Effective adsorption of toxic brilliant green from aqueous solution using peat of Brunei Darussalam: isotherms, thermodynamics, kinetics and regeneration studies

Abstract

Peat, a natural adsorbent, has been successfully used for the removal of the hazardous water-soluble cationic dye, brilliant green (BG). Characterization of peat was carried out by determining its physical and chemical compositions such as moisture, ash, carbon%, hydrogen%, nitrogen%, and sulphur% (CHNS), crude protein and crude fat. X-ray fluorescence (XRF) was used for the determination of the percentage of elements present in peat. Functional groups present in peat were analyzed using Fourier transform infra-red (FTIR). Changes in the surface morphology of peat before and after treatment with BG were studied using Scanning Electron Microscopy (SEM). The optimization time required for establishment of an adsorption equilibrium is determined to be 2.0 h. The ambient pH of BG was used throughout the study. Adsorption isotherm models such as Langmuir, Freundlich, Halsey, Temkin, Redlich–Peterson (R–P) and Sips were simulated to fit with the experimental equilibrium data. Based on linear regression, simulated isotherm models and error analyses, the R–P isotherm fitted well for the adsorption of BG by peat. Adsorption kinetics was found to follow the pseudo second order model, with a rate constant of 0.39 g mmol⁻¹ min⁻¹. BG-loaded peat was successfully regenerated using 0.01 M sodium hydroxide (NaOH) solution for up to 5 consecutive cycles, while maintaining high adsorption ability of 98% even after the 5^th cycle.

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1. Introduction

Industrial applications of dyes and pigments have increased tremendously over the past few decades,^1,2 and consequently, industrial wastewater is contaminated with such substances. Among many methods, adsorption is an alternative technology for the removal of dyes from wastewater due to its low cost, ease of operation, energy saving potential and high efficiency.³ This technique has been successfully used for the removal of various dyes on low-cost adsorbents namely methylene blue by peels of breadnut and Tarap,^4,5 reactive navy blue dye by powdered orange waste,⁶ methyl violet by Casuarina equisetifolia needles,⁷ direct red by Pseudomonas putida,⁸ Bismark brown R by hen feathers⁹ and acid red 14 on surface soils.¹⁰

In this study, peat was selected as an adsorbent for brilliant green (BG) in wastewater. BG, structure shown in Fig. 1, is a triarylmethane cationic dye and in dry form is an odourless yellowish-green solid powder. It is commonly used as a biological stain, dermatological agent, veterinary medicine, and as an additive to poultry to prevent mould, intestinal parasites and fungus.¹¹ BG is also used as an indicator which changes color from yellow to green at pH 2.6.¹² Human contact with BG might cause irritation to human gastrointestinal tract, respiratory tract, nausea, vomiting, diarrhoea, causing coughing and shortness of breath. BG can also cause skin irritation which in turn might cause redness and pain to human.^11,13

Fig. 1 Chemical structure of brilliant green (BG).

In Brunei Darussalam, it is reported that approximately 100 000 ha (18%) of land area is made up of peat soils and is mostly untouched.^14,15 Peat is a dark brown soil and is made up of vegetative remains of the partial decomposition of leaves, plants, tress or mosses. As peat soil in Brunei Darussalam is low cost and easily available, its adsorption properties on BG were explored in this research. To date, no study has been reported on the use of peat to remove BG. Parameters such as contact time and pH were determined for the most efficient removal of the dye. The pH of dye solutions is an important parameter to be considered in an adsorption process because many reactions, such as ionization, protonation, dissociation of functional groups on the active sites of the adsorbent are pH dependent.^13,16,17 Various salts present in dye wastewater often lead to high ionic strength and could affect the adsorption capacity of an adsorbent. Therefore, there is a need to study the effect of ionic strength on the adsorption of BG on peat. Equilibrium studies were carried out by comparing the adsorption data with isotherm models, such Langmuir, Freundlich, Halsey, Temkin, Redlich–Peterson (R–P) and Sips by using linear and non-linear functions. Determination of various error functions was performed to identify the isotherm model with the best precision. Kinetics studies of BG were examined by using pseudo first order, pseudo second order, Elovich, intraparticle diffusion and Boyd models.

Dye-loaded adsorbents are of no use after adsorption and are either discarded in landfill or incinerated. Both ways are not economical and could be hazardous, toxic, flammable or even explosive if incinerated. Therefore, it is more economical to reuse the dye-saturated adsorbent through regeneration. This in turn will reduce the cost of disposal and subsequently minimize the cost for adsorbent treatment. A superb adsorbent should not only have high adsorption capacity but also high reusable ability which could be recycled or regenerated for various cycles at the same time maintaining its high dye removal capacity. Finally, BG-loaded peat was regenerated using various solvents for the determination of its reusability as a spent adsorbent.

2. Background

Various isotherm models were used in this study for the adsorption of BG on peat. The widely used Langmuir adsorption isotherm is based on the assumption that adsorption takes place at the homogeneous sites and that the adsorbent's surface is saturated after monolayer adsorption.¹⁸ Langmuir isotherm is important for the determination of the adsorption capacity of an adsorbent. The values of q_max and k_L are calculated from the linear plot of Langmuir isotherm, where q_max (mmol g⁻¹) is the adsorption capacity of peat and the Langmuir constant k_L (L mmol⁻¹) is related to both the adsorption capacity and the rate of adsorption.

Langmuir isotherm model also showed that the feasibility of adsorption is based on the dimensionless adsorption intensity (R_L)¹⁹ value given in eqn (1):

where C₀ (mg L⁻¹) is the highest initial concentration of dye and k_L (L mmol⁻¹) is the Langmuir constant. The R_L value indicate the adsorption to be either favorable (0 < R_L < 1), linear (R_L = 1), unfavorable (R_L > 1) or irreversible (R_L = 0).

