Introduction

Colour-imparting chemicals are widely employed in numerous industries such as pharmaceuticals, cosmetics, textiles, plastic, printing and food. These chemicals are also used in biotechnology as a staining fluorescent (Jain et al. 2007). Hence, a large amount of untreated wastewater containing toxic dyes is produced across the globe (Sarwar and Khan 2022). The indiscriminate discharge of industrial effluents containing dyes can cause devastating environmental pollution (Ukaogo et al. 2020). Among these colour-imparting chemicals is rhodamine B (RhB). RhB is a basic cationic dye that belongs to the xanthene class (Wu and Gao 2009). It is also one of the most extensively used water-soluble dyes in various industries.

RhB has an intravenous median lethal (LD50) of 89.5 mg kg−1; hence, it tends to be harmful if consumed beyond the threshold concentration (Mittal and Mishra 2014). RhB can cause irritation to the eyes, skin and respiratory tract if a small amount is swallowed by man. On the other hand, high exposure to this RhB may cause neurotoxicity, adverse reproductive and developmental effects, and cancer (Marking 1969; Skjolding et al. 2021). Photosynthetic activity in the aquatic ecosystem is hindered due to the high tinctorial value of RhB. The complexity of RhB molecules confers great stability on it against biodegradation, oxidizing agents and photo-degradation. This makes it difficult to eliminate from the aquatic ecosystem.

To avert the health implication of RhB on both man and aquatic organisms. Stakeholders have intensified their effort towards the design of a robust and effective technique for the elimination of RhB from wastewater. Several physicochemical techniques have been assessed and reported for the sequestration of RhB from the aquatic ecosystem and they include adsorption (Hayeeye et al. 2017; Wang et al. 2017; Zamouche and Hamdaoui 2012), electrochemical oxidation (Du et al. 2012), chemical oxidation (Chang et al. 2009b), electro-oxidation and photocatalysis (Zhao and Zhu 2006) and photocatalytic oxidation (Jain et al. 2007). A comparison of the aforementioned techniques presented adsorption as an alternative method for the adsorption of RhB from aqueous solution and this could be due to its user-friendliness, rapidness, low cost and superior efficiency. Meanwhile, the effectiveness of different adsorbents to eliminate RhB from wastewater has been investigated and these adsorbents include activated carbon (Kadirvelu et al. 2005), CNT (Kumar et al. 2014), fly ash (Gupta et al. 2000), cedar con (Zamouche and Hamdaoui 2012), Fe3O4 magnetic nanoparticles (Mittal and Mishra 2014), montmorillonite (Selvam et al. 2008), bentonite (Hou et al. 2011) kaolinite (Khan et al. 2012) and biomass (Das et al. 2006),

Recently, nano-scaled materials have gained the attention of experts and stakeholders in water remediation practices. The ability of nanomaterials to sequester dyes from wastewater is generally attributed to their large surface area, high number of adsorption sites and reduced diffusion resistance (Liu et al. 2013; Vyawahare et al. 2014). Nanomaterials such as multi-walled carbon nanotubes (MWCNTs) demonstrated superior capacity for the adsorptive removal of both organic and organic and inorganic pollutants from industrial effluent (Abdullah et al. 2022; Adewoye et al. 2021; Lim et al. 2023). This excellent uptake capacity MWCNTs could be attributed to their superior physicochemical properties (Vinod Kumar Gupta et al. 2013; Vinod K Gupta and Saleh 2013). Buchholzia coriacea is commonly called wonderful kola and is mostly cultivated in Africa. Previous reports showed that this plant has great medicinal value, but its pod is often discarded as waste. The pod as biomass can be converted into useful material if modified and applied as adsorbents for wastewater remediation. Previous study showed that the combination of biomass and MWCNTs birth nanocomposites with superior capacity for environmental remediation practice (Hoang et al. 2022; Khalatbary et al. 2022). In this research, a novel nanocomposite (WKM) was fabricated from Buchholzia coriacea pod (WK) and f-MWCNTs. The applicability of WKM and WK in RhB adsorption was assessed in view of operational adsorptive variables, kinetics models, isotherms models and thermodynamics.

