Introduction

Although the Industrial Revolution substantially boosted standards of living, it also birthed persistent externalities, notably the release of hazardous heavy metals into aquatic environments1,2,3. In many developing nations, untreated wastewater is discharged directly into terrestrial and aquatic ecosystems, leading to the systemic contamination of both soil and groundwater resources. Clean water constitutes a fundamental human right and remains a critical requirement for agricultural irrigation and various industrial applications4,5,6,7. Among the diverse array of contaminants present in wastewater, heavy metals represent a critical threat to human health and ecological integrity due to their toxicity and persistence. Disposed wastewater in water bodies is dispersed by connecting water resources, which are directly used for irrigation purposes by canals8,9,10,11. Metal-containing water used for irrigation accumulates in the vegetable, grain, and food chain. However, the demand for clean water for the ever-growing population is increasing every day. In the near future, existing potable water reserves are projected to be inadequate to meet the escalating global demand9,10,11,12. In addition, the hydrological cycle of nature has been disturbed to a considerable extent due to the release of pollutants. Additionally, the groundwater level is decreasing to a concerning level13.

A recent study suggests that nanotechnology can significantly mitigate current water pollution challenges14. Nanomaterials are defined as particles with at least one dimension smaller than 100 nm. For the removal of organic and inorganic water pollutants, adsorption-based techniques have proven to be highly cost-effective and efficient15. Since adsorption is a surface-dependent process, an increase in available surface area directly enhances adsorption capacity. The smaller particles show significant enhancement in their physicochemical properties as compared to those of the bulk16. Currently, a wide range of single and composite nanomaterials are being utilized to remove environmental pollutants. Composite nanomaterials are observed to be more effective than single nanomaterials. Such performance enhancements are often ascribed to synergistic interactions within the composite matrix17. Heavy metals constitute a paramount environmental challenge, given their deleterious impacts on both ecological health and human physiology. Bioaccumulation of heavy metals such as cadmium and lead poses a severe threat to the physiological integrity of internal organs in various organisms18,19,20. Specifically, chromium (Cr) inhibits growth and impairs nutrient uptake in both plants and animals, making it a highly toxic environmental pollutant. Among the various oxidation states, Cr(VI) is the most dangerous, as it can cause cancer3,21,22,23. Hexavalent chromium, often known as chromium-6, is a highly hazardous and carcinogenic industrial waste that may be ingested, inhaled, or come into contact with the skin. It is known to induce oxidative stress and DNA damage, as well as lung cancer, gastrointestinal problems, and reproductive problems. While long-term exposure results in respiratory problems and skin ulcers, acute exposure produces excruciating agony. In light of these toxicological risks, this work focuses on the synthesis and deployment of CoMnZnONs for the efficient sequestration of Cr(VI)24,25.

Nabeel Jarrah et al. used Bent-CoAl for the adsorption of Cr(VI) from water. Adding bentonite to layers of CoAl significantly improved the surface area and better Cr(VI) adsorption (211.86 mg/g). In another work, Md. Aminul Islam et al. studied Burneside, pyrolusite, boehmite, and Mn-Al binary oxide as adsorbents for Cr(VI) in varying pH and Cr(VI) concentrations. It was observed that the adsorption capacity varies with the surface characteristics, pH levels, and Cr(VI) concentrations9.

Bimetallic Fe/Cu nanoparticles (Fe/Cu NPs) were successfully synthesized and used by Ahmed S. Mahmoud et al. to treat chromium-polluted water. Fe/Cu NPs absorbed 68% and 33% of Cr at 1 and 9 mg/L, respectively, during working circumstances. At pH 3, Fe/Cu removal was achieved with a 0.6 g/L dosage, 200 rpm swirling speed, 20 min contact time, and 20 °C constant temperature. The highest uptake was 20.5 mg/g of Cr. The PSO mechanism’s kinetic findings demonstrated that it uses chemisorption as the adsorption mechanism, with the least margin of error (0.098)26.

