Introduction

Environmental pollution refers to the contamination of the environment due to the introduction of pollutants that can cause harm and damage not only to the environment but also to human and other living organisms. It occurs because the rate of introduction of elements into the nature increases at a rate faster than natural processes can destroy them. Environmental pollution became significant since the industrial revolution in the nineteenth century and has become a major concern to the world today due to its negative effects on human health and environment and is increasing at a fast rate day by day.

One of the major environmental pollutions that cause significant damage to the environment and human health is water pollution. The main causes of water pollution are from industrial activities, agricultural activities and residences as they release harmful and toxic substances such as synthetic dyes and heavy metals into the waterbodies. Since water is a survival necessity to all living organisms including human, water pollution can directly affect our health or even endanger our life. In addition, water pollution also harms aquatic organisms which in turn affects the food chain and also creates water-borne diseases such as diarrhoea, cholera, typhoid fever and malaria.

Since waterbodies are the main reservoirs of water pollutants, wastewater treatment becomes important in addressing the issues of water pollution. Of the many wastewater treatments, adsorption has become the more popular and widely used method to remove pollutants such as dyes in wastewater due to its simplicity in design, low operating cost, high removal efficiency and flexibility of the adsorption method. In addition, adsorption method often does not produce harmful side-products. Adsorption refers to the enrichment or accumulation of a substance from a liquid phase onto the surface of a solid (Worch 2012). The solid onto which adsorption occurs on the surface is the adsorbent whilst the substance that adsorbs onto the surface of adsorbent is the adsorbate. Many researchers in this field have reported various low-cost adsorbents with high adsorption capability in the removal of dyes in wastewater, some of which are agricultural or industrial solid wastes (Mo et al. 2018; Hussain et al. 2018; Adeniyi and Ighalo 2019; Adeyemo et al. 2017; Bharathi and Ramesh 2013; Kooh et al. 2018).

Synthetic dyes are one of the major organic pollutants that affect waterbodies. They are chemicals that have colouring properties as they attach themselves to the surface of fabrics or other substances to impart colour (Yagub et al. 2014). Synthetic dyes are widely used in textile, food processing, paper and pulp, paint and cosmetics industries for colouring purposes. These industries then discharge the dye wastes into waterbodies which cause water pollution.

In this study, the inedible stems of Ipomoea aquatica (IA) was investigated as an adsorbate for the removal of a dye in aqueous solutions by adsorption method because being inedible, the stems are usually thrown away. IA, a semi-aquatic tropical plant that grows well on moist soil or in water, belongs to the family of Convolculaceae or morning glory and is commonly found in the tropical or subtropical regions around the world. It is a fast-growing and perennial or annual plant that grows in a creeping or sprawling manner. The stems are 2–3 m long and they usually spread over the ground, on surface of water or intertwine with other plants for support. IA is commonly known as water spinach in general, and as “kangkong” in Southeast Asian countries. It is a popular leafy vegetable where the young leaves and shoots are used as an ingredient in Southeast Asian dishes. In Brunei Darussalam, it is often stir-fry with spicy sambal belacan (fermented shrimp paste) sauce. Apart from being used in culinary dishes, IA is also an herbaceous plant where the leaf extracts are used to treat jaundice and nervous debility whilst the juice is used for nosebleed and high blood pressure (Prasad et al. 2008).

Methyl violet 2B (MV2B), an adsorbate investigated in this study whose structure, is shown in Fig. 1 and is also known as Basic violet 1. MV2B is a cationic dye and is soluble in water giving a deep violet colour, although the colour of MV2B in solid powder form is dark green. It is often used as a colouring dye for textiles, inks and paints. In addition, MV2B is an active ingredient present in Gram’s biological stain which is used for bacterial classification (Conn 1940). MV2B was reported to cause inhibition on the growth of bacteria and photosynthesis of aquatic plants (Kooh et al. 2016). Furthermore, MV2B is also a potential carcinogen and can cause severe irritations to the eye, skin and respiratory tracts in human (Vachalkova et al. 1996). Due to these harmful effects of MV2B, its removal from waterbodies is important. To date and to the best of our knowledge, there has only been one report on the use of IA as an adsorbent in the removal of Auramine O dye (Lu et al. 2020). Being fast-growing in nature, IA is readily available in abundance and therefore a potentially good low-cost adsorbent.

