Synthesis and catalytic evaluation of silver@nickel oxide and alginate biopolymer nanocomposite hydrogel beads

In this study, silver–nickel oxide/calcium alginate (Ag@NiO/Alg) hydrogel beads were synthesized by incorporating the nanomaterial into the polymer solution followed by their extrusion from a syringe to coagulating solution for crosslinking. The Ag@NiO/Alg beads formed through this procedure were characterized by different instrumental techniques. The catalytic efficiency of Ag@NiO/Alg was evaluated for the reduction of rhodamine B (RhB) and methyl orange (MO), in the presence of sodium borohydride. After applying various kinetic order equations to the experimental data, the reactions were found to follow pseudo-first order kinetics using the Ag@NiO/Alg catalyst. The apparent rate constant (kapp) was determined as 0.33358 min−1 and 0.20882 min−1 for the catalytic reduction of RhB and MO, respectively. Furthermore, reduction reactions were studied at varying dye concentration and catalyst dosages. The recovered catalyst was reused up to five cycles without significant drop in the activity.


Introduction
Hydrogels with 3-D interwoven structure and highwater retention capacity has found much attention among various researchers' groups (Al-Mubaddel et al. 2017;Aljohny et al. 2021). Biopolymer based hydrogels have introduced a new thought of green and sustainable catalytic support systems (Zhuang et al. 2016;Sarkar et al. 2017;Liu et al. 2018;Thakur et al. 2018;Fan et al. 2020). The intermolecular attractive forces (such as covalent bonding, hydrogen bonding etc.) between the polymer chains are held responsible for the uniform immobilization of metallic nanoparticles inside the polymeric matrix . Hydrophilic nature of the hydrogels renders quasi-homogeneous properties to the nanoparticles, hence, enhancing its catalytic efficiency. Different biopolymer-based hydrogels such as carboxy methyl cellulose, chitosan, agar, bacterial cellulose, and alginate etc. have been studied so far for academically and applied in various applied fields (Chen et al. 2017a;Xu et al. 2017b;Silva et al. 2019;Kamal et al. 2019a;Khan et al. 2019b;Chanthiwong et al. 2020;Khalil et al. 2020). Amongst them, alginate is one of the most extensively used polymers owing to its highly biocompatible and biodegradable nature. For instance, Gan et al. (2018) utilized sodium alginate hydrogel beads as a support for graphene oxide and investigated its efficiency for removal of organic dyes and bisphenol A in aqueous solution. Lam et al. (2017) utilized TiO 2 nanoparticles immobilized in calcium alginate beads for photocatalytic degradation of methylene blue. The chemical structure of alginate consists of covalently bonded 1,4-linked a-l-guluronic acids (G) and b-D-mannuronic acids (M) as main constituents. Unlike other biopolymers which require toxic organic crosslinking agents, alginates can be ionotropically crosslinked. Owed by the presence of divalent cations (e.g., Ca 2? ), ionic bridges are formed between the adjacent alginate chains which lead to its transformation into hydrogel beads in the aqueous medium (Asadi et al. 2018).
Metal nanoparticles with particle size in the range of nanometers, exhibiting excellent mechanical and optic-electric properties, have found applications in heterogeneous catalysis. Various semiconductor metal oxides and noble metals have been studied for its applications as an efficient catalyst. Recent studies emphasize on the immobilization of noble metals (such as Ag, Au, Pt and Pd) onto the surface of semiconductor metal oxides (such as NiO, CuO etc.) for considerable enhancement in its catalytic activity (Kao et al. 2017;Xu et al. 2017a;Chen et al. 2017b;Jiang et al. 2019). Noble metal nanoparticles act as an electron sink for the electron generated in the semiconductor. Due to excellent optical and catalytic properties of Ag among the various noble metals, it remains pre-dominant over others. Up-to-date different approaches have been put forward for the synthesis of Ag and NiO nanocomposites and studied for its application in photocatalysis, energy storage, sensors and supercapacitors etc. (Ngo et al. 2017;Nagamuthu and Ryu 2019;Karimi-Maleh et al. 2020). Even though, Ag@NiO is a good photocatalyst but similar to other nanoparticles catalyst, it is easily lost during the reaction due to its small size. Thus, there is a need of system in which its loss could be minimized. The Ag@NiO encapsulation inside a hydrogel can be a great way to address this problem. Moreover, the employment of a catalyst embedding in a hydrogel host instead of using the dispersed form of the nanoparticles is beneficial as the catalyst could be easily recovered for reusing purpose. Therefore, an alginate-based biopolymer was chosen as it could be ionically crosslinked without needing an environmentally toxic agent.
Dyes are widely used for several applications in various industries such as textile, dyeing, plastic, pharmaceutical and many others. These organic dyes, when discharged into the running water in untreated form causes water pollution due to their highly stable and non-biodegradable nature. Different physical, chemical and biological methods have been employed for the removal of these toxic organic pollutants such as adsorption, coagulation, reverse osmosis, ultrafiltration, biodegradation, oxidation, reduction etc. (Katheresan et al. 2018;Jamee and Siddique 2019;Ali et al. 2020a, b). Amongst them catalytic reduction is considered as primary choice, owing to less time required for removal of organic pollutants. In addition, the reduction products obtained as a result of this process have found numerous applications in different fields (Ali et al. 2018;Kamal et al. 2019b).
In the current study, we have reported the synthesis of Ag@NiO by deposition of silver nanoparticles on the surface of nickel oxide prepared by precipitation method. To ensure its stability and avoid leaching out in the aqueous medium, Ag@NiO was immobilized in calcium alginate (Alg) hydrogel beads. To the best of our knowledge, immobilization of Ag@NiO onto calcium alginate hydrogel beads is not yet reported. Also, no literature has been found on utilization of Ag@NiO for catalytic reduction of organic pollutants. The catalytic performance of as-prepared Ag@NiO/ Alg hydrogel beads were evaluated for catalytic reduction of organic dyes, namely, rhodamine B (RhB) and methyl orange (MO).

