Rhodamine B dye sequestration using Gmelina aborea leaf powder

Abstract

Chemically prepared activated carbon derived from Gmelina aborea leaves (GALAC) were used as adsorbent for the removal of Rhodamine B (Rh–B) dye from aqueous solutions. The adsorptive characteristics of activated carbon (AC) prepared from Gmelina aborea leaves (GAL) were studied using SEM, FTIR, pH point of zero charge (pH_pzc) and Boehm Titration (BT) techniques respectively. The effects of pH, contact time, initial dye concentration and solution temperature were also examined. Experimental data were analyzed using four different isotherm models: Langmuir, Freundlich, Temkin and Dubinin-Radushkevich. Four adsorption kinetic models: Pseudo-first-order (PFO), Pseudo-second-order (PSO), Elovich and Intraparticle diffusion models to establish the kinetics of adsorption process. The RhB dye adsorption on GALAC was best described by Langmuir isotherm model with maximum monolayer coverage of 1000 mg g⁻¹ and R² value of 0. 9999. The EDX analysis revealed that GALAC contained 82.81% by weight and 91.2% by atom of carbon contents which are requisites for high adsorption capacity. Adsorption kinetic data best fitted the PSO kinetic model. Thermodynamic parameters obtained for GALAC are (ΔG^o ranged from -22.71 to -18.19 kJmol^-1; ΔH^o: 1.51 kJmol^-1; and ΔS^o: 0.39 kJmol^-1 K⁻¹respectively) indicating that the RhB dye removal from aqueous solutions by GALAC was spontaneous and endothermic in nature. The cost analysis established that GALAC is approximately eleven times cheaper than CAC thereby providing a saving of 351.41USD/kg. Chemically treated GAL was found to be an effective absorbent for the removal of RhB dye from aqueous solution.

Biochemistry; Environmental science; Analytical chemistry; Surface Chemistry; Rhodamine B dye; kinetics; Gmelina arborea leaf; Isotherms.

1. Introduction

Pollution emanating from dye effluents has become a serious environmental problem in the last decade owing to the increasing and fast growing usage of dyes in different applications. Some dyes are resistance to fading even on exposing to light, water and various chemicals [1, 2, 3]. Dyes are chemically and/or thermally stable. Discharging dye effluents into the rivers or aqueous solutions is very dangerous because of the toxicity of such compounds to the living organism. Majority of contaminants (both dyes and chemical substances) present in aqueous solutions are from dye industries and manufacturing processes [3]. Dyes even at very low concentration(s) have negative influence on aquatic lives and food web. Ingestion of these dyes even at low concentration in aqueous media results in severe health complications that affect the central nervous system, reproductive system, brain and liver. In some cases they are mutagenic and carcinogenic [4, 5, 6, 7, 8]. Therefore, due to their harmful effects to both human and aquatic life, the removal of these colors from aqueous effluents become problematic because it poses a serious ecological damage due to its ability to extend to agricultural farm lands and water bodies. Therefore, dye effluents should not be directly discharge into the water bodies [1, 3, 9, 10, 11]. The treatment of dyes becomes imperative since a concentration as low as 1.0 mg/L in drinking water can impart intense color to the water and make it unsuitable for human consumption [3].

Over the years, applications of dyes in different industries including paper, textile, printing, plastics, food, rubber, etc have raised global concerns due to increasing contaminations to the ecosystems thereby raising issues to public health. The discharges of the effluents produced from these industries have detrimental consequences to aquatic life and humans [12, 13, 14, 15]. Also large volumes of water are required by textile industries in their operations, which in turn lead to the release of large volumes of effluents [3,4]. The distinctiveness in the composition of wastewater(s) from textile industries are such that they non-biodegradable and containing numerous dye materials [1, 4, 6, 16, 17]. This makes treatment of contaminated water demanding, since the colour tends to reoccur even after the removal process using conventional methods [18, 19, 20, 21, 22].

Numerous procedures for treating dye effluents include adsorption, coagulation, flocculation, oxidation, precipitation, electrolysis, reverse osmosis, liquid membrane separation, etc. By comparison, all these methods except adsorption have some inherent restrictions and drawbacks likes high operating cost, generation of by-products which are extremely hazardous, energy required is drastically intensive. Dyes are very stable to light and heat as well as have complex polymeric structures, thus making the biological method less flourishing or successful. Therefore ACs has become known as the only promising alternative to these conventional technologies in dyes and wastewaters treatment techniques. Based on this, among all physical techniques available, adsorption have been remains the superior method for treating both wastewater aqueous solution owing to its simplicity of design, lower cost, ease of operations, re-usability and insensitivities to toxicants [23]. Adsorption process has been confirmed as one of the prevalence wastewater treatment technologies in the world, making use of wide number of adsorbents [24, 25, 26].

However, in view of the low cost, regeneration potential and high adsorption capacity, agricultural wastes have been employed [3,24, 25, 26]. Various researchers have exploited adsorbents such as ackee apple (Blighia sapida) seeds [15], oil palm fruit [7,17,27], Imperata cylindrical [28], durian seed [29, 30] watermelon rinds [31], lime peel [32], Okra [33], fly ash [34, 35, 36], banana stalk [37, 38], etc for this purpose. A capable approach is increasing the adsorbents’ porosity sites for enhancement of interactions between adsorbents and dyes. Recently, many researchers have looked into modifying the surface of adsorbents for total or complete removal of various dyes from the aqueous solution and generally reported that modifying agents increase the adsorption site(s) for both cationic and anionic dyes. Examples includes: guava leaf [39] bamboo [40, 41], plant leaf [42], coconut leaves [43, 44], kola nut and bean husk [45, 46], ficus racemosa [47, 48], Prunus Dulcis [49],durian Leaf [50], dead Leaves of plane trees [51], prunus dulcis [49], Moringa oleifera leaf [52, 53], C. camphora leaves [54], berry leaves [55], Glossogyne tenuifolia leaves [56]. Several authors have demonstrated the peculiarity of agricultural-based materials to remove various dyes having diverse molecular structures. Nevertheless, fabricating structures of such kinds of functionalized cellulosic adsorbents for efficient dyes removal remains a hot topic of consideration globally.