Fig. 1
figure 1

The effect of contact time on adsorption MV2B on IA at different dye concentrations [50 mg L−1 ( ), 100 mg L−1 ( ), 200 mg L−1 ( ) and 500 mg L−1 ( )]

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Hence, this study aims to evaluate IA as a potential low-cost adsorbent towards the removal of MV2B dye. Important parameters such as the time required for the IA-MV2B system to reach equilibrium, the effects of pH and presence of salts on the adsorption of dye will be studied. Investigation of the adsorption characteristics of IA will also be carried out and the data obtained from adsorption isotherm and kinetics will be fitted to various models in order to help provide insight into the adsorption process. Regeneration studies will also be performed in order to determine if it is feasible for IA to be regenerated and reused.

Materials and methods

Chemicals and reagents

MV2B (C24H28N3Cl, 393.959 g mol−1), sodium phosphate monobasic dihydrate (BioUltra grade, ≥99% purity), NaCl (BioXtra grade, ≥99.5% purity) and NaNO3 (ACS grade, ≥99% purity) were purchased from Sigma-Aldrich Chemical Company. HCl, KNO3 and NaOH were purchased from Merck. KCl (≥99.5% purity) was purchased from May & Baker Ltd. Distilled water was used in the preparation of all stock solutions and throughout all experiments. The stock solution of 1000 mg L−1 MV2B was prepared by dissolving accurate amount of MV2B in distilled water, and the other concentrations of MV2B were prepared by diluting the MV2B stock solution. All adjustments of pH were done using 0.1 M NaOH and 0.1 M HCl solutions. All chemicals and reagents were used as received and without any further purification.

Preparation of the adsorbent

IA was purchased from local vegetable markets and supermarkets. The edible parts were picked whilst the leftover inedible stems were used as the adsorbent in this investigation. The stems were first washed and cleaned thoroughly with running tap water numerous times to remove all soils and dirt and then washed with distilled water. After washing, the cleaned stems were spread out on paper towels and air-dried to evaporate off surface water which will quicken the drying process in the oven. The stems were then dried in the Memmert oven at 65 °C for 3 days until a constant mass was obtained. The dried stems were blended using the Panasonic MX-GM1001 dry mill and sieved through a 355-μm laboratory stainless steel test sieve to obtain powdered IA of particle size <355 μm. This was used as the adsorbent throughout all experiments in this investigation. The powdered IA was stored in an air-tight glass bottle to prevent oxidation and contamination.

Adsorption experiments

MV2B dye stock solution of 1000 mg L−1 was prepared, and the solution was sonicated to ensure proper dissolution of MV2B in the distilled water. In all the batch adsorption experiments, the ratio of adsorbent: dye was kept at 1:500 (mass:volume). All adsorbent-dye solution mixtures were shaken using the Stuart SSL1 orbital shakers set at 250 rpm at room temperature for the shaking time determined from the effect of contact time experiment, unless otherwise stated. After shaking, the solutions were filtered through a metal sieve into clean labelled glass vials. The absorbance of diluted filtrates throughout all experiments was measured using the Thermo Scientific Genesys 20 UV-Visible spectrophotometer at 584 nm which is the maximum absorbance wavelength of MV2B. All experiments were performed in duplicates.

The optimising parameters were carried out for contact time (30–180 min), medium pH (pH 2–10) and ionic strength (0–1.0 mol dm−3) using NaCl, NaNO3, KCl and KNO3. The desired medium pH was adjusted using 0.1 M NaOH and 0.1 M HCl, and measurement of pH was done using the EDT instruments GP353 ATC pH meter.

