Comparative study on the characteristics, operational life and reactivity of Fe/Cu bimetallic particles prepared by electroless and displacement plating process

Abstract

In this study, an electroless (electrode-less) copper plating technology was developed to prepare the high-reactive and robust iron–copper (Fe/Cu) bimetallic particles. First, effect of pretreatment and key preparation parameters (e.g., complexant, H₃BO₃, NiSO₄·7H₂O, pH and plating time) on the reactivity of Fe/Cu bimetallic particles were investigated, respectively. Their reactivity was evaluated according to the obtained K_obs for PNP removal. Also, the characteristics of Fe/Cu bimetallic particles prepared by electroless plating and displacement plating were comparatively observed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The analysis results show that when Fe/Cu bimetallic particles were prepared by electroless plating, copper was uniformly deposited on the surface of Fe⁰ substrate, and only a few bulky particles were observed. However, plenty of loose copper blocks were heterogeneously distributed on the surface of Fe⁰ substrates when the Fe/Cu bimetallic particles were prepared by the conventional displacement plating. Furthermore, the operational life and reactivity of Fe/Cu bimetallic particles prepared by electroless and displacement plating process were comparatively investigated by the recycling experiment. The results suggest that the new Fe/Cu bimetallic particles have a longer operational life than that of the conventional Fe/Cu bimetallic particles. Meanwhile, it can be seen from the control experiments that the new Fe/Cu bimetallic particles have a stronger reactivity for the PNP removal both in Fe/Cu/air and Fe/Cu/N₂ processes. As results, the new electroless plating for the preparation of Fe/Cu bimetallic particles is superior to the conventional displacement plating. In other words, the electroless copper plating is a promising technology to prepare the high-reactive and robust Fe/Cu bimetallic particles.

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1. Introduction

Nowadays, toxic and refractory industrial wastewater containing aromatic pollutants still represents a big challenge for researchers. p-Nitrophenol (PNP) and its derivatives are typical aromatic pollutants discharged from pesticides, plasticizers, dyes and explosives.^1–4 All of these industrial wastewaters are difficult to treat directly by a conventional biological system,⁵ and some of them are considered hazardous wastewaters.⁶ Advanced oxidation processes (AOPs) including Fenton process,⁷ microwave assisted oxidation,⁸ ozonation^9,10 and electrochemical oxidation¹¹ are usually used to treat refractory and toxic wastewater containing PNP and its derivatives. However, all of these processes suffer from the limitation of high costs or low efficiency.

Zero-valent iron (ZVI) has been used to treat the refractory and toxic wastewater contaminated with chlorihated organic compounds (COCs), nitroaromatic compounds (NACs), heavy metals, nitrate, dyes, phenol and etc.⁶ And it is an environmentally friendly water and wastewater treatment process. In recent years, it has been found that iron–copper (Fe/Cu) bimetallic particles prepared by plating Cu layer on Fe⁰ surface have a stronger reactivity for reduction rates for organic matter transformation of the refractory and toxic pollutants in wastewater, due to the high potential difference (0.78 V) between Cu and Fe.^12–15 In addition, in Fe/Cu/air system, Cu is used as a catalyst to enhance the corrosion rate of Fe⁰, which could facilitate the generation of H₂O₂ and trigger Fenton-like reaction (eqn (1) and (2)).^16–18 Also, Fe/Cu bimetallic particles have a stronger reactivity or lower cost than other bimetallic particles (e.g., Fe/Pd, Co/Fe, Pt/Fe, Au/Fe or Fe/Ag) in wastewater industry.^13,19 Therefore, Fe/Cu bimetallic particles system is a promising technology for wastewater treatment and needs to be investigated thoroughly.

Fe²⁺ + H₂O₂ → ˙OH + Fe³⁺ + OH⁻ (2)

In literature, deposition of Cu on Fe⁰ surface by displacement plating is the prevailing method to prepare the Fe/Cu bimetallic particles.^13,14,20,21 In our previous work, the reactivity and operational life of Fe/Cu bimetallic particles could be improved through optimizing the preparation conditions (e.g., temperature, Cu²⁺ concentration, pH, etc.) has been found.²² Nevertheless, there is still some loose, flocculent or lumpy copper on the Fe⁰ surface, which is easy to be dropped off due to the external shearing force during the wastewater treatment process. In other words, the performance of Fe/Cu bimetallic particles prepared by the displacement plating would be limited by the abscission of Cu layer. In the displacement plating process, Cu²⁺ ions are reduced and deposited on Fe⁰ surface, meanwhile plenty of Fe²⁺ ions are rapidly generated and released which would affect the deposition of Cu. In addition, the deposited Cu would inhibit the further reduction of Cu²⁺ by Fe⁰. Furthermore, the consumption of Cu²⁺ during the preparation process should be further decreased to cut the preparation cost. Thus, it is necessary to develop a technology to avoid or reduce the limitation of the present preparation technology for Fe/Cu bimetallic particles.

