Investigating the parameters influencing floc formation

Flocculation, as the formation of insoluble organo-metallic complexes, has been studied considerably in the field of soil science, as it emerges e.g. from podzolization where a soil layer with reduced permeability is created; but also in the field of water treatment, as a means to remove natural organic matter (NOM). It is based on the process where positively charged metal species – Aluminium (Al), Iron (Fe) and Zirconium (Zr) in this study – react with the negatively charged surface of dissolved organic matter (DOM) and precipitate to form flocs. Since pH affects the surface charge of the organic matter (OM) particles and the form in which the coagulants are present in solution, controlling physico-chemical parameters, such as the pH, would significantly improve the flocculation process and help to enhance the efficiency of water purification strategies and soil permeability reduction from a geo-engineering perspective. It was shown that (1) there are ideal intervals in terms of metal to Carbon ratio (Me/C), in which the coagulants should be added to cover the surface of the DOM particles and these vary with metal type and pH; (2) at pH = 1, the OM forms flocs only with protons when mixed with Al, while with Fe some, and with Zr all, metal is associated with the solid phase; (3) Zr generally forms larger flocs than Al and Fe, even at acidic pH = 1, but also at higher concentrations; and (4) as the Me/C increases, the floc size does as well. • Floc formation differs between metal type with coagulant vs. organic matter dose, and pH. • At pH = 1, organic matter flocculates with H + when mixed with Al, but Fe and Zr (partially) associate with the solid phase. • Zr forms larger flocs than Al and Fe. • Floc size increases with increased coagulant dose.