biochar composite for sulfonamide antibiotics degradation through peroxymonosulfate activation

Environmental Pollution xxx (xxxx) xxx

Contents lists available at ScienceDirect

Environmental Pollution journal homepage: www.elsevier.com/locate/envpol

Yeast biomass-induced Co2P/biochar composite for sulfonamide antibiotics degradation through peroxymonosulfate activation* Yuanyuan Peng, Wenhua Tong, Yi Xie, Wanrong Hu, Yonghong Li, Yongkui Zhang, Yabo Wang* School of Chemical Engineering, Sichuan University, Chengdu, 610065, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 June 2020 Received in revised form 12 October 2020 Accepted 24 October 2020 Available online xxx

Advanced oxidation processes (AOPs) based on peroxymonosulfate (PMS) activation have attracted increasing attention in recent years for organic pollutants removal. Herein, we put forward a facile method to form cobalt phosphide/carbon composite for PMS activation. Combining impregnation approach with pyrolysis treatment enabled the formation of Co2P/biochar composites using baker’s yeast and Co2þ as precursors. The as-synthesized products exhibited excellent catalytic activity for sulfamethoxazole (SMX) degradation over the pH range 3.0e9.0 b y activating PMS. For example, 100% of SMX (20 mg L1) removal was achieved in 20 min with catalyst dosage of 0.4 g L1 and PMS loading of 0.4 g L1. Near zero Co2þ leaching was observed during catalytic reaction, which remarkably lowered the toxic risk of transition metal ion in water. Meanwhile, the reusability of catalyst could be attained by thermal treatment. SMX degradation intermediates were identified by liquid chromatography-mass spectrometry (LC-MS), which facilitated the proposal of possible SMX degradation pathways. Ecological Structure Activity Relationships (ECOSAR) analysis indicated that SMX degradation intermediates may not pose ecological toxicity to the environment. Further investigation verified that Co2P/biochar composites could set off PMS activation not only for the degradation of SMX but also for other sulfonamides. In this study, we not only developed a facile method of utilizing environmental-benign biomass for transition metal phosphide/carbon composite formation, but also achieved highly efficient antibiotic elimination by PMS-based AOP. © 2020 Elsevier Ltd. All rights reserved.

Keywords: Impregnation Biomass Cobalt phosphide Advanced oxidation process Sulfonamide

1. Introduction Antibiotics are widely used in medical treatment, animal husbandry and aquaculture, but whose excessive use can cause harm to the environment (Bu et al., 2013). Sulfonamides (SAs) with the structure of p-aminophenolphthalamide are commonly applied as a prominent class of antibiotics for veterinary and human medicines. SAs can interfere with the enzyme system of bacteria; inhibit the growth of bacterial frontal lobe by competing with paraaminobenzoic acid for binding dihydrofolate synthetase. On account of its inexpensiveness, wide antibacterial spectrum, convenience and good efficacy, SAs are widely employed all over the world to prevent animal diseases and promote body growth (Wong et al., 2016). However, it is disappointing that SAs are not

* €rg Rinklebe. This paper has been recommended for acceptance by Jo * Corresponding author. E-mail address: [email protected] (Y. Wang).

completely absorbed by the body, frequently excreted in urine and feces, and finally discharged into the soil and water environment through a series of environmental behaviors (Oh et al., 2017). In recent years, with the continuous improvement of analysis and detection technology, the existence of SAs has been detected in rivers, lakes, oceans and other water bodies, soil and other environments, when an increasing number of evidences show their risks to the ecological environment and human health (Zhang et al., 2015). Thus, it is of great importance and necessary to conduct a comprehensive study taking into account of efficient and economical technologies for SAs removal. Advanced oxidation process (AOP) is a burgeoning and promising technology which utilizes free radicals with strong oxidation activity produced from reactions, such as sulfate radical ( ), hydroxyl radical (OH) and singlet oxygen (1O2), as driving force to oxidize organic pollutants in the environment so as to eliminate contaminants (Yang et al., 2019). Particularly, AOP based on peroxymonosulfate (PMS) is attractive owing to the strong redox

https://doi.org/10.1016/j.envpol.2020.115930 0269-7491/© 2020 Elsevier Ltd. All rights reserved.

Please cite this article as: Y. Peng, W. Tong, Y. Xie et al., Yeast biomass-induced Co2P/biochar composite for sulfonamide antibiotics degradation through peroxymonosulfate activation, Environmental Pollution, https://doi.org/10.1016/j.envpol.2020.115930

