Magnetically recoverable Pd-loaded BiFeO3 microcomposite with enhanced visible light photocatalytic performance for pollutant, bacterial and fungal elimination

Journal Pre-proofs Magnetically recoverable Pd-loaded BiFeO3 microcomposite with enhanced visible light photocatalytic performance for pollutant, bacterial and fungal elimination Zeeshan Haider Jaffari, Sze-Mun Lam, Jin-Chung Sin, Honghu Zeng, Abdul Rahman Mohamed PII: DOI: Reference:

S1383-5866(19)33709-8 https://doi.org/10.1016/j.seppur.2019.116195 SEPPUR 116195

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Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

19 August 2019 28 September 2019 8 October 2019

Please cite this article as: Z. Haider Jaffari, S-M. Lam, J-C. Sin, H. Zeng, A. Rahman Mohamed, Magnetically recoverable Pd-loaded BiFeO3 microcomposite with enhanced visible light photocatalytic performance for pollutant, bacterial and fungal elimination, Separation and Purification Technology (2019), doi: https://doi.org/ 10.1016/j.seppur.2019.116195

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Magnetically recoverable Pd-loaded BiFeO3 microcomposite with enhanced visible light photocatalytic performance for pollutant, bacterial and fungal elimination Zeeshan Haider Jaffari a, Sze-Mun Lam a,c,d,e,*, Jin-Chung Sin b,c,d,e, Honghu Zeng c, Abdul Rahman Mohamed d a

Department of Environmental Engineering, Faculty of Engineering and Green Technology, Universiti Tunku Abdul Rahman, 31900 Kampar, Perak, Malaysia

b

Department of Petrochemical Engineering, Faculty of Engineering and Green Technology, Universiti Tunku Abdul Rahman, 31900 Kampar, Perak, Malaysia

c

College of Environmental Science and Engineering, Guilin University of Technology, Guilin 541004, China d

Guangxi Key Laboratory of Theory and Technology for Environmental Pollution Control, Guilin University of Technology, Guilin 541004, China e

Collaborative Innovation Center for Water Pollution Control and Water Safety in Karst Area, Guilin University of Technology, Guilin 541004, China f

School of Chemical Engineering, Universiti Sains Malaysia,

Engineering Campus, 14300 Nibong Tebal, Pulau Pinang, Malaysia

*Corresponding author: Tel: +605-4688888, ext. 4724 Email: [email protected] (Sze Mun Lam) 1

Abstract Herein, magnetically recoverable coral-like Pd-loaded BiFeO3 microcomposites were successfully synthesized using a two-steps hydrothermal technique. The structural, magnetic, optical and textural characteristics of the microcomposite were analyzed by numerous characterization techniques. The Pd nanoparticles loaded on the coral-like BiFeO3 surfaces were quasi-sphere shape with sizes of 15‒25 nm. The photodegradation of malachite green dye and phenol were assessed under exposure of visible light to evaluate the catalytic ability of the microcomposite. The findings indicated that Pd-BFO microcomposite exhibited the higher photoactivity than that of pure BFO or commercial TiO2. The enhanced photoactivity was accredited to appropriate Pd content that could improve the light absorbance in the visible region and efficiently improved the photogenerated charge carriers separation/migration abilities as confirmed by the UV-vis diffuse reflectance, photoelectrochemical and photoluminescence analyses. The intermediates formed during the phenol degradation were also detected. The cyclic test showed that the Pd-BFO microcomposite has excellent recyclable ability together with a minimal leakage of palladium ions after six runs. Based on the radical trapping test, hydroxyl radicals, hole and hydrogen peroxide were the chief reactive species contributing to the photodegradation. Furthermore, for the first time, antimicrobial activity towards Escherichia coli, Staphylococcus aureus, Enterococcus faecalis and Aspergillus niger were explored, and the consequence revealed that the Pd-BFO microcomposite presented exceptional antimicrobial properties.

Keywords: BiFeO3; Pd-loaded; Microcomposite; Visible light; Photocatalytic; Antimicrobial

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Introduction The organic pollutants present in the industrial wastewater pose a significant threat to the water bodies because of the excessive release of the contaminants, such as phenols, organic dyes, nitroaromatic compounds and fertilizer wastes [1,2]. Among these contaminants, organic dyes and phenol were extremely difficult to remove by natural degradation process due to their higher chemical and biological stabilities [3]. Removal of such organic pollutants were carried out using different purification techniques such as adsorption, biodegradation, chemical reduction, ozonation, coagulation, electrochemical, Fenton oxidation and etc. [4-7]. However, each of this technique has its own degree of effectiveness. These conventional treatment processes did not completely mineralize the effluents, which then produce toxic residuals as they only transfer the effluents from one form to another without elimination [8]. On the contrary, advanced oxidation processes (AOPs) including photocatalysis provided a better solution towards the treatment of wastewater as it can efficiently mineralize a large number of effluents to comparatively nontoxic final products such as water (H2O) and carbon dioxide (CO2), and thus improving the overall status of secondary products [9,10]. Recently, perovskite bismuth ferrite (BiFeO3, BFO) has been emerged as one of the most auspicious visible light active catalyst owing to its smaller energy band gap (2.2-2.7 eV) [11,12], good chemical stability, non-toxic nature, ferroelectric and ferromagnetic characteristics at the ambient temperature [13,14]. Nevertheless, the photoactivity of BFO has been still restrained due to the fast e−−h+ pairs recombination and low position of conduction band (CB) [15,16]. To overcome these shortcomings, different BFO nanostructures such as nanorods [17], nanobelt [18], nanofibers [19], nanoplates [20], mat-shape and mash-shaped [11] have been successfully synthesized using sol-gel and solvothermal/hydrothermal techniques. Recently, three