Gmelina aborea leaf is an agricultural waste, ubiquitously available all through the year at a no cost. Gmelina aborea grows on various localities and most especially on moist fertile valley. Gmelina aborea is a rapid growing tree which attains moderate to large height up to 40 m and 140 cm in diameter [57]. It occurs naturally in major part of India at altitudes up to 1,500 m and naturally in Thailand, Cambodia, Myanmar, Vietnam, Laos and some China provinces. Due to rapid growth of Gmelina aborea majorly for log purposes, this plant is planted expansively in Nigeria, Sierra Leone and Malaysia [57]. The high rate of leaf wastes generated from Gmelina aborea litters the ground thus acidifying the soil at some point during its decay processes thereby preventing the invasion of another plant species. Interestingly, Gmelina aborea leaf is a brittle lignocellulosic material with high carbon content, these properties enhances its processing into powdery form thus making it suitable for adsorption of various contaminants.

Rhodamine B (Rh–B) dyes are water soluble basic red cationic dyes and common water tracer fluorescent owing to their excellent photophysical properties. Rhodamine dyes, which belong to the xanthene class of dyes, are among the oldest synthetic dyes used for the dyeing of fabrics and in food industries. it is established to be harmful when swallowed, with acute oral toxicity, causes serious eye damage or irritation, hazardous to the aquatic environment with long-term effect [58, 59, 60]. Adsorption method using different adsorbents has been used severally for removing RhB dye from aqueous solution [59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76]. In addition, other method including photocatalytic degradation using MOFs, SiO₂@TiO₂ nano-spheres, titania films, titania nanorod thin films, ZnO nanorods, TiO₂ photonic crystals, UV/S₂O₈^2-, PbCrO₄/TiO₂, gelatin/CuS/PVA nanocomposites, MnO₂ nanorods have been used for RhB dye degradations [77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89].

This study is aimed at preparing, characterizing and applying Gmelina aborea leaf (GAL) activated carbon as adsorbent for the removal of RhB dye from aqueous solutions and to study the isotherm, kinetic and thermodynamic parameters governing the adsorption process. The uniqueness of GAL to remove RhB dye from aqueous solution has never been reported in this field, likewise the GAL morphological properties, surface characteristics and pH coupled with its carbonaceous content exemplified by the large surface area, high carbon contents and well-developed pores favors its affinity towards RhB dye adsorption. The influences of operational parameters are also studied for the first time on ability of chemically prepared AC from GAL to adsorb RhB dyes. Comparing with other reported work on RhB dye, this study is majorly focused on the use of functionalized leaf-based adsorbents from Gmelina aborea for RhB dye removal from aqueous media. Such study is necessary due to the particularities of these adsorbents in dye adsorption processes, numerous availability at a relative low cost, and numerous activation possibilities. These particularities have been highlighted in this study. Moreover, to the best of our knowledge, this is the first time a study on the utilization of the Gmelina aborea leaf adsorbent for RhB dye removal was investigated.

2. Materials and methods

Gmelina aborea leaf (GAL) was collected from LAUTECH Ogbomoso, Nigeria, washed with distilled water and oven dried to constant weight. A carefully weighed 25 g of GAL was then pulverized into tiny particle sizes and was activated chemically using 500 cm³ of 0.3 mol dm⁻³ of ortho-phosphoric acid (H₃PO₄) and was stirred. This was afterward heated at a temperature of 300 °C for 60 min in an oven and then allowed to cool. It was then washed with distilled water and later oven dried at a temperature of 105 °C for 4 h to constant weight. The adsorbent was then sieved to 150 μm mesh size and kept in air tight containers for further use.

2.1. Characterization

The characterization of Gmelina aborea leaf raw (GALR) and activated samples (GALAC) were carried out using Fourier Transform Infrared (FTIR) FTIR-2000 (Shimadzu Model IRPrestige-21 Spectrophotometer). The spectroscopic analysis was used to study the surface chemistry of GALR and GALAC powder. The FTIR spectra gave detailed characteristics' functional group(s) on the surfaces of both GALR and GALAC. Scanning Electron Micrograph (SEM) was employed to study the surface characteristics and the morphological features of the GALR and GALAC samples. It produces both the quantitative and qualitative information which are further analyzed by a range of detectors to give three-dimensional image(s). Elemental analysis of both GALR and GALAC were carried out using Energy Dispersive X-ray (EDX) to determine the elemental compositions present in the samples before and after acid activation. The resulting elemental analysis line spectrum corresponds to specific elemental composition. The intensities of the characteristics’ line are proportional to the elemental compositions which are quantitative in nature.

2.1.1. Determination of oxygen-containing functional groups

This study used Boehm titration method to determine the oxygen containing functional groups [90, 91]. Four portions of 1.0 g each of raw and GALAC samples were kept in contact with separate solutions of 10–15 ml of 0.1 M NaOH, 0.1 M NaHCO₃ and 0.05 M Na₂CO₃ for determining acidic groups and 0.1 M HCl for the basic groups composites respectively at temperature of 48 h. Afterwards, the resulting aqueous solutions were back-titrated with 0.1 M HCl for acidic and 0.1 M NaOH for basic groups. The types and numbers of acidic sites were calculated using previous procedure [45, 53]. Briefly, for the determination of the numbers and types of acidic sites, certain considerations were made that NaOH neutralizes carboxylic, lactonic and phenolic groups, Na₂CO₃ neutralizes carboxylic and lactonic groups and that NaHCO₃ neutralizes only carboxylic groups. The amount of oxygen-containing-functional groups, F_x, is calculated using the relation:

$$F_x=\frac{(V_{bx}-V_{ex})}{m_x}\times M_t\times D_F$$ (1)

$$D_F=\frac{initialvolume}{selectedvolumefortitration}$$ (2)

where “F_x (mmolg⁻¹) is the amount of oxygen containing functional groups, V_bx is the volume of titrant used to titrate the blank, V_ex is the volume of the titrant used to titrate the extract, M_t is the molarity of the titrant used, D_F is the dilution factor”.

