Dehydration of Cloudy-diesel Using Poly(Acrylamide-co-sodium Polyacrylate) Hydrogel Grafted Onto Cellulose

doi:10.21203/rs.3.rs-5020126/v1

This article discusses the synthesis and application of a novel hydrogel to remove water from diesel. The poly(sodium acrylate-co-acrylamide) grafted onto cellulose hydrogel combines natural and synthetic polymers in a single novel formulation. The synthesized of the hydrogel was evaluated using a 2³ full factorial design with two initiation methods (redox or thermal), two cellulose types (cellulose nanocrystal - CNC or hydroxyethyl cellulose - HEC) and two cellulose loadings (4 or 10%) as independent factors. The hydrogel swelling degree and final water concentration on diesel were used as responses. The best performing hydrogel was made with CNC using the redox initiation system. This hydrogel was further analyzed for its kinetic of water uptake from diesel and compared with ungrafted poly(sodium acrylate-co-acrylamide) hydrogels for their maximum swelling degree and water removal from diesel. The cellulose loading on the hydrogel was not found to be significant for the analyzed conditions. The cellulose-grafted hydrogel made with 4% CNC using the redox system achieved a swelling degree of around 139 g.g^− 1 and reduced the water content in diesel from 5,000 to 150 mg.kg^− 1. Compared to pure ungrafted hydrogel, the cellulose grafted hydrogel swelled less but removed more water from diesel. The hydrogel treatment also reduced the fuel turbidity and made it more compatible with standard requirements. This work demonstrates the successful synthesis of hydrogels of poly(SA-co-AAm) grafted onto cellulose and proves that these novel hybrid compounds can be used as fuel desiccants.

cellulose nanocrystal

hydroxyethyl cellulose

hydrophilicity

water uptake

fuel treatment

water-in-oil

Diesel may absorb water during production, transfer, and storage stages (Gonçalves et al. 2021), a phenomenon that compromises its quality and represents a major obstacle in fuel processing (Arthus et al. 2023a). When it surpasses the limits specified by technical standards, water may corrode metallic surfaces, and form foam and sludge (Arthus et al. 2023b). The presence of free water in oil may also favor the proliferation of microorganisms that use diesel components as an energy source and produce acidic metabolites (Estevam et al. 2023b). The acidification of the fuel triggers sludge formation, promotes internal corrosion of tanks, and compromises engine components (Gonçalves et al. 2021).

Several industrial separation technologies are currently used to reduce the fuel water content. The optimal choice, both technically and economically, depends on how much water must be removed, and on the type and volume of oil being treated (Atadashi et al. 2012; Arthus et al. 2023b). Unit operations such as gravity separation, centrifugation, and coalescing filters are commonly used for oils containing high water content (Perissinotto et al. 2020), while mass transfer operations such as distillation and salt beds are preferred to treat oils containing mainly dissolved water (Arouni et al. 2019). Despite being well established, these methods are limited by high operational costs, substantial energy consumption, generation of secondary pollutants, and inefficient elimination of the dissolved water (Atadashi et al. 2012; Perez et al. 2022; Estevam et al. 2023b). Consequently, the development of novel technologies for removing water from fuels is of utmost importance.

The use of hydrogels has shown promising results for the removal of free, soluble, and emulsified water (Arthus et al. 2023a). Hydrogel treatments are simple, consume low energy, and can undergo multiple regeneration cycles (Fregolente et al. 2023). These hydrophilic polymers can be made through various pathways that determine their structures, morphologies, mechanical stability, and water retention capacity (Estevam et al. 2023b). Generally, ionic polymers, such as sodium polyacrylate-based hydrogels, can efficiently remove water from oil (Fregolente et al. 2023; Arthus et al. 2023b). Grafting polymers or particles can also significantly enhance the structural stability of hydrogels (Delgado et al. 2023; Zhao et al. 2023).

Grafting biopolymers through free radical polymerization to a substrate is a convenient way to enhance the properties of the latter while preserving its original characteristics (Bayramgil 2018). Grafting hydrophilic polymers onto cellulose yields benefits to both the structural and hydrophilic attributes of the material, potentially leading to increased water absorption and retention capacity (Al Abdallah et al. 2024). This procedure involves the creation of free radicals on the cellulose chains, followed by growing grafts through polymerization (Estevam et al. 2023a). The grafting polymerization process begins with the activation of the initiator species, typically through the abstraction of hydrogen atoms from the cellulose chains by thermal initiation or through the use of a redox pair (Bayramgil 2018). This generates reactive free radicals, to which monomers are added via propagation reactions (Beyaz et al. 2019).

The type of cellulose used in the hydrogel synthesis may lead to distinct properties in the resultant materials (Estevam et al. 2023a; Al Abdallah et al. 2024). Cellulose nanocrystal (CNC) is obtained through acidic or enzymatic hydrolysis of cellulose fibers. This process cleaves the amorphous regions while preserving the crystalline domains of the cellulose, making a structure with a high surface area (Matsuo et al. 2024). However, due to the abundance of hydroxyl groups in a linear chain arrangement, the CNC presents extensive intra- and intermolecular hydrogen bonds, creating a highly ordered and crystalline structure that hampers its solubility in water (Rop et al. 2020). Water-soluble cellulose derivatives, such as hydroxyethyl cellulose (HEC), can be obtained by reacting cellulose hydroxyl groups with organic species like methyl and ethyl groups (Estevam et al. 2023a). The addition of hydroxyethyl groups reduces the number of hydroxyl groups available for bond the cellulose chains. This disruption decreases the crystallinity, enhancing its water solubility (Beyaz et al. 2019). HEC is commonly used in hydrogel synthesis due to its high water solubility and abundance of hydroxyl and carboxyl groups, which allow easy graft polymerization with hydrophilic vinylic monomers (Tudoroiu et al. 2021).