Y. Peng, W. Tong, Y. Xie et al.

Environmental Pollution xxx (xxxx) xxx

2. Materials and methods

potential of radicals, wide pH applicability and non-selective oxidation ability. However, the degradation of pollutants by sole PMS is not satisfactory, while the degradation rate and efficiency of pollutants can be significantly improved with specific conditions (ultrasonic, ultraviolet light, or catalysts: transition metal ions, transition metal oxides, transition metal phosphides, carbon materials, etc.) introduced to activate PMS (Ding et al., 2019; Do et al., 2009; Duan et al., 2018b; Xu et al., 2020; Yang et al., 2019). The fact that Co2þ possesses the highest activation ability for PMS inspires researchers to develop a series of cobalt-based heterogeneous catalyst (supported or unsupported Co3O4, CoP, Co2P, etc.), considering the advantages of easy operation, recyclability and small secondary pollution (Luo et al., 2017; Tong et al., 2019, 2020; Xu et al., 2019). Transition metal phosphides (TMPs) are emerging materials with potential applications for hydrorefining, electrocatalysis, solar energy conversion and environmental pollutant removal. Typically, TMPs could be synthesized by hydrothermal/solvothermal method, thermal decomposition method, temperature-programmed reduction method, low temperature phosphating method, etc (Guan et al., 2009; Shi and Zhang, 2016; Zhang et al., 2017a). One critical factor affecting TMPs synthesis is phosphorus (P) precursor. Organophosphorus such as trioctylphosphine, triphenylphosphine, tri-n-octylphosphine oxide, are commonly adopted as P precursor for the advantages of relatively moderate reaction temperature (Read et al., 2016; Tallapally et al., 2016). Inorganic P precursors of hypophosphite (NaHPO2, NH4HPO2, etc.) and red phosphorus are also considered for TMPs synthesis (Barry and Gillan, 2009; Pu et al., 2014). However, most of above-mentioned P precursors bring potentially hazardous risk or high cost, and may cause secondary pollution to the environment. In 2017, Zhang et al. (2017b), for the first time adopted phosphorus in biomass as an environmental-benign and cost-effective ingredient to prepare TMP by hydrothermal-pyrolysis process, proving the feasibility of microbial P-enabled synthesis of metal phosphide composites. Recently, our group adopted P-enriched yeast biomass (Candida utilis) as P and C precursor to prepare Co2P/C composite for AOP (Tong et al., 2019). Detailed synthesis process analysis confirmed that the in-situ generated reducing gases (CO, CH4, PH3, etc) were key factors, which determined the formation of Co2P. The utilization of biomass for TMP preparation not only eliminates the hazardous risk of organophosphorus, but also bring new insights for the value-added conversion of biomass. Besides, the re-utilization of P in biomass is also meaningful for the preservation of P element, since P is non-regenerable and facing depletion in the near future (Reijnders, 2014; Roy, 2017). In this study, we aim to further simplify the synthesis procedure of cobalt phosphide and investigate the ability of cobalt phosphide for antibiotics degradation through PMS activation. A facile impregnation method was introduced to load Co2þ precursor on the surface of yeast biomass. The baker’s yeast (Saccharomyces cerevisiae) was chosen as P precursor for its easy availability and robustness. Co2þ-loaded biomass was pyrolyzed under inert atmosphere to achieve phosphide formation. The catalytic performance of as-synthesized products was evaluated by activating PMS to degrade a model antibiotic of sulfamethoxazole (SMX). Effects of Co2þ loading amount and pyrolysis temperature on phosphide formation and SMX degradation were systematically investigated. The generation of radicals was confirmed by scavenging tests and electronic paramagnetic resonance (EPR) measurement. SMX degradation intermediates were also analyzed, which enabled the proposal of SMX degradation pathway. Furthermore, ecotoxicity of SMX degradation intermediates was preliminary predicated by Ecological Structure Activity Relationships (ECOSAR) program.

2.1. Chemicals Dry yeast powder (Saccharomyces cerevisiae) was purchased from Angel Yeast Co., Ltd. Co(NO3)2$6H2O and L-histidine was purchased from Maclin. Glucose, acetone, glutaraldehyde, methanol and tert-butyl alcohol were supplied by Chengdu Kelong reagent Co., China. Sulfamethoxazole (SMX), sulfathiazole (STZ) and sulfanilamide (SA) were obtained from Aladdin. Peroxymonosulfate (PMS) was purchased from Damas-beta. All chemicals (except methanol) were of analytical grade and used as received without further purification unless otherwise stated. 2.2. Synthesis of cobalt phosphide/biochar composite Yeast biomass was adopted as both P and C precursor to fabricate cobalt phosphide/biochar composite. Firstly, 5 g of dry yeast powder was suspended in 100 mL glucose aqueous solution (25 g L1). After incubating at 30  C for 30 min, the suspension was centrifuged at 4000 rpm for 5 min to collect yeast cells. The yeast cells were washed by acetone for 3 times and then suspended in 3% glutaraldehyde solution by constant stirring overnight. Washing by de-ionized water and lyophilizing in a freeze-drier (FD-1A-50, Boyikang, Beijing, China) were conducted to obtain dry yeast biomass. P content in yeast was measured by a spectrophotometric method as described in previous study (Tong et al., 2019). Secondly, Co2þ ions were loaded on the surface of yeast cells by a facile impregnation method. Typically, 0.882 mL of 1.4 mol L1 Co(NO3)2 aqueous solution was mixed with 2 g of dry yeast biomass. After thoroughly grinding for 30 min, the mixture was dried in an oven at 60  C. Thirdly, Co2þ-loaded yeast cells were undergone a thermal treatment procedure in a tube furnace (OTF-1200x, Hefei Kejing, China) at 900  C for 4 h with a heating rate of 5  C min1 under an argon atmosphere. After naturally cooling to room temperature under argon flow, the black powder was grinded to obtain the final product. The Co2þ loading amount was varied by adjusting the concentration of Co(NO3)2 solution. The thermal treatment temperature was also investigated in the range of 700e1000  C. Detailed preparation conditions and sample names could be found in Table 1. Besides, the yields of Co2P/biochar composites are listed in Table S1. 2.3. Material characterizations Powder X-ray diffraction (XRD) patterns were recorded at room temperature using an X-ray diffractometer (Rigaku D/max-TTR III)

Table 1 Synthesis conditions and Co2þ leaching in reaction for various samples. Sample Co2þ loading amount (wt%) a

Co:P in precursor (molar ratio) b

Pyrolysis temperature (oC)

Leached Co2þ amount (mg L1)

S-1 S-2 S-4 S-8 S-700 S-800 S-1000 B-1000

1:1 1:2 1:4 1:8 1:4 1:4 1:4 1:4

900 900 900 900 700 800 1000 1000

14.11 2.99 0.42 n.d. 0.15 0.50 0.27 n.d.

a

18.19 9.10 4.55 2.27 4.55 4.55 4.55 0.00

c

calculated based on precursor. P content in yeast was measured by a spectrophotometric method. Co2þ leaching amount after 60 min reaction measured by ICP-OES; n.d: not detected. b c

2

Y. Peng, W. Tong, Y. Xie et al.

Environmental Pollution xxx (xxxx) xxx

equipped with Cu Ka radiation (l ¼ 1.54056 Å). X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo Scientific ESCALAB 250Xi spectrometer (Kratos Axis Ultra DLD Al-Ka X-ray source). The morphology of sample was observed by field emission scanning electron microscope (FESEM, JEOL JSM 7610 F) and transmission electron microscope (TEM, FEI Tecnai G2 F20 S-TWIN). Cobalt ion concentration in reaction solution was measured by inductively coupled plasma optical emission spectrometer (ICPOES, Agilent 730). N2 physisorption measurement was conducted at 196  C on a Quantachrome QuadraSorb apparatus. The specific surface area was calculated by Brunauer-Emmett-Teller (BET) method. Fourier transform infrared (FTIR) spectra were recorded on IR Prestige 21 (Shimadzu). Energy-dispersive X-ray spectroscopy (EDX) was observed by field emission scanning electron microscope (FESEM) with energy dispersive spectrometer (Oxford IE450X-Max80).

