In situ formation of MnO2/Ni(OH)2@nickel foam with porous architecture for triggering persulfate-based advanced oxidation process

MnO2/Ni(OH)2 was grown in situ on three-dimensional nickel foam (NF), which was then used to activate potassium persulfate (PS) for orange II (OII) degradation. The structure and morphology of the prepared materials were characterized by X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy. The removal efficiency of OII was 98.68% with MnO2/Ni(OH)2@NF/PS system in 40 min. The three-dimensional hierarchical structure of mesoporous material promoted the efficient and fast electron transfer during catalytic activity. Based on classical quenching experiments and the electron spin resonance experiments, ·OH and SO4·− were identified as the active species in MnO2/Ni(OH)2/NF/PS system for OII degradation and a degradation mechanism was proposed for this system. Compared with traditional micro-nano particle catalysts, 3D MnO2/Ni(OH)2@NF has the advantages of convenient recovery and good stability. MnO2/Ni(OH)2@NF catalyst could be recycled for at least three times, and the degradation rate was 89.06% after three cycles. This MnO2/Ni(OH)2@NF is expected to be used as a green heterogeneous catalyst.


Introduction
In the past several decades, nano-materials and technologies for advanced water treatment have attracted a lot of attention in academic and industrial circles. In the actual water treatment process, nano-materials often have bottlenecks such as easy agglomeration and deactivation, difficult operation, potential safety risks, etc. One of the most effective strategies to deal with these bottlenecks is by using large-size inert carriers to fix nano-particles and prepare composite nano-materials with confined structure. Studies in chemistry, materials science and other related disciplines show that nano-materials after confinement are often significantly different from bulk materials in structure and performance [1,2], that is, nano confinement effect [1]. This effect has also been partially confirmed in the research of advanced water treatment, but the scientific mechanism research on confinement effect of composite nano-materials in water treatment is just in its infancy [3].
Advanced oxidation technology based on persulfate activation (PS-AOP) has attracted much attention because of its high removal efficiency, simple operation and strong versatility [4][5][6]. Many studies have shown that hydroxyl radicals and sulfate radicals are the key species to activate persulfate (PS) systems [7]. However, PS-AOP technology still faces challenges such as low selectivity of target pollutants removal and difficult catalyst recovery, both of which limit the practical application of PS-AOP. Therefore, it is necessary to explore a suitable catalyst carrier to solve the problems of catalyst recovery and surface area increase. Because of its three-dimensional channel and good thermal conductivity, metal foam has attracted extensive attention for use as catalyst carriers in various fields [8][9][10].
Ever increasing number of research papers on activation and application of persulfate indicate that sulfateradical (SO 4 ·− ) based Advanced Oxidation Processes (SR-AOP) have been widely studied in recent years. Persulfate 1 3 activation can generate sulfate-radicals through ultraviolet irradiation, heat or metals, and the latter constitutes a new oxidation technology [11,12]. Zhou et al. [13] investigated MnCuS as as an efficient active heterogeneous catalyst for activating persulfate for orange II removal. Dou et al. [14] reported that the catalytic degradation of methylene blue using the CoO/NaHSO 3 was 99.4%. Although cobalt/PMS systems were proved to be the most effective way to produce sulfate-radicals [6,15], cobalt with high toxicity was easy to leach out in homogeneous and heterogeneous reactions, which is a disadvantage as it might cause secondary pollution. Therefore, the application of sulfate radicals produced by non-cobalt-based heterogeneous catalysts in environmental remediation is particularly useful [16].
Polyvalent manganese oxides are abundant in natural environments such as soil, ocean, rocks and fresh water, usually in the form of oxides of manganese (II), manganese (III) and manganese (IV), which are environmentally friendly and far less toxic than cobalt ions [17]. Manganese oxide has the advantages of low cost, easy availability, good environmental compatibility, high catalytic activity and so on, and is expected to be a substitute for cobalt-based catalysts [18]. Ni is the ninth most abundant element in the Earth's crust and has a lower toxicity than Co, and hence Ni-based materials have attracted special attention to activate peroxydisulfate (PDS, S 2 O 2− 8 ) or peroxymonosulfate (PMS, HSO − 5 ) and proved to be efficient catalysts. The synthesis method of nickel hydroxide (Ni(OH) 2 ) is simple and has a broad application prospect in catalytic system. Yue et al. [19] first demonstrated the role of α-Ni(OH) 2 as a highly effective nickelbased heterogeneous catalyst for sulfate radical activation.
The monolithic catalyst with three-dimensional hierarchical structure can not only keep the bulk materials easy to separate and reduce the secondary pollution caused by leaching of metal ions, but also avoid the agglomeration of micro/nano particles and expose more active sites, thus significantly improving the catalytic performance [20]. Among many porous foam materials, nickel foam (NF) has high application value because of its unique advantages, including three-dimensional interconnected texture, outstanding mechanical firmness, heat resistance, low economic cost and good environmental stability [21,22].
As far as we know, previously reported Ni(OH) 2 or MnO 2 /Ni(OH) 2 Nanomaterials on NF were mainly designed for supercapacitors [23][24][25], but the catalytic performance of hybrids for persulfate has never been investigated. Herein, we report a very simple, low temperature and green route to prepare Ni(OH) 2 nanosheets and MnO 2 nano flowers at the same time on the surface of NF by in situ oxidation-reduction method. The morphology, crystal structure and specific surface area of the catalyst were characterized. The obtained catalyst showed excellent activity and good stability in peroxodisulfate-based Fenton-like reactions. In addition, the mechanism of catalytic oxidation of OII by MnO 2 /Ni(OH) 2 / NF/PS heterogeneous system was further studied.

