Chitosan@Carboxymethylcellulose/CuO-Co2O3 Nanoadsorbent as a Super Catalyst for the Removal of Water Pollutants

In this work, an efficient nanocatalyst was developed based on nanoadsorbent beads. Herein, carboxymethyl cellulose–copper oxide-cobalt oxide nanocomposite beads (CMC/CuO-Co2O3) crosslinked by using AlCl3 were successfully prepared. The beads were then coated with chitosan (Cs), Cs@CMC/CuO-Co2O3. The prepared beads, CMC/CuO-Co2O3 and Cs@CMC/CuO-Co2O3, were utilized as adsorbents for heavy metal ions (Ni, Fe, Ag and Zn). By using CMC/CuO-Co2O3 and Cs@CMC/CuO-Co2O3, the distribution coefficients (Kd) for Ni, Fe, Ag and Zn were (41.166 and 6173.6 mLg−1), (136.3 and 1500 mLg−1), (20,739.1 and 1941.1 mLg−1) and (86.9 and 2333.3 mLg−1), respectively. Thus, Ni was highly adsorbed by Cs@CMC/CuO-Co2O3 beads. The metal ion adsorbed on the beads were converted into nanoparticles by treating with reducing agent (NaBH4) and named Ni/Cs@CMC/CuO-Co2O3. Further, the prepared nanoparticles-decorated beads (Ni/Cs@CMC/CuO-Co2O3) were utilized as nanocatalysts for the reduction of organic and inorganic pollutants (4-nitophenol, MO, EY dyes and potassium ferricyanide K3[Fe(CN)6]) in the presence of NaBH4. Among all catalysts, Ni/Cs@CMC/CuO-Co2O3 had the highest catalytic activity toward MO, EY and K3[Fe(CN)6], removing up to 98% in 2.0 min, 90 % in 6.0 min and 91% in 6.0 min, respectively. The reduction rate constants of MO, EY, 4-NP and K3[Fe(CN)6] were 1.06 × 10−1, 4.58 × 10−3, 4.26 × 10−3 and 5.1 × 10−3 s−1, respectively. Additionally, the catalytic activity of the Ni/Cs@CMC/CuO-Co2O3 beads was effectively optimized. The stability and recyclability of the beads were tested up to five times for the catalytic reduction of MO, EY and K3[Fe(CN)6]. It was confirmed that the designed nanocomposite beads are ecofriendly and efficient with high strength and stability as catalysts for the reduction of organic and inorganic pollutants.


Introduction
Water pollution and its treatment is one of the most serious issues worldwide. Effluent discharge is produced from human activities including industrialization, which introduces a considerable amount of wastewater into nature [1]. Organic contaminates are the most noticeable water pollutants (e.g., organic dyes and nitrophenol compounds), which have deteriorating effects on human health and other living organisms due to their carcinogenic effect on nature [2]. Beside organic contaminations, inorganic pollutants are also considered as the most serious environmental problem including heavy metal ions. According to the literature, the accumulation of toxic metal ions in wastewater can cause genetic alteration, which can affect hormone metabolism and lead to serious diseases such as cancer and fetal  6 ]. The prepared nanocomposite beads and the beads were characterized by various analytical techniques, including FT-IR, SEM, EDX and XRD. The surface morphology of the prepared materials, CuO-Co 2 O 3 , CMC, CMC/CuO-Co 2 O 3 , Cs@CMC/CuO-Co 2 O 3 and Ni/Cs@CMC/CuO-Co 2 O 3 , was examined by utilizing FE-SEM, as seen in Figure 1. The low-magnification and high-magnification images for the prepared nanocomposite beads are represented on the left and right sides in Figure 1. The CuO-Co 2 O 3 picture indicates the particles of CuO-Co 2 O 3 , in Figure 1a,b. The pure CMC beads showed flat surfaces with less porosity [4,35], as seen in Figure 1c,d. On the other hand, CMC/CuO-Co 2 O 3 images illustrated that CuO-Co 2 O 3 was planted very well on the CMC surface, as seen in Figure 1e,f. As seen in Figure 1g,h for Cs@CMC/CuO-Co 2 O 3 , chitosan was coated and filled the porous surface of the CMC/CuO-Co 2 O 3 . Moreover, Figure 1i,j illustrates that Ni was rooted and dispersed on Cs@CMC/CuO-Co 2 O 3 , covering most of the Cs@CMC/CuO-Co 2 O 3 surface. The surface area of the metal increased due to the presence of some functional groups such as COO-on CMC, and N-H 2 and O-H on chitosan [36].

Energy Dispersive X-ray (EDX) Analysis
For more confirmation of the prepared nanocomposite beads, EDX analysis was applied. As clearly seen from Figure 2, the Cu and Co elements were present in the powder CuO-Co 2 O 3 and in all prepared beads (CMC/CuO-Co 2 O 3 , Cs@CMC/CuO-Co 2 O 3 and Ni/Cs@CMC/CuO-Co 2 O 3 ), confirming the successful synthesis of proposed materials. The carbon and oxygen elements were present in EDX images for CMC/CuO-Co 2 O 3 , Cs@CMC/CuO-Co 2 O 3 and Ni/Cs@CMC/CuO-Co 2 O 3 due to chitosan and CMC functional groups [37]. Al element was observed in all beads because AlCl 3 was used as a cross-linking agent in the formation of the beads.

X-ray Diffraction (XRD) Analysis
The crystal structures and phase purity of nanocomposite beads were examined by XRD analysis. The X-ray diffraction patterns were collected in the 2θ range of 10-80 • , as clearly indicated in Figure 3. As seen in Figure 3a, the XRD pattern of pure CMC showed a broad peak located at 2θ = 22 owing to the amorphous CMC crystal [4]. The spectrum of XRD for the CuO-Co 2

Energy Dispersive X-Ray (EDX) Analysis
For more confirmation of the prepared nanocomposite beads, EDX analysis was applied. As clearly seen from Figure 2, the Cu and Co elements were present in the powder CuO-Co2O3 and in all prepared beads (CMC/CuO-Co2O3, Cs@CMC/CuO-Co2O3

