The nature and kinetics of 2,4-dimethylphenol adsorption in aqueous solution on biochar derived from Sargassum boveanum macroalgae

The effluents of different industries such as petrochemical, oil refining, plastics, pharmaceuticals, insecticides, fungicides, dyes, and rubber chemicals are the major source of phenol pollutants (Mohammadi et al. 2015; Sun et al. 2015). Owing to their high toxicity, persistence, and possible bioaccumulation in the environment, they have been listed as pollutants of priority concern by the US Environmental Protection Agency (USEPA) and they have received special attention from scientists and researchers. Discharging these compounds without treatment not only contaminates drinking water and groundwater but also seawater and different aquatic environments which lead to serious health risks to animals, aquatic systems, as well as humans (Snoeyink et al. 1997; Sun et al. 2015). Therefore, there are many proposed methods for treatment of contaminated water which contains phenol compounds. These methods are (i) chemical oxidation, (ii) biodegradation, (iii) adsorption, (iv) membrane separation, (v) ion exchange, (vi) solvent extraction, (vii) ozonation, (viii) electrochemical oxidation, etc. (Ahmaruzzaman 2008; Laura et al. 2016). Among these methods, the adsorption process is one of the most efficient and promising methods applied widely for removal of contaminants from water bodies, and is simple, low cost, and capable of removing most forms of organic substituted phenol compounds (Li et al. 2015). This has led to researchers having more interest to develop adsorbent from different sources which are more efficient, cheaper, renewable, and more abundantly available than the commercially available materials such as activated carbon. Adsorption using naturally available macroalgae has shown promising results in removing many pollutants such as heavy metals (Nazal 2019) and phenolic compounds (Eugenia et al. 2006) from different aqueous media. These bioadsorbents are ecofriendly, inexpressive, and freely available. The brown seaweed macroalgae (Phaeophyta) are a potential source of bioadsorbent. One of these brown seaweeds are the Sargassum boveanum macroalgae which are abundant and freely available in the Arabian Gulf coastal area. They have many advantages as a bioadsorbent due to their cell wall structure and components, which contains cellulose, alginate, and other polysaccharides. These compounds have many chemical functional groups such as carboxylic acid, hydroxyl, and amine that could be active adsorption sites with high affinity, capacity, and selectivity to phenolic compounds. In addition, bioadsorption of phenolic compounds using brown macroalgae could be achieved through hydrophobic and donor–acceptor interactions (Podgorskii et al. 2004). Moreover, modifying the macroalgae by deriving a biochar and activating their surface and structure is also expected to improve their adsorption capacity for removal of phenolic compounds.

To the best of our knowledge, there is no information available concerning the adsorption of 2,4-dimethylphenol (DMP) compound onto a biochar adsorbent derived from S. boveanum marine macroalgae. Therefore, in this study, a biochar from S. boveanum macroalgae was prepared, characterized, then used as a biosorbent for the removal of DMP phenolic compound from aqueous solution. The effect of adsorption conditions such as the solution pH, adsorbent mass, and adsorbate concentration were investigated. The adsorption isotherms were studied by fitting the experimental data with the common isotherm models. The adsorption kinetics for this phenolic compound were studied. In addition, the thermodynamic parameters for the DMP adsorption on the prepared adsorbent were calculated and the reusability of the adsorbent for the removal of DMP from seawater was also tested.

2,4-dimethylphenol (DMP) was selected for the current study. All chemicals were analytical reagent (AR) grade and purchased from Chem Service, Inc., USA. Dichloromethane (99.9% purity, Fisher German) and a deionized water were used.

The S. boveanum macroalgae (MA) were collected from the Arabian Gulf coastal area and washed thoroughly with tap water then by distilled water to remove the water-soluble impurities. The washed material was dried at 110 °C for 48 hours. A biochar-derived S. boveanum macroalgae (BCM) was prepared by carbonizing the clean MA in a tube furnace at 500 °C under argon atmosphere (inert gas) and heating rate of 10 °C/min for 2 hours then cooled down to room temperature. The prepared adsorbent was preserved in a tightly closed vial for characterization and later for use in the adsorption experiments.

