Doğal organik maddenin (DOM) ana altgrubunu oluşturan hümik asitler (HA), temel olarak heterojen ve polidispers makromoleküllerden oluşmaktadır. Bu maddelerin özellikle dezenfeksiyon yan ürünü oluşturma gibi bazı olumsuz özellikler göstermeleri, içme suyu arıtımında karşılaşılan en önemli problem olarak değerlendirilmektedir. Bu çalışmada, humik maddelere ozon oksidasyonu, fotokatalitik oksidasyon ve bu iki oksidasyon sisteminin arka arkaya uygulanmasıyla geliştirilen ardışık oksidasyon sistemi uygulanmış, ardından da üç değerlikli katyonlarla (Al(III) ve Fe(III)) komplex oluşturma kapasiteleri değerlendirilmiştir. Oksidasyon sistemlerinin uygulanmasının humik asitlerin kompleks oluşturma verimlilikleri üzerine olan etkileri, UV görünür bölge parametrelerine (UV254 ve Renk436) ve ÇOK (çözünmüş organik karbon) içeriklerine göre normalize edilmiş Al(III) ve Fe(III) dozları şeklinde değerlendirilmiştir. Ham HA için her iki parametrede de 0.23 ve 0.93 gibi yüksek Al(III)/ÇOK ve Fe(III)/ÇOK oranlarına ulaşılmıştır. Bu dozlar, Al(III) için %95 UV254, %98 Renk436 ve Fe(III) için %94 UV254, %94 Renk436 giderim verimliliği sağlamıştır. Al(III) ve Fe(III) kullanarak fotokatalik olarak reaksiyona sokulmuş hümik asit için elde edilen giderim verimlilikleri, ozon oksidasyonu uygulanmış hümik asit çözeltilerinden daha yüksektir. Ham HA için olduğu gibi, ardışık oksidasyon sistemi için de ön oksidasyon uygulanmış formlara bağlı olarak, Fe(III)/ÇOK ve Al(III)/ÇOK oranlarında çok önemli değişimler elde edilmiştir. Ardışık olarak oksitlenmiş hümik asit numunesi için Al(III)/ÇOK aralığı 0.13 – 0.38 bulunmuş ve bu dozlar %27 - %64 UV254 ve %34 - %74 Renk436 giderimleri sağlamıştır. En düşük organik madde giderim verimliliği, başlangıçtaki hümik asit konsantrasyonundan bağımsız olarak, UV254 ve Renk436 cinsinden her iki üçdeğerli katyon içinde, ardışık olarak oksitlenmiş hümik asitte gözlemlenmiştir. Ham ve oksitlenmiş HA’in moleküler boyut dağıtım profillerinin etkisi de belirlenmiş ve parametrelerle açıklanmıştır. Anahtar Kelimeler: Hümik asit, ozon oksidasyonu, fotokatalitik oksidasyon, üçdeğerli katyonlar.Comprising of a continuum of simple organic molecules to highly polymeric complex organic compounds, natural organic matter (NOM) is introduced to aquatic environment from both allochthonous and autochthonous sources. Humic acids (HA) which constitute the major subgroup of natural organic matter (NOM) are mainly composed of heterogeneous and polydisperse macromolecules. The main significant concern related to the presence of humic material in drinking water reservoirs is its reactivity with oxidants during water treatment. Oxidative treatment of NOM is mainly achieved by the application of ozone as a pre- and/or post treatment agent to aid in other unit operations. Ozone attacks high molecular weight organic molecules, cleaving aromatic units and unsaturated double (Rekhow et. al., 1986). Heterogeneous photocatalysis using titanium dioxide as a semiconductor is one of the advanced oxidation processes that could be applied to drinking waters (Uyguner and Bekbolet, 2007). The degradation of the substrates proceeds through a free-radical mechanism utilizing reactive oxygen species (ROS) generated through the adsorption of light (E Ebg=3.2 eV). The main advantage of photocatalysis is the non-selective oxidation mechanism which is even capable of reaching to the total mineralization of the organic compounds. Application of a sequential oxidation system composed of ozonation followed by photocatalysis would also bring further substantial effects on the humic structure. Selective and nonselective oxidation mechanisms could result in a certain degree of mineralization bringing about a recalcitrant DOC (Dissolved organic carbon) fraction. For sequential oxidation, 50% reduction in Color436 was obtained by applying ozone and the preozonated humic acid solutions were then oxidized photocatalytically. Humic substances could bind to metal ions such as aluminum and iron through chelation between the carboxylic/phenolic groups of humic fractions and the central cation (Manahan, 2004). Upon addition of trivalent cations (Fe(III) and/or Al(III)), humic acid macromolecules undergo shrinkage and contraction leading to aggregate formation via intramo-lecular and intermolecular attractive forces. Considering the reactivities of the humic moieties towards the common trivalent cations during physico-chemical treatment of drinking water, different pre-oxidation conditions could exhibit various changes in humic acid structure leading to diverse reactivities towards the trivalent cations. The aim of this study was to evaluate the effects of the oxidative pre-treatment schemes applied by ozonation, TiO2 (Degussa P-25) mediated photocatalysis and sequential oxidation using ozonation followed by TiO2 mediated photocatalysis on the complexation efficiencies of humic acids with trivalent cations. The induced alterations in pre-oxidized humic acid solutions were assessed in relation to the changes in the UV-vis spectroscopic properties (UV254 and Color436) and DOC (Dissolved organic carbon). First order rate constants for the photocatalytic oxidation were 1.88×102 and 1.39× 102 min1 based on Color436 and UV254, respectively, under the specified experimental conditions (Bekbolet et al., 1998; Bekbolet et al., 2002). Jar tests were performed using appropriate concentrations of trivalent cations (alum, Al(III) and ferric chloride, Fe(III)) that were added to raw and treated humic acid solutions (pH=5.5). Sequential oxidation effect was found to be significantly different than both of the pre-oxidation systems for both of the trivalent cations. Higher Al(III)/DOC and Fe(III)/DOC ratios were attained for raw HA as 0.225 and 0.933 for both of the parameters. At this loading conditions 95% UV254 and 98% Color436 removals for Al(III) application and 94% removal for both of UV254 and Color436 was attained for Fe(III) application. However, Al(III)/DOC range was determined as 0.13 - 0.38 mg/mg presenting removal efficiencies as 27% - 64% for UV254 and 34% - 74% for Color436 for squentially oxidized humic acid sample respectively. In addition, for sequentially oxidized humic acid sample, 22% - 78% for UV254 and 33% - 80% for Color436 removal efficiencies were determined upon application of 1.30 - 2.59 mg/mg Fe(III)/DOC respectively. The reasons for these changes were attributed to the alterations in the molecular structure of humic acid due to ozonation and photocatalysis. Keywords: Humic acid, ozonation, photocatalytic oxidation, trivalent cations