2.1.2. pH and point of zero charge (pHpzc) determinations

For the determination of the pH_pzc of the GALAC, 0.05 g of GALAC was added into solution containing 100 ml of 0.1M NaCl with known initial pH. NaOH or HCl was used to adjust the pH value. The sample holder was corked and placed in a shaker, agitated for 24 h at 250 rpm, the final pH was then determined. A graph of pH difference, ΔpH (final pH − initial pH) versus the initial pH was used to determine the pH_pzc. The pH_pzc exists when pH does not change upon a contact with the adsorbent(s).

2.2. Batch equilibrium studies

Batch adsorption method was carried out using UV-Visible spectrophotometer at wavelength of 554 nm. Studies such as the effects of initial dye concentration at five different concentrations: 200, 400, 600, 800 and 1000 mg/L were investigated. Also, the effects of contact time, adsorbent dosage, and solution temperatures (at 303 K, 313 K and 323 K) were studied using standard methods [53, 92]. The adsorbent dosage used throughout the adsorption process was 0.1 g of GALAC. The adsorption process was allowed to proceed in the water bath shaker to equilibrium up to 120 min. Five (5) sets of 100 ml Erlenmeyer flasks containing the mixture of 0.1 g of the sample and the RhB dye solution of different initial dye concentrations were carefully arranged in the shaker, it was 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 in order to make the solution temperature uniform to that of the shaker at specified temperature until equilibrium was reached. Sample solutions were withdrawn at a pre-determined time intervals for the determination of residual concentrations using a UV-Vis spectrophotometer. The amount of RhB dye uptake and percentage removal at equilibrium were calculated using Eqs. (3) and (4) respectively:

$$q_e=\frac{(\text{Co}–\text{Ce})\text{V}}{\text{m}}$$ (3)

$$\text{\% removal}=\left[\frac{\left(\text{Co}–\text{Ce}\right)}{\text{Co}}\right]x100\%$$ (4)

where, C_o and C_e are respective initial and equilibrium dye concentrations (mg.L⁻¹), V is the volume of solution (dm³), m is the mass of adsorbent (g), Q_e is the amount of dye adsorbed (mg.g⁻¹).

2.3. Mathematical modeling

2.3.1. Isothermal studies and kinetic models

Four different adsorption isotherm models: Langmuir [93, 94], Freundlich [95], Temkin [96] and Dubinin–Radushkevich (D–R) [97] were employed to test the adsorption data. The detailed isotherm parameters for RhB dye adsorption on Gmelina aborea leaf (GALR) are listed in Table 1. Different kinetic models were used to test the adsorption data, they are: pseudo-first-order (PFO) [98], pseudo-second-order (PSO) [99], Elovich [100, 101] and intraparticle diffusion (IPD) [102] models. Table 1 shows the detailed kinetic model parameters for RhB dye adsorption on Gmelina aborea leaf (GAL).

Table 1.

Adsorption isotherm and kinetics parameters.

Adsorption model	Type	Equation	Expression of the equation	Refs.
Isotherm	Langmuir	$\frac{C_e}{q_e}=\frac{1}{q_m{C}_e}+\frac{1}{{\text{K}}_{\text{L}}{q}_m}\left(5\right)$$R_L=\left[\frac{1}{\left(1+{\text{ K}}_{\text{L}}\text{Co}\right)}\right]\left(6\right)$	Ce is the adsorbate concentration at equilibrium (mg/L), qe is the amount of adsorbate adsorbed per unit mass of adsorbent (mg.g−1), qm is the maximum monolayer adsorption capacity of the adsorbent (mg.g−1), KL; the Langmuir adsorption constant (L.mg−1). Co is the highest initial solute concentration; whereas, RL value implies the adsorption is unfavorable {($R_L$>1), linear ($R_L$ = 1), favorable (0 <$R_L$< 1), or irreversible ($R_L$ = 0)}.	[[93], [94]]
	Freundlich	$\text{ln}{q}_e=\frac{1}{n}ln{C}_e+ln{K}_f\left(7\right)$	Kf is the Freundlich isotherm constant ((mg.g−1) (L.mg−1)); n, the heterogeneity factor. The slope of 1/n ranging between 0 and 1 is a measure of adsorption intensity, which becomes more heterogeneous as the values get closer to zero.	[95]
	Temkin	$q_e=Bln{K}_T+Bln{C}_e\left(8\right)$	B = RT/b is the constant related to the heat of adsorption (L.mg−1); T is absolute temperature; R is universal gas constant (8.314 J/mol K); KT, equilibrium binding constant (L.mg−1).	[96]
	D–R	$ln{q}_e=ln{q}_m+\text{β }{\epsilon }^2\left(9\right)$$\epsilon =\text{ RT }[1+\frac{1}{C_e}]\left(10\right)$$E=\frac{1}{\sqrt{2\beta }}\left(11\right)$	β a constant related to the adsorption energy (mol2/kJ2); Ɛ is the Polanyi potential, E is adsorption energy (when value of E is between 1 and 8 kJ/mol, it implies a physical adsorption while value between 9 and 16 kJ/mol means a chemical adsorption.	[97]
Kinetics	PFO	$\ln \left(q_e-q_t\right)=ln{q}_e-K_1\left(12\right)$	qtis the amount of solute adsorb per unit weight of adsorbent at time t (mgg−1), K1 is the pseudo-first order kinetics rate constant (min−1).	[98]
	PSO	$\frac{\text{t}}{{\text{q}}_{\text{e}}}=\frac{1}{{\text{K}}_2{\text{q}}_{\text{e}}}+\frac{1}{{\text{q}}_{\text{e}}\text{ t}}\left(13\right)$	K2 is pseudo-second order kinetics rate constant (min−1).	[99]
	Elovich	$q_t=\frac{1}{\text{β}}\ln \left(\alpha \beta \right)+\frac{1}{\text{β}}\ln t\left(14\right)$	α is the initial desorption rate [mg. (g min)−1], β, the desorption constant (g.mg−1). The 1/b value denotes the number of available sites for adsorption and the value of (1/β); (ln αβ) shows quantity of adsorption when (ln t) equal to zero.	[100,101]
	IPD	$q_t+K_{diff}{t}^{1/2}+C\left(15\right)$	C is the intercept and reflects the boundary layer effect of the plot qtagainst t1/2. Kt (mg/g h1/2) denotes the intra-particle diffusion rate constant; t1/2, the half-adsorption time (h1/2). For intra-particle diffusion to be the only rate determining step, then the regression of qtagainst t1/2 must be linear and should pass through the origin, otherwise, it then implies that the intra-particle diffusion is not the only rate-controlling step	[102]