The batch adsorption isotherm experiments were carried out using MV2B dye concentrations ranging from 0 to 1000 mg L−1. Thermodynamics studies were investigated at 298–343 K and 50, 100, 200 and 500 mg L−1 MV2B were used for kinetics studies.

Regeneration of spent adsorbent

Regeneration study was carried out on IA following the method as outlined by Dahri et al. (2015), but using HCl instead of HNO3. A total of five consecutive cycles were carried out. The dye-loaded IA were treated using four different methods, namely 0.1 M HCl, 0.1 M NaOH, washing with distilled water and quick rinse with distilled water. An untreated spent adsorbent experiment was also carried out as control for comparison purposes.

Point of zero charge

The IA adsorbent (0.05 g) was weighed accurately into ten 150-mL Erlenmeyer flasks, and each flask was labelled properly. 0.1 M KNO3 solution (25.0 mL) at desired initial pH was added to each flask and was shaken for the optimum contact time. The desired initial pH were pH 2, 4, 6, 8 and 10 which were adjusted using 0.1 M NaOH and 0.1 M HCl. The solutions were shaken for 24 h at 250 rpm at room temperature using an orbital shaker. After shaking, the solutions were filtered through a metal sieve into clean labelled glass vials and the pH of each filtrate was measured using a pH meter. The change in pH was calculated and a graph of change in pH against initial pH was plotted to determine point of zero charge (pHpzc).

Scanning electron microscope analysis

A small amount of IA, before and after adsorption of MV2B, was collected and dried in the oven at 65 °C overnight and then analysed using the JSM-7610F field emission scanning electron microscope (SEM) to determine the surface morphology of the adsorbents.

Fourier transform infrared (FTIR) spectroscopy

A small amount of IA, before and after adsorption of MV2B, was collected and dried in the oven at 65 °C overnight and then analysed using the Agilent Cary 630 FTIR spectrometer in the range of 650–4000 cm−1 to determine the functional groups present in the adsorbent which may be responsible for the removal of MV2B.

Results and discussion

Effect of contact time

The effect of contact time is one of the essential optimising parameters to be determined in all adsorption studies as it establishes the optimum contact time for an adsorbent–adsorbate system to reach an equilibrium state. Since adsorption studies are often applied to wastewater treatment, the determination of contact time is also important in signifying the efficacy and efficiency of an adsorbent in the removal of dyes in wastewater treatment (Lim et al. 2017). In this investigation, the effect of contact time for IA to adsorb MV2B was determined using 50–500 mg L−1 dye solutions with shaking time of every 30-min intervals up to 3 h. The results of the effect of contact time are as shown in Fig. 1. As observed, a steep increase in the average percentage removal can be seen in the first 30 min with an average percentage removal of 69% for MV2B. Thereafter, the average percentage removal increased slightly and then slowed down to a plateau at 120 min. The rapid adsorption in the first 30 min is due to the vast availability of vacant active sites on the surface of the adsorbent where adsorption of MV2B occurs (Lim et al. 2015). However, as time progresses, more active sites on the adsorbents became preoccupied with MV2B and the number of vacant active sites became limited. Eventually an observed plateau signifying a saturation point has been reached and the adsorbent–adsorbate system is in an equilibrium state.

Effect of medium pH

In adsorption studies, the effect of medium pH is also one of the important controlling parameters as different medium pH can affect the adsorption of adsorbate on adsorbent by affecting the electrostatic attraction between adsorbate and adsorbent. This is due to and largely depends on the cationic or anionic structure of the adsorbate as well as the surface charge of the adsorbent at that particular medium pH. At the same time, as adsorption is a popular method in wastewater treatment, it is important to determine the effect of different medium pH on the adsorption capacity of an adsorbent as wastewater often contains substances or pollutants that may change the pH of the waterbodies.

