New lignin-based polyurethane foam for wastewater treatment

Abstract

Utilization of renewable feedstock for the development of alternative materials is rapidly increasing due to the depletion of petroleum resources and related environmental issues. Lignin, the second major constituent of lignocellulosic biomass, is catching the attention of researchers for the synthesis of various value-added materials due to its renewable and biodegradable nature, large abundance, non-food value and high functionality. In the present work, lignin was extracted from pine needles, a considerable bio-waste material, and was used as the polyol for the synthesis of polyurethane foam (PUF). The synthesized lignin-PUF (LPUF) was characterized for its physical and thermal properties, and employed as an adsorbent of dyes. The results demonstrated that LPUF is an efficient material to remove a cationic dye, malachite green, and was better in comparison to an anionic dye, methyl orange, from their aqueous solutions. Dye adsorption was a spontaneous and endothermic process. The adsorption kinetics and isotherms fitted well the pseudo second-order model and Langmuir adsorption isotherm, respectively, with a maximum adsorption capacity of 80 mg g⁻¹. In addition, LPUF was reusable for a number of repeat cycles with a cumulative adsorption capacity of 1.33 g g⁻¹ after twenty regeneration cycles.

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1. Introduction

Rapid industrialization and urbanization have increased the problems of water pollution to become an issue of major global environmental concern. A number of industries such as textile, paper, plastic, printing, leather, etc. make use of various organic dyes and discharge the used dye solutions as major water pollutants.¹ It is estimated that about 20% water pollution is due to dyes only.² The presence of dyes in water poses adverse effects both to the environment and human health since most of the dyes are highly toxic and can cause mutagenic or carcinogenic effects even at low concentrations. Moreover, it is very difficult to bioremediate wastewater containing dyes due to their complex chemical structure.³ Furthermore; dyes retard photosynthetic activity and inhibit growth of aquatic plants by blocking light penetration. Therefore, the removal of dyes from wastewater needs to be addressed on massive scale. Among different technologies used for wastewater treatment, adsorption is the most attractive technique since it is an easy, economic, highly efficient process with simple design, convenient operation including easy regeneration and the wide availability of adsorbents.⁴

Recently, the adsorbents synthesized from low-cost and renewable natural resources are drawing great attention. Lignin, the second most abundant component (15–35%) of lignocellulosic biomass, is the largest natural source of aromatic compounds on earth which mainly consists of three phenylpropionic alcohol monomers, i.e., p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol.⁵ The annual production of lignin is ∼50 million tons as byproduct of the paper and pulp industry and ethanol biorefineries.^6,7 However, most of it is burned as low-cost fuel and only ∼2% is commercialized for making value-added industrial applications.^5,8 Since, it possesses aromatic rich polymeric structure having different types of functional groups, especially the phenolic and aliphatic –OH groups, so it can be exploited by their proper functionalization to obtain different products to be used as green alternatives to petrochemical-based polymers. These attributes of lignin have led researchers to focus on lignin utilization for developing alternate products and there are reports in literature where lignin has been used to prepare lignin-modified phenolic^9,10 and epoxy resins,^11,12 and polyurethane foams (PUF).^8,13–21 The latter is highly crosslinked polymer generally synthesized through a polyaddition reaction between polyols and polyisocyanate linked by urethane linkages.¹⁷ PUF has rapidly grown as one of the most widely used synthetic polymers with a growing global market for diverse applications. Currently, the polyisocyanates and polyols are mostly derived from petroleum resources e.g., polyethylene glycol or glycerin. Furthermore, polyols obtained from sucrose and sorbitol have also been used which have food value.¹⁹ Hence, research focus should be shifted toward polyols produced from lignocellulosic biomass for sustainable development.