X-Ray Diffraction (XRD) Analysis
The crystal structures and phase purity of nanocomposite beads were examined by XRD analysis. The X-ray diffraction patterns were collected in the 2θ range of 10-80°, as clearly indicated in Figure 3. As seen in Figure 3a, the XRD pattern of pure CMC showed a broad peak located at 2 = 22 owing to the amorphous CMC crystal [4]. The spectrum of XRD for the CuO-Co2O3 nanocomposite (  [4]. The diffraction peaks at (311), (400), (511), (220) and (440) are responsible for the Co2O3 phase [38]. Therefore, CMC/CuO-Co2O3, Cs@CMC/CuO-Co2O3 and Ni/CMC/CuO-Co2O3 have the same diffraction peaks of the CuO-Co2O3 nanocomposite, indicating the successful preparation of the beads, as clearly seen in Figure 3b-d. Moreover, the XRD patterns of Ni/Cs@CMC/CuO-Co2O3 beads showed peaks located at (111), (200) and (220), which are attributed to the Ni phase on the surface of Ni/Cs@CMC/CuO-Co2O3 beads.      (Figure 4a) illustrated a band at 400-600 cm −1 , which was assigned to the metal oxides (M-O); besides a broad band for O-H (bending and stretching) [4]. FT-IR spectrum of CMC (Figure 4b) exhibited broad bands at 3300-3500 cm −1 , 1422 and 1607, and 1000 and 1200 cm −1 , which were assigned to the stretching of -OH groups, symmetrical and asymmetrical stretching vibrations of the COOgroups and -C-O stretching on the polysaccharide skeleton, respectively [4,39]. All the bands were present in CMC/CuO-Co 2 O 3 (Figure 4c), while for chitosan coating beads Cs@CMC/CuO-Co 2 O 3 (Figure 4d) new bands at 1155 and 1654 cm −1 appeared, which were attributed to the saccharide and (-NH 2 ) amine group in chitosan polymer [40]. All the peaks that appeared in the Ni/Cs@ CMC/CuO-Co 2 O 3 beads are clearly seen in Figure 4e.   Figure 4 represents the FT-IR spectra of all prepared beads, pure CMC, CMC/CuO-Co2O3, Cs@CMC/CuO-Co2O3 and Ni/Cs@CMC/CuO-Co2O3 along with CMC/CuO-Co2O3 powder. The spectrum of CuO-Co2O3 ( Figure 4a) illustrated a band at 400-600 cm −1 , which was assigned to the metal oxides (M-O); besides a broad band for O-H (bending and stretching) [4]. FT-IR spectrum of CMC (Figure 4b) exhibited broad bands at 3300-3500 cm −1 , 1422 and 1607, and 1000 and 1200 cm −1 , which were assigned to the stretching of -OH groups, symmetrical and asymmetrical stretching vibrations of the COO-groups and -C-O stretching on the polysaccharide skeleton, respectively [4,39]. All the bands were present in CMC/CuO-Co2O3 (Figure 4c), while for chitosan coating beads Cs@CMC/CuO-Co2O3 ( Figure 4d) new bands at 1155 and 1654 cm −1 appeared, which were attributed to the saccharide and (-NH2) amine group in chitosan polymer [40]. All the peaks that appeared in the Ni/Cs@ CMC/CuO-Co2O3 beads are clearly seen in Figure 4e.

Metal Uptake Study
To evaluate the amount of metal adsorbed on the surface of Cs@CMC/CuO-Co 2 O 3 and CMC/CuO-Co 2 O 3 beads, distribution coefficient (K d ) and uptake capacity (q e ) were calculated using the following Equations (1) and (2) [41]: where C i and C e are the concentration of the metal ions before and after adsorption by Cs@CMC/CuO-Co 2 O 3 and CMC/CuO-Co 2 O 3 nanocomposite beads, respectively. V refers to the solution volume (L), and m is the mass of beads (g). As can be seen from Table 1, Ni has the highest removal percentage with Cs@CMC/CuO-Co 2 O 3 compared to other metals, (66,86.06, 70 and 79%) for Ag(I), Ni(II), Zn(II) and Fe(II), respectively. When compared the adsorbed metals, Ag(I) has the highest adsorption (%) with CMC/CuO-Co 2 O 3 up to 95%. This result indicates that Ag(I) was more adsorbed by CMC/CuO-Co 2 O 3 beads, and Ni(II) was more adsorbed by Cs@CMC/CuO-Co 2 O 3 beads. However, the Cs@CMC/CuO-Co 2 O 3 adsorbent was more effective than the CMC/CuO-Co 2 O 3 beads toward all metals, as shown in Figure 5. The reason for this is that chitosan has strong chelating proprieties toward metal ions due to the high content of amino and hydroxyl groups present in the composition of chitosan, which act as active sites [15,42,43].
where Ci and Ce are the concentration of the metal ions before and after adsorption by Cs@CMC/CuO-Co2O3 and CMC/CuO-Co2O3 nanocomposite beads, respectively. V refers to the solution volume (L), and m is the mass of beads (g). As can be seen from Table 1, Ni has the highest removal percentage with Cs@CMC/CuO-Co2O3 compared to other metals, (66,86.06, 70 and 79%) for Ag(I), Ni(II), Zn(II) and Fe(II), respectively. When compared the adsorbed metals, Ag(I) has the highest adsorption (%) with CMC/CuO-Co2O3 up to 95%. This result indicates that Ag(I) was more adsorbed by CMC/CuO-Co2O3 beads, and Ni(II) was more adsorbed by Cs@CMC/CuO-Co2O3 beads. However, the Cs@CMC/CuO-Co2O3 adsorbent was more effective than the CMC/CuO-Co2O3 beads toward all metals, as shown in Figure 5. The reason for this is that chitosan has strong chelating proprieties toward metal ions due to the high content of amino and hydroxyl groups present in the composition of chitosan, which act as active sites [15,42,43].    As shown in Figure 6a, the influence of different concentrations of Ni(II) ions was tested to evaluate the adsorption isotherms. According to literature, the adsorption efficiency decreases with the increase in Ni(II) concentration as proved in most studies on heavy metals removal. The possible explanation is that the adsorbent surface sites are enough to accommodate the metal ions in solution and the sorption rate gets faster. However, when the metal concentrations are increased, the adsorbent surface sites will not be enough to capture all the metal ions present in the solution [44].

Effect of pH of Ni(II) Solution
The effect of pH was the most significant controlling parameter in the adsorption study. Adsorption of heavy metals depends on the pH and the type of ions solution. For heavy metal adsorption, a pH range of 5.0-8.0 is usually sufficient. Due to the decrease in H + concentration, heavy metal ions exist as free ions with an initial pH range of 4.0-5.0 and can be adsorbed onto chitosan at higher pH values. Because H + concentration is high at lower pH values, protonation of amino groups can cause electrostatic repulsion between protonated group and heavy metal ions. The net negative charge on the surface of chitosan increases when the pH value rises, and the ionic point of ligands such as -COOH, -OH and -NH 2 groups becomes free, enhancing binding with the heavy metal ions. It has been reported that at pH values less than 6.5, chitosan has strong cationic charges and strong anionic charges at pH higher than 6.5. In our adsorbent beads, the carboxylic (-COOH) and amino (-NH 2 ) groups existing in the beads are responsible for the binding of Ni(II). Therefore, various values of pH for Ni ions solution were examined using the adsorbent Cs@CMC/CuO-Co 2 O 3 nanocomposite beads, as illustrated in Figure 6b. It was clearly found that the adsorption of Ni ions increased with an increase in pH from 3 to 7 (neutral); and the removal efficiency was 1%, 29% and 83% for pH values 3, 5 and 7, respectively. However, the adsorption then reduced greatly with an increase in the pH from 7 to 9. The reduction in adsorption at high pH may be either due to aggregation of chitosan polymer because of the hard protonation of its amino groups, or may be due to the precipitation of metal ions or Ni ions in the alkaline medium as Ni(OH) 2 . [45]. Therefore, the neutral pH was selected as an optimal condition for further experiments. The same effect was reported in the literature [26,46,47].

