Effect of iron particle formation on the transformation of Al in drinking water distribution system: implications for...

Aluminum (Al), a common residual matter in drinking water distribution system (DWDS), can undergo transformation, accumulation and re-release, posing challenges for maintaining stability of distributed water quality. However, the interactions between various Al species and iron oxides (FeO_x) mainly generated from pipe corrosion remain poorly characterized. This study investigated the transformation behaviors of Al under different FeO_x formation processes and water chemistry conditions. Results showed that in-situ formed FeO_x significantly enhanced the transformation of inorganic Al into stable particulate forms, with up to 81.8 % reduction in soluble Al after 48 h, compared to only 18.0 % with preformed FeO_x at an initial Al concentration of 200 μg/L. Inorganic Al was substantially incorporated into FeO_x particles during in-situ formation through isomorphic substitution into layered double hydroxide (LDH) structures, resulting in stable, particle-associated Al. In contrast, preformed FeO_x exhibited limited Al adsorption capacity, thereby forming less stable surface association. Notably, organically complexed Al exhibited strong resistance to transformation and largely remained in the dissolved phase, while also inhibiting FeO_x formation. Acidic conditions (pH 4.5) triggered significant Al release from preformed FeO_x-Al composites. Elevated pH and increased dissolved organic carbon (DOC) concentrations further promoted Al remobilization, particularly from surfaces of preformed FeO_x. Validation experiments using real tap water confirmed the dominant role of in-situ FeO_x formation in promoting the immobilization of inorganic Al into particulate form. These findings provide critical mechanistic insights into FeO_x-mediated Al transformation pathways and inform strategies for mitigating Al accumulation and remobilization in DWDS.