The effect of pH and dissolved inorganic carbon on the properties of iron colloidal suspensions

Discoloured water resulting from suspended iron particles is a relatively common drinking water consumer complaint. These particles result from the oxygenation of Fe(II), and this study shows that pH and dissolved inorganic carbon (DIC) have important effects on their properties. Bench scale tests were conducted at a single oxygen concentration over a broad pH range. Increasing the DIC concentration increased the turbidity and apparent colour of a fixed concentration of iron suspension below pH∼8.7. Inorganic carbon was incorporated into the particle structure at these pH values. Above pH∼8.7 and in higher DIC waters, an intermediate green solid that contained Fe(II), Fe(III) and inorganic carbon formed for a brief period prior to complete oxidation to an Fe(III) solid. The green solid apparently was green rust, Fe₄^IIFe₂^III(OH)₁₂CO₃. After complete oxidation of the intermediate solid to an Fe(III) solid, the iron particles did not include measurable amounts of inorganic carbon and appeared to maintain the physical structure of the green rust. Above pH∼8.7, DIC did not affect suspension colour and turbidity and inorganic carbon was not incorporated in the particle structure. The colour and turbidity of iron suspensions were largely related to particle size distribution.