Electroless (electrode-less) plating is a variety of chemical deposition technology, involving the deposition of metals from bath onto surfaces without applying an external electric voltage.²³ It is carried out via the redox reaction of an oxidizer and a reductant in an electrolyte solution.²⁴ For example, the nontoxic sodium hypophosphite (NaH₂PO₂·H₂O) is usually used as a reductant to reduce the metal ions (e.g., Cu²⁺, Ni²⁺, Co²⁺ and etc.), and then the metals are deposited on the substrates.^25–27 If the Fe/Cu bimetallic particles were prepared by using the electroless plating, the Fe⁰ substrates would not be corroded to release plenty of Fe²⁺/Fe³⁺ ions, which would facilitate the deposition of Cu on the Fe⁰ substrates and enhance the bonding force between Cu and Fe⁰ substrates. In addition, the deposition rate of Cu could be controlled through adjusting the preparation conditions or additive agents. Therefore, the electroless plating might be much better than the displacement plating. Although this technology has been widely used for the production with uniform, less porous, adherent metal film in many industrial applications (e.g., printed circuit board industry,^28–30 aerospace and automotive industry³¹), there is no report on the preparation of Fe/Cu bimetallic particles for environmental application by using the electroless plating.

In this study, the electroless plating was used to synthesize Cu-coated Fe⁰ particles (i.e., Fe/Cu bimetallic particles). A promising electroless plating process would be obtained for Fe/Cu bimetallic particles preparation by experiments and analyses. First, effect of the preparation parameters (e.g., plating time, initial pH and ingredient of plating bath) on the reactivity of Fe/Cu bimetallic particles were investigated and evaluated according to their reduction rates for PNP in aqueous solution. Furthermore, comparative study on operational life of the Fe/Cu bimetallic particles prepared by electroless plating and displacement plating was carried out. Also, to confirm the superiority of the Fe/Cu bimetallic particles prepared by the electroless plating, comparative study on the reactivity of the two different Fe/Cu bimetallic particles under different condition of air or N₂ were carried out by 4 different batch experiments. Finally, characteristics of Fe/Cu bimetallic particles were observed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS).

2. Experimental

2.1 Reagents

In the experiment, the analytical reagents including PNP, CuSO₄ and Na₂SO₄ 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 98%. Other chemicals used in the experiment were of analytical grade. Deionized water was used in all experiments.

2.2 Preparation of the Fe/Cu bimetallic particles

Two different Fe/Cu bimetallic particles were prepared in this study. One was prepared by the conventional displacement plating, and the other was prepared by the new electroless plating. The preparation method and optimal parameters of electroless plating were listed in ESI,† while the preparation method of displacement plating has been reported in our previous work.^1,22 Meanwhile, their surface characteristics were comparatively investigated through the analyses of secondary electron imaging (SEI), back scattered electron imaging (BES), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS).

2.3 Experiment process

2.3.1 Operational life of the Fe/Cu bimetallic particles. To confirm the longer operational life of the Fe/Cu bimetallic particles prepared by electroless plating, a control experiment (using Fe/Cu bimetallic particles prepared by the conventional displacement plating) was setup. Under the same conditions, two different Fe/Cu bimetallic particles were repeatedly used to remove PNP in aqueous solution, and then their operational life could be evaluated according to the PNP removal.

When the Fe/Cu bimetallic particles were repeatedly used to treat PNP wastewater, their operational life could be evaluated by the observed pseudo-first-order degradation rate constant (K_obs, min⁻¹) for PNP removal because the PNP removal efficiency obtained by the Fe/Cu particles could be described by the pseudo-first-order equation:³²

where C is the PNP concentration at instant t (mg L⁻¹), C₀ is the initial PNP concentration, K_obs is the observed pseudo-first-order degradation rate constant (min⁻¹) and t is the treatment time (min). Finally, the PNP removal experiments and operating conditions can be presented as follows:

500 mg L⁻¹ PNP stock solution was prepared by 50 mmol L⁻¹ Na₂SO₄ addition used as an electrolyte.¹ Also, the PNP aqueous solution was not buffered, and its initial pH was adjusted to 7.0 by adding sodium hydroxide solutions (5 mol L⁻¹). First, 400 mL PNP aqueous solution and 30 g L⁻¹ Fe/Cu bimetallic particles were added in a 500 mL flat bottom beaker, and the slurry was mixed by a mechanical stirrer (300 rpm). This process was performed at 30 ± 1 °C by water batch heating. To detect the residual PNP concentration, samples were taken from the reactive beaker at intervals (5, 10, 15, 20, 30 and 40 min) by withdrawing 1 mL of sample solution. The obtained samples were filtered through the hydrophilic polyethersulfone (PES) syringe filters (0.45 μm) to remove Fe/Cu bimetallic particles, and then their PNP concentrations were determined by using HPLC (High Performance Liquid Chromatography). This batch experiment was continuously repeated 5 times, and 400 mL fresh PNP aqueous solution was treated in each batch experiment. In addition, 30 g L⁻¹ Fe/Cu bimetallic particles added at the first batch experiment were repeatedly used in the whole 5 times batch experiment, and there were no fresh Fe/Cu bimetallic particles to be complemented.

