Powdered activated carbon-catalyzed chlorine oxidation of bisphenol-A and methylene blue: Identification of the free radical and effect of the carbon surface functional group

Single-step synthesis of nitrogen and phosphorus co-doped biochar and its application in dye removal: synergistic effects of adsorption and peroxymonosulfate activation

In the field of advanced oxidation processes (AOPs), the development of catalysts with environmental friendliness and economic benefits faces multiple difficulties, mainly reflected in the catalytic efficiency, selection specificity, and complexity of the synthesis process. This study, we reported a nitrogen and phosphorus co-doped carbon catalyst (CANP800-1) synthesized by a one-step pyrolysis method. The co-doped catalyst was able to achieve 100 % removal of Acid Orange 7 (AO7) in about 30 min and had a high apparent rate constant (kobs = 0.125 min-1), which is better than unmodified carbon and other single-doped comparative materials. Structural analyses pinpointed that N, P co-doped enhanced specific surface area (1179 m2/g), introduced abundant mesopores, and created a wealth of active sites (such as graphitic nitrogen, C-P bonds) synergistically promoting adsorption and peroxymonosulfate (PMS) activation. The CANP800-1/PMS system had significant adaptability to various water matrices, including pH, coexisting ions, natural organic matter, and real water conditions. A mechanistic investigation confirmed that singlet oxygen (1O2) was essential to the reaction process, while electrochemical studies and DFT simulations validated that N/P-induced enhancement of electron transfer and PMS adsorption took place. This study established an innovative metal-free catalytic system that exhibited remarkable effectiveness in sustainable water treatment, providing distinctive solutions and a theoretical basis for ongoing technical difficulties in industrial wastewater treatment.

Novel calcium hypochlorite/ferrous iron as an ultrafiltration membrane pretreatment process for purifying algae-laden water

Algal fouling has become one of the most critical factors hindering the large-scale development of membrane processes in algae-laden water treatment. Herein, novel calcium hypochlorite (Ca(ClO)2)/ferrous iron (Fe(II)) process was proposed as an ultrafiltration (UF) membrane pretreatment technology, and its effects on membrane fouling and water properties were systematically studied. Results showed that the terminal specific fluxes were significantly elevated to 0.925 and 0.933, with the maximum removal ratios of reversible resistance reaching 99.65% and 96.99% for algae-laden water and extracellular organic matter (EOM), respectively. The formation of cake filtration was dramatically delayed, accompanied by a significant reduction of the adhesion free energy, and the contaminants attached to the membrane surface were effectively decomposed. With respect to water quality, the removal ratios of OD685 and turbidity achieved 81.25-95.31% and 90.16-97.72%, individually. The maximum removal rates of DOC, UV254 and fluorescent organics in influent water reached 46.14%, 55.17% and 75.77%, respectively. Furthermore, the generated reactive species (e.g., •OH, Cl•, Cl2•- and ClO•) could efficiently degrade EOM, which appreciably reduced the electrostatic repulsion between the algal foulants while ensuring the integrity of algal cells. At the Ca(ClO)2/Fe(II) dosage of 0.04/0.24 mM, the zeta potential changed from -32.9 mV to -10.8 mV, and a large range of aggregates was formed. The macromolecules in the algal solution were significantly removed, and the proportion of micromolecular organics was increased to some extent. Coagulation of in-situ formed Fe(III) dominated the membrane fouling mitigation, and the reactive species also contributed to the improvement of filtration performance. Overall, Ca(ClO)2/Fe(II) pretreatment has an exceptional prospect for efficient degradation of algal pollutants and enhancement of UF capability.

Rapid As(III) oxidation mediated by activated carbons: Reactive species vs. direct oxidation

Activated carbon (AC) is widely used in pollutant removal, due to its adsorption capacity, conductivity and catalytic performance. However, few studies focus on the redox activity of AC and its role in pollutant transformation. In this study, we found that AC could efficiently mediate the oxidation of As(III) and the process of As(III) oxidation was pH and oxygen concentration dependent. In general, the presence of O₂ promoted As(III) oxidation at pH 3.0-9.5. Acidic and alkaline conditions favored As(III) oxidation regardless of whether there was oxygen, but the mechanisms involved were quite different when there was oxygen. At pH 3.0, reactive species (H₂O₂ and ·OH) were generated and accounted for As(III) oxidation; at pH 9.5, As(III) was directly oxidized by O₂ (electron transfer from As(III) to O₂ mediated by carbon matrix) under aerobic conditions. Pre-oxidation and cyclic experiments results indicated the ability of AC to oxidize As(III) at pH 9.5 was sustainable and recyclable. This study provided a new insight in pollutant oxidation by AC in the environment.

Disinfection by-product formation and toxicity evaluation for chlorination with powered activated carbon

With the deterioration of source water quality, pre-chlorination and pre-addition of powdered activated carbon (PAC) have been widely applied to improve water treatment efficiency, which would lead to PAC exposure to chlorine. Although previous studies reported that some emerging carbon materials (e.g., graphene) could potentially act as disinfection by-product (DBP) precursors, there were few studies paying attention to the interaction between chlorine and the most commonly used carbon material-PAC on the DBP formation. In this study, the DBPs formed by chlorination with and without PAC were investigated, and the DBP toxicities in different systems were evaluated. The results showed that the PAC could react with chlorine and form trihalomethanes (THMs) and haloacetic acids (HAAs). The amount of surface oxygen groups of the PAC increased during the chlorination, with these oxygen groups, especially the meta-positioned -OH groups, facilitating the formation of THMs and HAAs. In the presence of NOM, lower concentrations of THMs and HAAs were observed in the systems with PAC than in those without PAC, demonstrating the critical role of PAC adsorption towards DBP control. The cytotoxicity evaluation indicated that more toxic reaction products between PAC and chlorine were formed besides conventional DBPs. Moreover, the PAC with higher BET surface area and more lactonic function groups formed less toxic DBPs during chlorination, which might reduce health risk for treatment processes with pre-chlorination.