Ambient-temperature fabrication of melamine-based sponges coated with hydrophobic lignin shells by surface dip adsorbing for oil/water separation

Abstract

A melamine-based sponge with a highly hydrophobic surface, high porosity and low density, is facilely fabricated through a simple surface adsorption technique that consisted of dipping in a tetrahydrofuran solution containing renewable alkali lignin and carbodiimide-modified diphenylmethane diisocyanate. The entire process can be performed at ambient temperature without consuming a large amount of energy. The resultant sponge, coated with a highly hydrophobic lignin-based shell, retained most of the inner space for the storage of oil and organic liquids and was similar to a sealed container with highly hydrophobic walls. The prepared sponge, with a highly hydrophobic surface (water contact angle of 145.9°), exhibited excellent absorption capacities for organic liquids, with a volume-based absorption capacity of up to 98% of its original volume. Moreover, the absorbed oil can be recovered by squeezing the oil-absorbed sponge with a good compressibility, which maintained above 86.96% of the mass-based absorption capacity even after 100 absorption/squeezing cycles. Due to their low cost, simple fabrication process and high oil/water separation performance, melamine-based sponges coated with lignin have a great potential in applications for oil spill cleanup and organic contamination removal.

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1 Introduction

Effective absorbent materials are urgently required to deal with the oil spill accidents frequently occurring in oceans, which have a bad impact on the surrounding environment with the increasing industrial development.¹ Facing the challenge of oil/water separation, three-dimensional (3D) porous materials with a high porosity and enormous space (such as polymer materials,⁶ nanotube,^7,8 3D graphene,^9,10 assembling nanoparticles,¹¹ and aerogel composites¹²) are potential candidates for water purification and oil storage.^2–5 To date, the preparation of 3D porous hydrophobic materials, which have been widely developed for the effective absorption of oil and organic liquids from oil-contaminated water, has become simpler.^13–15 Many macroporous nanocomposites, showing superhydrophobic properties and high absorption capacities prior to modification, can be directly used as oil absorbents.¹⁶ Moreover, there is a need for multi-functionalized absorbents due to the occurrence of new problems.¹⁷ Despite the excellent absorption capacities for oils and organic liquids, the absorbents should be resistant to fire and compressible for oil recovery.¹⁸ However, most of the successful fabrications of these hydrophobic porous materials entail high cost, high energy consumption and complicated production conditions and devices.

Nowadays, owing to their high porosity, low density, excellent compressibility and low price, melamine sponges are employed as absorbents for oil and organic liquids.^19–21 Due to their amphilicity, hydrophobic sponges might be obtained through different hydrophobic modifications, including the introduction of low surface energy chemicals, increase in the surface roughness and treatment at high temperatures.²² Typically, hydrophobic molecules with low surface energy are grafted onto the sponge to form stable hydrophobic sponges with an excellent adsorption performance.^23,24 The hydrophobic product was obtained by immobilizing the nanoparticles on the sponge to achieve a high oil/water separation capacity.^25,26 With a low cost, facile hydrophobic modification and excellent adsorption capacities towards oils and organic solvents, these sponge-derived oil absorbents can possibly be fabricated on a large scale.

Recently, biomass resources have received a significant attention, with an increased interest in its study and utilization, due to the shortage of non-renewable resources and serious environmental damage.²⁷ Lignin, a phenolic biopolymer, is widely applied to fabricate 3D porous materials due to its environmentally friendly and bio-renewable nature. Low-cost lignin (obtained from the paper industry) with numerous reactive hydroxyl groups (–OH) could be easily modified to form the target products.^28,29 Xu et al.³⁰ successfully used bacterial cellulose to toughen the lignin–resorcinol–formaldehyde aerogels, forming a blackberry-like structure that resulted in a good reversible compressibility.