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dimensional (3D) with well-defined structures had attained considerable attention because of the distinctive advantages, such as anti-aggregation ability, improved solar light harvesting, good recyclability and a large amount of active sites, which led to the higher photoactivities [21–23]. Besides the specific morphologies, loading of noble metals (Au, Pt, Pd and Ag) have also successfully applied to ameliorate the visible light absorption properties and e−−h+ pair separation efficiency [24,25]. The metallic energy levels of Ag, Pt and Pd were found to be below than the CB position of BFO, so they can act as an e− sink under the light irradiation, which led to the improvement in the photoactivity [26,27]. Interestingly, in some studies, Pdbased semiconductors showed higher photoactivities compared with the Ag, Pt and Au-based catalysts [28,29]. Diak et al. [28] synthesized the noble metals (Ag, Au, Pt and Pd) loaded TiO2 using a hydrothermal synthesis method and applied towards the degradation of phenol under the irradiation of UV light. The results presented that the Pd-TiO2 had the highest photoactivity than those of the remaining Ag-TiO2, Pt-TiO2, Au-TiO2 and pure TiO2. The higher photoactivity of Pd-TiO2 was credited to the much improved light absorption ability. Kumar and Thangadurai, [30] synthesized nanorods shaped Pd-V2O5 to improve the photoactivity of V2O5 for the degradation of Rhodamine 6G dye under the irradiation of visible light. The appropriate loadings and smaller size of Pd nanoparticles were the key factors for the enhancement of photodegradation activity. Unfortunately, the wastewater also contained a lot of pathogenic microorganisms, including bacteria and fungi that can threaten human health as well as ecological stability. The Pd-based semiconductor catalysts were also reported to effectively disinfect the microorganisms. For instance, Erkan et al. [31] suggested that the Pd-TiO2 exhibited higher visible light antibacterial and antifungal activities against E. coli, S. aereus, S. cerevisiae, and A. niger microbes than the

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pure TiO2. In another study, Gondal et al. [32] fabricated irregular shaped Pd-WO3 microcomposite, and they found that Pd-WO3 catalyst exhibited three times higher antibacterial performance towards the disinfection of E. coli than pure WO3. Nevertheless, to our best knowledge, there has been no report available in the literature on photocatalytic, antibacterial and antifungal properties using the Pd-loaded BFO catalyst. Based on the earlier argument, Pd-BFO microcomposite may increase the photocatalytic, antibacterial and antifungal performance. In this regard, coral-like Pd-loaded BFO (Pd-BFO) were synthesized via two-steps hydrothermal approach. The as-synthesized Pd-BFO microcomposites were characterized using X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), high resolution transmittance election microscope (HRTEM), energy dispersive X-ray (EDX), EDX mapping, UV-vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), magnetic hysteresis (M-H) loop and photoluminescence (PL) spectroscopy. The photoactivities of Pd-BFO microcomposite were investigated by the degradation of malachite green (MG) dye and phenol under the exposure of visible light. The ferromagnetic properties of the Pd-BFO microcomposite can be helpful in its removal from the aqueous solution, which led to the high recycling potential. The possible mechanism for enhanced photoactivity of the Pd-BFO microcomposite was investigated using photoelectrochemical, radical scavengers studies and hydroxyl (•OH) radical analyses. Moreover, the intermediate products of phenol were analyzed to obtain detail understanding on the photodegradation mechanism. The calculated electrical energy per order (EEO) as a power efficiency indictor was also estimated to evaluate the electrical energy consumption on the organics photodegradation using as-synthesized Pd-BFO. Finally, for

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the first time, antibacterial and antifungal activity of the Pd-BFO microcomposite was tested towards the E. coli, E. faecalis S. aureus and A. niger under visible light irradiation.

2.0 Material and methods 2.1 Chemical reagents Bismuth nitrate (Bi(NO3)3.5H2O, purity ≥98%) and iron nitrate (Fe(NO3)3.9H2O, purity ≥98%) were bought from Fisher Scientific. Palladium chloride (PdCl2, purity ≥99%), urea (purity ≥99.5%), potassium hydroxide (KOH, purity ≥85%) and malachite green (MG) dye were supplied from Alfa Aesar. Phenol (purity ≥99%), nitric acid (HNO3, purity ≥65%) and ethyl alcohol (purity ≥98%) were provided by ChemSoln. Nutrient Agar (28.0 g/L), nutrient potato dextrose agar (39.0 g/L) and nutrient broth (13.0 g/L) were purchased from HIMEDIA. The radical scavengers, such as potassium iodide (KI, purity ≥99%), catalase (10,000 units/mg), benzoquinone (BQ, purity ≥98%) and isopropanol (IPA, purity ≥99.7%) were purchased from Merck. All reagents applied in the current study were of analytical grade and used without additional purification.

2.2 Hydrothermal preparation of coral-like Pd-BFO microcomposite The coral-like BFO was synthesized at 125oC using hydrothermal synthesis method as reported in our previous report [24]. In the Pd-BFO synthesis, a certain amount of PdCl2 was dissolved separately in 30 mL of methanol and the mixture was placed in the ultrasonic system with 40 kHz/100 W for 15 min at room temperature. Afterwards, as-synthesized 1 g of coral-like BFO was mixed in the above mixture and ultrasonicate for further 20 min. The mixture was then 6

shifted into a Teflon lined reactor for 240 min at 100oC and allowed to cool down naturally after completion of the hydrothermal reaction. Afterwards, the products were collected, washed and dried at 80oC in an oven for 4 h for further characterization. The schematic illustration of PdBFO microcomposite preparation is shown in Scheme 1.

2.3 Characterization XRD patterns were performed using a Philips PW 1820 X-ray diffractometer using Cu Kα radiation with the accelerating voltage of 40 kV and an applied current of 40 mA. The scan rate was 0.02o sec−1 in the 2θ range from 20o to 80o. The FESEM images were taken using a Quanta FEG 450 together with an EDX detector. TEM and HRTEM analyses were executed at an accelerating voltage of 200 kV using a Philips CM-12 and JEOL JEM2100 microscope, respectively. The XPS spectra were performed using an Omicron els 5000 photoelectron spectrometer using Al Kα at 1480 kV as an incident X-ray source. The UV-Vis DRS spectra of the synthesized products were monitored through a Perkin Elmer Lambda 35 in the range from 200 to 800 nm. The M−H loop was assessed via a Quantum Design MPMS-5S vibrating sample magnetometer (VSM). The PL spectra were obtained using a Perkin Elmer S55 spectrometer at the room temperature with 325 nm as excitation wavelength. The Pdo and Fe3+ leaching contents of the Pd-BFO microcomposite were identified through a Perkin Elmer AA-6650 inductively coupled plasma mass spectrometry (ICP-MS).