2.3.2. Thermodynamic studies

Thermodynamic parameters which explain the feasibility, spontaneity and the nature of adsorbate-adsorbent interactions (ΔG°, ΔH° and ΔS°) were calculated from thermodynamic equations (Eqs. (16), (17), and (18)) [53]. These were used to explain the adsorption process at different temperatures (303, 313 and 323 K respectively):

$$\text{ΔG}°=-\text{RT ln}{K}_L$$ (16)

$$\text{ln}{K}_L=\frac{\text{ΔS}°}{\text{R}}-\frac{\text{ΔH}°}{\text{RT}}$$ (17)

The values of ΔS and ΔH were obtained from the intercept and slope of Van't Hoff plot of ln K_L against 1/T. Values of K_L (Langmuir constant in L.mol⁻¹) are calculated at 303 K, 313 K, and 323 K respectively. Arrhenius equation was used to calculate the adsorption energy of activation which represents the minimum energy needed by the reactants for the interaction to occur (Eqn 18):

$$\text{ln}{K}_2=\text{ln A}-\frac{\text{Ea}}{\text{RT}}$$ (18)

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

3. Results and discussion

3.1. Characterization

3.1.1. Scanning electron microscopy (SEM)

Fig. 1 shows the morphologies of the GALR and GALAC. It can be seen in Fig. 1b that several pores were found on the surface of GALAC with honeycomb shapes as opposed to that found in the raw sample (GALR) with smooth and closed surfaces (Fig. 1a). This indicates that acid activation is more effective in creating well-developed porous surfaces on GALR sample. These pores provide excellent surfaces for trapping and adsorption of RhB dye molecules [4,103,104]. The porosities observed here is a function of pores available for RhB dye uptake which further enhanced the sorption capacity in the GALAC sample. This is due to the breakdown of the lignocellulosic material at high temperature followed by evaporation of volatile compounds present in GALR. The presence of pores and internal surfaces are requisite for effective adsorbent. The simple mechanisms of activation involve decomposition of the tissues of GALRs (which are the carbon precursor) and creation of new pores and voids. H₃PO₄ permeates tiny pores or voids, thereby increasing the contact between the H₃PO₄ in the carbon precursors which further promotes the release of volatile materials from the GAL thus widening the micropores which are finally converted to mesopores.

Fig. 1.

SEM image of (a) Raw Gmelina aborea Leaf (b) Activated Gmelina aborea Leaf.

3.1.2. Energy Dispersive X-Ray

Elemental analysis of both GALR and GALAC were obtained from EDX spectra and data. The spectra of both samples are presented in Fig. 2. The data of EDX analyses for both GALR and GALAC are shown in Table 2. This revealed the amount of carbon and oxygen contents present in the two samples. The GALAC was very high in carbon content and significantly low in oxygen content. The low oxygen contents and high carbon contents are the major requisites for high adsorption capacity. Table 2 revealed that GALAC contained 82.81% by weight and 91.2% by atom of carbon while the GALR contained 74.53% by weight and 85.72 % by atom with oxygen contents higher of than that of GALAC. This implies that GALAC was richer in carbon contents than the GALR owing to the effects of acid activation on GALR sample. Since GALAC is richer in carbon content, it expected that it is an efficient material for dye removal. The lower the oxygen contents the higher the carbon contents of the samples under study, the more efficient the adsorbent. This observation is consistent with studies reported previously [52].

Fig. 2.

EDX spectrum of (a) GALR (b) GALAC.

Table 2.

EDX results of GALR and GALAC.

Element	Wt%	At%	K-Ratio	Z	A	F
GALR
C	74.53	85.72	0.4569	1.0154	0.6037	1.0001
O	9.18	7.93	0.0164	0.9963	0.1795	1.0001
Al	2.14	1.1	0.0171	0.9209	0.863	1.0022
Si	2.52	1.24	0.0217	0.9409	0.9147	1.0021
Ca	11.62	4.01	0.1081	0.9146	1.0167	1
Total	100	100
GALAC
C	82.81	91.2	0.327	1.013	0.3897	1
O	3.95	3.27	0.0074	0.994	0.1881	1.0001
P	11.94	5.1	0.1077	0.906	0.9952	1.0005
Ca	1.29	0.43	0.0119	0.9121	1.0058	1
Total	100	100

3.1.3. Fourier transform infra –red (FTIR)

The FTIR spectra analysis of both the raw and the activated Gmelina aborea leaf are presented in Fig. 3 and Table 3 showing different bands and assigned functional groups. It is evidence from Fig. 3b that the peaks were marginally shifted in comparism with Fig. 3a. These shifts in peak values are attributed to the influence of H₃PO₄ used for modification. The FTIR spectroscopic characteristics are shown in Table 3. The FTIR spectroscopic analysis demonstrated a wide band at 3755.40 cm⁻¹; representing the O–H stretching of alcohol. The bands observed at 3437–3417cm⁻¹ showed H-bonded of alcohol. The peaks observed between 2854 and 2922cm⁻¹ attributed to the C–H stretching of alkanes. The bands at 2376-2378cm⁻¹ representing H–C=O stretch of aldehydes. The peaks at 1913-1867cm⁻¹ were assigned to aromatic –C≡ C-stretch of alkynes and those observed at1720- 1705cm⁻¹ indicating a C=O stretch of lactones, ketones and carboxylic anhydrides. The bands around 1641-1633cm⁻¹ show the presence of N=H bonds of primary amines. N–H asymmetric stretch was observed at 1529- 1512cm⁻¹. The band around 1458-1442cm⁻¹ shows that C–H bending of alkanes is present in the sample. The bands observed at 1386-1384cm⁻¹ represents C–H bending of alkanes. The peak shown at 1259cm⁻¹ represents O–H wag (-CH₂X) of alkyl halides. The band observed between 1159-1176cm⁻¹ represents C–N stretch of aromatic amine and C–N stretch of aliphatic amine was observed at 1066cm⁻¹. The corresponding band at 665–696 cm⁻¹ was assigned to C≡C bending alkynes. The peak observed between 437-441cm⁻¹ shows C–Br stretch of alkyl halides. As shown in Table 3, spectra analysis before and after activation. Gmelina aborea leaf demonstrated that mostly O–H groups, C–N stretch, C–Br stretch and C–H stretching were engaged during the activation step. Reduction and broadening of bands validate the effectiveness of the activation process. The changes seen in GALAC FTIR spectra confirmed the effects of H₃PO₄ activation of GALR giving good indications of the suitability of the GALAC for effective RhB dye removal from aqueous solutions. Similar observations have been reported previously [52].

