Synthesis And Surface Modication of Cellulose Aerogel from Coconut Peat for Oil Adsorption

16 Oil spillage is one of the world’s biggest environmental problems, its various impacts including 17 shifting the balance of the ecosystem, affecting marine animals, and inhibiting economical activities. 18 Therefore, the efficient resolution of this issue is a topic of great interest. In this work, the solution of 19 choice is an adsorption method using aerogels made from coconut peat. Cellulose coconut peat aerogels 20 (CCPA) are synthesized by cross-linking method with poly(vinyl alcohol) (PVA) and freeze-drying 21 technique to form the porous structure. The CCPA are furthermore dip-coated in poly(dimethylsiloxane) 22 (PDMS) to obtain PDMS-coated cellulose coconut peat aerogel (CCPA-P) with hydrophobic properties 23 for the studying of oil adsorption. The characteristics of CCPA and CCPA-P are evaluated by density 24 and porosity, specific surface area following Brunauer-Emmett-Teller (BET) theory, scanning electron 25 microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), energy 26 dispersive X-ray analysis (EDX), and water contact angle (WCA) measurements. Results showed that 27 CCPA-10 with the mass ratios of cellulose to PVA 10:1 had the lowest density of 28.21 mg/cm 3 , highest 28 porosity of 98.15 %. Furthermore, the modified CCPA-P10 had maximum adsorption capacity of up to 29 2.083 and 2.452 mg/mg for the static adsorption model and dynamic adsorption model, respectively. 30 This indicates that coconut peat is a viable material for aerogel synthesis in oil adsorption applications. 31


INTRODUCTION 33
With the growth of the automotive industry, oil demand has skyrocketed, leading to numerous oil 34 spill incidents. Recent  to be dependent on the surface area and porosity of adsorbents (Feng et al. 2016). On the other hand, 51 coconut peat, otherwise known as agricultural waste, is causing agricultural land unproductivity during 52 the wet season due to tannins and phenols from the coconut peat seeping into the soil (Zhang et al. 2019). 53 While recognizing the escalating problem that peat wastes can cause to the environment, their rich 54 cellulose content means coconut peat has potential in the synthesis of green adsorbents for oil spillage 55 treatment. This has implored coconut peat to be chosen as the raw material to synthesize cellulose 56 aerogels for oil adsorption (Yue et al. 2018). 57 In this study, 3D network cellulose coconut peat aerogels (CCPA) were fabricated through cross-58 linking method with poly(vinyl alcohol) (PVA) as a cross-linker, followed by a freeze-drying technique. 59 Additionally, to provide water resistance, CCPA were modified via dip-coating in 60 poly(dimethylsiloxane) (PDMS) to obtain PDMS-coated cellulose coconut peat aerogel (CCPA-P). 61 Characteristics of CCPA and CCPA-P were investigated by density and porosity, specific surface area 62 following Brunauer-Emmett-Teller (BET) theory, scanning electron microscopy (SEM), Fourier-63 transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), energy dispersive X-ray analysis 64 (EDX), and water contact angle (WCA). The oil adsorption test of CCPA-P was performed through 65 static and dynamic models. Finally, to analyze the adsorption mechanism of the CCPA-P, the pseudo-66 first-order and the pseudo-second-order models were considered. The schematic illustration of the 67 synthesis route and adsorption process of the CCPA-P is shown in Figure 1.

Preparation of CCPA 75
Firstly, coconut peat was soaked in NaOH solution (2 M) with a mass to volume ratio of 1:30 g:mL, 76 then heated at 80 °C for 2 hours. 30 mL of H2O2 (30 wt.%) was added, and the mixture was heated at 77 80 °C for 2 hours and washed with DI water to pH 7 then drying at 60 °C for 24 hours to gain the 78 cellulose material. Subsequently, the obtained cellulose was soaked in PVA solution according to the 79 cellulose to PVA ratios presented in Table 1. The mixture was elevated to 80 °C for 2 hours to promote 80 cross-linking, then sonicated to homogenize the mixture and to remove air bubbles. Diffraction analysis was performed using XRD patterns (Bruker XRD D8, Germany) with operating 97 parameters including CuKα irradiation X  0.154 nm) in the range of 0-80° with a scanning speed of 98 2°/minute; maximum operating temperature of 30 °C; and maximum humidity of 70%. The water 99 contact angle of CCPA-P was measured (DATAPHYSICS OCA-20, Germany) with a magnification of 100 0.7-4.5 times. The specific surface area and pore size following BET theory was determined via the 101 nitrogen adsorption/desorption curves at p0 = 756 mmHg and 77.35 K. 102

Density and porosity 103
The mass and dimension of CCPA were measured using a four-digit balance (CPA225D, Germany) 104 and electronic clamp (VOREL-15240 -150 mm, Germany), respectively. Densities of CCPA were 105 calculated by Equation (1): 106 while m (mg) is the weight of the materials and V (cm 3 ) is the aerogels volume obtained by Equation 107 (2): 108 where d (mm) and h (mm) represent the diameter and height of the aerogels, respectively. 109 The porosities of CCPA and CCPA-P (θ) were determined according to Equation (3): 110 while ρ is the density of the aerogels and ρ is the density of the solid material. 111

