Selectivity Performance and Antifouling Properties of Modified Chitosan Composites ^†

Abstract

This study investigated the functionality and antifouling capabilities of the chitosan–silver nanoparticle–graphene oxide (CS/AgNPs/GO) composite membrane. An increase in the molecular interaction between the membrane surface and bovine serum albumin solution enhanced the flow recovery rate (FRR) due to the presence of amide -NH₂ and -OH groups. The modified CS composite showed a strong ability to prevent fouling, achieving over 77.5% due to greater interfacial intermolecular bonding. In addition, the tensile strength of the membrane composite improved from 42.7 to 49.6 MPa with an increase in the concentration of dimethylacetamide employed as a plasticizer. Therefore, efficient molecular interactions within the polymer matrix would significantly influence the membrane’s flux recovery rate, tensile strength, and ability to prevent fouling.

1. Introduction

Membrane separation technology employs biofouling control as one of its numerous water remedies. However, the presence of contaminants on membrane surfaces has been widely recognized to have significant adverse effects, including the decrease in membrane separation resistance, the reduction in membrane lifespan, and the impairment of membrane functionalities [1,2]. Chitosan (CS) is a biopolymer that is highly intriguing and is frequently used in the membrane industry for water remediation. This prevalence is attributable to its inherent antibacterial properties, biodegradability, and nontoxicity [3,4,5]. Furthermore, the substantial interest in CS as a potent adsorbent for the removal of contaminants from water and effluent is due to its abundance of amino and hydroxyl groups [6,7]. However, the mechanical resilience of the preponderance of CS polymer membranes utilized in water treatment systems is insufficient, rendering it a significant area of research. In order to improve the mechanical properties and antifouling capabilities of the CS matrix, graphene oxide (GO) and silver nanoparticles (AgNPs) were incorporated in this study. As a result, the aim of this study was to facilitate the enhancement of modified CS membrane wettability and reduction in surface contamination.

2. Materials and Methods

A gram of CS with a molecular weight of 198 kDa and a 75% deacetylation was dissolved in a 1% acetic acid (HAc) aqueous solution at ambient temperature. The mixture was subjected to vigorous agitation for a duration of 60 min. In order to neutralize the HAc, 0.5 mL of 2% (w/v) NaOH was introduced to the reacting mixture. The reaction was continuously stirred at 150 rpm for 24 h to ensure the complete removal of air pockets. A modified version of Tollens’ method was employed to prepare silver nanoparticles (AgNPs) in the presence of graphene oxide (GO) solution. This was achieved by directly reducing silver nitrate (AgNO₃) with trisodium citrate (C₆H₈O₇Na). In summary, 0.5 g of GO suspension was combined with 0.4 g of AgNO₃ and agitated gradually for 60 min. Subsequently, 80 mL of CS solution was combined with 0.5 mL of a 10 wt% AgNPs/GO aqueous solution after one hour of stirring. The composite membranes were prepared by the phase inversion method and dried at ambient temperature in a desiccator to avoid contamination.

3. Results

3.1. Membrane Surface Hydrophilicity

The water contact angle measurement was used to determine the surface wettability properties of the modified CS membrane composite surface using a contact angle goniometer (OCA 15 Pro, Data Physics, Filderstadt, Germany) equipped with a dpiMAX 20P image-processing software via the sessile drop method. Figure 1a illustrates the comparative wettability behavior of the CS membrane composites every 5 s. Throughout the 25 s period, all of the samples exhibited a gradual decrease in contact angles. The analysis determined that the initial wettability of the modified composite was 85.7° for the CS/GO composite sample, 81.3° for the CS/AgNPs, and 71.4° for the CS/AgNPs membrane samples after 5 s. The contact angle of the matrix further decreased in the following order after 25 s: CS/GO (71.4°), CS/AgNPs (61.9°), and CS/AgNPs/GO (47.3°). Previous research has shown that proper integration of membrane composites would improve fluid transport and decrease contact [8]. The water permeation flux was improved by the incorporation of GO and AgNPs into CS composite membranes, as demonstrated in Figure 1b,c. Nevertheless, the permeation flux of the entire membrane samples decreased gradually over a three-hour period while the system was operated at ambient temperature with a transmembrane operating pressure of 0.3 MPa.

Where

CS/GO (chitosan/graphene oxide);

CS/AgNPs (chitosan/silver nanoparticles);

CS/AgNPs/GO (chitosan/silver nanoparticles/graphene oxide).