Freundlich isotherm²⁰ is an empirical equation which provides the relationship between the adsorption of peat and the equilibrium concentration of the dye based on multilayer adsorption. The value of k_F (mmol g⁻¹) represents the Freundlich isotherm constant describing the amount of dye adsorbed on the adsorbent at equilibrium stage, while n is related to the adsorption intensity of an adsorption process.

Halsey isotherm²¹ model was applied to provide better insight on the multilayer adsorption of adsorbate on adsorbent.

Temkin isotherm²² assumes the heat of adsorption decreases linearly with the coverage due to the adsorbate–adsorbent interactions and the adsorption is characterized by uniform distribution of binding energies (up to some maximum binding energies). k_T is the equilibrium binding constant (L mmol⁻¹) corresponding to the maximum binding energy, constant B is related to the heat of adsorption, R is gas constant (8.314 J mol⁻¹ K⁻¹) and T is absolute temperature at 298 K.

Redlich–Peterson (R–P) isotherm²³ is another three parameters isotherm, which combines both Langmuir and Freundlich equations, in which the mechanism of adsorption is a hybrid and does not follow ideal monolayer adsorption. The equation reduces to Freundlich isotherm at high concentration and reduces to Henry's equation at low concentration. R–P isotherm constants are obtained by maximizing the R² value of the linear plot, ln[k_R(C_e/q_e) − 1] vs. ln C_e through k_R, using Microsoft Excel. k_R (L g⁻¹) and a_R (L mmol⁻¹) represent the R–P isotherm constants and g is the exponent lies between 0 and 1.

Sips isotherm²⁴ is a three parameter isotherm model, combining both Langmuir and Freundlich isotherms. At low dye concentrations, it reduces to Freundlich isotherm and at high dye concentration, the equation turned to Langmuir isotherm, predicting a monolayer adsorption.

Table 1 lists standard equations and linearized forms of the six isotherms. Five different error functions, namely average relative error (ARE), sum square error (ERRSQ), hybrid fractional error function (HYBRID), sum of absolute error (EABS), Marquardt's percent standard deviation (MPSD) and Non-linear chi-square test (χ²) of non-linear regression used in this study are given in Table 2.

Table 1 Different isotherm models and its linearized forms

Isotherm model	Equations	Linearized	Plot
Langmuir			Ce/qe vs. Ce
Freundlich			ln qe vs. ln Ce
Halsey			ln qe vs. ln Ce
Temkin		qe = B ln kT + B ln Ce	qe vs. ln Ce
R–P
Sips

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Table 2 List of different error functions^a

Type of errors	Equations
a qe,calc and qe,meas are the calculated and measured adsorption capacity; n is the number of data points; p is the number of parameters.
Average relative error (ARE)
Sum square error (ERRSQ)
Hybrid fractional error function (HYBRID)
Sum of absolute error (EABS)
Marquardt's percent standard deviation (MPSD)
Non-linear chi-square test (χ^2)

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Kinetics models such as pseudo first order,²⁵ pseudo second order,²⁶ Elovich,²⁷ Weber–Morris intraparticle diffusion²⁸ and Boyd¹⁵ models were used to investigate the mechanism of adsorption system in this study. The kinetics equations are summarized in Table 3, where q_e and q_t (mmol g⁻¹) are the amount of dye or heavy metal adsorbed at equilibrium and time (t), k₁ (min⁻¹), k₂ (g mmol⁻¹ min⁻¹) and k_id (mg g⁻¹ min^−1/2) are the rate constants of pseudo first order, pseudo second order and intraparticle diffusion models. α (mmol g⁻¹ min⁻¹) is the initial sorption rate constant (rate of chemisorption) and β (g mmol⁻¹) is related to surface coverage (desorption constant) for Elovich model. C value for intraparticle diffusion model represents the thickness of the boundary layer effect.

Table 3 Equations for kinetics models

Kinetics models	Linear equations	Plot
Pseudo first order		log(qe − qt) vs. t
Pseudo second order		t/qt vs. t
Elovich		qt vs. ln t
Intraparticle diffusion model	qt = kidt^1/2 + C	qt vs. t^1/2
Boyd model	Bt = −0.4977 − In(1 − F)	Bt vs. t

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3. Materials and methods

3.1. Materials

Representative peat samples were collected from Damit (04°N, 114°E) in the Belait District in Brunei Darussalam. Peat was dried in an oven at 60 °C until a constant mass was obtained. Dried samples were sieved to obtain particle sizes between 355 to 850 μm. All experiments in this study were carried out in triplicate at 298 K, unless otherwise stated.

3.2. Chemicals and reagents

The adsorbate, BG was purchased from Sigma-Aldrich [C.I.: 42 040, chemical formula: C₂₇H₃₄N₂O₄S, M.W.: 482.63, λ_max: 625.0 nm]. A stock solution of BG (1000 mg L⁻¹) was prepared by dissolving accurately weighed amount of dye in double-distilled water. Dye solutions of lower concentrations ranging from 10 to 1000 mg L⁻¹ were obtained by dilution with double-distilled water. The pH of solutions was adjusted using 0.1 M NaOH and 0.1 M HNO₃ purchased from Fluka. All chemicals were used without further purification.

