Adsorptive removal of 2,4-chlorophenol from water by micronized oil shale

doi:10.21203/rs.3.rs-4841803/v1

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Adsorptive removal of 2,4-chlorophenol from water by micronized oil shale

https://doi.org/10.21203/rs.3.rs-4841803/v1

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Chlorophenols, classified as priority water pollutants, can bioaccumulate in aquatic organisms, potentially leading to their enrichment throughout the food chain. This study investigated the utilization of a unique oil shale as a sorbent for the removal of 2,4-dichlorophenol (2,4-DCP) from aqueous solutions. The influence of various process parameters, including the contact time, sorbent/liquid ratio, temperature, and pH, on the sorption process was evaluated.

The results indicated the near-complete sorption of 2,4-DCP within 24 h. Favorable sorption was observed at a sorbent/liquid ratio of 1:10, elevated temperatures, and lower pH values within the examined range. Both the Langmuir and Freundlich isotherm models were in good agreement with the equilibrium sorption data. However, the Freundlich isotherm provides a superior fit, suggesting a multilayer sorption mechanism. Kinetic studies revealed a two-stage process: intraparticle diffusion dominated the initial stage, whereas other rate-limiting mechanisms may have contributed to the second stage. The first- and second-order kinetic models suggested a combined mechanism involving physisorption and chemisorption. Physisorption appeared to be more prevalent, particularly in the second stage, whereas chemisorption governed the initial stage.

These findings demonstrate the potential of the investigated oil shale as an unconventional and cost-effective sorbent, potentially serving as a substitute for activated carbon in 2,4-DCP removal.

4-dichlorophenol

oil shale

sorption

kinetics

isotherm models

cost-effective sorbent

Among the various pollutants in aquatic ecosystems, phenols, especially chlorinated ones, are considered priority pollutants because of their harmful effects on organisms, even at ppb concentrations; thus, they carry high ecological risks. The European Union and the Environmental Protection Agency of the United States list many of these chemicals as priority pollutants (Ma et al. 2023). They originate from the wastewater generated by industrial processes and are widely used as intermediates in the synthesis of pesticides, medicines, dyes, and plastics (Yadav et al. 2023). Furthermore, chlorophenols are frequently used as disinfectants or preservatives for wood, paints, natural fibers, and leather owing to their antimicrobial properties (Zhang et al. 2021). 2,4-dichlorophenol (2,4-DCP) was selected as a model compound in this study because it is representative of chlorophenols with antimicrobial properties. This compound can readily accumulate in nature owing to its persistent nature, which hinders its disappearance over time. The contamination of water with 2,4-DCP also poses a threat to the ecosystem, considering the toxic effects of the compound. This could lead to biomagnification of organisms across various food chains. Therefore, it is essential to remove 2,4-DCP from industrial effluents of interest to the whole ecosystem.

Removal from aqueous media is complicated because of its high solubility and low air-water partition coefficients. Several processes have been developed for the removal of chlorophenols, with one of the leading methods being sorption by activated carbon (Garba et al. 2019). However, because of the high cost of activated carbon, a number of cost-effective materials, such as fly ash and sawdust, have been investigated as substitutes for activated carbon sorbents (Chikri et al. 2020; Kobya et al. 2024).

Oil shale is also one of the promising materials, which can be used as a sorbent even in spent form (Abu-Surrah and Al-Degs 2022; Acar et al. 2015). Oil shale is a sedimentary rock of natural origin with no artificial chemicals. It consists mainly of organic materials that are present in an amorphous form, clay, carbonate minerals, and a smaller amount of gypsum. Oil shale formed in lakes filled with non-active volcanic craters or closed sea lagoons, resulting in two distinct types: maar- and lagoon-type oil shale, depending on the location of formation. Oil shale is abundant worldwide, and because of its versatile nature, it can be utilized as a cost-effective sorption material.

A special maar-type oil shale was used in this study, whose organic material content was primarily derived from residues of Botryococcus braunii microalgae. The goal of this work was to investigate how effectively oil shale can be utilized as a sorption material to remove the chlorinated phenol model chemical 2,4-DCP.

