Abstract
Raw and citric acid chemically treated date palm stone agro-waste biomass (RDSB and CA-MDSB) powders were used to remove an important class of emerging industrial pollutants, i.e., 2,4,6-trichlorophenol (2,4,6-TCP) from aqueous solutions towards sustainable waste utilization to develop cost-effective technology for treating wastewater. The biomass characterization was performed by using different analytical techniques such as CHN elemental analysis, particle size, BET, FTIR, and SEM-EDX, TGA analysis. The FTIR spectral analysis revealed that the main chemical groups (N–C, O=C, H-O, H-C, and O–C) were involved in trapping 2,4,6-TCP. The highest adsorption was achieved with a contact time of 150 and 120 min, an initial concentration of 50-200 mg/L, and a biosorbent dosage ranging from 0.1 to 1.0 g/L RDSB and CA-MDSB, respectively. The experimental kinetic data of the adsorption process for both adsorbents (RDSB and CA-MDSB) fitted very well with the pseudo-second-order kinetic model and Langmuir equilibrium data. The 2,4,6-TCP maximum monolayer adsorption capacities were 53.7, and 123.8 mg/g for RDSB and CA-MDSB, respectively. The present research confirms that the date palm stone biomass could be used as an effective and low-cost biosorbent for the remediation of 2,4,6-TCP from an aqueous environment.
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Remediation of 2,4,6-Trichlorophenol from Aqueous Solution by Raw and Chemical Modified Date Palm Stone Biomass: Kinetics and Isotherms Studies
Nadavala Siva Kumar,a,* Mohammad Asif,a Anesh Manjaly Poulose,b Ebrahim H. Al-Ghurabi,a Shaddad S. Alhamedi,a and Janardhan Reddy Koduru c
Raw and citric acid chemically treated date palm stone agro-waste biomass (RDSB and CA-MDSB) powders were used to remove an important class of emerging industrial pollutants, i.e., 2,4,6-trichlorophenol (2,4,6-TCP) from aqueous solutions towards sustainable waste utilization to develop cost-effective technology for treating wastewater. The biomass characterization was performed by using different analytical techniques such as CHN elemental analysis, particle size, BET, FTIR, and SEM-EDX, TGA analysis. The FTIR spectral analysis revealed that the main chemical groups (N–C, O=C, H-O, H-C, and O–C) were involved in trapping 2,4,6-TCP. The highest adsorption was achieved with a contact time of 150 and 120 min, an initial concentration of 50-200 mg/L, and a biosorbent dosage ranging from 0.1 to 1.0 g/L RDSB and CA-MDSB, respectively. The experimental kinetic data of the adsorption process for both adsorbents (RDSB and CA-MDSB) fitted very well with the pseudo-second-order kinetic model and Langmuir equilibrium data. The 2,4,6-TCP maximum monolayer adsorption capacities were 53.7, and 123.8 mg/g for RDSB and CA-MDSB, respectively. The present research confirms that the date palm stone biomass could be used as an effective and low-cost biosorbent for the remediation of 2,4,6-TCP from an aqueous environment.
DOI: 10.15376/biores.19.2.3543-3570
Keywords: Adsorption; Date palm stone; Characterization; 2,4,6-trichlorophenol; Kinetics and Isotherm modeling
Contact information: a: Department of Chemical Engineering, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia; b: Department of Chemical Engineering, SABIC Polymer Research Centre, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia; c: Department of Environmental Engineering, Kwangwoon University, Seoul 01897, Republic of Korea; *Corresponding author: snadavala@ksu.edu.sa
INTRODUCTION
Industries have a significant impact on our natural environment, especially aquatic ecosystems. Chemicals used in the manufacturing process, such as petrochemicals, paints, plastics, pharmaceuticals, oils, detergents, pesticides, herbicides, and paper-making release industrial organic effluents that contain phenol and chlorophenol derivatives (Priyanka et al. 2014; Soltani Goharrizi et al. 2019; Nazal et al. 2021). These chemicals are hazardous to human health and the environment. 2,4,6‐Trichlorophenol (2,4,6‐TCP) is particularly dangerous due to its special chemical structure, high level of toxicity, strong odor emission, persistence in the environment, and carcinogenic properties. The United States Environmental Protection Agency has listed most phenolic pollutants as a potential threat to living organisms (US EPA 1977). Therefore, it is crucial to remove these pollutants from contaminated sites and treat wastewater before discharging it into the water circulatory system.
