Cobalt-loaded cherry core biochar composite as an effective heterogeneous persulfate catalyst for bisphenol A degradation

Persulfate (PS)-based advanced oxidation processes have drawn tremendous attention for the degradation of recalcitrant pollutants, and cobalt composites are effective for PS activation to generate reactive species. In this study, composites of cobalt species loaded on cherry core-derived biochar (Co/C) were prepared with a one-step pyrolysis method. The Co/C catalyst was applied as a catalyst for PS activation to degrade bisphenol A (BPA). Factors influencing the degradation efficiency were examined, including the ratio of raw materials, Co/C and PS dosages, temperature, and solution pH. The Co/C catalyst prepared when the ratio of raw material was 1 : 1 (Co/C-50) could efficiently activate both peroxymonosulfate (PMS) and peroxydisulfate (PDS). When the initial concentration of BPA was 20 mg L−1, complete removal of BPA was achieved in the Co/C-50-PMS and Co/C-50-PDS systems within 8 min and 10 min, respectively. More than 70% of BPA could be mineralized in the Co/C-50-PS system. The free radical quenching experiments demonstrated that in the Co/C-50-PS system, the degradation of BPA was achieved through free radical, surface-bound free radical, and non-free radical pathways. The successful preparation of the Co/C-50-PS catalyst broadens the application of cobalt-based carbon materials in the activation of PS to remove organic pollutants.


Text S6 Effects of reaction temperature
Text S7 Reusability of Co/C-50 catalyst Text S8 Effect of actual water matrix Text S9 Degradation performance for different target pollutants Table S1 Dosages of raw materials for preparation of different Co/C composites    Text S1 Chemicals and reagents The cherry stones were supplied by a local food processing plant (Yantai,

Text S2 Material characterization
The surface morphologies of the composite materials were characterized by a high-resolution transmission electron microscope (HR-TEM, Talos F200x, FEI Corp., USA), and the materials were dispersed in ethanol and the mixture was dropped into copper mesh. The elemental composition and chemical valence states of were analyzed using X-ray photoelectron spectroscopy (XPS, Escalab Xi + , Thermo Scientific, UK) under monochromatic Al Kα radiation, and C 1s peak was used as the internal standard calibration peak at 284.8 eV. Fourier transform infrared (FT-IR) spectra were taken in KBr pressed pellets on a Nicolet iS10 FT-IR Spectrometer (Thermo Fisher Scientific, USA), and analyzed the surface functional groups of the material. The crystal structure of the material was analyzed by X-ray diffractometer (XRD, Ultima IV, Thermo Fisher Scientific, USA) with scanning range of 10-80° and scanning speed of 10°/min. Raman spectrum (in Via reflex, Renishaw PLC, UK) was used to analyze the graphitization degree of the material with an excitation wavelength of 532 cm -1 and a test range of 500 -2500 cm -1 .

Text S3 Magnetism of material
The magnetic properties of the materials were analyzed by vibrating sample magnetometer (VSM, BKT-4500). The magnetic properties of Co/C-50 were studied by hysteresis loops, and the results are shown in Fig. S8a. Coercivity of Co/C-50 (27.6Oe) and remanence (2.81 emu/g). Furthermore, the material has a maximum saturation magnetization of 104.0227 emu/g and is large enough to ensure that the catalyst can be easily separated from the solution with magnets. At the same time, the magnetic properties of the prepared materials were proved by Rb-Fe-B magnets (Fig.   S8b). Co/C-50 can be completely separated in a few minutes when a Rb-Fe-B magnet is placed next to the reaction solution. If the magnet is removed, the material will disperse again in the solution. Specific magnetic properties facilitate Co/C-50 collection, regeneration and reuse.

Text S4 BET characterization of materials
The specific surface area of the material and biochar was measured using a Micromeritics TriStar II 3020 instrument. According to Eq. S1 and Fig. S2, we can get that the specific surface area of Co/C-50 was 262.5032 m²/g, and that of biochar was 122.6786 m²/g. Co/C-50 had a large specific surface area, which could provide a large number of active sites and promote ion diffusion, which was conducive to the activation of PS and the degradation of organic pollutants. (S1) Where: NA is the Avogadro constant; σ is the cross-sectional area of each adsorbent molecule; W is the adsorbent mass (g), and 22400 is the volume (mL) of 1mol gas in the standard state.

Text S5 Mineralization of BPA and analysis of metabolite intermediates
The degree of mineralization reflected by the TOC removal rate was another important index to evaluate the performance of the catalyst [1]. The mineralization degree of BPA in Co/C-50-PS catalyst system was quantitatively studied by measuring TOC content. As shown in Fig showed that most BPA was oxidized to CO 2 and H 2 O finally in Co/C-50-PS system, although some intermediates were generated in the process [2] . The Co/C-50-PS catalyst system not only had good degradation efficiency of BPA, but also had a good mineralization capacity.

