Abstract
Abstract. Advanced oxidation processes, such as the Fenton's reagent, are powerful methods for decontamination of different environments from recalcitrant organics. In this work, the degradation of paraquat (PQ) pesticide was assessed (employing the commercial product gramoxone) directly inside the pipes of a pilot scale loop system; the effect of corroded cast iron pipe and loose deposits for catalysing the process was also evaluated. Results showed that complete degradation of paraquat ([PQ]0= 3.9 × 10−4 M, T = 20–30 °C, pH0 = 3, [H2O2]0 = 1.5 × 10−2 M and [Fe (II)] = 5.0 × 10−4 M,) was achieved within 8 h, either in lab scale or in the pilot loop. Complete PQ degradation was obtained at pH 3 whereas only 30% of PQ was degraded at pH 5 during 24 h. The installation of old cast iron segments with length from 0.5 to 14 m into PVC pipe loop system had a significant positive effect on degradation rate of PQ, even without addition of iron salt; the longer the iron pipes section, the faster was the pesticide degradation. Addition of loose deposits (mostly corrosion products composed of goethite, magnetite and a hydrated phase of FeO) also catalysed the Fenton reaction due to presence of iron in the deposits. Moreover, gradual addition of hydrogen peroxide improved gramoxone degradation and mineralization. This study showed for the first time that is possible to achieve complete degradation of pesticides in situ pipe water system and that deposits and corroded pipes catalyse oxidation of pesticides.
Highlights
Contamination of raw waters with pesticides is recognised as a problem in many countries
It was found that Paraquat degradation can be done in the pilot loop, achieving similar results to those obtained in a lab scale reactor, i.e., homogeneous Fenton’s reaction is an effective process in both scales for the pesticide degradation
Results showed complete paraquat degradation if initial pH was 3 and little degradation rate (30 % within 24 h) if initial pH was 5. It was for the first time shown that distribution system pipes can work as a catalyst for Fenton reaction providing complete paraquat degradation – the size of metallic pipes has a big influence in the oxidation process; the larger the pipe, the bigger is the contribution of the heterogeneous process
Summary
Contamination of raw waters with pesticides is recognised as a problem in many countries. Pesticides may pass water treatment plants and over long periods accumulate in water distribution pipes (Klamerth et al, 2010; Kralj et al, 2007; Sanches et al, 2010). During an accident or deliberate contamination large concentrations may enter the system. Due to sorption in biofilm or on the surfaces of the pipes their removal by network flushing is not efficient. Advanced oxidation processes (AOP) are wellknown for generating highly reactive and non-selective hydroxyl radical species, which are used to degrade (and mineralize into water, carbon dioxide and mineral salts) most of organics present in water and wastewater – cf Reactions (R1)
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