The composite of magnetic activated carbon derived from peanut shells functionalized titanium dioxide (Fe3O4/TiO2/AC) has been successfully synthesized. The composite was employed to remove indigosol green and Cr(vi) under ultraviolet (UV) and visible light. In this work, the activated carbon was synthesized from a sustainable source of peanut shell by carbonization and activation method employing NaOH as the activating agent. Magnetite was prepared by chemical co-precipitation technique using FeCl3·6H2O and FeSO4·7H2O, and then, the deposition of TiO2 was performed under ultrasonic irradiation. A variety of material characterization, consisting of Fourier transform infrared, X-ray diffraction, and scanning electron microscopy-energy dispersive X-ray, was used to analyze the physicochemical properties of the composite. The effects of pH, irradiation time, and composite mass during optimization performance were investigated. The characterizations represent the dispersed TiO2 in the anatase phase with the existence of magnetic particles. The activity tests revealed the superiority of the composite for applications involving adsorption and photocatalysis under visible light source compared to UV light. It was found that Fe3O4/TiO2/AC yields the efficiency for the removal of indigosol green and Cr(vi) from Batik liquid waste of 92.91 and 76.92%, respectively.
Batik is a traditional art unique to Indonesian culture. The number of Batik industries that exist in Indonesia is estimated at 6,120 units with an aggregate production value of around 4.89 trillion rupiahs per year. The Batik industries typically produce liquid waste that contains organic and inorganic pollutants, and has a pungent smell and a thick color. The use of chemicals in the Batik coloring process results in the liquid waste produced having high chemical oxygen demand (COD) and Biological Oxygen Demand (BOD) values of 6972.01 mg/L and 2161.62 mg/L, which can damage the aquatic ecosystem. Relatively, Regional Regulation Number 7 of 2016 of the Special Region of Yogyakarta Province concerning the contamination threshold for Batik liquid waste states the acceptable BOD and COD levels are 85 mg/L and 250 mg/L, respectively. Additionally, Batik liquid waste commonly contains heavy metals such as chromium (Cr), lead (Pb), nickel (Ni), copper (Cu), and manganese (Mn), which can induce cancer in organisms [1,2,3,4,5].
Various methods have been developed to treat Batik liquid waste so that it may be safely released into the surrounding environment, one of which is adsorption using activated carbon which can be derived from natural materials [6,7,8]. Peanut shells have a fairly high cellulose content of 44.80% with active –OH groups that can bind harmful pollutants in Batik liquid waste . However, it should be noted that adsorbents can lack effectiveness and efficiency in terms of their recovery and time required for the adsorption process.
One of the means to increase the efficiency of activated carbon as an adsorbent is to modify it using magnetite (Fe3O4) . Such modification is designed to facilitate and speed up the separation of the adsorbent from the bulk to the help of the liquid using a magnetic field. Fe3O4 can have multiple function due to its high magnetic properties as well as good absorption capacity; thus, it is widely used as an adsorbent [11,12,13].
Likewise, the incorporation of photocatalysis alongside adsorption can increase the effectiveness of Batik liquid waste treatment of the contaminants contained within . Titanium dioxide (TiO2) is a semiconductor material having a relatively large band gap energy (3.2 eV), non-toxic, affordable, and abundantly available in nature. The band gap of TiO2, however, is too wide causing it to be only active in ultraviolet (UV) light and have limited application in the visible light region. As such, modification of TiO2 structure is thus needed to reduce the band gap energy to allow for effective adsorption of visible lights [15,16,17,18].
The present research focuses on the synthesis of activated carbon composite from peanut shell waste modified with Fe3O4 and TiO2 (Fe3O4/TiO2/AC). To the authors knowledge, there is yet any specific research that examines the synthesis and activity of Fe3O4/TiO2/AC as an adsorbent as well as a photocatalyst for degradation of indigosol green and Cr(vi) from Batik liquid waste. Activated carbon modified with Fe3O4 and TiO2 showed good activity in treating Batik liquid waste under visible light source. The characteristics of TiO2 on AC defined its suitability for photocatalytic removal of dyes and heavy metals via photocatalytic reaction. In addition, the existence of magnetic properties of Fe3O4 promoted to AC improves the recyclability for the application capabilities of this material in real life.
