Waste treatment by waste: High-value utilization of superabsorbent polymer in disposable diapers as efficient adsorbent for heavy metal

The consumption of disposable diaper has increased drastically over the past decades in virtue of its cost effectiveness, user-friendly nature, and excellent skin compatibility. Nevertheless, used disposable diapers are typically fed to landfills with no treatment. This is a huge threat to the environment as superabsorbent polymer (SAP) in disposable diapers are essentially non-degradable under natural conditions. Unfortunately, recycling of SAP remains extremely challenging owing to its crosslinked network structure and ultra-high water absorption capacity. In this study, we propose a novel route for high-value utilization of SAP in used disposable diapers in a 'waste treatment by waste' manner. Specifically, the SAP was modified by polyethyleneimine (PEI) in a facile way (grafting and crosslinking under mild conditions) to develop the SAP-PEI, which is, to the best of our knowledge, the first modified-SAP-based adsorbent reported to date. The results demonstrated that the asprepared SAP-PEI was superior to most low-cost adsorbents in terms of maximum adsorption capacity towards Cr(VI) (527.52 mg/g). Meanwhile, adsorption isotherms and kinetics revealed that Cr(VI) adsorption by the SAP-PEI can be effectively described by the pseudo-second-order model and the Langmuir model. Meanwhile, electrostatic attraction plays a key role in Cr(VI) adsorption by the SAP-PEI, and Cr(VI) are partially reduced to Cr(III) in this process. Additionally, the SAP-PEI exhibited excellent reusability as its Cr(VI) removal efficiency decreased by less than 5 % after seven cycles. Overall, the SAP-PEI is a high-performance adsorbent and its application in removal of heavy metals such as Cr(VI) serves as a green and cost-effective approach for high-value utilization of SAP in used disposable diapers.