Functional Ag-EDTA-modified MnO2 nanocoral reef for rapid removal of hazardous copper from wastewater

A novel MnO2@EDTA-Ag nanocoral reef was constructed via a simplified redox reaction followed by EDTA and Ag nanoparticles impregnation to capture hazardous copper (II) from wastewater. A comprehensive characterization of the synthesized materials was conducted. The morphology of MnO2@EDTA-Ag in the form of a nanocoral reef was constructed of two-dimensional nanoplatelets and nanorod-like nanostructures. The optimal adsorption conditions proposed by the Plackett–Burman design (PBD) that would provide a removal % of 99.95 were pH 5.5, a contact time of 32.0 min, a Cu(II) concentration of 11.2 mg L−1, an adsorbent dose of 0.05 g, and a temperature of 40.3 °C. The loading of Ag nanoparticles onto MnO2@EDTA improved the adsorption capability of MnO2@EDTA-Ag. Additionally, the recyclability of MnO2@EDTA-Ag nanocoral reef was maintained at 80% after three adsorption–desorption cycles, and there was no significant change in the XRD analysis before and after the recycling process, implying its stability. It was found that nanocoral reef-assisted EDTA formed a chelation/complexation reaction between COO− groups and C–N bonds of EDTA with Cu(II) ions. In addition, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) analysis proved the synergistic effect of the electrostatic interaction and chelation/complexation was responsible for the removal mechanism of Cu(II). Also, the results demonstrated no significant variation in MnO2@EDTA-Ag removal efficiency for all the tested real water samples, revealing its efficacy in wastewater treatment. Therefore, the current study suggests that MnO2@EDTA-Ag has substantial potential to be used as a feasible adsorbent for probable hazardous metals remediation. Supplementary Information The online version contains supplementary material available at 10.1007/s11356-023-30805-0.


Introduction
One of the most serious challenges facing the globe today is water scarcity, and resource depletion in some regions is leading to significant international conflict and human suffering (Asim et al. 2021;Lee et al. 2021).Wastewater treatment has been recognized as an essential concern over the past few decades, along with the growing industrialization of society (Fu et al. 2019;Elfiad et al. 2020).Heavy metal ions are frequently released by industrial waste, particularly in the metal coating, leather tanning, and petroleum refining industries.Clean water technologies have faced a major barrier for removing highly hazardous and ultra-dilute contaminants of concern.Due to its hazardous and non-biodegradable properties, copper is one of the most prevalent heavy metal contaminants in urban and agricultural water (Hama Aziz et al. 2023).Copper is derived from numerous industrial wastewaters, such as printed circuit boards (PCBs) and electroplating effluent (Chen and Xie 2020; Duan et al. 2023).The World Health Organization (WHO) has established a limit of 2 mg L −1 for copper ions in drinking water; however, the United States Environmental Protection Agency (USEPA) has set an allowed maximum of 1.3 mg L −1 for copper ions in industrial effluents (Al-Saydeh et al. 2017).A variety of physicochemical and biological methods, including electrodialysis, coagulation and flocculation, membrane filtration, and adsorption, have been used to decontaminate water that has been contaminated with potentially dangerous metals (Dastkhoon et al. 2018;Guo et al. 2021;Shi et al. 2021;Kuang et al. 2022).However, all approaches have deficiencies, such as the requirement for extensive amounts of chemicals and energy, limited separation selectivity, partial pollutant removal, and the production of sludge that must be safely disposed of (Markovski et al. 2014).Adsorption, one of the many methods for removing metal ions from water, is a quick and affordable process that makes it simple to regenerate and recover the separation medium (Hasanpour and Hatami 2020).
Owing to their enormous surface area, varied morphologies, and significant activity, nanosized metal oxides (NMOs) have a promising capacity to remove heavy metals from aquatic environments (Shi et al. 2021).Moreover, with their excellent adsorption capacity, selectivity, and oxidation abilities, nanostructured manganese oxides (MnO 2 ) have been extensively employed for the removal of hazardous pollutants (Wei et al. 2019;Husnain et al. 2020).One of the most diverse groups of porous materials, MnO 2 includes nanorods, nanowires, nanoflowers, nanotubes, nanoparticles, and nanosheets among its many different structures and morphologies (Mallakpour and Motirasoul 2017).It has numerous crystallographic forms, such as α, β, γ, δ, ε, and λ-type, that are different in the spatial arrangement of MnO 6 octahedral units (Ghosh 2020).Due to their exceptional structural flexibility and new physical and chemical properties, manganese dioxides and their derivative compounds have received a lot of attention.Therefore, they can be utilized in many scientific and industrial applications, such as fuel cells, energy storage devices (supercapacitors and rechargeable batteries), catalysis, sensors, and adsorbents (Zhang et al. 2015;Zhao et al. 2017;Liu et al. 2018;Husnain et al. 2020;Bigiani et al. 2020;Hao et al. 2020;Asim et al. 2021;Claros et al. 2021).However, MnO 2 nanostructures have a significant tendency to adhesion and aggregation because of their high energy surface, small size, and electrostatic forces that limit their performance.Surface enhancements that are chemical and physical can solve this problem (Mallakpour and Motirasoul 2017).Therefore, we employed EDTA to modify the surface of MnO 2 with the coordination of carboxylic groups (-COOH) to the surface of MnO 2 .EDTA is a metal-chelating agent with high intrinsic selectivity that is commonly attached to substrate surfaces for application.Chen et al. prepared Fe 3 O 4 -MnO 2 -EDTA magnetic nanoparticles for targeted Cu(II) adsorption in a complex system (Chen and Xie 2020).EDTA-modified composites have comparable separation challenges as traditional adsorbents.However, common chelating agent EDTA has the ability to form stable chelates with metal ions and has good degradability in certain environmental systems, which can prevent secondary pollution (Repo et al. 2010;Panahandeh et al. 2021).Noble metals have recently received a lot of attention for their outstanding adsorption abilities.Due to their unique qualities, such as closely packed structure, varied crystallographic facets, and high surface-to-volume ratio, Ag nanoparticles (NPs) have been widely used as a possible adsorbent for industrial applications (Song et al. 2016;Vicente-Martínez et al. 2020).However, its reusability in large-scale wastewater treatment applications is constrained by NPs aggregation and challenging recovery.Therefore, employing effective assistance for Ag NPs may help in resolving these issues.An interesting candidate for use as an adsorbent material is the direct incorporation of Ag NPs on other active materials such as MnO 2 .However, the direct immobilization of Ag NPs onto active materials, such as MnO 2 , causes repulsive forces that need to be overcome to address stability.The issue of repulsive forces can be solved by incorporation an organic buffer layer between the two active materials, which will also provide a strong synergy in the hybrid structure, allowing it to be employed as an efficient adsorbent (Sharif et al. 2019).
Herein, a novel MnO 2 @EDTA-Ag nanocoral reef was effectively synthesized via a facile redox reaction followed by impregnation with EDTA and silver nanoparticles for rapid removal of hazardous copper (II) from real water samples.The dissolution of the MnO 2 adsorbent was avoided by impregnation with EDTA and Ag nanoparticles.Therefore, the feasibility of our strategy provides essential support for stability evaluation towards Cu(II) ions with acceptable adsorption properties.First, coral reef-like MnO 2 nanoparticles were prepared via a rapid redox reaction between KMnO 4 and Mn(II) sulfate.The dissolution of MnO 2 adsorbent was avoided by impregnation with EDTA and Ag nanoparticles as well.Statistical methods of experimental design, including factorial design and response surface analysis, have been employed in different methods for heavy metals' removal because of their ability to reduce the limitations of the time-consuming single-factor conventional and classical methods.For example, the Plackett-Burman design is a quick and useful screening technique to find the significant parameters amid a large number of variables, ensuring that each parameter is well understood, and saving time as well (Taylor et al. 2012;El-naggar et al. 2018).Therefore, the influences of all studied parameters on the removal efficiency of Cu(II) using MnO 2 @EDTA-Ag were investigated systematically using the Plackett-Burman design.A statistical analysis of the obtained results was carried out using ANOVA to evaluate the inference and impact of the studied parameters on copper removal.In addition, the impact of operating parameters, such as the initial pH, contact time, dose of adsorbent, initial metal concentration, temperature, and interfering ions towards Cu(II) removal was explored by adopting batch mode.To further assess the sorption behavior of MnO 2 @EDTA-Ag, the kinetics, rate-controlling mechanisms, isotherms, and thermodynamic characteristics of the sorption process have also been examined.Additionally, the desorption study, stability evaluation, and feasible mechanistic pathway for Cu(II) removal were elucidated.

