Experimental application of a zero-point charge based on pH as a simple indicator of microplastic particle aggregation

Micro/nanoplastics – a useful but threatening material – continuously require fundamental research on its behaviors and properties for aggregation. Zeta potential (ζ) has been using as an indicator to determine the optimal aggregation for particle removal in water treatment processes. In the field work, however, an alternative method for streamlining these tasks and reducing the variability in processing efficiency is necessary. To improve practical utility in the field work, this study aimed at investigating applicability of the zero-point charge (ZPC) of the isoelectric point (IEP; ψ_pI) as an alternative indicator for aggregation in place of ζ. For the purpose, this study conducted laboratory experiments and model simulations. The experiments measured ψ_pI of microplastics in a trivalent-electrolyte aqueous solution using various concentrations of polyaluminum chloride (PAC) for reproducing the behavior of microplastics in natural water environments. As a result, ψ_pI for polyethylene (PE) and polyvinylchloride (PVC) were found to be pH 6.59 and 6.43, respectively. The removal rates (r) depended on the aggregation at the initial pH and optimal PAC concentration. The experimental attachment efficiency (α_E), 0.14 to 0.4, showed a good correlation of over 95% with r, 0.04 to 0.84, both based on the pH change and PAC concentration and differing slightly with the type and size of the plastic. The highest α_E was achieved with the highest r when ψ_pI was close to zero in the pH range of 6–8 using the optimized PAC concentration. Based on the experimental results, the model confirmed the applicability of ψ_pI instead of ζ as an indicator of the aggregation by simulating α_E based on ψ_pI and ionic strength, which are themselves based on the change in pH. Therefore, this study provides some insights into behaviors of microplastics by using the isoelectric point (IEP, ψpI) as an indicator of aggregation of microplastics in place of ζ. The IEP method is limited by initial pH, optimal dosage of coagulant, and type and size of microplastics, but it will increase practical utility in the field.