Hydrothermal Carbon-Mediated Fenton-Like Reaction Mechanism in the Degradation of Alachlor: Direct Electron Transfer from Hydrothermal Carbon to Fe(III)

As Fenton systems suffer from the undesirable Fe­(III)/Fe­(II) cycle, great efforts were made to realize the effective reduction of Fe­(III) to Fe­(II). The effects of hydrothermal carbon (HTC) on the Fe­(III)/H₂O₂ Fenton-like reaction and the subsequent degradation of alachlor in water was systematically investigated, and the results indicated that HTC could enhance alachlor degradation in Fe­(III)/H₂O₂ by promoting the Fe­(III)/Fe­(II) cycle via electron transfer from HTC to Fe­(III) ions. The apparent alachlor degradation rate constant in the HTC-G/Fe­(III)/H₂O₂ system (7.02 × 10^–2 min^–1) was about 3 times higher than that in the Fe­(III)/H₂O₂ system (2.25 × 10^–2 min^–1). The electron spin resonance spectra analysis revealed that HTC consists of abundant carbon-centered persistent free radicals to act as the electron donor. Meanwhile, the hydroxyl groups on the surface of HTC also played an important role in the enhanced alachlor degradation because the decrease in the surface hydroxyl groups on HTC significantly decreased the degradation of alachlor. On the basis of these results, an Fe­(III) complex with surface hydroxyl groups on HTC was proposed to favor the electron transfer from the hydroxyl groups to Fe­(III), and then, the simultaneously produced Fe­(II) could accelerate the catalytic decomposition of H₂O₂ to facilitate alachlor degradation. These findings shed new light on the possible roles of carbon materials in a natural aquatic environment and provide a new pathway for environmental pollutant control and remediation of organic contaminants by HTC

Protocatechuic Acid Promoted Alachlor Degradation in Fe(III)/H₂O₂ Fenton System

In this study, we demonstrate that protocatechuic acid (PCA) can significantly promote the alachlor degradation in the Fe­(III)/H₂O₂ Fenton oxidation system. It was found that the addition of protocatechuic acid could increase the alachlor degradation rate by 10 000 times in this Fenton oxidation system at pH = 3.6. This dramatic enhancement of alachlor degradation was attributed to the complexing and reduction abilities of protocatechuic ligand, which could form stable complexes with ferric ions to prevent their precipitation and also accelerate the Fe­(III)/Fe­(II) cycle to enhance the ·OH generation. Meanwhile, the Fe­(III)/PCA/H₂O₂ system could also work well at near natural pH even in the case of PCA concentration as low as 0.1 mmol/L. More importantly, both alachlor and PCA could be effectively mineralized in this Fenton system, suggesting the environmental benignity of PCA/Fe­(III)/H₂O₂ Fenton system. We employed gas chromatography–mass spectrometry to identify the degradation intermediates of alachlor and then proposed a possible alachlor degradation mechanism in this novel Fenton oxidation system. This study provides an efficient way to remove chloroacetanilide herbicides, and also shed new insight into the possible roles of widely existed phenolic acids in the conversion and the mineralization of organic contaminants in natural aquatic environment