Powdered and beaded sawdust materials modified iron (III) oxide-hydroxide for adsorption of lead (II) ion and reactive blue 4 dye

The problems of lead and reactive blue 4 (RB4) dye contamination in wastewater are concerns because of their toxicities to aquatic life and water quality, so lead and RB4 dye removals are recommended to remove from wastewater before discharging. Sawdust powder (SP), sawdust powder doped iron (III) oxide-hydroxide (SPF), sawdust beads (SPB), and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB) were synthesized and characterized with various techniques, and their lead or RB4 dye removal efficiencies were investigated by batch experiments, adsorption isotherms, kinetics, and desorption experiments. SPFB demonstrated higher specific surface area (11.020 m2 g−1) and smaller pore size (3.937 nm) than other materials. SP and SPF were irregular shapes with heterogeneous structures whereas SPB and SPFB had spherical shapes with coarse surfaces. Calcium (Ca) and oxygen (O) were found in all materials whereas iron (Fe) was only found in SPF and SPFB. O–H, C–H, C=C, and C–O were detected in all materials. Their lead removal efficiencies of all materials were higher than 82%, and RB4 dye removal efficiencies of SPB and SPFB were higher than 87%. Therefore, adding iron (III) oxide-hydroxide and changing material form helped to improve material efficiencies for lead or RB4 dye adsorption. SP and SPB corresponded to Langmuir model related to a physical adsorption process whereas SPF and SPFB corresponded to the Freundlich model correlated to a chemisorption process. All materials corresponded to a pseudo-second-order kinetic model relating to the chemical adsorption process. All materials could be reused more than 5 cycles with high lead removal of 63%, and SPB and SPFB also could be reused more than 5 cycles for high RB4 dye removal of 72%. Therefore, SPFB was a potential material to apply for lead or RB4 dye removal in industrial applications.

Melon peels Lead (Pb 2+ ) 1.5 g L −1 1 h 30 7 10 98.50 19 Melon peels Copper (Cu 2+ ) 1.5 g L −1 1 h 30 6 10 99.10 19 Melon peels Cadmium (Cd 2+ ) 1.5 g L −1 1 h 30 6 10 99.20 19 Cordia trichotoma sawdust Crystal violet 0. 8  www.nature.com/scientificreports/ modifications of rice bran with SnO 2 /Fe 3 O 4 , bagasse with zinc oxide, and lemon peels beads with iron (III) oxidehydroxide have been applied for removing RB4 dye [11][12][13] . Moreover, the stability of the material is another point for applying industrial application, so the changing of material form from a powder form to a bead form in many studies has also been reported with supporting the increase of heavy metal or dye removal efficiencies [6][7][8][12][13][14] . Therefore, this study attempts to synthesize sawdust materials modified with iron (III) oxide-hydroxide in powder and bead materials, compare their lead or RB4 dye removal efficiencies through batch experiments, and verify whether adding metal oxide or changing form helped to improve a material efficiency for lead or RB4 adsorption. The study aimed to synthesize four types of adsorbent materials which were sawdust powder (SP), sawdust powder doped iron (III) oxide-hydroxide (SPF), sawdust beads (SPB), and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB). Several characterized techniques of Brunauer-Emmett-Teller (BET), field emission scanning electron microscopy and focus ion beam (FESEM-FIB) with energy dispersive X-ray spectrometer (EDX), and Fourier transform infrared spectroscopy (FT-IR) were used to investigate their specific surface area, pore volume, pore size, surface morphologies, chemical compositions, and chemical functional groups. Lead or RB4 dye removal efficiencies of SP, SPF, SPB, and SPFB were examined by batch experiments with varying doses, contact time, temperature, pH, and concentration. Moreover, linear and nonlinear adsorption isotherms of Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models and kinetics of pseudo-first-kinetic, pseudo-second-kinetic, Elovich, and intraparticle diffusion models were used for investigating their lead or RB4 dye adsorption patterns and mechanisms. Finally, the desorption experiments were also investigated for confirming the reusability of sawdust materials for lead or RB4 dye adsorption.

Result and discussion
The physical characteristics of sawdust materials. The   www.nature.com/scientificreports/ Characterizations of sawdust materials. BET analysis. The specific surface area, pore volume, and pore diameter size of sawdust powder (SP), sawdust powder doped iron (III) oxide-hydroxide (SPF), sawdust beads (SPB), and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB) determined by the Brunauer-Emmet and Teller technique (BET) with N 2 adsorption-desorption isotherm at 77.3 K and degas temperature of 80 °C for 6 h, and the results of specific surface area and pore volume by Brunauer-Emmett-Teller (BET) and pore size by Barrett-Joyner-Halenda (BJH) method are reported in Table 2.