Throughout the filtration procedure, the average water flux for the samples decreased. The maximum average flux was seen in CS/AgNPs/GO (51.02 Lm⁻²h⁻¹), followed by CS/AgNPs (48.93 Lm⁻²h⁻¹) and CS/GO (38.77 Lm⁻²h⁻¹). The potential cause of differences in permeation flux may be variations in the chemical composition of AgNPs/GO components in the modified CS membranes.

3.2. Characterization of Antifouling Performance

The modified CS composite membrane’s ability to resist fouling was assessed by quantifying the flow rates of a 100 parts per million bovine serum albumin (BSA) solution at a pressure of 0.25 MPa. The antifouling performance summary of the modified CS is presented in

Table 1. Flux recovery rate and antifouling capabilities.

Sample	FRR (%)	Rt	Rir	Rr
S1 (CS/AgNPs/GO)	77.52 ± 0.2	78.24 ± 0.31	10.31 ± 0.15	45.41 ± 0.43
S2 (CS/AgNPs)	68.21 ± 0.4	70.73 ± 0.34	9.82 ± 0.17	52.12 ± 0.51
S3 (CS/GO)	65.11 ± 0.5	66.51 ± 0.23	7.22 ± 0.11	58.71 ± 0.42

. A higher flux recovery ratio (FRR) value indicates a positive antifouling characteristic [9]. S1 (CS/AgNPs/GO) demonstrated a maximal flux recovery ratio of 77.5%, which is more noteworthy than those of S2 and S3. The antifouling capabilities and flux recovery rate of these membranes may have been influenced by the wettability of the modified CS composites and the presence of AgNPs on the membrane surface, which were likely enveloped by graphene oxide–epoxy clouds within the CS matrix. Additionally, the increased surface contact between the BSA and the membranes may be responsible for higher FRR. The enhanced contact could be a result of the increased abundance of -NH₂ groups from the DMAc group and -OH groups from the CS, which facilitated hydrogen bonding and electrostatics, which in turn increased the affinity for the negatively charged BSA molecules.

The fouling resistance of composite membranes was evaluated by evaluating the reversible (Rr) and irreversible (Rir) fouling ratios, as well as the total impurity ratio (Rt). In comparison to other samples, the observed increase in the overall fouling ratio in S1 (CS/AgNPs/GO) may have been attributed to an increase in the reversible fouling ratio (Rr), resulting in considerable fouling prevention. This high efficacy could be attributed to the potential interaction between BSA molecules and the membrane surface. This connection effectively prevented fouling on the membrane surface. In contrast, it is evident that the Rr fouling ratios of S2 and S3 were lower, which could have contributed to the increased frequency of fragmentation on their respective membrane surfaces.

3.3. Mechanical Properties of the Composite Membrane

The mechanical properties of modified CS composite samples were examined to assess the durability of the filtration procedure. The tensile strength and elongation at rupture of CS/AgNPs/GO composite membranes prepared with a fixed quantity of 15% by weight DMAc content are depicted in Figure 2a. The tensile capacities of all composite membrane samples are between 46.4 and 50.6 MPa.

The CS/AgNPs/GO membrane sample exhibited the lowest maximal elongation at break, with a value of 25.8% possibly due to stronger hydrogen bond formation. Also the polymer chains’ mobility might have been influenced by the intensive electrostatic interactions between the CS composite and the AgNPs, which restricted the elongation at break of the composite film. Figure 2b illustrates the influence of fluctuations in DMAc concentrations on the tensile strength of the modified CS. The mechanical properties of the composite samples were improved by an increase in the mole fraction of the solvent. The tensile strength of the CS/AgNPs/GO sample was 46.9 MPa at a maximal concentration of 7.5 wt% of DMAc, which was greater than that of CS/AgNPs (40.2 MPa) and CS/GO (31.9 MPa).

4. Conclusions

The incorporation of GO and AgNPs into CS improved membrane permeation flux and increased the tensile properties due to effective interfacial molecular interactions within the modified composite. The flux recovery rate and antifouling capabilities of these membranes were influenced by the wettability and the presence of silver nanoparticles on the membrane surface, which were surrounded by graphene oxide–epoxy clouds within the CS matrix. However, the mobility of polymer chains was restricted by the existence of strong electrostatic interactions, which subsequently decreased the elongation at break of the composite film.