Influence of the apparent molecular size of humic substances on the efficiency of coagulation using Fenton's reagent

Abstract

This work used Fenton's reagent as a coagulating agent in the treatment of water samples with high true colour caused by humic substances (HS) extracted from peat. In addition, the effects of the apparent molecular size of HS on coagulation, flocculation and flotation were studied. To that end, four distinct water samples having the same true colour were prepared using HS with different molecular sizes, which were obtained by ultrafiltration fractioning. Through optimisation of coagulant dosage and coagulation pH, as well as posterior construction of coagulation diagrams for each water sample, it was verified that the sample prepared with the smallest apparent molecular size of HS was the most difficult to treat, requiring higher coagulant (Fenton's reagent) dosages than samples prepared with larger HS molecular sizes. Furthermore, filtration experiments after dissolved air flotation (DAF) were carried out in an attempt to simulate conventional treatment. The most representative results in filtered water were: apparent colour ≤ 3 HU; turbidity < 0.5 NTU; and residual iron concentration < 0.005 mg/L. The absorbance and total organic carbon values of filtered water samples were also very low, suggesting that the formation of disinfection by-products with chlorine would likely be insignificant.