The manufacture of textiles, leather, paper, pharmaceuticals, food, synthetic rubber, plastics, and paints produces a considerable amount of wastewater. Typically, this water contains dye effluents and is directly discharged into the environment without treatment. Dyes can be of many different structural varieties, generally divided into cationic, nonionic, or anionic types [1
]. Safranin O (SO) is a red cationic dye and belongs to the azine group, which is extensively used in histology, cytology, bacteriology, and so forth. Safranin O has a complex organic structure and is frequently found in trace amounts in the industrial discharge waters. The release of synthetic dyes such as SO into the water environment not only obstructs light penetration into the surface waterbodies, thus reducing photosynthetic activity and endangering aquatic life, but is also toxic to aquatic microorganisms and seriously affects human health if not treated properly [2
Conventional wastewater treatment removes coloration through membrane filtration, coagulation-flocculation, advanced oxidation, electrochemical methods, or microbial degradation methods. However, such approaches often have low efficiency, are high-cost, and not environmentally safe [3
]. Adsorption has been considered as one of the most effective removal methods for effluents containing synthetic dyes, thanks to its flexibility and simplicity in design [3
]. In recent years, many more studies have focused on adsorption treatment methods by using low-cost adsorbents, such as biochar, instead of activated carbon. Compared to activated carbon, biochar is less energy- and cost-intensive due to lower temperatures used in its production [4
Biochar is a porous carbon-rich solid product resulting from the thermal degradation (pyrolysis) of lignocellulosic-derived biomass under an almost oxygen-free environment. Biomass waste materials appropriate for biochar production include grass, wood chips, wheat straw, seed husk, and bagasse.
Vietnam, with a large agricultural sector, has significant biomass residue potential for biochar production. Mekong River delta is the key rice-producing region in Vietnam. With an estimated rice production of approximately 43 million tons/year (GSO, 2021), rice straw residues offer a promising source for biochar production.
Biochar has been evaluated as a potential adsorbent for water treatment due to the multi-functional properties of the material. Its high porous structure and stability in water are considered favorable for its adsorption capacity. Additionally, the presence of several functional groups on its surface mainly include carboxyl (–COOH) and hydroxyl (–OH), which can form complexes with many classes of dyes [5
]. Several applied studies have investigated the biochar adsorption of dyes in solution [6
]. However, as with other powdered adsorbents, the powdered biochar after being used would cause secondary pollution in water bodies. This often requires filtration and centrifugation processes in further treatment steps. Difficulty in separating powdered biochar hindered its application on a large scale.
The synthesis of biochar-based magnetic composites is of increasing interest for its properties of being quickly separated from water by using an external magnet. Pyrolysis, coprecipitation, and calcination are the methods frequently used in the preparation of magnetic biochar [8
]. Among them, the co-precipitation method is mostly used to synthesize magnetic biochar, as it requires an easier process. The coprecipitation procedure describing the preparation of magnetic biochar from Fe2+
salt solution is summarised by the following equation:
Fe2+ + 2Fe3+ + 8OH− → Fe3O4↓ + 4H2O.
For the magnetization of biochar, different magnetic medium can be used. The majority of previous studies have selected iron-based materials such as FeCl3
, or Fe3
as the magnetic medium. The evaluation of the characteristics of the magnetic biochar can be determined by X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy [9
]. Magnetic biochar overcomes problems related to filtration of non-magnetic biochar, as they can be removed from the water after its adsorption by using powdered external magnetic fields. Magnetic biochar applications have already been practiced. For example, the ultrafine magnetic biochar/Fe3
adsorbed 62.7 mg/g of carbamazepine [10
]. Another magnetic biochar, produced by the precipitation of strontium hexaferrite (SrFe12
) onto sewage biochar surfaces, exhibited high removal malachite green capacity (388.65 mg/g) [11
]. Magnetic biochar from alkali-activated rice straw adsorbed 53.66 mg/g of rhodamine B [12
Rice straw conversion into biochar and magnetic biochar is a growing field of interest due to a variety of potential applications, including dye adsorption from aqueous solution. Several rice straw precursors have been used to synthesize magnetic biochar-based composites for removal of rhodamine B [12
] or crystal violet [13
]. It has been reported that with magnetic coating, the adsorption capacity of magnetic biochar to organic pollutants from aqueous solution is increased as compared to their precursor biochar. For example, corncob magnetic biochar produced by using low-temperature hydrothermal methods showed a higher adsorption capacity for Methylene blue to magnetic biochar (163.93 mg/g) than the pristine biochar (103.09 mg/g) [14
]. The co-precipitation of Fe2+
on orange peel powder has been shown to have adsorption capacities for hydrophobic organic compounds and phosphate than the non-magnetic biochar [15
]. Furthermore, corn stalk biochar coated with magnetic Fe3
nanoparticles by co-precipitation achieved adsorption capacity for crystal violet close to 20 times greater than that of precursor biochar [16
]. However, no study has investigated the removal of SO dye using magnetic biochar from rice straw or a comparative evaluation adsorption capacity of SO between magnetic biochar and precursor biochar. Therefore, in this study, an exploration of the physicochemical characteristics in addition to the comparative evaluation of both biochar and magnetic biochar for Safranin O adsorption from solution was examined. Batch experiments were performed using different variables, including pH solution, adsorbent dosage, initial SO concentration, and contact time, whilst the isotherms and kinetics were studied to explore the adsorption mechanism of SO adsorption.
