Development of fast and high-efficiency sponge-gourd fibers (Luffa cylindrica)/hydroxyapatite composites for removal of lead and methylene blue

2.1 Materials

Sponge-gourd fibers (Luffa cylindrica) were obtained from a local shop in Giza, Egypt. Calcium hydroxide (Ca (OH)₂), phosphoric acid (H₃PO₄, 85%), acetic acid, and ammonium hydroxide (30%) were purchased from (Sigma-Aldrich, St. Louis, MO, USA). Hydrogen peroxide (H₂O₂, 30%) was obtained from (S.D fine Chem Limited Mumbai, India). Sodium chlorite (NaClO₂) was supplied by (Carl Roth, GmbH & Co. Kg). Potassium hydroxide (KOH) was obtained from (Honeywell, GmbH, Germany).

2.2 Isolation of cellulose from luffa fibers (LF)

Cellulose was isolated from sponge-gourd fibers (Luffa cylindrica) following the method described by Oun and Rhim (Oun and Rhim, 2016). Briefly, luffa fibers were cut into small pieces and washed several times with tap water to remove impurities and attached dust, then dried in an air oven at 100 °C for 24 h. The dried luffa fibers were ground into fine powder for further use. Thirty grams of dried luffa fibers powder were dispersed into 1000 mL of sodium chlorite solution 1.4% (w/v) with adjusting the pH to 4 using 5% acetic acid and heated at 70 °C/5 h with stirring to remove lignin. The mixture was washed with distilled water several times until the filtrate became neutral. The residues were collected and oven-dried until constant weight to calculate lignin content from the difference in weights. After removing lignin, hemicellulose was removed by soaking holocellulose (hemicellulose and α-cellulose) into 600 mL of 5% KOH solution for 24 h at room temperature with stirring, then heated at 90 °C /2h. Obtained cellulose was washed and dried to calculate the percentage of hemicellulose and cellulose. The resulted chemical composition of original luffa fibers was cellulose 60%, hemicelluloses 26.6%, lignin 13.4%. This result in agreement with results previously reported by Hong et al. (2020) and Taimur-Al-Mobarak et al. (2018b).

2.3 Preparation of oxidized-fibers and oxidized-nanocellulose.

Ground luffa fibers and isolated cellulose were used for isolation of oxidized-fibers and oxidized-nanocellulose, respectively. For this, 5 g of raw luffa fibers or isolated cellulose separately were added into 100 mL of hydrogen peroxide (30%) then heated at 90 °C for 5 h with stirring. The suspensions were washed with distilled water several times to pH ∼ 6. Oxidized fibers and oxidized-nanocellulose suspensions were dried in an air oven at 100 °C until constant weight for calculating the yields, which were 54% and 50%, respectively.

2.4 Synthesis of luffa/hydroxyapatite composites

Hydroxyapatite (HAp) was synthesized in the presence of different luffa forms following the method reported by Niamsap, Lam, and Sukyai (Niamsap et al., 2019) with modification. One gram of each luffa form (i.e., oxidized-fibers, cellulose, or oxidized nanocellulose) was redispersed into 100 mL of distilled water using a homogenizer (Stuart, SHM2 /EURO, USA) at 5000 rpm until completely dispersed. Then, 0.741 g of calcium hydroxide was added to the previous suspension and sonicated in water bath sonication at 60 °C/ 30 min. The pH of the suspension was adjusted to 10 using acetic acid and stirred for 60 min/ 60 °C. Phosphoric acid (0.410 mL into100 mL water) was added dropwise into the mixture, and the pH was adjusted to 10 by ammonium hydroxide with stirring at 60 °C/ 3 h. Finally, the mixture was aged at room temperature for 24 h, then washed using distilled water until pH became 7–8, and dried at 80 °C. Obtained powders were called oxidized-fiber/HAp (S4), cellulose/HAp (S5), and oxidized-nanocellulose/HAp (S6), as illustrated in Fig. 1.