In our previous work,³¹ a superhydrophobic aerogel with excellent self-cleaning performance was obtained through the cross-linking reaction of lignin with carbodiimide-modified diphenylmethane diisocyanate. Although the preparation process was simple, this product exhibited low absorption capacity. We also prepared oil absorbents via the pyrolysis of melamine/lignin.³² The sponge with an excellent oil/water separation performance was obtained at the expense of large amount of energy consumption. Inspired by these achievements, we prepared melamine–formaldehyde (MF) sponges with a highly hydrophobic surface decorated with renewable lignin via a facile, low-cost and environmentally friendly surface dip-adsorbing technique. MF sponges with hydrophobic lignin-based shells (LMF) retained a large proportion of storage space, which is similar to a sealed container with highly hydrophobic walls. Compared with the total modification, this design can retain the compressibility and most of the oil storage space at a lower cost. Most importantly, the absorbed oil in the unmodified inner space can be easily recovered. Interestingly, the as-prepared sponge showed a great water repellence and an excellent oil/water separation capability with a volume-based adsorption capacity of up to 98% with respect to the original MF sponge. Moreover, with great compressible performance, the LMF sponge maintained a high oil/water separation capability after 100 absorption/squeezing cycles. With a low cost, simple process and high oil/water separation performance, the LMF sponge has a great potential to be an ideal absorbent for oil spill treatment, wastewater purification and environmental protection.

2 Experimental

2.1. Materials

Melamine–formaldehyde (MF) sponges were supplied by Guangzhou Green Care Co. Ltd., China. These were cut into cubes of a suitable size and washed with alcohol and water. Alkali lignin, with hydroxyl groups (2.19 mmol g⁻¹) and a high average molecular weight (about 9000), was obtained from Shixian Papermaking Co. Ltd. (Jilin, China). Carbodiimide-modified diphenylmethane diisocyanate (MMDI, Lupranate MM103C), as shown in Fig. S1,† was a gift from BASF (China). Tetrahydrofuran (THF) was purchased from Guangzhou Chemical Reagents Factory, China.

2.2. Preparation of LMF sponges

Various LMF sponges, listed in Table 1, were fabricated. A typical experiment was performed as follows: MMDI (0.2 g) was dissolved in 20.0 mL of a THF solution containing lignin (0.2 g); then, the six surfaces of the monolithic MF sponges (2 × 2 × 3 cm) were immersed into the solution, one at a time, for surface adsorption. The immersion height of every surface, which determined the shell thickness of the LMF sponges, was controlled within the range from 0.1 to 0.4 cm, and the immersion time was about two seconds to avoid unwanted absorption of modified lignin. The LMF sponges, with a highly hydrophobic surface, were obtained after drying at ambient temperature for 2 h.

Table 1 LMF sponges prepared with different concentrations of lignin and MMDI

Sample	Lignin (g mL^−1)	MMDI (g mL^−1)	Water contact angle (°)	Compressive stress (kPa)
1	0.005	0.005	68	28.0
2	0.01	0.01	145.9	34.6
3	0.015	0.015	136	54.4
4	0.02	0.02	129	119.5
5	0.025	0.025	124	214.9
6	0.01	0.005	<60	23.4
7	0.01	0.015	140	47.5
8	0.01	0.02	139	70.7

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2.3. Oil absorbency

The LMF sponges were immersed in various oils and organic solvents for about 2 minutes. The absorption capacities of the LMF sponges were calculated by the difference in the sponge weights after and before absorption.

The reusability of the LMF sponges was evaluated based on the number of absorption/squeezing cycles: firstly, oils were absorbed by the LMF sponges, and then released from the sponges by directly squeezing. The entire procedure was repeated 100 times. The absorption capacity was measured after each absorption/squeezing cycle.

2.4. Characterization

Scanning electron microscopy (SEM) images were obtained by a Zeiss EVO 18 scanning electron microscope. The wetting property was measured by a CAST 2.0 contact angle analysis system (Dataphysics, Germany) at room temperature. Fourier-transform infrared (FT-IR) spectra were obtained by a German Vector 33 & Nicolet 6700 FT-IR using KBr disks. Compressive stress–strain tests were performed using a Shimadzu AG-X plus system at room temperature.

3 Result and discussion

Renewable lignin with numerous hydroxyl groups on its side chains was used as the modifying agent (Fig. S2†).³³ The commercially available MF sponges with excellent compressibility, high porosity and low density were the ideal candidates for oil absorption. We used the MF sponge as a template to fabricate a highly hydrophobic product with excellent compressibility. The simple production process of lignin-modified MF sponges (LMF) is shown in Fig. 1a. All the surfaces of the MF sponge were dipped into the THF solution (containing lignin and MMDI), followed by drying at ambient temperature to generate the LMF sponge with a hydrophobic surface.

Fig. 1 (a) Experimental process and (b) schematic of the fabrication of LMF sponges.

