Introduction

The contamination of water caused by the release of various organic compounds has emerged as a widespread problem that has been well-documented in recent years. This hazardous problem was attributed completely to the growing activities in the petrochemicals, chemical-based, agricultural, and medical sectors1,2. The impact of medicinal residues, petroleum-derived chemicals, detergents, pesticides, and dyes on human health, aquatic ecosystems, and biodiversity has recently become a crucial topic of extensive research3,4. Numerous types of pesticides that are frequently employed in agricultural practices have been identified and classified as soluble toxic chemicals in aquatic ecosystems5,6. Recent studies have categorized most pesticides as highly hazardous and carcinogenic compounds, regardless of their significant ability to enhance agricultural yields7,8. Pesticides may be categorized into a number of categories according to their chemical composition, including pyrethroids, carbamates, organochlorines, and organophosphates9.

Organophosphates (OPh) are widely used as pesticides in agriculture because of their high efficacy and affordability10,11. OPh pesticides were employed to control the growth of insects and weeds for agricultural crops such as rice, vegetables, cotton, fruits, and sorghum. They operate by hindering cholinesterase enzymes along with the function of acetylcholine esterase. This enables control of the growth of pests throughout the cycle of cultivation of these crops12,13. The chemical constituents of OPh insecticides possess potent toxic and tumor-causing characteristics, particularly whenever they exist as water-soluble chemicals in both groundwater and surface water sources4,14,15. Moreover, the OPh chemicals display excellent chemical stability with half-life range from 1.5 to 15 days in addition to their considerable water solubility which increase their risk potentiality16,17. Chlorpyrifos (CPS) is a widely used insecticide that falls within the OPh group. It is often used in agriculture to manage pests in both urban and household settings18,19,20. The emergence of contaminants throughout the water supply results in the rise of several illnesses. The compound has multiple adverse effects, including immune suppression, neurobehavioral illnesses, genotoxicity, hindering acetylcholinesterase activity, endocrine disorders, carcinogenic effects, neurotoxicity, and impaired sensory reactions18,21,22. Consequently, the US Environmental Protection Agency and the World Health Organization (WHO) recommended against using CPS at concentrations beyond 0.08 µg/L and 0.05 µg/L, respectively23,24. As a result, there is an urgent need for studies on the removal of CPS, as well as its associated leftovers and metabolites, from the environment.

Previous security and toxicity issues, in addition to OPh's remarkable ability to migrate into groundwater supplies through the soil's profile, have all served as potent warnings for the development of suitable elimination methods for these types of compounds24,25. Various techniques, such as photocatalytic degradation, membrane filtering, coagulation, and adsorption, could efficiently eliminate pesticides26,27. However, researchers currently recommend adsorption approaches to eliminate dangerous organic residues, aiming to minimize the negative effects of chemical residues released during advanced oxidation and catalytic disintegration processes28,29,30. Moreover, adsorptive removal is an inexpensive, productive, safe, accessible, and recoverable technique which is frequently used in many different industries31,32,33. Various factors, including manufacturing prices, production methods, precursor availability, adsorption efficiency, recyclability, uptake rate, biodegradability, uptake specificity, safety, long-term reliability, and reactivity, determine the selection of a suitable material for adsorbing34,35. Therefore, researchers have conducted a thorough evaluation to establish new adsorbents using widely accessible and affordable ingredients found in the earth’s resources3,36,37. Researchers strongly recommend using recently introduced adsorbents that utilize earth's resources, such as various types of minerals and rocks, as they offer significant environmental and economic benefits38.

Several recent investigations have assessed the performance and effectiveness of a variety of naturally mined coal and carbonaceous materials as adsorbents against different species of organic pollutants39,40,41. This demonstrates the capacity of coal's functional units to act as binding receptors for many different kinds of organic molecules. In mineralogy, the term "coal" refers to a type of sedimentary rock that contains a high amount of organic compounds, particularly cellulose, lignite, and resin, along with inorganic components42,43. Natural coal is a mix of aromatic polycyclic hydrocarbons mostly made up of aromatic ring structures and many oxygenated groups (carbonyl, phenolic, hydroxyl, and carboxyl) that have strong adsorption properties44,45,46. The highly reactive nature of these oxygen-rich chemical groups qualifies coal for use as adsorbents or as constituents in nanocomposites containing various inorganic and organic compounds44,46,47. Several recently published studies have applied a variety of chemical and physical approaches to enhance the surface chemistry and physicochemical properties of coal41,48. Chemical activation, demineralization, thermal activation, and metal oxide exterior hybridization was the approaches used40,41,47. Previous studies have shown that chemical or surface treatment approaches significantly enhance the properties of carbon-based materials. The techniques described previously entail the activation of essential functional groups, the incorporation of new reacting groups (particularly oxygen-rich groups), and the transformation of coal into activated carbon with a greater surface area39,47,49.

Insufficient research has been conducted on the production of polymer-based composites using coal and its derivatives, particularly with regard to their application as adsorbents. It was expected that the production of different hybrids and composites using chemically treated or physically activated coal would result in highly efficient adsorbents comprising an extensive variety of functional groups that serve as adsorption sites. Polyaniline has garnered the attention of researchers due to its appealing characteristics, including its remarkable surface area, significant stability, impressive adsorption power, non-toxic nature, inexpensive cost, and notable environmental benefits50,51. The PANI interface exhibits strong adsorption affinities for dissolved organic compounds via multiple mechanisms, including pi-pi interactions, electrostatic attraction, and hydrogen bonds52,53. Moreover, the polymeric composition of PANI comprises a large number of amine and imine chemical groups, each possessing a lone pair of electrons, which contribute to its effectiveness in adsorption54. Consequently, the use of PANI frameworks as a covering surface over various materials in previous studies led to the formation of enhanced structures that demonstrated remarkable adsorption effectiveness in the removal of common pollutants54,55. This involved the integration of it with magnetite, copper oxide, carbon nanotubes (CNTs), graphene, and zeolite51,55,56. However, the majority of these structures lack significant natural abundance are still expensive, have poor affinities for organic molecules, and need complex synthesis techniques, in contrast to the framework of coal and its derivative forms.

The combination of coal and polyaniline can lead to the development of easily manufactured, affordable hybrids with greatly improved adsorption qualities and decontamination characteristics for various organic contaminants. These hybrids can be effectively used for the elimination of pesticides, and their performance can be further improved by utilizing modified varieties of coal. As a result, the main objective of this study is to conduct comprehensive research on the activation and hybridization of natural coal with polyaniline, generating multifunctional adsorbent that possesses enhanced adsorption properties for chlorpyrifos (CPS) residues from water sources. This entails a comprehensive assessment of several coal-related materials, particularly raw coal (CA), coal-derived activated carbon (AC), coal blended with polyaniline (PANI/CA), and activated carbon blended with polyaniline (PANI/AC), implementing the essential experimental parameters. Furthermore, the effects of the various transformation procedures have been evaluated employing steric and energetic aspects determined through advanced equilibrium modeling, in accordance with the principles of statistical physics theories.