Effect of Initial Concentration of Ni(II) Solution
As shown in Figure 6a, the influence of different concentrations of Ni(II) ions w tested to evaluate the adsorption isotherms. According to literature, the adsorpt efficiency decreases with the increase in Ni(II) concentration as proved in most studies heavy metals removal. The possible explanation is that the adsorbent surface sites enough to accommodate the metal ions in solution and the sorption rate gets fas However, when the metal concentrations are increased, the adsorbent surface sites w not be enough to capture all the metal ions present in the solution [44].

Effect of pH of Ni(II) Solution
The effect of pH was the most significant controlling parameter in the adsorpt study. Adsorption of heavy metals depends on the pH and the type of ions solution. F heavy metal adsorption, a pH range of 5.0-8.0 is usually sufficient. Due to the decrease H + concentration, heavy metal ions exist as free ions with an initial pH range of 4.0and can be adsorbed onto chitosan at higher pH values. Because H + concentration is h at lower pH values, protonation of amino groups can cause electrostatic repuls between protonated group and heavy metal ions. The net negative charge on the surf of chitosan increases when the pH value rises, and the ionic point of ligands such a COOH, -OH and -NH2 groups becomes free, enhancing binding with the heavy me ions. It has been reported that at pH values less than 6.5, chitosan has strong catio

Effect of Ni(II) Adsorption Contact Time
The effect of contact time is a significant factor in adsorption study. In fact, the adsorption property is very dependent on the time required for equilibrium between the adsorbent and adsorbate. The adsorption of Ni ions was carried out by applying different contact times (10, 30, 60, 120, 240 min). As clearly seen from Figure 6c, 48.6% of Ni(II) was removed in 10 min and reached 88.7% in 60 min before it gradually decreased to 62% at 240 min. The explanation for this phenomenon is that in the first 60 min, the surface of the Cs@CMC/CuO-Co 2 O 3 was filled with the Ni(II), and the whole surface was occupied, but the adsorption process was gradually reduced after 60 min due to the saturation of active sites on the beads' surface. This result is in accordance with previous published findings [48].

Effect of Adsorbent Dose
The adsorbent dose is also an important factor in adsorption. To evaluate the effect of the adsorbent amount on Ni removal, three different doses of CS@CMC/CuO-Co 2 O 3 beads (2.5, 5 and 10 mg) were used for a fixed initial Ni(II) concentration (5 mg L −1 ) at 25 • C with a contact time of 60 min. As clearly seen in Figure 6d, the removal percentage of Ni(II) was increased from 53% to 83% when the number of beads increased from 2.5 to 5 mg, respectively. However, the removal percentage was then decreased with an increase in bead dosage from 5 to 10 mg. The best explanation for this phenomenon is that the CS@CMC/CuO-Co 2 O 3 beads have more active sites, which remined unsaturated during the adsorption procedure. Therefore, 5 mg was fixed for further study as the optimum adsorbent dose.
Moreover, the isotherm and kinetic adsorption were studied as follows: The two isotherm models, Langmuir and Freundlich, are given in Table 2 (Equations (4) and (5)) to model the adsorption process of our system. According to the data obtained, the correlation coefficient (R 2 ) values for isotherm models, Langmuir was discovered to be a suitable model to represent the sorption system, which assumed a monolayer of analyte establisher on a homogeneous surface of the adsorbent. Linearity of plotting C e /q e vs. C e was achieved with R 2 of 0.9433. The Langmuir constant (q m ) was calculated to be 12.00 mg g −1 , which is close to the experimental value of the adsorption capacity (11.00 mg g −1 ). The Langmuir constant (b) is equal to 0.08 L mg −1 , which explains the strong affinity of the Ni(II) ions to the adsorbent beads. The essential factor R L was calculated using Equation (3) [44].
where C o is the initial concentration of Ni(II) (mgL −1 ), and b is the Langmuir constant. The calculated R L was found to be 0.50, which is in the range of 0 < R L > 1, referring to favorable adsorption. Thus, this confirmed that the adsorption of Ni(II) ions by Cs@CMC/CuO-Co 2 O 3 is favorable, as shown in Table 3. Table 2. Mathematical equations and isotherm models [49][50][51] used in this study.

Models Linear Equations Plot
Isotherm Langmuir vs. t Table 3. Data of isotherm models for Ni(II) adsorption using Cs@CMC/CuO-Co 2 O 3 .

Langmuir Model Freundlich Model
Metal Ion 12.00 0.943 0.50 0.08 0.553 9.10 5.06 The pseudo-first-order model and second-order model were applied to the adsorption system for an explanation of kinetics, as shown in Table 2 (Equations (6) and (7)). Both slope and intercept were calculated using the plot log (q e − q t ) vs. t, and t/qt vs. t for the pseudo-first order and the pseudo-second order, respectively. Based on the calculated values, the pseudo-first-order more suitably explained the adsorption. The pseudo-secondorder model was close to the adsorption capacities obtained from experiments; Langmuir isotherm and pseudo-second-order kinetic models were compatible, as illustrated in Table 4. The obtained data were compared with other studies for removal of Ni(II), as represented in Table 5.

Adsorption Mechanism
The possible adsorption mechanism is illustrated in Figure 7. The adsorption of heavy metals with Cs@CMC/CuO-Co 2 O 3 might be due to strong attraction of metal ions to nanocomposite beads, which contain active sites (COO-, OH, Cu-O, Co-O, -O-and -NH 2 ). These groups can easily attract and combined with metal ions. However, the amino group in chitosan has a significant role in the adsorption because the chitosan completely coats the surface of CMC/CuO-Co 2 O 3 . The chemical nature of chitosan, hydrophilicity due to the large number of (-OH) and the presence of (-NH 2 ) can determine the adsorption of chitosan toward the heavy metal. According to the literature, the adsorption of heavy metal ions by chitosan functional groups can occur based on different mechanisms (e.g., electrostatic attraction and chelation). Chitosan (NH 2 ) groups are responsible for the adsorption of metal cations by a chelation mechanism. In fact, the adsorption can be affected by the pH of the metal ion solution where the NH 2 group (free-electron doublet on nitrogen) can attract cations at neutral pH [24,54,55].