According to Jiwei Xu et al., Aspergillus niger strain (A1) demonstrated robust activity after being exposed to 500 mg L−1 of Cr6+. A1 exhibited the highest Cr6+ adsorption effect at pH 4, a 60-h culture duration at 40 °C, according to the optimized findings. When A1 was introduced into the chromium-contaminated red soil, the Cr6+ amount was significantly reduced compared to the inoculation of exogenous microbial agents, and the amount of high-toxicity chromium decreased while the amount of low-toxicity chromium increased27.

Ilyasse Loulidi et al. prepared activated carbon to study the adsorption properties of Cr (VI) from solutions. According to the analysis of the impacts of the operating settings, 0.15 g/100 mL at pH 6 adsorb 95% of the Cr (VI). The exothermic and spontaneous nature of the adsorption process is shown by the negative Gibbs free energies, which reduces disorder at solid–liquid interfaces28.

In this work, the co-precipitation approach is used to synthesize a new nanocomposite material, CoMnZnONs. This research highlights the enhanced performance of the CoMnZnONs nanocomposite by benchmarking its Cr(VI) sequestration capabilities against various adsorbent materials documented in recent studies. The synthesized CoMnZnONs as a viable option for treating industrial effluent are highlighted by this comparative analysis. The CoMnZnONs potential for wastewater treatment is highlighted by the study of its recyclability, which shows a negligible decline in adsorption performance across several cycles. The demand for economic and ecologically friendly adsorbents is addressed in this section of the study as well. The CoMnZnONs nanocomposite exhibited an excellent Qmax of 294.11 mg/g and good reusability. Our work justifies the SDGs goal 6, which ensures the availability and sustainable management of water and sanitation for all.

Experimental

Materials and methodology

For this experiment, double-distilled water (DDW) and chemicals of analytical reagent grade were utilized. Sigma-Aldrich provided the phthalic acid, potassium dichromate (K2Cr2O7), and 1,5-diphenyl carbohydrazide (DPC). Phthalic acid (12 g) and 0.75 g of 1,5-diphenyl carbohydrazide (0.75 g) were dissolved in 300 ml of ethanol to create the DPC reagent. The Jeol JEM 2100 Plus HR-TEM, Bruker D8-Advance XRD instrument, Hitachi SU8010 Field FE-SEM and EDX, conducted on the same device at 30 keV, Bruker Alpha FTIR, LABMAN Digital pH-Meter, 2 KG, LMPH10, Shimadzu UV mini-1240 spectrophotometer, were used to characterize the adsorbent. First, different batches of Cr (VI) solutions were mixed with different concentrations of the CoMnZnONs material (0.20 to 2 g/l). After that, these solutions were shaken at 100 revolutions per minute for varying contact durations, from 5 to 90 min. As described in previous studies, the concentrations of Cr(VI) was determined spectrophotometrically by the diphenylcarbazide method, monitoring the absorbances at 540 nm respectively, on a UV/Vis spectrophotometer.

Synthesis of CoMnZnONs nanocomposites

Co-precipitation was used to create CoMnZnONs nanocomposites in which almost equal weight proportion (almost equal supporting amounts of Co₃O₄, MnO₂, and ZnO) taken to synthesis the nanocomposite. To create a homogenous sol, Cobalt chloride (CoCl2), Magnesium chloride (MnCl2), and Zinc Chloride (ZnCl2) was gradually added to deionised water while being stirred and heated. The few drops of tween 80 surfactant added in the above solution to avoid the formation fine nanocomposite. The pH index was gradually raised to roughly 9 by adding NaOH till the pH reached upto 14. The above solution heated and stirred upto 3 h for the homogenous precipitation of all three-metal hydroxide. The filtered metal hydroxide was washed and dried at 100 °C for five hours. In order to achieve the appropriate composition of nanocomposites, the finished product was calcined at 350 °C for three hours or until a solid phase of the nanocomposites, CoMnZnONs, was reached.