This manuscript addresses the use of poly(sodium acrylate-co-acrylamide) hydrogel grafted onto cellulose for the removal of water from automotive diesel. Three variables were investigated during the hydrogel synthesis: cellulose type (CNC or HEC), cellulose loading (4 or 10%), and free radical initiation method (redox or thermal). The resulting hydrogels were evaluated based on their swelling degree and ability to remove emulsified water from automotive diesel. Thus, this study merges the characteristics of synthetic and natural polymers to develop a novel hydrophilic material that can efficiently and simply dehydrate diesel.

Materials

The hydrogels were made using ultrapure acrylamide (AAm), sodium acrylate 97% (SA), ultrapure N,N’-methylene-bis-acrylamide (MBAAm), potassium persulfate 99% and N,N,N,N’-tetramethylethylenediamine (TEMED) 99%. The cellulose used in this study was derived from wood pulp and met analytical-grade standards. The cellulose nanocrystal (CNC) was provided by Innotech Alberta (batch no. Comp181121-H), while the hydroxyethyl cellulose (HEC) was sourced from Sigma-Aldrich with a purity greater than 92%.The diesel sample was purchased at a local gas station located in Edmonton, Alberta, Canada, and may contain up to 5% of biodiesel. The distillation curve of the diesel (HT-SimDis analysis) is presented in Fig. S1 in the Supplementary Information.

Preparation of diesel emulsion and measurement of water content

The water-in-diesel emulsion was prepared with 0.5% of distilled water by volume of oil and homogenized using an Ultra Turrax ultradisperser (digital IKA T65) with a constant rotation speed of 4,000 rpm for 10 min. The water content in the diesel (prior to and after hydrogel treatment) was measured by the Coulometric Karl Fischer method, following the ASTM D6304 standard (D6304-20 ASTM 2021).

Synthesis of hydrogels

The cellulose-grafted poly(SA-co-AAm) hydrogels were made by free radical polymerization. A full factorial design (Table 1) was used to assess the optimal conditions for hydrogel synthesis regarding type (HEC or CNC) and loading (4 or 10%) of cellulose in the hydrogel, as well as the initiation method (redox or thermal). The mass percentage of cellulose was calculated based on the overall mass of monomers (acrylamide - AAm and sodium acrylate - SA) used in the hydrogel synthesis.

The hydrogels were made in a 250 mL glass reactor at 20–25°C. A solution with 1.25 g of SA and 1.5 g of AAm (monomers), 0.079 g of MBAAm (crosslinker), cellulose, and 45 mL of distilled water was stirred at 200 rpm for about 10 minutes under nitrogen atmosphere. Meanwhile, 0.02 g of potassium persulfate initiator (KPS) was dissolved in 5 mL of water.

Two free radical initiation methods were compared: thermal and redox. The thermal system used only potassium persulfate (KPS) at 70°C, while the redox system used KPS and TEMED at 25°C. For the thermal initiation mechanism, the solution with cellulose, monomers, and crosslinker was heated to 70°C under nitrogen atmosphere until the temperature stabilized, after which the initiator was added to the mixture under continuous stirring at 200 rpm until gel formation. For the redox initiation system, 1 mL of TEMED (0.57 mol.L^− 1) was added to the aqueous solution containing cellulose, monomers, and crosslinker. The mixture was homogenized for 10 min at 200 rpm and 25°C under nitrogen atmosphere. KPS was then added to the mixture, which resulted in the formation of a gel.

Once the gel was formed (for both initiation methods), the reactor was sealed using parafilm and kept without stirring for 24 h. The hydrogels were then transferred from the reactor to a 500 mL Becker with 200 mL of distilled water, followed by immersion in ethyl alcohol to eliminate unreacted monomer molecules. After that, the hydrogels were cut into cubes and classified through sieves, with an average diameter of 3.5 mm. Finally, the samples were dried in an oven at 65°C until their masses were constant.

Table 1

Independent variables and levels for the experimental design (2³ with duplicates).
Independent variables (factors)	Level
Independent variables (factors)	-1	+ 1
Cellulose type	HEC	CNC
Cellulose percentage	4%	10%
Initiation system	Thermal	Redox

Two response factors were used: the maximum swelling degree (W_max) and the final diesel water content (mg.kg^− 1). The hydrogel W_max was quantified by immersing 0.1 g of hydrogel in 150 mL of distilled water at 25°C for to 144 h, removing excess water by filtering the hydrogel through a nylon mesh and the mass variation used to calculate the swelling degree (Fregolente et al. 2018). The water removal effectiveness of the hydrogels was evaluated by mixing 1 g of hydrogel with 50 mL of diesel contaminated with water (at an initial water concentration of 5,000 mg.kg^− 1) and stirring the samples at 200 rpm and 20–25°C for 120 min. The final diesel water content was determined using the Coulometric Karl Fischer method (D6304-20 ASTM 2021). The experiments were performed in duplicate and the results evaluated using the Statistica10.0® software, considering pure error and a 95% confidence level.

Hydrogel characterization

The hydrogels, HEC, and CNC were characterized by Fourier Transform Infrared Spectroscopy (FTIR) and X-ray diffraction (XRD). FTIR analyses were used to identify the chemical bonds of the materials (from 550 to 4,000 cm^− 1 with a spectral resolution of 2 cm^− 1). The XRD analyses measured the crystallinity of the samples across a 2θ range spanning from 10° to 60°, scan rate of 2°.min^− 1, step size of 0.02°, and a Cu kα radiation source (λ = 1.5406 Å) at settings of 30 kV and 20 mA. These analyses were performed using dry hydrogels processed in a mill until they turned into powders and follows the conditions stablished in the literature and recommended in the equipment guidelines.