Lignin is a promising PUF precursor due to its non-food agricultural based economy, large abundance, low-cost, renewable, nontoxic, moderately biodegradable, eco-friendly nature and high functionality of –OH groups, both phenolic and aliphatic –OH groups which can provide good reacting sites toward isocyanates for the preparation of PUF with good adsorption properties. Hence, lignin is an important material of choice in PUF synthesis, directly or after modification. Cateto et al. have reported the synthesis of PUF using lignin along with sorbitol based polyol.¹⁹ There are some reports in which PUFs have been prepared from lignin after its modification by processes like oxypropylation.^8,14,21 Jia et al. prepared lignin-based polyurethane film using lignin as the only hydroxyl group containing polymer.²²

In the present study, lignin extracted from pine needles was used in the pristine form without modification to develop PUF. The as-prepared lignin-based PUF (LPUF) was evaluated as adsorbent for the removal of dyes; malachite green (MG, C₂₃H₂₅ClN₂) and methyl orange (MO; C₁₄H₁₄N₃NaO₃S) from their aqueous solutions. MG and MO are extensively used in textile, paper, printing, and food industry.^23,24 MG is also used as antibacterial and anti-parasitical agent in fish culture, but due to its high accumulation within edible fish tissues it causes toxic transmuting effects.²⁵ Hence many countries have banned its use in all categories of food-producing fish.²⁶ It not only deteriorates water quality, but poses adverse effects to human health also.²⁷ It causes irritation and pain on contact with skin and eyes, while its oral consumption is toxic, hazardous, carcinogenic and mutagenic.²⁸ MO also leads to serious health hazards such as breathing problem, vomiting, diarrhea and nausea.²⁴ Therefore, the treatment of wastewater containing MG or MO before being discharged to the environment is essential because of their detrimental effects. In literature, there are some reports on the removal of dyes using PUFs.^29,30 However, to the best of our knowledge no work is reported on the removal of MG or MO by LPUFs. The adsorption efficiency of the synthesized LPUF for the removal of dyes was investigated in a parametric study along with the evaluation of reusability, kinetics, isotherms and thermodynamics of the adsorption process. The results showed that LPUF is an efficient material in removing cationic dyes from their aqueous solutions with excellent reusability. Therefore, the resultant LPUF can be utilized as an efficient, economic and renewable adsorbent for dye removal.

2. Experimental

2.1. Materials

Pine needles (Pinus wallichiana, Shimla, India), NaOH, H₂SO₄, dimethylformamide, 4,4′-diphenylmethanediisocyanate (MDI, Sigma-Aldrich), triethylamine (HiMedia, India), triethanolamine (Sigma-Aldrich), silicone oil (Sigma-Aldrich), malachite green and methyl orange (HiMedia, India). All the chemicals were of reagent grade and were used as received.

2.2. Extraction of lignin from pine needles and synthesis of polyurethane foam

Lignin was extracted from needles of Pinus wallichiana by alkali treatment (4 wt% NaOH) at reflux temperature for 4 h.³¹ Then insoluble cellulose was filtered and the black colored filtrate containing lignin was acidified with conc. H₂SO₄ to pH 1.5 and heated at 100 °C for 1 h with continuous stirring to precipitate out lignin.^32,33 Thereafter, lignin was filtered and washed with hot water till neutral pH and air dried. Extracted lignin (1.0 g), as polyol, was dissolved in dimethylformamide via ultrasonification (Cole-Parmer) and mixed with triethylamine (0.05 g, as catalyst), triethanolamine (0.05 g, as co-catalyst), silicone oil (0.06 g, as surfactant) and distilled water (1.0 g, as blowing agent) by mechanical stirring for 5 min. Then MDI (1.5 g) was added and the reaction mixture was stirred vigorously until foam formation.^8,15,17,20 The foaming reaction between lignin polyol and MDI was very rapid and exothermic which resulted in solid foam. The as-synthesized foam, represented as LPUF, was cured at room temperature before further characterization and application.

2.3. Characterization of LPUF

The synthesized LPUF was characterized using different techniques to get the evidence of its synthesis. Fourier transform infrared (FTIR) spectra were recorded on FTIR spectrophotometer (Perkin Elmer) between 4000 and 400 cm⁻¹ using KBr disks. The surface morphology of the samples was observed by scanning electron microscopy (SEM) recorded on JEOL JSM-7000F. Powder X-ray diffraction (XRD) measurements were carried out by X-ray diffractometer (Philips PAN Analytical XPERT-PRO) using a wavelength of 1.54060 Å (Cu-Kα radiation) with diffraction angle 2θ varied from 10 to 80°. Thermal stability data was obtained using the TGA/DTA (Mettler Toledo) thermogravimetric analyzer within the temperature range 25–750 °C at a rate of 10 °C min⁻¹ under nitrogen atmosphere. The compressive strength of lignin polyurethane foam (LPUF) was measured using an Instron model 4466 universal testing machine in according with ASTM D-1621 (Standard test method for compressive properties of rigid cellular plastics) using a load cell of 10 kN and crosshead speed of 0.5 mm min⁻¹. The dimensions of samples were 30 mm × 30 mm × 15 mm. Bluehill 2 software was used to determine the compressive strength and compressive modulus of the sample. The results were averaged from three test specimen data.