Batch adsorption studies

The effectiveness of the synthesized CoMnZnONs in removing Cr(VI) ions from synthetic solutions was assessed using batch adsorption experiments. To determine the optimal adsorption conditions for maximum efficiency, a methodical approach was employed. First, different batches of Cr (VI) solutions were mixed with different concentrations of the CoMnZnONs material (0.20 to 2 g/l). After that, these solutions were shaken at 100 revolutions per minute for contact durations ranging from 5 to 90 min. To determine how quickly the ions were being absorbed by the adsorbent material, the kinetics of chromium adsorption were continuously observed. Furthermore, by varying the pH of the solutions between 2.0 and 10 while maintaining a constant adsorbent dose and contact time, the impact of solution pH on the adsorption was investigated. To make sure the results were reliable and reproducible, each experiment was carried out three times. By employing appropriate analytical techniques to analyze the concentration of Cr(VI) ions left in the solution following adsorption, the adsorption capacity of the nanocomposite material under various conditions was ascertained. The adsorption behavior was better understood thanks to these batch adsorption investigations, which helped to direct its possible uses in wastewater treatment and environmental remediation procedures.

Kinetic, isotherm, and thermodynamic parameters study

To comprehend the mechanism of an adsorbent’s adsorption properties, thermodynamic and kinetic isotherms are crucial tools29,31,32,32. The supplement file contains the equations needed for this investigation.

Statistical study

To investigate and identify the adsorption layers of the adsorbent, statistical physics-based models are employed. The use of process simulation and mathematical modelling can be very beneficial for measurement and the interpretation of experimental results33.

$$Q = \frac{{nD_{m} }}{{1 + \left( {\frac{{C_{1/2} }}{C}} \right)^{n} }}$$
(Model 1)
$$Q = n.D_{m} \frac{{\left( {\frac{C}{{C_{1} }}} \right)^{n} + 2\left( {\frac{C}{{C_{2} }}} \right)^{2n} }}{{1 + \left( {\frac{C}{{C_{1} }}} \right)^{n} + \left( {\frac{C}{{C_{2} }}} \right)^{2n} }}$$
(Model 2)

C1/2 is the concentration at half-saturation;

NM = receptor site density on the nano adsorbent;

n = number of adsorbate molecules per site of the nano adsorbent.

The concentrations \({C}_{1}\) and \({C}_{2}\) are at the first- and second-layer half-saturation respectively.

$$Q_{a} = \frac{{\left[ \begin{aligned} n*D_{M} * & \frac{{ - 2\left( {\frac{C}{{C_{1} }}} \right)^{{2n}} }}{{\left( {1 - \left( {\frac{C}{{C_{1} }}} \right)^{n} } \right)}} + \frac{{\left( {\frac{C}{{C_{1} }}} \right)^{n} \left( {1 - \left( {\frac{C}{{C_{1} }}} \right)^{{2n}} } \right)}}{{\left( {1 - \left( {\frac{C}{{C_{1} }}} \right)^{n} } \right)^{2} }} \\ & + 2\frac{{\left( {\frac{C}{{C_{1} }}} \right)^{n} \left( {\frac{C}{{C_{2} }}} \right)^{n} \left( {1 - \left( {\frac{C}{{C_{2} }}} \right)^{{nN_{2} }} } \right)}}{{\left( {1 - \left( {\frac{C}{{C_{2} }}} \right)^{n} } \right)}} \\ & - \frac{{\left( {\frac{C}{{C_{1} }}} \right)^{n} \left( {\frac{C}{{C_{2} }}} \right)^{n} \left( {\frac{C}{{C_{2} }}} \right)^{{nN_{2} }} N_{2} \left( {1 - \left( {\frac{C}{{C_{2} }}} \right)^{n} } \right)}}{{\left( {1 - \left( {\frac{C}{{C_{2} }}} \right)^{n} } \right)}} \\ & + \frac{{\left( {\frac{C}{{C_{1} }}} \right)^{n} \left( {\frac{C}{{C_{2} }}} \right)^{{2n}} \left( {1 - \left( {\frac{C}{{C_{2} }}} \right)^{{nN_{2} }} } \right)}}{{\left( {1 - \left( {\frac{C}{{C_{2} }}} \right)^{n} } \right)^{2} }} \\ \end{aligned} \right]}}{{\frac{{\left( {1 - \left( {\frac{C}{{C_{1} }}} \right)^{{2n}} } \right)}}{{\left( {1 - \left( {\frac{C}{{C_{1} }}} \right)^{n} } \right)}} + \frac{{\left( {\frac{C}{{C_{1} }}} \right)^{n} \left( {\frac{C}{{C_{2} }}} \right)^{n} \left( {1 - \left( {\frac{C}{{C_{2} }}} \right)^{{nN_{2} }} } \right)}}{{\left( {1 - \left( {\frac{C}{{C_{2} }}} \right)^{n} } \right)}}}}$$
(Model 3)

Here, the complete adsorbed layers can be denoted as Nc = 1 + N2 (Layer).

