Bismuth-Doped Nano Zerovalent Iron: A Novel Catalyst for Chloramphenicol Degradation and Hydrogen Production.

In this study, we showed that doping bismuth (Bi) at the surface of Fe0 (Bi/Fe0, bimetallic iron system)-synthesized by a simple borohydride reduction method-can considerably accelerate the reductive degradation of chloramphenicol (CHP). At a reaction time of 12 min, 62, 68, 74, 95, and 82% degradation of CHP was achieved with Fe0, Bi/Fe0-1 [1% (w/w) of Bi], Bi/Fe0-3 [3% (w/w) of Bi], Bi/Fe0-5 [5% (w/w) of Bi], and Bi/Fe0-8 [8% (w/w) of Bi], respectively. Further improvements in the degradation efficiency of CHP were observed by combining the peroxymonosulfate (HSO5 -) with Bi/Fe0-5 (i.e., 81% by Bi/Fe0-5 and 98% by the Bi/Fe0-5/HSO5 - system at 8 min of treatment). Interestingly, both Fe0 and Bi/Fe0-5 showed effective H2 production under dark conditions that reached 544 and 712 μM by Fe0 and Bi/Fe0-5, respectively, in 70 mL of aqueous solution containing 0.07 g (i.e., at 1 g L-1 concentration) of the catalyst at ambient temperature.

Introduction

Environmental pollution and energy crises are the key issues that need urgent and global solutions. Among various environmental and water pollutants, the pharmaceuticals in the form of antibiotics pose a serious threat to human beings and other living species. Antibiotics are frequently used for inactivating and killing of micro-organisms. More than 250 different antibiotics/pharmaceuticals are extensively used for the treatment of humans, animal husbandry, and variety of plant diseases.[1,2] They, therefore, frequently enter into the fresh water bodies from pharmaceuticals industries, hospitals, husbandries, and homes. However, the presence of these antibiotics/pharmaceuticals in the fresh water bodies poses serious problems to terrestrial and aquatic life.[3] Among these widely detected antibiotics, chloramphenicol (CHP) is a broad spectrum antibiotic, used for deactivation of both Gram-positive and Gram-negative bacteria.[4] It has been reported that CHP is not fully digested in the human body and is defecated in urine and feces.[5] Because of its low molecular weight and high hydrophilicity, CHP cannot be completely eliminated from water by conventional methods such as coagulation, sedimentation, and filtration.[6,7] Efforts are going on continuously for the development of effective degradation methods of antibiotics. These methods mainly include advanced oxidation processes (AOPs) such as Fenton, photo-Fenton, and UV/H2O2, which mainly produce hydroxyl radicals (•OH) for the removal of these contaminants.[8−10] However, recently, sulfate radical (SO4•–)-based AOPs have gained researcher’s interest for the degradation of organic contaminants because of their higher removal efficiency. SO4•–, having a redox potential of 2.5 V, is a strong and selective oxidant. Generally, peroxymonosulfate (PMS, HSO5–) and persulfate (S2O82–) are used as oxidants for the production of SO4•–. PMS could produce both •OH and SO4•– on activation. PMS can be activated by UV, heat, and transition metals.[11−14]

As far as the energy crisis is concerned, the green energy production in the form of H2 from water splitting could provide a possible solution to this problem. H2 is an ideal, clean and renewable energy source with outstanding advantages of high energy storage densities and environment-friendly (CO2-free) reaction products. Different methods such as water gas shift (CO + H2O ↔ CO2 + H2) and biogas dry reforming reactions through Pt/Ni-supported semiconductor catalysts have been applied for hydrogen production.[15,16] However, the replacement of the precious noble metal-based catalysts by the abundantly available cheap iron for economic production of H2 is the need of the day.

Recently, nano zerovalent metals have emerged as inexpensive and effective activators for the activation of HSO5–.[17−19] In this regard, nano zerovalent iron (Fe0) has gained considerable attraction because of its high reactivity for PMS activation and efficient reductive degradation of different contaminants such as heavy metals, pharmaceuticals, dyes, and pesticides.[20−22] Moreover, the application of Fe0 for the production of H2 has also been well documented.[23−25] The H2 production from water decomposition by Fe0 involves reduction of water and oxidation of Fe0, that is, a redox reaction (reaction ).[23]

Reaction could also be considered as hydrogen evolution reaction (HER). The mechanism of HER is reported to be consisting of (i) formation of atomic H at the surface of catalyst, (ii) combination of H-atoms, and finally (iii) desorption of H2 molecules.[23] The precipitate, Fe(OH)2, is unstable under anaerobic conditions and may undergo a disproportionation reaction at temperatures >80 °C, that is, Schikorr reaction, leading to the formation of hydrogen and magnetite (Fe3O4) (reaction ).[23,24,26]

Fe(OH)2 conversion to hydrogen and magnetite via reaction is generally very slow or even not at all at room temperature. However, this reaction can be catalyzed by Fe0.[24,27,28]

Thus, the overall redox reaction of water decomposition by Fe0 can be summarized in the form of reaction .

In summary, the overall stoichiometry of H2 formation to Fe0 could vary from 1:1 (reaction ) to 4:3 (reaction ). As the aforementioned HER could play an important role in clean fuel production through H2 generation, recently, Fe0 has received extensive attention of the researchers for its possible use for green energy production in addition to its use in water treatment processes for the effective removal of contaminants.

Furthermore, the magnetic characteristics of Fe0 make it more attentional because of its easy separating and recycling ability from the aqueous solution under the external magnetic field.[29] However, because of its small size and surface effects, Fe0 can easily be agglomerated via van der Waals and magnetically attractive forces, leading to the formation of bigger particles with significant loss in their reactivity.[29] In order to overcome this problem, many attempts have been made, for example, doping of Fe0 with a second metal such as Pd, Au, Ag, Pt, Bi, and Cu.[30,31]

In the present study, we paid attention to synthesize bismuth-doped Fe0 (Bi/Fe0) as a novel and effective catalyst for the degradation of CHP from water as well as H2 production from reductive decomposition of water. Bismuth (Bi) was selected as a dopant on account of its exceptional properties such as highly anisotropic Fermi surface, small effective mass, low carrier density, and long carrier mean free path.[30] Therefore, it is anticipated that the synthesized Bi/Fe0 would effectively improve the reactivity of Fe0 toward reductive degradation of CHP and activation of PMS. To date, HSO5– has been mainly activated by Fe2+.[32,33] However, recently, the activation of HSO5– by Fe0 has shown superior performance because of the maintainable formation of Fe2+.[34]

Experimental Section

Materials

Iron(II) sulfate (FeSO4·7H2O) was purchased from Sigma-Aldrich. CHP (C11H12Cl2N2O5), oxone (KHSO5·0.5KHSO4·0.5K2SO4), ammonium chloride (NH4Cl), methanol, and ethanol were provided by Sigma-Aldrich; sodiumborohydride (NaBH4) and perchloric acid (HClO4) 60% were purchased from Daejung, and bismuth nitrate (BiNO3) was provided by Acros. All chemicals were used as received without further purification.