The functional groups on the surface of MA and BCM adsorbents were identified using the Fourier transform infrared (FTIR) (Nicolet 6700 Thermo Electron) instrument, and the thermal stability of the material under air and nitrogen was tested using thermal gravimetric analysis (TGA Q500, TA Instruments, USA). The adsorbent's surface morphology and its elemental composition were characterized using a scanning electron microscope coupled with energy dispersive X-ray (SEM-EDX) (Jeol 6700LV). An automated gas sorption analyzer (Autosorb iQ Quantachrome USA) was used to analyze the surface area and porosity of the adsorbent.

A stock solution of DMP (1,000 mg/L) was prepared in deionized water. Subsequent test solutions were prepared by appropriate dilution of the stock solution with deionized water for the adsorption experiments or seawater to approach the real application. The pH of the test solutions was adjusted using nitric acid HNO₃ (2 M) and potassium hydroxide KOH (1 M), and the pH was measured using a pH meter (model PC 2700, Oakton Instruments, USA).

The DMP molecules were extracted from 5 mL of the filtered aqueous sample, before and after the adsorption, using 5 mL of dichloromethane solvent, then a 1 μL from the extract was analyzed using a gas chromatography coupled with flame ionization detector (GC-FID) instrument (Agilent Technology 6890N). The used column and the GC-FID parameters are summarized in Table 1.

The adsorption of DMP on the adsorbent was studied at a shaking speed of 180 rpm using the batch mode experiment. The effect of solution pH on DMP adsorption was studied at 24 ± 0.1 °C. Ten mg/L of DMP was prepared in deionized water having different pH values (i.e., 1, 2, 5, 7, 8, and 10). 50 ± 0.1 mg of adsorbent was added to a 50 mL solution in a screw-capped conical flask then agitated for 16 h. The effect of the adsorbent dosage on the removal of DMP (20 mg/L) from its solution was investigated by varying the added mass of BCM adsorbent from 5 to 150 ± 0.1 mg at a constant temperature of 24 ± 0.1 °C for 16 h. The effect of DMP concentration (i.e., 1, 5, 10, 15, 20 30, and 40 mg/L) on its removal efficiency using 50 ± 0.1 mg of adsorbent was studied at 24 ± 0.1 °C. For studying the adsorption kinetics of DMP (15 mg/L), 50 ± 0.1 mg of adsorbent were used for different time intervals from 5 to 960 minutes. The adsorption thermodynamics of DMP (5 mg/L) on 50 ± 0.1 mg of adsorbent was studied at different temperatures (i.e., 24, 30, 35, 40, 50, and 60 ± 0.1 °C). For approaching the real environmental application, 200 ± 0.1 mg of BCM adsorbent was used for decontaminating seawater that had 30 mg/L of DMP pollutant. Finally, the reusability of the prepared BCM adsorbent for removing DMP from seawater was evaluated for four cycles. In all the experiments, the samples were filtered using Whatman filter papers 42 to remove the adsorbent. To estimate the loss of DMP due to the evaporation or adsorption on the wall of the conical flask, control samples were placed together with the test samples in a water bath shaker (BS-31, Lab Companion, Korea). In each experiment, these samples had the same tested concentration and pH of the DMP solution but without adsorbent. The results of the control samples showed that there was no loss of DMP due to the experimental procedure. The precision of experiments was estimated by calculating the RSD % for five replicates of the test samples and it was 10.9%.

If the value of R_L is 0 then the adsorption is reversible, 0 < R_L < 1 favorable, R_L = 1 linear, and if R_L > 1 this indicates the adsorption is unfavorable.

In the above-mentioned isotherm models, q_e is the adsorption capacity at equilibrium in mg/g, C_e is the concentration of DMP in the solution at equilibrium in mg/L.

