Adsorption of Lead (II) Ions onto Goethite Chitosan Beads: Isotherms, Kinetics, and Mechanism Studies

: Lead is a highly toxic heavy metal that creates a water pollutant. It can be released from industrial processes, agricultural chemistry, and community wastes, affecting creatures and human health even at a low concentration. As a result, it is advised that lead be removed before releasing wastewater into the environment. This study synthesized three chitosan bead materials from shrimp shell wastes which were chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) for removing lead in an aqueous solution. Their surface area, pore volumes, and pore sizes were explored according to Brunauer– Emmett–Teller, and their crystalline formations were investigated using an X-ray diffractometer.


Introduction
Heavy metal pollution in water is becoming a major environmental concern around the world because it can be released from an increasing number of industries; so, wastewater from pollutant sources varies depending on the type of industry [1,2]. Lead is the most widely used raw material in many industries such as batteries, paint and pigments, petroleum, and ceramics [3]. Lead is highly toxic even at a low concentration and can affect organisms via bioaccumulation and biomagnification [4,5], including human health in the form of respiratory, gastrointestinal, and nervous system problems and carcinogens [6,7]. As a result, the World Health Organization (WHO) guideline for the clinical management of exposure to lead has specified provisional guidelines to address and prevent the effects of contamination and exposure to lead in drinking water at 10 µg/L [3], and the United States Environmental Protection Agency (USEPA) has established the maximum lead ion contaminant level in drinking water as being 0.015 mg/L [8].
At present, lead in wastewater is treated using several processes such as chemical precipitation, coagulation-flocculation, membrane filtration, adsorption, electrochemical treatment process, membrane separation process, ion exchange, photocatalysis, oxidation, and biological treatment [9,10]. However, before deciding to implement these methods, factors such as operation and disposal costs must be considered. As a result, adsorption is regarded as one of the most effective and intriguing strategies for increasing the efficiency of heavy metal removal in water [11][12][13]. The adsorption method depends on many factors, and one of the key factors is the adsorbent. As a result, the optimum adsorbent should have high removal efficiency and be specific to the pollutant being treated, while being inexpensive. For heavy metal removals, several adsorbents from commercial, natural, and waste sources are used, such as activated carbon, resin, silica, bagasse, lemon peel, rice husk, eggshells, creeps, and crustacean shells [14][15][16][17][18][19]. This is due to the increasing waste problem, especially food waste, which is a result of the increasing consumption of the population. Due to the increasing waste problem, especially food waste from the increasing consumption of the human population, the use of natural waste materials for new benefits is an interesting idea because this concept is useful for waste management, recycling waste, and treating wastewater. Among the food waste management options, an interesting choice is using crustacean and mollusk shell waste as raw materials for chitosan extraction and using the structure and properties of chitosan to remove heavy metals. Chitosan can remove several kinds of heavy metals from wastewater, including cadmium (Cd), copper (Cu), lead (Pb), arsenic (As), chromium (Cr), mercury (Hg), and nickel (Ni), according to previous studies [20][21][22][23][24]. However, investigations into how to improve the adsorption efficiency of chitosan specific to wastewater pollutants are still interesting.
Many previous studies have used various metal oxides of iron (II or III) oxide (Fe 3 O 4 or Fe 2 O 3 ), iron (III) oxide-hydroxide, zinc oxide (ZnO), titanium dioxide (TiO 2 ), magnesium oxide (MgO), aluminum oxide (Al 2 O 3 ), manganese oxide (MnO), and goethite (FeO(OH)) for improving the effectiveness of heavy metals, dye, or acid removals from wastewater because they increase surface area, pore volume, and the functional group of active sites [25][26][27][28][29][30][31][32][33][34][35][36]. Therefore, adding metal oxides to adsorbents promotes an increase in the efficiency of removing metal ions from wastewater, as similarly found in another report in Table 1. However, the direct use of metal oxides is not practical in industrial continuous flow systems due to clogging problems, pressure drops, and difficult separation after treatments [37,38]. To obtain high adsorption effectiveness for eliminating a particular target pollutant, it is therefore a good idea to add metal oxides to the effective adsorbent material. Additionally, the development of a long-lasting adsorbent form may aid in long-term use and reduce operating expenses. As a result, the goal of this study was to increase the effectiveness of chitosan material for lead removal from wastewater for upcoming industrial applications by including metal oxide and altering the material's form by employing shrimp shell waste as a raw material. The main objective of the present study was to synthesize chitosan beads modified with FeO(OH) (goethite), which were chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF), for removing lead from artificial wastewater. Their surface area, pore volumes, and pore sizes were explored according to Brunauer-Emmett-Teller, and their crystalline formations were investigated using an X-ray diffractometer. Their surface structures were studied via field emission scanning electron microscopy and the focus ion beam, and their chemical compositions were determined using an energy dispersive X-ray spectrometer. Their chemical functional groups were identified via Fourier-transform infrared spectroscopy. Furthermore, batch experiments were performed to compare the efficacy of their lead removals, adsorption isotherms and kinetics were verified to know the properties of their adsorption, and the reuse potentials of the CB, CFB, and CBF were estimated via desorption experiments.

