The effect of pressure on transition metals in seawater

Abstract

The effect of pressure ( P) on the activity coefficients ( γ i P / γ i 0) of ions ( i) in seawater can be estimated from the partial molal volumes and compressibilities of the ions in water ( V ¯ 0 and κ ¯ 0 ) and seawater ( V ¯ ⁎ and κ ¯ ⁎ ) ln ( γ P / γ 0 ) = ( V ¯ ⁎ − V ¯ 0 ) P / R T − 0. 5 ( κ ¯ ⁎ − κ ¯ 0 ) P 2 / R T where R and T have their normal meaning. One can also use the partial molal volumes and compressibilities of ions to estimate the effect of pressure ( K P / K 0) on ionic equilibria constants M 2 + + X 2 − = MX , K = [ MX ] / [ M 2 + ] [ X 2 − ] from the changes in the volume for the chemical reaction in seawater Δ V ⁎ = V ¯ ⁎ ( MX ) − V ¯ ⁎ ( M 2 + ) − V ¯ ⁎ ( X 2 − ) , Δ κ ⁎ = κ ¯ ⁎ ( MX ) − κ ¯ ⁎ ( M 2 + ) − κ ¯ ⁎ ( X 2 − ) using ln ( K ⁎ P / K ⁎ 0 ) = ( Δ V ⁎ / R T ) P − 0. 5 ( Δ κ ⁎ / R T ) P 2 The densities and sound speeds of a number of transition metal chlorides have been determined in 0.725 m NaCl solutions (the ionic strength of average seawater, S∼35) at 25 °C. These results have been used to determine the partial molal volumes and compressibilities of transition metal ions (Mn 2+, Co 2+, Ni 2+, Cu 2+, Zn 2+ and Cd 2+). These results and our earlier measurements (Mg 2+, Ca 2+, Sr 2+ and Ba 2+) were used to develop correlations between the values in water and NaCl. These correlations were used to predict the values of other divalent ions in water and NaCl (Be 2+, Pb 2+, Hg 2+ and Fe 2+) that were difficult to directly measure. These studies allow one to make estimates of the effect of pressure on the activity coefficients of these ions in seawater and ionic equilibria in the deep oceans. The lower stability constants will result in increased concentrations of free transition metals in the deep ocean.