Several techniques have been employed to eradicate phenolic derivative compounds from wastewater, including biodegradation, chemical, biological, electrochemical, photochemical, and catalytic processes (Baker et al. 1980; Ahmaruzzaman 2008; Wang et al. 2012; Villegas et al. 2016; Nazal et al. 2021; Zada et al. 2021; Grace Pavithra et al. 2023). However, some of these techniques are costly and have high energy consumption due to the complexity of treatment processes and the production of toxic by-products or specific conditions. The adsorption technique has become a popular option due to its ease of operation, cost-effectiveness, environmental compatibility, flexibility, lack of secondary pollution, and efficient performance in treating phenolic wastewater (Crini et al. 2019; Ho 2022; Abdulrahman et al. 2023; Hyder et al. 2023). As a result, many researchers are selecting suitable adsorbents from industrial and agricultural waste products for the remediation of phenolic pollutants (Kurnik et al. 2015; Dehghani et al. 2023). Agro-waste biomass adsorbents have the potential to remove heavy metals, dyes, phenol, and phenolic derivatives from effluents due to the presence of functional groups that can bind pollutants (Kumar et al. 2018a, b; Kadhom et al. 2020; El-Bery et al. 2022; Haque et al. 2022; Jabeen et al. 2022; Hambisa et al. 2023; Sen 2023). Among the various types of agro-waste biomass that are locally and abundantly available, date palm is one of the best candidates for this study purpose.
Date palm is a highly significant fruit crop in Saudi Arabia, producing almost 1.58 million tons of dates annually, which accounts for nearly 20% of the global date production. Date fruit is made up of fruit pulp (85 to 90% w/w) and a seed or pit (10 to 15% w/w) (Li et al. 2020). In 2021, Saudi Arabia ranked first in the world in date exports. Across the Middle East region, the total number of date palm trees in Saudi Arabia is almost 36 million, which includes 27 million fruitful trees as of 2020. The Riyadh Region has the highest number of date palm trees in Saudi Arabia, with a total of 9 million. The region produces more than 400,000 tons of dates, which represents 24% of the Kingdom’s total production (Santos Rocha et al. 2018; Nourani and Pegna 2022). The date pit material is not only cheap and renewable but is also considered an environmental problem in many regions, especially in Saudi Arabia and Mediterranean countries. Many studies carried out in the past few years have utilized date palm stone waste biomass (Ahmed 2016) due to its high carbon content, effectiveness, widespread availability, and low cost for removing heavy metals (Al-Ghouti et al. 2010; Hilal et al. 2012; Shafiq et al. 2018), dyes (El Messaoudi et al. 2016; Pathania et al. 2016), and phenolic derivative compounds (Al-Mutairi 2010; Okasha and Ibrahim 2010; Ahmed and Theydan 2012). Therefore, the usage of date palm stone agro-waste biomass as an adsorbent can be an effective and eco-friendly solution for the remediation of 2,4,6-TCP from wastewater.
This study investigated the effectiveness of date palm powder in its raw form and after chemical modification as an adsorbent for the remediation of chlorophenol compound (2,4,6-TCP). The study also examined how various operating conditions (such as initial pH, contact time, sorbent dosage, and adsorbate concentration) affect the performance of the adsorbent. Batch equilibrium studies were conducted at each stage of the study, and the samples were thoroughly characterized using a particle-size analyzer, elemental analyzer (CHN), BET surface area, SEM-EDX, FTIR, and TGA analysis, to better understand the relationship between the properties of the adsorbent and its ability to remove contaminants.
EXPERIMENTAL
Chemicals
The 2,4,6-trichlorophenol compound was obtained from Sigma-Aldrich (USA), and the stock solution was prepared by dissolving 1.68 g in 1000 mL of double-deionized water. For the batch experiments, diluted solutions of low concentrations ranging from 50 to 200 mg/L were prepared from the stock solution. The pH of the solution was adjusted using 0.1 M HCl and 0.1 M NaOH solutions.
Collection and Preparation of Date-Palm Stone Biomass
Agro-waste raw date palm stones were gathered from the date palm orchards surrounding Riyadh City, Kingdom of Saudi Arabia. To remove impurities, the date palm stone (DPS) was initially washed with water and then dried for 30 h at 100 °C. The DPS was then ground using a DLC multifunctional grinder, and the resulting powder was sieved. The collected sieved powder was then subjected to ball-milling using Fritsch Pulverisette 7 Premium line, Germany, with zirconia ceramic and steel balls at 400 rpm for 24 h. Additionally, POWTEQ Laboratory’s Micro Ball mill (GT300, Beijing, China) was utilized for 30 min at 1500 rpm. The raw micro ball-milled date palm stone sample was designated as RDSB.
