A novel cationic-modified chitosan flocculant efficiently treats alkali‒surfactant‒polymer flooding-produced water.

The existence of fine oil droplets that are difficult to coalesce greatly reduces the oil–water separation efficiency of alkali‒surfactant‒polymer (ASP) flooding-produced water. In order to improve the oil–water separation efficiency and promote the reinjection of ASP-produced water, a novel cationic graft copolymer CDC was obtained by ultrasonically assisted copolymerization of chitosan and dimethyl diallyl ammonium chloride under the initiation of cerium ions. The copolymer CDC was characterized by FT-IR, XRD, TGA and SEM, and it was confirmed that the grafting reaction proceeded successfully. The flocculation effect of copolymer CDC on ASP-produced water (Nan 4–8, pH = 7.83; Sanyuan-6, pH = 9.95) was investigated. Copolymer CDC has better oil removal rate (90.5%) and light transmittance (95.25%) to Nan 4–8 produced water compared with Sanyuan-6-produced water. It has a better oil removal effect when the initial pH of produced water is 6–7 and the temperature is 30 °C. After the addition of CDC, the large-diameter oil droplets in produced water will coalesce and separate. It may be that under weak acid or neutral conditions, the electrostatic attraction of produced water is enhanced, compressing the double electron layer of oil droplets, and destroying the stability of oil droplets. In addition, the hydrophobic effect of –CH=CH2 in CDC molecules increases the hydrophobic association between molecules and increases the probability of oil droplet collision during the flocculation process. [ABSTRACT FROM AUTHOR]