CO₂ Introduced The Coagulation-Flocculation Of Oil Acidized Wastewater

In the process of oil exploitation,acid fracturing technology used to increase production is very common,so oil wastewater has high acidity and strong corrosive effect.The method for titrating calcium ions with ethylenediaminetetraacetic acid（EDTA）shows that the concentration of Ca²⁺in the petroleum wastewater collected in this research is as high as 6413mg·L^-1.The Ca²⁺in wastewater consumes OH^-,resulting in the difficulties of p H adjustment and water treatment.So far,the conventional treatments include flocculation,neutralization,oxidation and activated carbon adsorption.But the problem was still incompletely solved on account of wastewater of low p H.Under acidic conditions,the low flocculation effect of polyacrylamide（PAM）is mainly due to the poor molecular chain extension of PAM.As an alkaline additive,Na₂CO₃ can increase the extension of the PAM molecular chain and react with Ca²⁺to form insoluble Ca CO₃precipitates.However,the cost of Na₂CO₃ is relatively high,and it cannot be directly used to treat wastewater in actual factories.Therefore,this study aims to induce the formation of Ca CO₃flocculation nuclei by introducing CO₂ into the wastewater,and cooperate with PAM to improve the coagulation-flocculation of oil acidized wastewater.The order in which CO₂ and Na OH were added affected the floc structures.Compared with CO₂-Na OH-PAM,the flocs of Na OH-CO₂-PAM were more compact and more Ca CO₃crystals were formed.The aqueous Ca²⁺involved in the reaction reached 20%,and CO₂utilization was enhanced.The settling time was shortened by half（from 20 to 3 min）,and Na OH consumption was reduced by one-tenth（from 0.03 to 0.003 mol）.Hence,the cost can be significantly reduced when applied to actual operation.Due to the higher settling rate and shorter contact time,Na OH-CO₂-PAM flocs have poor adsorption effect on oil,benzene series（BTEX）and polycyclic aromatic hydrocarbons（PAHs）.After treatment,the residual oil was124 mg·L^-1,the removal of benzene and ethylbenzene were 74%and 97%,respectively.And the total removal of PAHs in wastewater was 76%.However,the removal of suspended solids（SS）by Na OH-CO₂-PAM is as high as 93%,and the concentration of SS in the wastewater drops to 38 mg·L^-1.In addition,the surface free energy of the flocs produced by Na OH-CO₂-PAM is higher,improving the adhesion of the flocs to SS in wastewater.However,after XDLVO analysis,it was found that the second minimum energy level of the flocs produced by Na OH-CO₂-PAM was less than that of CO₂-Na OH-PAM,indicating that the flocs produced by Na OH-CO₂-PAM had slightly poorer adsorption firmness to pollutants.Based on the paraffin-embedded-section observation of the sample,it was found that the flocs produced by Na OH-CO₂-PAM are tight,with small gaps,and a large amount of SS is adsorbed on the surface of the flocs,thereby changing the surface free energy of the flocs.As a promising new type of adsorption material,Ca CO₃-PAM can improve the coagulation-flocculation effect of oil acidized wastewater.CO₂ gas can also be used as pollutants in mineralization wastewater,and the Ca CO₃ precipitation produced is harmless and stable to the environment.