Influence of the hydrophobicity of the starch-based coagulant on the coagulation of various pollutants in water and the study of its mechanism

Abstract

<p indent="0mm">Parts of surface water in China, including some rivers and lakes, show the characteristics of micropollution containing excessive dissolved organic matter, which has become a key factor in restricting the improvement of water quality. Coagulation is one of the most popular technologies in the purification of surface water owing to its low cost and simple operation. However, traditional coagulants, including inorganic and organic synthetic polymer coagulants, usually show a low efficiency in removing organic micropollutants from surface water and pose the risk of secondary pollution. Furthermore, the molecular structures of organic micropollutants contain hydrophilic and a large number of hydrophobic groups. Considering these facts, in this study, starch (St), a natural polymer with the characteristics of environmental friendliness, wide source, and low cost, was used as the raw material and acrylamide and various quaternary ammonium salts containing different lengths of carbon chains were used as modifiers. Accordingly, four modified starch coagulants (CS-DMRCs) with the same charge density and good water solubility but different degrees of hydrophobicity were designed and fabricated using graft copolymerization. Kaolin particles, sodium humate (NaHA), and bovine serum albumin (BSA) were used as the simulated sources of inorganic colloids and different organic pollutants in the micropolluted turbid surface water. The effects of the hydrophobicity of four St-based coagulants on the removal of kaolin particles, NaHA, and BSA from their single, binary, and ternary pollutant aqueous systems were investigated systematically. Based on the apparent coagulation performance, the floc properties, including floc size, compactness, surface morphology, breakage and recovery behaviors, and the zeta potentials of the supernatants after coagulation, the coagulation mechanisms associated with the structural features of the St-based coagulants and treated pollutants were explored and discussed in detail. The St-based coagulants with a higher hydrophobicity showed better coagulation performance owing to the synergistic effects of charge neutralization, bridging flocculation, and hydrophobic association. In addition to these simulated water samples, a real micropolluted turbid surface water sample from the Jiuxiang River of Nanjing was tested and compared with the simulated water samples, further confirming the practicality of the hydrophobically modified cationic St-based coagulants. In summary, CS-DMRCs should have broad prospects in practical applications owing to their easy and simple fabrication and high efficiency in the purification of organic micropolluted turbid surface water.