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Fig. 1

Schematic illustration for preparation of luffa/HAp composites for removal of methylene blue and lead ions.

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2.5 Characterization of luffa and luffa/hydroxyapatite

Morphology and dimensions of oxidized-fibers (S1) and isolated cellulose (S2) were observed using field emission scanning electron microscopy (Quattor S, Thermo Scientific, USA). While, high-resolution transmission electron microscopy (HR-TEM, Tecnai G20, FEI, Netherland) was used for oxidized-nanocellulose (S3), oxidized-fibers/HAp (S4), cellulose/HAp (S5), and oxidized-nanocellulose/HAp (S6) imaging. A drop of S3, S4, S5, and S6 samples (0.05%) was placed onto a nickel grid (300 mesh) then left to dry at room temperature for TEM observation. The average width of 20–30 fibers and the length and width of about 50 CNCs and HAp were measured at different places using internal measurement software of SEM and TEM instrumentation.

Fourier-transform infrared spectroscopy (FTIR- 6100 Jasco, Japan) was used to test the change in the chemical structure of sorbents over a range of 4000–400 cm⁻¹ at room temperature with a resolution of 4 cm⁻¹.

X-ray diffraction (XRD) of sorbents was performed using XRD diffractometer with reflection mode (XRD − X’Pert PRO PANalytical, Netherland), which operated at 45 kV and 30 mA using X-ray source “Cu Kα radiation” (λ = 1.5404 Å) and high score plus software for peaks matching and analysis. Dry samples were scanned in the range of 2θ = 10 − 80° with a scanning rate of 0.4°/min at room temperature. The crystallinity index (CI) of samples was calculated using the following equation after subtraction of the background (Eq. (1)) (French, 2014).

2.6 Adsorption studies

3.1 Morphology properties

Morphology properties of different luffa samples with or without hydroxyapatite nanoparticles (HAp) are shown in Fig. 2. The structure of oxidized luffa fibers (S1) and isolated cellulose (S2) was observed using the field emission scanning electron microscope (FE-SEM) Fig. 2 (S1 and S2). The apparent shape of raw luffa fibers was light brown (Fig. 1) and converted to white with a rough and flat surface after treatment with H₂O_2, as shown in an inset image (Fig. 2 S1 and S2). This mainly was due to the removal of wax, lignin, non-cellulosic materials, and other extractives by H₂O₂ via oxidative degradation process (hydroperoxide anion (HOO⁻) and radical species) (Taimur-Al-Mobarak et al. (2018b)). A similar result was observed when rice straw treated with 30% H₂O₂ (Oun and Rhim, 2018). Samples S1 and S2 showed a web-shape structure with long fibrils and average width 17.5 ± 3.1 µm and 20.1 ± 2.0 µm for S1 and S2, respectively.

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Fig. 2

SEM images of (S1) Oxidized-luffa fibers and (S2) Isolated cellulose from Luffa; TEM images of (S3) Oxidized-nanocellulose, (S4) Oxidized-luffa fibers/HAp, (S5) Cellulose/HAp, and (S6) Oxidized-nanocellulose/HAp.

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The microstructure and dimensions of oxidized-nanocellulose (S3), oxidized-fibers/HAp (S4), cellulose/HAp (S5), and oxidized-nanocellulose/HAp (S6) are determined using TEM imaging, as shown in Fig. 2 (S3-S6). The S3 sample was obtained via oxidation of isolated cellulose by H₂O₂, resulting in a needle-shaped structure with an average length of 192 ± 37 nm and a width of 25 ± 6 nm. Reportedly, Koshani et al. (2018) used H₂O₂ for isolation carboxylated cellulose nanocrystals with an average width of 23 nm and length of 263 nm (Koshani et al., 2018).

