Removal of Lead by Merlinoite Prepared from Sugarcane Bagasse Ash and Kaolin: Synthesis, Isotherm, Kinetic, and Thermodynamic Studies

This study introduces a merlinoite synthesized from sugarcane bagasse ash (SBA) and kaolin and evaluates its application as an adsorbent to remove lead from wastewater. The synthesis was performed via the hydrothermal method, and optimal conditions were determined. The adsorption of Pb by merlinoite was also optimized. Determination of the Pb2+ remaining in the aqueous solution was determined by atomic absorption spectroscopy (AAS). Adsorption isotherms were mainly studied using the Langmuir and Freundlich models. The Langmuir model showed the highest consistency for Pb adsorption on merlinoite, yielding a high correlation coefficient (R2) of 0.9997 and a maximum adsorption capacity (qmax) of 322.58 mg/g. The kinetics of the adsorption process were best described by a pseudo-second-order model. Thermodynamic studies carried out at different temperatures established that the adsorption reaction was spontaneous and endothermic. The results of this study show that merlinoite synthesized from kaolinite and SBA is an excellent candidate for utilization as a high-performance adsorbent for lead removal from wastewater.


Introduction
Water pollution is a significant problem globally. This type of pollution is an increasing international concern, affecting economic growth and the health of people. Lead (Pb) is well known to be a highly toxic metal, and as such it is a pollutant of great concern. The release of Pb into the environment from industrial processes and the resulting impact on ecosystems and human health continues to be a serious issue [1]. The effects of lead toxicity are known to include kidney damage, brain damage, nervous system damage, anemia, and hypertension [2]. The World Health Organization (WHO) sets the maximum permissible level of Pb in drinking water at 0.01 mg/L [3]. The maximum allowable contaminant level of Pb in wastewater effluent from factories in Thailand is 0.2 mg/L, based on the Industrial Effluent Standard [4]. Strict and comprehensive management is essential for controlling the release of lead into the environment and preventing potentially devastating damage [1,2]. Lead can cause renal, gastrointestinal, and cardiovascular problems in humans. Various treatment techniques such as ion exchange [5], electrocoagulation [6], chemical precipitation [7], membrane separation [8], and adsorption [9] have been developed for removing lead from wastewater. However, adsorption is the most extensively used treatment technique because of its simplicity, effectiveness, and low cost [10]. The most common adsorbent materials are alumina, silica, activated carbon, chitosan, natural substances, and zeolites [10,11].
Zeolites are microporous crystalline alumina silicates in which SiO 4 and AlO 4 tetrahedra are covalently bound together by oxygen atoms [12]. The tetrahedra are arranged

Materials
Sugarcane bagasse ash was provided by Kaset Thai Bio Power Co., Ltd., Nakhon Sawan, Thailand. Kaolin came from Ranong, Thailand. The chemical compositions of these materials, as determined by X-ray fluorescence spectrometry, are listed in Table S1.

Merlinoite Preparation
In this study, the merlinoite sample was prepared using the method that has been reported previously [23]. Sugarcane bagasse ash and kaolin were dried in an oven at 100 • C. The hydrothermal method was used for the synthesis of the merlinoite. Sugarcane bagasse ash, kaolin, and 2 M KOH (10 g:10 g:80 mL) were combined. The following parameters were investigated in order to determine optimal conditions for preparing the zeolite: reaction temperature (180 or 200 • C), reaction time (4, 8, 12, or 16 h), concentration of KOH solution (1, 2, 3, or 4 M), ratio of sugarcane bagasse ash to kaolin (1.1, 1:2, or 2:1), and ratio of raw materials to KOH solution (1:2, 1:3, 1:4, or 1:5). The mixture was placed in a stainless-steel PPL-lined autoclave and kept at the given temperature for the given amount of time in an oven under autogenous pressure. After synthesis, the material was washed with hot deionized water to remove excess KOH and then filtered. The material was dried at 105 • C for 16 h in an oven prior to analysis.