All equations related to the statistical physics models have been clearly presented, and the variables used in these equations have now been explicitly defined in the text. Additional explanations have also been added to clarify the physical meaning of the model parameters, including the number of adsorbed molecules per site (n), receptor site densities (C₁ and C₂), and adsorption energies.

Results and discussion

Characterization of CoMnZnONs

HRTEM

The nanoscale CoMnZnONs were characterized by their morphology, particle size, and lattice structure using high-resolution transmission electron microscopy (HRTEM) (Model: JEOL JEM 2100 plus). The CoMnZnONs are made up of quasi-spherical to truncated cuboidal nanoparticles that are primarily evenly sized, as seen by the HRTEM image in Fig. 1a at 200 nm34. Due to the high surface energy, with probably occurring some agglomeration of the CoMnZnONs nanoparticles, the estimated average particle size was 20–30 nm as seen by the HRTEM image in Fig. 1b at 200 nm35.

Fig. 1
Fig. 1The alternative text for this image may have been generated using AI.Fig. 1The alternative text for this image may have been generated using AI.
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Morphological characterization results of CoMnZnONs adsorbent, (a) HRTEM image, low magnification, (b) HRTEM image, high magnification, (c) SEAD, (d) Mapping Images, (e) EDX spectra, (f) XRD spectra, (g) FTIR spectrum.

SAED pattern

The polycrystalline nature of the material is supported by the accompanying SAED pattern (Fig. 1c), which displays concentric diffraction rings with individual spots superimposed36,37. Further supporting the multiphase component are the diffraction rings’ locations, which may be indexed to the typical planes of Co2O3 (110, 220), MnO₂ ([200], [111], and [220]), and ZnO (002), (100), and (112)38,40,40. While the rings show nanoscale characteristics typical of polycrystalline domains, the crisp and bright SAED spots indicated good crystallinity41,42.

Mapping

The oxygen (O) distribution throughout the sample is depicted in the mapping image in Fig. 1d. We can verify that oxygen is evenly distributed because the entire image is blue. This was anticipated as the sample contained many metal oxides (TiO₂, FeO₃, and SnO₂).

The titanium (Mn) distribution is displayed in the top left Mn Kα (Red map). The uniform distribution of the red signal indicates that MnO₂ is widely distributed throughout the composite matrix. The zinc (Zn) distribution is shown in the bottom left-Zn Kα (yellow Map), which also shows no indication of phase segregation or unevenness in total MnO₂ mixing, indicating sufficient mixing or co-precipitation was accomplished during synthesis. The homogeneous dispersion of the yellow dots indicates that the ZnO nanoparticles have been uniformly incorporated into the composite.

The distribution of cobalt (Co) is shown in the bottom right, Co Kα (green Map). Once more, the sample’s consistent distribution of green colour shows that the Co2O3 was successfully integrated into the MnO₂ and ZnO matrix.

Therefore, the TEM-EDS mapping verifies that all four elements—O, Zn, Co, and Mn—exist and are distributed evenly. No notable elemental clusters or inhomogeneities are found. The homogeneous phase composition necessary to achieve uniformity in attributes related to surface reactivity, charge transfer, and adsorption performance is demonstrated by the uniform distribution, which also shows that the CoMnZnONs nanocomposite was synthesized appropriately.

EDS

As anticipated for a CoMnZnONs nanocomposite, the elemental composition of Co, Mn, Zn, and O is also confirmed by EDS spectra (Fig. 1e). With the effective incorporation of Co2O3, MnO₂, and ZnO, the predominant phase is CoMnZnONs. The material’s oxide nature is confirmed by the presence of O. No contaminants were found, which facilitates the creation of a high-purity nanocomposite.