Chemical Modification of Raw Date-Palm Stone Biomass
The raw date palm stone biomass (RDSB) was chemically modified using a method similar to the one previously described by Xu et al. Yin Kuo et al. and Wang et al. with minor adjustments in reaction temperature and time (Kuo et al. 2015; Wang et al. 2016; Xu et al. 2016). To prepare CA-MDSB, 10 grams of RDSB were added to 300 mL of 0.7 mol/L citric acid (CA) solution and stirred at 400 rpm for 2 h at 30 °C. The date palm impregnated with CA was dehydrated at 70 °C for 24 h, then heated to 110 °C and kept for 120 min before cooling down to room temperature. The obtained chemical-modified RDSB by CA was washed with double distilled water several times to neutralize pH, remove excess CA, and finally dried at 80 °C for 24 h.
Batch Studies
To investigate the adsorption equilibrium, batch experiments were conducted. An amber glass reagent bottle with a capacity of 125 mL was used. Inside the bottle, 0.1 g of RDSB and CA-MDSB were added into a 100 mL solution containing 2,4,6-TCP at a concentration of 50-200 mg/L. The samples were stirred using a shaking water bath at 175 rpm and kept the temperature constant at 30 ± 1 °C until the samples reached a steady state. The pH of the solution was varied from 2 to 10. To measure the concentration of 2,4,6-TCP, a UV-Vis spectrophotometer (Model: Shimadzu UV-1900, Tokyo, Japan) was used, and it was operated at a wavelength of 296 nm. To determine the sorption capacity (qe, mg/g) and the removal efficiency, following equations were employed (Kumar et al. 2021),
(1)
(2)
where C0 is the 2,4,6-TCP concentration (mg/L) at time = 0, qe is the equilibrium adsorption capacity, Ce is the 2,4,6-TCP concentration (mg/L) at equilibrium, M is the RDSB and CA-MDSB biomass (g), and V is the volume (L) of the solution.
During batch kinetics studies, the 2,4,6-TCP sorption, qt (mg/g), at time t was calculated as follows,
(3)
where Ct is the concentration of solute (mg/L) at time, t (min).
Validation of Kinetic and Equilibrium Models
The normalized standard deviation of and the Chi-square (χ2) (Kumar et al. 2018a, 2019) obtained from Eqs. 4 and 5 were utilized to determine the most suitable fit of kinetics and equilibrium isotherm models. The mathematical expression for this is in the supplementary material.
Characterization of RDSB and CA-MDSB Materials
Various techniques were employed to study the RDSB and CA-MDSB material. These techniques included particle size, elemental analysis (C, H, N), and BET analysis to determine specific surface area, pore volume, and average pore size. In addition, the FTIR method was used to identify the adsorbent’s surface functional groups. SEM-EDX was utilized to analyze the biomass morphology. Thermogravimetric analysis (TGA) measures weight changes of a sample with respect to temperature and time. A detailed explanation of these techniques is in the Supplementary Material section 2.6 in the Appendix.
RESULTS AND DISCUSSION
Particle Size Data Analysis
The crushed samples of date palm stones (RDSB) were sieved using standard sieves to obtain uniform samples. These samples were then subjected to ball-milling to decrease their size and improve their specific surface area. Two different ball-milling strategies were used. The first strategy involved conventional ball-milling for a long duration of 24 h to achieve maximum size reduction.
The second strategy used micro ball-milling for 30 min. The particle size distribution of both strategies was analyzed and shown in Fig. 1(a-b). A significant difference in size reduction was found between the two strategies. The average particle size of the RDSB, which was initially over 180 µm, was reduced to 7.08 µm with the 24-h ball-milling strategy. However, the same sample undergoing 30-min micro ball-milling was reduced to 1.12 µm, which is 84% greater than the reduction achieved with conventional ball-milling.
These findings conclusively demonstrate that micro ball-milling is an effective technique for producing small-sized adsorbent samples that cannot be achieved through conventional ball-milling.