The swelling kinetics of the hydrogels was quantified by immersing approximately 0.1 g of dry hydrogel in 150 mL of distilled water at 25°C. Mass gain was measured over time (from 5 to 8,640 min) and swelling was calculated according to Eq. 1. The water diffusion into the hydrogel was described with a power-law model (Eq. 2) applied to the exponential phase of the swelling kinetics (Hertle et al. 2010).

$$\:{W}_{t}=\:\frac{{w}_{t\:}-\:{w}_{i\:}}{{w}_{i\:}}$$

1

$$\:\frac{{W}_{t\:}}{{W}_{eq\:}}=k.{t}^{n}$$

2

In Eqs. 1 and 2, W_t is the swelling degree (g.g^− 1) at time t, W_eq is the swelling degree (g.g^− 1) at equilibrium, w_i and w_t are the masses (g) of the hydrogels at the beginning and after time t of swelling, respectively, k is the kinetic swelling constant, n is the diffusional exponent that indicates the transport mechanism (dimensionless), and t is the swelling time (min).

Based on the experiment design results (Section 2.3), the hydrogel with the best performance (regarding swelling degree and water removal from diesel) was selected for further characterizations by Scanning Electron Microscopy (SEM) and X-Ray Microscopy (XRM) analyses. The SEM analysis used fragments of the oven-dried hydrogel, which were imaged at different magnifications at 15 kV and 9,800 nA. The XRM analysis determined the three-dimensional structure of a hydrogel sample with an average diameter of 3.5 mm. The sample was scanned with a voltage of 30 kV and a power of 2 W. More than 1,000 images of the sample were acquired at 4x magnification and the 3D model of the sample was built and processed by the Dragonfly Pro software from the company Object Research Systems.

Kinetics of water removal from diesel

The hydrogels that exhibited the most favorable performance concerning swelling and water removal in diesel (identified in the study outlined in Section 2.3) were assessed regarding their water removal kinetics. In these experiments, 1.0 g of hydrogel was mixed with 50 mL of diesel contaminated with 5,000 mg.kg^− 1 of water (in emulsion) at 25°C and 200 rpm. The water content in the diesel was measured after 120, 180, 240, and 300 minutes of contact with the hydrogel. Each experiment was performed in duplicate. Besides, control flasks (emulsion without hydrogel) were subjected to identical experimental conditions to identify possible water losses not related to the removal of water by the hydrogel.

Effect of cellulose addition

Cellulose-grafted hydrogels were compared with pure poly(SA-co-AAm) hydrogel for their maximum swelling degree and effectiveness in removing water from diesel. To carry out the assessment, the hydrogel was made as described in Section 2.3 without addition of cellulose using the redox initiation system. The poly(SA-co-AAm) hydrogel was subjected to the same swelling conditions (described in Section 2.4) as the cellulose-grafted hydrogel. Similarly, water removal from diesel was performed as described in Section 2.5 with a fixed contact time of 120 minutes. The analyses were carried out in duplicate.

Hydrogel synthesis method

Table 2 presents the full factorial design (2³ with duplicates) levels and responses for the hydrogel syntheses. The effects of each factor and their interactions were evaluated using Pareto charts and graph of marginal means for the maximum swelling degree (Fig. 1) and the final concentration of water in diesel (Fig. 2). The hydrogel with the highest W_max value and lower final water concentration in diesel was selected as the best formulation. The analysis of variance (ANOVA) and the predictive equations for the responses are presented in the Supplementary Information (Table S1 and S2 and Equations S1 and S2).

Table 2

Dependent and independent variables of the full factorial design (2³ + duplicates) used to evaluate the hydrogel synthesis method.
Factor and levels			Responses
Type of cellulose	Cellulose percentage (%)	Initiation system	W_max (g.g^− 1)		Final water content in diesel (mg.kg^− 1)
Type of cellulose	Cellulose percentage (%)	Initiation system	1st run	2nd run	1st run	2nd run
HEC	4	Thermal	44.88	49.21	443.3	431.8
CNC	4	Thermal	43.42	46.48	246.1	264.9
HEC	10	Thermal	52.53	68.38	430.4	372.3
CNC	10	Thermal	48.91	49.68	171.4	206.5
HEC	4	Redox	124.72	132.93	266.0	244.0
CNC	4	Redox	121.08	157.75	136.1	165.1
HEC	10	Redox	101.82	107.42	227.0	254.0
CNC	10	Redox	129.66	145.94	158.7	170.0

Using the W_max as response, the statistically significant factors were those with p-value greater than 0.05 (Fig. 1-a), evaluated with 95% confidence. The most significant factor was the type of initiation system. As this effect was positive, the response is higher when the initiation system is at level + 1 (Redox initiation). The interaction effect between the cellulose type and initiation system was also significant and positive; thus, when both factors were at level + 1 (CNC and Redox initiation), the produced hydrogel had a higher degree of swelling. Although the interaction between the cellulose type and initiation system is significant, it is close to the baseline p-value of 0.05, and the effect of cellulose type was not statistically significant.

The marginal means plots (Fig. 1-b) aligns with the results from the Pareto chart and confirm that the highest swelling degree was achieved with CNC and redox initiation. This plot also indicates that the initiation system is the most significant factor, as hydrogels synthesized using redox initiation attained a significantly higher swelling degree regardless of the type or amount of cellulose used. Additionally, similar results were obtained for the hydrogels grafted onto CNC and HEC, highlighting the reduced impact of this factor for the swelling degree. Using 4% HEC yielded a swelling degree comparable to that of 4% and 10% CNC. However, there was an overlap in the standard deviation ranges for 4% and 10% HEC or CNC, suggesting that the percentage of cellulose did not have a significant effect under the analyzed conditions.

