Coagulation is a critical step in the water treatment process. As an important component of coagulation technology, the development and application of coagulants have always been a core focus of the industry. To address the challenges posed by complex water quality and improve the efficiency of coagulation, titanium-based coagulants are evolving from single-component formulations towards composite formulations. By optimizing synthesis conditions (such as molar ratio, alkalinity, reaction temperature, and reaction time) to alter the structure of single-component titanium-based coagulants, composite titanium salt coagulants can be prepared. These composite variants not only effectively overcome the limitations of single titanium salts and combine the coagulation characteristics of multiple reagents but also achieve efficient removal of various pollutants through synergistic effects between components. Consequently, composite titanium salt coagulants demonstrate superior coagulation performance.

This article provides a comprehensive review of the preparation, classification, application, current challenges, and future development strategies of composite titanium salt coagulants. It begins with an overview of current preparation methods, including slow alkali titration (SAT), electrodialysis (ED), stepwise/copolymerization methods, and the sol-gel method. Each technique has its unique characteristics in terms of control precision, product performance, and suitability for large-scale application. The SAT method is simple to operate, low-cost, and easily scalable, making it the most commonly used method for laboratory and industrial preparation of composite titanium salt coagulants. The ED method allows precise control over hydrolysis and polymerization processes, producing products with excellent performance; however, its higher cost and operational complexity have so far limited its application to laboratory and pilot-scale stages. Copolymerization/stepwise polymerization is suitable for preparing titanium salt-silicate composite coagulants with controlled structures, while copolymerization can produce titanium salt-metal salt composites with stronger synergistic effects. The sol-gel method can prepare dry gel coagulants that are convenient for storage and use, combining both coagulation mechanisms and adsorption. However, this technology is still in the laboratory research stage, and its cost and control techniques are key factors for future large-scale application.

Based on compositional differences, composite titanium salt coagulants can be classified into several types: titanium salt-metal salt, titanium salt-silicate, and titanium salt-organic polymer composite coagulants. Titanium salt-metal salt composite coagulants mainly include liquid or conventional composite titanium salt coagulants prepared by techniques such as SAT and ED, as well as dry gel-form composite titanium salt coagulants prepared by the sol-gel method. Both types of coagulants form titanium-containing bimetallic or multimetallic composite systems by combining titanium salts with metal salts such as aluminum, iron, and zirconium, thereby incorporating the coagulation advantages of multiple metals. Titanium salt-silicate composite coagulants are formed by copolymerizing/stepwise polymerizing titanium salts and polysilicic acid (PSiA). These coagulants combine the charge neutralization capacity of titanium salts with the adsorption and bridging ability of PSiA, significantly enhancing coagulation efficiency. Titanium salt-organic polymer composite coagulants are a category of composite titanium-based coagulants formed by combining titanium salts with organic polymer compounds (such as polyacrylamide, chitosan, starch, etc.). These coagulants integrate the highly efficient charge neutralization capacity of titanium salts with the adsorption and bridging capabilities of organic polymers, thereby significantly improving coagulation performance. Composite coagulants effectively overcome the problems associated with single titanium salts, such as significant pH fluctuations and poor storage stability. They demonstrate superior performance compared to traditional coagulants in specific areas, including ultrafiltration pretreatment, sludge conditioning, treatment of low-temperature and low-turbidity water, and removal of micropollutants. The sludge generated from their use can serve as a raw material for producing TiO₂, providing a new pathway for resource recovery in water treatment processes and highlighting their unique application prospects. However, their adaptability in real water bodies, economic feasibility, and long-term ecological safety still require systematic evaluation.

Finally, this study outlines the challenges faced in transitioning composite titanium salt coagulants from laboratory research tolarge-scale engineering applications and proposes corresponding strategies to address them.