Effect of Iron Particle Formation on the Transformation of Al in Drinking Water Distribution System: Implications for Al Deposition Control

Abstract

Aluminum (Al), a common residual in drinking water distribution systems (DWDS), can undergo transformation and accumulation, posing challenges for long-term water quality management. However, the interactions between various Al species and iron oxides (FeO_x) generated from pipe corrosion remain insufficiently characterized. This study quantified the transformation efficiency of Al under different FeO_x formation pathways and water chemistry conditions. 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 hours, 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, forming less stable surface associations. 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 actual 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.