Study on Reduction of 4-nitrophenol by Magnetic Maize Straw Supported with Copper Nanoparticles

In this work, magnetic maize straw was prepared by the amidation process using renewable maize straw as starting material. After magnetic succinylated maize straw (Mag-S-MS) was mixed with cupric ions aqueous solution, Cu (II) could be captured by the amino and carboxylate groups. Then, the bonded Cu (II) was converted to valuable Cu nanoparticles (Cu NPS). It was characterized by SEM-EDS, XRD, XPS, and TGA, which indicated Cu NPS were formed successfully on Mag-S-MS without self-aggregation and oxidation. The above nanocomposites could be employed as a catalyst for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The effect of the dosage of Cu NPS loaded-Mag-S-MS, the initial concentrations of NaBH 4 and 4-NP were investigated, and a possible mechanism was discussed. The catalyst maintained relatively high catalytic activity after ve cycle tests. Due to its superparamagnetic nature, it could be quickly collected from the aqueous solution under a magnetic eld. These results could provide a method for using agricultural waste in nano catalytic reaction.


Introduction
In recent decades, water pollution has been a global environmental problem because of the indiscriminate disposal of industrial wastewater containing heavy metals and toxic organics. 4nitrophenol (4-NP) is one of the essential agents in pharmaceuticals, dyes and pesticides. As it is di cult to degrade in the environment and harmful to human health, it has been classi ed as a priority pollutant  Wang et al. 2020). Among them, Cu nanoparticles (Cu NPS) are relatively cheaper than the other noble metals. It is becoming a promising catalyst than those noble metals. Copper (II) is introduced into water by some industrial activities such as sensors, electronics, and biomedicines, which can cause severe damage to ecological systems. Therefore, it is a sustainable and green strategy to reduce Cu (II) in the industrial e uents to Cu NPS, which can be applied as a catalyst in transforming 4-NP to 4-AP.
Cu NPS are easy to form aggregation and be oxidized (Petri et al. 2009), which limited its practical applications. Much attention has been paid to obtain an effective solid support material to anchor Cu NPS. Over the years, numerous attempts have been focused on nding non-toxic, inexhaustible, and biodegradable composites from plant biomass (Akhtar et al. 2020;Su et al. 2019). Furthermore, when some functional groups such as carboxyl, amino, and hydroxyl are grafted into the supporting material, metal ions are immobilized spontaneously by electrostatic interactions, which can disperse the metal ions excellently to prevent self-aggregation (Liu et al. 2017). Maize straw (MS) is an abundant resource of agricultural waste in China, and millions of tons are produced every year. However, most of them are incinerated or abandoned, causing signi cant waste and pollution. Hence, the application of maize straw has drawn much attention to removing the toxic metal, which is one of the effective ways to make full use of the abundant bioresource. Based on our previous research, the carboxylic acid functions on succinylated maize straw (S-MS) could adsorb heavy metal ions from the aqueous solution (Guo et al. 2015), which inspired us to serve S-MS as Cu NPS carrier.
It is worth emphasizing that the sustainability of the catalyst can be assessed by the possibility of its recovery. It is necessary to impart magnetic properties to the catalyst to be separated effectively by applying an external magnetic eld (Reddy et al. 2013). Herein, we bonded amine-functionalized magnetite nanoparticles (NH 2 -Fe 3 O 4 ) with S-MS by the amidation process to obtain magnetic succinylated maize straw (Mag-S-MS). Then its remanent carboxylic acid functions were deprotonated by Na 2 CO 3 to obtain the sodium salt of the carboxylates (Mag-NaS-MS) to capture Cu(II). Finally, the bonded Cu(II) ions on Mag-NaS-MS were converted into valuable Cu NPS by NaBH 4 , which presented a catalytic activity to reduce 4-NP into 4-AP.