2.4. Evaluation of LPUF as dye adsorbent

The removal of dyes from their aqueous solutions by the as-synthesized LPUF was carried out using a batch adsorption process. A piece of 0.025 g of LPUF was added to 25 mL of dye solution (50 mg L⁻¹, 7.0 pH) prepared in distilled water at 25 °C. Then, after specific time intervals, the concentration of the unabsorbed dye was determined at 618 nm and 464 nm for MG and MO, respectively, with UV-Vis spectrophotometer (Photolab 6600, WTW GmbH, Weilheim, Germany) using a calibration curve for the known dye concentrations. Effect of different parameters on dye adsorption was studied as a function of time (5–240 min), temperature (25–65 °C), pH (2.0–9.0) and initial dye concentration (10–250 mg L⁻¹). The amount of dye adsorbed onto LPUF was calculated in terms of adsorption capacity (q) and % removal (P_r) using the following equations:³¹

where, q (mg g⁻¹) is the amount of dye adsorbed onto unit dry mass of the adsorbent, C_o and C_e are the initial and equilibrium dye concentration (mg L⁻¹), V is the used volume of dye (L) and w is the weight of LPUF (g).

The reusability studies were carried out by regenerating the adsorbent with 0.1 N NaOH and the regenerated sample was used again. The point zero charge (pH_pzc) i.e., the pH at which the net charge of the adsorbent is zero, was determined by adjusting the initial pH of 0.01 M NaCl solution from 2.0 to 9.0 with 0.1 M HCl or 0.1 M NaOH.^28,34 Then, 0.10 g LPUF was added to 20 mL of 0.01 M NaCl solution of different pH values and the final pH of the solution was measured after 24 h of agitation. The difference between the initial pH (pH_i) and final pH (pH_f) i.e., (pH_f–pH_i) was plotted against the initial pH (pH_i), and the pH_pzc was recorded as the point where pH_f–pH_i = 0.

The experimental data of MG adsorption on LPUF was treated to understand the kinetics and mechanism of the adsorption process using different kinetic models viz. pseudo-first order, pseudo-second order, Elovich and Weber Moris intraparticle diffusion, and three isotherm models: Langmuir, Freundlich and Temkin (Table 1).^31,34,35 The spontaneous or non-spontaneous nature of the adsorption process is evaluated from thermodynamic parameters calculated from van't Hoff equation (Table 1).

Table 1 Different kinetic models, thermodynamic equations and adsorption isotherms

Model	Equation	Parameters
Adsorption kinetic models
Pseudo-first-order	ln(qe − qt) = ln qe − k1t	qe (mg g^−1) = equilibrium adsorption capacity
		qt (mg g^−1) = adsorption capacity at time t
		k1 (min^−1) = pseudo-first order rate constant
Pseudo-second-order	t/qt = 1/k2qe^2 + (1/qe)t	k2 (g mg^−1 min) = pseudo-second-order rate constant
Elovich		α (mg g^−1 min^−1) = initial adsorption rate
		β (g mg^−1) = relationship between the degree of surface coverage and the activation energy
Weber Moris intraparticle diffusion	qt = kit^1/2 + Ci	ki (mg g^−1 min^−1/2) = intraparticle diffusion constant
		Ci = boundary layer thickness, if Ci = 0, the adsorption kinetics is controlled by intraparticle diffusion only otherwise, adsorption process is also controlled by the thickness of the boundary layer
Thermodynamic equations
van't Hoff equation		ΔS° = entropy change
		ΔH° = enthalpy change
		R = universal gas constant (8.314 J mol^−1 K^−1)
		T (K) = absolute temperature
		Kc (L g^−1) = thermodynamic equilibrium constant, expressed by
	ΔG° = −RT ln Kc	ΔG° (J mol^−1) = Gibbs free energy change, If ΔG° = −ve, adsorption process is spontaneous
Adsorption isotherms
Langmuir		qm = maximum monolayer adsorption capacity of the adsorbent
		kL = energy constant
		RL = separation factor which gives an idea about Langmuir isotherm such that unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0) isotherm
Freundlich		kF (mg g^−1) = Freundlich constant
		n = intensity of adsorption, n > 1 indicates a favourable and heterogeneous adsorption
Temkin	qe = β ln kT + β ln Ce where, β = RT/b	β (L mg^−1) = related to the heat of adsorption
		kT (mg L) = Temkin constant
		b (J mol^−1) = energy constant

