3.1 Effect of biochar content
To determine the effect of biochar content on PTH removal, the evolution of PTH removal in MW/PS system with different doses of W400 was initially investigated (Fig. 1a). The removal efficiency of PTH increased with biochar content, changing from 75.35–88.78% after adding 3% biochar for 80 min of MW irradiation at 60 oC. Consistent with the removal of PTH, the observed rate constants kobs significantly increased with biochar content (Fig. 1b and S1a). Moreover, the decomposition rate of PS was calculated by detecting the residual PS content after the reaction (Fig. S1b). The PS decomposition rate was 55.72% with 3% biochar, which was about 15% higher than that without biochar addition. Therefore, the biochar promoted the PS decomposition, which increased the removal of PTH.
In the water bath-PS systems, with the increase in the dosage of biochar, the rate of PTH removal only increased from 22.5–31.2% after 80 min (Fig. 1c). The efficiency of the PS/MW system in combination with biochar was much higher than that associated with the WB. As discussed above, the enhanced PTH transformation efficiency may be due to that carbonaceous materials have the possibility to absorb the MW energy and raise the reaction temperature. To explore this hypothesis, we studied the effect of biochar addition on the increasing of soil temperature (Fig. 1d). The heating rate of soil was obvious increased in the biochar/MW system as the biochar dosage increased. Compared with control experiment (no biochar), the heating time was reduced by 25% and 50% with the increasing biochar dosage of 0.5% and 3%, respectively. Therefore, biochar could absorb a large amount of MW energy to rapidly raise soil temperature (Liu et al. 2004). MW irradiation can directly heat the inside of the biochar through constant dipole rotation and ionic conduction loss (Zhang et al. 2018). These obtained results suggested that the high content of biochar could absorb more MW energy, which was favorable to activate PS and enhance the utilization rate of PS, and thus efficiently remove PTH from soil.
3.2 Effect of biochar types
Five types of biochar were performed to explore the potential influence for PTH removal in PS/MW system. As shown in Fig. S2, approximately 88.78%, 87.35%, 86.93%, 84.54%, and 82.04% of PTH were removed after 80 min with the addition of W400, W500, W600, W300, and W700, respectively. Previous study showed that the difference in contaminant removal capacity of biochar is mainly due to the content of PFRs in the biochar system (Fang et al. 2015). From the PFRs spectra (Fig. 2a), we can know that the g-factors of five biochars were all < 2.0030 (Fig. S3). This kind of free radical was classified as carbon-centric free radicals (Zhao et al. 2019). The concentrations of PFRs (Fig. 2b) were in the order of W400 > W500 > W600 > W300 > W700. This result suggested that the concentration of PFRs was the dominant element controlling the activation of PS by biochar (Fang et al. 2015).
3.3 Effect of activation temperatures
Four different temperatures were set to investigate the influence of activation temperature on PTH removal (Fig. 2c). The removal efficiency of PTH increased significantly from 34.2–88.78% with the MW activation temperature increased from 25 oC to 60 oC. However, the removal efficiency of PTH was only increased by nearly 6%-10% when temperature increased from 60 oC to 100 oC. These findings showed that the removal of PTH in biochar/PS/MW systems was significantly influenced by temperature (Zrinyi and Pham 2017). The activation ability of PS is mainly attributed to the generation of active species, such as SO4−• and •OH, which have a strong oxidizing capacity to remediate refractory organic pollutants (Zhang et al. 2023). Figure 2d depicts a clear EPR signal was observed with the characteristic peak of 1:2:2:1 classified as DMPO-OH, and the signal peak with the characteristic peak of 1:1:1:1:1:1 observed among the DMPO-OH signals was considered to be DMPO-SO4. Moreover, both of their signal intensity increased with MW temperature. These results indirectly confirmed that temperature could promote PS activation and accelerate the generation of free radicals in the MW/biochar environment, which was conducive to the degradation of PTH.
3.4 The role of activate radical species
The type and content of activate radicals in biochar/PS/MW system were thoroughly investigated. Figure 3a illustrates that even though SO4−• and •OH production was observed in the presence of PS without biochar, their peak intensities were not particularly high. On the other hand, the addition of biochar caused the peak intensities of DMPO-SO4 and DMPO-OH to increase dramatically, indicating that biochar prompte the formation of SO4−• and •OH. Furthermore, the DMPO-OH signal was detected in biochar suspensions without PS, which might be attributed to a single electron transfer from the PFRs of biochar to oxygen, resulting in the generation of •OH via Fenton-like processes (Wu et al. 2023). These results suggested that PS might be effectively activated by biochar to generate SO4−•.
The spin density of DMPO-OH and DMPO-SO4 increased rapidly from 8.229×1011/g to 85.14×1011/g, from 85.1×1011/g to 263.3×1011/g in the MW environment, respectively (Fig. 3b and 3c). The spin density of free radicals in the MW environment was significantly higher than that of water bath and room temperature treatment, indicating that biochar/MW activated PS was more reactive than that in WB and room temperature treatments. It has been reported that biochar can transfer electrons to O2 to generate superoxide radical anion (O2−•), and then reduce persulfate ion to generate SO4−• (Fang et al. 2015). Therefore, persulfate ions reduction by O2−• might represent another significant process that cause SO4−• formation from PS activation by biochar. The formation of O2−• was observed in the MW system (Fig. 3d). A six-line EPR signal, a hallmark of a DMPO-OOH spin adduct (αN = 14.0 G, αHβ = 9.8 G, and αHβ = 1.4 G), was simultaneously detected in the suspension of the reacted soil in DMSO. Results indicated that O2−• participated in the generation of SO4−•, which in turn improved the removal efficiency of PTH.
3.5 Degradation pathways
To investigate which free radicals played a significant role in the PTH degradation process, inhibition experiments were performed (Fig. S4). With the addition of TBA, IPA, and BQ, the removal efficiency of PTH decreased from 88.78–52.49%, 38.94%, and 65.56% within 80 min, respectively. At the same time, free radical spectra at different times were detected under inhibition experiment. In the presence of IPA, the peak intensity of SO4−• and •OH signals decreased (Miao et al. 2020). Both BQ and TBA individually decreased the O2−• and •OH intensity in solution. These results indicated that free radicals, including SO4−•, •OH, and O2−• were significant contributors to the PTH degradation process.
To further identify the potential degradation pathway of PTH, the intermediates and by-products after 80 min of treatment were detected by GC-MS (Fig. 4). The formation of paraoxon (P1) was found, which was mainly due to the oxidation of -P = S to -P = O by oxidizing agent, such as SO4−•, •OH, and O2−•. PTH was transformed into p-nitrophenol (P2) with the O-P bond cleavage under MW energy, which was the major transformation product (Tabassum et al. 2014). After that, the -NO2 function group was subjected to Redox reaction by free radicals to nitrobenzene (P3), generate phenol (P4) and hydroquinone (P5). Meanwhile, the -OH function group of hydroquinone (P5) was oxidized by active free radicals to generate p-benzoquinone (P6). The results suggested that free radicals, such as SO4−•, •OH, and O2−•, formed by the combination of biochar, MW and PS, can effectively promote the transformation of PTH in soil.