Synthesis of Fe0 and Bi/Fe0 Nanomaterials

The Fe0 and Bi/Fe0 nanomaterials were synthesized by a simple borohydride reduction method. For Fe0, 1 g of NaBH4 was dissolved in 40 mL of deionized (DI) water, and ferrous sulfate solution was prepared by dissolving 1 g of FeSO4 in 25 mL of 25 vol % aqueous ethanol solution (18.75 mL DI water and 6.25 mL ethanol). The NaBH4 solution was taken in a burette and that of FeSO4 was transferred to the three-neck flask. The NaBH4 solution was poured in a dropwise manner to the FeSO4 solution under nitrogen gas purging with continuous stirring. The appearance of black particles indicated the formation of Fe0. After completion of the reaction, the solution was stirred for 20 min. The Fe0 particles were separated from the solution by centrifugation at 5000 rpm. The particles obtained were washed three times with ethanol to remove the impurities. The reduction of FeSO4 by NaBH4 occurs in accordance with reaction .[29]

For the synthesis of Bi/Fe0 nanoparticles, the same process was followed except that the dopant precursor, bismuth nitrate (BiNO3), was added in different proportions to FeSO4 solution and stirred for 30 min for thorough mixing. The Bi/Fe0 nanoparticles were then separated and washed three times with ethanol. The as-synthesized bare Fe0 and Bi/Fe0 nanoparticles were dried in a vacuum oven to avoid their oxidation. The samples were stored in ethanol prior to their characterization and testing. Different weight % of Bi, that is, 1, 3, 5, and 8 wt %, were doped at the surface of Fe0 and were designated as Bi/Fe0-1, Bi/Fe0-3, Bi/Fe0-5, and Bi/Fe0-8, respectively.

Characterization

The morphologies of the as-synthesized Fe0 and Bi/Fe0 nanoparticles were investigated by scanning electron microscopy (SEM) (JEOL, JSM-5910) and transmission electron microscopy (TEM) (JEM-2011F, JEOL, Japan). Energy-dispersive X-ray (EDX) spectroscopy analysis was conducted by EDX (EX-2300BU, Jeol) for elemental analysis of the as-synthesized nanoparticles. To investigate the crystallinity of the as-synthesized nanoparticles, X-ray diffraction (XRD, PANalytical) analysis was performed using a Rigaku D/max-RB instrument with Cu Kα radiation (λ = 1.54 Å), operated at 45 kV and 100 mA. Thermogravimetric analysis (TGA) was conducted on a TG-DTA, PerkinElmer, system. The ultraviolet diffuse reflectance spectroscopy was performed on a PerkinElmer Lambda 35 UV–vis spectrometer (Shelton, CT, USA) using BaSO4 as a reference. X-ray photoelectron spectroscopy (XPS) was conducted through PHI-5300, ESCA in which Al Kα was used as the source of X-ray excitation for studying the elemental composition and oxidation states of Fe and Bi. A quantachrome analyzer was used during the investigation of Brunauer–Emmett–Teller (BET) specific surface areas (SBET), BJH pore size, and pore volume of the Fe0 and Bi/Fe0 nanoparticles.

Catalytic Activities of Fe0 and Bi/Fe0 Nanoparticles and the Analytical Procedure

The catalytic performance of Fe0 and Bi/Fe0 was investigated by the degradation of CHP in the presence and absence of HSO5–. The degradation experiments were performed in a 50 mL beaker containing 30 mL of reaction mixture. Unless stated otherwise, the initial concentration of CHP was 10 mg L–1, the catalyst load was 1 g L–1, pH = 6.8 and, when used, HSO5– was 1 mM. To ensure uniform mixing, the beaker was placed on a magnetic stirrer with constant stirring. The pH of the mixture was tuned with either perchloric acid (HClO4) or sodium hydroxide (NaOH) when required. At predetermined time intervals, 0.5 mL of sample was taken for qualitative/quantitative analysis. The catalyst was filtered out before analyzing the samples. High-performance liquid chromatography (HPLC, Agilent 1200 series) was used for quantification of CHP. HPLC was equipped with a quaternary pump, a Eclipse XDB-C18 column (150 mm × 4.6 mm, particle size 5 μm), and a variable wavelength detector set at 270 nm. The mobile phase was pure methanol and water in the v/v ratio of 70:30 at a flow rate of 1 mL min–1 under isocratic mode. The injection volume was 20 μL, and the column temperature was 25 °C.

The cumulative concentrations of SO4•– and •OH during the reaction process were quantified indirectly using hydroxybenzoic acid (HBA) and benzoic acid (BA) as chemical probes of SO4•–[35] and •OH,[36] respectively. It has been reported that SO4•– on reaction with HBA results in the formation of hydroquinone which immediately transforms to stable 1,4-benzoquinone (BQ) in the presence of excess of PMS, while •OH on reaction with BA gives p-HBA.[29] Each HBA and BA were used at 0.5 mM concentration (to ensure the complete conversion of SO4•– and •OH into BQ and p-HBA, respectively) in the presence of 1 mM HSO5– and 1 g L–1 catalyst load, at pH = 6.8. At 2 min time intervals, 0.5 mL of samples were withdrawn, filtrated, and quenched with 100 μL EtOH. The generated BQ and p-HBA were detected and quantified by the same HPLC used for CHP quantification. The mobile phase was a mixture of acetonitrile and ultrapure water (50:50, v/v) at a flow rate of 1.0 mL min–1 for both BQ and p-HBA. The detection wavelength was set at 246 nm for BQ and 265 nm p-HBA.[37] The amount of generated BQ and p-HBA was thus used to calculate the concentration of SO4•– and •OH in the reaction system. The concentrations of SO4•– and •OH were calculated according to the stoichiometric ratio of SO4•– and BQ (1:1) and •OH and p-HBA (5.9:1), respectively.[38]

The qualitative determination of the degradation products (DPs) of CHP was carried out by using liquid chromatography- mass spectrometry (LC/MS, Agilent Technologies), equipped with a C-18 column (150 × 4.6 mm, 1.7 μm particle size). The mass spectrometer was the Quattro Premier XE tandem quadrupole mass spectrometer having an electrospray ionization source. The mobile phase was composed of acetonitrile and methanol (1:3 v/v) with a flow rate of 1.5 mL min–1. Electrospray interface was used at full scan mode from 50 to 500 m/z. The other mass constraints were established as follows: source temperature 110 °C, capillary voltage 2.2 kV, desolvation gas flow rate 635 L h–1, and desolvation temperature 280 °C.