3.3. Surface characterization and instrumentation used

Dried peat sample was characterized for its moisture, ash, pH, bulk density, elemental composition (CHNS), crude protein and crude fat using the standard procedures.²⁹ Crude fat and protein of peat sample were analyzed using GerhardT Soxtherm multistate/SX PC, GerhardT Fibretherm FT12 and GerhardT VAP 50S Nitrogen Protein Analyzer, respectively. Elemental percent compositions for C, H, N and S for peat was obtained from the Elemental Analysis Laboratory at the National University Singapore using Elementar Vario Micro Cube. Elemental percent composition of peat before and after BG adsorption were determined by PANalytical Axiosmax X-ray fluorescence (XRF). The pH of the solution was measured using Thermo Scientific Orion 2 Star pH Benchtop pH meter. Morphological characteristics of adsorbent surface were carried out using Tescan Vega XMU Scanning Electron Microscope (SEM) operated at 20 kV accelerated voltage, and SPI-MODULETM Sputter Coater was used to coat the adsorbent.

The UV-absorbance of dye solution was measured using Ultraviolet-Visible (UV-Vis) spectrophotometer (Shimadzu/Model UV-1601PC). Fourier transform infra-red (FTIR) spectrophotometer (Shimadzu Model IRPrestige-21) was used to determine the functional groups present in the sample. Morphological characteristics of adsorbent surface were carried out using Tescan Vega XMU Scanning Electron Microscope (SEM) operated at 20 kV accelerated voltage, and SPI-MODULETM Sputter Coater was used to coat the adsorbent.

Surface chemistry of peat was determined by the point of zero charge (pH_pzc) using the solid addition method.³⁰ Solutions of different pHs were prepared in 0.1 M KNO₃ solution, adjusting its pH at 2, 4, 6, 8, 10 and 12 using 0.1 M HNO₃ and 0.1 M NaOH. KNO₃ solution (25.0 mL) at each pH was then separately mixed with a pre-weighed peat sample (0.050 g). The solutions were agitated on an orbital shaker at 250 rpm for 24 h at room temperature and the final pH was recorded using a pH meter. A graph of ΔpH(pH_final − pH_initial) against pH_initial was plotted where pH_pzc of peat was taken as the point that passes through the x-axis.

3.4. Adsorption experiments for peat on BG

3.4.1. Optimization process. Parameters, such as contact time and pH of the dye solution, were optimized by mixing peat samples (0.050 g) with 10 mg L⁻¹ of BG solution (25.0 mL) by varying the parameter to be optimized while keeping the other parameters unchanged. All adsorption experiments were performed at room temperature (298 K) using an orbital shaker fixed at 250 rpm, unless specified.

Optimum contact time was obtained by shaking the mixture at different time ranging from 30 to 240 min. Effect of pH was studied by using the pre-determined contact time periods. For this purpose, pHs of peat-dye suspensions were adjusted by drop wise addition of 0.1 M HNO₃ and 0.1 M NaOH solution at different pH values between 2 and 10. Each solution was filtered using fine metal sieve and the filtrate was analyzed.

The mass of adsorbed dye molecules per unit mass (g) of solid at the equilibrium state, q_e (mmol g⁻¹) was calculated using the eqn (2):

where C₀ (mg L⁻¹) is the initial dye concentration of BG, C_e (mg L⁻¹) is the equilibrium concentration of dye after adsorption, V is the volume of the dye solution (L), m (g) is the mass of peat sample (adsorbent) and M_r is the molecular weight of BG.

The percentage removal (%) of BG was calculated using eqn (3).

The effect of ionic strength was carried out using different concentrations of KNO₃ solutions ranging from 0.01 M to 1.0 M mixed with BG solution of 100 mg L⁻¹. Peat (0.050 g) was treated with each of the KNO₃ solutions and shaken for optimum contact at room temperature. The mixture was then filtered and the filtrate was analyzed for remaining dye concentration.

3.4.2. Effect of temperature and thermodynamics. Peat (0.050 g) was mixed with BG solution (100 mg L⁻¹, 25.0 mL) and shaken on a temperature-controlled water-bath shaker adjusted to 298, 314, 324, 334 and 344 K using pre-determined contact time, settling time without any adjustment to the pH of solution. BG solution was filtered and the filtrate was analyzed to determine the extent of adsorption. Standard thermodynamic parameters, such as Gibbs free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) were calculated.

3.4.3. Adsorption isotherms and kinetics for the adsorption of BG. Peat (0.050 g) was mixed with 25.0 mL of dye solution of known concentration ranging from 10 to 1000 mg L⁻¹ and shaken for optimum shaking and settle time periods at room temperature. The solution mixture was then filtered and the filtrate was analyzed for its dye content. For kinetics, peat (0.050 g) was mixed with dye solution (500.0 mg L⁻¹, 25.0 mL) and the mixture was shaken. Samples were withdrawn at every one minute interval until equilibrium was reached. The mixture was filtered and filtrate was analyzed for the dye content.

3.5. Regeneration of BG-loaded peat

Regeneration was studied by agitating peat (0.500 g) with 100 mg L⁻¹ BG solution (250 mL) using the optimized contact time so that the active sites were fully saturated with BG. The dye-loaded peat was washed with doubly distilled water to remove the excess BG on the surface and dried overnight at 60 °C. Regeneration was done by using 250 mL of 0.1 M HNO₃, 0.01 M NaOH and water. Control was kept to compare the adsorption capacity with the regenerated peat. Regeneration was carried out for five successive adsorption–regeneration cycles and the absorbance of the BG solution was analyzed using UV-Vis spectrophotometer.

