Adsorption of Methylene Blue from Aqueous Solutions Using Parkia speciosa Pod-based Magnetic Biochar

This study was aimed to investigate the use of Parkia speciosa pod (petai pod) in the form of magnetic biochar as an efficient bioadsorbent to remove methylene blue (MB) in batch mode. The adsorption onto the magnetic biochar achieved about 99% of removal for all the initial concentrations studied (25 mg/L – 250 mg/L). The adsorption processes were studied using the adsorption isotherms, which were analysed using Langmuir and Freundlich model. The adsorption using magnetic biochar followed Freundlich model, indicating the heterogeneous surface of the magnetic biochar. Thus, the study showed that the Parkia speciosa pod (PSP) as agricultural waste has the potential to be used as a low-cost adsorbent.


Introduction
Adsorption is one of the efficient techniques in wastewater treatment. Contaminants, such as dyes, pesticides, metal ions and others in water can be removed using adsorption in a cost-effective manner. Textile dyes are the most polluting agent and are directly discharged into the water bodies by irresponsible industrialists [1]. Dye contaminations in the natural water bodies reduce the sunlight penetration into the water, impacting aquatic plants and animals. The water quality also decreases due to the synthetic dyes which contain harmful chemicals. Thus, this study focuses on the batch adsorption of methylene blue (MB) using Parkia speciosa pod-based (PSP; petai pod) magnetic biochar.
Recently, the utilisation of agricultural wastes reduces waste accumulation and follows the 'waste to energy' concept. One of the agricultural wastes is petai pods which are disposed pods after the seeds have been consumed [2]. To the best of our knowledge, there are few researches on the utilisation of the petai pods as adsorbents for dye removal [3]. Functional groups, like carboxyl, hydroxyl, sulfhydryl, and amino, and the components, such as hemicellulose, lignin, lipids, proteins, carbohydrates, water, hydrocarbon, and starch found in the agricultural wastes actually cause the binding of the contaminants' ions on the surface of the adsorbent. However, based on some studies, petai pods and seeds have been utilised as bioadsorbents [4,3]. Nevertheless, there are limited studies on adsorption onto magnetic biochar derived from petai pods.
In this work, petai pods were pretreated with FeCl3 before carbonisation in order to produce magnetic biochar which had high reductive reactivity, large surface area and large surface adsorption sites. Magnetic biochar contains a metallic iron core and iron oxides layers or shells. The iron 2 component of magnetic biochar helps improve the efficiency in adsorption. The iron oxides layer plays a positive role in enhancing the adsorption capacity of magnetic biochar [5]. The aim of the study is to investigate the potential use of magnetic biochar as adsorbents to remove MB dyes in aqueous solutions.

Preparation of Magnetic Biochar
The petai pods were washed with distilled water to remove the dirt from the surface. Then, the petai pods were sun dried to eliminate the moisture content. The sun-dried petai pods were cut into small pieces and oven dried at 100°C for 24 hours to completely dry them. Then, the samples were grinded into powder. The powder was sieved in order to obtain 75 -125 µm particles. Sieved petai pod powder was pretreated with 0.5 M FeCl3.6H2O. The ratio of powder and FeCl3.6H2O was 1:8; 10 g of powder was mixed with 80 mL of 0.5 M FeCl3.6H2O [6]. The mixture was stirred for 30 minutes and heated for 30 minutes at 70°C for aging process. Then, the liquid was filtered out and the resulting solid was dried in the oven for 17.45 hours at 70°C. Then, the dried samples were placed in crucibles and carbonized at 800°C for 2 hours at 10°C/min to produce magnetic biochar [7].

Preparation of Stock Solution
The stock solution of MB at the concentration of 1000 mg/L was prepared by dissolving 0.5 g of MB dye powder in 500 mL of distilled water. The concentration of the dye was determined at 660 nm using the Thermo Scientific GENESYS 20 Visible Spectrophotometer [8].