The Synthesis of Three Chitosan Materials (CB, CFB, and CBF)
The synthesis of three chitosan materials which were chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) was modified from Ngamsurach, P et al., 2022 [30] and is illustrated in Figure 1b  Shrimp shell wastes were collected from the Banglamphu market, Muang Khon Kaen, Khon Kaen, Thailand. They were washed with tap water several times to remove pieces of tissue and other impurities and then rinsed with distilled water (DW). Next, they were dried at 80 • C for 24 h in a hot air oven (FED 53, Binder, Bohemia, NY, USA), and they were ground to a flake. Finally, they were kept in a desiccator before use.
Step 2: Demineralization First, 10 g of shrimp shell flake was added to 200 mL of 2 N HCl, and then it was mixed using a hot plate (C-MAG HS 7, IKA, Rawang, Malaysia) at 50 • C with a magnetic stirrer of 150 rpm for 2 h. Secondly, it was washed with tap water until obtaining a neutral pH. Thirdly, it was rinsed with distilled water and dried in a hot air oven at 40 • C. Finally, it was blended and kept in a desiccator until use, and was called the demineralized flake.
Step 3: Deproteinization First, 10 g of demineralized flake from Step 1 was added to 200 mL of 2 N NaOH, and then it was mixed using a hot plate at 100 • C with a magnetic stirrer of 150 rpm for 5 h. Secondly, it was filtered and washed with tap water until obtaining a neutral pH. Thirdly, it was rinsed with distilled water and dried in a hot air oven at 40 • C. Finally, it was blended and kept in a desiccator until use, and was called chitin.
Step 4: Deacetylation First, 10 g of chitin from Step 2 was added to 200 mL of 50% NaOH, and then it was mixed using a hot plate at 100 • C with a magnetic stirrer of 150 rpm for 5 h. Secondly, it was filtered and washed with tap water until obtaining a neutral pH. Thirdly, it was rinsed with distilled water and dried in a hot air oven at 40 • C. Then, chitosan flakes were blended and filtered through a sieve of 125 µm. Finally, it was kept in a desiccator before use and was called chitosan powder (CP).

The Synthesis of Three Chitosan Materials (CB, CFB, and CBF)
The synthesis of three chitosan materials which were chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) was modified from Ngamsurach, P et al., 2022 [30] and is illustrated in Figure 1b-d.
For the CB, 10 g of CP was added to 200 mL of 2% NaC 6 H 7 O 6 solution, and a hot plate at 60 • C with a magnetic stirrer of 150 rpm was used for the mixing for 1 h. After that, they were contained in a 10 mL syringe and were added dropwise into 225 mL of 0.1 M CaCl 2 solution. After that, they were soaked in 0.1 M CaCl 2 solution for 12 h for setting the beads. Finally, they were filtered, and deionized water (DI water) was used for rinsing many times. After that, they were dried for 12 h and kept in desiccators until use.
For the CFB, 10 g of CP and 10 g of FeO(OH) were added to 200 mL of 2% NaC 6 H 7 O 6 solution, and a hot plate at 60 • C with a magnetic stirrer of 150 rpm was used for the mixing for 1 h. After that, they were contained in a 10 mL syringe and were added dropwise into 225 mL of 0.1 M CaCl 2 solution. After that, they were soaked in 0.1 M CaCl 2 solution for 12 h for setting the beads. Finally, they were filtered, and DI water was used for rinsing many times. After that, they were dried for 12 h and kept in desiccators until use.
For the CBF, 10 g of CP was added to 200 mL of a 2% NaC 6 H 7 O 6 solution, and they were homogeneously mixed using a hot plate at 60 • C with a magnetic stirrer of 150 rpm for 1 h. Then, they were contained in a 10 mL syringe and were added dropwise into 225 mL of 0.1 M CaCl 2 solution, were filtered, and DI water was used for rinsing many times. Then, the bead samples were added to 200 mL of 5% FeO(OH) solution and shaken for 3 h at room temperature via an orbital shaker (GFL, 3020, Germany) at 150 rpm. Finally, they were filtered, and DI water was used for rinsing many times. After that, they were dried for 12 h and kept in desiccators until use.

Material Characterizations
The specific surface area, pore volumes, and pore sizes of the chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) were determined according to Brunauer-Emmett-Teller (BET) (QUADRASORB evo™, Austria) via isothermal nitrogen gas (N 2 ) adsorption-desorption at 77.3 K with a degas temperature of 80 • C for 6 h. An X-ray diffractometer (XRD) (Bruker, D8 Advance, Switzerland) in the range of 2θ = 5-80 • was used to analyze the crystalline structures of the commercial chitosan standard (STD), chitosan powder (CP), CB, CFB, and CBF. With the use of field emission scanning electron microscopy and a focus ion beam (FESEM-FIB) with an energy dispersive X-ray spectrometer (EDX) (FEI, Helios NanoLab G3 CX, USA), we investigated the surface morphologies and chemical compositions of the CB, CFB, and CBF. Finally, the chemical functional groups of STD, CP, CB, CFB, and CBF were identified via Fourier-transform infrared spectroscopy (FTIR) (Bruker, TENSOR27, Hong Kong) in a range of 4000-600 cm −1 .