Experimental work

Materials

The studied adsorbents were synthesized by using a low- to medium-grade type of highly volatile sub-bituminous coal. This coal was promptly obtained from the El-Maghara natural coal mine ___location in Sinai, Egypt. Table S1 displays the assessment of the main composition and fundamental chemical properties of the unprocessed coal sample being studied, measured employing the ultimate-proximate chemical detection technique. The activation process and hybridization with polyaniline entailed the integration of aniline (C6H7N), NaOH pellets, methanol, ammonium persulfate ((NH4)2S2O8), HCl, ethanol, phosphoric acid (85%), and HNO3 (65%). All of these constituents had been purchased from the Sigma-Aldrich Company in Egypt. The chlorpyrifos insecticide was obtained from Sigma-Aldrich Company with a purity level of 99%, which met the required analytical standards. It was acquired specifically for conducting the adsorption investigations.

Preparation of the coal based adsorbents

Synthesis of coal derived activated carbon (AC)

The activation procedure was executed in accordance with Shaban et al.41. For 48 h, the ground coal had been subjected to an oxidation process in an open environment at 150 °C, aiming to prevent the existing resinous components from generating a plasticizing impact. Subsequently, the material underwent carbonization at a temperature of 500 °C over a duration of 2 h. A total of 5 g of carbonized coal had been blended with 10 mL of phosphoric acid (85%) and subjected to heating within a water bath for a duration of 24 h at a temperature of approximately 85 °C. Next, the obtained blend was dried and reactivated at 800 °C for 2 h. Following that, the activated yields were washed multiple times with a dilute NaOH solution and warm distilled water. This was done until the pH value of the water-carbon combination reached a range of 6 to 7. Subsequently, the product underwent vacuum drying at a temperature of 100 °C for a duration of 24 h, resulting in the formation of porous carbon products.

Synthesis of coal and activated carbon based composites with polyaniline

Activated coal (AC) and untreated coal (CA) have been blended with the polyaniline polymer using the approach described by Sayed et al.57. Under the influence of ultrasound vibrations, the monomers (0.1 M) have been dissolved in a diluted version of HCl (0.5 M) to yield two distinct aniline solutions. Afterwards, the CA and AC (1 g) had been effectively dispersed throughout the two different aniline solutions as separate tests and synthesis units using collaborative mixing techniques involving magnetic stirring along with an ultrasound generator. Subsequently, the two mixtures were immediately treated with the oxidative agent (NH4)2S2O8 (0.15 M), and continuous stirring was maintained for an entire day in order to promote the polymerization procedures. The resulting hybrids had to be separated employing a centrifuge (15 min at 3000 rpm), thoroughly washed, and efficiently dried over 12 h at 50 °C. The blended particles were preserved for subsequent studies and adsorption activities and labeled as PANI/AC and PANI/CA.

Analytical techniques

The crystalline structures and crystallography qualities of the assessed materials were examined and evaluated using a PANalytical-Empyrean X-ray diffractometer, which has a measurement range of 0 to 70°. The changes in the structural chemical groups throughout the modifying procedures have been identified by evaluating their FT-IR spectra using a Fourier transform infrared spectrometer (FTIR-8400S; Shimadzu) throughout a detection limit of 400 cm−1 to 4000 cm−1. The surface geometry of the coal-based frameworks that were successfully produced was analyzed using scanning electron microscopy (Gemini, Zeiss Ultra 55) after these materials had been covered with an extremely thin layer of gold. The internal features of the produced structures were evaluated using HRTEM photographs captured by a transmission electron microscope (JEOL-JEM2100) operated at a 200 kV accelerating voltage. After the samples were carefully degassed, their porosity and surface area were determined utilizing a surface area analyzer (Beckman Coulter SA3100).

Adsorption studies

The adsorption of chlorpyrifos pesticide (CPF) using CA, AC, PANI/CA, and PANI/AC has been studied in a batch approach. The variables that were evaluated were pH values that varied from 3 to 9, time frames extending from 20 to 720 min, and CPF concentrations fluctuating from 100 to 700 mg/L. The investigation additionally explored variations in temperature that varied from 293 to 313 K as factors affecting adsorption. Furthermore, the other vital assessment factors were carefully maintained at consistent values throughout the whole testing. In particular, the solid amount was constantly kept at 0.4 g/L, and the overall volume was kept at a constant level of 200 mL. The testing approach has been assessed in three distinct groups, using the mean findings of all measured concentrations throughout all the subsequent calculations and assessments. After establishing stabilization for all experiments, the particulates of CA, AC, PANI/CA, and PANI/AC were separated from the CPF solutions using filtering methods. A Merck/Hitachi high-performance liquid chromatography (HPLC) system had been employed to measure the residual levels of CPF. Equation 1 has been applied to compute the amounts of CPF adsorbed (Qe) onto CA, AC, PANI/CA, and PANI/AC (mg/g). The formula included the following: the volume of CPF-polluted solutions utilized (V), the blend amounts of CA, AC, PANI/CA, and PANI/AC (m), the initial levels of CPF (Co), and the residual levels of CPF (Ce).

$${Q}_{e (mg/g)}=\frac{{(C}_{o}-{C}_{e})V}{m}$$
(1)

Theoretical traditional and advanced equilibrium studies

The numerical modeling of adsorption properties has utilized classical kinetic, classic isotherm, and other types of advanced equilibrium models (Table S2). The concepts of statistical physics theory established the significance of these new equilibrium models. Nonlinear regression fitting methodologies were used to tackle kinetic and conventional equilibrium modeling by employing the mathematical formulas that correspond to each of these models. The significance of the matching levels has been established using the results of the corresponding coefficients of correlation (R2) (Eq. 2) and Chi-squared (X2) (Eq. 3). The alignment and matching degrees of the uptake activities and the examined advanced equilibrium equations were evaluated by utilizing the correlation coefficient (R2) and root mean square error (RMSE) (Eq. 4). The variables m′, p, Qical, and Qiexp represent the results of the examinations, the factors analyzed, the expected binding capacity of CPF, and the measured adsorption capacity, respectively.

$${\text{R}}^{2}=1-\frac{\sum ({Q}_{e, exp}-{Q}_{e, cal}{)}^{2}}{\sum ({Q}_{e, exp}-{Q}_{e, mean}{)}^{2}}$$
(2)
$${\upchi }^{2}=\sum \frac{({\text{Q}}_{\text{e},\text{ exp}}-{\text{Q}}_{\text{e},\text{ cal}}{)}^{2}}{{\text{Q}}_{\text{e},\text{ cal}}}$$
(3)
$${\text{RMSE}} = \sqrt {\frac{{\mathop \sum \nolimits_{{{\text{i}} = 1}}^{{\text{m}}} ({\text{Qi}}_{{{\text{cal}}}} - {\text{Qi}}_{{{\text{exp}}}} )^{2} }}{{{\text{m}}^{\prime } - {\text{p}}}}}$$
(4)

