Introduction

Although two-thirds of the Earth form the hydrosphere, the availability of fresh and quality water decreases as urbanization is globally encouraged. The essentiality of quality water to all living organisms cannot be over-emphasized (Namasivayam and Sangeetha 2006; Parab et al. 2009; Gupta et al. 2010; Khan et al. 2011; Wang and Chu 2011; Jain et al. 2015; Rani et al. 2017). Since water is a universal solvent, it readily dissolves any water-soluble substance either solutes or pollutants; therefore, the quality of water becomes altered or contaminated (Namasivayam et al. 2007; Sureshkumar and Namasivayam 2008; Hayeeye et al. 2014; Patel 2018; Sydorchuk et al. 2019). Solubility property of dyes makes them one of the common water contaminants (Yadav et al. 2013; Bello et al. 2015; Goyal et al. 2015; Ojedokun and Bello 2017). Colorful materials are eye catchers, so manufacturing industries employ more of commercially available synthetic dyes that are toxic than non-toxic natural dyes to color their products. These dyes are used widely in paints, leather, plastics, paper and textile industries (Thirumalisamy and Subbian 2010; Bello et al. 2015). The stability of the ecosystem is affected due to effluents discharged from different industries gaining access into water bodies (Hameed et al. 2007; Tan et al. 2008). Furthermore, majority of these dyes pose allergic reactions, dermatitis and skin irritations which in addition lead to genetic mutations and cancer in humans as a result of their toxic nature (Adegoke and Bello 2015; Bello et al. 2015, 2017a; Ahmad et al. 2016; Adeyemo et al. 2017); industrial effluents or domestic sewage with the small quantity of dye concentration have severe effects on aquatic organisms owing to its toxicity and ability to inhibit penetration of light (Adegoke and Bello 2015).

Rhodamine-B (Rh-B) dye is an amphoteric dye; though often listed as a basic dye, to the class of xanthene dye, it is noted to be harmful when swallowed, with acute oral toxicity (Namasivayam and Kanchana 1992; Wilhelm and Stephan 2007; Hema and Arivoli 2007; Vasu 2008; Parab et al. 2009; Li et al. 2010; Gupta et al. 2012; Ashkarran et al. 2013; Gong et al. 2013; Suc and Kim Chi 2017; Cheng et al. 2017; Singh et al. 2017; Adegoke et al. 2019). Rh-B dye causes serious eye damage or irritation, hazardous to aquatic environment with long-term effects (Hema and Arivoli 2007; Sureshkumar and Namasivayam 2008; Sadasivam et al. 2010; Gupta et al. 2012; Inyinbor et al. 2015; Dahri et al. 2016; Dharmendirakumar et al. 2016; Fu et al. 2016; Inyinbor et al. 2016; Kooh et al. 2016; Goswami and Phukan 2017; Iqbal et al. 2017; Adegoke et al. 2019). Consequently, water treatment is one of the global campaign exercises, which demands scientific investigation. Several techniques adopted in wastewater treatment are: oxidative techniques, precipitation, reverse osmosis, ion exchange, ozonation, ultrafiltration, flocculation, coagulation, etc. (Namasivayam and Sangeetha 2006; Parab et al. 2009; Gupta et al. 2010; Khan et al. 2011; Wang and Chu 2011; Adegoke and Bello 2015; Jain et al. 2015; Rani et al. 2017). Among different conventional methods used for water treatment, adsorption process by activated carbon (AC) remains one of the sustainable approach to removing various pollutants (Deschaux et al. 2011; Adegoke and Bello 2015; Bello et al. 2015). This method is not complex for an average skilled technician to master and also requires only limited resources; hence, industries can adopt this noble method (Kooh et al. 2016). Commercial activated carbon (CAC), used conventionally for adsorption processes and other varieties of application, is scare, expensive and non-renewable (Namasivayam and Sangeetha 2006; Parab et al. 2009; Gupta et al. 2010; Khan et al. 2011; Wang and Chu 2011; Jain et al. 2015; Rani et al. 2017). The need for suitable replacements opens an opportunity for the effective use of agricultural wastes as adsorbent, and some of these largely available and inexpensive adsorbents have been reported for dye removal including cocoa pod husk (Olakunle et al. 2018) Moringa oleifera seed pod (Bello et al. 2017b), scrap tires (Li et al. 2010) Raphia hookerie fruit epicarp (Inyinbor et al. 2016), rambutan seed (Ahmad et al. 2016), Durian seed (Ahmad et al. 2015), sugarcane bagasse (Saad et al. 2010), bengal gram seed husk (Somasekhara Reddy et al. 2017), walnut shell (Ojo et al. 2019) among others.

