Preparation of a novel zwitterion flocculant with the characteristics of quaternary ammonium salt cation and dithiocarbamate anion and its application in produced water treatment

In this paper, a novel flocculant with the characteristics of both quaternary ammonium salt cation and dithiocarbamate anion was synthesized. Firstly, a cationic polymer having secondary amine side groups was synthesized by using cationic monomer and diallyl amine (DAA) as monomers via aqueous solution polymerization, and then the secondary amine was modified by carbon disulfide (CS2) to prepare the zwitterion flocculant. By using the oil removal as index, the effects of cationic monomer type, monomer concentration, mass ratio of cationic monomer to DAA, initiator concentration, polymerization temperature and time on the copolymerization results were investigated. The recommended rational condition was as follows: cationic monomer was methacryloxyethyltrimethyl ammonium chloride (DMC), monomer concentration was 50 wt%, mass ratio of DMC:DAA = 4:1, initiator was ammonium persulfate (APS) and its concentration was 0.4 wt%, and polymerization temperature was 60 °C for 8 h. The zwitterion flocculant obtained at the recommended condition had a removal of 99.5% at the dosage of 300 mg/L for produced water obtained from an offshore oilfield. In addition, the flocculation mechanism of the zwitterion flocculant was studied. According to the results of zeta potential measurement and floc morphology, it can be found that the quaternary ammonium salt unit in the zwitterion flocculant can bridge and flocculate oil droplets by destroying the stability of oil droplets in produced water through electrostatic neutralization, while the dithiocarbamate unit can interact with Fe2+ in produced water to form a network structure to catch flocculated oil droplets, and the synergistic effect of quaternary ammonium salt unit and dithiocarbamate unit is helpful for achieving the good flocculation result. The results provide a new selection for produced water treatment.