2.3.2 Degradation of PNP by Fe/Cu/air or Fe/Cu/N₂ processes. To confirm the superiority of the Fe/Cu bimetallic particles prepared by the electroless plating, comparative study on the reactivity of the two different Fe/Cu bimetallic particles under different conditions of air or N₂ was carried out by 4 different batch experiments. In particular, (I) Fe/Cu/air process, Fe/Cu particles were prepared by electroless plating, (II) Fe/Cu/air process, Fe/Cu particles were prepared by displacement plating, (III) Fe/Cu/N₂ process, Fe/Cu particles were prepared by electroless plating, (IV) Fe/Cu/N₂ process, Fe/Cu particles were prepared by displacement plating. In addition, the operation conditions of 4 batch experiments were as following: [PNP]₀ = 500 mg L⁻¹, initial pH = 7.0, Fe/Cu dosage = 30 g L⁻¹, TML_Cu (theoretical Cu mass loading) = 1.74%, stirring speed = 300 rpm and gas (air or N₂) flow rate = 1.5 L min⁻¹. To investigate the mechanism of PNP degradation, samples were taken from the reactive beaker at intervals (5, 10, 15, 20, 30 and 40 min) by withdrawing 2 mL of sample solution. The obtained samples were filtered through the hydrophilic polyethersulfone (PES) syringe filters (0.45 μm) in order to remove Fe/Cu bimetallic particles. COD, UV-vis and HPLC of them were detected after that. In addition the oxidation–reduction potential (ORP) of solution was detected at intervals.

2.4 Analytical method

The concentration of p-nitrophenol (PNP), p-aminophenol (PAP), p-benzoquinone (BK) and hydroquinone (HC) in the samples were 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⁻¹. Detection was performed using a G1365MWD UV detector set at 317 nm, 273 nm, 246 nm and 288 nm for PNP, PAP, BK and HC respectively. Characteristics of the prepared Fe/Cu bimetallic particles were observed by SEM (JSM-7500F, JEOL, Japan). Both of SEI and BES were observed to identify the morphology and the distribution of Cu on the Fe⁰ surface. In addition, the surface elementary composition of Fe/Cu bimetallic particles was analyzed by EDS. X-ray photoelectron spectroscopy (XPS) was used to identify the chemical states of Fe, Cu and O in the fresh or reacted Fe/Cu bimetallic particles (prepared by electroless plating under the optimal conditions). XPS analyses were similar to those reported in literature.^21,33 In particular, freshly prepared Fe/Cu bimetallic particles were mounted onto stainless steel sample stubs using carbon tape, and were placed in a sealed container prior to removal from the anaerobic glovebox. And then the samples were sent to the XPS instrument. Details of the analysis can be found elsewhere.³⁴ UV-vis absorption spectra of the samples were carried out in 10 mm quartz cuvettes. The simples were diluted 10 times before the analysis, and they were recorded from 190 to 500 nm. COD was determined by using COD (Chemical Oxygen Demand) analyzer (Lianhua, China). The oxidation–reduction potential (ORP) of solution was detected by a redox electrode assembly (Sinomeasure, China). The pH was measured by a pHS-3C meter (Rex, China).

3. Results and discussion

3.1 Fe/Cu bimetallic particles prepared by the electroless plating

In electroless copper plating process, Cu mass loading of the prepared Fe/Cu bimetallic particles could be directly affected by the plating time. In addition, nickel (Ni) is mandatory for the electroless copper deposition with sodium hypophosphite as a reducing agent due to its catalytic effect on the anodic oxidation reaction.^35,36 Furthermore, the copper deposition rate could be affected by the initial pH and the concentration of H₃BO₃, Ni²⁺ and complexants in the plating bath.^25,37 Therefore, it is necessary to optimize the key parameters including plating time, initial pH and the concentration of Ni²⁺, H₃BO₃ and complexants.

In addition, prior to the electroless plating process, pretreatment of the substrate materials was required to increase the surface roughness of the substrate materials.^26,38 After the proper pretreatment, the evenly distributed concave holes could be formed on the surface of substrate materials, which could improve the adhesion between the metal plating and substrate materials by the anchor effect.^39,40

Details pertaining to the batch experiments used to optimize the key parameters are provided in the ESI (Fig. S1–S5†). Meanwhile, Fig. S6 and S7 (ESI†) show that pretreatment of Fe⁰ particles (i.e., aeration oxidation and acid-washing) before the electroless copper plating could significantly improve the reactivity of the prepared Fe/Cu bimetallic particles. As a result, the optimal key plating conditions (i.e., Na₂EDTA·2H₂O = 10 g L⁻¹, H₃BO₃ = 30 g L⁻¹, NiSO₄·7H₂O = 0.5 g L⁻¹, pH = 9.5, plating time = 1 min, and pretreatment of Fe⁰ particles) could be concluded. Under the optimal conditions, a high PNP removal efficiency of 98.1% was obtained by the prepared Fe/Cu bimetallic particles, and its K_obs reached 0.095 min⁻¹. Furthermore, the other conditions (i.e., CuSO₄·5H₂O = 11.25 g L⁻¹, Fe = 30 g L⁻¹, NaH₂PO₂·H₂O = 50 g L⁻¹, T = 70 °C and stirring speed = 250 rpm) were chosen for the subsequent experiments.