2.1. Materials

Sorption experiments were conducted on mining-raw oil shale obtained from the Pula mine (Hungary). Before using oil shale, the raw material was air-dried, milled, and sieved. The fraction with a particle size of Ø < 360 µm was homogenized and subsequently used.

Sigma Aldrich supplied the 2,4-DCP (99%). Sodium chloride and mono- and disodium phosphate, which provided pH and ion strength stability, were purchased from Reanal Laborvegyszer Kft. Solutions of varying concentrations of 2,4-DCP were prepared in distilled water.

2.2. Sorption experiments

Experiments were performed using a batch system. Five grams of oil shale powder were weighed into 250 mL stoppered glass Erlenmeyer flasks. When the effect of the sorbent/liquid ratio was investigated, the amount of oil shale ranged from 0.5 to 15 g. Half to fifteen milliliter(s) of Milli-Q water (one mL water per gram of oil shale) was added to this oil shale powder and left to swell overnight at room temperature. 50–50 mL of various initial solute concentrations (10 to 100 mg/L (61.3 to 613 µmol/L)) of 2,4-DCP, also containing 0.01 mol/L NaH₂PO₄, Na₂HPO₄ buffer system and 0.1 mol/L NaCl, were added to the swollen oil shale powder samples. The pH of these solutions was adjusted to three relevant environmental values: 5, 7, and 8, and the pH dependence was investigated. In all the other cases, the pH was consistently set at 7. The pH of the liquid phase was measured using a Radelkis combination pH electrode. These suspensions were shaken for 24 h at 20°C (for different durations or at the indicated temperatures) on an orbital shaker (200 rpm) in an incubated environment. Subsequently, approximately 1 mL of the supernatant was sampled and centrifuged at 6000 rpm for 20 min. After achieving perfect separation, the concentration of 2,4-DCP in the liquid phase was determined by a combination of high-performance liquid chromatography and UV/Vis spectroscopy (HPLC-UV). All samples were analyzed in triplicate. Throughout each set of experiments, a blank sample was processed without solute.

2.3. Analytical methods

2.3.1. Instruments for characterization of the utilized oil shale

The particle-size distribution of the oil shale was measured using a Malvern Mastersizer 2000 laser diffraction system. The total organic carbon (TOC) was determined using a Tekmar-Dohrman Apollo 9000 TOC analyzer. Porosimetric measurements, including Brunauer-Emmett-Teller (BET) surface area and pore size determination, were conducted using CO₂ gas on a Quantachrome iQ-MP-AG instrument with a bath temperature of 0°C. The main mineral fractions were analyzed using a Philips PW3710 X-ray diffractometer (K filter, Cu radiation (50 kV, 40 mA)). The crystalline phases were identified using X’Pert Highscore Plus software. Scanning electron microscopy (SEM) measurements were performed using a Thermo Fisher APREO SEM equipped with EDX units operating at an accelerating voltage of 20 kV.

2.3.2. Determination of the concentration of 2,4-DCP by HPLC-UV

The concentrations of 2,4-DCP were measured using a Merck Hitachi LaChrom HPLC system equipped with a Waters Nova-Pak C18 column (75 × 3.9 mm, 4 mm, 60 Å) and a programmable UV detector. The mobile phase consisted of 55% acetonitrile in Milli-Q water. The flow rate was 1 mL/min. An autosampler injected a 10 µL sample into the column, and the absorbance values of 2,4-DCP were then recorded at 218 nm. The retention times were approximately 1.3 minutes. The concentration of the samples was calculated from the slope of the calibration curve generated by measuring 2,4-DCP standards in the concentration range of 1 to 50 mg/L (6.13 to 307 µmol/L).

3.1. Characterization of the applied oil shale

The porosimetry and key characteristics of the oil shale are listed in Table 1.

Table 1

Some main characteristics of the applied oil shale
Property	Value
pH	7.23
Total organic carbon	24.4%
Characteristic particle size	50–160 µm
BET surface	73.8 m²/g
Average pore size	0.955 nm

Figure 1 shows a SEM image of the micronized oil shale. The image clearly shows the well-preserved cell walls of fossilized Botryococcus colonies, which withstood diagenesis without any significant morphological changes. The cup-shaped and chimney-like remnants of the colonies formed mesh-like structures. Chemically resistant cell walls formed a porous structure, resulting in a relatively high specific surface area.

