A comparison of the coagulation performance of PAFC and FeSO₄ for the treatment of leach liquor from Stevia processing

Stevioside, mainly extracted from Stevia rebaudiana, is an important non-caloric additive for food, beverages and flavoring, and an alternative natural sweetener for people with diabetes and obesity. Pretreatment of Stevia rebaudiana, involving coagulation and filtration, is used to remove impurities from the leach liquor of stevia. Commercially, ferrous sulfate (FeSO₄) is commonly used as a coagulant in the factory, but large amounts of soluble Fe(Ⅱ)-protein complexes remain in the filtrate, which is potentially of health concern. In this study, the removal of impurities (particles and organic compounds) in leach liquor of stevia by polymeric aluminium ferric chloride (PAFC) was compared to FeSO₄, through simulated coagulation and membrane filtration processes. The results showed that PAFC has a superior coagulation efficiency to FeSO₄, achieving a very high removal of the impurities in the leaching solution without complexation, and which was not affected by the solution pH. In the subsequent membrane filtration, the flux was found to be influenced (reduced) by the interaction between organic matter (e.g. proteins) and nanoparticles in the cake layer, which formed a dense and compact structure with a lower permeability. Although using FeSO₄ had a greater membrane flux, the results have shown significant advantages in using PAFC as an alternative coagulant to FeSO₄ in the pre-treatment of leach liquor from Stevia processing. The higher treatment performance and acceptable membrane flux could be obtained by adjusting the pH of the leaching solution to 7 (addition of CaO), at the optimal PAFC dose (5000 mg/L).