Interactions and transport of hexavalent chromium with microplastics in detergent-dissolved water

The ubiquitous co-existence of microplastics and hexavalent chromium (Cr⁶⁺) causes their interactions, which are heavily influenced by the composition of wastewater. However, microplastic−bound vector transportation of Cr⁶⁺ in the presence of different species in water remains poorly understood. To address this paucity, Cr⁶⁺ adsorption onto pristine and aged polyethylene (PE) microplastics with four different sorption media was accessed to understand their influence. Thus, the sorption behavior of PE microplastics at different solution pHs (3–10), contact time (for 7 days), and concentration of Cr⁶⁺ (1−30 mg L⁻¹) was tested. The effect of ionic species (0.1 M of NaNO₃), dissolved organic matter (1.5 mg L⁻¹ of humic acid (HA)), and detergent-dissolved water (4% v/v) on Cr⁶⁺ adsorption was also examined. The highest adsorption capacities of Cr⁶⁺ (0.62−1.00 and 0.31−0.60 mg g⁻¹ for detergent and NaNO₃-dissolved water, respectively) showed at the pH range of 2.0–3.0 except for HA-dissolved water. Detergent-dissolved water demonstrated remarkable adsorption than ultrapure water, NaNO₃ and HA-dissolved water. Hydrophobic and electrostatic interactions governed the surface complexation of Cr⁶⁺ on PE microplastics in detergent-dissolved water. Pseudo-second-order kinetic model best-fitted for pristine microplastics indicating the chemisorption of Cr⁶⁺. Best fitting with fractional power and intra-particle-diffusion kinetic models with aged microplastics indicated a diffusion-controlled physisorption. Isotherm data modeling demonstrated non-linear corporative adsorption behavior compiled with Hill model for both PE microplastics. This study deduced that detergent mixing could boost the vector transport ability of PE microplastics for Cr⁶⁺ migration in aquatic environments.