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3. Results and discussion

3.1. Synthesis and characterization of LPUF

PUF synthesis involves an exothermic reaction between polyol and diisocyanate (MDI) via urethane (–NHCOO–) linkages catalyzed by tertiary amines. In LPUF synthesis lignin acted as polyol as it has large number of aromatic as well as primary hydroxyl groups. These groups react with the diisocyanate (MDI) and form urethane, –NHCOO–, linkages in the presence of Et₃N. The schematics of the synthetic pathway of LPUF along with its subsequent use as dye adsorbent is represented in Scheme 1.

Scheme 1 Schematics of LPUF synthesis and application.

The FTIR spectra of the lignin, LPUF and MG-loaded LPUF are presented in Fig. 1. FTIR spectrum of the extracted lignin has bands at 3500–3300 cm⁻¹ (phenolic and aliphatic –OH stretching), 2925 and 2852 cm⁻¹ (–C–H stretching), 1701 cm⁻¹ (–C=O stretching in carbonyl groups), and 1605, 1516 and 1458 cm⁻¹ (aromatic ring vibrations).^36,37 The bands at 1270 and 1046 cm⁻¹ are due to guaiacyl units, and 1217 and 1128 cm⁻¹ due to syringyl units of lignin. Synthesis of LPUF was confirmed from its FTIR spectrum as different bands are present due to vibrations of urethane (–NHCOO–) group at 3355 cm⁻¹ (–N–H stretching), 1541 cm⁻¹ (–N–H bending), 1718 cm⁻¹ (–C=O stretching), 1654 cm⁻¹ (–CO–NH stretching), 1231 cm⁻¹ (–C–N stretching) and 1093 cm⁻¹ (–C–O stretching). Also, the bands at 2952 cm⁻¹, 1408 cm⁻¹ and 1308 cm⁻¹, respectively, were observed due to stretching, bending and wagging vibrations of –CH₂ group, along with a band at 1597 cm⁻¹ due to –C=C stretching in aromatic ring of MDI. The change in band intensities in the FTIR spectra of MG-loaded LPUF confirmed the adsorption of MG onto LPUF.

Fig. 1 FTIR spectra of lignin, LPUF and MG-loaded LPUF.

The XRD patterns of extracted lignin, LPUF and MG-loaded LPUF are presented in Fig. 2. The amorphous nature of lignin is indicated by the broad peak in its XRD pattern. The presence of a broad peak at around 20° in the XRD diffraction patterns of LPUF and the corresponding MG-loaded LPUF also indicated their amorphous character which is generally observed for PUFs synthesized using aromatic isocyanates.³⁸

Fig. 2 XRD patterns of lignin, LPUF and MG-loaded LPUF.

The SEM image of LPUF indicated its highly porous structure that will allow the better adsorption of dye molecules (Fig. 3a). The surface of MG-loaded LPUF looks rough with closed pores which confirm the adsorption of dye onto LPUF (Fig. 3b).

Fig. 3 SEM images of (a) LPUF and (b) MG-loaded LPUF.

Porosity (p) of ∼75% was obtained for LPUF using liquid displacement method reported elsewhere.³⁹ Ethanol was used as the displacement liquid since it easily penetrates into the pores without inducing shrinkage or swelling of the polymers. LPUF was immersed in a cylinder containing a known volume of ethanol (V₁) and was kept for 5 min. Then LPUF was pressed to force air from it and allow the ethanol to penetrate and fill the pores and volume of ethanol was recorded as V₂. After that LPUF was removed from the cylinder and the residual ethanol volume was recorded as V₃. The porosity of LPUF was expressed as