Characterization
The structural features of the merlinoite and the raw materials were analyzed using X-ray diffraction (XRD) (D2 PHASER, Bruker, Billerica, MA, USA). The X-ray diffraction patterns were recorded for a 2θ scan angle between 5 and 60 • , with steps of 0.2 • and a 0.02 deg/min scan rate. The chemical composition of the raw materials was analyzed using wavelength-dispersive X-ray fluorescence (WDXRF) (S8 TIGER, Bruker, Billerica, MA, USA). The surface topography and elemental analysis of merlinoite were investigated by scanning electron microscopy (SEM-EDS) (LEO 1455 VP, Leo, Sauerlach, Germany). Functional groups of merlinoite were identified by Fourier transform infrared spectroscopy (FTIR) (Spectrum GX, PerkinElmer, Waltham, MA, USA). Lead concentrations were determined by flame atomic absorption spectrophotometry (FAAS) (contrAA 800F, Analytik Jena, Jena, Thuringia, Germany).

Adsorption Experiments
The adsorption of Pb by the synthetic merlinoite zeolite was evaluated using a batch system. Different operating conditions were investigated including the solution pH (2-6), which was adjusted using 0.5 M HNO 3 or 0.5 M NaOH, contact time (5-300 min), initial concentration (500-1200 ppm), and temperature (30-60 • C). In general, 50 mL of Pb solution was mixed with 0.1000 g of merlinoite using a temperature-controlled shaker (Thermo Fisher Scientific, Waltham, MA, USA) at a speed of 200 rpm. The suspension was then filtered using Whatman No. 5 filter paper. Flame atomic absorption spectrometry (FAAS) was used to determine the Pb concentration of the resulting supernatant. Air-acetylene was used as the flame type. A fuel flow of 65 L/h was used. A burner height of 5-9 mm was selected. The wavelength of 217 nm was used. All experiments were performed in triplicate, and the results were averaged.
The Pb adsorption percentage, Pb adsorption capacity (q t ), and equilibrium amount of Pb adsorption (q e ) were calculated using the following equations: where C o (mg/L) is the initial Pb concentration, C t (mg/L) is the Pb concentration at time t, C e (mg/L) is the equilibrium concentration, V (L) is the volume of the solution, and W (g) is the weight of merlinoite zeolite [27][28][29].

Application of Merlinoite for Pb Adsorption in Wastewater Samples
Wastewater samples were collected from three factories manufacturing batteries, paint, and religious statues in Samutprakarn and Phitsanulok, Thailand. The wastewater was filtered with Whatman No. 5 filter paper to eliminate suspended particles and then spiked with the stock Pb solution to a final Pb concentration of 50 ppm. A 50 mL sample and 0.1000 g of merlinoite were then mixed and shaken at 200 rpm on a temperature-controlled shaker at 30 • C for 150 min. After that, the suspension was filtered with Whatman No. 5 filter paper. FAAS was used to determine the Pb concentrations of both the initial solution and the supernatant that resulted after adsorption.

Merlinoite Synthesis Results
The synthesis of merlinoite from SBA and kaolin via hydrothermal conversion was found to be affected by the synthesis conditions investigated in this study: the hydrothermal temperature, hydrothermal time, concentration of KOH solution, ratio of sugarcane bagasse ash to kaolin, and ratio of raw materials to KOH solution. Figure 1 shows the XRD patterns for the merlinoite synthesized at hydrothermal temperatures of 180 and 200 • C, compared with the XRD patterns of the reference merlinoite phase (JCPDS 29-0989) and the two raw materials. All diffraction peaks of the MZ synthesized at 200 • C corresponded perfectly to those of the reference merlinoite phase, indicating complete conversion to merlinoite from the raw materials. However, for the merlinoite synthesized at 180 • C, there was a peak remaining from the raw materials, corresponding to the quartz phase at 26.7 • , indicating incomplete conversion at the lower temperature. Thus 200 • C was used as the temperature for subsequent hydrothermal synthesis. paint, and religious statues in Samutprakarn and Phitsanulok, Thailand. The wast was filtered with Whatman No. 5 filter paper to eliminate suspended particles an spiked with the stock Pb solution to a final Pb concentration of 50 ppm. A 50 mL and 0.1000 g of merlinoite were then mixed and shaken at 200 rpm on a tempe controlled shaker at 30 °C for 150 min. After that, the suspension was filtered with man No. 5 filter paper. FAAS was used to determine the Pb concentrations of b initial solution and the supernatant that resulted after adsorption.

