Integrated chitosan-based polyelectrolyte membranes and microwave-assisted advanced oxidation processes for sustainable coir retting wastewater treatment

Abstract

Coir retting effluent, rich in lignocellulosic and phenolic compounds, poses serious environmental challenges due to its high chemical oxygen demand (COD), conductivity, and salinity. This study evaluates the effectiveness of microfiltration membranes and microwave-assisted advanced oxidation processes in treating coir retting effluent. It explores the potential of combining both methods to achieve enhanced pollutant removal efficiency. An integrated treatment system was developed, combining low-pressure filtration using chitosan/poly(acrylic acid) (CHI/PAA) multilayer membranes with the MW-Fenton (MW-F) process for efficient remediation of coir retting effluent. The CHI/PAA multilayer membranes were fabricated via a layer-by-layer (LbL) assembly method and tested under varying pH conditions and bilayer numbers. At 5.5 bilayers, a COD reduction of 42.30% and a flux of 72.84 m³ m⁻² per day at native pH were achieved along with significant rejection of dissolved pollutants. Attenuated Total Reflectance Fourier-Transform Infrared (ATR-FTIR) analysis confirmed the adsorption of organic compounds on the membrane surface. Most of the phenolic compounds identified in the feed via Ultra-Performance Liquid Chromatography–Quadrupole Time-of-Flight Mass Spectrometry (UPLC-Q-ToF-MS) were effectively removed using this approach. Treatment of the coir retting effluent by MW-F alone, with an optimal H₂O₂ dosage of 200 mM and a fixed Fe²⁺ concentration of 0.18 mM, resulted in a COD reduction of 38.46% along with substantial decreases in conductivity, TDS, and salinity at near-neutral pH. Integration of membrane filtration with MW-F at an optimal H₂O₂ dosage of 200 mM significantly improved performance, resulting in a COD reduction of 76.92%, a colour removal of 97.51%, and a flux of 212.07 m³ m⁻² per day at neutral pH. This combined system offers a sustainable, efficient, and economically viable solution for treating complex lignocellulosic wastewater without the need for pH adjustment, making it particularly suitable for decentralized applications in the coir processing industry.