Catalytic Degradation of Bisphenol A in Water by Poplar Wood Powder Waste Derived Biochar via Peroxymonosulfate Activation

Abstract

A series of biochar materials was prepared through pyrolyzing poplar wood powder waste under different pyrolyzing temperatures, which were afterwards characterized in detail. Then, the poplar powder biochar (PPB) was used to degrade bisphenol A (BPA) in water via activating peroxymonosulfate (PMS). The results indicate that the activation efficiency of the prepared PPB was correlated with its surface functional groups, which were regulated by its pyrolyzing temperature. Specifically, the biochar pyrolyzed at 600 °C (PPB-600) exhibited the optimal BPA removal activity, in which 0.5 g/L of PPB-600 could remove 0.02 mM of BPA within 120 min. From the results of scavenging tests, ESR analysis and probe pollutant degradation tests, it was inferred that the BPA was degraded by non-radical singlet oxygen in the PPB/PMS system. Since PPB consumed its surface oxygen functional groups and structural defects to activate PMS, the catalytic performance of PPB was gradually reduced after several cycles. This study can provide new insight for the design and preparation of metal-free biochar catalysts from waste wood precursor for the highly-efficient removal of refractory organic pollutants in water.

1. Introduction

In recent years, due to the continuous development of urbanization and industrialization, the ecological environment and human health have been seriously threatened by various refractory organic chemicals (ROCs) that have stable structures, easy bioaccumulation and long half-lives in urban sewage [1,2,3]. Since these pollutants are difficult to completely remove by activated sludge processes, they are commonly drained off into natural water bodies with the secondary effluents [4,5,6]. Most adsorption materials have strong adsorption selectivity, which also cannot effectively eliminate ROCs, especially water-soluble ones [7]. Therefore, advanced oxidation processes (AOPs) based on transitional metal/persulfate systems have received extensive attentions in the field of ROC degradations in water [8], due to the merits of room-temperature activation and rapid active radical generation rates [9]. However, dissolved metal ions cannot be effectively separated from water after the degradation process, which might cause secondary pollution to the water [10].

Therefore, numerous studies have tried to explore the persulfate activation capability of carbo-catalysts for ROC degradation via transferring stabilized electrons from the condensed ring structures on the surface of carbonaceous materials to persulfate [11]. Amongst these, the wood-derived biochar catalysts have received tremendous interest due to the advantages of the abundant availability of precursors and their ease of preparation [12]. Besides this, biochar has rich fused ring structures with various oxygen-containing functional groups, which can form effective interactions with persulfate and generate abundant active species for ROCs degradation [13]. Wood powder, obtained from the residue of wood processing and furniture products, has been widely applied in the fabrication of artificial boards and wood plastic composites [14,15]. However, enormous amounts of wood powder waste are produced and released into the environment without proper post-processing. Meanwhile, few studies have explored the potential of introducing wood powder waste into fine processing and high-added-value applications [16]. Thus, it is worth exploring the application potential of wood powder waste-derived biochar as a carbo-catalyst for ROC degradation in water.

Consequently, a series of biochar carbo-catalysts derived through pyrolyzing poplar wood powder waste under different temperatures were prepared, and were used to activate peroxymonosulfate (PMS) for ROC degradation in this study. The impacts of different environmental parameters as well as ROC degradation mechanisms were studied in detail.

2. Results and Discussion

The morphology of the prepared PPB samples was investigated first. As shown in Figure 1a, the original poplar powder was optically light gray, and then turned to black after pyrolysis treatment. The SEM images displayed in Figure 1b show that the average particle size of the poplar powder was about 10 μm. After pyrolyzing, the obtained PPB samples maintained the original shape of the wood powder precursor. In the meantime, the particle size decreased after carbonization, which was caused by the removal of functional groups from the wood powder structure. The average particle size of the obtained PPB sample became smaller with the increase in the pyrolysis temperature (~7 μm for PPB-400 and ~3 μm for PPB-900), which indicates that more biomass component in the wood powder was decomposed and converted to carbon at higher pyrolysis temperatures.