2.5. Preliminary acute and chronic toxicity assessment The acute and chronic toxicities of SMX and its degradation intermediates were predicted using ECOSAR program (version 2.0) to estimate the toxicity of the intermediates and SMX to green algae, daphnid and fish from the molecular structure. The acute toxicity was expressed by LC50 values (the concentration for 50% mortality of fish and daphnid after 96 h and 48 h of exposure, respectively) and EC50 values (the concentration for 50% growth inhibition of green algae after 96 h). The chronic toxicity values (ChV) on the three aquatic organisms were also predicted by the same program. 3. Results and discussion 3.1. Characterization of materials XRD patterns of various samples are shown in Fig. 1. It is found that calcination temperature plays a decisive role for the formation of cobalt phosphide crystal phase. No obvious diffraction peaks are found for S-700 sample obtained at 700  C. Along with the increase of calcination temperature, typical diffraction peaks (located at 2q of 40.7, 41.0 , 43.3 , 44.1 and 52.1 ) belonging to Co2P (JCPDS card No. 32e0306) appears. And peak intensity becomes stronger at higher temperature, implying the good crystallinity of S-1000 sample. Reducing gases generated from biomass upon pyrolysis are considered to be critical for the formation of metal phosphide. Recent studies suggested that a calcination temperature of >800  C is necessary for the generation of CO, CH4, PH3, etc (Tong et al., 2019, 2020). Therefore, only when the calcination temperature reaches 800e1000  C, well crystallized Co2P phase is formed. The influence of Co2þ loading amount on the formation of Co2P is not remarkable. Under the investigated Co:P molar ratio from 1:1 to 1:8, Co2P exists as the sole crystal phase for all samples (S-1, S-2, S-4, S-8). In the absence of Co2þ, the as-obtained B-1000 sample shows only a broad peak located at 2q of 44.3 , implying the possible formation of partially graphitized carbon (Song et al., 2012). To further confirm the formation of Co2P, XPS measurement was

2.4. Catalytic performance evaluation Sulfamethoxazole (SMX) was adopted as a model sulfonamide antibiotic to evaluate the performance of as-obtained cobalt phosphide/biochar composite. Degradation experiments were conducted in a 250 mL glass beaker covered with aluminum foil. The beaker contained 100 mL SMX stock solution (20 mg L1) and 0.4 g L1 catalyst. After continuous stirring at 600 rpm for 30 min to reach an adsorption-desorption equilibrium, 0.4 g L1 PMS was added into the suspension to promptly initiate catalytic reaction. At regular time intervals, 0.5 mL mixture was taken out, immediately quenched with equal volume of methanol (MeOH) and then filtered with a 0.22 mm syringe membrane filter. The residual SMX was quantified by high performance liquid chromatography (HPLC, AllTech), equipped with a UV detector and a C-18 column (250  4.6 mm, 5 mm, Grace Davison Discovery Sciences, Maryland, USA). The detection wavelength for SMX was set at 264 nm. The mobile phase was a mixture of 46% acetonitrile and 54% 0.1 mol L1 oxalic acid aqueous solution, which was running at a flow rate of 1 mL min1. An injection volume of 20 mL was used. In recycle study, the used sample was washed by ethanol and de-ionized water. After drying at 60  C, the sample was re-used for the next run. To collect enough sample for the next run, parallel experiments were conducted. Scavenging experiments were conducted by employing MeOH, tert-butyl alcohol (TBA) or L-histidine (L-His) as quenching reagents. Except for the addition of quenching reagent, other reaction conditions were kept the same as described above. Electronic paramagnetic resonance (EPR) measurements were also conducted to identify the active species. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) and 2,2,6,6-tetramethyl-4-piperidinol (TEMP) were selected to capture (or OH) and 1O2, respectively. The SMX degradation intermediates were detected by liquid chromatography-mass spectroscopy (LC-MS, Thermo TSQ quantum ultra). Detailed information could be found in the Supplementary Material. Besides of SMX, other sulfamide antibiotics including SA and STZ were also considered as catalytic degradation targets. Catalytic reaction conditions were kept the same as described above. For quantifying the residual STZ and SA, the detection wavelength was set at 270 nm and 278 nm in HPLC, respectively. The mobile phase was a mixture of 30% acetonitrile and 70% ultrapure water for STZ, while for SA, the mobile phase was a mixture of 70% methanol and 30% ultrapure water. Both mobile phases were running at a flow rate of 1 mL min1. The injection volume for detection was 20 mL.

Fig. 1. XRD patterns of as-prepared samples.