4. Results and discussion

4.1. Characterization and surface chemistry of peat

As shown in Table 4, peat contains approximately 7.11% crude protein and 1.12% crude fat. Elemental characterization using XRF, as shown in Table 5, indicated that the major elements found in peat are oxygen (O), iron (Fe), zinc (Zn) and silicon (Si) in addition to many other minor elements.

Table 4 Characterization of peat³³

Analysis	Values
Moisture (%)	84.60
Bulk density (kg m^−3)	37.90
C (%)	50.00
H (%)	5.22
N (%)	2.16
S (%)	0.50
Crude protein (%)	7.11
Crude fat (%)	1.12
Crude fibre (%)^33	21.14
Total phenolic (mg GAE g^−1)^33	6.35

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Table 5 Characterization of elements present in peat before¹⁵ and after BG adsorption¹⁵

Element	Concentration (%)
	Peat	BG-loaded peat
O	34.95	28.28
Fe	20.16	28.18
Zn	19.01	32.85
Si	10.74	2.08
Ca	6.36	2.95
Al	2.03	0.63
S	1.96	1.78
Ru	1.41	1.77
K	0.87	0.12
P	0.76	0.51
Cl	0.65	0.46
Mg	0.5	0.16
Na	0.47	—
Br	0.09	0.18
Sr	—	0.07

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After adsorption with BG, great reduction was seen for Ca (6.36 to 2.95%) and Si (10.74 to 2.08%), while the elements Al, Cl, K, Mg, P and S showed little decrease after BG adsorption. Such decrease could be due to metal replacement. Previous reports have attributed decrease in metal ions after adsorption process as a result of replacement by adsorbates such as cationic dyes.^4,5,15 On the other hand, the element Zn increased after dye adsorption and explanation to this finding is currently beyond the scope of this work. However, similar observations have been reported where an increase in Zn after adsorption process was observed for adsorbents such as Stenochlaena palustris (Burm.f.) Bedd³¹ and Artocarpus camansi.³²

The point of zero charge (pH_pzc) is the pH at which there is no net charge on the surface of the adsorbent.³⁴ The pH_pzc of peat is determined to be 2.80 from the plot of ΔpH vs. pH_initial (Fig. 2), indicating that at pH < 2.80, the surface of the peat would be positively charged and at pH > 2.80, it would be negatively charged. The pH_pzc of peat experimentally determined in this research is similar to that of algal biomass,³⁵ which also has a high organic matter content. Above the pH_pzc, the pH of the peat suspension if greater than the initial pH, suggesting that there is a tendency for the peat surface to become negatively charged with exposed hydroxyl groups, promoting the adsorption of cationic dyes, such as BG. However, this trend is altered after pH = 8, probably due to the denaturing effect of peat at high basic values.

Fig. 2 Point of zero charge of peat using 0.10 M KNO₃ solution.

4.2. Optimization of parameters

4.2.1. Effect of contact time and settling time. In adsorption study, the effect of contact time relates to the amount of dye adsorbed onto an adsorbent with respect to the amount of time to reach equilibrium.¹³ The variation of the percentage removal against contact and settling times at an initial BG concentration of 10 mg L⁻¹ is shown in Fig. 3. It is clear that the adsorption of BG was rapid at the initial stage (0 to 60 min) of the contact period and gradually slows down when it reaches equilibrium beyond 60 min. The maximum removal of 99% was observed at 60 min.

Fig. 3 Effect of shaking time (♦) on the adsorption of BG on peat (355 μm < d < 850 μm; concentration of dye: 10 mg L⁻¹; mass of peat: 0.050 g; volume of solution: 25.0 mL).

Rapid adsorption at the initial stage is due to the availability of large amount of surface sites for adsorption, resulting in BG molecules being adsorbed onto the external surface of peat through boundary layer adsorption.³⁶ As the contact time increases, the surface of peat gets saturated. Hence, there is limited number of surface sites available for dye adsorption, due to the presence of some repulsive forces forming between BG molecules on the surface of peat and BG molecules present in the solution.^11,13 In the second stage, BG molecules diffused deeper into the micropores of peat, which makes the process slower, thereby BG molecule is aggregated at higher contact time.¹¹ Therefore, the optimum adsorption equilibrium for peat – dye system is established at a contact period of 2.0 h.

The contact time of for the removal of BG molecules on other natural adsorbents such as Citrus limetta peel (4 h),³⁷ rice husk ash (5 h),¹⁶ bottom ash (6 h),¹² deoiled soya (6 h)¹² and Saklikent mud (6 h),³⁸ are longer than that of peat, indicating that peat is efficient for dye removal if applied in real life wastewater treatment.

4.2.2. Effect of pH. The UV-Vis spectra for different pH of BG solutions were measured as shown in Fig. 4. From the spectra, BG is stable at initial ambient pH (4.90), and maximizes at 5 but become unstable when the solution pH is either increased above 10 or decreased below 3, causing the structural changes in BG which destabilized the solution.¹¹ Similar findings were reported for NaOH treated saw dust,¹¹ rice husk ash¹⁶ and binary oxidized cactus fruit peel.³⁹ Decrease of absorbance was observed at pH 2 and 12, indicating structural change of BG occurred which reduces the dye intensity. Therefore, pH 2 and 12 were not included in this study.

Fig. 4 UV-Vis spectra of BG solution at different pHs.

