Introduction

From green and sustainable chemistry perspectives, researchers have made many efforts to formulate an efficient and economical chemical process using heterogeneous sorbent for treated water (Alavinia et al. 2023; Heidari et al. 2023; Izadkhah et al. 2023; Rahimi et al. 2021; Gharehkhani et al. 2021). In recent years, chemical contaminants in aqueous solutions have become a major concern due to their potentially harmful effects on living organisms (Tizro et al. 2019). Among the chemical pollutants, fluoride is one of the water-soluble ions that enters the water from natural sources such as minerals, geochemical sediments, and products (Bhaumik et al. 2012). Nitrates are another pollutant for human health in wastewater that enters water through farm animals, chemical fertilizers, and water produced by food-producing plants. Water contaminated with nitrate easily passes from soil to soil (Bishayee et al. 2022). Therefore, it is necessary to develop effective methods for the removal of fluoride and nitrate from aqueous solutions.

Conventional water and wastewater treatment methods are ineffective at completely removing chemical contaminants and can only reduce their concentration in water (Couto et al. 2019). This failure is due to the nature of these compounds, as they are mostly known to be non-biodegradable products. Reverse osmosis (RO), chemical oxidation, coagulation, and their combinations are effective candidates for treating polluted water (Samokhvalov 2020; Ahmadi et al. 2022). Chemical and biological methods, as more efficient than the physical ones, still suffer serious challenges, including the need for high-cost equipment, producing by-products, and using a high amount of energy (Daraei et al. 2019). In this respect, the adsorption method seems an efficient alternative to deal with undesired drawbacks of biological and chemical techniques. Therefore, synthesizing a cost-effective adsorbent for removing hazardous pharmaceutical active compounds is of great necessity.

Adsorbents are materials that can remove contaminants from water through surface adsorption. Agricultural residues, such as oak fruit, are a potential source of adsorbents due to their high carbon content (Zhang et al. 2013). The synthesis of activated carbon-based nanocomposites from agricultural residues can be a practical method in green chemistry (Babamoradi et al. 2021). Activated carbon-based nanocomposites have received much attention in several applications, including heterogeneous catalysis, gas storage, separation, toxic chemical removal, and drug delivery systems (Gao et al. 2020; Li et al. 2022; Chen and Feng 2022). Activated carbon is widely used as an effective adsorbent for removing pollutants from water. However, high cost and limited availability of this material have led researchers to explore using natural organic adsorbents such as biomasses (Marandi et al. 2021; Oyekunle et al. 2022; Prasetya et al. 2023). In this respect, activated carbon modification with inorganic material has been suggested as an efficient technique to enhance compatibility, reactivity, and physicochemical properties (Halder et al. 2018; Nasrollahzadeh et al. 2021; Taghizadeh and Tahami 2022). Also, carefully choosing the source and synthesis conditions is critical in activated carbons’ electronic, chemical, and topological properties (Ghosh et al. 2023). Furthermore, activated carbon modification with ammonium chloride leads to novel active sites, thereby improving the adsorption capacity of activated carbon.

In this study, we evaluated the removal efficiency of fluoride/nitrate from aqueous solutions using modified carbon of oak fruit. In this regard, we design and characterize a new heterogeneous sorbent consisting of carbon of oak fruit and ammonium chloride (COF/NH4Cl). Overall, using modified carbon oak fruit as natural organic adsorbents shows promising results for removing fluoride/nitrate from aqueous solutions. The findings of this study provide important insights into the development of cost-effective and environmentally friendly methods for the removal of fluoride/nitrate from aqueous solutions. Further research is needed to optimize the adsorption conditions and to evaluate the feasibility of using modified carbon of oak fruit for the removal of other contaminants from aqueous solutions. The process presented in this study is a green strategy, where the reactions takes place in a short time. As the sorbent consists of crushed carbon of oak fruit, the sorbent becomes more eco-friendly, non-toxic, and biodegradable.