For the specific surface area of SP, SPF, SPB, and SPFB, they were 0.328, 1.551, 1.960, and 11.020 m 2 g −1 , respectively which SPFB demonstrated the highest specific surface area than other materials. In addition, their pore volumes were 0.075, 0.356, 0.250, and 2.532 cm 3 g −1 , respectively, and their pore diameter sizes were 4.256, 3.940, 4.068, and 3.937 nm, respectively. As a result, the addition of iron (III) oxide-hydroxide into sawdust materials (SPF and SPFB) increased the specific surface area and pore volume while the pore diameter size was decreased. In addition, all three parameters increased from changing material form from SP to SPB. Since their pore size was in a range of 2-50 nm, they were classified as mesoporous by the classification of International Union of Pure and Applied Chemistry (IUPAC) 35 .
For the BET comparison, SP had a lower specific surface area than all studies reported in Table 2 whereas SPF, SPB, and SPFB had higher values than the studies of Chen et al. and Houshangi et al. 29,34 . The studies of Chen et al. and Houshangi et al. had been reported that the modifications of sawdust materials by sodium hydroxide (NaOH) or triethanolamine (C 6 H 15 NO 3 ) or iron dioxide (Fe 2 O 4 ) helped to increase specific surface area similar to this study 29,34 .
FESEM-FIB analysis. The surface morphologies of sawdust powder (SP), sawdust powder doped iron (III) oxide-hydroxide (SPF), sawdust beads (SPB), and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB) by FESEM-FIB analysis at 500 × magnification with 400 µm for a surface and at 100 × magnification with 1 mm for a bead illustrated in Fig. 2a-f. SP and SPF were irregular shapes with heterogeneous fiber structures demonstrated in Fig. 2a,b similar reported of sawdust morphologies by other studies 36,37 . For SPB, it had a spherical shape with a coarse surface at 100 × magnification with 1 mm shown in Fig. 2c, and its surface was a rough surface when zoomed at 500 × magnification with 400 µm demonstrated in Fig. 2d. Finally, SPFB had a spherical shape with a coarse surface at 100 × magnification of 1 mm shown in Fig. 2e, and its surface was an irregular shape with the heterogenous surface when zoomed at 500 × magnification with 400 µm illustrated in Fig. 2f.
EDX analysis. The chemical compositions of sawdust powder (SP), sawdust powder doped iron (III) oxidehydroxide (SPF), sawdust beads (SPB), and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB) were analyzed by using EDX analysis represented in Table 3, and the elemental mapping of SP, SPF, SPB, and SPFB demonstrated in Fig. 3a-d which showed the dispersions of chemical elements of each material on the surface. Two main chemical components of carbon (C) and oxygen (O) were found in all materials while copper (Cu) was only found in powder materials of SP and SPF. For calcium (Ca), it was detected in SPB and SPFB. For sodium (Na) and chloride (Cl), they were observed in all materials except SP. In addition, iron (Fe) was found in the materials with the addition of iron (III) oxide-hydroxide which were SPF and SPFB to confirm the successful adding Fe into SP and SPB. For SP and SPF, the mass percentages by weight of C and O were decreased when iron (III) oxide-hydroxide was added to SP whereas Cu was increased. Moreover, Na, Cl, and Fe were detected in SPF which might be from using chemicals in a process of adding iron (III) oxide-hydroxide by ferric chlo- 6H 2 O) and sodium hydroxide (NaOH) for synthesizing SPF. For SP and SPB, the mass percentage by weight of C was deceased while O was increased after changing the material to a bead form. In addition, the mass percentages by weight of Ca, Na, and Cl were also detected in SPB by using sodium alginate (NaC 6 H 7 O 6 ) and calcium chloride (CaCl 2 ) in a bead formation. For SPF and SPFB, the mass percentages by weight of C, O, and Na were decreased. While the mass percentages by weight of Ca, Cl, and Fe were increased when SPF was changed to a bead form. The increases of Ca and Cl might be from using chemicals of CaCl 2 in a www.nature.com/scientificreports/ process of a bead formation similar to SPB. Therefore, the addition of iron (III) oxide-hydroxide and the changing material form affected the increases of Ca, Na, Cl, and Fe contents in sawdust materials. Lead removal efficiencies of all materials were increased with the increase of material dose which might be from the increase of active sites of materials 7 . Their highest lead removal efficiencies were 85.12%, 96.11%, 89.57%, and 100% at 2 g, 1 g, 1.5 g, and 0.5 g for SP, SPF, SPB, and SPFB, respectively. Therefore, they were optimum doses of sawdust materials that were used for studying the contact time effect. For RB4 dye removal, six different doses from 0.5 to 3 g were used for investigating the dose effect of RB4 dye adsorption by sawdust beads (SPB) and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB), and the results are demonstrated in Fig. 5b. The control condition was the RB4 dye concentration of 50 mg L −1 , a sample volume of 200 mL, a contact time of 12 h, pH 7, a temperature of 60 °C, and a shaking speed of 150 rpm. RB4 dye removal efficiencies of all materials were increased with the increase of material dose which might be from the increase of active sites of materials 12 . The highest RB4 dye removal efficiency of SPB was found at 3 g with 89.65% while the highest RB4 dye removal efficiency of SPFB was found at 1.5 g with 94.10%. Therefore, they were the optimum dosages of sawdust materials that were used for studying the contact time effect.
