Research On The Multi-mechanism Synergistic Removal Of Pollutants From Water By Alkali Metal Salt Modified Biocha

Plant biomass straw,as a carbon-based material with immense potential,can be transformed into biochar via methods such as pyrolysis.This biochar can be used for water purification and pollutant removal,thus realizing the recycling of resources.Alkali and alkaline earth metals such as potassium,calcium,sodium,and magnesium exert certain influences on the pyrolysis process of biomass and the modification process of biochar.This study employs strategies of alkali metal activation/modification to optimize the properties of biochar,thereby enhancing and expanding its functional capabilities.（1）By modification with sodium salt（Na HCO₃）,we developed a type of carbonate biochar（CBC）possessing dual characteristics of adsorption and flocculation.The process and mechanism of the modified biochar were determined through adsorption isotherm experiments at different p H levels,adsorption kinetics experiments with various initial concentrations,and flocculation experiments.The biochar exhibited a maximum removal capacity of 482.1 mg/g for heavy metal Cd²⁺in water under initial p H conditions of 6.The removal mechanisms include surface coprecipitation,surface chelation,ion exchange,Cd-πinteractions,and flocculation（adsorptive charge neutralization）,among which,surface coprecipitation and adsorptive charge neutralization are the leading factors,accounting for over 70%of the total Cd²⁺removal effect.The research found that the flocculation process of CBC was realized by Olation reaction between OH^-produced from carbonate hydrolysis and Cd²⁺,forming cadmium-containing nanochains with high aspect ratios and positive charges on the surface of the biochar.These nanochains formed a carbon-cadmium alunite flower with biochar as the core on the surface,achieving an"adsorption-flocculation"integrated function.Moreover,after acid washing,CBC could regenerate the adsorption-flocculation characteristics and simultaneously recover heavy metal ions during the regeneration process.（2）Through activation and modification with calcium salt（Ca Cl₂）,we developed a type of biochar（Ca BC）that has adsorption and catalytic oxidation characteristics for organic pollutants.This biochar demonstrated not only strong adsorption ability for organic pollutants in water,but also could catalyze the degradation of organics by oxygen.The occurrence of advanced catalytic oxidation was validated through adsorption experiments under anaerobic conditions and oxidation quenching experiments.Compared to the original biochar,Ca BC enhanced the removal effect on organic pollutants by approximately 364.5%.It was discovered that after Ca BC’s adsorption of organic pollutants reached saturation,aeration in the water further reduced the concentration of organic pollutants by 25.7%.Quenching experiments confirmed that this process involved Ca BC catalyzing and activating oxygen,thus accomplishing pollutant degradation through oxidation.Electron paramagnetic resonance（EPR）measurements verified that reactive oxygen species（such as·OH,·O₂^-,·CH₃,·CH=OH,and CH₃C（=O）OO·）were only generated when aerating the reaction system in the presence of Ca BC.In addition,high-performance liquid chromatography-mass spectrometry（HPLC-MS）identified the degradation products of BPS,such as 4-methylsulfonylphenol and para-toluenesulfonic acid,further confirming the existence of the oxidation process.（3）Using methods such as molecular dynamics,density functional theory（DFT）calculations,and molecular orbital analysis,we deeply investigated the microscopic mechanisms of biochar’s adsorption-catalytic oxidation degradation of pollutants.With the aid of molecular dynamics simulations,we analyzed the specific effects of biochar’s adsorption sites and pore spacing on pollutant adsorption,revealing the differential effects of biochar pore size on the adsorption of various types of pollutants.Through DFT calculations,we elucidated the adsorption capacity of Cd and Cd hydroxide nanochains on biochar,attributing the significant increase in biochar’s adsorption ability to heavy metal ligands to the unique structure and electronic distribution characteristics of the nanochains.Additionally,modeling and simulation results for micropores and mesopores suggest that micropores with diameters less than 2 nm significantly intercept and block bisphenol organic matter,hence,enhancing the proportion of mesopores can effectively improve the adsorption capacity for bisphenol pollutants.Moreover,by employing molecular orbital analysis,we explored the possible degradation paths of pollutants during oxidative degradation and predicted the potential reaction sites of free radicals using molecular orbital theory,ESP,and CDD methods.This research utilizes alkali metal salt modification/activation of biochar,significantly enhancing the biochar’s pollutant removal effectiveness.The study enriches the understanding of biochar’s pollutant removal mechanisms from the perspectives of flocculation and catalytic oxidation.The flocculation characteristics of biochar not only accomplish efficient removal of heavy metals but also achieve regeneration of flocculation characteristics and successful recovery of heavy metals.The ability of biochar to catalyze oxygen aids effective degradation of organics without introducing new substances.Therefore,this study provides a theoretical basis for the efficient and environmentally friendly application of biochar in pollutant removal,bearing significant practical implications.