The effect of medium pH on the adsorption of 100 mg L−1 MV2B on IA in this investigation was limited to within a range of pH 2–10. This is because a report has shown that huge reductions in the colour intensity of MV2B occurred at extreme pH of the medium prior to addition of adsorbent (Kooh et al. 2016). The results of the effect of medium pH are presented in Fig. 2. At pH 2, the average percentage removal of MV2B by IA was 31%, the lowest among the other pH values of the medium. This is because at low acidic pH, large amounts of H+ ions are present in the medium which compete with the cationic MV2B for the vacant active sites on the surface of the adsorbent (Musyoka et al. 2014). This results in a decrease in the adsorption capacity of the adsorbent. Figure 2 also shows that the average percentage removal of MV2B by IA in medium pH > 4 remained more or less similar at 65–75%. This may be due to a lower amount of H+ ions present in the medium which decreases the competition of cationic MV2B with H+ for the active sites. Furthermore, basic condition of the medium can also lead to deprotonation of the functional groups on the adsorbent’s surface hence enhancing the electronic attraction between the cationic MV2B and the now anionic surface of the adsorbent. This further increases the adsorption capacity of the adsorbents. As the average percentage removal of MV2B by IA in medium of pH > 4 remained similar with no significant differences compared to the untreated, ambient medium pH of 4.7, the medium pH for all other experiments in this investigation was kept without adjustment of pH for the simplicity and efficiency of the adsorption process.

Fig. 2
figure 2

The effect of medium pH on the adsorption of 100 mg L−1 MV2B on IA

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Effect of ionic strength

The determination of the effect of ionic strength in adsorption studies is another important parameter as wastewater from industries and other sources often contain numerous salts which can influence an adsorbent’s adsorption efficiency of dyes in wastewater treatment. Results have shown that the interactions between adsorbent and dye adsorbate were due to electrostatic and hydrophobic interactions (Hu et al. 2013). As discussed previously, a change in medium pH affects the electrostatic interactions between adsorbent and adsorbate. At the same time, a change in the ionic strength of medium can also affect the electrostatic and hydrophobic interactions between adsorbent and adsorbate. An increase in the ionic strength of medium reduces the electrostatic interaction whilst increases the hydrophobic interaction (Dahri et al. 2016).

The effects of ionic strength on the adsorption of 100 mg L−1 MV2B on IA using different concentrations of NaCl, NaNO3, KCl and KNO3 ranging from 0.01 to 1.0 M were determined. The highest percentage removal of MV2B by IA (76%), as shown in Fig. 3, was in the absence of any salts whereby as the concentration of each salt increases, the percentage removal of MV2B by IA generally decreases. The average percentage removal was 60%, 23%, 51% and 46% for 1.0 M NaCl, NaNO3, KCl and KNO3, respectively. The reduction in dye removal as the concentration of salts increases (hence ionic strength increases) was due to the presence of positively charged ions such as Na+ and K+, which compete with cationic MV2B for the vacant active sites on the surface of adsorbents. In addition, an increase in the ionic strength of the medium causes buildup of cations on the adsorbent’s surface which leads to electronic repulsion between the adsorbents and cationic MV2B (Dahri et al. 2015). As a result, the adsorption capacity of the adsorbents decreased. Since the adsorption capacity of IA for removal of MV2B was the highest when no salts were added to the medium, all the other experiments in this investigation were carried out with no addition of any salts.

Fig. 3
figure 3

The effect of ionic strength on the adsorption of 100 mg L−1 MV2B on IA using NaCl, NaNO3, KCl and KNO3 in a concentration range of 0.01–1.0 mg L−1

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Adsorption isotherm studies

Adsorption isotherm can be considered the most significant and vital experiment in any adsorption studies as it not only describes the interaction between an adsorbent and adsorbate in an adsorption process but it also provides information regarding the adsorption capacity of an adsorbent for an adsorbate. In this study, six adsorption isotherm models namely Langmuir (Langmuir 1918), Freundlich (Freundlich 1906), Temkin (Temkin and Pyzhev 1940), Dubinin-Radushkevich (D-R) (Dubinin 1947), Redlich-Peterson (R-P) (Redlich and Peterson 1959) and Sips (Sips 1948) were used to describe the adsorption process of MV2B onto IA. Their isotherm equations in nonlinear and linear forms are listed in Table 1.

