Porous Materials for Multicomponent Oily Wastewater Purification: Detection, Removal and Recovery of Contaminants

Studies on materials for water treatment and quality monitoring are usually limited to single functions and/or energy-consuming purification processes. This PhD thesis reports the development of a reusable and multifunctional porous material for the detection, removal and recovery of contaminants though gravity-driven filtration. To this aim, poly(sodium) acrylate (PSA) based nanocomposite cryogels containing a functional nanofiller, namely graphitic carbon nitride nanosheets (CNNs), were developed. The obtained PSA/CNN cryogels present appropriate properties for the single-step purification of multicomponent oily wastewater, for the detection of mercury (Hg2+) ions, and for the photodegradation of the sorbed organic pollutants under daylight irradiation. The nanocomposite cryogels were fabricated via cryopolymerization of a mixture of PSA and CNNs, with the CNNs content optimized towards the different functions. Although the tested CNNs concentrations (0.5-3% w/w with respect to the monomer) do not significantly affect the morphology, swelling, or wetting, as well as the mechanical properties of the cryogels, they do influence their optical properties, rendering the cryogels fluorescent and photocatalytically active. In the filtration process, the purification is mainly attributed to the PSA matrix, which exhibits underwater superoleophobicity, allowing for the water-oil separation, and contains the appropriate functional groups for the effective adsorption of the methylene blue (MB) cationic dye, through electrostatic interactions, and of the Hg2+ ions, through Na+-Hg2+ ion exchange. We prove that the PSA/CNN cryogel can efficiently and rapidly clean oily wastewater, and its excellent performance is maintained after diverse filtration cycles, whereas the co-presence of Hg2+ ions, oil and MB does not affect the separation efficiencies of any of the pollutants. The sensing properties of the PSA/CNN cryogels are attributed to the CNNs’ fluorescence quenching phenomenon occurring upon interaction with the Hg2+ ions. Through the optimization of the CNNs concentration within the cryogel, sufficiently low amounts of Hg2+ ions can be detected upon the gravity-driven filtration of the wastewater and the subsequent fluorescence intensity evolution monitoring. The sorbed Hg2+ ions can be desorbed from the nanocomposite cryogel through a pH-responsive mechanism, and the regenerated cryogel maintains its excellent Hg2+ ions separation performance in diverse sorption-desorption cycles. Finally, the photocatalytic self-cleaning property of the composite cryogels, attributed to the CNNs’ semiconductive properties, enables it to be regenerated through the degradation of the sorbed organic dye upon exposure to the visible light irradiation for reuse in multiple filtration cycles. Overall, the herein presented work opens an ample space for the development of multifunctional materials for the cost and energy-efficient integrated water treatment and quality monitoring, as well as for the recovery of the trapped contaminants and the regeneration of the spent filters, going towards a greener alternative to the conventionally used sophisticated and secondary-pollution-causing processes related to water protection.