Introduction
Rapid industrialization has accelerated water environment contamination, posing potential threats to human health [1]. For instance, a previous study indicated that organic contaminants pose serious non-carcinogenic and carcinogenic hazards to humans at a Chinese decommissioned pesticide manufacturing site [2]. Dyes are used in textiles and leather industries; among the wide number of dyes, Rhodamine B (RhB) is a chromophore from the family of xanthene dyes [3]. There are various methods for removing RhB, such as photocatalytic degradation of RhB by a TiO2- coated activated carbon catalyst [4] and magnetic bentonite adsorption of RhB [5]. Nowadays, various physicochemical methods have been extensively employed for the removal of organic contaminants, including adsorption, electrochemical, and advanced oxidation processes (AOPs) [6-8]. In AOPs, various oxidants are used for the degradation of organic contaminants, such as Fenton reagent, Fenton-like reagent, persulfate, and potassium permanganate [9- 11]. Among them, Fenton oxidation is commonly used in water remediation due to the presence of reactive oxygen species (ROS) (e.g., hydroxyl radicals (∙OH), superoxide radicals (O2•–), singlet line oxygen (1O2), etc.) [12-14]. ROS are capable of oxidizing organic pollutants until mineralization, i.e., converting organic matter into CO2, water, and inorganic ions [15-17]. However, the degradation effects of the Fenton system are greatly limited due to the alkaline environment, sludge generation, the difficulty in H2O2 storage, and high costs [18, 19]. Therefore, it is of great significance for the selection of oxidants with broader pH compatibility and metal-free catalysts.
Sodium percarbonate (Na2CO3•1.5 H2O2, SPC) has emerged as a novel oxidant and is garnering more attention in AOP remediation [20, 21]. When SPC is dissolved in water, carbonate, water, and oxygen form, as delineated in (Eqs. (1)-(2)). SPC as an alternative to H2O2 has various advantages in storage, transportation, and a wide pH range [22]. In addition, SPC is effective in mitigating water acidification and has excellent environmental and biocompatibility. SPC can be used as an oxidizing agent to degrade organic contaminants. However, the efficacy of free radical formation in the atmosphere is hindered without catalytic activation.
Various methods have been employed for SPC activation, such as ultraviolet (UV), carbon materials, transition metals, heat, and microwave activation [23- 26]. To date, previous studies have focused primarily on the activation of SPC by transition metals, iron complexes, and UV [27]. Nevertheless, transition metals induce secondary contamination, and both heat and UV treatments are expensive and impractical for engineering-scale remediation. As an environmentally friendly catalyst, biochar has gained continuous attention due to its low cost and broad application [28]. However, limited studies focus on biochar activating SPC for pollutant removal. Previous studies have indicated that, as a typical biomass material, coconut shell biochar (CSBC) can be used as a catalyst. For example, CSBC doped with α-MnO was used to activate PMS (peroxymonosulfate) for BPA (bisphenol A) degradation [29]. However, the catalytic performance of biochar to activate SPC for contamination removal has been rarely investigated.
Therefore, the main purpose of this study was to:
(1) verify the possibility of CSBC activating SPC for the degradation of a target pollutant; (2) compare the catalytic performance of various CSBC; (3) investigate the factors influencing the degradation effects of the CSBC/SPC system; and (4) explore the underlying mechanism of free radical formation in the CSBC/SPC system. The present results could provide a new novel technical method for water remediation.
Materials and Methods
Materials
Rhodamine B (RhB), SPC, concentrated hydrochloric acid (HCl), concentrated sulfuric acid (H2SO4), sodium pyrophosphate (H3PO4), tert-butyl alcohol (TBA), para-benzoquinone (BQ), and 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) were obtained from Sinopharm Chemical Reagent Co. Coconut shell raw materials were purchased from Hainan Wenchang.
Biochar Preparation and Characterization
The biochar samples were referred to as YK300, YK500, and YK700 for coconut shell pyrolyzed at 300, 500, and 700ºC for 4h, respectively. The biochar samples were referred to as YK1, YK2, and YK3 for coconut shell pyrolyzed at 700ºC for 1, 2, and 3h, respectively.