Results and discussion

Characterization of the used adsorbent

XRD analysis

The XRD diffraction properties of the synthesized CA, AC, PANI/CA, and PANI/AC were compared to the patterns of the original components, which consisted of unprocessed coal and PANI (Fig. 1). The diffraction patterns of unprocessed coal exhibit broad peaks, which are characteristic of amorphous or non-crystalline carbon-based components or frameworks of carbonaceous materials (Fig. 1A). The correlated peaks are situated at angles between 8° to 30° (002) and 40° to 50° (101). The activated product (AC) exhibits strong diffraction peaks around 23.22° and 26.53°, which correspond to the crystalline forms of graphite with basal spacing of 3.35 Å and an average crystallite size of 31 nm (Fig. 1B)58. The PANI that was used has a semi-crystalline structure, as shown by the X-ray diffraction pattern, which has two broad peaks that can be seen clearly at about 20° and 26° (Fig. 1C). The PANI/CA hybrid's observed peaks suggest that the polymerization of PANI in the presence of coal particles strongly binds the carbonaceous parts of the coal with the PANI molecules which appeared in shifting the positions of coal peaks (002) and (101) in addition to the decoction of an extra peak at approximately 27.5° (200), which match the fundamental peak of PANI (Fig. 1D)59. Similar features have been identified throughout the evaluation of the PANI/AC pattern (Fig. 1E). The interaction between PANI and the primary chemical groups of AC led to a noticeable shift in the prominent peaks of the formed graphite, which correlated with the activated coal. This was followed by the appearance of comparable peaks for both PANI and crystallized graphite.

Fig. 1
figure 1

XRD patterns of raw coal (CA) (A), activated coal (AC) (B), PANI polymer (C), synthetic PANI/CA composite (D), and synthetic PANI/AC composite (E) (G symbol refers to graphite crystalline phase).

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FT-IR analysis

The alterations in the basic chemical composition were successfully confirmed by an examination of the FT-IR spectra for CA, AC, PANI, PANI/CA, and PANI/AC (Fig. 2). The untreated coal specimen exhibited distinctive bands corresponding to the principal chemical components that comprise unrefined coal (Fig. 2A). The chemical units identified in coal include carboxylic and/or alcoholic OH groups that result from the hydrogen bindings inside the coal structure (3000–3600 cm−1). In addition, the identification of COOH groups was determined by the presence of a band around 1716 cm−1. Other distinguishable groups in the spectrum include symmetrical aromatic C–H vibrations (500–900 cm−1), C–O bonding vibrations (1000–1200 cm−1), C–H bonding vibrations inside the methylene group (1450 cm−1) and methyl group (1372 cm−1), aromatic C–C vibrations (1616 cm−1), aliphatic C–H vibrations (2940 cm−1), and asymmetrical aliphatic C–H vibrations (2858 cm−1) (Fig. 2A)42,60. During the conversion of coal into activated carbon (AC), a significant decrease in the intensity of absorption bands occurred, accompanied by a change in their exact positions (Fig. 2B). The significantly diminished bands are comparable to aliphatic C–H (2940 cm−1), asymmetrical aliphatic C–H (2858 cm−1), aromatic C–C (1616 cm−1), and methylene C–H bonding (1450 cm−1) (Fig. 2B). The dramatic decrease and complete disappearance of such bands indicate the substantial changes in existing functional groups and structural units following the activation process of the organic components of coal into activated carbon, which contains graphite as a crystalline phase.

Fig. 2
figure 2

FT-IR spectra of raw coal (CA) (A), activated coal (AC) (B), PANI polymer (C), synthetic PANI/CA composite (D), and synthetic PANI/AC composite (E).

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The FT-IR spectra of PANI/CA and PANI/AC were evaluated and compared with the spectra of PANI, CA, and AC. The spectral properties of PANI display distinct bands that correspond to its polymer structure. The spectral bands observed for PANI correspond to its distinctive chemical constituents, including aromatic C–H (2918 cm−1), N–H (3401 cm−1), C–N (1105 cm−1), C=C within quinoid rings (1467 cm−1), C–H (789 cm−1), C=C within benzenoid rings (1301 cm−1), and C–H within the para-aromatic rings (587 cm−1) (Fig. 2C)50,51. The spectra of PANI/CA (Fig. 2D) and PANI/CA (Fig. 2E) exhibit intricate bands due to the interactions between the exteriors of CA and AC and the integrated PANI. Identification of the distinct bands of the two blended components at different positions verified the effective integration of PANI. Spectral analysis of both samples showed that they had unique bands around 1550 and 1450 cm−1. These bands were connected to C=C bonds in the benzenoid and quinoid rings of PANI, respectively59,61. Furthermore, there are additional specific PANI bands situated at about 2300 cm−1, which correspond to the C–N binding. Furthermore, additional bands appeared at about 1300 and 1130 cm−1, which correspond to the C–N binding within aromatic amine59,62. Moreover, there is a notable overlap between the corresponding bands of some functional groups of PANI and CA, as well as AC. The presence of distinguishable C–H bands at around 670 cm−1, exhibiting greater intensity, indicates a successful incorporation of PANI into the surfaces of CA and AC.

SEM and HRTEM analyses

The obtained SEM and HRTEM images clearly showed noticeable changes in the coal’s surface properties due to the activation step and the PANI hybridization process (Fig. 3). The coal grains were recognized for their characteristic forms as compact layers encompassing massive or angular-like fragments that can be assigned to the compression of their internal components, more specifically wood tissues and macerals (Fig. 3A). In terms of the morphology of the derived activated product (AC), the SEM images revealed significant changes in the shapes and surficial features compared to the raw coal (Fig. 3B and C). The morphology of the AC particles exhibit morphology ranges from highly porous massive particulates to well-developed flakey grains of spherical or elliptical outlines corresponding to the formed graphite phases (Fig. 3B). During the activation procedures, the intersection between the formed graphite flakes typically results in a flower-like shape with interstitial pores in addition to the existing pores (Fig. 3C). Both PANI/CA (Fig. 3D, E, and F) and PANI/AC (Fig. 3G, H, and I) display a significant presence of PANI particulates over the surfaces of the coal-layered or AC flakes, like tiny particles. The existence of PANI has been regularly recognized by its intricate network of curved nanorods. The growth of crossing or intersection-curved nanorods promotes the formation of a porous framework consisting of interstitial nanopores and uneven topographical characteristics of both CA and AC, resulting in significant increases in surface area. The HRTEM images of the materials under investigation confirmed the existence of PANI tabular or rod-like clusters, which are intersecting nanoparticles with network-like structures embedded into the smooth exterior layers of CA (Fig. 3F) and AC (Fig. 3I). The surface area of coal (5 m2/g) increased to 283.4 m2/g, 27.7 m2/g, and 296.2 m2/g after undergoing morphological changes due to the activation, synthesis of PNAI/CA, and production of PANI/AC, respectively.