Batch Adsorption Experiments
The lead removal efficiencies of chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) were examined via a series of batch experiments for the affecting factors of dose, contact time, pH, and concentration. To compare their lead removal efficiencies in batch experiments, the effects of dose (0.1-0.5 g), contact time (0-24 h), pH (3, 5, 7, and 9), and concentration (25-125 mg/L) were investigated. The control conditions of the lead concentration of 50 mg/L, a sample volume of 200 mL, a temperature of 25 • C, and a shaking speed (New Brunswick™, Innova 42, USA) of 200 rpm were applied. An atomic absorption spectrophotometer (AAS) (PerkinElmer, PinAAcle 900 F, USA) was used to analyze the lead concentration in all samples. The optimum condition was chosen by the lowest value of each affecting factor whilst obtaining the highest lead removal efficiency, and that value was applied to the next affecting factor studies. The lead removal efficiency in the percentage was calculated using Equation (1).
where C o and C e (mg/L) are the initial and final lead concentrations in the solution (mg/L).

Adsorption Isotherms
Adsorption isotherms were used to investigate the concentration or quantity of the adsorbed solute via the weight of adsorbents for homogeneous or heterogeneous adsorbent surfaces on a monolayer or multilayer, which are generally analyzed using linear Langmuir and Freundlich isotherms following Equations (2) and (3): Langmuir isotherm: C e /q e = 1/K L q max + C e /q max (2) Freundlich isotherm: log q e = log K F + 1/n (log C e ) where q e is the equilibrium adsorption capacity (mg/g), q max is the maximum adsorption capacity (mg/g), K L is the Langmuir constant (L/mg), C e is the equilibrium residual adsorbate concentration (mg/L), K F is the Freundlich constant (mg/g)(L/mg) 1/n , and 1/n is the measure of intensity. For the adsorption isotherm experiments, 0.5 g of CB, 0.5 g of CFB, or 0.4 g of CBF were added to 500 mL Erlenmeyer flasks with a variable series of lead concentrations of 25, 50, 75, 100, and 125 mg/L. The control conditions were a sample volume of 200 mL, a contact time of 15 h for CB, 15 h for CFB, or 14 h for CBF, pH 5, a temperature of 25 • C, and a shaking speed of 150 rpm.

Adsorption Kinetics
Adsorption kinetics were used to explain the mechanisms of lead adsorption on chitosan bead materials. The adsorption characteristics of chitosan materials were demonstrated using the pseudo-first-order and pseudo-second-order kinetic equations following Equations (4) and (5).
Pseudo-first-order kinetic model: ln(q e − q t ) = ln q e − k 1 t Pseudo-second-order kinetic model : t/q t = t/q e + 1/ k 2 q 2 e (5) where q e is the equilibrium adsorption capacity (mg/g), q t is the amount of the amount adsorbed at equilibrium at a time (t)(mg/g), k 1 is the pseudo-first-order rate constant (1/min), t is the time for adsorption (min), and k 2 is the pseudo-second-order rate constant per min (g/mg·min). For the kinetic adsorption experiments, 0.5 g of CB, 0.5 g of CFB, or 0.4 g of CBF were added to 1000 mL of the breaker with a lead concentration of 50 mg/L. The control conditions were a sample volume of 200 mL, a contact time of 16 h for CB, 16 h for CFB, or 14 h for CBF, pH 5, a temperature of 25 • C, and a shaking speed of 150 rpm.

Desorption Experiments
The desorption experiments were used to verify the reusability of the chitosan materials. In adsorption, 0.5 g of chitosan powder beads (CB), 0.5 g of chitosan powder mixed with goethite beads (CFB), or 0.4 g of chitosan powder beads coated with goethite (CBF) were added to 500 mL Erlenmeyer flasks with a lead concentration of 50 mg/L. The control conditions were a sample volume of 200 mL, a contact time of 16 h for CB, 16 h for CFB, or 14 h for CBF, pH 5, a temperature of 25 • C, and a shaking speed of 150 rpm. In desorption, the saturated chitosan materials were rinsed several times with deionized water and added to 500 mL Erlenmeyer flasks with 0.5 M HNO 3 . The control conditions were a sample volume of 100 mL, a contact time of 16 h for CB, 16 h for CFB, or 14 h for CBF, a temperature of 25 • C, and a shaking speed of 150 rpm. After that, they were washed with deionization water and dried at room temperature, and CB, CFB, and CBF were ready for the next adsorption cycle.

The Physical Characterizations
The physical characterizations of the commercial chitosan standard (STD), chitosan powder (CP), chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) are shown in Figure 2a For CB, they were cream-color beads similar to the color of CP, as illustrated in Figure 2c, whereas the CFB were dark-yellow-color beads, as shown in Figure 2d. Finally, the CBF were orange-yellow-color beads, as shown in Figure 2e.

BET
BET analysis was used to identify the specific surface area, pore volume, and pore size of chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF). The results from the BJH method are demonstrated in Table 2, and their BJH pore size distribution is illustrated in Figure 3. The specific surface areas of the CB, CFB, and CBF were 1.453, 2.305, and 1.959 m 2 /g, respectively, whereby the CFB had the largest size of surface area. The pore volumes of the CB, CFB, and CBF were 0.0049, 0.0115, and 0.0054 cm 3 /g, respectively, and the CFB had the larger pore volume. The pore sizes of the CB, CFB, and CBF were 3.01, 1.80, and 1.81 nm, respectively, and the CFB had the smallest pore size. Therefore, the addition of goethite into the chitosan beads helped to increase the specific surface area and pore volume, promoting more active sites for capturing more lead [52], while the pore size was decreased, and these characteristics were good and similar to the lead adsorption reports from previous investigations [26,29,42,43,53]. Moreover, according to the International Union of Pure and Applied Chemistry (IUPAC) classification of pore size based on diameter, the CFB and CBF were micropores (2 nm), and the CB were mesopores (2-50 nm) [54].
The physical characterizations of the commercial chitosan standard (STD), chitosan powder (CP), chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) are shown in Figure 2a-e. CP was a cream-color powder matching the color of STD, as demonstrated in Figure 2a,b. For CB, they were cream-color beads similar to the color of CP, as illustrated in Figure 2c, whereas the CFB were dark-yellow-color beads, as shown in Figure 2d. Finally, the CBF were orange-yellow-color beads, as shown in Figure 2e.