The contents of carbon (C) and hydrogen (H) in CSBC were measured using an elemental analyzer (Unicube Vario, Elementar, Germany). The specific surface area of CSBC was determined using the Teller (BET) nitrogen adsorption technique (ASAP 2460, Micromeritics, USA). The surface functional groups of CSBC were identified using Fourier-transform infrared (FTIR) spectroscopy (Nicolet iS20, Thermo Scientific, USA). The microstructure characteristics were analyzed using scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS) (GeminiSEM 300, ZEISS, Germany). The valence states of carbon and oxygen in CSBC were characterized using X-ray photoelectron spectroscopy (XPS) (5000 VersaProbe III, PHI, Japan). The free radicals in the CSBC/SPC system were detected using electron paramagnetic resonance (EPR) spectroscopy (EMXplus-6/1, Bruker, Germany).
Catalytic Experiments
Batch experiments were performed in flasks containing 250 mL of 20mg/L RhB dye aqueous mixture. The oxidation process was initiated by SPC and CSBC addition. The mixture was agitated at 180 rpm and 25ºC for 4 h. Control experiments without biochar or SPC were also carried out under the same reaction conditions. The efficacy of the CSBC/SPC system in degrading RhB was evaluated following a comparative study of different reaction systems. Furthermore, radical quenching tests were conducted using different quenchers (TBA, BQ) based on a similar procedure as described above. Samples were collected at predetermined intervals for further analysis. After filtering through 0.45 μm membranes, the sample was analyzed using a UV-Vis spectrophotometer (Cary 3500, Agilent) at a wavelength of 554 nm [30, 31]. The concentration of aqueous RhB was determined using a UV–vis spectrophotometer by measuring its absorbance. After each degradation experiment, CSBC was recovered, washed with deionized water, and dried thoroughly at 80ºC for the subsequent degradation experiment to determine the recyclability and stability of CSBC [32].
The dye decomposition percentage was calculated using Equation (3), where C0 and C signify the initial and remaining concentration [30]. All the experiments were performed three times to ensure result stability, and the results were reported as mean with standard deviations (SD).
Results and Discussion
Characterization of Biochar
Elemental Analysis
Table 1 shows the elemental composition of CSBC prepared under different pyrolysis temperatures and times. When the pyrolysis temperature increased from 300ºC to 700ºC, C contents increased from 70.40% to 83.73%. Conversely, O and contents decreased from 19.6% to 9.00% and from 4.06% to 0.97%, respectively. The mass loss of O and H could be attributed to the cleavage and rupture of weak oxidative bonds within biochar [33]. The H/C ratio was usually related to the aromatic degree of CSBC. The H/C ratio increased as pyrolysis temperature increased, indicating a significantly higher aromatic degree of CSBC. In addition, dehydration, decarboxylation, and demethylation might enhance the carbonization and stability of biochar [34, 35]. A loss of C and H elements was observed in CSBC with prolonged time, due to the release of gases such as methane (CH4) and hydrogen (H2) [35, 36]. Conversely, N content remained relatively constant.
The BET Analysis of CSBC
Fig. 1 presents N2 adsorption–desorption isotherms of CSBC. The N2 adsorption curves of CSBC were consistent with Type I isotherms, and the hysteresis loop type belonged to type H4, indicating the presence of lots of microporous structures in CSBC. The surface area and pore size of CSBC were 314.5231 m²/g and 2.1747 nm for YK1h, 339.7825 m²/g and 3.4086 nm for YK2h, 500.7433 m²/g and 3.0610 nm for YK3h, 575.6408 m²/g and 3.4757 nm for YK700, 1.8508 m²/g and 53.0390 nm for YK300, 117.7411 m²/g and 4.9598 nm for YK500, respectively. These results indicated that with increasing pyrolysis temperature and time, the specific surface area of CSBC progressively increased [37], which increased active sites and enhanced activation efficiency.
