Removal of p-Nitrophenol in Aqueous Solution by Mixed Fe0

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Removal of p-nitrophenol (PNP) in aqueous solution by the mixed Fe0/(passivated Fe0) fixed bed filters Yi Ren, Jun Li, Donghai Yuan, and Bo Lai Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02082 • Publication Date (Web): 27 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017

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Industrial & Engineering Chemistry Research

Removal of p-nitrophenol (PNP) in aqueous solution by the mixed Fe0/(passivated Fe0) fixed bed filters Yi Ren1, Jun Li1, Donghai Yuan2, Bo Lai1* 1. Department of Environmental Science and Engineering, School of Architecture and Environment, Sichuan

University, Chengdu 610065, P. R. China

2. Key Laboratory of Urban Stormwater System and Water Environment, Ministry of Education, Beijing Climate

Change Response Research and Education Center, Beijing University of Civil Engineering and Architecture, Beijing,

P. R. China

Corresponding authors. ∗

Tel./fax: +86 18682752302; E-mail address: [email protected] (Bo Lai).

Abstract: Fe0 particles were passivated by concentrated nitric acid, and a Fe0/(passivated Fe0) system was setup for p-nitrophenol removal. First, the characteristics of passivated Fe0 particles were analyzed. The results suggest that the passivated Fe0 particles have iron oxides passivation film on their surface with inertness and high electrode potential (0.57 V). Besides, the optimal conditions were obtained according to the significant parameters optimization. In addition, control experiments were set up to investigate the advantage of reactivity and operational life of the Fe0/(passivated Fe0) system, and the results confirmed that the new system had higher reactivity and longer operational life. Meanwhile, the reaction mechanism of Fe0/(passivated Fe0) system for PNP removal was proposed. Finally, with the analysis of preparation cost, the Fe0/(passivated Fe0) system could also be seemed as a cost-effective technology. Consequently, the developed 1

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Fe0/(passivated Fe0) system in this study is a promising technology for contaminated water treatment.

Keywords: zero valent iron (Fe0); passivated Fe0; fixed bed; characteristics; performance; mechanism

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Table of Contents:

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1. Introduction

There is an increasing interest for researchers in the study of wastewater treatment and natural water restoration by zero valent iron (ZVI or Fe0) based fixed bed in recent years. Fe0, as an environmentally material for contaminated water treatment, could remove nitrate and heavy metal ions, and transform organic pollutants in fixed bed1, 2. However, low effect, narrow available pH range and short operational life severely restrict the application of this material in fixed bed2, 3. In order to overcome these shortcomings of Fe0 technique, some new Fe0 based systems were developed. Quartz sand is usually added in the fixed bed as inert materials to relieve harden and caking and prolong operational life, but the efficiency is limited due to the stratification caused by the difference mechanical property of quartz sand and Fe04. The system with medium materials of mixture of Fe0 and transition metal particles (e.g.,Cu0) was also studied thoroughly5-7. This system has high treatment efficiency for the pollutants and long operational life, because the transition metal particles are inert and have similar mechanical property with Fe0 particles, and Fe0 and transition metal particles could form high potential difference galvanic cell to enhance the corrosion rate of Fe08. However, the high cost of transition metal particles (e.g., Cu0, Pd0 and Ni0) extremely limits the practical application of the Fe0/(transition metal) system. Besides, the risk of heavy metal secondary pollution due to the dissolution of transition metal particles also should be considered when the wastewater is treated by this system. Therefore, it is necessary to develop a new Fe0 based fixed bed system with good performance, low negative effects, long operational life and low costs. 4


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Passivation of iron and steel has been investigated and applied widely in anti-corrosion project. In particular, concentrated sulfuric acid, concentrated nitric acid, some oxidant solution and other passivation solution could be used to passivate iron9, 10. After passivation process, some iron oxides and other iron compounds (e.g. iron chromium compounds) may form densely on the surfaces of iron, which could reduce the reactivity and enhance the potential (i.e., 0.5-1 V) of iron extremely11. If this technology was used in environment field for Fe0 passivation, the passivated Fe0 with high potential and low reactivity could be obtained. If Fe0 and passivated Fe0 were used as medium materials in fixed bed for contaminated water treatment, high potential difference between Fe0 and passivated Fe0 might result in strong reactivity and wide available pH range for pollutants reduction. Furthermore, passivated Fe0could be used as inert materials in the system due to low reactivity of passivated Fe0, which could relieve harden and caking phenomena. In particular, the costs of system filling with Fe0 and passivated Fe0 would be relatively low compared with that filling with Fe0 and transition metal particles. Besides, the passivation process and passivated Fe0 by concentrated nitric acid are totally different from the case that has been reported in environmental field. In environment field, the passivated Fe0 would be formed and inhibit the treatment efficiency of the micro-electrolysis system after long-term running, and it is a fatal shortcoming of Fe0 based system7, 12. In contrast, the passivated Fe0 investigated in this study would have dense and uniform passive film with high electrode potential, which could enhance the reactivity of Fe0 and restrict harden and caking. 5



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In this study, concentrated nitric acid was used to passivate Fe0 particles, and the passivated Fe0 particles were mixed with Fe0 particles to form Fe0/(passivated Fe0) system. First, characteristics of passivated Fe0 were observed by using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmet-Teller (BET) and electrochemical workstation., respectively. Furthermore, p-Nitrophenol (PNP) was selected as a model pollutant to evaluate the treatment efficiency of the Fe0/(passivated Fe0) system under the different conditions. Meanwhile, comparative study on the treatment efficiency of 5 different systems (i.e., Fe0 alone, passivated Fe0 alone, Fe0/(acid-washed Fe0), Fe0/(aging Fe0) and Fe0/Cu0) was carried out thoroughly. In addition, operational life of Fe0/(passivated Fe0), Fe0 alone and Fe0/Cu0 was also investigated comparatively. Finally, the reaction mechanism of Fe0/(passivated Fe0) system for PNP removal was proposed and the materials expenses were analyzed.

