Multifunctional superhydrophilic/underwater superoleophobic lignin-based polyurethane foam for highly efficient oil-water separation and water purification

Abstract

Superwettability can affect the performance of oil separation from water during the treatment of oily wastewater. Among various types of materials developed for cleaning oil pollution, foam is a popular choice due to its attractive lightweight properties and adjustable porosities. Foams with superwetting surfaces (superhydrophilic/superoleophobic underwater) are ideal for oil/water separations. In this study, lignin-based polyurethane foams (LPUFs) were synthesized first, and then polydopamine particles were deposited on the surface of the foam by in situ polymerization under weak alkaline conditions to increase its surface roughness. Afterwards, phytic acid was used to modify the foam to achieve surface superhydrophilicity and underwater superoleophobicity. Successful loading of polydopamine (PDA) particles and coating of phytic acid (PA) onto the foam was demonstrated by SEM, EDS, FTIR, XPS, and thermogravimetric analysis measurements. It was shown in the cyclic compression tests that LPUFs had good mechanical properties. Under 75 % compression strain, the maximum stress in the first cycle of LPUF/PDA/PA was 105.92 kPa. After 30 cyclic compression tests, the maximum stress of LPUF/PDA/PA under 75 % compression strain was 105.63 kPa, demonstrating a high mechanical stability. The contact angle of the foam modified by PDA and PA was 0° for water, 166.7° for chloroform, and 158.4° for hexane, which also demonstrated excellent underwater anti-oil adhesion performances. Oil-water separation tests by using PDA and PA modified lignin-based foam with mixtures of water and n-hexane, cyclohexane, toluene, and pump oil indicated a separation efficiency of over 99 % for the types of mixtures tested together with excellent repeatability. In addition, the PDA and PA modified lignin-based foams were able to adsorb 67.1 mg/g of methylene blue, 96.1 mg/g of rhodamine B, and 98.2 mg/g of copper sulfate. The lignin-based foam could completely degrade under weak alkaline conditions after usage. This study highlighted a novel strategy for synthesizing environmentally friendly high-performance adsorbents to efficiently treat polluted wastewater using lignin as a raw material.