SMART Stimuli-Responsive Adsorptive Membranes for Sustainable Water Desalination

Abstract

This study explores the development of green and sustainable pH-responsive smart adsorptive membranes for wastewater treatment, highlighting innovations in material science and adsorption technologies. An extensive literature review has been conducted on wastewater treatment technologies, which aim to mitigate the risks posed by toxic dyes and heavy metals, with adsorption being a widely used and effective method. The review focuses on pH and temperature smart adsorbents, exploring the mechanisms, isotherms, kinetics, and thermodynamics of the adsorption process for each type of stimuli-responsive material. It also highlights strategies for performance optimization and concludes with future perspectives and key considerations for these materials in wastewater treatment.

Next, novel pH-responsive smart adsorptive membranes were developed by incorporating modified mangrove-based materials into polylactic acid (PLA) ultrafiltration membranes. Two types of fillers, activated carbon-modified mangrove (MAC) and MXene-modified mangrove (MXAM), were integrated into the membranes using the non-solvent induced phase separation (NIPS) technique. The addition of 3 wt.% MAC improved water permeability from 1336.4±96.4 L.m⁻².h⁻¹.bar⁻¹ in pristine PLA to 2306.6±9.7 L.m⁻².h⁻¹.bar⁻¹ in 3MAC-PLA. Rejection of heavy metal ions increased from 18.15%, 17.31%, and 17.50% for Cu²⁺, Pb²⁺, and Ni²⁺ at pH 4 to 99.95%, 100%, and 99.95% at pH 10 in 3MAC-PLA. Similarly, incorporating 1.5 wt.% MXAM enhanced water permeability to 1855.6 L.m⁻².h⁻¹.bar⁻¹, achieving rejection rates of 79%, 91.5%, and 99.9% for polystyrene microplastics at pH 4, 7, and 10, respectively. Adsorption behavior of heavy metals followed the Freundlich isotherm model, with maximum adsorption capacities of 4.2 mg.g⁻¹ for Cu²⁺, 5.0 mg.g⁻¹ for Ni²⁺, and 6.8 mg.g⁻¹ for Pb²⁺ at pH 7, indicating multilayer adsorption on heterogeneous surfaces. In contrast, microplastic adsorption adhered to the Langmuir isotherm model, with a maximum monolayer adsorption capacity of qₘ = 23.5 mg.g⁻¹, suggesting uniform surface adsorption sites.

The effectiveness of these membranes was further demonstrated in a dual-stage filtration system for treating raw wastewater. The configurations 1.5MXAM-3MAC (C1) and 3MAC1.5MXAM (C2) were tested in dead-end and cross-flow modes. The C2 configuration exhibited superior performance, with rejection rates of 99.7% for Cu, 87.9% for Pb, 47.2% for Ni, 62.4% for PO₄³⁻-P, 47.8% for NH₄⁺-N, 89.8% for BSA, 76% for COD, 98.7% for polystyrene microplastics, and 95.2% for bacteria by the 10th cycle. Long-term cross-flow filtration over 70 hours maintained a J/J₀ value of 0.665, with controlled fouling dominated by pore blocking transitioning to cake layer formation. These findings highlight the potential of cascaded PLA-based membranes as an effective and scalable solution for wastewater treatment, making them promising for industrial applications.

Date of Award	8 May 2025
Original language	American English
Supervisor	SHADI Hasan (Supervisor)