Merlinoite Synthesis Results
The synthesis of merlinoite from SBA and kaolin via hydrothermal conversi found to be affected by the synthesis conditions investigated in this study: the hyd mal temperature, hydrothermal time, concentration of KOH solution, ratio of sug bagasse ash to kaolin, and ratio of raw materials to KOH solution. Figure 1 shows th patterns for the merlinoite synthesized at hydrothermal temperatures of 180 and compared with the XRD patterns of the reference merlinoite phase (JCPDS 29-098 the two raw materials. All diffraction peaks of the MZ synthesized at 200 °C corresp perfectly to those of the reference merlinoite phase, indicating complete conver merlinoite from the raw materials. However, for the merlinoite synthesized at 1 there was a peak remaining from the raw materials, corresponding to the quartz p 26.7°, indicating incomplete conversion at the lower temperature. Thus 200 °C wa as the temperature for subsequent hydrothermal synthesis.  Figure 2a shows the XRD patterns for the merlinoite prepared using reaction of 4, 8, 12, and 16 h. Unconverted quartz was still detected at 4 h, but single-phase noite formed at 8, 12, and 16 h. The reaction time of 8 h was selected as optimal, sin was sufficient time for the reaction to be completed. Figure 2b shows the XRD patte the merlinoite synthesized using SBA/kaolin ratios of 1:1, 1:2, and 2:1. The me formed with high intensity and phase purity when an SBA/kaolin ratio of 1:1 wa but using a ratio of 1:2 resulted in low intensity and the presence of a quartz phas impurity. Merlinoite formed with high intensity when an SBA/kaolin ratio of 2:1 wa however, there were also quartz and microcline phases. An SBA/kaolin ratio of 1 thus chosen as optimal for the synthesis of highly crystalline merlinoite. Figure 2c the XRD patterns for the merlinoite synthesized using KOH concentrations of 1, 2,  Figure 2a shows the XRD patterns for the merlinoite prepared using reaction times of 4, 8, 12, and 16 h. Unconverted quartz was still detected at 4 h, but single-phase merlinoite formed at 8, 12, and 16 h. The reaction time of 8 h was selected as optimal, since this was sufficient time for the reaction to be completed. Figure 2b shows the XRD patterns for the merlinoite synthesized using SBA/kaolin ratios of 1:1, 1:2, and 2:1. The merlinoite formed with high intensity and phase purity when an SBA/kaolin ratio of 1:1 was used but using a ratio of 1:2 resulted in low intensity and the presence of a quartz phase as an impurity. Merlinoite formed with high intensity when an SBA/kaolin ratio of 2:1 was used; however, there were also quartz and microcline phases. An SBA/kaolin ratio of 1:1 was thus chosen as optimal for the synthesis of highly crystalline merlinoite. Figure 2c shows the XRD patterns for the merlinoite synthesized using KOH concentrations of 1, 2, 3, and 4 M. The results show that raising the KOH concentration to 2 M caused complete dissolution of quartz, and only merlinoite was detected. Furthermore, at 4 M, an additional unknown phase emerged. The principle of minimal chemical use makes 2 M the optimal KOH concentration for merlinoite synthesis. Figure 2d shows the XRD patterns for the merlinoite synthesized using raw-material-to-KOH ratios of 1:2, 1:3, 1:4, and 1:5. A quartz phase was still detected at ratios of 1:2 and 1:3. Single-phase merlinoite was obtained at ratios of 1:4 and 1:5. Choosing the minimal amount of chemicals, a ratio of 1:4 was used for merlinoite synthesis.
Molecules 2021, 26, x FOR PEER REVIEW 5 of 12 4 M. The results show that raising the KOH concentration to 2 M caused complete dissolution of quartz, and only merlinoite was detected. Furthermore, at 4 M, an additional unknown phase emerged. The principle of minimal chemical use makes 2 M the optimal KOH concentration for merlinoite synthesis. Figure 2d shows the XRD patterns for the merlinoite synthesized using raw-material-to-KOH ratios of 1:2, 1:3, 1:4, and 1:5. A quartz phase was still detected at ratios of 1:2 and 1:3. Single-phase merlinoite was obtained at ratios of 1:4 and 1:5. Choosing the minimal amount of chemicals, a ratio of 1:4 was used for merlinoite synthesis.

