Rice husk charcoal and acid modified rice husk charcoal for efficient adsorption of methyl orange

Globally, industrial and urban developments are tremendously increasing due to rising population growth, which significantly enhances environmental pollution (Carolin et al. 2021). Diverse industrial activities release large amounts of wastewater, which finally goes to the aquatic and terrestrial environments, which is a matter of serious concern (Radoor et al. 2021). Dyes are widely used in many industries and discharged as colored effluent in aqueous environments where they can cause many difficulties, including reduction of the rate of photosynthesis, hindering sunlight penetration, chelating metal ions, and generating diseases for aquatic flora and fauna (Carolin et al. 2021; Zaman et al. 2021). Additionally, dyes can create human health hazards, such as skin and eye problems, and diarrhea, and act as mutagenic and carcinogenic agents (Chakraborty et al. 2021). Depending on their chemical structure, there are three dye types: cationic, anionic, and nonionic. MO is an anionic azo-dye that is commonly used as a coloring agent (Radoor et al. 2021), the textile industry alone producing 7 × 10⁵ tonnes/year of toxic dyes and associated byproducts (Kubendiran et al. 2021). Though different physicochemical properties of MO including lower degradation rate, aromatic ring structure, and high retaining capacity make it difficult to remove from effluents but it must be free from effluents before discharges into the receiving environment.

Different treatment technologies including oxidation, membrane filtration, reverse osmosis, coagulation and flocculation, ion exchange, adsorption, nanofiltration, ultrafiltration, etc., have been used to remove toxic dyes from effluent (Bensalah et al. 2020), where adsorption is superior to other techniques in terms of cost-effectiveness, ease of operation, energy input, and low probability of further byproduct formation (Zaman et al. 2021). High production costs reduce the use of commercial activated carbon as an adsorbent material. Furthermore, researchers are trying to produce cost-effective alternative adsorbents from biomaterials such as solid waste products, natural discard materials, and agro-processing byproducts (Ghosh et al. 2020; Zaman et al. 2021), including cotton waste, sawdust, silica gel, bark, rice husk, natural coal, peat, chitosan, jute stick, activated carbon, activated clay, and hen feathers (Chakraborty et al. 2020; Zaman et al. 2021).

Recently, agro-processing byproducts (e.g., rice husks) have received consideration due to their ready availability, low or zero cost, and natural properties (Leng et al. 2015). They also contain high proportions of cellulose (55–65%), SiO₂ (15–20%), and 20% lignin (Fu et al. 2019). Rice husk has no good nutritional status and is rarely used as cattle food, but is used as cooking, paving, and landfilling materials.

Rice husk charcoal (RHC) was produced by burning rice husks, and acid modified rice husk charcoal (AMRHC) was formed by treating RHC with acid to increase the sorption capacity. RHC was used as an adsorbent for textile effluent treatment in this study. The objectives were (i) to assess the performance of RHC and AMRHC for MO dye removal from simulated wastewater with diverse experimental conditions (pH, adsorbent dose, contact time, and initial dye concentration) and (ii) to estimate the adsorption actions of RHC and AMRHC using adsorption kinetics and isotherms.

Rice husk was collected in Chougacha, Jashore, soaked in distilled water for 1.5 h, and washed with distilled water numerous times. It was then dried in an oven (Labtech LDO-150F, Korea) for 48 h at 80 °C, and then in a muffle furnace (SXT-10, Shanghai Shuli Instrument and Meters Co., Ltd, China) for carbonization at 400 °C for 10 min. The charcoal was pulverized using a mortar and pestle, and sieved to get the 0.1–0.5 mm-sized particles. The charcoal was kept in a sealed, borosilicate glass bottle for further use.

The rice husk was washed and dried in the oven, before a grinder was used to grind and sieve it to get the 0.1–0.5 mm material. Next, 50 g of the material were mixed with 50 mL of concentrated (18M) H₂SO₄ (impregnation ratio = 1:1) and stored for 12 h at room temperature. After 12 h, it was heated to 70 °C for 1.5 h using a hot plate, then diluted with distilled water numerous times until the pH was 7, and finally left for 24 h in an oven at 80 °C and stored in a borosilicate glass bottle.

MO dye was acquired from Sigma-Aldrich, USA. The molecular weight of MO is 327.34 g/mol, its chemical structure C₁₄H₁₄N₃NaO₃S, pH range 3–4.4, and purity, in this case, 85%. This adsorbate was used as-received. All chemicals used were analytical grade. The required amount of MO was dissolved in double-distilled water to prepare the MO stock solution (400 mg/L) from which the desired working solution was prepared. The experimental pH was adjusted using a 0.1 N HCl/0.1 N NaOH solution, and determined with a pH meter (Ezdo 6011, Taiwan). A UV-visible spectrophotometer (HACH DR 3900, USA) was used to measure the MO concentration in the solution at 464 nm wavelength.

The adsorption isotherm tests were conducted using 250 mL of MO solutions in several 500 mL beakers at diverse MO concentrations ranging from 10 to 100 mg/L, with the following common experimental conditions: RHC and AMRHC dose (10 g/L), rotation speed (180 rpm), pH (3), temperature (25 °C), and contact time (150 min). Adsorption isotherms deliver a complete knowledge of the types of associations by demonstrating how the adsorbate behaves with sorbent materials. In this study, the Langmuir, Freundlich, Elovich, and Halsey isotherm models were used, as detailed in Supplementary Table S1.

