Effect And The Mechanism Of Modificated Coal-based Humin On Cadmium Passivation And The Stabilization

Coal-based humin is the main component of residue after extracting humic acid from lignite and weathered coals.Due to the lack of relevant utilization technologies and standards,the landfill and disorderly discharge of these waste residues have caused serious environmental pollution,and the recycling and utilization of resources have become an urgent problem to be solved in production.Additionally,cadmium（Cd）pollution has always been considered as the main pollution category of farmland soil in China.It is necessary to prepare high efficiency,environmental protection and low-cost adsorption materials to control cadmium pollution.Based on two key scientific problems of serious cadmium pollution and low utilization rate of coal-based humin in waste residue,this study mainly used the weathering coal and lignite from Shanxi（S）,Heilongjiang（D）and Inner Mongolia（N）,the main producing areas of humic acid raw materials in China,coal-based humin（HM）that after extraction of brown and fulvic acid from raw materials was used as the research object.Calcium-based humin（Ca-SHM,Ca-DHM and Ca-NHM）were prepared using Ca（OH）₂and ultrasonic-assisted hydrothermal method.The prepared calcium-based humin materials were used as heavy metal cadmium passivator,applied in aqueous solution and soil-plant system to explore its adsorption and desorption characteristics of cadmium,soil cadmium passivation stabilization effect and cadmium pollution evaluation.In addition,using SEM/EDS,BET-surface,Zeta potential,FTIR,¹³C-CPMAS-NMR and XPS to reveal the mechanism of cadmium passivation stabilization.The results provide an important reference for promoting the recycling and utilization of coal-based humin and the development of new materials for soil cadmium passivation and stabilization.The main results obtained are as following:（1）Comparison of characteristics of coal-based humin from different sources.Coal-based humin that from Shanxi,Heilongjiang and Inner Mongolia,were characterized and analyzed.The results showed that:Coal-based humin materials are environmentally friendly,the elements mainly consist of C,H,O,N and S;minerals are mainly quartz,kaolin,pearl clay,silica and other components;contains hydroxyl,aliphatic,carbonyl,carboxyl and ether oxygen and other functional groups,and are large molecular weight,high aromatic degree.Additionally,the total humic acid content and oxygen functional group content of coal-based humin in Shanxi were the highest,accounting for 11.48 and 45.39%,respectively.The organic matter content of coal-based humin of Shanxi and Inner Mongolia were 97.35 and 98.49%,respectively.The oxygen content of coal-based humin of Heilongjiang was the lowest,accounting for 16.79%.（2）Preparation and optimization of coal-based humin modified materials.Preparing the mercapto,calcium and iron-based humin,through comparing the specific surface area,pore size,infrared spectrum and cadmium adsorption performance of coal-based humin before and after modification,found that the adsorption effect of calcium-based humin on Cd²⁺was the best.The BET specific surface of calcium-based humin was 6.48 m²/g,the total pore volume was 0.025 cm²/g,and the average pore size was 15.37 nm.The optimum preparation conditions were as follows:the ratio of coal-based humin and Ca（OH）₂was 1:1,the hydrothermal temperature was 85℃,and the ultrasonic time was 3 h.After optimization,the cadmium removal rate of calcium-based humin reached 86.43%,which increased by 22.09%.