Preparation of a Magnetic Polysilicate Composite Flocculant for Dye Wastewater Treatment

In this study, a magnetic composite flocculant containing FeO magnetic nanoparticles was prepared using sodium silicate, aluminium sulfate, titanium sulfate, and sodium alginate as raw materials. Then, the effects of preparation conditions and flocculation parameters on the adsorption of Congo red were investigated. Specifically, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, thermogravimetric analysis, scanning electron microscopy–energy-dispersive X-ray spectroscopy, transmission electron microscopy, and vibrating sample magnetometry were used to confirm the successful synthesis of the composite flocculant. One-way experiments revealed that a flocculant dosage of 40 mg/L yielded the highest Congo red removal rate of 95.43%. Compared with traditional flocculants, the flocs formed by the magnetic composite flocculant could settle faster; further, they could be magnetically separated for reuse, thereby reducing secondary pollution caused by the flocs and improving the utilisation rate. When the magnetic flocculant and flocs were mixed with dye wastewater in an appropriate ratio, recycling experiments showed that removal rates are still good, with no significant change in the adsorption capacity. The flocculation mechanism was dominated by chelation, adsorption bridging, net trapping, and sweeping effects, while charge neutralisation played a minor role in the process. The maximum adsorption of Congo red dye from aqueous solution by a magnetic iron tetraoxide–poly(aluminium–titanium silicate)–sodium alginate (SA) composite flocculant was calculated to be 1710.6 mg/g according to the Langmuir isotherm model. The negative value of ∆G° and positive values of ∆H° indicated spontaneity and heat absorption of the flocculation process.