Plasma-assisted MnO surface engineered activated carbon felt for enhanced heavy metal adsorption

Optimisation of plasma surface modification parameters

Experiments with different plasma power (watts) and exposure times to determine the optimum plasma parameters and validate the optimal KMnO₄ concentration for Cu(II) and Cd(II) adsorption on modified ACF. It is apparent from the results, which are presented in Fig. 2a,b, that the maximum adsorption of the heavy metals was accomplished with a plasma power of 150 W and a 5-minute exposure duration for the adsorption of Cu(II) and Cd(II). Increasing the plasma power over longer exposure durations may provide larger discharge effects of plasma species, which could also cause the ACF’s overall porosity structure to be over-etched and collapsed¹⁶. These findings agree with prior studies on ACF plasma treatment, highlighting that different applications and target pollutants have considerably different plasma specification needs^17,18,19. The incorporation of oxygen-containing functional groups, the opening of micropores, and the increment of surface roughness are all the results of the plasma treatment that are attributable to the enhancement in adsorption capacity^20,21. Figure 2c showed that 0.1 M of KMnO₄, pre-treatment, significantly improved the adsorption capacity of plasma-treated ACF compared to concentrations ranging from 0.01 M to 1 M. This might be due to a lack of active sites available for surface oxidation to effectively produce enough reactive species to modify the ACF surface with MnO species^20,22. Conversely, the excessive KMnO₄ concentrations can lead to redox reactions at the surface, potentially causing ACF structure dissolution or deterioration, resulting in loss of structural integrity, reduced surface area, and reduced adsorption capacity²³. The optimum plasma parameters, which include 0.1 M KMnO₄, pre-treatment, 5 min of exposure duration, and 150 W of power, provide crucial results for effectively removing heavy metals.

Optical emission spectroscopy from the plasma sheath was used to identify prominent electroactive species involved in the surface modification of the ACF after MnO pre-treatment. According to Fig. 2d, atmospheric nitrogen, along with Ar and O₂, effectively embeds functional groups on carbon surfaces, indicating the creation of nitrogen-rich electroactive species in the plasma. The plasma-enhanced surface modification of the ACF is driven by the presence of singlet oxygen and the metastable Ar* species, which are primarily abundant in the plasma sheath, according to the OES spectra²⁴.

Adsorption capacities of Cu(II) and Cd(II) on modified ACF with (a) plasma power variation, (b) plasma exposure time, (c) concentration of KMnO_4, and (d) OES spectra of active plasma species.

Mechanism of plasma reaction

Modified ACF undergoes a series of surface modification processes by plasma that enriches the material’s surface and core with functional groups. The overall reaction mechanism is illustrated in Fig. 3. The first step is assumed to be (i) Formation of defects – The modified ACF structure experiences defects due to active plasma species, reactive oxygen and nitrogen species, free electrons, Frenkel-pair defects, and interstitial atom vacancies, enhancing its reactivity and surface characteristics, which are depicted in equations (2 to 5)^25,26. Secondly, (ii) Breaking of π bonds – plasma treatment causes π bonds in modified ACF structure to rupture due to high energy electrons and reactive species, potentially creating new reactive sites on the modified ACF surface. Followed by (iii) Creation of dangling bonds and reduction of MnO species – Plasma treatment reduces MnO species on modified ACF surfaces to Mn₃O₄ and MnO₂, forming reactive sites and functional groups, resulting in increased dangling bonds²⁷. These dangling bonds act as fresh sites for further reactions, as shown in equations (5 to 11). Finally, (iv) Creation of oxygen and nitrogen-containing functional groups – Plasma reactive species lead to the creation of oxygen-containing functional groups, such as hydroxyl (-OH) and carbonyl (-C = O), as well as nitrogen-containing functional groups, such as amine (-NH₂) and nitrile (-C ≡ N). The abundant MnO species on the modified ACF surface facilitate these reactions, further promoting the formation of Mn-based functional groups²⁸. Plasma treatment and MnO pre-treatment coating activate oxidation, modifying ACF surface, producing functional groups, breaking π bonds, reducing MnO species, and introducing defects, improving ACF’s reactivity and adsorption capability. Additionally, the equations from (2 to 11) are conjectured to comprehend the chemical processes that are taking place^10,29.

