The influence of coagulation process conditions on theefficiency of microplastic removal in water treatment

The objective of this study was to investigate the influence of coagulation conditions on the removal efficiency of microplastics (polyethylene – PE, polyvinyl chloride – PVC) and textile microfibers (synthetic and a mix of synthetic and cotton fibers) from different water matrices: Danube river water, municipal wastewater effluent (MWW) from a wastewater treatment plant (WWTP), washing machine wastewater effluent, and a synthetic matrix. The coagulants used were ferric chloride (FeCl₃) and polyaluminum chloride (PACl). Additionally, the effects of pH adjustment, microsand addition, and the influence of reuse of coagulation sludge on process efficiency were examined. Furthermore, the impact of microplastic presence on the removal of 4-methylbenzylidene camphor (4-MBC) was analyzed, as well as the potential leaching of phthalates from PVC present in WWTP sludge considering that PVC is commonly reported in literature as a frequent component of such sludge and is known for its tendency to release additives into the environment. Coagulation efficiency results showed that PVC was the most effectively removed in all tested matrices, particularly when using FeCl₃ (up to 100% in washing machine wastewater effluent), whereas PE exhibited the lowest removal susceptibility, especially in more complex matrices (wastewaters), with PACl achieving only 16–31% removal. Both types of textile fibers were efficiently removed (up to 99%) using FeCl₃, except in MWW effluent where the removal efficiency was slightly lower (~70%). PACl proved more effective in simpler water matrices, such as the synthetic one, particularly at lower coagulant doses. Compared to PACl, FeCl₃ consistently achieved better results, especially in water matrices rich in organic matter. pH adjustment to pH = 5 significantly enhanced coagulation efficiency using FeCl₃, particularly in the synthetic matrix at lower doses (PE removal increased to ~90%), marking a significant improvement over the initial condition. In Danube river water, the improvement from pH correction was observed only for PE, whereas in MWW effluent the effect was negligible, aside from a slight decrease in efficiency for synthetic textile fibers. In washing machine wastewater effluent, pH adjustment led to improved removal of most tested materials, particularly for PE and synthetic textile fibers, highlighting the importance of pH under conditions with surfactants and organic matter. Microsand addition did not significantly affect PE and PVC removal in the synthetic matrix but notably improved textile fiber removal, especially for mixed fibers (up to 98%) compared to results without microsand. In Danube river water, microsand improved PE removal at lower PACl doses, while its influence on other materials was limited. In MWW effluent, microsand significantly enhanced PE (up to 98%) and synthetic fiber (up to 91%) removal, whereas the effect was limited in washing machine wastewater. The results suggest the selective effectiveness of microsand addition, particularly for lowdensity materials (like PE) or those with specific structures (like textile fibers). Although the reuse of coagulation sludge is under-researched, results suggest it may enhance pollutant removal efficiency. Results from this research indicate that sludge reuse (in combination with freshly added FeCl₃) contributed to the highest removal of PVC and PE in washing machine wastewater effluent (up to 100%), as well as PVC in the synthetic matrix (77%). In matrices containing organic matter, such as Danube river water and wastewater, enhanced particle aggregation was observed. Conversely, this method was not effective for removing both types of textile fibers, likely due to coagulant adsorption onto the fibers and unfavorable physical properties such as low density and poor sedimentation. Moreover, the results showed that coagulation using FeCl₃ and PACl could successfully remove 4-MBC from various water matrices, with the highest efficiencies achieved in real samples—MWW effluent and washing machine wastewater effluent (up to 100%). FeCl₃ generally resulted in higher removal efficiencies, especially at higher coagulant doses and in the presence of PE and textile fibers. Although the presence of the tested materials generally reduced 4-MBC removal efficiency, some materials like textile fibers, due to their structure, contributed to improved flocculation. The lowest 4- MBC removal efficiencies were observed in the synthetic matrix with PE and PVC present, while in some cases, PE presence improved 4-MBC removal at low FeCl₃ doses. The results emphasize the importance of selecting the appropriate coagulant and optimizing conditions depending on the water matrix and particles present. In environments rich in natural organic matter, such as sludge, phthalate release - particularly DEHP (Di(2-ethylhexyl) phthalate) - from PVC was predominantly controlled by resistance in the aqueous boundary layer. This was confirmed by the application of the ABLD (Aqueous boundary layer diffusion) model and low Biot number values (Biₘ ≪ 1). The calculated boundary layer thickness (δ ≈ 440 μm) greatly exceeds typical values (10–30 μm), indicating that the presence of organic matter, poor mixing, and possible formation of organic coatings reduce mass transfer and lead to slower additive release from PVC under such conditions. Based on the obtained results, it was concluded that coagulation and flocculation are effective methods for removing microplastics and textile fibers from water. Key parameters include coagulant type and dosage, pH value, and characteristics of the water matrix. Additional measures such as microsand addition and sludge reuse after coagulation can further improve the process, especially for low-density materials. The findings also contribute to understanding the mechanisms of additive release from microplastics in organically rich environments, which is important for predicting their long-term impact and optimizing wastewater treatment processes.