Biochar-ZnO Composites from Petung Bamboo Stems to Increase Adsorption and Photocatalytic Degradation of Methylene Blue in Wastewater

The rapid development of industrial sector worldwide has contributed greatly to environmental pollution. In many countries, surface and groundwater are polluted by industrial wastewater effluent, such as synthetic dyes. Dyes are difficult to decompose naturally and cause disruption to aquatic ecosystems. To eliminate risks associated with the presence of synthetic dyes, photocatalytic-adsorption is still considered the most environmentally friendly and effective for dye removal. In this work, a novel biochar-ZnO composite was synthesized from petung bamboo stems having a high cellulose content, and used to eliminate methylene blue dye in the water. Biochar-ZnO composite was prepared from bamboo stems via a two-step pyrolysis process in a furnace with nitrogen flow at 400°C and 700°C for one hour. The addition of ZnO into biochar enhanced methylene blue removal up to 94.6% at an initial concentration of 10 mg.dm-3, pH 3, and contact time of 180 minutes, under UV light. It can be concluded that biochar-ZnO composite is highly feasible for dye removal from wastewater.


Introduction
The rapid development of the industrial sector worldwide has contributed greatly to environmental pollution.In many countries, surface and groundwater are polluted by industrial wastewater effluent.For example, the textile industry's effluent contains 10-50 mg/L of dyes such as methylene blue [1].Besides causing eutrophication, methylene blue has severe effects on human health, including gastrointestinal disturbances, methemoglobinemia, vomiting, necrosis, and nausea [1, 2], which renders the necessity of removing it from wastewater unquestioned.A number of techniques, such as adsorption, photocatalytic membrane, advanced oxidation processes, and microbial degradation, have been 2 employed to lessen the intensity of dyestuffs in water [3-6].Among them, adsorption is considered the most effective technology for removing dyes in wastewater due to its simple design, low cost, environmentally friendly, and high efficiency [3].Recently, the removal of dyestuff from aqueous solution using photocatalytic technology has also gained more interest [7,8].However, each of these processes may still be constrained by issues including high regeneration expenses, insufficient photocatalytic performance, and potential for secondary contaminants.In order to address the shortcomings of the single technology and increase the efficiency of the process, it has been suggested that adsorption and photocatalytic technologies be combined.In this method, adsorption increases the contact between pollutant molecules and the photocatalyst so that the photocatalytic process is more effective.In addition, the separation and transfer of photoexcited charge carriers can be accelerated by adsorbents having strong electron transfer characteristics.Thus, the adsorbed dyes can be effectively degraded by photocatalysis, thereby increasing the removal of the dyes.
Recently, metal oxide-biochar composites have been widely used in photocatalytic-adsorption technologies.To make metal oxide-biochar composites, the raw material often needs to be treated with metal salt solutions such as ZnCl2 and FeCl3 before the pyrolysis process [6, 9].In addition, pure biochar can load metal oxides onto its surface through precipitation or reduction [10,11].Loading of metal components before or after pyrolysis is the main difference between these two production techniques.However, metal salt doses that are not optimal can cause aggregation or uneven distribution of metal oxides on the biochar surface, so it needs to be investigated further.This study aims to develop biochar-ZnO composites from bamboo stems for the adsorption of methylene blue dye in an aqueous solution.

Materials and Chemicals
Petung bamboo stem sawdust was collected from Magelang.Bamboo sawdust was dried in the sun for one day.The dried material was further sieved using an 18-mesh sieve and dried again using a Memmert oven type UN55 (Germany) at 110 o C until the weight was constant.Methylene blue (MB) dye, HCl, NaOH, and ZnCl2 were bought from Merck (Germany).

Biochar-ZnO Composite (BZC) Synthesis
In this work, BZC preparation was divided into two steps.In the first procedure, a colloidal solution was made by mixing 30 grams of ZnCl2 with 250 mL of aquadest in a 500 ml beaker glass.Then, 6 grams of bamboo sawdust was added to the colloid solution and stirred for 24 h.The sample was filtered to separate the solid from the solution, then the solid was dried at 110°C for 5 h.The dried sample was pyrolyzed at 400°C under a nitrogen flow rate of 0,1 L/min for an hour.The resulting material was washed several times with 1 M HCl and aquadest until the pH was 7. The sample was filtered and dried at 110°C until the weight was constant.For the second process, the obtained biochar was impregnated with ZnCl2 solution for 24 hours using a biochar to ZnCl2 ratio of 1/5 w/w.After the filtering process, the solid was dried at 110°C and heated at 700°C under a nitrogen flow rate of 0,1 L/min for an hour to acquire the composite.The resulting material was washed with 1 M HCl and aquadest several times until the solution pH was 7. The sample was filtered and dried at 110°C until the weight was constant.

Photodegradation Test
The photodegradation test was conducted using the batch photoreactor equipped with three UV lamps (Phillps).Methylene blue (MB) and the composite were combined in the trials at a specific dosage (0.1 g).The initial concentration of MB solution (10, 20, 30, 40, 50, 100, 200, 300, 400, and 500 mg/L), contact duration (5, 10, 20, 30, 40, 50, 60, 120, 180, and 240 minutes), and pH (3, 5, 7, 9, 11, 13) were the three variation conditions used in the photodegradation test.Erlenmeyer filled 50 mL of methylene blue solution and 0.1 g adsorbent was placed in a water bath and shaken mechanically at 115 rpm.The adsorption was carried out at a certain pH with the addition of 0.1 N HCl solution or 0.1 N NaOH solution.After the equilibrium was attained, the suspended material was filtered, and the filtrate was examined using a Genesys 10 UV-visible spectrophotometer (Thermo Scientific, USA) at a wavelength (max) of 664 nm to determine the concentration of methylene blue.