2.4 Photoactivity experiments The photoactivity performance of the synthesized Pd-BFO microcomposite was investigated by the degradation of aqueous MG dye and phenol solutions under a 105 W visible light (Hazzle, 7

Malaysia). In a typical experiment, 0.1 g sample particle was placed in 100 mL of 10 ppm MG dye or 5 ppm phenol in a glass beaker. During the photocatalytic test, air was provided in the aqueous solution with a continuous flow rate of 2 mL min−1. The aqueous solution was constantly stirred using a magnetic stirrer for 30 min in the dark to attain adsorption-desorption equilibrium. Then, the solution was exposed to the visible light irradiation from a fixed distance of 12 cm above the aqueous suspension surface. After specific time intervals, the samples were taken from the system and the catalyst was separated using a small magnetic bar. The residual MG dye or phenol concentrations in the aqueous solution were monitored via a Hach DR6000 UV-vis spectrophotometer at the maximum absorbance wavelength of 616 nm and 270 nm, respectively. The intermediate products generated during phenol degradation was also investigated using a Shimadzu high performance liquid chromatography (HPLC) equipped with a C18 column (150 mm length × 4.6 mm internal diameter × 5µm particle size) using an UV vis detector at 254 nm wavelength. The mobile phase was water and acetonitrile in the volumetric ratio of 70:30 with a flow rate of 1 mL min-1 and the temperature was 40oC. The recyclability of Pd-BFO microcomposite was evaluated for several successive cycles. After each cycle, the catalyst was recovered with the help of a small magnetic bar, washed thoroughly using distilled water, dried in an oven for 3 h at 120oC and again inserted into the fresh solution of MG or phenol for the succeeding cycle.

2.5 Photoelectrochemical analyses The Photoelectrochemical analyses of the photocatalysts were performed using a threeelectrode quartz cell on the Gamry Interface 1000 electrochemical workstation, with platinum 8

(Pt) wire and Ag/AgCl electrode applied as counter and reference electrodes, respectively. The fluorine-doped tin oxide (FTO) glass loaded with the synthesized photocatalysts was utilized as the working electrodes. The 105 W lamp (Hazzle, Malaysia) provided a source of visible light irradiation. The 0.5 M Na2SO4 aqueous solution was utilized as an electrolyte. The photocurrent was estimated under the visible light irradiation (30 sec on/off cycles) at a 0.4 V vs. Ag/AgCl bias potential. Linear sweep voltammetry (LSV) were measured using the electrodes at a scan rate of 50 mV/sec.

2.6 Detection of active species The significance of photogenerated •OH, •O2− radicals, positively charged h+ and H2O2 in the photocatalytic reaction were studied by using 2 mM of various trapping agents in a similar way as described in the photocatalytic test. The BQ was applied to identify •O2─ radical, catalase was used to find the H2O2, IPA was employed to identify the •OH radical, whereas the importance of h+ was detected by KI. In another test, 5 × 10−4 M terephthalic acid (TA) was added in the 2 × 10−3 M NaOH aqueous solution to confirm the significance of •OH radicals [33]. The TA-PL technique was applied at 315 nm excitation wavelength using a Perkin Elmer LS-55 spectrophotometer.

2.7 Antibacterial and antifungal assays The antimicrobial activity of as-synthesized pure BFO and 2 wt% Pd-BFO microcomposite were investigated against gram-negative bacteria (E. coli ATCC 11778), gram-positive bacteria (E. faecalis ATCC 29212 and S. aureus ATCC 25923) and fungi (A. niger ATCC 1015) under visible light irradiation. Before starting the test, all the glassware and culture solutions were 9

disinfected using an autoclave for 20 min at 120oC. The photocatalytic antimicrobial experiment was executed using the following procedure. Initially, 1 mL of microbes with a fixed 107 CFU mL−1 initial concentration and 99 mL of saline water were inserted in a 250 mL glass beaker and annealed at 37oC temperature for the best microbial growth environment. Afterwards, 0.1 g of microcomposite was inserted into the solution and stirred continuously for 30 min in the dark before the light exposure. After a fixed intervals of time, the microbes were separated, washed and again diluted using a similar process as reported by Zyoud et al. [34]. The 0.5 mL bacterial solutions were poured into the nutrient agar plates, while A. niger solution was placed on the nutrient potato dextrose agar plates. Then, these plates were incubated at 37oC for 24 h. The nutrient agar plates were finally witnessed for the bacterial colonies.

3.0 Results and discussion 3.1 Characteristics of the Pd-BFO Fig. 1a displays the XRD patterns of as-synthesized pure BFO and Pd-BFO microcomposites at varied Pd wt% contents. It can be seen that all the diffraction peaks were sharp, narrow and well crystalline. All the diffraction peaks obtained in the samples can be indexed as a single-phase rhombohedral perovskite structure with the R3c space group (JCPDS file: 86–1518) [11,35]. The calculated analogous lattice parameters of BFO were a = b = 5.5774 Å and c = 13.8667 Å. For Pd-BFO microcomposites, the enlarged portion in Fig. 1b showed the low intensity diffraction peaks appeared at 40.1o (111) and 46.9o (200) can be corresponded to the cubic phase structure of metallic Pd (JCPDS No. 88−2335) [36,37]. The XRD patterns presented that the as-synthesized microcomposites consisted of the two-phase structure of BFO and metallic Pd. In addition, the XRD peaks of BFO did not show any variation in the patterns of 10

Pd-BFO microcomposites, suggesting that the metallic Pd did not substitute in the BFO lattice structure. The morphology of the as-synthesized Pd-BFO microcomposite was investigated using FESEM. Fig. 2a shows that the pure BFO had a high yield generation of coral-like BFO structure. The coral-like nanoparticles were linked with each other to form a three dimensional (3D) structure. The synthesized coral-like BFO particles had a smooth and uniform surface with an average size ranging between 110 to 130 nm. Fig. 2b depicts the microscopic image of 0.5 wt% Pd-BFO microcomposite. Incorporation of metallic Pd did not alter the coral-like morphology and they were uniformly dispersed to the BFO surface. With increasing Pd contents, more Pd nanoparticles were incorporated on the coral-like BFO, and the surface became rough. Further increasing the Pd loadings to 3 wt%, the coral-like BFO surface became rougher as witnessed in Fig. 2e. Fig. 2f presents the EDX analysis of the 2 wt% Pd-BFO microcomposite. The EDX spectrum showed the existence of Bi, Fe, O and Pd, confirming the presence of Pd nanoparticles in the Pd-BFO microcomposite. The presence of C peak was due to the applied carbon tape. Moreover, the EDX mapping and the related elemental distribution of 2 wt% PdBFO microcomposite are displayed in Figs. 2g-h. It can be found that the Bi, Fe, O and Pd elements were homogeneously dispersed, which was of great importance for the charge carrier transfer. The presence of metallic Pd on the BFO surface was also confirmed using TEM and HRTEM analyses as recorded in Fig. 3. As shown in Fig. 3a, TEM image agreed well with the results of FESEM analysis in terms of morphologies and dimensionalities. The microscopic image also witnessed that some tiny Pd nanoparticles dispersed on the 3D coral-like BFO surface as illustrated using the red-colour circles. The metallic Pd nanoparticles were observed in a 11