Figure 1: Pareto chart (a) and plots of marginal means (b) indicating the effects of the parameter evaluated in the full factorial design (2³ + duplicates) with W_max (g.g^-1) as response.

For diesel final water content response, the lower the values, the more effective the water removal process. According to the Pareto chart (Fig. 2-a), the most significant factors were cellulose type and initiation system, both with negative effects. The interaction between initiation system and the cellulose type was also significant, with a positive effect. This indicates improved outcomes with redox initiation system and CNC. The dependence between these factors can also be observed in the plot of marginal means (Fig. 2-b). Regarding the cellulose loading, the graph of marginal means indicates an overlap in the responses obtained for each level evaluated, confirming the small significance of this factor, as predicted in the Pareto’s chart. Therefore, under the conditions analyzed, the use of a smaller loading of biopolymer is sufficient for the intended objective of removing water from diesel.

Figure 2

Pareto chart (a) and graph of marginal means (b) indicating the effects of the parameters evaluated in the full factorial design (2³ + duplicates) with the final water content on diesel (mg.kg^− 1) as response.

The use of different initiation systems affected both the swelling degree and the water removal capacity. The redox initiation instantly generates a large number of free radicals. According to Huang et al., (Huang et al. 2017) this leads to a polymerization onto a range of sites in the cellulose chain, resulting in a more tightly entangled hydrogel structure with evenly dispersed crosslinking centers. Meanwhile, the thermal initiation requires the gradual decomposition of KPS into HO₃SO• at 70°C, resulting in the formation of larger polymeric chains more widely dispersed onto the cellulose chain (Huang et al. 2017). Therefore, the hydrogel prepared by redox initiation may exhibit a higher crosslinking density compared to those prepared by thermal initiation. Kabiri et al., (Kabiri et al. 2003) explains that for strongly hydrophilic hydrogels, a higher crosslinking density can lead to faster water diffusion. This could explain the increased swelling degree and more efficient water removal from diesel when using hydrogels prepared by redox initiation.

Regarding the type of cellulose, the hydrogels synthesized with CNC swelled more and removed more water from diesel than the HEC hydrogels. These differences may be due to the higher surface area of CNC nanoporous structure, which enables higher absorption of water molecules (Tammelin et al. 2015). In addition, CNC is crystalline and has a higher concentration of exposed hydrophilic groups on its external surface, which may also improve water absorption (Tammelin et al. 2015; Bayramgil 2018; Das et al. 2021; Wong et al. 2021).

The percentage of cellulose used in the hydrogel synthesis had no significant impact on water removal under the conditions examined. However, it is worth noting that the amount of cellulose used can affect the network density and potentially lead to the formation of more compact structures (Arthus et al. 2024). Thus, the lack of significance attributed to this factor largely stems from the levels used in this study and the responses that were evaluated.

Hydrogel characterization

Figure 3 shows the FTIR spectra of the prepared hydrogels. The hydrogels were named according to cellulose type, cellulose percentage, and initiation type (“R” for redox and “T” for thermal). For example, “HEC-4%-R” is the hydrogel produced with 4% HEC using the redox system. The spectra of poly(SA-co-AAm) and pure HEC and CNC were added as references. The peaks in the poly(SA-co-AAm) spectrum are similar to those in spectra of the cellulose-grafted hydrogels, but the peaks in the spectra of pristine HEC and CNC were absent in the spectra of cellulose-grafted hydrogels, indicating interactions between cellulose chains and monomer molecules. The 1,556 cm^− 1 and 1,450 cm^− 1 bands present in the cellulose-grafted hydrogels correspond to the N-H bond and the σ-bond from the homolysis of π-bonds of the monomers, respectively, confirming that the grafting of acrylamide and sodium acrylate onto the cellulose chains was successful (Dourado et al. 2021). Table 3 identifies the peak and corresponding functional groups in the hydrogels.

Figure 3

FTIR spectra of pure HEC and HEC-grafted hydrogels (a), and pure CNC and CNC-grafted hydrogels (b). “R” corresponds to redox and “T” refers to thermal initiation system.

Table 3

Correspondence between the absorbance peaks obtained in the FTIR spectrum with the functional groups of the hydrogels and cellulose polymers (Foo et al. 2017; Beyaz et al. 2019; Jayaramudu et al. 2019; Li et al. 2019; Boruah et al. 2023).
Wave number (cm^− 1)	Corresponding functional group	Polymer in which it was identified
3,345-3,180	O-H, N-H	poly(AS-co-AAm) and cellulose-grafted hydrogels
3,313-3,367	O-H	CNC and HEC
2,928-2,854	C-H	CNC, HEC, poly(AS-co-AAm) and cellulose-grafted hydrogels
1,659	C = O	poly(AS-co-AAm) and cellulose-grafted hydrogels
1,635-1,640	O-H	CNC and HEC
1,556	NH2	poly(AS-co-AAm) and cellulose-grafted hydrogels
1,420-1,449	C-H	CNC, HEC, poly(AS-co-AAm) and cellulose-grafted hydrogels
1,316-1,319	C-H	CNC and HEC
1,158	C-C	poly(AS-co-AAm) and cellulose-grafted hydrogels
1,045 − 1,058	C-O	CNC and HEC
669–885	O-H	CNC and HEC

Figure 4 shows the diffractograms obtained from the XRD analysis. HEC displayed a peak at 2θ = 22°, indicating a slight degree of crystallinity accompanied by amorphous regions (El Fawal et al. 2018). In the CNC diffractograms, the peaks at 14°, 16°, 22°, and 34° confirm the crystalline nature of this material (Boruah et al. 2023). All cellulose-grafted hydrogels exhibited a single peak at 2θ = 22° with varying intensities. This observation implies that both cellulose types were affected by the grafting reactions, which changed their original crystalline structures (Mukerabigwi et al. 2016).

