BTO rapport - BTO 2020.059

Effect of iron(hydr)oxide formation on micropollutant removal in drinking water treatment


Both in oxic and in anoxic water iron redox reactions can occur. During these reactions reactive components are formed, either via an oxidative or a reductive pathway, depending on conditions. According to the oxidative pathway Fenton-like processes may occur, in which sunlight and surfaces play a crucial role. During these cycling processes reactive compounds like HO▪, HO2▪, ROO▪, Fe2+ and Fe3+ are formed. Although they are present in low concentrations (order of magnitude ng/L), cycles may occur 30 times per hour. Apart from chemical and physical parameters, also biological processes may be involved in the reactions that occur because of iron redox cycling in surface water and produce reactive compounds.
According to the reductive pathway, which occurs in anoxic groundwater, electrons are the main reactive species, and here too surfaces play a crucial role.
All processes mentioned above may be involved in the degradation of organic micropollutants. However, very limited information is available on the actual role of iron redox cycling on the degradation of OMPs, both under oxic and anoxic circumstances. For the Dutch reservoirs, the possible effect of Fenton-like reactions on the degradation potential of OMPs was estimated, assuming a light penetration depth of about 10 cm. It showed that depending on the local circumstances, the degradation may vary between 0 and 100 %, which is in accordance with practical findings, although it is not known whether iron redox cycling actually plays a role in this. And the important question here is whether the degradation of OMPs can be improved by adding additional iron.
However, it was concluded that it certainly would be worthwhile to further investigate this possibility, and establish the role of Fenton-like reactions on OMP concentrations, how FeCl3 or FeSO4 dosing may affect the iron concentration in water and the light penetration depth of the water.

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