A comparative study on the treatment of 2,4-dinitrotoluene contaminated groundwater in the combined system: efficiencies, intermediates and mechanisms

Ultrasound-Assisted Mineralization of 2,4-Dinitrotoluene in Industrial Wastewater Using Persulfate Coupled with Semiconductors

Oxidative degradation of 2,4-dinitrotoluenes in aqueous solution was executed using persulfate combined with semiconductors motivated by ultrasound (probe type, 20 kHz). Batch-mode experiments were performed to elucidate the effects of diverse operation variables on the sono-catalytic performance, including the ultrasonic power intensity, dosage of persulfate anions, and semiconductors. Owing to pronounced scavenging behaviors caused by benzene, ethanol, and methanol, the chief oxidants were presumed to be sulfate radicals which originated from persulfate anions, motivated via either the ultrasound or sono-catalysis of semiconductors. With regard to semiconductors, the increment of 2,4-dinitrotoluene removal efficiency was inversely proportional to the band gap energy of semiconductors. Based on the outcomes indicated in a gas chromatograph-mass spectrometer, it was sensibly postulated that the preliminary step for 2,4-dinitrotoluene removal was denitrated into o-mononitrotoluene or p-mononitrotoluene, followed by decarboxylation to nitrobenzene. Subsequently, nitrobenzene was decomposed to hydroxycyclohexadienyl radicals and converted into 2-nitrophenol, 3-nitrophenol, and 4-nitrophenol individually. Nitrophenol compounds with the cleavage of nitro groups synthesized phenol, which was sequentially transformed into hydroquinone and p-benzoquinone.

Insights into the impacts of chloride ions on the oxidation of 2,4-dinitrotoluene using ferrous activated persulfate: Removal efficiency, reaction mechanism, transformation pathway, and toxicity assessment

Persulfate/Fe²⁺-based advanced oxidation processes are widely used to treat water contaminated with 2,4-dinitrotoluene (DNT). However, the oxidation of DNT by persulfate/Fe²⁺ in the presence of the chloride ion (Cl⁻) has not been addressed, and the transformation pathways and toxicities of the intermediate products remain unclear. In this study, the effect of different Cl⁻ concentrations on the oxidation of DNT was investigated by persulfate/Fe²⁺. After the addition of 1.0 mM Cl⁻ and 6 h of oxidation, the removal efficiency of DNT increased by 68.5%. Scavenging experiments and an electron spin resonance analysis suggested that Cl⁻ caused hydroxyl radicals to increase in content in the persulfate/Fe²⁺ system, thus promoting the removal of DNT. Eight intermediate products of DNT were accurately detected using high-resolution mass spectrometry, and the transformation pathways of DNT were proposed, including hydroxylation/oxidation, elimination of the nitro group, and chlorination process. The acute and chronic toxicities of the intermediate products decreased during the oxidation process, but chlorinated by-products posed a higher toxicological risk. This result is vital for the practical application and environmental safety evaluation of persulfate/Fe²⁺-based advanced oxidation.

Preparation and properties of the persulfate gel materials and application for the remediation of 2,4-dinitrotoluene contaminated groundwater