Fig. 3.

(a) FTIR spectra of the raw gmelina aborea leaf (GALR). (b) FTIR spectra of acid activated gmelina aborea leaf (GALAC).

Table 3.

Functional group presents in GALR and GALAC.

Wavenumber (cm−1)			Band
GALR	GALAC	Differences
3755	3755	0	O–H stretch of alcohol
3437	3417	-19	O–H stretch of alcohol
2922	2922	0	C–H stretch of alkanes
---	2854	0	C–H stretch of alkenes
2376	2378	2	H–C=O stretch of aldehydes
1913	1867	-46	-C≡C- stretch of alkynes
1720	1705	-15	C=O stretch of β- unsaturated
1641	1633	-8	-C=C-stretch of alkenes
1529	1512	-17	C–C stretch of aromatics
1458	1442	-15	C–H bend of alkanes
1386	1384	-2	C–H bend of alkanes
1259	---	---	C–H wag of alkyl halides
1066	---	---	C–N stretch of aliphatic amines
665	---	---	C–Br stretch of alkyl halides
437	441	4	C–Br stretch of alkyl halides

3.1.4. Boehm Titration and pH_pzc

The surface functional groups and pH_pzc are significant parameters governing RhB dye adsorption onto GALAC. These parameters are used to explain the adsorbents' acidities and/or basicity, types of AC (H- or L-type) and also the carbon's net surface charge in solution. It must be noted that AC contained both acidic and basic groups. The acidic functional groups include phenolic, lactonic and carboxylic [105] while the basic groups are the oxygen-containing species including pyronic, ketonic, chromenic, and p-electron system(s) of carbon basal plane(s) [106]. The nature and preparation condition of AC determines the density of surface functional groups of the precursors. In this study, H₃PO₄ greatly increased the density of GAL surface functional groups thereby improving the functional group interactions with polar solute from the solutions. The Boehm technique reveals the surface chemical properties of the adsorbents. Table 4 presents a summary of the properties of the surface functional groups. To evaluate the surface acidity and basicity of GAL; two assumptions were made: (i) acidic groups would only be neutralized by NaOH, Na₂CO₃ or NaHCO₃ and (ii) all basic groups are neutralized by HCl. The concentration of the acidic and basic groups are shown in Table 4, The basic group value was lower than the acidic group indicating that the adsorbent surface is predominantly acidic, Similar result was obtained in the adsorption of Ba²⁺ and Fe²⁺ ions [107] from aqueous solutions using activated carbon produced from mazot ash [108]. Acidic functional groups results in increased adsorption of RhB dye molecules [109,110].

Table 4.

Different functional groups on GALAC.

Adsorbent	Carboxylic groups (mmol/g)	Lactones (mmol/g)	Basicity (mmol/g)	Acidity (mmol/g)
GALAC	0.2000	0.075	0.094	0.12500

The pH_pzc (which is the pH where the net carbon surface charge equals zero) was determined by the combined influences of all the AC functional groups. The GALAC surface charge was determined by pH of the RhB dye solution. It has been established that a net positive charge on carbon surface are obtained when the pH of the solution is lower than the pH_pzc and a negative charge predominates when the pH of the solution is higher than pH_pzc. To determine pH_pzc, ΔpH was plotted against pH_o as shown in Fig. 4. The final pH_f plateau was obtained in Fig. 4 corresponding to a pH where no net H⁺ or OH⁻ adsorptions, implying that at this point, the difference between final [H⁺] and initial [OH⁻] is zero. The pH_pzc (Fig. 4) was observed to be 5.75 for GALAC. Cationic adsorption is favored when pH_pzc is lesser than pH while adsorption of anions are enhanced at pH less than pH_pzc [111, 112]. The total acidic groups determined by Boehm titration for the GALAC is 0.125 mmol/g while the basic group was 0.094 mmol/g (Table 2), this suggests that the adsorbent surface was predominantly acidic. Therefore, the acidic groups resulted in greater number of oxygenated-functional groups on the surface of the GALAC [109]. Therefore, RhB dye‒adsorption increased at pH higher than the pH_pzc.

Fig. 4.

Plot of point zero charge of acid Activated Gmelina aborea Leaf (GALAC).

3.2. Operational parameters

3.2.1. Initial RhB dye concentration and contact time

The RhB dye uptake by GALAC as a function of different initial Rh.B dye concentrations (200–1000 mg/L) and temperatures of 303, 313, and 323K were investigated. The RhB dye uptake at 303K is shown in Fig. 5. The amount of RhB dye adsorbed increased with increased initial RhB dye concentrations. The increase in initial dye concentrations gave the driving force required to break the mass transport barriers that resist the interactions of RhB dye between the solid and aqueous phases. Thus, the comparative patterns were observed at higher temperatures. The RhB dye uptake (q_t) was very rapid at the initial stages of the contact period around 0–15 min but after 15 min it became very slow, it reaches equilibrium before 40 min. This was attributable to the availability of larger numbers of vacant sites accessible for Rh.B dye adsorption during the initial stages [10]. After a lapse of time, the adsorption became slower as a result of the decreasing number of vacant sites since these surface sites were almost fully occupied by dye molecules. The progressive slow adsorption toward equilibrium was most likely due to the saturations of active sites. It can be inferred here that the contact time required for Rh.B dye with initial concentrations of 200–600 mg/L to achieve equilibrium was 35 min. Conversely, for 800–1000 mg/L initial concentrations, the equilibrium was reached between 95-105 min. This is so because, the Rh.B dye molecule(s) needs to firstly encounter the boundary layer effects first prior to a diffusion onto the adsorbent's surfaces, thereafter, it diffuses into the adsorbent's porous structures [113]. In fact, at higher initial dye concentrations, the higher Rh.B dye uptake was obtained as the numbers of dye molecules contending for the sites available on the GALAC surface are higher. This means that Rh.B dye solutions having higher concentration (for this study; 800–1000 mg/L) would require longer time period to reach equilibrium. The adsorption uptakes of Rh.B dye at equilibrium shows that the process of adsorption is majorly initial Rh.B dye concentrations dependent [39, 53, 113].