Finally, details pertaining to the recycling experiments used to evaluate the operational life of the electroless plating bath are provided in the ESI (Fig. S8†). The results suggest that the reactivity of Fe/Cu bimetallic particles were not affected seriously by the recycling of the electroless plating bath. In other words, electroless copper plating was an effective and robust process to prepare the Fe/Cu bimetallic particles.

3.2 Characteristics of the prepared Fe/Cu bimetallic particles

Table S2 (ESI†) shows that under the above optimal conditions and parameters, TML_Cu of Fe/Cu bimetallic particles prepared by electroless plating was about 1.74% (w/w). In other words, only a small amount of Cu was used as catalyst to be deposited on Fe⁰ surface. Meanwhile, TML_Cu was calculated according to the Cu²⁺ consumption of plating bath. Only little TFe (i.e., sum of Fe²⁺/Fe³⁺ ions concentration) was detected in the electroless plating bath after a Fe/Cu preparation process, which suggests that Fe⁰ substrates were hardly to be corroded and consumed during the Fe/Cu preparation process. According to the reaction mechanism of electroless copper plating (eqn (4)), Fe⁰ substrates would not be consumed.³⁶ In the displacement process, however, TFe reached 0.471 g L⁻¹, which was mainly attributed to Fe–Cu displacement reaction. The rapidly released Fe²⁺/Fe³⁺ ions might seriously affect the deposition of Cu, meanwhile the high reaction rate (i.e., high Cu deposition rate) would also affect the distribution characteristics of Cu on the Fe⁰ surface.^41,42

To further compare the surface characteristics of Fe/Cu bimetallic particles prepared by two different methods, they were observed by using BES and SEM-EDS in this study. Fig. 1(a)–(c) and 2(a)–(c) shows that copper was uniformly deposited on the surface of Fe⁰ substrate, and only a few bulky particles were observed, when Fe/Cu bimetallic particles were prepared by electroless plating. Specially, it could observe from Fig. 1(c) and 2(c) that plenty of nano-copper particles were deposited on the surface of Fe⁰ substrates, which would significantly increase the contact area between Fe and Cu. The high contact area means that plenty of Fe–Cu galvanic cells would be formed, which would enhance the reactivity of Fe/Cu bimetallic particles.^13,21 Meanwhile, the nano-copper particles would have an adhesion on the surface of Fe⁰ substrates,⁴³ which would avoid the abscission of copper and improve the operational life of Fe/Cu bimetallic particles. Furthermore, the nano-copper particles have a high catalytic activity for the iron corrosion, which would improve the reactivity of Fe/Cu bimetallic particle. However, Fig. 1(d)–(f) and 2(d)–(f) shows that plenty of loose copper blocks were heterogeneously distributed on the surface of Fe⁰ substrates when the Fe/Cu bimetallic particles were prepared by the conventional displacement plating. Fig. 1(d) also shows that some copper blocks have dropped off from Fe⁰ substrate. These characteristics would inhibit their reactivity from two aspects, (i) the loose copper blocks would decrease their contact area with Fe⁰ substrates, which would limit the formation of Fe–Cu galvanic cells, (ii) the loose copper blocks were easy to be dropped off by the external shearing force when they were used to treat wastewater under the higher rotation speed conditions.²² Subsequently, both their reactivity and operational life would be limited seriously. For example, their K_obs for PNP removal rapidly decreased from 0.077 to 0.053 min⁻¹ after only two cycles (Fig. 3(b)), while K_obs for PNP removal only decreased a little from 0.095 to 0.090 min⁻¹ when the Fe/Cu bimetallic particles prepared by electroless plating were used (Fig. 3(a)).

Fig. 1 Backscattered electron (BSE) imaging of Fe/Cu bimetallic particles prepared by (a–c) electroless plating, (d–f) displacement plating.

Fig. 2 EDS mapping of Fe/Cu bimetallic particles prepared by (a–c) electroless plating, (d–f) displacement plating.

Fig. 3 Operational life of the Fe/Cu bimetallic particles prepared by electroless plating (a) and displacement plating (b), and the dependence of K_obs on the number of cycles for the two different Fe/Cu particles (c). ([PNP]₀ = 500 mg L⁻¹, initial pH = 7.0, Fe/Cu dosage = 30 g L⁻¹, TML_Cu = 1.74% and stirring speed = 300 rpm).