On the other hand, the resulted luffa forms (S1, S2, and S3) were used as a template for loading HAp. It can be seen that HAp nanoparticles were successfully synthesized on the surface of samples, as shown in Fig. 2 (S4-S6). The average length and width of HAp synthesized on the surface of S4 were (56 ± 17 nm and 19 ± 4 nm), S5 (53 ± 14 nm and 22 ± 5 nm), and S6 (32 ± 5 nm and 14 ± 3 nm). It is interesting to note that HAp nanoparticles are presented in abundance but relatively agglomerated on the surface of the S4 sample. However, it is densely and uniformly attached to the surface cellulose (S5). While in the case of sample S6, it showed less content of HAp, which probably was related to the small dimensions of prepared oxidized-nanocellulose. Also, it looks that the presence of HAp helped in an agglomeration of the oxidized-nanocellulose (S6) compared to without HAp (S3). Similar agglomeration behavior was observed when HAp prepared with nanocellulose at different ratios (Lu et al., 2019). It has been reported that the presence and concentration of cellulose nanofibrils play a significant role in nucleation, growth, and regulation (size and morphology), subsequently distribution or agglomeration of HAp on the surface of cellulose film (Narwade et al., 2017). In this study, it can be concluded that the preparation method and size of luffa samples played a significant role in synthesizing, attaching, and distributing HAp on their surfaces.

3.2 FTIR analysis

The change in chemical structure of luffa samples with or without HAp was performed via FTIR analysis, and the results are shown in Fig. 3. The characteristic absorption peaks of oxidized-Luffa fibers (S1), cellulose (S2), and oxidized nanocellulose (S3) are observed at 3334 cm⁻¹ (O—H stretching), 2898 cm⁻¹ (CH2 groups of cellulose), 1631 cm⁻¹ (C1—O—C4), 1429 cm⁻¹ (O—H groups) (Niamsap et al., 2019), peak at 1054 cm⁻¹ (C—O stretching vibrations), 1029 cm⁻¹ (C1—O—C6), and 896 cm⁻¹ associated with β-glycosidic linkages between glucose units (Hernández-Francisco et al., 2020). The FTIR spectra of luffa fibers (S1) and oxidized-nanocellulose (S3) samples exhibited a new absorption peak at 1733 cm⁻¹, resulted from the introduction of C⚌O groups. The introduction of carbonyl groups (C⚌O) in acid form is due for the oxidation of C6 in the glucose ring from the cellulose structure after H₂O₂ treatment (Oun and Rhim, 2018).

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Fig. 3

FTIR spectra of (a) Oxidized-luffa fibers (S1) and Oxidized-luffa fibers/HAp (S4), (b) Cellulose (S2) and Cellulose/HAp (S5), (c) Oxidized-nanocellulose (S3) and Oxidized-nanocellulose/HAp (S6), (d) Control HAp.

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The characteristic peaks of HAp are observed at 1036 cm⁻¹ corresponded to ʋ₃ PO₄³⁻ group (Niamsap et al., 2019), 873 cm^−1, and 1425 cm⁻¹ related to the presence of CO₃^2– (Yu et al., 2013). The samples of S4, S5, and S6 containing HAp showed some typical HAp peaks at 871 cm⁻¹ (ʋ₂ CO₃^2–) and 1026 cm⁻¹ (ʋ₃ PO₄³⁻). Also, it can be seen that the peak intensity of O—H and C—H groups in samples S4, S5, and S6 were significantly decreased, probably due to the interaction of these groups with HAp, which helped in attaching HAp on the surface of CNCs (Narwade et al., 2017).

3.3 XRD analysis

XRD analysis was used to determine of crystalline structure and chemical composition of luffa samples with and without HAp. The XRD diffraction patterns of Luffa and luffa/HAp samples are shown in Fig. 4. The characteristic peaks of cellulose were observed at lattice planes (1 1 0), (2 0 0), and (0 0 4), which are related to native cellulose structure (Oun and Rhim, 2016). Oxidized-luffa fibers (S1), isolated cellulose (S2), and oxidized-nanocellulose (S3) showed different diffraction pattern intensities due to different chemical treatments. Treatment of luffa fibers with H₂O₂ presented fewer peak intensities than samples S2 and S3, probably due to the role of H₂O₂ in removing lignin only. While in the case of sample S2, the peak intensity was increased, which may be due to the removal of non-cellulosic parts (hemicellulose and lignin). Compared to sample S1 and S2, sample S3 presented the highest peak intensity, perhaps due to the removal of non-cellulosic parts and amorphous regions in cellulose fibers (Oun and Rhim, 2018). The crystallinity index (CI) was calculated using Eq. (1), and the results were 73.5%, 82.4%, and 84.4% for samples S1, S2, and S3, respectively.