The effect of solution pH on BG adsorption can be partially explained using the information of pH_pzc of peat (2.80) and pK_a (4.93 and 2.62) values of BG, which are related to the ionization of the dye functional groups.⁴⁰ When pH is decreased from 4 to 2, the fact that pH < pH_pzc = 2.80, leads to the competition of H⁺ ions with bulky BG molecules for limited surface sites causing the reduction in dye removal. Increase in the extent of removal when pH is increased is due to the opposite effect as pH > pH_pzc leads to negative charges on the surface of the adsorbent, promoting uptake of cationic dyes and the dye cations are positively charged when pH > pK_a. Little decrease observed at pH = 10 (95%) can be attributed to high OH⁻ concentration which would inhibit the dye to adsorb onto the peat surface as was reported for other dyes with Arundo donax root⁴¹ and Citrus limetta peel.³⁷

Effect of pH was carried out by adjusting BG solution pH from 3 to 10 and 98 to 99% removal of the dyes was obtained between 4 and 9 as shown in Fig. 5. Because the ambient pH at 4.9 leads to the maximum removal of 99% under the experimental conditions employed, no adjustment to pH is recommended for the adsorption study. A similar result¹⁷ was found for the removal of BG by white rice husk ask, where no adjustments of pH were needed.

Fig. 5 Effect of pH on the removal of BG by peat (355 μm < d < 850 μm; concentration of dye: 10 mg L⁻¹; mass of peat: 0.050 g; volume of solution: 25.0 mL).

4.2.3. Effect of solution temperature and thermodynamics. The effect of solution temperature for dye removal on peat was investigated at various temperatures (298, 314, 324, 334 and 344 K) at initial dye concentrations of 100 mg L⁻¹. The results showed that the removal of BG increases from 88% to 99% as the temperature rises from 298 K to 344 K. Increasing temperature could also enhance the rate of diffusion of the dye molecules across the external boundary layer and internal pores of peat which reduces the viscosity of BG solution.^37,38,42

Thermodynamic parameters such as Gibbs free energy (ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°) were estimated for BG adsorption on peat. ΔG° values at different temperatures ranging 298 to 344 K were calculated using eqn (4),

ΔG° = −RT ln K_c (4)

where ΔG° is the Gibbs free energy (kJ mol⁻¹), R is the universal gas constant (8.314 J mol⁻¹ K⁻¹), T is the absolute temperature (K) and K_c is the equilibrium constant (C_s/C_e). C_s is concentration of dye on adsorbent (mg L⁻¹) and C_e is the equilibrium concentration of dye in solution phase (mg L⁻¹). The enthalpy (ΔH°) and entropy (ΔS°) values can be calculated from the Van't Hoff eqn (5):

Enthalpy (ΔH°) and entropy (ΔS°) values of BG on peat was determined from slope (−ΔH°/R) and intercept (ΔS°/R) of the plot of ln K_c vs. 1/T (Fig. 6). The calculated parameters are given in Table 6.

Fig. 6 Thermodynamics plot of ln K_c vs. 1/T.

Table 6 Thermodynamic parameters for the adsorption of BG onto peat at different temperatures

T (K)	ΔG° (kJ mol^−1)	ΔS° (J mol^−1 K^−1)	ΔH° (kJ mol^−1)
298	−4.8	175.7	47.0
314	−8.9
324	−10.7
334	−11.0
344	−13.2

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The negative values of ΔG° indicated that the adsorption process of BG on peat is spontaneous and feasible within the temperature range of investigation. Further having ΔG° between 0 to −20 kJ mol⁻¹ signifies that the adsorption of BG on peat is a physisorption process, similar to the adsorption of BG on red clay⁴³ and basic dye using Luffa cylindrical.⁴⁴ Positive value of ΔH° (47.0 kJ mol⁻¹) indicates that the reaction is endothermic. Having ΔH° > 8 kJ mol⁻¹, also verifies that the adsorption of BG on peat is a physical process.^13,43 This is also supported by the increase of dye uptake (100 mg L⁻¹ BG: 88 to 99%) with the rise of solution temperature (298 to 344 K), which further supports the endothermic behavior described earlier.

4.2.4. Effect of ionic strength. The effect of ionic strength was studied by adding 0 to 1.0 M KNO₃ solution to each BG solution (100 mg L⁻¹) and agitated at predetermined time periods at ambient temperature. Increases in the concentration of KNO₃ decreased removal of BG from 67% (0 M KNO₃) to 54% (1.0 M KNO₃) (Fig. 7). This decrease could be due to the increased K⁺ and NO₃⁻ ions surrounding peat molecules and blocking BG molecules being attracted towards the peat surface. However, this reduction is considered minimal whereas adsorbents such as breadnut peel^4,32 and duckweed⁴⁵ showed a decrease of more than 40% when the concentration of salt increases. With respect of this, peat has great potential to be applied in real life for the removal of BG. Similar results have been reported for Saraca asoca leaf³⁴ and breadnut peel.⁴

Fig. 7 Effect of ionic strength on the removal of BG by peat (355 μm < d < 850 μm; concentration of dye: 100 mg L⁻¹; mass of peat: 0.050 g; volume of solution: 25.0 mL).

4.3. Adsorption isotherms

Sorption isotherm equations were used to estimate the amount of peat sorbent to sorb dye molecules in solution and providing explanation for the interaction of peat and dye molecules. Different isotherm models such as the Langmuir, Freundlich, Halsey, Temkin, Redlich–Peterson (R–P) and Sips were applied in this study for BG solution of initial concentration between 0 and 1000 mg L⁻¹. The isotherms equations and the linearized form used for regression analysis are shown in Tables 1 and 2. The relationship between the extent of adsorption and the initial concentration is shown in Fig. 8.

Fig. 8 Adsorption isotherm of BG by peat at 298 K (355μm < d < 850 μm; mass of peat: 0.050 g; volume of solution: 25.0 mL).