Schematic anticipated illustration of the reaction of plasma species with ACF coated with MnO (modified ACF).

Characterisations of plasma surface modified ACF

The X-ray Photoelectron Spectroscopy (XPS) analyses were conducted to discern structural and chemical modifications in Activated carbon felt (ACF), as presented in Fig. 4. Examination of survey peaks in Fig. 4a unequivocally indicates the successful origination of Mn peaks after the modification process³⁰. A detailed deconvolution of C1s + K2p spectra unravelled intriguing insights. The emergence of K2p at ~ 292.85 eV and ~ 295.6 eV, accompanied by a relatively stronger peak area of C = O at ~ 287.7 eV, denotes a synergistic modification induced by KMnO₄, molecules and plasma treatment³¹. In Fig. 4b, the O1s spectra unveil compelling evidence of structural alterations. The appearance of additional peaks along with C-OH/C = O and H-OH for K-OH at ~ 531.0 eV and for Mn-O at ~ 529.7 eV substantiates the reduction of Mn species and the formation of new chemical species in the presence of plasma. These findings underscore the intricate interplay between KMnO₄ and plasma treatment, providing a nuanced understanding of the modifications occurring in ACF.

XPS analysis of raw and modified ACFs (a) survey peaks of ACF before and after modification; deconvoluted peaks of (b) C1s + K2p; and (c) O1s.

Furthermore, Fig. 5a shows the FTIR results for the modifications of adsorbent materials at various parameters. It was evident that the pre-treatment with MnO species and then plasma treatment had significant variations in the vibrational stretching of the functional groups on the adsorbent material. Notably, the FTIR spectrum exhibited intense peaks after plasma treatment in comparison to only MnO coating owing to the -OH group stretching vibrations that appeared in the range of 3800 –3400 cm⁻¹, suggesting the introduction of hydroxyl (-OH) groups onto the adsorbent surface that was accredited to the improved adsorption of Cu(II) and Cd(II)²⁹. Secondly, C = O stretching vibrations detected in the range of 2350 –2200 cm⁻¹ are allegedly responsible for increasing the adsorption capacity of the modified ACF and the C-O group³². The appearance of ester or etheric carbonyl group (C-O) vibrations in the region of 1800 –1530 cm⁻¹ was prominent evidence in the graph indicating successful modification of the carbon skeleton structure of modified ACF by the addition of oxygen from the plasma species³³. These findings suggest that the surface modification process involving MnO and plasma treatment has successfully introduced oxygen-rich functional groups such as hydroxyl, carbonyl, and ether/ester functional groups, making the material a promising candidate for boosted up adsorption of Cu(II) and Cd(II). Based on the data in Table S1, it is evident that atmospheric pressure plasma impacts the physical characteristics of adsorbents. The raw ACF was very porous in nature, with a surface area of 836.6 m²/g; however, after plasma treatment, there was a small decrease in surface area, which can be ascribed to surface defects imparted by high-power collisional plasma molecules.

Furthermore, the MnO-ACF treated material was observed to reduce the highest amongst the group with a surface area of 588 m²/g, which is suspected to be due to the clogged pores of the porous ACF material due to deposition of MnO particles. Lastly, the modified ACF had 692 m²/g, indicating the clogged pores were exposed to high-power plasma particles, which resulted in more porous surface characteristics in the material. According to the IUPAC classification, the BET adsorption-desorption graph in Fig. 5b showed a pattern that resembled type II isotherm, which denotes multilayer adsorption that is reversible and suggests the presence of meso- and macropores in the adsorbent material³⁴. Therefore, it is certain that the plasma treatment was quite efficient in increasing surface area, presumably because of the production of surface defects and new pores. On the other hand, the MnO pre-treatment, which involves a chemical reaction, reduced the surface area.