The effect of contact time. For lead removal, the different contact times from 1 to 6 h were used for studying the contact time effect on lead adsorptions by sawdust powder (SP), sawdust powder doped iron (III) oxidehydroxide (SPF), sawdust beads (SPB), and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB), and the results are demonstrated in Fig. 5c. The control condition was the lead concentration of 50 mg L −1 , a sample volume of 200 mL, pH 5, a temperature of 25 °C, a shaking speed of 200 rpm, and the optimum dose of 2 g (SP) or 1 g (SPF) or 1.5 g (SPB) or 0.5 g (SPFB). Lead removal efficiencies of all materials were increased with the increase of contact time similar to the dose effect. Their highest lead removal efficiencies were 86.74%, 97.58%, 90.12%, and 100% at 5 h, 3 h, 4 h, and 2 h for SP, SPF, SPB, and SPFB, respectively. Therefore, they were the optimum contact time of sawdust materials that were used for studying the pH effect. www.nature.com/scientificreports/ For RB4 dye removal, the different contact times from 3 to 18 h were used for studying the contact time effect on RB4 dye adsorptions by sawdust beads (SPB) and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB), and the results are demonstrated in Fig. 5d. The control condition was the RB4 dye concentration of 50 mg L −1 , a sample volume of 200 mL, pH 7, a temperature of 60 °C, a shaking speed of 150 rpm, and the optimum dose 3 g (SPB) or 1.5 g (SPFB). RB4 dye removal efficiencies of all materials were increased with the increase of contact time similar to the dose effect. Their highest RB4 dye removal efficiencies were found at 12 h www.nature.com/scientificreports/ with 88.15% for SPB and 9 h with 93.76% for SPFB. Therefore, they were the optimum contact times of sawdust materials that were used for studying the pH effect.
The effect of temperature effect. Only dye removal investigated the effect of temperature whether the changing temperature affects RB4 dye removal by sawdust beads (SPB) and sawdust powder doped iron (III) oxidehydroxide beads (SPFB). The different temperatures from 40 to 80 °C were used for studying the temperature effect on RB4 dye adsorptions by sawdust materials, and the results are demonstrated in Fig. 5f. The control condition was the RB4 dye concentration of 50 mg L −1 , a sample volume of 200 mL, pH 7, a shaking speed of 150 rpm, and the optimum dose of 3 g (SPB) or 1.5 g (SPFB) and contact time of 12 h (SPB) or 9 h (SPFB). RB4 dye removal efficiencies of all materials were decreased with the increase of temperature, and the highest RB4 dye removal efficiencies were at a temperature of 40 °C with 87.78% for SPB and 30 °C with 95.12% for SPFB. Therefore, they were the optimum temperatures of sawdust materials that were used for studying the pH effect. . Lead removal efficiencies of all materials were increased with the increase of pH values from 1 to 7, then they were decreased. Their highest lead removal efficiencies of all materials were found at pH 5 with lead removal at 86.21%, 98.15%, 91.45%, and 100% for SP, SPF, SPB, and SPFB, respectively which corresponded to other previous studies reported the highest lead removal efficiency at pH > 4 7,8,18 . Therefore, pH 5 was the optimum pH of sawdust materials that were used for studying the concentration effect. For RB4 dye removal, the effect of pH was studied by varying pH values of 1, 3, 5, 7, 9, and 11 represented the acid, neutral, and base conditions on RB4 dye adsorptions by sawdust beads (SPB) and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB), and the results are demonstrated in Fig. 5h. The control condition was the RB4 dye concentration of 50 mg L −1 , a sample volume of 200 mL, a shaking speed of 150 rpm, and the optimum dose 3 g (SPB) or 1.5 g (SPFB), contact time of 12 h (SPB) or 9 h (SPFB), and temperature of 40 °C (SPB) or 30 °C (SPFB). RB4 dye removal efficiencies of all materials were increased with the increase of pH values from 1 to 3, then they were decreased. Their highest RB4 dye removal efficiencies of all materials were found at pH 3 with RB4 dye removal at 89.12% and 95.96% for SPB and SPFB which corresponded to other previous studies reported the highest RB4 dye removal efficiency found at acidic conditions 6,14 . Therefore, pH 3 was the optimum pH of sawdust materials that were used for studying the concentration effect.     Fig. 5i. The control condition was a sample volume of 150 mL, and the optimum dose of 3 g (SPB) or 1.5 g (SPFB), contact time of 12 h (SPB) or 9 h (SPFB), temperature of 40 °C (SPB) or 30 °C (SPFB), and pH of 3. RB4 dye removal efficiencies of all materials were decreased with the increase of concentration because RB4 dye ions were more than the available active sites of sawdust materials similar to the report by other studies 12,14 . RB4 dye removal efficiencies from 30 to 70 mg L −1 of SPB and SPFB were 84.35-89.65% and 88.43-96.12%. For the RB4 dye concentration of 50 mg L −1 , RB4 dye removal efficiencies of SPB and SPFB were 87.96% and 92.84%, and SPFB demonstrated the highest RB4 dye removal efficiency of other materials.