XRD

The prepared CoMnZnONs nanocomposite X-ray diffraction patterns are shown in Fig. 1f. They exhibit notable peaks at several angles, including 12.2°, 13.5°, 16.1°, 18.2°, 21.3°, 22.0°, 25.9°, 31.9°, 32.77°, 33.4°, 35.6°, 39.1°, 49.7°, and 58.58°. These peaks correspond to reflection from the CoMnZnONs which may be indexed to the typical planes of Co2O3 (110, 220), MnO₂ (200, 111, and 220), and ZnO (002, 100), and (112) miller planes42,43.

FTIR

The FT-IR spectra of CoMnZnONs scanned between 4000–500 cm−1 are displayed in Fig. 1e. The middle-strong band deformations of the CO stretching vibrations at 1545 and the spectral band at 3378.80 cm−1 indicate the presence of the -OH group in the nanocomposite. Metal oxide bands in FTIR typically appear in the fingerprint region (below 1000 cm⁻1), reflecting metal–oxygen (M–O) bond vibrations, which are 991.5, 834.9, 784.6 cm−144.

Adsorbent dose, contact time, and pH

To streamline the adsorbent mass and investigate adsorption properties, various parameters were varied. This work effectively removes Cr(VI) using adsorbent inputs at different concentrations (200–2000 mg) with a constant concentration of Cr(VI) (100 mg/L) at 25 °C. The impact of adsorbent on the removal efficiency of Cr(VI) in relation to the mass loaded is depicted in Fig. 2a. For Cr(VI) evaluation, 200 mg of the adsorbent yields the best productivity of 92.1%.

Fig. 2
Fig. 2The alternative text for this image may have been generated using AI.
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Adsorption properties of the CoMnZnONs (a) Cr(VI) dose optimization, (b) contact time optimization, and (c) pH.

Since the nanocomposite is saturated in about 60 min, as shown in Fig. 2b, an ideal contact period of 60 min is chosen (at 200 mg/L and 200 mg). A Cr(VI) adsorption rate of 87.09% was achieved by quicker adsorption due to the availability of many adsorption sites. Using pH estimates, Cr(VI) is adsorbed onto the outer layer of CoMnZnONs. The adsorption capacity of Cr(VI) particles is controlled by a number of factors, including pH, electrostatic attraction, porosity, and practical hydrogen gatherings45. An experiment using 200 mg of adsorbent and 100 mg/L of starting Cr(VI) focus at 25 °C, is conducted to determine the effect of pH on the evacuation of Cr(VI).

The optimal pH value for improved adsorption is 4.0, as shown in Fig. 2c. As pH rises, there is a progressive decrease in Cr(VI) adsorption. This effect is caused by the presence of hydroxyl groups on the surface of CoMnZnONs in an aqueous solution, and the quantity of hydroxyl groups varies with pH. At the point of zero charge, the surface charge of an adsorbent reaches neutrality. The adsorption of Cr(VI) is reduced when the pH is raised because more OH − ions are produced, which compete with Cr(VI) species (CrO42−) for the adsorption sites. The adsorbed HCrO4 − and CrO42− are released into the solution due to an increase in electrostatic repulsion46,47. Depending on the pH of the fluid, the chromium ion can take on several forms. The most prevalent form of dichromate (Cr2O72-) is chromate (CrO4-), while the main form of hydrogen chromate (HCrO4-) above pH 6.0 is chromate (CrO4-) at pH 2.0–3.0. The many chromate particles seem to play a significant role in the chromium adsorption component. As shown in Fig. 2c, the maximum chromium adsorption was achieved at pH 4.049.

Kinetics and isotherm

The adsorption relationship between the pollutant Cr(VI) and the adsorbent (CoMnZnONs) is determined by several active models. To investigate the synthesized CoMnZnONs for Cr(VI) adsorption to evaluate pseudo first order (PFO) and pseudo second order (PSO), models. The results (Table 1. and Fig. 3a) confirmed that the adsorption of Cr(VI) on the CoMnZnONs followed the pseudo-second-order kinetic model (R2 = 0.999).