2. Materials and Methods
All chemicals including ferric chloride (FeCl3
O), ferrous chloride (FeCl2
O), hydrochloric acid (HCl), and sodium hydroxide (NaOH) were provided by Merck (Darmstadt, Germany). Safranin O (SO) was supplied by Sigma-Aldrich and was used without any pretreatment. The stock solution of SO was prepared (1000 mg/L), and the initial concentration was adjusted to the desired concentration by diluting with deionized distilled water. The chemical structure of SO is demonstrated in Figure 1
2.2. Preparation of Biochar and Magnetic Biochar from Rice Straw
Biochar preparation: The raw rice straw collected from the Vietnamese Mekong Delta was first air-dried and cut into small pieces of ca. 2–4 mm size, then formed into cylindrical granules. Pyrolysis occurred in a furnace (Model VMF 165, Yamada Denki, Adachi, Tokyo, Japan) at 500 °C, with a heating rate of 10 °C·min−1 for 2 h in the absence of oxygen. After cooling, the biochar was crushed and sieved (grain size <0.075 mm). The sieved biochar was then washed with a solution of 0.1 M HCl and distilled water until the pH was between 6.0 and 7.0. The biochar was finally dried overnight at 80 °C and stored until use or further magnification. The samples were identified as rice straw biochar (RSB).
Magnetic biochar preparation: Magnetic rice straw biochar was synthesized using the high-temperature co-precipitation method. Here, biochar was mixed with a suspension of various Fe-chlorides under very alkaline conditions (pH = 10) in order to precipitate Fe-hydroxides, according to protocol summarized by Sun et al. (2015). Thereafter, it was precipitated using FeCl3·6H2O and FeCl2·4H2O with a ratio of 3:1, where the pH was adjusted to 10 using a 5 mol/L NaOH solution. The mixture was stirred on a magnetic stirrer with RSB at 80 °C and for 1 h, then was separated by centrifugation at 3000 rpm for 10 min. The collected precipitate was finally oven-dried at 60 °C to constant weight. The samples were identified as magnetic rice straw biochar (MRSB).
2.3. Characterization of Adsorbents
A scanning electron microscopy SEM (Hitachi S-4800, Hitachi Ltd., Tokyo, Japan) was used to visualize the microstructure of biochar-based adsorbents, coupled with an energy-dispersive X-ray (EDX) spectroscopy (Hitachi, Japan) to analyze the elemental surface composition of the biochar-based adsorbent samples. Fourier transform infrared spectroscopy FTIR (FTIR-PerkinElmer Spectrum 10.5.2, Buckinghamshire, UK) was used to determine the functional groups on the surface samples. X-ray diffraction (XRD) for crystal phase recognization was performed using Bruker D8 Advance (Bruker AXS GmbH, Karlsruhe, Germany). The BET-specific surface area was calculated on the basis of low-temperature nitrogen adsorption isotherm measured using Nova Station A (Quantachrome Instruments version 11.0, Miami, FL, USA). The detailed sample preparation for SEM, EDX, FTIR, and BET analysis is provided in the Supplemental Materials
2.4. Batch Adsorption Experiments
The adsorption performance of RSB and MRSB against SO in solution was evaluated in the batch adsorption experiments. All the adsorption tests were conducted in triplicate and the average results were taken. In a typical experiment, the fixed quantity of adsorbents and 10 mL SO solution were added into #15 mL conical centrifuge tubes. The flasks were agitated in a shaker (HS 250 Basic, IKA Labortechnik, Ho Chi Minh, Vietnam,) at 120 rpm at room condition (25 ± 2 °C) for a fixed time. The solutions were filtered by Whatman No. 6 filter paper (pore size, 3 μm). After filtering, the residual SO in solution was analyzed by measuring the SO at an optimal wavelength of 530 nm (corresponding to the maximum adsorption capacity for SO), using UV-Vis spectroscopy (Shimazdu, Japan).
To determine the optimal experimental conditions, a series of preliminary tests were performed with variation of solution pH (2–10), adsorbent dosage (1–5 g/L), SO concentration (10–200 mg/L), and contact time (1–720 min). The optimum results obtained were then used for kinetic and isotherm studies.
The amount of SO adsorbed (qe
, mg/g) was determined according to the following equation:
(mg/L) is the initial dye concentrations; Ce
(mg/L) is the dye concentrations at equilibrium; m (g) is the mass of adsorbent; and V (mL) is the volume of the solution.
This study explored the physicochemical characteristics and comparatively evaluated Safranin O adsorption capacity between biochar and magnetic biochar from rice straw using batch experiments. Our results suggest that magnetic biochar improved adsorption properties over non-magnetic biochar, since magnetization helps in improving the specific surface area, increasing the number of functional groups, all which may provide more SO adsorption sites. The theoretical maximum adsorption capacities of the biochar and magnetic biochar were 31.06 and 41.59 mg SO per gram of dry adsorbent, respectively. The adsorption capacity of magnetic biochar for SO is therefore close to 1.4 times greater than its precursor. Their experimental data are well-described by both Freundlich and Langmuir isotherms, with correlation coefficients higher than 0.96. In addition, their kinetic data fit well with the pseudo-second-order kinetic model, as well as the intra-particle diffusion model, which suggested that the collective physical and chemical forces may account for the adsorption mechanism of Safranin O molecules by both adsorbents, including porous diffusion, H-bonding, the π-π interaction, π+-π interaction, and electrostatic attraction. Under the action of a magnetic field, the magnetic biochar material is easily collected from water after adsorption.
From an industrial point of view, the removal of pollutants including dye ions using continuous flow systems is found very useful and reliable. Therefore, it is necessary to examine the practical applicability of the biochar/magnetic biochar adsorbents from this work for real-world textile wastewater in the continuous-flow mode.
Regeneration of an adsorbent is of crucial importance in industrial practices for the removal of dye pollutants from textile wastewaters. Regeneration generally provides useful information to allow for the economic design of an overall operation and to make the adsorption process more feasible and practical. Therefore, regeneration studies and desorption modelling steps should be considered in future work.