The TGA/DTG thermograms were recorded between 25 °C and 750 °C under nitrogen atmosphere at a heating rate of 10 °C min⁻¹ (Fig. 4 and Table S1†). In lignin and LPUF, the initial weight loss of <8% in the temperature range of 50–150 °C is attributed to loss of moisture. Lignin is moderately stable at elevated temperatures due to its highly aromatic backbone. It undergoes major weight loss between 200 and 600 °C.⁴⁰ The peaks located at 245 °C and 314 °C in DTG are attributed to the weight loss due to the degradation of the phenylpropane side chains of lignin and the elimination of carbon dioxide, water, etc. and peak at 500 °C corresponds to the decomposition of its aromatic rings (Fig. 4a). The TGA/DTG curve revealed the three step degradation of LPUF (Fig. 4b). The first stage of decomposition at 242 °C corresponds to dissociation of urethane bonds. A peak at 320 °C corresponds to the second stage of decomposition due to cleavage of the aliphatic segments derived from the polyol. In the third stage of decomposition, the peak located at 565 °C corresponds to the decomposition of aromatic rings derived from lignin and diisocyanate (MDI).⁴¹ From DTG, the final decomposition temperature of LPUF was observed at 565 °C in comparison with the lignin (500 °C). From TGA it was observed that LPUF was stable upto 700 °C as compared to pristine lignin which totally decomposes at 600 °C. Therefore it can be inferred that LPUF formation enhances thermal stability of the pristine lignin. It can be seen from Table S1† that the decomposition temperatures for 10–100% weight losses are higher for LPUF in comparison to pristine lignin which clearly indicate high thermal stability of the foam material.

Fig. 4 TGA and DTG analysis curves of (a) lignin and (b) LPUF.

The compressive load versus strain curve is presented in Fig. S1.† The LPUF showed moderately good compression strength and compression modulus of 0.08911 MPa and 1.0911 MPa, respectively, which can be primarily attributed to the rigid and aromatic structure of lignin polyol.^15,18 Also, the high functionality of –OH groups in lignin leads to the more cross-linked network of LPUF which is responsible for higher compressive strength and modulus. Higher value of compression modulus indicates that LPUF is rigid having regular structure.

3.2. Analysis of dye adsorption by LPUF

3.2.1. Comparative adsorption of dyes by LPUF. Synthesized LPUF was applied as adsorbent for the removal of dyes from their aqueous solutions. For the sake of comparison a cationic (MG) and an anionic dye (MO) were chosen as the candidate dyes (Fig. 5). It was observed that cationic dye (MG) was removed quite effectively and rapidly than the anionic dye (MO). After 120 min, LPUF removed ∼75% (36.1 mg g⁻¹) of MG while removal of anionic dye (MO) was <10% (3.8 mg g⁻¹) which is negligible in comparison to MG removal. Such adsorption behaviour emanates from the surface structure of the foam. The pH_pzc of LPUF was found to be 5.53. The surface of LPUF is negatively charged at pH > pH_pzc and positively charged at pH < pH_PZC.³⁵ Since, the surface of PUF is negatively charged at pH 7.0, so the cationic dye, MG was adsorbed more in comparison to anionic dye, MO. Therefore, for further adsorption studies only MG was used due to its better adsorption by LPUF. For comparison purpose PUF prepared from cellulose nanowhiskers (CNW-PUF), in our earlier reported work was used for removal of MG from its aqueous solution.⁴² Under similar conditions ∼85% MG was removed by CNW-PUF after 120 min which is higher as compared to LPUF because nano-dimensional CNWs were used as polyol for PUF synthesis in the reference material (Fig. S2†).

Fig. 5 % removal (P_r) of dyes by LPUF with time (inset) adsorption capacity (q) of dye removal with time.

3.2.2. Parametric study of MG adsorption by LPUF. The MG removal was quite rapid and it increased with time and equilibrium was attained after 120 min (Fig. 5). The higher initial adsorption of MG within 120 min is due to the high availability of active adsorption sites on LPUF and after that equilibrium is attained due to the saturation of the active sites.⁴³

Thermodynamic consideration of an adsorption process is very important to conclude whether the process is spontaneous or non-spontaneous. Adsorption process is greatly dependent on temperature since the thermodynamic parameters such as heat of adsorption, free energy and entropy which predict the behavior of adsorption depend on temperature. The effect of temperature on MG removal was studied between 25 and 65 °C (Table 2). Adsorption capacity was observed to increase from 36.1 to 40.7 mg g⁻¹ with temperature (25–45 °C) and equilibrated after 45 °C. The initial increase of adsorption with temperature indicated the endothermic nature of dye adsorption onto PUF. This increase in adsorption capacity can be attributed to the decrease in the viscosity of the solution and increase in pore size and rate of diffusion of dye molecules across the surface of LPUF with rise of temperature.⁴⁴

