Effects of different modifying agents on the preparation and properties of coagulants derived from excess sludge

The contents and kinds of oxygen-containing functional groups are very significant when preparing cationic hydrochar coagulants via graft copolymerization. Herein, the hydrothermal conditions to produce sludge-based hydrochar (SBC) precursors were optimized by introducing different kinds and amounts of modifying agents (i.e., HCOOH, citric acid (CA), H₂SO₄, and H₂O₂), then the surface properties and flocculation performance of derived cationic coagulants (SBC-g-DMC) were studied. Results showed that the utilization of four modifiers raised the acidic groups on the SBC surface; thereinto, the presence of CA could evidently increase the content of phenolic hydroxyl groups. After DMC monomer grafting, the formed coagulants possess positive zeta potentials over a wide pH range (i.e., 3.0 ~ 11.0), showing a typical cationic property. The grafting ratio and efficiency, as well as the cationic degree of coagulants prepared with different SBC precursors follow a descending order of SBC_CA-g-DMC > [Formula: see text]-g-DMC > SBC_HCOOH-g-DMC > [Formula: see text]-g-DMC; thus, SBC_CA-g-DMC coagulant with the best grafting result shows a superior flocculation performance. When a dosage of 4 mg/L was adopted, the average turbidity removal rate of SBC_CA-g-DMC could reach up to 94.44%. Meanwhile, due to the possible and robust oxidation with the initiator, H₂O₂ seems not a perfect modifier for SBC preparation. This study could provide an essential reference for the optimal synthesis of SBC and its based coagulants for organic matter recovery and pollutant removal.