Oil adsorption test 112
The oil adsorption capacity of CCPA-P was investigated via a static model. The samples were placed 113 on the surface of the oil-water mixture with time from 1 to 10 minutes. After the adsorption test, the 114 samples were removed and weighed. The oil adsorption capacity was calculated via Equation 4. 115 where Q (mg/g) is the oil adsorption capacity of the sample, Ws (mg) and Wt (mg) are the mass of the 116 sample before and after being tested, accordingly. 117 To investigate the adsorption behaviors of CCPA-P, the pseudo-first and pseudo-second-order 118 adsorption kinetics models were employed -shown in Equations 5 and 6, respectively. 119 by plotting ln against time, the gradient of the best fit yields a k1 value. 120 Similarly, by plotting t Q t against time, the gradient of the best fit gives both t Q t and 1 k 2 Q m 2 , which are 121 then used to determine k2 by rearranging the equation. Qm and Qt (mg/mg) are the oil adsorption 122 capacities of the aerogel at equilibrium and at the investigated time t (minutes), respectively. The rate 123 constants k1 and k2 were determined from the diagrams for pseudo-first and pseudo-second-order 124 models.   The composition of elements in CCPA determined by the EDX spectra is represented in Figure 4. 151 The major elements consisting of carbon and oxygen corresponded to the two oscillating signals with 152 binding energies having the highest intensity. Some common minor elements (Mg, Si, Ca, etc.) are also 153 observed in insignificant amounts (below 1%) for the oscillating tips with lower intensity. The 154 composition of elements in CCPA is categorized into mass ratio and elemental ratio (as shown in Table  155 3). More specifically, carbon and oxygen accounted for roughly 99.02 % (mass ratio) and 99.61 % 156    Figure 5 shows the FTIR spectrum of four distinct samples including coconut peat, cellulose 162 extracted from coconut peat, CCPA, and CCPA-P. The first three samples exhibit two adsorption peaks 163 in the wavenumber region from 3325 to 3600 cm -1 and from 2920 to 2960 cm -1 , indicating OH 164 stretching and CH stretching, respectively (Gori et al. 2013). The characteristic peak of OH stretching, 165 in particular, is intensified from the spectrum of original coconut peat to that of CCPA due to the 166 exposure of the cellulose content after eliminating lignin and hemicellulose, along with the formation 167 of more hydroxyl groups after the addition of PVA. However, the peak at around 1600-1640 cm -1 is 168 assigned to C=O carbonyl stretching of the acetyl groups in hemicellulose or of the α-keto carboxylic  The characteristic peak at 2θ = 22.38°, corresponding to the (200) plane, oscillates with the highest 182 intensity, signifying the orderly crystal structure in cellulose (French 2014). However, the XRD pattern 183 of CCPA exhibits two characteristic peaks at around 20 o , indicating the (110) and (020) planes of 184 cellulose type II crystal structure. Therefore, the obtained material might be partially involved cellulose 185 type II along with cellulose type I crystal structure. This partial conversion of cellulose type II from 186 cellulose type I might be resulted from NaOH treatment. Moreover, XRD pattern of CCPA also 187 possesses a substantial reduction in intensity of the three characteristic peaks which shows that the 188 presence of PVA in CCPA has led to the disorder of the cellulose crystal lattice. And this disorder could 189 lead to the disappearance of the (11 0) plane of cellulose type II peak. After being coated with PDMS, 190 the emergence of two characteristic peaks involving a sharp peak at 2θ = 12 ° and a broad peak at 2θ =  PVA are slowly immersed into the oil layer. The CCPA-P quickly started to adsorb the oil; after 120 209 seconds, the oil on the water surface is completely adsorbed. The oil adsorption mechanism is elucidated 210 in Fig.8b. After being immersed into the oil/water mixture, the CCPA-P floated on the oil layer surface 211 due to its low density and hydrophobicity. As the oil particles were adsorbed into the porous structure, 212 air is consequently expelled from the porous structure until an equilibrium is reached. 213

CONCLUSION 233
In this work, 3D-networked CCPA were synthesized from coconut peat via combining cross-linking 234 with freeze-drying methods. The obtained CCPA were successfully modified into hydrophobic CCPA-235 P by dip-coating in PDMS and accordingly applied for studying of oil adsorption. The results 236 demonstrated that CCPA-10 with a ratio of cellulose to PVA 10:1 possessed the best properties, with 237 the lowest density of 28.21 mg/cm 3 and the highest porosity of 98.15 %. The maximum adsorption 238 capacity of CCPA-P10 for dynamic models (Qm = 2.452 mg/mg) is found to be higher than static models 239 (Qm = 2.083 mg/mg). Studying the adsorption kinetics showed the second-order adsorption kinetic 240 model to more accurately predict the oil adsorption behavior of CCPA-P than the first-order adsorption 241 kinetic model. Therefore, it is reasonable to conclude that CCPA-P10 is promising to become a potential, 242 biodegradable, and eco-friendly oil-absorbing material in the future. 243

ACKNOWLEDGEMENT 244
We acknowledge the support of time and facilities from Ho Chi Minh City University of Technology 245 (HCMUT), VNU-HCM for this study. 246

DECLARATION OF CONFLICTS 247
We confirm that this work is original and has not been published elsewhere, nor is it currently under 248 consideration for publication elsewhere. 249

CONFLICTS OF INTEREST 250
We have no conflicts of interest to disclose. This paper was written by listed authors who are all 251 aware of its content and approve its submission. 252

ETHICAL STANDARDS STATEMENTS 253
This study does not involve any human subjects and no animal or human studies were carried out by 254 the authors. 255 Please address all correspondence concerning this manuscript to corresponding author at 256 nhhieubk@hcmut.edu.vn.