Figure 4

Diffractograms obtained by XRD analysis of pure HEC and HEC-grafted hydrogels (a), and pure CNC and CNC-grafted hydrogels (b). “R” corresponds to redox and “T” refers to thermal initiation system.

The swelling kinetics of the different hydrogels (Fig. 5) were influenced by the initiation method. Notably, all hydrogels prepared by the redox method showed higher swelling across all conditions. Additionally, the HEC hydrogel with the lowest percentage of cellulose presented the largest swelling degree. This phenomenon may be attributed to the less dense structure and higher chain mobility associated with the lower cellulose content (Nath et al. 2023). During the initial stages of swelling the hydrogels were grafted onto CNC using 4% cellulose resulted in a greater swelling degree compared to those with 10%. However, the hydrogels with both CNC loadings achieved similar results at equilibrium. This indicates that the early phases of swelling are more affected by the decreased chain mobility caused by the addition of CNC, requiring longer periods to reach the maximum swelling degree for the grafted hydrogel. In addition, all prepared hydrogels reached equilibrium swelling after 1,440 min and those formulated with CNC exhibited slightly superior performance (up to 139 ± 25 g.g^− 1) but showed higher standard deviations compared to those formulated with HEC (up to 128 ± 5 g.g^− 1).

Figure 5

Swelling kinetics of cellulose-grafted hydrogels prepared with HEC (a) and CNC (b).

The power-law model was used to evaluate the physical mechanism of water retention during the exponential phase of the swelling process. The fit of the data to the model is depicted in Fig. 6, while Table 4 shows the fit and parameters of the power-law model for each hydrogel formulation. During the swelling, the water diffusion into the hydrogel results in the relaxation and reorganization of the polymer chains. The exponent n of the power-law model provides information about the physical mechanism of water retention in the hydrogel by the relation between the rate of water diffusion and chain relaxation. This mechanism can be divided in Fickian, anomalous or non-Fickian. In Fickian diffusion swelling, water diffuses into the hydrogel more rapidly than the polymer network can reorganize. Conversely, in non-Fickian diffusion swelling, the rates of polymer network relaxation are higher than the rate of water diffusion. Anomalous swelling is marked by a complex interplay of diffusion and relaxation processes, where water diffusion and polymer chain relaxation impact the swelling at the same order of magnitude (Ostrowska-Czubenko et al. 2015; Arthus et al. 2023b). When n < 0.5, swelling occurs by Fickian diffusion; for 0.5 ≥ n ≥ 1.0, the water transport mechanism is anomalous; and when n > 1, it indicates non-Fickian diffusion (Hertle et al. 2010).

Most of the synthesized hydrogels showed water transport governed by non-Fickian diffusion (Table 4), indicating that the water diffusion is much faster than the relaxation of the polymeric chains. The faster water uptake process may be related to the amount of hydrophilic groups arranged in a denser network (Arthus et al. 2023b). In the hydrogel formulation that used thermal initiation in the grafting of CNC, an anomalous water transport mechanism was identified (Table 4). In this case, Fickian diffusion and relaxation of the hydrogel chains occurs at similar rates (Jayaramudu et al. 2019). Similar results were obtained by Arthus et al. (2023b), who reported non-Fickian diffusion in hydrolyzed polyacrylamide hydrogels, and anomalous diffusion of polyacrylamide hydrogels. Gonçalvez et al. (2021), also reported anomalous diffusion in hydrogels formulated with poly(acrylamide-co-sodium acrylate) and poly(acrylamide-co-acrylic acid), with n value higher than 0.9, which may indicate a strong influence of the relaxation of the polymeric chains.

Table 4

Swelling kinetic fit and parameters.
Hydrogel formulation	R²	n	K	Predicted water transport mechanism
HEC-4%-T	0.9904	1.4928	-0.1008	Non-Fickian
HEC-4%-R	0.9969	1.3719	-0.8144	Non-Fickian
HEC-10%-T	0.9979	1.0848	-0.3471	Non-Fickian
HEC-10%-R	0.9958	1.5483	-1.8014	Non-Fickian
CNC-4%-T	0.9845	0.8196	0.6791	Anomalous
CNC-4%-R	0.9882	1.3507	-0.7936	Non-Fickian
CNC-10%-T	0.9585	0.7058	0.9561	Anomalous
CNC-10%-R	0.9898	1.4952	-1.7064	Non-Fickian

The CNC-4%-R hydrogel was selected based on the results obtained in the experimental design (detailed in Section 3.1) and further characterized by SEM and MRX analyses. Through SEM analysis (Fig. 7), it was possible to verify that the material has a rough surface, which may be related to the presence of CNC. This observation suggests that a substantial portion of the CNC is dispersed within the hydrogel matrix, facilitated by intermolecular hydrogen bonding with the monomers (AAm and SA) of the crosslinked network (Jayaramudu et al. 2019). Besides, the material presents regions where the CNC formed some aggregated, as highlighted in Fig. 7-b.

Figure 7: SEM images of CNC-4%-R hydrogel at 1000x (a) e 2000x (b) magnification.

The XRM analysis (Fig. 8) agrees with the SEM observations, as both methods confirm that the prepared hydrogel is not porous. Instead, the hydrogel features spherical voids that are not interconnected. These voids stem from entrapments that occurred during the hydrogel synthesis process due to the increase in the viscosity of the medium during gel formation, which hampers the exit of nitrogen gas bubbles. To mitigate this phenomenon, it is advisable to enhance the process of nitrogen gas bubbling during the synthesis.

