Bioinspired Phototropic Endogenous Heating for Low-Temperature Biological Wastewater Treatment

Abstract Biological wastewater treatment in cold regions faces a low-efficiency and high-energy paradox due to low-temperature inhibition. Inspired by plant phototropism, this study reports bioinspired phototropic endogenous heating (Bio-PEH) based on the underwater opto-thermomechanical response. Upon illumination, cryopolymerized macroporous reduced graphene oxide@poly(N-isopropylacrylamide) fibers undergo asymmetric contraction, which reorients the illuminated surface to be normal to the light vector, so as to maximize photothermal flux under oblique incidence. Meanwhile, flash heating further enhances water-confined heat transfer and expands the effective heating domain. Bio-PEH arrays raise bulk wastewater from 7.8 to 30.2 °C, which improves NH4+ removal by 2.8-fold, accounting for 25.3–169.7% enhancement compared with Joule heating (JH), direct solar heating (DH), and static endogenous heating (SEH). Global simulations further indicate multifold thermal compensation at high latitudes with substantial carbon and energy cost reduction. This study provides a biophysically inspired strategy by utilizing opto-thermomechanical response to realize heating of low-temperature wastewater, making wastewater treatment in cold regions more efficient, economical, and sustainable.