The microstructure characteristics of CSBC
Fig. 1 shows SEM micrographs of CSBC. YK700 was characterized by fine porosity with roughened surface texture and stratified morphology. The YK700 after catalysis retained the basic structure of the pristine biochar, but became more brittle and porous with increased pore size and damaged surface structure. Compared with the pristine YK700, the surface structure of the used YK700 was damaged; the previous continuous layered structure was transformed into a disintegrated discrete structure. Moreover, YK300 and YK500 lacked definitive regularity and showed irregular arrangements with heterogeneous and macropore structures. In contrast, YK700 showed higher graphitization and a larger specific surface area, which was conducive to electron transfer. Many previous studies have reported that as the charge transfer medium, the graphitized structure of CSBC significantly contributed to electron transfer from oxidants to organic contaminants [38]. Therefore, compared to YK300 and YK500, YK700 might have better catalytic ability.
Table 1.
The contents of C, O, H, and N in CSBC under different pyrolysis temperatures and times.
CSBC had lots of O-containing functional groups, which were also considered to be the main catalytic active sites [39]. Both the pyrolysis temperature and time were the key parameters influencing OFG formation [40]. Fig. 1g) and h) show FTIR spectroscopy of CSBC. As shown in Fig. 2e), the stretching vibration at 3432.73 cm-1 belonged to -OH groups, while the stretching vibration at 1068.10 cm-1 showed the presence of C-O bonds, indicating the presence of -OH groups in CSBC [41, 42]. The peak at 1612.74 cm-1 might be related to the vibration of -C=C. In addition, the peak at 1630 cm-1 might be due to the stretching vibration of -C=O. The disappearance of the infrared peak for C=O in YK700 indicated that -C=O might be an essential functional group in SPC activation by CSBC. As shown in Table 1, the peak at 2230-2358 cm-1 may be due to the stretching vibration of -C≡N, due to 18% of N in CSBC. The peak at 1385.12 cm-1 showed that CSBC contained -OH bonds, which could produce •OH from H2O2 by electron transfer [43]. Previous studies have shown that some OFGs had functional oxidation and reduction effects (e.g., quinones or phenolic hydroxyl groups), which could transfer electrons to cooperatively catalyze oxidation [44]. The results showed that YK700 has abundant OFGs and more active sites with stronger peak intensity, indicating excellent catalytic performance.
The XPS Analysis of CSBC
Fig. 1i) and j) show the XPS spectra of CSBC. Three distinct peaks of the C1s spectrum of YK700 at 284.8 eV, 287.8 eV, and 292.7 eV corresponded to -C=C, -C=O, and π-π*, respectively. The results indicated that pyrolytic YK700 showed a graphitic carbon structure. In the O 1s spectrum, two peaks at 530.8 eV and 532.6 eV corresponded to -C=O and -C-O, respectively. Moreover, it was observed that the percentage of -C=O peak areas decreased in the C 1s spectrum, suggesting that -C=O groups were involved in the reaction.
The Selectivity of CSBC Based on Their Catalytic Performance
Adsorption experiments were performed to explore the possibility of SPC activation by CSBC for RhB degradation, as depicted in Fig. 2a); the adsorption capacities for YK300, YK500, and YK700 were 2.07%, 1.01%, and 16.02%, respectively. Among them, YK700 showed the highest adsorption capacities, due to the larger specific surface area and more abundant porous structure. The increase in temperatures enhanced adsorption performance, due to the formation and development of pores in CSBC [45]. In a single SPC system, the degradation efficiency of RhB was 7.76% within 240 min. In the YK300/SPC system, the degradation efficiency of RhB decreased to 3.57% within 240 min. Conversely, in the YK500/SPC and YK700/SPC systems, the degradation efficiency of RhB increased to 4.81% and 45.01% within 60 min, respectively, and reached 11.64% and 96.11% after 240 min, respectively. YK500/SPC systems showed a slight increase in the degradation efficiency of RhB compared to a single SPC system. Nevertheless. The removal efficiency of RhB in the CSBC/SPC system was comparable to what AOPs. For the 700YK/SPC system, the degradation efficiency of RhB increased significantly by 88.35% compared to a single SPC. The volatilization of cellulose and lignin from the CSBC might enhance the degradation performance of RhB, which likely improved interaction between active sites on the CSBC surface and SPC and thus resulted in a higher generation of •OH radicals for RhB degradation. Furthermore, CSBC volatilization led to an increase in specific surface area and porosity [36].