Preparation of material
3D NF sheet ( Fig. 1a) was first cut to a size of 2 cm × 2 cm × 0.16 cm and was ultrasonically cleaned in 6 mM HCl for 30 min followed by absolute ethanol and deionized water for 15 and 15 min. Thus, clean 3D NF sheet (Fig. 1b) was obtained and it was then placed in the Teflon liner of 100 mL stainless-steel hydrothermal vessel.
Then, MnCl 2 ·4H 2 O with molar mass of 198 g/mol, 44 mL deionized water and 6 mL 30% H 2 O 2 solution were added into 100 mL beaker, and magnetically stirred for 30 min to form a light pink transparent precursor solution. The precursor solution was added into the Teflon liner of hydrothermal vessel. The hydrothermal vessel was sealed and treated at 90 °C for 3 h in an oven. The hydrothermal vessel was removed from oven and naturally cooled to room temperature. Then the autoclave was opened, and it was found that except for the foam nickel which changed from silver to black as shown in Fig. 1c below and the rest was a light green transparent solution. The surface of foamed nickel Fig. 1 a 3D NF sheet, b 3D NF sheet after ultrasonic treatment, and c MnO 2 /Ni(OH) 2 @NF material at 90 °C was rinsed with ultrapure water until the solution is neutral, and dried at 80 °C for 12 h to obtain MnO 2 /Ni(OH) 2 @NF material (Fig. 1c). In order to obtain the highest catalytic efficiency of MnO 2 /Ni(OH) 2 @NF material, the synthesis conditions were optimized by changing the amount of raw materials, hydrothermal temperature and the amount of H 2 O 2 . When the reaction temperature was lower than 90 °C, the characteristic peaks of Ni(OH) 2 could not be found in the synthesized sample, which indicated that the temperature was too low. Although higher reaction temperatures (100 and 110 °C) were also investigated, the three-dimensional reticulated skeleton of NF prepared at these temperatures became less firm and was easily crushed under subsequent stirring.
It is inferred from the experimental phenomena observed during the preparation of MnO 2 /Ni(OH) 2 @NF that the possible formation mechanism is as follows:

Characterization
In this study, all samples used for characterization are synthetic composite particles scraped from NF by stainless steel spoon. The crystallinity and phases of the materials were analyzed by X-ray diffraction (XRD) with Cu Kα (λ = 0.15418 nm) radiation using D/max-2500 unit from Rigaku Co., Japan). Morphology and sizes of the materials were observed by the scanning electron microscope (SEM, JEOL, JSM-6360LA, Japan) and transmission electron microscope (TEM, FEI-G20). The Brunauer-Emmeet-Teller (BET) surface area and pore size distribution of the catalyst was measured by using ASAP 2460 adsorption device from Micromeritics. The mineralization of organic pollutants was quantitatively determined with an analyzer (multi N/C 2100 S, Analytikjena) for total organic carbon (TOC). To investigate the reactive species generated in the heterogeneous MnO 2 /Ni(OH) 2 @NF/PS system, ESR spectra were collected by ESR spectrometer (Bruker, EMX PLUS) to capture the different types of free radicals generated in the reaction system.

Catalytic degradation studies of Orange II
The prepared samples have been evaluated for degradation of orange II (OII) under different conditions using the following procedure. The degradation experiments were carried out in a 100 mL beaker containing 50 mL OII dye solution (30 mg/L) while stirring with a magnetic stirrer at the rotating speed of 100 rpm, and the experimental temperature for dye degradation was 15 ± 2 ℃. Here, the sample dosage is a piece of MnO 2 /Ni(OH) 2 @NF material (Fig. 1c), and the particles scraped off the surface are about 20 mg. The initial solution pH was adjusted to the determined value by using NaOH (1 M) and HCl (3 M). Firstly, the prepared MnO 2 / Ni(OH) 2 @NF material was immersed in OII dye solution, and the degradation process was started by adding PS. After the reaction, about 1.5 mL solution was extracted by syringe at regular time intervals, and then filtered by 0.22 μm filter membrane. The filtered solution (1mL) was added into the colorimetric tube and diluted 10 times. The absorbance of OII dye at its maximum absorption wavelength of 484 nm was determined by UV-Vis spectrophotometer. All the experiments were conducted in dark conditions and were repeated at least twice.

