Synthesis of biomass-supported CuNi zero-valent nanoparticles through wetness co-impregnation method for the removal of carcinogenic dyes and nitroarene

Abstract Stabilization of zero-valent CuNi nanoparticles (NPs) supported on Cicer arietenum (CP) is reported here for the reduction and removal of persistent organic pollutants. The functional groups and interactions of NPs with the CP were determined by ATR-FTIR. The crystallinity, morphology, and the elemental composition of the samples were determined through powder XRD, FESEM, and EDS techniques, respectively. The XRD spectrum displayed a sharp crystalline peak at 43.9 for CuNi. The Cu and Ni zero-valent NPs displayed a peak at almost the same region, and thus, both the peaks are merged and appeared as a single peak. The chemical reduction/degradation of eight model pollutants, viz., 2-nitrophenol (ONP), 3-nitrophenol (MNP), 4-nitrophenol (PNP), 2,4-dinitrophenol (DNP), methyl orange (MO), congo red (CR), methylene blue (MB), and rhodamine B (RB) were carried out in the presence of NaBH4. The k app value of 0.1 mM 4NP was highest which was 1.8 × 10−1 min−1 while the slowest rate was observed for CR and RB with k app 5.5 × 10−3 and 5.4 × 10−3 min−1 respectively. This article helps in the removal of toxic organic pollutants through green supported NPs.


Introduction
The zero-valent metal NPs are much more reactive than their bulk materials due to their high Fermi potential resonance and small quantum size effects. These characteristics of the zero-valent metal NPs make them one of the most versatile materials in the field of catalysis [1]. Zero-valent metal NPs have a large surface-to-volume ratio. This high surface area provides new and novel features to these materials. The field of zero-valent metal NPs is the emerging research at the industrial level as it is used in the field of catalysis, photocatalysis, biological sectors, water splitting, sensors, and many others [1][2][3][4][5][6][7][8]. Literature has reported many methods for the synthesis of zero-valent metal NPs including chemical, physical, and biological. Chemical fabrication involves organic solvents and other reducing agents to support and stabilize the NPs, but it is not eco-friendly [6]. Physical methods involve thermal evaporate, plasma arcing, pulsed laser, ball milling, ultra-thin films, spray pyrolysis, lithographic techniques, desorption, diffusion flame, sputter deposition, and molecular beam epistaxis [3]. Chemical methods involve electrode position, sol-gel process, chemical vapor deposition, chemical solution deposition, Langmuir-Blodgett method, soft-chemical method, hydrolysis, catalytic route, wet-chemical method, and co-precipitation [3,9]. However, excessive chemical use, radiation, and nonecofriendly methods limit the use of these practices [10][11][12]. Therefore, a more reliable, costeffective, and eco-friendly procedure is required [4,[13][14][15][16]. Plant and plant-derivative materials are suitable candidates for such processes. Plant materials with their secondary metabolites are responsible for the support and stabilization of NPs [17,18]. In this article, the Cicer arietinum was used for the stabilization of CuNi NPs. The C. arietinum is a legume of the family Fabaceae and subfamily Faboideae and is largely used for making different foodstuffs. In South Asia, Bengal grams and chickpeas are used to make curry, which is one of the most popular vegetarian food served with steamed rice and bread in the Diaspora community of various countries [16]. In Indian cuisine, popular dishes are made with chickpeas flour such as mirapakaya mirchi bada. The unripe chickpeas leaves are eaten as food vegetable salad and as a raw snack [12]. The C. arietinum-supported CuNi NPs can be applied for the removal of toxic organic pollutants from the effluents. Nitrophenols and dyes are organic pollutants that have detrimental effects on human health and other living organisms; hence, their removal is inevitable [17]. The toxicity of nitrophenols is adverse to such an extent that various nitrophenols have been listed as "Priority Pollutant" by the U.S. Environmental Protection Agency and strict laws have been framed to control the release of these hazardous chemicals into the natural waters (U.S. EPA, 1976). Principles have been made, and strict constraints are imposed on the industries to control the release of these pollutants into the environment (U.S. EPA, 1988) [18][19][20]. The removal of these pollutants is of utmost importance due to their large scale use in the agriculture and industrial sectors [18][19][20][21][22]. From both sectors, excess concentration of these pollutants find their way into the water resources and thus contaminate the natural water bodies. Nitrophenols have some disturbing effects on liver, kidney, stomach, and central nervous system [19,23]. Dyes are the other pollutants, which is a matter of serious concern at the global level because even the presence of meagre concentration in water makes it colorful. The presence of unwanted color reduces the penetration of air, oxygen, and sunlight into the water bodies, which has a detrimental effect on the aquatic flora and fauna. Approximately, 10,000 dyes are commercially available and are largely used in the textile, paper, hides, and skin industries, which are also used for other coloring purposes [24]. Unfortunately, it is easier for industries to release their waste materials in the surroundings rather than treatment [25,26]. The removal of these unwanted chemicals and dyes is the pressing need of time [27]. In this research work, an attempt has been made on the zero-valent metal NPs, which is one of the most emerging research for the removal of hazardous chemicals from the effluents [28][29][30]. The use of CP powder for the stabilization of CuNi NPs has not yet been reported, and to the best of authors' knowledge, no publications have been reported yet.