Characterization of Merlinoite
The morphology of the synthetic merlinoite was examined by SEM. Figure 3 shows the SEM photos at 5000× magnification. The morphology was characterized by bundles of columns with uneven lengths. EDS was used to analyze the synthetic merlinoite's elemental components. The constituent elements are shown by weight percent in Figure 3b.
The results indicate that the merlinoite consists mainly of silicon, aluminum, potassium, and oxygen.

Characterization of Merlinoite
The morphology of the synthetic merlinoite was examined by SEM. Figure 3 shows the SEM photos at 5000× magnification. The morphology was characterized by bundles of columns with uneven lengths. EDS was used to analyze the synthetic merlinoite's elemental components. The constituent elements are shown by weight percent in Figure 3b.
The results indicate that the merlinoite consists mainly of silicon, aluminum, potassium, and oxygen.
In addition, it should be noted that the Al/Si ratio was quite small (0.46). However, even using this low Al/Si ratio, the resultant merlinoite still showed a high Pb removal efficiency, as shown later. This agrees well with results reported in the literature [25,30]. Molecules 2021, 26, x FOR PEER REVIEW 6 of 12 In addition, it should be noted that the Al/Si ratio was quite small (0.46). However, even using this low Al/Si ratio, the resultant merlinoite still showed a high Pb removal efficiency, as shown later. This agrees well with results reported in the literature [25,30].
The TEM micrographs ( Figure S1a,b) of the synthesized merlinoite showed columnlike morphology. The SEAD pattern in Figure S1c exhibits a ring-like pattern suggesting the monocrystalline nature of the sample [31]. In addition, the HR-TEM micrograph in Figure S1d shows the lattice fringe with a d-spacing of 0.54 nm. This is due to the reflection from the (121) plane of the merlinoite.
The specific surface area is an important factor affecting the adsorption efficiency of an adsorbent [32]. Therefore, the textural properties of the synthesized merlinoite were investigated. The nitrogen adsorption-desorption isotherms and the BJH pore size distribution of the sample ( Figure S2) showed a type IV isotherm following the IUPAC classification. A BET surface area of about 20 m 2 /g was obtained. The sample showed the mesoporous nature of the resultant merlinoite.
The FTIR spectrum of the synthesized merlinoite is shown in Figure S3. The bands in the range of 3428-3608 cm −1 are valence vibrations of O-H bonds. The O-H band at 1635 cm −1 was assigned to deformation vibrations of adsorbed water molecules. The strongest peak at around 1007 cm −1 was seen in zeolite as asymmetric stretching vibrations of Si-O-Si and Si-O-Al, whereas the peak at about 753 cm −1 represented the symmetric stretching vibration. The peak at 427 cm −1 was attributed to the internal vibration of Si-O-Si and Si-O-Al bending [33]. These spectra indicate that the synthesized product had Si-O-Si or Si-O-Al groups, as a specific fingerprint of zeolite. In addition, it should be noted that the FTIR spectra of the fresh and the used merlinoite (after Pb adsorption) are similar. However, the slight lowering of the transmittance after Pb adsorption correlates well with that shown in previous work [27], where a small vibrational band shift was also detected, indicating the attachment of Pb 2+ on the adsorbent.

Effect of Solution pH
The pH is an important factor affecting adsorption capacity because the solution pH influences both the surface properties of the merlinoite and the ionic forms of the lead ions in solution. The effect of pH on Pb adsorption is displayed in Figure S4. The results show that Pb adsorption was poor at low pH levels. At low pH, the merlinoite surface functional groups are protonated, leading to many positively charged species and a decrease in the number of active adsorption sites [34]. Electrostatic repulsion between the positively charged functional groups and Pb could also inhibit the binding of Pb onto the surface of the merlinoite, thus lowering adsorption. When the pH is higher, there are more deprotonated functional groups and thus more active adsorption sites available. This promotes The TEM micrographs ( Figure S1a,b) of the synthesized merlinoite showed columnlike morphology. The SEAD pattern in Figure S1c exhibits a ring-like pattern suggesting the monocrystalline nature of the sample [31]. In addition, the HR-TEM micrograph in Figure S1d shows the lattice fringe with a d-spacing of 0.54 nm. This is due to the reflection from the (121) plane of the merlinoite.
The specific surface area is an important factor affecting the adsorption efficiency of an adsorbent [32]. Therefore, the textural properties of the synthesized merlinoite were investigated. The nitrogen adsorption-desorption isotherms and the BJH pore size distribution of the sample ( Figure S2) showed a type IV isotherm following the IUPAC classification. A BET surface area of about 20 m 2 /g was obtained. The sample showed the mesoporous nature of the resultant merlinoite.
The FTIR spectrum of the synthesized merlinoite is shown in Figure S3.  [33]. These spectra indicate that the synthesized product had Si-O-Si or Si-O-Al groups, as a specific fingerprint of zeolite. In addition, it should be noted that the FTIR spectra of the fresh and the used merlinoite (after Pb adsorption) are similar. However, the slight lowering of the transmittance after Pb adsorption correlates well with that shown in previous work [27], where a small vibrational band shift was also detected, indicating the attachment of Pb 2+ on the adsorbent.

