Adsorption of Pb(II) from Aqueous Solution Using Dendritic Fibrous Type SBA-15 (DFSBA-15)

– The presence of heavy metals in the environment is undeniably harmful and toxic to living things, where the presence of Pb(II) inside human foods and drinks results in various diseases. The use of adsorption as a method of wastewater treatment is widespread, and effective adsorption performance depends on appropriate adsorbent selection. In this research, dendritic fibrous type SBA-15 (DFSBA-15) was prepared and used for lead (Pb(II)) adsorption. The physicochemical properties of the DFSBA-15 were characterized using TEM, BET, and FTIR. The characterization analyses confirmed the formation of fibrous morphology, moderate surface area, and bulk -OH. Several factors, including contact time (min ) , adsorbent dosage (g/L), pH, and initial concentration (mg/L), were examined for Pb(II) adsorption. The best adsorption performance (89.88%) was attained at 180 min, 1 g/L of adsorbent dosage, pH 5, and 100 mg/L of Pb(II) initial concentration. Pseudo-second-order reaction type and Langmuir isotherm provided good fits to the experimental data, with R 2 ≥ 0.9 943 and R 2 = 0.9982, respectively. In short, the DFSBA-15 exhibits great potential for excellent Pb(II) removal.


INTRODUCTION
Hazardous substances and heavy metals frequently contaminate water bodies that serve as a source of drinking water during rainy runoff. Heavy metals in aquatic environments have fatal long-term effects, non-biodegradability, and toxic effects [1,2]. The maximum acceptable lead (Pb(II)) concentration in drinking water is less than 0.015 mg/L, and a high concentration of Pb(II) could result in a variety of adverse health effects [3]. Therefore, removing Pb(II) from wastewater is crucial, pushing scientists to continue developing novel strategies for its removal.
Several treatment techniques, including adsorption [4], photocatalytic degradation [5], precipitation [6], and ultrafiltration by the membrane [7], were reported. Among others, adsorption has attracted considerable attention and is widely used to treat industrial wastes, effluents, and water supplies owing to its efficiency and practicability [8]. Additionally, the adsorption method has advantages in terms of its adaptability, does not produce any toxic by-products, low energy requirement, and is inexpensive [9]. However, an appropriate selection of adsorbents is vital for excellent adsorption performance.
Fibrous nano-silica (KCC-1) was firstly discovered in 2010 and has attracted considerable attention in numerous applications attributable to its attractive properties, such as wide pore diameter and high surface area. Unlike the typical pore-based silica materials, KCC-1 is surrounded by a vast amount of dendrimer, thus forming fibrous morphology on it. The unique morphology of KCC-1 renders abundant accessible active sites, which subsequently enhance its performance in several applications. The remarkable properties of KCC-1 induced the attempt to modify conventional silica materials into fibrous-type.

Adsorption Procedure
The adsorption procedure was conducted following the method described in the literature [4,14]. The stock solution was produced by adding lead nitrate (99%, Sigma-Aldrich) to distilled water at different concentrations (50 mg/L -400 mg/L). The Pb(II) solution was continuously stirred while adding DFSBA-15 powder. Hydrochloric acid (HCl, Merck) was used to alter the pH of the solution to acidic, while sodium hydroxide (NaOH, Merck) was employed to modify the solution to alkaline. The samples were collected, followed by centrifugation (8 min, 12,000 rpm) and UV-VIS analysis with the addition of a dithizone reagent. The Pb(II) adsorption was calculated using the following Equations.
C 0 and C t (mg/L) represent the concentration at initial and at any time, respectively. The amount of an adsorbed Pb(II) at any given time was measured by Q t (mg/g). The weight of the adsorbent was represented by m (g), and the Pb(II) solution's volume was indicated by V (L).

Adsorbent Characterizations
Figure 1(A) depicts the FTIR spectra of DFSBA-15. The peaks at 798 cm -1 and 1047 cm -1 indicated the Si−O−Si band's symmetrical and asymmetrical stretching, respectively [15], while the peak at 437 cm -1 was assigned to the vibration of bending Si−O [16]. The band at 3351 cm -1 was credited to the bulk −OH stretching vibration on the Si−OH bond. During the hydrothermal preparation of DFSBA-15, water was utilized as a solvent and assisted in forming −OH on the surface [17]. The properties of the synthesized DFSBA-15 are comparable with the findings described by Chong et al. [11]. Figure 1(B) shows the TEM image of DFSBA-15. As illustrated, DFSBA-15 consists of dendritic dendrimer fibres, similar to KCC-1's fibrous nano-silica [18]. This dendrimer's structure, with its open hierarchical fibrous channel, will benefit the reactant mass transfer and increase active site accessibility [18]. The BET analysis discovered that the surface area (SA), pore volume (V p ), and pore diameter (D p ) of the DFSBA-15 are 156 m 2 /g, 0.31 cm 3 /g, and 7.95 nm, respectively. The moderate SA, large V p , and broad D p of DFSBA-15 will be advantageous in the adsorption of Pb(II). journal.ump.edu.my/jceib ◄