Experimental
All materials and the characterization methodologies are provided in the electronic supplementary information (ESI).

Synthesis of MnO 2 nanostructures
A simple, ultrafast, economical, and environmentally friendly redox reaction was developed for the construction of nanocoral reef-like MnO 2 .In a 500-mL beaker, 4.61 g of MnSO 4 was combined with 100 mL of isopropyl alcohol (IPA).The solution was vigorously stirred while being heated to around 60 °C.After being thoroughly dissolved in 150 mL of double-distilled water, 3.00 g of KMnO 4 was quickly added to the previously mentioned boiling solution while being stirred for 10 min.Immediately, a significant amount of brown-black precipitate was collected.After being centrifuged, the mixture was cleansed with doubledistilled water and oven-dried for 6 h at 90 °C.Under the same conditions, various solvents were used to create additional MnO 2 structures.Typical solvents included glycerol and ethylene glycol.

Synthesis of Ag-EDTA-modified MnO 2 nanostructures
The above-obtained brown-black precipitate MnO 2 (100 mg) prepared by isopropyl alcohol (IPA) as a solvent was dispersed in 20 mL H 2 O and followed by 100 mg of EDTA.The mixture was mixed for 2 h and then agitated for 48 h at room temperature.The resulting EDTA-modified MnO 2 nanostructures were labelled as MnO 2 @EDTA.A simple sonochemical method was developed to construct embedded Ag nanoparticles on MnO 2 @EDTA.In order to create the Ag-NH 4 + solution, 150 mg of AgNO 3 was combined with 25 mL of NH 4 OH (5%) in 150 mL of water.Afterward, 150 mL of Ag-NH 4 + solution was sonicated with 50 mg of MnO 2 @EDTA for 10 min at room temperature.Finally, the grayish-black solid product (MnO 2 @EDTA-Ag) was centrifugated, washed with dilute HNO 3 , and double-distilled water several times, then dried at a temperature of 70 °C.

Statistical experimental design
For studying Cu(II) removal using MnO 2 @EDTA-Ag, the removal percentage of metal could depend on the medium pH, the initial Cu(II) concentration (C o ), the contact time (t), the adsorbent dose (m), and the temperature.Plackett-Burman design was employed to determine the significant parameters for Cu(II) removal.The low and high levels are given in Table 1.Design expert statistical software program, version 12, was employed for the experimental design and data statistical analysis.