Effect of Solution pH
The pH is an important factor affecting adsorption capacity because the solution pH influences both the surface properties of the merlinoite and the ionic forms of the lead ions in solution. The effect of pH on Pb adsorption is displayed in Figure S4. The results show that Pb adsorption was poor at low pH levels. At low pH, the merlinoite surface functional groups are protonated, leading to many positively charged species and a decrease in the number of active adsorption sites [34]. Electrostatic repulsion between the positively charged functional groups and Pb could also inhibit the binding of Pb onto the surface of the merlinoite, thus lowering adsorption. When the pH is higher, there are more deprotonated functional groups and thus more active adsorption sites available. This promotes enhanced interaction between Pb and the functional groups, so higher levels of Pb removal can be expected [35]. The optimal initial solution pH for Pb adsorption was found to be pH 6, which yielded 93.63% Pb removal. On the other hand, using a pH higher than 6 causes Pb precipitation [28,36].

Effect of Contact Time
The effect of contact time on Pb adsorption is shown in Figure S5. The results show that as Pb contact time increased from 5 to 150 min, adsorption also increased from 81% to 99%, leveling off after that. Initially, vacant active sites are abundantly available on the merlinoite surface, so the uptake of Pb can occur readily. At around 150 min, the situation changes. The vacant active sites have been filled, and equilibrium has been reached. Thus, a contact time of 150 min was selected for use in the subsequent experiments.

Effect of Co-Existing Metal Ions
The effect of various co-existing metal ions on Pb adsorption is shown in Figure S6. The results show that the adsorbent is selective for removing Pb 2+ in the solution, in comparison to other metal ions. The effect of the co-existing metal ions on lead removal can therefore be neglected. An efficiency of about 80-98% was achieved.

Adsorption Kinetics
The adsorption kinetics and rate constants of Pb adsorbed on merlinoite were determined using kinetic models. In investigations based on equilibrium adsorption, the pseudo-first-order and pseudo-second-order kinetic models are expressed as follows: ln(q e − q t ) = ln q e − k 1 t (4) where q e and q t represent the amounts of Pb ions adsorbed onto the merlinoite (mg/g) at equilibrium and at time t, respectively, and k 1 and k 2 are the rate constants for the pseudo-first-order and pseudo-second-order kinetics, respectively [37].
The linear plots of the pseudo-first-order and pseudo-second-order models for Pb adsorption process are shown in Figure S7, and the model parameters obtained from the fitting are listed in Table 1. It was observed that the coefficient of regression value (R 2 ) determined from the pseudo-second-order model (0.9999) was higher than that from the pseudo-first-order model (0.8430). In addition, the experimental adsorption capacity (q e, exp ) of 247.21 mg/g clearly fitted well with the calculated adsorption capacity (q e,cal ) of 250.00 mg/g from the pseudo-second-order model. The results indicate that Pb adsorption onto merlinoite can be described very well using a pseudo-second-order model, which means the kinetics of adsorption are those of chemical adsorption. The positively charged Pb cation reacts with negatively charged terminal hydroxyl groups on the merlinoite surface.