Kinetics of water removal from diesel

Based on the outcomes of the experimental design (presented in section 3.1), the hydrogels synthesized with CNC using the redox initiation method were chosen for further assessment, while the amount of cellulose was not significant. Therefore, both CNC-4%-R and CNC-10%-R hydrogels were evaluated regarding the kinetics of water absorption from cloudy diesel. The obtained results are summarized in Table 5, revealing that both cellulose-grafted hydrogels exhibited a water removal efficiency of approximately 94% at all contact times assessed. Efficiency was calculated from the water content in control flasks (cloudy diesel without hydrogel) and treatment flasks (cloudy diesel with hydrogel). Worth highlights that using either 4% or 10% CNC yielded similar water removal capacities. This discovery aligns with predictions from the experimental design and underscores that the percentage of CNC cellulose did not substantially impact water removal from diesel under the conditions examined.

Table 5

Kinetics of water removal from diesel using CNC-grafted hydrogels prepared by redox initiation system. Experimental conditions: 1 g of hydrogel, 50 mL of water-in-oil emulsion, 5,000 mg.kg^-1 of initial water content in diesel, and 25°C.
Contact time (min)	Water content (mg.kg^− 1) in control flask	Water content (mg.kg^− 1) in hydrogel treated diesel
Contact time (min)	Water content (mg.kg^− 1) in control flask	CNC-4%-R	CNC-10%-R
120	2862.02 ± 368.04	150.63 ± 14.53	164.38 ± 5.68
180	2231.90 ± 248.88	131.85 ± 2.65	114.80 ± 0.60
240	1944.54 ± 223.99	114.20 ± 3.80	115.45 ± 0.85
300	1897.12 ± 194.70	110.91 ± 5.10	111.80 ± 5.60

The water content within diesel holds a critical significance as it is a mandatory parameter for the proper utilization of the fuel. This research seeks to treat the diesel to meet the specification established by the Canadian government, which allows a maximum water content of 200 mg.kg^-1 (National Standard of Canada 2020). Through the utilization of cellulose-grafted hydrogel, this study facilitates the reduction of water concentration in diesel to levels below the specified limit. The visual appearance of the control and treatment flasks provides compelling evidence (Fig. 9). The control flask exhibits cloudy diesel, whereas the treatment flask has clear diesel. This visual contrast confirms the effectiveness of the hydrogel in absorbing water from the fuel.

Effect of cellulose addition

The cellulose-grafted hydrogels and the poly(SA-co-AAm) hydrogel were compared concerning their swelling degree and water removal capacity from diesel. This evaluation aimed to discern the impact of cellulose addition on the crosslinked structure. The recorded W_max values (depicted in Fig. 10-a) indicate that the poly(SA-co-AAm) hydrogel displayed a greater swelling degree than the cellulose-grafted hydrogels. This difference can be attributed to the potential effect of CNC in increasing the stiffness of the polymeric chains, causing an increase in resistive forces that oppose the swelling process (Jayaramudu et al. 2019). Nonetheless, cellulose-grafted hydrogels were able to remove more water from diesel than poly(SA-co-AAm) (Fig. 10-b). This phenomenon may be related to the presence of more hydrophilic groups available to interact with the water present in diesel. It is noteworthy that the comparison between the hydrogels was accessed in just one contact time (120 min) and the water contents might tend to equalize as equilibrium is achieved. Nevertheless, a shorter contact time can confer certain advantages due to the reduced time spent in the system.

Figure 10

Maximum swelling degree (a) and final water content in diesel (b) using poly(AS-co-AAm) hydrogel and CNC-grafted hydrogels.

The synthesis of poly(SA-co-AAm) grafted onto cellulose hydrogel was successful and the resulting materials were efficient in removing water from diesel. The hydrogels, prepared using CNC or HEC, presented semicrystalline and amorphous regions, in addition to functional groups characteristic of poly(SA-co-AAm) hydrogel. The hydrogels presented a rough surfaces with some cellulose aggregated and an internal structure containing spherical voids not interconnected. The hydrogels synthesized with CNC and the redox initiation system presented superior performance both for maximum swelling degree and water removal from diesel at 95% confidence. It may be related to the generation of a large number of free radicals instantly in the redox initiation system, as well as the higher surface area and more -OH groups exposed to the nanoporous structure of the CNC. The percentage of cellulose used had no significant effect on the results obtained under the analyzed conditions, reaching a water removal efficiency of around 94%. The hydrogel treatment also led to a reduction in the visual turbidity of the diesel, making it clear rather than cloudy. When comparing cellulose-grafted hydrogels with poly(SA-co-AAm) hydrogels, it was observed that the addition of cellulose slightly reduced the swelling degree (from 160 to 139 g.g^− 1) due to the increase in the density and rigidity of the hydrogel’s networks. However, the cellulose-grafted hydrogel showed superior performance in removing water from diesel, which can be attributed to the addition of hydrophilic functional groups of cellulose to the crosslinking matrix. Using the CNC-4%-R hydrogel, the water concentration in the diesel reduced from 5,000 to 150 mg.kg^− 1 after just 2 h of treatment, making it possible to fit the fuel to the requirements set by the Canadian government. Thus, this work shows that poly(SA-co-AAm) grafted onto cellulose hydrogels can be used to remove water from fuels and points out the best conditions for the synthesis of the hydrophilic polymer.

Funding

This research received financial support from the Institutional Internationalization Program of the Coordination for the Improvement of Higher Education Personnel (CAPES– PrInt), grant numbers 88887.682593/2022-00 and 88887.936650/2024-00; the National Council for Scientific and Technological Development (CNPq), grant number 140819/2019; São Paulo Research Foundation (FAPESP), grant numbers 2015/20630-4, 2017/12120-1 and 2021/03472-7; Petrobras S.A. grant number 5462; and the Human Resources Training Program for the Oil Sector, Natural Gas and Biofuels (PRH-ANP), grant number 50/2015 (PRH 29.1 – Referring to notice No. 1/2018/ANP-PRH; FINEP/FUNCAMP/FEQ Agreement Ref. 5485).