Table 1 The nonlinear and linear forms of the six isotherm models
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The Langmuir isotherm model assumes that the maximum adsorption of an adsorbate occurs homogeneously on the surface of an adsorbent resulting in the formation of a saturated monolayer. On the other hand, the Freundlich isotherm model assumes a multilayer adsorption in which the adsorbate can still adsorb on a saturated surface of the adsorbent and that the adsorption occurs heterogeneously on the surface of the adsorbent (Li et al. 2010). The Temkin isotherm model describes the linear decrease in adsorption heat with increasing coverage of the adsorbent’s surface as a result of the adsorbent–adsorbate interactions (Li et al. 2010). The D-R isotherm model postulates that the adsorption follows a pore-filling mechanism rather than a layer-by-layer surface coverage mechanism (Hutson and Yang 1997), and this model can generally be applied to adsorption that occurs homogeneously or heterogeneously on the surface of the adsorbent (Chen 2015). The R-P isotherm model is a combination of Langmuir and Freundlich isotherm models which can be applied to homogeneous or heterogeneous systems due to its flexibility. In addition, the R-P isotherm model follows the Freundlich isotherm model at high adsorbate concentration whilst at low adsorbate concentration, it is in accordance with the Langmuir isotherm model (Foo and Hameed 2010). Similar to R-P, the Sips isotherm model is also a combination of Langmuir and Freundlich isotherm models and is used to predict heterogeneous systems. At low adsorbate concentration, the Sips isotherm model assumes the Freundlich isotherm model indicating a multilayer adsorption whilst at high adsorbate concentrate, it assumes the Langmuir isotherm model indicating a monolayer adsorption (Ayawei et al. 2017).

Herein, the best-fit isotherm model for the adsorption of MV2B onto IA is determined based on the highest linear regression coefficient (R2), lowest error functions and fitting of nonlinear simulated isotherm plots to the experimental isotherm curve. The six error functions in the error analysis of the isotherm models are listed in Table 2.

Table 2 Types of error functions and their expressions
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In this investigation, adsorption isotherm studies on IA were carried out using a range of MV2B concentrations from 10 to 1000 mg L−1 under optimised conditions. Using the linear plots from the isotherm equations as shown in Table 1, the isotherm parameters of IA are calculated and presented in Table 3. The linear regression coefficient (R2) in decreasing order is as follows: Sips > Temkin > D-R > Freundlich > Langmuir > R-P. The invalidity of R-P model in describing the adsorption of MV2B by IA can be further confirmed by its simulation plot, as shown in Fig. 4. Although the D-R model has good R2 yet its overall error values are the largest and this is further confirmed by the deviation of the D-R simulated isotherm plot in comparison with the respective experimental isotherm curve.

Table 3 The linear regression coefficients (R2) and isotherm parameters of Langmuir, Freundlich, Temkin, D-R, R-P and Sips for adsorption of MV2B onto IA
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Fig. 4
figure 4

Simulated isotherm plots of IA in comparison with experimental isotherm curve for adsorption of MV2B

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Of the four models left, the Langmuir and Temkin models gave the lowest R2 and highest error values, respectively, whilst even though the Freundlich model has good R2, but when compared with the Sips model, the latter has the highest R2 with the lowest errors. Hence, the Sips isotherm model is chosen as the best fit isotherm model for the adsorption of MV2B on IA, assuming multilayer adsorption onto a heterogeneous surface of IA at low MV2B concentration and monolayer adsorption at high MV2B concentration.

According to the chosen best fit Sips isotherm model, the qmax of IA is 267.9 mg g−1, whilst based on the monolayer Langmuir isotherm model is 371 mg g−1. This qmax value is considered high compared to other reported adsorbents as shown in Table 4. IA can also serve as a good adsorbent as its production is low cost since it involves oven drying only. Further, its qmax value is high when compared to many adsorbents such as orange and banana peel with qmax values of 11.5 and 12.2 mg g−1, respectively.