3.2. Investigation of the sorption of 2,4-DCP on oil shale under different conditions

To determine the optimum sorption conditions of 2,4-DCP on oil shale, the environmental conditions, such as contact time, sorbent/liquid ratio, temperature, and pH were varied.

3.2.1. Effect of contact time on sorption

The quantity of sorbed compounds significantly depends on the contact time (Krime et al. 2023); thus, the removal efficiency of 2,4-DCP was examined within the range of 0–38 h to assess its impact (Fig. 2a).

The results indicated that 80% of the maximum 2,4-DCP removal occurred within the first 2 h. The removal in this stage is significant and rapid, as there is still a considerable amount of available free sorption space on the sorbent. As a result, the concentration gradient is high at the surface, and 2,4-DCP can rapidly bind to the surface of oil shale via sorption. A further increase in the contact time caused a slight increase in the sorbed quantity. At this stage, the sorbed molecules extensively cover the sorption sites, making it challenging for additional molecules to occupy the remaining vacant active sites. Consequently, the sorption process slows. In the third stage, the extent of sorption did not change significantly as the removal efficiency saturated with the filling of active sites, reaching dynamic equilibrium.

3.2.2. Effect of the sorbent/liquid ratio on sorption

One of the key parameters influencing the liquid-phase sorption is the sorbent/liquid ratio. The specific sorbed amount of solute, defined as the quantity of dissolved substance sorbed per unit mass of sorbent, typically decreases as the mass of the sorbent increases in a constant volume and concentration of the solute, according to Kroeker’s rule (Pernyeszi et al. 2019). The practical significance of this is in the design of water purification processes during the planning of the operational parameters. It is crucial to determine the optimal amount of sorbent required for the sorption process under given conditions to achieve the desired efficiency.

To determine the optimal solid sorbent/liquid ratio, a Kroeker isotherm was constructed using a solution volume of 50 mL and an initial nominal 2,4-DCP concentration of 50 mg/L (307 µmol/L) (Fig. 2b). The weighed mass of the oil shale ranged between 0.5 and 15 grams. If the quantity of the sorbent reached or exceeded 80%, sorption could be considered efficient.

The results indicate that the specific sorbed amount and equilibrium concentration of the solute significantly decreased as the mass concentration of oil shale increased in the 0.5 and 5 g sorbent mass range, using a constant initial concentration of the solute. These results are in accordance with Kroeker's rule. In the range above 5 g of oil shale, the decrease in the specific sorbed amount and equilibrium concentration was less significant.

Depending on the sorbed quantity of the solute in percentage as a function of the sorbent mass, similar conclusions can be drawn (Fig. 2c). In the 0.5 and 5 g sorbent mass range, the increase in the sorbent quantity significantly enhanced the sorption efficiency. This increment goes beyond linear proportion. When 5 g of oil shale was used, the sorption efficiency reached 95% at the applied solution concentration. Further increasing the quantity of the sorbent under constant conditions did not lead to significant additional efficiency enhancement in sorption. Therefore, in further experiments, a 1:10 of sorbent/liquid ratio (5 g of oil shale per 50 mL of supernatant) was used.

3.2.3. Effect of temperature on sorption

Temperature also affects the sorption process. It influences the physicochemical properties of the solution and the surface of the sorbent, as well as the movement of the dissolved substances. Upon increasing the temperature, the viscosity of the solution decreased, leading to an increase in the diffusion rate of the sorbate molecules between the external boundary layer and the internal pores of the sorbent particles. Additionally, it alters the equilibrium capacity of the sorbent (Sathishkumar et al. 2009).

The effect of temperature on the sorption capacity of oil shale was examined at 2, 10, 15, 20, 30, and 40°C using a 50 mg/L (307 µmol/L) concentration of 2,4-DCP. The results are shown in Fig. 2d. It can be observed that the increase in temperature enhanced the removal of the substance, indicating an endothermic sorption process. When the temperature of the system increased from 2°C to 40°C, the sorption capacity of the oil shale changed from 2.9046 µmol/g to 3.2967 µmol/g, representing a 13.5% increase.

According to the literature, physisorption processes are dominant and are generally exothermic (Weber 1973). However, in this case, the sorption capacity improved with increasing temperature. The potential reasons for the increase in sorption capacity are as follows (Rápó and Tonk 2021):

At higher temperatures, cracks and pores may open on the surface of the sorbent, leading to an increase in the number of active sites and resulting in enhanced sorption efficiency. However, this depends on the size of the sorbing molecule and pore diameter of the sorbent.

At elevated temperatures, the chemisorption process could become significant for solute sorption. In the case of the investigated substance, sorption may involve not only surface accumulation, but also surface reactions, leading to chemical bonds between the surface and sorbate molecules.

As a result of hydrophobic hydration (in an aqueous environment), the water structure changes in the vicinity of poorly soluble compounds and surfaces with poor wettability (hydrophobic properties). Water is less mobile and has lost some degrees of freedom. From this state, it can only escape with the help of activation energy, which increases with increasing temperature. Hydrophobic molecules come closer to the surface, allowing van der Waals attractions to be exerted, thereby increasing the sorbed amount (Chen et al. 2003; Petersen et al. 2009 ; Tielrooij et al. 2010).

The increase in sorption capacity in the higher temperature range could also be attributed to the fact that 2,4-DCP molecules easily penetrated the external boundary layer and diffused into the pores owing to the elevated temperature. This reduces the diffusion resistance involved in the sorption process. A higher temperature decreases the viscosity of the solution; consequently, the molecules diffuse more easily through the external liquid film layer surrounding the oil shale particles.

3.2.4. Effect of pH on sorption

The environmental pH has a crucial impact on the sorption process of pollutants. The pH of the investigated sorption system plays a significant role in sorption at the solid-liquid interface, influencing both the surface and chemical properties of the sorbate, particularly in the case of ionic compounds. A change in pH can affect the properties of sorbents, substances, and aqueous buffered solutions, thereby altering the interactions formed among them. The extent of sorption dependence on pH in the case of complex sorbents, such as oil shale, also depends on its organic content and composition. Increasing the pH could increase the amount of dissolved humic substances originating from the oil shale, thereby enhancing the solubility of pesticides, which, in turn, reduces the extent of sorption (Ertli et al. 2004). Another important parameter is the strength of the buffering system in the liquid phase, which determines the initial pH of an aqueous buffer solution. Consequently, proton transfer reactions occurring between the components of oil shale and the supernatant during the sorption process could significantly shift the initial pH of the solution, and altered pH values might affect the sorption process.

The velocities of the buffer reactions in the pH range of 5–8 were measured, as shown in Fig. S1 in the Supplementary Information (SI). These reactions occurred relatively quickly during the initial sorption phase. Moreover, during the same sorption stage, a large amount of solute was sorbed by the sorbent, as discussed earlier. However, these final pH values deviated from the initial values because of the strong buffering capacity of the sorbent (Table S1 in the SI).

The pH of the sorption system fundamentally influences the surface charge of the sorbent and the degree of ionization (protonated versus deprotonated form) of the phenolic sorbate. Therefore, it is important to consider the variation in the final pH for each sorbent. When the sorbent has a negative surface charge, the sorption of phenolic compounds is more favorable at a lower pH. At higher pH values, repulsive forces become dominant between 2,4-DCP and the surface of the sorbent owing to the anionic form of the solute. This means that neutral phenolic molecules can bind to the negatively charged sorbent to a greater extent, whereas phenolate anions are capable of binding to a lesser extent. The pKa value of 2,4-DCP is approximately 7.85. Thus, at pH of 7.85 or lower, the sorption process, predominantly physisorption, primarily took place via hydrophobic interactions and hydrogen bonds.

The pKa value of 2,4-DCP is approxymetly 7.85, so under pH = 7.85, the sorption process occured by mainly hydrophobic interactions and/or hydrogen bonds (Ertli et al. 2004). When the ionic form of 2,4-DCP was predominant (pH > pK_a = 7.85), the electrostatic repulsion between the sorbent and solute became more significant. Thus, the extent of 2,4-DCP sorption decreased.