Materials and methods

Rhodamine B powder (98.98%), sulphuric acid (H2SO4) (98%), multi-walled carbon nanotubes (95%), triethylenetetramine (99.99%), nitric acid (HNO3) (99.9%), sodium hydroxide (NaOH) (99.98%), sodium chloride (NaCl) (> 95%) and hydrochloric acid (HCl) (99.9%) were purchased from Sigma-Aldrich.

Samples and sample preparation

Buchholzia coriacea were purchase from Orieugba market (5° 53′ 71ʺ North, 7. 7° 49′ 97ʺ East) in Umuahia Abia state, Nigeria. The plant was declared not endangered. Thereafter, the pod was washed using deionized water, oven-dried at 80 °C and pulverized to a fine powder using an electric grinder and stored in an airtight container.

Functionalization of MWCNTs

About 0.5 g of MWCNTs was immersed in 25 cm3 of 6 M HCl at room temperature for 1 h on stirring and thereafter diluted 10 times using deionized water and then filtered under vacuum. The product obtained was subsequently treated with 6 M NHO3 using a similar procedure. The purified MWCNTs (p-MWCNTs). 0.25 g of p-MWCNTs was immersed in a concentrated (HNO3:H2SO4) 100 cm3 of a mixture of acid (1:3 by volume) at room temperature. The mixture was stirred for 12 h and afterwards diluted 20 times its initial volume using deionized water and then filtered under vacuum. The product obtained was washed under a vacuum until the pH of the water was obtained. The black product (f-MWCNTs) obtained was vacuum-dried (80 °C) for 24 h and thereafter stored in an airtight container for future application.

Preparation of nanocomposite

About 0.5 g of f-MWCNTs and 2 g of WK were weighed into a beaker containing 50 cm3 of deionized water, the mixture was stirred at room temperature for 3 h. Subsequently, triethylenetetramine (1 cm3) was added to the mixture (f-MWCNTs/WK). At 90 °C, the mixture was then agitated to dryness, vacuum oven-dried, designated as WKM and stored in an airtight container for further use.

Characterization

The specific surface area of WKM and WK was investigated using the BET analysers (Micromeritics Instruments Corp., USA). WKM and WK were characterized using the Fourier-transformation infrared spectroscopy (FTIR) (Termo Nicolet-870 spectrophotometer, USA). The field-emission scanning electron microscope (ZEISS ultra plus, USA) was used to characterize the surface morphology of the adsorbents. The thermal behaviour of the nanocomposite was examined using the PerkinElmer simultaneous thermal analyser (STA6000 instrument, USA).

pH point of zero charges (pHPZC)

To determine the pHPZC of WKM and WK, by using the solid addition method (Balistrieri and Murray 1981; Belachew and Hinsene 2020; Zubrik et al. 2017). Eleven Erlenmeyer flasks containing 50 cm3 of 0.1 mol dm−3 NaCl solution adjusted to the pH range of 2–12 were contacted with 0.1 g of WKM or WK. The mixtures were stoppered and agitated for 48 h at room temperature. The pHPZC of WKM and WK were evaluated from the plot of the final pH versus the initial pH of the mixture.

Batch adsorption

The removal of RhB by WKM and WK was achieved using the batch adsorption technique. About 25 cm3 (100 mg dm−3) of RhB was contacted with WKM or WK in a 100 cm3 stoppered glass bottle for 180 min. The mixture was equilibrated using a pre-set thermostatic water bath shaker at room temperature for 180 min and filtered using filter paper (Whatman No 1). The effect of pH (solution pH varied from 1 to 10 (using 0.1 M HCl and 0.01 M NaOH) with mass; 0.05 g, time: 180 min and concentration: 100 mg dm−3), contact time (time was varied from 5 to 180 min with mass; 0.05 g, pH: 3 and concentration: 100 mg dm−3), adsorbent dose (dosage was varied from 0.01 to 1 g with solution pH: 3, time: 180 min and concentration: 100 mg dm−3) and initial concentration (initial RhB concentration was varied from 10 to 100 mg dm−3 with mass; 0.05 g, pH: 3 and contact time: 180 min) were examined. The implication of solution temperature was also assessed within the temperature range of 298–318 K. The experiments were performed in duplicates. The equilibrium concentration of RhB was estimated by making use of a UV–Visible spectrophotometer at 554 nm. The uptake potential and the % adsorption of WKM and WK were calculated using the equation displayed in the supplementary information (see Eq S1 & S2).