2 Materials and methods
The chemicals utilized for this research consisted of potassium hydroxide (KOH 99%), ethanol 96%, iron(iii) chloride hexahydrate (FeCl3·6H2O 99%), iron(ii) sulfate heptahydrate (FeSO4·7H2O 99%), titanium dioxide (TiO2 99%), sodium hydroxide (NaOH 98%), hydrochloric acid (HCl 37%), indigosol green IB, potassium dichromate (K2Cr2O7 99%), sodium nitrite (NaNO2 99%), nitric acid (HNO3 65%), liquid waste obtained from a Batik industry in Yogyakarta, peanut shell waste, and buffer solution pH 4, 7, and 10.
2.2 Preparation of Fe3O4/TiO2/AC
Dried peanut shells were crushed and sieved to 100 mesh. A total of 25 g of peanut shell powder was added to 150 mL of 0.1 N NaOH and then heated at 160°C for 36 h. The peanut shell powder was then rinsed with distilled water and dried at a temperature of 110°C for 24 h to obtain activated carbon powder.
A total of 5 g of activated carbon was added with 6.66 g FeCl3·6H2O and 3.66 g FeSO4·7H2O, dissolved in distilled water and heated at 85°C for 1 h, and then added with 20 M KOH until pH 10–11. The mixture was then stirred for 1 h and cooled to room temperature. Then, the precipitate was rinsed with distilled water and then dried at 75°C overnight in an oven to obtain Fe3O4/AC material .
Subsequently, a total of 2.5 g of TiO2 was homogenized in 20 mL of ethanol through ultrasonication for 10 min; 1.25 g of Fe3O4/AC previously synthesized was then added to the TiO2 solution and then stirred at 110°C for 1 h. The mixture was later calcined using a furnace at 400°C for 2 h. After, it was washed with distilled water and dried in an oven at 100°C for 24 h to obtain Fe3O4/TiO2/AC composite . Physicochemical characterizations to investigate the chemical changes that occurred during the synthesis of the composite material was carried out using a Fourier transform infrared spectrophotometer (FTIR, Nicolet Avatar 360 IR), an X-ray diffractometer (XRD, Bruker D2 Phaser), and a scanning electron microscope-energy dispersive X-Ray (SEM-EDX, Phenom Desktop ProXL) (Figure 1).
2.3 Optimization of Fe3O4/TiO2/AC
Determination of optimum conditions was carried out on 100 mL of 100 ppm indigosol green solution with pH 4, 5, 6, 7, and 8 dispersed with 100, 150, and 200 mg of Fe3O4/TiO2/AC composite with stirring at 150 rpm for 60 min and irradiation time of 1, 2, and 3 h under UV light. The composite in model solution was then centrifuged at 3,000 rpm for 10 min. In the determination of optimum pH, irradiation time, and composite mass, the indigosol green solution was analyzed using a UV-Visible spectrophotometer (Hitachi UH5300).
Determination of optimum conditions was carried out on 100 mL of 5 ppm Cr solution with pH 4, 5, and 6 dispersed with 100, 150, and 200 mg of Fe3O4/TiO2/AC composite with stirring at 150 rpm for 60 min and irradiation time of 1, 2, and 3 h under UV light. The composite in model solution was then centrifuged at 3,000 rpm for 10 min. In the determination of optimum pH, irradiation time, and composite mass, the Cr solution was analyzed using an atomic absorption spectrophotometer (AAS, Perkin Elmer PinAAcle 900 T).
2.4 Application of Fe3O4/TiO2/AC
A total of 100 mL of Batik liquid waste sample was set at the optimum pH, added with the optimum Fe3O4/TiO2/AC composite mass, and stirred at 150 rpm for 60 min under UV light and visible light at the optimum irradiation time. The composite material was separated with the help of a magnet from the outside of the container. Samples were centrifuged for 10 min at 3,000 rpm and analyzed using a UV-Vis spectrophotometer for indigosol green solution and an AAS for Cr solution, followed by analysis of COD levels based on SNI 6989.2:2019 and BOD levels based on SNI 6989.72:2009.
3 Results and discussion
3.1 Physicochemical characterization of Fe3O4/TiO2/AC
Based on the FTIR spectra in Figure 2, the activated carbon showed the formation of a wide peak at the wave number 3338.95 cm−1, denoting the stretching vibration of the –OH group from the peanut shell cellulose structure. Meanwhile, the peak that appeared at the wave number 1630.28 cm−1 indicated the presence of C═O of carboxylic acid contained in the peanut shell. The bands at 1419.90 cm−1, 1313.14 cm−1, and 1030.80 cm−1 were ascribed to –CH3, –CH2–, and C–O groups, respectively. This is in accordance with previous research stating that peanut shells contain –OH, C═O, C–H, and C–O functional groups . Significant changes in the intensity and shifts of the absorption wave numbers of the –OH and C═O groups in the Fe3O4/TiO2/AC spectra indicated successful modification by Fe3O4 and TiO2 on the activated carbon surface .