Coconut husk (Cocos nucifera), an agricultural waste product from the coconut tree, is widely grown worldwide, for consumption, beautification and/or for erosion control. The husk is the large covering part of the fruit at the point of harvest. After consuming the white edible part of the fruit, the outer cover is thrown away, constituting a serious nuisance to the environment. However, to salvage the environment from the resulting mess, a non-conventional adsorbent is made from these coconut husks, thus converting these wastes into useful adsorbents. Activated carbon is extensively used adsorbent in many industrial processes because it composes of microporous and mesoporous structures and high surface areas (Mittal et al. 2010; Jawad et al. 2016; Rashid et al. 2018). Currently, research into finding sustainable alternative to replace CAC has been given more attention (AlOthman et al. 2014; Adegoke and Bello 2015; Bello et al. 2017a). Exploring this sustainable and eco-friendly adsorbent offers numerous usages for future industrial scale-up applications (Bello et al. 2017a; Ojedokun and Bello 2017). The costs of ACs derived from biomaterials and agricultural wastes are realistically lower in comparison with CACs (Ahmad et al. 2016; Adegoke et al. 2017; Bello et al. 2017a, c). In this study and for the first time, coconut husk was modified using orthophosphoric acid and its capability to remove rhodamine-B dye from simulated water was tested. Adsorption kinetic and batch equilibrium studies were employed to investigate the kinetics, isotherms,and kinetic data of the adsorption process. Adsorption mechanisms and thermodynamic parameters governing the sorption of Rh-B dye onto the modified coconut husk were studied.

Materials and method

Adsorbent pre-treatment and activation

Coconut husks were obtained in Ogbomoso, Oyo State, Nigeria, and then washed thoroughly with clean tap water. To remove the suspended impurities, the husks were further washed with distilled water and double-distilled, respectively. The husk’s fiber dust and cuticle were mashed and sieved using a different mesh-sized sieve. Particle size of 120 μm was selected for characterization prior experimentation. Twenty-five grams of the powdered sample was weighed, activated with orthophosphoric acid and then heated in 500 mL of 0.3 M H3PO4 until paste was formed. The resulting paste was transferred into an evaporating dish and then allowed to cool. The cooled paste was carbonized in a furnace at 350 °C for 90 min to establish the reaction between the carbon and the activating agent in breakdown of the lignocellulosic materials at this temperature. After cooling, the resulting ACs were then washed with distilled water to obtain a pH 6.8. Activated coconut husk (CHA) was then dried at a temperature of 105 °C for the purpose of removing the moisture content. The CHA sample is kept for further use in an airtight container.

Adsorbate preparation

1000 mg L−1 of stock solution containing rhodamine-B (Rh-B) dye was prepared by dissolving an accurately weighed 1 g of analytically grade Rh-B in 1000 cm3 of double distilled water. Other concentrations for batch equilibrium studies were prepared from the stock by serial dilution method.

Characterization of adsorbent

Fourier transform infrared (FTIR)

The FTIR spectra of both raw (CHR) and acid activated coconut husk (CHA) were analyzed using FTIR-2000 with KBr disk technique (Shimadzu Model IRPrestige-21 Spectrophotometer). The spectroscopic analyses enable the study of the surface chemistry of CHR and CHA powder. The FTIR spectra revealed the detail about the characteristics functional group(s) on the surfaces of both raw (CHR) and activated (CHA).