Fig. 3
figure 3

SEM images of raw coal (A), activated carbon with porous structure (B), the resulted graphite flakes in the activated samples (C), SEM image of PANI/CA composite (D and E), HRTEM images of PANI/CA composite (F), SEM images of synthetic PANI/N.CA composite (G and H), and HRTEM image of PANI/N.CA composite (I) (red arrows refer to coal flakes and blue arrows refers to PANI).

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Adsorption studies

Effect of pH solution

Adsorption of water-soluble substances is strongly regulated by the pH of their solutions, which are being studied. The ionizing characteristics of soluble chemicals are significantly influenced by pH as well as the charges that exist on the surfaces of the materials63,64. Experiments were conducted in the presence of CA, AC, PANI, PANI/CA, and PANI/AC to determine the influence of pH on the characteristics of CPF elimination. The pH range that had been evaluated ranged from 3 to 9, while keeping all other variables constant. The volume employed was 200 mL, the concentration of CPF was 200 mg/L, the time continued for 60 min, the dose applied was 0.4 g/L, and the temperature had been kept constant at 293 K. The observed trends confirm that higher pH levels have a significant positive impact on CPF removal using CA, AC, PANI, PANI/CA, and PANI/AC (Fig. 4). At pH 3, the CPF had an adsorptive capacity of 3.2 mg/g (CA), 10.4 mg/g (PANI/CA), 8.7 mg/g (AC), and 17.2 mg/g (PANI/AC). However, at pH 9, the capacity greatly enhanced to 44.3 mg/g (CA), 59.2 mg/g (PANI/CA), 49.8 mg/g (AC), and 78.5 mg/g (PANI/AC) (Fig. 4). Therefore, the materials stated previously could serve as effective adsorbents in actual treatment procedures, in accordance with the acceptable pH range between 6 and 9, as recommended by the US Environmental Protection Agency (EPA), for treating industrial effluents65. The results indicate a strong correlation between pH and the ionizing tendencies of CPF, as well as the surface charges of AC, AC, PANI/CA, and PANI/AC64,66,67. As the alkalinity level of the CPF solutions increased the reactive functions of CA, AC, PANI/CA, and PANI/AC underwent deprotonation causing their exteriors to become completely charged with negative charges68,69. As a result of the high density of negative charges that exist on their exteriors, CA, CA, PANI/CA, and PANI/AC serve as powerful interfaces for the electrostatic attraction of the positively charged CPF35,70.

Fig. 4
figure 4

The influence of pH on the adsorption of CPF pesticide by CA, AC, PANI/CA, and PANI/AC.

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Kinetic studies

Effect of contact time

Experiments were performed to investigate the adsorption properties of CA, AC, PANI, PANI/CA, and PANI/AC with respect to the duration of CPF elimination. The examination lasted between 20 and 1080 min. After confirming the adapted levels of key variables, such as CPF content (200 mg/L), pH (9), volume (200 mL), temperature (20 °C), and dosage (0.4 g/L), we assessed the specific influence of various time frames. The effective performance of CA, AC, PANI, PANI/CA, and PANI/AC in eliminating CPF is evident from the dramatic rise in both the amount of CPF trapped and the reported elimination rates throughout the course of the tests. Furthermore, it is essential to realize that the duration of the tests has a major effect on the detected improvements in the uptake characteristics until about 480 min (Fig. 5A). However, following the specified times of contact, neither the rates of CPF removal nor the quantity of CPF retained demonstrated any significant changes or increases. These results indicate that the CA, AC, PANI, PANI/CA, and PANI/AC attained a stable state after the previously specified time intervals, reflecting their equilibrium states as adsorbents (Fig. 5A). The equilibrium adsorption properties of CPF using CA, PANI/CA, AC, and PANI/AC were 65.5 mg/g, 107.3 mg/g, 84.8 mg/g, and 125.8 mg/g, respectively. At the start of the investigations, an extensive variety of interacting and unbound receptors existed over the CA, AC, PANI, PANI/CA, and PANI/AC structures, which correlated with notable enhancements and raises in both the rate of CPF elimination and the quantity of CPF retained71. As the evaluation period expands, there is a significant drop in the number of vacant receptors. The extended binding of CPF, which subsequently occupies the aforementioned receptors and minimizes the total number of vacant sites, may be the main reason for this behavior. As a result, the rate of CPF adsorption decreased significantly over a specific period of time. Additionally, CPF binding using CA, AC, PANI, PANI/CA, and PANI/AC showed minimal improvement or s` characteristics. It is possible to detect the equilibrium states of CA, AC, PANI, PANI/CA, and PANI/AC when all sites are fully occupied72.

Fig. 5
figure 5

The influence of contact time on the adsorption of CPF (A), Intra-particle diffusion curves during the uptake of CPF (B), and the kinetic modeling of the CPF uptake reactions (C (Pseudo-First order model) and D (Pseudo-Second order model).

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Intra-particle diffusion behavior

Examining the trend lines in intra-particle diffusion might potentially be used as a technique to investigate the uptake properties of CPF using CA, AC, PANI, PANI/CA, and PANI/AC. The displayed curves exhibit three distinct segments with varying slopes (Fig. 5B). The evaluated curves display variations from their initial settings, suggesting the existence of several adsorption processes in combination with the diffusing process of CPF67,73. The activities generally include three main stages: (1) the interactions between CPF and the vacant receptors throughout the exterior interfaces of CA, AC, PANI, PANI/CA, and PANI/AC (boundary); (2) the sequential layered retaining of CPF, along with the diffusion behavior of CPF; and (3) the impact of saturating and stabilizing conditions74. The first outcomes of this investigation suggest that the main processes that trigger the uptake of CPF onto the surfaces of CA, AC, PANI, PANI/CA, and PANI/AC (external adsorption) comprised the most significant pathways recognized throughout all phases of uptake activity (Fig. 5B). The efficacy of CPF adsorption during this phase is dependent on the overall quantity of sites situated throughout the contact surfaces of CA, AC, PANI, PANI/CA, and PANI/AC75. The efficiency of additional-layered adsorption processes was instantaneously confirmed by raising the length of time since all external sites were completely filled (Fig. 5B)74,76. Furthermore, these recent efforts consider the impacts of CPF-spreading occurrences. The last mechanistic processes involved in CPF binding using CA, AC, PANI/CA, and PANI/AC exhibit a significant effect after attaining equilibrium levels. This implies that CPF molecules effectively saturated all the interaction sites67,77. During this stage, molecule-interacting and interionic-attracting mechanisms facilitate CPF removal69.