Combining pre-treatment with MnO and plasma treatments resulted in moderate surface area, with structural modifications examined through X-ray diffraction analysis. Referring to Fig. 5c, the XRD study showed no distinct crystalline Mn species peaks before plasma treatment and MnO coating, suggesting non-crystalline manganese species in the material, but significant changes were observed after plasma treatment. The reduction of the manganese species and subsequent generation of the Mn₃O₄ and MnO crystalline phases indicated the plasma treatment promoted the successful transformation of Mn species. The highly energetic characteristics of the plasma treatment process trigger a series of chemical reactions on the material surface, culminating in structural and functional changes³⁵. The MnO coating significantly enhanced the reduction of manganese species in the modified ACF material, with plasma treatment playing a crucial role in initiating the redox process. The spectrum revealed six distinct peaks at 2θ = 11.4°, 21.4°, 25.4°, 31.5°, 35.7°, 41.3°, and 57.9°. The broad peak at 21.4° corresponds to amorphous carbon and the (101) reflection of tetragonal MnO₂, while the other five peaks are assigned to tetragonal MnO₂ (JCPDS reference card number 18–0802). These results align with previous research on the impact of plasma treatment on material properties³⁶. Thus, conclusively, XRD analysis helps to gain valuable insights into the structural changes induced by the plasma treatment and MnO coating process, confirming the successful reduction and crystallisation of the Mn species.

Graphs of (a) FTIR spectra, (b) plots of BET surface adsorption and (c) XRD graph for ACF, MnO-ACF, and modified ACF.

The investigations conducted by FE-SEM showed invaluable insights into the morphological changes of the ACFs under various treatments. As seen in Fig. 6a,b, it can be observed that the surface of raw ACF exhibited a remarkably smooth surface, which revealed a pristine, unaltered structure characterised by a lack of surface irregularities. In the next stage, upon coating the ACF with MnO (MnO-ACF), there was a noteworthy transformation observed from the smooth surface coating; there was a smooth formation of spherical spheres onto the surface, as observed in Fig. 6c,d. The deposition of MnO significantly alters the surface chemistry of ACF, resulting in a distinct change in surface morphology after plasma treatment. Also, it was found that small pits and broken surfaces on modified ACF surfaces were due to plasma’s disruptive effect, as shown in Fig. 6e,f. Pre-treatment with MnO and plasma resulted in significant modifications to the surface morphology of the fibre. As shown in Fig. S1, EDX mapping revealed the elemental composition of the treated fibre, confirming the presence of carbon (75.84%), nitrogen (4.78%), oxygen (11.39%), and manganese (7.98%). These changes highlight the impact of the pre-treatment on the material’s surface properties³⁷. Conclusively, these FE-SEM results underscore the dynamic changes in ACF morphology throughout the experimental process. The transformation from a smooth surface to the formation of evenly coated spherical spheres of MnO molecules that were exposed to the high-power disruptive effects of plasma resulted in the subsequent surface reduction of MnO₄ molecules, which reformed into oxygen-rich species within the surface layers.

SEM image of (a, b) raw ACF, (c, d) MnO ACF and (e, f) modified ACF.

Impact of pH

The impact of pH on the adsorption capabilities of Cu(II) and Cd(II) using modified ACF is depicted in Fig. 7a. The raw and modified ACF’s pHzpc (zero-point charge) is shown in Fig. S2. After plasma surface modification, the modified ACF from pH 2 to pH 8 has a positive surface charge, which modulates ion exchange and electrostatic interaction^38,39. pHzpc 8 facilitates species adsorption through both positive and negative surface charges, whereas between 4 and 6 pH, there is competition between protons and metal ions, increasing affinity for Cu(II) and Cd(II). Thus, pH 5 is optimal for metal removal due to abundant active sites created by oxygen-rich functional groups^1,38,40. At pH levels above 5, precipitation dominates, reducing metal ion adherence to adsorbent.