quasi-sphere structure with the sizes of 15‒25 nm. The HRTEM images of 2 wt% Pd-BFO microcomposite are shown in Figs. 3 b and c. The measure lattice fringe of 0.27 nm and 0.39 nm were assigned to the (110) and (111) crystal planes of perovskite rhombohedral BFO, respectively, while the lattice spacing of 0.22 nm was well coincided with the (011) crystal plane of metallic Pd [15,25,38]. The TEM and HRTEM findings confirmed that the Pd-BFO microcomposite was good crystalline material, which was expected to reveal its photocatalytic potential in catalytic reaction. Chemical states and elemental chemical compositions of the 2 wt% Pd-BFO microcomposite has also executed through XPS analysis. The XPS spectrum for Bi 4f is presented in Fig. 4a. Two peaks appeared at 164.1 and 155.8 eV could be acknowledged as the binding energies of Bi 4f7/2 and Bi 4f5/2, respectively, confirming the form of bismuth (Bi3+) in the Pd-BFO microcomposite [39,40]. Fig. 4b depicts the narrow scan XPS spectrum of Fe 2p. Two peaks centered at 710.8 and 724 eV could be attributed to the Fe 2p3/2 and Fe 2p1/2 respectively, showing the interaction of a spin-orbital. Another peak around 718.8 eV, 8 eV above the main 2p3/2 peak could be associated as a typical Fe oxidation state [41,42]. The presence of the satellite peak specified that the element Fe could be in the Fe3+ valance state on the microcomposite. The O 1s spectrum in Fig. 4c displayed that the two binding energies positioned around 529.6 and 530.6 eV could be credited to the oxygen-metal bond and surface adsorbed O2, respectively [43]. Moreover, the binding energies at 336.7 and 341.8 eV could be assigned to the metallic Palladium (Pd0) as exhibited in Fig. 4d [37,44]. The XPS findings further realized the presence of metallic Pd on the microcomposite, which was well consistent with the XRD and HRTEM findings.

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Fig. 5a displays the UV-vis DRS spectra of pure BFO along with various wt% Pd contents on Pd-BFO microcomposite. It was observed that the introduction of metallic Pd into BFO significantly affected the light absorption abilities of the Pd-BFO microcomposite. In contrast with pure BFO, the Pd-BFO microcomposite showed boosted light absorption abilities up to 800 nm. The improved light absorption abilities of the microcomposite would be beneficial for the visible light photoreaction. The band-gap (Eg) energies were calculated using the following transformation equations based on Kubelka-Munk (K─M) function [33,45]. F(R) = (1─R)2/2R

(1)

Eg = hc/λ

(2)

where F(R) is the K-M function, R is the reflectance (%), Eg is the energy band-gap (eV), hc is the Planck constant (4.135667 × 10−15 eVs), c is the velocity of light (3 × 108 m/s) and λ is the onset absorption wavelength (nm). The energy band gaps for pure BFO, 0.5 wt% Pd-BFO, 1 wt% PdBFO, 2 wt% Pd-BFO and 3 wt% Pd-BFO were determined to be 2.40, 2.28, 2.20, 2.10 and 2.22 eV, respectively as displayed in Fig. 5b. These outcomes suggested that the band gap values of BFO can be altered by different contents of metallic Pd, which caused the enhancement in optical properties than that of pure BFO. However, the band gap value increased with further increasing Pd contents to 3 wt%. This phenomenon might be attributed to additional contents of metallic Pd on the BFO surface, which declined the light penetrating ability, conceivably scattering it [46]. To clarify the trapping, migration and recombination of e−−h+ pair in catalyst, PL method was carried out on pure BFO and Pd-BFO microcomposites (as recorded in Fig. 6). The PL intensities of Pd-BFO microcomposites presented intense diminution than pure BFO, indicating the introduction of Pd onto coral-like BFO surface greatly ameliorated the separation efficiency of e−−h+ pairs [47-49]. Moreover, the PL spectra can be discerned at different wt% Pd on Pd13

BFO microcomposites and their intensities of the microcomposites were strongly influenced by the modification of metallic Pd. The 2 wt% Pd-BFO showed lowest PL intensity among the samples, deducing the photogenerated e−−h+ pair separation was efficiently quenched in the microcomposite. The PL findings suggested that 2 wt% Pd-BFO microcomposite could hold the sufficient capacity in transporting charge carriers, which could greatly conducive for the enhancement of its photoreaction activity. The magnetic properties of pure BFO and 2 wt% Pd-BFO microcomposite were investigated via the M-H loop as depicted in Fig. 7. The saturation magnetization values for BFO and PdBFO microcomposite were measured around 4.84 and 4.06 emu/g, respectively. The magnetic properties of Pd-BFO microcomposite were marginally reduced due to the incorporation of metallic Pd on the BFO surface. However, the Pd-BFO magnetization was still several times higher than other doped BFO literature reports [50–52]. The magnetic separation of Pd-BFO microcomposite was tested by placing a small magnet near a sample bottle as presented in the inset of Fig. 7. Clearly, the sample particle was fast and conveniently separated by the magnet, which demonstrated its technical adaptability and environmental-friendliness in the practical application.

3.2 Degradation of MG dye and phenol using Pd-BFO microcomposite The MG is a cationic dye, which is typically found in the discharged wastewater of several industries, such as paints, textile, paper and pulp, cosmetic, leather and etc. Utilization of MG in extensive applications caused harmful effects on mammals gill, intestine, liver, kidney and gonadotrophic cells [53,54]. Hence, MG dye was taken as a target contaminant to investigate the

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Pd-BFO photocatalytic performance. Fig. 8a displays the variation in UV-vis absorption spectra of MG dye degradation via 2 wt% Pd-BFO microcomposite. The two main spectral peaks centered at 417 and 616 nm were attributed to the azo group (–N=N–) and the benzene ring, respectively [24,55]. The peak intensity gradually decreased and diminished after 240 min, indicating the complete degradation of azo conjugated benzene ring responsible for the characteristic colour of MG dye. Fig. 8b depicts the MG dye degradation over Pd-BFO microcomposite in the presence of visible light. For comparison purposes, the photoactivities of pure BFO and commercial TiO2 were also studied under similar condition. The findings revealed a negligible decrease in the MG dye concentration in the absence of catalyst, indicating that the dye was extremely stable. Under dark condition, Pd-BFO microcomposite also presented a small decrease in the MG dye concentration. On the contrary, the Pd-BFO microcomposite exhibited an improvement in MG degradation with increasing in the Pd contents. It can be seen that 2 wt% Pd-BFO microcomposite displayed an excellent activity (95.7% degradation) in comparison with other wt% Pd-BFO microcomposites. While, the MG degradation for pure BFO and commercial TiO2 was 72.3% and 78.6%, respectively. The enhanced photoactivity could be credited to the appropriate Pd contents that enhanced the e− trapping capacity, which was helpful in the generation and transmission of the generated e−−h+ pair [33,56]. Nonetheless, with further increasing the metallic Pd contents to 3 wt%, deterioration in the photoactivity was observed. This phenomenon could be explicated by the formation of excess metallic Pd may shield the BFO surfaces and obstacle the light absorption, which led to a declination number of photogenerated of e−−h+ pairs by BFO [46]. Moreover, the excessive metallic Pd could cover a part of the active sites on the BFO surface and reduced the adsorption of MG molecules on the 15