Materials and methods

In the current study, all reagents were purchased from Sigma-Aldrich (Spain) and Merck (Germany). Oak fruit sorbent was used as an adsorbent, and ammonium chloride, zinc chloride, and phosphoric acid were utilized for optimization and the adsorbent efficiency in removing fluoride/nitrate from synthetic aqueous solutions was examined. The powder X-ray diffractometry (XRD) patterns were obtained using an X'Pert Pro Panalytical diffractometer applying a 30 mA current and a 40 kV voltage with Cu-Kα radiation (λ = 1.5418 Å). The morphology and size of the prepared nanosorbents were investigated using SEM images obtained from the FESEM-TESCAN MIRA3 instrument. Nanosorbents' chemical composition was determined by EDX using SEM analysis. Prior to the surface area analysis, the samples were activated in a high vacuum at 80 ℃ for 12 h. All adsorption and desorption measurements were performed on a Micromeritics TriStar 3020 version 3.02 (N2) system and measured at 77 K. The pore size distributions were calculated from the adsorption–desorption isotherms. Wavelength-dispersive X-ray spectroscopy (WDX) was performed using a TESCAN mira3.

Synthesis of modified carbon of oak fruit sorbent (COF/NH4Cl)

The crushed carbon of oak fruit (COF) was placed inside the furnace at 100 °C for 24 h. For carbonization, the materials prepared from the previous step were heated to 600 °C for 1 h in the oven. After carbonization, ammonium chloride (1:1) was used for activation. To this end, 4 and 4 g of these chemicals were separately dissolved in distilled water (20 Ml), and then the prepared carbon material (4.0 g) was added to the solutions. The as-synthesized solution was sintered at 900 °C for 2 h. Finally, the resultant suspension was cooled, filtered, and washed with excess ethanol and water. The obtained solid material was dried at 100 °C for 2 h and used for adsorption (Scheme 1).

Scheme 1
scheme 1

General procedure for modified carbon of oak fruit sorbent (COF/NH4Cl)

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Design of experiment

Choice of Materials: Before optimizing the conditions, the carbon of oak fruit sorbent and modified carbon of oak fruit (COF/NH4Cl) were investigated in the same conditions (m = 100 mg; extraction time = 200 min) for pollutant adsorption. The COF/NH4Cl exhibited the highest adsorption efficiency among the studied sorbents. The functionalized solid COF/NH4Cl maintains sufficient porosity and stability to act as an excellent sorbent. The functionalization changes the properties favorably. Therefore, the COF/NH4Cl was selected for the next experiments. The pollutant concentration was measured by spectrophotometric analysis using UV/Vis spectrophotometer DR-5000 device manufactured by JASCO Co (Japan). Furthermore, the adsorption capacity and regeneration are calculated using Eqs. (1) and (2), respectively:

$$q = \frac{{\left( {C_{0} - C_{e} } \right) \times V}}{m}$$
(1)
$${\text{Re}} {\text{generation }}\% = \frac{{\left( {C_{0} - C_{e} } \right)}}{{C_{0} }} \times 100\%$$
(2)

Results and discussion

Characterization

Figure 1 presents the FT-IR absorption spectra of COF and COF/NH4Cl. The stretching vibrations at 3400/cm indicate the symmetric modes of the O–H bonds (Fig. 1a) (Babamoradi et al. 2022). The stretching vibrations of C–O were observed at 1054/cm. Comparing spectra of carbon of oak fruit (Fig. 1a) with modified COF/NH4Cl (Fig. 1b) confirmed the successful functionalization. The broad absorption band at 3420–2900/cm is related to N–H and O–H groups. Also, the absorption band at 1104/cm is related to the stretching vibrations of ammonium groups. These results showed that the ammonium chloride species was connected to the COF. In the FT-IR of spectrum of COF/NH4Cl, because of overlapping, slight shifts in peak positions, variations in peak intensities were detected.

Fig. 1
figure 1

FT-IR spectrum of the COF (a) and COF/NH4Cl (b)

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Figure 2 displays the XRD pattern for the carbon of oak fruit and modified carbon of oak fruit (COF/NH4Cl). The figure shows two main diffraction peaks at 23.7° and 42.3° in the 2θ range from 10° to 90°, which are the characteristic peaks of activated carbon. The crystalline peaks are observed in both materials. These peaks are mainly due to the presence of the diamond structure of carbon materials (Fig. 2a). The presence of new diffraction peaks at 28.3°, 36.1°, and 40.5° at COF/ NH4Cl composite is related to the ammonium chloride species which is in good agreement with the Joint Committee on Powder Diffraction Standards (JCPDS) card no. 96-221-5686 (Fig. 2b). XRD analysis shows no specific change in the morphology of the material after functionalization of the activated carbon. The XRD analysis is used to detect crystalline and amorphous materials. Likewise, according to the comparison of the spectra, it can be observed that the presence of peaks in 2θ of 38° and 41° confirms the presence of NH4Cl functional group.