In conclusion of lead removal, 2 g, 5 h, pH 5, 50 mg L −1 , 1 g, 3 h, pH 5, 50 mg L −1 , 1.5 g, 4 h, pH 5, 50 mg L −1 , and 0.5 g, 2 h, pH 5, 50 mg L −1 were the optimum conditions in dose, contact time, pH, and concentration of SP, SPF, SPB, and SPFB, respectively, and they could be arranged in order from high to low of SPFB > SPF > SPB > SP. As a result, both changing material form and adding iron (III) oxide-hydroxide helped to improve material efficiency for lead adsorption.
In conclusion of RB4 dye removal, 3 g, 12 h, 40 °C, pH 3, 50 mg L −1 and 1.5 g, 9 h, 30 °C, pH 3, 50 mg L −1 were the optimum conditions in dose, contact time, temperature, pH, and concentration of SPB and SPFB, respectively. As a result, the changing material form and adding iron (III) oxide-hydroxide helped to improve material efficiency for RB4 dye adsorption than only changing material form.
Therefore, sawdust materials could be removed both lead and RB4 dye in aqueous solutions, and SPFB demonstrated the highest removal efficiency on both pollutants. Finally, SPFB was recommended to be applied for lead or RB4 dye removal in the wastewater treatment system in the future.
Adsorption isotherms for lead and RB4 dye adsorptions. The adsorption patterns of sawdust powder (SP), sawdust powder doped iron (III) oxide-hydroxide (SPF), sawdust beads (SPB), and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB) for lead adsorption and sawdust beads (SPB) and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB) for RB4 dye adsorption were investigated through various adsorption isotherms of Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models in both linear and nonlinear models. For linear models, Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherms were plotted by C e /q e versus C e , log q e versus log C e , q e versus ln C e , and ln q e versus ε 2 , respectively. For nonlinear models, all isotherms were plotted by C e versus q e. The plotting graphs of lead and RB4 dye adsorption demonstrated in Figs. 6a-h and 7a-f, respectively, and their isotherm parameters were illustrated in Tables 4 and 5, respectively. Generally, the best-fit isotherm model for explaining the adsorption pattern of material is chosen from the high regression value (R 2 ) which is close to 1 12 .
For lead adsorption, the adsorption patterns of SP and SPB corresponded to Langmuir isotherm with relating to a physical adsorption because their R 2 values in both linear and nonlinear had higher than Freundlich, Temkin, and Dubinin-Radushkevich models. Therefore, Langmuir parameters of q m and K L values were used for explaining the adsorption pattern. Since q m and K L values of SPB were higher than SP, SPB had possibly higher lead removal efficiency with a high adsorption rate than SP correlated to the results of the batch experiment. For SPF and SPFB, their adsorption patterns corresponded to Freundlich isotherm with relating to a physiochemical adsorption because their R 2 values in both linear and nonlinear had higher than Langmuir, Temkin, and Dubinin-Radushkevich models. Therefore, Freundlich parameters of K F and 1/n values were used for explaining the adsorption pattern. K F refers Freundlich adsorption constant which SPFB represented the highest K F value, so SPFB had a higher adsorption rate than SPF. For a 1/n value, it is a constant depiction of the adsorption intensity which 0 < 1/n < 1 means the favorable adsorption isotherm, so both materials were favorable adsorption since their 1/n values in this range.
For RB4 dye adsorption, the adsorption pattern of SPB corresponded to Langmuir model relating to a physical adsorption whereas the adsorption pattern of SPFB corresponded to the Freundlich model correlated to a physicochemical adsorption from choosing the highest R 2 value isotherm or closely to 1. These results corresponded to lead adsorption patterns that Langmuir and Freundlich isotherms were best-fitted models for explaining lead adsorption patterns of SPB and SPFB. Therefore, both lead and RB4 dye adsorption patterns of SPB and SPFB were physical and physicochemical adsorption processes, respectively.