Materials
Sodium alginate (NaC 6 H 7 O 6 ) was obtained from Shanghai Aibi Chemistry Preparation Co., Ltd. Calcium chloride (CaCl 2 ), Siver nitrate (AgNO 3 ) and Nickel nitrate hexahydrate (NaNO 3 Á6H 2 O) were purchased from Sigma Aldrich. Sodium borohydride (NaBH 4 ) was purchased from BDH Chemical Laboratories, Poole, England. Rhodamine B (RhB) and Methyl orange (MO) were purchased from Merck. All the chemicals used were of analytical grade and were used without any further purification process. All solutions were prepared in deionized water obtained from PCSIR laboratories, Peshawar, Pakistan.

Synthesis of Ag@NiO nanocomposite
Firstly, nickel oxide nanoparticles were prepared by precipitation method. Briefly, 10 g of NiNO 3 Á6H 2 O was dissolved in 500 mL of deionized water, followed by drop-wise addition of dilute ammonia water solution, until the pH reaches to 8. The precipitates formed were then filtered, washed with deionized water for several times. The filtered precipitates were then allowed to dry at 100°C and calcined at 800°C for 24 h.
For the synthesis of Ag@NiO nanocomposite, 0.5 g of the preformed NiO nanoparticles was welldispersed in water under ultrasonic treatment. Then 0.1 M AgNO 3 was added to the NiO suspension and sonicated for 1 h. The deposited Ag ? ions were then reduced with 0.1 M NaBH 4 aqueous solution. The resultant Ag@NiO was then filtered, washed, and dried at 80°C.

Synthesis of Ag@NiO/calcium alginate hydrogel beads
For the synthesis of Ag@NiO/Alg hydrogel beads, Ag@NiO nanomaterial was dispersed well in 20 mL of distilled water and added to already prepared alginate solution (with percent composition of 2% w/v) under vigorous stirring. 0.2 M CaCl 2 solution was prepared by dissolving 14.7 g into 500 mL. For the preparation of hydrogel beads, Ag@NiO/alginate suspension was taken in 5 cc medical syringe and added drop by drop to CaCl 2 solution from 10 cm height with continuous stirring at a speed of 50 rpm. As soon as the drop fell into the Ca 2? ions solution, it attained a solid spherical shape. The same procedure was employed for the formation of pure calcium alginate beads without the addition of Ag@NiO nanomaterial.

