Characterization of materials
The morphology of Ginkgo biloba biochar was checked by SEM, and the results are shown in Fig. 1. Comparing Fig. 1a, b, and Fig. 1c, it can be found that the biochar with phytic acid is more likely to form a honeycomb shape during the pyrolysis process, and the morphology is neater. But BC500 is irregularly columnar with more surface folds and no pores visible. PBC300 is irregularly columnar with rough side surfaces, uneven and honeycomb-like at the cross-section, and the cross-section contains a large number of small pores of about 10 μm in diameter. With the increase of pyrolysis temperature, it gradually formed a regular and smooth column, and the cross-section showed a large number of small pores. PBC500 section had small pores in the range of about 10–15 μm, and the inner surface of small pores was very smooth, and the inner surface also had a large number of pores about 2–3 μm in diameter. PBC700 section had pores in the range of 5–30 μm in diameter, and the inner wall of the section also contained a large number of transversely distributed small pores with a pore diameter of about 200 nm.
The phytic acid already affected the pyrolysis of biochar at 300 °C. It indicated that the single layer of carbon on the surface of the phytate modified biochar started to peel off when the temperature increased from 300 to 500 °C, and the comparison of the pore diameters of PBC300 and PBC500 sections showed that the carbon layer on the inner surface of the small pores of biochar also started to shed at the pyrolysis temperature of 500 °C, so the surface gradually became smooth, and the pore diameter became larger. When the temperature continued to rise to 700 °C, all the carbon layers originally contained in the surface at 500 °C shed, and the surface became very smooth.
In this study, the phosphorus spectrum of biochar was scanned using XPS and the P2p track was divided into 3 peaks (Fig. 2). The peak P1 at a binding energy intensity of 136.8 eV is –O–PO3/–P2O6, the peak P2 at a binding energy intensity of 134.12 eV is O–P/C–PO3, and the peak P3 at a binding energy intensity of 131.8 eV is C–P (Valero-Romero et al. 2017). The intensity of the P1 peak was the highest in PBC500 and changed very significantly, which indicated that more metaphosphoric acid attached to oxygen atoms was formed in PBC500. And the compounds –O–PO3/–P2O6 and O–P/C–PO3 underwent a mutual transformation during the pyrolysis process. The changing patterns of P1, P2, and P3 peaks indicated that the chemical state of the P atom changes continuously during the pyrolysis process, with the C–PO3 chemical bond first changing to C–O–PO3 and then to C–PO3 and C–P. C–P–O bonds are not very stable at high temperatures (Mckee et al. 1984), and C–O–P bonds will be more stable than C–P–O bonds under high-temperature conditions (Lee and Radovic 2003). And phosphate is often attached to the surface of biochar through C–O–P bonds (Puziy et al. 2008; Lee and Radovic 2003).
As is shown in Fig. 3. two characteristic peaks appear in the Raman spectrum around wavelengths 1329 cm−1 and 1583 cm−1, D peak appearing around 1300 cm−1, and G peak appearing around 1580 cm−1 (Li et al. 2021). So, they are corresponding to the D and G peaks of graphite, the D peak represents the defect peak, which is caused by the low symmetry or irregularity of the carbon material and corresponds to doping, defects, wobbly bonds, and bent graphite layers in the graphite structure. The G peak represents the graphite peak, whose peak reflects the degree of graphitization and is caused by the sp2 vibration of the graphite hexagonal structure. Raman plots of all four biochars showed distinct first-order peaks at 1340 cm−1 (D-Band) and 1590 cm−1 (G-Band). The intensity ratio of D-peak to G-peak reflects the orderliness of biochar. The ID/IG ratios of PBC300, PBC500, and PBC700 were 0.553, 0.812, and 1.1, respectively. It can be seen that the orderliness of biochar increased with the increase of pyrolysis temperature. PBC500 was more ordered than BC500. The possible reasons are as follows: firstly, the catalytic dehydration of acids; secondly, the enhancement of aromatization of carbon atoms; thirdly, the increase of hole formation rate; exceeded. The ID/IG ratio of PBC700 exceeds 1.04, so there may be defective activation of PBC700 during catalytic PMS.