Fig. 2
figure 2

XRD spectrum of the COF (a) and COF/NH4Cl (b)

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The morphology of COF and modified COF (COF/NH4Cl) was confirmed through SEM analysis. SEM image of COF is presented in Fig. 3a and b, showing the well-dispersed mesoporous structure of the synthesized COF.

Fig. 3
figure 3

FESEM images of COF (a and b)

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The SEM image of the COF/NH4Cl sorbent exhibits the rough and mesoporous morphology, suggesting the uniform immobilization of ammonium chloride species on the COF surface. SEM analysis shows no specific change in the morphology of the material after the functionalization of activated carbon (Fig. 4a and b).

Fig. 4
figure 4

FESEM images of COF/NH4Cl (a and b)

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The elemental analysis of the prepared COF is presented in Fig. 5. Also, the elemental mapping of the synthesized COF/NH4Cl sorbent is depicted in Fig. 6. As can be seen, C, O, K, N, Mg, P, and Cl elements are present in the structure of synthesized support. The elemental analysis of the COF/NH4Cl is presented in Fig. 7. In addition, Fig. 8 illustrates the elemental mapping of synthesized sorbent. Based on this figure, C, Zn, C, Fe, Mg, Cl, P, Zn, and O elements are present in the structure of the synthesized adsorbent. In this sense, the ammonium chloride distribution on the COF surface greatly influences the adsorption efficiency by increasing the active sites for electrostatic interaction. Therefore, the results obtained with the proposed method confirm those obtained with the EDX analysis.

Fig. 5
figure 5

Energy dispersive X-ray analysis (EDX) of the synthesized activated carbon (COF)

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Fig. 6
figure 6

Elemental mapping of the synthesized activated carbon (COF

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Fig. 7
figure 7

Energy dispersive X-ray (EDX) analysis of the COF/NH4Cl sorbent

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Fig. 8
figure 8

Elemental mapping of the COF/NH4Cl sorbent

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The porosity of the COF and COF/NH4Cl was measured using the N2 adsorption/desorption isotherms at 77 K. The results showed a type-IV isotherm (because of the mesoporous materials) and type-H3 hysteresis loops (defined by IUPAC). The surface areas for fresh carbon (Fig. 9a) and modified COF/NH4Cl (Fig. 9b) were found to be 159.62 and 20.03 m2/g, respectively. Functionalization with only ammonium chloride changed the surface area slightly. In this respect, the porosity of materials plays a key role in ammonium chloride adsorption. The BET results indicate the arrival of ammonium chloride on carbon by bonding it with the surface carbon or the surface functional groups. The results of BET analysis are presented in Table 1. Furthermore, according to the shape of BJH for the COF/NH4Cl, most of the pores are 4.6 nm in size.

Fig. 9
figure 9

Nitrogen adsorption–desorption isotherm and BJH of mesoporous COF (a) and COF/NH4Cl (b)

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Table 1 Results of the Langmuir and BET measurements of A) COF, B) COF/NH4Cl
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Adsorption statistics

Adsorption kinetic

Table 2 summarizes the kinetic model parameters, which are fitted to the pseudo-first-order and the pseudo-second-order kinetic models. Moreover, Eqs. (3) and (4) express the pseudo-first-order and the pseudo-second-order kinetic models, respectively.

$$q_{{{t}}} = q_{{{e}}} \left( {1 - {\text{exp}}\left( { - k_{1} t} \right)} \right)$$
(3)
$$q_{{{t}}} = {{q_{{{e}}}^{2} k_{2} t} \mathord{\left/ {\vphantom {{q_{{{e}}}^{2} k_{2} t} {(1 + q_{{{e}}} k_{2} t)}}} \right. \kern-0pt} {(1 + q_{{{e}}} k_{2} t)}},$$
(4)

where \({q}_{t}\) and \({q}_{e}\) stand for the adsorption capacity of the pollutant on the surface of the adsorbent at \(t\) and at equilibrium time, respectively. Moreover, \({K}_{1}\) and \({K}_{2}\) show equilibrium constants for pseudo-first-order and pseudo-second-order kinetic models, respectively.

Table 2 Kinetic parameter of fluoride and nitrate adsorption
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The accuracy of the prepared models was examined using the coefficient of determination (\({R}^{2}\)). Although both have the same \({R}^{2}\), the pseudo-second-order kinetic model fits the pollutant adsorption results more precisely. The average relative error is low for the pseudo-second-order kinetic model, and the maximum adsorption capacity obtained from the model is in acceptable agreement with that obtained from the Langmuir isotherm.

