Introduction

Dyes are widely used in various industries such as textile, leather, paper, printing, food, cosmetics, paint, pigments, petroleum, solvent, rubber, plastic, pesticide, wood preserving chemicals, and pharmaceutical industry. Over 10,000 of different commercial dyes and pigments exist currently and more than 7 × 105 tonnes are produced annually worldwide [1–3]. Discharge of dye-bearing wastewaters into the natural environment from textile, paper and leather industries causes a serious threat for the aquatic life [4]. On the other hand, limited aquatic resources and increasing demand for safe water require efficient water treatment methods [5]. Synthetic dyes are generally resistant to biodegradation and physicochemical techniques for their removal [6, 7], such as adsorption, chemical oxidation, electrocoagulation and advanced oxidation processes (AOPs) have been extensively used to comply with more and more stringent legislation regarding the maximum allowable dye concentration in wastewaters [7–10]. Methylene blue (MB) is a thiazine cationic dye with widespread applications, including coloring paper, dyeing cottons, wools and coating for paper stock. It is also used in microbiology, surgery and diagnostics and as a sensitizer in photo-oxidation of organic pollutants. Although it has low toxicity, it can cause some specific harmful effects for the human health such as heartbeat increase, vomiting, shocks, cyanosis, jaundice and tissue necrosis [11, 12]. Hence, its removal from wastewaters is an important issue for the environmental protection [13]. The conventional methods have been extensively used for treating waters contaminated with heavy metal and dyes [14–16]. However, these methods present some disadvantages such as high cost, low removal efficiency and production of excessive toxic sludge [17]. Recently, inexpensive, ecofriendly and not pathogenic organisms have been used for the dye removal [18]. In this respect, the biosorption process has attracted a great interest in this context, and seems a good alternative for the removal of dyes and other pollutants from wastewaters [19, 20], as a replacement for costly commercially biosorbents [21]. It can be defined as sequestering of organic or inorganic compounds by alive or dead biomasses or their derivatives; the biomass can consist of bacteria [22], fungal [19], yeasts [22], algae [23], seaweeds and even industrial or agricultural wastes [24, 25]. Different vegetal biomasses have been used such as Opuntia ficus indica [26], Sugar beet pulp [21], Stoechospermum marginatum [24], Scolymus hispanicus L. [27], Palm kernel [28], Pinus brutia Ten. [29], Waste orange peel [30], Posidonia oceanica L. [31], Cyperus rotundus [32], Date stones and Palm-trees waste [33].

The present study examines a new dye biosorbent namely the Luffa cylindrica fiber and its feasibility for the removal of methylene blue from aqueous solution. It is inexpensive and easily available in many regions of Algeria. Luffa cylindrica is composed of 60 % cellulose, 30 % hemicelluloses and 10 % lignin and is classified as lignocellulosic material [34]; the Luffa products are natural and biodegradable. The biosorption of methylene blue onto Luffa cylindrica fiber is carried out by batch biosorption experiments. The influence of the contact time, initial pH, biosorbent dose, initial MB concentration, particle size and temperature is investigated. Furthermore, the isotherm and kinetic models are evaluated and the thermodynamic data are determined.

Materials and methods

Preparation of the biosorbent

The Luffa cylindrica plant was naturally collected in July, from Algeria. The plant was repeatedly washed with distilled water to remove dirt particles, dried at 80 °C for 48 h, crushed in grinder and sieved to obtain particle sizes in the range (63–630 μm). The powdered biosorbent was stored in an airtight container until use.

Point of zero charge (pHpzc)

The point of zero charge (pHpzc) of the Luffa cylindrica fiber was evaluated by the solid addition method using KNO3 (0.01 M) solution [36]. The experiments were carried out in 100 mL erlenmeyer flasks with stopper cork containing 50 mL of KNO3 solution (10−2 M). The initial pH (pHi) in each flask was adjusted between 3 and 11 by adding NaOH or HCl solutions (0.1 M). Then, 0.5 g of the Luffa cylindrica was added to each flask which are kept for 48 h with intermittent manual shaking to reach the equilibrium. The difference of the initial and final pH (pHi, pHf) was plotted against the initial pH. The point of intersection of the resulting curve with the abscissa axis, for which ΔpH = 0, gives pHpzc (Fig. 1).

