Highly-efficient degradation of organic pollutants by oxalic acid modified sludge biochar: Mechanism and pathways.

The application of sludge biochar (SC) materials as efficient catalysts for organic pollutants mineralization via advanced oxidation process meets the good strategy of "make waste profitable". The catalytic oxidations of methyl orange (MO) and pyrene by oxalic acid modified sludge biochar (SC-OA) with and without H 2 O 2 were carried out. The analysis of Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), electronic paramagnetic resonance spectrometer (EPR) and free radical quenching experiment were performed and the definite relationships between persistent free radicals (PFRs) type and specific reactive oxygen species (ROS) were made clear. It is suggested for the first time that carbon-centered type PFRs in SC-OA without H 2 O 2 could form O 2 •- and •OH from COOH groups, while oxygen-centered type PFRs induced H 2 O 2 to produce •OH. The degradation intermediates of MO and pyrene were identified and the transformation pathways were proposed. SC-OA, possessing good sustainable utilization and clean catalytic property, is expected to be popularized and applied in the mineralization of organic pollutants, especially in the in-situ remediation of contaminated soil where is no continuous supply of H 2 O 2. It is known that the g-factors for oxygen-centered radicals are larger than 2.0040, while g-factors less than 2.0030 correspond to carbon-centered radicals. As shown in Fig. 4a, the g-factors for SC, SC-OA, and SC-OA deg were 2.1326, 1.9914, and 2.1330, indicating that carbon-centered PFRs played a key role in the catalytic degradation of MO. From the above results, we can speculate that SC has the intrinsic oxygen-centered type PFRs, and OA monoanion on the SC-OA can convert to carbon-centered PFRs (SC–OH/HOOC•) under the activation of the oxygen-centered PFRs followed by radical decarboxylation to give CO 2 and formate. Then, SC-OH/HOOC• stimulates the generation of ROS. [Display omitted] • The relationships of PFRs type and ROS generation was made clear. • Carbon-centered type PFRs in SC-OA prefer to form O 2.•- and •OH without H 2 O 2. • Oxygen-centered type PFRs in SC contribute to induce H 2 O 2 to produce.•OH. • SC-OA possess good sustainable utilization and clean catalytic property. [ABSTRACT FROM AUTHOR]