Catalytic Reduction Study
The catalytic ability of all prepared catalysts, including CuO-Co 2 O 3 , CMC/CuO-Co 2 O 3 , Cs@CMC/CuO-Co 2 O 3 and Ni/Cs@CMC/CuO-Co 2 O 3 , was examined for the reduction of two anionic dyes (MO and EY). MO dye was chosen as a model dye for this study. MO solution (0.01 mM) was placed into a UV-cuvette and mixed with a reducing agent, NaBH 4 . Further, each catalyst was added to the mixture of MO and NaBH 4 as mentioned previously. Afterward, the reduction reaction of MO was examined by the UV-Vis spectrophotometer every min as the reaction proceeded. Initially, pure MO solution (0.01 mM) was recorded by UV-Vis, and two absorbance bands appeared at λ max = 460 nm and 270 nm, as shown in Figure 8. There was no change observed when the reducing agent (NaBH 4 ) was added because NaBH 4 cannot reduce the dye even with an excess amount, as reported in the literature [56]. However, after the addition of both reducing agent and catalysts, the color of MO dye disappeared gradually from orange to colorless. The reason for this that MO was converted to hydrazine derivatives by breaking the azo bond (-N=N-) and transforming it to -NH 2 (amino). During the reduction, the peak at l max = 460 nm was decreased gradually [57,58]. The reduction percentage of MO was 92 % in 4 min with Ni/Cs@CMC/CuO-Co 2 O 3 while it reached to 85%, 90% and 33.5 % with CuO-Co 2 O 3 , CMC/CuO-Co 2 O 3 and Cs@CMC/CuO-Co 2 O 3 in 5, 20 and 14 min, respectively. Moreover, the reduction of MO was studied by using the Ag/CMC/CuO-Co 2 O 3 under the same conditions. It was found that Ag/Cs@CMC/CuO-Co 2 O 3 can reduce 88% of MO in 6 min. chitosan toward the heavy metal. According to the literature, the adsorption of heavy metal ions by chitosan functional groups can occur based on different mechanisms (e.g., electrostatic attraction and chelation). Chitosan (NH2) groups are responsible for the adsorption of metal cations by a chelation mechanism. In fact, the adsorption can be affected by the pH of the metal ion solution where the NH2 group (free-electron doublet on nitrogen) can attract cations at neutral pH [24,54,55].