Adsorption experiments
Batch adsorption experiments were conducted at a constant rate of 150 rpm in a thermostatic shaking water bath (AHAAM, Egypt).The impacts of several parameters were studied, such as initial pH (2-6), contact time (1-120 min), initial Cu(II) concentration (10-50 mg L −1 ), the adsorbent dose (0.001-0.1 g), and temperature (25-55 °C) using 10 mL of Cu(II) solutions.The pH values of the copper solutions were altered as required through the addition of drops of HCl (0.1 mol L −1 ) or NaOH (0.1 mol L −1 ).The values of the point of zero charges (pH PZC ) of the prepared adsorbents were calculated using the salt addition method (Bakatula et al. 2018).After a measured time interval, samples were taken out and filtered through a membrane syringe filter (0.22 μm) to be analyzed to determine the residual Cu(II) concentration using the modified sodium diethyldithiocarbamate method (Ali and Mohamed 2017).
The removal efficiency (R%) and the amount of Cu(II) adsorbed (q e , mg g −1 ) were calculated using the following equations.(Mozaffari et al. 2020b, c): where C o and C e (mg L −1 ) represent the initial and equilibrium copper concentrations, respectively; V is the volume (L) of the copper solution, and m denotes the mass (g) of the adsorbent.

Desorption and reusability
After adsorption of 10 mL Cu(II) (10 mg L −1 ) using 0.01 g of MnO 2 @EDTA-Ag, the loaded adsorbent was filtered and dried.The adsorbed Cu(II) was desorbed using different concentrations (0.001, 0.005, and 0.01 mol L −1 ) of different acids (HCl, HNO 3 , and H 2 SO 4 ).The desorbed Cu(II) was separated and analyzed as described in the "Adsorption experiments" section.Desorption efficiency (E Des ) was estimated using Eq.(3) (Ali et al. 2020): (1) (2) where the amounts of adsorbed and desorbed Cu(II) are represented by A Ads and D Des , respectively.The regenerated adsorbent was employed in the following adsorption tests for reusability, and the adsorption-desorption cycle was repeated with the same sample.The produced adsorbent's ability to capture Cu(II) ions from a variety of real water samples was next tested analytically.Several water samples were used, including tap water, groundwater, and Nile water.
On the other hand, the diffraction peak at about 28.5° for the MnO 2 sample corresponded to the reflection plane of ( 110) of a pyrolusite-type (β-MnO 2 ) of manganese oxide (JCPDS 24-0735) (Devaraj and Munichandraiah 2008;Bai et al. 2016;Raheem and Al Sammarraie 2020;Sun et al. 2020).It can be observed that there is no significant difference in the XRD pattern of MnO 2 @EDTA upon the modification of the MnO 2 sample with EDTA, demonstrating that the phase structure was almost completely conserved.However, with the incorporation of Ag nanoparticles, it can be seen that the peaks (3) E Des = D Des A Ads × 100 became sharper and more intense (Fig. 1b), reflecting the higher crystallinity of MnO 2 @EDTA-Ag sample.In addition, the XRD pattern of MnO 2 @EDTA-Ag lacked the expected Ag diffraction peaks, which may be attributable to the great dispersion of Ag nanoparticles.
The surface functional groups of MnO 2 , MnO 2 @EDTA, and MnO 2 @EDTA-Ag nanocoral reef were verified by FT-IR spectra as shown in Fig. 2. The peaks located around 500 and 720 cm −1 are associated with the Mn-O vibration of [MnO 6 ] octahedra (Asim et al. 2021).A new peak at 1409 cm −1 is observed in MnO 2 @EDTA and MnO 2 @ EDTA-Ag samples, and it is attributed to the symmetric stretching vibration of C-N bonds and COO − groups derived from EDTA (Chen and Xie 2020).This new peak is absent in the MnO 2 sample, indicating that EDTA was successfully anchored on MnO 2 @EDTA.The non-existence of this peak in MnO 2 sample is evidence of the successful anchoring of EDTA in the MnO 2 @EDTA sample.In addition, a small peak that appeared at 450 cm −1 is assigned to Ag, confirming that Ag nanoparticles have been successfully embedded into MnO 2 @EDTA (Sharif et al. 2019).The bending vibrations of the -OH groups associated with the Mn atoms appeared at a peak of 1634 cm −1 (Dinh et al. 2020).The broad vibration band around 3382 cm −1 represents O-H stretching vibration, which is from water molecules that are physically adsorbed to the surface of samples (Revathi and Kumar 2017;Cao et al. 2021).
The surface morphologies of MnO 2 @EDTA and MnO 2 @ EDTA-Ag structures were characterized by FESEM.As displayed in Fig. 3a-b, MnO 2 @EDTA shows a nanocoral reef composed of a two-dimensional nanoplatelet structure with the formation of some nanorods out of the plate structure.Figure 3c-d clearly show the in situ deposition of Ag (NPs) on the surface of the MnO 2 @EDTA nanocoral reef.The Fig. 1 XRD patterns of MnO 2 , MnO 2 @EDTA, and MnO 2 @ EDTA-Ag nanocoral reef elemental mapping analysis (Fig. 3e-j) indicated that Mn, O, C, N, and Ag elements were homogeneously distributed in the hybrid structure.The EDX spectrum of MnO 2 @EDTA-Ag is shown in Fig. 3g, and the elemental composition confirms the existence of Mn, O, C, N, and Ag elements.In the meantime, the TEM images (Fig. 4a-b) also confirmed that the obtained MnO 2 @EDTA-Ag was a nanocoral reef composed of two-dimensional nanoplatelet with nanorod-like structure, which was the same as the SEM results.The average diameter of a nanoplatelet is 60 nm, and the diameters of nanorods are about 10 to 80 nm and 200 nm in length.In addition, the embedded Ag NPs were observed and attached to the surface nanorods (Fig. 4b).HRTEM image (Fig. 4c) of MnO 2 @EDTA-Ag nanocoral reef exhibited lattice fringes of 0.24 and 0.31 nm.These spacings are related to the (131) lattice plane of the γ-MnO 2 phase and the (110) lattice plane of the β-MnO 2 nanostructures, as shown in Fig. 4c.It can be concluded that the Ag NPs were successfully immobilized on the MnO 2 @EDTA nanocoral reef to form a hybrid structure.Figure 4 d illustrates the SAED pattern of the MnO 2 @ EDTA-Ag nanocoral reef, which demonstrates the polycrystalline nature of the hybrid structure.

