Highlights

AbstractThe efficient separation of oil from complex industrial wastewaters, particularly hydraulic fracturing flowback, remains a critical environmental challenge. This study presents an innovative approach to utilizing cationic polyacrylamide (CPAM) synergistically modified with hydrophobic nano silica (SiO2) and 3-(methacryloyloxy)propyltrimethoxy silane (KH570) to enhance oil/water separation efficiency. Comparative evaluation of unmodified CPAM against CPAM-SiO2-KH570 composites at varying SiO2 loadings (10 %, 13 %, and 16 %) revealed significant performance improvements. Unmodified CPAM achieved a peak transmittance of 89 % at pH 7 (0.4 g/L), with reduced clarity in acidic and alkaline conditions. In contrast, the nano-modified composites demonstrated superior and more robust flocculation. The 10 % SiO2 composite exhibited excellent clarity (90–94 % at pH 7) and the broadest dose tolerance (0.3–0.4 g/L), offering high operational flexibility. The 13 % SiO composite achieved the highest and most robust clarity (95–98 % at pH 7) over a broad dosage window (0.25–0.4 g/L), maintaining strong performance across varying pH, making it ideal for high-specification polishing. While the 16 % SiO2 composite delivered clarification (∼99 %) at lower dosages (0.1–0.2 g/L), it exhibited a narrow optimal window and high pH sensitivity. These enhancements result from the hybrid flocculant's increased hydrodynamic diameter 16–25 μm vs. ∼7 μm for CPAM and sustained positive charge. Additionally, stronger electrostatic and hydrophobic anchoring via the KH570 alkyl shell, combined with enhanced bridging capability due to rigid silica nodes, leads to more stable flocs. This research demonstrates that hydrophobic nano-modified CPAM offers a highly efficient, fast-kinetics, and structurally robust solution for oilfield wastewater treatment, enabling high separation efficiency with reduced chemical consumption and promoting sustainable water management practices.

The rapid growth of industrialization and increased oily discharges from petroleum and petrochemical industries threaten human health and the environment . Oil contamination in produced water primarily results from emulsified or free oil [3]. Free oil can be removed through gravimetric separation and mechanical processes, but emulsified oil is challenging to separate due to its stability in water and environmental risks . Hydraulic fracturing fluids, consisting of approximately 94 % water, 5 % proppants, and 1 % additives, are essential for unconventional oil and gas extraction [8,9]. The stability of hydraulic fracturing flowback water (HFW) is influenced by several factors, especially high salinity and pH affects. Recent advances in HFW treatment focus on coagulation/flocculation, which destabilizes colloidal particles and enhances aggregation for efficient separation, and adsorption techniques. Negative charges on oil droplets are crucial for the stability of oil-water emulsions]. They create electrostatic repulsion that prevents coalescence and maintains the dispersed state of oil in wate. In this context, the fundamental mechanisms governing flocculant–emulsion interactions have been extensively explored in recent literature. Dey et al. highlighted the role of charge neutralization and polymer bridging as dominant destabilization mechanisms in colloidal systems, which remain central to both natural and synthetic flocculant applications. These insights provide a theoretical framework for the design of high-efficiency polymeric flocculants in complex aqueous environments.

Cationic polyacrylamide (CPAM) is a water-soluble polymer synthesized through methods like solution polymerization, inverse emulsion/microemulsion polymerization, and water dispersion polymerization. Water dispersion polymerization is a fascinating technique used for synthesizing polymers. Unlike solution polymerization, where the polymer remains dissolved, or emulsion/microemulsion polymerization which often uses organic solvents, water dispersion polymerization creates a dispersion of polymer particles directly in an aqueous medium]. CPAM is widely used as a sludge dewatering agent due to its high positive charge density, intrinsic viscosity, and tunable molecular weight. Its positive charge attracts negatively charged oil droplets, forming flocs. CPAM is typically produced by copolymerizing acrylamide (AM) with cationic monomers like acryloyloxyethyltrimethyl ammonium chloride (DAC), 2-methacryloxyethyltrimethyl ammonium chloride (DMC), diallyl dimethyl ammonium chloride (DMDAAC), and quaternary ammonium monomers. While DMDAAC copolymerization with AM faces challenges in achieving high molecular weights due to steric hindrance, DAC and DMC exhibit better reactivity, enabling the production of high-molecular-weight polymers. A strong CPAM copolymer, PAMA, was synthesized by reacting AM with methacrylamido propyl trimethyl ammonium chloride (MAPTAC) under UV radiation]. However, these polymers are prone to shrinking, hydrolysis, and flocculation, with limited compatibility in high-pH environments.The CPAM copolymers face limitations in high-salinity and high-pH environments where polymer chain coiling or hydrolysis reduces flocculation efficiency. This challenge has also been observed in metal removal applications. For instance, Ageenko et al. investigated the influence of polyacrylamide-based flocculants on cadmium cementation from aqueous solutions and found that polymeric additives significantly affected reaction kinetics, morphology, and separation behavior, particularly under elevated ionic strength. Their findings underline the importance of designing CPAM-based systems that remain effective under high-salinity conditions similar to those in hydraulic fracturing wastewater. Nanotechnology offers innovative solutions for water treatment, including membrane functionalities and filtration processes for freshwater and seawater desalination. Nanoparticles have been used to enhance oil-water separation with CPAM. Fe3O4 nanoparticles, with magnetic properties, enable separation via magnetic fields and simplify treatment. Surface modification of Fe3O4 with KH570 enhances both magnetic separation and oil adsorption efficiency. Modified nano-SiO2, with its high surface area and adjustable properties, can be functionalized with organic compounds like 3-(methacryloyloxy) propyltrimethoxy silane (KH570) for targeted contaminant adsorption], providing additional oil removal beyond CPAM-induced flocculation .The DMC cationic monomer has a positively charged center beyond its double bond, enabling the production of high-molecular-weight copolymers. Cheng et al. synthesized the cationic copolymer P(AM-DMC) using AM and DMC through inverse emulsion polymerization. According to the literature, our research work is particularly significant because it introduces innovative cationic polyacrylamide (CPAM) system, synthesized by acrylamide (AM) and 2-methacryloxyethyltrimethyl ammonium chloride (DMC), modified with nano-SiO2 with KH570 silane agents, offering an advanced and efficient approach to Kaolin based oil/water separation. The incorporation of nano materials enhances the adsorption and charge neutralization capabilities, whereas KH570 improved the interaction of the polymer with suspended particles, leading to more effective flocculation at reduced dosages. By optimizing the polymer composition and concentration, this study presents a scalable solution for oilfield wastewater treatment, thereby mitigating the environmental footprint of hydraulic fracturing operations. The ability to achieve high separation efficiency while minimizing chemical consumption renders this study highly relevant for industrial applications, ensuring cleaner water discharge and promoting sustainable water management practices in the oil and gas sector.