Preparation of CuNi NPs
First, CP fine powder was obtained by blending CP with the help of an electric blender for about 20 min. Afterward, 1 g of this powder was added to a 100 mL solution comprising 50 mL of 0.2 M solution of NiCl 2 ‧6H 2 O and 50 mL of 0.2 M solution of CuSO 4 ‧5H 2 O. For complete saturation of the adsorptive sites on CP, the powder was allowed to stir in the solution for 6 h. After the adsorption of the salts, the color of the CP changed, which is indicative of the metal salts adsorption. The CP adsorbed metal salts were collected and washed several times with double distilled water to remove any nonadsorbed metal salts. It was then dried at normal temperature and stored in clean tubes. The CP-adsorbed CuNi ions were treated with X mL of 1 M NaBH 4 solution, which turned the Cu 2+ and Ni 2+ ions into CuNi zero-valent NPs, which turned the color of the sample black. The appearance of black color is the first indication of the formation of CuNi zero-valent NPs. The synthesized CuNi NPs were then used for the reduction of nitro groups in nitrophenols and discoloration of various dyes in the presence of NaBH 4 .

Catalytic reduction/removal reactions
For the reduction/removal of all the pollutants, 2.5 mL of each pollutant was treated with 0.5 mL of NaBH 4 and 10 mg of the NPs and then the rate of reduction/removal and degradation of the pollutants was monitored in a time-dependent UV-Vis spectrophotometer.
The following formula (equation (1)) was used for the decrease in the original concentration of the pollutants: and initial concentration (C 0 ) of the targeted pollutants.
The percent reduction/removal of all the pollutants as a function of contact time was deduced from the following formula (equation (2)): In equation (1), A t is the final absorbance of the pollutant and A 0 is the initial absorbance of the model pollutants.
While the k app value of the reduction and degradation of nitrophenols and dyes were inferred from the first-order kinetics presented in equation (3) as follows: where ln is the natural logarithm and C t and C 0 indicate the final and initial concentrations of the documented pollutants.

Instrumental characterization
The sharp fine crystalline peaks of the CuNi NPs were obtained through powdered XRD spectrum (Thermo scientific diffractometer) of Cu Kα radiations having λ = 0.154 nm from 2θ = 10-80°. Morphology (shape and particle size) of NPs were resolute by Field Emission-Scanning Electron Microscope (FESEM) JEOL (JSM-7600F, Japan). Similarly, the functional group analysis in both plant and NPs were observed by ATR-FTIR spectroscopy (Nicolet 50 ATR-FTIR spectrometer Thermo Scientific). The batch experiment for the reduction and degradation of all the pollutants was done through a UV-visible spectrophotometer (Shimadzu, MultiSpec1501).

Results and discussion
The FTIR spectrum was recorded from 500 to 4,000 cm −1 range as shown in Figure 1a. A sharp intense peak appeared at 3,392 cm −1 for -OH stretching vibration. The sharpness of the peak suggested the multiple H-bonding in the CuNi@CP NPs. However, a less intense peak for -OH which appeared at 3,283 cm −1 was also observed for the pure CP. The difference in both peaks might be due to the involvement of H-bonding in the NPs formation. The absorption at 1,643 cm −1 reflects the bending vibration mode of -OH. Other important peaks at 2,929 and 2,855 cm −1 were due to the C-H stretching vibrations. Other noticeable peaks in the NPs and pure CP were observed at 1157, 1011 cm −1 due to C-O bending vibration. M-O bond in the NPs appeared at 656 cm −1 [19,25].
The XRPD spectrum indicated a broad hump at 2θ = 26.2°for the pure CP, confirming the amorphous nature of CP. However, after stabilizing the CuNi NPs with the help of CP, a sharp peak at 2θ = 43.9°indicated the crystalline nature of CuNi, while the peak at 2θ = 24.7°is due to the metal oxide [6]. Along with the crystalline peaks, a hump at 2θ = 18.5°appeared for the amorphous nature of CP. The stong interaction of NPs with the CP has shifted the amorphous peak of the CP as depicted in Figure 1b. The XRD analysis was carried out after threerun, and it was observed that the intensity of the peak was decreased possibly the leaching of the NPs.
The FESEM was carried out for the surface morphology of the CuNi NPs. The low-and highresolution images are indicated on the left and right sides of Figure 2a and d, respectively. The low resolution was carried out at 10 μm and high at 100 nm, respectively. The low resolution of pure CP not showing any NPs; at high resolution, the pure CP does not show any NPs. However, CuNi@CP indicated the presence of NPs. The high-resolution FESEM images of CuNi@CP indicated a bright spot of NPs, which suggested that NPs are strongly interacting with the chemical constituents of CP.
EDS spectrum indicating the purity of the sample and used for the elemental analysis of the prepared sample (Figure 2e and f)