The mechanism for the fabrication of highly hydrophobic LMF sponges is illustrated in Fig. 1b. Lignin with –OH groups first reacted with the –NCO groups of MMDI when they were mixed in THF solution. The cross-linking reaction took place at ambient temperature when the mixed solution was adsorbed on the surfaces of the monolithic sponge until the –NCO groups were consumed completely. The phase separation occurred due to the self-assembly of cross-linked lignin upon volatilization of THF. Lignin became more hydrophobic due to the loss of free –OH groups. Finally, the numerous lignin aggregates generated the rough skeleton surface that formed the hydrophobic sponge shell.

The cross-linking reaction of lignin and MMDI can be confirmed by FT-IR analysis (Fig. 2). The prominent peak at 2250 cm⁻¹ for the –NCO groups was absent in the LMF sponge because the –NCO groups of MMDI had been consumed completely by the –OH groups of lignin. The number of –OH groups of lignin (2.19 mmol g⁻¹) was higher than the –NCO groups of MMDI (1.6 mmol g⁻¹).³¹ However, the cross-linked lignin with less hydroxyl groups was less hydrophilic, resulting in a polymer phase separation in polar THF.

Fig. 2 FT-IR spectra of lignin, MMDI, MF and LMF sponges.

Due to the hydrophobic coating of the monolithic sponge, the surfaces of the as-prepared product exhibited high lipophilicity and hydrophobicity, as displayed in Fig. 3a. The water droplets could stand on the lignin-modified rough surface, which was wetted by toluene dyed with Sudan I. However, the untreated inner domain was wetted by both water and toluene. The original MF sponge sank in water due to its open-pore structure and hydrophilicity, whereas the LMF sponge floated on water, which showed a greater water repellence due to its high surface hydrophobicity and low density (Fig. 3b). The LMF sponges had a low density, ranging from 25 to 27 mg cm⁻³. The cubic LMF sponge with a volume of about 12 cm³ could easily stand on a piece of clover (Fig. 3c).

Fig. 3 (a) Water and toluene standing on the modified outer surface and the untreated inner surface of the LMF sponge and water contact angle test in the inset. (b) Image of a LMF sponge floating on water and a MF sponge sinking in water. (c) A 12 cm³ LMF sponge standing on a clover. SEM images at different magnifications of (d and e) the original MF sponge and (f and g) the modified LMF sponge.

The surface microstructures of the LMF and original MF sponges were observed by SEM (Fig. 3d–g). The original MF sponge, with a high porosity of over 99% and a large pore size of about 100–200 μm, showed a perfect open-pore structure (Fig. 3d and e) and clean surface. However, the surfaces of the LMF sponges were covered by many lignin aggregates, which formed a dense rough shell (Fig. S3a†). The unmodified inner core and modified shell had a legible interface (red lines in Fig. 3f). While taking a closer look (Fig. 3g), it can be observed that the rough shell had some lignin membranes with many micropores in the skeleton. The lignin aggregates were hardly removed after a few minutes of ultrasonication. Compared to the original MF, the porosity of the product (96.5%) slightly decreased due to the surface loading of crosslinked lignin and would increase even close to 99% by the increasing volume of the whole sample as the result of the increasing volume fraction of unmodified inner space. This phenomenon might be mainly attributed to the phase separation of cross-linked lignin to form the membranes with micropores. In general, the rough lignin-based aggregates, on the surfaces of the monolithic LMF sponge, improved hydrophobicity.²⁵

The effects of lignin concentration on the LMF shell structure were investigated, and the concentration ratio of MMDI and lignin was fixed to 1 : 1. At a lignin concentration of 0.02 g mL⁻¹, the lignin membranes in the modified shell skeleton were thick with a few micropores of 1–20 μm, and the LMF sponge had a low porosity (Fig. 4a). At 0.015 g mL⁻¹, the lignin membranes became a little thinner with many micropores of 1–10 μm (Fig. 4b). At 0.01 g mL⁻¹, the lignin membranes were formed with many interconnected nano- or micropores with a size of less than 2 μm (Fig. 4c). The number of micropores and the porosity of the LMF sponges increased as the lignin concentration decreased from 0.02 to 0.01 g mL⁻¹. However, on decreasing the lignin concentration down to 0.005 g mL⁻¹, the surfaces of the monolithic sponge cannot be fully covered by homogeneous lignin-based aggregates (Fig. S3b†).