2. Experimental 2.1 Reagents

In the experiment, the analytical reagents including PNP, concentrated nitric acid(65%, w/w), zero valent iron (Fe0) and zero valent copper (Cu0) were purchased from Chengdu Kelong chemical reagent factory. Meanwhile, zero valent iron (Fe0) particles used in this study have a mean particle size of approximately 120 µm, and their Fe content is above 99%. Zero valent copper (Cu0) particles have a mean particle size of approximately 50 µm, and their Cu content reaches above 99%. Quartz sand has a mean 6


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particle size of approximately 1 mm, and their SiO2 content reaches above 95%. Other chemicals used in the experiment were of analytical grade. Deionized water was used in all experiments. 2.2 Preparation of passivated Fe0 particles

The micron-scale Fe0particles were used as the substrate material for the preparation of passivated Fe0 particles. The passivated Fe0 particles were prepared via adding 100 mL concentrated nitric acid to 10 g Fe0 particles keeping certain plating temperature (25±1 o

C) and stirring speed (250 rpm) in a flat bottom beaker for 10 min. Then the prepared

passivated Fe0 particles were separated from the supernatant immediately after passivation process. Finally, the separated passivated Fe0 particles were rinsed three times with deionized water, three times with ethanol, and then they were dried under vacuum protection at 50±1 oC for 2 hours. In addition, acid-washed Fe0 were pretreated by dilute hydrochloric acid (20%) for 0.5 min, and aging Fe0 were pretreated by water (open to the air) for 2 hours, respectively. Other conditions were same with that for passivated Fe0 preparation. 2.3 Experimental procedures

The experiments of PNP reduction were conducted in a column reactor with a diameter of 30 mm and total length of 150 mm. Quartz sand was used as fillers on the bottom height of 50 mm, and 20.00 g medium materials (Fe0 and passivated Fe0, Fe0 alone, passivated Fe0 alone, Fe0 and acid-washed Fe0, Fe0 and aging Fe0, Fe0 and Cu0) were fully mixed, and then added them above the quartz sand in the column. 200 mL PNP solution([PNP]0 = 50 mg/L) was used in one batch experiment as model 7



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contaminated water, and the initial pH was adjusted by adding NaOH solution (0.1 mol/L) or H2SO4 solution (0.1 mol/L). The feed water was circulated through the packed column by a peristaltic pump, and the empty bed hydraulic retention time was calculated by the medium materials volume and treatment time. The column reactor was performed at 25±1 ºC by water batch heating, and the flow rate of pump was 50 mL/min. The configuration of the reactor is same as that in our previous study13. To investigate the Fe0 and passivated Fe0 particles as medium materials thoroughly, the PNP solution with different initial pH (3.0, 4.0, 5.0. 6.0, 7.0 and 9.0) and the medium materials with different mass percentage of Fe0 (100%, 60%, 55%, 50%, 45%, 40%. 35% and 0%) were studied. In addition, to confirm the advantage of Fe0/(passivated Fe0) system in fixed bed, 5 control experiments, (a) Fe0 system, (b) passivated Fe0 system, (c) Fe0/(acid-washed Fe0) system, (d) Fe0/(aging Fe0) system and (e) Fe0/(Cu0) system were setup. The control experiments have same mass Fe0 ratio of medium materials and initial pH of PNP solution with the Fe0/(passivated Fe0) system under the optimal conditions. Samples were taken from the system at intervals to detect the residual PNP concentration by withdrawing 1 mL of sample solution. The obtained samples were filtered through the hydrophilic polyethersulfone (PES) syringe filters (0.45 µm) to remove particles, and then the PNP concentrations were determined by using HPLC (High Performance Liquid Chromatography). Furthermore, the operational life of Fe0/(passivated Fe0) system, Fe0 system and Fe0/(Cu0) system were also investigated. A certain amount of PNP solution ( [PNP]0 = 50 mg/L) was used as feed water once, and it was treated with the best hydraulic retention 8


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time (reaching 99.0% PNP removal in 200 mL PNP solution experiments). The operational conditions were same as them in the control experiments. Feed water was changed, and residual PNP concentration in solution was detected after each circulation. 2.4 Degradation kinetics for PNP by different systems

When different systems were used to treat PNP wastewater, the PNP was mainly transformed to p-aminophenol (PAP)14, 15, and the efficiency could be evaluated by the observed pseudo-first-order degradation rate constant (kobs, min-1), because the PNP removal could be described by the pseudo-first-order equation16: ஼

ln ஼ = −݇௢௕௦ ∙ ‫ܴܶܪ‬

(1)