Characterization
The functional group analysis of Alg, Ag@NiO/Alg and Ag@NiO was conducted by Fourier transform infrared spectroscopic technique using Perkin Elmer spectrometer at a frequency range of 450 to 4000 cm -1 with a resolution of 4 cm -1 and 0.2 scanning speed. X-ray Diffraction technique was used to determine the crystalline nature of Alg, Ag@NiO/ Alg and Ag@NiO with the help of X-ray diffractometer (model: JDX-3532, JEOL, Japan) using Nickel filtered Cu-Ka radiation of 1.5418 Å in the 2h range of 5°to 90°. The morphology of Alg, Ag@NiO/Alg and Ag@NiO was evaluated by scanning electron microscopic (Model; JSM5910, JEOL, Japan) and energydispersive X-ray diffractometric techniques (Oxford 7353 EDX, Oxford Instruments, Abingdon, UK).

Catalytic evaluation of Ag@NiO/Alg hydrogel beads
In order to evaluate the catalytic efficiency of Ag@NiO/Alg hydrogel beads, catalytic reduction of RhB and MO was studied. For this purpose, 2.5 mL of RhB (0.05 mM) and MO (0.1 mM) dye solution was taken in a quartz cuvette separately, followed by addition of 0.1 M NaBH 4 solution. Then depending on the required reaction conditions, a known amount of the Ag@NiO/Alg hydrogel beads was added into the reaction system. The decrease in absorbance at the kmax of the dyes was regularly noted with the help of UV-Vis spectrophotometer after each 2 min.

Results and discussion
Scheme 1 shows the general overview of this research work. The first part consists of the preparation of Ag@NiO nanomaterial and Ag@NiO/Alg nanocomposite beads (catalyst preparation) and the second part consists of the utilization of Ag@NiO/Alg nanocomposite beads in the reduction of RhB and MO dyes (catalytic evaluation). At first, nickel salt solution was precipitated by drop-wise addition of ammonia until pH 10 has been reached. The precipitates of Ni(OH) 2 thus formed were filtered, dried, and calcined to form NiO nanoparticles. In order to deposit Ag ? ions on NiO nanoparticles, they were mixed with AgNO 3 solution in a sonicator, followed by the addition of sodium borohydride as a reducing agent. By the addition of NaBH4, Ag ? ions were reduced into Ag 0 , thus forming Ag@NiO. The prepared Ag@NiO nanocomposite was then mixed with sodium alginate solution under vigorous stirring. Finally, the mixture was put into Ag@NiO/Alg hydrogel beads by crosslinking with Ca 2? ions. After preparation of the Ag@NiO nanomaterial, pure Alg, Ag@NiO/Alg nanocomposite beads, they were first subjected to various instrumental techniques as given in the following text.
Scheme 1 Preparation of hydrogel beads and its catalytic evaluation for toxic dyes reduction FTIR analysis FTIR spectra of Alg hydrogel, Ag@NiO and Ag@NiO/Alg are shown in Fig. 1. The spectral peaks at 475 cm -1 and 568 cm -1 corresponds to stretching vibrations of NiO as shown in Fig. 1a (Nagamuthu and Ryu 2019). Ag nanoparticles synthesized by chemical approach do not exhibit peak in the infra-red region. No appearance of any absorption peak for Ag confirms the synthesis of metallic silver only in this approach, without the formation of oxides of silver (Beura et al. 2021). The broad absorption band at 3225 cm -1 corresponds to the presence of hydroxyl group. The sharp peaks at 1586 cm -1 and 1413 cm -1 are attributed to asymmetric and symmetric stretching vibrations of the carboxylate groups, respectively. Peaks observed at 1294 cm -1 , 1076 cm -1 , and 1025 cm -1 are associated with stretching vibrations of C-O functional group of glycosidic linkage between b-D-mannuronic and R-L-guluronic acid and is an indication of the degree of stability of the linear chain in the alginate (Simonescu et al. 2020). In the FTIR spectrum of Ag@NiO/Alg all peaks characteristic of Ag@NiO and Ca-Alg can be seen, as represented by Fig. 1b.