Table 1 The characteristics of the CoMnZnONs nanocomposite’s PSO kinetics, isotherm, ∆G°, ∆H°, and ∆S° for Cr(VI) adsorption
Full size table
Fig. 3
Fig. 3The alternative text for this image may have been generated using AI.Fig. 3The alternative text for this image may have been generated using AI.
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(a) PSOKM, (b) Langmuir, (c) Freundlich, (d) Temkin, (e) RL for CoMnZnONs nanocomposite.

To characterize the adsorption equilibrium, the experimental results were analyzed using three common models: Langmuir, Freundlich, and Temkin. Figure 3b-e shows the linearized isotherm and separation factor for adsorption. The computed isotherm parameters for Cr(VI) adsorption onto CoMnZnONs derived from linearized isotherm models are also summarized in Table 1.. Since the R2 values are close to 1, the adsorption behavior of the CoMnZnONs as found by the Freundlich, Langmuir, and Temkin is consistent with their linearized isotherm model. The R2 values for the Freundlich, Langmuir, and Temkin isotherm models are 0.9958, 0.9937, and 0.9853, respectively. The high regression coefficient (R2) for the Freundlich isotherm indicates a superior fit, suggesting that the adsorption process involves multilayer formation on a heterogeneous surface. Physisorption, or physical adsorption, on heterogeneous surfaces is mostly described by the Freundlich isotherm. The findings suggest that during compound harmony, Cr(VI) is continuously cycled on the outer layer of CoMnZnONs. According to the Langmuir isotherm model, there is only one type of communication between the CoMnZnONs and each adsorbate, and all adsorption locations on the adsorbent’s outer layer are identical. CoMnZnONs has a Qmax of approximately 294.11 mg/g for Cr(VI), according to calculations.

In the meantime, an adsorption system’s “favourable” or “unfavourable” status is predicted using a dimensionless constant (RL)42 (Fig. 4d), also referred to as the separation factor (all equations are presented in the Supplementary material). RL can be used to characterize the adsorption process as follows: i. unfavorable (RL > 1), ii. linear (RL = 1), and iii. favorable (0 < RL < 1).

Fig. 4
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Double-layer model at 298, 308, and 318 K temperatures.

Thermodynamic parameters

Researchers investigated the effects of temperature on Cr(VI) adsorption onto CoMnZnONs between 25 and 45 °C. According to the thermodynamic parameters of Cr(VI) are computed. The Cr(VI) adsorption onto the surface of CoMnZnONs is investigated under advanced circumstances at three distinct temperatures of 25, 35, and 45 °C. The negative value of ΔG° suggests an unlimited path for Cr(VI) adsorption. Table 1. indicates that the adsorption process is exothermic due to the negative value of ΔH°. Furthermore, the observed positive value of ΔS° suggests that entropy increased at the solid/solution interface following the elimination of Cr(VI).

Interpretation of results of statistical models

Figure 4 displays the interpretation of physics models for Cr(VI) adsorption using CoMnZnONs nanocomposite at various temperatures. Here, the Cr(VI) adsorption on the CoMnZnONs nanocomposite best fits the double-layer model (Model 2) data. Table 2 shows that parameters like n, NM, Qsat, ε1, and ε2 are used in the double-layer model (Model 2) for Cr(VI) adsorption on CoMnZnONs.

Table 2 Model 2 fitting obtained values.
Full size table

• n < 0.5: In this instance, the adsorbate was able to interact with at least two adsorption sites due to the parallel adsorption orientation on the CoMnZnONs adsorbent surfaces.

• 0.5 < n < 1: The dye molecule can be adsorbed with mixed orientation (parallel and non-parallel orientations) with two distinct percentages when there are between 0.5 and 1 connected dye molecules per adsorbent surface.

• n > 1: This suggests that the dye molecules Cr(VI) can be adsorbed in a non-parallel form, meaning that the adsorbate can interact with a single adsorption site.

The horizontal orientation and multi-docking mechanism were demonstrated by the n values of 0.40, 0.45, and 0.46 that were determined at 25, 35, and 45 °C, respectively. Table 2 displays the findings of mathematical comparisons, and Fig. 5a illustrates how the value of n varied with temperature (Lotfi and D. Franco et al., 2019).