BFO surfaces, resulting in a decrease in photoactivity. Similar findings were also reported in the literature [37,57]. Additionally, the plots of ln (Co/C) vs time were revealed to be a linear correlation as shown in Fig. 8c, exhibiting that the photocatalytic reaction follows a first-order kinetic model. The experimental rate constants (k) values were calculated using the following equation.

ln(Co/C) = kt

(3)

where Co was the initial pollutant concentration (ppm) and C was the remaining pollutant concentration (ppm) at time, t (min). Based on the kinetic model, the k values were 0.004588, 0.006148, 0.00755, 0.01259 and 0.009136 min−1 for the pure BFO, 0.5 wt% Pd-BFO, 1 wt% PdBFO, 2 wt% Pd-BFO and 3 wt% Pd-BFO microcomposite, respectively. The 2 wt% Pd-BFO microcomposite had the maximum value of k, which was 2.75 and 1.83 times higher than those of pure BFO as well as commercial TiO2, respectively as displayed in the inset of Fig. 8c. Furthermore, all the Pd-BFO microcomposites also presented improved values of rate constants, which was well consistent with the photodegradation activities. Phenol was chosen as another model pollutant in this study owing to its strong corrosive and highly stable to the light irradiation and biodegradation [58]. Fig. 8d reveals the change in UVvis absorption spectra of phenol in the presence of 2 wt% Pd-BFO microcomposite under exposure of visible light. As can be seen, a major characteristic peak for phenol molecule was recorded at 270 nm. The intensity of absorption peak decreased consecutively as the illumination time extended. After 240 min of light illumination, the peak was entirely vanished, suggesting that the complete degradation of the benzene ring to other aromatic and aliphatic compounds. Fig. 8e depicts the time dependent phenol degradation curves of synthesized Pd-BFO 16

microcomposite under irradiation of visible light. The photoactivities suggested that Pd-BFO microcomposite were also highly effective towards the phenol degradation. With increasing in Pd contents in the BFO microcomposite, the k values ascended initially and reached the maximum of 0.01819 min−1 using 2 wt% Pd-BFO, which was 3.28 and 3.16 times higher than those of pure BFO and commercial TiO2 respectively, and then decreased with further increasing of the Pd contents as exhibited in Fig. 8f. The photocatalytic data proved that the synthesized PdBFO microcomposite could degrade the organics due to its excellent catalytic ability. Chemical oxygen demand (COD) technique was employed to gain insight into the mineralization extent of MG dye and phenol over pure BFO and 2 wt% Pd-BFO microcomposite. Fig. 9 shows the COD removal efficiencies of MG dye and phenol after 240 min of visible light exposure. In comparison to pure BFO, the Pd-BFO microcomposite displayed 1.35 and 1.74 times higher mineralization efficiency towards MG dye and phenol, respectively. The enhanced COD removal of the phenol and MG dye were the consequence from effective degradation of the organic pollutants. Hence, the decline in both COD and UV-vis absorbance values further confirmed the decomposition of phenol and MG dye molecules.

3.3. Intermediates detection upon phenol degradation To further investigate the phenol degradation pathway over 2 wt% Pd-BFO microcomposite under visible light irradiation, intermediate products were also identified. Fig. 10a presents the HPLC profiles during phenol degradation as a function of time. In the HPLC profiles, the phenol presented a distinctive bend at retention time (RT) of 5.6 min. After 240 min of visible light exposure, the peak intensity assigned to phenol was entirely vanished, indicating the complete degradation of phenol. In addition to the initial compound, muconic acid at RT 1.4 min, 17

pyrogallol at RT 1.7 min, resorcinol at RT 2.6 min, and benzoquinone at RT 3.3 min were the generated intermediate products, when correlated with the standard chemicals. These findings were also comparable with other literature findings [59–61]. The HPLC data suggested that the phenol degradation pathway involved complex multistage processes as exhibited in Fig. 10b. The phenol was initially oxidized to various aromatic intermediate products. These aromatic intermediates were then further oxidized to induce ring cleavage to produce aliphatic acids such as muconic, maleic, oxalic, formic and eventually converted to the CO2 and H2O [62]. Moreover, the initial solution pH in this study shifted from pH 5.6 to pH 4.5, revealing that the acidic compounds with the smaller molecular weight were produced during the photodegradation of phenol.

3.4 Recyclability of the Pd-BFO microcomposite Stability and recyclability of any catalyst played an integral role in practical applications. Hence, in the present study, recycling experiment was performed to ascertain the extent of reusability of the 2 wt% Pd-BFO microcomposite. Fig. 11a depicts that the Pd-BFO microcomposite still maintained high photoactivity even after the sixth cycle. The photocatalytic activity of the Pd-BFO microcomposite for MG dye degradation reduced from 95.7% to 89.8% after six cycling runs. Similarly, phenol degradation slightly declined from 100% to 94.2%. The leaching of different ions, such as Fe3+ and Pd0 in Pd-BFO microcomposite were also estimated for each cycle using an ICP-MS technique. The results indicated no leaching for Fe3+ ions, which was also consistent with the literature report available on BFO [63,64]. Small concentration of metallic Pdo was detected in the range from 0.0327 ppm to 0.0237 ppm during the six recycling experiments, however, it was still lower than the literature reports [65,66]. 18

The stability of reused Pd-BFO microcomposite was further analyzed. Fig. 11b clearly showed that the XRD pattern of recycled Pd-BFO microcomposite was almost unchanged, suggesting the high structural crystallinity. Moreover, the coral-like Pd-BFO morphology was well maintained under the microscopic image as recorded in Fig. 11c. Figs. 11d-g depict the results of EDX mapping on recycled Pd-BFO microcomposite. The results demonstrated that the Pd, Bi, Fe and O elements were dispersed on the Pd-BFO. Thus, the synthesized magnetic PdBFO microcomposite can be regarded as a promising candidate of recyclable and stable catalysts to eliminate the environmental organics during practical applications.