3

Y. Peng, W. Tong, Y. Xie et al.

Environmental Pollution xxx (xxxx) xxx

Fig. S3b, which is beneficial for catalytic reaction. A further highmagnification image shown in Fig. S3c indicates that Co2P nanoparticles are surrounded by graphitic carbon layer, implying the partially graphitization of biochar (Li et al., 2018a). The textural properties of as-obtained samples are summarized in Table 2. It is found that pyrolysis temperature affects the specific surface area (SSA) remarkably. Sample of S-700 obtained at 700  C possesses a low SSA of only 2.28 m2 g1. Increasing pyrolysis temperature to 800  C and 900  C continually increases SSA to 390.88 m2 g1 and 551.21 m2 g1 for S-800 and S-4 sample, respectively. Gases of CO, CO2, NH3, PH3, etc. are produced at relatively high temperature (Tong et al., 2019, 2020), which facilitate the formation of pores in biochar and eventually increases SSA. However, further raising pyrolysis temperature to 1000  C leads to a decreased SSA, which may be ascribed to the expanded pore diameter. Changing of Co:P ratio in precursor does not bring remarkable difference of SSA. High SSAs of 526e673 m2 g1 were found for S-1, S-2, S-4 and S-8 samples. Sample without Co2P loading (B-1000) shows a SSA of 178.11 m2 g1.

conducted to characterize S-1000 sample. As illustrated in Fig. S1a, the Co 2p spectrum is deconvoluted into four peaks located at 777.0 eV (Co2þ in Co2P), 778.9 eV and 780.8 eV (Co 2p3/2), followed by a satellite peak at 782.8 eV, respectively (Liu et al., 2019). For P element (Fig. S1b), the peaks at 130.2 eV and 133.7 eV correspond to phosphide for Co2P (PeCo bond) and surface oxidized P (PeO bond), respectively (Liu et al., 2019). Therefore, both XPS and XRD results confirm the successful formation of Co2P. In addition, XPS C 1s and N 1s spectra are deconvoluted as illustrated in Figs. S1ced. Typical CeN bond (285.7 eV) is found in C 1s spectrum (Du et al., 2019). Pyridinic N (398.6 eV), pyrrolic N (399.5 eV) and graphitic N (401.6 eV) are identified in N 1s spectrum (Zhuang et al., 2016). Therefore, N doping into carbon framework is suggested. The N content measured by XPS is about 3.16% (Fig. S1). Considering the high N content in yeast biomass, the above results are reasonable. The intrinsic C, N and P elements in biomass are utilized to obtain Co2P/N-doped biochar composite. Fig. S2 presents the morphology of as-obtained samples. Under the investigated synthesis conditions, the original sphere-like structure of yeast cell could be preserved in all samples, which should be ascribed to the relatively high tolerance of yeast cell components to thermal treatment. Similar structures are also found in other studies adopting yeast cell as support or biochar precursor (Gong et al., 2015; Xie et al., 2020; Zhang et al., 2017b). Co2P nanoparticles are well dispersed and anchored on the surface of Co2P/biochar composites. For S-1 sample with high Co2þ loading, slightly larger size of Co2P nanoparticle is observed, comparing with other samples (Figs. S2beg). There is not distinct morphological difference between other Co2P/biochar composites. Since no Co2þ precursor was loaded, only spherical biochar with smooth surface is observed for B-1000 sample (Fig. S2h). Detailed morphological observation was further conducted by TEM for S1000 sample. As shown in Fig. S3a, spherical Co2P nanoparticles with size of 30e50 nm are embedded on the surface of biochar. Meanwhile, typical mesopores are observed for biochar as shown in

3.2. PMS activation for SMX degradation Co2þ ion, cobalt-containing metal oxide are typical catalysts for PMS activation, leading to the generation of oxidative species (OH, , etc.) for organic pollutants degradation (Luo et al., 2017; Xu et al., 2019). Recent studies show that Co2P and CoP are also active for PMS activation (Luo et al., 2017; Tong et al., 2019, 2020). To evaluate the catalytic performances of as-obtained Co2P/biochar composites, SMX, a prominent antibiotic widely used in veterinary and human medicines, is chosen as a model pollutant for investigation. Fig. 2aeb shows SMX degradation efficiencies over different catalyst systems. In the absence of catalyst, PMS alone could only degrade 23.97% of SMX in 60 min. After adding Co2P/biochar composite as catalyst in the reaction system, remarkably enhanced SMX removal efficiencies are achieved. Typically, 100% SMX removal efficiency is achieved in 20 min using S-1 and S-2 sample as catalyst, corresponding to the apparent first order reaction rate constant (k) of 0.6012 min1 and 0.4877 min1, respectively (Table S3 in Supplementary Material). A longer reaction time up to 60 min is required for S-4 sample to achieve 100% removal efficiency (k ¼ 0.0755 min1). And S-8 sample could degrade 97.33% of SMX in 60 min (k ¼ 0.0325 min1). On the one hand, the variation of Co2þ loading amount should be one reason for the difference of SMX degradation efficiency. Higher Co2þ loading content could provide more reactive sites. On the other hand, leached Co2þ ions contribute to the fast degradation of SMX. As measured by ICP-OES, the leached Co2þ ions reach 14.11 mg L1 and 2.99 mg L1 (Table 1)

Table 2 Textural properties of Co2P/biochar composite samples. Sample

Specific surface area (m2 g1)

Average pore diameter (nm)

Total pore volume (cm3 g1)

S-1 S-2 S-4 S-8 S-700 S-800 S-1000 B-1000

673.18 526.40 551.21 630.87 2.28 390.88 107.63 178.11

2.62 2.95 2.90 2.90 13.33 2.12 4.11 2.48

0.44 0.39 0.40 0.46 0.01 0.21 0.11 0.11

Fig. 2. Effects of Co2þ loading amount (a), pyrolysis temperature (b) and initial solution pH (c) on SMX degradation. (Catalyst ¼ 0.4 g L1; PMS ¼ 0.4 g L1; SMX ¼ 20 mg L1, and temperature ¼ 25  C). 4