బ

where C is the PNP concentration at instant HRT (mg/L), C0 is the initial PNP concentration, kobs is the observed pseudo-first-order degradation rate constant (min-1) and HRT is hydraulic retention time (min). 2.5 Analytical method

The concentration of p-nitrophenol (PNP) in the samples was determined by using reversed-phase HPLC (Agilent, USA) equipped with the Athena C18-WP (5 µm, 250×4.6 mm) (CNW Technologies GmbH, Germany). The binary phase were (A) water with 0.1% H3PO4 and (B) acetonitrile, and the eluent was A and B (1:1, v/v) with a flow rate of 1.2 mL/min. Detection was performed using a G1365MWD UV detector set at 317 nm14. Characteristics of the particles were observed by SEM (JSM-7500F, JEOL, Japan). In addition, the surface elementary composition of particles was analyzed by EDS (JSM-7500F, JEOL, Japan). In addition, the Fe0 particles and passivated Fe0 particles were further investigated by XRD EMPYREAN diffractometer (PANalytical B.V., 9



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Holland). XPS (AXIS Ultra DLD, Kratos Co., UK) was used to identify the chemical states of Fe and O in the Fe0 particles and fresh passivated Fe0 particles. XPS analyses were similar to those reported in literature17, 18. In particular, passivated Fe0 particles were mounted onto stainless steel sample stubs using carbon tape, and were placed in a sealed container prior to removal from the anaerobic glove box. And then the samples were sent to the XPS instrument. Details of the analysis can be found elsewhere19. The structure of the particles were detected based on the BET method using a surface area analyzer (Micromeritics, ASAP2020, USA). Besides, the electrode potential of passivated Fe0 also need to be detected, because it is an important parameter for galvanic cell formation and harden and caking restriction in Fe0/(transition metal) system. Since it is difficult to detect the electrode potential of the passivated Fe0 particle, passivated Fe0 sheet was prepared in the same way with passivated Fe0 particle without stirring. The electrode potential of passivated Fe0 sheet by concentrated nitric acid was detected by electrochemical workstation (Autolab, PGSTAT 302N, Switzerland). A platinum foil electrode and a saturated calomel electrode were used as the counter electrode and the reference electrode in potassium ferricyanide/potassium ferrocyanide aqueous electrolyte. The passivated Fe sheet was formed galvanic cell with standard electrodes, and the electrode potential was detected. The pH was measured by a pHS-3C meter (Rex, China).

3 Results and discussion 3.1. Characteristics of passivated Fe0

The surface morphology, elementary composition, chemical states, electrode potential 10


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of the passivated Fe0 particles were observed and analyzed by SEM, EDS, XRD, XPS, BET and electrochemical workstation, respectively. As shown in Figure 1, a schistose and dense film was formed on the surface of Fe0 particles after the passivation process, and it could not be remove even if the passivated Fe0 particles were washed by HCl (20%, w/w) for 5 min (see Figure 1(g-i)). In addition, it could be observed that the sponginess morphology of Fe0 particle did not change seriously, while surface structure of the particles is obvious different due to the formation of passive film

after passivation process. Furthermore both sponginess morphology and

surface structure of the passivated

Fe0 particles did not change seriously after

acid-washed process (see Figure 1(a), (d) and (g)). These results suggest that only a thin film was formed on Fe0 surface after the passivation process. In previous literature, it was also reported that a passivation film could be formed on the surface of iron, which could effectively prevent corrosion of Fe020. In other words, the dense coating film on the surface of Fe0 particles could effectively prevent corrosion of the particles. Figure 2 and Figure S1 show EDS spectra and mapping of Fe0, passivated Fe0 and passivated Fe0 after acid-washing. According to Figure 2 (a), only Fe element was detected on the surface of the Fe0 particle (without considering of C element). However, O (5.29%, w/w) and Fe (94.71%, w/w) was detected on the surface of passivated Fe0 particle (see Figure 2(b)), which indicate that the passive film mainly consist of iron oxides. In addition, the high mass percentage of Fe (94.71%, w/w) was mainly attributed to the very thin passive film that could be penetrated completely by X-ray during the EDS analysis process. The results also confirm the above assertion that the sponginess 11



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morphology of Fe0 particle did not change after passivation process because only a thin passive film was formed on Fe0 surface. Furthermore, the mass percentage of O (3.08%) of passivated Fe0 particles after it was acid-washed by HCl was not decreased seriously, which suggest that the passive film was hard to be removed by acid-washing. Moreover, Figure S1 (Supporting Information) shows that O element distributed uniformly on the surface of passivated Fe0 particle even if it was acid-washed by HCl, which suggest that the passive film was uniform and dense. The chemical state of Fe and O of passivated Fe0 particles was examined by XPS, and the results are shown in Figure 3. The survey scan of passivated Fe0 particles reveals the presence of Fe (2p), O (1s) and carbon whose C 1s peak (284.5 eV) was used to calibrate the acquired spectra, as seen in Figure 3(a). Further detailed scan have performed for Fe 2p and O 1s core level spectra to determine charge state of elements present in the particles. All XPS core level spectra were fitted using Shirley background. The XPS survey scan of the 2p of the particles is shown in Figure 3(b). It can be observed that Fe 2p3/2 and Fe 2p1/2 binding energy positions are 711.0 and 724.5 eV, which are very close to Fe3+ state in Fe2O3 at 710.7 and 724.3 eV, respectively21, 22. Also, it has been reported that the peaks of Fe 2p1/2 and Fe 2p3/2 for iron oxides (i.e., hematite, maghemite and magnetite) are 723.2-724.8 eV and 710.6-711.2 eV, respectively23. The high-resolution scan of the O 1s core level is depicted in Figure 3(c). The spectrum shows the presence of two peaks at 531.1 eV and 529.7 eV. According to literatures, the peak at 529.7-530.1 eV corresponds to O2-, which is attributed to the formation of iron oxide (i.e., Fe2O3, Fe3O4 etc.)24.The peak at 531.1-531.7 eV can be attributed to 12