3.5 Mechanism underlying the photoactivity of Pd-BFO 3.5.1 Photoelectrochemical analyses The charge carriers generation and separation/transfer are the key factors in the photocatalytic process. Herein, the separation/transfer of e−−h+ pairs using as-synthesized photocatalysts were investigated under on/off circulation of a 105 W visible light by transient photocurrent measurements and presented in Fig. 12a. It can be clearly seen that Pd-BFO microcomposite possessed the highest current intensity of 2.59 µA, which was 1.6 times higher than the pure BFO. These results explicitly revealed that the loaded metallic Pd on the BFO surface would highly expedite the generation and separation/transfer of charge carriers, which validated the Pd-BFO microcomposite had an improved photocatalytic ability to degrade the organic pollutants. Moreover, linear sweep voltammetry (LSV) was also performed to compare the electrochemical proton reduction capabilities of pure BFO and Pd-BFO microcomposite. Fig. 12b depicts the cathodic current enhanced sharply with the increasing applied potential, which 19

suggested that the synthesized photocatalysts had the capabilities to perform the proton reduction reaction [67]. As presented in Fig. 12b, Pd-BFO microcomposite exhibited a lowest overpotential for the proton reduction and a highest photocurrent density compared with the pure BFO, signifying the incorporation of metallic Pd on BFO surface can offer distinguished kinetics for reduction reaction [67,68]. The higher current density and small overpotential exhibited that the Pd-BFO microcomposite had highly accelerated the transfer of charge carriers [69].

3.5.2 Role of the active species To investigate the degradation mechanism of MG dye and phenol using Pd-BFO microcomposite, principal reactive species generated during the degradation process was determined using the radical trapping experiment. In the radical trapping experiment, IPA, BQ, KI and catalase were applied as a trapping agents for •OH, •O2−, h+ and H2O2, respectively. Fig. 13a shows the photodegradation efficiencies of MG dye using various scavengers after 240 min of light irradiation. It can be found that MG dye degradation was considerably declined with the addition of KI, IPA and catalase, indicating the importance roles of h+, •OH and H2O2 in PdBFO microcomposite photocatalytic system. However, the insertion of BQ did not cause any significant decrease in the photoactivity of Pd-BFO microcomposite, signifying the •O2− radicals only had a slight influence on the MG dye degradation. Inhibitory effect of various scavengers towards the degradation of phenol was also studied. Fig. 13b displays that the degradation of phenol also followed a similar trend using same radical scavengers. With the addition of KI, IPA and catalase, the phenol degradation efficiency significantly decreased, while the introduction of BQ also had a slight impact on the photocatalytic activity of Pd-BFO microcomposite. These results clearly stated that the MG dye

20

and phenol degradation mechanism was mainly happened because of the collective efforts of •OH radicals, h+ and H2O2 generated by Pd-BFO microcomposite.

3.5.3 Hydroxyl (•OH) radical generation Based on the active species results, •OH radical was one of the key reactive radicals in the Pd-BFO photodegradation system. The TA-PL measurements were executed to identify the generation of •OH radicals on the Pd-BFO microcomposite. Fig. 13c presents the PL spectra changes witnessed using pure BFO and Pd-BFO microcomposite excited at 315 nm. The peak appearance around 425 nm implied the presence of 2-hydroxyterephthalic acid (2-HTA), which was formed through a chemical reaction among the •OH radical and TA molecules. All the PdBFO microcomposite presented superior PL intensities as compared to pure BFO, which proposed the incorporation of metallic Pd on BFO was an ideal route to enhance the •OH radical formation. Additionally, the 2-HTA intensities of Pd-BFO microcomposite were also varied with Pd contents. In particular, 2 wt% Pd-BFO microcomposite generated the highest peak intensity of 2-HTA, suggesting that the maximum generation of •OH radicals to partake in the photoreaction. To further detect the generation of •OH radical, 2-HTA spectra changes with time over 2 wt% Pd-BFO microcomposite was also studied (as illustrated in Fig. 12d). As expected, 2-HTA at 425 nm exhibited dramatically enhanced fluorescence intensity when prolonged the irradiation time. This result not only reflect more number of •OH radicals generated with exist of the Pd-BFO microcomposite, but also witnessed the •OH radical was indeed participated in the photodegradation reaction (Bharathkumar et al., 2015).

21

3.5.4 Photocatalytic mechanism In order to study the separation of e−−h+ pairs for the phenol and MG dye degradation under the irradiation of visible light, it was essential to obtain the valance band (VB) and conduction band (CB) positions of the Pd-BFO microcomposite at the point of zero charge, which can be predicted by the following equations [70,71]. EVB = X − Ee + 0.5Eg

(4)

ECB = EVB − Eg

(5)

where EVB is the VB edge potential (eV), ECB is the CB edge potential (eV), Ee is the free electron energy on hydrogen scale (around 4.5 eV), Eg is the band gap energy of catalyst (eV) and X was the absolute electronegativity of the BFO, which is around 6.04 eV [70]. The calculated ECB and EVB edge potentials for BFO were 0.49 and 2.71 eV, respectively. Based on the ECB and EVB edge potential values and the above discussed analyses, a potential degradation mechanism over PdBFO microcomposite could be proposed and shown in Scheme 2. Upon visible light irradiation, the photogenerated e− in the CB of BFO was excited and moved to VB, leaving the equal amount of h+ in the VB. . For pure BFO, the photogenerated e−−h+ pairs recombined quickly, resulting in the lower photocatalytic performance. Upon the incorporation of metallic Pd nanoparticles on BFO surface, the photogenerated e− in CB of BFO would rapidly transferred to the metallic Pd, through the interface generated among the BFO and Pd nanoparticles. The introduction of metallic Pd as co-catalyst would highly stimulate the separation/transfer of e−−h+ pairs due to the lower energy position of metallic Pd (+1.01 eV vs NHE) [72] than those of the CB edge potential of BFO (+0.49 eV vs NHE) [38]. Subsequently, the photogenerated e− in the BFO were trapped by absorbed O2 molecules to produce the H2O2 owing to the greater CB edge potential than those of the standard redox potential Eo (O2/H2O2) (0.682 eV vs NHE) [59,73]. In the 22

meantime, the photogenerated h+ in the VB of BFO oxidize the –OH ion on the surface of PdBFO microcomposites to produce •OH radicals Eo (•OH/H2O) (+2.68 eV vs NHE) [73,74]. The produced H2O2, h+ and •OH radical were the main reactive agents, which vigorously partook in the MG dye and phenol degradation process. In this way, the e−−h+ pairs were efficiently separated and take part in the photocatalytic process under visible light irradiation.