The results of the pH-dependent sorption of 2,4-DCP can be seen in Fig. 3. The final pH remained below the pKa value of 2,4-DKF in all the cases. Therefore, a significant portion of the solute was present in its molecular form during the sorption process. The molecules primarily adhere to the surface through van der Waals forces, and electrostatic repulsion may exert less influence on sorption. In the case of oil shale-based sorbents, comparing the results using solutions with pH values of 5 and 7, the effect of the solution pH was less significant; a slight difference in the sorbed amount was observed. When the initial solution pH was 8, the repulsive forces between the phenolate anions and negatively charged oil shale slightly diminished the sorption efficiency.

3.3. Fitting of the sorption models and fitting parameters

Sorption isotherms can elucidate the relationship between the sorbent and sorbate, and various mathematical models can be applied to describe them. Oil shale, as a sorbent for environmental pollutants, possesses numerous types of binding sites (Chianese et al. 2020; Huang et al. 2003), leading to the formation of rather complex sorption mechanisms.

Langmuir or Freundlich isotherms (Eqs. (1) and (2)), which provided a better fit (determined by the nonlinear least-squares fitting procedure) to the measured data, was applied. In all cases, the Freundlich isotherm was fitted to the data using the coefficient of determination (R²). The fitting parameters and R² values are presented in Table 2.

Langmuir isotherm: $\:{q}_{e}={q}_{max}\bullet\:\frac{{K}_{L}\bullet\:{c}_{e}}{1+{K}_{L}\bullet\:{c}_{e}}$ (1)

Freundlich isotherm: $\:{q}_{e}={K}_{f}\bullet\:{c}_{e}^{1/n}$ (2)

where q_e is the sorption capacity at equilibrium (µmol/g); c_e is the equilibrium solute concentration (µmol/L); q_max is the maximum amount of solute sorbed (µmol/g); K_L is the Langmuir constant (L/µmol); K_F (L^1/n /(µmol^{((1/n)− 1)}·g)) and n (dimensionless) are Freundlich constants.

Table 2

Fitting parameters of Langmuir and Freundlich isotherms
Initial pH	Langmuir isotherm			Freundlich isotherm
Initial pH	q_max	K_L	R²	K_F	1/n	R²
5	13.8	1.59·10^− 2	0.977	0.437	0.675	0.986
7	22.2	6.27·10^− 3	0.993	0.232	0.794	0.995
8	31.5	2.60·10^− 3	0.986	0.141	0.835	0.990

The Freundlich model assumes that the surface of the sorbent is heterogeneous and possesses various sorption sites (Sparks et al. 2024). The applicability of this model is owing to its consideration of multilayer sorption on heterogeneous surfaces. Heterogeneity is ensured by the diversity of sorption sites and functional groups present on the surface of the oil shale, with which the adsorbed molecules interact.

The K_f and n parameters of the Freundlich isotherm model represent the sorption capacity and sorption intensity, respectively. The K_f values calculated using the Freundlich model were consistent with the results and accurately reflected the variations in the sorbed quantities. The n parameter of the Freundlich equation reflects the sorption intensity, and a value of n > 1 indicates favorable sorption, suggesting that the surface and energy of the sorption sites are heterogeneous, and interactions occur among the sorbing molecules (Khan et al. 2022). In accordance with the sorption results, as the pH increased, the value of n decreased (1.48, 1.26, and 1.20 at pH = 5, 7, and 8, respectively).

Although the Freundlich isotherm had a better fit to the measured points, the difference in R² values were not too large compared to that of the Langmuir isotherm. As can be seen in Table 2, increasing the pH evoked an increase in the maximum sorption capacity (13.8, 22.2, and 31.5 µmol/g). This is seemingly a contradictory result, since increasing the pH diminished the sorption efficiency in the investigated concentration range. However, the Langmuir isotherm model assumes monolayer sorption. In reality, multilayer sorption may occur. Therefore, the extrapolation results were unreliable over a significantly larger concentration range. The K_L value of the Langmuir isotherm, which is related to the energy of sorption, is analogous to the n parameter obtained from 1/n of the Freundlich isotherm. As the pH increased, similar to the n value, K_L decreased, which suggests energetically unfavorable sorption.