Fig. 5.

Plot of Rh–B dye adsorption uptake against adsorption time at 323 K.

3.2.2. Effect of solution temperature

The Fig. 6 reveals the extent of adsorption of Rh.B dye against the solution temperature for the GALAC at 323 K. The amount of RhB dye adsorbed (q_m (mg/g)) increase from 142.86 mg/g to 1000 mg/g as the solution temperature increases from 303 K to 323 K. This means that; increasing the temperature resulted into rapid mobility of RhB dye to interact with the GALAC active sites and also facilitate the adsorbent pores, both on the internal boundary layers and on the surface (external boundary layers) as the solution viscosity decreased at high temperature. This thereby created a networked linkage between the pores which significantly enhanced the GALAC to trap more RhB dye molecules thereby enhancing the affinity of GALAC surface to trap RhB dye molecules. This observation indicates an endothermic process of adsorption. Adsorption process was favored by increase in temperature of the interaction. This was due to large available surface areas and aggregate pore volumes of GALAC. Therefore, since reactions at 323 K shown a prevalence and significant adsorption yields, it was selected as the optimum temperature for RhB dye adsorption onto GALAC. Similar pattern have been observed in other studies [29].

Fig. 6.

Effect of temperature on the adsorption of RhB dye onto GALAC.

3.2.3. Effect of pH on adsorption of RhB dye

pH of the solution has a profound influence on adsorption process; it determines both the extent of adsorption of the dye molecule and the surface charge of the adsorbent. The effect of pH on the uptake of RhB dye onto GALAC was investigated and the highest percentage of RhB dye adsorbed was obtained at pH 3 (91 %) (Fig. 4). A gradual decrease was observed at pH above 3. The lowest amount of RhB dye adsorbed was at pH 10 (30 %). RhB dye has a pK_a value of 3.7. At this pK_a, it exists as cationic, lactonic or zwitterionic forms depending on the solution media. At pH value lower than pH_pzc (5.75), the RhB dye are of cationic and monomeric molecular forms [114], thus the dye molecule can enter easily into the pore structure of GALAC. At pH value higher than pH_pzc, the zwitterionic forms of RhB dye exist in solution mixture. This form increases the aggregation of RhB dye molecule to form larger molecules (dimers). The increase in aggregation of the zwitterionic form is due to the attractive electrostatic interactions between the carboxyl and xanthene groups of the monomers [115]. These molecules are unable to enter the pores as a result of their size thereby resulting in lower percentage removal at high pH. Optimum adsorption at pH of 3 has been previously reported in one of our studies [53, 108].

3.3. Adsorption equilibrium and isotherms

Figs. 7(a-d) shows the plot of Langmuir, Freundlich, Temkin and D-R isotherms for the RhB dye uptake onto GALAC. Their corresponding isotherm parameters were determined from slope and intercept of each plot. The adsorption parameters obtained from Langmuir, Freundlich, Temkin and DBR isotherm plots are presented in Table 5. The best equilibrium was obtained on the basis of linear regression correlations (R²). Contrast to other isotherms, Langmiur model has the highest regression values (R² = 0.9941, R² = 0.9930, R² = 0.9999) at 303 K, 313 K, and 323 K respectively. Temperature of 323 K gave the best fit judging from the R² values. In the present study, Langmuir equation was also expressed in term of dimensionless separator factor (R_L). Increase in Langmuir isotherm separation factor (R_L) from 0.482 to 0.5 (Table 5) with increase in temperature indicates that the adsorption is favorable at higher concentration leading to a gradual decrease of K_L. This suggests that the process of adsorption was favored at higher initial Rh.B dye concentrations. The value of q_m value (q_m = 1000 mg/g) obtained for RhB dye adsorption onto GALAC was compared with other sorbents [47, 49, 50, 51, 53, 55, 58, 116, 117, 118, 119, 120, 121, 122] (Table 6), GALAC exhibited higher performance. These results show that acid modification largely enhance the adsorption capacity from 142.86 to 1000 mg L⁻¹ (Table 5) implying that H₃PO₄ modification resulted in the creation of several pores on GALAC thereby improving the adsorption capacity. The K_F values obtained from Freundlich isotherm demonstrate that a physical process was reasonably part of the sorption process. The higher the value of K_F, the higher the adsorption capacity. The “n” values were used to measure the linear deviation of the adsorption and the adsorption types. In this study, the “n” values were greater than 1 suggesting a favorable adsorption process [123]. The Temkin isotherm parameters are listed in Table 5; the R² for GALAC is 0.9918 at 323 K indicating that Temkin parameters account for GALAC-RhB dye interactions. The Temkin isotherm constants (B_T) were positive at all temperatures, this suggests that the system was an endothermic process. The values of E_a obtained from DBR isotherm were found to be 79.52, 22.36 and 74.54 at 303K, 313K, and 323K respectively confirming that the adsorption processes follows physio-sorption mechanisms. Comparing the correlation (R²) values for the four isotherms, parameters, one can easily conclude that Langmuir isotherm described the adsorption process most. The sequence is Langmuir (R² = 0. 9999) > Temkin (R² = 0. 9918) > Freundlich (R² = 0. 9752) > DBR (R² = 0. 9182) respectively.

Fig. 7.

