Facile and Green Synthesis of Pd-Decorated Alginate/ Nanofibrillated Cellulose Aerogel for Simultaneous Dye Degradation, Oil/Water Separation and Oil Collection

Water pollution caused by dyeing wastewater discharge, organic chemical reagent leakage and frequent oil spill incidents has aroused widespread international concern. Various methods have been proposed to remove single pollutant from wastewater. However, it is still a challenging task to develop advanced materials that can simultaneously and effectively remove various pollutants from wastewater. In this study, we successfully fabricated a novel multifunctional Pd-decorated alginate/nanofibrillated cellulose (Pd@ALG/NFC) aerogel through a facile and green in-situ reduction, followed by solution-aerogel conversion strategy. The as-prepared Pd@ALG/NFC aerogel exhibited superior catalytic activity toward methylene blue (MB) with a high degradation efficiency above 99% and excellent underwater superoleophobicity with OCA > 150° for various oils. It enabled rapid degradation of MB and efficient separation of different kinds of organic pollutants and oils from water by gravity, with an efficiency exceeding 98.3% and a water flux above 120.5 L·m− 2·min− 1. Moreover, the aerogel is compressible, shape stable and recyclable. These features make the hybrid aerogel a promising candidate for multi-pollutants wastewater treatment.


Introduction
Water pollution is a global environmental issue that poses serious threats to the ecological balance and human health.Among various sources of water pollution, dyeing wastewater discharge, organic chemical reagent leakage and frequent oil spill incidents are particularly prominent due to their widespread occurrence and persistent toxicity [1][2][3][4][5].Various functional materials, such as dye degradation materials [6][7][8], oil absorption materials [9][10][11], and oil/water separation materials [12][13][14][15][16][17], have been developed for inorganic catalytic nanoparticles that can efficiently remove multiple pollutants from wastewater.However, one of the major challenges to achieve stable and uniform immobilization of nanoparticles on the organic scaffold [33,34].Alginate aqueous system offers a promising solution for this challenge, as it provides circumstance for reducing the metal precursors to their corresponding elemental states, and also acts as stabilizer to prevent agglomeration of these metal nanoparticles [35][36][37].Therefore, an alginate aqueous solution containing highly dispersible metal nanoparticles can be easily obtained through the above general and green in-situ reduction, which can then be converted into an alginate-based aerogel with embedded inorganic catalysts by freeze-drying.
Herein, we present a novel multifunctional Pd-decorated alginate/nanofibrillated cellulose (Pd@ALG/NFC) aerogel for simultaneous oil/water separation and dye degradation.The Pd@ALG/NFC aerogel was fabricated by in-situ reduction of PdCl 2 precursor on ALG/NFC matrix, followed by solution-aerogel conversion strategy.The features of the Pd@ALG/NFC aerogel, including surface microstructures, palladium nanoparticles (Pd NPs) distribution, wetting behaviors, oil/water separation capacity and dye degradation performance were investigated in detail.A mixed system of methylene blue (MB)/kerosene/water served as a model to illustrate the oil/water separation and catalytic degradation performance of the Pd@ALG/NFC aerogel.The results showed that the hybrid aerogel could simultaneously and efficiently achieve dye degradation, oil/water separation and oil collection.Moreover, the aerogel maintained its remarkable durability even under high-salinity conditions, indicating its potential for multi-pollutants wastewater treatment.

Fabrication of Pd@ALG/NFC Composite Aerogel
First, a PdCl 2 -HCl aqueous solution was prepared by dissolving 150 mg of PdCl 2 in 100 mL of 20 mM HCl solution and heating it at 60 °C until complete dissolution of PdCl 2 [38].Then, this solution was added dropwise to an alginate aqueous solution (1 wt%, 150 mL) under vigorous magnetic stirring at 100 °C for 2 h [35].This resulted in the formation of Pd nanoparticles (NPs) by reducing Pd (II) ions.Next, the NFC suspension (1 wt%) was mixed with the alginate-Pd solution (volume ratio = 1:10) under vigorous stirring for about 30 min.After that, the mixture was transferred into a PTFE mould and lyophilized for 28 h in a programmable lyophilizer (LyoBeta 3PS, Telstar, Spain) to obtain the composite aerogel.Finally, the aerogel was immersed in CaCl 2 solution (4 wt%) to undergo the calcium ionic cross-linking process and air-dried at 40 °C for 12 h to produce the final product (Pd@ALG/NFC aerogel).