Kinetic modeling

Modeling the kinetics of adsorption is essential for studying the time-dependent effects and determining the physical processes involved, such as mass transfer pathways or chemical processes that control adsorption activities78. The study evaluated the kinetic properties of CPF-eliminating behaviors through the application of the conventional kinetic principles of pseudo-first order (P.F.) and pseudo-second order (P.S.) mathematical models utilizing CA, AC, PANI/CA, and PANI/AC. The PFO model has been applied to analyze the kinetics of uptake activities during the equilibrium phase to establish the relationship between the quantity of free sites and the speed at which the soluble ions entirely fill the binding sites. The PSO model could be applied to demonstrate the relationship between the performances of assessed adsorbents throughout a certain time frame. With regard to the two distinct hypotheses, the extent of agreement between the CPF adsorption behaviors and kinetic theories was evaluated using nonlinear fitting parameters that matched the corresponding equations. The appropriate degrees of matching were identified by analyzing the coefficients of determination (R2) and Chi-squared (X2) values (Table 1; Fig. 5C and D). The R2 values, when paired with the X2 statistics, indicate that the kinetic properties and basic assumptions of the P.F. theory provide a more accurate representation of the binding and uptake behaviors of CPF utilizing CA, AC, PANI/CA, and PANI/AC compared to the assessed P.S. theory.

Table 1 The mathematical parameters of the evaluated kinetic models.
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The results obtained from mathematical simulations using the P.F. model (70.6 mg/g (CA), 114.7 mg/g (PANI/CA), 91.7 mg/g (AC), and 134 mg/g (PANI/AC)) were approximately consistent with the measured equilibrium values (65.5 mg/g (CA), 107.3 mg/g (PANI/CA), 84.8 mg/g (AC), and 125.8 mg/g (PANI/AC)) (Table.1). The recognized concordance confirms the earlier reported results, which highlight the higher level of adequacy of the P.F. model during the kinetic assessments (Table 1). The P.F. theory suggests that physical mechanisms, such as van der Waals forces or electrostatic attraction, play an essential function throughout the adsorption of CPF utilizing CA, AC, PANI/CA, and PANI/AC79,80. The examined uptake requirements also demonstrate substantial adherence to the P.S. notion; nonetheless, the P.F. modeling provides a higher level of consistency. Previous literature has reported that particular chemical processes, such as hydrogen bonding, complexation, and hydrophobic bonds, may either enhance or possess minimal impact on the adsorption of CPF utilizing CA, AC, PANI/CA, and PANI/AC67,79. The earlier developed chemically bonded CPF layer might be acted as the basis for the development of further CPF adsorbing layers using physical processes81.

Equilibrium studies

Effect of concentrations

The investigation focused on determining the greatest quantities of CPF elimination activities using CA, AC, PANI/CA, and PANI/AC across the range of 100 to 700 mg/L. This was accomplished by studying the effects of starting CPF concentrations on the equilibrium state. The other factors influencing the retention of CPF were kept constant at specific levels, which included a total volume of 200 mL, a duration of 24 h, a dosage of 0.4 g/L, and temperatures ranging from 293 to 313 K. There is a possible correlation between increased CPF levels and the reported rise in the quantities of CPF adsorbed utilizing CA, AC, PANI/CA, and PANI/AC (Fig. 6A, B, C, and D). A commensurate increase in the concentration of CPF within a particular volume greatly improved the diffusion, driving forces, and mobility characteristics of dissolved CPF. This enhanced the ability to interact with a larger number of active uptake sites spread across the CA, AC, PANI/CA, and PANI/AC exterior surfaces. Consequently, the efficacy of CPF-retaining operations using CA, AC, PANI/CA, and PANI/AC significantly increased in relation to the examined CPF concentrations82. However, this relationship can only be observed within specific limits of CPF concentrations. Additionally, increasing the starting concentration of CPF does not seem to affect its adsorption using CA, AC, PANI/CA, and PANI/AC. Determining the appropriate, maximum retention effectiveness of CPF. The retention characteristics of CPF were measured by CA at temperatures of 293 K, 303 K, and 313 K, resulting in values of 130.8 mg/g, 118.5 mg/g, and 107.6 mg/g, respectively (Fig. 6A). The PANI/CA demonstrated adsorption qualities of 211.4 mg/g at 293 K, 183.5 mg/g at 303 K, and 155.7 mg/g at 313 K (Fig. 6B). The adsorption qualities of AC were 149.8 mg/g at 293 K, 132.1 mg/g at 303 K, and 116.3 mg/g at 313 K (Fig. 6C). The adsorption capacities of the PANI/AC material have been estimated to be 266.1 mg/g at 293 K, 239.3mg/g at 303 K, and 217.2 mg/g at 313 K (Fig. 6D). Various variables contribute to the improved uptake aspects observed using modified coal-based adsorbents: (1) an augmented surface area; (2) a significant increase in exterior reactivity; and (3) a significant rise in the total quantity of binding sites due to the incorporation of additional functional groups related to PANI. The reduction in CPF retention recognized while utilizing CA, AC, PANI/CA, and PANI/AC at different temperatures indicates that these processes have exothermic properties.

Fig. 6
figure 6

The influence of the CPF concentration on the uptake capacities of coal based adsorbent (A, B, C, and D), fitting of the CPF uptake behaviors with classic Langmuir model (E, F, G, and H), fitting of the CPF uptake behaviors with classic Freundlich model (I, J, K, and L), and fitting of the CPF uptake behaviors with classic D–R model (M, N, O, and P).

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Giles’s classification

The categorization of the CPF binding processes employing CA, AC, PANI/CA, and PANI/AC has been established according to the criteria set out by Giles et al.83. The study revealed that the analyzed curves demonstrated an L-type classification. The isotherm aspects of this class highlight the significant impacts arising from intermolecular attractive interactions throughout the entire process of removing CPF utilizing CA, AC, PANI/CA, and PANI/AC (Fig. 8A, B, C, and D). The aforementioned behaviors are intensified by the powerful interactions of CPF across the very reactive surfaces of CA, AC, PANI/CA, and PANI/AC84. Based on the L-type class criteria, it was assumed that the surfaces of CA, AC, PANI/CA, and PANI/AC particles could be completely capped with layers of adsorbed CPF85. Furthermore, the confirmed isothermal situations indicate that the CA, AC, PANI/CA, and PANI/AC particulates display a diverse range of vital and effective binding sites. Moreover, these binding sites exhibit substantial affinity for the CPF molecules, particularly when the beginning CPF levels are low.

