Aggregation state manipulation of small-molecule iron-based coagulant hydrolysates for efficient nanoplastics removal

Abstract

The pH value significantly influences the aggregation state of coagulant hydrolysis products, thereby impacting coagulation performance. However, prevailing methods primarily focus on regulating the initial pH of water bodies before coagulation (pre-coagulation pH regulation), posing challenges in controlling the aggregation morphology of coagulant hydrolysates and resulting in suboptimal NPs removal efficiency and elevated residual coagulant levels in water. In this study, we herein propose a novel in-coagulation pH regulation approach to manipulate the aggregation states of Fe3+-based hydrolysates, thereby markedly enhancing the NP removal efficiency. Specifically, the Fe3+ coagulant exhibits exceptional coagulation performance at a low dosage of 0.1 ∼ 0.2 mM, achieving removal efficiencies of 97.6 % and 93.5 % for polystyrene NPs with sizes of 500 nm and 100 nm, respectively. Moreover, the post-coagulation and sedimentation supernatant shows an ultralow residual Fe concentration of less than 0.1 mg/L, compliant with Chinese drinking water sanitary standards (GB 5749–2022). Such outstanding performance can be attributed to the structural transformation of Fe3+ hydrolysates from the low-branching linear structure to the high-branching cross-linked network, shifting the coagulation mechanism from the adsorption bridge to sweep coagulation, thus significantly enhancing coagulation efficiency. Furthermore, this strategy proves applicable to the Al3+ coagulants, indicating its great potential for practical implementation in drinking water treatment.