Humic acid modiﬁed Fenton reagent for enhancement of the working pH range A. Georgi a, * , A. Schierz a , U. Trommler a , C.P. Horwitz b , T.J. Collins b , F.-D. Kopinke a a UFZ-Centre for Environmental Research Leipzig-Halle, Department of Environmental Technology, Permoserstrasse 15, D-04318 Leipzig, Germany b Department of Chemistry, Carnegie Mellon University, 4400 5th Avenue, Pittsburgh, PA 15213-3890, USA Received 12 July 2006; received in revised form 10 October 2006; accepted 16 October 2006 Available online 13 November 2006 Abstract The suitability of the Fenton process for the remediation of soil and groundwater is limited by the necessity to acidify the reaction medium. This study examines the applicability of humic acid (HA) as an iron chelator in a modiﬁed Fenton system with the aim of extending the optimum pH range for this process towards neutral conditions. Addition of HA at a concentration of 50–100 mg L 1 greatly enhances the rate of oxidation of organic compounds in a catalytic Fenton system in the range of pH 5–7. Similar rates at pH 5 in the presence of HA can be achieved as at pH 3 for a typical Fenton process in the absence of HA (k 0 =9  10 3 min 1 for benzene degradation at c H 2 O 2 ¼ 0:13 M). A comparison of the relative reactivities of various model compounds supported the hypothesis that OH  radicals are the main reactive species in the HA-modiﬁed Fenton system. In contrast, however, another type of chelated Fe-catalyst (Fe-TAML) proved to be more selective than expected for OH  radicals. A long- term study revealed that the HA itself is oxidized and thereby loses its ability to enhance the degradation of the pollutant molecules. Therefore, the HA-modiﬁed Fenton system is effective for degrading pollutants which are at least as reactive towards OH  radicals as the HA itself, such as BTEX, phenols or PAHs. The results obtained indicate that the HA-modiﬁed Fenton system is also applicable for compounds with a high sorption tendency towards HA. # 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen peroxide; Humic substances; Fenton; Iron chelators; AOP 1. Introduction Oxidation processes that involve the production of the highly reactive hydroxyl radical (OH  ) are of current interest for the destruction of organic pollutants in surface and groundwaters as well as industrial wastewaters. The Fenton reagent, consisting of H 2 O 2 and ferrous iron, has been shown to be effective in the degradation of a wide spectrum of organic and inorganic pollutants [1,2]. The hypothesis of Haber and Weiss [3] that the Fenton reaction involves the formation of OH  radicals as the actual oxidants (Eq. (1)) is widely accepted. However, the participation of high-valent iron-oxo species has also been discussed, especially for systems where the iron is complexed [4,5]. The general mechanism for a free radical chain involves the following key steps: H 2 O 2 þ FeðIIÞ! FeðIIIÞþ HO  þ OH  (1) H 2 O 2 þ FeðIIIÞ! FeðIIÞþ HO 2  þ H þ (2) OH  þ H 2 O 2 ! HO 2  þ H 2 O (3) OH  þ FeðIIÞ! FeðIIIÞþ OH  (4) FeðIIIÞþ HO 2  ! FeðIIÞþ O 2 þ H þ (5) FeðIIÞþ HO 2  þ H þ ! FeðIIIÞþ H 2 O 2 (6) HO 2  þ HO 2  ! H 2 O 2 þ O 2 (7) Fe(II) and Fe(III) are representative for all species present in solution in each respective oxidation state. The hydroxyl radical formed in reaction (1) can react with organic com- pounds by H abstraction or addition to alkenes, alkynes or aromatic rings. In technical applications, iron is usually used in much lower concentration than hydrogen peroxide (catalytic www.elsevier.com/locate/apcatb Applied Catalysis B: Environmental 72 (2007) 26–36 * Corresponding author. Tel.: +49 341 2352419; fax: +49 341 2352492. E-mail address: anett.georgi@ufz.de (A. Georgi). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.10.009