Use of pipe deposits from water networks as novel catalysts in paraquat peroxidation

The present work intends to evaluate the possibility of using real pipe deposits as peroxidation catalysts (i.e., in the heterogeneous Fenton-like oxidation process) to degrade the pesticide paraquat in water. The valorisation of these loosely or firmly attached materials, often discarded after networks cleaning or maintenance, respectively, is envisaged in off-line waste water treatment applications. They may however be also applicable in situ, for instance in case of a contamination event of water networks. Four iron-containing real deposits, herein called S1, S2, S3 and S4, were considered. The results revealed that two of them (samples S1 and S2) exhibit good catalytic performance in the pesticide degradation and mineralization. Another one (sample S3) has goethite and also some metals in its composition that can decompose the hydrogen peroxide, compromising the peroxidation process. The calcium rich deposit, S4, is only useful as catalyst when the medium pH is adjusted to the required acidic values of Fenton's reaction. Even so, other deposits showed good performances even at neutral pH values, which is related with their low pH(pzc) values. It was found that depending on the solid nature, the catalytic performance can be predominantly due to the presence of the solid phase, but in some cases the homogenous process resulting from leached ions play an important role. With the deposit that exhibited the best performance, an exhaustive study of the pH effect was done, showing the stability of these materials at neutral conditions. (C) 2012 Elsevier B.V. All rights reserved.