Isotherm and kinetics and models

The kinetics responsible for the adsorption of RhB onto WKM and WK were assessed by making use of the pseudo-first-order, pseudo-second-order, intraparticle diffusion and Elovich kinetics models as displayed in Table 1. On the other hand, the experimental data obtained from initial RhB concentration experiment over a known temperature range (298–313 K) were fixed into two isotherm models which include Freundlich and Langmuir (see Table 2).

Table 1 Kinetics models investigated for the adsorption of RhB onto WKM and WK
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Table 2 Isotherm models used to describe the uptake of RhB onto WKM and WK
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Regeneration study

Reusability of the spent WKM and WK was achieved via the adsorption–desorption technique. The aforementioned adsorption procedure was used to load RhB onto WKM and WK and thereafter desorbed using aqueous ethanol. About 0.05 g of the spent adsorbent (WKM–RhB or WK–RhB) was contacted with 10 cm3 of absolute ethanol in a 100 cm3 stoppered glass bottle for 20 min at room temperature. The regenerated WKM and WK were cleaned with deionized water and dried in preparation for the next cycle. The percentage uptake of WKM and WK for the next cycle was calculated using Eq. S2 (see supplementary information).

Results and discussion

The microstructure and surface morphology of the as-prepared and RhB-loaded adsorbent was assessed using FESEM analysis. The FESEM micrograph of the pristine biomass (WK) was noticed to have an agglomerate of irregular-shaped mass with apertures of varied diameter. Meanwhile, the RhB-loaded WK was noticed to have a smooth surface sustaining irregular sheets (see Fig. 1). On the other hand, the nanocomposite was observed to exhibit a lump-like shape that is clustered with a filament-like long rod having uniform diameters. This shows a successful fabrication of the nanocomposite, it also reflects the homogeneity of WKM. The nanocomposite exhibited swelling characteristics after the adsorption of RhB molecules. This shows that RhB molecules were adsorbed onto the surface of the WK and WKM. This outcome is consistent with earlier reports. (Kumar et al. 2014; Mubarak et al. 2016).

Fig. 1
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FESEM micrographs of (a) WK, b WK–RhB, c WKM and d WKM–RhB

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To determine the surface properties of WK and WKM, the nitrogen adsorption–desorption isotherm and Barrett–Joyner–Halenda (BJH) pore-size distribution methods were used. To achieve this, about 0.25 g of WK and WKM were degassed at 105 °C prior to BET run and the data obtained were analysed with Tristar II 3020 version 2 software and plotted on OriginPro 8.0. As displayed in Fig. 2, the N2 isotherm is similar to type III, in that it has a high adsorption capacity but has broader pore-size distributions, narrower mesopores and wider micropores. The modest hysteresis loop at a relative pressure of 0.8 < P/P0 < 1 in the N2 adsorption–desorption isotherm indicates capillary condensation, suggesting the dominance of a mesoporous structure. The BET surface area of WKM from the N2 adsorption analysis was 81.1184 m2 g−1, which is far greater than that of WK (0.7925 m2 g−1) (see Table 3). The large surface area of WKM was due the incorporation MWCNTs (modifier) to WK. Therefore, the increased surface area of the WKM provides a large storage area for RhB adsorption. A similar trend was noticed for the pore volume and pore diameter of both adsorbents. Hence, easy pore entrapment of RhB into the cavities of the nanocomposite could enhance the removal of RhB.

Fig. 2
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N2 adsorption/desorption isotherm of WK and WKM

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Table 3 Surface characteristics of WK and WKM
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Thermogravimetric analysis

To establish the composition of the WKM and the behaviour of the nanocomposite at higher temperatures, the thermogram of the novel adsorbent (WKM) was acquired within the temperature range of 25–600 °C under a N2 atmosphere. TG analysis of WKM revealed that the weight (4%) loss at temperatures below 100 °C was associated with the loss of moisture (see Fig. 3). Weight loss between 100 and 200 °C may be attributed to degradation of the crosslinker employed in the fabrication of the nanocomposite. The second pyrolytic stage may be attributed to the decomposition of the WK in the nanocomposite and the formation of volatile components (CO, CO2, H2 and CH4) which accounted for almost 30% of the weight loss within the temperature range of 200–366 °C. The last stage (350–600 °C) accounted for 5% weight loss and can be attributed to partial oxidation of the MWCNTs. This result is in line with past work on the thermal behaviour of multi-walled carbon nanotube-based adsorbents (Sadegh et al. 2016; Zhan et al. 2018).

