Synergistical enhancement by Ni2+ and Tween-80 of nanoscale zerovalent iron dechlorination of 2,2',5,5'-tetrachlorinated biphenyl in aqueous solution

Dechlorination of PCBs in oil transformers by microwaves catalysed by Fe0, glycerol, and NaOH

Dechlorination of Polychlorinated Biphenyls (PCBs), carcinogenic substances used as oil in electrical transformers, remains an environmental challenge. This study aims to investigate the dechlorination of PCBs-contaminated transformer oils using microwave catalytic (Fe0, glycerol) pyrolysis and to study the kinetics of PCBs dechlorination. After determining the composition of PCBs-contaminated oil using Gas Chromatography with an Electron Capture Detector (GC-ECD), response surface methodology through a central composite design was used to optimize dechlorination factors (irradiation time, microwave power, NaOH). Finally, a kinetic study of PCBs dechlorination under optimal conditions was conducted. The results showed that the concentration of PCBs in the studied transformer oil was 526 ± 0.01 mg/kg, composed of 14 congeners, with a dominance of hexa-PCBs (70 %) and penta-PCBS (18 %). The optimal PCBs dechlorination yield of 98.87 % was obtained under the following conditions: microwave power of 700 W, irradiation time of 8 min, and 0.3 g of NaOH. The kinetic study showed that PCBs degradation under optimal conditions follows a first-order reaction. These findings suggest that microwave-assisted catalytic pyrolysis is an effective and promising method for PCBs dechlorination in transformer oil, offering a potential solution for environmental remediation.

Interaction between pollutants during the removal of polychlorinated biphenyl-heavy metal combined pollution by modified nanoscale zero-valent iron

Modified nanoscale zero-valent iron (nZVI) is a promising functional material for the remediation of combined pollutants involving polychlorinated biphenyls (PCBs) and heavy metals. However, the interaction between the two types of pollutants has not been systematically studied for this method of treatment. In this study, 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153), Cu²⁺, and Ni²⁺ were selected as the target pollutants. To understand the interaction between pollutants, the efficiencies of nZVI, sulfidated nZVI (S-nZVI), and carboxymethylcellulose stabilized nZVI (CMC-nZVI) were investigated for removal of PCB153, Cu²⁺/Ni²⁺, and combined pollution system (PCBs-Cu²⁺/Ni²⁺). Results showed that the removal kinetics of the two types of pollutants by the three materials fitted a pseudo-first-order model well and that the reaction mechanisms were similar. Among the three materials, CMC-nZVI showed the highest reactivity to degrade PCB153 (pseudo-first-order kinetic constants (k_obs) = 2.7 × 10^-4 min^-1) and remove Cu²⁺ (k_obs = 2.890 min^-1), while S-nZVI showed higher affinity for the removal of Ni²⁺ (k_obs = 0.931 min^-1). For the combined pollution system, PCB153 had little effect on the removal of heavy metals by the three materials, while the effect of heavy metals on PCB153 degradation was related to the types of heavy metals and the materials. Cu²⁺ had no significant effect on PCB153 degradation by the three materials, while the coexistence of Ni²⁺ promoted PCB153 degradation by nZVI and CMC-nZVI. XPS and electrochemical analysis showed that Cu⁰ and Ni⁰ were produced on the surface of the three materials. Ni is a more effective catalyst and promoted the electron transfer efficiency of the materials and had a positive impact on the dechlorination reaction.