Reduction of isomeric nitrophenols
Nitrophenols are extremely toxic organic compounds and are ranked among the most hazardous pollutants. According to the U.S. Environmental Protection Agency, the PNP, ONP, and 2,4-dinitrophenol are the most dangerous pollutants and listed as "Priority Pollutants." Environmental protection agency is up for implementing strict restrictions to control the concentrations of these hazardous pollutants in natural waters to <10 ng/L (U.S. EPA, 1976). Nitro group-containing compounds are distressing to liver, kidney, and central nervous system of living organism especially human beings. Therefore, careful and robust technology is the need of time to eradicate these pollutants from the wastewater. For this reason, CuNi@CP NPs were developed for the reduction of isomeric ONP, MNP, and PNP as well as DNP to the amino group.
Initially, the ONP is treated without the CuNi@CP NPs, with which no change was observed in the absorbance. After that, NaBH 4 redshift has been observed in the λ max of ONP and has been shifted from 350 to 415 nm. NaBH 4 abstract the proton from the -OH group of ONP which results in the formation of the o-nitrophenolate anion. It is well known that the negative charge becomes more stable on the most electronegative atom; therefore, the negative charge on the oxygen becomes more stable and delocalized more as compared with the lone pair electrons on -OH group. Furthermore, delocalization weekens the bonds and increases the bond length, that is why after the addition of NaBH 4 , redshift has been observed in the UV-Vis spectrum of ONP.  Furthermore, the reduction of nitrophenols to aminophenol with NaBH 4 as a reducing agent is a thermodynamically favorable reaction, but kinetically unfavorable reaction. Therefore, an efficient and effective catalyst is required for this reduction reaction. In the next stage, 10 mg of the CuNi@CP NPs was introduced to the ONP solution along with the NaBH 4 in the UV cuvette to make this reduction reaction kinetically favorable by minimizing the activation energy barrier. The reduction of ONP in the presence of NaBH 4 and catalyst is diagrammatically represented in Scheme 1. The data of PNP are compared with the data reported in the literature as depicted in Table 1. A 2.5 mL of 0.5 mM of a solution of all the ONP, MNP, and DNP were treated against 10 mg of the CuNi@CP NPs and monitored the reduction of nitrophenols to aminophenol in the time-dependent UV-Vis spectrophotometer. The concentration of PNP was 0.28 mM because the high concentration of PNP is going out of the UV-Vis range.
The λ max of ONP, MNP, PNP, and DNP was ∼350, ∼290, ∼318,0 and ∼350 nm; however, after reaction with NaBH 4 , ONP, MNP, PNP, and DNP, the corresponding λ max changed to 415, 328, 400, and 430 nm, and all these nitrophenols turned pale to sharp yellow color. The decrease in the concentration of various nitrophenols with contact time is represented in Figure 3a-d. The reason for their redshift was due to their extended conjugation. The explanation is that when negative charge resides on the most electronegative atom, it is more stabilized that is why the lone pair of -OH not so delocalized as compared to their corresponding anions.
The CuNi@CP (10 mg) catalyst is used for the reduction of 3 mL of 0.5 mM solution of ONP, MNP, and DNP while the same amount of the catalyst was used against 0.28 mM of 2.5 mL solution of PNP. To each solution, 0.5 mL of 1 mM aqueous NaBH 4 was added. With the addition of NaBH 4, redshift was observed in all the nitrophenols. The decrease in the concentration of ONP, MNP, PNP, and DNP was monitored at λ max 415, 328, 400, and 430 nm as a function of contact time. The change in the original concentration was deduced from equation (1).
The % reduction of the ONP, MNP, PNP, and DNP indicated a continuous increase in reaction progress. All the pollutants were allowed to reduced till 100% or close to 100% and then the time was noted. It was observed that 90.99% of ONP is reduced to o-aminophenol in 31 min, 94.91% of 0.28 mM of PNP is reduced to p-aminophenol in 28 min, and 92.0% of DNP is reduced to di-aminophenol in 48 min. However, only 60.7% of MNP is reduced in 8 min, and no further reduction was observed as it became constant shown in Figure 3f.
The rate of reaction of different nitrophenols was determined from ln C t /C 0 .

