Mechanism And Strategy Of Photochemical And Electrochemical Regulation Of Reactive Chlorine Species For The Oriented Transformation Of Ammonium Nitrogen To Nitrogen In Wastewater Treatment

In recent years,environmental problems caused by reactive nitrogen species have become increasingly severe.Ammonia nitrogen（NH4⁺-N）,as one of the most harmful forms of reactive nitrogen,has received considerable attention from governments and administrations.The total amount of NH4⁺-N emissions in China exceeds the environmental capacity of natural waters.In the"11th Five-Year Plan,"NH4⁺-N surpassed chemical oxygen demand（COD）as the primary indicator of surface water environmental quality in China.The latest"14th Five-Year Plan"proposes reducing the total emission of NH4⁺-N by more than 8%（compared with the baseline year）for pollutant prevention and control.Conventional water treatments such as coagulation,sedimentation,and filtration have low efficiency in removing and transforming NH4⁺-N in wastewater due to its high solubility and stability.Numerous approaches have been developed to address this issue,but most techniques used in wastewater treatment plants rely on high pH,making it difficult to achieve effective conversion of NH4⁺-N to harmless N2 through traditional biological or physicochemical treatments due to the inert reactivity of protonated NH4⁺in highly acidic conditions.Breaking the pH limit is crucial for promoting conversion efficiency.Relative studies have shown that chlorine free radical species exhibit high reaction selectivity towards transforming NH4⁺-N to N2.Photocatalytic and electrocatalytic technologies are effective in regulating free radical reactions.This study aims to unravel the kinetic and thermodynamic principles of free radical-initiated NH4⁺-N transformation under acidic solutions,focusing on HO^·,Cl^·,Cl2^·-,and ClO^·radicals.Using photocatalytic and electrocatalytic technologies capable of regulating free radicals,several in-situ photoelectrochemical catalytic systems utilizing self-contained substances like Cl^-in wastewater were established to promote the formation of highly active chlorine species conducive to oriented transformation of NH4⁺-N to N2.The main research findings are elaborated below.Kinetic studies showed that ClO^·and Cl^·had higher reaction rates with NH4⁺in acidic solution(k_NH4+-ClO=5.3×10⁷ M^-1 S^-1 andk_NH4+-Cl=5.2×10⁶ M^-1 S^-1,respectively).The reaction rates of Cl2^·-and HO^·with NH4⁺were below the instrument detection limit,indicating poor reactivity between these free radicals and NH4⁺.Thermodynamic calculations revealed that the reaction between NH4⁺and Cl^·proceeds via hydrogen abstraction（HAA）,while the reaction between NH4⁺and ClO^·involves a free radical addition（RAF）reaction.Poor reactivity of Cl2^·-and HO^·with NH4⁺primarily results from their high reactivity with H3O⁺in acidic solutions,preventing effective reaction with NH4⁺,thereby limiting reaction possibilities.Based on the effective oxidation of NH4⁺-N by ClO^·and Cl^·,a new method was proposed for in-situ photogeneration of chlorine radicals using bismuth oxychloride（BiOCl）under UV irradiation.This method successfully promoted the oriented transformation of NH4⁺-N to N2 in Cl^--laden solution with a pH range of 1.0-6.0.All reactions followed pseudo-zero-order kinetics(with rate constants of 0.14-0.32 mg L^-1 min^-1),indicating complete transformation of NH4⁺by reacting with reactive species.The principle lies in the high susceptibility of interlayer Cl^-in BiOClto be oxidized by valence-band holes（VB h⁺）,leading to formation of various free chlorine and corresponding chlorine radicals.Disengaged interlayer Cl^-can be offset by Cl^-in solution,while conduction-band electrons（CB e^-）are responsible for H2O or O2 reduction.This process prevents self-oxidation and self-reduction of lattice Bi^III,suppressing catalyst deactivation,and realizing selective oxidation of NH4⁺in acidic wastewater.To improve selectivity of chlorine evolution reaction（ClER）at the anode interface,BiOClwas loaded onto the surface of a commercial mixed metal oxide（MMO）electrode due to its special Cl^-adsorption mechanism.The BiOCl@MMO anode increased current efficiency of NH4⁺-N degradation by～3-4 times and decreased average energy consumption by～70%.Electron paramagnetic resonance（EPR）and transient spectrum confirmed that the BiOCl@MMO anode exhibited excellent performance in electrochemical generation of chlorine free radical species,with Cl^·playing a key role in NH4⁺-N oxidation.Physical and chemical characterization methods,combined with theoretical calculations,confirmed that the BiOCl（110）crystal surface with a channel-like structure improved Cl^-transfer,reducing Gibbs free energy to only 1.48 e V,much lower than for the MMO catalytic layer（at 4.01 e V）.Differential charge density distribution analysis showed that electrons could effectively transfer from the BiOClcatalytic layer to the MMO surface,serving as the ohmic contact layer,achieving efficient electron transfer during the BiOClcatalytic layer and ClER process.A continuous flow electrochemical reaction device with BiOCl@MMO anodes was constructed,and a model based on the NH4⁺-N removal rate predicted the relationship between NH4⁺-N concentration and hydraulic retention time,providing a long-term,efficient,stable,and continuous flow operation mode for practical treatment of NH4⁺-N in wastewater.A photoelectrocatalysis（PEC）system was developed for efficient and stable NH4⁺-N wastewater treatment by combining photocatalysis and electrocatalysis reactions.This system effectively enriched free radical species,leading to improved NH4⁺-N transformation efficiency.An integration with a solar automatic tracking system（STS）further boosted the efficiency of photoelectric catalytic reactions,resulting in a 59.5%and 22.8%increase in NH4⁺-N oxidation efficiency under dark and fixed light modes,respectively.The STS also utilized self-sufficient solar energy,achieving zero energy consumption in the process of treating NH4⁺-N in wastewater.This study explores the generation of chlorine radicals for NH4⁺-N wastewater treatment through the construction of photo/electrocatalysis systems,with the objective of overcoming the pH limit.The principles governing the interaction between different types of free radicals and NH4⁺are revealed,and various strategies for regulating free radicals through different reaction systems and operation modes are proposed.The findings provide theoretical support and practical guidance for achieving efficient and low-consumption treatment of NH4⁺-N wastewater.