Scanning electron micrograph (SEM)

This is a versatile imaging technique based on electron–material interaction, capable of producing images of the sample surface. The principle is based on the fact that an electron beam bombards the surface of the sample to be analyzed which thereby re-emits certain particles; the electrons then interact with atoms in the sample, thus providing quantitative and qualitative information pertaining to particle morphology and surface appearance of samples. Various detectable signals contain specific information concerning the samples’ surfaces topology and compositions which are analyzed by a range of detectors to give three-dimensional image(s). This technique was employed to study the surface characteristics and the morphological feature of the adsorbent materials for both the raw and the activated samples.

Energy-dispersive X-ray (EDX)

The EDX analysis was carried out on both the raw and activated coconut samples, to determine the component elements before and after acid activation. Each elemental analysis line(s) spectra correspond to specific element composition. The intensities of the characteristics’ line are proportional to the element concentration; these analyses are quantitative in nature.

Oxygen-containing functional group(s) determination

Functional groups containing oxygen were determined using the Boehm titration analysis method (Boehm 2002; Ekpete and Horsfall 2011). Four portions of 1.0 g each of the raw and activated samples were kept in contact with 10–15 mL separate solution of 0.1 M NaHCO3, 0.05 M Na2CO3 and 0.1 M NaOH for an acidic group composite and 0.1 M HCl for a basic group composite, respectively, at an ordinary temperature for 48 h. Afterward, the resulted solutions were back-titrated with 0.1 M HCl for acidic and 0.1 M NaOH for basic groups. The numbers and types of acidic sites were calculated using our previous procedure (Bello et al. 2017b, c).

pH and point of zero charge pH (pHpzc) determination

To determine pHpzc of the adsorbent, 0.05 g of activated coconut husk (CHA) was added to the 100 mL solution of 0.1 M NaCl of a known initial pH; the pH was adjusted with NaOH or HCl. The sample holder was corked and placed in a shaker, made to be agitated at 250 rpm for 24 h. The final pH was then measured. In order to determine the pHpzc, a graph of pH difference, ΔpH (final pH − initial pH) was plotted against the initial pH. The pHpzc exists when pH does not change upon a contact with the adsorbent(s).

Batch adsorption experiments

Rhodamine-B dye removal was investigated using the batch technique at various temperatures (303 K, 313 K and 323 K). The effects of operational parameters such as initial dye concentration, contact time, adsorbent dose and solution temperatures were studied. The adsorbent dosage used throughout the adsorption process was 0.1 g of CHA. Adsorption process was studied at five initial dye concentrations: 200, 400, 600, 800 and 1000 mg L−1, respectively. The process was carried out in a water bath shaker and allowed to proceed to equilibrium at 120 min. Five sets of 100-mL Erlenmeyer flasks containing the mixture of 0.1 g of the sample and the Rh-B dye solution of different initial dye concentrations were carefully arranged in the shaker and then agitated at 120 rpm. The shaker used is a thermostatic water bath shaker filled with water to the level of the arranged flask’s solution, so as to maintain a uniform temperature to that of the shaker at specified temperatures until equilibrium was reached. Withdrawals of sample solutions were done at predetermined time intervals for the determination of residual concentrations using a UV–Vis spectrophotometer at the maximum wavelength of 554 nm. Amount of Rh-B dye uptake and Rh-B dye percentage removal at equilibrium were calculated using Eqs. 1 and 2, respectively:

$$ q_{\text{e}} = \frac{{\left( {C_{\text{o}} {-} C_{\text{e}} } \right)V }}{m } $$
(1)
$$ \% \;{\text{removal }} = \left[ {\frac{{\left( {C_{\text{o}} {-} C_{\text{e}} } \right)}}{{C_{\text{o}} }}} \right] \times 100\% $$
(2)

where “Co and Ce are respective initial dye concentration and equilibrium concentration (mg L−1), V is the volume of solution (mL), m is the mass of adsorbent (g), Qe is the amount of dye adsorbed (mg g−1)”.

However, the interaction between the adsorbate and the adsorbent was analyzed via four isotherm models: Freundlich, Langmuir, Dubinin–Radushkevich (D–R) and Temkin models. Adsorption kinetic study offers useful information on the pathways and reaction mechanisms of the reaction as it relates rate of the adsorption with the adsorbate concentration in the solution. Kinetic of adsorption of Rh-B dye onto CHA was studied via pseudo-first-order (PFO), pseudo-second-order (PSO), Elovich and intraparticle diffusion (IPD) models. Isotherm and kinetics parameters for adsorption of Rh-B dye onto CHA are presented in Table 1.

