Treatment of halogenated phenolic compounds by sequential tri-metal reduction and laccase-catalytic oxidation

• Tri-metallic reduction (TMR) dehalogenates HPCs and improves their biodegradability.
• Dehalogenated product can be completely removed by laccase-catalytic oxidation (LCO).
• TMR and LCO processes are combined to sequentially remove HPCs from water.
• Most TOC and toxicity of HPCs are efficiently removed by TMR-LCO coupling technique.
• TMR-LCO system hardly loses activity and stability after 256 h continuous reaction.

Halogenated phenolic compounds (HPCs) are exerting negative effects on human beings and ecological health. Zero-valence metal reduction can dehalogenate HPCs rapidly but cannot mineralize them. Enzymatic catalysis can oxidize phenolic compounds but fails to dehalogenate efficiently, and sometimes even produces more toxic products. In this study, [Fe|Ni|Cu] tri-metallic reduction (TMR) and laccase-catalytic oxidation (LCO) processes were combined to sequentially remove HPCs, including triclosan, tetrabromobisphenol A, and 2-bromo-4-fluorophenol in water. The kinetics, pH and temperature dependences of TMR and LCO were obtained. The detailed TMR, LCO, and TMR-LCO transformation pathways of three HPCs were well described based on the identification of intermediate products and frontier molecular orbitals (FMOs) theory. The results showed that the two-stage process worked synergically: TMR that reductively dehalogenated HPCs followed by LCO that completely removed dehalogenated products. TMR was proven to not only improve biodegradability of HPCs but also reduce the yield of potential carcinogenic by-products. Furthermore, a TMR-LCO flow reactor was assembled and launched for 256 h, during which >95% HPCs and >75% TOC were removed. Meanwhile, monitored by microorganism indicators, 83.2%–92.7% acute toxicity of HPCs was eliminated, and the genotoxicity, produced by LCO, was also avoided by using TMR as pretreatment process.

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