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surface-adsorbed oxygen species (i.e., O22-, O-, OH-)24, and it was also found that adsorbed oxygen species on the surface could also contribute to the formation of passive film11. Therefore, the observed Fe 2p peak position suggests that the passivation film on Fe0 are mainly composed of Fe2O3 and Fe3O4. The Fe0 particles and passivated Fe0 particles were also analyzed by XRD (Figure S2 in Supporting Information). However, there was no big different between these two patterns, and only Fe0 was detected in both of these two samples. This results indicated that the passivated film is very thin, so the iron oxides were difficult to be detected by XRD. In addition, the Fe0 particles and passivated Fe0 particles were analyzed by BET as well (Figure S3 in Supporting Information). Calculating according to the results, the specific area of Fe0 particles and passivated Fe0 particles were 10.2 and 14.4 m2/g, respectively. This indicated that the passivation process could increase the specific area of Fe0 particles due to the formation of passive film, which could enhance the connection between passivated Fe0 particles and Fe0 particles as well as particles and solution, and thus, the treatment efficiency could be improved25. Furthermore, the electrode potential of passive film on surface of passivated Fe0 by was also detected by electrochemical workstation. The electrode potential of passive film was 0.57 V. Thus, the electrode potential of Fe0could be significantly enhanced from -0.44 to 0.57 V after passivation process due to the formation of a dense passive film. In literature, a high potential difference between Fe0 and other materials (e.g., Cu) could form galvanic cell and enhance corrosion rate of Fe0, which could improve the pollutants reduction remarkably26, 27. In particular, the potential difference between Fe0 particles and 13



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passivated Fe0 particles (1.01 V) is much higher than that of Fe0 and Cu0 (0.78 V). Thus, passivated Fe0 could significantly improve the corrosion rate of Fe0. In other words, mixed Fe0/(passivated Fe0)fixed bed filters would have a high treatment efficiency for the PNP removal. 3.2. Effect of initial pH of solution

It has been reported that the initial pH is a key influencing factor for the treatment efficiency of Fe0 based systems

28

. Thus, effects of the initial pH (3.0-9.0) on the PNP

removal by Fe0/(passivated Fe0) or Fe0 alone were evaluated thoroughly in this study. Meanwhile, the mass percentage of Fe0 particles was 45% in Fe0/(passivated Fe0) system. Figure 4(a) and (b) show the logarithmic plots of residual concentration of PNP in solution versus the hydraulic retention time with different initial pH, and a good linear fitting was observed in the treatment system. The observed pseudo-first-order degradation rate constant (kobs) of Fe0/(passivated Fe0) increased from 3.08 min-1 to 7.21 min-1, and that of Fe0 system increased from 0.78 min-1 to 3.87 min-1when the initial pH decreased from 9.0 to 3.0. Figure 4(c) shows the ratio of kobs measured for Fe0/(passivated Fe0) to the kobs measured for Fe0 alone (i.e., kobs(Fe0/(passivated Fe0))/kobs(Fe0)). It can be observed that kobs(Fe0/(passivated Fe0))/kobs(Fe0) rose from 1.86 to 4.50 when the initial pH increased from 3.0 to 7.0, and it decreased a little to 3.95. The results indicate that the reactivity of Fe0/(passivated Fe0) was not limited seriously by the higher pH (i.e., neutral and alkali conditions),while the reactivity of Fe0 alone was inhibited seriously under neutral or alkali conditions. It is well known that the high reactivity of Fe0 alone only could be obtained under the acidic conditions (pH＜ 14


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4.0)29. In addition, the high reactivity of Fe0/(passivated Fe0) was mainly attributed to the high potential difference (1.01 V) between Fe0 and passivated Fe0, which could improve the corrosion rate of Fe0. In other words, passivated Fe0 (0.57 V) have a similar performance (high potential) with the transition metals (e.g., Cu with a potential of 0.34 V), and the similar phenomenon had been found in previous work when the Fe/Cu bimetallic was used to treat PNP in aqueous solution30. Therefore, the optimal initial pH of 7.0 was selected for the Fe0/(passivated Fe0) system in the subsequent experiments. 3.3. Effect of mass percentage of Fe0 of medium materials