Table 1 Adsorption Isotherm and Kinetic Equations
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Kinetic model fitness test

In addition to the correlation regression (R2) value common to all the kinetic models, the model fitness or applicability can be tested by using the sum of error squares (SSE, %). Adsorption rate of Rh-B dye molecule onto CHA was determined at various initial dye concentrations. However, all kinetic models employed for the kinetic studies of the adsorption processes were verified by SSE (%) calculated using Eq. 14:

$$ {\text{SSE }}\left( \% \right) = \sqrt {\sum \frac{{(q_{{{\text{e}},{ \exp }}} - q_{{{\text{e}},{\text{calc}}}} )^{2} }}{N}} $$
(14)

where “N is the number of data points, qe,exp and qe,calc are the amount of the adsorbed Rh-B-dye, obtained experimentally and by calculation (mg g−1). The lower the value of SSE (%), the higher R2 value, the better the kinetic model fitted” (Bello et al. 2017b).

Adsorption thermodynamic studies

The changes in Gibbs free energy (ΔGo), enthalpy (ΔHo) and entropy (ΔSo) are the thermodynamic parameters used as actual indicators for practical applications in this study (Eqs. 15–17). These were used to evaluate the thermodynamics of adsorption at different temperatures under study (303, 313 and 323 K):

$$ \Delta G^\circ = - {\text{RT}}\;{ \ln }\;K_{\text{L}} $$
(15)
$$ { \ln }\;K_{\text{L}} = \frac{\Delta S^\circ }{R} - \frac{\Delta H^\circ }{\text{RT}} $$
(16)

The values of ΔS and ΔH were obtained from the intercept and slope of van’t Hoff plot of ln KL against 1/T. The values of KL (Langmuir constant in L mol−1) are calculated from the relation lnqe/Ce at different solution temperatures (303 K, 313 K and 323 K), respectively. Arrhenius equation was employed to calculate the adsorption energy of activation which represents the minimum energy needed by the reactants for reaction to occur (Eq. 17):

$$ { \ln }\;K_{2} = { \ln }\;A - \frac{{E_{\text{a}} }}{\text{RT}} $$
(17)

where K2 is the PSO rate constant (g (mg h)−1), Ea is the Arrhenius energy of activation of Rh-B dye adsorption, (kJ mol−1) with A being the Arrhenius factor, R. The plot of ln K2 versus 1/T gives a straight line graph with the slope of −Ea/R.

Results and discussion

Characterization of activated carbon prepared

The FTIR Spectral analysis

Figure 1 shows the FTIR spectra of raw coconut husk (RCH) (Fig. 1a) and activated coconut husk (CHA) (Fig. 1b). The comparable features of absorption bands for each FTIR spectrum are presented in Table 2. The spectra of both samples revealed the existence of different functional groups with either a disappearance, reduction or broadening of the peaks after the process of acid activation (Fig. 1a). The stretched band width observed at 3433.29 cm−1 was assigned to O–H stretching vibration of hydroxyl groups such as hydrogen bonding. The aliphatic C–H stretch was observed for the band seen at 2926.01 cm−1. Some major detectable peaks at bandwidths 2376.30 cm−1, 1705–1739.79 cm−1 and 1624.06–1618.28 cm−1 were assigned to alkyne group (i.e., C≡C stretching), carboxylic C=O stretching vibrations of lactone, ketone and carboxylic anhydride and C=C aromatic rings, respectively. The band observed at 1377.17 cm−1 was attributed to C–H stretching alkane or alkyl groups. The disappearance of phenol and ether in CHA samples showed a thermal instability of functional groups (Deschaux et al. 2011; Ahmad et al. 2015).