Fig. 5
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Plot of (a) n, (b) Nm, (c) Qesat, and (d) Energies vs T (K).

Cr(VI) adsorption occurs on the nanoparticle surface in a horizontal direction. The temperature-dependent thermal movement parameter n for Cr(VI)-CoMnZnONs adsorption is found and documented in a decremental order (0.40 to 0.46). It should be noted that at 318 K, the Cr(VI)-CoMnZnONs aggregates in solution break apart, which inhibits the formation of layers on the surface of the adsorbent.

Figure 5b illustrates how temperature affects the density of receptor sites (Nm). This boundary revealed the boundary n’s opposite example. In essence, the CoMnZnONs surface may have a larger accessible region, thereby reducing the number of fortified Cr(VI) particles at dynamic locations due to the availability of additional dynamic sites.

In the two systems being studied, temperature enhanced the adsorption receptor site density (NM) of Cr(VI) and CoMnZnONs. The CoMnZnONs structure’s temperature-dependent expansion effect, which produced new active sites, caused NM to rise (Asma et al. 2014). This result is consistent with the exothermic effect found by analyzing the experimental data. The control of the new unoccupied CoMnZnONs dynamic destinations by the adsorbed particles was mirrored by a rise in the NM value that coincided with an increase in the adsorbed quantity, as shown in Fig. 5b. This shows that CoMnZnONs dynamic destinations are the more specific sites for Cr(VI) adsorption.

The adsorption of Cr(VI) from aqueous solutions onto CoMnZnONs is investigated using the Langmuir model. According to this standard concept, Cr(VI) can only be adsorbed by the creation of an adsorbed sheet, regardless of the adsorbent. Finally, the number of soaked CoMnZnONs particles represents the final intriguing physicochemical boundary. The explanation of trial data now confirms that Qsat (e.g., Qesat = N*Nm and the model with two-fold layer Qsat = 2N*Nm) declines with increasing temperature, as shown in Fig. 5c. The Cr(VI) expulsion process can be better understood by calculating the adsorption energy of two useful groupings.

The following formulas are used to calculate the energy of adsorption.

$$\varepsilon_{1} = RT \ln \left( {\frac{{C_{S} }}{{C_{1} }}} \right)$$
(1)
$$\varepsilon_{2} = RT \ln \left( {\frac{{C_{S} }}{{C_{2} }}} \right)$$
(2)

where T is the temperature, R is the ideal gas constant, and Cs is the solubility of Cr(VI). When El > E2, these energies in both functional classes indicate an exothermic chemisorption.

Figure 5d displays the adsorption energies (E) values for each of the solution temperatures (25, 35, and 45 °C). The El showed values of 20.61, 22.25, and 23.26 kJmol−1 at 25, 35, and 45 °C (Table 2). Because of the adsorption energy (i.e., the Qsat) and the corresponding temperature trend, the active sites of the CoMnZnONs adsorbent were therefore better equipped to absorb hexavalent Cr ions. A physical component to the adsorption phenomena was also suggested by the correlation between the Cr(VI) ion adsorption onto CoMnZnONs and absorption energies that were all 40 kJmol−1. It was clear that a higher adsorption energy signifies the binding contact of the second functional group, demonstrating their important function in the removal of Cr(VI) ions. Table 2 compares the CoMnZnONs and Cr(VI) adsorbents that have been previously reported in the literature.