Fig. 1(a-b). Particle size distribution curve of (a) 24 hrs ball-milled (b) 30 min micro ball-milled date palm stone biomass (RDSB)
The elemental composition of RDSB and CA-MDSB biomass samples, in terms of carbon, hydrogen, and nitrogen (C, H, N), is presented in Table 1. The analysis reveals that the C, H, and N content of the RDSB and CA-MDSB was similar to that of the local Sukkary date palm stones, which have 47.1% carbon, 6.63% hydrogen, and 0.9% nitrogen (Nasser et al. 2016). Another study that used Tunisian date-stone also reported similar values, with 46.8% C, 6.66% H, and 0.74% N (Rezma et al. 2019). The results indicate that the ball milling process caused less than 3% loss in the carbon content of the micro ball-milled RDSB sample, with a corresponding increase in oxygen content. This is because the fraction of other elements, i.e. hydrogen and nitrogen, remained unaffected during the milling process (Daffalla et al. 2020). After the chemical activation of the micro ball-milled biomass (RDSB) sample using citric acid, a slight decrease in the carbon, hydrogen, and nitrogen contents was observed.
Table. 1. Elemental Data (C, H, N) Analysis on RDSB and CA-MDSB Adsorbent
FTIR Analysis
The FTIR spectra of RDSB, CA-MDSB, and their compositions after 2,4,6-TCP adsorption were analyzed. The bands in the range of 4000 to 400 cm-1 were examined and shown in Fig. 2(a-d). The highest broadband peak at around 3425 cm–1 in Fig. 2a mainly reflects the stretching vibration of hydroxyl groups (–OH) (Li et al. 2018). The peak at 2929 cm–1 is assigned to the C–H stretching vibration of methyl groups. In addition, the remaining peaks are C=O (carbonyls), C=C (alkene), CH2 cellulose, lignin, C-H cellulose, hemicellulose stretching vibrations at 1736, 1628, 1442, 1365, and C-O stretching 1069 cm–1, respectively (Xu et al. 2016). In Fig. 2(c), spectra of the after chemical modification of CA-MDSB biomass peaks confirmed the presence of O-H, C-H stretching band vibrations at 3410 cm–1 and 2922 cm–1. Other peaks were due to C=O, C=C, and C-O-C, stretching vibration frequencies at 1748, 1644, and 1069 cm–1, respectively (Yang et al. 2010), indicating the presence of carbonyl group, alkene functional groups. Some bands in the range 600 to 800 cm–1 were due to the vibrations of O–H groups. After modification, the hydroxyl group peak intensity was decreased. The peak intensity of the carboxyl group significantly increased, which was evidently due to the introduction of citric acid (Gong et al. 2008; Wang et al. 2016). After adsorption, there was little and negligible change in the intensity of the spectrum peaks, as seen in Fig. 2(b and d). These changes could be attributed to specific interactions between hydroxyl and carboxyl groups with 2,4,6-TCP molecules.
Fig. 2. FTIR analysis of date palm stone biomass (a) RDSB (b) RDSB–after adsorption of 2,4,6-TCP (c) CA–MDSB (d) CA–MDSB after adsorption of 2,4,6-TCP
BET Analysis
The supplementary figures (S1 through S4) show the BET characterization of raw date-palm, 24-h ball-milled, micro ball-milled, and chemically treated with citric acid biomass samples in terms of N2 adsorption/desorption isotherm curves and cumulative pore size and pore volume (Inset). The BET surface area, pore volume, and pore size for all samples are presented in Table 2. The ground DPS had the lowest surface area, pore volume, and pore size. When the raw biomass sample was subjected to 24-h ball-milling, there was a significant enhancement of the surface area in most cases, with a rather insignificant increase in the pore volume and a substantial decrease in the pore size. Using micro ball-milling further improved the specific surface area for DPS. In fact, there was almost a hundred-fold increase in the surface area for DPS after micro ball-milling, mainly due to the reduction of the sample size. However, the pore volume showed a decrease. After micro ball-milling, the samples were subjected to chemical modification using citric acid (CA). In this case, there was a substantial reduction in the surface area since the micro ball-milled sample had 0.4574 m²/g before modification. The reduction in the surface area was accompanied by a simultaneous reduction in the pore volume, indicating the pore blockage due to the chemical modification of the micro ball-milled DPS sample (RDSB). Similar results were reported for the lowering of BET surface area in surfactant-modified montmorillonite and surface-modified lignocellulosic material (Krishna et al. 2000; Thamilarasi et al. 2018).