The adsorption kinetic studies involved adding 10 g/L charcoal to a 350 mL solution, which had 20 mg-MO/L at pH 3 and was agitated at 180 rpm at 25 °C. At specific times – 1, 2, 3, 5, 7, 10, 30, 60, 90, 120, 150, 180, 210, and 240 – samples were taken from the solution, filtered, and analyzed. Adsorption kinetics includes details of the rate of MO adsorption by the adsorbents, the amount of time needed to complete the process, and the reaction mechanism. The kinetic behavior was identified using Ho and McKay's pseudo-second-order model and Lagergren's pseudo-first-order model, whereas the intraparticle diffusion model and Boyd model were employed to observe the possible adsorption process rate-controlling step and diffusion mechanism, as detailed in Supplementary Table S1.

The values of ΔH and ΔS were determined from the slope and intercept of the plot of ln K_d versus 1/T, respectively, and the values of ΔG from the K_d value at each temperature.

Huang et al. (2017) proposed a similar type of clarification for MO removal using protonated, cross-linked chitosan. Conversely, in a basic environment, the electrostatic repulsion between the MO ion and the adsorbent surface charge reduces the adsorption performance of RHC and AMRHC.

Figure 4(b) shows that the MO removal rate decreases (96–43% for RHC and 98–93% for AMRHC) as the initial MO dye concentration increases from 10 to 100 mg/L, because, at any fixed dose, the adsorbent's outer layer is covered by dye molecules. While the adsorption rate of MO gradually rises (0.96–4.29 mg/g for RHC and 0.98–9.29 mg/g for AMRHC) with rising MO concentration (10–100 mg/L) at constant RHC and AMRHC dose (10 g/L) (Figure 4(b)), the higher association between MO molecules and adsorbent (RHC and AMRHC) improved the major dynamic to shift a greater mass of MO from the liquid to the solid phase in an aqueous environment (Huang et al. 2017).

To identify the diffusion mechanism, the interparticle diffusion (IP) model is used in this study. The IP plot did not pass through the origin (Figure 7(c)), and had high intercept values (C = 0.14 for RHC and 1.269 for AMRHC) (Table 2), indicating that the diffusion might be external and film site of the adsorbent (Chakraborty et al. 2021). While the Boyd plot showed (Figure 7(d)) that film diffusion was the rate-controlling step for MO adsorption onto RHC and AMRHC.

Desorption is applied to assess the possibility of further contamination when the treated adsorbent enters the environment. The process is wholly controlled by chemical bonding (ionic, covalent, Van der Waals' forces, or dipole–dipole interaction) between adsorbent and adsorbate (Chakraborty et al. 2021). In this study, desorption was studied at pH values from 4 to 10. Figure 8(b) shows that the proportional desorption from RHC (13–19%) and AMRHC (21–38%) was low; instead of increasing pH, might be the strong and weak chemical bonding occurs between MO molecules and RHC and AMRHC, respectively. Chakraborty et al. (2021) also found similar types of results where they suggesting that strong and weak binding forces may have existed between the adsorbent (Mahagoni wood charcoal and Mahagoni bark charcoal) and the reactive red 120.

The efficiency of RHC and AMRHC for MO removal from industrial wastewater was assessed using the optimized adsorption experimental conditions. A textile effluent sample was collected from an industrial area in Saver, Dhaka, Bangladesh. Before the experiment, the pH (8.3), EC (731 μS/cm), TDS (353 mg/L), salinity (0.3 g/L), and MO concentration (221.28 mg/L) of the wastewater were determined. Figure 8(c) shows the proportional MO removal using the two adsorbents. The studies were conducted at pH 3, volume of 150 mL, and adsorbent dose of 10 g/L. The mixture was separated by filtration after stirring for 150 min for RHC and 120 min for AMRHC. The agitation speed was 180 rpm. Proportional MO removal was 50% for RHC and 60% for AMRHC, indicating that both adsorbents are appropriate for MO removal from polluted water.

This study focuses on MO removal from aqueous solutions and wastewater using RHC and AMRHC. The optimal pH (3) gave maximum MO adsorption for both adsorbents. The maximum adsorption capacities of the adsorbents were 4.57 and 11.53 mg/g, respectively. The Freundlich and Langmuir isotherms gave the best-fits with the experimental data for RHC and AMRHC, respectively. The kinetic investigation showed that the pseudo-second-order kinetic model gave the better fit with a multi-step diffusion process. The FTIR analysis indicates that –OH group and heteroatoms are responsible for adsorption where SEM analysis also shows lots of active adsorption sites influences this adsorption. A thermodynamic study showed that the RHC and AMRHC adsorption processes are endothermic and favorable. It is concluded that both RHC and AMRHC could be effective in MO removal from aqueous solutions and wastewater because of their availability, low cost, adsorption capacity, and good kinetics.

We would like to thank the Department of Environmental Science and Technology, Jashore University of Science and Technology, Bangladesh, for providing the necessary support.

The authors would like to thank the Ministry of Science and Technology, Bangladesh, for the research grant (R&D) award.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.