Fig. 3
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The TGA thermograms of WKM

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The chemical moieties of pristine and RhB-loaded adsorbents were investigated using FTIR (see Fig. 4). From the spectra, several peaks were noticed, indicating the presence of different chemical moieties on the surfaces of the WK and WKM. Among these peaks include bands at 3312–3410 cm−1 and 2920–2933 cm−1 which were assigned to –OH and aliphatic C–H vibrational stretches, respectively (Rasana and Jayanarayanan 2022). Meanwhile, peaks at 1604–1648 cm−1, 1382–1415 cm−1, 991–1008 cm−1 and 751–784 cm−1 were attributed to –C=O, –C=O,–C–O–C–, –C-halogen, vibrational stretching (Alam et al. 2016; Omer et al. 2018), respectively. Meanwhile, a comparison of spectra acquired for used and unused adsorbents revealed a shift in bands for spent adsorbents (WKM–RhB). Hence, a physicochemical interaction between the adsorbent (nanocomposite (WKM) and RhB molecule occurred. This corroborates with the deduction made on the FESEM analysis of WKM–RhB. Following the adsorption of RhB onto the nanocomposite, a shift in the band was observed as revealed by the spectra of WKM-Cr. This shows that the nanocomposite provided active sites for the uptake of the target adsorbate (RhB). In addition, the bands associated with WKM–RhB were shifted to lower frequencies as observed in the acquired spectrum (Table 4). This indicated the successful incorporation of the RhB to the surface of MWK.

Fig. 4
figure 4

The FTIR spectra of WK, WKM and WKM–RhB

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Table 4 The observed FTIR spectral bands (cm−1) and assignments
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The crystalline structure of the plant biomass (WK) and nanocomposite (WKM) was examined by means of powder XRD analysis. The samples were scanned from 15° to 90° using 2θ range with a step size of 0.02°, Cu Kα (λ = 1.54050 Å) radiation and a step rate of 0.454° s−1. The biomass showed no resolute peaks in its diffractogram. On the other hand, diffraction peaks with 2θ values of 26.01° and 44.23°, 56.09° and 77.08° for WKM were obtained and indexed to the (002) and (100) (004) and (110) reflections of graphite from MWCNTs, respectively (ICDD No. 01-074-2379) (see Fig. 5). These peaks reflect the incorporation of the modifier (MWCNTs) to the nanocomposite. Hence, the nanocomposite was successfully synthesized.

Fig. 5
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X-ray diffraction spectral for WK and WKM

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Effect of pH

A cardinal factor of adsorption is the solution pH and this is because of its capacity to influence the surface chemistry of the adsorbent, ionization of adsorbate and impacts the mode of interaction (Batzias and Sidiras 2007; Gherbi et al. 2022). The surface charge of all adsorbents is known to be null, negative or positive. To validate the nature of charges on the adsorbent surface, point of zero charge (pHPZC) experiment was performed. pHPZC defines the pH for which the surface of the adsorbent will be zero. At pH values lower or higher than the pHPZC of WKM and WK, the surfaces of these adsorbents (WKM and WK) will be positively charged and negatively charged, respectively. As shown in Fig. 6, the pHPZC of WKM and WK were estimated as 5.08 and 6.68, respectively. The effect of pH on the uptake of RhB onto WKM and WK was assessed. The results revealed that the uptake of RhB by WKM and WK was favoured at lower pH values. The adsorbents were observed to be most effective at pH of 3 (see Fig. 6). As the pH increased from 4 to 10, a decrease in the capacity of WKM and WK to eliminate RhB from aqueous solution was observed (see Fig. 7). The high uptake of RhB by WKM and WK could be attributed to the monomeric nature of RhB at solution pH less than 3.7 (Zamouche and Hamdaoui 2012). This may enhance the pore entrapment of the cationic Rhodamine B (RhB+) into the pore of WKM and WK in addition to hydrophobic interactions (π–π stacking) of the dye molecules with the adsorbent surface.