Figure 5 shows the PNP removal by the Fe0/(passivated Fe0) system with different mass percentage of Fe0, which demonstrates that all of them followed the pseudo first-order kinetics model (Figure 5(a)). Figure 5(b) shows that kobs increased rapidly to the maximum (4.63 min-1) when mass percentage of Fe0 increased from 0 to 45%, and then it began to decrease rapidly with the further increase of mass percentage of Fe0 (from 45% to 100%). Meanwhile, at a very short hydraulic retention time (0.7 min), a high PNP removal (99.0%) could be obtained by the Fe0/(passivated Fe0) system with an optimal mass percentage of Fe0 (45%, w/w). A similar result had been found in our previous work, when wastewater from acrylonitrile–butadiene–styrene (ABS) resin manufacturing was pretreated by Fe/C micro-electrolysis system31. In particular, the highest COD removal efficiency was obtained by Fe/C micro-electrolysis system with a Fe/C ratio of 1:1 (v/v), which facilitate the formation of macroscopic galvanic cell between Fe and C31. In the Fe0/(passivated Fe0) system, the optimal mass percentage of Fe0 (45%, w/w) would also facilitate the 15



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formation of macroscopic galvanic cell between Fe0 and passivated Fe0. Subsequently, the increase of macroscopic galvanic cell would improve the corrosion rate of Fe0, which could enhance the treatment efficiency of the Fe0/(passivated Fe0) system. As a result, a mass Fe0 percentage of 45% (w/w) was chosen as the optimal condition in the following experiments. 3.4. Control experiments

To investigate the advantage of Fe0/(passivated Fe0) system, 5 control experiments including (a) Fe0alone, (b) passivated Fe0 alone, (c) Fe0/(acid-washed Fe0), (d) Fe0/(aging Fe0), and (f) Fe0/Cu0 were setup. Meanwhile, the same PNP aqueous solution (initial pH of 7.0, and 50 mg/L) and the mass percentage of Fe0 (45%, w/w) were used in the 5 control experiments. Figure 6(a) and (b) show the following kobs values for PNP removal trend toward: Fe0/(passivated Fe0) system (4.63 min-1)> Fe0/(Cu0) system (2.97 min-1)> Fe0/(acid-washed Fe0) system (1.96 min-1)> Fe0/(aging Fe0) system (1.64 min-1)> passivated Fe0 system (1.26 min-1)> Fe0 system (1.03 min-1). The results suggest that the reactivity of Fe0/(passivated Fe0) system was much higher than that of other control experiments. The highest reactivity was mainly attributed to the highest potential difference (1.01 V) between Fe0 and passivated Fe0, which was much higher than that (0.78 V) between Fe0 and Cu0. In addition, kobs (1.64 min-1) of Fe0/(aging Fe0) system was only about one third of that (4.63 min-1) of Fe0/(passivated Fe0) system, which suggest that only the dense passive film of Fe0 prepared by concentrated nitric acid has a high potential (0.57 V). Therefore, Fe0/(passivated Fe0) system superior to other 5 control system, and it was a high efficiency process for PNP removal. 16


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3.5 Operational life

In order to confirm the advantage of Fe0/(passivated Fe0) system for long-term running, the operational life of Fe0/(passivated Fe0), Fe0 alone and Fe0/Cu0 were investigated comparatively. A certain amount of PNP aqueous solution was used as feed water once, and it was treated with the obtained optimal hydraulic retention time (PNP removal reached 99%) of Fe0/(passivated Fe0) (HRT=0.7 min), Fe0 alone (HRT=4 min) and Fe0/Cu0 (HRT=1.2 min), respectively (See Figure S6). Meanwhile, other operation conditions were initial pH of 7.0 and Fe0 mass percentage of 45%. Feed water was replaced after optimal hydraulic retention time treatment, and PNP concentration in solution were detected after each circulation. In addition, when the PNP removal decreased to 10%, the operational life experiment of each system was stopped. Figure 6(c) shows that all PNP removal efficiencies of the 3 treatment system decreased gradually with the increasing of amount of treated wastewater during the whole operational life experiment process. However, the operational life of Fe0/(passivated Fe0) was much higher than that of other two control experiments. In particular, wastewater volume treated by Fe0/(passivated Fe0) system (i.e., 15040 bed volume wastewater) was much higher than that of Fe0/Cu0 system(i.e., 11750 bed volume wastewater) or Fe0 system (i.e., 6171 bed volume wastewater) before the PNP removal was below 10%. Furthermore, the characteristics of Fe0/(passivated Fe0) after operational life experiment were analyzed by SEM and EDS (Figure S7 in Supporting Information). Passivated Fe0 and Fe0 particles were difficult to distinguished after long-term used, and both of them were observed like roundness rather than sponginess (see Figure 2(a) and 17