Fig. 1
figure 1

FTIR spectra of a raw coconut husk (RCH) and b activated coconut husk (CHA)

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Table 2 FTIR spectrum band assignment for CHR and CHA
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The scanning electron micrograph (SEM)

The SEM images of CHR and CHA are shown in Fig. 2a, b. It shows clearly that the pores surfaces of CHR (Fig. 2a) were not well developed; this surface morphology hinders the internal penetration of dye molecules, whereas in Fig. 3b, there are formations of several well-developed pores on the CHA (Fig. 2b), owing to the effects of activating agent at high temperature which broke down the lignocellulosic materials followed by volatilization of volatile compound(s) (Bello et al. 2012, 2017c). This demonstrated that H3PO4 activation leads to creation of well-developed pores on the precursor surfaces, thereby leading to AC with large porous surface areas and structures. The development of pores coupled with an enhanced surface area is requisite properties of an effective adsorbent (Ahmad et al. 2016).

Fig. 2
figure 2

SEM images of a CHR (Mag × 500) and b CHA (Mag × 500)

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Fig. 3
figure 3

EDX spectrum of a CHR and b CHA

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Energy-dispersive X-ray

Elemental analysis of both CHR and CHA was investigated by EDX to quantitatively determine the element(s) presents in both samples. Table 3 shows the differences between the amounts of carbon and oxygen in both samples. Similarly, Fig. 3a, b shows the EDX spectra of raw and activated samples, respectively. It was observed that CHA has a higher percentage by atom of carbon (87.13%) and lower percentage by atom of oxygen (4.99%) when compared with those obtained from CHR. This shows that the sample is predominantly carbonaceous in composition. We have previously reported that only the samples that are richer in carbon and lower in oxygen contents proved to be efficient adsorbents for removing dyes and other pollutants from the aqueous solutions (Bello et al. 2017a). This finding is consistent with other studies (Xiong et al. 2013; Kooh et al. 2016; Lim et al. 2017).

Table 3 Elemental analysis from EDX spectra of CHR and CHA
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Table 4 Boehm titration values of acid-activated coconut husk
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Determination of oxygen-containing functional groups

The Boehm titration technique was carried out to characterize the surface chemical properties of the acid activated adsorbent. Two assumptions were made before the surface acidity and basicity could be evaluated: (1) Acidic groups are neutralized by NaHCO3, NaOH or Na2CO3 and (2) HCl neutralized basic groups. Table 4 summarizes the properties of the surface functional groups obtained from the Boehm titration analysis using the previous procedures (Bello et al. 2017c, b). The concentration of acidic sites for AC produced from coconut husk (CHA) is 2.244 mmol g−1, while the basicity groups are very low given a corresponding value of 0.096 mmol g−1. It was observed that the adsorbent prepared has more acidic than basic functional groups after acid modification. This considerable increase in acidic groups in comparison with the basic groups suggests that the bulk of functional group(s) on the surface of the CHA is acidic. This is similar to result obtained in a study conducted on adsorption of barium and iron ions from the aqueous solution by AC obtained from mazot ash (Hilal et al. 2013).

The pH point of zero charge (pHpzc,) determination

The pHpzc of activated carbon produced from coconut husk was calculated by determining the value at which the point of the resulting curve cuts through the pHo axis, i.e., [plotting the pH difference, ΔpH (final pH − initial pH) against initial pH to determine the pHpzc] (Dahri et al. 2016). As shown in Fig. 4, the pHpzc was determined to be at 3.69. This implies that adsorption of cations was improved at pH values higher than pHPZC, while anions adsorption is favored at pH value less than pHPZC (Farahani et al. 2011; Bello et al. 2017b; Olakunle et al. 2018). It can be deduced from Fig. 4 that the combined influences of all the AC functional groups determine the pHpzc, i.e., the pH at which the net surfaces charge on carbon was zero. At pH less than pHpzc (pH < pHpzc), the carbon surface has a net positive charge whereas at pH greater than pHpzc (at pH > pHpzc); the surface has a net negative charge (Al-Degs et al. 2000). This is consistent with Boehm titration results, indicating that acidic groups are dominant on the CHA surface. Since the pHPZC of CHA was 3.69, it thus points out that the optimum amount of dye adsorbed will occur at pH > 3.69. Conversely, the experimental data disagreed with the concepts of pHPZC, thereby suggesting an involvement of additional forces of attraction including the possibilities of dominance of hydrophobic–hydrophobic interactions than electrostatic interactions.