Statistical experimental design
Plackett-Burman design (PBD) using two-level factors was utilized to analyze the removal of Cu(II) ions employing MnO 2 @EDTA-Ag taking into consideration different parameters: pH (A), contact time (B), initial Cu(II) concentration (C), adsorbent dose (D), and temperature (E).Table 2 summarizes the different operating parameters along with the experimental results of R% for Cu(II) removal using MnO 2 @ EDTA-Ag according to PBD experiments.The data listed in the table indicated a substantial variation in Cu(II) removal efficiency.The lowest R% of 8.34% was achieved using a MnO 2 @EDTA-Ag dose of 0.005 g, 1 min as contact time, 50 mg L −1 Cu(II), pH 3, and at 55 °C.In contrast, the highest R% (95%) was reached with a MnO 2 @EDTA-Ag dose of 0.05 g, 90 min as a contact time, 10 mg L −1 Cu(II), pH 6, and at 55 °C.The residuals between the practical (actual) and predicted (model) values were found to be small, indicating that both the observed and predicted responses had satisfactory agreement.
Analysis of variance (ANOVA) was employed to assess the appropriateness of the model.The ANOVA of R% of Cu(II) using MnO 2 @EDTA-Ag is exhibited in Table 3.The model is highly significant in this study, as shown by the F value of 273.61, and there was only a 0.03% possibility that the results were only due to noise.The correlation coefficient (R 2 ) manipulated to reveal the relationship between the experimental (actual) and predicted responses was 0.9986, as confirmed by the model.This implies that the process parameters analyzed justify 99.86% of the removal efficiency variability.Moreover, as displayed in the table, the value of the predicted R 2 (0.9791) is in acceptable agreement with the adjusted R 2 (0.9950), i.e., the deviation is < 0.2.Also, model terms with P-values < 0.05 imply that they are statistically significant.The dose (D) of MnO 2 @EDTA-Ag was the most significant parameter affecting Cu(II) removal, followed by Cu(II) concentration (C) > pH (A) >> time (B) and their interactions (AB and BC).On the other hand, temperature (E) and interaction (CE) had no significant impact on Cu(II) removal.As demonstrated in Table 3, the respective contributions of the investigated parameters, including pH, time, Cu(II) concentration, adsorbent dose, and temperature, were 31.7,30.51, 25.17, 12.44, and 0.18%, respectively.In addition, a Pareto chart was employed to affirm the impact of each parameter on Cu(II) removal (Fig. 5a).As demonstrated, pH and adsorbent dose had positive effects on the Cu(II) removal efficiency, whereas time and Cu(II) concentration had negative effects.The following equation can be utilized to figure out the model equation of R% for copper removal: Figure 5b displays the plot between the actual values and predicted values of the removal efficiency of Cu(II).As demonstrated, both values are in reasonable agreement.
In this investigation, there was just one response taken into account, and the objective was to achieve the highest possible R%.The input parameters were each given values that were within a predetermined range (Fig. 6), while the Fig. 2 FTIR spectra of MnO 2 , MnO 2 @EDTA, and MnO 2 @EDTA-Ag nanocoral reef response (R%) was considered to achieve a maximum.The ramp desirability is shown in Fig. 6, which was generated using a total of 112 starting points using numerical optimization using the Design Expert software.As observed, the experimental conditions of pH = 5.5, time = 32.0 min, Cu(II) conc.= 11.2 mg L −1 , dose = 0.05 g, and temp.= 40.3°C would achieve a R% of 99.95 and desirability of 1.000.

Effect of the solvent type
To investigate how the kind of solvent affects the prepared adsorbents' ability to adsorb, MnO 2 nanostructures were fabricated using different solvents, including isopropyl alcohol,  glycerol, and ethylene glycol, where they were denoted as IPA, EG, and GL, respectively.These materials were independently used to prepare MnO 2 @EDTA structures and were labelled IPA-EDTA, EG-EDTA, and GL-EDTA, respectively.These prepared materials were employed to capture Cu(II) ions, and the results are illustrated in Fig. S1.
Obviously, there is a slight difference in their removal efficiencies towards Cu(II) ions, where MnO 2 and MnO 2 @ EDTA prepared via isopropyl alcohol exhibited the highest removal percentage.Therefore, MnO 2 @EDTA-based isopropyl alcohol was used to prepare MnO 2 @EDTA-Ag, and both were employed in the succeeding investigations.