Isotherm parameters calculated from its linear plot are shown in Table 7. High R² (0.92) showed that the Langmuir isotherm model fitted well to the adsorption process between BG and peat, obtaining a maximum adsorption capacity (q_max) of 0.55 mmol g⁻¹ (265.47 mg g⁻¹). The R_L values calculated for the adsorption of BG for many concentrations lie between 0 and 1, suggesting the adsorption of BG on the surface of peat was favorable.

Table 7 Isotherm parameters for the removal of BG by peat at 298 K

Model	Parameter
Langmuir	qmax (mmol g^−1)	0.55
	kL (L mmol^−1)	0.02
	R^2	0.92
Freundlich	kF (mmol g^−1)	0.04
	n	2.52
	1/n	0.40
	R^2	0.99
Halsey	n	2.52
	kH (L mmol^−1)	0.0003
	R^2	0.99
Temkin	kT (L mmol^−1)	0.62
	bT (J mol^−1)	32 314.25
	R^2	0.86
R–P	kR (L g^−1)	8.00
	g	0.60
	aR (L mg^−1)	200.31
	R^2	0.99
Sips	qmax (mmol g^−1)	1.00
	kS (L mmol^−1)	0.04
	n	1.98
	1/n	0.51
	R^2	0.96

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According to the adsorption data in Table 7, the Freundlich isotherm model also fits well (R² = 0.99) to the adsorption data, having n > 1, indicating a favorable adsorption. The surface heterogeneity of an adsorbent is represented by using 1/n value, in which the adsorption is considered favorable and heterogeneous when the value of 1/n lies between 0 and 1, the adsorption is homogenous in which there is no interaction among the adsorbed species when 1/n = 1 and the adsorption is unfavorable when 1/n > 1.⁴⁶ In this case, the value of 1/n is 0.40; this value implies only 40% of active adsorption sites having equal energy level, indicating that there is a possibility of multilayer adsorption of BG on the heterogeneous sites of peat.⁴⁷ As implied by Mane and Babu,¹¹ 1/n = 0.40 implied that there are 40% of active adsorption sites having equal energy level. Hence, the surface heterogeneity of peat could be due to the presence of different types of cations, surface charges and crystal edges on peat.⁴⁸

Similar to the Freundlich isotherm model, the Halsey isotherm model also fits well to the adsorption data (R² = 0.99), signifying that BG follows multilayer adsorption on peat. This phenomenon is further confirmed by evaluating the coverage factor,^43,49 which is the fraction of the peat surface covered by BG molecules, using eqn (6) and (7).

where q_max (0.55 × 10⁻³ mol g⁻¹) is the Langmuir maximum adsorption capacity, N is the Avogadro' constant (6.022 × 10²³ molecules mol⁻¹), S (365.64 m² g⁻¹) is the specific surface area of peat, M.W. (482.62 g mol⁻¹) is the molecular weight of BG, ρ (0.55 g cm⁻³)⁴³ is the density of BG. Value of θ > 1 indicates the formation of double or multilayer of dye on peat.⁴³ The coverage factor (θ) for BG is 1.27 hence, confirming the formation of multilayer adsorption on peat.

Low R² values for Temkin (0.86) isotherm model indicates that this adsorption model was not suitable to fit to the experimental data. Linear plot of R–P model showed the best fit (R² = 0.99) for the adsorption of BG on peat compared to Langmuir, Freundlich, Halsey, Temkin isotherm models. The g value (0.60) falls between 0 and 1, suggesting that BG adsorption on peat is favorable and physiosorptive¹⁶ and suggesting that the adsorption of BG on peat was favored towards Freundlich instead of Langmuir. This also implies that the sorption of BG on peat is not monolayer, but possibly by complexation or ion-exchange interaction between peat and dye molecules. The order of isotherm models is explained in the following order: R–P (0.99), Freundlich and Halsey (0.98), Sips (0.96), Langmuir (0.92) and Temkin (0.86), based on R² values. This trend was found to be similar to red clay,⁴³ modified mesoporous clay,⁴⁷ sugarcane dust⁵⁰ and NaOH treated saw dust.¹¹

The simulated isotherm plot for the adsorption of BG on peat is shown in Fig. 9 for the determination of the best fit isotherm models. The error values calculated from error functions are listed in Table 8.

Fig. 9 Different isotherm models for adsorption of BG peat at 298 K (355 μm < d < 850 μm; concentration of dye: 0 to 1000 mg L⁻¹; mass of peat: 0.050 g; volume of solution: 25.0 mL).

Table 8 Values of different error values of isotherm models for BG onto peat at 298 K

Isotherms	R^2	ARE	ERRSQ	HYBRID	EABS	MPSD	χ^2
Langmuir	0.92	31.61	0.06	1.98	0.83	42.77	0.28
Freundlich	0.99	9.05	0.02	0.31	0.38	11.38	0.11
Halsey	0.99	9.05	0.02	0.31	0.38	11.16	0.11
Temkin	0.86	49.31	0.08	4.77	0.99	94.78	0.57
R–P	0.99	9.10	0.02	0.33	0.38	11.85	0.11
Sips	0.96	12.90	0.03	0.61	0.49	17.46	0.11

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The Freundlich, Halsey and R–P appeared to fit the data best, with high R² and the lowest error function values. From the linear plots, R–P isotherm has the highest R² value (0.99) compared to Freundlich (0.99) and Halsey (0.99) isotherms. Besides high R² values, lowest error values were found for Freundlich and Halsey isotherms. Similar error values were found for R–P isotherm. Alternatively, the experimental data in Fig. 9 fitted very well with the calculated Freundlich, Halsey and R–P isotherms. Of all the isotherms, Temkin model did not fit the experimental data due to low R² values together with high ARE, ERRSQ, HYBRID, EABS, MPSD and χ² values for the adsorption of BG on peat. Both Langmuir and Sips isotherms have slightly higher R² values of 0.92 and 0.96, respectively. However, the error values calculated for both Langmuir and Sips were also very high when compared with the error values of Freundlich, Halsey and R–P isotherms. Therefore, with respect to the R² values, based on the error values and simulated isotherms, the suitable isotherm models to represent the adsorption data of BG on peat are Freundlich, Halsey and R–P isotherm models.

