Green polycaprolactone/sulfonated kraft lignin phase inversion membrane for dye/salt separation

Abstract

Exploring environmentally friendly, renewable, and cost-effective raw materials is essential in sustainable membrane fabrication. This study presents a facile and scalable method for fabricating a green and biodegradable tight ultrafiltration membrane for dye/salt separation. This involves simply blending biodegradable polycaprolactone (PCL) with the low-cost biobased sulfonated kraft lignin (SKL) additive. Additionally, we employed acetic acid as a green alternative solvent to enhance the sustainability of the membrane fabrication process. The incorporation of hydrophilic SKL into the PCL matrix resulted in increased hydrophilicity (water contact angle changed from 72° to 56°), surface roughness (increased from 29 nm to 43.5 nm), and enhanced negative electrostatic charge of the membrane (−40 mV to −45 mV). The optimized PCL/SKL membrane (M3) exhibited excellent water flux (∼45 LMH) under 40 psi hydraulic pressure coupled with ∼98 % and ∼10 % rejection rates for reactive red (RR) dye and NaCl, respectively. Moreover, the M3 membrane maintained its exceptional dye/salt fractionation performance while separating the mixtures at low salt concentrations. However, with increasing salt concentration (1–50 g/L), the membrane's RR dye rejection declined from ∼90 % to ∼50 %, with a significant reduction in NaCl and Na2SO4 salts rejection (from ∼14 % to ∼1 % and ∼22 % to –∼1 %, respectively). The M3 membrane exhibited remarkable antifouling properties during dye and humic acid filtration with a high flux recovery ratio (>98 %) and low flux decline rate (<7 %). The PCL/SKL membrane also showed excellent stability and maintained consistent separation performance over a long period. Overall, the novel biodegradable PCL/SKL membrane prepared in this study presents a promising avenue toward sustainable membrane fabrication for wastewater treatment applications.