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Figure S7 in Supporting Information). Fe of 41.93% and O of 58.07% was detected on the Fe particle, respectively. According to the wastewater treatment volume and PNP removal efficiency, the PNP removal per gram filler for different systems could be calculated, respectively. Figure 6(d) shows that the PNP removal per gram filler in Fe0/(passivated Fe0) system (i.e., 423.7 mg PNP/g filler) was higher than that of Fe0/Cu0 system (i.e., 323.2 mg PNP/g filler) or Fe0 system (i.e., 160.0 mg PNP/g filler). As a result, Fe0/(passivated Fe0) system had a longer operational life and a stronger treatment capacity than other two control systems, which could be explained from the following aspects, (i) The highest electrode potential of passivated Fe0 (0.57 V), it was much higher than that of Cu0 (0.34 V), which has a stronger catalytic activity to enhance the corrosion rate of Fe0. Also, the faster iron was corroded, the higher PNP removal was obtained32. (ii) Passivation and caking of Fe0 filler after long-term running, it could impair the reactivity of Fe0 and inhibit the operational life of Fe0 system seriously33, 34. However, these drawbacks could be relieved effectively due to the added passivated Fe0 with a dense passive film (and high electrode potential of 0.57 V) in Fe0/(passivated Fe0) system. In particular, it has a same performance of quartz sand that could achieve the suitable permeability to relieve harden and caking of Fe0 filler35. In addition, improvement of iron corrosion rate resulted from the high potential difference between Fe0 and passivated Fe0 (1.01 V) also relieve this problem. Ma and his colleagues also found the similar phenomenon when the wastewater was treated by Fe0/Cu0 system36. (iii) the similar physical characteristics (i.e., density and particle size) of Fe0 and 18


Page 19 of 37

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passivated Fe0, which facilitate their uniform mixing in fixed bed. Thus it could avoid

the stratification of the fillers, which usually occurred in the fixed bed with quartz sand and Fe0 due to their difference physical property4. The uniform mixing of the fillers could remarkably improve their contact area, which could increase the number of macroscopic galvanic cells formed between Fe0 and passivated Fe0. Also, the more macroscopic galvanic cells, the higher PNP removal could be obtained by Fe0/(passivated Fe0) system. The same phenomenon was also found in our previous work when the industrial wastewater was treated by Fe/C micro-electrolysis system31. In this study, therefore, we could found that operational life of Fe0/(passivated Fe0) system was longer than that of Fe0/Cu0 system, which was mainly attributed to the higher electrode potential of passivated Fe0 with a dense passive film (0.57 V). 3.6 Reaction mechanism of Fe0/(passivated Fe0) system

In literature, Fe0/(transition metal) system reduce PNP through three pathways, (i) indirect reduction by the generated reactive atomic H ([H]abs) on the active sites (i.e., transition metal)37, 38, (ii) indirect reduction by the new generated Fe2+39, (iii) direct reduction on the catalytic activity sites by accepting the released electrons from the iron corrosion16. In this study, the prepared passivated Fe0 has a similar performance with the transition metal to improve iron corrosion, and its reaction mechanism inFe0/(passivated Fe0) system for PNP removal is shown in Figure 7. As shown in Figure 7, its reaction mechanism was as follows. First, plenty of macroscopic galvanic cells would be formed between Fe0 and passivated Fe0 due to their high potential difference (i.e., 1.01 V), which could improve iron corrosion seriously. 19



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Page 20 of 37

And then plenty of electrons released from iron corrosion process could be transported to passive film on the surface of passivated Fe0 due to their high potential difference. The formation of galvanic cell could decrease the activation energy of reaction, Therefore, it could be expected that there will be an activation process when passivated Fe0 particles were used. On the one hand, PNP could be reduced directly through trap the released electron from iron corrosion on the surface of Fe0 of passivated Fe0. On the other hand, the active sites on the surface of passive film could utilize the accepted electrons and catalyze H2, produced by the oxidation of Fe0, dissociation to generate [H]abs, which facilitate the hydrogenation effect of the pollutants. The similar mechanism has been reported when the pollutants were reduced by Fe/Cu bimetallic particles17,

40,

41

.Meanwhile, it was also reported that the higher electrode potential of the catalytic

activity sites could result in better effect of these two pathways42. Besides, the new generated Fe2+ was also a strong reducing agent that could reduce the pollutants effectively. The higher iron corrosion rate, the higher concentration of Fe2+ was released, which facilitate the PNP removal in Fe0/(transition metal) system. 3.7 Analysis of preparation cost

The comparations of preparation cost of Fe0/(passivated Fe0) system, Fe0 system and Fe0/(Cu0) system were presented in Table 1. The prepared cost (~220 USD/t) of passivated Fe0 particles can be calculated according to the market price of Fe0 particles (~200 USD/t) and concentrated nitric acid (~150 USD/t). Therefore, the costs of the fillers in the fixed bed of Fe0/(passivated Fe0), Fe0 alone and Fe0/(Cu0) were about 210 USD/t, 200 USD/t and 2290 USD/t, respectively. Also, it could be calculated that about 20


Page 21 of 37

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424 kg, 160 kg and 323 kg PNP could be removed by per ton Fe0/(passivated Fe0), Fe0 and Fe0/Cu0, respectively. Subsequently, the materials expenses in Fe0/(passivated Fe0) system, Fe0 system and Fe0/(Cu0) system for PNP removal are about 500 USD/t PNP, 1250 USD/t PNP and 7085 USD/t PNP, respectively. As a result, Fe0/(passivated Fe0) system was a cost-effective technology for contaminated water treatment.