The maximum adsorption capacities (q_max) of BG by various reported adsorbents are listed in Table 9. Generally, the q_max of peat with BG are higher compared to some natural adsorbents as well as some of the activated carbon, indicating peat is suitable to be used as an effective low cost adsorbent in real life wastewater treatment.

Table 9 Comparison of maximum adsorption capacity (q_max) for the removal of BG on selected adsorbents based on the best experimental condition of each work

Adsorbent	qmax (mg g^−1)	Isotherm model	Temperature (K)	References
Damit peat	265.4	Langmuir	298	This work
Ashoka leaf powder	125.0	Langmuir	298	34
Kaolin	65.4	Langmuir	299	13
Rice husk ash	25.1	Langmuir	303	16
Waste brewery's yeast	141.9	Langmuir	303	51
Binary oxidized cactus fruit peel	166.7	Langmuir	293	39
Acorn activated carbon	2.1	Langmuir	303	40
Saklikent mud	1.2	Langmuir	298	38
Neem leaf powder	133.6	Langmuir	300	52
NaOH treated saw dust	58.5	Langmuir	303	11

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4.4. Scanning electronic micrograph studies (SEM)

The morphological features and surface characteristics of the peat sample before and after treatment with BG analyzed using SEM at 1000× magnification (Fig. 10a) showed that the surface of the untreated peat are uneven, rough and contains pores which provide suitable binding sites for BG molecules. However, identification of binding sites for BG is beyond the task of this research. Adsorption of the dye molecules leads the coverage of most of the available pores present in peat causing the surface of peat sample to become saturated as shown in Fig. 10b. Nevertheless, it is not expected that BG molecules, owing to its relatively larger size, would transfer to the bulk of the peat matric, and consequently, compositional changes are not expected.

Fig. 10 Scanning electron microscope (SEM) micrographs of peat (a) before adsorption and (b) after adsorption with BG at 1000× magnification.

4.5. FTIR studies

FTIR spectra provide useful information for characterization of the functional groups. The infrared spectra of peat before and after treatment with 1000 mg L⁻¹ BG are presented in Fig. 11. The band at 3386 cm⁻¹ for untreated peat indicates the presence of –OH and –NH stretching on the surface of peat both untreated and treated peat. The bands observed at 2920 and 2850 cm⁻¹ are related to the asymmetric stretches of –CH group and symmetric stretching vibrations of –CH group respectively for untreated and dyes treated peat. Absorption band at 1717 cm⁻¹ is attributed to –COOH groups. The bands at 1626, 1512 and 1460 cm⁻¹ indicate the presence of –COO, –C=O and –NH groups for both untreated and treated peat. The C=O band at 1626 cm⁻¹ for untreated peat was shifted to 1617 cm⁻¹ and the band formed at 1581 cm⁻¹ for BG treated peat was associated with the existence of aromatic rings. Another band appearing at 1421 cm⁻¹ represents the –O–H bending vibration. The peak at 1376 cm⁻¹ for untreated peat indicated the asymmetrical bending vibration of –CH₃ and –CH₂. This peak has become broader and sharper for BG treated peat at 1329 cm⁻¹. Peak present at 1242 cm⁻¹ indicates the presence of O–C–H, C–C–H and C–O–H groups bending vibration. Band at 1035 cm⁻¹ implies the presence of bending vibration of –OH group and stretch vibration of C–O–C in lignin structure of peat.

Fig. 11 FTIR spectrum of (a) untreated peat, (b) peat treated with 1000 mg L⁻¹ BG.

4.6. Kinetics studies

Kinetics models are used to investigate the mechanism of adsorption and its potential rate-controlling step which include the mass transport and chemical reaction processes. Dye adsorption on solid adsorbents can be understand through the two distinct mechanisms where binding of dye molecules on the surface of sorbent was initially rapid and slowed at the intraparticle diffusion stage. The kinetics data of peat treated with 500 mg L⁻¹ BG were analyzed using the kinetics expression (Table 3) and the kinetics parameters are tabulated in Table 10. The best models were selected by the determination of R² and by comparing the adsorption capacity values q_e,calc and q_e,exp.

Table 10 Kinetics parameters for adsorption of BG onto Damit peat at RT

Pseudo first order	BG
qe,exp (mmol g^−1)	0.28
qe,calc (mmol g^−1)	0.16
k1 (min^−1)	0.03
R^2	0.98
Pseudo second order
qe,calc (mmol g^−1)	0.27
k2 (g mmol^−1 min^−1)	0.39
h (mmol g^−1 min^−1)	0.03
R^2	0.99
Elovich
α (mmol g^−1 min^−1)	0.34
β (mmol g^−1)	29.16
R^2	0.81
Intraparticle diffusion
C	0.10
kid (mmol g^−1 min^−1)	0.02
R^2	0.94

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In order to determine the best fit kinetics model to describe the sorption process of BG on peat, comparison of R² values (Table 10) showed that pseudo second order model gave the best R² (0.99), suggesting that this model provides a better correlation for the adsorption of BG on peat. In addition, the q_e,calc value (0.27 mmol g⁻¹) fitted very well with its q_e,exp value (0.28 mmol g⁻¹), further confirming the suitability and validity of the pseudo second order model for the adsorption process for BG on Damit peat. The initial adsorption rate, h (mmol g⁻¹ min⁻¹) is 0.03 mmol g⁻¹ min⁻¹ and the rate constant, k₂, is 0.39 g mmol⁻¹ min⁻¹. The rate of adsorption, k₂, for the adsorption of BG on peat was high, indicating that less time is required to reach equilibrium, which is cost beneficial if applied in real life wastewater treatment.