Moreover, the results of both linear and nonlinear Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models of all sawdust materials were agreed with each other, so the plotting of both linear and nonlinear isotherm models were also recommended for correct data translations [42][43][44] .
Finally, the comparison of the maximum adsorption capacity (q m ) value for lead and RB4 dye adsorptions by various adsorbents is illustrated in Table 6. For the comparison of lead removal, all sawdust materials in this study had higher q m values than q m values from Picea smithiana sawdust, sawdust-based cellulose nanocrystals, lemon peels, pomelo peels, and onion peels. In addition, SPFB had a higher q m value than all studies in Table 6   www.nature.com/scientificreports/ Therefore, all sawdust materials in this study were highly efficient materials for lead and RB4 dye adsorptions, and they are potential materials for application in industrial applications in the future, especially SPFB.
Adsorption kinetics for lead and RB4 dye adsorptions. The adsorption rates and mechanisms of lead adsorption by sawdust powder (SP), sawdust powder doped iron (III) oxide-hydroxide (SPF), sawdust beads (SPB), and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB) and RB4 dye adsorption by sawdust beads (SPB) and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB) were investigated through various the adsorption kinetics of pseudo-first-order kinetic model, pseudo-second-order kinetic model, Elovich model, and intraparticle diffusion in both linear and nonlinear models. For linear models, they were plotted by ln (q e − q t ) versus time (t), t/q t versus time (t), q t versus ln t, and q t versus time (t 0.5 ) for pseudo-first-order kinetic, pseudo-second-order kinetic, Elovich, and intraparticle diffusion models, respectively. For nonlinear models, they were plotted by q t versus time (t). The plotting graphs of lead and RB4 dye adsorptions illustrated in Figs. 8a-h and 9a-f, respectively, and their adsorption kinetic parameters were demonstrated in Tables 7 and  8, respectively. Generally, the best-fit isotherm model for explaining the adsorption rate and mechanism of material is chosen from the high regression value (R 2 ) which is close to 1 12 . For lead adsorption, the adsorption rate and mechanism of sawdust materials corresponded to a pseudosecond-order kinetic model with relating to the chemisorption process because their R 2 values in both linear and nonlinear pseudo-second-order kinetic models were higher than pseudo-first-order kinetic, Elovich, and intraparticle diffusion models. Therefore, the adsorption kinetic parameters of q e and k 2 were used for explaining the adsorption rate and mechanism. Their adsorption capacities (q e ) of a pseudo-second-order kinetic model were arranged in order from high to low of SPFB > SPF > SPB > SP correlated to the results of batch experiments and adsorption isotherm. For a k 2 value, it is the pseudo-second-order kinetic rate constant in which SPFB demonstrated the highest value than other adsorbents. As a result, SPFB had higher lead adsorption with a fast reaction than other materials.
For RB4 dye adsorption, the adsorption rate and mechanism of SPB and SPFB corresponded to a pseudosecond-order kinetic model similar to lead adsorption. In addition, q e and k 2 values of SPFB had also higher than SPB, so SPFB had higher RB4 dye adsorption than SPB corresponding to lead adsorption of SPB and SPFB. Therefore, both lead and RB4 dye adsorption rates and mechanisms of SPB and SPFB were explained by a physicochemical adsorption process.
Finally, the results of both linear and nonlinear pseudo-first-order, pseudo-second-order kinetic, Elovich, and intraparticle diffusion models of all sawdust materials were consistent with each other, so the plotting graphs of both linear and nonlinear kinetic models were also recommended for protecting mistake data translations 42-44 . Desorption experiments for lead and RB4 dye adsorptions. The desorption experiments were used to investigate the feasibility of the reuse of sawdust powder (SP), sawdust powder doped iron (III) oxide-hydroxide (SPF), sawdust beads (SPB), and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB) because this is a necessary point to estimate the cost and economic feasibility of industrial applications.
For Lead adsorption, SP, SPF, SPB, and SPFB for 5 cycles of adsorption-desorption were applied to confirm their abilities, and their results are illustrated in Fig. 10a. For SP, it could be reused in 5 cycles with high adsorption and desorption in ranges of 63.33-82.16% and 60.54-81.76%, respectively which adsorption and desorption were decreased by approximately 19% and 21%, respectively. For SPF, it also confirmed to be reusability in 5 cycles with high adsorption and desorption in ranges of 81.83-95.23% and 78.30-94.93%, respectively which adsorption and desorption were decreased by approximately 13% and 17%, respectively. For SPB, it could be reused in 5 cycles with high adsorption and desorption in ranges of 71.58-88.62% and 68.33-88.27%, respectively which adsorption and desorption were decreased by approximately 17% and 20%, respectively. For SPFB, it also confirmed to be reusability in 5 cycles with high adsorption and desorption in ranges of 89.45-100% and 86.33-99.75%, respectively which adsorption and desorption were decreased by approximately 11% and 13%, respectively. Therefore, sawdust materials are potential materials for lead adsorption with the reusability of more than 5 cycles by more than 63%, and they can be further applied to industrial applications.
