Molecular-Level Explanation of AOM Removal by a Composite Coagulant Based on the Subdivision of Organic Components

Abstract

Algal organic matter (AOM), comprising intracellular organic matter (IOM) and extracellular organic matter (EOM), poses a significant challenge to drinking water safety. Coagulation serves as an effective method for removing algae, and investigating the binding sites of coagulant hydrolyzates is crucial for comprehending the coagulation mechanism. In this study, a novel polyferric titanium sulfate (PFTS) composite coagulant was prepared and used to remove AOM. Characterization techniques, including Fourier infrared (FTIR), X-ray photoelectron spectroscopy (XPS), hydrolysis polymerization curves, and chemical species analysis were utilized to identify the hydrolyzates of PFTS. The results revealed the formation of Fe–Ti copolymers through the interaction between Fe hydroxyl and Ti hydroxyl, facilitated by a Fe–O–Ti bond. Under optimal coagulation conditions (50 mg/L dosage at neutral pH), PFTS demonstrated superior performance in treating EOM and IOM compared to PFS and PTS, achieving significant DOC removal efficiencies of 46.69% and 56.8%, respectively. Furthermore, the removal characteristics of organics were investigated at the molecular level using Fourier transformation cyclotron resonance mass spectrometry (FT-ICR MS). It was found that organic compounds with unsaturated (H/C < 1.0) and oxidized (O/C > 0.5) substances containing carboxyl groups in AOM could be preferentially removed by PFTS for the carboxyl group demonstrate a higher affinity to Fe and Ti hydroxyl formed in PFTS coagulation. XPS and water contact angle analysis were conducted to gain deeper insights into the interaction between the hydrolyzates of PFTS and AOM. The findings demonstrated that Fe–Ti hydrolyzates could bind with EOM and IOM by forming coordination bonds and H–O···O and H–O···N hydrogen bonds with its −COOH, −NH₂, and −OH groups through bonding reactions. This study highlights the potential of composite coagulants as alternatives to conventional coagulants for the purification of algae-laden water.