Classic isotherm models

Standard isotherm examinations of adsorption procedures were applied to assess the dispersion of soluble contaminants across the water-based solutions as well as the adsorbents beyond the equilibrium state. Traditional equilibrium modeling methodologies have a substantial influence on the illustration of adsorption reactions. The typical isotherm functions provide valuable insights into three main aspects: (a) the sorbate's affinity toward the reacting surfaces of adsorbents; (b) the potential quantities of water-soluble molecules that might bind to the exteriors of the adsorbent; and (c) the maximum binding capacities. The isotherm characteristics of CPF adsorption qualities were assessed employing the Langmuir (Fig. 6E, F, G, and H), Freundlich (Fig. 6I, J, K, and L), and Dubinin–Radushkevich (D–R) (Fig. 6M, N, O, and P) equilibrium concepts. Non-linear regression approaches have been employed to evaluate the level of agreement between the hypothesized equilibrium assumptions stated by the preceding models and the observable CPF retention characteristics. The investigation involved analyzing the correlation coefficient (R2) and the Chi-squared (X2) values. Analysis of R2 and X2 indicates that the CA, AC, PANI/CA, and PANI/AC particles exhibit CPF-adsorbing characteristics that align better with Langmuir's theory than the Freundlich assumption (Table 2). The detected equilibrium activity indicates that CPF exhibits homogeneous and consistent affinity for binding to the reacting and free sites of CA, AC, PANI/CA, and PANI/AC particulates, leading to the development of a single or monolayer of retained CPF79,80. The research also showed that the particles of CA, AC, PANI/CA, and PANI/AC were good at absorbing CPF, as shown by RL values below 175,77. The mathematical modeling revealed that the maximum adsorption capacities (Qmax) of CPF employing CA were 135.9 mg/g at 293 K, 123.46 mg/g at 303 K, and 109.9 mg/g at 313 K (Table 2). The reported values for PANI/CA were 235.8 mg/g at 293 K, 191.2 mg/g at 303 K, and 154.4 mg/g at 313 K (Table 2). The predicted values obtained using AC are 156.9 mg/g at a temperature of 293 K, 131 mg/g at 303 K, and 116.13 mg/g at 313 K (Table 2). The projected values using PANI/AC were 309.7 mg/g at a temperature of 293 K, 263.8 mg/g at a temperature of 303 K, and 227.2 mg/g at a temperature of 313 K (Table 2).

Table 2 The mathematical parameters of the evaluated classic isotherm models.
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The equilibrium features of the D–R theory provide a thorough comprehension of the energy fluctuations generated by CA, AC, PANI/CA, and PANI/AC particles during CPF-eliminating behaviors, regardless of their level of homogeneity or heterogeneity86. The investigation of the D–R modeling findings provides valuable insights into the determination of Gaussian energy (E) and its role in interpreting fundamental mechanisms, whether chemical or physical. The energy levels associated with binding processes could be classified into three categories: below 8 kJ/mol, within 8 and 16 kJ/mol, and beyond 16 kJ/mol. At these levels of energy, the main mechanisms are strong physical actions, chemical-based activities, or a mix of chemical and physical mechanisms, and strong chemical-based activities in that order77. The measured levels of energy (E) corresponding to CPF-sequestrating mechanisms using CA, AC, PANI/CA, and PANI/AC have been determined to be within the approved energy limit (less than 8 kJ/mol) for physical activities (Table 2).

Advanced isotherm modeling

Utilizing statistical physics approaches to simulate the equilibrium qualities of binding behaviors may provide an in-depth analysis of the distinctive features of adsorption processes. The study's mathematical models evaluate the interactions between the water-soluble contaminants and external reacting functionalities that operate as interactive receptors across the adsorbing agents' surfaces. The mathematical equations implemented in this investigation provide reliable mathematical factors that precisely depict underlying mechanisms, including energetic and steric factors. The models encompass various steric variables, including the total quantity of occupied adsorption sites over the interfaces of CA, AC, PANI/CA, and PANI/AC (Nm), the number of trapped CPF molecules by each site (n), and the highest possible uptake effectiveness of CPF by using CA, AC, PANI/CA, and PANI/AC during total saturation (Qsat). The energetic aspects comprise internal energy (Eint), entropy (Sa), adsorption energy (E), and free enthalpy (G). Non-linear regression analyses were used to evaluate the stated theories of the advanced models. The previous investigation was successfully completed employing multivariable nonlinear regression methods in combination with the Levenberg–Marquardt iterative method. The obtained fitting levels were subsequently employed to assess and characterize the adsorption reactions. The highly fitted model—specifically, the monolayer model of a single energetic site—were used for this (Fig. 7A, B, C, and D). Table 3 presents the computed factors.

Fig. 7
figure 7

fitting of the CPF uptake behaviors with advanced monolayer isotherm model with one energy site (A (CA), B (PANI/CA), C (AC), and D (PANI/AC).

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Table 3 The steric and energetic parameters of the evaluated advanced isotherm models.
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1. Steric properties

a. Number of adsorbed CPF (n) per each site

The numerical findings of the n(CPF) factor provide adequate proof of the arrangement behavior of the adsorbed CPF ions occupying the exterior surfaces of CA, AC, PANI/CA, and PANI/AC. This includes both the vertical as well as the horizontal orientations. Additionally, these outcomes are significant in understanding the mechanisms that regulate the reactions, such as multi-docking or multi-interactions. The capture of one CPF ion via multiple uptake sites is one of the reactions that are most significantly impacted by multi-anchorage or multi-docking operations. The retention reactions with values less than one are associated with the ions' horizontal orientation. Conversely, activities that display values above 1 indicate the retaining of CPF within non-parallel geometries together with a vertical orientation. The uptake processes in such systems are mainly mediated by multi-molecular pathways, whereby a single site captures several dye ions77,87. The determined values of n, which represent the number of bound CPF by a single site of CA, range from 3.4 to 3.7 (Fig. 8A). The quantity of CPF molecules maintained by a single site throughout the interfaces of PANI/CA (with a range of n(CPF) = 2.07–3.63) (Fig. 8A), AC (with a range of n(CPF) = 2.53–3.8) (Fig. 8B), and PANI/AC (with a range of n(CPF) = 1.8–2.9) (Fig. 8B) was found to be consistently greater than 1 as determined for CA. As a result, the CPF was adsorbed by a complex mechanism involving many molecules. Each retaining site within the CA, PANI/CA, AC, and PANI/AC particulates had the ability to accommodate multiple ions that were organized in vertical configurations with distinct non-parallel characteristics. The individual receptors within the exterior surface of PANI/AC can hold up to 3 CPF ions in comparison with 4 using CA, PANI/CA, and AC, and PANI/CA. The calculations for n(CPF) for CA, PANI/CA, AC, and PANI/AC demonstrate a substantial increase whenever the temperature increases from 293 to 313 K (Fig. 8A and B; Table 3). The observed result might be explained by the assumed enhancement in the aggregation properties of the CPF throughout its uptake by CA, PANI/CA, AC, and PANI/AC at high temperatures82. This also suggests the existence of thermal activation processes earlier than CPF uptake processes36,88.

Fig. 8
figure 8

change in the steric parameters during the uptake of CPF by coal based adsorbents including the number of adsorbed CPF per site (A and B), the occupied active sites density (C and D), and the adsorption capacities as the saturation states (E and F).