4. Conclusions

The model PNP contaminated water was effectively treated by the Fe0/(passivated Fe0) (passivated by concentrated nitric acid) system. The characteristic of passivated Fe0 particles was observed, and according to the analysis results of SEM, EDS, XRD, XPS, BET and electrochemical workstation, it could concluded that passive film were uniformly deposited on the surface of Fe0 substrate after Fe0 were treated by the passivation process. Meanwhile, O (5.29%, w/w) was detected on the surface of passivated Fe0 particles, and the main composition of the film was iron oxides (i.e., Fe2O3, Fe3O4 etc.). In addition, it is difficult to remove the film by regular acid-washed, which indicated that this film is inert and could be seem as inert materials to achieve the suitable permeability to relieve harden and caking of Fe0 filler, and thus the operational life of Fe0/(passivated Fe0) system could be enhanced. Also, the passivation Fe film has high potential (0.57 V), which could form high potential difference (1.01 V) galvanic cell with Fe0, and thus improve the reactivity of Fe0 and degradation rate of PNP. Besides, under the optimal conditions (initial pH of 7.0 in solution and mass Fe0 percentage of 45% of medium materials), Fe0/(passivated Fe0) system could obtain high 21



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Page 22 of 37

PNP (＞99.0%) with short hydraulic retention time (0.7 min) and high kobs value (4.63 min-1), whose ability was much higher than that of 5 control experiments (i.e., Fe0 system, passivated Fe0 system, Fe0/(acid-washed Fe0) system, Fe0/(aging Fe0) system and Fe0/(Cu0) system). Furthermore, Fe0/(passivated Fe0) system also shows the obvious advantage of operational life compared with Fe0/(Cu0) system and Fe0 system. Meanwhile, the reaction mechanism of Fe0/(passivated Fe0) system for PNP removal was also proposed. Finally, according to the preparation cost analysis, the materials expenses in Fe0/(passivated Fe0) system (500 USD/t PNP) for PNP removal were also much cheaper than that in Fe0 system (1250 USD/t PNP) and Fe0/(Cu0) system (7085 USD/t PNP). As conclusions, Fe0/(passivated Fe0) system can be considered as a cost-effective, feasible and robust Fe0 based system for contaminated water treatment.

Supporting Information

EDS mapping imaging of Fe0, passivated Fe0 and passivated Fe0 after acid washing by HCl (20%, w/w) (Figure S1); XRD patterns of Fe0 particles and passivated Fe0 particles (Figure S2); N2 adsorption and desorption isotherms of Fe0 particles and passivated Fe0 particles (Figure S3); Effect of initial pH on the PNP removal by Fe0/(passivated Fe0) system and Fe0 system (Figure S4); Effect of mass percentage of Fe0 on PNP removal by Fe0/passivated Fe0 system (Figure S5); Effect of different treatment systems with different initial pH conditions on PNP removal (Figure S6); SEM-EDS of the particle from Fe0/(passivated Fe0) system after operational life experiments (Figure S7). 22


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Acknowledgements

The authors would like to acknowledge the financial support from National Natural Science Foundation of China (No. 21207094), and Fundamental Research Funds for the Central Universities (No. 2015SCU04A09).

23



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Page 24 of 37

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(ABS)

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Pd/Fe nanoparticles: Comparisons with other bimetallic systems, kinetics and mechanism. Sep and Purif Technol 2010, 76, 206-214.

29



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Page 30 of 37

Table 1 Comparation of Fe0/(passivated Fe0) system, Fe0 system and Fe0/(Cu0) system

Hydraulic retention time Systems

kobs values

costs of the fillers

PNP removal

materials expenses for PNP removal

(min-1)

(USD/t)

(kg/t filler)

(USD/t PNP)

(obtained 99% PNP removal) (min)

Fe0/(passivated Fe0)

0.7

4.63

210

424

500

Fe0 alone

4.0

1.03

200

160

1250

Fe0/(Cu0)

1.2

2.97

2290

323

7085

30


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(a)

(d)

(g)

(b)

(e)

(h)

(c)

(f)

(i)

Figure 1. SEM images of (a-c) Fe0, passivated (d-f) Fe0and (g-i) passivated Fe0 after acid-washing by HCl (20%, w/w).

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

(a)

Item

Fe

Weight (%) 100.00

Page 32 of 37

Total 100

Fe

Fe Fe

0

1

2

3

4

5

6

7

8

9

10

8

9

10

8

9

10

Energy (Kev)

(b)

(b)

Item

Fe

O

Total

Weight (%)

94.71

5.29

100

Fe

Fe O Fe 0

1

2

3

4

5

6

7

Energy (Kev)

(c)

(c)

Item

Fe

O

Total

Weight (%)

96.92

3.08

100

Fe

Fe O Fe 0

1

2

3

4

5

6

7

Energy (Kev)

Figure 2. SEM-EDS of (a) Fe0, (b) passivated Fe0 and (c) passivated Fe0 after acid-washing by HCl (20%, w/w) (without considering of C element).

32


Page 33 of 37

Intensity (a.u.)

(a)

O

Fe

1000

800

C

600

400

200

0

Binding energy (eV)

(b)

(c)

Fe 2p

O 1s

711.0 eV

724.5 eV

Intensity (a.u.)

531.1 eV

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


740

730

720

Binding energy (eV)

710

700

529.7 eV

540

538

536

534

532

530

528

526

524

522

Binding energy (eV)

Figure 3. XPS spectra of passivated Fe0 particles: (a) survey scan, (b) Fe 2p core level and (c) O 1s core level.

