reaction,the energy cost of the system was 0.4978 k Wh/L,showing an economic advantage to some extent.