The number of studies published by other authors investigating the sorption of 2,4-DCP using oil shale is infinitesimal. To compare the obtained values, these results were compared with the adsorption results obtained for crude soil. However, the number of these studies cannot be considered numerous (Cea et al. 2007; Liao 2004). In these studies, the sorbed quantity of 2,4-DCP in the crude soil was only a fraction of that observed in the present study. Liao (2004) found a similar sorption quantity of 2,4-DCP after modification of the studied soil. This means that the investigated oil shale, which is compositionally similar to soils, could sorb significantly greater quantities of 2,4-DCP without modifying the sorbent.

3.4. Sorption kinetics

Data for the specific sorbed amount (q) and time (t) were utilized to determine the rate equations and to investigate the role of diffusion as the rate-determining step in sorption. The kinetics of the sorption process was characterized by linear fitting of the models.

3.4.1. Weber-Morris model

The role of intraparticle diffusion as the rate-determining step was examined using the Weber-Morris model (Weber and Morris 1963) (Eq. (3)).

$$\:{q}_{t}={k}_{WM}\bullet\:{t}^{\frac{1}{2}}+c$$

where q_t is the sorbed amount of solute at elapsed time t (µmol/g); k_WM is the internal diffusion rate constant (µmol/(g·hour^1/2)); c is the intercept (µmol/g).

If intraparticle diffusion is involved in the sorption process, plotting q_t vs. t^1/2 results in a linear relationship according to the model. If this line passes through the origin, intraparticle diffusion was the rate-determining step of the sorption process (Li et al. 2022). The Weber-Morris kinetic plot for 2,4-DCP sorption is shown in Fig. 4a. Two-step kinetic behavior was observed, and the plot exhibited a linear relationship passing through the origin. Rapid initial sorption was followed by a slower process that gradually reached equilibrium. The first stage exhibited a high determination coefficient (R² = 0.997), and the fitted line passed through the origin, indicating that intraparticle diffusion predominated in the initial stage of sorption. In the second stage, the points aligned on a straight line (R² = 0.951), but other rate-determining processes may have contributed.

3.4.2. Pseudo-first- and second-order kinetics

Lagergren introduced a first-order kinetic rate equation, its linear form is expressed by the following equation (Eq. (4)):

$$\:\text{log}\left({q}_{e}-{q}_{t}\right)=log{q}_{e}-\frac{{k}_{1}\bullet\:t}{2.303}$$

where k₁ is the rate constant for the pseudo-first-order kinetic model (1/hour), the other symbols have been previously defined.

According to the model, physisorption played a significant role in the sorption of the substance, but there might be other determining processes in the binding (Abd Wahab Sha’arani et al. 2019).

The first-order model was less applicable to the data in the initial stage, where R² was lower (0.944) (Fig. 4b) in the linear representation. This may be because the boundary layer limits the sorption process during rapid sorption. However, the model fit well with the experimental data from the second stage, indicating that the first-order model is applicable for a slower sorption process (R² = 0.996).

The linear pseudo-second-order model (represented by Eq. (5)) is well suited for investigating the sorption kinetics of 2,4-DCP over the entire range, according to the R² value (0.999) (Fig. 4c).

$$\:\frac{t}{{q}_{t}}=\frac{1}{{k}_{2}\bullet\:{q}_{e}^{2}}+\frac{1}{{q}_{e}}\bullet\:t$$

where k₂ is the pseudo-second-order rate constant (g/(µmol·h)), and all other symbols have been defined previously.

From the good fit of the pseudo-second-order model in the linear representation (t/q_t vs. t), it can be inferred that molecules bound to the sorbent via chemisorption as well, and presumably, this process might be the rate-determining step in sorption (Ho and McKay 1999; Obike et al. 2018).

3.4.3. Non-linear representation of the kinetic models

Usually, the fit of the pseudo-second-order model is better for the experimental data points (Febrianto et al. 2009; Plazinski 2013). However, it is possible that this is because the models were fitted to the experimental data points in a linear representation of the kinetic equations. Nevertheless, the accuracy of this linearization can be questioned (Plazinski, 2013). Therefore, the nonlinear forms of the abovementioned models (Table S2 in the SI) were fitted to the measurement data in a conventional manner (q_t vs. t) (Fig. 5). The fitting parameters of these models and the R² values are listed in Table 3.