Therefore, Elovich kinetics model which assumes the solid surface active sites are heterogeneous in nature and exhibit different activation energies for chemisorption is applied to provide better understanding on the adsorption process. The rate of chemisorption and the available adsorption surface (α) for the sorption of BG on peat is 0.34 mmol g⁻¹ min⁻¹, while the surface coverage (β) of peat is 29 mmol g⁻¹.

Intraparticle diffusion and Boyd kinetics expression were both applied to identify whether the adsorption mechanism is particle diffusion or film diffusion. Particle diffusion indicates that the adsorbate transport within the pores of the solid adsorbent while film diffusion is to the external surface of adsorbent. The intraparticle diffusion plot (Fig. 12) did not pass through the origin, suggesting that this could control the overall rate of the adsorption process. The thickness of the boundary layer (C) is 0.10, indicating that the surface adsorption and intraparticle diffusion along the boundary layer diffusion were simultaneously operating during the adsorption of BG on peat.

Fig. 12 Intraparticle diffusion plot.

The kinetics data was fitted into Boyd kinetics expression¹⁵ to provide information on the actual slow step involved in adsorption. The equation is given in eqn (8).

andwhere F is the fraction of solute adsorbed at any time (t) and B_t is mathematical function of F.

Rearranging the eqn (8) gives:

and

B_t = −0.4977 − In(1 − F) (11)

The B_t values at different time, t were calculated and a plot of B_t vs. t for the adsorption of BG on peat in Fig. 13 gave linear plots with high R² value (0.98). According to Fig. 13, the plot gives a straight line passing through the origin, indicating that the adsorption process is governed by intraparticle diffusion mechanism according to the mathematical relationship given in Table 3.

Fig. 13 Boyd plot for the adsorption of 500 mg L⁻¹ BG on peat.

The effective diffusion coefficient, D_i (cm² s⁻¹) is calculated using eqn (12).

where r (cm) is the radius of adsorbent calculated by sieve analysis by assuming as spherical particles. In this case, r is 0.0425 cm. The average D_i values for BG is estimated to be 1.18 × 10⁻⁴ cm² s⁻¹.

4.7. Regeneration of dye-loaded adsorbent

Peat loaded with 100 mg L⁻¹ BG solution for predetermined contact time was regenerated with 0.01 M HNO₃, 0.01 M NaOH and doubly distilled water for five adsorption–regeneration cycles, with a control (no washing) to compare the adsorption capacity with the regenerated peat. The removal efficiencies (%) of BG-loaded peat for five adsorption–regeneration cycle are shown in Fig. 14. For BG-loaded peat which was regenerated through washing with doubly distilled water and control showed that the adsorbent lost its removal efficiency by almost 50% after four consecutive cycles of adsorption–regeneration and was unable to adsorb more dye at the 5^th cycle. The reduction of removal efficiency after several cycles could be due to the collapse of porosity on the surface of peat after several adsorption–regeneration cycles, which induce pore blockage and therefore restricts the adsorption of dye molecules onto the surface. For acid regeneration, BG-loaded peat gave an initial adsorption of 60% removal efficiency and thereafter increased to 69% at 5^th cycle, showing a slight improvement of removal efficiency through acid regeneration. However, regeneration with 0.01 M NaOH was the best regeneration method compared to regeneration using acid and water. The removal efficiency increased from 59 to 98% from cycle 1 to 2 and remained 98% through 5 cycles. Hence, base regeneration is the most suitable regeneration method for peat in real life application in wastewater treatment which enabled the reusability of peat for a longer period of time while maintaining its high removal efficiency.

Fig. 14 Regeneration of BG-loaded peat adsorbent using control, water, 0.1 M HNO₃, 0.01 M NaOH for 5 consecutive cycles.

5. Conclusion

In this study, peat native to Brunei Darussalam was demonstrated to remove BG from solution. Maximum removal of BG was observed at the ambient pH (4.9), hence no pHs adjustments were required throughout the experiments. Thermodynamics studies show that the adsorption process of peat and BG was favorable, spontaneous and endothermic in nature. Among the six isotherm models investigated in this study, R–P isotherm model with the highest R² value of 0.99, is the suitable model. This was further confirmed by error analysis and comparison of the simulated isotherms. As it is a combination of the Langmuir and Freundlich models, the maximum adsorption capacity of 0.55 mmol g⁻¹ (265 mg g⁻¹), determined from the Langmuir model is a reasonable prediction for the BG-peat system. The adsorption kinetics fitted well to the pseudo second order model, with the rate constant of 0.39 g mmol⁻¹ min⁻¹. BG-loaded peat was regenerated and reused 5 times while maintaining high removal efficiencies of >90% using 0.01 M NaOH as the desorption agent. Further testing in pilot scale of side stream applications with manufacturer wastewater is recommended.

Acknowledgements

The authors would like to thank the Government of Negara Brunei Darussalam and the Universiti Brunei Darussalam (UBD) for their financial support in carrying out this research. The authors are also grateful to the staff at the CAMES at UBD for the use of XRF.

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