Y. Peng, W. Tong, Y. Xie et al.

Environmental Pollution xxx (xxxx) xxx

for S-1 and S-2 sample, respectively, mainly due to the acidic environment brought by Hþ releasing from PMS molecule (Li et al., 2018b). Such high concentration of free Co2þ ion further promotes PMS activation reaction. However, the cytotoxic effect of Co2þ ion to the environment can not be ignored (Jyothi et al., 2019). In contrast, S-4 and S-8 sample show high stability with only 0.42 mg L1 and almost zero Co2þ leaching after 60 min reaction (Table 1). The leached amount complies well with the regulatory limit (1.0 mg L1, environmental quality standards for surface water, GB 3838e2002, China). Considering the good stability and relatively high activity, the Co:P molar ratio of 1:4 (eg., sample S-4) is adopted to investigate the influence of pyrolysis temperature (Table 1). Fig. 2b illustrates the effect of pyrolysis temperature on SMX removal. Although not being crystallized (Fig. 1), the cobaltcontaining nanoparticles on the surface of S-700 sample contributes to SMX degradation with a removal efficiency of 87.76% in 60 min. Increasing pyrolysis temperature to 800e1000  C promotes the crystallization and formation of Co2P nanoparticles (Fig. 1). Since well crystallized Co2P could effectively transfer electrons, gradual enhancement of SMX degradation efficiency is found with increasing pyrolysis temperature. Particularly, S-1000 sample could achieve 100% SMX removal in 20 min with a reaction rate constant of 0.1808 min1 (Table S3 in Supplementary Material). It is worth to mention that all samples investigated in Fig. 2b show good stability with less than 1 mg L1 Co2þ leaching in 60 min reaction (Table 1). Non-metal catalysts (eg., g-C3N4, graphene, nano-diamond, Ndoped carbon, etc.) are also reported active for PMS activation (Liang et al., 2017). Since the as-obtained samples in this study contain biochar, it is necessary to figure out the contribution of biochar for SMX degradation. As shown in Fig. 2b, control sample of B-1000 without Co2þ loading in precursor could degrade 28.79% of SMX in the presence of PMS, which is slightly better than that of bare PMS (Fig. 2a). Therefore, the contribution of biochar is not remarkable in this study. It is worth mentioning that compared with other reported catalysts (Table S4), S-1000 sample prepared in this study showed comparable or ever higher efficiency for the removal of sulfonamide antibiotics. Co2P nanoparticles embedded in biochar possess the advantageous features of good dispersion, high crystallinity and good stability, which are key factors for its superior catalytic activity. Solution pH is an important factor which should be investigated from the viewpoint of practical application. As revealed in Fig. 2c, under the investigated initial solution pH range of 3.0e8.9, S-1000 sample performs well with negligible difference in terms of SMX degradation efficiency and reaction rate. A 100% SMX removal efficiency is obtained in 20 min. Such results indicate that the developed S-1000 sample has good pH applicability and possesses great potential for organic pollutant removal in a relatively broad solution pH range. Catalyst reusability is also important for heterogeneous catalysis. S-1000 sample is chosen to evaluate the reusability by recycle study. As shown in Fig. 3, S-1000 sample could degrade 100% of SMX in 20 min in the first run. After washing by de-ionized water and ethanol, S-1000 sample is reused in the second run. However, its catalytic activity obviously decreases with only 39.13% SMX removal in 30 min. Based on previous studies, one highly possible reason of deactivation is reaction intermediates covering on the surface of catalyst, hindering the contact between SMX and active sites (Xie et al., 2018). We attempt a facile thermal treatment approach to recover the activity of S-1000. Fortunately, the SMX degradation capability of S-1000 is almost completely restored after re-calcination at 1000  C for 2 h under an argon atmosphere. Comparing with the first run, similar catalytic reaction efficiency is observed for reactivated sample in the third run. To further investigate the stability of S-1000 sample and understand its behavior in

Fig. 3. SMX degradation efficiency in recycle study using S-1000 sample. (Catalyst ¼ 0.4 g L−1; PMS ¼ 0.4 g L−1; SMX ¼ 20 mg L−1, and temperature ¼ 25 °C).

recycle study, XRD, FTIR, and total survey spectra of XPS characterization for catalyst before and after regeneration are conducted. As shown in Fig. S4, no distinct changes are found from XRD patterns and XPS survey spectra for samples before and after regeneration. Therefore, the main composition of sample changes little, verifying its good stability. However, organic functional groups are found from FTIR spectrum of sample before regeneration, which should be originated from the intermediate products covering on the surface of sample. Such kind of coverage of catalyst will hinder the contact between SMX and active catalytic sites, leading to the decrease of reaction rate and SMX removal efficiency. After regeneration, relevant FTIR peak intensity remarkably decreases, indicating the loss of functional groups and exposure of active sites. And eventually, the activity of catalyst is resumed. Therefore, the developed sample in this study is stable could be reused after proper reactivation treatment. Commonly, PMS activation will induce various oxidative species for organic pollutants degradation, for example OH, and 1O2 (Luo et al., 2017; Zhang et al., 2014). Quenching experiment using radical scavenging reagent and EPR measurement using spin trapping agent could provide strong evidence for such oxidative species. Herein, methanol (MeOH), tert-butyl alcohol (TBA) and Lhistidine (L-His) were used as scavenging reagents for both OH and , only OH and 1O2, respectively. As depicted in Fig. 4a, the addition of scavenging reagents remarkably hinders the SMX degradation efficiency, indicating the presence of all three kinds of oxidative species. Further EPR spectra shown in Fig. 4b and c clearly demonstrate the characteristic peaks belonging to DMPO-OH, DMPOand TEMP-1O2 products (Cheng et al., 2017; Duan et al., 2018a). Therefore, OH, and 1O2 species are all involved in Co2P/biochar-induced PMS activation process. Based on above analysis and previous studies, a possible mechanism of PMS activation by Co2P/biochar composite is proposed as shown in equations (1)e(8) (Luo et al., 2017; Zhang et al., 2014). (1)

(2)

(3) 5

Y. Peng, W. Tong, Y. Xie et al.

Environmental Pollution xxx (xxxx) xxx

Fig. 4. Effect of scavengers on SMX degradation (a); detected EPR signals by using DMPO (b) and TEMP (c) as the spin trapping agent. (Catalyst ¼ 0.4 g L1; PMS ¼ 0.4 g L1; SMX ¼ 20 mg L1, and temperature ¼ 25  C).