Fig. 1
figure 1

The chemical structure of the methylene blue

Full size image

Methylene blue solution

The dye used in all experiments was methylene blue, a basic cationic dye supplied by (Biochem company, Algeria). MB was chosen because of its various applications. MB has a molecular weight of 319.85 g mol−1, which corresponds to methylene blue hydrochloride with three water molecules, the structure is shown in Fig. 2.

Fig. 2
figure 2

The determination of the point of zero charge (pHPZC)

Full size image

The FT-IR spectra were recorded over the range (400–4000 cm−1) using a Shimadzu FTIR-8400S spectrometer. The scanning electron microscopy (SEM) was performed with a JEOL-JSM 6360 Microscope.

Batch biosorption experiments

The biosorption was conducted in Pyrex 500 mL conical flasks at a constant agitation speed. The experiments were carried out by varying the biosorbent particle size over the range (63–630 µm), contact time (5–160 min), biosorbent dosage (0.5–8 g L−1), pH (2–10), initial dye concentrations (20–300 mg L−1) and temperatures (20–60 °C). The temperature was controlled with an isothermal shaker. After each biosorption test, the sample was centrifugated (6000 rpm, 10 min) for solid–liquid separation; the residual MB concentration was analyzed by a UV–Vis spectrophotometer (2120 UV Optizen III, South Korea) at λ max = 663 nm. The equilibrium, kinetic and thermodynamic study were performed by determining the optimum biosorption conditions. The amount of MB biosorbent q t (mg g−1) was calculated from the relation (1):

$$q_{t} = \frac{{\left( {C_{0} - C_{t} } \right)}}{m}V$$
(1)

where C 0 is the initial dye concentration (mg L−1), Ct the concentration of dye at time t (mg L−1), V the volume of the solution (L) and m the mass of biosorbent (g). The dye removal percentage is calculated as:

$$R\,\left( \% \right) = \frac{{\left( {C_{0} - C_{t} } \right)}}{{C_{0} }}100$$
(2)

Statistical evaluation of the kinetic and isotherm parameters

To determine the best-fit model for the biosorption, the linear curve fitting by the software OriginPro 8.5 was employed to simulate and to confirm the fitting of the biosorption kinetic and isotherm models to the experimental data. The statistical significance of variables was evaluated from the analysis of variance ANOVA (Fisher function, F value, and probability, P value), while the adjusted correlation coefficient (Adjusted R 2) was used to assess the adequacy of the fitting [35]. F value and Adjusted R 2 were calculated as:

$$F\,{\text{value}} = \frac{{\left( {\sum\limits_{i = 1}^{n} {\left( {q_{i,cal} - \bar{q}_{i,\exp } } \right)^{2} } } \right)/p - 1}}{{\left( {\sum\limits_{i = 1}^{n} {\left( {q_{i,\exp } - q_{i,cal} } \right)^{2} } } \right)/n - p}}$$
(3)
$${\text{Adjusted }}R^{2} = 1 - \frac{{\left( {\sum\limits_{i = 1}^{n} {\left( {q_{i,\exp } - q_{i,cal} } \right)^{2} } } \right)/n - p}}{{\left( {\sum\limits_{i = 1}^{n} {\left( {q_{i,\exp } - \bar{q}_{i,\exp } } \right)^{2} } } \right)/n - 1}}$$
(4)

where q i,exp is each value of q i measured experimentally, q i,cal is each value of q i predicted by the fitted model, \({\bar{\text{q}}}_{\text{e,exp}}\) is the average of q i experimentally measured, n is the number of experiments performed and p is the number of parameter of the fitted model.

Desorption

MB solution (100 mg L−1) was mixed with Luffa cylindrica at pH 6 for 4 h. The residual MB concentration was measured. The MB loaded Luffa cylindrica was dried at 80 °C. Four eluting solvents (100 mL): H2O, HCl (0.1 M), NaOH (0.1 M), and NaCl (0.1 M) each one containing 0.2 g of MB loaded Luffa cylindrica at room temperature. The percentage of desorbed dye from the adsorbent was calculated (=100× desorbed mass/adsorbed mass).