Conversely, pH levels below 5 show intense competition between H⁺ and Mn²⁺ species, reducing adsorption effectiveness. The electrostatic attraction between Cd(II)/Cu(II) and Mn²⁺ decreases surface positive charge, reducing metal repulsion. High solution pH strengthens this attraction, decreasing adsorption efficiency. In conclusion, pH greatly influences the optimal adsorption condition at pH 5, while lower and higher pH values lead to decreased efficiency due to competition with H⁺ ions and metal ions and precipitation, respectively.

Batch adsorption studies for 20 mg/L of Cu(II) and Cd(II) on 0.5 g/L dose of modified ACF for 720 min, pH 5, at 298 K respectively; (a) pH studies; (b and c) Contact time studies; and (d and e) Isotherm modelling plots between the concentrations of 10–200 mg/L (f) Vant-Hoff’s plot.

Contact time and initial concentration effect

Adsorption kinetics explain the solute’s rate of adsorption as well as the adsorbates’ duration of residence on the solid-liquid interface. These offer perceptions of the adsorption process’s dynamics. Popular kinetic models are pseudo-first-order (PFO), pseudo-second-order (PSO), and Weber-Morris intraparticle diffusion (IPD) models^39,41. PFO implies that adsorption sites are occupied in proportion to empty sites, PSO chemisorption includes electron exchange, and the Weber-Morris intraparticle diffusion model looks at rate-limiting processes.

The adsorption kinetics for various ACF treatments are shown in Fig. S3. The most effective method involves sequential modification of KMnO₄, pre-treatment and plasma surface modification, while sole KMnO₄, dipping, or plasma treatment alone is less effective. The combined treatments (KMnO₄ + plasma) improve the adsorption efficiency for Cu(II) and Cd(II) due to embedding functional groups on carbon surfaces, validating the dual-modification process for heavy metal removal in water remediation. Thus, Fig. 7b,c shows the results of an investigation into the adsorption kinetics of Cu(II) and Cd(II) onto modified ACF utilising different contact times. The study reveals the mechanisms of adsorption and the influence of Mn²⁺ ions on the adsorption process. The graph’s trend shows that up until 600 min, the occupancy of the active adsorption sites grew; afterwards, it seemed to be gradually reaching equilibrium. Thus, the anticipated equilibrium time was fixed to be 720 min for further studies.

Furthermore, three models were employed to forecast the adsorption’s kinetics. Cu(II) and Cd(II) adsorption on modified ACF was found to fit the pseudo-second-order (PSO) model, revealing a chemisorption process that contributes to the emergence of strong chemical interactions between the metal ions and the ACF external surface, as shown in Table 1. The obtained Q_e values are closely related to theoretical Q_e values, and the R² value is less than 1^41,42. The Weber-Morris model of intraparticle diffusion (IPD) revealed the involvement of electron transfer and formation of stable bonds between modified ACF and contaminant metals, supporting the assumption that Mn²⁺ ions on the modified ACF surface can act as binding sites, promoting rapid chemisorption through the formation of complexes with metal ions^43,44. Although the IPD model predicts that adsorption occurs mostly within the pores, the presence of Mn²⁺ ions provide more active sites and facilitates metal ion accessibility to the modified ACF intraparticle pores, enabling faster adsorption. This all indicates the creation of strong bonds during the chemisorption process due to the Mn²⁺ ions, which contributes to a better removal efficiency of Cd(II) and Cu(II)^38,40,45,46. The adsorption process is further aided by the rapid intraparticle diffusion within the modified ACF, which allows efficient usage of the surface area available. Conclusively, the findings of this investigation amply validate that the adsorption of Cu(II) and Cd(II) onto modified ACF conforms to the pseudo-second-order model, signifying chemisorption accompanied by the creation of strong bonds. The presence of Mn²⁺ ions on the modified ACF surface serves as active binding that promotes access within the intraparticle pores, enhancing adsorption through intraparticle diffusion, as confirmed by the IPD model.