Also, SEM-EDS analyses of Fe/Cu bimetallic particles prepared by the two different methods are shown in Fig. S9(a) and (b).† It is clear that Cu (2.29 wt%) on Fe/Cu surface prepared by electroless plating detected by EDS was much higher than that (1.96 wt%) prepared by displacement plating, when the Fe/Cu bimetallic particles had same TML_Cu (1.74%, w/w). The results suggest that only a part of Cu could be deposited on the surface of Fe⁰ substrates, and other Cu would be dropped off during the preparation process by using the displacement plating. This phenomenon has also been found in our previous works.^1,22 The lower Cu mass loading on Fe⁰ surface would directly decrease the reactivity of Fe/Cu bimetallic particles. For example, K_obs (0.077 min⁻¹) obtained by the Fe/Cu bimetallic particles (prepared by displacement plating) was much lower than that (0.095 min⁻¹) obtained by Fe/Cu bimetallic particles (prepared by electroless plating).

Fig. S9(a) and (b)† also show that there was no oxygen detected when Fe/Cu bimetallic particles were prepared by electroless plating, while oxygen (6.97 wt%) was detected by EDS when they were prepared by displacement plating. In the electroless plating process, the excessive NaH₂PO₂·H₂O (50 g L⁻¹) were used as reducing agents to reduce the Cu²⁺, and they could also protect Fe⁰ substrates from oxidation even if this process was exposure to air. Thus, iron oxidation was hard to be generated on Fe⁰ substrates. In the displacement plating process, however, Fe⁰ substrates were easy to be oxidized if there is no nitrogen protection. In addition, the nitrogen protection would increase the operational difficulty and costs of the displacement plating. Furthermore, the generated iron oxidations would impair the reactivity of Fe/Cu bimetallic particles. According to the above comparative study, it could be concluded that the electroless copper plating process was much superior to the displacement plating process.

3.3 Operational life of Fe/Cu bimetallic particles

Fig. 3 shows that the Fe/Cu bimetallic particles prepared by the two different methods were respectively used to remove 500 mg L⁻¹ PNP in aqueous solution under the same conditions. Fig. 3(a) and (b) show that K_obs (0.095 min⁻¹) for PNP removal obtained by the fresh Fe/Cu bimetallic particles prepared by electroless plating was much higher than that (0.077 min⁻¹) obtained by the fresh Fe/Cu bimetallic particles prepared by conventional displacement plating. The results suggest that the new developed method could significantly improve the reactivity of Fe/Cu bimetallic particles.

Fig. 3 also shows that two different Fe/Cu bimetallic particles were respectively recycled to remove the 500 mg L⁻¹ PNP in aqueous solution. Although their K_obs for PNP removal were all decreased gradually with the increase of number of cycles, K_obs (0.068–0.095 min⁻¹) obtained by Fe/Cu (prepared by electroless plating) were always much higher than those (0.039–0.077 min⁻¹) obtained by Fe/Cu (prepared by displacement plating) (see Fig. 3). Fig. 3(c) shows that the data was plotted as the K_obs against the number of cycles. When Fe/Cu bimetallic particles prepared by the electroless plating were used to remove PNP in aqueous solution, the K_obs and number of cycles have a good linear relationship and the correlation of determination (R²) reaches 0.99, which suggests that the K_obs is proportional to the number of cycles and K_obs can be decreased by elevating the number of cycles. This phenomenon can be explained that the Fe/Cu dosage used to remove PNP was consumed gradually with the increasing number of cycles. The decrease of Fe/Cu dosage would result in the reduction of surface area, [H]_abs, active site and galvanic couple, which would inhibit their removal efficiency for the pollutants.^1,44 However, when Fe/Cu bimetallic particles prepared by displacement plating were used to remove PNP in aqueous solution, the K_obs and number of cycles do not have a good linear relationship (R² = 0.78). In particular, its K_obs rapidly decreased from 0.077 to 0.053 min⁻¹ after only two cycles (Fig. 3(b)), which suggests that the reactivity of Fe/Cu was decreased seriously. This phenomenon might be mainly attributed to the abscission of Cu coating from Fe/Cu bimetallic particles. In other words, adhesion between Cu coating and Fe⁰ particles of the Fe/Cu bimetallic particles (prepared by conventional displacement plating) might be much less than that prepared by the electroless plating.

In addition, SEM-EDS analysis of Fe/Cu bimetallic particles recycled to remove the 500 mg L⁻¹ PNP in aqueous solution were also investigated. Fig. S9(c) and (d)† show that after 5 recycling treatment of the 500 mg L⁻¹ PNP aqueous solution, oxygen (11.53 wt%) on the surface of Fe/Cu (prepared by electroless plating) was much lower than that (32.03 wt%) of Fe/Cu prepared by displacement plating. Meanwhile, Cu (3.77 wt%) of the former was much higher than that (2.93 wt%) of the later. The results show that Fe/Cu bimetallic particles prepared by the electroless plating were not easy to be passive by the iron corrosion products, which could have a good reactivity and operational life.