XRD analysis
In order to study the crystalline nature of prepared samples, pure Alg, Ag@NiO and Ag@NiO/Alg were characterized by X-ray diffractrometry technique. Figure 2a represents the XRD pattern of Alg hydrogel beads. No diffraction peak can be observed which confirms non-crystalline nature of Alg. Figure 2c indicates the XRD pattern for Ag@NiO nanomaterial. The diffraction peaks at 2h = 37.22°, 43.25°, 62.83°a nd 75.34°corresponds to the presence of NiO (JCPDS card No-73-1523) in the sample. Similarly, the peaks at 2h = 38.26°, 44.47°, 64.71°, 77.74°and 81.91°are in good agreement with the presence of metallic silver (JCPDS card No-87-0719) in the Ag@NiO sample. XRD peaks for nickel oxide and metallic silver coincides with (111), (200), (220), (311) and (222) hkl planes, thus indicating their cubic crystal structures. All the characteristic peaks were observed in the XRD pattern of ALG/Ag@NiO hydrogel beads, which confirms the successful incorporation of Ag@NiO into the polymer matrix, as can be seen from Fig. 2b.

SEM analysis
The surface features and morphology of pure Alg beads, Ag@NiO nanoparticles and Ag@NiO/Alg composite beads were characterized by scanning electron microscopy (SEM) as can be seen in SEM photographs (Fig. 3). Figure 3a shows the SEM photographs of Ag@NiO nanoparticles. No sign of large particles was observed in the low magnification image (Fig. 3a, right side). It can be seen in a left side photograph that all the particle sizes were below 100 nm. Pure Alg hydrogel beads with sizes in the range of 1.5 to 2 mm were prepared and subjected to morphological analysis by SEM. As shown in the leftside SEM photograph in Fig. 3b, the pure Alg beads were not completely spherical and indicated the oval shapes with some dents. The high magnification photograph of the pure Alg beads is given in the right-side of the Fig. 3b. It shows porosity in the bead's surface. Figure 3c (left-side) shows the SEM photograph of Ag@NiO/Alg nanocomposite bead. As compared to the pure Alg beads, the Ag@NiO/Alg nanocomposite bead surface has small white spots. High magnification SEM photographs in Fig. 3c on left side reveals that the beads surface has small particles. Thus, the SEM analysis of the Ag@NiO/Alg nanocomposite bead suggest the successful immobilization of Ag@NiO onto Ca 2? -Alg hydrogel beads.

EDS analysis
To explore the elemental constituents of the prepared samples, EDS analyses were carried out for Ag@NiO nanoparticles, Alg beads and Ag@NiO/Alg beads. The EDS elemental mapping analyses on the Ag@NiO nanocomposite reveal the presence of Ag, Ni and O elements, as can be seen from Fig. 4a. No other elements were detected which confirms that Ag@NiO was formed in pure form without any external impurities. The EDS spectrum of pure Alg