Catalytic Reduction Study
The catalytic ability of all prepared catalysts, including CuO-Co2O3, CMC/CuO-Co2O3, Cs@CMC/CuO-Co2O3 and Ni/Cs@CMC/CuO-Co2O3, was examined for the reduction of two anionic dyes (MO and EY). MO dye was chosen as a model dye for this study. MO solution (0.01 mM) was placed into a UV-cuvette and mixed with a reducing agent, NaBH4. Further, each catalyst was added to the mixture of MO and NaBH4 as mentioned previously. Afterward, the reduction reaction of MO was examined by the UV-Vis spectrophotometer every min as the reaction proceeded. Initially, pure MO solution (0.01 mM) was recorded by UV-Vis, and two absorbance bands appeared at max = 460 nm and 270 nm, as shown in Figure 8. There was no change observed when the reducing agent (NaBH4) was added because NaBH4 cannot reduce the dye even with an excess amount, as reported in the literature [56]. However, after the addition of both reducing agent and catalysts, the color of MO dye disappeared gradually from orange to colorless. The reason for this that MO was converted to hydrazine derivatives by breaking the azo bond (-N=N-) and transforming it to -NH2 (amino). During the reduction, the peak at lmax = 460 nm was decreased gradually [57,58]. The reduction percentage of MO was 92 % in 4 min with Ni/Cs@CMC/CuO-Co2O3 while it reached to 85%, 90% and 33.5 % with CuO-Co2O3, CMC/CuO-Co2O3 and Cs@CMC/CuO-Co2O3 in 5, 20 and 14 min, respectively. Moreover, the reduction of MO was studied by using the Ag/CMC/CuO-Co2O3 under the same conditions. It was found that Ag/Cs@CMC/CuO-Co2O3 can reduce 88% of MO in 6 min.   The kinetic behavior of four prepared catalysts toward the reduction of MO dye was evaluated by applying the pseudo-first-order kinetics. The rate constants were calculated from the slope of lnC t/ C 0 vs. time, as seen in Figure 8f. The rate constant K value per second and R 2 correlation coefficient of decolorization of MO dye with Ni/Cs@CMC/CuO-Co 2 O 3 was 1.02 × 10 −2 s −1 and 0.964, respectively, which is higher than other catalysts, using Cs@CMC/CuO-Co 2 O 3 (4.4 × 10 −4 and 0.953), CMC/CuO-Co 2 O 3 (2.6 × 10 −3 and 0.915) and CuO-Co 2 O 3 (6.11 × 10 −3 and 0.868). This clearly indicates that Ni/Cs@CMC/CuO-Co 2 O 3 is the most active catalyst among other prepared catalysts toward the reduction of MO dye.
Moreover, the catalytic reduction was tested toward the degradation of EY. The decolorization of EY was conducted by using the same procedure described previously for catalytic reduction of MO (Figure 9). EY had an absorbance band at 510 nm, which gradually decreased. During the reduction, the EY color changed from orange to pale yellow and then turned to colorless, indicating the formation of ESH 2 [59]. Ni/Cs@ CMC/CuO-Co 2 O 3 was the highest efficient catalyst toward EY. According to data obtained, around 90% of EY was decolorized in 9 min by Ni/Cs@CMC/CuO-Co 2 O 3 , while reduction of 71%, 94% and 35% were obtained in 24, 15 Figure 10 shows the possible reduction mechanism of MO and EY. The reduction occurs mainly through the transfer of electrons via nanocatalyst facilitation. Firstly, the NaBH4 dissociates to BH4 -ions and Na + , in which BH4 − acts as a source of eand H + . Further, the catalyst Ni/Cs@CMC/CuO-Co2O3 transfers efrom the BH4 -ion to dye molecules for catalytic reduction. For MO dye, the azo bonds are activated by the electron transfers by BH4 -ion via Ni/Cs@CMC/CuO-Co2O3 nanocomposite beads. The MO molecules are bounded to the nanocomposite beads by oxygen and sulfur atoms. Thus, the first step is the conversion of the -N=N-bond into -HN-NH-bond followed by bondbreaking to form aromatic amines. In fact, this happens because eare accepted from nanocomposite beads catalyst and H + from BH4 − . Thus, the orange color of the MO dye is turned colorless, indicating the completion of the reduction of MO. On the other hand, the EY is adsorbed on the surface of Ni/Cs@CMC/CuO-Co2O3 nanocomposite beads because of the electrostatic attractive force between Ni/Cs@CMC/CuO-Co2O3 nanocomposite and anionic dye. Afterward, the electron is transferred by Ni/Cs@CMC/CuO-Co2O3 from BH4to EY for its catalytic reduction [4].     Figure 10 shows the possible reduction mechanism of MO and EY. The reduction occurs mainly through the transfer of electrons via nanocatalyst facilitation. Firstly, the NaBH 4 dissociates to BH 4 − ions and Na + , in which BH 4 − acts as a source of e − and H + . Further, the catalyst Ni/Cs@CMC/CuO-Co 2 O 3 transfers e − from the BH 4 − ion to dye molecules for catalytic reduction. For MO dye, the azo bonds are activated by the electron transfers by BH 4 − ion via Ni/Cs@CMC/CuO-Co 2 O 3 nanocomposite beads. The MO molecules are bounded to the nanocomposite beads by oxygen and sulfur atoms. Thus, the first step is the conversion of the -N=N-bond into -HN-NH-bond followed by bond-breaking to form aromatic amines. In fact, this happens because e − are accepted from nanocomposite beads catalyst and H + from BH 4 − . Thus, the orange color of the MO dye is turned colorless, indicating the completion of the reduction of MO. On the other hand, the EY is adsorbed on the surface of Ni/Cs@CMC/CuO-Co 2 O 3 nanocomposite beads because of the electrostatic attractive force between Ni/Cs@CMC/CuO-Co 2 O 3 nanocomposite and anionic dye. Afterward, the electron is transferred by Ni/Cs@CMC/CuO-Co 2 O 3 from BH 4 − to EY for its catalytic reduction [4]. Additionally, an evaluation of CuO-Co 2 O 3 , CMC/CuO-Co 2 O 3 , Cs@CMC/CuO-Co 2 O 3 and Ni/Cs@CMC/CuO-Co 2 O 3 as catalysts toward the catalytic reduction of 4-NP was performed. By using the same procedure mentioned previously, the reaction of 4-NP was performed by utilizing CuO-Co 2 O 3, CMC/CuO-Co 2 O 3 , Cs@CMC/CuO-Co 2 O 3 and Ni/Cs@CMC/CuO-Co 2 O 3 as catalysts in the presence of NaBH 4 . In the beginning, the absorbance band of 4-NP appeared at λ max = 317 nm. As observed, the yellow color of 4-NP changed directly to dark yellow in the presence of (0.5 mL) NaBH 4 with a new UV-Vis band appearing at 400 nm. This indicates the transformation of 4-NP to 4-nitrophenolate. Then, the CuO-Co 2 O 3, CMC/CuO-Co 2 O 3 , Cs@CMC/CuO-Co 2 O 3 and Ni/Cs@CMC/CuO-Co 2 O 3 catalysts were added and tested separately for the reduction of 4-nitrophenol. The band at λ max = 400 nm disappeared gradually, and a new absorbance band at λ max = 320 nm appeared along with the disappearance of dark yellow color, proving the formation of 4-AP because of 4-NP reduction. It was found that among all the catalysts, the prepared Ni/Cs@CMC/CuO-Co 2 O 3 was the most effective catalyst because 4-NP was completely reduced to 4-AP in 13 min, while it was reduced in 19 and 20 min by using CuO-Co 2 O 3 and CMC/CuO-Co 2 O 3 , respectively. However, the reduction of 4-NP by Cs@CMC/CuO-Co 2 O 3 took 20 min (Figure 11a). Additionally, an evaluation of CuO-Co2O3, CMC/CuO-Co2O3, Cs@CMC/CuO-Co2O3 and Ni/Cs@CMC/CuO-Co2O3 as catalysts toward the catalytic reduction of 4-NP was performed. By using the same procedure mentioned previously, the reaction of 4-NP was performed by utilizing CuO-Co2O3, CMC/CuO-Co2O3, Cs@CMC/CuO-Co2O3 and Ni/Cs@CMC/CuO-Co2O3 as catalysts in the presence of NaBH4. In the beginning, the absorbance band of 4-NP appeared at max = 317 nm. As observed, the yellow color of 4-NP changed directly to dark yellow in the presence of (0.5 mL) NaBH4 with a new UV-Vis band appearing at 400 nm. This indicates the transformation of 4-NP to 4nitrophenolate. Then, the CuO-Co2O3, CMC/CuO-Co2O3, Cs@CMC/CuO-Co2O3 and Ni/Cs@CMC/CuO-Co2O3 catalysts were added and tested separately for the reduction of 4-nitrophenol. The band at max = 400 nm disappeared gradually, and a new absorbance band at max = 320 nm appeared along with the disappearance of dark yellow color, proving the formation of 4-AP because of 4-NP reduction. It was found that among all the catalysts, the prepared Ni/Cs@CMC/CuO-Co2O3 was the most effective catalyst because 4-NP was completely reduced to 4-AP in 13 min, while it was reduced in 19 and 20 min by using CuO-Co2O3 and CMC/CuO-Co2O3, respectively. However, the reduction of 4-NP by Cs@CMC/CuO-Co2O3 took 20 min (Figure 11a).
The rate constant and R 2 were found to be 4.26 × 10 −3 s −1 and 0.912, 17 × 10 −5 s −1 and 0.824, 2.62 × 10 −3 s −1 and 0.842 and 2.7 × 10 −3 s −1 and 0.855 for Ni/Cs@CMC/CuO-Co2O3, Cs@CMC/CuO-Co2O3, CMC/CuO-Co2O3 and CuO-Co2O3, respectively, as shown in Table  6 and Figure 11b. The data for 4-NP reduction was compared with other catalysts and is illustrated in Table 7.   Figure  12a. Based on findings, the catalytic reduction reaction of K 3 [Fe(CN) 6 ] follows the pseudofirst-order, as seen in Figure 12b. Subsequently, the rate constant and R 2 were found to be 5.   Table 6 and Figure 11b. The data for 4-NP reduction was compared with other catalysts and is illustrated in Table 7.
The catalytic reduction of K 3 [Fe(CN) 6 ] was also examined to evaluate the catalytic activity of CuO-Co 2 O 3 , CMC/CuO-Co 2 O 3 , Cs@CMC/CuO-Co 2 O 3 and Ni/Cs@CMC/CuO-Co 2 O 3 . The UV-vis absorption of the catalytic reduction of K 3 [Fe(CN) 6 ] was monitored every minute to check the progress of the K 3 [Fe(CN) 6 ] reduction. As the catalytic reaction proceeded in the presence of NaBH 4 and the catalyst, the absorption band of K 3 [Fe(CN) 6 ] at λ max = 420 nm gradually decreased in 6 min when using Ni/Cs@CMC/CuO-Co 2 O 3 along with disappearance of the yellow color, indicating the reduction of K 3 Figure 12a. Based on findings, the catalytic reduction reaction of K 3 [Fe(CN) 6 ] follows the pseudo-first-order, as seen in Figure 12b. Subsequently, the rate constant and R 2 were found to be 5.1 × 10 −3 s −1 and 0.975, 1.8 × 10 −3 s −1 and 0.909, 1.69 × 10 −3 s −1 and 0.835 and 3.6 × 10 −3 s −1 and 0.964 for Ni/Cs@CMC/CuO-Co 2 O 3 , Cs@CMC/CuO-Co 2 O 3 , CMC/CuO-Co 2 O 3 and CuO-Co 2 O 3, respectively, as shown in Table 6.
The catalytic reduction of K3[Fe(CN)6] was also examined to evaluate the catalytic activity of CuO-Co2O3, CMC/CuO-Co2O3, Cs@CMC/CuO-Co2O3 and Ni/Cs@CMC/CuO-Co2O3. The UV-vis absorption of the catalytic reduction of K3[Fe(CN)6] was monitored every minute to check the progress of the K3[Fe(CN)6] reduction. As the catalytic reaction proceeded in the presence of NaBH4 and the catalyst, the absorption band of K3[Fe(CN)6] at max = 420 nm gradually decreased in 6 min when using Ni/Cs@CMC/CuO-Co2O3 along with disappearance of the yellow color, indicating the reduction of K3[Fe(CN)6] to K4[Fe(CN)6] [69]. In contrast, the reduction reaction took longer times of 8, 13 and 18 min when using CuO-Co2O3, CMC/CuO-Co2O3 and Cs@CMC/CuO-Co2O3, respectively. The efficient transformation of K3[Fe(CN)6] to K4[Fe(CN)6] was obtained by applying the Ni/Cs@CMC/CuO-Co2O3 catalyst (91%), while 85, 73.5 and 83% reduction were obtained by using Cs@CMC/CuO-Co2O3, CMC/CuO-Co2O3 and CuO-Co2O3, as shown in Figure  12a. Based on findings, the catalytic reduction reaction of K 3 [Fe(CN) 6 ] follows the pseudofirst-order, as seen in Figure 12b. Subsequently, the rate constant and R 2 were found to be 5.1 × 10 −3 s −1 and 0.975, 1.8 × 10 −3 s −1 and 0.909, 1.69 × 10 −3 s −1 and 0.835 and 3.6 × 10 −3 s −1 and 0.964 for Ni/Cs@CMC/CuO-Co2O3, Cs@CMC/CuO-Co2O3, CMC/CuO-Co2O3 and CuO-Co2O3, respectively, as shown in Table 6. The possible mechanism for the reaction of K 3 [Fe(CN) 6 ] in the presence of both catalyst beads and NaBH 4 is illustrated in Figure 10. Based on the reported studies, the catalytic reduction of [Fe(CN) 6 ] −3 to form [Fe(CN) 6 ] −4 is an electron-transfer route, as shown in the reaction below [3]. Therefore, the catalytic reaction mechanism of K 3 [Fe(CN) 6  The effect of concentration was examined for all compounds (4-NP, EY, MO and K 3 [Fe(CN 6 )]) by using a certain catalyst (Ni/Cs@CMC/CuO-Co 2 O 3 ) since it is more effective than others (Cs@CMC/CuO-Co 2 O 3, Cs@CMC/CuO-Co 2 O 3 , CMC/CuO-Co 2 O 3 and CuO-Co 2 O 3 ). As clearly seen in all figures below (Figure 13), when the concentration of pollutants was increased the time taken for reduction was increased. This finding indicates that the concentration of the pollutants has an essential role and Ni/Cs@CMC/CuO-Co 2 O 3 was found to be more efficient catalyst with a low concentration of MO, EY, 4-NP and K 3 [Fe(CN) 6 ], which was found to have similar effect as reported in literature [70]. effective than others (Cs@CMC/CuO-Co2O3, Cs@CMC/CuO-Co2O3, CMC/CuO-Co2O3 and CuO-Co2O3). As clearly seen in all figures below (Figure 13), when the concentration of pollutants was increased the time taken for reduction was increased. This finding indicates that the concentration of the pollutants has an essential role and Ni/Cs@CMC/CuO-Co2O3 was found to be more efficient catalyst with a low concentration of MO, EY, 4-NP and K3[Fe(CN)6], which was found to have similar effect as reported in literature [70].