Fig. 1.
N2 adsorption–desorption isotherm a); pore size distribution curve b); SEM images of YK300 c), YK500 d), YK700 e), and YK700 after catalysis f); FTIR of YK700 g) and after catalysis h); YK700 about high-resolution XPS spectra of C 1s i) and O 1s j).
Fig. 2b) illustrates the degradation performance of RhB by SPC activation by CSBC. The degradation efficiency of RhB was 8.5% for YK1h, 8.57% for YK2h, and 16.23% for YK3h, respectively. The extended pyrolysis time enhanced RhB degradation. The kinetic degradation test of RhB was conducted to determine the inherent activity of CSBC. Fig. 2c) and d) present the kinetic degradation results of RhB. The degradation kinetics of RhB could be better fitted by the pseudo-first-order kinetic model. Overall, YK700 exhibited the best catalytic performance, compared with other CSBCs, since it might have more active sites [46].
YK700 almost completely degraded RhB after 4 h of reaction. CSBC pyrolyzed at 700°C for 4 h demonstrated the best catalytic performance in the RhB degradation by SPC. Therefore, YK700 was used in all the subsequent tests.
Key Factors Affecting RhB Degradation in the YK700/SPC System
Effect of CSBC Concentration
Fig. 3a) shows the RhB degradation results with CSBC dosages. CSBC dosages had significant influences on the degradation efficiency of RhB by SPC. Fig. 3b) and c) present the degradation kinetics results of RhB in the CSBC/SPC system. The degradation kinetics data of RhB could be better fitted by pseudo-first-order kinetics at lower CSBC dosages, while that could be better fitted by pseudo-second-order kinetics at higher CSBC dosages. Intriguingly, the degradation rates of RhB at 2 g/L of CSBC were 20 times higher than those at 1 g/L of CSBC. The degradation rates of RhB increased rapidly, probably due to the limited SPC content. Therefore, the number of active sites on the CSBC surface dominated RhB degradation. The C=O groups and defect structures on the YK700 surface could serve as catalytic active sites to promote the generation of free radicals [47].
Fig. 2.
Degradation curves of RhB by SPC, which is catalyzed by CSBC at various pyrolysis temperatures a); Degradation curves at varying pyrolysis durations b); First-order kinetics at different pyrolysis temperatures c); First-order kinetics across various pyrolysis durations d). Experimental conditions: [RhB] = 20 mg/L, [SPC] = 1 g/L, [BC] = 1 g/L, time= 240 min, and T = 25ºC.
Fig. 3.
Degradation curves of RhB by YK700/SPC at different catalyst concentrations a), kinetics of the YK700/SPC pseudo-primary reaction at different concentrations b), kinetics of the YK700/SPC pseudo-secondary reaction, and c), experimental conditions: [RhB] = 20 mg/L, [SPC] = 1 g/L, time = 240 min, and T = 25ºC.
Effect of SPC Concentration
Fig. 4 shows the RhB degradation results with SPC concentration. As shown in Fig. 4a), the degradation efficiency of RhB increased from 49.8 to 99.63% as the SPC concentration increased from 0.25 to 4 g/L, indicating a positive correlation between SPC concentration and RhB degradation. The increase in SPC concentration enhanced the formation of more surface radicals in the solution, resulting in higher RhB degradation efficiency [48]. However, the degradation efficiency did not significantly increase at the SPC concentration of 2 g/L compared to that of 1 g/L. The results might be because excess SPC under the same CSBC concentration did not generate lots of •OH radicals. Therefore, the optimal dosage ratio of SPC to YK700 was 1:2, based on the economic perspective.