For RB4 dye adsorption, SPB and SPFB for 5 cycles of adsorption-desorption were applied to confirm their abilities, and their results are illustrated in Fig. 10b. For SPB, it could be reused in 5 cycles with high adsorption and desorption in ranges of 72.45-87.84% and 69.36-88.09%, respectively which adsorption and desorption were decreased by approximately 15% and 19%, respectively. For SPFB, it also confirmed to be reusability in 5 cycles with high adsorption and desorption in ranges of 82.85-92.63% and 80.15-92.43%, respectively which adsorption and desorption were decreased by approximately 10% and 12%, respectively. Therefore, SPB and SPFB are potential materials for RB4 dye removal with the reusability of more than 5 cycles by more than 72%, and they can be applied to industrial applications in the future.

The possible mechanisms of lead and dye adsorptions by sawdust materials
The possible mechanisms of lead and RB4 dye adsorptions on sawdust materials are demonstrated in Fig. 11a,b. For lead adsorption, the cellulose, hemicellulose, pectin, lignin, a hydroxyl group (-OH), methyl groups (C-H), aromatic ring represented lignin (C=C), and alcohol and carboxylic acid of lignin and hemicellulose (C-H) were the main structure and chemical functions groups of sawdust materials. The carboxyl group (-COOH) was also demonstrated in sawdust beads (SPB and SPFB) by forming the complex compound between SP or SPF with sodium alginate. In addition, the complex compound between the surface of SPF or SPFB and iron (III) oxidehydroxide was formed to be SP•Fe(OH) 3 or SPB•Fe(OH) 3 by a process of electron sharing with hydroxyl groups of sawdust. Therefore, the possible mechanism of lead adsorption by sawdust materials might occur from donating a proton (H + ) from carboxyl groups (-COOH) or hydroxyl groups (-OH) or SP•Fe(OH) 3 Fig. 11a. For RB4 dye adsorption, the main structure and chemical groups of SPB and SPFB were the same as mentioned above, but the possible mechanism of RB4 dye adsorption used a different explanation based on Ngamsurach et al. 12 shown in Fig. 11b. Three possible mechanisms of electrostatic attractions, hydrogen bonding interactions, and n-π bonding interactions were used for explaining RB4 dye adsorptions by SPB and SPFB. For

Conclusion
Four sawdust materials of sawdust powder (SP), sawdust powder doped iron (III) oxide-hydroxide (SPF), sawdust beads (SPB), and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB) were successfully synthesized for lead or RB4 dye removals in an aqueous solution. SPFB demonstrated higher specific surface area (11.020 m 2 g −1 ) and smaller pore size (3.937 nm) than other materials, and the results illustrated that the addition of iron (III) oxide-hydroxide into sawdust materials increased the specific surface area and pore volume while the pore diameter size was decreased. The surface morphologies of SP and SPF were irregular shapes with heterogeneous fiber structures whereas SPB and SPFB had spherical shapes with coarse surfaces. Carbon (C) and oxygen (O) were found in all materials whereas iron (Fe) was only found in the materials with the addition of iron (III) oxide-hydroxide (SPF and SPFB).  Since SPFB demonstrated the highest lead or RB4 dye removal than other materials, adding iron (III) oxide-hydroxide and changing material form helped to improve material efficiencies for lead or RB4 dye adsorptions. For adsorption isotherms, SP and SPB corresponded to Langmuir model correlated to physical adsorption whereas SPF and SPFB corresponded to the Freundlich model relating to a physicochemical adsorption process. For the kinetic study, all materials corresponded to a pseudo-second-order kinetic model related to a chemisorption process with heterogenous adsorption. For desorption experiments, all materials could be reused more than 5 cycles with high lead removal of 63%, and SPB and SPFB also could be reused more than 5 cycles with high RB4 dye removal of 72%. Therefore, all sawdust materials were high potential materials for lead or dye adsorption in an aqueous solution, and SPFB demonstrated the highest lead and RB4 dye removals. Therefore, SPFB was suitable for wastewater treatment in industrial applications. For future works, the real wastewater with contaminated lead or RB4 dye should be investigated to confirm the ability of sawdust materials, and the continuous flow study also needs to study for further industrial applications.

Materials and methods
Raw material. Sawdust (Pterocarpus indicus) was obtained from a local sawmill in Khon Kaen province, Thailand.  Fig. 12a-c and the details were clearly explained below.