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Fig. 4

XRD spectra of (S1) Oxidized-Luffa fibers, (S2) Cellulose (S3) Oxidized-nanocellulose, (S4) Oxidized-luffa fibers/HAp, (S5) Cellulose/ HAp, (S6) Oxidized-nanocellulose/HAp, and HAp (inset figure).

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After loading of HAp on the surface of luffa samples, the intensity of cellulose peaks was significantly diminished, as shown in Fig. 4 (S4-S6). The CI of composite samples was decreased to 70.2%, 66.6%, and 55.9% for S4, S5, and S6, respectively, compared to samples without HAp. The reduction in the CI of composite samples was probably due to covering of characteristic cellulose peaks by HAp. Similar results were observed when metallic nanoparticles such as AgNPs, CuONPs, and ZnONPs formed on the surface of regenerated cellulose (Shankar et al., 2018).

The inset figure (Fig. 4) shows the XRD analysis of HAp and luffa/ HAp composite samples. It worth noting that new peaks have been observed in luffa/HAp composites at 2θ = 26.1°, 32.3°, 40.1°, 47.1°, 49.9°, 52.9°, and 64.2° (Narwade et al., 2017). These new peaks indicate the formation of HAp onto the surface of different forms of luffa samples with different peak intensities, which fit with the peaks of control HAp (Niamsap et al., 2019).

3.4 Adsorption studies

3.4.1

3.4.1 Effect of sorbent type

Experiments were carried out to study the effect of prepared sorbent type (i.e., oxidized-fibers S1, cellulose S2, oxidized-nanocellulose S3, oxidized-fibers/HAp S4, cellulose/HAp S5, and oxidized-nanocellulose/HAp S6) on the removal efficiency of methylene blue (MB) and lead ions (Pb²⁺). For this, 0.4 g of the prepared sorbents were added to 100 mL aqueous solutions of MB (25 mg/L) and Pb²⁺ (100 mg/L) for 120 min at 25 °C, as shown in Fig. 5.

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Fig. 5

Removal efficiency for (a) methylene blue (MB, C_o = 25 mg/L, pH = 7.3) and (b) lead ions (Pb²⁺, C_o = 100 mg/L, pH = 5.3) by prepared sorbents (0.4 g/100 mL) at 25 °C for 120 min.

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The data shows that the removal efficiency of MB by S1 (85%), S2 (89.6%), and S4 (83.8%) sorbents is higher than S3 (61.5%), S5 (71.1%), and S6 (73.0%) (Fig. 5a). The web-shape structure of the samples probably played a significant role as a net and trapped the dye molecules compared to the samples with short and needle-shaped structures. This appeared clearly between sample S1 or S2 (in micrometer lengths) and sample S3 (192 nm), as presented in Fig. 2. Boudechiche et al. (2016) studied the effect of raw luffa powder size (63–630 μm) on the removal efficiency of MB and found that the sorption of MB increased with decreasing sample size, but the size of their samples was still in the micrometer size range (Boudechiche et al., 2016). Whereas the difference in MB removal efficiency between HAp-loaded samples (S4, S5, and S6) may be related to the presence of free functional groups that did not interact with HAp.

