Functionalized biochar derived from novel Neolamarchia cadamba leaf extracts for the adsorption of Congo Red dye: kinetics, optimization, and reusability studies

In the present scenario, pollution due to industrialization is one of the most significant issues against the ecological equilibrium. Textile, leather tanning, printing, food coloring, cosmetics, and other industrial sectors utilize synthetic dyes extensively which in turn are the primary source of wastewater production and increase pollutant content in the water bodies. The effluent dyes are of major concern considering their harmful effects which in turn affect the water quality adversely (Yaseen & Scholz 2019). The efffects of these poisonous dyes is a big issue for the aquatic life as well as life on land because water is utilized for copious purposes like irrigation and domestic usage, which intentionally or unintentionally affects all potential life forms occurring in the world. Toxins are a primary source of cutaneous allergies, respiratory sickness, and changes in the BOD and COD of water, producing health problems for all living species on land and in aquatic bodies (Bruno et al. 2019).

Congo Red (CR) dye is a compound with IUPAC name as the sodium salt of benzidinediazo-bis-1-naphthylamine-4-sulfonic acid. It is used for coloring cotton, silk, and for staining biological samples for the microscopic inspection at tissue-level organization in histology. CR is highly toxic even at low concentration exposure as it inhibits survival as well as the fertility of the exposed individuals of certain species. Thus, CR is prohibited in many nations because of their harmful effects; however, it is still being used by many nations, and upon leaking it causes adverse health effects on the exposed individuals. Therefore, the remediation of the wastewater containing CR is necessary before its discharge into the water bodies (Hernández et al. 2016).

In this study, Neolamarchia cadamba (NC) leaf extract was combined with zinc nanoparticles to form a composite material. This composite was then subjected to carbonization, and the resulting material was optimized as an adsorbent for potential applications in pollutant removal. Several other techniques like biological degradation (Bayramoglu et al. 2019), micro-filtration, electrokinetic treatment, and photodegradation (Ahmed et al. 2022) are also utilized for the remediation of different pollutants present in the water. On the other hand, adsorption itself is an extremely efficient process as it can remove different types of chemicals effectively such as aromatic hydrocarbons (Rezagholizade-Shirvan et al. 2024), toxic dyes (Arica et al. 2017; Bayramoglu & Yakup Arica 2021), and heavy metals (Sabir et al. 2021) from a solution. Adsorption is one of these methods that is increasingly used to accomplish this task because it is suitable from an economic prospect, simple to use, and efficient at eliminating a variety of contaminants from wastewater. Numerous adsorbents for the extraction of pollutants from its aqueous solution have been used such as Fe₃O₄@SiO₂@Zn-TDPAT (Wo et al. 2019), m-cell/Fe₃O₄/ACCS (Zhu et al. 2011), modified sorbent from waste Irvingia gabonensis seed husk (Abugu et al. 2023a), Fe₃O₄/GO composite (Namvari & Namazi 2015), chemically treated Lagenaria breviflora seeds (Abugu et al. 2023b), ZrO₂-CdZrO₃-S (Tavakoli-Azar et al. 2022), ZnFe₂O₄@ZnO-Chrysanthemum spp. floral waste (Nguyen et al. 2023), ZnMg@PH (Paul et al. 2024).

NC or Kadam or burflower belongs to the Rubiaceace family. It is a South Asian native plant used for cattle feeding and timber production. NC is rich in phytochemicals like flavonoids, glucosides, terpenoids, saponins, etc., and secondary metabolical chemicals. It also has antioxidant, antipyretic, anti-inflammatory, and anti-cancerous properties, which makes it a suitable biological resource to use as an adsorbent. The NC leaf extract has hypoglycemic effects.

The production of nanoparticles via a greener approach using biological sources such as plants, bacteria, and fungi is safe and eco-friendly. Metal nanoparticles synthesized by the bottom-up approach method do not require high pressure, temperature, or toxic chemicals. Various metal nanoparticles synthesized by plant extract have been studied for the removal of pollutants such as copper nanoparticles synthesized by citrus juice having a particle size between 10 and 60 nm, Chrysanthemum spp. floral extract for ZnFe₂O₄-ZnO nanoparticles (Pouran et al. 2015), Hyptis suaveolens extract for copper nanoparticles, egg albumen/Sr-doped/Fe₂O₃ nanoparticles (Lajevardi et al. 2019), Fe₂O₃ nanoparticles from Leucas Aspera and Jatropa podagrica leaf extract (Nouri et al. 2020) showing good adsorption behavior for the pollutants. Zinc oxide nanoparticles are effectively used as a material in various applications such as biosensors, cosmetic ingredients, dental fillings, and environmental applications due to their nontoxic behavior, ease of cost, ease of preparation, and material stability. The plant extract with metal oxide as nanoparticles is a promising adsorbent as they are safe, easily available, nontoxic, and presents various phytochemicals that help in the adsorption of pollutants (Raza et al. 2022).

