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It is difficult to adjust the pH of oil acidized wastewater rich in Ca2+, thus hindering the polyacrylamide (PAM) flocculation. This study aims at accelerating the flocculation process by introducing CO2 into the water to induce the formation of CaCO3 nuclei. The order in which CO2 and NaOH were added affected the floc structures. Compared with CO2-NaOH-PAM, the flocs of NaOH-CO2-PAM were more compact and more CaCO3 crystals were formed. The aqueous Ca2+ involved in the reaction reached 20%, and CO2 utilization was enhanced. The settling time was shortened by half (from 20 to 3 min), and NaOH consumption was reduced by one-tenth (from 0.03 to 0.003 mol), hence significantly reducing the costs. Due to the higher settling rate and shorter contact time, the NaOH-CO2-PAM flocs adsorbed less so that the residual oil was 124 mg·L−1, while in the case of CO2-NaOH-PAM it was 88 mg·L−1. As a promising coagulation aid, CO2 can also be used to mineralize pollutants in wastewater.

CO2 introduced the coagulation-flocculation of oil acidized wastewater: pollutant removal and cost analysis