The XRD profile in Figure 3 showed a diffraction peak of amorphous activated carbon at 2θ = 21.89° (002) and 51.15° (100). Fe3O4/AC diffraction peaks at 2θ = 30.12° (220); 35.79° (311); 43.32° (400); 53.56° (422); 57.32° (511); and 62.78° (440) satisfy the diffractions of the cubic and orthorhombic phase of magnetic materials (JCPDS No. 19e0629). Fe3O4/TiO2/AC diffraction peaks at 2θ = 25.33° (101); 27.47° (111); 37.82° (004); 48.05° (200); 53.90° (105); 55.06° (211); 62.86° (204); 68.77° (116); 70.29° (220); 75.27° (215); and 82.68° (312) signified TiO2 in its anatase phase, while the diffraction peaks at 2θ = 35.79° (311); 53.56° (422), and 62.78° (440) indicated magnetic particles . Based on the Debye Scherrer equation with the formula D = Kλ/β cos θ (K = 0.9 and λ = 0.154056 nm), the average size of the Fe3O4/TiO2/AC composite crystal was 70.22 nm.
The morphology of the material presented in Figure 4 showed open pores of the activated carbon. The pores act as the entry point for the adsorbate. Moreover, the surface structure of Fe3O4/AC showed white aggregates of Fe3O4, which were unevenly distributed on the surface . Meanwhile, the morphology of the Fe3O4/TiO2/AC composite showed a coating of TiO2 particles which resulted in a smoother surface structure. The presence of Ti in the Fe3O4/TiO2/AC composite was confirmed using EDX analysis, which showed an elemental peak of Ti (0.35 wt%).
Table 1 lists the content of C, O, Fe, and Ti over the materials. The elemental chemical analysis evidenced that AC have been modified by Fe3O4 and TiO2. However, it can be noticed that the elemental content of Ti was relatively low. These results are also in agreement with elemental mapping images shown in Figure 5. Furthermore, these images confirm that TiO2 was unevenly dispersed on the surface of Fe3O4/TiO2/AC composite.
|Sample||Element content (wt%)|
3.2 Performance test of Fe3O4/TiO2/AC
Determination of optimum pH, irradiation time, and composite mass was aimed to specify the best conditions to yield the best removal efficiency in adsorption and photocatalysis of indigosol green and Cr(vi) using Fe3O4/TiO2/AC composites. Figure 6 shows that the optimum pH for the adsorption and photocatalysis of indigosol green was obtained at pH 4 with removal efficiency of 93.94%. The lower the pH, the greater the number of H+ ions in the OH group that allow for the formation of ˙H and ˙OH radicals so as to neutralize the negatively charged composite surface [21,22]. Meanwhile, the optimum pH for the adsorption and photocatalysis of Cr(vi) was obtained at pH 5 with removal efficiency of 27.75%. In the Cr solution, pH variations of 7 and 8 such as that in indigosol green solution were not applied because TiO2 is able to reduce Cr(vi) in acidic conditions between pH 1–5 .
The duration of irradiation affected the ability of Fe3O4/TiO2/AC composite in degrading indogosol green and Cr. It was shown that photocatalytic activity increased until an optimum condition was reached where equilibrium had occurred . Figure 7 shows the reduction of indigosol green levels in relation to irradiation time, in which effective treatment was established with 2 h of irradiation with removal efficiency of 95.04%. Alternatively, irradiation time for 1 h can optimally remove Cr(vi) by 88.61%.
Figure 8 demonstrates that the use of 100 mg Fe3O4/TiO2/AC composite mass resulted in optimum performance for the removal of indigosol green and Cr(vi) with effectiveness of 97.07 and 76.85%, respectively. The more composites added, the less effective the results obtained. It is likely that the bulk mass of composite used hindered optimal irradiation of the composite particles and resulted in more composite being suspended . Accordingly, Fe3O4/TiO2/AC composite showed an outstanding removal of indigosol green at pH 4, a composite mass of 100 mg, and an irradiation time of 2 h. In addition, the optimum removal of Cr(vi) was established at pH 5 with a composite mass of 100 mg and an irradiation time of 1 h.