Effect of solution pH and temperature on the degradation
The results of the influence of pH on degradation are shown in Fig. 4. The ability of BC500 to catalyze PMS degradation of Ponceau 2R did not change significantly at different pH ranges, and the reaction equilibrium was reached at 10 min, and at most 5% of Ponceau 2R could be degraded. The possible reason was that the insufficient active sites exposed on the surface of BC500 could catalyze PMS. PBC300 had the best catalytic capacity at acidic pH and could remove 90% of Ponceau 2R from the solution at a reaction time of 5 min. The maximum removal of Ponceau 2R, 99%, was reached at a reaction time of 15–20 min and the maximum removal of Ponceau 2R by PBC300 under both neutral and alkaline conditions was 62%. The catalytic degradation ability of PBC500 was not significantly affected by pH, and it could efficiently treat Ponceau 2R under acidic, neutral, and alkaline conditions, achieving 99% degradation and removal in 15–20 min of reaction time. As the temperature increases, the lignin fraction within the biomass continues to fracture, enhancing the richness of catalytic active sites on the surface of PBC500 (Meng et al. 2020). The catalytic ability of PBC700 was stronger at pH 3 and 7 but weaker at pH 11. More than 99% of Ponceau 2R was removed at pH 3 and 7 within 20–25 min, while the maximum removal rate of PBC700 was 90% at pH 11 at 10–15 min. It indicated that for PBC700 acidic and neutral conditions did not affect the ability of biochar to catalyze PMS to produce ROS, but the ability of biochar to produce ROS became significantly weaker under alkaline conditions.
As indicated in Fig. 5, the ability of BC500 to catalyze the degradation of Ponceau 2R by PMS was not significantly affected by the change of temperature, and it can be inferred from the changing trend that BC500 had almost no ability to catalyze the degradation of Ponceau 2R by PMS, and it mainly relied on the adsorption to remove the Ponceau 2R from the solution. The ability of PBC300 to catalyze PMS degradation of Ponceau 2R was weaker at low temperature and enchanted at 25 °C and 30 °C. At 25 °C and 30 °C, 57% of Ponceau 2R can be removed in 2 min and reached the equilibrium in 5–10 min, and the maximum removal of Ponceau 2R was 62%. This indicates that PBC300 needs a certain temperature in catalytic PMS degradation of Ponceau 2R, and the catalytic ability decreases below 25 °C. The ability of PBC500 and PBC700 to catalyze PMS degradation of Ponceau 2R was affected by temperature in a similar trend. The reaction speed was slower at 20 °C, and the reaction equilibrated at 10–15 min to reach the maximum degradation, and the reaction at 25 °C and 30 °C reached the maximum degradation at 5–10 min. It shows that low temperature is also unfavorable for PBC catalyzed PMS degradation of Ponceau 2R. And it is consistent with the conclusion that persulfate has different activation effects at different temperatures from other studies (Nie et al. 2014).
In summary, it is not yet clear that the reason for the difference in pH on PBC300 and PBC700. The oxidation activity of PBC500 was stronger than that of PBC300 under all conditions, which showed that the increase of lysis temperature did help to increase the metaphosphoric acid on the surface of biochar and increase the active sites. However, the continual increase of temperature (700 °C) resulted in increasing aromatization of carbon atoms, shedding of carbon layers and creation of structural defects, which may facilitate electron transfer.
Mechanism of degradation of environmental Ponceau 2R by BC-PMS system
To further investigate the main degradation pathway of Ponceau 2R in the BC-PMS system, the degradation intermediates were analyzed by LC-MS, and the results are shown in Fig. 6. The results showed that C6H6O2 and C8H8O were the main degradation products. The mass spectra of Ponceau 2R at 15 min (Fig. 7) indicated that the main proton molecules m/z were 83.0809 and 338.3409 and the relative molecular weight was 84; the molecular formula was presumed to be C5H8O. The possible degradation pathways of Ponceau 2R are shown in Fig. 8.