33



0.5

0.5

(a) Fe0/(passivated Fe0) pH=3.0 kobs=7.21 R2=0.98

0.0

0.0

(b)

pH=3.0 kobs=3.87 R2=0.93

Fe0 alone

pH=4.0 kobs=2.32 R2=0.99

2

pH=4.0 kobs=6.33 R =0.99

-0.5

2 2

-1.0

2

-1.5

pH=6.0 kobs=5.09 R =0.99

ln(C/C0)

pH=7.0 kobs=4.63 R =0.99 2

-1.5

pH=5.0 kobs=1.91 R2=0.99

-0.5

pH=5.0 kobs=5.43 R =0.99

-1.0

ln(C/C0)

pH=9.0 kobs=3.08 R =0.98

-2.0

pH=6.0 kobs=1.51 R2=0.99 pH=7.0 kobs=1.03 R2=0.97 pH=9.0 kobs=0.78 R2=0.99

-2.0 -2.5 -3.0

-2.5

-3.5

-3.0 0.0

0.2

0.4

0.6

0.8

1.0

-4.0

0

1

Hydraulic retention time (min) 6

kobs(Fe0/(passivated Fe0))/kobs(Fe0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 37

2

3

4

Hydraulic retention time (min)

(c)

5

4.5 3.95

4 3.37

3 2

2.73

2.84

1.86

1 0 3

4

5

6

7

9

Initial pH value

Figure 4. Effect of initial pH on the PNP removal by Fe0/(passivated Fe0) or Fe0alone: (a) PNP removal efficiency described by pseudo-first-order equation for Fe0/(passivated Fe0), (b) PNP removal efficiency described by pseudo-first-order equation for Fe0 alone and (c) bar graph displaying the ratio of kobs measured for Fe0/(passivated Fe0) to the kobs measured for Fe0alone (i.e., kobs(Fe0/(passivated Fe0))/kobs(Fe0)) (Experiment conditions: initial PNP concentration=50 mg/L, mass percentage of Fe0in Fe0/(passivated Fe0)=45% and total mass of Fe0 and passivated Fe0=20 g)

34


Page 35 of 37

0.5

5

100% kobs=1.03 R2=0.96

(a)

(b)

60% kobs=3.17 R2=0.99

0.0

2

55% kobs=4.01 R =0.99

-0.5

4

50% kobs=3.93 R2=0.99 40% kobs=4.31 R2=0.99

-1.5

35% kobs=3.78 R2=0.99 0%

-2.0

kobs=1.26 R2=0.96

kobs (min-1)

45% kobs=4.63 R2=0.99

-1.0

ln(C/C0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

2

-2.5 -3.0 -3.5

1 0.0

0.5

1.0

1.5

2.0

2.5

Hydraulic retention time (min)

3.0

3.5

0

20

40

60

80

100

Mass percentage of Fe0 (%)

Figure 5. Effect of mass percentage of Fe0 on PNP removal by Fe0/(passivated Fe0) system: (a) PNP removal efficiency described by pseudo-first-order equation and (b) kobs for PNP removal as a function of mass percentage of Fe0 (Experiment conditions: initial PNP concentration=50 mg/L, initial pH=7.0 and total mass of Fe0 and passivated Fe0=20 g).

35



0

6

Fe0/(acid-washed Fe0) system kobs=1.96 R2=0.96

(a)

Fe0/(passivated Fe0) system kobs=4.63 R2=0.99 0

0

Passivated Fe0 system kobs=1.26 R2=0.96

-2

-1

Fe0 system kobs=1.03 R2=0.96

4.63

4

Fe0/(Cu0) system kobs=2.97 R2=0.98

kobs (min )

ln(C/C0)

-1

(b)

5

2

Fe /(aging Fe ) system kobs=1.64 R =0.97

-3

3

1

2

2.97

1.96 1.64 1.03

0

0

A: Fe0/(passivated Fe0) system B: Fe0 system C: Passivated Fe0 system D: Fe0/(acid-washed Fe0) system E: Fe0/(aging Fe0) system F: Fe0/(Cu0) system

2 1

-4

A

3

B

1.26

C

D

E

F

Hydraulic retention time (min) 500

(c) 0

0

Fe /(passivated Fe ) system Fe0 system Fe0/(Cu0) system

80 60 40 20 0

PNP removal (mg/ g filler)

100

PNP removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(d)

423.7

400 323.2 300

200

160.0

100

0

0

2500

5000

7500 10000 12500 15000

Fe0/(passivated Fe0)

Fe0 alone

Fe0/(Cu0)

Bed volume

Figure 6. PNP removal by the different treatment systems (i.e., Fe0/(passivated Fe0), passivated Fe0, Fe0/(acid-washed Fe0), Fe0/(aging Fe0) and Fe0/Cu0), (a) PNP removal efficiency described by pseudo-first-order equation, (b) kobs for PNP removal by the different system, (c) PNP removal as a function of bed volume of wastewater, (d) PNP removal per gram filler (mg/g filler) (Experiment conditions: initial PNP concentration=50 mg/L, mass percentage of Fe0 particles=45%, initial pH=7.0,total mass of Fe0 and other fillers=20 g)

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Figure 7. Reaction mechanism of the Fe0/(passivated Fe0) system for PNP removal

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Removal of p-Nitrophenol in Aqueous Solution by Mixed Fe0

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