Figure 7d, e reveals essential adsorption isotherms that determine interactions between adsorbent and adsorbate, optimal adsorption capacity, and molecule distribution. Common models are Langmuir, Freundlich, and Temkin isotherms^47,48. The Langmuir model assumes monolayer adsorption on a surface with identical sites, leading to a hyperbolic isotherm. The Freundlich model describes adsorption on heterogeneous surfaces with varying affinities, resulting in a logarithmic isotherm. The Temkin model accounts for interactions between adsorbate and adsorbent, assuming a linear decrease in the heat of adsorption with coverage, resulting in a linear isotherm at moderate concentrations⁴⁹.

Table 2 shows that the Q_max maximum adsorption capacities of Cu(II) and Cd(II) on modified ACF were 163.39 and 214.59 mg/g, respectively. The study reveals substantial metal ion adsorption capacity, with Mn²⁺ species creating more active sites for binding, following the Langmuir model, but with potential heterogeneity and secondary forces. The modified ACF surface may have influenced adsorption, but reduced R² values suggest adsorption heterogeneity. The Langmuir model fits experimental data, while Freundlich predicts favourable adsorption⁵⁰.

The Temkin isotherm model reveals surface heterogeneity and energetics of the adsorption process. The equilibrium binding constants (K_T (L/mg)) for Cu(II) and Cd(II) were 24.05 and 101.99 respectively, indicating the modified ACF’s significant affinity for the metal ions. The parameter B (J/mol), representing heat of adsorption, was 13.99 for Cd(II) and 1.846 for Cu(II), suggesting different levels of adsorption strength. These findings support the assumptions of the Temkin model, which reasonably describes the adsorption process for both metal ions on the modified ACF. The Langmuir and Temkin adsorption models explain adsorption behaviour but are not as effective as the Freundlich models in modifying ACF’s diverse adsorption affinity⁵¹. Recent studies on the adsorption of Cu(II) and Cd(II) from aqueous solutions have investigated various adsorbents, as summarised in Table 3. Among them, the modified ACF exhibits heterogeneous adsorption affinity due to its diverse functional groups. This characteristic highlights its versatility in removing heavy metals, particularly Cu(II) and Cd(II), under varying pH and dosage conditions.

Thermodynamical aspect of adsorption

The thermodynamics of the adsorption process for Cu(II) and Cd(II) utilising modified ACF were investigated and are plotted in Fig. 7f using the standard Gibbs free energy change (G°), enthalpy change (H°), and entropy change (S°), insights into spontaneity, heat effects, and change in disorderliness with rising temperature were assessed. The thermodynamic parameters for Cd(II) and Cu(II) adsorption on modified ACF were calculated and shown in Table S2. ΔG° values ranged from − 32.35 to -38.04 kJ/mol for Cd(II) and from − 30.55 to -33.88 kJ/mol for Cu(II). The negative values indicate spontaneous adsorption, supported by similar findings with MnO₂ and the presence of active sites, including the Mn²⁺ ions, on the ACF surface, enhancing metal ion binding^52,53.

The positive ΔH° values of 52.64 kJ/mol and 19.02 kJ/mol for Cd(II) and Cu(II) respectively, on modified ACF indicate an endothermic process where environmental energy is involved^54,55. Heat energy overcomes energy barriers, and Mn²⁺ ions enhance strong bond formation and chemisorption, contributing to the reaction’s endothermic nature. Adsorption onto modified ACF shows an increase in disorder, indicated by positive ΔS° values for the Cd(II) = 286.03 J/K.mol and Cu(II) = 166.53 J/K.mol. Various mechanisms, including chemisorption and complex formation, contribute to this disorder. Positive ΔS° readings demonstrate increased entropy, resulting from faster movement of Cu(II) and Cd(II) ions at higher temperatures. This implies higher mobility and entropy during adsorption⁵¹.

