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Revealing the Roles of Microplastics and Dissolved Organic Matter in Phosphorus Recovery via Struvite Crystallization in Batch and Fluidized-Bed Reactors
Summary
Researchers found that microplastics act as nucleation seeds that actually speed up struvite crystallization — a process used to recover phosphorus fertilizer from wastewater — while the organic compound humic acid slows it down and promotes formation of an undesirable mineral byproduct. The presence of both contaminants also altered the crystal structure and purity of the recovered struvite. This matters because wastewater increasingly contains microplastics, and if they contaminate recovered fertilizers, those microplastics could enter agricultural soils.
Struvite crystallization, a promising technology for nutrient recovery from wastewater, is ever more encountering challenges due to the presence of emerging contaminants such as microplastics, which are ubiquitous in wastewater. In this study, we investigate the roles of microplastics and humic acid in struvite crystallization in batch and fluidized bed reactors, with emphasis on crystallization kinetics and physicochemical properties of struvite crystals. Batch crystallization kinetic experiments were conducted with synthetic wastewater with varying concentrations of microplastics and humic acid. The results showed that microplastics expedited the nucleation and growth rates of struvite (e.g., 1.43 times the blank suspension in the presence of 30 mg L−1 of zinc loaded polyethylene terephthalate particulates), while humic acid hindered the formation of struvite. Besides, X-ray diffraction analysis and the Rietveld refinement revealed that the presence of microplastics and/or humic acid can result in quite many changes in phase compositions of the reclaimed precipitates in batch and fluidized bed reactors. The characterization analysis demonstrated that microplastics act as seeds of struvite nucleation, spurring the formation of well-defined struvite, while humic acid favors the formation of newberyite rather than struvite in both the batch and the fluidized bed reactors. These findings highlight the need for a more comprehensive understanding of the interactions between emerging contaminants and struvite crystallization processes to optimize nutrient recovery strategies for mitigating their adverse impact on the quality and yield of struvite-based fertilizers.