The novelty of this work lies in the development of an eco-friendly method for synthesizing zinc oxide nanoparticles embedded in biochar using NC leaf extract and zinc nitrate solution, followed by carbonization. The resulting NC@ZnC adsorbent offers efficient removal of azo dyes like CR while being environmentally safe. Key operating factors such as temperature, dosage, pH, time, and concentration are optimized to enhance dye removal, making this approach both sustainable and effective.

The NC@ZnC nano-adsorbent was prepared in a three-step procedure, first, the extract of NC leaves was prepared by adding 20 g of washed and crushed leaves in 100 mL of deionized water and the solution temperature was extended up to 60 °C to make sure that no denaturation of the biochemicals found in the leaves occurs (Kanase et al. 2020). This extract was reduced to 20 mL. In the second step, the extract was transferred into an RB flask with Zn(NO₃)₂·6H₂O in a 1 g/10 mL ratio to the extract. This mixture was allowed to stir on a magnetic stirrer at 60 °C for 90 min. A slurry was produced and dehydrated in a hot air oven until all the moisture was lost, which then was cooled down to a moderate environment. The last step includes transferring of the slurry solution into the silica crucible followed by carbonizing it at a required time of 120 min at 400 °C in a high-temperature furnace for the synthesis of biochar, namely NC@ZnC as an adsorbent. It is later used for adsorption studies (Saemi et al. 2021).

The optimization of CR dye removal using the NC@ZnC adsorbent was conducted through response surface methodology (RSM), employing a Box–Behnken Design (BBD) to systematically investigate the effects of key operating variables, including concentration and pH. The statistical significance of these variables and their interactions were analyzed using Analysis of Variance (ANOVA), providing a robust model to predict and optimize the adsorption efficiency under various conditions.

After the adsorption of CR on NC@ZnC at optimum conditions, the adsorbent was then weighed and transferred in 0.1 N, 25 mL solution of ethanol, hydrochloric acid, and sodium hydroxide each for 120 min. The effect of adsorption–desorption was then analyzed utilizing the UV–Visible spectrophotometer by examining the supernatant.

To analyze the rate of adsorption, various kinetic models, including pseudo-first-order (PFO), pseudo-second-order (PSO), intraparticle diffusion (IPD), and Elovich models, were applied to the experimental data. The results indicated that the PFO did not accurately describe the adsorption of CR, as shown by a lower regression coefficient and a mismatch between calculated (⁠⁠) and experimental (⁠⁠) equilibrium values (Figure 7(b) and 7(c), Table 1). In contrast, the PSO model provided a better fit, with a higher regression value of 0.9988 and value in coordination with value indicating that the adsorption process follows second-order kinetics.

The IPD model was also used to explore the diffusion mechanism. The IPD model revealed that the rate-controlling mechanism consists of three distinct phases: (1) the diffusion of CR molecules from the bulk solution to the adsorbent surface, (2) the penetration of CR into the pores of NC@ZnC, and (3) the final equilibrium stage where adsorption stabilizes. The linear portion of the IPD plot, which does not pass through the origin, along with the intercept value, suggests that IPD is not the sole rate-limiting step. The magnitude of the boundary layer effect can be inferred from the intercept, with larger intercepts indicating greater boundary layer resistance.

The Elovich model was employed to further characterize the adsorption mechanism, indicating that both chemisorption and physisorption processes are involved in the reaction. This model's suitability reinforces the idea that multiple adsorption mechanisms are at play during the CR sorption on NC@ZnC. The higher value of A depicts the higher adsorption rate over the rate of desorption. In conclusion, the kinetic evaluation shows that the PSO model provides the best fit for the adsorption of CR on NC@ZnC, with IPD playing a significant role in the rate-determining step, while the Elovich model highlights the combined effects of chemisorption and physisorption. PSO kinetic model is best suited for the adsorption of CR on NC@ZnC as it can easily be seen (Figure 11) in the adsorption mechanism how the interaction of adsorbent–adsorbent takes place.