The fitting of the adsorption kinetics is shown in Fig. 10. The figure presents the linear diagram of the pseudo-first-order for the pollutant adsorption model on COF/NH4Cl. Based on the results (Fig. 10), the pseudo-second-order kinetic model gives a straight line with a high R2 of 0.990.

Fig. 10
figure 10

Pseudo-second-order kinetics fitting

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Based on Fig. 11, the equilibrium time for nitrate (Fig. 11a) and fluoride (Fig. 11b) can be reported as 90 and 30 min, respectively. Moreover, the absorption capacity of nitrate and fluoride at these times was 26.53 and 20.00 mg/g, respectively.

Fig. 11
figure 11

Experimental points and fitted curves from pseudo-second-order kinetic model for nitrate (a) and fluoride (b) (reaction conditions: amount of adsorbent, 100 mg, reaction time: 200 min, and initial concentration: 50 mg/L in 100 mL)

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Adsorption isotherm

The adsorption capacity on the surface of the COF/NH4Cl surface was investigated to understand the possible effect of π–π stacking, acid–base interaction, electrostatic forces, and hydrogen bonding. The Langmuir, Freundlich, and Dubinin–Radushkevich models were investigated. The models and their formulas are listed in Table 3, respectively.

Table 3 Isotherm models
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Comparing the R2 values in Table 4 reveals that the Langmuir and Redlich–Peterson isotherm models are more compatible with the experimental data. This result suggests a further closure of the pores of adsorbent structure with the adsorption and a reduced effect of secondary factors on the adsorption. As the \(\beta\)-value in the Redlich–Peterson equation is close to 1, the Langmuir isotherm can better fit the adsorption results of nitrate and fluoride. In the Langmuir isotherm, the constant \({K}_{L}\) value indicates the adsorption power of the adsorbent. The maximum value of this constant for catalyst was 0.015 and 0.142 L/mg for fluoride and nitrate, respectively. Other isotherms, including the Freundlich and Temkin models, are further being investigated. However, the low correlation coefficients indicate a non-significant fit with experimental data. Based on the mentioned points, the maximum adsorption capacity (\({q}_{\mathrm{max}}\)) in the Langmuir model for fluoride and nitrate via the as-prepared ternary composite was 243.9 and 38.0 mg/g (Figs. 12a and 13a). Furthermore, the Freundlich model fits with the experimental equilibrium data (Figs. 12b and 13b). The results exhibited that both Langmuir and Freundlich isotherms efficiently represented the adsorption data regarding their high adjusted-R2 of 0.980 for Freundlich models. Hence, the Freundlich model was selected to represent experimental data on pollutant removal with COF/NH4Cl.

Table 4 Isotherm parameters for fluoride and nitrate adsorption
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Fig. 12
figure 12

Experimental point and fitted curves from the Langmuir isotherm (a) and Freundlich isotherm (b), and Dubinin–Radushkevich Temkin (Reaction conditions: amount of adsorbent, 100 mg, reaction time: 150 min for nitrate, and initial concentration: 50 mg/L in 100 mL)

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Fig. 13
figure 13

Experimental point and fitted curves from the Langmuir isotherm (a) and Freundlich isotherm (b), and Dubinin–Radushkevich Temkin (Reaction conditions: amount of adsorbent, 100 mg, reaction time: 200 min for fluoride, and initial concentration: 50 mg/L in 100 mL)

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As shown in Fig. 14, the removal percentage of nitrate from 32.00 to 14.09 and the fluoride removal percentage from 32.0 to 11.0 decreased with an increase in ionic strength from 20 to 100 mg/L of calcium ions.

Fig. 14
figure 14

Effect of ionic strength on the removal efficiency from the Langmuir isotherm (a) Freundlich isotherm and (b) and Dubinin–Radushkevich Temkin (reaction conditions: the amount of adsorbent, 100 mg, reaction time: 200 min for nitrate, and initial concentration: 50 mg/L in 100 mL)

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The pH effect

The pH has a significant effect on the absorption of acetaminophen and ibuprofen. As shown in Fig. 15, the pH effect on pollutant adsorption was estimated to be in the range of 3–9. The results showed that the adsorption efficiency of the fluoride/nitrate on the synthesized sorbent decreased with increasing pH. As can be observed, the absorption rate of nitrate increased from 15.01 to 28.74 mg/g, and the absorption rate of fluoride decreased from 24.5 to 10.5 mg/g (Fig. 15a). Moreover, the capacity of the adsorbent used decreases with increasing pH for fluoride adsorption (Fig. 15b). The result can be related to the acidic nature of the solution containing the pollutant and the charges on the adsorbent surface due to its ionization.