Fig. 6
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pHPZC plots of WK and WKM

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Fig. 7
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Effect of initial solution pH on the uptake of RhB onto WKM and WK

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Kinetics of adsorption

Essential information (removal rate and mechanism of adsorption) needed to develop an adsorption process is partly deduced from the kinetic analysis (Haro et al. 2021). The effect of contact time on the adsorption of RhB onto WKM and WK was assessed using a fixed concentration of 100 mg dm−3 (25 cm3), agitation times ranging from 5 to 180 min and 0.05 g adsorbent dose at room temperature (see Fig. 8). An increase in the uptake capacity of WKM and WK with increased agitation time was observed in the adsorptive removal of RhB. This could be attributed to increase in site occupation by RhB with time. Meanwhile, about 80.32 mg and 57.71 mg of RhB were sequestered per gram of WKM and WK after 100 min of contact. Thereafter, no substantial removal of RhB was observed for WKM and WK. As a result, 100 min was chosen as the best adsorption time. However, to ensure a complete adsorptive process, 180 min was used for further experiments. The kinetic modelling for the adsorptive removal of RhB by WKM and WK was evaluated using the intraparticle diffusion (ID), pseudo-first-order (PFO), Elovich and pseudo-second-order (PSO) model (see Fig. 9). Table 5 shows the parameters acquired from the kinetic model plots. For the adsorption of RhB onto WKM, it is clear that the procedure was closely expressed by the Elovich kinetic model based on the least value for the sum of squared residuals (SSR) and the residual standard error (RSE) when compared to other models. Meanwhile, this validates the report of most researchers on the uptake of RhB onto other adsorbents (Wang et al. 2017). This suggests a chemisorptive adsorption process. For the adsorption of RhB onto WK, PFO was noticed to best describe the acquired experimental data. Conventional kinetics models such as PFO and PSO lack the basics to offer information on the mechanism of diffusion of RhB into the pores of the WKM and WK. Values of intercepts extrapolated from the plots of t1/2 versus qeq indicate the inclusion of another rate-controlling step in the uptake of RhB onto WKM and WK (see Table 5). Meanwhile, owing to the fact that the plot did not go through the starting point suggests that intraparticle diffusion is not the only rate-determining mechanism but comprises some degree of boundary layer diffusion. This report is in good agreement with the result obtained for the uptake of RhB onto activated carbon composite (Hayeeye et al. 2017) and rice husk ash (Suc and Kim Chi 2017).

Fig. 8
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Effect of agitation time on the uptake of RhB by WK and WKM

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Figure. 9
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Comparison of the various kinetics models fitted to the experimental data of RhB adsorption onto WK and WKM

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Table 5 Kinetics parameters for RhB adsorption onto WK and WKM
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Effect of dosage

The influence of adsorbent dosage was examined at pH 3 using 25 cm3 of 100 mg dm−3 RhB solution and varying the WKM or WK dosage from 0.01 to 0.1 g. As displayed in Fig. 10, it was observed that as the adsorbent dosage was increased from 0.01 to 0.1 g, the percentage removal of RhB also increases up to 97.17% and 71.47% for WKM and WK, respectively. This could be ascribed to the availability of a sufficient number of active sites. It was further observed that at 0.5 g, more than 75% of RhB was eliminated from the aqueous solution. Hence, 0.05 g was accepted as the working adsorbent dose. On the other hand, the uptake capacity of WKM and WK was noticed to reduce from 192.34 to 23.17 mg g−1 and 91.19 to 19.37 mg g−1, respectively. This phenomenon could be associated with the agglomeration of adsorbents that leads to binding site deprivation.