Characterization of material
The XRD patterns of prepared MnO 2 /Ni(OH) 2 @NF samples are presented in Fig. 2  The surface morphologies of MnO 2 /Ni(OH) 2 @NF were observed by SEM at different magnifications. As can be seen from Fig. 3, the overall framework of the material is in a network structure, petal-shaped particles are scattered on the surface of the flakes, and the pores between the flakes are large and evenly distributed. It can also be observed from Fig. 3b that flake nanosheets are very dense and evenly distributed. It shows that MnO 2 /Ni(OH) 2 composite material has been successfully deposited on the surface of NF. TEM images of MnO 2 /Ni(OH) 2 @NF sample are shown in Fig. 3c, d. Its continuous lattice planes at different locations represent good crystallinity of different phases. The interplanar spacings are 0.779 nm, 0.464 nm, 0.203 nm, which correspond to (003), (111), (111) crystal plane of Ni(OH) 2 , MnO 2 , Ni, respectively. Figure 4 shows N 2 adsorption/ desorption isotherm and pore size distribution of MnO 2 / Ni(OH) 2 scraped from NF surface. The BET surface area of MnO 2 /Ni(OH) 2 nanostructure is 5 m 2 /g. According to IUPAC classification, the adsorption isotherm of the sample is type IV isotherm, and the adsorption H3 hysteresis loop appears in the middle section, and the pore size of the porous material is between 2 and 50 nm, which belongs to mesoporous material characteristics.

Preliminary study on degradation of OII
The degradation of OII in different systems is shown in Fig. 5. With only the NF (Fig. 1b) in the system, the degradation rate was 13.65%. It can be seen from Fig. 5 that the degradation rate is basically unchanged after 10 min, indicating that the adsorption-desorption equilibrium has been reached and the adsorption capacity of MnO 2 / Ni(OH) 2 @NF sample is obviously higher than that of NF. The degradation rate was 34.09% when only the oxidizing agent, PS was added into the above reaction system. When MnO 2 /Ni(OH) 2 @NF sample and PS were added to OII solution, we found that 98.68% of OII was degraded within 40 min. In addition, the mineralization degree of pollutants should be determined by TOC index. Figure 6 shows the decrease of TOC in OII solution during 40 min reaction. The results demonstrated that the TOC value of OII solution decreased by nearly 30% after 40 min.

Effect of PS dosage
In the presence of MnO 2 /Ni(OH) 2 @NF catalyst, the degradation of OII under different PS dosages was investigated (Fig. 7). Without PS, because of the specific surface area of the catalyst, about 50% of OII was adsorbed on MnO 2 /Ni(OH) 2 @NF surface within 40 min. When MnO 2 / Ni(OH) 2 @NF and PS existed in the reaction system at the same time, it can be seen from Fig. 6 that with the increase of PS dosage from 1 to 2 g/L, the degradation rate of OII was obviously improved, and this result is consistent with previous studies [20,26,27]. This may be due to PS accelerating activation for the generation of SO 4 ·− -and ·OH radicals. In addition, the electronic exchange Mn 3+ , Mn 4+ , Ni 2+ , Ni 3+ in MnO 2 and Ni(OH) 2 may be another main reason for the catalytic reaction. However, the degradation of OII decreased with the further increase of PS dosage (3 g/L). Excess PS reacted with SO 4 ·− and ·OH radicals and generated sulfurpentaoxide radical (SO 5 ·− ) (Eqs. 4, 5), which is a weaker oxidant comparing to SO 4 ·− radical [28][29][30]. Therefore, 2 g/L appears to be the optimum dosage of PS for OII degradation.

Effect of initial solution pH
The effect of the initial solution pH from 3.0 to 11.0 on the OII degradation by MnO 2 /Ni(OH) 2 @NF/PS was investigated and the corresponding results are illustrated in Fig. 8. showed that with the increase of initial pH, the removal rate of OII decreased, which is consistent with the previously reported results [20,27]. Obviously, acidic conditions are favorable for the formation of SO 4 ·− /·OH radicals as can be seen from Eq. (12) and the generated free radicals accelerate the degradation of OII. In addition, under acidic conditions, the redox potential of free radicals (SO 4 ·− and ·OH) is relatively high, which also helps to promote the faster degradation of OII. With the initial pH increasing from 3.0 to 11.0, the removal rate of OII decreased from 98.96 to 74.22%.