Separation and Catalytic Performance
To obtain various types of oil/water systems, equal volumes of kerosene, soybean oil, n-hexane, toluene and crude oil were mixed with water, respectively.Then, 4 mg of methylene blue (MB) and 100 mg of NaBH 4 were added into 200 mL of the above oil/water systems to prepare the complex wastewater.A Pd@ALG/NFC aerogel (40 mm in diameter, 10 mm in thickness) was completely wetted by water and then carefully placed between two glass tubes.The complex wastewater was poured into the upper glass tube and allowed to permeate through the aerogel, achieving simultaneous oil/water separation, oil collection and dye degradation.
In Eq. ( 1), m a and m b represent the mass of the oil or organic solvent before and after separation, respectively.The water flux (F) of the Pd@ALG/NFC aerogel, which is defined as the volume flow of water passing through the aerogel, was calculated using Eq. ( 2): In Eq. ( 2), V is the volume of the collected water, ∆t is the time required for the separation and A is the effective filtering area of the aerogel.
The concentration of methylene blue (MB) before and after filtration was quantified by UV-vis spectroscopy.The degradation efficiency (η d ) was calculated using Eq.(3).
In Eq. ( 3), C 0 and C denote the MB concentration before and after separation, respectively.

Characterizations
The microstructure and dispersion of Pd NPs in alginate solution were characterized by a JEM-2100 F transmission electron microscopy (TEM).The surface morphologies of Pd@ALG/NFC aerogel were examined by a Philips Sirion 200 field emission scanning electron microscope (FE-SEM).The X-ray diffraction (XRD) patterns of the Pd@ALG/NFC aerogel were obtained using a Bruker AXS D8 Advance Diffractometer equipped with Cu Ka radiation, and the scanning angle ranged from 20° to 80°.The wettability of the aerogel was measured by a Kruss-DSA30 contact angle measurement system.The amount of elemental Pd in the aerogel was characterized by a Perkin Elmer Sciex ELAN DRC II inductively coupled plasma mass spectroscopy (ICP-MS).The concentration of methylene blue (MB) was quantitatively confirmed by a Perkin Elmer Lambda-750 UV-vis spectrometer.The total pore surface area and pore size distribution were measured by an AutoPore IV 9510 the mercury porosimetry.

Results and Discussion
The preparation of the Pd@ALG/NFC aerogel is schematically illustrated in Fig. 1.First, alginate solution was well mixed with PdCl 2 /HCl solution and gently heated for in situ formation of Pd NPs.Next, NFC was added as a reinforcing filler into the above solution.Then, the ALG/NFC solution with highly dispersible Pd NPs was transformed to the prototype aerogel by a simple freeze-drying method.
Finally, the aerogel was ionically cross-linked with Ca (II) ions and air-dried to obtain the final product (Pd@ALG/ NFC aerogel).
The morphology and crystallinity of the Pd NPs synthesized by SA reduction were investigated by TEM, HRTEM, and SAED. Figure 2a and b illustrate the color change of the alginate solution from light yellow to black after gentle heat treatment, which indicates the successful reduction of Pd (II) to Pd NPs.The Pd NPs with an average diameter of 10 nm were obtained and uniformly dispersed in the alginate solution (Fig. 2c and d).The high crystallinity of the Pd NPs was confirmed by HRTEM analysis, which displayed clear and uniform lattice fringes with a d-spacing of about 0.224 nm (Fig. 2e), corresponding to the (111) planes of the face-centered cubic (FCC) phase of metallic Pd (0.23 nm).This result was consistent with previous reports [37].The selected area electron diffraction (SAED) pattern of the Pd To analyze the pore distribution of the Pd@ALG/NFC aerogel, mercury porosimetry was applied to the aerogel.The results of mercury intrusion analysis (Fig. 3f) show that the aerogel has a broad pore distribution ranging from about 1 to 100 μm.The total pore area is relatively low (2.28 m 2 /g) and the average pore diameter is about 73.95 μm, which corresponds to the macropore structure observed in SEM the image (Fig. 3b).
The wettability of Pd@ALG/NFC aerogel was determined by the contact angle (CA) tested in air and underwater, respectively.As illustrated in Fig. 4a and b, both the CA values of oil (kerosene) and water on the aerogel surface are close to 0°, demonstrating its excellent superamphiphilic property in air.Nevertheless, when immersed in water, the aerogel shows excellent underwater superoleophobicity with OCA of 159° (Fig. 4c).This phenomenon might NPs (Fig. 2f) exhibited discrete spots in the ring pattern, indicating that most of the particles were single crystalline materials oriented along their (111), ( 200) and (220) Pd directions, as typically observed for the FCC structured Pd crystal lattice.
After freeze-drying, ionic cross-linking and air-drying processes, the Pd@ALG/NFC aerogel with black color (Fig. 3a) was obtained.The apparent density of the hybrid aerogel was measured to be 26.21mg cm − 3 .The surface morphologies of the developed Pd@ALG/NFC aerogel were studied using FE-SEM.As illustrated in Fig. 3b, the as-prepared aerogel displayed well-organized and interconnected porous 3D structures.Numerous micropores ranging in size from 30 to 100 μm were observed in the hybrid aerogel, which could enable continuous liquids transportation and separation in a gravity-driven process.The high magnification SEM image (Fig. 3c) shows micropores in the aerogel have a corrugated rough surface, which is essential for achieving underwater superoleophobicity.
The structure of Pd NPs embedded in the Pd@ALG/NFC aerogel were investigated by X-ray diffraction (XRD) analysis.The XRD pattern taken using Cu Kα target in the range 20-80° of the aerogel is shown in Fig. 3e.The Pd NPs peaks are located at 40.28°, 46.78° and 68.1°, which are indexed selectively retained by the Pd@ALG/NFC aerogel, while the water containing MB passed through the aerogel and the MB was decomposed by Pd NPs in the aerogel along with the disappearance of blue color.No noticeable oil and color were observed in the collected water, indicating that dye degradation and oil/water separation processes were simultaneously and effectively achieved.More importantly, the whole purification procedure was driven by gravity and did not require any external energy consumption, following the principle of energy conservation and environmental protection.
The separation efficiency (η s ) calculated by Eq. ( 1) is up to 99.8% for kerosene and more than 98.3% for other oils (Fig. 6).These results are mainly attributed to the porous structures and outstanding underwater superoleophobicity of the Pd@ALG/NFC aerogel.The water flux of the Pd@ ALG/NFC aerogel obtained through Eq. ( 2) is approximately 120.5 L•m − 2 •min − 1 and the degradation of methylene blue (MB) calculated from Eq. ( 3) is as high as 99.5%, demonstrating that the aerogel is extremely effective and powerful in removing organic dye from wastewater.
The oil/water separation mechanism of the Pd@ALG/ NFC aerogel can be explained by the liquid wetting model below.
be attributed to the corrugated rough structure caused by Pd NPs and NFC on the pore walls and the massive hydrophilic groups in the aerogel.Moreover, the underwater OCA of other oils and organic solvents were also measured and the resultant values are all higher than 150° (Fig. 4d), suggesting the excellent anti-oil-fouling property of the aerogel.Additionally, the salt-tolerance of the Pd@ALG/NFC aerogel was also investigated through immersing it in seawater.After immersion for four weeks, the Pd@ALG/NFC aerogel still maintains underwater superoleophobic property, demonstrating its remarkable salt-tolerance in the hypersaline environment.
To demonstrate the feasibility of the proposed application, the as-prepared Pd@ALG/NFC aerogel was employed for complex wastewater treatment.A mixture of methylene blue (MB), kerosene, and water was chosen as a model system to evaluate the catalytic degradation performance and oil/water separation capability of Pd@ALG/NFC aerogel.The schematic diagram of the experimental setup for water purification and separation process is shown in Fig. 5.The process of oil/water separation and dye degradation is also presented in Movie S1.After adding MB and NaBH 4 to the water, it turned blue, and the oil (kerosene) was dyed red for easy observation.When the complex wastewater was poured into the experimental setup, the oil (colored red) was  4), when θ > 150°, ∆P > 0. This means that the water-wetted aerogel can withstand a certain oil intrusion pressure, and thus water can penetrate the aerogel while oil is blocked.Therefore, the developed Pd@ALG/ NFC aerogel can effectively achieve oil/water separation.
To clearly demonstrate the degradation process, a piece of Pd@ALG/NFC aerogel was immersed in the MB-NaBH 4 Where γ denotes the surface tension, l represents the perimeter of the pore, ∆P signifies the theoretical intrusion pressure, θ indicates the contact angle of the liquid, and A refers to the area of the pore.More interestingly, the Pd@ALG/NFC aerogel exhibited excellent shape memory function in water, which enabled its reuse for multiple cycles.After being thoroughly rinsed with water, the aerogel can be reused for the next cycle.As shown in Movie S3, the aerogel can be rubbed into any shape in the cleaning process and restored to its original state immediately, indicating its good mechanical stability.The oil/water separation efficiency and the degradation efficiency of the aerogel were measured for four cycles.The results showed that the separation efficiency decreased slightly from 99.6 to 98.5%, while the degradation efficiency reduced from 99.5 to 98.2% as the degradation time increased from 37 to 80 s (Fig. 7b).The increase in degradation time was probably related to the saturation of adsorption sites and the reduction of catalytic activity of the Pd NPs after repeated use.The Pd@ALG/NFC aerogel demonstrated good reusability, which was mainly attributed to its superior catalytic activity of the Pd NPs.
To evaluate the performance of the Pd@ALG/NFC aerogel in high salinity conditions for dye degradation and oil-water separation, the aerogel was immersed in seawater and sampled every two days for testing using the filtration device shown in Fig. 5.After each test, the aerogel was mixture solution to study the catalytic reaction kinetics.Meanwhile, the ALG/NFC aerogel without Pd NPs was also employed for comparison.As displayed in Movie S1, the color of MB solution gradually lightens and finally becomes completely colorless within 1 min.This phenomenon can also be reflected by the changes of absorption peaks in UVvis absorption spectra.As illustrated in Fig. 7a, the typical absorption peak of MB (665 nm) is gradually disappearing within 30s, suggesting an excellent dye removal efficiency.Moreover, the UV-vis results also show that the maximum absorption rate occurs in the final 10s, which may be due to the full contact between MB and the Pd catalysts on the wall of aerogel, accelerating the degradation process.In contrast, the discoloration of the MB solution in the presence of NaBH 4 proceeds very slowly in the absence of Pd decorated ALG/NFC aerogel (Movie S2).It takes almost two or three hours to completely decolorize without Pd catalysts.Based on the above analysis, the effective degradation of MB by the developed Pd@ALG/NFC aerogel can be attributed to the following two factors.(i) The highly dispersible Pd NPs in the aerogel facilitate the electron transfer from the electron donor NaBH4 to the target pollutant MB.NaBH4 serves as a nucleophilic reagent and is oxidized to BO2 − by donating electrons to Pd [40].Then, the electrons are transferred to MB through direct contact with Pd, which is an electrophilic reagent that leads to MB degradation.

Conclusions
A facile and green approach to fabricate the multifunctional Pd-decorated alginate/ nanofibrillated cellulose (Pd@ALG/ NFC) aerogel for simultaneous dye degradation, oil/water separation and oil collection was reported in this work.The highly dispersible Pd NPs not only endowed the aerogel with excellent catalytic performance, but also increased the surface roughness of the aerogel and accordingly improved its special wettability.By combining the superior wetting behavior and catalytic property, the as-prepared Pd@ALG/ NFC aerogel could simultaneously and efficiently achieve dye degradation, oil/water separation and oil collection.Furthermore, the hybrid aerogel exhibited good stability and rinsed with water and stored in seawater for next use.As shown in Fig. S1, after 10 cycles of use for 20 days, there was no significant change in degradation efficiency and oil-water separation efficiency.This indicates that the Pd@ ALG/NFC aerogel has good stability and durability in seawater, which is mainly attributed to the three-dimensional structure of sodium alginate crosslinked by calcium ions.This network is stable and salt-resistant because calcium ions have a higher affinity for guluronic acid (G) units than sodium ions.Therefore, even if there are a large number of sodium ions in the solution, they cannot replace calcium ions and destroy the crosslinking.

Fig. 1
Fig. 1 Schematic illustration of the preparation of the Pd@ALG/NFC aerogel

Fig. 2
Fig. 2 Color of the ALG/PdCl 2 solution (a) before and (b) after in-situ reduction; (c) and (d) TEM images of Pd NPs in different magnification; (e) HRTEM images of Pd NPs; (f) SAED pattern of the Pd NPs.

Fig. 3
Fig. 3 (a) Photo image of the Pd@ALG/NFC aerogel; (b) Low and (c) high magnification SEM pictures of the Pd@ALG/NFC aerogel; (d) EDS of the Pd@ALG/NFC aerogel (PdL is assigned to Lβ line of Pd

Fig. 4
Fig. 4 Contact angles of Pd@ALG/NFC aerogel with different liquids.(a) Water contact angle in air; (b) kerosene contact angle in air; (c) kerosene contact angle underwater; (d) contact angles of other oils and organic solvents underwater

Fig. 5
Fig.5 Polluted water purification process using the Pd@ALG/NFC aerogel: simultaneous gravity-driven oil/water separation, oil collection and dye degradation

Fig. 6
Fig. 6 Separation efficiency of the Pd@ALG/NFC aerogel for various oil/water mixtures