Adsorption of Cr(VI) and Cr(III) on layered pipe scales and the effects of disinfectants in drinking water distribution systems

Pipe scales in drinking water distribution systems (DWDS) potentially adsorb chromium (Cr). Meanwhile, the fate of Cr in pipe scales and water could be influenced by the disinfectants used in DWDS since they might influence the valence state of Cr. Therefore, the adsorption of Cr (Cr(VI) and Cr(III)) on pipe scales, the transformation between different valence states, and the effects of disinfectants present in DWDS are important research topics for improving tap water quality but have not yet been sufficiently investigated. This study investigated the properties of layered pipe scales and conducted adsorption kinetic experiments in single and binary Cr(VI) and Cr(III) systems, as well as experiments related to the oxidation and adsorption of Cr(III) under the influence of decaying disinfectants. According to the results, pipe scales exhibited distinct layered structures with varying mechanisms for the adsorption of Cr(VI) and Cr(III). Cr(VI) was adsorbed through surface complexation on the surface and porous core layers, while redox reactions predominantly occurred on the shell-like layer. Furthermore, Cr(III) was adsorbed via surface precipitation on the three-layer pipe scales. Importantly, disinfectants promoted the transformation of Cr(III) to the less readily released Cr(VI) in pipe scales, reducing the Cr exposure risk from the pipe scale phase. Pipe scales also decreased the Cr(VI) concentration in water (almost 0mg/L), enhancing the safety of DWDS. This study provides theoretical guidance on the safe operation of DWDS. Environmental Implication This study is highly relevant to the protection of the environment and human health as it addresses the safety of drinking water, particularly focusing on the harmful characteristics of the heavy metal Cr. The research elucidates the mechanisms of Cr adsorption and the transformation of different valence states by the three-layer pipe scales. Additionally, the study explores the valence transformation of Cr between pipe scale and water phases in the presence of disinfectants. The findings offer valuable insights to mitigate environmental risks linked to heavy metal contamination in drinking water distribution systems, contributing to the control of secondary water pollution and ensuring the safety of water quality.