Competing Interests

The authors have no relevant financial or non-financial interests to disclose.

Declaration of generative AI and AI-assisted technologies in the writing process

During the preparation of this work the authors used Grammarly® in order to improve readability and language. After using this tool, the authors reviewed and edited the content as needed and takes full responsibility for the content of the published article.

Data availability statements

Data will be made available upon reasonable request.

Ethics approval and consent to participate

Not applicable.

Author Contributions

Isadora Dias Perez: conceptualization, methodology, validation, data Curation, formal analysis, investigation, writing—original draft, and visualization.

Bianca Ramos Estevam: investigation, validation, writing—original draft, and visualization.

João B.P. Soares: conceptualization, supervision, validation, and writing—review and editing.

Melissa Gurgel Adeodato Vieira: conceptualization, supervision, validation, and writing—review and editing.

Leonardo Vasconcelos Fregolente: conceptualization, supervision, resources, funding acquisition, project administration, and writing—review and editing.

Al Abdallah H, Tannous JH, Abu-Jdayil B (2024) Cellulose and nanocellulose aerogels, their preparation methods, and potential applications: a review. Cellulose. https://doi.org/10.1007/s10570-024-05743-w
Arouni H, Farooq U, Goswami P, et al (2019) Coalescence efficiency of surface modified PBT meltblown nonwovens in the separation of water from diesel fuel containing surfactants. Results Eng 4:100048. https://doi.org/10.1016/j.rineng.2019.100048
Arthus L, Estevam BR, Maciel MRW, Fregolente LV (2024) Design of mechanically stable polyacrylamide/cellulose hydrogel with high performance for biodiesel dehydration. Ind Crops Prod 218:118859. https://doi.org/10.1016/j.indcrop.2024.118859
Arthus L, Fregolente PBL, Maciel MRW, Fregolente LV (2023a) Hydrogels for the Removal of Water Content from Liquid Fuels. In: Hydrogels, 1st edn. Taylor & Francis, p 24
Arthus L, Ramos Estevam B, Jova Aguila Z, et al (2023b) Facile tuning of hydrogel properties for efficient water removal from biodiesel: An assessment of alkaline hydrolysis and drying techniques. Chem Eng Sci 282:119224. https://doi.org/10.1016/j.ces.2023.119224
Atadashi IM, Aroua MK, Abdul Aziz AR, Sulaiman NMN (2012) The effects of water on biodiesel production and refining technologies: A review. Renew Sustain Energy Rev 16:3456–3470. https://doi.org/10.1016/j.rser.2012.03.004
Bayramgil NP (2018) Grafting of hydrophilic monomers onto cellulosic polymers for medical applications. In: Biopolymer Grafting: Applications, 1st edn. Elsevier Inc., pp 81–114
Beyaz K, Vaca-Garcia C, Vedrenne E, et al (2019) Graft copolymerization of hydroxyethyl cellulose with solketal acrylate: Preparation and characterization for moisture absorption application. Int J Polym Anal Charact 24:245–256. https://doi.org/10.1080/1023666X.2019.1567085
Boruah P, Gupta R, Katiyar V (2023) Fabrication of cellulose nanocrystal (CNC) from waste paper for developing antifouling and high-performance polyvinylidene fluoride (PVDF) membrane for water purification. Carbohydr Polym Technol Appl 5:100309. https://doi.org/10.1016/j.carpta.2023.100309
D6304-20 ASTM A standard (2021) Standard Test Method for Determination of Water in Petroleum Products, Lubricating Oils, and Additives by Coulometric Karl Fischer Titration
Das D, Prakash P, Rout PK, Bhaladhare S (2021) Synthesis and Characterization of Superabsorbent Cellulose-Based Hydrogel for Agriculture Application. Starch 73:1–10. https://doi.org/http://dx.doi.org/10.1002/star.201900284
Delgado JF, Salvay AG, Arroyo S, et al (2023) Effect of Dissolution Time on the Development of All-Cellulose Composites Using the NaOH/Urea Solvent System. Polysaccharides 4:65–77. https://doi.org/10.3390/polysaccharides4010005
Dourado MD, Fregolente PB, Wolf Macie l MR, Fregolente L. (2021) Screening of Hydrogels for Water Adsorption in Biodiesel using Crosslinked Homopolymers. Chem Eng Trans 81:1129–1134
El Fawal GF, Abu-Serie MM, Hassan MA, Elnouby MS (2018) Hydroxyethyl cellulose hydrogel for wound dressing: Fabrication, characterization and in vitro evaluation. Int J Biol Macromol 111:649–659. https://doi.org/10.1016/j.ijbiomac.2018.01.040
Estevam BR, Perez ID, Moraes ÂM, Fregolente LV (2023a) A review of the strategies used to produce different networks in cellulose-based hydrogels. Mater Today Chem 34:101803. https://doi.org/10.1016/j.mtchem.2023.101803
Estevam BR, Vieira FF dos S, Gonçalves LH, et al (2023b) Cellulose hydrogels for water removal from diesel and biodiesel: Production, characterization, and efficacy testing. Fuel 347:128449. https://doi.org/10.1016/J.FUEL.2023.128449
Foo ML, Tan KW, Wu TY, et al (2017) A characteristic study of nanocrystalline cellulose and its potential in forming pickering emulsion. Chem Eng Trans 60:97–102. https://doi.org/10.3303/CET1760017