Effect of initial pH
Figure 7a illustrates the effect of pH on the Cu(II) adsorption onto the MnO 2 @EDTA and MnO 2 @EDTA-Ag nanocoral reef at initial pH values that varied from 2 to 6. Furthermore, the pH PZC values for MnO 2 , MnO 2 @EDTA, and MnO 2 @EDTA-Ag nanocoral reef (Fig. 7b) were calculated, and they were discovered to be 4.7, 5.3, and 4.3, respectively.It can be noted that as the pH of the Cu(II) solution was raised from 2 to 6, the removal efficiency of the two adsorbents gradually increased.The trend can be interpreted in terms of the electronegativity property of the adsorbents' surfaces.At low pH values, extra protons and Cu(II) ions competed for the binding sites of the MnO 2 @EDTA and MnO 2 @EDTA-Ag nanocoral reef, resulting in low removal efficiency.As pH values increased, the functional groups on the adsorbents were deprotonated, thereby rendering them more accessible to adsorb Cu(II) ions through electrostatic attraction forces.Based on these observations and the pH PZC values, it can be presumed that electrostatic attraction forces were not the only mechanism for Cu(II) adsorption on the prepared adsorbents.It is possible that several sorption processes, including surface complexation, ion exchange, and precipitation, may be involved in addition to the electrostatic attraction process.These processes have been reported before (Touihri et al. 2021).Hence, pH 6 was utilized in the experiments that followed.

Effect of contact time and initial concentration
Figure 7 c shows the removal efficiency (R%) of Cu(II) of MnO 2 @EDTA and MnO 2 @EDTA-Ag nanocoral reef as a function of time and constant pH 6.Initially, the Cu(II) adsorption onto MnO 2 @EDTA and MnO 2 @EDTA-Ag was fast.In the first 5 min, 47.4% and 82.3% of Cu(II) were adsorbed onto MnO 2 @ EDTA and MnO 2 @EDTA-Ag, respectively.Further, the uptake % of Cu(II) increased gradually up to 120 min with the increasing extension of contact time between adsorbent and adsorbate and afterward sustained a plateau with 86% and 98% removal using MnO 2 @EDTA and MnO 2 @EDTA-Ag, respectively.The rapid initial adsorption rate is responsible for the considerable number of accessible active sites in terms of functional groups and pores on the surface of nanocoral reef in the early stages of the adsorption process.The Cu(II) molecules build up on the surface of the adsorbent as the process of adsorption progresses, blocking molecules from diffusing into the pores and causing a slow adsorption rate (Choudhary et al. 2020).In addition, an improved adsorption behavior from 86 to 98% was achieved using MnO 2 @ EDTA and MnO 2 @EDTA-Ag, respectively.Moreover, the initial Cu(II) concentration, which varied between 10 and 50 mg L −1 at pH 6.00, had a substantial impact on the adsorption behavior.As shown in Fig. 7 d, when the Cu(II) concentration rose from 10 to 50 mg L −1 , the Cu(II) removal efficiency (%) onto MnO 2 @EDTA and MnO 2 @EDTA-Ag decreased from 86 and 98% to 18% and 33%, respectively.Insufficient active sites on the adsorbent or a lack of equilibrium time for higher concentration adsorption can be ascribed to the decline in the percentage of Cu(II) removal with increasing initial concentration.

Effect of adsorbent dosage
The removal performance (%) of Cu(II) was further investigated using different dosages of adsorbent in the range of 0.5-10 g L −1 .The Cu(II) concentration was initially 10 mg L −1 and was adjusted for an optimal pH of 6.00 in 90 min.
As shown in Fig. 7 e, increasing the dosage of adsorbent from 0.5 to 10 g L −1 enhanced the removal efficiency (%) of Cu(II) from 30.86 to 95.40% and from 51.9 to 99.8% using MnO 2 @EDTA and MnO 2 @EDTA-Ag, respectively.The elimination (%) of Cu(II) remained essentially constant as the adsorbent dosage was increased to 10 g L −1 .It was evident that the adsorption efficiency considerably increased with elevating the amount of adsorbent due to the higher number of active sites and porous surfaces (Zare et al. 2015).

Effect of ionic strength and interfering ions
Figure 7f presents the effect of ionic strength on the removal (%) of Cu(II) from an aqueous solution.The removal rate (%) dramatically decreased as the NaCl concentration changed, demonstrating the considerable influence of ionic strength on Cu(II) uptake.The effectiveness of Cu(II) adsorption decreased .Contrarily, Na + , K + , Mg 2+ , and Ca 2+ significantly hindered Cu(II) adsorption; as a result, the percentage of Cu(II) removed from the sample reduced from 98 to 93, 91, 89, and 85%, respectively.The competition between these cations for adsorption sites on the MnO 2 @EDTA-Ag surface was the cause of the observed inhibitory effect.

