Chitosan/MXene/Polymer Hybrid as a new Filter with Enhanced Hydrophobicity and degradation Properties

Technical Summary: Chitosan/MXene/Polymer Hybrid as a New Filter with Enhanced Hydrophobicity and Degradation Properties

Problem Statement
The primary objective of this study is to address the need for cost-effective, high-performance filtration systems capable of reducing the Chemical Oxygen Demand (COD) in wastewater. While surface hydrophobicity is critical for applications such as oil–water separation and self-cleaning, commercially available hydrophobic substrates are often expensive. Conversely, common substrates like cellulose filter paper are inherently hydrophilic due to abundant hydroxyl groups, limiting their utility in hydrophobic filtration processes. The study aims to transform low-cost, hydrophilic cellulose filter paper into a robust, hydrophobic composite filter capable of both physical filtration and oxidative degradation of organic contaminants.

Methodology
The research employed a three-layer hierarchical fabrication strategy:

1. Substrate Modification (Hydrophobic Coating):

   • Commercial cellulose filter paper was modified using a dip-coating method.
   • A solution of Polydimethylsiloxane (PDMS) and Polyvinylidene fluoride (PVDF) was prepared in a THF/DMF solvent mixture.
   • The study systematically varied the PDMS/PVDF weight ratios (2:1, 4:1, 6:1, and 8:1) and immersion times (35, 50, 65, 80 minutes, and 24 hours) to optimize surface hydrophobicity.
   • Post-coating, filters were washed in a reverse-bath method to remove residual solvents and dried.

2. Active Layer Fabrication (MXene/Chitosan Hybrid):

   • Ti₃AlC₂ MAX phase powder was etched using 7% hydrofluoric acid (HF) for 24 hours to produce MXene nanosheets.
   • Chitosan was dissolved in an acetic acid/water solution.
   • The MXene nanosheets were dispersed in water via sonication and combined with the chitosan solution. The mixture was sonicated and stirred to create a stable, homogeneous hybrid solution.

3. Final Assembly:

   • The optimized hydrophobic filter paper (coated with PDMS/PVDF) was dipped into the Chitosan/MXene solution to deposit the active top layer.
   • The final product is a multilayer composite: a cellulose support, a PDMS/PVDF intermediate layer, and a Chitosan/MXene top layer.

4. Characterization and Testing:

   • Surface Properties: Water contact angles were measured to determine hydrophobicity.
   • Structural Analysis: FTIR, XRD, FESEM, EDS mapping, and UV-vis DRS were used to analyze morphology, crystallinity, elemental distribution, and optical properties.
   • Performance: COD reduction was tested under varying pH levels (4, 7, 10), contact times (20–120 min), and H₂O₂ concentrations (20–50 ppm) using the standard dichromate oxidation method.

Key Results

• Optimization of Hydrophobicity:

  • The contact angle was highly dependent on both the polymer ratio and immersion time.
  • The PDMS/PVDF ratio of 4:1 yielded the highest hydrophobicity. Specifically, the sample immersed for 65 minutes achieved a maximum contact angle of 120.12° (Sample B3).
  • Lower ratios (2:1) resulted in lower contact angles (<90°), while higher ratios (6:1 and 8:1) showed a decline in hydrophobicity, attributed to potential phase separation, surface smoothing, or reduced surface roughness caused by excess PDMS.
  • Immersion times beyond 65 minutes generally led to a slight decrease or stabilization in contact angle, suggesting an optimal coating thickness and surface texture.

• Structural and Morphological Characterization:

  • FTIR: Confirmed the interaction between MXene and chitosan, showing overlapping O–H/N–H bands (~3445 cm⁻¹) and Ti–O/Ti–C vibrations, indicating strong hydrogen bonding.
  • XRD: The composite showed a shift in MXene peaks to lower angles and reduced intensity, indicating intercalation of chitosan molecules between MXene layers and an increase in d-spacing.
  • FESEM/EDS: Revealed a rough, hierarchical surface with stacked MXene nanosheets embedded in a chitosan matrix. EDS mapping confirmed the uniform distribution of Ti, C, O, and N, and a distinct multilayer architecture with strong interfacial adhesion to the cellulose substrate.
  • Optical Properties: DRS analysis indicated that the Chitosan/MXene composite exhibited higher light absorption across the wavelength range compared to MXene alone, suggesting enhanced electronic or optical properties.

• COD Reduction Performance:

  • The filter demonstrated significant COD removal capabilities.
  • Time Dependency: At pH 4, COD removal increased from 3.3% (20 min) to ~59.8% (120 min). At pH 10, removal reached ~62.5% after 120 minutes.
  • pH Influence: Alkaline conditions (pH 10) showed slightly higher removal efficiency than acidic conditions, likely due to enhanced hydroxyl radical formation.
  • H₂O₂ Influence: At neutral pH (7) and 120 minutes, increasing H₂O₂ concentration from 20 to 50 ppm significantly improved COD removal (from 7.5% to 43.6%), indicating the system operates in a radical-limited regime within this range.

Significance and Claims
The paper claims that the primary significance of this work lies in the successful fabrication of a low-cost, high-performance filtration system using commercially available cellulose filter paper as a substrate. By eliminating the need for expensive commercial hydrophobic membranes, the study provides a scalable strategy for wastewater treatment.

The authors assert that the multilayer architecture functions synergistically:

1. The PDMS/PVDF layer converts the hydrophilic substrate into a hydrophobic surface, regulating water permeation and increasing residence time.
2. The Chitosan/MXene top layer acts as both an adsorption barrier and a photocatalytic agent. The MXene nanosheets facilitate the generation of reactive oxygen species (ROS) for the oxidative degradation of organic pollutants, while the chitosan matrix prevents nanosheet restacking and enhances interfacial compatibility.
3. The system achieves simultaneous physical filtration and chemical degradation, resulting in effective COD reduction without the requirement for complex or expensive materials.

The study concludes that controlling polymer composition and immersion time is critical for tuning surface wettability, and the resulting hybrid filter offers a viable, scalable solution for improving filtration efficiency in wastewater treatment applications.