Fig. 4 SEM images of LMF sponges prepared with different lignin concentrations: (a) 0.02 g mL⁻¹, (b) 0.015 g mL⁻¹, (c) 0.01 g mL⁻¹.

The corresponding water contact angles of the LMF sponges for the different lignin concentrations are shown in Fig. 5a. The LMF sponge made with 0.005 g mL⁻¹ lignin had a low water contact angle of 68° because of low modification level. The hydrophobicity of the LMF sponges decreased upon increasing the lignin concentrations from 0.01 to 0.02 g mL⁻¹ and the water contact angles decreased from 145.9° to 129°. From Fig. 4, the decrease in the lignin concentration, to a certain range, was beneficial for the formation of the rough surfaces, which improved the hydrophobicity and reduced the surface energy.³⁴ Compressibility is an inherent property of the MF sponge. The LMF sponges were the derivative of the MF sponge modified by lignin. The compressibility of the LMF sponges by varying the lignin concentrations was investigated (Fig. S4†). Compressive stress of the LMF sponges increased from 28.0 to 214.9 kPa upon increasing the lignin concentrations from 0.005 to 0.025 g mL⁻¹ (Fig. 5b). This phenomenon is attributed to that the increasing concentration of lignin which reduced the porosity and pore volume of the surface, leading to the main dissipation of mechanical energy through the breakage of skeletons and lignin-based substrates at the same compressible deformation,^35,36 resulting in worse compressible recoverability.

Fig. 5 Water contact angles (a) and compressive stress–strain curves (b) of LMF sponges with different concentrations of lignin.

Similar trends for the effects of the MMDI to lignin ratio (with a 0.01 g mL⁻¹ lignin concentration) on the water contact angles and the compressive properties were observed. The water contact angle decreased from 145.9° to 138.5° with the increasing MMDI to lignin ratio from 1 : 1 to 2 : 1 (Fig. S5a†). When the ratio was 0.5 : 1, the extra unmodified lignin, rich in hydroxyl groups, slowed down the phase separation and contributed to the aggregation of polymer instead of forming dispersive cross-linked lignin due to the existence of abundant hydrogen bonds. As a result, the surface of the MF sponges was not fully covered by the lignin-based substrate. The uncovered part was wetted by water droplets, and the covered part, with a water contact angle under 60°, was hydrophilic, mainly owing to the extra hydroxyl groups of lignin (Fig. S3c†). Compressive stress of LMF increased from 23.4 to 70.7 kPa on increasing the MMDI to lignin ratio from 0.5 : 1 to 2 : 1 (Fig. S5b†), whereas the compressive recoverability became worse (Fig. S6†). This phenomenon might be ascribed to the increased cross-linked density of lignin with the increase in number of crosslinked agents, and led to less space to trap air, which decreased the hydrophobicity. However, the hydrophilic groups decreased because of the increasing cross-linked agents. The slight decrease of water contact angles was achieved by the combined effects of both surface structures and the chemical composites. Simultaneously, more cross-linked agents increased the cross-linked density, which in turn increased the compressive stress at the expense of damaging the compressive recoverability.³⁷ In brief, a 1 : 1 ratio of MMDI to lignin at 0.01 g mL⁻¹ was optimal to fabricate a LMF sponge with a highly hydrophobic surface and with an excellent compressive recoverability.

The LMF sponge also had an excellent compressive recoverability. The compressive stress decreased slightly from 34.6 to 30.2 kPa (87.3% of the original value) after 1000 cycles of compression tests at a strain value of 60% (Fig. 6a and b). The structural robustness of the LMF sponge was close to that of the cellular aerogels and carbon nanotube sponges.^38,39 The porous 3D network structure of the LMF sponge remained mostly intact with only a few skeletons broken down after a 1000-cycle compression testing (Fig. 6c and d). The mirror-like phenomenon (Fig. S7†) showed that the LMF sponge still retained its high hydrophobicity after these compression tests.

Fig. 6 (a) Sequential images of the LMF sponge during the compression process. (b) Compressive stress–strain curves and (c and d) SEM images of the LMF sponge after 1000 cycles of compression tests at a strain value of 60%.