Fig. 4.
RhB degradation curves by different concentrations of oxidants a); Kinetics of oxidant-catalyzed SPC primary reactions b). Experimental conditions: [RhB] = 20 mg/L, [BC] = 1 g/L, time = 240 min, and T = 25ºC, RhB’s degradation curves of SPC catalyzed by CSBC at different pH c), Kinetics of SPC-catalyzed primary reaction of CSBC at different pH d). Experimental conditions: [RhB] = 20 mg/L, [SPC] = 1 g/L, [BC] = 1 g/L, time = 240 min, and T = 25ºC.
Effect of pH
Fig. 4c) shows the RhB degradation results with pH. The YK700/SPC system exhibited good degradation efficiency for RhB in an alkaline environment. The degradation efficiency of RhB at pH = 11.72 is higher than that at pH = 3.93 by 29.44%. This might be because SPC activation by CSBC resulted in the formation of •OH and H+. The continuous consumption of H+ in an alkaline environment increased the number of free radicals and thus enhanced RhB degradation [49]. In addition, RhB is a cationic dye and is positively charged in an acidic environment. In alkaline environments, the carboxylate group of RhB dye is deprotonated to the negatively charged –COO− state [50]. Under alkaline conditions, the CSBC/SPC system showed increased interaction with RhB and enhanced SPC generation for •OH [51]. The carbonates contained in SPC change to bicarbonate in an acidic environment. Bicarbonate has a significant inhibitory effect on •OH, so RhB degradation is significantly reduced in acidic environments [52].
Effect of RhB Concentrations
Fig. 5a) illustrates the RhB degradation results with RhB concentrations. An increase in RhB concentration resulted in a decrease in the degradation efficiency of RhB. The degradation efficiency of RhB was relatively low at the initial RhB concentration of 30 mg/L, whereas that notably increased at the initial RhB concentrations of 20 mg/L (69.56%) and 10 mg/L (100%), respectively.
Effect of Inorganic Anions
Fig. 5c) illustrates the RhB degradation results with inorganic anions. In the presence of Cl-, SO42-, and PO43-, the degradation efficiency of RhB was 54.59%, 59.16%, and 51.17%, respectively. Without inorganic anions, the degradation efficiency of RhB was 73.75%. These results indicated that Cl-, SO42-, and PO43- could inhibit RhB degradation, possibly because the inorganic anions in reaction with free radicals led to radical consumption [53].
CSBC Reusability
Fig. 5d) shows the RhB degradation results with CSBC reusability. The degradation efficiency of RhB decreased by 56% from the 1st to the 5th cycles. However, CSBC could still effectively degrade RhB, verifying the stability and recyclability of the CSBC. The results were probably because the decomposition intermediates on the CSBC surface inhibited the interaction between CSBC and SPC [54].
Fig. 5.
CSBC catalyzed SPC degradation profiles of RhB at different initial concentrations a); Kinetics of the CSBC-catalyzed SPC primary reaction with different initial concentrations of RhB b). Experimental conditions: [SPC] = 1 g/L, [BC] = 1 g/L, time = 240 min, and T = 25ºC. Degradation curves in the presence of different anions [RhB] = 20 mg/L, [SPC] = 1g/L, [BC] = 1g/L, [anions] = 20 mM/L c); Number of recycling times after YK700 catalysis [RhB] = 20 mg/L, [SPC] = 1 g/L, [BC] = 1 g/L, T=25ºC d).
Fig. 6.
Effect of quenchers on RhB removal a); Electron paramagnetic resonance (EPR) spectra of radical adducts trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) of the SBC catalyst b); The change diagram of degradation and TOC in the CSBC/SPC reaction system at different reaction times: initial [RhB] = 20 mg/L, [SPC] = 1 g/L, [BC] = 1 g/L.