FIGURE 1. X-ray Diffraction of biochar-ZnO composites
Functional group analysis using Fourier Transform Infrared (FTIR) was performed to determine the availability of functional groups on the biochar-ZnO surface.The result of the FTIR analysis is shown in Figure 2.An absorption peak at wave number 3396.79 cm -1 indicates the presence of vibrations of the O-H stretching groups of the carboxylic acid (RCOOH) and alcohol (ROH) groups [17].The wave numbers 1121.65 cm -1 and 909.48 cm -1 indicate the presence of stretching C-O vibrations [17].The absorption peak at wave numbers 434 cm -1 to 583.49 cm -1 indicates the existence of ZnO in biochar [18].After the adsorption of methylene blue, the O-H absorption peak shifted from 3396.79 cm -1 to 3409.33 cm -1 .It indicates a change in the absorption peak which may be caused by methylene blue

Influence of Solution pH on Methylene Blue Adsorbed
The solution pH greatly affects the effectiveness of the adsorption-photodegradation process.In this work, the influence of solution pH was examined in the range 3-13.The results showed that the effective pH for the biochar−methylene blue system and biochar-ZnO− methylene blue was 13 and 3 respectively.In the absence of ZnO, the removal of methylene blue was only through the adsorption process.At high pH (i.e., 13), the biochar surface tends to be negatively charged resulting in electrostatic interactions with methylene blue which is a cationic dye.

Influence of Concentration toward Methylene Blue Adsorbed
The influence of the initial concentration on the amount of adsorbed methylene blue is shown in Figure 5.As shown in Figure 5, the higher the concentration of the solution, the more methylene blue is adsorbed.In addition, the interaction between the adsorbent and the dye molecules also increases until equilibrium is reached.It is because the higher the concentration of the adsorbate, the greater the driving force, so the movement of the dye molecules causes the rate of mass transfer to be faster.

Influence of Contact Time toward Methylene Blue Adsorbed
The influence of contact time on the adsorption capacity of methylene blue dye is shown in Figure 6.As shown in Figure 6, the removal of methylene blue takes place quickly but gradually slows down until it reaches equilibrium.This is because there are still many empty active sites in the initial empty stage, so the adsorbed methylene blue is quite high [33], and the remaining empty active sites are difficult to occupy because of the repulsive forces between solute molecules in the solid phase and the bulk phase.
The longer the contact time, the active site reaches a saturation point so that it is no longer able to adsorb the adsorbate, or an equilibrium occurs where the amount of adsorbed is equal to that which is desorbed.

Conclusion
The findings of this work demonstrate that the biochar-ZnO composite was created and used to successfully remove the dye methylene blue (MB).The decolorization efficiency reaches more than 95% at pH 3, an initial concentration of 400 mg/l, for 180 minutes.Li R, Wang J J, Gasto L A, Zhou B, Li M, Xiao R, Wang Q, Zhang Z, Huang H, Liang W, Huang H, Zhang X 2018 An overview of carbothermal synthesis of metalebiochar composites for the removal of oxyanion contaminants from aqueous solution Carbon 129 674-687 [10] Shi J, Wang J, Liang L, Xu Z, Chen Y, Chen S, Xu M, Wang X, Wang L 2021 Carbothermal synthesis of biochar-supported metallic silver for enhanced photocatalytic removal of methylene blue and antimicrobial efficacy J.

FIGURE 3 .
FIGURE 2. FTIR Spectra of biochar-ZnO composites before and after the adsorption process

FIGURE 6 .
FIGURE 6. Influence of Contact Time toward Methylene Blue Adsorbed (pH 3 for Biochar-ZnO, pH 13 for Biochar, Initial Concentration 10 mg/L, Dose of Adsorbent 0,1 gram) Hazard.Mater.401 123382 [11] Goncalves M G, Veiga P A S, Fornari M R, Peralta-Zamora P, Mangrich A S, Silvestri S 2020 Relationship of the physicochemical properties of novel ZnO/biochar composites to their efficiencies in the degradation of sulfamethoxazole and methyl orange Sci.Total Environ.748 141381 [12] Abdessemed A, Rasalingam S, Abdessemed S, Djebbar K E Z, and Koodali R 2019 Impregnation of ZnO onto a vegetal activated carbon from algerian olive waste: A sustainable photocatalyst for degradation of ethyl violet dye Int.J. hotoenergy 2019 4714107 [13] Kamal A, Saleem M H, Alshaya H, Okla M K, Chaudhary H J, and Munis M H F 2022 Ballmilled synthesis of maize biochar-ZnO composite (MB-ZnO) and estimation of its photocatalytic ability against different organic and inorganic pollutants J. Saudi Chem.Soc.26(3) 101445 [14] Sangon S, Hunt A J, Ngernyen Y, Youngme S, and Supanchaiyamat N 2021 Rice straw-derived Conversely, at low pH, the surface of the biochar is protonated into O-H 2+ or O-OH + resulting in repulsion with methylene blue dye.It is what causes the effective pH to be at 13. Different things happen when there is ZnO in biochar and ultraviolet (UV).e − in ZnO can recombine with h + where this migration of charge carriers confers redox capability.The strong reducing ability of e − in ZnO can degrade methylene blue, and certain e − in the conduction band can interact with O2 and H + , giving H2O2.The accumulation of h + from ZnO oxidizes OH − and H2O to •OH, which plays an important role in the photodegradation process of methylene blue.Radical species (i.e., •O 2-and •OH) resulting from the oxidation-reduction process then interact with methylene blue molecules to produce simple single-ring molecule chains that are environmentally friendly, such as H2O, Cl − , CO2, SO4 2− , and NO3 − .It is what causes the adsorption-photocatalytic process to be more optimal in acidic conditions.The photocatalytic reaction mechanism is described as follows: