Vertical distribution of new and weathered microplastics in the water column: Implications for developing reliable sampling methods.

Microplastics (MPs) exposure to environmental conditions results in weathering, alters their physicochemical properties, and subsequently influences their mobility in aquatic systems. This study aims to investigate the interrelated roles of MP type and weathering conditions on their terminal settling/rise velocities and subsequently distribution in the water column under turbulent flow to further inform sampling methodologies. Low density polyethylene (LDPE), polyvinyl chloride (PVC), and ground tire rubber (GTR) were subjected to accelerated photodegradation and sedimentation processes, and biofilm was grown on LDPE and GTR MPs. The variations in their density, surface chemistry, morphology, and hydrophobicity following weathering were studied. The analyses revealed an increasing density for photodegraded and sedimented PVC MPs. Moreover, photodegradation enhanced the hydrophilicity of LDPE and GTR MPs. Weathering decreased the terminal rise velocity for LDPE, while it increased the settling velocities for PVC and GTR MPs due to the variations in their density and hydrophilicity. Weathering-induced changes in MP physicochemical properties, such as increased hydrophilicity and density, alter their settling and suspension behavior within the water column. These behavioral changes lead to a more dispersed vertical distribution, affecting the efficiency of grab sampling methods that rely on sampling at limited depths. The MP's total recovery through grab sampling from the top, middle, and bottom of the water tank revealed the order of new LDPE (16.40%)>new PVC (7.93%)>new GTR (6.7%). MP weathering mostly reduced their total recovery. The reduced recovery of weathered MP may be attributed to increased hydrophilicity and density, which promoted their settling and suspension within the water column and decreased their likelihood of being captured during sampling. This study underscores the urgent need to account for MP type and weathering when designing sampling protocols. Moreover, their non-uniform distribution suggests that a single-point or single-type grab sample may underestimate the MPs content in the aquatic system.