Table 2. BET data of a Grinding Sample of Date Palm, 24 h Ball-milling, and 30 min Micro Ball-milling Biomass Material
Fig. 3. SEM-EDX images of raw date palm stone (RDSB) biomass
Fig. 4. SEM-EDX images of CA-chemical modified date palm stone (CA–MDSB) biomass
SEM Analysis
Scanning electron microscopy shows the surface morphology of RDSB and CA-MDSB biomass. The images in Figs. 3(a) and 4(a) show particles with an irregular shape and a porous surface. Figure 3(a) shows that the morphology of the particle was bulky and irregular, with a slightly agglomerated, smooth surface. Complete separation had yet to occur, and the particles were held together by weak physical force. Figure 4(a) shows that the surface structure of the chemically modified biomass was porous, with rough surface, fracture, and contours like pores. The EDX elemental composition analysis confirmed the ultimate composition (C, H, N) of RDSB and CA-MDSB, as shown in Figs. 3, 4 (b-c). The results of the semi-quantitative analysis agreed with the elemental analysis of C, H, and N.
Thermogravimetric Analysis (TGA)
The thermal analysis of the raw and modified date palm stone (RDSB and CA-MDSB) biomass is depicted in Fig. 5. The thermal behavior of biomass materials depends on their chemical composition, structure, and crystallinity degree. In general, decomposition of lignocellulose biomass can occur in four distinct phases, i.e., i) moisture evaporation, ii) hemicellulose degradation, iii) cellulose degradation, and iv) lignin degradation (Shaikh et al. 2021). The temperature at the onset of decomposition can be used to evaluate stability. The first small decomposition curve at around 100 ℃ represented the elimination of adsorbed water or moisture content, which was nearly 3.31% and 6.07% for RDSB and CA-MDSB, respectively. Then, samples displayed thermal stability with negligible mass loss depending on the nature of the samples. The RDSB showed the temperature at the onset of decomposition at about 221 ℃, while CA-MDSB samples showed it was at 240 ℃. This difference could be attributed to the chemical modification of biomass. The CA-MDSB sample showed a relatively high onset degradation temperature. The mass loss beyond 420 °C was due to the decomposition of lignin and char components, which took place over a wide temperature range, up to 600 °C. Similar results were observed in earlier reported studies (El may et al. 2012; Parthasarathy et al. 2022).
Fig. 5. TGA analysis of RDSB and CA–MDSB biomass
Effect of Solution pH
The effect of pH value on the removal capacity of 2,4,6-TCP was investigated by varying the pH of the solution from 2 to 10. Figure 6 shows the pH effect on the adsorption efficiencies for RDSB and CA-MDSB. There was a strong correlation between the solution pH and the uptake capacity of 2,4,6-TCP. As illustrated in Fig. 6, the sorption capacity of both adsorbents for 2,4,6-TCP increased as the pH of the solution increased from 2 to 7. However, when the pH was increased beyond 7, the sorption capacity decreased. This is because 2,4,6-TCP is a weak acid with a dissociation constant of 6.23, so an acidic pH below this value helps it remain un-dissociated, and dispersion interactions become more effective (Kumar et al. 2019). The maximum adsorption of 2,4,6-TCP on RDSB and CA-MDSB was observed at pH values of 6.0 and 7.0, respectively. Recent studies have shown that similar behavior occurs during the adsorption of 2,4,6-TCP on agro waste adsorbents (Kuśmierek et al. 2017; Kumar et al. 2018b; Ahmad et al. 2023). Thus, experiments for 2,4,6-TCP were conducted at pH 6.0 and 7.0 on RDSB and CA-MDSB, respectively.
Effect of RDSB and CA-MDSB Dosage
The amount of sorbent used in the adsorption process is an essential factor in determining its effectiveness. To examine the impact of sorbent dosage, experiments were conducted by varying the dosage of sorbent from 0.1 to 1.0 g in 100 mL of a test solution containing 100 mg/L of 2,4,6-TCP at an optimum pH of 6.0 and 7.0 for 4 h at a temperature of 30 ± 1 °C. Results shown in Fig. 7(a-b) indicated that the removal percentage of 2,4,6-TCP increased from 80.9% to 87.5% and 83.8% to 94.0% with an increasing amount of RDSB and CA-MDSB dosage, respectively. This is due to the presence of more active sites on the surface of the adsorbent, which makes it easier for 2,4,6-TCP molecules to penetrate into these sites. However, the sorption capacity of RDSB and CA-MDSB for 2,4,6-TCP molecules decreased significantly. This may be due to the agglomeration of the adsorbent or interactions between sorbent atoms, which reduces the availability of active sorption sites (Kumar et al. 2021; Siva Kumar et al. 2023). Additionally, there was hardly any improvement in the removal efficiency of 2,4,6-TCP beyond 0.6 and 0.5 g/L of RDSB and CA-MDSB, respectively.