Fe3O4/TiO2/AC composite performance on Batik liquid waste treatment was tested at the optimum conditions as previously specified. Figure 9 presents that Batik liquid waste treatment using Fe3O4/TiO2/AC composite can remove indigosol green and Cr(vi) levels with effectiveness of 92.91 and 76.92%, respectively, under visible light. The activated carbon coated with Fe3O4 was magnetic, making the surface layer denser and stronger as well as easy to separate. The optimum photocatalysis was established with irradiation in the visible light region attributable to the overall neutral surface charge of the peanut shell-based adsorbent, whereby negative and positive charges exist in tandem. Correspondingly, aggregation of immobilized TiO2 can be prevented by the neutral nature of the porous material, thus maintaining better catalytic activity under visible light than UV light .
Some synergistic research work for comparison with this study is summarized in Table 2. The various treatment methods, such as adsorption, photocatalysis, and the combination of adsorption and photocatalysis can remove dyes and heavy metals in high percentages. In addition, it shows the success of Fe3O4/TiO2/AC for the removal indigosol green and Cr(vi) in Batik liquid waste.
|Material||Method||% Removal efficiency||Reference|
|Clam shell catalyst||Adsorption-photocatalysis||Methylene blue (99.60%)|||
|Congo red (83.30%)|
|PEG-MoS2||Photocatalysis||Rhodamine B (97.30%)|||
|Methylene blue (98.05%)|
|Calcined CoFe-LDH/g-C3N4||Adsorption-photocatalysis||Cr(vi) (100%)|||
|Ag–Co NPs||Photocatalysis||Congo red (88.09%)|||
|Ti/Fe/AC||Photocatalysis||Reactive black (95%)|||
|Dried water hyacinth roots||Adsorption||Cr(vi) (95.43%)|||
|Magnetite NPs modified by sodium alginate||Adsorption||Acid red 18 (97%)|||
|Fe3O4/TiO2/AC||Adsorption-photocatalysis||Indigosol green (92.91%)||This study|
Fe3O4/TiO2/AC composite was able to reduce the levels of positively charged indigosol green cationic dye through adsorption process on the negatively charged surface (attributable to –OH groups from cellulose) of activated carbon through electrostatic (ionic) interactions. Meanwhile, the decrease in Cr(vi) was due to the replacement of metal cations in exchange for H+ ions from the hydroxyl and carboxylic functional groups [34,35]. In addition, the photocatalytic activity of TiO2 caused the excitation of electrons from the valence band to the conduction band which caused a vacancy (hole), which enables reaction with H2O in solution to form ˙OH which can degrade indigosol green compounds into CO2 and H2O as well as reduce Cr(vi) to Cr(iii) [36,37]. The reactions involved during the photocatalytic process are presented in equations (1–3) for indigosol green and equations (4–5) for Cr(vi) (Figure 10).
The liquid waste quality standard test showed that the COD and BOD values in the treated Batik liquid waste were 291.5 and 45.77 mg/L, respectively. Based on the threshold detailed in the Regional Regulation No. 7 of 2016 of the Special Region of Yogyakarta Province, the BOD value has met the quality standard, but the COD value still exceeded the specified quality standard. This is likely because the dye had not been completely degraded or that desorption occurred so as to release new compounds back into the solution.
Fe3O4/TiO2/AC composite material prepared from peanut shells containing –OH, C═O, C–H, and C–O groups had the potential to be used as both adsorbent and photocatalyst. XRD results showed the formation of TiO2 anatase phase in the composite with magnetic properties. Fe3O4/TiO2/AC composite showed an outstanding removal of indigosol green at pH 4, a composite mass of 100 mg, and an irradiation time of 2 h. In addition, the optimum removal of Cr(vi) was established at pH 5 with a composite mass of 100 mg and an irradiation time of 1 h. Batik liquid waste treatment at optimum conditions using Fe3O4/TiO2/AC composite was able to remove indigosol green and Cr(vi) under visible light with removal efficiency of 92.91 and 76.92%, respectively. The results showed that Batik liquid waste treatment using Fe3O4/TiO2/AC composite is a highly prospective method to be developed due to its excellent performance and environmentally friendly in that peanut shell waste can be utilized. The adsorbent is also highly recyclable and can be used to photodegrade contaminants under visible light.
The authors are grateful to the Ministry of Education, Culture, Research, and Technology, Indonesia for funding this work through the Students Creativity Program in research field.
Author contributions: M.U. – conceptualization, methodology, writing and data curation; H.A.Z. – investigation and formal analysis; K. – investigation and formal analysis; T.A.D. – investigation and formal analysis.
Conflict of interest: The authors declare no conflicts of interest.
Ethical approval: The conducted research is not related to either human or animal use.
Data availability statement: The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.
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