Surfactant-facilitated dechlorination of 2,2',5,5'-tetrachlorinated biphenyl using zero-valent iron in soil/sediment solution: Integrated effects of plausible factors

Surfactants are used to assist the zero-valent iron-mediated reductive dechlorination (ZVI-RD) of hydrophobic organic contaminants (HOCs). Although the effect of surfactants has been investigated in single-factor systems, the relationships between the surfactant and the matrix properties during RD are not well understood. Thus, an orthogonal experiment and post-experiment characterization of ZVI were conducted in the present study to estimate the integrated effects of plausible factors. The results showed that the introduction of surfactants significantly influenced the reduction of 2,2',5,5'-tetrachlorinated biphenyl (PCB-52) by altering the contact between ZVI and PCB-52. An anionic surfactant was able to alleviate the adverse impact of high amounts of non-ionic surfactants and humic acid (used as representative soil organic matter) by changing their sorption behaviors, which were also influenced by the initial pH value. However, the reduction of ZVI by humic acid decreased the electron transfer efficiency of ZVI, and also reduced the contact between ZVI and PCB-52 by generating FeCO₃. These results suggest that the rate-limiting process for the ZVI-RD of HOCs in the soil/sediment solution is the contact between ZVI and HOCs, which can be improved by the addition of surfactants at concentrations corresponding to the maximum adsorption capacity of HOCs on the ZVI surface.

Zerovalent iron in conjunction with surfactants to remediate sediments contaminated by polychlorinated biphenyls and nickel

Dredging and disposal is commonly used for cleanup of contaminated sediments, leaving the relocated sediments still in need of remediation. In this study, the feasibility of two approaches to using zerovalent iron (ZVI) in conjunction with surfactants to remediate sediments contaminated by polychlorinated biphenyls (PCBs) and Ni was investigated. Approach A is surfactant desorption followed by ZVI treatment and approach B is a simple mixture of ZVI and sediment in surfactant solution. Results of approach A show that 65.24% of PCBs and 2.12% of Ni were desorbed by 1% Envirosurf; however, the sequential ZVI-mediated reductive dechlorination (ZVI-RD) was ineffective due to micelle sequestration by high contents of surfactants while Ni could be almost completely removed. For approach B, less than 1% of coexisting Ni was released to aqueous solution, and 47.18%-76.31% PCBs could be dechlorinated by ZVI with the addition of 0.04% surfactants (Tween-80 and Envirosurf). Results of dechlorination kinetics and ZVI morphologies reveal that surfactants at the concentrations as low as 0.04% were able to enhance the contact of sediment-bound PCBs with ZVI, and also to alleviate ZVI passivation. The PCB mixtures in sediment were continuously desorbed and dechlorinated, yielding lower substituted homologues that are less toxic and less hydrophobic. Thus, a simple mixture of ZVI and contaminated sediments without dewatering appears to be a promising alternative to the remediation of PCBs-contaminated sediments.

Nanoscale zerovalent iron-mediated degradation of DDT in soil

Nanoscale zerovalent iron (nZVI)-mediated degradation of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) was investigated in a spiked soil under different conditions (iron sources, iron dosage, soil moisture, temperature, and soil types) and DDT-contaminated field. The degradation efficiency of p,p'-DDT by nZVI and nZVI coated with sodium oleate (SO-nZVI) was much higher than that by nZVI coated with polyimide (PI-nZVI). The rapid degradation of p,p'-DDT by nZVI only occurred in flooded soil. The degradation half-life of p,p'-DDT decreased significantly from 58.3 to 27.6 h with nZVI dosage from 0.5 to 2.0% and from 46.5 to 32.0 h with temperature from 15 to 35 °C. The degradation efficiency of p,p'-DDT by nZVI differed in Jinhua (JH), Jiaxing (JX), Xiaoshan (XS), Huajiachi (HJC), and Heilongjiang (HLJ) soils. A good correlation was found between the degradation half-life of p,p'-DDT and multiple soil properties. The probable nZVI-mediated degradation pathway of p,p'-DDT in soil was proposed as DDT → DDD/DDE → DDNS → DDOH based on the metabolites identified by GC-MS. The in situ degradation efficiency of residual DDTs in a contaminated field was profoundly enhanced by the addition of nZVI as compared to the control. It is concluded that nZVI might be an efficient agent for the remediation of DDT-contaminated soil under anaerobic environment.