Effects of Parameters for Pb(II) Adsorption
Figure 2 depicts Pb(II) adsorption employing a DFSBA-15 as an adsorbent. Pb(II) adsorption efficiency was 38 % after 60 minutes and progressively raised to 47 % after 180 min. Beyond 180 minutes, the system reached an equilibrium where the adsorption rate remained constant, and a steady-state assumption was predicted. The obtained result is comparable to the study described by Teong et al., wherein the Pb(II) removal rapidly escalated and then gradually raised and remained constant until it reached an equilibrium point [4]. The significant increase in Pb(II) adsorption is owing to the abundant active sites on the DFSBA-15. However, the sluggish adsorption rate above 180 minutes might be due to the less active site availability, and the saturation was reached. Furthermore, the fibrous morphology structure of DFSBA-15 was found to provide a highly accessible active site and allow more Pb(II) pollutants attached to the pores of DFSBA-15, as revealed by the high percentage of Pb(II) removal. This good performance could be ascribed to the moderate SA, large V p , and broad D p of the prepared DFSBA-15, as confirmed by BET analysis.  As anticipated, attributable to the abundance of active sites, the Pb(II) removal rate increased as the number of adsorbents DFSBA-15 increased. The Pb(II) adsorption was 48% when the DFSBA-15 dosage was 0.1 g/L but increased to 67% with 1 g/L of DFSBA-15. This is owing to the considerable amount of available binding sites and large surface area, resulting in an increasing number of OH radicals that performed strong interaction with the contaminants molecules (Pb(II)) [19]. On the other hand, as the DFSBA-15 dosage increased from 1 g/L to 5 g/L, the adsorption rate slightly decreased from 67% to 65%. This could be due to the excessive adsorbent of DFSBA-15, where the remaining Pb(II) molecules were unavailable for the adsorption interactions. Hence, 1 g/L of DFSBA-15 is considered the best adsorbent dosage. A similar result was obtained by Song et al., in which Pb(II) adsorption efficiency became higher as the adsorbent dose was added, but no apparent alterations in adsorption percentage after reaching saturation point [20]. In brief, the amount of DFSBA-15 adsorbent loading is directly proportional to the overall adsorption reaction rate as the amount of adsorption is below the saturation stage. journal.ump.edu.my/jceib ◄  Figure 4 demonstrates the effect of multiple pHs for Pb(II) adsorption from an aqueous phase. It is evident that modifying the pH from 3 to 5 increases the removal effectiveness from 52% to 77.65%, respectively. This is because the amount of Pb(II) molecule was in cationic forms, and no complexation could occur with less HCl addition [21]. Thus, allowing more Pb(II) to bind to the adsorbent sites. However, the adsorption rate drops progressively in the alkali phase, from 68.24% to 64.14% for pH 8 and 10, respectively. This is owing to OH radicals competing with Pb(II) on the DFSBA-15 surface, slowing down the Pb(II) breakdown and forming Pb(OH) 2 precipitate. As observed, pH 5 is considered the best pH for Pb(II) adsorption using DFSBA-15, with Pb(II) removal of 77.65%. The influence of pH followed a similar pattern described by Zhao et al. [22] for Pb(II) adsorption using amorphous Zr-MOG-12 and CzBPOF.  Figure 5 presents the effect of various initial Pb(II) concentrations on the Pb(II) adsorption. The figure depicts that increasing initial concentration reduced the efficiency of Pb(II) removal. As indicated, 100 mg/L illustrated the higher Pb(II) removal, followed by 200 mg/L. Nevertheless, a further increase in the initial concentration to 400 mg/L lowered the Pb(II) adsorption percentage, which is 78.11%. This could be due to the less Pb(II) pollutant at a lower concentration (100 mg/L) compared to a higher concentration (400 mg/L), and surface-active sites have become saturated with the pollutant [4]. For the above adsorption study using one-factor-at-one-time (OFAT), the best adsorption performance (89.88%) was attained at 180 min, 1 g/L of adsorbent dosage, pH 5, and 100 mg/L of Pb(II) initial concentration.