The R² value for the Weber-Morris model was very low, indicating that the removal rate did not follow this model over the entire range. However, the experimental data were well fitted to the pseudo-first- and second-order models according to their R² values, with that of the second-order kinetic data being the closest to 1 among the three models. Comparing the above results, it can be concluded that intraparticle diffusion was significant only in the first stage of the sorption of 2,4-DCP on oil shale and not in the entire range of the sorption process. The two main processes involved in the sorption were physisorption, which was more prominent in the second, slower stage of sorption, and chemisorption, which was the primary determining process. The sorption of 2,4-DCP on the oil shale followed pseudo-second-order kinetics.

Table 3

Fitting parameters of the three non-linear kinetic models
Weber-Morris model			pseudo-first-order kinetic			pseudo-second-order kinetic
k_WM	c	R²	q_e	k₁	R²	q_e	k₂	R²
0.8079	0	0.216	2.685	1.5171	0.974	2.875	0.7506	0.997

This study aimed to investigate the effects of contact time, sorbent/liquid ratio, temperature, and pH on the sorption of 2,4-DCP using unconventional oil shale as a sorbent. The results indicated that the sorption equilibrium was practically reached within 24 h. When the sorbent/liquid ratio reached 1:10, any further increase in the sorbent ratio had an almost negligible effect on the sorption equilibrium. Surprisingly, the sorption capacity increased with increasing temperature, which might be attributed to the sorbent characteristics and/or the binding process via chemisorption. The highest sorption of 2,4-DCP was observed at pH 5, the lowest pH value in the investigated pH range. The Freundlich isotherm provided a better fit to the measured data, indicating a multilayer sorption process. The applied kinetic models indicated that the sorption process can be divided into two stages. The first stage involved faster sorption, in which intraparticle diffusion was the rate-determining step. In the second stage, additional rate-determining processes might play a significant role. Both physisorption and chemisorption took part in the solute binding. This study suggests that the efficient removal of 2,4-DCP from the liquid phase can be achieved under optimal conditions using oil shale as a sorbent. In the future, other chlorinated aromatic compounds should be investigated using the same sorbent to broaden their applicability.

Funding

The authors did not receive support from any organization for the submitted work.

Competing interests

The authors have no competing interests to declare that are relevant to the content of this article.

Author Contribution

Renáta Rauch: Conceptualization, Methodology, Investigation, Writing - Original Draft, Review & Editing, Visualization, Supervision. Éva Lukács: Investigation. Miklós Molnár: Investigation, Writing - Original Draft, Review & Editing, Visualization. All authors read and approved the final manuscript.

Acknowledgements

The authors acknowledge Dr. Miklós Jakab (University of Pannonia) for the SEM image.

Data Availability

The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Information files. Should any raw data files be needed in another format they are available from the corresponding author upon reasonable request.

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Adsorptive removal of 2,4-chlorophenol from water by micronized oil shale

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Abstract

Figures

1. Introduction

2. Materials and methods

2.1. Materials

2.2. Sorption experiments

2.3. Analytical methods

2.3.1. Instruments for characterization of the utilized oil shale

2.3.2. Determination of the concentration of 2,4-DCP by HPLC-UV

3. Results and discussion

3.1. Characterization of the applied oil shale

3.2. Investigation of the sorption of 2,4-DCP on oil shale under different conditions

3.2.1. Effect of contact time on sorption

3.2.2. Effect of the sorbent/liquid ratio on sorption

3.2.3. Effect of temperature on sorption

3.2.4. Effect of pH on sorption

3.3. Fitting of the sorption models and fitting parameters

3.4. Sorption kinetics

3.4.1. Weber-Morris model

3.4.2. Pseudo-first- and second-order kinetics

3.4.3. Non-linear representation of the kinetic models

Conclusion

Declarations

Funding

Author Contribution

Acknowledgements

Data Availability

References

Additional Declarations

Supplementary Files

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