The regression coefficient value is high in the Temkin isotherm model. When the heat of sorption indicated by B₁ is lower than 20–40 KJ/mol, then the process is predicted as physiosorption. On comparison, when the value of B₁ lies in the range of 80–240 KJ/mol, then the mechanism interprets chemisorption (Aldahash et al. 2022; Eze et al. 2024) and when the condition is within 20–40 KJ/mol, then the sorption is both physical and chemical in nature as shown in Figure 8(c) and Table 2. The value of B₁ below 20–40 KJ/mol indicates the physical nature of the process which can also be seen by the value of E from D–R isotherm. Using the constant β as shown in Figure 8(d), the D–R isotherm provides an approximated account of the mean free energy E of adsorption per adsorbate molecule (Özcan et al. 2006; Ezugwu et al. 2023). This was also observed by Aldahash et al. (2022) and Aldahash et al. (2023). The value of mean free energy is less than 8 KJ/mol indicating physical nature, whereas E > 8 KJ/mol indicates chemical nature. According to Table 2, the value of E is 1.294 KJ/mol, indicating a physical process. Furthermore, the equilibrium adsorption capacity (q_e) at 298 K after a 90-min incubation period was determined to be 59.78 mg/g, which matches closely with the experimental value of 60 mg/g. This strong agreement between the experimental values demonstrates consistency between the thermal and isothermal studies, confirming the reliability of the adsorption process under the specified thermal conditions (Eze et al. 2022). The thermal behavior shows that the adsorption process at fixed temperature shows physiosorption.

The desorption of the CR dye from NC@ZnC was carried in a 0.1 M solution in 25 mL of hydrochloric acid, sodium hydroxide, and ethanol each as shown in Supplementary material, Figure S1. The desorption of CR from NC@ZnC was highest by sodium hydroxide which was about 98%. The desorption % decreased with each consecutive cycle, and the desorption rate was considerably efficient for up to four cycles as shown in Supplementary material, Figure S1. The decline in the desorption capacity was because of the deposition of CR in the pores of NC@ZnC.

The adsorption of CR onto the NC@ZnC surface is mainly governed by interactions between specific functional groups. Before adsorption, a Raman peak at 1,601 cm⁻¹ is observed, corresponding to lysine present in NC. This peak disappears after adsorption, indicating that lysine plays a key role in the adsorption process.

The interaction is primarily between the NH₄⁺ ion in lysine and the O⁻ ion in the sulfonate group of CR. Lysine, an amino acid, contains amino groups (NH₄⁺) that are positively charged. CR, on the other hand, contains negatively charged sulfonate groups (O⁻). The strong electrostatic attraction between these opposite charges results in the adsorption of CR onto the NC@ZnC surface.

Additionally, hydrogen bonding may also contribute to the interaction, where the NH₄⁺ groups in lysine could form bonds with oxygen atoms in the CR molecule. Together, these electrostatic and hydrogen bond interactions result in the effective binding of CR to NC@ZnC, facilitating its removal from the solution. The graphical representation for the synthesis of NC@ZnC, adsorption of CR, and possible adsorption pathway is given in Figure 11.

Numerous bionanoadsorbents were utilized to extract CR from aqueous solutions have been taken into account for the comparative investigation as shown in Table 6.

An efficient bio-nano-adsorbent NC@ZnC was prepared for the removal of CR. The FE-SEM images showed the deposition of CR on NC@ZnC, the extraction of CR can be altered by changing certain parameters like pH, concentration of the solution, and contact time to reach a higher efficiency. The optimum stasis for the extraction of CR was at a pH of 5 for 90 min given the calculated Langmuir adsorption capacity of 303.03 mg/g and experimental adsorption capacity of 59.78 mg/g. Kinetic studies reveal that the adsorption proposes a PSO reaction mechanism. The equilibrium data are nearby and follows Langmuir and Freundlich isotherm. The thermodynamic analyses indicate that the process exhibits spontaneity and is characterized by endothermicity. The desorption capacity of the adsorption is about 98% under the effect of eluent sodium hydroxide (0.1 M), and the adsorbent can be reprocessed four times until it is completely exhausted which gives it an economical advantage. The results were optimised by BBD-RSM. Thus, the prepared bioadsorbent is an efficacious tool to remove industrial waste like CR from water.

This article does not contain any studies with humans and animals.

A.S.P. and S.S. contributed to data analysis and investigation; S.S. conceptualized the study; S.S., S.H.K., A.S.P. contributed to writing, reviewing, and editing.

The article was not supported by any funding.

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

The authors declare there is no conflict.