On the other hand, the data also showed that samples S4, S5, and S6 have the highest removal efficiency of lead ions (Pb²⁺) over other sorbent materials, which reached 96.9%, 97.8%, and 96.3%, respectively (Fig. 5b). While the removal efficiency of samples S1, S2, and S3 (without HAp) was 44.1%, 52.7%, and 49.5%, respectively. The higher removal % of S4, S5, and S6 can be attributed to the presence of hydroxyapatite nanoparticles (HAp) which have a great affinity to adsorb the heavy metal ions (Mohammad et al., 2017). This result indicates that the removal efficiency of luffa samples to Pb²⁺ ions increased 2-folds after incorporating HAp. The possible reactions between HAp and Pb²⁺ ions probably due to ion exchange, surface adsorption, and precipitation, suggesting dissolution of HAp then formation of Pb apatites (hydropyromorphites) with the occupation of Ca sites by Pb²⁺ ions (Bailliez et al., 2004).

Among all samples, sample S4 (oxidized-fibers/HAp) showed the most effective sample on the removal of MB (83.8%) and Pb²⁺ ions (96.9%). This may be due to its web structure and presence of active groups like hydroxyl groups, and also carboxyl groups resulted after H₂O₂ treatment, as well as the presence of HAp.

3.4.1.1

3.4.1.1 Effect of contact time

Samples were mixed with MB and Pb²⁺ ions at different times (2–120 min) to determine the adequate contact time to reach the equilibrium state. Fig. 6 displays the effect of contact time on the removal efficiency of sorbents for methylene blue (Fig. 6a) and lead ions (Fig. 6b). According to primary experiments, samples S1(oxidized-fibers), S2 (cellulose of Luffa), and S4 (oxidized-fibers/HAp) showed the best efficient sorbents in removal of MB, as shown in Fig. 5a. While samples S4 (oxidized-luffa fibers/HAp), S5 (cellulose/ HAp), and S6 (oxidized-nanocellulose/HAp) were the best in removal of Pb²⁺ ions (Fig. 5b). For this reason, these samples were chosen to test the effect of contact time on their removal efficiency.

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Fig. 6

The removal efficiency of sorbents of (a) methylene blue (25 mg/L), and (b) lead ions (500 mg/L), Pseudo-second-order adsorption fitting of (c) methylene blue, and (d) lead ions, using 0.4 g of sorbents as a function of contact time (2–120 min).

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It can be seen that a quick removal within the first 5 min of the adsorption process was obtained with the removal amount of more than 87% for MB and more than 99.5% for Pb²⁺ ions. Then, a slower sorption step continued until reaching a state of equilibrium. This behavior possibly is due to the availability of sufficient active sites at the beginning of the reaction; after that, the active sites became occupied by MB and Lead ions. Generally, the sorption process pass first by fast adsorption capacity due to sufficient active sites, then slow adsorption rate until it reaches the equilibrium state at a specific time, followed by constant adsorption state even with increase the reaction time, which the sample could not adsorb more (Khadir et al., 2021). In removal of MB, the samples showed equilibrium time after only 5 min for samples S1 and S2. Whereas in sample S4, quick removal for MB in the initial 5 min, then slow adsorption 5–30 min, after that no change in the removal efficiency was observed after 30 min.

While in the case of Pb²⁺ ions, a quick removal was observed within the first 5 min, followed by the equilibrium state, which reached after 5 min, 15 min, and 60 min for samples S5, S6, and S4, respectively. It has been reported that the equilibrium time of removal MB was 120 min using luffa sponge-based (Li et al., 2018a,b) and 60 min by luffa/sodium dodecyl sulfate (Abbasi and Asgari, 2018). While the Pb²⁺ ions reached an equilibrium state after 600 min using luffa charcoal (Umpuch et al., 2011) and 15 min by Luffa activated carbon-doped chitosan (Gedam and Dongre, 2016). This study found that the samples reached equilibrium time within 5 min, which is relatively short compared to reported literature.