XRD was used to analyze the main phases of the as-prepared samples. It can be seen in Figure 2a that the pristine poplar had three sharp diffraction peaks at 2θ = 15°, 22° and 35°, which represent the Cellulose I structure, suggesting that poplar powder was mainly composed of cellulosic biomass [17]. After thermal treatment, these characteristic peaks disappeared from the PPB samples, whereas two new peaks at 2θ = 24° and 44° corresponding to the (002) and (100) planes of the carbon material appeared [18], which indicates that the poplar powder was transformed into carbon-based biochar containing graphitized structures. Moreover, with the increase in the pyrolysis temperature, the peak intensities of the corresponding PPB-600 and PPB-900 became higher, revealing that the increased pyrolysis temperature brought about a higher graphitization degree to the biochar samples.

Figure 2b shows the Raman spectra of the PPB samples. It can be clearly observed that all three PPB samples had two remarkable peaks located at ~1360 cm⁻¹ and 1580 cm⁻¹, which represent the defect level of sp³ hybrid carbon (D band) and the graphitization peak of the sp2 hybrid carbon (G band) in a carbonaceous material, respectively. Furthermore, the defect and disorder level of the biochar could be described through the intensity ratio between these two peaks (I_D/I_G), in which a lower I_D/I_G value is attributed to a higher graphitization degree [19]. Therefore, it can be seen that the I_D/I_G values of the PPB samples decreased from 1.00 to 0.67 with the annealing temperature increasing from 400 °C to 900 °C, indicating again that higher pyrolysis temperature could lead to higher graphitization degree in the PPB.

The core-level C1s XPS spectra of the PPB samples are illustrated in Figure 3. As was revealed, the C1s peak could be deconvoluted into three components at the binding energies of 284.8, 286.2 and 288.8 eV, which correspond to C=C, C-O and C=O bonding, respectively.

Table 1. Carbon bonding concentrations of PPB samples based on deconvoluted C1s XPS peak.

Sample	C1s Bonding Energy, eV
	284.8 (C=C)	286.2 (C-O)	288.8 (C=O)
PPB-400	50.8%	41.7%	7.5%
PPB-600	58.6%	30.1%	11.3%
PPB-900	64.3%	26.5%	9.2%

summarizes the bonding concentrations of PPB samples based on the deconvoluted C1s XPS peaks. Due to the conversion from oxygen-containing functional groups to carbon bonds, the peak proportion of C=C increased remarkably, demonstrating that the poplar powders were transformed into a graphitized structure during the pyrolysis process [20]. It was found that the proportion of C=C bonds increased, while C-O bonds gradually disappeared, when the annealing temperature increased from 400 °C to 900 °C, confirming that the oxygen-containing groups in the main chemical ingredients among the three compositions of wood powder (cellulose, hemi-cellulose, lignin) were gradually removed, and the skeleton of the lignocellulosic biomass was converted into a more stable graphitic structure. After the pyrolysis treatment at 900 °C, the intensities of the C-O and C=O bonds in the poplar powder both decreased, while the intensity of the graphitized C=C bond increased, which suggests that a higher pyrolysis temperatures could contribute to a higher graphitization degree. These results are consistent with the analysis of XRD and Raman spectra.

Figure 4a displays the performances of PPB catalysts in regard to BPA degradation via activating PMS. Clearly, PMS alone exhibited a negligible effect on the BPA degradation, and the adsorption removal efficiency of pristine PPB-600 was relatively limited. In the presence of both PPB and PMS, the degradation rate of BPA increased rapidly. Among all prepared catalysts, PPB-600 showed the best activation ability, in which 0.5 g/L of catalyst could remove BPA (0.02 mM) completely within 120 min. Conversely, PPB-900 did not show the optimal BPA degradation capacity, which demonstrates that the graphitization degree is not the only thing determining the PMS activation rate. Compared with many previously reported carbo-catalysts used for BPA degradation, the PPB-600 showed analogous efficiency [21,22]. The TOC results indicate that all the PPB catalysts could remove more than 35% of the TOC from the system, in which PPB-600 also exhibited the optimal TOC removal efficiency (

Table 2. TOC removal of BPA in PPB/PMS system.