Kinetic Study
Pseudo-first-order (PFO), pseudo-second-order (PSO), and Elovich kinetic models were deployed under the effect of initial concentrations (C o = 50 mg/L -400 mg/L). The Elovich model explained the kinetics of heterogeneous chemisorption, while PFO and PSO defined adsorption behavior in physisorption and chemisorption, respectively [23]. The Equation of the models is described as follows: PSO model: Elovich: The amount of adsorbed Pb(II) at the time, t, and at equilibrium (mg/g) is described by q t and q e , respectively. k 1 and k 2 signify rate constants of the PFO (L/min) and PSO (g/mg·min). While, α represent adsorption rate constant (mg/(g·min)), and β indicates desorption constant (g/mg). Table 1 lists the linear regression coefficient, R 2 , and other calculated parameters. The PSO model was the best model representing the adsorption data owing to its highest R 2 (≥ 0.9943) and closer q e , cal values with q e , exp values. The close match of data with the PSO kinetic model indicates that the reaction is chemisorption, and the reaction rate is proportionate to the number of active sites on the DFSBA-15's surface. Therefore, the fascinating textural properties of DFSBA-15 that consist of fibrous structure could provide better distribution of active sites, thus contributing to the excellent adsorption performance of DFSBA-15. Moreover, the presence of bulk −OH on the DFSBA-15 surface, as evidenced by FTIR analysis (Figure 1(A)), could contribute to the chemisorption adsorption. A comparable finding was also reported by Zhang and a co-worker, where the adsorption of Pb(II) by magnetic polyethyleneimine lignin (M-Lignin-PEI) best fit with the PSO reaction type [24].

Isotherm Study
The isotherm models of Freundlich, Temkin, Dubinin-Radushkevich, and Langmuir [25] were applied to analyze the experimental data in this investigation. In a nutshell, Temkin was utilized to explore the impact of adsorbate-adsorbent interactions on the adsorption process [26]. Meanwhile, utilizing the heterogeneous surface theory, the Dubinin-Radushkevich isotherm was ascribed to the adsorption process. In contrast, Langmuir and Freundlich's isotherm explained that monolayer adsorption happens on the homogenous adsorbent's surface and multilayer adsorption over the heterogeneous adsorbent's surface, respectively [27]. These isotherms' linearized forms were stated in Equation (6,7,8,9): Freundlich: = log + 1 log Temkin: = ln + ln (8) Where adsorption capacity at equilibrium and maximum is described as q e and q m (mg/g), respectively. n indicates an empirical constant, and C e (mg/L) signifies the Pb(II) concentration at equilibrium. In contrast, Langmuir and Freundlich's constants are represented by K L (L/mg) and K F ((mg/g)(L/mg) 1/n ), respectively. B signifies Temkin constant, A (L/g) indicates Temkin equilibrium constant, K DR (mol 2 /kJ 2 ) signifies Dubinin-Radushkevich constant, and ɛ (J/mol) represents Polanyi potential. Table 2 depicts the calculated regression coefficient, R 2 , and kinetic parameters of the studied models. Data analysis revealed that the Langmuir is the close-matched model for this study, represented by the highest R 2 (0.9982). The close match of data with the Langmuir indicates monolayer adsorption on a uniform surface of DFSBA-15. A similar kinetic model was also reported in the literature [28], where the adsorption of Pb(II), Cd(II), and Cu(II) by SiO 2 /kaolinite/Fe 2 O 3 was best-fitted Langmuir isotherm.

CONCLUSION
The DFSBA-15 was successfully prepared using the microwave-assisted microemulsion method. The characterization of DFSBA-15 using FTIR, BET, and FTIR confirmed the formation of fibrous morphology, moderate surface area (156.46 m 2 /g), large pore size (7.95 nm), and bulk -OH. Several factors, including time (min), adsorbent dosage (g/L), initial pH, and initial Pb(II) concentration (mg/L), were examined for Pb(II) adsorption, and the best adsorption performance was attained at 180 min, 1 g/L DFSBA-15 dosage, pH 5, and 100 mg/L initial Pb(II) concentration, with 89.88% adsorption of Pb(II). The Langmuir model was a good match for adsorption isotherm, indicating monolayer adsorption on the homogeneous DFSBA-15's surface. The adsorption data closely matched the PSO kinetic model, which revealed that the reaction rate is proportionate to the number of active sites on the DFSBA-15's surface. In short, DFSBA-15 has demonstrated remarkable performance in treating wastewater containing hazardous heavy metals.