Experimental determination of nitrogen isotope fractionation associated with NH3 degassing at 0-70 ºC

Abstract

Ammonia degassing is a common process in natural alkaline water systems (e.g., hydrothermal vents). Nitrogen isotope fractionation factor is an important parameter to quantitatively assess the nitrogen cycle in these systems, but still not constrained yet. In this study, we carried out laboratory experiments to examine the nitrogen isotope behavior during partial degassing of ammonia from an ammonium sulfate solution. The experiments started with ammonium sulfate solution with excess sodium hydroxide. The reaction can be described as: NH4+ + OH- (excess) → NH3•nH2O → NH3 (g)↑. Nitrogen isotopic ratios were analyzed on remaining ammonium. Two sets of experiments, one under static conditions and the other with N2 gas bubbling, were carried out at 2, 21, 50, and 70 ºC. The results indicate that the data from the bubbling experiments fit well with a Rayleigh distillation model, suggest that a kinetic isotope fractionation occurred during partial degassing of ammonia. Modeling results gave a fractionation factor of 0.9898 at 2 °C, 0.9918 at room temperature (~21 °C), 0.9935 at 50 °C and 0.9948 at 70 °C. These isotope fractionation factors increase with temperature. A linear fitting yields a relationship between nitrogen isotope fractionation factor and temperature as 103lnɑNH3 (g)-NH3 (aq) = 14.6 – 6.8 ·1000/T . On the other hand, nitrogen isotopic data from experiments under static conditions do not fit either equilibrium isotope fractionation (the batch model) or kinetic isotope fractionation (the Rayleigh distillation model). One possible cause is the back dissolution of the degassed ammonia, which is likely associated with another unconstrained isotopic effect. These experimental results provide important insights into the understanding of the alkaline system in the field.