Adsorption of Reactive Red 120 dye from simulated wastewater using rice straw charcoal: isotherm and kinetic studies Open Access

Releasing numerous toxic chemicals into aquatic and terrestrial environments has become a serious global concern (Zaman et al. 2021). Water pollution from dyes and pigments used across various industries - such as textiles, printing, leather, plastics, food and cosmetics production, electroplating, rubber manufacturing, and pesticide creation - poses a significant threat to the aquatic environment (Chakraborty et al. 2020; Çelekli et al. 2019). Over 70% of the synthetic dyes are used in textile and other industries but more than 50% dyes fail to adhere to fabrics, and these are released as colored effluent from industrial manufacturing and operation steps. Compared to other dye types, reactive azo dyes undergo rapid hydrolysis in alkaline environments, leading to the loss of more than 30% of these dyes during the dyeing process. As a result, these dyes are released into aquatic systems as toxic-colored contaminants (Dasgupta et al. 2015). Elevated dye concentration levels in aquatic environments interfere with light absorption, disrupt the biological metabolism of aquatic organisms, and affect photosynthetic activity (Chakraborty et al. 2020). By chelating metal ions, the dye also causes microtoxicity in fish and other aquatic lives. Additionally, certain dyes that break down products can cause cancer, diarrhea, eye burns, skin irritation, and other mutagenic or carcinogenic effects on living things, including humans (Ghosh et al. 2020). One typical anionic dye, Reactive Red 120 (RR120), has azo (–N = N–) groups in structure, which reduces its biodegradability and increases the risk to human health and the environment. So, getting rid of these dyes from effluents before dumping them into waterways is very important to protect both people's health and the ecosystems (Jawad et al. 2020). The removal of dyes from wastewater can be achieved through various treatment technologies such as membrane separation, coagulation and flocculation, exchange of ions, reverse osmosis, and other advanced oxidation processes, including oxidation/ozonation, photocatalysis, ultrafiltration, electrochemical, adsorption, and biosorption (Ghosh et al. 2018; Chakraborty et al. 2021, 2024; Zaman et al. 2021). Adsorption stands out as the most effective method due to its operational simplicity, low cost, high efficiency, abundant sources, absence of sludge, adaptability, and resistance to harmful substances. Although commercial activated carbon is also used as an adsorbent, it is highly expensive (Ghosh et al. 2018). Consequently, there is a growing interest among researchers in synthesizing new, cost-effective, and eco-friendly adsorbents that are highly effective and made from discarded materials for the removal of colored wastewater. Various bio-adsorbents have been employed to eliminate dye from wastewater including Mahagoni wood charcoal and Mahagoni bark charcoal (Chakraborty et al. 2021); activated carbon by Swietenia mahagoni bark (Ghosh et al. 2020); jute stick charcoal (Zaman et al. 2022; Chakraborty et al. 2020); jute stick charcoal (Uddin et al. 2024), rice husk charcoal, and acid-modified rice husk charcoal (Zaman et al. 2023); municipal organic solid waste charcoal (Chakraborty et al. 2023a), jute fiber, rice straw (Chakraborty et al. 2023b), and charcoal (Patel 2018). Agricultural wastes are now a feasible alternative for wastewater treatment since they are affordable and environmentally benign adsorbents. For instance, one of the main crops farmed worldwide is rice. Rice straw (RS) is an agro-industrial byproduct that is readily available in the majority of rice-producing nations (Fathy et al. 2013). Due to its high carbohydrate content, rice straw is fed to ruminants; nonetheless, its usefulness as a source of power is restricted (El-Maghraby & El Deeb 2011). Most people see RS as a waste material that is burnt often to create rice straw charcoal (RSC). Therefore, it is crucial to employ rice straw as an inexpensive adsorbent in underdeveloped nations like Bangladesh. Recently research on improved rice straw with phosphoric acid and citric acid, lacking base pretreatment to produce potential adsorbents for the removal of dyes from aqueous solutions, was conducted (Gong et al. 2006, 2007, 2008). Chakraborty et al. (2011) settled NaOH-modified rice husk as a low-cost adsorbent for the adsorption of crystal violet dye. Bangladesh and numerous other nations that produce rice have access to the RS in large quantities at little or no cost. Furthermore, the use of agro-industrial wastes as adsorbents exhibits potential as a sustainable alternative or augmentation of commercially accessible adsorbents, especially in regions lacking access to organized wastewater treatment facilities (Ghosh et al. 2018). Therefore, the objectives of this study were (i) to produce RSC using inexpensive methods and evaluate the performance of RSC for RR120 dye removal from simulated wastewater using diverse operational conditions such as pH, time of contact, adsorbent dose, and initial dye concentrations; (ii) to explore the adsorption mechanism using diverse models, namely, isotherm and kinetic, and (iii) to assess the performance of RSC for real wastewater using the optimum experimental conditions, respectively.

Analytical grade chemicals and reagents were used in the whole experiment and purchased from Sigma-Aldrich (Germany), namely RR120, NaOH, and HCl. Table 1 represents the RR120 dye's chemical structure and characteristics. Whole experiments were conducted with double distilled water.

Table 1

General characteristics of RR120 (source: Chakraborty et al. 2021)

Properties .	RR120 .
Color index name	RR120
Molecular formula	C44H24Cl2N14O20S6Na
Molecular weight	1,469.98 g/mol
CAS number	61951-82-4
Water solubility	70 g/L
λmax	535 nm
Class	Diazo (−N = N-bond)
Color	Red blue light
Chemical structure

Properties .	RR120 .
Color index name	RR120
Molecular formula	C44H24Cl2N14O20S6Na
Molecular weight	1,469.98 g/mol
CAS number	61951-82-4
Water solubility	70 g/L
λmax	535 nm
Class	Diazo (−N = N-bond)
Color	Red blue light
Chemical structure

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The starting and final equilibrium levels of RR120 in mg/L are C_o and C_e, respectively. q_e provides an equilibrium value for RR120 adsorption in mg/g. The mass of the adsorbent is m (g), and its solution volume is V (L).