Figure. 10
figure 10

Effect of dosage on the uptake efficiency (%) and the uptake capacity (qeq) of RhB adsorption by WK and WKM

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Adsorption isotherm

To validate the nature and affinity of sorbent-sorbate interaction and the characteristics of the adsorbent (WK and WKM) surface, isotherm modelling of the adsorptive removal of RhB by WK and WKM becomes exigent. To establish the aforementioned adsorptive characteristics, the effect of initial RhB concentration experiment was evaluated and presented in Fig. 11. With the increase in the RhB concentration from 10 to 80 mg dm−3, we observed a continuous increase in the adsorption capacity of WK and WKM. Above 80 mg dm−3, equilibrium was noticed for the uptake of RhB onto WK at all temperatures investigated. A similar observation was made for WKM with the exception of higher solution temperatures (313 K and 318 K). Hence, 100 mg dm−3 was selected for further optimization process in order to guarantee maximum use of the active sites on WK and WKM. The increase in absorption capacity with concentration could be attributed to an increase in driving force caused by the greater concentration gradient, which improves RhB interaction with the active sites of WK and WKM. This result is in good agreement with the report on the adsorption of RhB onto carbon nanotube–cobalt ferrite nanocomposites (Oyetade et al. 2015) and gelatin/activated carbon composite beads (Hayeeye et al. 2017).

Fig. 11
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Effect of initial RhB concentration and solution temperature on the adsorption capacity of WK and WKM for RhB

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The experimental data from the initial RhB concentration experiment were modelled using the Langmuir and Freundlich isotherms, as stated in the supplemental document. The estimated isotherm parameters obtained from the models are presented in Table 6. Isotherm models that best describe the uptake of RhB onto WKM and WK were selected because they had the least SSR and RSE. As observed, for the adsorption of RhB onto WKM and WK, the Freundlich model was noticed to best describe the experimental data acquired for both adsorbents. As a result, multilayer adsorption of RhB onto heterogeneous WKM and WK surfaces is suggested. (Herbert Freundlich 1907). The deduction from the Freundlich model (heterogeneous uptake of RhB onto WKM and WK) was confirmed by FESEM images of the WKM and WK, which revealed an uneven surface morphology with sheet-like particles of diverse sizes and shapes. The affinity between the RhB and the novel adsorbent (WKM and WK) can be deduced from the Freundlich n value. Meanwhile, n values between 1 and 10 indicate a favourable adsorption process (Chen et al. 1999; Fo and Odebunmi 2010). Hence, the uptake of RhB onto WKM and WK was favourable. On the other hand, the maximum uptake capacity as deduced from Langmuir adsorption Isotherm was 67.58 mg g−1 and 67.82 mg g−1 for WKM and WK, respectively. These values showed the superior potential of WKM and WK when compared with the monolayer adsorption capacity of other adsorbents (see Table 8). Hence, WKM and WK have demonstrated the potential for environmental remediation of RhB from contaminated water.

Table 6 Isotherm parameters for RhB uptake onto WK and WKM
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Adsorption thermodynamics

We also computed thermodynamic parameters (i.e. standard free energy change (ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°)) to analyse critical information regarding the adsorption process's exothermic/endothermic nature, degree of unpredictability and feasibility (see Table 7). These parameters were estimated using equations S3 and S4 as described in the supplementary information. The thermodynamic analysis for the adsorption of RhB onto WKM and WK was noticed to be significant at elevated solution temperature (see Fig. 11). This could be attributed to the enhanced kinetic energy of the RhB molecules, leading to frequent interaction with the adsorption site that could result to an effective collision. It could also be due to the enlargement of sorbent pores resulting to increase RhB removal (Al-Rashed and Al-Gaid 2012). This result is consistent with the report on the adsorption of RhB onto kaolinite (Khan et al. 2012) and agricultural by-products (Gad and El-Sayed 2009). It is apparent that the uptake of RhB molecule onto WKM and WK was an endothermic process. Table 7 presents the thermodynamic parameters estimated for the adsorption of RhB onto WKM and WK. Negative ΔG° values were estimated at all temperatures for the adsorption of RhB onto WKM and WK. This indicates the potential of WKM and WK to decontaminate RhB-loaded wastewater. This is consistent with the report on the uptake of RhB onto kaolinite (Khan et al. 2012). Meanwhile, positive ΔS° values of 352.5 J K−1 mol−1 and 101.1 J K−1 mol−1 suggested a rise in randomness at the RhB-WKM and RhB-WK interfaces, respectively, during the adsorption process. It has been reported that ΔH° values higher than 40 kJ/mol and ΔH° values less than 40 kJ/mol indicate chemisorption and physisorption adsorptive process, respectively. Hence, the adsorptive removal of RhB by WKM and WK was dominated by chemisorption and physisorption, respectively. This further corroborates with the SEM results and the isotherm analysis obtained for WKM (see Table 6).

