The electrokinetic properties of gamma-irradiated Cryptosporidium oocysts in the presence of
coagulants (
ferric chloride and
alum) and
coagulant aids (DADMAC based
cationic polyelectrolytes) have been studied. The zeta potential of the oocysts was unaffected by the addition of
ferric chloride at all pH values (3-10) studied. Addition of
alum resulted in reversal of the oocysts charge, which suggests that the initial stage in the coagulation process leading to floc formation proceeds via the adsorption of hydrolysed
aluminium species. The cationic
polyelectrolyte Magnafloc LT35 was adsorbed onto
iron flocs at doses of 0.1 mg/L even against an electrostatic barrier. The cationic
polyelectrolyte only adsorbed and caused charge reversal at the oocyst surface at around 0.4 mg/L, suggesting a lower affinity for this surface. These results indicate that the oocysts, unlike inorganic colloidal materials such as
metal oxides, appear to possess a lower surface density of active or charged sites. The lower density of sites, combined with the rapid precipitation of
iron salts, may be responsible for the lack of specific adsorption of either hydroxylated ferric species or primary
iron hydroxide particles on the oocysts. Further, this suggests that a process of sweep flocculation, where oocysts are engulfed in flocs during coagulation and floc formation, is the more likely mechanism involved. By comparison, it is likely that the specific interaction of hydrolysed
aluminium species with the oocysts surface would result in a stronger link at the oocyst-floc interface and that the flocculation process may initially proceed via charge neutralisation.