Kinetic studies
The rate at which Cu(II) ions are removed using the prepared adsorbents from aqueous solutions can be investigated using kinetic studies, which also provide valuable data for identifying the adsorption mechanism.In this regard, several models are employed, including pseudo-first-order, pseudo-second-order, and intra-particle diffusion models.The linearized forms of the employed kinetic models (Sun et al. 2021) are given in Table S1.
The kinetic constants and correlation coefficients (R 2 ) for the applied models were evaluated (Lima et al. 2019), and their corresponding values are demonstrated in Table 4. Also, RMSE, ΔQ, and χ 2 (Table S2) were employed to assess the consistency of the computational models and experimental points (Khadhri et al. 2019).As demonstrated by the low values of RMSE, ΔQ, and χ 2 and the high values of R 2 (Table 4), pseudo-second-order kinetics (Fig. 8a) rather than pseudo-first-order govern Cu(II) removal onto MnO 2 @ EDTA and MnO 2 @EDTA-Ag.Furthermore, the determined q e values using the pseudo-second-order model are more consistent with the experimental q e values, revealing that the pseudo-second-order kinetics effectively describe the Cu(II) removal using MnO 2 @EDTA and MnO 2 @EDTA-Ag.
Although the pseudo-second-order model provides a sufficiently accurate description of the overall adsorption kinetics, it provides no details on the rate-regulating steps.Therefore, kinetic data were investigated with the Weber and Morris intraparticle diffusion model.To estimate k i and C values, the intercept and slope of the plot of q t versus t 0.5 were utilized.The plots of this model are shown in Fig. 8b, which reveals the occurrence of two stages in the case of Cu(II) removal using MnO 2 @EDTA-Ag.The initial stage was the film diffusion attributed to the external mass transfer of Cu(II) ions from the bulk solution to the MnO 2 @EDTA-Ag's surface, while the second stage involved the Cu(II) ions' passage through the pores of adsorbents (Ali et al. 2020).However, Cu(II) removal using MnO 2 @EDTA comprises three stages, the initial was for film diffusion, while the second was attributed to Cu(II) ions diffusion through MnO 2 @EDTA pores, followed by the last stage at which the system has reached equilibrium.The two Fig. 6 Predicting points of actual profiles regarding the best settings for copper removal plots do not cross the origin point, demonstrating that film diffusion in addition to intra-particle diffusion may play a role in the rate-determining stage for controlling Cu(II) removal using MnO 2 @EDTA or MnO 2 @EDTA-Ag.

Adsorption isotherms
The Cu(II) ions' equilibrium concentrations in the solid and liquid phases were expressed using a variety of models, including the Freundlich, Langmuir, Dubinin-Radushkevich (D-R), and Temkin isotherm models (Mozaffari et al. 2020a;Azzam et al. 2022) These models can be expressed in linearized forms, as demonstrated in Table S3.
The isotherms' constants of the Langmuir and Freundlich models were estimated from their linear plots by plotting C e /q e vs. C e and log q e vs. log C e , respectively.Also, plotting ln q e vs. ε 2 and q e versus ln C e enables one to deter- mine the constants K DR , B, and E for the D-R model and the constant b T for the Temkin model, respectively.Fig. S2 shows the fitting of the experimental data using the applied isotherm models.The calculated parameters of the applied models, along with their R 2 , RMSE, ΔQ, and χ 2 values, are summarized in Table 5.
It can be inferred from the high values of R 2 and the low values of RMSE, ΔQ, and χ 2 (Table 5) that the experimental equilibrium results of Cu(II) ions using MnO 2 @EDTA or MnO 2 @EDTA-Ag are better described by the Langmuir model than the Freundlich model.Also, the R L values (between 0 and 1) demonstrated favorable adsorption of Cu(II) onto MnO 2 @EDTA or MnO 2 @EDTA-Ag (Mohamed et al. 2023).Based on the Langmuir isotherm, single-layer adsorption of Cu(II) ions takes place on an even surface of MnO 2 @EDTA or MnO 2 @EDTA-Ag with a limited number of sites for adsorption.In contrast, the Freundlich model is employed for investigating the interaction of adsorbed molecules on heterogeneous surfaces.
The maximum adsorption capacity (q max ) of MnO 2 @ EDTA-Ag for copper removal was also compared to those of previous studies, as illustrated in Table 6.As concluded, MnO 2 @EDTA-Ag can be regarded as an effective adsorbent based on comparison with other adsorbents.
To identify the kind of adsorption-physisorption or chemisorption-the equilibrium data were modeled applying the D-R isotherm.As noticed from Table 5, Cu(II) adsorption onto MnO 2 @EDTA and MnO 2 @EDTA-Ag was physical in nature because the obtained values of E were < 8 kJ mol −1 Table 4 The estimated parameters of the examined kinetic models for Cu(II) removal using MnO 2 @EDTA or MnO 2 @EDTA-Ag