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b. Occupied active sites density (Nm)

It is feasible to accurately determine the total number of effective adsorption receptors (Nm(CPF)) across the outermost surfaces of CA, AC, PANI/CA, and PANI/AC particles during the entire duration of the treatment by examining the density of CPF-occupied sites (Fig. 8C and D; Table 3). The calculated values corresponding to the Nm(CPF) at various temperatures for CA are as follows: 39.6 mg/g at 293 K, 36.2 mg/g at 303 K, and 29.2 mg/g at 313 K (Fig. 8C). After the activation steps (AC), the levels demonstrated a substantial rise up to 62 mg/g at 293 K, 37.85 mg/g at 303 K, and 30.5 mg/g at 313 K (Fig. 8D). The degree of improvement encountered an impressive rise after the PANI hybridization steps, regardless of whether it had been performed for CA (PANI/CA) or AC (PANI/AC). The PANI/CA samples exhibited Nm values of 113.5 mg/g at 293 K, 66.7 mg/g at 303 K, and 43.8 mg/g at 313 K (Fig. 8C). The use of PANI/AC resulted in an enormous rise in these levels, with Nm values of 169.7 mg/g (293 K), 115 mg/g (303 K), and 78.3 mg/g (313 K) (Fig. 8D). The previous study shows that the activation and incorporation steps of PANI lead to a significant increase in the number of functional adsorption sites. The detected results might be related to the effect of the activation process on triggering the porosity of the produced structure, thereby enhancing the exterior area and contact interface, and therefore available active sites. Furthermore, the inclusion of PANI enhances the surface area and incorporates additional active sites within the hybrid structures throughout the uptake process. The Nm (CPF) levels for CA, AC, PANI/CA, and PANI/AC show thermal-dependent reversible variations as a consequence of temperature responses. The trends that have been identified for CA, AC, PANI/CA, and PANI/AC align with the established trends for n (CPF), since the enhanced aggregating qualities of CPF result in a decrease in the overall number of occupied sites.

c. Adsorption capacity at the saturation state of (Qsat)

The saturation adsorption characteristics of CA, AC, PANI/CA, and PANI/AC (Qsat) exhibit the greatest quantities of tolerance and CPF being sequestered. The estimation of Qsat values is influenced by two main variables: the specified density of the filled sites (Nm) and the total amount of CPF molecules acquired through each single site (n). CA as an unprocessed product has a CPF capacity of 135.8 mg/g at 293 K, 123.4 mg/g at 303 K, and 109.9 mg/g at 313 K (Fig. 8E). The maximum efficiencies of activated carbon (AC) were determined to be 156.95 mg/g at 293 K, 135.5 mg/g at 303 K, and 116.1 mg/g at 313 K (Fig. 8F). The effectiveness of PANI/CA was improved, with reported values of 235.8 mg/g at 293 K, 191.2 mg/g at 303 K, and 158.9 mg/g at 313 K (Fig. 8E). The PANI/AC hybrid had the highest CPF binding capacities, with values of 309.75 mg/g at 293 K, 263.8 mg/g at 303 K, and 227.2 mg/g at 303 K (Fig. 8F). The adverse impact of temperature could reflect the exothermic nature of CPF adsorption using CA, AC, PANI/CA, and PANI/AC. These findings demonstrate that greater uptake temperatures lead to higher levels of thermal collisions, which in turn decrease the effectiveness of CPF binding87. Furthermore, the temperature-related discernible behaviors of Qsat exhibit resemblance to the behaviors described by Nm (CPF) versus n (CPF). This result suggests that the main factor influencing the effectiveness of CPF retention is the quantity of interacting sites.

2. Energetic properties

a. Adsorption energy

The energetic changes (ΔE) detected throughout the adsorption reactions of CPF may provide beneficial insights into the underlying mechanisms, irrespective of whether they correspond to either chemical or physical activities. Physical processes exhibit energies less than 40 kJ/mol, while chemical tracks possess energetic levels exceeding 80 kJ/mol. The adsorption energies serve as an efficient metric for categorizing various physical mechanistic behaviors. The values of adsorption energy characterized various physical approaches. These involve van der Waals forces (4–10 kJ/mol), hydrophobic bonding (5 kJ/mol), dipole bonding forces (2–29 kJ/mol), and hydrogen bonding (< 30 kJ/mol). The theoretical determination of the CPF elimination energies (ΔE) was performed employing Eq. 5, integrating the solubility measurement of CPF in water (S), the gas constant (R = 0.008314 kJ/mol.K), the CPF extents under half saturation situations of CA, AC, PANI/CA, and PANI/AC, and the exact temperature (T)89.

$$\Delta E=RT \,ln\left(\frac{S}{C}\right)$$
(5)

The CPF uptake energy values for all CA, AC, PANI/CA, and PANI/AC samples range from approximately − 11 kJ/mol to roughly − 14 kJ/mol, as shown in Table 3. Therefore, the predominant mechanisms accountable for the adsorption of CPF using CA, AC, PANI/CA, and PANI/AC comprised physical activities, mostly involving van der Waals forces (4–10 kJ/mol), dipole bonding (2–29 kJ/mol) and hydrogen binding (< 30 kJ/mol). Furthermore, the negative signals accomplished for the predicted ΔE values throughout the sequestration of CPF using CA, AC, PANI/CA, and PANI/AC align with previous experimental findings demonstrating the exothermic nature of these processes. The energetic results are highly compatible with the previously described adsorption mechanisms for both coal and polyaniline. Also, these mechanistic findings are in agreement with the mechanisms corresponding to such hydrophobic structures39,90,91. The CPF is mostly absorbed by polyaniline by hydrogen bonding, electrostatic interactions, and pi-pi interactions52,53. The adsorption mechanisms of coal involve the attraction of CPF through electrostatic forces, the formation of hydrogen bonds between the free hydrogen on the exterior of coal and nitrogen atoms that exist within the framework of CPF, and π–π interactions between the aromatic rings of CPF and the coal framework. These reactions are associated with dispersion as well as dipole/dipole interactions39,92. The suggested mechanism was presented schematically in Fig. 9.

Fig. 9
figure 9

schematic diagram for the adsorption of CP molecules by the prepared composite from coal and polyaniline.

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3. Thermodynamic functions

a. Entropy

The entropy (Sa) corresponding to the CPF retention processes employing CA, AC, PANI/CA, and PANI/AC provides clear evidence of the ordered and disordered characteristics displayed by the outer interfaces of the coal-based adsorbents utilized with various concentrations of CPF ions and reaction temperatures. The characteristics of Sa were validated using the outcomes resulting from Eq. 6, involving the previously calculated values of Nm and n, in addition to the predicted rest concentrations of CPF at the stages of half-saturation of CA, AC, PANI/CA, and PANI/AC (C1/2).

$$\frac{{S}_{a}}{{K}_{B}}=Nm\left\{\mathit{ln}\left(1+{\left(\frac{C}{{C}_\frac{1}{2}}\right)}^{n}\right)-n{\left(\frac{C}{{C}_\frac{1}{2}}\right)}^{n} \frac{ln\left(\frac{C}{{C}_\frac{1}{2}}\right)}{1+{\left(\frac{C}{{C}_\frac{1}{2}}\right)}^{n }}\right\}$$
(6)