The recovery of the catalysts was done by using a magnet to separate catalysts from the aqueous media. The catalyst was then dried at 60 °C for 1 h and then applied for the next run.

Toxicological Assessment

To examine the toxicological (eco-toxicity) assessment of CHP and its DPs, Ecological Structure Activity Relationship (ECOSAR) build on computerized structure activity relationships was applied. According to the ECOSAR program, the acute and chronic toxicities of CHP and its DPs were estimated toward fish, daphnia, and green algae.[1] The acute toxicities (related to the adverse effects of a substance that result either from a single exposure or from multiple exposures in a short period of time) were expressed in terms of LC50 and EC50, while chronic toxicities (defined as the adverse effects that result from long-term exposure) were measured from ChV values. LC50 can be defined as the concentration of pollutant that can demise 50% of daphnia and fish when applied for 48 and 96 h, respectively, whereas EC50 is the concentration of toxin liable for 50% inhibition of green algae growth when interacted for 96 h.[1]

Hydrogen Evolution Experiments

The H2 production experiments were performed in a 100 mL Pyrex reaction cell containing 70 mL of DI water. In a typical H2-production experiment, the Fe0 and Bi/Fe0-5 powders, at the desired concentrations, were added to the reaction cell at ambient temperature. Of note, DI water was purged with pure nitrogen gas for 45 min to remove the dissolved oxygen before the addition of Fe0 or Bi/Fe0-5 powders. During the experiment, the catalyst/DI water suspension was stirred at 300 rpm on a magnetic stirrer. The hydrogen gas was detected by gas chromatography (Agilent 7890 A) equipped with a 5 Å molecular sieve column and a thermal conductivity detector employing nitrogen as a carrier gas.

Results and Discussion

Morphological, Structural, and Optical Characteristics of Fe0 and Bi/Fe0 Nanoparticles

Figure represents the morphological characteristics of the as-synthesized Fe0 and Bi/Fe0 samples. It can be noticed that the as-synthesized Fe0 particles are spherical in shape and are agglomerated most probably because of their magnetic nature (Figure A). However, the extent of agglomeration is decreased with increase in Bi content in Bi/Fe0 nanoparticles along with reduction in particle size and the appearance of a cage type morphology (Figure B,C). Figure D shows a ring-shaped structure consisting of well-dispersed nanoplates (Figure D inset). Interestingly, when the Bi content was further increased to 8 wt % (Bi/Fe0-8), the agglomeration again started to develop.

Figure 1

SEM images of Fe0 (A), Bi/Fe0-1 (B), Bi/Fe0-3 (C), Bi/Fe0-5 (D), and Bi/Fe0-8 (E).

To get more details and in-depth morphological and structural information of the as-synthesized nanoparticles, (high resolution) TEM [(HR)-TEM] analysis was performed. TEM analysis shows that Bi/Fe0-5 particles have a worm/vertical plate-like structure (Figure A). Furthermore, the particles are dense in distribution, and the dark color spots indicate the superposition of the crystal. This phenomenon anticipates that there would be a strong contact among particles and consequently would be able to show good adsorption. HR-TEM analysis showed that the interplanar space between two consecutive fringes was about 0.202 nm, which corresponds to the (110) plane of Fe0 (Figure B).[39] Furthermore, the HR-TEM result indicates that the as-synthesized Bi/Fe0-5 has a good single-crystal structure. The existence of a considerable amount of Fe0 along with the calculated amount of Bi in the Bi/Fe0-5 sample is well confirmed by EDX analysis (Figure S1). However, a small amount of carbon is also observed in EDX which could be coming from the glassware used for the sample preparation. The peak for sodium (Na) arises due to the use of sodium borohydride as the reducing agent in the synthesis process.

Figure 2

TEM image of the as-synthesized Bi/Fe0-5 samples (A) and the corresponding HRTEM image (B).

Figure depicts the XRD patterns of the as-synthesized Fe0 and Bi/Fe0 samples. In all the patterns, a prominent peak at 2θ = 44.9° appears which corresponds to (110) facets of iron (Fe) having a cubic crystal system (JCPDS = 06-0696). Furthermore, it can be seen that while moving from Fe0 to Bi/Fe0-1 and then to Bi/Fe0-3, the peak of Fe0 becomes more pronounced, which slightly decreases for Bi/Fe0-5, suggesting that Bi doping increases the crystallinity of Fe0. However, for the Bi/Fe0-8 sample, the satellite peaks in the 2θ range of 25–40° were observed, which are ascribed to the formation of oxides on the surface of the Fe0.[40] Moreover, for the Bi/Fe0-8 sample, the peaks at 2θ = 43.2, 53.32 and 57.36° correspond to magnetite (Fe3O4) (JCPDS = 01-1111) suggesting that excessive loading of Bi has oxidized Fe0. The structural properties of the Fe0 and Bi/Fe0 samples are summarized in Table . It can be seen that the BET surface areas (SBET) of the Fe0, Bi/Fe0-1, Bi/Fe0-3, Bi/Fe0-5, and Bi/Fe0-8 samples correspond to 46.53, 58.02, 70.22, 112.31, and 46.69 m2 g–1, respectively. The results showed that while going from Fe0 to Bi/Fe0-5, the SBET increases from 46.53 to 112.31 m2 g–1, indicating that Bi doping up to the optimum level inhibits the crystal growth of Fe0.[1] However, the SBET decreases to 46.69 m2 g–1 for the Bi/Fe0-8 sample possibly because of the agglomeration of Fe0 particles at higher concentration of Bi. The same is also evidenced from their corresponding particle size (Table ). The crystal sizes of the as-synthesized Fe0, Bi/Fe0-1, Bi/Fe0-3, Bi/Fe0-5, and Bi/Fe0-8 samples were measured by a well-known Scherrer’s equation[41] and found to be in the decreasing order of Bi/Fe0-3 (2.02 nm) > Fe0 (1.76 nm) > Bi/Fe0-1 (1.72 nm) > Bi/Fe0-8 (1.53 nm) > Bi/Fe0-5 (1.45 nm) (Table ). The porosity of the as-synthesized Fe0, Bi/Fe0-1, Bi/Fe0-3, Bi/Fe0-5, and Bi/Fe0-8 samples corresponds to 4.45, 12.28, 18.91, 25.37, and 13.29%, respectively (Table ). The high porosity and also surface area of Bi/Fe0-5 make it favorable for strong adsorption of CHP on its surface, and thus, it is anticipated that the catalytic properties of Bi/Fe0-5 would be excellent.