Results and discussion

Characterization

FT-IR analysis of the biosorbent

The FT-IR spectrum of the Luffa cylindrica was plotted to obtain information about the nature of functional groups at the surface. The spectrum (Fig. 3) shows a dominant peak at 3450 cm−1 attributed to O–H stretching vibrations in hydroxyl groups, involved in hydrogen bonds. The bands observed at 2944 cm−1 are assigned to asymmetric C–H bonds, present in alkyl groups. The absorption peaks at 1737 cm−1 correspond to stretching of carboxyl groups. The strong absorption band at 1639 cm−1 is indicative of OH bending vibrations, while that at 1401 cm−1 is due to C–O stretching. The band at 1322 cm−1 is assigned to C–O groups on the biomass surface, whereas that at 1160 cm−1 corresponds to antisymmetric bridge C–OR–C stretching (cellulose) [37, 38]. The band at 1107 cm−1 is attributed to anhydroglucose ring (cellulose) [38]. The peaks at 1058 cm−1 are indicative of C-OR stretching (cellulose), while the band at 884 cm−1 could be attributed to antisymmetric, out of phase ring stretching [37].

Fig. 3
figure 3

FTIR spectrum of Luffa cylindrica

Full size image

SEM–EDS analysis

The morphology of the Luffa cylindrica was observed by SEM. The fibers, formed by fibrils glued, are disposed in a multi-directional array, forming a natural mat (Fig. 4a); the diameters of single fibers are in the range (63–125 µm). To observe the inner fibrils and further investigate the complicated physical structures in the natural Luffa cylindrica, a crude fiber was observed at high magnification (Fig. 4b). The SEM image shows that the fiber has a heterogeneous appearance with an outer rich lignin layer around the fibers. The internal fibrils cannot be seen due to the lignin layer. At higher magnification (Fig. 4c, d), the SEM image displays a rougher surface with lots of waxy and gummy substances on the untreated Luffa cylindrica fiber; the internal fibrils cannot be observed [38]. The EDS spectrum is shown in Fig. 5 and the contents of each element are listed in Table 1. The energy dispersive X-Ray microanalysis (SEM/EDS) of the Luffa cylindrica fibers indicates mainly the presence of carbon (65.68 %) and oxygen (30.13 %). However, as the EDS analysis is less sensitive for light elements (Z ≤ 10) [39], the carbon and oxygen content were quantified by ultimate analysis. Their concentrations suggest the presence of high amount of different oxygenated groups on the carbon surfaces, such as Cl, Ca, Na, Cu, Mg, K, Ni, Si and P whose contents are between 0.09 and 1.21 %. Similar results (carbon: 64.0 %, oxygen: 34.9 %) were already obtained by Tanobe et al. [38].

Fig. 4
figure 4

SEM micrographs of Luffa cylindrica

Full size image
Fig. 5
figure 5

EDS spectrum from the Luffa cylindrica

Full size image
Table 1 Principal elements identified on the biomass surface by SEM/EDS
Full size table

Biosorption

Effect of contact time and initial dye concentration

Experiments were undertaken to study the effect of the initial concentration of MB over the range (20–300 mg L−1) at 20 °C on the biosorption onto Luffa cylindrica at regular interval times. The rate of the MB removal by Luffa cylindrica was rapid, the maximum uptake was achieved in the first 20 min, accounting for 90–42 % biosorption, respectively, for MB initial concentrations of 20–300 mg L−1 (Fig. 6). The biosorption rate after this initial fast phase slows down significantly until it reaches a plateau after 60 min, indicating equilibrium of the system. The initial rapid phase may be due to an increase in the number of available vacant sites. The increase of the biosorption with raising the MB concentration is attributed to the fact that at higher concentrations, the ratio of the initial number of MB molecules to the available surface area is large; consequently, the fractional biosorption becomes dependent on the initial concentration. By contrast, at low concentrations, the available sites of biosorption are fewer and hence the MB removal depends upon their concentration [40].