(a)Plot of Langmuir isotherms for Rh–B dye adsorption onto GALAC. (b) Plot of Freundlich isotherm for Rh–B dye adsorption onto GALAC. (c) Plot of Temkin isotherm for Rh–B dye adsorption onto GALAC. (d) Plot of Dubinin-Radushkevich isotherm for Rh–B dye adsorption onto GALAC.

Table 5.

Different isotherm parameters for the adsorption of Rh.B dye unto GALAC.

Isotherms	Tempratures
	303 K	313 K	323 K
Langmiur
qm (mg/g)	142.86	250	1000
KL	0.0172	0.006	0.0003
RL	0.482	0.499	0.5
R2	0.9941	0.9930	0.9999
Freundlich
n	41.67	2.398	5.952
Kf	95.78	12.77	64.07
R2	0.987	0.959	0.9752
Temkin
A	0.091	2.852	2.195
Bt	106.2	56.4	0.946
R2	0.9650	0.9551	0.9981
DBR
B	-7.00E-05	-1.00E-03	-9.00E-05
qo	110.72	195.97	156.35
Ea	79.515	22.361	74.536
R2	0.9942	0.9910	0.9182

Table 6.

Comparison of adsorption capacities of different dyes onto various adsorbents.

Adsorbents	Dye	qmax (mg/g)	Refs
NaOH-modified durian leaf	Methylene Blue	125	[50]
NaOH-modified dead leaves of plane trees	Methylene Blue	145.62	[51]
Surfactant (C19H42BrN)-modified Prunus Dulcis leaves	Acid blue 113	97.09	[49]
NaOH-modified Ficusracemosa with	Acid blue 25	83.33	[47]
Citric acid-modified ricinuscommunis leaves	Methylene Blue	81.29	[116]
H3PO4-modified berry leaves with	Eriochrome Black T	133.33	[55]
Surfactant (HDTMA-Br)-modified pineappleleaf	Methylene blue	52.6	[117]
Surfactant (HDTMA-Br)-modified pineappleleaf	Methyl orange	47.6	[117]
HCl-modified Calotropis procera leaf	Methylene Blue	192.31	[118]
Sulfonic acid-modified tea leaf	RhodamineB	757.6	[119]
Bagasse pith	Rhodamine B	263.85	[120]
Aleurites moluccana seeds	Rhodamine B	117	[121]
Rice husk-based	Rhodamine B	478.5	[122]
KOH-treated Irvingia gabonensis	Rhodamine B	232	[58]
Moringa oleifera seed pod	Rhodamine B	1250	[53]
H3PO4-modified Gmelina aborea	Rhodamine B	1000	This study

3.4. Adsorption kinetics and mechanistic studies

The kinetics of adsorbate uptake is essential for selecting optimum operating conditions for the process design. These adsorption data were analyzed using PFO (Fig. 8a) [98], PSO (Fig. 8b) [99], Elovich (Fig. 8c) [100,101] and IPD (Fig. 8d) [102] models. As presented in Fig. 8b, and Table 7, the pseudo-second-order (PSO) kinetics best described the adsorption kinetic process of RhB dye onto GALAC. Elovich model was used to depict the second-order kinetics by assuming that the actual solid surfaces are energetically heterogeneous. Comparing the R² values obtained in the adsorption process, the suitability of the kinetic isotherms fit for the adsorption data follows the order: PSO > PFO > Elovich. The mechanism of adsorption was investigated using intraparticle diffusion (IPD) model. As indicated by IPD model, the values of q_t were observed to vary linearly with the value of t^1/2. The rate constant, k_diff presented in Table 7 was determined from the slope of the graph (Fig. 8d). The plots obtained were non-linear. The first adsorption phase is bulk diffusion while the second phase is intraparticle mass transfer resistance. The rate at which the equilibrium is attained is IPD-controlled [124]. The RhB dye uptake onto GALAC at different initial dye concentration (Fig. 8d) gave multilinear profiles which imply that the sorption process occurred in two stages. The first stage is the boundary layer diffusion of the RhB dye molecules onto GALAC, which is a steeper portion while the second stage is linear and gradual adsorption stage revealing that IPD was the rate determining step. However, it was observed that K_t2 parts were characterized by the IPD established to be the rate determining step. The plots with non-zero origin (C≠0) showed an occurrence of IPD in the adsorption process. The deviations from the origin show that IPD was not the only rate-controlling step. The intercept “C” showing a proportionality relationship with boundary layers having the observable extent of thickness at the highest temperature under study 323 K [53,125]. It was observed that the second adsorption stage is considered as the IPD that controls the rate of adsorption process.

Fig. 8.

Plots of (a) Pseudo-first order kinetic, Pseudo-second order kinetic, (c) Elovich and (d) Intraparticle diffusion for Rh–B dye adsorption onto GALAC at 323K.

Table 7.

Adsorption Kinetics studies parameters for the adsorption of Rh–B dye on acid activated GALR at 323K.

Kinetics	Initial Rh–B concentrations
	200 mg/l	400 mg/l	600 mg/l	800 mg/l	1000 mg/l
PFO
qeexp (mg/l)	126.87	155.08	187.59	154.2	186.44
qecal (mg/l)	61.93	63.31	100.08	74.59	72.39
k1 (min)	0.019	0.026	0.022	0.021	0.018
SSE%	12.494	17.99	16.84	15.32	21.94
R2	0.926	0.97	0.926	0.989	0.99
PSO
qeexp (mg/l)	126.87	155.08	187.59	154.2	186.44
qeca l (mg/l)	125	166.67	200	166.7	200.2
k2 (min)	1.33E-03	1.33E-03	6.76E-04	1.03E-03	1.09E-03
H (mg/gmin)	20.78	27.03	28.52	37.02	43.48
SSE (%)	0.352	3.954	13.821	12.467	22.57
R2	0.994	0.997	0.993	0.99	0.994
Elovich
Βl0.058	0.049	0.037	0.05	0.056
Α.0.773	0.221	0.304	0.804	1.72
R2	0.973	0.94	0.971	0.916	0.953
IPD
Kdiff	15.65	23.02	21.99	4.896	13.7
C	28.22	37.12	42.17	89.77	85.33
R2	0.945	0.931	0.945	0.827	0.946
Kdiff	4.92	4.78	9.42	24.14	6.04
C	68.74	103.8	85.77	22.23	115.4
R2	0.923	0.957	0.93	0.913	0.934