Text S6 Effects of reaction temperature
Reaction temperature was a significant factor in the reaction process of AOPs.
Three temperature values of 15, 30 and 45 ℃ were selected to study the effect of temperature on the BPA degradation in Co/C-50-PS system. It could be distinctly seen from Fig. S4 that increasing the reaction temperature could promote the degradation rate of BPA, indicating the reaction was endothermic. In the Co/C-50-PMS reaction system (Fig. S4a), when the temperature was set from 15 to 45 ℃, the k 1 value of BPA degradation increased from 0.325 to 0.793 min -1 . Similarly, the k 1 value increased from 0.424 to 0.610 min -1 in the Co/C-50-PDS system (Fig. S4b). The principal reasons for the enhancement of reaction rate at high temperature were as follows: (1) Higher temperature could enhance the thermal movement of PS and BPA, especially the transfer of BPA on Co/C-50 surface [3]; (2) Persulfate was more easily activated at elevated temperature [4]. The temperature dependence of the kinetic constants was evaluated by the Arrhenius equation as following (Eq. S2): Where k 1 (min -1 ) represents for the first-order kinetic constant, E a (kJ·mol -1 ) is reaction activation energy, R (8.314 J·mol -1 ·K -1 ) and T (K) are ideal gas constant and thermodynamic temperature, and C is constant related to pre-exponential factor. In the light of the illustration, the E a values of Co/C-50-PMS and Co/C-50-PDS were 22.70 kJ·mol -1 (R 2 = 0.9987) and 9.34 kJ·mol -1 (R 2 = 0.9896), respectively. The small E a value predicted the thermodynamic feasibility of the reaction, and 30 ℃ was enough to provide energy to activate the reaction, so the removal of BPA in Co/C-50-PS system could be carried out under relatively mild conditions.

Text S7 Reusability of Co/C-50 catalyst
Reutilization of heterogeneous catalyst was very significant to enhance water treatment process to become more cost-effective [5]. The magnetic separation and recovery of Co/C-50 material was carried out with an Rb-Fe-B magnet (Text S9, Fig.   S8), and then the catalysts were rinsed with SnCl 2 solution (SnCl 2 , as a reducing agent, can reduce Co 3+ to Co 2+ ) and pure water for the next experiment. Fig. S5 showed five recycling runs of Co/C-50 catalytic degradation of BPA. The degradation rate of BPA decreased a little after each cycle. In the Co/C-50-PMS catalytic system (Fig. S5a), the removal rates of BPA reduced from 100% (1st) to 98.76% (2nd), 97.21% (3rd), 95.86% (4th), and 92.22% (5th), respectively. In Fig. S5b, the removal rate of BPA in Co/C-50-PDS catalytic decreased from 100% to 91.16% after five cycles. The removal rate of BPA remained above 90% after five cycles. The reasons for the decrease of catalytic activity were as follows: (1) The intermediary products were adsorbed on the surface and covered with some active sites, and the intermediate products would affect the mass transfer between Co/C-50 and PS [6]; (2) Slight loss of the analysts in each recycle reduced the active sites [7]. In general, Co/C-50 had excellent stability and reusability for activation PS to degrade BPA.

Text S8 Effect of actual water matrix
The degradation of BPA by Co/C-50-PS system in ultrapure water, underground water from Dongmotang waterworks (Yantai, Shandong), surface water from Menlou reservoir (Yantai, Shandong), and seawater from Huanghai (Yantai, Shandong) was shown in Fig. S6. The solid impurities in the water sample were removed by pretreatment through static and filtration operations. At the same time, water quality conditions of several water samples were tested (Table S4). The complex water environment of groundwater and surface water of Dongmatang water plant limited the degradation of BPA by Co/C-50-PS system, which made the removal efficiency of BPA decrease a trifle. The reasons mainly included two aspects: (1) There were other organic pollutants in actual water, which would compete with BPA; (2) Carbonate species (HCO 3 -, CO 3

2-) and phosphate species (HPO 4 2-, PO 4 3-, H 2 PO 4 -) existed in
actual water samples would quench active radicals [8]. Although the degradation efficiency of BPA in real water sample decreased a little, the removal efficiency still reached almost 100%. Interestingly, because a large amount of Clin seawater could effectively activate PMS but not activate PDS [2,9], the degradation performance of BPA in seawater appeared different phenomenon in Co/C-50-PMS and Co/C-50-PDS.
The degradation rate in Co/C-50-PMS could reach 93.71% in 1 min, and attained 100% in 5 min, and seawater ranked the highest reaction rate among these actual water samples. While the degradation efficiency in Co/C-50-PDS showed conspicuous drops, and it was only 48.82% and 74.84% in 1 and 5 min, exhibiting lowest reaction rate. Fortunately, the final removal rate could still reach 96.04% in 30 min. This result indicated that Co/C-50-PS system still owned satisfactory degradation performance for BPA in actual water sample.

Text S9 Degradation performance for different target pollutants
Beside BPA, the removal performance of other refractory organic contaminants such as ibuprofen (IBP), phenol, sunset yellow (SY), and rhodamine B (RhB) was studied to investigate the versatility of Co/C-50-PS catalytic system. In Fig. S7(a, b), when the Co/C-50-PMS system was used, RhB and SY were completely degraded at Although Co/C-50-PDS system was not as good as Co/C-50-PMS system in degrading five organic contaminants (Fig. S7(c, d)), the removal efficiencies of five kinds of organics still attained 100% (BPA), 80.88% (IBP), 94.55% (phenol), 79.25% (SY), and 99.08% (RhB) after 30 min reaction, and the corresponding k 1 values were 0.586, 0.143, 0.258, 0.0789, and 0.486 min -1 , respectively. The diverse removal efficiencies of diverse pollutants may be due to the different molecular architecture [10], for example, -OH and -CH 3 groups in BPA make it vulnerable to be attacked by free radical, and heteroatoms in RhB is apt to be oxidized by the electrophilic attack [11], while the low removal efficiency of phenol resulted from its relatively stable structure containing the delocalized π 7 8 bond [12]. The results indicated that the Co/C-50-PS system could be used as a general method for degradation of various organic pollutants.