3.6 Energy requirement for the degradation of organics Besides the desired photoactivity, there were several important factors should be considered in the aqueous wastewater treatment, such as operating cost, safety, regulations, and etc [75]. As the photocatalysis is an electric-energy intensive process, electrical energy consumption can represent a main fraction of the operating cost. Bolton et al. [76] proposed a useful concept of EEO for pseudo-first-order kinetic reactions to calculate the electrical energy consumed during the photocatalytic process. The EEO can be expressed as the electrical energy required in kilowatt per hour (kWh) to decompose a pollutant by one order of degree in 1 m3 wastewater. Typically, EEO values can be estimated by the equations as below [77,78].

EEO 

log(

P  t 1000 C V  60  (log i ) Cf

(6)

Ci )  k t Cf

(7)

where, EEO is the electrical energy per order (kWh m−3 order−1), P is the electric power entering the photocatalytic system (kW), t is the light irradiation time (min), V is the volume (L) of wastewater, k is the pseudo first order rate constant (min−1), Ci and Cf are the initial and final concentration of pollutants (ppm). From equations (6) and (7), the EEO can be written as follow: 23

E EO 

38.4  P V k

(8)

By considering the total energy entering the photocatalytic system was consistent for each catalyst during the degradation reaction, the estimated EEO values were listed in Table 1. The obtained EEO values for MG dye and phenol degradation using Pd-BFO microcomposite were 3200 and 2115.38 kWhm−3 order−1, respectively. These values exhibited that the Pd-BFO microcomposite decreased the EEO values for the MG dye and phenol degradation as compared to those of pure BFO as well as commercial TiO2. The higher EEO value for MG dye than the phenol could be owing to its complex molecular structure. A few reports in the literature had also applied this useful figure of merit for different organic pollutants to make a cost comparison. For instance, Lv et al., [79] investigated the degradation of methyl orange using CdS-TiO2 and AuCdS-TiO2, where EEO value was lowered from 57900 to 11552 kWhm−3 order−1 after loading Au nanoparticles on CdS-ZnO surface. Li et al. [80] had also studied the degradation of various organic pollutants, including methyl orange, amoxicillin and 3-chlorophenol by TiO2 catalysts. These organic pollutants were effectively degraded with EEO values ranged from 73100 to 25200 kWhm−3order−1 depended upon the initial concentration. It can be seen that the calculated EEO values of Pd-BFO microcomposite in the current study were considerably lower than those presented in the literature, which further confirmed the Pd-BFO microcomposite was a potential cost-effective catalyst for the organics degradation.

3.7 Antibacterial and antifungal activities E. coli (gram-negative), S. aureus (gram-positive) and E. faecalis (gram-positive) are typically classified as the bacterial organism, while A. niger is known as a fungus. These 24

microorganisms were extremely causative agents for several waterborne and food sicknesses such as cholera, typhoid, diarrhea, urinary tract infection, blood infection, food poising, hepatitis and etc [81,82]. Therefore, the antibacterial and antifungal performances of pure BFO and 2 wt% Pd-BFO microcomposite were investigated towards E. coli, S. aureus, E. faecalis and A. niger microorganisms in the presence of visible light. Fig. 14 presents the antimicrobial performance of pure BFO and Pd-BFO microcomposite as a function of time irradiation. It can be observed that there was a small quantity of the microbial cells was still survived in the presence of pure BFO. Conversely, the antimicrobial activity of Pd-BFO microcomposite was apparently improved with more than half of microbial colonies were disinfected after 60 min visible light irradiation. When the system was further irradiated for 180 min, all the microbial colonies were almost completely disinfected. Compared with those in the blanks, the microbial colonies did not visibly reduce their survivals under photolysis condition (as recorded in Figs. S1). When the experiment was performed under the dark condition, negligible antimicrobial activities were also witnessed in Fig. S2. The photocatalytic performance of the metallic Pd loaded on BFO will play a vital role in the antimicrobial activity in conjunction with its heterojunction effect and light harvesting property. The UV light boosted the antimicrobial activities by suppressing e- and h+ using Pd dopant and released highly reactive species, such as h+, •OH and H2O2 molecules and attacked at different locations of bacterial cells [79,80,81]. For example, the h+ and •OH radical would attack on the cell wall, and H2O2 would permeate the cell membranes and further breakdown the membrane integrity molecules [82,83]. In this way, the bacteria cells death and reduced the survival possibility. Finally, Table 2 summarizes the comparison of the current research with earlier published reports on the degradation of aqueous organics and antimicrobial activity with 25

other noble metals-loaded catalysts. As can be seen, the Pd-BFO system exhibited a better performance than most of other reported noble metal-based catalysts. It could also expectable that the photocatalytic and antimicrobial performance can be further improved by using natural sunlight in this Pd-BFO system.

4.0 Conclusion In summary, coral-like Pd-BFO at varied wt% Pd contents were fabricated via two-steps hydrothermal process and analyzed using various characterization approaches. The examination of visible light degradation of MG dye and phenol exhibited that the Pd-BFO activity was highly influenced by the Pd contents. Particularly, the 2 wt% Pd-BFO microcomposite exhibited the best photoactivity than the other Pd-BFO microcomposite, pure BFO and commercial TiO2. The COD results proved the unselective mineralization of MG dye and phenol molecules. The HPLC data also signified the effective and complete degradation of phenol and its intermediates. The outstanding photoactivities of Pd-BFO microcomposite were attributed to the effective separation of e−−h+ pair, which led to the enormous generation of the ROSs to take part in the reaction. Additionally, the Pd-BFO microcomposite still sustained high photoactivity and showed a minimal leakage of Pd0 ion after six cyclic runs. The efficient e−−h+ pair separation/transfer in the Pd-BFO microcomposite was validated using photoelectrochemical and PL analyses. By comparing the energy consumption, the Pd-BFO microcomposite was more economical and effectively on degradation of organic pollutants than pure BFO and commercial TiO2. The Pd-BFO microcomposite also exhibited outstanding antibacterial and antifungal activities towards E. coli, S. aureus, E. faecalis and Aspergillus niger microbes. This work provided a foundation for further development of magnetic Pd-BFO microcomposite with ease of 26

recovery and reusability properties towards more effective organics and antimicrobial abatements in the large-scale wastewater treatment plants.

Conflict of interest The authors do not have any conflicts of interest.

Acknowledgements The financial supports from Universiti Tunku Abdul Rahman (UTARRF/2019-C1/L03), Ministry

of

Higher

Education

of

Malaysia

(FRGS/1/2016/TK02/UTAR/02/1

and

FRGS/1/2019/TK02/UTAR/02/4) and Research funds of The Guangxi Key Laboratory of Theory and Technology for Environmental Pollution Control (1801K012 and 1801K013) were gratefully acknowledged.

