1 Introduction

Manufacturing activities from different industrial sectors produce heavy metals and consequently pollute water bodies. Remediation of water pollutants is essential because freshwater availability is a significantly increasing problem. Toxic heavy metals from different anthropogenic sources directly/indirectly affect the biota and humans [2, 21, 49]. Cadmium (Cd) occurrence in water effluents is sourced from industries such as electroplating, smelting, paint pigments, batteries, fertilizers, mining, and alloy [26]. The occurrence of Cd at higher concentrations in water bodies leads to human consumption that affects vital organs such as the liver, lungs, and kidneys [27]. As per the WHO guidelines, the maximum permissible limit concentration of Cd in drinking water is 1 μg/L. The separation of these harmful metal ions from industrial effluent is an indispensable task to reduce the ecological impacts of Cd ions and comply with the pollution rules and legislation [6]. Many scientists are currently paying attention to water decontamination and desalination methods to conserve water resources worldwide [14, 46, 47, 50, 54, 23].

There are numerous techniques used for removal of Cd from contaminated water such as precipitation [1, 9], ion-exchange [29], membrane separation [34], electro-coagulation [57], photo-catalysis [45], and compared to conventional methods, presented with some drawbacks in the treatment, including inefficient removal, higher operational cost, high power requirements, production of toxic sludge, etc. [8, 24]. Adsorption is considered a suitable method for the remediation of Cd containing effluent with the merits of strong affinity, economical, simple design, effective at low metal concentration, and higher adsorption capacity [10].

The adsorption technique has drawn enormous benefits in efficiency and suitability because of the distribution of different precursors for adsorbent preparations, lower cost, reusable, and easy modifications [5, 46, 47]. Cadmium’s removal using the adsorption technique has been reported with adsorbents of customized fibers of activated alumina, Fe oxide, resin, metal oxides, etc. [47, 59]. Furthermore, copper-based adsorbent synthesized from metal oxides or hydroxides showed high surface areas [17]. Recently, copper–ceria-based nano-composites have received significant attention because of the consistent, stable capability of the functional groups and selectivity [46, 47, 53].

The present research deals with the composite copper–ceria adsorbent prepared by sol–gel and co-precipitation methods. The adsorbents were assessed for their ability to remove Cd from an aqueous solution. Characterization of the prepared adsorbents was carried out using a particle size analyzer, BET surface area, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analysis. The adsorption process experimental data were fitted to the two-parameter isotherm model, and the contact time experimental data were analyzed using the adsorption kinetic models.

2 Material and methods

2.1 Chemicals

Chemical cerium nitrate hexahydrate copper nitrate trihydrates, citric acid, acetic acid, ethyl alcohol, NaOH (Merck at purity ~98.9%) were used without further purifications. A standard Cd solution of 1000 μg/ml (Accu Standard) was prepared to make different concentrations of Cd solutions using deionized water.

2.2 Preparation of adsorbent by sol–gel (U1) and co-precipitation (U2) methods

Copper–cerium oxide was prepared following a modified sol–gel method (U1) reported in the literature [18], in which citric acid (CA) was used as a complexing agent. First, cerium nitrate hexahydrate and copper nitrate trihydrate, mixed in proportions based on the composition, were dissolved in deionized water to form a 0.1 M solution of whole metal ions [43]. Copper–cerium oxide was prepared by the co-precipitation method (U2). Cerium nitrate, copper nitrate, and urea were dissolved in 50 ml of deionized water to get a transparent solution [18]. The initial urea–nitrate molar ratio was taken into account to measure the urea–nitrate stoichiometric molar ratio [43]. The precipitates usually have pH values in the range of 4.0 to 8.0 at ambient temperature.

2.3 Characterization

BET specific surface area measurements of adsorbents were determined using a Micromeritics ASAP 2020 analyzer. Particle size analysis of the adsorbents was determined using a laser diffraction (Helium–Neon Laser, 5 Mill) based particle size analyzer (ANKERSMID, CIS-50, USA). Fourier transform infrared spectroscopy (FTIR) of the synthesized adsorbents was recorded in the range of 400 to 4000 cm−1 on a Shimadzu 8400. X-ray measurement of the adsorbents was carried out using a Rigaku Ultima IV X-ray diffractometer (Germany). X-ray photoelectron spectroscopy (XPS) was used to monitor the component’s surface concentrations and chemical states and was performed using an Amicus spectrometer. Transmission electron microscopy (TEM) used a model Tecnai G2 20 TWIN, FEI Company of USA.