Fig. 6
figure 6

Effect of contact time on the biosorption kinetics of MB by Luffa cylindrica (biosorbent dose = 3 g L−1, initial pH = 5.80 and T = 20 °C)

Full size image

Effect of solution pH

The pH of the solution is a crucial controlling parameter in the biosorption [41, 42]. This is possibly due to its impact on both the surface binding sites of the biosorbent and ionization status of the MB molecule in water. Since the MB biosorption can dramatically change with changing pH, it has been stressed that not only it should be accurately reported but also the data for all comparative studies must be obtained at the same pH values. The effect of pH on MB biosorption was studied over the pH range (2–10) and the results are shown in Fig. 7. The equilibrium biosorption uptake presents a minimum at pH ~ 2 (6.16 %) and increases up to 5, then remains nearly constant (80.86 %) over the initial pH ranges (6–10). At low pHs, the surface charge is positively charged, and the H+ ions compete effectively with dye cations causing a decrease in the amount of adsorbed dye. At higher pH, the Luffa cylindrica fibers, mainly lignin and cellulose chains, become negatively charged, thus enhancing the cationic dye by electrostatic attraction forces [43, 44].

Fig. 7
figure 7

Effect of the solution pH on the MB removal (C 0 = 20 mg L−1, biosorbent dose = 1 g L−1 and T = 20 °C)

Full size image

Effect of biosorbent dose

The biosorbent dose is an important parameter because it determines the capacity of biosorbent for a given concentration of the adsorbate [45]. The effect of the biomass dosage (0.5–8 g L−1) on the MB biosorption was studied in 1 L MB solution (50 mg L−1) under optimized conditions of pH and contact time. The removal percentage of MB increases drastically from 12.77 to 96.16 % for biosorbent dosage of 0.5 and 8 g L−1, respectively (Fig. 8). This is due to the availability of more binding sites as the dose of biosorbent increases. It is due to the high number of unsaturated biosorption sites during the biosorption process [46]. Similar results were previously reported by some researchers [45, 47].

Fig. 8
figure 8

Effect of the biosorbent dose on the MB biosorption by Luffa cylindrica (C 0 = 20 mg L−1, initial pH 5.80 and T = 20 °C)

Full size image

Effect of biosorbent particle size

The particle size of the biosorbent can greatly influence the external surface of the biosorbent, thus impacting on its interaction with the solution through the effect of resistance to the film diffusion. As a consequence, a variation in the biosorbent particle size modifies the accessibility and the availability of reactive groups present on its surface [13]. The biosorption of MB was studied at four different domains (63–125, 125–250, 250–400 and 400–630 μm) of the biomass fibers. As expected, it was found that the MB biosorption decreases with increasing the size of the biosorbent (Fig. 9). This is due to larger surface area of smaller particles for the same amount of the biosorbent. For larger particles, the diffusion resistance to the mass transport is higher, and most of the internal surface of the particle is not utilized for biosorption. Consequently, the amount of MB adsorbed is small. Similar results were reported by other researchers with coniferous brown macroalga Stoechospermum marginatum [24] and Scolymus hispanicus L. [27], Pinus brutia Ten. [29].

Fig. 9
figure 9

Effect of the particule size on the MB removal (C 0 = 10 mg L−1, initial pH 5.80, biosorbent dose = 0.5 g L−1 and T = 20 °C)

Full size image

Effect of temperature

The temperature is well known to play an important role in the biosorption process [48]. The biosorption of MB on Luffa cylindrica fiber was investigated over the range (20–60 °C). A slight decrease in the dye biosorption with raising temperature was observed from Fig. 10, suggesting an exothermic process.

Fig. 10
figure 10

Effect of the temperature on the MB biosorption (C 0 = 50 mg L−1, initial pH 5.80, and biosorbent dose = 0.5 g L−1)

Full size image

Biosorption isotherms

The isotherm describes the equilibrium between the concentration of the adsorbate on the solid phase and the concentration in the liquid phase. The equilibrium biosorption data have been analyzed using the Langmuir, Freundlich, Dubinin–Radushkevich and Tempkin models. Such analysis is important to develop a relation that accurately represents the experimental results and could be used for design purposes [49].

