A mimosa-inspired photothermal-responsive multifunctional hydrogel for passive solar-driven efficient water purification

Abstract

Water scarcity is threatening human health and draining precious energy resources as a result of water pollution from small molecule dyes, heavy metals, oil, yeast, and bacteria. However, it remains a challenge to synthesize a multifunctional passive solar-driven hydrogel with excellent decontamination and bactericidal properties for wastewater purification while maintaining the super-hydrophilicity, broad solar energy absorption, strong photothermal conversion, and durable compressibility of the materials. Herein, inspired by the contraction-response of mimosa, a multifunctional passive solar-driven hydrogel (PNIPAm/CMC/CMCNT hydrogel) with excellent reversible compression, decontamination, and bactericidal properties, photothermal conversion-induced energy saving, and high-water production rate capabilities is proposed to purify wastewater using only natural sunlight, and includes poly(N-isopropylacrylamide) (PNIPAm), carboxymethyl chitosan (CMC), and carboxylic multi-walled carbon nanotubes (CMCNTs). Owing to the unique photosensitive effect, CMCNTs can be applied as photothermal conversion materials for solar-driven volume shrinkage by using the lower critical solution temperature to achieve repeatable water release. Based on the dual effects of the PNIPAm/CMC/CMCNT hydrogel's unique three-dimensional structure and the polyfunctional groups (from CMC and CMCNTs), the hydrogel has an excellent water production rate, decontamination (removal rate R > 90.89%), and bactericidal properties (bactericidal rate P > 98.2%). With superior elasticity and interpenetration, the PNIPAm/CMC/CMCNT hydrogel exhibits highly effective and stable water transport with a water purification rate of 9.83 kg m⁻² h⁻¹ under one sun irradiation, the highest purification rate reported so far. Thus, our study provides meaningful insight into the rational design of solar-driven purification materials for sustainable clean water production.