Therefore, the electroless copper plating is a promising technology to prepare the high-reactive and robust Fe/Cu bimetallic particles.

3.4 Degradation of PNP by Fe/Cu/air or Fe/Cu/N₂ process

3.4.1 PNP and COD removal efficiencies. In Fe/Cu/air process, Fig. 4(a) and (b) show that PNP and COD removal efficiencies obtained by using Fe/Cu bimetallic particles prepared by electroless plating were much higher than those obtained by using Fe/Cu bimetallic particles prepared by displacement plating. For example, the former PNP and COD removal efficiencies reached about 83.1% and 32.1% after 40 min treatment, while the later only reached about 65.0% and 21.0%, respectively. And as shown in Fig. 4(e), the ORP values obtained during Fe/Cu/air process by using Fe/Cu bimetallic particles prepared by electroless plating were also higher than those obtained by using Fe/Cu bimetallic particles prepared by displacement plating.

Fig. 4 Comparative study on the reactivity of Fe/Cu bimetallic particles prepared by electroless and displacement plating under the conditions of N₂ or air. (a) PNP removal by Fe/Cu/air process, (b) COD removal by Fe/Cu/air process, (c) PNP removal by Fe/Cu/N₂ process, (d) COD removal by Fe/Cu/N₂ process, (e) ORP of Fe/Cu/air process, (f) ORP of Fe/Cu/N₂ process ([PNP]₀ = 500 mg L⁻¹, initial pH = 7.0, Fe/Cu dosage = 30 g L⁻¹, TML_Cu = 1.74%, stirring speed = 300 rpm and air or N₂ flow rate = 1.5 L min⁻¹).

In Fe/Cu/N₂ process, Fig. 4(c) and (d) show that PNP and COD removal efficiencies obtained by using Fe/Cu bimetallic particles prepared by electroless plating were also higher than those obtained by using Fe/Cu bimetallic particles prepared by displacement plating. For example, the former PNP and COD removal efficiencies reached about 98.2% and 3.9% after 40 min treatment, while the later only reached about 91.0% and 3.4%, respectively. In addition, both of these two systems show lower ORP values during the treatment process compared with that of influent (see Fig. 4(f)).

In literature, it has been reported that oxidation from the Fenton-like reaction plays a leading role in Fe/Cu/air process, while the pollutants removal is mainly attributed to reduction in Fe/Cu/N₂ process.^16,45 Fig. 4 also shows that the higher COD removal was obtained in Fe/Cu/air process, while the higher PNP removal was obtained in Fe/Cu/N₂ process. The results of PRO during treatment process were also indicated that Fe/Cu/air process has stronger oxidation potential, which contributed to COD removal, while Fe/Cu/N₂ process was a reduction system, which was conducive to PNP removal.¹⁸ Furthermore, it can be seen from Fig. 4 that Fe/Cu bimetallic particles prepared by electroless plating have a higher reactivity both in Fe/Cu/air process and Fe/Cu/N₂ process. The results also suggest the superiority of the new preparation method of electroless plating.

3.4.2 Intermediates of PNP degradation. To further investigate PNP degradation process, some intermediates were determined during 40 min treatment process of Fe/Cu/air or Fe/Cu/N₂. As shown in Fig. 5(a) and (b), three intermediates including p-aminophenol (PAP), p-benzoquinone (BK) and hydroquinone (HC) were determined in Fe/Cu/air process with different Fe/Cu bimetallic particles. Furthermore, carbon mass balance during treatment process by these two different systems is shown in Fig. S10(a).† The concentration of the three intermediates all increased to the maximum after about 20 min treatment, and they all began to decrease gradually in the following 20 min treatment (i.e., 20–40 min). The results suggest that the intermediates with benzene ring could be further decomposed (i.e., open ring) in Fe/Cu/air process, which mainly attributed to the oxidation of Fenton-like reaction.¹⁶ However, concentration of intermediates obtained by the Fe/Cu bimetallic particles prepared by electroless plating was much higher than that of the Fe/Cu bimetallic particles prepared by displacement plating. For example, the maximum of PAP, BK and HC at about 20 min obtained by the former Fe/Cu bimetallic particles were 85.1 mg L⁻¹, 3.9 mg L⁻¹ and 4.1 mg L⁻¹ (Fig. 5(a)), while only 43.0 mg L⁻¹, 5.5 mg L⁻¹ and 0.7 mg L⁻¹ were obtained by the later Fe/Cu bimetallic particles (Fig. 5(b)). These intermediates could be further mineralized to CO₂, which lead to the reduction of COD. Therefore, these results suggest that the new Fe/Cu bimetallic particles prepared by electroless plating have a higher reactivity to enhance the Fenton-like reaction in Fe/Cu/air process.