The Possible Catalytic Mechanism of SPC Activation by CSBC
In YK700/SPC, the degradation efficiency of RhB was 98.37%. As a superoxide radical (O2•_) scavenger, BQ addition led to a complete removal of 100% within 120 min, due to the established role of O2•_ as an intermediate in the reaction system. OFGs in CSBC could facilitate electrons transferred to dissolved oxygen, resulting in superoxide radicals (O2•_) formation. These O2•_ radicals could react with H+ to form H2O2, which could be further converted into •OH. Due to O2•_ consumption, BQ addition led to the formation of more •OH radicals and further destroyed the RhB degradation equilibrium [55, 56].
As a •OH quenching agent, TBA was introduced into the CSBC/SPC system, resulting in the RhB degradation efficiency of 12.01% with 120 min. TBA addition significantly inhibited RhB degradation, confirming that the main free radical in the CSBC/SPC system was •OH, which was consistent with previous studies [31]. Furthermore, the EPR spectrum (Fig. 6b) demonstrated that the CSBC/SPC generated an •OH signal. In contrast, no discernible peak was observed in the single SPC system, indicating that the main active radical in RhB oxidation was •OH [57].
In addition, the partial degradation of RhB after TBA addition suggested the potential role of CSBC as an electron transfer medium. The XPS spectra indicated the presence of sp2 hybridized carbon π-π* transitions in YK700. Previous studies have shown that π-π* stacking structures benefited from the graphitized structure of biochar and enhanced long-distance electron transfer [58]. The disappearance of the π-π* signal peak in the C1s spectra indicated that YK700 enhanced electron transfer between SPC and RhB and thus increased RhB degradation efficiency. These results indicated that YK700 might serve as an electron transfer medium, enhancing the electron transfer between SPC and RhB through a non-radical pathway. However, the nonradical pathway was not dominant based on the results of quenching tests. RhB degradation was primarily facilitated by the attack of •OH radicals.
Based on the above analysis, the CSBC/SPC system likely enhanced RhB degradation through the •OH radical. Graphitized carbon in CSBC showed an enhanced potential for electron acceptance, which might play a key role in SPC activation [59]. The active sites for SPC activation and catalysis mainly included defects, oxygen-containing functional groups, and the graphite-carbon structure of CSBC [60, 61]. The RhB degradation mechanism in the YK700/SPC system might involve:
(1) The transfer of electrons from surface functional groups (e.g., -COOH, -OH, and -C=O) on CSBC to SPC generated •OH radicals, as shown in Equations (4)-(6) [62]. (2) Defect structures and persistent free radicals could activate SPC to generate •OH radicals [63].
In order to further explain the degradation of RhB in the reaction system, the degradation rate and TOC in the reactive solution were detected. As shown in Fig. 6c), when the reaction time increases, the degradation rate of RhB solution and the removal rate of TOC increase gradually. This fiinding shows that the CSBC/SPC system effectively removes RhB and mineralizes into CO2 and H2O.
Conclusion
A simple method for preparing a CSBC free of metal catalysts was proposed and applied as an SPC activator to eliminate contaminations. The use of YK700 in SPC activation was more advantageous than that of the YK300 and YK500 due to faster and higher RhB removal. The process exhibited satisfactory activity toward RhB degradation over the alkaline environment, and the degradation increased by 29.44% compared with an acidic solution. The addition of Cl-, SO42, and PO43- anions to the system showed that all these anions inhibited the degradation of RhB. EPR results indicated that the primary free radicals involved in the degradation of RhB in the system are •OH. CSBC primarily activated SPC to generate reactive oxygen species for pollutant degradation through radical pathways, involving active sites like OFGs, PFRs, and defect structures on its surface. Furthermore, nonradical pathways in YK700/SPC also promoted electron transfer for the degradation of pollutants. Overall, the results suggested that the CSBC/SPC system can effectively treat groundwater contamination. This study provided an efficient and green degradation pathway for the treatment of recalcitrant organic in the groundwater and offered new strategies for the resource utilization of coconut shell waste.