2.4 Adsorption of Cd

The adsorption on the Cd was studied via varying primary Cd concentration (10 to 50 μg/ml) through 0.71 g/100 ml of CuO/CeO2 adsorbent and a complete solution amount of 100 ml in a 250 ml container at pH 6.6 and stirring speed 120 rpm. Aliquots (0.5 ml) were removed at regular intervals, and the concentration of Cd was calculated using an atomic absorption spectroscopy (AAS) analyzer (Perkin Elmer, Analyst 800).

3 Results and discussion

3.1 BET surface area and particle size analysis

The standard technique for evaluating the BET surface area is based on the physical adsorption on the concrete surface. The results are 70 m2/g for the sol–gel preparation method and 60 m2/g for the co-precipitation preparation method, as shown in Table 1. The copper–ceria adsorbent prepared using the sol–gel method has a larger surface area than that made using the co-precipitation methods. The properties of the copper–ceria-based adsorbent prepared using the sol–gel method lead to better Cd removal than the adsorbent made using the co-precipitation method. While particle size decreases, the experimental scattering angle increases logarithmically. Scattering intensity is also dependent on particle size, diminishing with particle volume [43]. Consequently, large particles scatter light at narrow angles with high intensity, and small particles scatter at wider angles but with lower intensity.

Table 1 Characteristics of the Cu–Ce composite adsorbent
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3.2 FTIR analysis

The FTIR spectra of the CuO/CeO2 adsorbent are shown in Fig. 1. The broad bands (3700 to 3000 cm−1) for cerium oxide represent the stretch vibration of the hydroxyl group in chemisorbed water [40, 42, 46]. The significant peaks between 900 and 1629 cm−1 after calcination at 500 °C can be seen in the case of cerium oxide, indicating some remaining organic constituents available in the copper–cerium oxide adsorbent. These special functional properties of the copper–ceria-based adsorbent lead to better Cd removal from wastewater. The absorption band’s essential improvements (499–1059 cm−1) show cerium oxide formation [55]. The significant peaks of Cd adsorbed by the copper–cerium oxide adsorbent are at wavenumber 3437 cm−1 (O–H strong), 2927 cm−1 (C–H medium to strong), 1624 cm−1 (C=C weak to medium), 1114 cm−1 (polysacrides), 1049 cm−1 (C–O bond), 975 cm−1, and 470 cm−1 (halogens group). After the adsorption of Cd ions, some peaks and shifting of peaks also take place during the adsorption. Other researchers also reported analogous peaks of Cu particles at 3442, 1631, 1019, and 519 cm−1 subsequent to chemical groups such as hydroxyl, ether, Cu–O, and other groups, correspondingly [22].

Fig. 1
figure 1

FTIR of Cu–Ce oxide using U1 and U2, respectively, and Cd ions adsorbed on adsorbent

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3.3 XRD analysis

The XRD spectra have been compared with the JCPDS files. The XRD spectra of the CuO/CeO2 composite powder are presented in Fig. 2. The major intense reflections of cerium oxide at a 2θ value of 28.56° and (h, k, l) values (1, 1, 1) are evidently visible, and generally intense reflections of copper oxide are present at a 2θ value of 47.56° and (h, k, l) values (2, 0, 2) are visible [40, 42, 47]. Because of these special properties, the copper–ceria-based adsorbent leads to better Cd removal from aqueous solution. The micrographs exhibit distinctive peaks (111) of a fluorite-like cubic structure and active surfaces in the adsorbent [56].

Fig. 2
figure 2

XRD spectra of the composite Cu/CeO2 prepared by U1 and U2, respectively

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3.4 XPS analysis

Figure 3 shows the XPS spectra of cerium 3d5/2 in composite copper–ceria adsorbent: the different methods are coded as sol–gel (U1) and co-precipitation (U2). The high-resolution O 1s spectra of cerium oxide material were deconvoluted into three peaks: two oxygen species on the surface zone [33] oxygen moieties jump with Ce(III) and Ce(IV) [40, 42]. The cerium(III) ions composition may be immediate to the oxygen opening on the ceria surface and influence ceria movement such as catalyzing ozonation of aniline, which is straight depending on the increase of cerium(III) species on cerium oxide surfaces [39].