Effect of PNP concentration
The effect of concentration of the pollutants was checked against the CuNi@CP NPs, different concentrations such as 0.28, 0.2 and 0.1 mM of PNP were selected and to each one, 10 mg of CuNi@CP NPs was added in all the experiments. It was observed that the rate of reaction increased with the decrease in the PNP concentration from 0.28 to 0.1 mM. The 0.28 mM PNP was reduced to p-aminophenol in 28 min, 0.2 mM in 15 min, and 0.1 mM in 7 min. This reflects that by increasing the concentration of analyte, the active sites of the catalyst saturate sooner as compared with the low concentration of the analyte (Figure 3e). The ln C t /C 0 indicating the highest reduction rpm for 0.1 mM PNP, which is 1.8 × 10 −1 min −1 while the R 2 value is 0.91449 (Figure 3g). Similarly, the k app and R 2 values for other nitrophenols are presented in Table 2.

Discolorization of dyes
The degradation of two anionic and two cationic dyes was considered in this experiment. The anionic dyes were CR and MO, and the cationic dyes were MB and RB. The same protocol was followed as discussed for the nitrophenols. Three milliliters of 0.05 mM of all the dyes was taken in a quartz cuvette with the addition of 0.5 mL of 1 mM NaBH 4 solution. Like nitrophenols, the NaBH 4  alone does not degrade the dyes, it is the catalyst that actually takes over the activation energy barrier and makes this reaction kinetically feasible. NaBH 4 changed the azo groups of MO and CR dyes to their hydrazine form and MB and RB dyes to their luco MB and luco RB form as shown in the inset of Figure 4. The λ max of CR, MO, MB, and RB before and after the addition of NaBH 4 appeared at 500, 465, 665, and 454 nm, respectively. After the addition of 10 mg of the catalyst to their respective solution, the absorbance of each dye decreased as a function of time. It was observed that CR degraded in 8.3 min (Figure 4a), MO in 2.3 min (Figure 4b), MB in 5 min (Figure 4c), and RB in 9 min (Figure 4d). The decrease in the original concentration of the dyes, the percent removal of all the dyes, and the rate of reaction were deduced from equations (1) Table 2. The k app and R 2 values of all the dyes were determined from the ln C t /C 0 as indicated in the inset of Figure 4g.

Mixed system
The catalytic activity of the CuNi@CP NPs was checked against the simultaneous removal of PNP, CR, and MB dyes to check the selectivity of the reaction. The mixed system is extremely important in case where the simultaneous degradation of pollutants is required. Three different attempts were made for the simultaneous removal of PNP and dyes. In the first attempt, PNP-CR (Figure 5a)  400, 500, and 760 nm. These λ max values indicated that dyes and nitrophenol remained mixed up without any chemical reaction.
Three milliliters of 0.05 mM CR and 3 mL of 0.1 mM PNP solution was mixed in a clean vial, to this solution 1 mL of 1 mM NaBH4 solution was added. From this solution, 3 mL of the solution in a quartz cuvette was taken and 10 mg of the catalyst was added. Figure 5a clearly indicates that CR degraded first as compared to PNP; however, after 6 min, the whole solution degraded. The similar experimental protocol was followed for PNP-MB and PNP-CR-MB, where MB is degraded before PNP (Figure 5b), and PNP and CR (Figure 5c), respectively. This shows that MB is selective over both CR and PNP and CR is selective over PNP.

Conclusion
The CuNi NPs were supported on the C. arietinum and were used for the removal of nitrophenols and organic dyes. The rate of reaction was determined through firstorder kinetics, and among all the nitrophenols, the removal rate was highest for ONP followed by ONP, DNP, and MNP, respectively. The highest rate of removal was recorded for MO followed by MB and RB and CR. The synthesized NPs were also applied for the simultaneous removal of nitrophenols and dyes.