Fregolente LV, Gonçalves HL, Fregolente PBL, et al (2023) Sodium polyacrylate hydrogel fixed bed to treat Water-Contaminated cloudy diesel. Fuel 332:125953. https://doi.org/10.1016/j.fuel.2022.125953
Fregolente P, Gonçalves H, Wolf Maciel MR, Fregolente L (2018) Swelling Degree and Diffusion Parameters of Poly (Sodium Acrylate-Co-Acrylamide) Hydrogel for Removal of Water Content From Biodiesel. Chem Eng Trans 65:445–450. https://doi.org/https://doi.org/10.3303/CET1865075
Gonçalves HL, Bogalhos Lucente Fregolente P, Wolf Maciel MR, Fregolente LV (2021) Formulation of hydrogels for water removal from diesel and biodiesel. Sep Sci Technol 56:374–388. https://doi.org/10.1080/01496395.2019.1709506
Hertle Y, Zeiser M, Hasenöhrl C, et al (2010) Responsive P(NIPAM-co-NtBAM) microgels: Flory–Rehner description of the swelling behaviour. Colloid Polym Sci 288:1047–1059. https://doi.org/10.1007/s00396-010-2232-8
Huang C, Li Y, Duan L, et al (2017) Enhancing the self-recovery and mechanical property of hydrogels by macromolecular microspheres with thermal and redox initiation systems. RSC Adv 7:16015–16021. https://doi.org/10.1039/C7RA00317J
Jayaramudu T, Ko HU, Kim HC, et al (2019) Swelling behavior of polyacrylamide-cellulose nanocrystal hydrogels: Swelling kinetics, temperature, and pH effects. Materials (Basel) 12:. https://doi.org/10.3390/ma12132080
Kabiri K, Omidian H, Hashemi SA, Zohuriaan-Mehr MJ (2003) Synthesis of fast-swelling superabsorbent hydrogels: effect of crosslinker type and concentration on porosity and absorption rate. Eur Polym J 39:1341–1348. https://doi.org/10.1016/S0014-3057(02)00391-9
Li W, Cai G, Zhang P (2019) A simple and rapid Fourier transform infrared method for the determination of the degree of acetyl substitution of cellulose nanocrystals. J Mater Sci 54:8047–8056. https://doi.org/10.1007/s10853-019-03471-2
Matsuo Y, Sato R, Tabata K, et al (2024) Hybrid composite cellulose nanocrystal, hydroxyapatite, and chitosan material with controlled hydrophilic/hydrophobic properties as a remineralizable dental material. Cellulose. https://doi.org/10.1007/s10570-024-05763-6
Mukerabigwi JF, Lei S, Fan L, et al (2016) Eco-friendly nano-hybrid superabsorbent composite from hydroxyethyl cellulose and diatomite. RSC Adv 6:31607–31618. https://doi.org/10.1039/C6RA01759B
Nath PC, Debnath S, Sharma M, et al (2023) Recent Advances in Cellulose-Based Hydrogels: Food Applications. Foods 12:350. https://doi.org/10.3390/foods12020350
National Standard of Canada (2020) Diesel fuel containing low levels of biodiesel (B1 – B5) - CAN/CGSB-3.520-2020
Ostrowska-Czubenko J, Gierszewska M, Pieróg M (2015) pH-responsive hydrogel membranes based on modified chitosan: water transport and kinetics of swelling. J Polym Res 22:153. https://doi.org/10.1007/s10965-015-0786-3
Perez ID, dos Santos FB, Miranda NT, et al (2022) Polymer hydrogel for water removal from naphthenic insulating oil and marine diesel. Fuel 324:124702. https://doi.org/10.1016/j.fuel.2022.124702
Perissinotto RM, Monte Verde W, Perles CE, et al (2020) Experimental analysis on the behavior of water drops dispersed in oil within a centrifugal pump impeller. Exp Therm Fluid Sci 112:109969. https://doi.org/10.1016/j.expthermflusci.2019.109969
Rop K, Mbui D, Karuku GN, et al (2020) Characterization of water hyacinth cellulose-g-poly(ammonium acrylate-co-acrylic acid)/nano-hydroxyapatite polymer hydrogel composite for potential agricultural application. Results Chem 2:100020. https://doi.org/10.1016/j.rechem.2019.100020
Tammelin T, Abburi R, Gestranius M, et al (2015) Correlation between cellulose thin film supramolecular structures and interactions with water. Soft Matter 11:4273–4282. https://doi.org/10.1039/C5SM00374A
Tudoroiu E-E, Dinu-Pîrvu C-E, Albu Kaya MG, et al (2021) An Overview of Cellulose Derivatives-Based Dressings for Wound-Healing Management. Pharmaceuticals 14:1215. https://doi.org/10.3390/ph14121215
Wong LC, Leh CP, Goh CF (2021) Designing cellulose hydrogels from non-woody biomass. Carbohydr Polym 264:118036. https://doi.org/10.1016/j.carbpol.2021.118036
Zhao C, Liu G, Tan Q, et al (2023) Polysaccharide-based biopolymer hydrogels for heavy metal detection and adsorption. J Adv Res 44:53–70. https://doi.org/10.1016/j.jare.2022.04.005

No competing interests reported.

Dehydration of Cloudy-diesel Using Poly(Acrylamide-co-sodium Polyacrylate) Hydrogel Grafted Onto Cellulose

Status:

Version 1

Abstract

Figures

INTRODUCTION

MATERIALS AND METHODS

Materials

Preparation of diesel emulsion and measurement of water content

Synthesis of hydrogels

Hydrogel characterization

Kinetics of water removal from diesel

Effect of cellulose addition

RESULTS AND DISCUSSION

Hydrogel synthesis method

Hydrogel characterization

Kinetics of water removal from diesel

Effect of cellulose addition

CONCLUSION

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1