Contaminated microplastics: adsorption/desorption of Rhodamine B and phase separation by electrocoagulation-flotation

The excessive use of plastics and inadequate waste management result in significant environmental challenges, including the pollution of water, air, and soil. Microplastics (MPs), defined as particles smaller than 5 mm, are ubiquitous in nature and persist in aquatic environments. Moreover, they can carry other inorganic and/or organic pollutants, such as Rhodamine B (RhB) dyes, which are widely used and pose potential risks to human and animal health when present in aquatic systems. Research indicates that wastewater treatment plants (WWTPs) are among the primary sources of MPs in aquatic environments, as they discharge domestic effluents with high concentrations of polymers. However, new treatment methods are being developed to remove these materials. Given this context, this study aimed to understand the adsorption and desorption behavior of RhB dye on different polymers, in both pristine and aged conditions, to elucidate pollutant transport and to assess the efficacy of Electrocoagulation-Flotation (ECF) in the removal of MPs from effluents. The study was conducted in four main phases: (1) characterization of the MPs; (2) preliminary adsorption study between polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC) with RhB; (3) analysis of the MP with the highest adsorption capacity to investigate the RhB adsorption/desorption process; and (4) application of ECF to evaluate the removal of MPs with RhB generated in the previous phase. The results revealed that, in phases 1 and 2, pristine PVC (PVCp) exhibited lower crystallinity, higher roughness, and a point of zero charge (PZC) of 5.0, indicating a greater capacity to adsorb pollutants compared to PE and PP. In phase 3, aged PVC (PVCe) displayed rougher and more porous surfaces, with a higher RhB adsorption capacity (10. 46 mg g-1) compared to pristine PVC (6.23 mg g-1), attributed to photodegradation and the presence of oxygenated groups. Desorption was lower in PVCe, indicating stronger binding with the dye. In the final phase, the study employing Central Composite Rotational Design (CCRD) demonstrated that lower electrical currents and longer electrolysis times improved the removal of PVCp and turbidity. The ECF process was effective, with costs ranging from 0.83 to 16.67 kWh m-3. The optimal conditions applied to PVCp, PVCe, PVCp + RhB, and PVCe + RhB resulted in MPs removal exceeding 96.4% for PVCe + RhB. It was concluded that MPs, such as PVC, have a high capacity to transport pollutants due to their properties, with aged MPs showing stronger binding with toxic substances. ECF emerged as a promising technique for removing MPs with dyes from wastewater, but it requires optimization to balance efficiency and costs, ensuring economic viability.