467 Int. J. Environ. Res., 9(2):467-474, Spring 2015 ISSN: 1735-6865 Received 25 June 2014; Revised 26 Aug. 2014; Accepted 23 Dec. 2014 *Corresponding author E-mail: jelena.molnar@dh.uns.ac.rs Groundwater Treatment using the Fenton process: Changes in Natural Organic Matter Characteristics and Arsenic Removal Molnar, J. * , Agbaba, J., Watson, M., Tubić, A., Kragulj, M., Maletić, S. and Dalmacija, B. University of Novi Sad Faculty of Sciences, Trg Dositeja Obradovića 3, 21000 Novi Sad, Serbia ABSTRACT: This work investigates the use of Fenton’s process (pH 5.5 and 6; 0.10-0.50 mM Fe(II); molar ratios Fe(II) to H 2 O 2 of 1:5-1:20) combined with flocculation for the treatment of groundwater which is rich in natural organic matter (NOM) (10.6±0.37 mg C/L) and arsenic (110±6.7 μg/L). Changes to the character of the residual NOM fractions were followed, as well as the removal efficiencies of NOM and arsenic. Under optimal reaction conditions at pH 5.5, 55% of the Dissolved Organic Carbon (DOC) was removed, whilst removals of specific disinfection by-product precursors of trihalomethanes (THM), haloacetic acids (HAA) and haloacetonitriles (HAN) were 80%, 75% and 98%, respectively. Total arsenic concentrations were reduced to below 5 μg/L under all investigated experimental conditions. After the Fenton treatment, the percentage of the hydrophobic NOM fraction (humic acid and fulvic acid fraction) decreased, and the percentage of the hydrophilic fraction (acidic and non-acidic fractions) increased. The hydrophilic fraction, which was the most abundant in the treated water, was also the most reactive fraction towards THM and HAA formation, whereas the residual fulvic acid fraction contributed the most to the formation of HAN. Key words: Natural organic matter, Arsenic, Water treatment, Fenton’s process, Disinfection by-products INTRODUCTION Natural Organic Matter (NOM) is present in all surface, ground and soil waters. The NOM present in drinking water sources plays an important role during drinking water treatment, influencing the efficiency of many water treatment processes, including the performance of unit processes (i.e. oxidation, coagulation and adsorption) and the application of Disinfectants. The role of NOM as a precursor to Disinfection By-Products (DBPs) formation is particularly significant. More than 600 DBPs have been identified in drinking water to date, and many have been found to be hazardous to human health, in particular showing carcinogenic effects. The chemistry associated with oxidation/disinfection in water is complex due to differences which exist in NOM structure, particularly the distribution of the hydrophobic and hydrophilic NOM fractions, as well as the presence of bromide/iodide, water alkalinity etc. (Von Gunten, 2003; Krasner et al., 2006; Matilainen et al., 2011; Molnar et al., 2013). Two classes of DBPs, the trihalomethanes (THMs) and the haloacetic acids (HAAs), are considered to be the most commonly found chlorinated DBPs in drinking water (Krasner et al., 2006; Bond et al., 2009; Sarathy and Mohseni, 2010). The Nitrogenous DBPs (N-DBPs) such as haloacetonitriles (HANs), halonitromethanes (HNMs) and haloacetamides also occur in drinking water in lower concentrations than the THMs and HAAs, but are all far more cytotoxic and genotoxic (Bond et al., 2012). Furthermore, NOM can play an important role in increasing the mobility of arsenic in aquifers, such that many groundwaters which are naturally contaminated with arsenic also contain high levels of NOM. NOM is a complex and variable mixture of organic compounds of biological and terrestrial origin, so it is therefore expected to show different reactivities for DBPs formation, depending upon its origin and structure. Knowledge of the identity of DBP precursors can allow the selection of appropriate processes for their removal (Bond et al ., 2009). Generally, the hydrophobic NOM fraction is thought to be the most important precursor for THM and HAA formation (Kitis et al., 2002; Liang and Singer, 2003; Tubić et al., 2013). In addition, some researchers have reported that the hydrophobic fraction is not always the primary source of THM precursors, with the hydrophilic/polar fraction bearing the highest amount of THM precursors of all the NOM fractions