The Langmuir model is based on an the assumption that the biosorption occurs on specific homogeneous sites of the biosorbent and the monolayer biosorption onto a surface containing a finite number of uniform sites with no transmigration of adsorbate in the plane of the surface [50]; the isotherm is expressed by Eq. (5).

$$q_{\text{e}} = \frac{{q_{\hbox{max} } K_{\text{L}} C_{\text{e}} }}{{\left( {1 + K_{\text{L}} C_{\text{e}} } \right)}}$$
(5)

where C e is the equilibrium dye concentration (mg L−1), q e the amount of biosorbed dye (mg g−1), q max the amount for a complete biosorption monolayer (mg g−1), and K L the constant related to the affinity of the binding sites and energy of biosorption (L mg−1).

$$\frac{{C_{\text{e}} }}{{q_{\text{e}} }} = \frac{1}{{q_{\hbox{max} } K_{\text{L}} }} + \frac{{C_{\text{e}} }}{{q_{\hbox{max} } }}$$
(6)

A dimensionless constant separation factor (R L) of the Langmuir isotherm was used to determine the favorability of the biosorption process. R L is defined using Eq. (7); its value indicates the type of isotherm: irreversible (R L = 0), favorable (0 < R L < 1), linear (R L = 1) or unfavorable (R L > 1) [50].

$$R_{\text{L}} = \frac{1}{{\left( {1 + K_{\text{L}} C_{0} } \right)}}$$
(7)

The Freundlich expression is an empirical equation based on the biosorption onto a heterogeneous surface. The equation generates an exponential shaped theoretical equilibrium curve [51] and is represented as follows:

$$q_{\text{e}} = K_{\text{F}} \,C_{\text{e}}^{{^{{\frac{1}{nF}}} }}$$
(8)
$$\ln \,q_{\text{e}} = \ln K_{\text{F}} + \frac{1}{{n_{\text{F}} }}\ln \,C_{\text{e}}$$
(9)

where K F (mg g−1 L(1/n )F  mg−(1/n )F ) is the Freundlich constant and (1/n F) the heterogeneity factor, related to the capacity and the biosorption intensity.

The Dubinin–Radushkevich (D–R) model does not assume a homogeneous surface or a constant biosorption potential [52]. The biosorption characteristic is related to the porous structure of the biosorbent [53].

$$q_{\text{e}} = q_{\text{D - R}} \,\exp \left( { - \beta \varepsilon^{2} } \right)$$
(10)

The Polanyi potential (ε) is equal to:

$$\varepsilon = RT\,\ln \left( {1 + \frac{1}{{C_{\text{e}} }}} \right)$$
(11)

where ε is a constant related to the mean free energy of biosorption per mole of biosorbate (mol2 J−2), qD–R (mg g−1) the theoretical saturation capacity, R (J mol−1 K−1) is the universal gas constant, and T (K) the absolute temperature.

The energy E is defined as the free energy change (kJ mol−1), required to transfer 1 mol of ions from the solution to the solid:

$$E = \left( {2\beta } \right)^{ - 1/2}$$
(12)
$$\ln \,q_{\text{e}} = \ln \,q_{\text{D - R}} - \beta \varepsilon^{2}$$
(13)

Tempkin and Pyzhev have considered the effects of indirect adsorbate/adsorbate interactions on the biosorption isotherms and suggested that the heat of biosorption of all molecules on the layer should decrease linearly with the coverage [26]. The Temkin isotherm is shown in Eq. (14) [54, 55]:

$$q_{\text{e}} = \left( {\frac{RT}{{b_{\text{T}} }}} \right)\,\ln \left( {A_{\text{T}} C_{\text{e}} } \right)$$
(14)

Equation (14) can be expressed in its linear form :

$$q_{\text{e}} = \frac{RT}{{b_{\text{T}} }}\,\ln \left( {A_{\text{T}} } \right) + \frac{RT}{{b_{\text{T}} }}\,\ln \left( {C_{\text{e}} } \right)$$
(15)

where A T is the equilibrium binding constant corresponding to the maximum binding energy (L mg−1) and b T (J mol−1) the Tempkin isotherm constant related to the heat of biosorption.