BET
BET analysis was used to identify the specific surface area, pore volume, and pore size of chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF). The results from the BJH method are demonstrated in Table 2, and their BJH pore size distribution is illustrated in Figure 3. The specific surface areas of the CB, CFB, and CBF were 1.453, 2.305, and 1.959 m 2 /g, respectively, whereby the CFB had the largest size of surface area. The pore volumes of the CB, CFB, and CBF were 0.0049, 0.0115, and 0.0054 cm 3 /g, respectively, and the CFB had the larger pore volume. The pore sizes of the CB, CFB, and CBF were 3.01, 1.80, and 1.81 nm, respectively, and the CFB had the smallest pore size. Therefore, the addition of goethite into the chitosan beads helped to increase the specific surface area and pore volume, promoting more active sites for capturing more lead [52], while the pore size was decreased, and these characteristics were good and similar to the lead adsorption reports

XRD
The XRD technique was used to study the crystalline formations of commercial chitosan standard (STD), chitosan powder (CP), chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF), and their XRD patterns are described in Figure 4a-e. All materials were semicrystalline structures, and the diffraction patterns of the chitosan materials were changed by modifying the forms and adding goethite. The crystalline structure of CP matched the diffraction pattern of STD at 2θ of 20 • representing the hydrogen bonds in the chitosan structure, shown in Figure 4a,b, similar to that found in other studies [30,55]. The CB had a lower crystalline structure than CP in a range of 2θ = 20 • , while the pattern at 2θ = 13 • was more crystalline than CP and was demonstrated to contain sodium alginate, indicating that the crystalline structure of the chitosan material was altered by the bead forms shown in Figure 4b,c [56]. For the CFB and CBF, they represented more amorphous structures than the CB and demonstrated the goethite diffraction pattern at 2θ = 21 • , 33 • , and 36 • , shown in Figure 4d,e [57,58]. (CBF).

XRD
The XRD technique was used to study the crystalline formations of commercial chitosan standard (STD), chitosan powder (CP), chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF), and their XRD patterns are described in Figure 4a-e. All materials were semicrystalline structures, and the diffraction patterns of the chitosan materials were changed by modifying the forms and adding goethite. The crystalline structure of CP matched the diffraction pattern of STD at 2θ of 20˚ representing the hydrogen bonds in the chitosan structure, shown in Figure 4a,b, similar to that found in other studies [30,55]. The CB had a lower crystalline structure than CP in a range of 2θ = 20°, while the pattern at 2θ = 13° was more crystalline than CP and was demonstrated to contain sodium alginate, indicating that the crystalline structure of the chitosan material was altered by the bead forms shown in Figure 4b,c [56]. For the CFB and CBF, they represented more amorphous

FESEM-FIB
The surface structures of chitosan material chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) were investigated using FESEM-FIB analysis at 2500× magnification with 50

FESEM-FIB
The surface structures of chitosan material chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) were investigated using FESEM-FIB analysis at 2500× magnification with 50 µm for a surface structure and 100× magnification with 1 mm for a bead form, as shown in Figure 5a-f. The surface structures of chitosan materials were heterogeneous with spherical shapes. The CB were uneven, rough, and a little wavy, as shown in Figure 5a,b. Furthermore, because of the presence of a group of goethite mixed into the surface of the material, the surfaces of the CFB and CBF were rougher than the CB. In Figure 5c,d, the CFB were demonstrated as being lumpy and sparsely needle-shaped, while the CBF were presented with a surface with dense needle-shaped patterns, as shown in Figure 5e,f.

EDX
The chemical compositions of the chitosan material chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) were analyzed using the EDX technique as demonstrated in Table 3. The six main chemical compositions of oxygen (O), carbon (C), calcium (Ca), nitrogen (N), chlorine (Cl), and sodium (Na) were detected in all chitosan materials, whereas the iron (Fe) was observed in only the CFB and CBF because of the addition of goethite. O, C, and N were the main chemical elements of the chitosan material from shrimp shells, as similarly found in previous studies [30,59]. For Ca, Cl, and Na, they were from the sodium alginate (NaC 6 H 7 O 6 ) and calcium chloride (CaCal 2 ) used for bead formation. In addition, the addition of goethite in CFB and CBF affected the mass percentages of the chemical compositions by decreasing O, C, Ca, N, Cl, and Na by the substitution of Fe, whereby the addition of goethite via the coating method of the CBF found a higher percentage by weight of Fe than the mixing method of the CFB. As a result, the CBF had a higher decrease in all chemical compositions than the CFB, except for Na.