The synthesis of sawdust powder (SP). Firstly, sawdust was washed with tap water to remove contaminants, and then it was dried overnight in a hot air oven (Binder, FED 53, Germany) at 105 °C. Then, it was ground and sieved to a size of 125 µm. Finally, it was kept in a desiccator before use called sawdust powder (SP).
The synthesis of sawdust powder doped iron (III) oxide-hydroxide (SPF). Firstly, 5 g of SP were added to 500 mL of Erlenmeyer flask containing 160 mL of 5% FeCl 3 ·6H 2 O, and they were mixed by an orbital shaker (GFL, 3020, Germany) of 200 rpm for 3 h. Secondly, they were filtrated and air-dried at room temperature for 12 h. Then, they were added to 500 mL of Erlenmeyer flask containing 160 mL of 5% NaOH, and they were mixed by www.nature.com/scientificreports/  Figure 8.   where C 0 is the initial lead concentration (mg L −1 ), and C e is the final lead concentration (mg L −1 ).
Batch experiments for RB4 dye removal. A series of batch adsorption experiments were designed to investigate the effect of dose, contact time, temperature, pH, and concentration on RB4 dye removal efficiency by sawdust beads (SPB) and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB). The differences in dose from (1) Lead removal efficiency (%) = (C 0 − C e )/C 0 × 100 Table 7. The comparison of linear and nonlinear kinetic parameters for lead adsorptions on sawdust powder (SP), sawdust powder doped iron (III) oxide-hydroxide (SPF), sawdust beads (SPB), and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB). www.nature.com/scientificreports/ 0.5 to 3 g, contact time of 3, 6, 9, 12, 15, 18 h, temperature from 30 to 80 °C, pH values of 1, 3, 5, 7, 9, 11, and RB4 dye concentration from 30 to 70 mg L −1 with the control condition of initial RB4 dye concentration of 50 mg L −1 , a sample volume of 200 mL, a shaking speed of 150 rpm, and a contact time of 12 h were applied. The lowest value of each affecting factor with the highest RB4 dye removal efficiency was selected as the optimum value, and that value was applied to the next affecting factor study. Dye concentrations were analyzed by UV-Vis spectrophotometer (Hitachi, UH5300, Japan) at a maximum wavelength of 595 nm, and triplicate experiments were conducted to confirm the results. Dye removal in the percentage (%) to calculate the following Eq. (2).
where C 0 is the initial dye concentration (mg L −1 ), and C e is the final dye concentration (mg L −1 ).
Adsorption isotherms. The adsorption patterns of sawdust powder (SP), sawdust powder doped iron (III) oxide-hydroxide (SPF), sawdust beads (SPB), and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB) are investigated by adsorption isotherms for explaining that are the adsorption process of monolayer or multi-layer or heat or thermodynamic. Linear and nonlinear Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models are used to analyze followed Eqs. (3)-(10) 65-68 . Langmuir isotherm: Freundlich isotherm: (2) Dye removal efficiency (%) = (C 0 − C e )/C 0 × 100 (3) Linear: C e /q e = 1/q m K L + C e /q m (4) Nonlinear: q e = q m K L C e /1 + K L C e where C e is the equilibrium of lead or dye concentration (mg L −1 ), q e is the amount of adsorbed lead or dye on sawdust materials (mg g −1 ), q m is indicated as the maximum amount of lead or dye adsorption on adsorbent materials (mg g −1 ), K L is the adsorption constant (L mg −1 ). K F is the constant of adsorption capacity (mg g −1 ) (L mg −1 ) 1/n , and 1/n is the constant depicting the adsorption intensity. R is the universal gas constant (8.314 J mol −1 K −1 ), T is the absolute temperature (K), b T is the constant related to the heat of adsorption (J mol −1 ), (5) Linear: log q e = log K F + 1/n log C e (6) Nonlinear: q e = K F C 1/n e (7) Linear: q e = RT/b T ln A T + RT/b T ln C e www.nature.com/scientificreports/ and A T is the equilibrium binding constant corresponding to the maximum binding energy (L g −1 ). q m is the theoretical saturation adsorption capacity (mg g −1 ), K DR is the activity coefficient related to mean adsorption energy (mol 2 J −2 ), and ε is the Polanyi potential (J mol −1 ). Graphs of linear Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherms were plotted by C e /q e versus C e, log q e versus log C e , q e versus ln C e , and ln q e versus ε 2 , respectively whereas graphs of their nonlinear were plotted by q e versus C e . For adsorption isotherm experiments, 2 g of SP or 1 g of SPF, or 1.5 g of SPB or 0.5 g of SPFB was added to 200 mL Erlenmeyer flasks with variable lead concentrations from 30 to 70 mg L −1 with the control condition of sample volume of 200 mL, a shaking speed of 200 rpm, pH 6, a temperature of 25 °C, and a contact time of 5 h for SP, 3 h for SPF, 4 h for SPB, and 2 h for SPFB for studying lead adsorption. For studying RB4 dye adsorption, 3 g of SPB or 1.5 g of SPFB was added to 200 mL Erlenmeyer flasks with variable RB4 dye concentrations from 30 to 70 mg L −1 with the control condition of a sample volume of 200 mL, a shaking speed of 150 rpm, pH 7, a temperature of 40 °C for SPB and 30 °C SPFB, and a contact time of 12 h. Adsorption kinetics. The adsorption mechanisms of sawdust powder (SP), sawdust powder doped iron (III) oxide-hydroxide (SPF), sawdust beads (SPB), and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB) are determined by various adsorption kinetics which were linear and nonlinear pseudo-first-order kinetic, pseudo-second-order kinetic, Elovich, and intraparticle diffusion models calculated by Eqs. (11)-(17) [69][70][71][72] .