This study aims to develop persulfate new gel sustaining-release material (PGSR) and gelatin-gel sustaining-release material (G-PGSR) that can be injected into aquifers and slowly release S₂O₈^2- to groundwater. Compatibility and miscibility of colloidal silica gels and gelatin with S₂O₈^2- were tested. Morphologies of the as-prepared PGSR and G-PGSR were observed by scanning electron microscope (SEM) and Fourier transform infrared spectrometer (FT-IR). Release characteristics of PGSR containing variable persulfate concentrations (from 1.25 wt% to 5 wt%), silica sol (from 30 wt% to 40 wt%), and gelatin (from 0.5 wt% to 2.0 wt%) were monitored. Viscosities of PGSR solution increased from 5 to 112 cP with increasing silica sol from 30 wt% to 40 wt% during the first 10 min. Viscosities of PGSR solution in 40 wt% silica sol increased to 346 cP within the 30 min and rapidly increased to 8000 cP within the next 30 min followed by the gelation phase. Gelation rates of the PGSR solution increased with increased persulfate concentrations from 1.25 wt% to 5.0 wt%. The maximum release rates achieved at 5 h in G-PGSR were 1.98 mg of S₂O₈^2- per min similar to that in PGSR. The release persulfate concentrations in G-PGSR suggested that gelatin and colloidal silica were both compatible and miscible with S₂O₈^2-. Meanwhile, the PGSR exhibits a characteristic two-phase increase in viscosity with increased silica sol concentrations, persulfate concentrations, and gelatin concentrations. Compared with the persulfate only system, the degradation efficiency of 2,4-dinitrotoluene (2,4-DNT) was achieved 91.5 % within 3 h, while 78.6 % and 66.9 % degradation efficiency were shown in PGSR and G-PGSR, respectively. The PGSR and G-PGSR both could create persistent oxidation degradation of 2,4-DNT. Results suggested that colloidal silica and gelatin could be used to create PGSR and G-PGSR for persistent oxidation in groundwater remediation.

Development of novel persulfate tablets for passive trichloroethylene (TCE)-contaminated groundwater remediation

In this study, a biodegradable binder, hydroxypropyl methyl cellulose (HPMC), was used for the first time to mix with persulfate powder for developing novel persulfate-releasing tablets to remediate trichloroethylene (TCE)-contaminated groundwater. To obtain feasible parameters for the preparation of persulfate tablets, different pressures, HPMC/tablet mass ratios, and persulfate dosages were evaluated. The results showed that the persulfate tablet released 2868 mg-persulfate/day for 12 days under the optimal manufacturing parameters of HPMC/tablet mass ratio of 0.5 and pressure of 4.90 × 10⁸ N/m². Persulfate diffusion and gel layer erosion were dominant mechanisms for controlling the persulfate released in water. The persulfate release time and rate can be controlled by adjusting the persulfate dosage at the optimal HPMC/tablet ratio. In the column experiment, TCE with an initial concentration of 70 mg/L reached 55% removal efficiency by the tablet, which showed that the developed tablet was capable of degrading highly concentrated TCE. The results of electron spin resonance (ESR) spectroscopy showed that both SO₄^-· and ·OH were responsible for the oxidation of TCE. During 150 days of incubation, the biodegrading efficiency of HPMC by microbes in soil and activated sludge was 67% and 80%, respectively, under aerobic conditions, while 58% of HPMC was removed by soil bacteria under anaerobic conditions. The results showed that persulfate tablets could be used as a passive groundwater remediation system. There is no waste generated after persulfate is completely released during groundwater remediation. The developed persulfate tablets are environmentally friendly and meet the green remediation aspect.

Iron-carbon microelectrolysis for wastewater remediation: Preparation, performance and interaction mechanisms

Rapid industrialization and urbanization have produced a lot of hazardous substances in water and wastewater, which has turned into a crucial issue to the environment and the public health. Recently, iron carbon microelectrolysis (IC-ME) has attracted extensive attention in environmental remediation due to its low costs and excellent performance. Nevertheless, there is still a lack of a more systematic review on IC-ME preparation methods, their performance, and the interaction mechanisms of IC-ME in the remediation of wastewater. Herein, this work summarizes the synthetic methods, application of IC-ME materials, and the mechanism of pollutant removal by IC-ME. A variety approaches have been applied to prepare IC-ME materials, and the preparation methods and conditions have a certain influence on the properties of IC-ME materials, thus affecting the performance of pollutant removal. The mechanisms of IC-ME for contaminants removal are very complex, including adsorption, coprecipitation, reduction, surface complexation, and oxidation. Moreover, research vacant fields and problems that existed in the application of IC-ME are proposed. At last, the problems to be addressed to adapt IC to future applications are introduced. This paper reviews and prospects IC-ME wastewater remediation technology, which provides a reference for further scientific research and engineering applications.