Catalytic reduction of RhB
After the successful synthesis of Ag@NiO/Alg and its confirmation by different instrumental techniques, its catalytic efficiency was evaluated by RhB dye reduction. Despite of its useful aspects, it has adverse effects on the environment, when discharged in untreated form into the water bodies. In order to decrease its toxic effects, rendered to it by the presence of -N(C 2 H 5 ) sites in its structure, its abatement has become crucial. For this purpose, 0.05 mM aqueous solution of RhB was processed. Before going for the actual catalytic experiments, absorption spectrum of RhB was observed in the presence of Ag@NiO/Alg only, without the addition of reducing agent. Figure 5b confirms that no adsorption has been taken place with Ag@NiO/Alg. In another batch of experiment, reduction of RhB was studied in the absence of Ag@NiO/Alg using NaBH 4 as a reducing agent. Figure 5a shows UV-Vis absorption spectra for reduction of 2.5 mL RhB (0.05 mM) dye solution and 1 mL of 0.1 M NaBH 4 . No notable decrease in the absorption spectrum was observed for upto 1 h which indicates that RhB does not undergo self-hydrolysis with mere addition of reducing agent. Catalytic reduction is an electron transfer process between NaBH 4 and dye molecule, where NaBH 4 acts as an electron donor and dye molecule serves as an acceptor. The results from Fig. 5a indicate that this electron transfer process is thermodynamically feasible but kinetically un-favorable. For actual practice, 2.5 mL of RhB aqueous solution with molar concentration of 0.05 mM, 1 mL of 0.1 M NaBH4 solution and 0.1 g of Ag@NiO/Alg hydrogel beads was placed into a quartz cuvette with a total capacity of 4 mL. The decrease in absorption maxima was repeatedly recorded at 554 nm, after an interval of 2 min. Figure 5c shows the time dependent UV-Vis spectra for catalytic reduction of RhB. The disappearance of absorption maxima at 554 nm indicates the complete conversion of RhB into leuco-rhodamine B (Abay et al. 2017). The rate of reaction k app was calculated from the linear slope of lnC t /C 0 versus time as can be seen from Fig. 5d. k app value obtained was found to be equal to 0.33358 min -1 or this reaction. As NaBH 4 is used in excess, the reduction reaction is assumed to follow pseudo-first order kinetics. The linear coefficient value of R 2 = 0.96 for the correlation between lnC t /C 0 and time, validate pseudo-first order kinetics for the reduction process.
For the purpose to investigate the effect of RhB concentration on the reduction kinetics, three different dye concentrations, say, 0.025 mM, 0.05 mM and 0.1 mM were studied while keeping all other condition constant at 0.1 M NaBH 4 and 0.1 g of hydrogel beads. Figure 6a shows kinetic plot of lnC t /C 0 versus time at varying dye concentration. The calculated apparent rate constant values were 0.38469 min -1 , 0.33358 min -1 and 0.19429 min -1 for dye concentration of 0.025 mM, 0.05 mM and 0.1 mM, respectively. R 2 values were found in the range of 0.96-0.99, which refers to goodness of the fit. Decrease in reaction rate was observed with increasing dye concentration. This is because of the fact that catalyst acts as a limiting reagent, when dye concentration, hence number of dye molecules is increased. Our results are similar to the literature reports of Chook et al. studied the RhB concentration effect on the reaction time and rate constant while using CNF-AgNPs as a catalyst (Chook et al. 2015). The reaction time was reported to be 5, 12 and 90 min for the reduction of 2.0 9 10 -4 , 1.0 9 10 -4 and 4.0 9 10 -5 M dye concentration.
In the same manner, reduction of 0.05 mM RhB dye was studied at three different catalyst amounts, that is 0.05 g, 0.1 g and 0.15 g, concerning to evaluate the effect of catalyst dosage on the reaction rate. Figure 6b shows kinetic plot of lnC t /C 0 with time for reduction of 0.05 mM RhB at varying dye concentration. The k app values at 0.05 g, 0.1 g and 0.15 g were calculated to be 0.13011 min -1 , 0.33358 min -1 and 0.4528 min -1 , respectively. These values clearly depict that rate of reaction increased with an increase in catalyst quantity. This increase in k app value is associated with the increase in number of active sites for the reaction to take place. Similar results were reported by Maryami et al. (2017) for the catalytic reduction of Rh B (2.09 9 10 -5 M) in the presence of 5.0 mg, 7.0 mg and 10.0 mg Pd/Perlite nanocomposite catalyst. A decrease in the reaction time was observed in the order of 45 min (5.0 mg), 5 min (7.0 mg) and 60 s (10 mg) (Maryami et al. 2017). Further, Zhang et al. investigated the effect of Ag dosage on the catalytic reduction of RhB dye with an initial concentration of 20 m/L (Zhang et al. 2021). An increase in the k app values was reported which were 0.12 min -1 , 0.17 min -1 , and 0.21 min -1 for Fe 3 Pt-60 mL Ag, Fe 3 Pt-90 mL Ag and Fe 3 Pt-120 mL Ag, respectively.