Effect of Dose
Chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) were investigated for the effect of dose from 0.1 to 0.5 g, as shown in Figure 7a, with a lead concentration of 50 mg/L, a sample volume of 200 mL, a contact time of 24 h, a shaking speed of 150 rpm, and a temperature of 25 °C as the control conditions. The lead removal efficiencies of the CB, CFB, and CBF increased with increasing dosages because of the increase in the material active sites [30], and the highest lead removal efficiencies of the CB and CFB were found at 0.5 g for 99.14% and 99.32%, respectively. For the CBF, the highest lead removal efficiency of 99.57% was found at 0.4 g. Therefore, the optimum dosages of the CB, CFB, and CBF were 0.5 g, 0.5 g, and 0.4 g, respectively, and were used for the contact time effect.

Effect of Dose
Chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) were investigated for the effect of dose from 0.1 to 0.5 g, as shown in Figure 7a, with a lead concentration of 50 mg/L, a sample volume of 200 mL, a contact time of 24 h, a shaking speed of 150 rpm, and a temperature of 25 • C as the control conditions. The lead removal efficiencies of the CB, CFB, and CBF increased with increasing dosages because of the increase in the material active sites [30], and the highest lead removal efficiencies of the CB and CFB were found at 0.5 g for 99.14% and 99.32%, respectively. For the CBF, the highest lead removal efficiency of 99.57% was found at 0.4 g. Therefore, the optimum dosages of the CB, CFB, and CBF were 0.5 g, 0.5 g, and 0.4 g, respectively, and were used for the contact time effect.

Effect of Contact Time
Chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) had their contact time effect studied from 2 to 24 h, shown in Figure 7b, with a lead concentration of 50 mg/L, a sample volume of 200 mL, a shaking speed of 150 rpm, a temperature of 25 °C, and the optimum dose from Section 3.3.1 as the control conditions. The lead removal efficiencies of the CB, CFB, and CBF increased with increasing contact time from 2 to 24 h, and their saturated lead adsorptions were found at the highest contact time with the constant lead removal efficiency. The highest lead removal efficiencies of 99.09%, 99.25%, and 99.62% were found at 16 h for CB and CFB, whereas for CBF it was found at 14 h, respectively. Therefore, the optimum contact time of the CB, CFB, and CBF was 16 h, 16 h, and 14 h, respectively, and was used for the pH effect.

Effect of pH
Chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) were examined for their pH effects of 3, 5, 7, and 9, shown in Figure 7c, with a sample volume of 200 mL, a shaking speed of 150 rpm, a temperature of 25 °C, and the optimum dose and contact time from Sections 3.3.1 and 3.3.2 as the control conditions. The lead removal efficiencies at pH 3 of the CB, CFB, and CBF were 97.75%, 99.01%, and 99.30%, respectively. At a low pH, the active sites of

Effect of Contact Time
Chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) had their contact time effect studied from 2 to 24 h, shown in Figure 7b, with a lead concentration of 50 mg/L, a sample volume of 200 mL, a shaking speed of 150 rpm, a temperature of 25 • C, and the optimum dose from Section 3.3.1 as the control conditions. The lead removal efficiencies of the CB, CFB, and CBF increased with increasing contact time from 2 to 24 h, and their saturated lead adsorptions were found at the highest contact time with the constant lead removal efficiency. The highest lead removal efficiencies of 99.09%, 99.25%, and 99.62% were found at 16 h for CB and CFB, whereas for CBF it was found at 14 h, respectively. Therefore, the optimum contact time of the CB, CFB, and CBF was 16 h, 16 h, and 14 h, respectively, and was used for the pH effect.