Table 7 Thermodynamic parameters for the uptake of RhB onto WK and WKM
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Desorption and reusability

In wastewater treatment practice, economically viable adsorbents are materials with sustainable uptake potential that possesses excellent regeneration and reuse characteristics (Dai et al. 2019; Dutta et al. 2019). Besides, the application of non-toxic and inexpensive eluting solvents in the regeneration process was observed. The sustainability of a high uptake potential after several cycles is also a vital criterion in assessing the value of any adsorbent. The desorption of RhB from the surface of the adsorbent has been investigated using different concentrations of HNO3, HCl and NaOH solutions (Suc and Kim Chi 2017). Hence, the choice of NaOH as eluent was selected. The preliminary desorption showed about 90% of RhB desorbed using 0.1 M NaOH, this also suggests an anion exchange mechanism during the adsorption and desorption processes. Figure 12 shows the reusability of the WKM and WK for RhB adsorption after five adsorption–desorption cycles. We observed a modest decrease in the adsorption efficiency of RhB from 85.60 to 80.73% and 54.78 to 41.43% for WKM and WK from the initial uptake to the fifth cycle of reuse, respectively. Hence, WKM has demonstrated a superior capacity for environmental radiation practices.

Fig. 12
figure 12

The RhB removal efficiency of WK and WKM after different cycles

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Mechanism of adsorption

In order to establish the mechanism responsible for the adsorption of RhB on WKM, the physicochemical properties of spent and pristine of WKM were assessed by making use of SEM and FTIR spectroscopic techniques. Besides the spectroscopic techniques, adsorption parameter, such as solution pH, was also considered for the elucidation of mechanism responsible for the uptake of RhB onto WKM. The influence of solution pH on the speciation of RhB gives insight into chemistry of RhB during uptake. RhB is in the zwitterionic state (pH > pKa with pKa = 3.7). The zwitterionic form of RhB in water may promote aggregation of RhB to create a bigger molecular form (dimer) and become impossible to enter the pore of WKM at pH values greater than 3.7. The favourable electrostatic interactions between the carboxyl and xanthene groups of the monomers cause the zwitterionic form to aggregate more. Hence, favourable removal of RhB in the lower pH can be attributed to pore entrapment. This could be responsible for the swelling characteristics observed on using SEM. Meanwhile, FTIR revealed shift in –OH and –C=C– peaks, this suggests the inclusion of intermolecular hydrogen bonding or Yoshida H-bonding and π–π interaction, respectively. This may be resposible for the superior capacity of WKM (see Table 8). Hence, a collection of reaction path was responsible for the adsorption of RhB onto WKM (see Fig. 13).

Figure. 13
figure 13

Mechanisms of interaction that could exist between WKM and RhB

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Table 8 Comparison of the Langmuir maximum adsorption capacities for RhB onto AMC, WK and WKM with those of other sorbents
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Conclusions

This study demonstrated the fabrication of nanocomposite using Buchholzia coriacea pod and MWCNTs. The adsorbents (WK and WKM) were examined as value-added adsorbents for RhB adsorption, and it was found that the uptake capacity of WK and WKM during RhB removal was higher than the potential of most adsorbents reported in the literature. Moreover, optimal conditions such as 100 min, solution pH 3.0 and adsorbent dose of 0.05 g were determined for the uptake of RhB onto WK and WKM. Meanwhile, maximum monolayer capacity (qmax) of 67.82 mg g−1 and 87.58 mg g−1 was determined for WK and WKM. The kinetics for the uptake of RhB onto WK and WKM was best described by pseudo-first-order and Elovich, respectively. On the other hand, the Freundlich isotherm models were noticed to describe the equilibrium data acquired for the uptake of RhB onto WK and WKM. The excellent performance of WK and WKM presents these materials as promising adsorbents for the sequestration of dye from textile wastewater.