Figure 3

XRD spectra of the as-synthesized Fe0, Bi/Fe0-1, Bi/Fe0-3, Bi/Fe0-5, and Bi/Fe0-8.

Table 1

Physiochemical Characteristics of the As-Synthesized Fe0, Bi/Fe0-1, Bi/Fe0-3, Bi/Fe0-5, and Bi/Fe0-8 Samples

samples	SBET (m2 g–1)	pore volume (cm3g–1)	pore size (nm)a	porosity (%)b	particle size (nm)c	crystal size (nm)d
Fe0	46.53	0.007	2.90	4..45	16.38	1.76
Bi/Fe0-1	58.02	0.021	2.45	12.28	13.14	1.72
Bi/Fe0-3	70.22	0.035	2.80	18.91	10.85	2.02
Bi/Fe0-5	112.31	0.051	3.15	25.37	6.78	1.45
Bi/Fe0-8	46.69	0.023	2.94	13.29	16.32	1.53

Calculated from the pore radius of the adsorption isotherm.

Porosity (%) = pore volume (cm3 g–1)/[pore volume (cm3 g–1) + solid catalyst volume without pore (cm3 g–1)] × 100.

Calculated from BET, using D = 6000/(ρ×SBET), where ρ = 7.87 g cm–3 of iron (Fe) density.

Measured from XRD data by applying Scherrer’s equation; D = 0.89 λ/(B × cos θ), where λ = 0.154 nm and B = full width at half-maximum (FWHM).

The XPS spectrum was recorded for better understanding the surface chemistry of the as-synthesized Bi/Fe0-5 material (Figure ). Figure A shows that the dominant species were Fe, O, Bi, and C on the surface of Bi/Fe0-5 particles. The presence of Na (sodium) might be from NaBH4 which was used for the reduction process in the synthesis of Bi/Fe0-5. The position of C 1s peak at 284.5 eV is designated to the remaining carbon from the sample and adventitious hydrocarbon from the XPS instrument.[1]Figure B depicts the high-resolution Fe 2p XPS spectrum. The appearance of peak at 706.5 eV confirms the existence of Fe0 (Fe 2p3/2).[42] Moreover, the existence of peak at 708.1 eV corresponds to the Fe3+ oxides (Figure B).[43] The presence of two peaks in the photoelectron spectrum of O 1s (at 530.3 eV and 531.9 eV) corresponds to the existence of hydroxyl bonded to metal (M – OH) and chemically or physically adsorbed H2O on the surface of Bi/Fe0-5, respectively (Figure C).[30] The high-resolution XPS spectrum of Bi 4f is separated into two large peaks at 159.3 and 164.1 eV corresponding to Bi 4f7/2 (Bi3+) and Bi 4f5/2 (Bi3+). Finally, the peaks at 162.6 and 158.1 eV are ascribed to Bi0 (Figure D).

Figure 4

Full survey XPS spectra of Bi/Fe0-5 (A), high-resolution Fe 2p high-resolution spectra (B), high-resolution O 1s high-resolution spectra (C), and high-resolution Bi 4f high-resolution peaks (D).

Figure depicts the TGA of the Fe0 and Bi/Fe0-5 particles in the temperature range from 40 to 600 °C. In the case of Fe0, it can be seen that there is gradual weight loss from 50 to 250 °C, which might be attributed to the evaporation of water and ethanol from the nanomaterial. However, at 430 °C, a dramatic increase in the weight of the Fe0 nanomaterial was observed possibly because of the oxidation of Fe0 (formation of oxides of iron). For the Bi/Fe0-5 particles, initially there was rapid weight loss from 50 to 140 °C, which is attributed to the evaporation of water and ethanol. However, unlike Fe0, no further prominent weight loss and gain was observed for Bi/Fe0-5 which suggests that the thermal stability of the Bi/Fe0-5 is increased due to Bi doping which prevents the formation of iron oxides and thus is in accordance with the XRD result.

Figure 5

TGA curves of the as-synthesized Fe0 and Bi/Fe0-5 materials.

Catalytic Degradation of CHP

The catalytic efficiencies of Fe0 and Bi/Fe0 nanoparticles were investigated for the degradation of CHP. Under the experimental conditions of [CHP]0 = 10 mg L–1, [catalyst load]0 = 1 g L–1, [pH]0 = 6.8, and reaction time of 12 min, 62, 68, 74, 95, and 82% degradation of CHP was achieved by Fe0, Bi/Fe0-1, Bi/Fe0-3, Bi/Fe0-5, and Bi/Fe0-8, respectively (Figure A). These results showed that the as-prepared nanomaterials follow the catalytic activity order in the sequence of Bi/Fe0-5 > Bi/Fe0-8 > Bi/Fe0-3 > Bi/Fe0-1 > Fe0. The higher catalytic degradation efficiency of Bi/Fe0-5 is due to its higher porosity and surface area which makes it more favorable for adsorption of CHP on its surface. Moreover, the formation of reactive oxygen species (ROS) on the surface of Bi/Fe0-5 is much higher as compared to that of Fe0, Bi/Fe0-1, Bi/Fe0-3, and Bi/Fe0-8 nanomaterials, as depicted from their corresponding photoluminescence (PL) spectra (Figure S2), which leads to the higher removal of CHP by Bi/Fe0-5 as compared to its counterparts.

Figure 6

Catalytic degradation performance of the as-synthesized Fe0, Bi/Fe0-1, Bi/Fe0-3, Bi/Fe0-5, and Bi/Fe0-8 materials (A); catalytic degradation of CHP by HSO5– alone, Bi/Fe0-5 alone, and HSO5–/Bi/Fe0-5 system (B) and comparison of the apparent rate constant (kapp) values of CHP degradation by Fe0, Bi/Fe0-1, Bi/Fe0-3, Bi/Fe0-5, Bi/Fe0-8, HSO5– alone, and HSO5–/Bi/Fe0-5 system (C). Experimental conditions: [CHP]0 = 10 mg L–1, [pH]0 = 6.8, [catalyst load]0 = 1.0 g L–1, [HSO5–]0 = 1.0 mM.