Table 2 Effect of temperature, pH and initial dye concentration on MG removal by LPUF

Temperature (°C)	q (mg g^−1)	pH	q (mg g^−1)	Initial MG concentration (mg L^−1)	q (mg g^−1)
25	36.1	2.0	0.25	10	4.1
35	38.1	3.0	9.35	25	16.2
45	40.7	4.0	22.5	50	40.5
55	39.8	5.0	31.3	75	58.4
65	39.2	6.0	40.5	100	70.2
		7.0	40.7	150	73.8
		9.0	39.4	200	75.4
				250	77.6

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The pH is one of the most important parameters which influence the adsorption process by affecting the surface charge of adsorbent and the pollutant. The effect of initial pH value for MG removal by LPUF was studied in a range of 2.0–9.0 at 45 °C using 50 mg L⁻¹ of MG and 0.025 g of LPUF for 120 min (Table 2). The q value increased from 0.25–40.5 mg g⁻¹ with increasing the initial pH of MG solution from 2.0 to 6.0 and equilibrated thereafter. The effect of pH on adsorption can be described on the basis of pH_pzc (5.53).^34,35 Since the surface of LPUF is positively charged at pH < pH_pzc, so because of electrostatic repulsions between the positively charged MG molecules and the surface of LPUF, the adsorption is very low. Also, concentration of H⁺ ions is high under the acidic pH medium and they compete with MG cations for vacant adsorption sites causing a decrease in dye adsorption. At high pH, when pH > pH_pzc, the adsorbent surface is negatively charged. Such surface structure favours the uptake of cationic dyes due to increased electrostatic force of attraction. Hence an increase in MG removal by LPUF was observed at pH ≥ 6.0.

The effect of the initial MG concentrations on its removal by LPUF from aqueous solution is shown in (Table 2). Results indicated that the q value increased from 4.1–70.2 mg g⁻¹ with increase in MG concentration from 10–100 mg L⁻¹ and became nearly constant after 100 mg L⁻¹. The increase of adsorption capacity with initial dye concentration was due to the high ratio of adsorbent sites available to the dye molecules and higher contact probability between LPUF and MG. At higher initial dye concentration the MG adsorption become almost constant due to the saturation of the limited number of active sites on the adsorbent surface which results in a limited increment in adsorption capacity.^34,44

3.2.3. Adsorption kinetics. The plots for different kinetic models are shown in (Fig. 6 and S3a–c†) and the values of different parameters determined from their slopes and intercepts are listed in Table S2.† The correlation coefficient (R²) obtained for pseudo-second-order kinetic model (0.987) was higher than the R² values obtained for the other kinetic models. In addition, the calculated q_e values obtained from pseudo-second-order kinetic model fit well with the experimental q_e values (Fig. 6). Moreover since the boundary layer thickness C_i = 0.846 ≠ 0, hence it is suggested that the adsorption of MG onto LPUF is not controlled by intraparticle diffusion only.³⁵ Therefore, it can be concluded that the kinetics of dye adsorption on LPUF followed a pseudo-second-order model which suggests that adsorption process involves chemisorption as the rate-limiting step.

Fig. 6 Comparison of different kinetic models for MG removal by LPUF (inset) pseudo-second-order plot.

3.2.4. Adsorption thermodynamics. The values of different thermodynamic parameters, ΔG°, ΔH° and ΔS° calculated from the slope and intercept of the plot between ln K_c versus 1/T are shown in Table S3.† The +ve values of ΔS° specify the increase in randomness at the solid-solution interface and the redistribution of energy between the adsorbate and adsorbent during the adsorption process which indicate the affinity of LPUF for MG. The +ve value of ΔH° and −ve values of ΔG°, respectively, indicated the endothermic and spontaneous nature of the adsorption of MG onto LPUF.³⁴

3.2.5. Adsorption isotherms. The plots for different isotherms are shown in (Fig. 7, S4a and b†) and the values of different parameters obtained from different isotherms are listed in Table 3. The R² value obtained for Langmuir isotherm (0.997) was higher than the other adsorption isotherms and, also, the calculated q_e values obtained from Langmuir isotherm fit well with the experimental q_e values (Fig. 7). Moreover, R_L values lie between 0.026 and 0.407 for MG which further indicated Langmuir isotherm as favourable adsorption isotherm for MG adsorption on LPUF (Fig. S4c†). Therefore, it can be concluded that MG adsorption on LPUF followed a monolayer adsorption process with maximum adsorption capacity (q_m) of 80 mg g⁻¹ calculated from Langmuir isotherm. The q_m values for MG removal by some adsorbents have been compared in Table 4.^{28–30,34,45–50}

Fig. 7 Comparison of different adsorption isotherms for MG removal by LPUF (inset) Langmuir isotherm.