Effect of NaBH4 Concentration
The impact of NaBH4 concentration on the catalytic reduction of pollutants is a very important parameter. Therefore, a range of NaBH4 concentrations (0.2, 0.1 and 0.05 M) were used to evaluate its effect on the reduction of target pollutants in the presence of

Effect of NaBH 4 Concentration
The impact of NaBH 4 concentration on the catalytic reduction of pollutants is a very important parameter. Therefore, a range of NaBH 4 concentrations (0.2, 0.1 and 0.05 M) were used to evaluate its effect on the reduction of target pollutants in the presence of Ni/Cs@CMC/CuO-Co 2 O 3 beads as a catalyst. As the data demonstrate, the reducing agent has an important role in the reduction reaction of pollutants in the presence of an effective catalyst. However, this reducing agent has no activity or ability to reduce toxic compounds even at high concentrations. Therefore, an effective catalyst should be added to enhance the reduction. A high concentration of NaBH 4 (0.2 M) in addition to an effective catalyst such as Ni/Cs@CMC/CuO-Co 2 O 3 can promote the reduction reaction of MO at a faster rate, decolorizing it in only 2 min, as shown in Figure 14a. However, when the reducing agent (NaBH 4 ) concentration was decreased, the reduction reaction rate was influenced and decreased. This means that the reduction reaction requires more time to complete. Indeed, MO was reduced in 4 min and 12 min when the NaBH 4 concentration was 0.1 and 0.05 M, respectively, as shown in Figure 14a. EY was also reduced in 6 min by using a high NaBH 4 concentration, while this reduction took 9 min and 14 min using 0.1 and 0.05 M NaBH 4 , respectively, as shown in Figure 16b. The same effect was observed for K 3 [Fe(CN) 6 ] (Figure 14c) and 4-NP (Figure 14d), and a similar impact was reported in literature [70].
influenced and decreased. This means that the reduction reaction requires more time to complete. Indeed, MO was reduced in 4 min and 12 min when the NaBH4 concentration was 0.1 and 0.05 M, respectively, as shown in Figure 14a. EY was also reduced in 6 min by using a high NaBH4 concentration, while this reduction took 9 min and 14 min using 0.1 and 0.05 M NaBH4, respectively, as shown in Figure 16b. The same effect was observed for K3[Fe(CN)6] (Figure 14c) and 4-NP (Figure 14d), and a similar impact was reported in literature [70].

Effect of Number of Ni/Cs@CMC/CuO-Co2O3 Beads
The influence of amount of Ni/Cs@CMC/CuO-Co2O3 was tested by utilizing three different amounts of Ni/Cs@CMC/CuO-Co2O3 bead catalyst (3 mg, 5 mg and 8 mg) in the presence of reducing agent (0.2 M NaBH4). This effect was tested using 0.01 mM MO and 0.05 mM K3[Fe(CN6)], as seen in Figure 15. In fact, the amount of catalyst is an important factor in the reduction reaction. The results demonstrate that a larger catalyst amount (8 mg of Ni/Cs@CMC/CuO-Co2O3) could enhance the reaction of MO causing decolorization by 98% in 2 min and 92% in 2 min using an amount of 5 mg. However, MO was reduced by 97% in 6 min by using a low amount of catalyst, as shown in Figure 15a. A similar effect was found for K3[Fe(CN6)], as clearly seen in Figure 15b.