（3）Characteristics and mechanisms of cadmium adsorption and desorption of calcium-based humin in water.The external factors such as p H value,initial concentration,adsorption time and temperature can affect the adsorption performance of coal-based humin for cadmium in water.Calcium-based humin can significantly increase the adsorption capacity of Cd²⁺.The Langmuir model was used to calculate the maximum adsorption capacity of SHM,Ca-SHM,DHM,Ca-DHM,NHM and Ca-NHM for Cd²⁺were 72.46,89.29,15.29,41.84,18.48 and 64.94 mg/g,respectively,at p H 5.5,298.15 K.The adsorption process was in accordance with Langmuir model and quasi-second-order kinetic model,which indicated that the adsorption of coal-based humin for Cd²⁺was dominated by monomolecular chemisorption,and intra-particle diffusion was the main rate-limiting step,and the liquid film had an effect on Cd²⁺adsorption rate by coal-based humin.Thermodynamic analysis showed that increasing the temperature can promote the adsorption reaction of coal-based humin to Cd²⁺.Coal-based humin adsorbed Cd²⁺mainly through ion exchange,electrostatic action,complexation reaction and physical adsorption.Calcium-based humin can improve the adsorption capacity of Cd²⁺.The first reason was increased the content of active functional groups such as Alkoxy C,Aromatic C,and Phenolic C on the surface of coal-based humin.Secondly,the corrosion of Ca（OH）₂resulted in the chemical bond rupture of oxygen-containing functional groups on the surface of coal-based humin,and Ca²⁺was successfully loaded onto the surface of coal-based humin.Thirdly,calcium hydroxide ionizes to produce OH^-,which can form hydroxide precipitation with Cd²⁺.Fourthly,the increase of pore size provided a channel for the diffusion of Cd²⁺into the coal-based humin.In addition,under different p H,temperature and ionic strength,calcium-based humin can significantly reduce the desorption capacity and desorption rate of Cd²⁺and improve the adsorption stability of Cd²⁺.（4）Soil cadmium passivation and stabilization effect and mechanism of calcium-based humin.Calcium-based humin has good passivation effect on soil cadmium.Through the adsorption isotherm fitting achieved the maximum adsorption capacity of coal-based humin for Cd²⁺was 8.62-10.23 mg/g in soil environment.Compared with coal-based humin,calcium-based humin can reduce the desorption rate of soil cadmium and enhance the stability of soil cadmium passivation.Acid leaching tests showed that compared with SHM,DHM and NHM,the leaching mass concentration of cadmium by Ca-SHM,Ca-DHM and Ca-NHM decreased by 3.85,12.90 and 2.44%,respectively,and improved the stability of soil cadmium.Combined with the analysis of soil physical and chemical properties,showed that coal-based humin can increase soil p H,conductivity,cation exchange capacity and organic matter content,and promote the passivation of soil cadmium.In addition,coal-based humin can passivate heavy metal cadmium by forming phosphate precipitation,carbonate precipitation,surface complexation of oxygen-containing functional groups,ion exchange and other functions,and the passivation effect is stable.（5）Bioavailability and comprehensive evaluation of calcium-based humin on soil heavy metal cadmium.Coal-based humin could increase chlorophyll content,photosynthesis,leaf numbers,plant height,leaf area and root length of Pakchoi under cadmium stress,and promote the growth of Pakchoi.SHM,DHM and NHM treatment groups compared with CK,aboveground fresh weight and underground fresh weight of pakchoi increased by 11.03,11.88,16.95%and 5.21,32.81,36.12%,respectively.The resistance coefficient relationship among different treatments of Pakchoi was as follows:Ca-NHM>Ca-DHM>Ca-SHM>NHM>SHM>DHM,that is,calcium-based humin could enhance the resistance of pakchoi to heavy metal cadmium,and reduce the cadmium content and enrichment ability of aboveground.The transport coefficients are all less than 1 after calcification,which alleviates the toxic effect of heavy metal cadmium.The comprehensive evaluation of calcium-based humin to cadmium passivation and stabilization showed that calcium-based humin has functionality,could reduce the activity coefficient,activity ratio,K_RSPvalue and RAC value of soil cadmium,and improved the passivation efficiency of soil cadmium.Compared with CK,soil cadmium passivation efficiency of SHM,Ca-SHM,DHM,Ca-DHM,NHM and Ca-NHM increased by 22.92,27.97,23.98,29.92,13.90 and 23.59%,respectively.Moreover,its production cost（3500 yuan/ton）is lower than the existing soil heavy metal passivator（4000-9000 yuan/ton）on the market,which is economical.Therefore,calcium-based humin can be used as a green environmental material for soil cadmium remediation.