Table 3 Parameters of different adsorption isotherms for MG removal by LPUF

Langmuir			Freundlich			Temkin
qm	KL	R^2	KF	n	R^2	KT	β	b	R^2
80.0	1.5 × 10^−1	0.997	28.5	4.56	0.667	4.4	12.6	208.8	0.738

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Table 4 Comparison of the maximum adsorption capacities (q_m) of various adsorbents for MG removal

S. no.	Adsorbent	Optimum conditions				Adsorption capacity (qm) (mg g^−1)	References
		Time (min)	Temp. (°C)	pH	Conc. (mg L^−1)
1	Carrot leaves powder	30	30	7.0	10–50	52.6	Kushwaha et al., J. Saudi Chem. Soc., 2014 (ref. 28)
2	p-tert-Butyl thiacalix(4)arene imbedded flexible polyurethane foam	30	25	6.0	30–80	58.82	Mohammadi et al., React. Funct. Polym., 2014 (ref. 29)
3	Polyurethane/chitosan composite foam	720	30	9.0	100	16.67	Li et al., J. Cell. Plast., 2015 (ref. 30)
4	Rice husk bio-char	720	30	>6.0	80	32.5–67.6	Leng et al., Fuel, 2015 (ref. 34)
5	Oxalic acid modified rice husk	480	20	7.0	20–450	54.02	Zou et al., J. Chem. Eng. Data, 2011 (ref. 45)
6	Bivalve shell-treated maize husk leaf	30	50	6.0	10–200	81.5	Jalil et al., Bioresour. Technol., 2012 (ref. 46)
7	Activated carbon produced from spent tea leaves	180	45	4.0	100	256.4	Akar et al., Ecol. Eng., 2013 (ref. 47)
8	Hydrothermally carbonized pine needles	180	30	7.0	25–450	52.91	Hammud et al., RSC Adv., 2015 (ref. 48)
9	Lignin sulfonate polymer	300	45	7.0	10–60	70.07	Tang et al., RSC Adv., 2015 (ref. 49)
10	Magnetic litchi pericarps	66.69	25	6.0	150	70.42	Zheng et al., J. Environ. Manag., 2015 (ref. 50)
11	Lignin-based polyurethane foam	120	45	6.0	100	80.0	Present study

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The regenerated LPUF was observed to be reusable for a number of repeat cycles and was efficient to remove ∼60% of MG even after 20 repeat cycles and it still had capacity to be used further (Fig. 8). The cumulative adsorption capacity for 20 repeat cycles is 1.33 g g⁻¹ of the foam. Hence LPUF is an excellent candidate for cationic dyes removal from the aqueous solutions.

Fig. 8 Reusability of LPUF for MG adsorption.

4. Conclusions

LPUF was synthesized from the lignin extracted from the forest biowaste. Hence, it has a renewable nature and is a low-cost adsorbent for the removal of dyes from their aqueous solutions. In the present study, MG dye was efficiently removed by LPUF and the adsorption process followed pseudo-second order kinetics and Langmuir isotherm with maximum monolayer adsorption capacity of 80.0 mg g⁻¹. In addition, LPUF has excellent reusability for numerous repeat cycles after regeneration as ∼60% of the dye removal efficiency can be realized even after twenty repeat cycles. These results show that LPUF is an ideal alternative material with proper utilization of bio-waste for the removal of toxic dyes from wastewater and can replace the conventional expensive adsorbents.

Acknowledgements

Sapana Kumari acknowledges the financial support by University Grants Commission (UGC), New Delhi, India, in the form of UGC – Basic Scientific Research (BSR) scholarship and facilities under Special Assistance Programme (SAP) – Departmental Research Support II (DRS II).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13308h

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