Changes in iron(II) and iron(III) content in a solution of humic acids during coagulation by means of monomeric iron(III) salts

This study discusses the results of laboratory analyses investigating the chemical coagulation of a model of humic acid solution with the use of monomeric iron(III) salts – chloride and sulfate – in a standard coagulation test environment. The objective of the study was to determine changes in iron compound concentrations in the tested solution as a result of coagulation. The following parameters were analyzed in solution samples: changes in COD, colour, turbidity, suspended solids, pH, and streaming potential in relation to the applied coagulant dose. The content of total iron (Fetotal) and iron(II) was determined in solution samples after coagulation. The lowest Fetotal concentrations were observed following the use of optimal coagulant doses, and higher doses led to a repeated increase in total iron levels. In purified solution samples, iron(III) was partially reduced to iron(II) with an 8-42% share of Fetotal, depending on salt type and the applied dose.