Fig. 6. Effect of pH on 2,4,6-TCP adsorption onto (a) RDSB and (b) CA–MDSB at 30±1ºC (C0 = 50 mg/L, Contact time = 4h, adsorbent dosage = 0.1 g, Agitation speed = 175 rpm)
Fig. 7 (a-b). 2,4,6-TCP adsorption onto (a) RDSB at pH = 6.0 (b) CA–MDSB at pH = 7.0 of adsorbent dosage level at 30±1ºC [C0 = 100 mg/L, Sorbent dosage = 0.1 – 1.0 g, Agitation speed = 175 rpm, Contact time = 4h]
Effect of Contact Time and Initial Concentration
The time for agitation plays a critical role in determining the amount of pollutant taken up by the adsorbent. At a temperature of 30 °C, the impact of contact duration and initial 2,4,6-TCP concentration (ranging from 50 to 200 mg/L) was measured on 2,4,6-TCP uptake. As shown in Fig. 8(a-b), the absorption capacity increased with time and initial concentration. The sorption rate was initially fast until an equilibrium constant was reached. For RDSB and CA-MDSB, this occurred after 150 and 120 min of contact, respectively. The fast absorption rate at the beginning of sorption was attributed to the greater affinity of the interacting groups and the availability of the adsorbent’s external surface. As shown in Fig. 8(a-b), the equilibrium sorption capacity (qe) increased from 28.58 to 140.47 and 30.36 to 157.06 mg/g as the initial concentrations of 2,4,6-TCP increased from 50 to 200 mg/L. The higher initial concentrations resulted in increased mass transfer driving force, leading to greater 2,4,6-TCP adsorption. As a result, the RDSB and CA-MDSB contact time for the batch experimental studies was set at 150 and 120 min, respectively.
Fig. 8 (a-b). Effect of 2,4,6-TCP adsorption onto (a) RDSB at pH = 6.0 and (b) CA–MDSB at pH = 7.0, contact time at 30±1ºC [(◆) C0= 50 mg/L, (●) C0= 100 mg/L, (▲) C0= 150 mg/L, (■) C0= 200 mg/L; adsorbent dosage = 0.1g; Contact time = 165 & 135 min, Agitation speed = 175 rpm]
Adsorption Kinetics
The pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models can predict the rate of contaminants adsorption onto RDSB and CA-MDSB. To understand the adsorption mechanism, the data was fitted using PFO and PSO models (Dada et al. 2021). Equations 6 and 7 in the supplementary section text 3.9 represent the models. Table 3 shows the kinetic results for the adsorption of 2,4,6-TCP by RDSB and CA-MDSB adsorbents. Additionally, qe (exp) is displayed to compare with the corresponding predicted values. The PSO model was found to provide better agreement in RDSB and CA-MDSB between experimental values and model predictions of qe, in the range of initial 2,4,6-TCP concentration (C0 = 50 to 200 mg/L) considered in the study. The results confirmed that the CA-MDSB was more effective in adsorbing 2,4,6-TCP than the RDSB adsorbent.
The Elovich and IPD (Intraparticle diffusion) models (Kumar et al. 2009; Siva Kumar et al. 2012a) have been utilized to explain the adsorption kinetics. The Elovich equation is used to calculate the contaminant uptake rate. However, the model does not suggest any clear mechanism governing the interaction between adsorbent and adsorbate; instead, it assumes that the adsorbent surfaces are energetically heterogeneous. However, from Table 3, the calculated qe values of the Elovich model do not show good agreement with experimental equilibrium values. On the other hand, the IPD model evaluates the relative significance of the intra-particle diffusion in the mass transport of the solute and calculates the kid values. As seen in Table 3, the RDSB and CA-MDSB intercept ranged from 23.20 to 111.87 and 25.26 to 112.61, respectively. Therefore, the intra-particle diffusion model was not valid in this case since a large intercept indicates that adsorption primarily occurs on the adsorbent surface, and the mass transport is controlled by external mass transport resistance throughout the movement of the solute from the liquid phase to the external surface of the adsorbent.