The biosorption isotherms are useful to describe the interaction adsorbate/biosorbent of any system. The parameters obtained from different models provide information on the biosorption mechanisms, the surface properties and affinities of the biosorbent [56]. Table 2 and Fig. 11 illustrate the isotherms for 160 min of contact time, initial MB concentration in the range (20–300 mg L−1), a pH of 5.80, a biosorbent dose of 3 g L−1 and a temperature of 20 °C. Based on the linear regression correlation coefficient (R 2), F and P values, the isotherm models fit well the experimental data in the following order:

Table 2 Constants of isotherm models for the biosorption of MB onto Luffa cylindrica fiber at various initial MB concentrations
Full size table
Fig. 11
figure 11

The isotherm plots: Langmuir biosorption isotherm (a), Freundlich biosorption isotherm (b), Dubinin–Radushkevich (c) and Tempkin biosorption isotherm (d)

Full size image

  • Langmuir R 2 > Tempkin R 2 > Freundlich R 2 > (D–R) R 2.

  • Langmuir F value > Tempkin F value > Freundlich F value > (D–R) F value.

  • Langmuir P value < Tempkin P value < Freundlich P value < (D–R) P value.

Table 3 presents the comparison of the maximum biosorption capacity (q max) of MB onto Luffa cylindrica fiber with those obtained by other researchers. It is clear that the Luffa cylindrica used in this work without any treatment has a relatively suitable biosorption capacity compared to other biosorbents in the literature. Therefore, raw Luffa cylindrica fibers seem to be competitive to other methylene blue sorbents and some optimizing treatments on this biomass might be interesting for further studies.

Table 3 Comparison of the maximum biosorption capacity of dyes for different absorbents
Full size table

Biosorption kinetics

The kinetic is important for understanding the treatment of aqueous solutions because it provides valuable information about the mechanism of biosorption processes and potential rate-controlling steps, such as the mass transport [56]. Experimental data of MB biosorption using Luffa cylindrica fibers were evaluated by the pseudo-first and pseudo-second-order kinetics and intra-particle diffusion models to understand the mechanisms of the biosorption process.

The pseudo-first-order rate expression of Lagergren [65] is generally described by the following equation [66]:

$$\log \,\left( {q_{\text{e}} - q_{t} } \right) = \log \,q_{\text{e}} - \frac{{k_{1} t}}{2.303}$$
(16)

where q e and q t are the amounts of dye adsorbed at equilibrium and at time t (mg g−1), respectively, and k1 the pseudo-first-order rate constant (min−1), k1 is obtained from the slope of the linear plot of log (q e − q t ) against t.

The pseudo-second-order kinetic model is expressed as [67]:

$$\frac{t}{q}_{t} = \frac{1}{{k_{2} q_{\text{e}}^{ 2} }} + \frac{1}{{q_{\text{e}} }}t$$
(17)

where k2 is the rate constant of second-order biosorption (g mg−1 min−1). If the second-order kinetic is applicable, the plot of t/qt against t of Eq. (17) should give a linear plot. The initial biosorption rate “h” (mg g−1 min−1) is expressed as [68]:

$$h = k_{2} q_{\text{e}}^{2}$$
(18)

The intra-particle diffusion model is used by Weber and Morris [69] and the rate constant (k int, mg g−1 min−½) is given by [41, 67]:

$$q_{t} = k_{\text{int}} t^{1/2} + C$$
(19)

C (mg g−1) is the intercept. The relation gives information about the thickness of the boundary layer and the plot of q t versus t ½ should yield a straight line passing by the origin if the biosorption process obeys to the intra-particle diffusion model [46, 70].

The kinetic parameters for the biosorption of MB onto Luffa cylindrica fiber are calculated and summarized in Table 4 and Fig. 12. We can observe that only the pseudo-second-order model gives the best fit, with low error probability (5.440 × 10−15 to zero), High F values of pseudo-first-order and high adjusted R 2 (0.9964 to 0.9999). Moreover, the calculated biosorption amount q e (cal) fits well with experimental one q e (exp).