Co-existing ions studies

The influence of various ions on Cu(II) and Cd(II) adsorption onto modified ACF was investigated. Specifically, the influence of cationic groups (potassium, calcium, magnesium, and sodium) and anionic groups (nitrates, sulfates, phosphates, and chlorides) on the adsorption efficiency of Cu(II) and Cd(II) was scrutinised. Anionic co-existing species impacted the removal of Cu(II) and Cd(II), as shown in Fig. S4a, b. Anionic species, particularly phosphates, negatively affected the adsorption of Cu(II) and Cd(II). The negative charge of phosphate groups can form complexes with metal ions, reducing their availability to bind to active sites on the modified ACF surface. As shown in Fig. S4c,d, different cations negatively affected adsorption. Mg(II) and Ca(II) had a greater impact on Cd(II) adsorption than Cu(II). Magnesium and calcium, divalent cations, compete with negatively charged surfaces for adsorption sites, inhibiting Cd(II) adsorption. Their higher charge density reduces electrostatic attraction, resulting in lower adsorption capacity¹⁰. The results emphasise the significance of the presence of phosphate groups and divalent cations, particularly Mg(II) and Ca(II), which have a substantial implication on the efficiency of adsorption by decreasing the number of available adsorption sites and interfering with the electrostatic interaction between the metal ions and the surface of the modified ACF, thereby influencing the adsorption process of Cu(II) and Cd(II).

Regeneration and recyclability studies

Traditional methods like rotation and centrifugation have limitations in mass transfer during recycling processes. Sonication offers faster transfer, efficient reactions, and improved dispersion, accelerating recycling studies⁵⁶. Thus, regeneration of adsorbed sites of modified ACF through sonication was used for effective adsorption. The study yielded results shown in Fig. S4e–f pointing that regeneration can be achieved by sonicating 3% nitric acid and rinsing with distilled water, restoring adsorption sites on modified ACF, and accelerating metal ion removal by disrupting adsorbed species and facilitating their release from the ACF matrix⁵⁷. Figure S4f shows the test results for different time intervals. Thus, 15 min of sonochemical disruption effectively regenerated and desorbed Cu(II) and Cd(II) ions from modified ACF⁵⁸. As observed from Fig. S5, the modified ACF was washed with distilled water to neutralise the pH and ensure optimal adsorption performance for over 5 cycles⁵⁹. Efficient desorption of Cu(II) and Cd(II) ions reveals the potential for continuous adsorption-desorption cycles, increasing sustainability and cost-effectiveness. This study concludes that with 3% nitric acid, sonication, and washing, the modified ACF can be reused for up to 5 cycles by breaking metal adsorbate bonds and releasing Cu(II) and Cd(II) from the surface.

Column studies

To assess breakthrough performance, the modified ACF’s adsorption capability was tested using three models (Thomas, Adams-Bohart, and Yoon-Nelson). The Thomas model was used to analyse the breakthrough curves during adsorption. Fig. S5a,b shows a strong fit and provides insights into equilibrium-driven processes and adsorption kinetics. Table 4 presents important parameters: rate constant (K_Th) for Cd(II) was 0.023 and for Cu(II) was 0.0220 (L/mg.day), and adsorption capacity (Q_Th) for Cd(II) was 0.217 mg/g and for Cu(II) was 0.233 mg/g. The Thomas model confirms the reversible nature of the adsorption process and accurately assesses the adsorption performance of the modified ACF in the column configuration without axial dispersion⁶⁰.

As seen in Fig. 8a,b, the Thomas model showed that the adsorbate’s adsorption process depended on initial concentration, contradicting the Adams-Bohart model. This suggests that factors other than adsorbent concentration and capacity control the adsorbent rate. Additionally, the Yoon-Nelson model revealed information about the interaction between the adsorbate and adsorbent, including internal and external mass transfers⁶¹. Fitting this model to breakthrough curves provided insights into the dynamic behaviour of the adsorption process.

