Development and testing of a new protocol to preconcentrate natural organic matter in surface water

The term natural organic matter (NOM) is generally defined as all organic compounds (in dissolved and particulate forms), except synthetic molecules such as organic micropollutants, present in aquatic or terrestrial environments. Hydrophobic acids form the major fraction of aquatic NOM, constituting more than half of the dissolved organic carbon (DOC) in water. These hydrophobic acids are often described as: (i) humic substances containing humic acids (HA), which are soluble in alkali, but insoluble in acid, (ii) fulvic acids (FA), which are soluble in both alkali and acid, and (iii) humins, which are insoluble in both alkali and acid [1]. Humic substances may constitute 95% of the total dissolved organic matter in acquatic system. These play an important role in aquatic chemistry and their presence can cause various environmental and health problems. For example, they bind with heavy metal ions facilitating their transporting in the water system [2]. In the last years, thallium contamination in water aroused interest due to its high toxicity and significant accumulation in human body [3]. Dissolved thallium can be found in two oxidation states, Tl (l) and Tl (III). Although Tl(I) is predicted to be more thermodynamically stable than Tl(III), photo-oxidation reactions and microbial activity, combined with the formation of stable hydroxo-complexes, contribute to the persistence of Tl(III) in surface waters. In 2017, in Department Environmental and Health, Water Quality and Health Unit, Italian National Institute of Health, a multi sequential ultrasonic assisted extraction to determinate different thallium fraction on inner pipe surface was developed. Third step (extraction in oxidizing conditions) provides organic fraction release. ICP-MS analyses detect thallium contained in organic fraction. Moreover, the extent of Tl (III) complexation by natural organic matter (fulvic and humic acids) is unknown [4]. The aim of this study is to develop a procedure to concentrate all organic substances in surface water without fractionation that could interact with thallium. Because of the low concentration of dissolved organic matter (DOM) in surface water, the concentration procedure is needed. In this work, the first step was a bibliographic research on preconcentration techniques applied to NOM and the analysis of different techniques to understanding the performance in terms of recovery. Then, several techniques, in series, to concentrate most of DOM were tested. The pooled techniques are microfiltration, nanofiltration and reverse osmosis. Additionally, cation exchange extraction was applied to remove cation interference, before nanofiltration. The concentration procedure has been tested on a real case considering samples of surface water. To this end, 25 litres surface water were sampled by pumping system and microfiltration (20, 1, 0.45 µm) was applied in field. The permeate solution was collected in glass tanks. In laboratory, total organic carbon was determined. Then, the permeate was passed through columns in series containing cation exchange resin, to remove Ca2+, Mg2+, Fe2+ and other interfering cations. Nanofiltration and reverse osmosis experiment were carried out on polyamide wrapped spiral membrane. After each filtration, DOC of permeate and concentrate solutions were measured by TOC analyser to assess recovery: concentrate solution was passed through membrane not less than three times. The use of pooled techniques allows for concentrate most of dissolved organic matter and at the same time, it allows for decrease interfering species and it prevents the formation of membrane fouling. This approach is the necessary basis to study organo-Tl complexes and to develop a method to characterize of the organic component that interact with thallium.