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• PPFS-PAD-RGO compound coagulants were successfully prepared. • The coagulability of PPFS-PAD-RGO has been enhanced. • PPFS-PAD-RGO has certain demulsification ability. • The oil content of oily sludge is reduced. During oil extraction, a substantial volume of oily sewage (water-in-oil emulsion) is generated. The existence of emulsified oil diminishes the effectiveness of conventional coagulants used for purification. Furthermore, coagulants frequently precipitate emulsified oil as impurities, leading to resource wastage. Hence, there is an urgent need to develop multifunctional coagulation/demulsification agents that are environmentally safe. In this study, we synthesized the branched-chain compound coagulant (PPFS-PAD-RGO) and investigated its coagulation and demulsification properties. The results of dynamic and static light scattering and FTIR show that chitosan grafted onto the surface of modified PPFS forms a dendritic flocculation system. In addition, PPFS-PAD is fixed to a reduced graphene oxide carrier with a high specific surface area, which improves the positive charge density and dispersion of the coagulant. The layered structure amplifies the effect of adsorption bridge formation and net aggregation. At a concentration of 80 mg/L, PPFS-PAD-RGO exhibited a turbidity removal rate of 96.15 % and demulsification rate of 74 %. By comparing the structure of PPFS, it was found that the floc volume of PPFS-PAD-RGO is larger. This demulsification ability is mainly attributed to an amine matrix in the chitosan extension chain, which destroys the oil–water interface of the oil-in-water emulsion and induces demulsification. The introduction of RGO increases the local concentration of coagulant molecules. Therefore, the coagulation effect of PPFS-PAD-RGO in oily sewage is attributed to the introduction of demulsifier groups and nanomaterials, which strengthen the synergistic effect of charge neutralization, bridge adsorption and sweeping. [ABSTRACT FROM AUTHOR]

• A method of gravity coagulation was proposed. • The covalently linked coagulant microcapsules was successfully prepared. • The coagulant capsule has able to release its contents slowly upon water impact. • The mechanism of The coagulant capsule was studied. If untreated, oilfield sewage generated during the oil exploitation process can lead to severe environmental pollution. The coagulation/flocculation process plays a pivotal role in the comprehensive treatment of oilfield sewage on a large scale, yet traditional methods exhibit significant limitations. To address the disparity between laboratory experiments and large-scale implementation, this study introduces an innovative coagulant microcapsule. Prepared under gentle conditions, these microcapsules effectively eliminate colloidal particles from water when introduced at the coagulation tank inlet. Featuring a core–shell structure, coagulant microcapsules demonstrate controlled content release upon contact with water. The coagulation process occurs as sewage enters the coagulation tank. The core of the coagulant microcapsule consists of the inorganic polymer coagulant (PASS-C16) with hydrophobic association capabilities. Upon contact with sewage at a specific flow rate, the coagulant is released gradually from the microcapsule. Upon reaching the bottom of the coagulation pool, the coagulant comes into full contact with colloidal particles, influenced by gravity. Ultimately, through integration with Fluent simulation and analysis of floc particle size changes, it was determined that the primary mechanisms of coagulant capsules in aquatic conditions involve charge neutralization and bridging adsorption. The sweeping action is effective only when the test water tends to be static. This technology is viable for application in the coagulation sedimentation tanks of diverse sewage treatment plants. Gravity coagulation demonstrates potential as a replacement for conventional mechanical mixing methods, offering increased efficiency and potential energy savings in various water treatment processes. [ABSTRACT FROM AUTHOR]