Degradation Of 2,4-DNT In Groundwater By Modified Zero-valent Iron Activate Persulfate Slow-release Gel

With the rapid development of industry and agriculture,groundwater in China has been generally exposed to the threat of serious organic pollution.Nitrobenzene compounds were widely used in chemical,pharmaceutical and dyeing industries as important organic synthesis materials.However,due to illegal emissions,leakage during production and storage,nitrobenzene compounds had become one of the main contaminants in soil and groundwater,posing a great risk to human health and production.As one of the representatives of nitrobenzenes,2,4-dinitrotoluene（2,4-DNT）was highly toxic,stable and difficult to be biodegraded.Therefore,it had been listed as one of the priority hazardous organic contaminants by the US,EU and China,and has become a hot spot in groundwater remediation.Traditional physicochemical and biological techniques were not effective remediate 2,4-DNT contaminated water,and it is urgent to develop new remediation technologies.Zero-valent iron（ZVI）had been widely used as an activator of persulfate（PS）in organic pollutant degradation due to its promising activation effect,environmental friendliness and low cost.However,the existence of inherent iron oxide shell produced during synthesis,storage and remediation reduced its reactivity.In this study,a novel persulfate activator oxalic acid-modified zero-valent iron（BZVI@OA）,was synthesized via ball milling ZVI with oxalic acid dihydrate.Scanning electron microscopy,X-ray diffraction spectroscopy,X-ray photoelectron spectroscopy,Fourier transform infrared spectroscopy and Time-of-flight secondary ion mass spectrometry confirmed that the original iron oxide shell was replaced by FeC₂O₄ shell.The accelerated proton transition and diminished corrosion potential of BZVI@OA were confirmed with electrochemical experiments,resulting in the significantly improved activation of persulfate.The batch experiments proved that the BZVI@OA/PS system had promising degradation effects on various organic pollutants including 2,4-DNT with low dosing and wide initial pH range.Electron paramagnetic resonance and quenching experiments confirmed that the FeC₂O₄ shell layer formed an inner sphere adsorption of PS and promoted the breakage of peroxide bonds.Variety of free radicals generated during the BZVI@OA/PS system,among which SO₄^·-played a dominant role in the degradation process.Reverse diffusion,trailing and rebound occurred during the in-situ injection remediation of groundwater,thus placing higher demands on the sustainable remediation ability of materials.Agarose and silica sol were widely used in environmental remediation due to their ability to change phase state and achieve encapsulation and release of remediation reagents.In this study,a novel slow-release gels（ASG）were prepared through mutual doping different ratios of agarose and silica sol to achieve the loading and controlled release of persulfate.Scanning electron microscopy,X-ray diffraction spectroscopy and Fourier transform infrared spectroscopy confirmed the existence of a large number of pore channels inside the gels and the storage of persulfate in the pores of the gels.The solidification rate and release time of the slow-release gels were significantly influenced by temperature.Encapsulation and slow release of persulfate was effectively achieved under complex environmental conditions.The mechanism of persulfate release was revealed by fitting various kinetics to the slow-release process.Degradation experiments were conducted using oxalic acid-modified zero-valent iron as an activator and agarose-silica slow-release gel as an oxidant to verify the performance of the coupled system.Batch experiments with different systems,different zero-valent iron and different ratios confirmed that the BZVI@OA/ASG system could stabilize the dissolved persulfate and significantly overcome the agglomeration and passivation of zero-valent iron.BZVI@OA/ASG_0.50 coupled system achieved effective degradation of 2,4-DNT at an initial concentration of 10.0 mg/L within 24 h reaction time.The kinetic fitting of the degradation process indicated that the reaction followed the pseudo-first-order kinetic,and the release rate of persulfate in the system was the dominant factor limiting the further increase of the reaction rate.The effects of environmental factors such as different temperatures,initial pH and anions on the degradation effect were further investigated.The successive experiments further confirmed that the combined system had reliable continuous degradation performance.This work clarified the theoretical support and empirical strategy for sustainable in situ remediation of organic pollutants in groundwater.