	TOC Removal Rate
PPB-400	36%
PPB-600	47%
PPB-900	41%

). This indicates that the degradation intermediates of BPA were further mineralized in the reaction system.

Subsequently, the identification of reactive species involved in the PPB-600/PMS system was conducted through employing MeOH, TBA and L-histidine as scavengers in the degradation system, respectively. As can be seen from Figure 4b, L-histidine exhibited a significant inhibition effect on BPA degradation, while adding MeOH and TBA hardly declined the removal rate. This indicates that singlet oxygen (¹O₂) instead of hydroxyl radical (·OH) and sulfate radical (SO₄^·−) was the main active species that dominated the BPA degradation in the PPB-600/PMS system. Hence, in order to verify this assertion, probe pollutant degradation tests were conducted, with the results shown in Figure 4c. As illustrated, the PPB/PMS system degraded PN rapidly and displayed a certain IBP degradation efficiency, but failed to remove BA. PN and IBP could be degraded by both radical and non-radical species; nevertheless, BA was only immune to ¹O₂, and the results suggest that ¹O₂ was the dominant active species in the oxidation of BPA [23].

ESR was further applied to identify the generation of the main reactive species in the PPB/PMS system. It could be observed from Figure 5a that when using DMPO as the trapping agent, the DMPO-SO4 and DMPO-OH adduct signals were not presented, revealing that the amount of SO₄⁻ and·OH produced in the oxidation process was negligible. Meanwhile, when the trapping agent was changed to TMP, a typical three-peak signal assigned to the classical spectrum of TMP-¹O₂ appeared (Figure 5b). These findings further prove that PPB activated PMS via a non-radical pathway, and ¹O₂ played a major role in BPA’s removal from the aquatic environment [24].

The effects of different water conditions on the degradation rate of BPA were then explored, with the results shown in Figure 6. As illustrated in Figure 6a, PPB-600 showed an incredible catalytic ability in a relatively wide pH range from 3 to 9. By means of activating PMS in a non-radical pathway, the pH value had a negligible impact on the PPB-600/PMS system, demonstrating its outstanding pH adaptability. Figure 6b displays the influences of inorganic nonmetal ions (Cl⁻, NO₃⁻, HCO₃⁻, H₂PO₄⁻) and humic acid (HA) on the BPA removal rate in a PPB-600/PMS system. Similarly, the presence of external substances hardly inhibited the BPA degradation process due to non-radical BPA oxidation. As a result, the PPB-600/PMS system exhibited remarkable degradation efficiency with the existence of various organic and inorganic compounds. The promising adaptability of the PPB-600/PMS system was further confirmed in removing BPA from actual water. It is clearly revealed in Figure 6c that regardless of whether one is using Xuanwu Lake water (Nanjing) or tap water as the pollutant solution medium, PPB-600 could exhibit a relatively stable BPA degradation, implying that PPB-600 has bright prospects in terms of its practical applications.

Finally, the cycling stability of PPB-600 was studied. It is obvious from Figure 7a that the BPA removal efficiency decreased as the cycle number increased, implying that PMS activation was not simply achieved through an electron transfer process. The XPS C1s spectrum for recycled PPB-600 was analyzed to identify active sites in the biochar. Compared with that of fresh-made PPB-600, the proportion of C=C bond increased significantly, while C=O bond declined after cyclic degradations (Figure 7b,

Table 3. Carbon bonding concentrations of fresh-made and recycled PPB-600 based on deconvoluted C1s XPS peak.

	284.8 (C=C)	286.2 (C-O)	288.8 (C=O)
Fresh PPB-600	58.6%	29.8%	11.6%
Used PPB-600	62.1%	30.1%	7.8%

), whereas the C-O bond remained unchanged after three recycles. The results demonstrate that the PPB activated PMS to generate non-radical ¹O₂ for BPA degradation via consuming its inner structural defects and C=O groups (Figure 7c), which gave rise to the decline of the catalytic capacity of the recycled PPB-600 [25].