Materials
Maize straw was harvested from Dalian, China. After being cut into 5 cm length pieces, it was mixed with 10% NaOH for 2 h at 25 ℃. The alkalized solid was washed with deionized water to the neutral and dried at 80 ℃ in the oven. The above obtained solid was smashed to pieces with sizes 50-500 µm. A amount of CuSO 4 ·5H 2 O was added into 100 mL distilled water to obtain the stock Cu(II) solution. All chemicals were analytical grade and were used as received.

Preparation of S-MS
S-MS was prepared by a chemical method according to (Guo et al. 2015). 1.0 g succinic anhydride and 0.5 g MS were rst mixed with 50 mL of xylene. Then, 1.4 mL of triethylamine was added to the reaction mixture and re uxed for 8 h. The resultant solid was ltered and washed with ethanol and water.
2.3 Preparation of amine-functionalized magnetic nanoparticles NH 2 -Fe 3 O 4 were prepared by a solvothermal method (Wang et al. 2006). After 3.0 g FeCl 3 ·6H 2 O, 2.0 g anhydrous sodium acetate, and 6.5 g 1,6-hexanediamine were dissolved in 30 mL of ethylene glycol, the solution was stirred vigorously at 50 ℃ for 30 min to obtain a transparent solution. This mixture was then transferred into a 50 mL Te on-lined stainless steel autoclave and reacted at 198 ℃ for 6 h. The products were washed with water and ethanol and nally dried in vacuum at 50 ℃.

Preparation of Mag-S-MS and Mag-NaS-MS
1g S-MS (5.8 mmol/g of carboxylic acid functions) and a certain amount of N,N′-diisopropylcarbodiimide (DIC) were added into a three-neck ask containing 50 mL dimethylformamide (DMF) and stirred at 30 ℃ for 2 h to activate the carboxylic acid functions. Then 0.3 g NH 2 -Fe 3 O 4 was added into the reaction mixture and stirred for another 15 h. After the resulting black solid was separated by magnetic decantation, it was washed with deionized water and 95% ethanol. Afterward, the product was dried overnight through desiccation and was named Mag-S-MS.
Mag-NaS-MS was prepared through the alkalization process. Mag-S-MS was mixed with sodium carbonate solution (0.01 mol/L) for 2 h at 25 ± 2 ℃. The product was washed with deionized water until the e uent was neutral. The sample, designated as Mag-NaS-MS, and was dried in vacuum at 50 ℃. The rate constant (k) was measured as a function of time by recording the change in the intensity of the absorption peak at 400 nm. To study the reusability of the catalyst, the black solid was separated under a portable magnet. After being washed with ultra-pure water, it was used consecutively in the same reaction system.

Characterization and measurements
The functional groups present in Mag-S-MS were characterized by the FTIR spectrum (Perkin-Elmer, USA).
The surface morphologies of all the specimens were obtained by a eld emission SEM (JSM-6460, JEOL, Japan), equipped with an energy dispersive X-ray spectroscope (Oxford, British). XRD patterns were acquired on a Shimadzu XRD-6100 diffraction with CuKα radiation (λ = 1.54060 Å) from 5º to 80º. The magnetic properties of all samples were measured with a Lake Shore 7410 VSM at room temperature. Thermogravimetric analysis (TGA, Perkin-Elmer, USA) was performed from 25 ℃ to 600 ℃ at a heating rate of 10 ℃ /min under nitrogen ow. The XPS spectra were obtained by an X-ray photoelectron spectroscopy (Thermo VG ESCALAB 250, USA ) equipped with an Al-Ka X-ray source (1486.6 eV).