On the other side, the kinetic studies and rate constants of MB and Pb²⁺ ions sorption by sorbents were elucidated after applying the pseudo-first-order, pseudo-second-order, intra-particle diffusion model, and Elovich model. The kinetic model's constants and correlation coefficients of MB and Pb²⁺ ions were calculated and presented in Tables 2 and 3, respectively. Interestingly from the data, the kinetics of sorption reaction was perfectly fitted to the pseudo-second-order model for both methylene blue (Fig. 6c) and lead ions (Fig. 6d), this attributed to the higher correlation coefficient value (R²), and the close matching between the experimental and calculated sorption capacities from this model (Kamal et al., 2019). This may indicate that the rate of solute adsorption is directly proportional to the square of the number of vacant binding sites (Choudhary and Paul, 2018). This result is consistent with previous studies, which reported that the adsorption kinetic of luffa materials generally follows a pseudo-second-order kinetic model in removing MB and Pb²⁺ ions (Anastopoulos and Pashalidis, 2020; Khadir et al., 2021).

Table 2 Constants of kinetic models and isotherm models for MB removal.

Constants of kinetics models												Constants of isotherms models
Pseudo-first-order												Langmuir isotherm
K1 (min−1)			qe(exp.) (mg/g)			qe (cal.) (mg/g)			R2			qmax (mg/g)			b (L/mg)			R2
S1	S2	S4	S1	S2	S4	S1	S2	S4	S1	S2	S4	S1	S2	S4	S1	S2	S4	S1	S2	S4
0.08	0.02	0.039	5.45	5.83	5.45	0.27	0.15	0.77	0.831	0.697	0.963	30.86	36.2	25.2	0.1	0.097	0.072	0.978	0.971	0.986

Pseudo-second-order												Freundlich isotherm
K2 (g/mg min)			qe (exp.) (mg/g)			qe (cal.) (mg/g)			R2			N			Kf			R2
S1	S2	S4	S1	S2	S4	S1	S2	S4	S1	S2	S4	S1	S2	S4	S1	S2	S4	S1	S2	S4
0.46	0.83	0.18	5.45	5.83	5.45	5.53	5.83	5.49	1	1	0.999	1.72	1.6	2	2.45	2.56	2.46	0.894	0.895	0.974

Intra-particle diffusion model									Temkin isotherm
Kp(mg. g−1min1/2)			C			R2			kt (mol/L)			B			R2
S1	S2	S4	S1	S2	S4	S1	S2	S4	S1	S2	S4	S1	S2	S4	S1	S2	S4
0.037	0.019	0.076	5.17	5.6	4.7	0.6452	0.8265	0.889	1.5	1.4	2.5	5.27	6.24	3.54	0.933	0.936	0.886

Elovich

(D–R) isotherm

β (mg. g−1min)

α (mg.g−1.min−1)

R2

qmax (mg/g)

Β

E (kJ/mol)

R2

S1

S2

S4

S1

S2

S4

S1

S2

S4

S1

S2

S4

S1

S2

S4

S1

S2

S4

S1

S2

S4

10.3

21.3

5.65

5.6*10^21

2.7*10^50

5.6*10^21

0.8179

0.9602

0.8745

11.3

11.96

9.25

7*10^−8

7*10^−8

1*10^−7

2.67

2.67

2.23

0.621

0.612

0.553

Table 3 Constants of kinetic models and isotherm models for Pb²⁺ ions removal.

Constants of kinetics models												Constants of isotherms models
Pseudo-first-order												Langmuir isotherm
K1 (min−1)			qe(exp.) (mg/g)			qe (cal.) (mg/g)			R2			qmax (mg/g)			b (L/mg)			R2
S4	S5	S6	S4	S5	S6	S4	S5	S6	S4	S5	S6	S4	S5	S6	S4	S5	S6	S4	S5	S6
0.044	0.12	0.09	125.2	126	125.9	31.8	1.0	7.9	0.996	0.703	0.957	625	714.5	714	0.028	0.21	0.027	0.989	0.999	0.994