Fig. 3
figure 3

XPS analysis of Ce 3d5/2 in Cu/CeO2 composite: U1 (a) and U2 (b)

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Figure 4 shows the XPS spectra of copper 2p3/2 in CuO/CeO2 composite: (a) U1, (b) U2, and binding energies of adsorbents prepared. The continuation of a strong metal-support contact between copper and cerium oxide can adapt the structural properties of copper, which could recover this system’s catalytic performance [51]. The copper was commenced into cerium oxide, and cerium(III) ions of copper–cerium calcined at the same condition were not completely oxidized to cerium(IV) ions, representing that copper loading leads to the preservation of more cerium(III) ions [32].

Fig. 4
figure 4

XPS analysis of Cu 2p3/2 in Cu/CeO2 composite: U1 (a) and U2 (b)

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3.5 TEM analysis

The TEM analysis showed the crystalline size of the CuO/CeO2 adsorbent. The typical TEM images of the CuO/CeO2 adsorbent are shown in Figs. 5 and 6. The TEM images show the different preparation methods, i.e., sol–gel and co-precipitation, with an abundance of roughly hexagonal-shaped CuO/CeO2 with dimensions ranging from 15 to 20 nm. A size allocation histogram of CuO/CeO2 corresponding to different TEM images showed that the highest CuO/CeO2 was in the range of 5 nm to 28 nm and having an average size distribution of 15.5 nm for sol–gel and 16.75 nm for co-precipitation (Figs. 5a and 6a). Selected area electron diffraction (SAED) spectra of CuO/CeO2 colloid are shown in Figs. 5b and 6b. The typical SAED pattern with bright circular rings revealed that the synthesized CuO/CeO2 was crystalline.

Fig. 5
figure 5

(a) and (b) TEM analysis of composite CuO/CeO2 prepared using the sol–gel method

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Fig. 6
figure 6

(a) and (b) TEM analysis of composite CuO/CeO2 prepared using the co-precipitation method

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3.6 Kinetics study of Cd removal

The adsorption of Cd on the CuO/CeO2 at various initial concentrations was studied. Figure 7 shows the varying of adsorbed Cd depending on the reaction time. Cadmium adsorption was more rapid in the sol–gel CuO/CeO2 adsorbent compared to the co-precipitation CuO/CeO2 adsorbent. It required less time to attain equilibrium because of the larger surface area of CuO/CeO2 prepared using the sol–gel method. More than 93% of the equilibrium adsorption capability was reached within 1 h for the sol–gel CuO/CeO2 adsorbent.

Fig. 7
figure 7

Kinetics of Cd removal by the composite Cu/CeO2 at initial concentrations 40 ppm, for adsorbent dose 3.35 g/500 ml, pH 6.5 ± 0.5, temperature 32 °C

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In comparison, it took approximately 60 min to attain 89% of the co-precipitation copper oxide ceria’s equilibrium adsorption capability. Because of the large surface area of CuO/CeO2 and its dispersed nature, Cd’s removal percentage was quite high. Other researchers [36] also investigated the Cd removal from chicken eggshell adsorbent for theoretical Cd removal effectiveness based on the produced model of 75.3% less removal within a 75 min contact time, which also required more time than the current study. The occurrence of the unique chemical groups such as hydroxyl, carbonyl, and others explains an elevated removal [36]. It contains dynamic binding sites that enhanced adsorptive components in heavy metals such as Cd. This means removal efficiency greater than 93% was achieved, which is more than the values (75%) found in a previous study. The time taken for removal was approximately 90 min, and the present study’s maximum time was 60 min [44, 48].