Effect of pH
Chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) were examined for their pH effects of 3, 5, 7, and 9, shown in Figure 7c, with a sample volume of 200 mL, a shaking speed of 150 rpm, a temperature of 25 • C, and the optimum dose and contact time from Section 3.3.1 and Section 3.3.2 as the control conditions. The lead removal efficiencies at pH 3 of the CB, CFB, and CBF were 97.75%, 99.01%, and 99.30%, respectively. At a low pH, the active sites of the chitosan materials were protonated, and the competition with proton (H + ) between the solution and the cation of lead might be the limit for lead removal [66]. Furthermore, many previous studies have reported that the point of zero charge (pH pzc ) of chitosan material was pH > 4 [46,63,67,68], and so lead removal might happen at a pH of solution more than pH pzc (pH > 4). At pH 5, the lead removal efficiencies of the CB, CFB, and CBF increased to 99.41%, 99.49%, and 99.68%, respectively, which were the highest lead removal efficiencies in the entire pH range studied. At pH 7, the lead removal efficiencies of the CB, CFB, and CBF slightly decreased to 99.11%, 99.39%, and 99.28%, respectively. At pH 9, the lead removal efficiencies of the CB, CFB, and CBF decreased to 79.79%, 99.25%, and 99.24%, respectively.
The lead speciation diagram in the aqueous system is displayed in Figure 8, modified from Nurchi, V.M. and Villaescus, I. 2011 and Wang X. et al. 2017 [69,70]. In pH 4-6, lead could be adsorbed by the chitosan materials because of its status of Pb (II) ions (aq). At pH > 7, the lead removal efficiencies might be limited due to an increase in hydroxyl products such as Pb(OH) + , which is possible to remove via adsorption. However, lead precipitations might occur at pH > 9 becoming an insoluble Pb(OH) 2 (s) product, and lead adsorption might not happen from the observation found in CB [69,70]. In the case of the addition of goethite in the CFB and CBF, lead adsorptions could occur at all pH values with high lead removal efficiencies, which might be from the high pH pzc of goethite (pH > 8) reported by previous studies [71][72][73]; so, the addition of goethite to the CFB and CBF improved the material efficiency for adsorbing lead in a wider pH range. Therefore, the optimum pH value of the CB, CFB, and CBF was pH 5 and was used for the initial lead concentration effect. the chitosan materials were protonated, and the competition with proton (H + ) between the solution and the cation of lead might be the limit for lead removal [66]. Furthermore, many previous studies have reported that the point of zero charge (pHpzc) of chitosan material was pH > 4 [46,63,67,68], and so lead removal might happen at a pH of solution more than pHpzc (pH > 4). At pH 5, the lead removal efficiencies of the CB, CFB, and CBF increased to 99.41%, 99.49%, and 99.68%, respectively, which were the highest lead removal efficiencies in the entire pH range studied. At pH 7, the lead removal efficiencies of the CB, CFB, and CBF slightly decreased to 99.11%, 99.39%, and 99.28%, respectively. At pH 9, the lead removal efficiencies of the CB, CFB, and CBF decreased to 79.79%, 99.25%, and 99.24%, respectively. The lead speciation diagram in the aqueous system is displayed in Figure 8, modified from Nurchi, V.M. and Villaescus, I. 2011 and Wang X. et al. 2017 [69,70]. In pH 4-6, lead could be adsorbed by the chitosan materials because of its status of Pb (II) ions (aq). At pH > 7, the lead removal efficiencies might be limited due to an increase in hydroxyl products such as Pb(OH) + , which is possible to remove via adsorption. However, lead precipitations might occur at pH > 9 becoming an insoluble Pb(OH)2(s) product, and lead adsorption might not happen from the observation found in CB [69,70]. In the case of the addition of goethite in the CFB and CBF, lead adsorptions could occur at all pH values with high lead removal efficiencies, which might be from the high pHpzc of goethite (pH > 8) reported by previous studies [71][72][73]; so, the addition of goethite to the CFB and CBF improved the material efficiency for adsorbing lead in a wider pH range. Therefore, the optimum pH value of the CB, CFB, and CBF was pH 5 and was used for the initial lead concentration effect.

Effect of Concentration
Chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) were investigated for their initial lead

Effect of Concentration
Chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) were investigated for their initial lead concentration effect from 50 mg/L to 125 mg/L, as shown in Figure 7d, with a sample volume of 200 mL, a shaking speed of 150 rpm, a temperature of 25 • C, and the optimum dose, contact time, and pH from Sections 3.3.1-3.3.3 as the control conditions. The lead removal efficiencies of the CB, CFB, and CBF did not affect the increasing lead concentrations, and they could remove lead from 25-125 mg/L with high lead removal efficiencies of more than 98%. However, their lead removal efficiencies at 50 mg/L were 99.27%, 99.52%, and 99.66%, respectively. Therefore, the CBF were a higher potential material than the CB and CFB.
Finally, 0.5 g, 16 h, pH 5, 50 mg/L, 0.5 g, 16 h, pH 5, 50 mg/L, and 0.4 g, 14 h, pH 5, 50 mg/L were the optimum conditions for dose, contact time, pH, and concentration of the CB, CFB, and CBF, respectively, and they could be arranged in order from high to low of CBF > CFB > CB. The CBF had a higher potential material than the CB and CFB because they required less material dosage and contact time.

Adsorption Isotherms
The adsorption isotherms of chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) were observed by plotting linear Langmuir (C e /q e vs. C e ) and linear Freundlich (log q e vs. log C e ) models, whereby their plotted graphs are presented in Figures 9a,b and 10, and the equilibrium of linear isotherm parameters are reported in Table 4. concentration effect from 50 mg/L to 125 mg/L, as shown in Figure 7d, with a sample volume of 200 mL, a shaking speed of 150 rpm, a temperature of 25 °C, and the optimum dose, contact time, and pH from Sections 3.3.1-3.3.3 as the control conditions. The lead removal efficiencies of the CB, CFB, and CBF did not affect the increasing lead concentrations, and they could remove lead from 25-125 mg/L with high lead removal efficiencies of more than 98%. However, their lead removal efficiencies at 50 mg/L were 99.27%, 99.52%, and 99.66%, respectively. Therefore, the CBF were a higher potential material than the CB and CFB. Finally, 0.5 g, 16 h, pH 5, 50 mg/L, 0.5 g, 16 h, pH 5, 50 mg/L, and 0.4 g, 14 h, pH 5, 50 mg/L were the optimum conditions for dose, contact time, pH, and concentration of the CB, CFB, and CBF, respectively, and they could be arranged in order from high to low of CBF > CFB > CB. The CBF had a higher potential material than the CB and CFB because they required less material dosage and contact time.

Adsorption Isotherms
The adsorption isotherms of chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) were observed by plotting linear Langmuir (Ce/qe vs. Ce) and linear Freundlich (log qe vs. log Ce) models, whereby their plotted graphs are presented in Figures 9a,b and 10, and the equilibrium of linear isotherm parameters are reported in Table 4.     [68,[74][75][76]. As a result, the adsorption patterns of the CB, CFB, and CBF were explained by the physiochemical process, which was presented to the heterogeneous surfaces of all of the materials. Figure 10 demonstrated that the adsorption capacities (qe) increased with the increase in lead concentrations in all materials, and their qe could be arranged from high to low with CBF > CFB > CB. Figure 10. The adsorption isotherm of adsorption capacity (qe) vs. lead concentration (Ce).

