Hydroxyl radical yields in the Fenton process under various pH, ligand concentrations and hydrogen peroxide/Fe(II) ratios

•OH yield differs not much after 3 h of reaction in the pH range 1.5-5.•OH yield increases significantly after 24 h of reaction in the pH range 2.7-3.5.•Pyrophosphate forms a stable complex with Fe(III) and terminates the Fenton process.•Sulfate decreases the OH yield in the Fenton process.•At the low sulfate concentration applied the Fenton process is not terminated.

The Fenton process, one of several advanced oxidation processes, describes the reaction of Fe(II) with hydrogen peroxide. Fe(II) is oxidized to Fe(III) that reacts with hydrogen peroxide to Fe(II) and again initiates the Fenton reaction. In the course of the reactions reactive species, e.g. hydroxyl radicals, are formed. Conditions such as pH, ligand concentrations and the hydrogen peroxide/Fe(II) ratio may influence the OH radical yield. It could be shown that at pH < 2.7 and >3.5 the OH radical yield decreases significantly. Two ligands were investigated, pyrophosphate and sulfate. It was found that pyrophosphate forms a complex with Fe(III) that does not react with hydrogen peroxide and thus, the Fenton reaction is terminated and the OH radical yields do not further increase. The influence of sulfate is not as strong as that of pyrophosphate. The OH radical yield is decreased when sulfate is added but even at higher concentrations the Fenton reaction is not terminated.