opened small molecules such as P12, P13, P14 and P15. (4) 3.4. Prediction of the ecotoxicity of SMX and its degradation intermediates (5) The acute and chronic toxicity of SMX and its degradation intermediates were predicted by using ECOSAR program. The toxic level of compounds can be classified according to the Globally Harmonized System of Classification and Labelling of Chemicals (GHS). Based on the predicted values listed in Table 3, SMX shows different toxic levels for different species. For example, it is toxic to daphnid, but not harmful to fish, in terms of acute toxicity. After a period of degradation by Co2P/biochar composite, a few intermediates are generated. The toxicity of most of the intermediates is significantly reduced, implied by the increased LC50, EC50 and ChV values. And many intermediates are even not harmful, especially for fish and algae. However, in terms of chronic toxicity, it is found that daphnid is the most sensitive species for SMX and its degradation intermediates. Although the ChV values of most of the degradation intermediates increased for daphnid, a few of them (eg., P1, P2, P5, P9, P10, P14) are still highly toxic. In addition, for fish and algae, some intermediates are more toxic than original SMX, such as P2, P9, P10 and P14. Among them, P10 is even classified as “very toxic” to the three aquatic organisms. Surprisingly, these four intermediates are derived from the same degradation pathway (pathway B in Fig. 5), which indicates that the potential risks of this SMX degradation pathway should be considered.

(6)

(7)

(8)

3.3. Possible pathways of SMX degradation Although SMX could be totally degraded in Co2P/biochar-PMS system (Fig. 2), a few new peaks are observed in HPLC analysis, indicating the existence of possible degradation intermediates. LCMS measurement is conducted to analyze intermediates, which are listed in Table S5 in Supplementary Material. Based on the detected 15 intermediates and previous studies, possible pathways of SMX degradation are proposed as shown in Fig. 5 (Du et al., 2018; He et al., 2020; Naraginti et al., 2019; Wang et al., 2019, 2020). In pathway A, the isoxazole ring of SMX is attacked by oxidative species, leading to the formation of ring-opening product of P1. In pathway B, the liable S7eN11 bond is cleaved on account of and OH, leading to the formation of P2 and P3. Further attacked by oxidative species, P2 is transformed to P7, P8 and P9. The coupling of N-centered radical derived from eNH2 group of P8 and P9 is attributed to form P10. Meanwhile, via OH substitution, P7 is first converted to P11, afterwards, to P12. In pathway C, the amine group in benzene ring is oxidized by reactive species ( , OH and 1O2). As a result, P4 (nitro-sulfamethoxazole, NO2-SMX) is detected, which is recognized as one of the most classical oxidative products during degradation process of SMX (Naraginti et al., 2019; Wang et al., 2019, 2020). Further hydroxyl substitution of P4 leads to the appearance of P6. In a similar way, P5 is transformed from SMX in pathway D. Finally, all benzene ring and isoxazole ring are oxidized to ring-

3.5. SMX analogues removal Considering the excellent catalytic capability of Co2P/biocharPMS system in SMX degradation process, the developed Co2P/biochar composite possesses great potential for the treatment of other sulfonamides antibiotics. Sulfathiazole (STZ) and sulfanilamide (SA) are chosen as the target pollutants for investigation. As shown in Fig. 6, S-1000 sample exhibits outstanding catalytic activity for STZ and SA degradation. In the absence of catalyst, sole PMS could only degrade 38.30% of STZ and 40.36% of SA in 30 min. After involving S-1000 sample as PMS activation catalyst, 100% STZ removal efficiency is obtained in 20 min, while 98.97% of SA is degraded in 30 min, indicating the essential role of S-1000 for PMS

6

Y. Peng, W. Tong, Y. Xie et al.

Environmental Pollution xxx (xxxx) xxx

Fig. 5. Proposed SMX degradation pathways over S-1000/PMS system.

nanoparticle are embedded on the surface of sphere-like biochar. When adopted as PMS activation catalyst, the developed Co2P/ biochar composite performs well for sulfonamide antibiotics (SMX, STZ, SA) degradation, including superior activity, good stability and recyclability, small metal leaching amount and pH-tolerance. Based on degradation intermediate identification, possible ways of SMX, STZ and SA degradation are also proposed. Preliminary ecotoxicity analysis by ECOSAR indicates that most of the SMX degradation intermediates possess less toxicity than that of the original SMX. This study not only provides a facile strategy for transition metal phosphide development using sustainable precursor, but also contributes a potential candidate for elimination of antibiotic contaminants from environment.

activation. Compared with other reported catalysts (Table S8), S1000 sample prepared in this study showed comparable or ever higher efficiency for the removal of sulfonamide antibiotics. Considering the superior activity, good stability and recyclability, broad applicability, the developed Co2P/biochar composite is considered as a good candidate for organic pollutants removal through PMS activation. 4. Conclusions We demonstrate a facile impregnation-pyrolysis approach to develop Co2P/biochar composite adopting yeast biomass as sole and environmental-benign P and C precursor. Well dispersed Co2P 7

Y. Peng, W. Tong, Y. Xie et al.

Environmental Pollution xxx (xxxx) xxx

Table 3 Predicted acute and chronic toxicity of SMX and its degradation intermediates using ECOSAR system.

a

Writing - original draft. Wenhua Tong: Data curation, Investigation. Yi Xie: Data curation, Visualization, Writing - review & editing. Yonghong Li: Supervision. Yongkui Zhang: Project administration. Yabo Wang: Conceptualization, Writing - review & editing, Supervision, Funding acquisition. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors gratefully acknowledge funding support from the National Natural Science Foundation of China (grant number 21808147). We also thank Prof. Hui Li from School of Chemical Engineering, Sichuan University, for his kind assistance on material characterization. Appendix A. Supplementary data 1

Fig. 6. STZ and SA degradation in PMS and S-1000/PMS system. (Catalyst ¼ 0.4 g L PMS ¼ 0.4 g L1; STZ or SA ¼ 20 mg L1, and temperature ¼ 25  C).