Because Fe0 is a strong reducing agent, it is expected that it could activate PMS (HSO5–) through an electron-transfer mechanism to generate reactive radicals in the form of hydroxyl and sulfate radicals (•OH and SO4•–). To test this hypothesis and further promote the catalytic activity of Bi/Fe0-5 nanoparticles, the degradation of CHP was studied by the Bi/Fe0-5/HSO5– system, and the results are depicted in Figure B. Interestingly, 98% CHP degradation was achieved by the Bi/Fe0-5/HSO5– system as compared to 81% by Bi/Fe0-5 under the reaction conditions of [CHP]0 = 10 mg L–1, [HSO5–]0 = 1 mM, [catalyst load]0 = 1 g L–1, [pH]0 = 6.8, and a reaction time of 8 min. Of note, only 8% CHP degradation was achieved by HSO5– alone under the same experimental conditions. It means that the sum of CHP degradation by Bi/Fe0-5 and HSO5–, when used separately, is 89% (81% by Bi/Fe0-5 + 8% by HSO5–) which is lower than that by the Bi/Fe0-5/HSO5– system (98%). These results clearly indicated the synergism between HSO5– and Bi/Fe0-5 which is possibly due to the involvement of reactive radicals in the Bi/Fe0-5/HSO5– system (reactions –12).[44,45]

The apparent rate constant (kapp) values are also depicted in Figure C and were found to be 0.0105, 0.2093, and 0.03253 min–1 for HSO5–, Bi/Fe0-5, and Bi/Fe0-5/HSO5–, respectively. These results reveal that the coupling of HSO5– with the Bi/Fe0-5 is an effective way for accelerating the antibiotics degradation in the aqueous medium. To further investigate the reactivity of •OH and SO4•– with CHP, second-order rate constants of •OH and SO4•– with CHP (k and kSO) were measured using competition kinetics according to our previous report.[1] The para-chlorobenzoic acid (p-CBA, k = 5.0 × 109 M–1 s–1) and meta-toluic acid (m-TA, kSO = 2.0 × 109 M–1 s–1) were used as competitors for •OH and SO4•–, respectively.[46,47] The values of k and kSO were measured to be 5.5 × 109 and 3.2 × 109 M–1 s–1, respectively, which suggest comparable and higher reactivity of both •OH and SO4•– with CHP.

Effect of pH on the Degradation of CHP by the Bi/Fe0-5/HSO5– System

pH is one of the critical environmental parameters that could affect the removal efficiency of a water treatment technology. To investigate the effect of pH on the degradation of CHP by the Bi/Fe0-5/HSO5– system, three different pH values (acidic, neutral, and alkaline) were studied, that is, 3.5, 6.8, and 10.5, respectively (Figure ). It can be seen that as the solution pH increases from acidic to neutral and then to alkaline, the removal efficiency of CHP decreases. At a reaction time of 6 min, 93, 87, and 52% degradation of CHP was attained at pH 3.5, 6.8, and 10.5, respectively. The results indicate the strong influence of the initial solution pH on the catalytic degradation of CHP by the Bi/Fe0-5/HSO5– system. This decrease in the catalytic degradation of CHP by the Bi/Fe0-5/HSO5– system with the rise in pH from 3.5 to 10.5 could be due to the following three reasons. First, the pH of point of zero charge (pHPZC) of Bi/Fe0-5 was calculated to be 3.6 (Figure S3) which suggests that at pH > 3.6, the surface of Bi/Fe0-5 is charged negatively. In other words, at pH > 3.6, the negative surface of Bi/Fe0-5 repels the negative PMS anion. As a result, at higher pH values, less concentration of PMS would be catalyzed by Bi/Fe0-5, which in turn led to the lower concentration of the reactive radicals. Second, the pKa value of CHP is 9.5.[48] This means that at pH ≥ 10, CHP would exist in its anionic form in the solution, and consequently, very few CHP would be adsorbed on the surface of Bi/Fe0-5 due to electrostatic repulsion. Third, the pH of solution affects the speciation of the PMS. The pKa1 and pKa2 of H2SO5 are less than 0 and 9.4, respectively. Therefore, at acid-neutral and basic conditions, the most dominant species of PMS are HSO5– and SO52–, respectively.[48] Thus, at pH = 10.5, the predominant species of PMS is SO52– which could scavenge both •OH and SO4•– (reactions and 14) and thereby reduce the degradation of CHP.[48] Of note, SO5•– is less reactive than •OH and SO4•–.

Figure 7

Effect of pH on the catalytic degradation of CHP by the HSO5–/Bi/Fe0-5 system. Experimental conditions: [CHP]0 = 10 mg L–1, [Bi/Fe0-5]0 = 1.0 g L–1, [HSO5–]0 = 1.0 mM.

Identification of Reactive Species and Activation Mechanism of Bi/Fe0-5/HSO5–

To investigate the participation of reactive species in the Bi/Fe0-5/HSO5– system, quenching experiments were conducted by applying different radical scavengers. It has been well documented that in addition to •OH and SO4•–, O2•– and 1O2 could form in the HSO5–-activated system.[44] Ethanol (EtOH) is considered to be a strong scavenger of both •OH (k = 1.2 to 2.8 × 109 M–1 s–1) and SO4•– (k = 1.6 to 7.7 × 107 M–1 s–1), whereas tert-butanol (TBA) is a well-known •OH scavenger (k = 3.8 to 7.6 × 108 M–1 s–1) but could not scavenge SO4•– efficiently because of its low rate constant with SO4•– (k = 4.0 to 9.1 × 105 M–1 s–1). Furthermore, BQ and l-histidine (LH) were employed to inhibit the oxidation of CHP by O2•– and 1O2, respectively, due to their high rate constants values (kO = 0.9 to 1.0 × 109 M–1 s–1 and k = 5.0 × 107 M–1 s–1).[29]

The results show that in the absence of a scavenger, 98% of CHP degradation was observed with kapp = 0.3253 min–1 at a reaction time of 8 min (Figure A). However, under the same experimental conditions, the % degradation of CHP reduced to 22 (kapp = 0.0307 min–1), 63 (kapp = 0.1292 min–1), 86 (kapp = 0.2439 min–1), and 92% (kapp = 0.3063 min–1) in the presence of EtOH, TBA, BQ, and LH (EtOH and TBA at 50 mM concentration, and BQ and LH at 1.0 mM concentration), respectively (Figure A). These results suggest that among •OH, SO4•–, O2•–, and 1O2, the major species involved in the degradation of CHP by the Bi/Fe0-5/HSO5– system is SO4•– followed by •OH with some minor contribution from O2•– and 1O2 as well.