The study compared the predictions of experimental and calculated data models for 2,4,6-TCP concentrations ranging from 50 to 200 mg/L using PFO, PSO, Elovich Kinetic, and Intraparticle diffusion equations. Supplementary Figures S5(a–d) and S6(e–h) depict the comparison results. Based on the data presented in Figs. S5 and S6, it can be concluded that the PSO kinetic model accurately predicted the adsorption of 2,4,6-TCP onto RDSB and CA-MDSB. However, the PFO kinetics did not provide a satisfactory description of the same process. The results fitted with PFO, PSO, Elovich, and IPD kinetic models of 2,4,6-TCP adsorption onto RDSB and CA-MDSB, their corresponding regression (R2), Chi-square (χ2) and Δqt (%) values are listed in Table 3 and 4. The R2 value for the PSO model was found to be greater than 0.993 to 0.999 and 0.993 to 1 for RDSB and CA-MDSB, respectively, which was much closer to unity than those of the PFO, Elovich, and IPD models. The calculated (qe, cal) values for RDSB and CA-MDSB showed an excellent fit with the experimental values (qt, exp) for the PSO kinetic model. The Chi-square (χ2) values ranged from 0.98 to 5.58 and from 0 to 2.64% for RDSB and CA-MDSB, respectively, indicating that the PSO model provided the best fit. Based on the investigative results, it appears that the PSO is more suitable option for adsorbing 2,4,6-TCP on RDSB and CA-MDSB. This result is consistent with recent studies that have reported similar kinetics of 2,4,6-TCP adsorption on other adsorbents, following the PSO kinetic model (Ahmad et al. 2023; Hyder et al. 2023; Nazal et al. 2021; Soltani Goharrizi et al. 2019).
Adsorption Isotherm Models
This study focused on three different empirical models: the Langmuir (Eq.10), Freundlich (Eq. 11), and Dubinin–Radushkevich (Eq. 13) models (Dubinin, M. M. and Radushkevich 1947; Freundlich 1906; Langmuir 1917). The equilibrium adsorption isotherm was analyzed using these models and detailed information is provided in the supplementary section. The Langmuir isotherm provides information on the maximum amount of solute that can be adsorbed per unit mass of the sorbent, and is concerned with adsorption energy. The Freundlich isotherm model can be used to correlate heterogeneous surfaces containing binding sites to their different energies. The Dubinin–Radushkevich isotherm has also been utilized to model the present equilibrium data. For adsorption by ion exchange, the value of E usually lies between 8 and 16 kJ/mol, while it is less than 8 kJ/mol for the case of physical adsorption. Correlation coefficients, parameter values, normalized standard deviation (Δqt (%)) and Chi-square (χ2) for all three isotherms, i.e., Langmuir, Freundlich, and D-R, are presented in Table 5 for comparison. In this study, the experimental data and predictions from three isotherm models (Langmuir, Freundlich, and D-R) were compared in supplementary Fig. S7(a-b). The Langmuir isotherm resulted in higher R2 values (0.9996 and 0.9992) for RDSB and CA-MDSB, lower Δqt (3.88 and 6.09%), and lower Chi-square (χ2) values (0.86 and 2.19). The Langmuir model provided a better fit for the experimental sorption data for RDSB and CA-MDSB than the Freundlich and D-R models. To compare this study with other studies, Table 6 summarizes the data of 2,4,6-TCP adsorption systems with different types of adsorbents.
Adsorption Mechanism
The adsorption process of 2,4,6-TCP onto raw (RDSB) and modified date palm stone biomass (CA-MDSB) may involve various mechanisms, such as ion exchange, surface complexation, and electrostatic interactions. In addition, the excellent fits of the kinetic data to the PSO equation, as shown earlier, implies that the rates of adsorption were controlled by a process of adsorption into a network of small pores (Hubbe et al. 2019). The FTIR data analysis (Fig. 2) indicates that the biomass surface contains different functional groups, such as hydroxyl, carbonyl, alkene, cellulose, lignin, hemicellulose carboxyl, and amine groups, which play an essential role in the sorption process.
Table 3. PFO and PSO Kinetic Parameters of 2,4,6-TCP on RDSB and CA-MDSB
Table 4. IDM and EKM Kinetic Parameters of 2,4,6-TCP on RDSB and CA-MDSB