Ferrate is believed to have a dual role in water treatment, both as oxidant and coagulant.
Few studies have considered the coagulation effect in detail, mainly because of the
difficulty of separating the oxidation and coagulation effects. This study aims to evaluate
the coagulation performance of ferrate together with its oxidation effect in water and
wastewater treatment processes, as it has not been studied thoroughly in previous work.
The study involves laboratory-based experiments that investigate the coagulation reaction
dynamically via a PDA instrument, between ferrate and a suspension of kaolin powder,
and humic acid solution at different doses and pH values, and comparing the observation
with the use of ferric chloride. The PDA output gives a comparative measure of the rate
of floc growth and the magnitude of floc formation.
The results of the tests with kaolin suspension show some similarities and significant
differences in the pattern of behaviour between ferrate and ferric chloride. Ferrate
demonstrated very similar coagulation characteristics to ferric chloride with regard to the
influence of pH and Fe dose. However, the magnitude of floc formation with ferrate was
always inferior to that with ferric chloride. The rate of floc growth with ferrate was
slower and sometimes less extensive than that with ferric chloride in most cases. It was
discovered that the reason for the inferior performance of ferrate was the slow formation
of Fe(III) species at neutral to high pH owing to the low degradation rate of ferrate in
aqueous solution at these pH values.
The interaction between HA and ferrate was extensively investigated in comparison to
ferric chloride in this study. The results reveal that ferrate achieved comparable or better
floc formation to ferric chloride over a much broader Fe dose range. However, the degree
of organics removal (DOC) was lower than ferric chloride (~5%). The results obtained
from model waters containing HA were consistent with those for samples of a natural
upland coloured water. Ferrate oxidation appears to increase the hydrophilic and
electronegative nature of the HA leading to an extended region of charge neutralization
and a consequent slightly lower HA removal compared to ferric chloride.