Figure 8

Effect of different scavengers (EtOH, TBA, BQ, and LH) on the catalytic degradation of CHP by the HSO5–/Bi/Fe0-5 system (A) and formation of [•OH] and [SO4•–] during HSO5–/Bi/Fe0-5 system at different reaction times (B). Experimental conditions: [CHP]0 = 10 mg L–1, [Bi/Fe0-5]0 = 1.0 g L–1, [HSO5–]0 = 1.0 mM, [pH]0 = 6.8, [EtOH]0 = [TBA]0 = 50 mM, and [BQ]0 = [LH]0 = 1.0 mM (A); [Bi/Fe0-5]0 = 1.0 g L–1, [HSO5–]0 = 1.0 mM, [pH]0 = 6.8, [HBA]0 = 0.5 mM (for SO4•– determination); [BA]0 = 0.5 mM (for •OH determination) (B).

To get more insights into the activation mechanism of the Bi/Fe0-5/HSO5– system, the concentrations of SO4•– and •OH during the reaction process were analyzed quantitatively. For this purpose, HBA and BA were chosen as representatives of SO4•–[35] and •OH,[36] respectively. Figure B depicts the concentration of SO4•– and •OH produced during the activation of HSO5– by Bi/Fe0-5. As shown in Figure B, the cumulative concentration of SO4•– and •OH steadily increased as the reaction proceed and reached 24.8 and 6.4 μM, respectively, at a reaction time of 10 min. Theoretically, 1000 μM (1 mM) of HSO5– could produce 1000 μM as the maximum cumulative concentration of SO4•– and •OH. This is because one HSO5– ion could produce one SO4•– or •OH according to reactions –8. However, reaction which is responsible for the cyclic generation of Fe2+ could reduce the formation of SO4•–/•OH as this reaction consumes one HSO5– ion without the production of SO4•–/•OH. On the other hand, reactions and 10 could lead to the formation of an additional •OH without the consumption of the HSO5– ion. Another reaction which could result in the conversion of SO4•– to •OH without changing the total concentration of SO4•– + •OH may result in the decrease of SO4•– concentration and corresponding increase of •OH concentration (reaction ).[44]

It is noteworthy to be mentioned here that SO4•– and •OH do not accumulate in the reaction mixture but rather continuously consumed by reacting with the target compounds. However, the products of the probe compounds, that is, BQ and p-HBA, may accumulate in the reaction mixture as long as their parent compounds, that is, HBA and BA, are available for their reaction with SO4•– and •OH. Therefore, the total concentrations of BQ and p-HBA were used to calculate the total concentration of SO4•– and •OH produced in the reaction system from 0 to 10 min of reaction time in the present study. Moreover, the calculated concentration of BQ and p-HBA at different time intervals could be used to find out the SO4•– and •OH concentration produced during the 2 min intervals (i.e., between 0 and 2, 2 and 4, 4 and 6 min, and so on) (see Table S1). It can be seen that the radical formation during the 2 min intervals is almost constant for both radicals, suggesting the steady-state formation of SO4•– and •OH during the course of reaction. The steady-state formation of these radicals is due to the availability of their precursor (i.e., PMS) and its activator (Bi/Fe0-5) in the reaction mixture up to the studied treatment time. The result of SO4•– and •OH concentration is consistent with the radical scavenging results of higher contribution of SO4•–.

On the basis of the above discussion, the detailed activation mechanism of HSO5– by Bi/Fe0-5 is illustrated in Scheme . The as-synthesized Bi/Fe0-5 through corrosion either by oxygen (O2) or by HSO5– results in the formation of Fe2+ and Fe3+. Afterward, Fe2+/Fe3+ results in the formation of •OH and SO4•–.[44] These produced •OH and SO4•– participate in the catalytic degradation of CHP; however, scavenging reactions may also occur if the concentration of these ROS (•OH and SO4•–) exceeds the optimum value. Furthermore, the in situ produced H2O2 also reacts with Fe2+ producing •OH. The other ROS such as 1O2 and O2•– are also generated by the reaction of HSO5– with H2O after a series of reactions.[44] These generated reactive species (•OH, SO4•–, 1O2, and O2•–) have sufficient ability to oxidize any target contaminants by a radical/nonradical way. Furthermore, various iron hydroxides such as Fe(OH)2 and Fe(OH)3 may also be generated during the reaction. These hydroxides could remove CHP and its DPs through adsorption.[49] It has been documented that these hydroxides are further transformed through dehydration and crystallization into Fe2O3, Fe3O4, and FeOOH, which have comparatively less adsorption capability,[44] although they could activate HSO5– to produce SO4•–.[44]

Scheme 1

Proposed Activation Mechanism of HSO5– by the As-Synthesized Bi/Fe0-5

Assessment of DPs and Their Ecotoxicity

An effective water treatment technology is the one which could reduce the overall toxicity of the treated water. Therefore, to find out the effectiveness of the Bi/Fe0-5/HSO5– system for the treatment of antibiotics, in this CHP, the DPs of CHP and their ecotoxicities toward three aquatic organisms, that is, fish, daphnia, and green algae, were assessed. A total of nine DPs were identified. The details about these DPs are summarized in Table S2 (Supporting Information) along with their structure, chemical formula, and m/z values. A possible degradation pathway of CHP was proposed based the identified DPs (see Scheme ). It can be seen that attack of •OH/SO4•– on CHP results in the formation of various hydroxylated DPs, in accordance with the previous study.[50] The first pathway (pathway-I) was initiated by the cleavage of the C–N bond with the loss of dichloroacetamide and thus result in the formation of DP2 with m/z = 195. DP2 is further dehydrated and gave rise to DP3 with m/z = 179. The second pathway (pathway-II) involves the elimination of the methanol group from the propylene glycol branch of CHP and gives rise to the formation of DP4 with m/z of 293, which further results in the formation of DP10 (dichloro-acetamide) with m/z = 128. Further attack of •OH/SO4•– on DP4 (pathway-IIB) oxidizes the lateral group and results in the substitution of −NO2 with the OH group leading to the formation of DP7 (4-HBA) with m/z = 138. DP8 (4-hydroxy benzaldehyde) with m/z = 122 is formed due to the dehydroxylation of 4-HBA (DP7). The formation of DP9 (formic acid) with m/z = 46 suggests the effective oxidation of CHP into shorter chain acids. DP4 can also proceed further through degradation route II-A which involves hydroxylation followed by decarboxylation and results in the formation of DP5 (4-nitro benzoic acid) with m/z = 167, which further through decarboxylation gives DP6 (4-nitrophenol) having m/z = 139. The degradation pathway-III suggests the dechlorination of CHP by the attack of •OH/SO4•– and results in the formation of DP1 with m/z = 270.