Adsorption Kinetics
The adsorption kinetic is used to study the rate of adsorption over time of chitosan material chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF). Their rate adsorptions of lead with chitosan materials were investigated using adsorption kinetic models which were pseudofirst-order and pseudo-second-order kinetic models, by plotting ln(qe − qt) versus time (t) For the Langmuir model, the maximum adsorption capacities (q max ) of the CB, CFB, and CBF were 322.58, 333.33, and 344.83 mg/g, respectively, whereby the maximum adsorption capacity (q max ) of the CBF was demonstrated to be higher than the other materials. The Langmuir adsorption constants (K L ) of the CB, CFB, and CBF were 1.72, 1.76, and 2.07 L/mg, respectively. For the Freundlich model, the measure of intensity or the adsorption strength (1/n) values of the CB, CFB, and CBF were 0.51, 0.52, and 0.55, respectively, whereby the measure of intensity (1/n) of the CBF had a higher value than the other materials. The Freundlich adsorption constants (K F ) of the CB, CFB, and CBF were 190.15, 204, and 233.29 (mg/g)(L/mg) 1/n , respectively, whereby the K F of the CBF demonstrated a higher value than the other materials. In general, the explanation of the adsorption pattern of each adsorption material is decided by the R 2 value of the adsorption isotherm model. Therefore, they are chosen with the correlation coefficient (R 2 ) value being especially close to 1. Finally, the CB, CFB, and CBF corresponded to the Freundlich model with an R 2 of 0.992, 0.992, and 0.994, respectively, which agrees with other studies [68,[74][75][76]. As a result, the adsorption patterns of the CB, CFB, and CBF were explained by the physiochemical process, which was presented to the heterogeneous surfaces of all of the materials. Figure 10 demonstrated that the adsorption capacities (q e ) increased with the increase in lead concentrations in all materials, and their q e could be arranged from high to low with CBF > CFB > CB.

Adsorption Kinetics
The adsorption kinetic is used to study the rate of adsorption over time of chitosan material chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF). Their rate adsorptions of lead with chitosan materials were investigated using adsorption kinetic models which were pseudo-first-order and pseudo-second-order kinetic models, by plotting ln(q e − q t ) versus time (t) for pseudo-first-order and t/q t versus time (t) for pseudo-second-order, whereby their plotted graphs are demonstrated in Figure 11a,b, and their equilibrium of kinetic parameters is presented in Table 5. for pseudo-first-order and t/qt versus time (t) for pseudo-second-order, whereby their plotted graphs are demonstrated in Figure 11a,b, and their equilibrium of kinetic parameters is presented in Table 5.  The results demonstrated that the correlation coefficients (R 2 ) from the adsorption kinetic models of the CB, CFB, and CBF corresponded to the pseudo-second-order kinetic model at 0.9997, 0.9999, and 0.9999, respectively, as similarly reported by other studies [40,47,55,67], whereby the adsorption mechanism and the rate-determining step were the chemisorption mechanisms on the surface of all of the materials [77]. Furthermore, Figure  12 demonstrates that the rate of lead adsorption of chitosan bead materials achieved the equilibrium state within 200 min, whereby the CBF represented the highest adsorption capacity (qe) at 25.25 mg/g, followed by the CFB at 20.10 mg/g and CB at 20.00 mg/g, respectively.  The results demonstrated that the correlation coefficients (R 2 ) from the adsorption kinetic models of the CB, CFB, and CBF corresponded to the pseudo-second-order kinetic model at 0.9997, 0.9999, and 0.9999, respectively, as similarly reported by other studies [40,47,55,67], whereby the adsorption mechanism and the rate-determining step were the chemisorption mechanisms on the surface of all of the materials [77]. Furthermore, Figure 12 demonstrates that the rate of lead adsorption of chitosan bead materials achieved the equilibrium state within 200 min, whereby the CBF represented the highest adsorption capacity (q e ) at 25.25 mg/g, followed by the CFB at 20.10 mg/g and CB at 20.00 mg/g, respectively.

Desorption Experiments
The adsorption and desorption experiments of chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) for lead adsorption were investigated in three cycles using 0.1 M HNO3 as the desorption eluent, and the results of the lead removal efficiency decreased gradually as the number of cycles increased, which is demonstrated in Figure 13.
For the CB, the lead adsorption efficiencies of three cycles were 99.49%, 97.71%, and 93.56%, respectively, while the lead desorption efficiencies of three cycles were 82.64%, 80.27%, and 75.32%, respectively. For the CFB, the lead adsorption efficiencies of three cycles were 99.60%, 97.89%, and 93.70%, respectively, while the lead desorption efficiencies of three cycles were 83.22%, 81.62%, and 73.70%, respectively. For the CBF, the lead removal efficiencies of three cycles were 99.62%, 97.88%, and 93.60%, respectively, while the lead desorption efficiencies of three cycles were 82.33%, 78.94%, and 75.21%, respectively. Moreover, their lead adsorption and desorption efficiencies after use in three cycles decreased by approximately 5.95% and 7.99%, respectively. As a result, they could be reused in more than three cycles with lead adsorptions of more than 73%.