Adsorption Isotherms
The adsorption isotherm shows the distribution of the adsorbate molecules at the liquid/adsorbent interface. In this study, the Langmuir and Freundlich isotherm models were investigated to characterize the Pb adsorption process on the merlinoite. The experimental data were fitted using these isotherms ( Figure S8), and the obtained parameters are presented in Table 2. The linear forms of the Langmuir and Freundlich isotherms are shown below in Equations (6) and (7).
lnq e = lnK F + 1 n (lnC e ) (7) where C e is the concentration of Pb solution at equilibrium (mg/L), q e is the quantity of Pb adsorbed by the merlinoite (mg/g), q m is the maximum adsorption capacity (mg/g), and K L is the Langmuir constant (L/mg). The terms K F and n are Freundlich constants which represent the adsorption capacity and the intensity of adsorption, respectively [38]. The experimental data clearly agree with the Langmuir adsorption isotherm (R 2 = 0.9997) better than with the Freundlich adsorption isotherm (R 2 = 0.9248), so adsorption of Pb should take place in a monolayer on the surface of merlinoite. In addition, the favorability of the adsorption process can be predicted by the separation factor (R L ), which can be calculated using Equation (8). The R L values for all Pb concentrations were found to be in the range of 0.002-0.005, indicating the favorability of the Langmuir isotherm.
where K L is the Langmuir constant and C o (mg/L) is the initial Pb concentration. The value of R L predicts the adsorption process: unfavorable (R L > 1), favorable (0 < R L < 1), linear (R L = 1) or irreversible (R L = 0) [39].
For comparison, Table S2 shows the q m values for various adsorbents used for Pb adsorption in previous studies. The merlinoite investigated in this study has superior adsorption capacity.
Furthermore, apart from these two models, other adsorption models were also used to fit the raw data regarding Pb adsorption [40][41][42][43]. These models included the Redlich-Peterson isotherm, the Temkin isotherm, and the Dubinin-Radushkevich isotherm. The results are shown in Figure S8. However, the Langmuir model still showed the best fit.

Adsorption Thermodynamics
Determination of the thermodynamic parameters is vital to understanding the spontaneity and feasibility of an adsorption process. These parameters include the change in Gibb's free energy of adsorption (∆G • ), the change in enthalpy of adsorption (∆H • ), and the change in entropy of adsorption (∆S • ), which were determined using Equations (9)-(12), respectively [34,35,44,45]. K e = q e /C e (9) where K e represents the equilibrium distribution constant, R is the ideal gas constant (8.314 J/K·mol), and T(K) is the temperature. The adsorption thermodynamics model was established by plotting the value of ln(q e /C e ) against the reciprocal of temperature (1/T), to calculate the values of ∆H • and ∆S • as well as the regression coefficient (R 2 ), as shown in Figure S9. The adsorption thermodynamic parameters are summarized in Table 3. The positive value of ∆H • (40.55 kJ/mol) indicates an endothermic process of Pb adsorption onto merlinoite. The negative value of ∆G • over all the studied temperatures indicates a spontaneous process, and the degree of spontaneity of the adsorption process increases with increasing temperature. As the temperature increases, the mobility of Pb increases and the retardation of the diffusing ions decreases. The positive value of ∆S indicates that an increase in the randomness of the adsorbed Pb at the merlinoite-solution interface during the adsorption is expected.  Table 4 shows the results of utilizing the merlinoite product in actual wastewater samples with 50 ppm Pb. The adsorption percentages of merlinoite in all the wastewater samples were in the range of 94.4-97.6%. The results indicate that the synthesized merlinoite is an effective adsorbent for the removal of Pb from wastewater. In addition, comparison of the lead adsorption found in the present work and those reported in the literatures [46][47][48][49][50][51] can be shown in Table S2.

Conclusions
Merlinoite was successfully synthesized from sugarcane bagasse ash and kaolin using a one-step hydrothermal method. The adsorption of lead by the merlinoite depended on the solution pH, contact time, lead concentration, and temperature. The experimental data in the adsorption process correlated well with the pseudo-second-order kinetic model and the Langmuir adsorption isotherm. The maximum monolayer adsorption capacity of 322.58 mg/g obtained from merlinoite was higher than that obtained from other adsorbents. The thermodynamic data revealed that the lead adsorption was a spontaneous endothermic process. Thus, this new merlinoite, synthesized inexpensively from kaolinite and sugarcane bagasse ash, is an excellent candidate for utilization as a high-performance adsorbent in the removal of lead from wastewater.