Geochemical Journal, Vol. 25, pp. 411 to 420, 1991 The speciation of cobalt in seawater and fresh waters at 25°C PUJING PAN' and NICHOLAS J. SUSAK2 Laboratory for Experimental Geochemistry and Modelling, Department of Geological Sciences, . McGill University, 3450 University Street, Montreal, Quebec', Canada 113A 2A7, and Department of Geology, University of New Brunswick, Fredericton, N.B.2 Canada E3B 5A3 (Received August 8, 1991; Accepted December 2, 1991) Cobalt(II) speciation in seawaterand fresh waters was calculated usingrecent spectrophotometrical ly determined stability data for cobalt(II) chloride and bromide complexes in aqueous solutions, and a theoretically estimated stability constant for the cobalt(II) bicarbonate complex. Calculations for seawater suggest that 56.25% of total dissolved cobalt(II) occurs as the free ion, Cot+, 39.53% as CoCl+, 2.76% as CoC12, 1.02% as CoSO4,and lessthan 1.0% as CoNH3+,CoCOO, CoHCO3 , CoCl3, CoF+ and CoBr+. In neutral to acidic fresh waters, 90% or more of the dissolvedcobalt(II) occurs as the free ion. CoCOO becomesimportant in basic fresh waters (pH > 8.5), and predominant in very basic fresh waters (pH > 9.5). Co(II) organic complexesare not included in the calculations, discussions sug gest that they are unlikely to be abundant in seawater. Co(II) organic complexes may, however, be im portant in fresh waters, and future work should investigatethis possibility. INTRODUCTION The speciation of a metal in natural waters is of primary significance, because it is the species of a metal which determine its chemical activity and toxicity. Although the concentration of cobalt in natural waters is very low (in seawater, about 1 x 10-10m, Ahrland, 1985; 6.87 X 10_9 m, Goldberg, 1980), as the principal constituent of Vitamin B12 and methymalonyl GA mutase in livers, cobalt is one of the essential trace elements for man and animals (Brooks, 1977). It is therefore important to understand the behavior of cobalt in the environment, especially in natural waters. Unfortunately, controversy exists over cobalt speciation in seawater. Horne (1965) suggests that the free ion, Co2+ and CoSO°a are the principal dissolved species of cobalt in seawater. The model of Turner et al. (1981) suggests that 88% of cobalt in seawater is pre sent as Co2+ and CoCI+ (see Table 1). Stumm and Brauner (1975) and Stumm and Morgan (1981) state that the principal cobalt species in seawater are the free ion, Co2+, and possibly, CoCO°. Ahrland (1985) and Byrne et al. (1988) made similar, but more complete inference for cobalt speciation in seawater (Table 1). Most of the above authors indicate that Co(II) is a fairly "hard" acceptor in the Pearson (1963) sense . Complexes of cobalt(II) with oxy-anions will be much more stable than with halides (excluding F-). Because sufficiently reliable stability con stants were not available at the time of these early studies, calculations of cobalt speciation in seawater were based mainly on theoretical estima tion. Because stability constants for Co(II) bicar bonate complexes are still not available, all the above speciation schemes of Co(II) in seawater do not take the bicarbonate complexes into ac count. Millero's (1974) review of trace elements such as Cu, Zn, Cd, Hg and Pb, which have been studied in more detail, indicates a few general points of agreement among authors: 1) only a small portion of trace metals exists in seawater as the free ion; 2) with few exceptions, chloride complexes are the dominant complexed species; 3) F-, Br and S02 4 are not important complex 411