In this study, the importance of charge interactions during flocculation of Fe3+ in the presence of particles and anions/cations at various pH values was investigated. SiO2, (s) and ZnO(s) were dosed as particles to promote charge interactions and/or serve as a nucleus to accelerate floc formation. In the pH range 6–9, SiO2, (s) is negatively charged, while ZnO(s) carries a positive charge. Ca2+ and HPO42− were selected to investigate charge interactions in the water phase. A significant delay in floc growth due to charge repulsion between negatively charged iron species was observed at pHini 9. For positively charged species at pHini 6, a delay in floc growth was observed as well, but to a lesser degree. These effects could be neutralized by either dosing (positively charged) ZnO(s) or Ca2+ at pHini 9, or (negatively charged) SiO2, (s) at pHini 6. The addition of phosphate did not hinder floc growth at pHini 6. While phosphate completely inhibited floc growth at pHini 7–9 in the presence of negatively charged SiO2, (s), the presence of positively charged ZnO(s) partly neutralized the detrimental influence of phosphate on floc growth. Similarly, dosing Ca2+ partly neutralized the effect of phosphate.

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