To gain an insight into the thermal stability of MA and BCM adsorbents in a temperature range from 25 to 800 °C, the thermal analysis was performed using a titanium pan with a sample weight of 15 mg under air with a flow rate of 30 mL/min and heating rate of 10 °C/min. The same experiment was carried again out under an inert atmosphere (nitrogen gas) to show the weight loss percentage in MA during the BCM preparation. Figure 1 shows the thermal oxidation and degradation of MA and BCM adsorbents under air and nitrogen atmosphere. It was found that under air MA adsorbent has three weight loss phases that are attributed to the oxidation, degradation, and decomposition. The first main step started at 213 °C and was completed at around 368 °C, with a loss of 51.2% of its original mass. The second step was in a temperature range between 424 and 482 °C with a 73.5% loss, while the third step was between 712 and 800 °C with 88.7% loss. However, BCM adsorbent shows two main oxidation and degradation steps. The first one started at around 344 °C and was completed at around 520 °C with a weight loss of 40%. The second degradation step was in a temperature range between 721 and 800 °C with a weight loss of 57.7%. Under nitrogen, the decomposition of MA material was between 211 and 800 °C with a weight loss around 75%, while for BCM adsorbent, its decomposition was between 600 and 800 °C with a weight loss around 41%. It may be concluded that the BCM shows more thermal stability under both atmospheres.

The functional groups on the surface of the adsorbents were identified using FTIR experiment. Figure 2 shows the IR spectra for the raw and carbonized macroalgae. The macroalgae contain cellulose, polysaccharides, fucoidans, alginates, and proteins. The absorption between 3,600 and 3,000 cm⁻¹ is due to the stretching, vibration and angular vibration of hydroxyl (OH) and amine (NH₂) groups. The weak and intense absorption bands in the ranges of 1,500 to 1,300 cm⁻¹ and 1,640 to 1,540 cm⁻¹ are due to the presence of proteins (Sekkal et al. 1993). However, their precise assignment is difficult because of the complexity of protein structures. The intense band between 1,000 and 1,100 cm⁻¹ is due to the presence of the polysaccharides (Ata et al. 2012). The intense bands at 1,100, 1,680, and 1,425 cm⁻¹ are attributed to the presence of the hemiacetals and carbonyl functional groups, respectively. These functional groups confirm the presence of polyalginates, which mainly contribute to the adsorption of organic pollutants (Navarro et al. 2008; Cuizano et al. 2009). Finally, the decrease in the peak intensity at around 3,440 and 1,620 cm⁻¹ and the intense absorption peak at 1,420 cm⁻¹ in the BCM material are due to the carbonization.

The dried macroalgae before and after carbonization were placed on aluminum stubs and sputter-coated with gold-palladium or platinum using a DESK V HP TSC cold sputter coater. Respectively, Figures 3 and 4 show the surface morphology and the elemental composition of MA and BCM adsorbents. According to the images, the surface heterogeneity of MA adsorbent is greater than the surface heterogeneity of BCM adsorbent. The energy dispersive X-ray (EDX) results, which are presented in Table 2, show the surface of MA covered with alkali metals to be more than those on the surface of BCM. However, the carbonization of MA removed these metals and produced surface morphology with small openings and holes having a larger contact area and more active adsorption sites. Moreover, the carbon and oxygen element percentages on the surface of BCM adsorbent were found to be 61.78 and 33.85%, respectively, and they are higher than the percentages of carbon (35.56%) and oxygen (12.81%) elements on the surface of MA material before carbonization which may contribute in increasing the adsorption capacity of BCM for DMP phenolic compound (Vidic et al. 1993).

The surface area and pore volume of the adsorbents were obtained at relative pressures between 0.10 and 1.00. The liquid nitrogen adsorption-desorption isotherms were measured after degassing all the adsorbents at 200 °C to a pressure of 6.5 × 10⁻⁵ Torr. The Brunauer–Emmett–Teller (BET) and the density functional theory (DFT) methods were used to calculate the surface area and total pore volume, respectively. The results show the surface area of the BCM adsorbent is small (3.8 m²/g) and the pore volume is 0.006 mL/g with an average pore size of 7.049 Å. However, that was larger than the surface area (1.34 m²/g) and the pore volume (0.005 mL/g) of the MA material but smaller than its average pore size (13.845 Å).

The effect of pH is one of the significant factors influencing the adsorptive removal of some pollutants from aqueous solutions. The solution pH influences the ionization of adsorbates and the functional groups that are present on the surface of the adsorbents. The removal percentage values were plotted against the corresponding initial pH values. Keeping in mind that the pK_a of DMP is 10.1 and pH_zpc of BCM adsorbent is 9.87, Figure 5 shows that at low pH values, from 1 to 2, the maximum removal efficiency is approximately 75% and it decreases to reach 30% at pH values higher than 5. This may be attributed to the decrease in the protonated functional groups and, as a result, the repulsion between the adsorbate and adsorbent surface increases. This is in line with the finding of Won et al. (2005).

