New insight to superoxide radical-mediated degradation of pentachlorophenate: Kinetic determination and theoretical calculations

Investigation of the photolysis process of benzo(a)anthracene (BaA) on polyvinyl chloride (PVC) and polystyrene (PS) microplastics: Plastics aging effect, transformation products and toxicity assessment

Microplastics (MPs) and persistent pollutants (POPs) are new pollutants that are extensively studied worldwide. To fill the gaps that the degradation processes and mechanisms of polycyclic aromatic hydrocarbons (PAHs) on the surface of most MPs are still unclear, the photochemical transformation of benzo(a)anthracene (BaA) on polyvinyl chloride (PVC) MPs and polystyrene (PS) MPs in water were investigated and compared. The photolysis of BaA on the surface of PS in water proceeded easier than that on PVC within the 48 h irradiation period, with the pseudo-first-order rate constant of 0.0489 min-1 and 0.0181 min-1, respectively, which can be ascribed to the smaller particle size and more OH production of PS MPs. Due to the light competition between the chromophore and BaA as well as the light-shielding effect, aged MPs showed an inhibitory effect on the degradation of BaA compared with pristine MPs. For BaA/PVC MPs system, the degradation of BaA in real water was not significantly affected by coexisting ions and humic acid (HA) (p < 0.05), while slight inhibitory effect on the degradation of BaA appeared for PS MPs in different water matrices (UP: 86.97 %, YR: 84.47 %, PR: 81.42 % and HR: 83.21 %). According to the electron paramagnetic resonance (EPR) test, quenching experiment and probe experiment, the relative contribution of direct photolysis (PVC: 82.02 %; PS: 69.54 %) and indirect photolysis (PVC: 17.98 %; PS: 30.46 %) was confirmed. A total of 14 products were identified, and the product types were not affected by plastics aging. The results of the toxicity assessment indicated that although some intermediate products remained toxic to aquatic organisms, the toxicity of most products was lower than that of BaA. This study provides new insights into the environmental fate of PAHs and the role of MPs in the photolysis process of contaminants in surface water.

Merits and Limitations of Radical vs. Nonradical Pathways in Persulfate-Based Advanced Oxidation Processes

Urbanization and industrialization have exerted significant adverse effects on water quality, resulting in a growing need for reliable and eco-friendly treatment technologies. Persulfate (PS)-based advanced oxidation processes (AOPs) are emerging as viable technologies to treat challenging industrial wastewaters or remediate groundwater impacted by hazardous wastes. While the generated reactive species can degrade a variety of priority organic contaminants through radical and nonradical pathways, there is a lack of systematic and in-depth comparison of these pathways for practical implementation in different treatment scenarios. Our comparative analysis of reaction rate constants for radical vs. nonradical species indicates that radical-based AOPs may achieve high removal efficiency of organic contaminants with relatively short contact time. Nonradical AOPs feature advantages with minimal water matrix interference for complex wastewater treatments. Nonradical species (e.g., singlet oxygen, high-valent metals, and surface activated PS) preferentially react with contaminants bearing electron-donating groups, allowing enhancement of degradation efficiency of known target contaminants. For byproduct formation, analytical limitations and computational chemistry applications are also considered. Finally, we propose a holistically estimated electrical energy per order of reaction (EE/O) parameter and show significantly higher energy requirements for the nonradical pathways. Overall, these critical comparisons help prioritize basic research on PS-based AOPs and inform the merits and limitations of system-specific applications.

Insights into oxidation of pentachlorophenol (PCP) by low-dose ferrate(VI) catalyzed with α-Fe2O3 nanoparticles

