Bioinspired mineralization and remediation of polystyrene nanoparticles by urease-induced calcite precipitation

Nanoplastics (NPs) are emerging contaminants, and their remediation is a challenge, as they escape most of the advanced treatment methods owing to their small size and mobility. Considering this, the present study highlights a novel application of enzyme-induced calcite precipitation (EICP) technique in removal of polystyrene nanoparticles (PSNPs) from the aqueous system using co-precipitation with calcite. A 96% removal of PSNPs was achieved as a calcite-PSNPs composite (precipitated product) within 0.5 h from an aqueous solution (pH 7.0) supplemented with 2% (w/v) urea, 76 U urease, and 50 mM CaCl2. In the solution, the negatively charged PSNPs interacted with available Ca2+ and formed the PSNP-Ca2+ complex. The electrostatic attraction between the complex and the CO32- (generated by ureolysis) resulted in the formation CaCO3-PSNPs composite, which finally precipitates down. Additionally, entrapment of free PSNPs during the precipitation of calcite also contributed to PSNPs removal. The incorporation of PSNPs in the calcite matrix was substantiated through scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, and thermogravimetric analysis. Optimum removal of PSNPs was detected at pH 7.0, and with the increase in precursors viz CaCl2, urease, and PSNP levels, the precipitation of PSNPs increased. Moreover, calcite-PSNP interaction was stable, as there was no significant polymer release upon ultra-sonication of the composites. Overall, the study provides a feasible and newer approach to sequester PSNPs from the aqueous system using the naturally occurring EICP method.