The calculation of kinetics is essential for the design of adsorption systems and reaction rate-controlling step, to ensure a chemical reaction occurs. The adsorption process’s nature depends on the adsorbent’s physicochemical characteristic and system conditions such as temperature [37]. The amount of metals adsorbed, qt, at time t was calculated using the following equation:

$$ {\mathrm{q}}_{\mathrm{t}}=\frac{\left({\mathrm{C}}_{0-}{\mathrm{C}}_{\mathrm{e}}\right)\ \mathrm{V}}{\mathrm{W}} $$
(1)

where qt is adsorption capacity at time t (mg/g), Co is the initial metal ion concentration (mg/ L), Ce is the metal ion concentration at equilibrium (mg/ L), V is the volume of solution (L), W is the weight of the bio-adsorbent (g), and qcd ion is the adsorption capacity at equilibrium (mg/ g).

Pseudo-first-order and pseudo-second-order models were used to fit the adsorption data, and four kinetic fitting curves were obtained. The pseudo-first-order equation of Lagrange is generally expressed as follows [16]:

$$ \log \left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)=\log {\mathrm{q}}_{\mathrm{e}}+\frac{{\mathrm{k}}_1}{2.303}\mathrm{t} $$
(2)

The pseudo-second-order rate expression is based on the solid phases’ sorption capacity (include citation), which has been applied for analyzing kinetics rate of chemisorption. It is given by:

$$ \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}=\frac{1}{{\mathrm{q}}_{\mathrm{e}}^2\ {\mathrm{k}}_2}+\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}} $$
(3)

After again computing the variation of Cd adsorption kinetics on the CuO/CeO2, the pseudo-first-order and pseudo- second-order kinetic data were utilized to understand the adsorption kinetics (Figs. 8 and 9). The kinetics analysis was achieved by fitting the adsorption data to the kinetic models and the results are shown in Table 2. As per the regression analysis results (R2), the experimental values fit the pseudo-first-order data superior to the pseudo-second-order data for Cd [40,41,42].

Fig. 8
figure 8

Adsorption kinetics of Cd by Cu/CeO2 adsorbent for pseudo-first-order and -second-order models

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Fig. 9
figure 9

Adsorption isotherm of Cd by CuO/CeO2 for Langmuir (a) and Freundlich (b) models

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Table 2 Kinetics parameters for Cd(II) adsorption using U1 and U2
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The nonlinear kinetic models described in Table 2 are shown in Fig. 8a, b, and computed coefficients are tabulated in Table 2. The constants of U1 and U2 were k1 0.074 per min and 0.0335 per min, respectively, for the first-order, and k2 were 0.00061 and 0.0129 (g/mg min), respectively, for the second-order kinetic model, and R2 values of U1 and U2 were 0.97 and 0.98 for the first-order and 0.67 and 0.45 for the second-order model, respectively. The calculated equilibrium capacities are 7.43 mg/g and 3.217 mg/g for CuO/CeO2 sol–gel and CuO/CeO2 co-precipitation methods, respectively, for the pseudo-first-order model, showing that the Cd adsorption to CuO/CeO2 sol–gel fitted to this model. Other studies, as shown in Table 2, used white sandstone as the base material for a nano-silica generation [28]. Meanwhile, the values of sol–gel CuO/CeO2 k2 were higher than those of co-precipitation CuO/CeO2 under similar conditions. Cadmium concentration reduction was better by the previous compared to the later one. The adsorption capability of composite CuO/CeO2 and several other adsorbents for Cd metal ions are collected from the literature [25, 60]. Cadmium composition in normal polluted waters is generally less. Therefore, it is more logical to estimate the adsorption performance of Cd at a small equilibrium Cd composition. With the lower equilibrium Cd composition, the adsorption value might better suit the French model. At the equilibrium Cd composition of 0.01 mg/L as the Cd standard for consumption water, Cd’s adsorption capability was approximately 6.86 mg/g for Cd [41, 58]. From this data, it is reasonably clear that the adsorption capacity of CuO/CeO2 is relatively high compared to other adsorbent resources [36].

Langmuir and Freundlich models are used to fit the adsorption isotherm, which is represented below:

The Langmuir isotherm assumes monolayer adsorption on a uniform surface with a finite number of adsorption sites. This model is presented by the following equation [30, 31]. The linear form of the Langmuir isotherm is described as:

$$ \frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}=\frac{1}{{\mathrm{q}}_{\mathrm{m}}{\mathrm{b}}_0}+\frac{\mathrm{t}{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{m}}} $$
(4)

Values of Langmuir parameters qm and b0 are calculated from the slope and intercept of the linear plot Ce/qe versus Ce.