Fig. 4
figure 4

Plot of pHpzc of activated coconut husk

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Batch equilibrium studies

Effects of contact time and initial dye concentration

Contact time and the initial dye concentrations have noticeable effects on the Rh-B dye adsorption onto CHA as shown in Fig. 5. The percentage of dye removed increases rapidly as contact time also increases; as the Rh-B dye molecules get locked-up in the free pores of the adsorbent, consequently, the trapping pores becomes limited. Thus, the amount of dye adsorbed decreased progressively and slowly until equilibrium is attained. There was relatively constant dye uptake at 25 min of the contact time, that is, 105–120 min. Our previous study has shown that the adsorption tends to be rapid in the first 10–15 min and steadily reduces until equilibrium is attained (Bello et al. 2017b). The reason for this was due to the large number of free pores on the surface of the activated coconut husk which enhanced the rapid uptake of Rh-B dye at the initial stage. Eventually, at a prolonged contact time, most of the free pores are almost occupied with dye molecules, because there are few free pores compared to the un-trapped Rh-B dye molecules; hence, the adsorption steadily reduces until equilibrium is attained. Adsorption is highly dependent on the initial dye concentration, because the percentage dye removal increases at lower initial dye concentrations and decreases at higher initial dye concentration; also the precise quantity of Rh-B dye adsorbed per unit mass of CHA increases with an increased in concentration of Rh-B dye (Hema and Arivoli 2007).

Fig. 5
figure 5

Effect of contact time and initial Rh-B dye concentration on CHA at 323 K

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Adsorption isotherm studies

At equilibrium, the experimental data obtained in this study were tested with four different isotherm models: Freundlich, Langmuir, DR and Temkin isotherms so as to determine the one that fitted most. The linear regression value (R2) and the maximum adsorption capacity (qm) were used as the major operational parameters to justify the requisite of isotherm model fitness. R2 values for the entire isotherm models were obtained from the plot a linear graph (Fig. 6), and their slopes and intercepts were used in calculating all other isotherm parameters. Linear plots were obtained at the temperatures studied. The results presented in Table 5 show that the adsorption data fitted most with the Langmuir isotherm model due to its highest value of qm (mg g−1) and R2 closer to unity at all temperature studied (Table 5). In this study, we also showed isothermal nonlinear plots to avoid error(s) arising from different estimations that might result from linearized regression of isotherm equations shown in Table 2 which could significantly affect the values of R2. Avoiding such error(s) became necessary so as to describe the adsorption isotherm for Rh-B dye uptake by CHA. Analysis via nonlinear method showed that saturation amounts of Langmuir are much closer to the experimental values with relatively low error functions than other isotherm models, thus confirming that Langmuir isotherm fitted most as presented in Fig. 7. This implies that the isotherm models used are valid and could effectively describe the equilibrium data. Figures 6 and 7 represent both linear and nonlinear models used in comparing experimental values of Rh-B dye adsorption onto CHA.

Fig. 6
figure 6

Linear isotherm plots of: a Langmuir, b Freundlich, c Temkin, d DB-R for adsorption at 323 K

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Fig. 7
figure 7

Nonlinear isotherm plots of: a Langmuir, b Freundlich, c Temkin, d DB-R for adsorption at 323 K

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Table 5 Isotherm parameters for Rh-B dye adsorption onto CHA at different temperatures
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Effect of temperature on Rh-B-dye adsorption

