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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
Published for geochemistry community from Geochemical Society of Japan.

Filtration and exposure to benzalkonium chloride or sodium chloride to preserve water samples for dissolved inorganic carbon analysis

Hiroshi A. Takahashi, Hiroko Handa, Ayumi Sugiyama, Makoto Matsushita, Miyuki Kondo, Hiroyuki Kimura, Maki Tsujimura
Geochemical Journal, Vol. 53, No. , P. 305-318, 2019

ABSTRACT

The concentration and carbon isotopic ratio (δ13C) of dissolved inorganic carbon (DIC) in natural waters is often altered by microbial activity, even over short durations, which can affect sample preservation. Hence, the influence of these biogenic processes should be limited to properly investigate DIC in natural waters. This study investigates the effectiveness of filtration and exposure to benzalkonium chloride (BAC) and sodium chloride (NaCl) for preserving natural water samples (seawater, groundwater, river, pond, hot spring, and brackish waters) in order to analyze the concentration and δ13C of DIC. We assessed six filter materials with pore sizes of 0.2–5.0 μm. In the waters filtered by polyether sulfone, polyvinylidene fluoride, or cellulose acetate filters, the number of microorganisms still increased during preservation. However, the 0.45-μm polytetrafluoroethylene filter provided effective microbe removal, yielding constant DIC concentration and δ13C even after three weeks of preservation. Although none of the filters studied could offer complete microorganism removal and constant δ13C, biogenic DIC changes in the water were successfully reduced by the use of filters with pore sizes of 0.45 μm or below. Exposure to BAC or NaCl promoted microbial exclusion in the freshwater samples, with the DIC concentration and δ13C maintained during preservation over the one month. These effects were degraded in the seawater and brackish water samples, with the DIC concentrations and δ13C only constant for 10–15 days when exposed to BAC, and 1–3 days when exposed to NaCl. Although NaCl diminished biogenic activity in the freshwater samples, its influence on DIC concentration must be corrected for, which requires accurate knowledge of the NaCl concentration. We also applied the BAC solution in a field campaign in eastern Thailand and compared the DIC concentrations and δ13C values with those of samples treated with HgCl2. The results of the field trial showed that, except for brackish water from a rivulet in a mangrove forest and seawater samples, BAC-treated samples were not significantly different from HgCl2-treated samples. BAC can be considered an effective disinfectant for natural water samples, with less efficacy in seawater and brackish water.

KEYWORDS

carbon isotope of DIC, biological influence, filtration, benzalkonium chloride, sodium chloride

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