Batch Adsorption Studies
The effects of adsorbent dosage were studied by using three different adsorbent dosages: 0.5 g, 1.0 g and 2.0 g. The optimum adsorbent dosage was used to investigate the effect of initial dye concentrations. The effect of initial dye concentrations was studied at 25, 50, 100, 150, 200, and 250 mg/L in a batch system. Then, 50 mL of dye was placed in contact with the optimum dosage of magnetic biochar in a conical flask. The mixture was stirred using glass rod. Based on the preliminary study, a duration of 50 minutes was fixed as the optimum time required to reach the optimum removal.
The adsorbed amount of dye or adsorption capacity was calculated using Equation 1 [9]: Where Co represents the initial concentration of dye, Ct is the concentration of dye at end time (mg/L), m indicates adsorbent mass (g) and V is the solution volume (L).
The percentage of dye removal was calculated using Equation 2 [9]: Where Co represents the initial concentration of dye (mg/L) and Ct is the concentration of dye at end time (mg/L).

Adsorption Isotherm
Where qe is the amount of dye adsorbed at equilibrium (mg/g), Ce represents the equilibrium concentration of the adsorbate (mg/L), qmax defines the monolayer adsorption capacity of sorbent (mg/g), and KL is the Langmuir constant (L/mg).

Freundlich Isotherm.
In Freundlich isotherm, the adsorbent has a heterogeneous surface. The isotherm is applicable to multilayer sorption of the surface. In this case, infinite surface coverage is predicted, with no saturation [9]. The general equation involved is as follows: Where KF is a constant value related to adsorption capacity (mg/g) and 1/n is the empirical parameter related to the adsorption intensity which depends on heterogeneity.

Effect of Adsorbent Dosage
As mentioned in section 2.3, three different dosages were used. Figure 1 shows the removal percentage of MB at three different adsorbent dosages. The results indicated that the removal percentage of MB increased with the increasing adsorbent dosage due to the increased number of binding sites for dye molecules [10]. We chose 1.0 g of magnetic biochar as the optimum dosage since 1.0 g of magnetic biochar was able to achieve about 99% of removal.  Figure 2 shows the removal time profile of MB by the magnetic biochar. At low initial MB concentrations, such as 25 mg/L and 50 mg/L, the equilibrium was reached in the first five minutes. As the initial MB concentration increased, the time taken to reach the equilibrium also increased ( Figure 2). However, zerovalent iron (ZVI), Fe3O4 and Fe2O3 formation on the magnetic biochar influenced the time taken to reach the equilibrium at high initial MB concentrations. The transformation process of iron into ZVI during pyrolysis and the quality of ZVI formation are influenced by the nature of biomass. However, the transformation process of iron into ZVI is still being studied, requiring further researches [11]. The formation of Fe3O4 and Fe2O3 (iron oxide layers) are the results of the spontaneous chemical reactions with limited oxygen, and high temperature during Studies have shown that the formation of Fe3O4 can be the dominant form of iron in the biochar during carbonisation at 700°C while evidence of ZVI formation has been observed for biochar produced at 700°C -900°C [11]. Based on the previous studies, Fe3O4 has higher magnetic properties and adsorption capacity compared to Fe2O3 [12]. Thus, the biochar surrounded primarily by Fe3O4 adsorb dye molecules rapidly even at higher initial concentrations. The removal percentage of MB by the magnetic biochar as illustrated in Figure 3 remained at around 99% for all the initial concentrations, indicating high adsorption sites and adsorption capacity (Refer Figure 4) of the magnetic biochar even though the rate of reaction varied according to the magnetic biochars' composition. As illustrated in Figure 4, the adsorption capacity of the magnetic biochar increased with the increasing initial dye concentration. Similar trend was also reported in other studies [13]. The increase in the initial dye concentrations led to an increase in the amount of MB dyes adsorbed onto the magnetic biochar due to the inclining collision rate between the dye molecules and magnetic biochar [14].    Table 1, the experimental data fitted well to the Freundlich isotherm, indicating that the magnetic biochar had heterogeneous solid surfaces. Based on the literature, the Freundlich equation is used for data fitting related to the adsorption of cations on iron oxides [15]. The Freundlich constants, KF and n are defined as the adsorption capacity and adsorption intensity, respectively [16]. The adsorption capacity obtained from the Freundlich model was good compared to Langmuir's and the adsorption intensity of n < 1 meant that the adsorption was a chemical process [16].