The analysis of the accomplished curves demonstrates a significant reduction in the entropy levels (Sa) whenever CPF is adsorbed using CA, AC, PANI/CA, and PANI/AC, particularly at higher concentrations (Fig. 10A, B, C, and D). As the assessed CPF concentrations rise, the trends observed indicate a notable decrease in the disorder characteristics of the outer interfaces of CA, AC, PANI/CA, and PANI/AC. The entropy characteristics also enhance the successful docking of CPF onto the unoccupied and active binding sites across CA, AC, PANI/CA, and PANI/AC surfaces, even when utilizing minimal CPF concentrations89,93. The maximum entropy levels have been determined at CPF equilibrium levels of 187.88 mg/L (293 K), 189.5 mg/L (303 K), and 190.2 mg/L (313 K) utilizing CA (Fig. 10A). The equilibrium concentrations that resulted in the highest level of entropy during the CPF elimination using AC were 183.04 mg/L at 293 K, 184.7 mg/L at 303 K, and 187.5 mg/L at 313 K (Fig. 10B). The use of PANI/CA demonstrates the highest degree of entropy at CPF concentrations of 151.1 mg/L (at 293 K), 155.08 mg/L (at 303 K), and 161.3 mg/L (at 313 K) (Fig. 10C). The concentrations at which PANI/AC can estimate the maximum entropy during CPF uptake were 143.7 mg/L at 293 K, 148.9 mg/L at 303 K, and 154.12 mg/L at 313 K (Fig. 10D). The concentrations estimated at the half-saturation states of CA, AC, PANI/CA, and PANI/AC approximate the corresponding equilibrium concentrations of maximum entropy. As a result, the remaining binding sites are unable to facilitate the docking of additional CPF ions. Furthermore, the significant decreases observed in the assessed entropy values suggest an extensive decrease in the quantity of accessible sites, along with a major decrease in the mobility and diffusion characteristics of the CPF ions93.

Fig. 10
figure 10

change in the thermodynamic functions during the uptake of CPF by coal based adsorbents including the entropy (A, B, C, and D), internal energy (E, F, G, and H), and free enthalpy (I, J, K, and L).

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b. Internal energy and free enthalpy

The analysis examined the internal energy (Eint) associated with CPF binding processes using CA, AC, PANI/CA, and PANI/AC. It also examined the properties of free enthalpy (G) and how variations in the CPF concentration and operation temperature can influence them. We performed the assessment using Eqs. 7 and 8, which calculated the values based on the previously established values of Nm, n, and C1/2, along with the translation partition (Zv)88.

$$\frac{{E}_{int}}{{K}_{B}T }= n \,{N}_{m}\left[\left( \frac{{\left(\frac{C}{{C}_{1/2}}\right)}^{n} ln\left(\frac{C}{{Z}_{v}}\right)}{1+{\left(\frac{C}{{C}_{1/2}}\right)}^{n}}\right)-\left( \frac{n\text{ln}\left(\frac{C}{{C}_{1/2}}\right) {\left(\frac{C}{{C}_{1/2}}\right)}^{n}}{1+{\left(\frac{C}{{C}_{1/2}}\right)}^{n}}\right)\right]$$
(7)
$$\frac{G}{{K}_{B}T }= n \,{N}_{m}\frac{\text{ln}\left(\frac{C}{{Z}_{v}}\right)}{1+{\left(\frac{{C}_{1/2}}{C}\right)}^{n}}$$
(8)

The measured levels of Eint in relation to CPF retention operations using CA, AC, PANI/CA, and PANI/AC exhibit negative signs. These results show a significant decrease in Eint when the temperature increases from 293 to 313 K (Fig. 10E, F, G, and H). This validation confirms the spontaneous and exothermic nature of the CPF sequestration mechanisms using CA, AC, PANI/CA, and PANI/AC. The enthalpy values and behaviors specified (Fig. 10 I, J, K, and L) exhibit identical characteristics and specifications. The G findings exhibit negative characteristics and possess a reversible relationship with the actual adsorption temperature. This implies a decrease in feasibility aspects and validates the exothermic activities and spontaneity of the CPF adsorption employing CA, AC, PANI/CA, and PANI/AC.

Comparison study

The adsorption capacities of the investigated coal-based adsorbents (CA, AC, PANI/CA, and PANI/AC) were compared with some of the recently studied adsorbents of chlorpyrifos in the literature (Table 4). The results demonstrate the significant efficiency of coal as a raw material in comparison with several investigated adsorbents. This highlights its suitability for application in commercials on a realistic scale considering its cost and natural availability as compared to other materials that actually have higher prices and complex fabrication processes (Table 4). The conversion of coal into nanoporous graphite as an activated product (AC) results in remarkable enhancements that are mostly of higher capacities than the structures presented, except nano-magnetite. Also, the integration of PANI (PANI/CA) induces the uptake performances to be strongly higher than all the reported values for the presented structures. This signifies the higher positive impact of PANI hybridization as a modification process on the adsorption performances of coal as compared to the activation process. The resulting product can be produced by facile polymerization steps without the need for thermal activation or power consumption. The composite of PANI and raw coal (PANI/CA) is of lower cost than the mentioned adsorbents, and its applications can achieve higher removal efficiency using low doses and within short time intervals. The integration between the activated product and PANI (PANI/AC) resulted in highly effective adsorbents with excellent uptake capacities as compared to all the structures. This qualifies the composite to be used in the remediation of industrial wastewater with an excessive content of chlorpyrifos residuals.

Table 4 Comparison between the adsorption properties of coal based adsorbents and other adsorbents in literature.
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Conclusion

Natural sub-bituminous coal was subjected to activation process into porous graphitic carbon (AC) and hybridized with polyaniline forming two composites with the raw coal (PANI/CA) and the activated product (PANI/AC). The raw samples and modified products were assessed as enhanced adsorbents for chlorpyrifos contaminants. PANI/AC achieved the best uptake performance (309.75 mg/g) as compared to PANI/CA (235.8 mg/g) and AC (156.9 mg/g), which are all higher than CA (135.8 mg/g). This was assigned to the textural and chemical effect of the modification steps in inducing the quantities of the existing active sites (Nm = 169.7 mg/g (PANI/AC), 113.5 mg/g (PANI/CA), 62 mg/g (AC), and = 39.6 mg/g (CA)). The uptake of target chlorpyrifos molecules by multi-molecular physical mechanisms involving dipole and hydrogen bonding processes based on the energetic studies (< 40 kJ/mol) and number of adsorbed ions per site (n > 1). The thermodynamic functions involving entropy, internal energy, and enthalpy validate the exothermic properties of operating uptake reactions and their spontaneous behavior. Based on the optimization conditions, the determined adsorption capacities, and recyclability values, the synthetic structures can be effectively applied as reliable adsorbents during the realistic removal of chlorpyrifos from industrial wastewater that is related to its commercial production or drainage water enriched in it out of the agriculture sectors.

Recommendation

Further studies will be conducted separately to follow the impact of each modification technique on the properties of coal, considering the tested conditions and the different applied techniques for the different modification procedures.