Due to its high porosity, highly hydrophobic surface and interconnected network structure, the LMF sponge could be used as a potential absorbent for the efficient separation of oil/water. The gravimetric adsorption performance of LMF sponges for n-hexane (dyed with Sudan I) on the surface of water is shown in Fig. 7a. The LMF sponge still floated on water surface after absorbing hexane due to its light weight and good water repellence. Furthermore, dichloromethane (dyed with Sudan I), which sank at the bottom of water, was fully absorbed by forcing the LMF sponge into the water for a few seconds (Fig. 7b). In general, oils of either low or high density could be absorbed by the as-prepared hydrophobic sponges.

Fig. 7 Images of the removal process with LMF sponges for (a) n-hexane (dyed with Sudan I) floating on water and (b) dichloromethane (dyed with Sudan I) sunk in water. (c) mass-based absorption capacities and (d) volume-based absorption capacities of the LMF sponges for various organic liquids.

In next experiment, various organic liquids were used for oil absorption. Mass-based absorption capacities of LMF sponges were obtained in the range of 18–51 g g⁻¹ (Fig. 7c), which was comparable to those of previous oil absorbents.^40–44 However, the volume-based absorption capacities might be more persuasive to characterize the absorption capacity of LMF sponges. The corresponding volume-based absorption capacities of LMF sponges for organic liquids were found to be in the range from 90% to 98% of the original volume (Fig. 7d), which was close to the volume-based absorption capacity of the original MF sponges with high porosity (>99%) and higher than that of other former absorbents (such as carbon nanofiber aerogels,⁴⁵ nitrogen-doped graphene frameworks and carbon aerogels^46,47). These results demonstrate that the LMF sponge displayed an almost full-space-utilization for oil-storage (the highly oleophilic walls occupied a little space). The organic liquids were forced onto the hydrophobic and oleophilic surface of the LMF sponges by capillaries, and then spread into the amphipathic inner pores of the sponge through the interconnected network structure, resulting in a high volume-based absorption capacity.⁴⁸

To verify the feasibility of the LMF sponges, n-hexane dyed with Sudan I was used as the adsorbate to investigate the oil/water separation performance, as shown in Fig. 8a. After absorbing the oil completely within few seconds, the sponge could still float on water. Moreover, it retained its original morphology and size with a little residual oil on the surfaces after the absorbed oil was recovered via the squeezing method. The absorbing/squeezing recovery method was also employed to deal with the absorbed oils such as gasoline, dodecane, liquid paraffin and peanut oil (Fig. 8b). It was found that the LMF sponge retained above 87% of its mass-based absorption capacity after 100 cycles for all oils. This result might be attributed to the fact that the skeletons were broken down to cause a non-elastic deformation during the absorbing/squeezing method, which led to a decreased volume of the whole LMF sponge as the number of cycles increased. However, its outstanding high hydrophobicity and compressibility were retained, resulting in a slightly decreased absorption capacity. Moreover, the absorbed oil could also be recovered with an injection syringe without breaking off the whole sponge (Fig. 8c). Through the injection syringe, the absorbed oil was easily suctioned out from the LMF sponge depending on the untreated inner space of the oil storage. This strategy would be beneficial to extend the service life of the LMF sponges with little material loss in the oil recovery process when compared with the absorbing/squeezing method.

Fig. 8 (a) Absorption and recovery through the squeezing of n-hexane (dyed with Sudan I). (b) Absorption recyclability of the LMF sponge after squeezing it to recover oils. (c) Recovery of absorbed n-hexane by a syringe.

4 Conclusions

A simple and low-cost technique was developed to fabricate a melamine sponge with a highly hydrophobic surface coated with cross-linked lignin at ambient temperature. As a result of the hydrophobic surface modification, the as-prepared LMF product retained excellent compressibility, low density and high porosity. It was also able to absorb oils and organic liquids from water, with a maximum volume-based absorption capacity of up to 98% of its volume. Moreover, the adsorbates could be recovered via an absorption/squeezing cycle from the highly compressible hydrophobic LMF sponge. We believe that this simple and low-cost fabrication method for absorbents would be suitable for a wide range of applications such as oil recovery and environmental protection.

Acknowledgements

The funding support for this study from the National Natural Science Foundation of China (21474032) and the Natural Science Foundation of Guangdong Province (2016A030311031) is acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: Photos, wettability and compressive stress–strain curves of LMF sponges. See DOI: 10.1039/c6ra18329h

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