Effect of Number of Ni/Cs@CMC/CuO-Co 2 O 3 Beads
The influence of amount of Ni/Cs@CMC/CuO-Co 2 O 3 was tested by utilizing three different amounts of Ni/Cs@CMC/CuO-Co 2 O 3 bead catalyst (3 mg, 5 mg and 8 mg) in the presence of reducing agent (0.2 M NaBH 4 ). This effect was tested using 0.01 mM MO and 0.05 mM K 3 [Fe(CN 6 )], as seen in Figure 15. In fact, the amount of catalyst is an important factor in the reduction reaction. The results demonstrate that a larger catalyst amount (8 mg of Ni/Cs@CMC/CuO-Co 2 O 3 ) could enhance the reaction of MO causing decolorization by 98% in 2 min and 92% in 2 min using an amount of 5 mg. However, MO was reduced by 97% in 6 min by using a low amount of catalyst, as shown in Figure 15a. A similar effect was found for K 3 [Fe(CN 6 )], as clearly seen in Figure 15b.

Recyclability of Ni/Cs@CMC/CuO-Co2O3 beads
Recyclability of the catalyst is a significant factor during a catalytic reduction study. Most of the catalysts become deactivated after the first or second use. In our study, Ni/Cs@CMC/CuO-Co2O3 was able to be reused up to five times without deactivation or any loss of the catalyst beads. Consequently, Ni/Cs@CMC/CuO-Co2O3 beads were tested

Recyclability of Ni/Cs@CMC/CuO-Co 2 O 3 Beads
Recyclability of the catalyst is a significant factor during a catalytic reduction study. Most of the catalysts become deactivated after the first or second use. In our study, Ni/Cs@CMC/CuO-Co 2 O 3 was able to be reused up to five times without deactivation or any loss of the catalyst beads. Consequently, Ni/Cs@CMC/CuO-Co 2 O 3 beads were tested in the reduction of MO, EY and K 3 [Fe(CN) 6 ] for several time to check the recyclability of the catalyst. As mentioned previously, the same procedures were followed, except the beads were washed by deionized water and then MeOH, followed by deionized water several times, and then dried for the next use. This process was repeated five times to assess the reusability of the Ni/Cs@CMC/CuO-Co 2 O 3 beads. Figure 16

Recyclability of Ni/Cs@CMC/CuO-Co2O3 beads
Recyclability of the catalyst is a significant factor during a catalytic reduction study. Most of the catalysts become deactivated after the first or second use. In our study, Ni/Cs@CMC/CuO-Co2O3 was able to be reused up to five times without deactivation or any loss of the catalyst beads. Consequently, Ni/Cs@CMC/CuO-Co2O3 beads were tested in the reduction of MO, EY and K3[Fe(CN)6] for several time to check the recyclability of the catalyst. As mentioned previously, the same procedures were followed, except the beads were washed by deionized water and then MeOH, followed by deionized water several times, and then dried for the next use. This process was repeated five times to assess the reusability of the Ni/Cs@CMC/CuO-Co2O3 beads. Figure 16 shows the time taken for each reduction cycle of MO, 4-NP and K3[Fe(CN)6] using Ni/Cs@CMC/CuO-Co2O3 beads.

Application to Real Samples
The catalytic activity of Ni/Cs@CMC/CuO-Co 2 O 3 beads was also assessed in four types of real samples with spiked MO (0.06 mM). The real samples used for this study were full-fat milk and three juice samples (orange, pineapple and apple), and they were obtained from a local market (Jeddah, Saudi Arabia). The preparation of real samples was performed by taking around 1 mL of each sample and diluting it in 100 mL of deionized water individually. Further, 2.5 mL of each real sample was placed into a UV-vis cuvette, and 0.5 mL of 0.06 mM MO was added followed by addition of 0.5 mL of 0.1 M NaBH 4 . Finally, 5 mg of Ni/Cs@CMC/CuO-Co 2 O 3 was added. The catalytic degradation of MO was monitored by a UV-vis spectrophotometer. As clearly seen from data presented in Table 8, full-fat milk was the only sample that took a longer time (15 min) with a very low reduction % (65%). This is due to the high interference found in the milk, which can influence the reduction of MO. In contrast, the reduction of MO in the three juice samples occurred in 5-6 min with 91-97%. The data confirm that the Ni/Cs@CMC/CuO-Co 2 O 3 is effective and reliable since it was able to decolorize and effectively reduce MO in real samples. O)) and 4-nitrophenol (4-NP, (≥99%)) were obtained from Sigma-Aldrich. Potassium hexacyanoferrate (III), (99%) and the reducing agent (sodium borohydride, (99%)) were obtained from Sigma-Aldrich. In all preparations, deionized water was utilized.

Preparation of CuO-Co 2 O 3 Nanocomposite
The nanocomposite, CuO-Co 2 O 3 was prepared by co-precipitation method. Firstly, 0.1M CuSO 4 ·5H 2 O was mixed with 0.1 M CoSO 4 ·6 H 2 O in a (50:50) ratio. Then, NaOH was added dropwise to adjust the pH, 10-11. The preparation was carried out at 80 • C with stirring for 4 h. Finally, the precipitate was collected via filtration, then washed several times with deionized water and dried overnight. Afterward, the CuO-Co 2 O 3 nanocomposite was calcined at 500 • C for 5 h.

Synthesis of Cs@CMC/CuO-Co 2 O 3 Beads
The novel nanocomposite beads were synthesized in two main steps. The method was reported by our group with some modifications [4,21]. The first step, CMC (0.5 g) was dissolved in (25 mL) with stirring for 2 h at 50 • C. Meanwhile, 60 mg of CuO-Co 2 O 3 powder was dissolved in 5 mL of deionized water and further sonicated for around 10 min for suspension of CuO-Co 2 O 3 . Subsequently, CuO-Co 2 O 3 dispersed solution was added into CMC solution with continuous stirring for 1 h at 50 • C, and half an hour at RT (24 • C). The mixture was transferred to 3 mL syringe and dropped into 0.2 M AlCl 3 solution for crosslinking and formation of beads. The beads were kept in AlCl 3 solution for 12 h and then collected and washed three times using deionized water. Secondly, Cs solution was prepared in 1% acetic acid distilled water mixture and stirred for 3 h at 50 • C. The washed beads were transferred to Cs solution and kept for 1 h. Finally, the Cs@CMC/CuO-Co 2 O 3 beads were separated and dried at RT for 24 h. Dry CMC/CuO-Co 2 O 3 beads were flat, while the dry Cs@CMC/CuO-Co 2 O 3 beads had a circle shape due to their chitosan coating, Figure 17. solution for crosslinking and formation of beads. The beads were kept in AlCl3 solution for 12 h and then collected and washed three times using deionized water. Secondly, Cs solution was prepared in 1% acetic acid distilled water mixture and stirred for 3 h at 50 °C. The washed beads were transferred to Cs solution and kept for 1 h. Finally, the Cs@CMC/CuO-Co2O3 beads were separated and dried at RT for 24 h. Dry CMC/CuO-Co2O3 beads were flat, while the dry Cs@CMC/CuO-Co2O3 beads had a circle shape due to their chitosan coating, Figure 17.

