Iron hydroxides are produced at various places during the production of drinking water. That results in the formation and breakdown of reactive oxygen compounds. There are indications that iron removal in the subsurface degrades organic microcontaminants. This study aims to determine whether that process is also associated with iron removal in surface water or treatment processes.
Iron hydroxides in drinking water production
Iron hydroxides are produced at various places in the production of drinking water during the treatment/pre-treatment of groundwater or surface water above or below the surface. In reservoirs, this is sometimes done by dosing iron and therefore removing phosphates from the surface water. An example of this can be found in the Afgedamde Maas river, from which Dunea extracts water before taking it to the dunes. Since 1976, the water company has been removing phosphates here by adding iron (II) sulphate. Another example is the De Lange Vlieter reservoir, where the Limburg Water Supply Company (WML) recently started to remove phosphates from the surface water using iron (III) chloride. Iron removal both above and below the surface is a feature of almost all groundwater extraction systems. The dissolved Fe(II) oxidises to Fe(III), which then precipitates in the form of iron hydroxides. Iron is also used in the treatment/post-treatment of water from bank filtration systems and dune water extraction.
Iron is a redox-sensitive element that – depending on the biogeochemical conditions – can undergo cyclic oxidation from Fe(II) to Fe(III) and reduce from Fe(III) to Fe(II). During these redox reactions and the associated solution and precipitation reactions of iron hydroxides, reactive oxygen species (ROS) are formed and broken down. These include, for example, the superoxide anion radical (O2-), hydrogen peroxide (H2O2) and the hydroxyl radical (OH·). An example of a process in which ROS are produced and broken down is the oxidation of Fe(II) with oxygen to Fe(III). This also happens during the reduction of Fe(III) with dissolved organic matter (DOM) under the influence of light. Reactive oxygen species can oxidise organic molecules such as DOM but they can also degrade organic microcontaminants. An example of a process in which reactive oxygen species are deliberately produced for this purpose is the Fenton process, which is used in water treatment. Water treatment based on the Fenton process doses hydrogen peroxide in addition to iron. The Fenton reaction (the oxidation of Fe(II) to Fe(III) with hydrogen peroxide involving the formation of a hydroxyl radical) also occurs without the addition of hydrogen peroxide because hydrogen peroxide is also formed during the oxidation of Fe(II) with oxygen to Fe(III). Of course, the ROS concentrations in that case are much lower than in water treatment based on the Fenton process. In addition, previous research looking at underground treatment has also shown that enrichment with iron hydroxides can further the removal of some organic microcontaminants that are relevant for drinking water production.
Effects of iron dosing on organic microcontaminants
The formation of iron hydroxides in drinking water production by the application of iron (III) chloride or iron (II) sulphate and aeration of groundwater can therefore (unintentionally) have a beneficial effect on the removal of organic microcontaminants. The cyclic reduction of iron hydroxides with DOM and the oxidation of Fe(II) in varying redox conditions can also further this process. Observations of organic microcontaminants in the Afgedamde Maas have identified degradation processes but it is unclear whether these are related to the iron (II) sulphate dosage.
The research is quite fundamental in nature and the aim is to map out the potential effects of iron (II) oxidation and iron hydroxide precipitation on the removal of organic microcontaminants during drinking water production. The results can be applied to use iron more specifically for this purpose during pre-treatment or possibly to make adjustments. Examples are improvements in subsurface treatment by using iron removal or the pretreatment of surface water in reservoirs.
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