Table 4 Comparison table for the adsorption capacity (qmax) of different adsorbents for the removal of MV2B
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Thermodynamics studies

Apart from room temperature, batch adsorption isotherm studies on the removal of MV2B by IA were also carried out in different temperatures, as shown in Fig. 5a. From the van Hoff’s plot in Fig. 5b, the thermodynamics parameters are calculated and presented in Table 5. The negative Gibbs free energy values (ΔG°) suggests that the adsorption process is favourable, whilst the negative enthalpy (ΔH°) suggests that the adsorption process is exothermic in nature. The entropy (ΔS°) is positive which suggests an increase in randomness in the system which may be due to “solvent-replacement” phenomenon, where the water molecules on the surface of the adsorbent needs to be replaced by the MV2B molecules in order to be adsorbed (Ruixia et al. 2004).

Fig. 5
figure 5

(a) Thermodynamics data of IA at 298–343 K and (b) van Hoff’s plot

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Table 5 Thermodynamics parameters for the adsorption of MV2B onto IA
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Kinetics studies

The study of kinetics in adsorption process is very useful in that it determines the rate of reaction of the system and provides valuable information on the adsorption mechanism (Zehra et al. 2016). In this investigation, two kinetics models were used to describe the adsorbent–adsorbate interactions and to determine the adsorption mechanism and they are the Lagergren’s pseudo-first-order kinetics model (Lagergren 1898) and Ho’s pseudo-second-order kinetics model (McKay and Ho 1999).

The nonlinear and linear equations of the kinetics models are listed in Table 6. The determination of the best-fit kinetics model is based on the calculated linear regression coefficient (R2) using linearised kinetics equations, error functions and the closeness in value between calculated qe from kinetics model and experimental qe from isotherm. Using the linearised kinetics equations, the parameters and error functions of the kinetics models for adsorption of MV2B on IA are calculated and presented in Table 7.

Table 6 Nonlinear and linear equations of pseudo-first-order kinetic and pseudo-second-order kinetic models
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Table 7 Parameters and error functions of pseudo-first-order kinetics model and pseudo-second-order kinetics models for adsorption of MV2B on IA
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According to Table 7, the R2 values for IA are the highest and closest to 1 in pseudo-second-order kinetics model for all the concentrations investigated whilst the pseudo-first-order kinetics model has lower R2 values. Furthermore, the errors from the six error functions (ARE, SSE, HYBRID, EABS, MPSD and χ2) for IA are the smallest in pseudo-second-order kinetics model. Comparison of the simulation plots of experiment data with the two models in Fig. 6 also shows that the Lagergren pseudo-first-order is not the right model. Hence, based on R2 and error values as well as comparison of the plots, it can be concluded that the best-fit kinetics model for the adsorption of MV2B onto IA is the pseudo-second-order kinetic model. This is further confirmed by the closeness in values between qe,calc from pseudo-second-order kinetics model and qe,expt from isotherm as tabulated in Table 7.

Fig. 6
figure 6

Comparison of experiment data (●) with the Lagergren pseudo-first-order kinetics ( ) and pseudo-second-order ( ) using (a) 50 mg L−1, (b) 100 mg L−1, (c) 200 mg L−1 and (d) 500 mg L−1 MV2B

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Regeneration studies

The purpose of regeneration in adsorption studies is to investigate the reusability and capability of a spent adsorbent to adsorb more adsorbate. Regeneration experiment is important and useful in wastewater treatment as an adsorbent with good regeneration capability can help reduce the cost (Dahri et al. 2015). In this investigation, the spent adsorbent (IA) was treated in four ways, namely acid (0.1 M HCl), base (0.1 M NaOH), distilled water and quick rinse with distilled water. A control was also prepared for comparison with the four treatment methods. The regeneration experiment proceeded through five consecutive cycles, and the results are presented in Fig. 7. Washing the spent IA with base resulted in the highest percentage removal of 63.7% at Cycle 5 followed by the washing with acid (56.8%). The high percentage removal of spent IA when it is treated with acid and base can be attributed to the acid and base that improved the roughness of the adsorbent’s surface and exposed the reactive functional groups on the binding-sites of the adsorbent (Dahri et al. 2015), as NaOH was known to remove natural fats, waxes and low molecular weight lignin (Kooh et al. 2015). On the other hand, the washing of spent IA with distilled water, quick rinse with distilled water as well as the control experiment gave lower percentage removal of 9.0%, 7.3% and 3.43%, respectively, at Cycle 5. However, it must be emphasised that the MV2B concentration used in this study is high (600 mg L−1) when compared to other reported studies. Therefore, IA has good regeneration capability, especially when it is treated with acid or base, and hence has great advantage as an adsorbent in wastewater treatment.