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Figure captions Fig. 1. XRD patterns of (a) pure BFO and Pd-BFO microcomposites at varied wt% Pd contents and (b) narrow-scan diffraction patterns from 39o to 48o. Fig. 2. FESEM images of (a) pure BFO, (b) 0.5 wt% Pd-BFO, (c) 1 wt% Pd-BFO, (d) 2 wt% PdBFO, (e) 3 wt% Pd-BFO, (f) EDX spectra of 2 wt% Pd-BFO microcomposite, EDX elemental mapping, (g) Bi, (h) Fe, (i) O and (j) Pd. Fig. 3. (a) TEM image, (b) and (c) HRTEM image of 2 wt% Pd-BFO microcomposite. Fig. 4. XPS spectra of 2 wt% Pd-BFO microcomposite (a) Bi 4f peaks, (b) Fe 2p peaks, (c) O 1s 40

peaks and (d) Pd 3d peaks. Fig. 5. (a) UV–vis DRS spectra for pure BFO and Pd-BFO microcomposites using various Pd wt% contents and (b) plot of (F(R)hv)1/2 vs the photon energy (E) of pure BFO and Pd-BFO microcomposites. Fig. 6. PL spectra of pure BFO and various wt% Pd contents on Pd-BFO microcomposites. Fig. 7. Room temperature M−H loop on pure BFO and 2 wt% Pd-BFO microcomposite, inset of the figure is the magnetic separation of 2 wt% Pd-BFO microcomposite. Fig. 8. UV-vis absorption spectra of 2 wt% Pd-BFO microcomposite (a) MG dye, (d) phenol, photoactivities of Pd-BFO microcomposite under different experimental conditions (b) MG dye, (e) phenol, corresponding first-order plot and rate constant k of (c) MG dye and f phenol. Fig. 9. COD removal efficiencies of MG dye and phenol by pure BFO and 2 wt% Pd-BFO microcomposite. Fig. 10. (a) HPLC profiles of phenol solution treated by the 2 wt% Pd-BFO microcomposite as a function of time irradiation and (b) phenol degradation with reaction pathway during the photocatalytic reaction. Fig. 11. (a) Degradation efficiencies and leakage of Pd0 ions using 2 wt% Pd-BFO microcomposite over six consecutive recycling runs, (b) XRD patterns of the recycled 2 wt% PdBFO microcomposite, (c) FESEM image of recycled 2 wt% Pd-BFO microcomposite, EDX mapping of recycled 2 wt% Pd-BFO microcomposite (d) Bi, (e) Fe, (f) O and (g) Pd. Fig. 12. (a) Transient-photocurrent curves for the pure BFO, 2 wt% Pd-BFO microcomposite and commercial TiO2 in 0.5 M Na2SO4 electrolyte solution under visible light irradiation and (b)

41

LSV curves for pure BFO, 2 wt% Pd-BFO microcomposite and commercial TiO2 in 0.5 M Na2SO4 solution. Fig. 13. Effects of different radical scavengers on the photocatalytic degradation of organic pollutants using 2 wt% Pd-BFO microcomposite (a) MG dye, (b) phenol, (c) fluorescence spectra of 2- HTA solution over pure BFO and Pd-BFO microcomposites at different Pd wt% contents and (d) fluorescence spectra changes as a function of time irradiation over 2 wt% PdBFO microcomposite. Fig. 14. Antibacterial and antifungal activities of pure BFO and 2 wt% Pd-BFO microcomposite against E. coli, S. aureus, E. faecalis and A. niger microorganisms on agar plates.

Scheme 1. Schematic diagram of the fabrication of coral-like Pd-BFO microcomposite. Scheme 2. Schematic illustration of photocatalytic degradation and antimicrobial mechanism of Pd-BFO microcomposite under exposure of visible light.

Table 1. EEO values of phenol and MG dye degradation systems using various catalysts. Table 2. Tabulated comparison of this study with photocatalytic and antimicrobial activities of noble metals-loaded catalysts.

42

Fig. 1

43

Fig. 2

44

Fig. 3

45

Fig. 4 46

Fig. 5

47

Fig. 6

48

Fig. 7

49

Fig. 8

50

Fig. 9

51

Fig. 10

52

Fig. 11 53

Fig. 12

54

Fig. 13 55

Fig. 14 56

Scheme 1.

57

Scheme 2.

58

Table 1. Pollutant MG dye

Phenol

Catalyst Pure BFO Pd-BFO Commercial TiO2 Pure BFO Pd-BFO Commercial TiO2

k (min−1) 0.0046 0.0126 0.0069 0.0054 0.0182 0.0057

59

EEO (kWhm−3 order−1) 8765.21 3200 5843.47 7466.66 2115.38 7073.68

Table 2 Catalyst Ag-g-C3N4

Ag-ZnO

Ag-TiO2

AuFe3O4/TiO2

Pd-BFO

Morphology

Pollutant

Experimental condition Sheets Amaranth 5 ppm of Amaranth; 0.5 g/L of catalyst loading; 500 W Xe lamp Fiber-like Phenol 5 ppm of phenol; 1.0 g/L catalyst loading; direct sunlight irradiation Rod-like MB, MO 20 ppm of pollutants; 2 g/L of catalyst loading; direct sunlight irradiation Microspheres MB 8 ppm of MB; 1.25 g/L of catalyst loading; 4 × 9 W UV light Coral-like MG, 10 ppm of MG; phenol 5 ppm of phenol; 1 g/L of catalyst loading; 105 W CFL lamp

Degradation activity

Microbe Type S. aureus

Antimicrobial activity 99.4% inactivation towards S. aureus within 4 h

85% removal achieved within 180 min

E. coli, S. aureus

Zone of inhibition (mm): 8.6 (E. coli) and 7.2 (S. aureus)

[88]

79.4% MB removal achieved within 150 min 78.5% MO removal achieved within 150 min 80% MB removal achieved within 240 min

S. aureus

99.9% inactivation towards S. aureus within 24 h

[89]

E. coli

89.3% inactivation towards E. coli within 120 min

[90]

95.6% MG removal achieved within 240 min 100% phenol removal achieved within 240 min

E. coli, S. aureus, E. faecalis, A. niger

More than 99% Current inactivation towards study all the selected microorganisms within 180 min

75% removal achieved within 240 min

60

Ref [87]

Graphical abstract

61

Highlights 

Coral-like Pd-BFO has been synthesized using two-steps hydrothermal method.



Loaded Pd could restrain charge recombination and prolong light absorption range.



Pd-BFO can be efficiently applied for visible light degradation of MG and phenol.



Pd-BFO can be magnetically separated after treatment without any ions leakage.



Pd-BFO can be also effectively disinfected several microbial in wastewater.

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