Thermodynamic studies
At various temperatures (298-328 K), the sorption process' thermodynamic characteristics were assessed.
The following equations were used to estimate the enthalpy change (∆H, kJ mol −1 ), entropy change (∆S, J mol −1 K −1 ), and free energy change (∆G, kJ mol −1 ) (Ali and Mohamed 2017): where R is the universal gas constant (8.314J K −1 mol −1 ), T is the temperature in Kelvin, and K d is the equilibrium constant.The slope and intercept of the plot of ln K d vs 1/T can be used to determine the values of ∆H and ∆S.
As shown in Table 7, for the adsorption of Cu(II) onto MnO 2 @EDTA and MnO 2 @EDTA-Ag, the positive and negative values of ∆H revealed that the nature of the adsorption process was endothermic and exothermic, respectively.When the value of ∆H is less than 80 kJ mol −1 , the adsorption process involves physisorption, such as van der Waals or electrostatic interaction, and when the value of ∆H is greater than 80 kJ mol −1 , the interaction is chemisorption (Ali and Mohamed 2017).
(5) The adsorption was favorable since the positive values for ∆S suggested a rise in randomness at the solid-solution interface.The disorder at the solid/liquid boundary (interface) was reduced throughout the adsorption process of the copper ions, as shown by the negative value of ∆S (− 128.82 J K −1 mol −1 ) obtained using MnO 2 @ EDTA-Ag.Because copper ions are more mobile at higher temperatures, ions from the solid phase were able to move into the liquid phase.Therefore, less amount of copper could be adsorbed (Rukayat and Usman 2021).
The negative ΔG values at the higher temperatures demonstrated the viability and spontaneity of the Cu(II) sorption process onto MnO 2 @EDTA, whereas the positive ΔG values at the lower temperatures reflected the nonspontaneous character of the adsorption process.On the contrary, Cu(II) adsorption onto MnO 2 @EDTA-Ag was spontaneous at the lower temperatures and non-spontaneous at the higher ones.

Desorption and reusability study
To evaluate the desorption behavior of MnO 2 @EDTA-Ag, different concentrations of strong acids, such as HCl, H 2 SO 4 , and HNO 3 , ranging from 0.001 to 0.005 mol L −1 are most commonly used to elute Cu(II).It was observed that increased concentration of strong acids (0.001-0.005 mol L −1 ) enhanced the desorption efficiency (%) of Cu(II) from 30.24 to 77.55%, 35.66 to 82.16%, and 77.50 to 92% using HCl, H 2 SO 4 , and HNO 3 , respectively (Fig. 9a).For economic reasons and environmental points of view, recycling of adsorbents has a significant role.Herein, 0.005 mol L −1 HNO 3 was used as a stripping agent for regeneration of MnO 2 @EDTA-Ag.After the adsorption process of Cu(II), MnO 2 @EDTA-Ag was recollected and mixed with 10 mL HNO 3 (0.005 mol L −1 ), shaken for 1 h, and separated.The collected adsorbent was washed with distilled water and dried to be reused again.The Cu(II) solution was added again to the collected adsorbent to evaluate the stability of the adsorbent.After completing three consecutive cycles of  adsorption-desorption, the efficacy of MnO 2 @EDTA-Ag for removing Cu(II) was slightly reduced, as shown in Fig. 9b.The removal efficiency % of the Cu(II) solution was reduced from 98 to 80%.In addition, there is no significant change in the XRD analysis before and after the recycling process, implying its stability (Fig. 9c).The results demonstrated that the MnO 2 @ EDTA-Ag nanocoral reef was comparably stable under the studied conditions, which is an advantage of using the adsorbent for environmental remediation and the industrial field.

Proposed removal mechanism of Cu(II) onto MnO 2 @ EDTA-Ag
The adsorption mechanism has been comprehensively elucidated using XPS and FTIR analysis.The chemical states of MnO 2 @EDTA-Ag nanocoral reef before and after Cu(II) adsorption were investigated by XPS analysis.Figure 10a illustrates the XPS survey spectra of both samples before and after Cu(II) removal, where the typical Mn, Ag, C, O, and N signals could be observed.Meanwhile, the newly appeared peak of the Cu(II) signal could be found in the sample after adsorption, confirming the uptake of Cu(II) ions onto the MnO 2 @ EDTA-Ag nanocoral reef.The high-resolution scan of the Mn 2p spectrum is given in Fig. 10b.The peaks located around binding energies of 642.14 and 653.74 eV are assigned to Mn 2p 3/2 and Mn 2p 1/2 .The Mn 2p 3/2 and Mn 2p 1/2 peaks showed a splitting of 11.55 eV, which confirms that the main oxidation state of Mn atoms is+4.The values of binding energy at 642.19 and 644.99 eV of the Mn 2p 3/2 spectrum are ascribed to Mn 3+ and Mn 4+ , respectively.After Cu(II) adsorption, the binding energy of the Mn 2p 3/2 spectrum is slightly shifted to 642.14 and 644.49eV, confirming the absence of Mn 2+ in the MnO 2 @ EDTA-Ag nanocoral reef.In addition, the Mn 3+ /Mn 4+ ratio of MnO 2 @EDTA-Ag nanocoral reef is slightly increased from 3.40 to 4.38% after the adsorption process, confirming the absence of redox reactions (Lin and Chen 2020).As shown in Fig. 10c, the O 1 s spectrum before and after adsorption was elucidated.The peak located at 529.77 eV is attributed to lattice oxygen of metal oxide (Mn-O-Mn bonds), the peak at 531.34 eV corresponds to Mn-OH and -COO − groups, and the peak at higher binding energy, 531.92 eV, associated with OH groups of water molecules or carbonyl groups (Claros et al. 2021).After adsorption of Cu(II) to the surface of MnO 2 @EDTA-Ag, the peaks of 531.34 and 531.92 eV are shifted to 531.39 and 532.12 eV, respectively.The relative area ratio for the peak of the Mn-O bond is slightly increased from 74.21 to 74.24% due to the binding of Mn-O, Cu-O, and the formation of bidentate mononuclear or multidentate complexes, instead of monodentate complexes during the adsorption.Meanwhile, the relative ratio of Mn-OH decreased from 13.06 to 10.58% due to the participation of the hydroxyl groups in the adsorption process (Lin and Chen 2020;Claros et al. 2021).Before the adsorption of Cu(II), the value of binding energy of N 1s spectrum (Fig. 10d) was 401.01 eV, which is slightly shifted to 399.84 eV after the adsorption process, suggesting that Cu(II) ions may be bound to N atoms of EDTA by chelation interaction (Cui et al. 2021).Therefore, the synergistic effect between the physisorption forces and the chelation interaction of MnO 2 @EDTA-Ag nanocoral reef enhanced Cu(II) removal.The Ag 3d spectrum (Fig. 10e) before and after Cu(II) adsorption exhibits only a couple of peaks at binding energies of 367.89 and 373.84 eV, corresponding to Ag 3d 5/2 and Ag3d 3/2 , respectively, showing the presence of metallic Ag (NPs) (Bai et al. 2016;Tang et al. 2019).The relative area ratio of Ag 3d 5/2 decreased from 52.91% (before adsorption) to 38.52% (after adsorption).This decrease may be due to the adsorption of Cu(II) onto MnO 2 @EDTA-Ag.The binding energies at 934.3 and 953.8 eV (Fig. 10f) are attributed to Cu 2p 3/2 and Cu 2p 1/2 , with a spin-orbit separation of 19.5 eV, implying the removal of Cu(II) ions onto MnO 2 @EDTA-Ag nanocoral reef (Claros et al. 2021).In addition, the "shake-up" satellite peaks in the range 940-945 eV are evidence for the presence of Cu 2+ state (Mulla and Rabinal 2021).
Additionally, Fig. 10g displays the MnO 2 @EDTA-Ag FTIR spectra before and after Cu(II) capture.Following Cu(II) adsorption, the peak at 1409 cm −1 ascribed to the symmetric stretching vibration of C-N bonds and COO − groups that originated from EDTA shifted to 1426 cm −1 .This implies that these groups participate in the formation of a coordination complex involving Cu(II).