The effect of adsorbent mass loading on the removal of DMP from its solution which contains 20 mg/L initial concentrations at pH 2 was studied. Figure 6 shows that the removal percentage of DMP molecules increases and the adsorption capacity of BCM adsorbents decreases with the increase of adsorbent dosage. This is attributed to the increase of adsorption active sites (i.e., carbonyl and acetal functional groups) with the increase of the adsorbent mass. The highest DMP removal efficiency (100%) was achieved by using 150 ± 0.1 mg of BCM adsorbent.

Figure 7 presents the adsorption isotherms of DMP using BCM adsorbent. The experimental data of DMP adsorption were fitted with Freundlich, Langmuir, and Temkin adsorption isotherms. It is clearly shown that the experimental data fit the Freundlich model, whereas the difference between the experimental and the calculated q_e is the lowest. The obtained isotherm parameters from the linearized models are summarized in Table 3.

In the Freundlich model, the n value is obtained from the slope of the linear least square fit of versus ⁠, while the K_F value is calculated from the intercept. Larger n and K_F values correspond, respectively, to greater heterogeneity on the adsorbent's surface and a higher adsorption capacity (Li et al. 2002). The n and K_F indicate that the adsorption of DMP phenolic compound is favorable on the BCM adsorbent. The squared correlation coefficient (R²) value of the linearized Freundlich equation for DMP adsorption on BCM adsorbent is higher than the obtained R² values of the other linearized forms (i.e., Langmuir and Temkin isotherm equations). In addition, the SRE of fitting the experimental data with Freundlich is lower than those obtained by the fitting with the other linearized forms. However, R² of the linearized Langmuir isotherm model for DMP adsorption on BCM is also high, but lower than of R² of the linearized Freundlich equation. This indicates that the adsorption mechanism of DMP on the BCM adsorbent is mixed and dominated by a physisorption mechanism, and the hydrogen bonding, Van der Waals as well as acceptor–donor interactions between the adsorbent surface and the DMP adsorbate contribute to the adsorption process and these molecules form multi-layers on the adsorbent. In addition, the separation factor (R_L) value (shown in Table 3) for the adsorption of DMP on BCM is between 0 and 1 which indicates the adsorption is favorable. It has been also found that, within the experimental conditions, the maximum adsorption capacity is 17 mg/g for DMP (shown in Figure 7). This value is comparable with most of those reported in the literature (shown in Table 4). It has been found that the adsorption capacity of BCM is lower than the adsorption capacity of the standard activated charcoal (Jain et al. 2002) by eight times while it is higher than the adsorption capacity of K-FAU-X (zeolite) (Vittenet et al. 2014) by 28 times.

The mechanism of the adsorption can be explored by studying the adsorption kinetics. The results presented in Figure 8(a) show that the adsorption rates of DMP reach equilibrium within 9 hours. The experimental adsorption capacity (q_e,exp) for DMP on the BCM adsorbent is given in Table 5. It has been observed that DMP molecules are slowly adsorbed and reach equilibrium after 9 hours. The initial fast adsorption is attributed to the availability of a large number of active adsorption sites, while the slowness at which the maximum adsorption is reached is due to the occupied adsorption sites and the repulsion between the free adsorbate molecules in solution and the adsorbed molecules on the BCM adsorbent. The linear least square fits of versus t and q_t versus ln(t) (shown in Figure 8(b) and 8(c)) with the adsorption data of DMP yield R² values of 0.973 and 0.907, respectively, that are lower than R² (0.990) value which is obtained from the linear least square fit of versus t (shown in Figure 8(d)). The calculated adsorption capacity (q_epred) of DMP from the pseudo-first order and the Elovich kinetics deviates largely from the experimental adsorption capacity (q_eexp). However, SRE value of the linear least squares fits of versus t (see Table 5) is the lowest. This clearly reveals that the adsorption process follows pseudo-second order kinetics, and the chemisorption is involved in the adsorption mechanism.