3.5. Thermodynamic studies

Thermodynamic parameters such as ΔG, ΔH and ΔS are significant features in adsorption systems [108]. The values of ΔG, ΔH and ΔS were calculated using Eqs. (16), (17), and (18). The values of ΔS° and ΔH° are obtained from the intercept and slope of Van”t Hoff plot of 1/T against In K_L as shown in Fig. 9 which agreed well with our previous studies on malachite green dye adsorption onto rambutan seed activated carbon [4]. The thermodynamic parameters (Table 8) indicate that the value ΔH was 11.51 kJ/mol; this positive value reveals an endothermic nature of the process of adsorption. The positive value of (ΔS, 0.388 kJ/mol K) revealed that the affinity of adsorbent for RhB dye uptake occurred with increasing randomness at the interface of GALAC-RhB dye system [108,126]. However, ΔG values were -22.71, -19.56, and -18.19 kJ/mol at 303K, 313K, 323K respectively, confirming that the process of adsorption was endothermic, with an increase in degree of disorderliness. This also depicts the feasibility and spontaneity of the process of adsorption having higher preference for the RhB dye uptake onto GALAC surface. This demonstrates the affinity of GALAC towards the RhB dye [2,38]. Likewise, the energy value (E_a) (i.e. minimum energy E_a needed for reaction to occur) obtained is in the range of 1–80 kJ/mol. Since the value of E_a obtained were less 80 kJ/mol, it suggests that the process of adsorption followed physio-sorption mechanism. These observations agreed with other findings reported in several studies [4, 39, 76, 108, 127, 128].

Fig. 9.

Van't Hoff plot of 1/T against In K_L for Rh–B dye adsorption onto GALAC.

Table 8.

Thermodynamic parameters for the adsorption of Rh.B dye unto GALAC at different temperatures.

ΔH (kJmol−1)	ΔS (kJmol−1K)	Ea (kJmol−1)	ΔG (kJmol−1)
			303 K	313 K	323 K
11.51	0.39	5.67	-22.71	-19.56	-18.19

3.6. Regeneration and recycling studies

Three different solvents were used for regeneration studies: HCl, NaOH and H₂O respectively. The effect of various solvents used for desorption studies indicated that HCl was the best reagent for regeneration studies because 50 mg/l, 40 mg/l and 30 mg/l of adsorbed RhB dye molecules were obtained after 15 h of contact between the loaded matrix and the regenerating agents, It was also observed that equilibrium was reached in 12 h (Fig. 8). Under acidic conditions, RhB dye is displaced by protons from their binding sites. This is expected because regeneration depends on size of the molecule, number of contact points, surface concentration, temperature and concentration of adsorbed species in solution [129]. The regenerated adsorbent was subjected to recycling studies through regeneration step in between. The regeneration efficiency increased from 80.26 to 92.74% when 0.5 M HCl was used in desorbing RhB dye from GALAC. There was a decrease in efficiency when the adsorbent was used without HCl treatment (Table 9).

Table 9.

Price difference between GALAC and CAC.

Cost Description	Price (USD)
	GALAC (1 kg)	CAC (1 kg)
Cost of purchase	-	376.45
De-ionized water	7.76	-
Electricity	3.22	-
o-phosphoric acid	14.89	-
Transportation	8. 02	11.3
Filter paper	2.45	-
Total	36.34	387.75
Difference (CAC- GALAC)	351.41

3.7. Cost analysis

The cost analysis presented in Table 9 provides a simple proof that GALAC is approximately eleven times cheaper than CAC providing a saving of 351.41 USD/kg. CAC costs 387.75 US$ per kg (transportation inclusive) in total, while GALAC preparation and transportation costs 36.34 US$ per kg. The low cost of preparing GALAC as stated in Table 9 gave detailed summary of prices from GAL transportation to filtration and washing of the AC. Ortho-phosphoric acid, deionized water and transportation are the major significant costs (Table 9).

4. Conclusion

The study showed that GALAC absorbent could be effectively used in the removal of RhB dye from aqueous solution. The GALR and GALAC adsorbents were characterized using SEM, FTIR, EDX, pH_pzc and Boehm Titration (BT techniques respectively. The adsorption of RhB dye on GAL was best described by Langmuir Isotherm model with maximum monolayer coverage of 1000 mg g⁻¹. Adsorption kinetics data was best described by pseudo-second-order kinetics model. Thermodynamic parameters obtained for GALAC (ΔG^o ranges from -22.71 to -18.19 kJmol^-1; ΔH^o 1.51 kJmol^-1; and ΔS^o 0.39 kJmol-1 K⁻¹) indicates that the removal of RhB dye from aqueous solution by GAL was spontaneous, thermodynamically feasible and endothermic in nature. The cost analysis established that GALAC is approximately eleven times cheaper than CAC. Acid treated Gmelina aborea leaf (GALAC) was found to be an effective absorbent for the removal of RhB dye from aqueous solutions.

Declarations

Author contribution statement

Olugbenga Solomon Bello: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Esther Oluwadamilola Alabi, Kayode Adesina Adegoke, Samuel Adewale Adegboyega, Adejumoke Abosede Inyinbor, Adewumi Oluwasogo Dada: Performed the experiments; Analyzed and interpreted the data.

Funding statement

This work was supported by The World Academy of Science (TWAS) in form of Research grants; Research Grant number: 11–249 RG/CHE/AF/AC_1_UNESCO FR: 3240262674 (2012), 15–181 RG/CHE/AF/AC_1_:3240287083 (2015) for the purchase of Research Equipments, NRF-TWAS Doctoral scholarship award given to the third author (UID: 105453 & Reference: SFH160618172220) respectively and LAUTECH 2016 TET Fund Institution Based Research Intervention (TETFUND/DESS/UNI/OGBOMOSO/RP/VOL. IX) given to the corresponding author respectively.

Competing interest statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.