Environmental applications
The analytical usefulness of MnO 2 @EDTA-Ag to be employed for water remediation was demonstrated by applying them to remove Cu(II) ions that were spiked in different water samples, including groundwater, Nile water, and tap water.This was carried out by shaking 0.01 g of MnO 2 @ EDTA-Ag for 1 h, at pH 6 with 10 mL of each water sample that was previously spiked with 10 mg mL −1 Cu(II).The results are presented in Fig. S3, which revealed no significant difference in the removal efficiency of MnO 2 @EDTA-Ag in any of the water samples tested, reflecting the effectiveness of the prepared samples in wastewater treatment.It is worth mentioning that the equilibrium Cu(II) concentration in all the tested samples was all less than 0.7 mg L −1 , demonstrating the remarkable effectiveness of MnO 2 @EDTA-Ag to remediate complex wastewater from heavy metals.

Conclusion
In this work, a novel MnO 2 @EDTA-Ag nanocoral reef was effectively synthesized via a facile redox reaction followed by impregnation with EDTA and silver nanoparticles for rapid removal of hazardous copper (II) from real water samples.The results showed that the nanocoral reef structure's Cu(II) removal efficiency (99.95%) employing Plackett-Burman design (PBD) was achieved at pH 5.5, a contact time of 32.0 min, a Cu(II) concentration of 11.2 mg L −1 , MnO 2 @EDTA-Ag dose of 0.05 g, and a temperature of 40.3 °C.The proposed model with PBD revealed that the dose of MnO 2 @EDTA-Ag, followed by Cu(II) concentration and pH, were the most significant parameters affecting Cu(II) removal.It was found that the loading of Ag NPs onto MnO 2 @EDTA improved the adsorption capability of MnO 2 @EDTA-Ag.The adsorption data were significantly correlated with the models of pseudo-second-order kinetics and Langmuir isotherm.Moreover, the thermodynamic study of the Cu(II) removal is energetically exothermic.XPS and FTIR analysis elucidated that the electrostatic interaction and chelation/complexation are responsible for the removal mechanisms of Cu(II).No significant loss in the Cu(II) removal efficiency using the real wastewater samples compared to the aqueous solution was observed, revealing its efficacy in water remediation.Additionally, the recyclability of MnO 2 @EDTA-Ag nanocoral reef for Cu(II) was maintained at 80% after three adsorption-desorption cycles, and the XRD data before and after the recycling process revealed no significant changes, suggesting its stability.According to these conclusions, the MnO 2 @EDTA-Ag nanocoral reef shows promise as a sorbent for the removal of potentially hazardous metals from wastewater.

Fig. 5 a
Fig. 5 a Pareto chart of the impacts of various parameters on the Cu(II) removal and b graphical representation of experimental values as a function of predicted values

Fig. 9 a
Fig. 9 a Effect of stripping agents for desorption study, b recycling of MnO 2 @EDTA-Ag for Cu(II) removal, and c XRD patterns of MnO 2 @EDTA-Ag before and after the third cycle

Fig. 10
Fig. 10 XPS and FTIR analysis before and after adsorption of Cu(II) onto MnO 2 @EDTA-Ag: a full survey, b Mn 2p, c O 1s, d N 1s, e Ag 3d, f Cu 2p, and g FTIR spectra

Table 1
Levels of the parameters tested in Plackett-Burman design

Table 2
Results of Cu(II) removal using MnO 2 @ EDTA-Ag using Plackett-Burman design Run no pH Time (min) Conc.

Table 5
The estimated parameters of the examined isotherm models for Cu(II) removal