Photocatalytic GO - TiO₂ - Incorporated TFN Nanofiltration Membranes for Antibiotic Removal from Wastewater

Technical Summary: Photocatalytic GO-TiO₂-Incorporated TFN Nanofiltration Membranes for Antibiotic Removal

Problem Statement
The rapid proliferation of emerging contaminants, particularly antibiotics like amoxicillin and ciprofloxacin, poses severe environmental and health threats due to their persistence and toxicity. Conventional wastewater treatment methods are often inadequate for removing these low-molecular-weight compounds. While Nanofiltration (NF) membranes offer a promising alternative for removing micro-pollutants, they face limitations such as low rejection efficiency for small antibiotics and susceptibility to fouling by natural organic matter and biofilms. Furthermore, traditional NF membranes lack the ability to degrade pollutants, merely transferring them to the concentrate stream. There is a critical need for membranes that combine high-efficiency separation with the capability to degrade contaminants in situ.

Methodology
This study aimed to develop and optimize a novel Thin-Film Nanocomposite (TFN) nanofiltration membrane incorporating Graphene Oxide (GO) and Titanium Dioxide (TiO₂) into the selective polyamide layer. The research employed a systematic Design of Experiment (DOE) approach using Response Surface Methodology (RSM) with a Central Composite Design (CCD).

• Membrane Fabrication: A Polyethersulfone (PES) support layer was prepared via phase inversion. The selective layer was formed through interfacial polymerization using m-phenylenediamine (MPD) and trimesoyl chloride (TMC). GO and TiO₂ nanoparticles were dispersed in the aqueous phase prior to polymerization.
• Experimental Variables: Three independent variables were screened and optimized:
  1. Nanomaterial loading (0.5 – 2.5 wt%).
  2. GO:TiO₂ ratio (ranging from 1:2 to 3:1).
  3. Interfacial polymerization reaction time (30 – 70 min).
• Performance Evaluation: Synthetic wastewater containing 30 mg/L of amoxicillin and ciprofloxacin was used. Performance was assessed based on water flux, antibiotic rejection efficiency, and photocatalytic degradation efficiency under UV irradiation.
• Characterization: The membranes were analyzed using Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and contact angle measurements to evaluate morphology, roughness, crystalline structure, chemical composition, and hydrophilicity.

Key Results
The study identified optimal synthesis conditions that balanced filtration performance with photocatalytic activity:

• Optimal Conditions: A nanomaterial loading of 1.5 wt%, a GO:TiO₂ ratio of 2:1, and an interfacial polymerization reaction time of 50 minutes.
• Rejection Efficiency: Under optimal conditions, the membrane achieved rejection rates of 96% for amoxicillin and 94% for ciprofloxacin.
• Water Flux: The optimized membrane exhibited a water flux of 47 L m⁻² h⁻¹, indicating that the incorporation of nanomaterials did not compromise permeability.
• Photocatalytic Degradation: The membrane demonstrated a photocatalytic degradation efficiency of 83% for the target antibiotics.
• Material Properties:
  • Morphology: SEM and AFM revealed a rougher "ridge-and-valley" surface structure (roughness ~56 nm) compared to pristine membranes, which increased the effective surface area.
  • Hydrophilicity: The contact angle decreased to 52°, attributed to oxygen-containing functional groups from GO and TiO₂, enhancing water flux and reducing fouling.
  • Structure: XRD confirmed the presence of the anatase phase of TiO₂ and graphene oxide within the membrane matrix. FTIR verified the formation of the polyamide layer and the presence of carboxyl and hydroxyl groups.
  • Surface Charge: The Point of Zero Charge (PZC) was determined to be 5.6, ensuring the membrane surface remains negatively charged at neutral pH (6–8), facilitating electrostatic repulsion of pollutants.

Key Contributions and Significance
The authors claim this research makes several distinct contributions to the field of membrane technology:

1. Simultaneous Evaluation: Unlike previous studies that focused solely on contaminant rejection, this work simultaneously evaluates standard nanofiltration performance and in-situ photocatalytic degradation of antibiotics.
2. Structure-Performance Link: The study establishes a link between the membrane's structural properties (GO-TiO₂ incorporation) and its dual performance (filtration and degradation), a relationship noted as previously absent in literature regarding TFN membranes.
3. Systematic Optimization: This research provides the first systematic DOE-RSM optimization specifically for GO and TiO₂-incorporated TFN nanofiltration membranes, identifying the precise synergistic ratios and reaction times required for maximum efficiency.
4. Novelty: The work presents a novel approach to handling antibiotic-contaminated wastewater by integrating a photocatalytic degradation mechanism directly into the filtration membrane, offering a potential solution to the limitations of conventional separation-only technologies.

The paper concludes that functionalizing nanofiltration membranes with GO-TiO₂ nanomaterials significantly enhances both degradation and filtration efficiency, offering a promising strategy for treating antibiotic-laden wastewater.