Freezing-induced acidification of sea ice brine.

• Published in: Science of The Total Environment
• Main Authors: Veselý, Lukáš, Štůsek, Radim, Mikula, Ondřej, Yang, Xin, Heger, Dominik
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier Science 2024
• Subjects: Acidification; Brine; Buffering capacity; Fractional crystallization; Sea ice
• Online Access: https://doi.org/10.1016/j.scitotenv.2024.174194; https://pubmed.ncbi.nlm.nih.gov/38925394

The acidity of sea ice and snow plays a key role in the chemistry of the cryosphere; an important example lies in the photochemical catalytic release of reactive bromine in polar regions, facilitated at pHs below 6.5. We apply in-situ acid-base indicators to probe the microscopic acidity of the brine within the ice matrix in artificial sea water at a range of concentrations (0.35-70 PPT) and initial pHs (6-9). The results are supported by analogous measurements of the most abundant salts in seawater: NaCl, Na2SO4, and CaCO3. In the research herein, the acidity is expressed in terms of the Hammett acidity function, H2-. The obtained results show a pronounced acidity increase in sea water after freezing at -15 °C and during the subsequent cooling down to -50 °C. Importantly, we did not observe any significant hysteresis; the values of acidity upon warming markedly resembled those at the corresponding temperatures at cooling. The acidity increase is attributed to the minerals' crystallization, which is accompanied by a loss of the buffering capacity. Our observations show that lower salinity sea water samples (≤ 3.5 PPT) reach pH values below 6.5 at the temperature of -15 °C, whereas higher salinity ices attain such values only at -30 °C. The ensuing implications for polar chemistry and the relevance to the field measurements are discussed.