In this study, the catalytic performance of α-Fe₂O₃ nanoparticles (nα-Fe₂O₃) in the low-dose ferrate (Fe(VI)) system was systematically studied through the degradation of pentachlorophenol (PCP). Based on the established quadratic functions between nα-Fe₂O₃ amount and observed pseudo first-order rate constant (k_obs), two linear correlation equations were offered to predict the optimum catalyst dosage and the maximum k_obs at an applied Fe(VI) amount. Moreover, characterization and cycling experiments showed that nα-Fe₂O₃ has good stability and recyclability. According to the results of reactive species identification and quenching experiment and galvanic oxidation process, the catalytic mechanism was proposed that Fe(III) on the surface of nα-Fe₂O₃ may react with Fe(VI) to enhance the generation of highly reactive Fe(IV)/Fe(V) species, which rapidly extracted a single electron from PCP molecule for its further reaction. Besides, two possible PCP degradation pathways, i.e., single oxygen transfer mediated hydroxylation and single electron transfer initiated polymerization were proposed. The formation of coupling products that are prone to precipition and separation was largely improved. This study proved that nα-Fe₂O₃ can effectively catalyze PCP removal at low-dose Fe(VI), which provides some support for the application of Fe(VI) oxidation technology in water treatment in the context of low-carbon emissions.

Enhanced paper sludge dewatering and in-depth mechanism by oxalic acid/Fe2+/persulfate process

As a typical advanced oxidation process, Fe²⁺-persulfate (PDS) oxidation technology has been widely and efficiently reported for enhancing sludge dewaterability. However, higher dosage of Fe²⁺ must be added, which will restrain the oxidation efficiency of Fe²⁺-PDS process. In this work, the oxalic acid (OA)/Fe²⁺-PDS process was studied to improve paper sludge dewatering. With the OA dosage of 6 μmol (g total solid (TS))^-1, Fe²⁺ dosage of 0.3 mmol (g TS)^-1, and PDS dosage of 0.6 mmol (g TS)^-1, sludge dewaterability was improved more efficiently. The specific resistance to filtration and water content of sludge cake were decreased by 70.7% and 8.0%, respectively. In comparison with Fe²⁺-PDS process, the viscosities of sludge suspension and supernatant were further reduced by 3.73% and 51.77%, respectively, and the contents of extracellular polymeric substances fractions were increased. The improved sludge dewaterability in OA/Fe²⁺-PDS process was mainly contributed by the synergistic effect of oxidative disintegration by free radicals and flocs re-flocculation, the contributions of which were the orders: metal cations > sulfate radical > hydroxyl radical. OA enhanced the efficient disintegration of sludge flocs, released more bound water, generated more Fe³⁺-oxalate complexes, and finally increased the sludge particle size significantly, forming a larger aggregation and obvious cracks. Additionally, the stabilization of several heavy metals was improved due to the chelating capacity of OA. These works will provide a novel approach for sludge dewatering in the PDS oxidation process.

The environmental fate of biomass associated polybrominated diphenyl ethers

The widespread use of polybrominated diphenyl ethers (PBDEs) inevitably leads to their occurrence in the atmosphere, soil, and sediment. Biomass, especially dry branches and fallen leaves, may act a large reservoir for PBDEs through atmospheric deposition or soil bioaccumulation. Thus, clarifying the sunlight-induced transformation behaviors of PBDEs on biomass is highly significant for our understanding on its natural self-purification process. In this work, the degradation kinetics and mechanisms of two common PBDEs congeners, decabromodiphenyl ether (BDE-209) and 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), on biomass were systematically studied under natural and simulated sunlight irradiation conditions. The highest photodegradation rate constant of BDE-209 and BDE-47 was observed on sour cherry (SC) and zoysia matrella (ZM), respectively, which was related to their larger light receiving area and poor crystallinity. Due to the higher apparent quantum efficiency, BDE-209 degrades faster than BDE-47 (0.063-0.223 vs 0.006-0.026 h^-1). The sunlight self-purification cycle of BDE-209 and BDE-47 on biomass were 6 and 14 days, respectively, with the corresponding sunlight contribution in the range of 0.12-0.51 ng mW^-1. Products analysis by GC-MS and HPLC-MS/MS revealed that the main reactions involved in the photodegradation of BDE-209 and BDE-47 on biomass were debromination, hydroxylation, cyclization, and C-O bond breaking reaction. Especially, it was firstly proposed that hydroxyl H in lignin from biomass participated in the formation of primary products, which were rationalized by density functional theory (DFT) calculations and control experiments.