Pseudo-second-order												Freundlich isotherm
K2 (g/mg min)			qe (exp.) (mg/g)			qe (cal.) (mg/g)			R2			N			Kf			R2
S4	S5	S6	S4	S5	S6	S4	S5	S6	S4	S5	S6	S4	S5	S6	S4	S5	S6	S4	S5	S6
0.004	0.3	0.04	125.2	126	125.9	126.58	126.6	126.6	0.999	1	1	3.78	3.78	3.74	100	153.2	107.4	0.854	0.626	0.54

Intra-particle diffusion model									Temkin isotherm
Kp (mg. g−1min1/2)			C			R2			kt (mol/L)			B			R2
S4	S5	S6	S4	S5	S6	S4	S5	S6	S4	S5	S6	S4	S5	S6	S4	S5	S6
3.3	0.16	0.86	94.04	124.7	118.5	0.876	0.365	0.659	5.41	14.15	5.42	67.1	82.3	67.1	0.963	0.814	0.821

Elovich

(D–R) isotherm

β (mg. g−1min)

α (mg.g−1.min−1)

R2

qmax (mg/g)

β

E (kJ/mol)

R2

S4

S5

S6

S4

S5

S6

S4

S5

S6

S4

S5

S6

S4

S5

S6

S4

S5

S6

S4

S5

S6

0.122

2.12

0.43

371591

6*10^113

8.5*10^21

0.988

0.594

0.888

444.8

594.5

338.9

4*10^−7

4*10^−7

4*10^−7

1.11

1.11

1.11

0.898

0.928

0.10073

3.4.1.2

3.4.1.2 Effect of initial MB and Pb²⁺ ions concentrations

The initial concentration of contaminants is one of the most critical factors in adsorption efficiency. The active sites on the adsorbent should be adequate to pollutant molecules for maximum removal efficiency. In this study, the removal efficiency was tested using 0.4 g of the selected sorbents at different MB and Pb²⁺ ion concentrations (Fig. 7). The data presented in Fig. 7a shows that with the increase in the initial concentration of MB from 5 to 225 mg/L, the sorption efficiency decreased gradually from ∼ 100% to 51%, 59%, and 40% for S1, S2, and S4, respectively. This behavior can be attributed to the saturation of the most active sites of the sorbents by MB molecules (Aksu and Tezer, 2005). While Fig. 7b shows a steady sorption efficiency of around 100% at a concentration range of Pb²⁺ ions (200–1000 mg/L) for samples S4, S5, and S6. After increasing the Pb concentration from 1000 mg/L to 4000 mg/L, it was found that the sorption efficiencies gradually decreased to 63%, 75%, and 70% for S4, S5, and S6, respectively. The samples exhibited a higher sorption ability to Pb²⁺ than MB. This is probably attributed to the small size of lead ions (ionic radius of Pb²⁺ ions 1.33 Å) rather than the large dye molecules (estimated area of MB molecule130-135 Å), which allows less adsorption competition on the available sorbent sites (Aljeboree et al., 2017).

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Fig. 7

Effect of initial concentrations of (a) MB (5–225 mg/L) and (b) Pb²⁺ ions (200–4000 mg/L) on removal efficiency. Langmuir adsorption isotherm fitting of MB (c) and Pb²⁺ ions (d). (Sorbents dose = 0.4 g, contact time = 120 min).

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On the other hand, to illustrate how the MB and Pb²⁺ ions interact with the sorbents, Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D–R) isotherm models were studied. The constants and correlation coefficients calculated from the isotherm models were listed in Table 2 and Table 3. Remarkably from the data and correlation coefficients, See Fig. 7 (c and d), the sorption of MB and Pb²⁺ ions were fitted with the Langmuir model, which assumes monolayer adsorption of the MB and Pb²⁺ ions onto active sites of the surface of sorbent (Gupta and Babu, 2009). The value of n greater than 1 in Freundlich and E < 8 in (D-R) model demonstrating that the adsorption is a physical process (Kumar et al., 2014). Moreover, the separation factor (RL) values were found to be in the range from 0 to 1, which proposing favorable adsorption between sorbents and sorbates.