The amount of adsorbate adsorbed per unit mass of adsorbent qe, (mg g−1) increases from 1111.11 to 1666.67 mg g−1 when temperature of the solution was increased from 303 to 323 K, respectively. This increase indicates the nature of the process of adsorption. The result shows that increase in temperature favors the mobility of Rh-B dye molecules onto adsorbent (Fig. 8). The change in solution temperature increases the degree of randomness of the dye molecules and hence increases its mobility. The pores on the adsorbents were enhanced by the increased in temperatures which finally altogether facilitated a spontaneous adsorption process. Chemical interaction between Rh-B and CHA was also observed at higher temperature which resulted in the creation of higher affinities between the active sites and Rh-B dye. More so, when temperature was changed, it altered the adsorbent equilibrium capacity (Hema and Arivoli 2007; Sureshkumar and Namasivayam 2008; Sadasivam et al. 2010; Gupta et al. 2012; Inyinbor et al. 2015; Dahri et al. 2016; Dharmendirakumar et al. 2016; Fu et al. 2016; Inyinbor et al. 2016; Kooh et al. 2016; Goswami and Phukan 2017; Iqbal et al. 2017; Adegoke et al. 2019). The capacity of adsorption for the most AC tends to increase with an increase in temperature, i.e., from 303 to 323 K (Hema and Arivoli 2007; Ahmad et al. 2016). Similar effects of temperature were observed from Rh-B dye removal using Corchorus olitorius-L leaves (Subasri et al. 2015) and R. hookerie fruit epicarp (Inyinbor et al. 2016).

Fig. 8
figure 8

Effect of temperature on the adsorption of Rh-B dye onto CHA

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Adsorption kinetic studies

Four different kinetic models employed in this study to determine the processes of adsorption (PFO, PSO, Elovich and IPD models) are shown in Table 6. Figure 9 shows linearized plots of the four kinetic models for the Rh-B adsorption onto CHA. The linearized forms of Eqs. 14–14 were employed for fitting the equilibrium data. The values of R2 obtained from isotherm models were correlated for the fitting the adsorption data. The closer the value of R2 to unity, the better the fit. PSO kinetic model gave the best fit (Fig. 9) judging from R2 value. R2 obtained shows consistent trends, and thus, the rate constant was found to decrease consistently as the initial Rh-B dye concentration increases. This implies that equilibrium is reached at lower initial Rh-B dye concentration than at higher Rh-B dye concentration. The reason could be as a result of low competitions for CHA surface sites at low concentrations, whereas at high concentration, the competition for the CHA active surface sites increased (Ahmad et al. 2016). More so, high R2 value close to unity suggests a better agreement between qe and qcal values. We had previously obtained similar results in the adsorption of synthetic dye onto durian seeds (Ahmad et al. 2015).

Table 6 Kinetic parameters for Rh-B dye adsorption onto CHA at 323 K
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Fig. 9
figure 9

Plot of pseudo-second-order kinetic model at 323 K for Rh-B dye adsorption onto CHA

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The adsorption capacity of CHA was compared with other non-conventional adsorbents as listed in Table 7. CHA proved to be a better and sustainable adsorbent for the removing Rh-B dye among others. This is consistent with the findings reported in the literature on rhodamine-B dyes (Namasivayam and Kanchana 1992; Wilhelm and Stephan 2007; Hema and Arivoli 2007; Sureshkumar and Namasivayam 2008; Vasu 2008; Parab et al. 2009; Li et al. 2010; Sadasivam et al. 2010; Gupta et al. 2012; Ashkarran et al. 2013; Gong et al. 2013; Inyinbor et al. 2015; Kooh et al. 2016; Dharmendirakumar et al. 2016; Dahri et al. 2016; Fu et al. 2016; Inyinbor et al. 2016; Iqbal et al. 2017; Singh et al. 2017; Suc and Kim Chi 2017; Cheng et al. 2017; Goswami and Phukan 2017; Adegoke et al. 2019).

Table 7 Comparison of adsorption capacities of Rh-B dye with various adsorbents
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Adsorption thermodynamic studies

Thermodynamic parameters such as ΔG, ΔH and ΔS are significant features in adsorption systems; these parameters are also key factors for adsorbent adsorption capacity (Tan et al. 2007). The values of ΔG, ΔH and ΔS were calculated using Eqs. 15–17. The positive values of ΔS (0.27625 kJ mol−1 K−1) revealed the affinity of adsorbent for the Rh-B dye uptake and increasing randomness at the solid–solution interface during Rh-B dye adsorption onto the active sites of CHA (Hema and Arivoli 2007). The negative ΔG (ranging from − 26.3762 to − 20.9291 kJ mol−1) obtained for the Rh-B dye adsorption onto CHA depicts the feasibility and spontaneity of the process of adsorption having higher preferences for the Rh-B dye onto CHA, and also the ΔH positive value (62.7707 kJ mol−1) revealed the endothermic nature of the adsorption process (Table 8).

