Understanding and Improving Microplastic Removal during Water Treatment: Impact of Coagulation and Flocculation

Abstract

The efficacy of plastic particle removal by municipal water treatment plants is currently uncertain, and the mechanisms involved in microplastic (MP) coagulation and flocculation have only been superficially investigated. The removal of pristine versus weathered plastic debris and the impact of plastic particle size on removal remain largely unexplored. In this study, coagulation, flocculation, and settling performances were investigated using pristine and weathered MPs (polyethylene (PE) and polystyrene (PS) microspheres, and polyester (PEST) fibers). Weathering processes that changed the surface chemistry and roughness of MPs impacted MP affinity for coagulants and flocculants. A quartz crystal microbalance with dissipation monitoring was used to identify the mechanisms involved during MP coagulation and flocculation. Measured deposition rates confirmed the relatively low affinity between plastic surfaces and aluminum-based coagulants compared to cationic polyacrylamide (PAM). In every case examined, coagulant efficiency increased when the plastic surface was weathered. Removals of 97 and 99% were measured for PEST and weathered PE, respectively. Larger pristine PE MPs were the most resistant to coagulation and flocculation, with 82% removal observed even under enhanced coagulation conditions. By understanding the interaction mechanisms, the removal of weathered MPs was optimized. Finally, this study explored the use of settled water turbidity as a possible indicator of MP removal.

Notes on Common Misconceptions in Microplastics Removal from Water

“…The positive effect of coagulant dosing enhancing microplastic particles removal was also confirmed [44]. It was reported that weathered microparticles revealed higher removal efficiency via coagulation and flocculation than fresh particles, due to changes on their surface roughness [45]. Aluminum coagulation is reported to be more efficient and studies on iron coagulation are not as intensively reported as aluminum coagulation.…”

Removal of Microplastics from Waters through Agglomeration-Fixation Using Organosilanes—Effects of Polymer Types, Water Composition and Temperature

“…Using a dosage of 1 mL n-butyltrichlorosilane, an almost complete removal could be achieved reaching 98.1%. Studies using aluminum, iron or polyacrylamide based flocculants also showed increased effectivity of higher dosages [13,21].…”

“…In comparison Lapointe et al 2020 tested aluminum and polyacrylamide based flocculants towards weathered and unweathered PE and could show that the introduction of polar hydroxyl and carboxylic acid groups by weathering could improve the interaction of the flocculants and microplastics leading to improved removal [21]. This result shows that coagulants with polar sorption sites can lead to enhanced flocculation of polar microplastics.…”

“…It is currently unclear whether the ini microplastics has an influence on the final size of the agglomerates, as data particle growth during the agglomeration process for all polymer types. In p ies testing flocculation of microplastics with different size compositions us minum and polyacrylamide based flocculants, larger microplastic particles s flocculation as they are more difficult to incorporate into the flocks [13,21]. The mean size of the microplastics and the agglomerates is shown in Figure 6.…”

“…Microplastics can consist of a multitude of different types of polymers with different properties and surface chemistries. These can have a strong influence on the interaction of flocculants and microplastics and make finding suitable flocculants even more challenging [13,21].Most commonly used flocculants are based on iron or aluminum and therefore have a limited adaptability [21]. Polyelectrolyte based flocculants are more adaptable, but due to their solubility they remain in the water and can cause harm to aquatic organisms and ecosystems [22,23].…”