The purpose of the Freundlich equation also suggests that sorption energy exponentially decreases upon the completion of the sorption centers of the adsorbent [20].

Freundlich equilibrium constants could be determined from the linear plot of ln qe and ln Ce according to the following equation:

$$ \ln {\mathrm{q}}_{\mathrm{e}}=\ln {\mathrm{K}}_{\mathrm{f}}+\frac{1}{\mathrm{n}}\ln {\mathrm{C}}_{\mathrm{e}} $$
(5)

The Freundlich isotherm is widely applied in heterogeneous systems, especially organic compounds or highly interactive species on activated carbon and molecular sieves [19].

From Table 3, it is proposed that Freundlich more adequately describes the data than other studied models. The finding suggests the surface of CuO/CeO2 sol–gel and CuO/CeO2 co-precipitation methods for Cd adsorption. The higher attraction capacity (6.86 mg/g) for Cd removal was higher than the CuO/CeO2 sol–gel preparation method. Table 3 compares the maximum attraction of Cd ions using CuO/CeO2 sol–gel and CuO/CeO2 co-precipitation methods with other used adsorbents in the literature and shows that both adsorbents are effective in the removal of the objective purity. Also, the R2 value of the Freundlich isotherm model for Cd ion adsorption using CuO/CeO2 sol–gel and CuO/CeO2 co-precipitation methods were 0.98 and 0.99, respectively, indicating that the Freundlich isotherm model has fair potential to investigate the equilibrium behavior of Cd ion adsorption by CuO/CeO2 sol–gel [52].

Table 3 Comparative equilibrium isotherm parameters of Cd adsorption on the U1 and U2
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The assessment of the copper oxide ceria’s adsorption capacity among other substances at lower equilibrium Cd composition is shown in Table 4. Figure 10a, b depicts the trend of variation in removal percentage of Cd with various doses and pH. Figure 10b depicts the effect of initial pH on the equilibrium adsorption capacity of biosorbent. It is observed that at pH 6.6, a maximum removal percentage (92.78%) was obtained at approximately 60 min using the copper–ceria prepared by sol–gel, after which it stabilizes. Thus, pH 6.6 is found to be the optimum pH at which maximum Cd removal takes place. At a very low pH, the adsorption decreases as the hydrogen ion itself competes with the Cd ions for adsorption [4, 13, 15]. As the pH increases, adsorption ability enhances due to an increase in the negative charge on the surface of the adsorbent walls. However, as we go toward alkaline pH, the adsorption again decreases as the metal ions in the solution precipitate out, obstructing the procedure. In general, a range of pH 2–8 is favorable for Cd ion adsorption [3, 11, 12, 15]. Rauf et al. [46] also investigated Cd removal with increasing pH, where the uptake efficiency elevated, and this trend continued until the initial pH of 6. Then with increasing pH, the efficiency reduces due to Cd(II) sequestration. As observed, with increasing initial Cd ion concentration, the adsorption efficiency decreases [46].

Table 4 Comparison of different adsorbents used for cadmium removal from water
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Fig. 10
figure 10

Cadmium removal (%) at different pH of synthetic contaminated solution with cadmium

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4 Conclusion

The copper–ceria adsorbent composites prepared using sol–gel and co-precipitation methods were effective for Cd removal from aqueous solution. Cd adsorption was higher and more rapid in the sol–gel CuO/CeO2 adsorbent compared to the co-precipitation CuO/CeO2 adsorbent. Because CuO/CeO2 prepared using the sol–gel method has higher surface area and small particle size, it required less time to attain equilibrium. The sol–gel CuO/CeO2 had more than 93% equilibrium adsorption efficiency, while the co-precipitation CuO/CeO2 had approximately 89% equilibrium adsorption efficiency. TEM showed that the sol–gel and co-precipitation methods made an abundance of approximately hexagonal-shaped CuO/CeO2 with size ranging from 15.0 to 20.0 nm and having an average size distribution of 15.45 nm for sol–gel and 16.79 nm for co-precipitation. The Freundlich kinetic model well-described the adsorption process of Cd using adsorbent based on the CuO/CeO2 sol–gel method. The adsorption kinetic models of Cd removal were better explained using the pseudo-first-order model.