The maximum MB and Pb²⁺ ions sorption capacities (q_max) of the selected sorbents calculated from the Langmuir model were compared with different sorbents in previous studies, as presented in Table 4 and Table 5. The data indicated that the prepared sorbents have a good ability to remove MB and Pb²⁺ ions from the solution. Also, the data showed that the luffa/HAp composite showed superior adsorption capacity, especially with Pb²⁺ ions compared to reported results.

Table 4 Comparison of adsorption capacity of methylene blue with several sorbents reported in the literature.

Sorbents	Adsorption capacity of MB (mg/g)	Contact time	Adsorbent dosage (g/L)	pH	Temperature (°C)	References
Luffa treated with sodium dodecyl sulfate	4.0	60 min	5.0	6.0	25	(Abbasi and Asgari, 2018)
Neem leaf powder	8.7	5.0 h	10.0	7.0	27	(Bhattacharya and Sharma, 2005)
Banana peel Orange peel	20.8 18.6	24 h	1.0	7.2	30	(Annadurai et al., 2002)
H2SO4 cross-linked magnetic chitosan	20.4	25 min	10.0	6–8	NA	(Rahmi and Mustafa, 2019)
Carbon-TiO2 composite	25.7	180 min	250	6.0	25	(Simonetti et al., 2016)
Luffa fibers treated with 0.1 M NaOH	49.0	120 h	0.8	NA	20	(Demir et al., 2008)
Luffa fibers	49.4	20 min	3.0	5.8	20	(Boudechiche et al., 2016)
Luffa/graphene oxide	63.3	250 min	NA	7.0	25	(Li et al., 2016)
Carbonized Luffa fibers under argon atmosphere at 800 °C	210.9	120 min	0.05	NA	15	(Li et al., 2018a,b)
Oxidized-fibers (S1)	30.8	120 min	4.0	7.3	25	Present work
Cellulose (S2)	36.2	120 min	4.0	7.3	25	Present work
Oxidized-fibers/HAp (S4)	25.2	120 min	4.0	7.3	25	Present work

NA: not available data.

Table 5 Comparison of adsorption capacity of lead ions (Pb²⁺⁾ with several sorbents reported in the literature.

Sorbents	Adsorption capacity of Pb2+ (mg/g)	Contact time	Adsorbent dosage (g/L)	pH	Temperature(°C)	References
Luffa fibers	4.6	30 min	10	4.0	40	(Saueprasea et al., 2010)
Luffa fibers treated with NaOH	13.4	30 min	10	4.0	40	(Saueprasea et al., 2010)
Luffa fibers treated with 4% NaOH	24.9	24 h	5.0	5.0	21	(Emene and Edyvean, 2019)
Cellulose acetate membrane	43.9	6 h	0.375	NA	NA	(Aquino et al., 2018)
Luffa fibers charcoal	51.0	12 h	1.0	5.0	25	(Umpuch et al., 2011)
Cellulose acetate/ polycaprolactone (10%) membrane	70.5	6 h	0.375	NA	NA	(Aquino et al., 2018)
Luffa cylindrica sponge	75.8	5 h	2.0	6.0	25	(Adewuyi and Pereira, 2017)
Raw clinoptilolite	80.9	4 h	2.5	4.5	22	(Günay et al., 2007)
Activated carbon from luffa cylindrica doped chitosan	112	20 min	0.1	5.0	27	(Gedam and Dongre, 2016)
Pretreated clinoptilolite	122.4	4 h	2.5	4.5	22	(Günay et al., 2007)
Nanohydroxyapatite	192.3	90 min	0.5	5.6	25	(Mohammad et al., 2017)
Oxidized fibers/HAp (S4)	625.0	120 min	4.0	5.3	25	Present work
Cellulose/HAp (S5)	714.5	120 min	4.0	5.3	25	Present work
Oxidized nanocellulose/HAp (S6)	714.5	120 min	4.0	5.3	25	Present work

NA: not available data.