Fig. 7
figure 7

Regeneration of IA through six consecutive cycles of adsorption using 600 mg L−1 MV2B and desorption using acid (0.1 M HCl), base (0.1 M NaOH), distilled water, quick rinse with water and control

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Characterisation of adsorbent

Point of zero charge

The pHpzc of an adsorbent is the pH at which the surface of the adsorbent has a net zero charge. The determination of pHpzc of adsorbents in adsorption studies is an important characterisation as it provides information on how a change in medium pH can affect the adsorption of adsorbate on adsorbent. The value of pHpzc of an adsorbent can determine from the x-intercept of a plot of change in pH (∆pH) against initial pH of the medium. This is the intercept of the plot where ∆pH is zero. The results from the determination of pHpzc of IA are as shown in Fig. 8 and accordingly, the pHpzc of IA was found to be at pH 6.81. This explains why IA showed a drastic reduction in removal of MV2B at pH 2 (Fig. 2). In a medium where pH < pHpzc, functional groups such as carboxyl, hydroxyl and amine present on the surface of adsorbents are protonated due to the availability of large numbers of H+ ions (Kooh et al. 2016). This causes the surface of the adsorbents to become predominantly positively charged which leads to electronic repulsion between the cationic MV2B and the now positively charged adsorbent (Lim et al. 2015).

Fig. 8
figure 8

A linear plot of ∆pH against initial pH of the medium for the determination of pHpzc of IA

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SEM analysis

A SEM works by scanning the surface of a solid sample using a focused electron beam to produce a magnified image showing the surface morphology of the sample. The surface morphology of IA, before and after adsorption of MV2B, was analysed using SEM at 500× magnification (Fig. 9). Prior to adsorption of MV2B, the surface of IA appeared to consist of curly fibrous-like strands, rough with many irregular-shaped pores and cavities.

Fig. 9
figure 9

Scanning electron micrographs of IA before (left) and after (right) adsorption of MV2B at 500× magnification

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Functional group analysis

FTIR spectroscopy was used to determine the functional groups present in IA which may be responsible for the adsorption process. In this investigation, the FTIR spectroscopy analyses of IA, before and after the adsorption of MV2B, are shown in Fig. 10, and the shifts in the bands were recorded. These shifts could be the functional groups that may be responsible for the adsorption of MV2B on the surface of the adsorbent (Table 8).

Fig. 10
figure 10

FTIR spectra of IA before (blue) and after (purple) adsorption of MV2B

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Table 8 Shifts in FTIR spectral bands and their responsible functional groups due to adsorption of MV2B on IA
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Conclusion

In this investigation, IA was successfully used as an adsorbent for the removal of MV2B. The adsorption isotherm of IA was found to be best-fitted to Sips model, giving qmax of 267.9 mg g−1, a value higher when compared to many other reported adsorbents. In kinetics studies, the adsorption process obeyed the pseudo-second-order kinetics. Furthermore, IA was investigated for its regeneration ability and was found to be able to maintain high adsorption capacity when treated with acid (HCl) and base (NaOH) even after 5 cycles. Due to the abundance and fast-growing nature of IA with high adsorption capacity and regeneration capability, IA is deemed as a potential low-cost adsorbent in the removal of MV2B in aqueous solutions such as wastewater to alleviate the issues of water pollution.