;

Supplementary data to this article can be found online at https://doi.org/10.1016/j.envpol.2020.115930.

Main findings

References

Co2P-embeded biochar composite derived from biomass exhibited great catalytic activity and low Co2þ leaching for pollutant degradation where most of intermediates possessed less ecotoxicity than SMX.

Barry, B.M., Gillan, E.G., 2009. A general and flexible synthesis of transition-metal polyphosphides via PCl3 elimination. Chem. Mater. 21, 4454e4461. Bu, Q., Wang, B., Huang, J., Deng, S., Yu, G., 2013. Pharmaceuticals and personal care products in the aquatic environment in China: a review. J. Hazard Mater. 262, 189e211. Cheng, X., Guo, H., Zhang, Y., Wu, X., Liu, Y., 2017. Non-photochemical production of singlet oxygen via activation of persulfate by carbon nanotubes. Water Res. 113, 80e88. Ding, M., Chen, W., Xu, H., Shen, Z., Lin, T., Hu, K., Kong, Q., Yang, G., Xie, Z., 2019. Heterogeneous Fe2CoTi3O10-MXene composite catalysts: synergistic effect of the ternary transition metals in the degradation of 2,4-dichlorophenoxyacetic

Author contribution Yuanyuan Peng: Conceptualization, Methodology, Investigation, 8

Y. Peng, W. Tong, Y. Xie et al.

Environmental Pollution xxx (xxxx) xxx Shi, Y., Zhang, B., 2016. Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction. Chem. Soc. Rev. 45, 1781-1781. Song, X., Zhang, Y., Chang, C., 2012. Novel method for preparing activated carbons with high specific surface area from rice husk. Ind. Eng. Chem. Res. 51, 15075e15081. Tallapally, V., Esteves, R.J.A., Nahar, L., Arachchige, I.U., 2016. Multivariate synthesis of tin phosphide nanoparticles: temperature, time, and ligand control of size, shape, and crystal structure. Chem. Mater. 28, 5406e5414. Tong, W., Xie, Y., Hu, W., Peng, Y., Liu, W., Li, Y., Zhang, Y., Wang, Y., 2020. A bifunctional CoP/N-doped porous carbon composite derived from a single source precursor for bisphenol A removal. RSC Adv. 10, 9976e9984. Tong, W., Xie, Y., Luo, H., Niu, J., Ran, W., Hu, W., Wang, L., Yao, C., Liu, W., Zhang, Y., Wang, Y., 2019. Phosphorus-rich microorganism-enabled synthesis of cobalt phosphide/carbon composite for bisphenol A degradation through activation of peroxymonosulfate. Chem. Eng. J. 378, 122187. Wang, H., Guo, W., Yin, R., Du, J., Wu, Q., Luo, H., Liu, B., Sseguya, F., Ren, N., 2019. Biochar-induced Fe(III) reduction for persulfate activation in sulfamethoxazole degradation: insight into the electron transfer, radical oxidation and degradation pathways. Chem. Eng. J. 362, 561e569. Wang, S., Xu, W., Wu, J., Gong, Q., Xie, P., 2020. Improved sulfamethoxazole degradation by the addition of MoS2 into the Fe2þ/peroxymonosulfate process. Separ. Purif. Technol. 235, 116170. Wong, K., Yoon, Y., Snyder, S.A., Jang, M., 2016. Phenyl-functionalized magnetic palm-based powdered activated carbon for the effective removal of selected pharmaceutical and endocrine-disruptive compounds. Chemosphere 152, 71e80. Xie, Y., Li, P., Zeng, Y., Li, X., Xiao, Y., Wang, Y., Zhang, Y., 2018. Thermally treated fungal manganese oxides for bisphenol A degradation using sulfate radicals. Chem. Eng. J. 335, 728e736. Xie, Y., Wang, X., Tong, W., Hu, W., Li, P., Dai, L., Wang, Y., Zhang, Y., 2020. FexP/ biochar composites induced oxygen-driven Fenton-like reaction for sulfamethoxazole removal: performance and reaction mechanism. Chem. Eng. J. 396, 125321. Xu, H., Zhang, Y., Li, J., Hao, Q., Li, X., Liu, F., 2019. Heterogeneous activation of peroxymonosulfate by a biochar-supported Co3O4 composite for efficient degradation of chloramphenicols. Environ. Pollut. 257, 113610. Xu, L., Wang, X., Sun, Y., Gong, H., Guo, M., Zhang, X., Meng, L., Gan, L., 2020. Mechanistic study on the combination of ultrasound and peroxymonosulfate for the decomposition of endocrine disrupting compounds. Ultrason. Sonochem. 60, 8. Yang, Q., Ma, Y., Chen, F., Yao, F., Sun, J., Wang, S., Yi, K., Hou, L., Li, X., Wang, D., 2019. Recent advances in photo-activated sulfate radical-advanced oxidation process (SR-AOP) for refractory organic pollutants removal in water. Chem. Eng. J. 378, 122149. Zhang, B., Zhang, Y., Teng, Y., Fan, M., 2014. Sulfate radical and its application in decontamination technologies. Crit. Rev. Environ. Sci. Technol. 45, 1756e1800. Zhang, J., Matsubara, K., Yun, G.N., Zheng, H., Takagaki, A., Kikuchi, R., Oyama, S.T., 2017a. Comparison of phosphide catalysts prepared by temperatureprogrammed reduction and liquid-phase methods in the hydrodeoxygenation of 2-methylfuran. Appl. Catal. Gen. 548, 39e46. Zhang, Q., Ying, G., Pan, C., Liu, Y., Zhao, J., 2015. Comprehensive evaluation of antibiotics emission and fate in the river basins of China: source analysis, multimedia modeling, and linkage to bacterial resistance. Environ. Sci. Technol. 49, 6772e6782. Zhang, T., Liu, J., Huang, L., Zhang, X., Sun, Y., Liu, X., Bin, D., Chen, X., Cao, A., Hu, J., Wan, L., 2017b. Microbial-phosphorus-enabled synthesis of phosphide nanocomposites for efficient electrocatalysts. J. Am. Chem. Soc. 139, 11248e11253. Zhuang, M., Ou, X., Dou, Y., Zhang, L., Zhang, Q., Wu, R., Ding, Y., Shao, M., Luo, Z., 2016. Polymer-embedded fabrication of Co2P nanoparticles encapsulated in N,P-doped graphene for hydrogen generation. Nano Lett. 16, 4691e4698.