Table 4 Kinetic parameters for the biosorption of MB onto Luffa cylindrica fiber at various initial MB concentrations
Full size table
Fig. 12
figure 12

The kinetic plots, pseudo-first-order (a) and pseudo-second-order (b) models

Full size image

An intra-particle diffusion model was used to identify the diffusion mechanism. The plots of q t versus t 1/2 (Fig. 13), are multi-linear, indicating the existence of three different stages during the biosorption process. The first sharp stage represents the transfer of MB from the solution to the outer surface of the biosorbent; the second gradual stage can be attributed to the penetration of MB into the interlayer of the biosorbent where the intra-particle diffusion is rate limiting. The third stage corresponds to the equilibrium phase and the weak biosorption is ascribed to the residual low MB concentration [70]. The intra-particle diffusion rate constants (k int) are gathered in Table 4. As the initial MB concentration increases, the amount of MB reaching the biosorbent surface increases and the intra-particle diffusion rate increases [40]. It can also be observed that the lines do pass by the origin (C = 0.737 to 28.789), and this indicates that the transfer mechanism is controlled not only by intra-particle diffusion but also by other mechanisms, such as boundary layer [57]. Similar results have been reported for the biosorption of MB onto activated carbons prepared from NaOH-pretreated rice [71], Luffa cylindrica fiber-activated carbons [72], sugar beet pulp [21] and low cost biomass material lotus leaf [73].

Fig. 13
figure 13

The intra-particle diffusion model of MB removal by Luffa cylindrica fiber at various initial MB concentrations

Full size image

Thermodynamic studies

The temperature presents a notable effect on the biosorption and the thermodynamic parameters such as change in the standard free energy (ΔG°), standard enthalpy (ΔH°), and standard entropy (ΔS°) are determined [74]:

$$\Delta G^\circ = - RT\ln K_{\text{d}}$$
(20)
$$\ln K_{\text{d}} = \frac{ - \Delta G^\circ }{RT} = \frac{\Delta S^\circ }{R} - \frac{\Delta H^\circ }{RT}$$
(21)

where R is the universal gas constant (8.314 J mol−1 K−1), T (K) the absolute temperature and K d (L g−1) the distribution coefficient for the biosorption calculated from the following relation [27]:

$$K_{\text{d}} = \frac{{q_{\text{e}} }}{{C_{\text{e}} }}$$
(22)

The plot of ln K d versus of 1/T yields a straight line form; ΔH° and ΔS° are calculated from the slope and intercept of the plot, respectively (Fig. 14, Table 5). The negative values of ΔG° and ΔH° indicate that the biosorption is spontaneous, exothermic and physical in nature, thus confirming the affinity of the biosorbent toward the MB molecule [75]. The negative entropy ΔS° reflects the decreased randomness at the solid/solution interface during the MB biosorption [75, 76]. Similar results were reported by Barka et al. [27] and Han et al. [77] where MB was adsorbed on Scolymus hispanicus L. and Fallen phoenix tree’s leaf, respectively.

Fig. 14
figure 14

The Vant Hoff’s plot for the determination of thermodynamic parameters

Full size image
Table 5 Thermodynamic parameters for the biosorption of MB onto Luffa cylindrica fiber
Full size table

Desorption study

Desorption studies help in deciding the mechanism of the biosorption process and recovery of adsorbent for the reuse. The MB desorption on the Luffa cylindrica (Fig. 15) is low for the four solvents (<10 %) at 293 K. The undesorbed MB in the biosorbate is due to the complex formation (MB—active site) of the biomass, and hence the inability of the eluting solvent to completely desorb the dye [78].

Fig. 15
figure 15

Batch desorption of MB from biomass using different eluting solvents

Full size image

Conclusion

The results obtained in the present work showed that the biomass derived from locally available material (Luffa cylindrica) can be readily used for the removal of methylene blue from aqueous solutions. In batch studies, the biosorption was strongly dependent on operating parameters such as the contact time, solution pH, particle size, biosorbent dose, initial MB concentration and temperature. The parameters were optimized and the experimental data were analyzed by various isotherm models; the results showed that the isotherm data were well correlated by the Langmuir model. The kinetic studies indicated that the pseudo-second-order model fits suitably the experimental data and suggest that the interlayer diffusion is not the rate-determining step in the MB biosorption mechanism. The maximum monolayer biosorption capacity was found to be 49.46 mg g−1 at 20 °C. Moreover, the thermodynamic parameters showed that the biosorption was spontaneous, exothermic and physical in nature. The biosorption experiments indicated that the Luffa cylindrica was an efficient biosorbent for the removal of MB and favorably compared with respect to most biomasses reported nowadays.