Catalytic reduction of MO
Similarly, catalytic efficiency of Ag@NiO/Alg hydrogel beads was investigated for MO dye reduction. MO is an anionic azo based dye having k max at 464 nm in its UV-visible spectrum at neutral pH. For MO reduction reaction, 2.5 mL of MO dye with molar concentration of 0.1 mM and 1 mL of 0.1 M NaBH 4 was taken into a quartz cuvette, followed by addition of 0.1 g of Ag@NiO/Alg hydrogel beads. As like in the case of RhB, before going for the actual experimentation, reduction of MO was investigated for selfhydrolysis with NaBH 4 . No reduction reaction was observed in the absence of catalyst, as can be seen from Fig. 7a. Figure 7b shows absorption spectra for MO dye in the absence of NaBH 4 , which illustrates that no adsorption of dye occurred on Ag@NiO/Alg surface. Figure 7c shows time dependent UV-Vis spectra for 0.1 mM MO dye in the presence of 0.1 M NaBH 4 and 0.1 g of Ag@NiO/Alg hydrogel beads. The progress of reaction was observed from decrease in the absorbance value at 464 nm. At the end, the observed peak at 464 nm is completely diminished, considered to be due to the breakage of the -N=N-(azo) bond. Apparent rate constant value was calculated from slope of the linear plot between lnC t /C 0 and time. Figure 7d shows kinetic plot of lnC t /C 0 versus In order to study the effect of dye concentration on rate of reaction, MO dye reduction was studied at three varying concentrations, say, 0.05 mM, 0.1 mM and 0.15 mM, keeping all the other parameters constant. Figure 8a shows kinetics plot of lnC t /C 0 against time for catalytic reduction of MO at varying initial concentrations. Rate constants for these reactions were found to be equal to 0.13201 min -1 , 0.20882 min -1 and 0.31388 min -1 , respectively. Hu et al. reported a decrease in % decolourization rate with an increase in the initial concentration of MO (10 mg/L, 20 mg/L, 30 mg/L, 40 mg/L and 50 mg/L) in the presence of 0.1 g of nano-MoS/TiO 2 composite catalyst (Hu et al. 2010) .
Similarly rate of reaction was studied at varying catalyst dosage, keeping concentration of MO and NaBH 4 constant. For this purpose, reduction of 0.1 mM MO with 0.1 M NaBH 4 was studied in the presence of three varying catalyst quantities, that is, 0.05 g, 0.1 g and 0.15 g. Figure 8b shows plot of lnCt/ C0 and time for MO dye reduction at varying catalyst dose. k app values were estimated to be 0.050487 min -1 , 0.20882 min -1 and 0.33705 min -1 at 0.05 g, 0.1 g and 0.15 g catalyst dosage, respectively. In a similar manner, Islam et al. reported apparent rate constant values of 1.27 9 10 -4 s -1 , 3.36 9 10 -3 s -1 and 4.58 9 10 -3 s -1 for MO reduction in the presence of pristine CF, CF-AuNPs-0.98 and CF-AuNPs-2.87 (Islam et al. 2017).
Furthermore, the spectral data of the catalytic reduction of the RhB and MO was also analyzed for the possibility to follow other type of reaction kinetics. In addition to the pseudo-first order reaction, the data were also analyzed using the zero, pseudo-first, first and second order kinetic equations. Table 1 summarizes k app and R-square values for the RhB and MO reduction at varying dye initial concentration and catalyst dosage. The highest R-square values for any given reaction suggest that the experimental data was well-fitted to the pseudofirst order kinetic equation.
A number of researchers' groups put forward different approaches for the successful abatement of RhB and MO. Table 2 summarizes the reaction conditions and k app values for the reduction of the above-mentioned toxic dyes in the presence of different catalytic systems, reported earlier in the literature. Zhang et al. (2021) reported a novel multifunctional Fe 3 Pt-Ag nanoparticles for the SERS detection and its catalytic reduction. The highest rate constant of 021 min -1 was observed which was lower than our study. Similarly, Wang et al. prepared porous carbon protected magnetite and AgNPs (Wang et al. 2013). Besides cell imaging, the Fe 3 O 4 @C-Ag hybrid NPs showed a good reduction rate constant 014 min -1 for the RhB. Mishra et al. synthesized a Fe 3 O 4 -MnO 2 nanocomposites by a green method for the sp 3 C-H functionalization of 2-methylpyridine and RhB reduction (Mishra et al. 2016). Their catalyst reduced the RhB with a rate constant of 0.08 s -1 . Bhargava et al. (2016) also used a green method of using Cladosporium oxysporum AJP03 for the synthesis of gold nanoparticles. The AuNPs were catalytically Similarly, many research studies used the MO as a model reducible dye to test the catalysts. In Ahsan et al. (2019), the authors prepared magnetic cobalt nanoparticles on a porous carbon support using metal organic frameworks as templates. The prepared magnetic C@Co nanocatalyst catalyzed the reduction of MO with a 0.041 s -1 . In Gupta et al. (2011), three different type of noble metal (Ag, Au and Pt) nanoparticles using a green tannic acid reducing agent. The MO dye reduction rate constant of 0.0029 min -1 was experimentally measured for the PtNPs.