acid based on peroxymonosulfate activation. Chem. Eng. J. 378, 122177. Do, S., Jo, J., Jo, Y., Lee, H., Kong, S., 2009. Application of a peroxymonosulfate/cobalt (PMS/Co(II)) system to treat diesel-contaminated soil. Chemosphere 77, 1127e1131. Du, J., Guo, W., Wang, H., Yin, R., Zheng, H., Feng, X., Che, D., Ren, N., 2018. Hydroxyl radical dominated degradation of aquatic sulfamethoxazole by Fe0/bisulfite/O2: kinetics, mechanisms, and pathways. Water Res. 138, 323e332. Du, Y., Pan, G., Wang, L., Song, Y., 2019. CoxNiyP embedded in nitrogen-doped porous carbon on Ni foam for efficient hydrogen evolution. Appl. Surf. Sci. 469, 61e67. Duan, X., Su, C., Miao, J., Zhong, Y., Shao, Z., Wang, S., Sun, H., 2018a. Insights into perovskite-catalyzed peroxymonosulfate activation: maneuverable cobalt sites for promoted evolution of sulfate radicals. Appl. Catal. B Environ. 220, 626e634. Duan, X., Sun, H., Wang, S., 2018b. Metal-free carbocatalysis in advanced oxidation reactions. Acc. Chem. Res. 51, 678e687. Gong, X., Liu, S., Ouyang, C., Strasser, P., Yang, R., 2015. Nitrogen- and phosphorusdoped biocarbon with enhanced electrocatalytic activity for oxygen reduction. ACS Catal. 5, 920e927. Guan, Q., Li, W., Zhang, M., Tao, K., 2009. Alternative synthesis of bulk and supported nickel phosphide from the thermal decomposition of hypophosphites. J. Catal. 263, 1e3. He, Y., Zhang, J., Zhou, H., Yao, G., Lai, B., 2020. Synergistic multiple active species for the degradation of sulfamethoxazole by peroxymonosulfate in the presence of CuO@FeOx@Fe0. Chem. Eng. J. 380, 122568. Jyothi, P., Koppu, S., Sumalatha, V., Ushaiah, B., Gyana Kumari, C., 2019. Synthesis, characterisation, cytotoxicity, DNA binding and antimicrobial studies of binary and ternary metal complexes of Co (II). Inorg. Chem. Commun. 110, 107590. Li, J., Liu, G., Liu, B., Min, Z., Qian, D., Jiang, J., Li, J., 2018a. An extremely facile route to Co2P encased in N,P-codoped carbon layers: highly efficient bifunctional electrocatalysts for ORR and OER. Int. J. Hydrogen Energy 43, 1365e1374. Li, J., Xu, M., Yao, G., Lai, B., 2018b. Enhancement of the degradation of atrazine through CoFe2O4 activated peroxymonosulfate (PMS) process: kinetic, degradation intermediates, and toxicity evaluation. Chem. Eng. J. 348, 1012e1024. Liang, P., Zhang, C., Duan, X., Sun, H., Liu, S., Tade, M.O., Wang, S., 2017. An insight into metal organic framework derived N-doped graphene for the oxidative degradation of persistent contaminants: formation mechanism and generation of singlet oxygen from peroxymonosulfate. Environ. Sci. J. Integr. Environ. Res.: Nano 4, 315e324. Liu, Y., Yang, F., Qin, W., Yang, G., 2019. Co2P@NiCo2O4 bi-functional electrocatalyst with low overpotential for water splitting in wide range pH electrolytes. J. Colloid Interface Sci. 534, 55e63. Luo, R., Liu, C., Li, J., Wang, J., Hu, X., Sun, X., Shen, J., Han, W., Wang, L., 2017. Nanostructured CoP: an efficient catalyst for degradation of organic pollutants by activating peroxymonosulfate. J. Hazard Mater. 329, 92e101. Naraginti, S., Yu, Y., Fang, Z., Yong, Y., 2019. Novel tetrahedral Ag3PO4@N-rGO for photocatalytic detoxification of sulfamethoxazole: process optimization, transformation pathways and biotoxicity assessment. Chem. Eng. J. 375, 122035. Oh, W., Dong, Z., Ronn, G., Lim, T., 2017. Surface-active bismuth ferrite as superior peroxymonosulfate activator for aqueous sulfamethoxazole removal: performance, mechanism and quantification of sulfate radical. J. Hazard Mater. 325, 71e81. Pu, Z., Liu, Q., Jiang, P., Asiri, A.M., Obaid, A.Y., Sun, X.P., 2014. CoP nanosheet arrays supported on a Ti plate: an efficient cathode for electrochemical hydrogen evolution. Chem. Mater. 26, 4326e4329. Read, C.G., Callejas, J.F., Holder, C.F., Schaak, R.E., 2016. General strategy for the synthesis of transition metal phosphide films for electrocatalytic hydrogen and oxygen evolution. ACS Appl. Mater. Interfaces 8, 12798e12803. Reijnders, L., 2014. Phosphorus resources, their depletion and conservation, a review. Resour. Conserv. Recycl. 93, 32e49. Roy, E.D., 2017. Phosphorus recovery and recycling with ecological engineering: a review. Ecol. Eng. 98, 213e227.

9