Influence of ageing on the catalytic activity of ferric sludge for oxidation of Fe(II)

The catalytic effect of freshly formed or added ferric iron on the Fe(II) o xidation has been demonstrated by previous studies. High Fe(III) concentrat ions significantly accelerate the oxygenation rate. High Fe(III) concentrat ions can be maintained only by sludge recycle which eventually leads to slu dge ages as high as 10 days. Therefore, the aim of this study was to determ ine whether ageing of ferric hydroxide sludge affects its catalytic effect on the oxidation of ferrous iron by aeration. In order to reach this aim four different groups of experiments were carrie d out using, ferric hydroxide sludge aged for a period of 0 to 10 days. Ini tial Fe(III) concentrations were varied within the range of 50 mg/l to 200 mg/l. It has been demonstrated that the catalytic effect of ferric hydroxid e sludge on the oxidation of ferrous iron by aeration increases with increa sing sludge age contrary to what is commonly expected. It has been conclude d that, catalytic oxidation rate constant k(cat) obtained from the batch sy stems can safely be used in design of continuous flow iron oxidation reacto rs with sludge recycle. (C) 1998 Published by Elsevier Science Ltd. All rig hts reserved.