Experimental And Mechanistic Studies On The Degradation Of P-nitrophenol By Biochar-based Modified Nanoscale Zero-valent Iron

With the rapid development of China’s economy,the issue of environmental pollution has become increasingly severe,with water pollution being a particularly pressing concern.Among them,p-nitrophenol(PNP)is particularly difficult to biodegrade in the environment and is one of the typical pollutants that are most difficult to be directly degrad,posing a large potential pollution risk to the ecological environment.It is therefore crucial to find rapid,efficient,and safe ways to remove PNP from the environment.Recent studies have shown that PNP can be reduced to para-aminophenol(PAP)which can be easily oxidized directly.Thus,the development of advanced reduction materials and degradation technology will pave the way for PNP degradation research.Among the various materials studied for this purpose,nanoscale zero-valent iron(nZVI)is a promising nanomaterial for water treatment and soil remediation due to its abundant raw materials,environmental friendliness,and high reduction efficiency.However,its small particle size and high reactivity can lead to oxidation,agglomeration,and loss in practical applications.In this thesis,nZVI was firstly optimized for addressing the issues of easy oxidation,agglomeration and loss,and nanoscale zero-valent iron composites were prepared with dual modification of biochar and sulfide.The reduction mechanism of PNP and the modification mechanism of nZVI were discussed based on the density functional theory(DFT).PNP was only reduced to PAP in the nZVI system,and although the toxicity of pollutants decreased,it still could not achieve the purpose of water purification.To address this issue,the S-nZVI/ABC composite was prepared as an electrochemical anode.The synergistic effect of electrocoagulation technology and electrochemical oxidation technology was used to rapidly reduce the PNP concentration and achieve the water purification effect.Experiments on PNP removal by modified nZVI revealed that biochar could effectively alleviate the agglomeration of nZNI.Due to the standard electrode potential difference between nZVI and biochar,the two could form a primary cell,and the internal electric field promoted the release of electrons from nZVI corrosion.The electrons provided by the reducing agent could be transferred to the contaminant through the primary cell.The addition of sulfide not only enhanced the efficiency and reaction rate of PNP removal but also delayed surface passivation and prolonged the service life.The optimal conditions for S-nZVI/BC for PNP removal are a nZNI/BC mass ratio of 1:1,S/Fe molar ratio of 0.15,material dosage of 0.5 g/L,and initial concentration of 100 mg/L.Dual modification could reduce the energy barrier of the PNP removal reaction and alleviate the surface passivation of nZVI.Biochar loading and sulfation could maintain the stability of the particles in water.When water molecules were adsorbed on the Fe(110)surface,the adsorption and dissociation of water molecules became increasingly difficult with higher sulfur coverage,resulting in a reduction of hydrogen precipitation corrosion and surface passivation of iron.Among the various conformations,the LB-30°-2 conformation had the largest absolute value of adsorption energy.The increase of electron density on nitro activated the N-O bond,which could facilitate the transition from nitro to amino.S-nZVI demonstrated an apparent advantage in PNP removal due to its easy adsorption of PNP and rapid transfer of electrons for reaction.The indirect hydrogen-induced dissociation path in the reduction of p-nitrophenol to p-aminophenol exhibited a low Gibbs free energy change,making it the most preferred pathway.The KOH chemical activation method could effectively increase the specific surface area of biochar.Among various types of anodes,the modified nZVI anode exhibited higher electrochemical activity.The highest efficiency of PNP removal by S-nZVI/ABC anode was achieved with an nZNI/ABC mass ratio of 5:1,an initial current density of 5 mA/cm2,and pH=7.The process of PNP removal by electrocoagulation could be described by synthesized first-order kinetics,while experiments with different initial mass concentrations were better described by the Langmuir-Freundlich isothermal model.The main component of the flocculant was Fe3O4,which was generated from the conversion of Fe(OH)2 and Fe(OH)3.The flocculation,oxidation and flotation all played an important role in removing PNP during the electrocoagulation process.Seed germination tests confirmed that the electrocoagulation materials synthesized in this thesis had almost no adverse ecological effects,and the electrocoagulation process had good degradation performance of PNP,which significantly attenuated the ecotoxicity of PNP in the aquatic environment to improve the water quality for irrigation and cultivation.