Pseudo-first-order kinetic model: (11) Linear: ln (q e − q t ) = ln q e − k 1 t Figure 11. Possible mechanisms of (a) sawdust powder (SP), sawdust powder doped iron (III) oxide-hydroxide (SPF), sawdust beads (SPB), and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB) for lead adsorption and (b) sawdust beads (SPB) and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB) for RB4 dye adsorption. where q e is the amount of adsorbed lead or dye on adsorbent materials (mg g −1 ), q t is the amount of adsorbed lead or dye at the time (t) (mg g −1 ), k 1 is a pseudo-first-order rate constant (min −1 ), and k 2 is a pseudo-secondorder rate constant (g mg −1 min −1 ) 73 . α is the initial adsorption rate (mg g −1 min −1 ) and β is the extent of surface coverage (g mg −1 ). k i is the intraparticle diffusion rate constant (mg g −1 min −0.5 ) and C i is the constant that gives an idea about the thickness of the boundary layer (mg g −1 ). Graphs of linear pseudo-first-order, pseudo-secondorder, Elovich, and intraparticle diffusion models were plotted by ln (q e − q t ) versus time (t), t/q t versus time (t), q t versus ln t, and q t versus time (t 0.5 ), respectively whereas their nonlinear graphs were plotted by the capacity of lead or dye adsorbed by sawdust materials at the time (q t ) versus time (t). For adsorption kinetic experiments, 10 g of SP or 5 g of SPF, or 7.5 g of SPB or 2.5 g of SPFB was added to 1000 mL of breaker with the control condition of the initial lead concentration of 50 mg L −1 , a sample volume of 1000 mL, a shaking speed of 200 rpm, pH 5, a temperature of 25 °C, and a contact time of 8 h for studying lead adsorption. For studying RB4 dye adsorption, 15 g of SPB or 7.5 g of SPFB was added to 1000 mL of breaker (12) Nonlinear: q t = q e (1 − e −k 1 t ) (13) Linear: t/q t = 1/k 2 q 2 e + t/q e (14) Nonlinear: q t = k 2 q 2 e t/ 1 + q e k 2 t (15) Linear: q t = 1/β ln αβ + 1/β ln t (16) Nonlinear: q t = β ln t + β ln α (17) Linear and nonlinear: q t = k i t 0.5 + C i Table 9. The chemical characteristic and structure of RB4 dye. Desorption experiments. The possible material reusability is an important factor for considering adsorbents of industrial applications, so the desorption experiments are designed to examine by studying five adsorption-desorption cycles to confirm the abilities of sawdust powder (SP), sawdust powder doped iron (III) oxidehydroxide (SPF), sawdust beads (SPB), and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB) for lead adsorption or sawdust beads (SPB) and sawdust powder doped iron (III) oxide-hydroxide beads (SPFB) for RB4 dye adsorption. For lead adsorption, the saturated sawdust materials were added to 500 mL of Erlenmeyer flask containing 200 mL of 0.5 M HNO 3 solution, then it was shaken by an incubator shaker (New Brunswick, Innova 42, USA) at 200 rpm for 6 h. Then, they were washed with deionization water and dried at room temperature, and sawdust materials are ready for the next adsorption cycle. For RB4 dye adsorption, the saturated sawdust materials were added to 500 mL of Erlenmeyer flask containing 200 mL of 0.01 M NaOH solution, then it was shaken by an incubator shaker at 150 rpm for 15 h with a temperature of 30 °C. Then, they were washed www.nature.com/scientificreports/ with deionization water and dried at room temperature, and sawdust materials are ready for the next adsorption cycle. The desorption efficiency in percentage is calculated following Eq. (18).

Properties and characteristics of dye
where q d is the amount of lead or dye desorbed (mg mL −1 ) and q a is the amount of lead or dye adsorbed (mg mL −1 ). www.nature.com/scientificreports/