The results of the content of acidic oxygen-containing functional groups are shown in Table 1, and it can be found that BC500 and PBC500 have a similar distribution of surface oxygen functional groups, however, according to Figs. 3 and 4, there is a big difference in the ability of BC500 and PBC500 to catalyze PMS degradation of Ponceau 2R. BC500 showed almost no degradation ability for Ponceau 2R, while PBC500 degraded 99% of Ponceau 2R in 10 min. PBC700, which had the least surface oxygen functional groups, had higher degradation capacity than PBC300 and PBC500, which had higher surface oxygen abundance and it can be inferred that the surface oxygen functional groups are not involved in the catalytic PMS degradation of Ponceau 2R.
To further investigate the main degradation factors of the BC-PMS system in the degradation of Ponceau 2R, the amount of Ponceau 2R removed by PMS under the action of pH, and the amount of adsorption and removal by biochar were analyzed. The role of various factors in the degradation of Ponceau 2R by PBC300, PBC 500, and PBC 700 is shown in Fig. 9. It can be seen that the degradation of Ponceau 2R was less affected by pH; the adsorption removal of biochar was not the main factor leading to the removal of Ponceau 2R. PMS degraded more Ponceau 2R under the action of modified biochar, which showd that the degradation of Ponceau 2R catalyzed by phytate-modified biochar was the main degradation factor. PBC 300 can efficiently catalyze PMS only under acidic conditions, but not under neutral and alkaline conditions. PBC 500 and PBC 700 can efficiently catalyze PMS not only under acidic conditions but also under neutral and alkaline conditions. This may be because there were enough active sites on the surfaces of PBC500 and PBC700 to catalyze PMS. At pH 3, 7, and 11, sufficient ROS can be produced to degrade Ponceau 2R. However, there were not enough active sites on the surface of PBC300. The ability to catalyze the production of ROS was inhibited under neutral and alkaline conditions, which cannot effectively degrade Ponceau 2R.
In this experiment, ROS were captured in three biochar-catalyzed PMS processes. It was characterized by the capture signal of EPR and quantified by using free radical standards. As shown in Fig. 10; Table 2, PBC500 produced the highest concentration of ROS in the catalytic PMS process and the type of ROS was hydroxyl radical; PBC700 produced ROS as singlet oxygen; PBC300 produced ROS as hydroxyl radical in the catalytic process and the lowest concentration. This explains the superior catalytic performance of PBC500 compared to the others. The Raman characterization of PBC700 showed that the ID/IG value of PBC700 was 1.1 (> 0.85), indicating that PBC700 can also generate electron transfer from defects in the structure and catalyze PMS finally.
To further infer whether the catalysts were catalyzed by persistent radicals, the three phytate-modified biochars were treated with anhydrous ethanol for 5 h to quench the persistent radicals in the biochars, and the changes in the catalytic capacity of PBC300, PBC500, and PBC700 were observed. The EPR spectra of three biochars after 5 h of treatment with anhydrous ethanol are shown in Fig. 11. It can be seen that the persistent radicals in PBC300 and PBC500 were completely burst after 5 h of treatment with ethanol, and the EPR signal can still be detected in PBC700, which can be considered as a structural defect in the biochar during pyrolysis. According to Fig. 12, there existed differences in the adsorption capacity of ethanol-treated PBC300, PBC500, and PBC700 and untreated biochars on the catalytic process of PMS. The adsorption capacity of the ethanol-treated biochar was higher than that of the untreated biochar, but the adsorption capacity was not very high compared with the total amount removed. The removal of Ponceau 2R mainly depended on the biochar to catalyze PMS to generate ROS for degradation. The removal amount of biochar treated with ethanol did not change significantly compared with that of biochar not treated with ethanol, indicating that the persistent radicals of biochar did not participate in the catalytic process of PMS. Therefore, it can be speculated that one of the catalytic pathways of PBC700 was caused by a defect in the structure.
In summary, PBC300, PBC500, and PBC700 may have metaphosphoric acid attached to oxygen atoms and metaphosphoric acid attached in a bridging manner on the surface of biochar catalyzed the production of hydroxyl radicals from PMS, and PBC700 may also catalyze the production of singlet oxygen from PMS through its structural defects, and singlet oxygen was the main catalytic product of PBC700.