Fig. 15
figure 15

pH effect on the removal efficiency for nitrate (a) and fluoride (b) (Reaction conditions: amount of adsorbent, 100 mg, reaction time: 200 min, and initial concentration: 50 mg/L in 100 mL)

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Different temperatures were investigated to obtain the appropriate solution temperature on the absorption rate (15, 25, 35, and 45 °C). As shown in Fig. 16, the temperature has little effect on the absorption efficiency. The adsorption capacity of the adsorbent decreased as the solution temperature increased from 15 to 45 °C. The absorption rate of nitrate decreased from 67.23 to 30.03 mg/g, and the absorption rate of fluoride decreased from 75.00 to 20.00 mg/g. The thermodynamic parameters resulting from the effect of temperature on the absorption rate of acetaminophen and ibuprofen are provided in Table 5.

Fig. 16
figure 16

Thermodynamics parameters

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Table 5 Thermodynamic parameters of the adsorption process
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Thermodynamic analysis

The enthalpy change (ΔH°), the Gibbs free energy change (ΔG°), and the entropy (ΔS°) are the main thermodynamic parameters of thermodynamic analysis. The ΔG° is a criterion to show whether a process is spontaneous, with its negative values indicating the presence of a spontaneous process. ΔH° illustrates the difference between chemical and physical processes and the endothermic or exothermic nature of the adsorption. Finally, ΔS° is a measure of changes in the adsorbent surface due to the observed reversibility changes. These parameters are measured in Eqs. (5) and (6):

$$\Delta G = - {\text{RT}} \ln K_{C}$$
(5)
$$\Delta G = \Delta H - T \Delta S$$
(6)
$$K_{C} = \frac{{q_{e} }}{{C_{e} }}$$

Thermodynamic experiments were conducted at four different temperatures of 283, 293, 303, and 313 K to study temperature influence on the fluoride/nitrate adsorption into COF/NH4Cl. Table 4 gives the thermodynamic parameters obtained and their estimated R2. Examining the fluoride results demonstrated that ΔG° ranged from 10 to 37.5 kJ/mol under constant reaction conditions, and the corresponding ΔH° and ΔS° values of 0.212 kJ/mol and 63,548.1 J/mol/K were obtained, respectively. Studying nitrate results demonstrated that ΔG° ranged from 15.01 to 33.61 kJ/mol under the constant reaction conditions, and the corresponding ΔH° and ΔS° values of 39.9 kJ/mol and 131.3 J/mol/K were obtained, respectively. The positive values of ΔH° indicate that the adsorption was processed as an endothermic phenomenon. In the present study, the ΔH value shows that the adsorption is of the chemical type. The increasing value of ΔG° with temperature rise shows a descending trend in the feasibility degree of fluoride/nitrate adsorption.

Functional groups containing oxygen and nitrogen molecules (e.g., amine, hydroxyl, and C=N) are important coordinating ligands affecting dye adsorption and metal complexation. Also, due to strong hydrogen bonding, abundant functional groups, and electrostatic interactions, it is possible to improve the interface compatibility in this structure (Alavinia et al. 2023).

Recyclability of adsorbent

Having the ability to recycle and reuse is one of the unique and interesting features of catalysts. Accordingly, the COF/NH4Cl sorbent was separated by filtration after the reaction, washed with water and ethanol, and then dried at 80 °C for 12 h. Next, the recovered sorbent was reused. The results showed that the prepared catalyst has the ability to perform both reactions up to seven cycles without significant loss of activity. After 7 times, as the FESEM image shows, the structure of the COF/NH4Cl did not significantly change (Fig. 17).

Fig. 17
figure 17

FESEM image of COF/NH4Cl after 7 times

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Conclusion

This study synthesized a novel modified carbon sesame straw via the reaction between activated carbon made from oak fruit and ammonium chloride species. The maximum absorbing capacity is 26.00 mg/g for fluoride and 26.53 mg/g for nitrate. Freundlich isotherm and the pseudo-second-order kinetic models gave the best fit for the experimental data and adsorption kinetics, respectively. Likewise, thermodynamic studies showed that the absorption of acetaminophen and ibuprofen using activated carbon is an exothermic process. Moreover, COF/NH4Cl was recycled for seven cycles for acetaminophen adsorption without a noticeable activity loss.