Fig. 5 Concentration of PNP and its intermediates in the aqueous solution during 40 min treatment process by Fe/Cu/air or Fe/Cu/N₂ process, (a) Fe/Cu/air process with Fe/Cu bimetallic particles prepared by electroless plating, (b) Fe/Cu/air process with Fe/Cu bimetallic particles prepared by displacement plating, (c) Fe/Cu/N₂ process with Fe/Cu bimetallic particles prepared by electroless plating, (d) Fe/Cu/N₂ process with Fe/Cu bimetallic particles prepared by displacement plating (operational conditions were same to those in Fig. 4).

The concentration of PNP and intermediated is shown in Fig. 5 carbon mass balance during treatment process by these two different systems is shown in Fig. S10(b).† As shown in Fig. 5(c) and (d), the extremely high PAP concentration was determined in Fe/Cu/N₂ process. When the Fe/Cu bimetallic particles prepared by electroless plating were used in Fe/Cu/N₂ process, the generated PAP reached 390.6 mg L⁻¹ after 40 min treatment. However, the lower yield of PAP (378.2 mg L⁻¹) was determined when the Fe/Cu bimetallic particles prepared by displacement plating were used in Fe/Cu/N₂ process. Meanwhile, a little of BK and HC (1–2 mg L⁻¹) were accumulated during the 40 min treatment process (see Fig. 5(c) and (d)). The results suggest that the PNP removal was mainly attributed to the reduction of NO₂– group on the molecular structure of PNP. In other words, PNP was reduced into PAP by Fe/Cu/N₂ process, and the generated PAP was hard to be further decomposed. This phenomenon has been reported in our previous work.^1,46 However, Fe/Cu particles prepared by electroless plating have a higher reactivity for the reduction of PNP.

As a result, the performance of the new Fe/Cu bimetallic particles was much stronger than that of the conventional Fe/Cu bimetallic particles under the different conditions of air or N₂.

3.4.3 UV-vis spectral analysis. UV-vis spectra of PNP in aqueous solution during the 40 min treatment by Fe/Cu/air or Fe/Cu/N₂ process were shown in Fig. 6 It has been reported that the peak at 227 nm is mainly attributed to the π–π* transition of benzene ring of monoaromatics,⁴⁷ and the peak at 317 is mainly attributed to the conjugation of benzene ring and chromophoric group (–NO₂).¹ Fig. 6(a) and (b) show that the absorbance intensity of all peaks decreased after 40 min treatment by Fe/Cu/air process. The results suggest that the benzene ring structure of PNP could be decomposed in Fe/Cu/air process. In particular, when the new Fe/Cu bimetallic particles prepared by electroless plating were used in Fe/Cu/air process, the intensity of two key peaks (at 227 nm and 317 nm) decreased rapidly to 0.879 a.u. and 0.624 a.u. after 40 min treatment (see Fig. 6(a)). However, it only decreased to 1.49 a.u. and 1.30 a.u. when the conventional Fe/Cu bimetallic particles were used in Fe/Cu/air process (see Fig. 6(b)). The results also suggest that the new Fe/Cu bimetallic particles were superior to the conventional Fe/Cu bimetallic particles in Fe/Cu/air process.

Fig. 6 Variation of UV-vis spectra for PNP aqueous solution during 40 min treatment process by Fe/Cu/air or Fe/Cu/N₂ process, (a) Fe/Cu/air process with Fe/Cu bimetallic particles prepared by electroless plating, (b) Fe/Cu/air process with Fe/Cu bimetallic particles prepared by displacement plating, (c) Fe/Cu/N₂ process with Fe/Cu bimetallic particles prepared by electroless plating, (d) Fe/Cu/N₂ process with Fe/Cu bimetallic particles prepared by displacement plating (operational conditions were same to those in Fig. 4).

In Fe/Cu/N₂ process, the intensity of peak at 317 nm decreased rapidly during the 40 min treatment process, while the peak at 227 nm dropped little in whole process (see Fig. 6(c) and(d)). Meanwhile, a new peak at about 272 nm was formed gradually during treatment process. The results indicate that PAP was generated and accumulated during the treatment process,⁴⁸ which are accord with the above analysis for intermediates by HPLC. In addition, when the new Fe/Cu bimetallic particles were used in Fe/Cu/N₂ process, the intensity of peaks at 317 nm decreased more quickly than that obtained by using the conventional Fe/Cu bimetallic particles. The results suggest that the reactivity of the new Fe/Cu bimetallic particles was also higher than that of the conventional Fe/Cu bimetallic particles even if they were performed in Fe/Cu/N₂ process.

As a result, it could be concluded that the electroless copper plating process was superior to the conventional displacement plating process.

3.5 XPS analysis for Fe/Cu bimetallic particles

The chemical states of Fe, Cu, O in fresh and reacted Fe/Cu bimetallic particles were examined by XPS, and the results are shown in Fig. 7. The survey scan of Fe/Cu bimetallic particles reveals the presence of oxygen (1s), copper (2p), iron (2p) and carbon whose C 1s peak (284.8 eV) was used to calibrate the acquired spectra, as seen in Fig. 7(a). Note that the binding energy of the residual carbon originating from the precursor and the XPS instrument itself was corrected by referencing C 1s to 284.5 eV.