Preparation of Mag-S-MS and Cu NPS-loaded-Mag-S-MS
The synthesis route of Mag-S-MS is shown in Fig. 1.The carboxylic acid functions of S-MS were used to synthesize Mag-S-MS. DIC was used as a coupling agent to activate the carboxylic acid carbonyl and amine-functionalized magnetic nanoparticles were introduced through the formation of an amide bond (Gurgel et al. 2009).
There are two new strong bands at 1550-1650 cm −1 in the FT-IR spectra of Mag-S-MS, which correspond to the presence of amide function (Fig. 2b). Mag-NaS-MS toward Cu (II) were investigated by the batch adsorption experiments. The adsorption behavior and mechanism were discussed in Section S1 and S2 in Supplementary information, respectively. The carboxylate groups on Mag-NaS-MS played a critical role in adsorbing Cu (II), which formed a monodentate structure with Cu (II) (Fig S6). The Cu (II) ions bonded through the amino and the carboxylate groups were well dispersed on the Mag-S-MS to inhibit the self-aggregation of the reduced Cu NPS.
The cupric ions adsorbed on Mag-NaS-MS were reduced in situ by NaBH 4 to Cu NPS, which were applied in the further catalytic degradation process.  (Fig. 4a, b). Fig. 4b displays the spherical structure of Fe 3 O 4 nanoparticles in diameter between 50-85 nm were dispersed on the surface of S-MS. After being reduced by NaBH 4 in-situ, it was found that the synthesized copper nanoparticles had ower shapes (Fig. 4c), and its diameter was approximately 220-800 nm. The Cu NPS were uniformly dispersed on the surface of Mag-S-MS without aggregation, which are the potential catalytic sites.

SEM investigation SEM images of S-MS and Mag-S-MS show that the Mag-S-MS surface is much rougher than S-MS
From the EDS spectra, it can be found that there was no Fe element exists in S-MS, while the proportion of Fe in Mag-S-MS was 21.70%, which was introduced by the amidation reaction ( Fig. 4d and Fig. 4e). There was no Cu element in Fig. 4d and Fig. 4e, but the distribution elements (Fe and Cu) can be observed in the EDS mapping (Fig. 4f inner image).   The inset of Fig. 7 shows that Cu NPS-loaded-Mag-S-MS could be dispersed homogeneously without a magnetic eld. However, it was drawn to the glass bottle side by a magnetic bar in 10 s. Therefore, Cu NPS-loaded-Mag-S-MS could be separated and collected easily from the treated solution via the use of an external magnetic eld, which is bene cial to develop a recyclable catalyst for its practical applications.

Mechanism of catalytic reduction of 4-NP
It is thermodynamically favorable process for the catalytic reduction of 4-NP to 4-AP in the presence of NaBH 4 because their standard electrode potential (E for 4-NP/4-AP = -0.76 V, H 3 BO 3 /BH 4 − = -1.33 V) is 0.67, which is larger than zero (Khan 2020).As soon as 4-NP was mixed with an aqueous solution of NaBH 4 , the color of the solution was changing from light yellow to yellowish-green immediately, which indicated phenolate anions of 4-NP were formed ). On the other hand, a strong absorption peak shifted from 317 nm to 400 nm, monitored by UV-Vis absorption spectroscopy, and it was almost unchanged in 1 h without the catalyst. However, the solution gradually became transparent within 10 minutes (Fig. 8) when Cu NPS-loaded-Mag-S-MS (4 mg) was added into the 4-NP solution.
Meanwhile, the peak intensity at 400 nm decreased gradually with the increase of the peak intensity of 4-AP at 303 nm. The two absorption peaks were an isosbestic point, indicating the successful reduction of 4-NP to 4-AP without any side reaction.
There were numerous bubbles on the catalyst's surface, which suggested that the hydrolysis of NaBH 4 could be stimulated e ciently by the catalyst. The reduction mechanism could be illustrated in Fig. 9 when BH 4 − was adsorbed by the catalyst, it could provide the electrons on the surface of Cu NPS-loaded-Mag-S-MS. Subsequently, the electrons could transfer to the nanocomposite and produce the hydrogen atom, which could attack the 4-NP molecules and reduce the nitro group to the amino group (Khan et al. 2017). Therefore, the catalyst assisted the electron in transforming from electron donor (NaBH 4 ) to acceptor (4-NP) to cut down the energy barrier and make the reaction possible. According to the reduction mechanism, the catalytic rate might depend on the amount of catalyst, the initial concentrations of NaBH 4 and 4-NP.