Effect of inorganic anions
It is well known that inorganic anions in industrial wastewater, such as nitrate ion (HCO 3 − ) and chlorine ion (Cl − ), which can react with active free radicals at high reaction rates and hence this reaction with anions inhibits the degradation of pollutants. Therefore, the influences of inorganic anions such as HCO 3 − and Cl − on the catalytic degradation process of OII was evaluated and shown in Fig. 9. It can be seen from Fig. 8 that the existence of HCO 3 − [at high pH values the bicarbonate form to carbonate affects the rates (3.2-4.2 × 10 8 M − 1 s − 1 and 1-6.1 × 106 M − 1 s − 1 , for hydroxyl and sulfate radicals, respectively [31])] and Cl − (which can scavenge both sulfate radicals and hydroxyl radicals with constant rate of 2.8 × 10 8 and 6 × 10 9 M − 1 s − 1 , respectively [32]) obviously reduced the degradation efficiency of OII, which is due to the fact that HCO 3 − and Cl − can react with free radicals to generate active substances with low activity, thus inhibiting the removal rate of OII (Eqs. 6-12) [33].

Material stability and recyclability
In order to evaluate the repeatability and stability of the catalyst, the whole MnO 2 /Ni(OH) 2 @NF catalyst material after degradation reaction was taken out of the solution. After rinsing with ultra-pure water, the used catalyst was cycled for the next reaction under the same reaction conditions. The cyclic degradation efficiency of OII solution by MnO 2 /Ni(OH) 2 @NF using the combination of MnO 2 /Ni(OH) 2 @NF and PS is depicted in Fig. 10. After three cycles, the degradation rate of OII could still reach 89.05%, which indicated that MnO 2 /Ni(OH) 2 @NF had good stability and high recoverability. The deactivation was mainly related to the occupation of active sites by the adsorbed degradation products of OII, which could not be fully removed by water washing but may be alleviated by drying or annealing [5]. It can be concluded that the high stability of MnO 2 / Ni(OH) 2 with three-dimensional hierarchical structure can effectively avoid the agglomeration phenomenon of nanocatalyst when it is used for many times.

Investigation of major free radicals
To study the free radicals produced in MnO 2 /Ni(OH) 2 @NF/ PS system, scavenging experiments were carried out in the oxidation process (Fig. 11). As we all know, P-benzoquinone (BQ) was used as a unique quenchant for superoxide anion radical [k= (0.9-1)×10 9 M − 1 s − 1 ]. When excessive BQ content (2 mM) is added into the system, the inhibition can be neglected. This may be due to the extremely low yield of superoxide anion radicals (O 2 ·− ) as well as their low reactivity to OII [34]. This indicates that O 2 ·− radical could not be the main active species for OII degradation. Methanol (CH 3 OH, MeOH) and tert-butyl alcohol (TBA) were used as the scavengers of the hydroxyl radicals (·OH) and sulfate radicals (SO 4 ·− ) and surface free radicals. However, the quenching effect of TBA on ·OH is particularly good. It can be seen from Fig. 10 that compared with TBA, MeOH has a greater inhibitory effect on the degradation rate of OII. When TBA and MeOH were added into the system, the degradation rates of OII were 67.20% and 51.04%, respectively. Therefore, the main active species to degrade OII could be SO 4 ·− , and the conversion of SO 4 ·− into ·OH may lead to the outstanding contribution of ·OH through (Eq. 17).
Furthermore, ESR test with 5,5-dimethyl-1-pyrolin-Noxide (DMPO)as spin capture agent was also carried out. The catalyst powder scraped from NF was added into OII solution together with DMPO and PS. Within 5 min before the reaction, the characteristic peaks of ·OH and SO 4 ·− were DMPO-·OH and DMPO-SO 4 ·− adducts (Fig. 12). Thus, it appears that ·OH and SO 4 ·− act together in this degradation reaction. Furthermore, it can be seen from Fig. 12 that the heights of the peaks are relatively high, indicating that the concentration of free radicals is also relatively high. It also verified the phenomenon of free radical scavenging experiments. (

Conclusions
In this work, a rapid and mild green preparation technology was proposed for catalyst preparation. The results show that mesoporous materials are successfully formed in situ on the NF carrier at the reaction temperature of 90 ℃ under hydrothermal condition and time of 3 h. Through the free radical quenching experiments and ESR test, it was determined that the active species such as ·OH and SO 4 ·− were mainly produced during the activation of PS by MnO 2 /Ni(OH) 2 @NF catalyst. The excellent degradation ability of the system may have potential application prospects in the catalytic oxidation of organic dyes or other organic pollutants.