Scheme 2

Pathways for the Degradation of CHP by the Bi/Fe0-5/HSO5– System

The % removal of total organic carbon (TOC) was investigated for Bi/Fe0-5 and Bi/Fe0-5/HSO5– processes, and the results are shown in Figure S4. At a reaction time of 240 min, the % removal of TOC in the case of the Bi/Fe0-5/HSO5– system was increased from 65 (Bi/Fe0-5) to 92%, which could be due to the production of additional SO4•– in the case of the Bi/Fe0-5/HSO5– process. Furthermore, it can be seen that in the case of the Bi/Fe0-5/HSO5– system, after a reaction time of 120 min, there is a rapid increase in the % removal of TOC from 36 to 58%; however, in the case of Bi/Fe0-5 process, it was just increased from 28 to 33% (Figure S4). This indicates the mineralization of persistent DPs of CHP by the Bi/Fe0-5/HSO5– system due to additional involvement of SO4•– along with •OH in the mineralization of CHP. The higher % removal of TOC for CHP by the Bi/Fe0-5/HSO5– system implies that this process can be successfully applied in batch-scale reactors for contaminated water treatments.

For the practical implementation of any treatment technique, it is necessary to investigate the cyclic catalytic performance. To study the cyclic catalytic performance of the as-synthesized Bi/Fe0-5 material coupled with HSO5–, twelve (12) consecutive cyclic degradation runs were performed, and the results are shown in Figure S5. After each run, the catalyst was washed and dried and then reapplied without any additional regeneration treatment. It can be seen that till the 6th run, the catalytic degradation performance of the proposed Bi/Fe0-5/HSO5– system was almost constant; however, afterwards, it slowly started to decrease from 93 to 85% at the 12th run (Figure S5). This decrease in the catalytic performance of Bi/Fe0-5/HSO5– was probably due to the adsorption of DPs of CHP on the active sites of Bi/Fe0-5, which prevents further adsorption and thus decreases the degradation of CHP over the surface of Bi/Fe0-5. However, these adsorbed DPs can easily be desorbed by vacuum drying of the as-synthesized material at 80 °C. The successful stable catalytic performance of the Bi/Fe0-5/HSO5– system till 6th runs further provides an evidence that the proposed treatment technique is not only efficient but also economical for the treatment of organic contaminants.

To further evaluate the practical applicability of the Bi/Fe0-5/HSO5– system, the ecotoxicity of these detected DPs toward fish, daphnia, and green algae was assessed using the ECOSAR program. The toxicity results are presented in Table and Figure . The results show that CHP and its DPs have varied levels of ecotoxicity toward different organisms, and most of the DPs have even higher toxicity than CHP. This opens a new window for environmental researcher to think about these DPs and to find new ways to effectively eliminate them. However, the appearance of DP9 (formic acid) through pathway-IIB has lowest toxicity, and thus, it shows that the catalytic degradation of CHP by the Bi/Fe0-5/HSO5– system through pathway-IIB is the most suitable and environmentally friendly degradation pathway.

Table 2

Calculation of Ecotoxicity of CHP and Its DPs toward Different Aquatic Organisms in the Units of mg/La

	acute toxicity			chronic toxicity
compound	fish (LC50) duration 96 h	daphnia (LC50) duration 48 h	green algae (EC50) duration 96 h	fish (ChV)	daphnia (ChV)	green algae (ChV)
CHP	883.0	643.0	185.0	62.0	81.5	70.2
DP1	2.04 × 104	1.31 × 104	2.01 × 103	1.01 × 103	1.09 × 103	492
DP2	405	298	90.5	29.3	39.1	35.5
DP3	107	82.7	31.8	8.83	12.7	14.7
DP4	910	660	185	63	82.2	69.1
DP5	2.60 × 103	1.43 × 103	949	245	129	233
DP6	21.9	9.11	1.92	2.15	0.98	3.99
DP7	491	162	42.3	45.7	15.9	77.7
DP8	17.3	17	9.19	3.61	0.16	3.20
DP9	6.13 × 103	2.77 × 103	807	458	144	127
DP10	55.8	80.3	1.07	29.2	85	1.06

Acute toxicities based on the European Union criteria (described in Annex VI of Directive 67/548/EEC) as LC50 > 100 or EC50 > 100 (not harmful), 10 < LC50 < 100 or 10 < EC50 < 100 (harmful), 1 < LC50 < 10 or 1 < EC50 < 10 (toxic), and LC50 < 1 or EC50 < 1 (very toxic). While chronic toxicities assessed based on the Chinese hazard evaluation criteria for new chemical substances (HJ/T154-2004), that is, ChV > 10 (not harmful), 1 < ChV < 10 (harmful), 0.1 < ChV < 1 (toxic), and ChV < 0.1 (very toxic).

Figure 10

Calculation of ecotoxicity of CHP and its DPs toward different aquatic organisms in the units of mg/L. Acute toxicities based on the European Union criteria (described in Annex VI of Directive 67/548/EEC) as LC50 > 100 or EC50 > 100 (not harmful), 10 < LC50 < 100 or 10 < EC50 < 100 (harmful), 1 < LC50 < 10 or 1 < EC50 < 10 (toxic), and LC50 < 1 or EC50 < 1 (very toxic), while chronic toxicities assessed based on the Chinese hazard evaluation criteria for new chemical substances (HJ/T154-2004), that is, ChV > 10 (not harmful), 1 < ChV < 10 (harmful), 0.1 < ChV < 1 (toxic), and ChV < 0.1 (very toxic).