Effect of each variable on 4-NP reduction
The where A 0 and A t are the absorbance of 4-NP at the initial time and instant time, respectively. k is the pseudo-rst-order rate constant (min −1 ), t is the reaction time (min).
Based on the linear relationship in Fig. 10a, it can be found that the catalytic kinetics of the reduction reaction t the pseudo-rst-order reaction (Liu et al. 2017). Table S5 showed the rate constants (k) calculated from the slopes with the different amounts of the catalyst. It is a positive correlation between amounts of Cu NPS-loaded-Mag-S-MS and the k value, which demonstrated that increasing the amount of catalyst would provide more active sites for reducing 4-NP and accelerate NaBH 4 hydrolysis more e ciently.
As shown in the mechanism of catalytic reduction (Fig. 9), NaBH 4 and 4-NP were the electron donor and acceptor, respectively. The effect of the concentration of NaBH 4 and 4-NP on the reaction was tested. The plots of ln(A t /A 0 ) against reaction time for different concentration of NaBH 4 and 4-NP were shown in Fig. 10b and Fig. 10c, respectively. From which it can be found that a linear tting curve was obtained, indicating that this reduction reaction followed a pseudo-rst-order kinetic reaction. The rate constants (k) calculated from the slope was shown in Table S5. The reduction rate was higher with the increase of NaBH 4 because the higher concentration of NaBH 4 could provide the more amounts of hydrogen atoms, which could accelerate the reduction of the nitro group. The reaction rate also depended on the concentration of 4-NP. It can be seen from Table S5 that increasing the 4-NP concentration reduces the reduction rate constant. When the 4-NP concentration was 0.017 mM, the reduction rate constant k was 0.46 min −1 , but when the initial 4-NP concentration was increased to 0.05 mM, the reduction rate constant k decreased to 0.24 min −1 . This might be due to the concentration of NaBH 4 in the reaction was constant, and as the concentration of 4-NP increased, there were not enough hydrogen atoms in the reaction, resulting in reduced catalytic e ciency and a prolonged reaction time.

Reusability of Cu NPS-loaded-Mag-S-MS
Repeated usability of the catalyst is one of the critical parameters for its practical applications. Under the same experimental conditions, the reusability study of Cu NPS-loaded-Mag-S-MS catalytic reduction of 4-NP was evaluated ve times. Fig S7 shows

Conclusion
In this study, a low-cost magnetic carrier for copper nanoparticles was fabricated. Cu (II) ions were captured from the aqueous solution by its amino and carbonyl groups. After in situ reduction of Cu (II) ions with NaBH 4 , the analysis of XRD, XPS and TGA, it proved that Cu 2+ was successfully reduced to metallic copper (Cu 0 ).SEM-EDS images showed ower-like Cu NPS was uniformly dispersed on the surface of Mag-S-MS with diameter of 220-800 nm. Cu NPS-loaded-Mag-S-MS exhibited remarkable catalytic performance in the catalytic reduction of 4-NP to 4-AP in the presence of NaBH 4 . The reduction process followed the pseudo-rst-order kinetic reaction. Increasing the amount of catalyst and NaBH 4 can accelerate the reduction reaction, while increasing the concentration of 4-NP has the opposite effect. Due to its superparamagnetism, the catalyst can be easily separated from the solution by external magnetic force. The catalyst recycling test showed that the catalytic activity did not decrease signi cantly after 5 cycles. The results of this study indicated that Cu NPS-loaded-Mag-S-MS had the characteristics of low cost, good reusability, convenient separation and high catalytic e ciency. Therefore, the concept of using agricultural waste maize straw and heavy metal Cu(II) pollutants in wastewater to prepare Cu NPSsupported-Mag-S-MS catalyst and use it for 4-NP reduction to 4-AP is consistent with the concept of human beings sustainable development.
Declarations Figure 8 Time-dependent UV-Vis absorption spectra for 4-NP with NaBH4 and the addition of Cu NPS-loaded-Mag-S-MS (4 mg). The inset illustrated the reduction of 4-NP with NaBH4 before and after catalyzed by Cu NPS-loaded-Mag-S-MS, which can be gathered by an external magnetic eld Figure 9