Fig. 7 XPS spectra of fresh and reacted Fe/Cu bimetallic particles (prepared by electroless plating), (a) survey scan, (b) Cu 2p core level, (c) Fe 2p core level, (d) O 1s core level.

In the core level XPS spectra of Cu 2p (Fig. 7(b)), the peaks corresponding to Cu 2p_3/2 and Cu 2p_1/2 are observed at around 933.6 eV and 953.8 eV, respectively. The characteristic peaks of Cu(0) and Cu(I) were identified both in Cu 2p_3/2 and Cu 2p_1/2 peak positions. The peaks at 932.6 eV and 952.2 eV are the indications of the presence of Cu(0). The peak for Cu(I) 2p_3/2 and Cu(I) 2p_1/2 was observed at 932.4 eV and 952.3 eV, respectively.^49–51 The results suggest that the chemical valences of Cu in fresh Fe/Cu bimetallic particles are mixture valence states of Cu(0) and Cu(I). The peak for O 1s of Cu(I) oxide was at about 530.20 eV,⁴⁹ but only very low intensity was observed at around 530.20 eV in the core level XPS spectra of O 1s (fresh Fe/Cu, Fig. 7(d)). The results suggest that Cu(0) was the prevailing component in the deposited copper coating on fresh Fe/Cu particles, only little Cu(I) might be generated during the preparation process by electroless plating. In addition, it could be observed from Fig. 7(b) that the similar chemical states of Cu was found in fresh and reacted Fe/Cu bimetallic particles. The results suggest that copper deposited on Fe⁰ substrates was only used as catalyst to enhance iron corrosion, and thus it could not be oxidized during the wastewater treatment process.

The peaks at 724.7 eV (Fe 2p_1/2) and 710.9 eV (Fe 2p_3/2) are assigned to the oxidized iron.⁵² Only very weak peaks were observed in the core level XPS spectra of Fe 2p_1/2 and Fe 2p_3/2 for the fresh Fe/Cu, while the strong peaks were observed for the reacted Fe/Cu (Fig. 7(c)). In addition, the peak intensity of O 1s was significantly enhanced when Fe/Cu bimetallic particles was used to remove PNP in aqueous solution (Fig. 7(d)). The results suggest that Fe⁰ in Fe/Cu particles was oxidized and generated corrosion products (e.g., FeOOH, Fe₂O₃, Fe₃O₄, etc.) during the PNP treatment process.

4. Conclusions

High-reactive Fe/Cu bimetallic particles were prepared by the electroless copper plating, and the optimal key plating conditions (i.e., Na₂EDTA·2H₂O = 10 g L⁻¹, H₃BO₃ = 30 g L⁻¹, NiSO₄·7H₂O = 0.5 g L⁻¹, pH = 9.5 and plating time = 1 min) were obtained in this study. Meanwhile, the rough Fe⁰ surface obtained by the pretreatment could facilitate the adhesion between the deposited copper and Fe⁰ substrates, which could improve the reactivity and operational life of Fe/Cu bimetallic particles. In addition, the reactivity and operational life of Fe/Cu bimetallic particles prepared by electroless plating were much better than those of Fe/Cu bimetallic particles prepared by displacement plating. The Fe/Cu bimetallic particles (prepared by electroless plating) can obtain higher PNP and COD removal efficiencies both in Fe/Cu/air (83.1% and 32.1%) and Fe/Cu/N₂ (98.2% and 3.9%) processes. In Fe/Cu/air process, the generated intermediates (e.g., PAP, BK and HC) can be further decomposed and obtained a higher COD removal efficiency (32.1%). In Fe/Cu/N₂ process, however, the intermediates (special for PAP) were hard to be further decomposed and accumulated during the whole treatment process. In other words, PNP was mainly reduced into PAP (390.6 mg L⁻¹) after 40 min treatment.

According to the analysis results of SEI, BES and SEM-EDS, it could concluded that nano-copper particles were uniformly deposited on the surface of Fe⁰ substrate when Fe/Cu particles were prepared by the electroless plating, while plenty of loose copper blocks were heterogeneously distributed on the surface of Fe⁰ substrates when the Fe/Cu bimetallic particles were prepared by the conventional displacement plating. Meanwhile, Cu (2.29 wt%) on Fe/Cu surface prepared by electroless plating detected by EDS was much higher than that (1.96 wt%) prepared by displacement plating. With regard to Fe/Cu bimetallic particles prepared by displacement plating, its copper coating was easy to be dropped off due the external shearing force during the wastewater treatment process. Finally, the XPS analysis suggests that Cu(0) was the prevailing component in the deposited copper coating on fresh Fe/Cu particles, and it was only used as a catalyst to improve the iron corrosion. As a result, it could be concluded that the electroless copper plating process was much superior to the conventional displacement plating process.

Acknowledgements

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

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11255b

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