Table 8 Thermodynamic parameters for adsorption onto CHA
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Mechanism of adsorption

Although several factors control the adsorption rate (Wu et al. 2001; Gerçel et al. 2008; Bello et al. 2017b; Ojedokun and Bello 2017), the mechanism of adsorption is the most significant factor governing the kinetics of adsorption in which there is occurrence of initial curved portion owing to a very fast surface adsorption and external diffusion of Rh-B dye onto CHA (Bello et al. 2017b). This present study shows multilinear plots which agreed with our previous study (Bello et al. 2017b). From Fig. 10, the Kt1 part is the sharper region called the boundary diffusion layer of Rh-B dye molecules while the Kt2 part is attributed to a slower and moderate phase revealing the intraparticle diffusion (IPD) as the slowest step called the rate-determining step (RDS). The masses transfers are controlled by various relationships: mechanism of adsorption, liquid–solid phase coupling and initial-to-boundary factors. It therefore connotes that equilibrium rate attainments was IPD-controlled (Bello et al. 2017b). Subsequent to the initial faster adsorption phase, there existed a stage where adsorption of Rh-B was relatively gradual with IPD being the rate controlled. Following this was the relatively slower adsorption process with a linear stability to approach plateau (equilibrium) called the maximum sorption stage. The IPD model constant values kt and C are determined from the qt (mg g−1) versus t1/2 (h1/2) plot as shown in Table 6. The nonlinearity of the qt (mg g−1) versus t1/2 (h1/2) plot obtained for Rh-B dye adsorption onto CHA (Fig. 10) with deviation from zero revealed that IPD was not the only rate-determining step. However, it was observed that Kt2 part characterized by the IPD was established to be the rate-determining step. The plots with nonzero origin (C≠0) showed an occurrence of IPD in the adsorption process but not the only controlling parameter for the reaction rate. The intercept “C” shows a proportionality relationship with boundary layers having the observable extent of thickness at 323 K (Bello et al. 2017b). More so, there is increased in the boundary layer effects with “C” values. This actually helped in knowing the adsorbent tendency to either adsorb Rh-B dyes or remain in the solutions. The obtained high “C” values depict an enhanced adsorption capacity. This observation agreed well with the existing literature (Tan et al. 2008; Bello et al. 2017b; Ojedokun and Bello 2017; Khasri et al. 2018).

Fig. 10
figure 10

Plot of intraparticle diffusion model for Rh-B dye adsorption onto CHA

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Cost analysis

The cost analysis presented in Table 9 provides a simple proof that CHA is six times cheaper than CAC. CAC costs 259.5 US$ per kg (transportation inclusive) in total, while CHA preparation and transportation cost 42.96 US$ per kg. The low cost of preparing CHA as stated in Table 9 gave detailed summary of prices from coconut husk transportation to filtration and washing of the AC. Orthophosphoric acid and deionized water account for most of the significant cost (Table 9).

Table 9 Price difference between CHA and CAC
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Conclusion

CHA, an economically viable material prepared from agricultural waste, is a good precursor for adsorbing rhodamine-B dye from its solutions. The adsorption data fitted most to Langmuir isotherm among all the models used with a maximum monolayer adsorption capacity of 1666.67 mg g−1 and a highest regression value of 0.99 which implies that CHA has greater affinity for the adsorption of Rh-B dyes owing to its pore development via acid activation resulting in higher adsorption capacity. The adsorption process was best explained by PSO kinetic model. The process of adsorption was described to be both endothermic and spontaneous. The positive value of ΔS0 (0.276 kJ mol−1 K−1) suggests increased randomness between adsorbent–adsorbate interactions. The adsorption of Rh-B dye onto CHA was highly dependent on operational parameters (contact time, initial dye concentrations and solution pH). This study revealed that CHA prepared form coconut husk waste material is a promising and sustainable sorbent for removing Rh-B dye from aqueous solution owing to its sustained availability.