The adsorption isotherm experiments were adopted from Chakraborty et al. (2023a). After slight modification, this study was carried out using the following experimental conditions: an experimental solution volume of 250 mL, the dye concentration varied from 5 to 70 mL, a contact time of 120 min, an adsorbent dose of 10 g/L, a pH of 3.0, and a rotational speed of 200 rpm (Jar-test instrument: JLT4, VELP Scientific, Italy) at room temperature (experimental condition: 25 ± 2 °C). Adsorption isotherms show how the adsorbate interacts with sorbent materials, giving a thorough understanding of the nature of association. This study used the Langmuir, Freundlich, Elovich, and Halsey isotherm models to investigate the nature of adsorption. Details are presented in Table S1.

The adsorption kinetic experiments, with slight modifications, were adapted from Chakraborty et al., 2023a. This study implemented the following modified experimental conditions for the kinetic analysis. RR120 dye was a working solution volume = 350 mL, an adsorbent dose = 10 g/L, RR120 dye concentration = 20 mg/L, pH 3.0 (Jar-test instrument: JLT4, VELP Scientific, Italy) at room temperature (experimental condition: 25 ± 2 °C). Samples from the solution were obtained, filtered, and analyzed at predetermined time intervals (1, 2, 3, 5, 7, 10, 15, 30, 60, 90, 120, 150, 180, and 210 min). Adsorption kinetics include information on the rate of RR120 adsorption by the adsorbents, how long it takes for the adsorption process to be finished, and the mechanism of the reaction. The kinetic behavior was determined using Ho and McKay's pseudo-second-order model and Lagergren's pseudo-first-order model; the possible rate-controlling step and the diffusion mechanism of the adsorption process were examined using the intraparticle diffusion model. The Boyd model was used to further examine the kinetic experiment data, and all kinetic models are presented in Table S1.

A similar statement was proposed by Chakraborty et al. (2021) for the removal of RR120 using Mahagoni wood charcoal and Mahagoni bark charcoal. On the other hand, when the pH increased, more anionic RR120 molecules and surplus OH⁻ ions competed for adsorption sites, which reduced the removal effectiveness. Cardoso et al. (2012) found about 95% removal of RR120 dye onto Spirulina platensis microalgae at pH 2.0.

Figure 5(b) shows that the removal percentage of RR120 is reduced from 94.58 to 58.94% with increasing initial RR120 dye concentration from 5 to 70 mg/Lm at the constant dose, the adsorbent's external surface is saturated and dye molecules block the pores. As a result, the initial dye concentration and contact time have a big impact on dye removal. The adsorption capacity of RSC increases from 0.47 to 4.13 mg/g by raising RR120 dye concentration from 5 to 70 mg/L at a fixed RSC dose of 10 g/L. This might be a result of the intense interaction between the molecules of the dye and the adsorbent surface, which increases the significant driving force to transfer a high mass of RR120 from the liquid to the solid phase in the aqueous solution (Zaman et al. 2021).

The lower adsorption capacity (q_m = 1.406 mg/g) and the R² value obtained from the Elovich isotherm indicate that this model is inappropriate for describing the adsorption process of RR120 onto RSC. A similar observation was reported by Munagapati et al. (2019) for the adsorption of RR120 dye by quaternary amine-modified orange peel powder and Jawad et al. (2020) for the adsorption of RR120 dye onto the hybrid crosslinked chitosan epichlorohydrin/TiO₂ nanocomposite surface. The absorption capacities of RSC and other adsorbents are given in Table S2.

Intraparticle diffusion was applied to investigate the diffusion mechanisms. An intraparticle diffusion plot passed through the origin (Figure 7(c)), indicating that mechanisms, such as intraparticle diffusion, bulk diffusion, and film diffusion, controlled these dye adsorption processes, which show the rate-limiting step of RR120 dye adsorption onto RSC. On the other hand, the Boyed plot showed that film diffusion was the rate-limiting step for the adsorption of RR120 onto RSC because the plot was linear and did not pass through the origin (Figure 6(d)). A similar study was reported by Ghosh et al. (2020).

In this study, only one adsorbate (RR120 dye) and one adsorbent (RSC) were considered. The FTIR technique is used for adsorbent characterization, where Brunauer-Emmett-Teller (BET), pore volume distribution, and other approaches (e.g. regeneration and cost analysis) were not conducted due to resource limitations and a lack of laboratory facilities. For real wastewater treatment experiments, only one type of textile industrial effluent was considered due to time, resource, and sampling difficulties. All limitations will be considered for further studies.

This present study explores the usability of RSC as a potential biosorbent for RR120 removal from the aqueous solution and industrial wastewater under batch adsorption experiments. The optimum pH was selected as 3 because of achieving maximum adsorption. The Langmuir isotherm had a better fit with experimental data for RSC, where the maximum adsorption capacity was 4.43 mg/g. The kinetic investigation demonstrated that the pseudo-second-order kinetic model was better fitted with a multi-step diffusion process. Numerous functional groups and lots of micro- and mesopores on the RSC enhanced the adsorption processes. So, it concludes that RSC could be an effective alternative for RR120 removal from the aqueous solution and textile effluent treatment, where a centralized wastewater treatment system is not accessible because of its rare 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 did not receive any research grant.

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

The authors declare there is no conflict.