3. Experimental

3.1. Reagents and Chemicals

Poplar powder (300 mesh) was provided by Yixing wood powder factory (Linyi, China). Potassium peroxymonosulfate (PMS, Oxone, KHSO₅·0.5 KHSO₄·0.5 K₂SO₄), L-histidine (99%), Biphenol A (BPA, 99%) and Ibuprofen (IBP, 98%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Phenol (PN, 99.5%), benzoic acid (BA, 99%), 5,5-dimethyl-1-pyrroline N-oxide (DMPO, 97%) and 2,2,6,6-tetramethyl-4-piperidine (TMP, 98%) were obtained from Aladdin (Shanghai, China). Other reagents and solvents were of analytical grade and used as received. The double distilled deionized water (H₂O) was used throughout the study.

3.2. Preparation of Wood Powder Biochar (PPB)

A certain amount of poplar powder was calcined in a tubular furnace with a heating rate of 5 °C/min under N₂ atmosphere from room temperature to 200 °C, stayed at 200 °C for 1 h, and then heated to the desired temperature, then stayed at this temperature for 3 h. After being cooled to room temperature, the biochar was washed with H₂O and ethanol 3 times each and dried in an oven at 60 °C for 24 h. The obtained poplar powder-derived carbo-catalysts were named PPB-x (poplar powder biochar), where x represents the calcination temperature.

3.3. Characterizations

The microstructure of biochar was observed by scanning electron microscope (SEM, JEOL SEM 6490, Tokyo, Japan). An X-ray diffraction diffractometer (XRD, Rigaku Smartlab, Tokyo, Japan) was employed to analyze the crystalline structure. The structural changes of biochar were studied using Raman spectroscopy (Raman, Horiba Jobin Yvon HR800, Tokyo, Japan). The chemical bond composition of samples was analyzed by X-ray photoelectron spectroscopy (XPS, Kratos Ultra DLD, Warwick, UK). The active oxygen species generated from the system were identified by electron spin resonance spectroscopy (ESR, Bruker EMX-10/12, Bremen, Germany), in which DMPO and TMP were used as the active species trapping agents. The concentration of the BPA was analyzed by high-performance liquid chromatography (HPLC, Dionex Ultimate 3000, Sunnyvale, CA USA) equipped with a Thermo C18 column (5 μm particle size, 250 × 4.6 mm). The total organic carbon (TOC) was measured by a MutiN/C 2100 analyzer (Jena, Germany).

3.4. Catalytic Degradation of ROCs in PPB/PMS System

If not specified, all the degradation experiments were conducted in a 200 mL beaker with magnetic stirring containing 100 mL pollutant solution at ambient temperature (25 °C). The initial pH values mentioned in the text were measured after the PMS and catalyst were added, which was adjusted by NaOH (1 mol/L) and H₂SO₄ (1 mol/L). Firstly, a certain amount of PPB was added into the pollutant solution, and sonicated in the dark for 20 min to achieved the adsorption/desorption equilibrium. After PMS was dosed to initiate the reaction, 1 mL of the reaction solution was taken from the reaction solution at pre-determined time intervals, filtered with a 0.22 μm polytetrafluoroethylene membrane, and immediately quenched with 0.5 mL methanol before analysis. Methanol (MeOH), isobutanol (TBA) and L-histidine were selected as the scavengers for scavenging tests. All the degradation tests were triplicated, and the data were the average values with standard deviations.

4. Conclusions

In this study, poplar wood-derived biochar samples (PPB) were prepared, which manifested superior PMS activation for BPA degradation. Amongst them, PPB-600 exhibited the best performance. Scavenging tests confirmed that ¹O₂ dominated BPA degradation in the PPB/PMS system. The consumption of C=O groups and structural defects resulted in the deterioration of the catalytic performance of PPB-600 after three cycles. Based on the above results, it seems possible to improve the catalytic capacity by means of heteroatom doping and/or metal compositing in future studies, which can provide inspiration for transforming forest waste into high-value-added environmental catalytic materials.