Figure 9(a) shows the plot of q_t versus t^0.5 for DMP on BCM adsorbent. These results imply that the adsorption processes involve more than a single kinetic stage or sorption rate (Weber & Morris 1963). The first linear part (first stage) may be attributed to the mass transfer of adsorbate molecules across the boundary layer and intra-particle diffusion, which produces a delay in the adsorption process. The second stage is attributed to the diffusion through small pores, which is followed by the establishment of equilibrium. Table 6 summarizes the calculated values of the diffusion constants for DMP on the BCM adsorbent.

The negative values of ΔG at all temperatures for the adsorption confirm that the adsorption process of DMP molecules on the BCM adsorbent is spontaneous and thermodynamically favorable. However, the positive value of ΔH and decreasing of ΔG value with increasing the temperature indicates that the adsorption process of DMP is endothermic. Zaghouane-Boudiaf & Boutahala (2011) reported that the adsorption process is physical if ΔG values range from −30 to 0 kJ mol⁻¹ and it is a chemical process if ΔG values are between −400 and −80 kJ mol⁻¹. Also, the magnitude of ΔH gives an idea into the nature of the adsorption as to if it is physical or chemical. The physical adsorption evolves heat in the range of 2.1 and 20.9 kJ mol⁻¹ while the chemical adsorption has a heat ranging between 80 and 400 kJ mol⁻¹(Saha & Chowdhury 2011). In this study, the calculated values of ΔG for DMP adsorption range between −19.65 and −5.84 kJ mol⁻¹ and the ΔH value is 108.10 kJ mol⁻¹. These values indicate that the adsorption of DMP molecules involves a mixed adsorption mechanism and is dominated by physical adsorption. The small positive value of ΔS (0.38 kJ mol⁻¹ K⁻¹) reflects the affinity of the adsorbent toward the adsorbate molecules and indicates that the randomness at the liquid/solid interface is increased.

To approach the real application and since the cost-effectiveness and regeneration of the adsorbents are significant factors, the removal of DMP from seawater (pH = 7.80 and salinity = 41 g/kg) has been tested. 50 mL of seawater was spiked with 30 mg/L DMP and 200 ± 0.1 mg BCM adsorbent was added. DMP concentration before and after the adsorption was measured. It has been found that the prepared adsorbent removes around 60% of DMP from the tested seawater. Furthermore, the reusability of the prepared BCM adsorbent has been tested. The method for BCM adsorbent regeneration was very simple. After the completion of the adsorption experiments, the BCM adsorbent loaded with DMP was filtrated and washed three times with distilled water to remove the stacked salts then heated at 250 °C for 1 day in air to remove the adsorbed phenolic molecules. The regenerated BCM was reused for the next adsorption, and the successive adsorption-desorption cycles were repeated four times. As shown in Figure 11, BCM remains, within the experimental error, nearly unchanged in their ability for DMP adsorption for at least four cycles.

In this study, for the first time, biochar derived from S. boveanum macroalgae as an adsorbent has been prepared, characterized, then tested for the removal of 2,4-dimethylphenol from aqueous solution and its adsorption isotherms and kinetics were investigated. The carbonization improved the removal capability of the macroalgae adsorbent for DMP compound with removal efficiency up to 100%. The adsorption equilibrium results showed that the adsorption was attained through a mixed mechanism dominated by physisorption and followed by pseudo-second order kinetics. Also, the adsorption of DMP molecules is spontaneous and thermodynamically favorable with a negative value of ΔG. The adsorption process of these molecules is endothermic, while the positive value of ΔS reflects the affinity of the adsorbent toward the DMP adsorbate and indicates increasing of randomness at the liquid/solid interface. The findings in this study reveal that the biochar derived from BCM adsorbent can be used in the removal of DMP phenolic compound from high salinity seawater with removal efficiency up to 60% and reusability for at least four times.

The authors highly acknowledge the Center for Environment and Water (CEW) at King Fahd University of Petroleum and Minerals (KFUPM) for the support and funding this research study. The authors declare that there is no conflict of interest regarding the publication of this paper.

All relevant data are included in the paper or its Supplementary Information.