Desorption Experiments
The adsorption and desorption experiments of chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) for lead adsorption were investigated in three cycles using 0.1 M HNO 3 as the desorption eluent, and the results of the lead removal efficiency decreased gradually as the number of cycles increased, which is demonstrated in Figure 13.
For the CB, the lead adsorption efficiencies of three cycles were 99.49%, 97.71%, and 93.56%, respectively, while the lead desorption efficiencies of three cycles were 82.64%, 80.27%, and 75.32%, respectively. For the CFB, the lead adsorption efficiencies of three cycles were 99.60%, 97.89%, and 93.70%, respectively, while the lead desorption efficiencies of three cycles were 83.22%, 81.62%, and 73.70%, respectively. For the CBF, the lead removal efficiencies of three cycles were 99.62%, 97.88%, and 93.60%, respectively, while the lead desorption efficiencies of three cycles were 82.33%, 78.94%, and 75.21%, respectively. Moreover, their lead adsorption and desorption efficiencies after use in three cycles decreased by approximately 5.95% and 7.99%, respectively. As a result, they could be reused in more than three cycles with lead adsorptions of more than 73%.

Possibility of Mechanisms via Lead Adsorption for Chitosan Materials
The lead removal processes of chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) are influenced by their chemical functional groups which consist of an amine group (-NH2 or N-H), a hydroxyl group (-OH), and a carboxyl group (-COOH). Moreover, the CFB and CBF also had the chemical functional group of goethite (FeO(OH)), which affected the capture of lead. The possible mechanisms for the lead adsorption of chitosan materials are demonstrated in Figure 14a-c, which modified the idea from the study of Ngamsurach, P et al. [30]. For CB, lead (II) ions (Pb 2+ ) will be adsorbed by the donating proton (H + ) of -NH2, -OH, and -COOH followed by Pb 2+ replacement via electrostatic interaction [60,62,63,65,78,79], as shown in Figure 14a. For CFB and CBF, lead adsorption might happen similarly to CB. In addition, lead adsorption might be from the chitosan materials and sodium alginate connected with FeO(OH) by covalent and hydrogen bonds [58,80], as illustrated in Figure 14b,c.

Possibility of Mechanisms via Lead Adsorption for Chitosan Materials
The lead removal processes of chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) are influenced by their chemical functional groups which consist of an amine group (-NH 2 or N-H), a hydroxyl group (-OH), and a carboxyl group (-COOH). Moreover, the CFB and CBF also had the chemical functional group of goethite (FeO(OH)), which affected the capture of lead. The possible mechanisms for the lead adsorption of chitosan materials are demonstrated in Figure 14a-c, which modified the idea from the study of Ngamsurach, P et al. [30]. For CB, lead (II) ions (Pb 2+ ) will be adsorbed by the donating proton (H + ) of -NH 2 , -OH, and -COOH followed by Pb 2+ replacement via electrostatic interaction [60,62,63,65,78,79], as shown in Figure 14a. For CFB and CBF, lead adsorption might happen similarly to CB. In addition, lead adsorption might be from the chitosan materials and sodium alginate connected with FeO(OH) by covalent and hydrogen bonds [58,80], as illustrated in Figure 14b

Conclusions
Shrimp shell wastes were successfully extracted to chitosan powder (CP) and were synthesized to the chitosan bead materials modified with or without goethite which were chitosan powder beads (CB), chitosan powder mixed with goethite beads (CFB), and chitosan powder beads coated with goethite (CBF) for lead removal in aqueous solution. They were semicrystalline structures, and the CFB and CBF were also found in the specific goethite peaks. The CBF illustrated the highest surface area and the smallest pore size compared to the CB and CFB. In addition, the pore sizes of the CFB and CBF were micropores, whereas those of the CB were mesopores. They had a spherical shape with heterogeneous surfaces. O, C, Ca, N, Cl, and Na were the main chemical components in all of the materials, and Fe was detected in the CFB and CBF due to the addition of goethite. N-H, O-H, C-H, C-O, and -COOH were the five main functional groups of all materials. For batch experiments, the lead removal efficiencies of all of the chitosan materials were more than 98% as seen via the optimum conditions of the CB, CFB, and CBF for lead removal efficiencies being 0.5 g, 16 h, pH 5, 0.5 g, 16 h, pH 5, and 0.4 g, 14 h, pH 5, respectively, indicating that the addition of goethite to chitosan materials improved their lead removal efficiencies. Moreover, all of the chitosan materials corresponded to the Freundlich model, which indicates the physiochemical process of their adsorption patterns. For adsorption kinetics, all of the chitosan materials corresponded to a pseudo-secondorder kinetic model which was the chemisorption mechanism. The CBF demonstrated the highest lead adsorption capacity of 344.83 mg/g. The CB, CFB, and CBF could be reused for more than three cycles, with lead removal efficiencies above 90%. For future work, continuous flow experiments for investigating chitosan materials' efficiencies for applying wastewater treatment plants and the competing ion studies such as Mg 2+ and Ca 2+ should be studied to determine whether the chitosan bead materials could specifically adsorb lead in the solution.
Author Contributions: T.S.: investigation, visualization, writing-original draft. P.P.: supervision, conceptualization, funding acquisition, investigation, methodology, validation, formal analysis, visualization, writing-original draft, writing-review and editing. All authors have read and agreed to the published version of the manuscript.
Funding: This work has been financial supported by the research capability enhancement program through the graduate student scholarship, Faculty of Science, Khon Kaen University.

Data Availability Statement:
The data presented in the study are available upon request from the corresponding author.

Conflicts of Interest:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.