Compositional and structural identification of organic matter contributing to high residual soluble aluminum after coagulation

Understanding the complexation of aluminum (Al) with dissolved organic matter (DOM) is of great significance for the control of residual Al in drinking water after treatment. Here we used high-resolution and accurate mass measurements to identify the composition and structure of DOM contributing to soluble organically-bound Al formation during coagulation at alkaline pH (pH 7.50). The results showed that the organic compounds that contribute to the soluble organically-bound Al were primarily phenolic compounds and aliphatic compounds. Among them, phenolic compounds with a sulfonic acid group could greatly enhance the hydrolysis of polymeric Al and the formation of high concentration of monomeric/oligomeric Al-DOM complexes. These organic molecules had a mass-to-charge ratio concentrated below 350. Based on the assumption that oxygen-containing functional groups providing unsaturation in the molecular structure were carboxyl groups, it was inferred that the maximum number of carboxyl groups in phenolic compounds and aliphatic compounds was concentrated between 1 and 2 and 2–4, respectively. The presence of these molecules was responsible for soluble organically-bound Al accounting for over 80 % of the total soluble Al in the supernatant after coagulation in this study. These findings deepen the understanding of the complexation of Al with DOM. In drinking water treatment plants, the combination of coagulation with processes that can remove such characteristic organics is beneficial for residual Al control. [Display omitted] •Phenolic compounds with a sulfonic group enhance hydrolysis of polymeric Al.•Phenolic compounds with 1–2 carboxyl groups are the main DOM.•Aliphatic compounds with 2–4 carboxyl groups are the main DOM.•Soluble organically-bound Al account for >80 % of residual Al.