Mechanism

To understand the adsorption mechanism, we must take into account a number of variables, including the adsorbent’s surface chemistry, the adsorbate’s chemical characteristics, and the impact of pH on the adsorption process. When we deal with removing Cr(VI) from the solution, significance of active surface sites which was correlated to CoMnZnONs nanocomposite. The surface charges of both CoMnZnONs and the Cr(VI) species are strongly impacted by pH. The CoMnZnONs nanocomposite’s surface is positively charged at lower pH values because surface hydroxyl groups have been protonated, whilst Cr(VI) ions are mostly present as chromate ions (CrO42-) and dichromate ions (Cr2O72-) depending on the pH. Adsorption is improved by the adsorbent’s positively charged surface, which makes it easier for the negatively charged Cr(VI) species to attract it electrostatically. Dichromate ions (Cr2O72-) and chromate ions (CrO42-) are the most common Cr(VI) species in acidic environments (pH < 7). Through electrostatic attraction, the negatively charged Cr(VI) species interact with the positively charged surface of the CoMnZnONs nanocomposite. Additionally, hydroxyl and surface oxide groups on the adsorbent may complex with Cr(VI) ions, enhancing the overall adsorption capacity. According to batch adsorption tests, the nanocomposite’s adsorption ability for Cr(VI) ions is affected by variables like temperature, adsorbent dosage, and contact time.

Regeneration and comparison

One of the important characteristics of a good absorbent is its reusability. Selecting economical adsorbents for disposal and reducing any possible environmental effects in relation to sustainable solid waste management are the study’s primary goals. Reusability investigations are carried out under the ideal experimental conditions for the adsorption of Cr(VI) on the surface of CoMnZnONs. After each adsorption procedure, the used adsorbent is successfully neutralized with 0.1 M NaOH and collected using a centrifuge machine and washed with 0.1 M HCl. Each adsorption cycle is followed by a similar procedure. It is shown that the adsorption effectiveness drops by less than 2% after using the adsorbent for up to five cycles (Fig. 6 and Table 3). For up to five cycles, the adsorbent’s regeneration performance remains nearly constant. This demonstrates that its affordability and ecological impact are significantly balanced. At the conclusion of each cycle, the percentage regeneration efficiency is determined using Eq. (3):

Fig. 6
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Reuse study.

Table 3 Comparative table of previously reported adsorbents with CoMnZnONs.
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$$Efficiency{\text{ }}of{\text{ }}regeneration{\text{ }}(\% ){\text{ }} = {\text{ }}(amount{\text{ }}desorbed/amount{\text{ }}absorbed){\text{ }}$$
(3)

In order to keep a hygienic atmosphere and conserve water through recycling, the synthesized CoMnZnONs are effectively used for the treatment of wastewater containing Cr(VI). 93% of Cr(VI) was adsorbed, with a Relative standard deviation (RSD) of less than 2%. Additionally, this study’s experimental data demonstrated the CoMnZnONs’ capability and effectiveness, making them a viable option for treating industrial effluents. Additionally, it has a higher adsorption capacity than other previously described adsorbents (Table 3).

Conclusion

This study successfully synthesized a novel adsorbent, CoMnZnONs, using the co-precipitation method and evaluated its effectiveness in adsorbing Cr(VI) in a mono-component system under various conditions. The results show that the adsorbent’s efficiency in removing Cr(VI) from solutions is highest at pH 3, with a Qmax. The adsorption kinetics were analyzed using PFO and PSO models, both of which demonstrated strong fits with R2 values near unity. The Qmax was determined to be 294.11 mg/g, with n values of 0.40, 0.45, and 0.46 at temperatures of 25, 35, and 45 °C, respectively. The R2 values for the Freundlich, Langmuir, and Temkin isotherm models were 0.9958, 0.9937, and 0.9853, respectively, indicating a robust fit. The findings suggest that Cr(VI) interacts primarily with the outer layer of CoMnZnONs during adsorption. Additionally, the active sites displayed a strong preference for Cr(VI) ions across all tested temperatures, indicating the selective nature of the adsorption behavior. The multi-docking interaction of Cr(VI) with CoMnZnONs was evident, particularly at 25 and 35 °C. Importantly, the adsorbent showed excellent recyclability, maintaining high adsorption efficiency with only a minor decline over five cycles. Such findings highlight the potential for the cyclical application of CoMnZnONs, offering a sustainable and cost-effective solution for large-scale industrial effluent treatment. The synthesized CoMnZnONs demonstrate significant promise for the effective removal of Cr(VI), supporting initiatives for water conservation and environmental sustainability. Ultimately, this study confirms the efficacy and reliability of CoMnZnONs as a viable solution for addressing Cr(VI) contamination in water sources, achieving an impressive 92.71% adsorption efficiency and a RSD of less than 2%.