Statistical analysis
In order to validate the experimental conditions, simple linear regression model, variance of coefficients and ANOVA has been applied to catalytic  Table S2. Adjacent R 2 value above 0.9 for all of the given data showed that the experimental data is highly significant and also indicated goodness of the fit. Furthermore, significance of the experimental data was validated by the ANOVA calculations, as can be concluded from p values (\0.05) in Table S1. Table S3 shows variance of coefficient for the reduction of RhB and MO under varying conditions of Alg@AgNiO and dye concentration. It can be concluded that the given table is highly significant (with lower p values than the chosen alpha values).

Reusability tests
Recyclability is a key factor for evaluating catalytic efficiency in heterogeneous catalysis. Although Ag@NiO/Alg exhibited high catalytic activity, it is crucial to study the stability of the nanocatalyst. Therefore, 0.1 g of Ag@NiO/Alg was utilized for reduction of 0.05 mM RhB and 0.1 mM MO (2.5 mL each in a separate batch of reactions) with 1 mL of 0.1 M NaBH 4 solution. After the completion of reaction, Ag@NiO/Alg hydrogel beads were recovered from the reaction media, washed with deionized water and reutilized in another series of reaction. The  same process was repeated for upto five cycles. % reduction for each successive cycle was calculated by using the formula; Percent reduction ¼ Initial dye concentration À Final dye concentration Initial dye concentration Â 100 Figure 9 shows that % reduction of the catalyst remained unaffected (96-97% for RhB and 87-88% for MO) for up to five consecutive cycles which confirms the high stability of Ag@NiO/Alg.

Conclusion
In summary, Ag nanoparticles were immobilized on NiO, followed by incorporation into calcium alginate hydrogel beads, by ionotropic crosslinking procedure. The as prepared Ag@NiO/Alg hydrogel beads were characterized by XRD, SEM, FTIR and EDS techniques. The catalytic activity of Ag@NiO/Alg was evaluated against catalytic reduction of two toxic organic pollutants, RhB and MO, in the presence of sodium borohydride as a reducing agent. The calculated k app values revealed it as an excellent catalyst. Furthermore, ease of its separation from reaction medium and its recyclability upto five cycles without any noticeable change in its catalytic activity, provided evidence for Ag@NiO/Alg as an efficient catalytic system. Acknowledgments This research was supported by Institutional Fund Projects under Grant No. (IFPIP: 329-130-1442). Therefore, the authors gratefully acknowledge technical and financial support from the Ministry of Education and King Abdulaziz University, DSR, Jeddah, Saudi Arabia.

Declaration
Conflict of interest The authors report no conflicting interest in any capacity, competing, or financial.