Metal Uptake Adsorption
In order to evaluate the selectivity of the prepared nanocomposite beads (Cs@CMC/ CuO-Co 2 O 3 and CMC/CuO-Co 2 O 3 ), adsorption methods of metal ions were developed toward certain metal ions including Ni(II), Ag(I), Zn(II), and Fe(II). Certain amounts of Cs@CMC/CuO-Co 2 O 3 and CMC/CuO-Co 2 O 3 (5.0 mg) were added individually into 5 mL of 5 ppm of sample solution of selected metal ions for 1 h at RT (25 ± 1 • C). The beads were separated from the solutions and dried at RT. The inductively coupled plasma−optical emission spectroscopy (ICP-OES) was employed to detect the concentration of each metal ion before and after adsorption on Cs@CMC/CuO-Co 2 O 3 and CMC/CuO-Co 2 O 3 beads. Optimization of parameters of the selected metal ions Ni(II) was carried out as mentioned later. Moreover, the Ag (I) beads were also kept after adsorption and converted to NPs for further studies.
For isotherm study, 5 mg of each adsorbent Cs@CMC/CuO-Co 2 O 3 and CMC/CuO-Co 2 O 3 beads was added to 5 mL of Ni solution with initial concentrations from (5-100 mg L −1 ). The pH value of Ni solution was adjusted to pH = 7, with mechanical shaking for 60 min.
Moreover, for kinetic study, 5 mg of each adsorbent Cs@CMC/CuO-Co 2 O 3 and CMC/CuO-Co 2 O 3 bead was added to 5 mL of 5 mgL −1 Ni solution. The pH value of Ni solution was adjusted to pH = 7. The concentration of Ni ions was tested at different times (10, 30, 60, 120, 240 min).

Formation of Zero-Valent Nanoparticles
Ni(II)/Cs@CMC/CuO-Co 2 O 3 and Ag(I)/CMC/CuO-Co 2 O 3 beads were collected and dried. These dried beads were later utilized for the synthesis of nanoparticles ( Figure 18) by loading them into a freshly prepared 0.05 M NaBH 4 aqueous solution for 20 min in order to complete the reduction of Ni(II) and Ag(I) to Ni(0) and Ag(0) zero-valent nanoparticles, respectively, as shown in the following Equations (9) and (10).

Metal Uptake Adsorption
In order to evaluate the selectivity of the prepared nanocomposite beads (Cs@CMC/CuO-Co2O3 and CMC/CuO-Co2O3), adsorption methods of metal ions were developed toward certain metal ions including Ni(II), Ag(I), Zn(II), and Fe(II). Certain amounts of Cs@CMC/CuO-Co2O3 and CMC/CuO-Co2O3 (5.0 mg) were added individually into 5 mL of 5 ppm of sample solution of selected metal ions for 1 h at RT (25 ± 1 °C). The beads were separated from the solutions and dried at RT. The inductively coupled plasma−optical emission spectroscopy (ICP-OES) was employed to detect the concentration of each metal ion before and after adsorption on Cs@CMC/CuO-Co2O3 and CMC/CuO-Co2O3 beads. Optimization of parameters of the selected metal ions Ni(II) was carried out as mentioned later. Moreover, the Ag (I) beads were also kept after adsorption and converted to NPs for further studies.
For isotherm study, 5 mg of each adsorbent Cs@CMC/CuO-Co2O3 and CMC/CuO-Co2O3 beads was added to 5 mL of Ni solution with initial concentrations from (5-100 mgL −1 ). The pH value of Ni solution was adjusted to pH = 7, with mechanical shaking for 60 min.
Moreover, for kinetic study, 5 mg of each adsorbent Cs@CMC/CuO-Co2O3 and CMC/CuO-Co2O3 bead was added to 5 mL of 5 mgL −1 Ni solution. The pH value of Ni solution was adjusted to pH = 7. The concentration of Ni ions was tested at different times (10, 30, 60, 120, 240 min).

Formation of Zero-Valent Nanoparticles
Ni(II)/Cs@CMC/CuO-Co2O3 and Ag(I)/CMC/CuO-Co2O3 beads were collected and dried. These dried beads were later utilized for the synthesis of nanoparticles ( Figure 18) by loading them into a freshly prepared 0.05 M NaBH4 aqueous solution for 20 min in order to complete the reduction of Ni(II) and Ag(I) to Ni(0) and Ag(0) zero-valent nanoparticles, respectively, as shown in the following Equations (9)

Catalytic Reduction Experiments
The catalytic ability of prepared catalysts, Ni/Cs@CMC/CuO-Co 2 O 3 , Cs@CMC/CuO- The catalytic procedure was performed by placing 2.5 mL of each pollutant (MO, EY, 4-NP and K 3 [Fe (CN) 6 ]) in a UV cuvette, and 0.5 mL of a freshly prepared solution of NaBH 4 (0.1 M) was added. Then, 5 mg of the prepared catalyst and NaBH 4 were both added to the mixture in the cuvette. Catalytic activity was continuously examined via UV-vis spectrophotometer with every 1 min interval. The recyclability of Ni/Cs@CMC/CuO-Co 2 O 3 beads was tested in the catalytic reduction of MO, EY and K 3 [Fe(CN) 6 ] . This catalyst was used up to five times after washing with deionized water and MeOH and dried for the next cycle.
The conversion (%) of all compounds was calculated by utilizing Equation (11): where C 0 (mgL −1 ) is the initial concentration of compounds, and C e (mgL −1 ) is the final concentration. The rate constant K and adjacent R 2 values were determined from pseudo-first-order kinetics as described below in Equation (12):

Characterization
The morphologies and structures of Ni/Cs@CMC/CuO-Co 2 O 3 , Cs@CMC/CuO-Co 2 O 3 , CMC/CuO-Co 2 O 3 and CuO-Co 2 O 3 were characterized by scanning electron microscope (SEM) (of JEOL, JSM-7600F, Japan). For SEM analysis, Ni/Cs@CMC/CuO-Co 2 O 3 , Cs@CMC/CuO-Co 2 O 3 , CMC/CuO-Co 2 O 3 and CuO-Co 2 O 3 were individually fixed on the stub using carbon tape as a binder and then sputtered with platinum for 15 s. In addition, X-ray diffraction (XRD) was employed to examine the phase structure of all prepared catalysts. Elemental analysis of Ni/Cs@CMC/CuO-Co 2 O 3 , Cs@CMC/CuO-Co 2 O 3 , CMC/CuO-Co 2 O 3 and CuO-Co 2 O 3 was analyzed by energy dispersive spectrometer (EDS). FTIR spectrometer was applied to analyze the spectra of all prepared materials. The catalytic reduction studies were recorded by UV-vis spectra, (Thermo Scientific TM Evolution TM 350 UV-vis spectrophotometer).

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.