(a and b) Column modelling data; after adsorption graphs (c) XRD analysis (d–i) FE-SEM images of Cd(II) and Cu(II) onto modified ACF respectively.

The 50% breakthrough time for adsorption was found to be 46 days and 51 days for Cd(II). and Cu(II), respectively. Table 4 presents the data of the breakthrough curve evaluation indicated by the equation variable and the sum square error for each model. Thomas and Yoon’s models showed excellent agreement with theoretical values, which were validated by low error levels^62,63. Plasma treatment enhances the adsorption efficiency of ACF by increasing surface area and introducing functional groups, allowing selective adsorption of contaminants like Cu(II) and Cd(II). Plasma treatment is an environmentally friendly, cost-effective, and sustainable alternative to traditional hazardous chemicals, potentially leading to advancements in adsorption technology.

Characterisations for after adsorption of Cu(II) and cd(II)

Figure 8c shows a decrease in peak intensity of Mn³⁺ and Mn²⁺ after adsorption of Cu(II) and Cd(II), indicating their role in the adsorption process. MnO coating on ACF substrate confirmed by XRD analysis, demonstrating adhesion of Mn species⁴⁵. The FE-SEM analysis shown in Fig. 8d–e elucidated prominent surface changes on the ACF after adsorption. Images of the modified ACF treated with MnO-PT revealed a relatively homogeneously coated layer of MnO and pits filled with MnO₂. Figure 8f–i indicates the formation of solid scales on the surface, indicating a complex interaction between the modified ACF and Cu(II) and Cd(II) ions. These scales affect solution pH during adsorption, causing chemisorption or surface precipitation. Modified ACF surface exhibits metal oxide precipitation, phase transition, and co-precipitation, enhancing adsorption capacity.

The XPS survey peaks in Fig. 9a present several key findings. The peaks of Cu(II) and Cd(II) confirm the successful binding of heavy metals to the modified ACF. The peaks of Mn3p, Mn3s, Mn2p, C1s, and O1s also indicate the involvement of these elements in the adsorption process. Deconvolution of the C1s + K2p spectra in Fig. 9b reveals sharp peaks at ~ 284 eV sp²C and ~ 285 eV sp³C, suggesting different carbon states. The peak at ~ 287 eV indicates the presence of C = O, while the peaks at ~ 293-296 eV confirm the presence of potassium (K) from KMnO₄, which reduces after adsorption. The C1s spectral peaks show a decrease in the functional groups C = O and sp²C, indicating their involvement in chemical adsorption. In the case of Cu(II) adsorption, hydroxide formation suggests the precipitation of Cu(OH)₂, aligning with surface complexation. Deconvolution of the O1s spectra in Fig. 9c shows distinct peaks corresponding to various oxygen-containing functional groups, including Mn-O, Mn-OH, K-OH, and H-OH. The increased peak areas of Mn-O, Mn-OH, K-OH, and H-OH after Cd(II) and Cu(II) adsorption indicate the involvement of Mn and K species, as well as hydroxyl groups, in enhancing the adsorption capacity³⁰. Figure 9d reveals the dominance of Mn^+ 4 and Mn²⁺ states in the adsorption process. Overall, the adsorption mechanism involves sp²C, C = O, Mn^+ 4, and Mn²⁺ in surface complexation and precipitation. Conclusively, Fig. 10 represents the adsorption mechanism of Cu(II) and Cd(II) onto the modified ACF, which is suggested to entail a complex interplay of ion exchange, co-precipitation, H-bonding, and complexation processes. This is corroborated by the high Q_max values, extended equilibrium time, and active involvement of MnO and oxygen-rich species observed in the XPS, XRD, and FE-SEM analyses⁶⁴.

XPS analysis of modified ACF (a) survey peaks of modified ACF before and after adsorption of Cu(II) and Cd(II); deconvoluted peaks of (b) C1s + K 2p; (c) O1s and (d) Mn 2p.

Anticipated Cu(II) and Cd(II) adsorption mechanism onto modified ACF.