Hydrogen Production

To achieve the desirable goal of green and clean energy production, the synthesized nano zerovalent iron (Fe0) and bismuth-doped Fe0 (Bi/Fe0-5) were investigated as cheap potential sources of H2 production from water. Figure depicts H2 production by Fe0 and Bi/Fe0-5. It can be seen that H2 production could reach to 544 and 712 μM by Fe0 and Bi/Fe0-5, respectively, in 70 mL of aqueous solution containing 0.07 g (i.e., at 1 g L–1 concentration) of the catalyst at ambient temperature. The theoretical hydrogen yield by Fe0 could be 1253.58 and 1671.44 μM at 1:1 (shown in reaction ) and 4:3 (shown in reaction ) stoichiometry of H2/Fe0, respectively. Similarly, the theoretical yield of hydrogen by Bi/Fe0-5 could be calculated as 1190.90 and 1587.87 μM at 1:1 and 4:3 stoichiometry of H2/Fe0, respectively, considering 95% Fe0 and 5% Bi in Bi/Fe0-5 and proposing the hydrogen evolution only from Fe0. In other words, the theoretical yield of hydrogen production from Bi/Fe0-5 could be 95% of the hydrogen production from Fe0. Using the above actual and theoretical yields of hydrogen production, one can find the H2 recovery efficiencies of 43.4 and 32.5% for Fe0 and 59.8 and 44.8% for Bi/Fe0-5 at 1:1 (shown in reaction ) and 4:3 (shown in reaction ) stoichiometry of H2/Fe0, assuming complete dissolution of 1 g L–1 Fe0 (i.e., 0.07 g of Fe0 and Bi/Fe0-5 in 70 mL of reaction solution). Because the above data was collected for 72 h of Fe0 and Bi/Fe0-5 dissolutions in water, this recovery efficiency can further be increased by allowing more time to Fe0 and Bi/Fe0-5 dissolutions. However, the abundant availability of Fe at extremely low cost compared to other H2-generator catalysts such as noble metals, the obtained H2 recoveries are still more than sufficient to persuade the green energy production companies and investors to boost up the practical applications of Fe0 for H2 production. Interestingly, the doping of a less active metal, that is, a metal with a higher redox potential, at the surface of iron (bimetallic iron system) has been demonstrated to considerably accelerate H2 production.[25,51,52] That is why Bi/Fe0-5 showed higher H2 yield than Fe0, that is, 712 versus 544 μmol. When attached to a metal having higher redox potential, Fe0 oxidizes more rapidly and therefore accelerates iron corrosion leading to higher H2 production. The iron-less active metal couple forms numerous galvanic cells. In such galvanic cells, Fe0 serves as an anode and readily loses electrons. Meanwhile, the less active metal (i.e., Bi in this case), acting as the cathode, is protected and therefore remains unchanged. The standard redox potentials of Fe/Fe2+ and Bi/Bi3+ couples relative to the standard hydrogen electrode are −0.44 and +0.308, respectively, suggesting the ability of Fe0 as the anode and Bi as the cathode when working as galvanic cell electrodes. Besides, the doping of Bi at the surface of Fe0 reduces the particle size and enhances the BET surface area. For example, in the present study, the particle size and BET surface area for Fe0 and Bi/Fe0-5 were calculated to be 16.38 and 6.78 nm and 46.53 and 112.31 m2 g–1, respectively. Thus, apart from the galvanizing effect produced by Bi in the crystal lattice of Fe0, the smaller particle and higher surface area of Bi/Fe0-5 than Fe0 could possibly be contributed to the relatively higher hydrogen production by Bi/Fe0-5 as compared to Fe0 (i.e., 712 vs 544 μmol). The present study underscores the importance of Fe0, a readily available and inexpensive reductant, for green energy production in addition to its use for water remediation. The ability of less active metals to improve the oxidizing capability of Fe0 opens new windows and paves the way for the roadmap of Fe0 research in the fields of water treatments and clean energy productions.

Figure 9

Hydrogen production from water decomposition by Fe0 and Bi/Fe0-5 nanoparticles. Experimental conditions: [Fe0]0 = [Bi/Fe0-5]0 = 1.0 g L–1, [pH]0 = 6.8, reaction solution = 70 mL (containing 0.07 g of catalyst).

Environmental Implications

The proposed Bi/Fe0 system could be not only beneficial as an effective water treatment protocol but also suitable for the production of hydrogen as clean fuel and thus makes it an environmentally benign system. In the present study, the magnetic behavior of the Bi/Fe0 system and its capability to produce ROS under dark without the need of any electricity/energy source has significantly enhanced its importance for the economical treatment of polluted water or subsurface environments. Furthermore, hydrogen, produced from the Bi/Fe0/water system, being a clean fuel (without the common impurities such as CO2 and CH4) with a high energy to mass ratio, is regarded as a promising candidate compared to alternative energy sources.[23] In this study, the higher hydrogen production by Bi/Fe0-5 as compared to Fe0 (i.e., 712 vs 544 μmol) suggests that the hydrogen production rate can be further increased by simply doping Fe0 with Bi at an appropriate ratio and thus serves as evidence that such technology can be sustainable and economic. The investigated Bi/Fe0 system operates under ambient temperature and pressure for hydrogen production and thus shows that it could be easily scaled-up for large-scale production.

Although, in the present study, the Bi/Fe0 system has shown great removal performance, however, in real engineering application, the Bi/Fe0 system may face a complex environment, and there are numerous conditions that may influence the applicability of the Bi/Fe0 system in a sustainable treatment process. Therefore, to widely and wisely apply Bi/Fe0 system in subsurface and water treatment processes, further studies, such as the influence of pollutant concentration, influence of natural organic matter, and degradation in real water and wastewater samples, are underway to reveal the whole picture.

Conclusions

The doping of bismuth into the surface of Fe0 is an effective way to enhance the reductive capability/activity of Fe0. The present study reveals that Bi/Fe0-5/HSO5– is an efficient alternative for the treatment of (waste)water containing antibiotics. Interestingly, the Bi/Fe0-5/HSO5– system shows appreciable activity at neutral pH albeit little lower than that at acidic pH. The scavenging experiments illustrate that both •OH and SO4•– are involved in the degradation of CHP by the Bi/Fe0-5/HSO5– process, although the role of SO4•– was more predominant. The competition kinetic studies showed that the second-order rate constants of •OH and SO4•– with CHP are 5.5 × 109 and 3.2 × 109 M–1 s–1, respectively. The toxicity assessment of CHP and its DPs showed that the route leading to the formation of formic acid (DP9) (pathway-IIB) is the most suitable and environmentally friendly degradation route. Furthermore, Bi doping at the surface of Fe0 has beneficial effect on hydrogen production from water decomposition.