| In the presence of sulfide, MoO{dollar}sb4sp{lcub}2-{rcub}{dollar} undergoes a series of reactions in which O is replaced by S. In order to predict the rate of formation of thiomolybdates in natural waters, the kinetics of the reaction between MoO{dollar}sb4sp{lcub}2-{rcub}{dollar} and H{dollar}sb2{dollar}S were investigated in aqueous solutions buffered with {dollar}rm NHsb4sp+/NHsb3{dollar} (pH = 8.3-8.5) and {dollar}rm Hsb2S/HSsp-{dollar} (pH 7.0) at {dollar}25spcirc{dollar}C, I = 0.7-2 M. Equilibrium constants were determined from UV-vis spectral data obtained during kinetic runs for reactions with the general form:{dollar}{dollar}rm MoOsb{lcub}x{rcub}Ssb{lcub}4-x{rcub}sp{lcub}2-{rcub}+Hsb2S sbsp{lcub}gets{rcub}{lcub}to{rcub} MoOsb{lcub}x-1{rcub}Ssb{lcub}5-x{rcub}sp{lcub}2-{rcub}+Hsb2O (x=1{lcub}-{rcub}4){dollar}{dollar}All reactions were found to be catalyzed by NH{dollar}sb4sp+.{dollar} Acid catalysis was not observed in H{dollar}sb2{dollar}S buffered systems. The uncatalyzed rate of reaction was investigated in unbuffered solutions (pH = 8.0-9.0) in both deionized and filtered Chesapeake Bay water. No differences were observed between the rates of formation in the natural water sample and those in deionized water. Rate constants for the formation of the di-, tri-, and tetra-thiomolybdate anions were calculated according to rate law: -{dollar}dlbrack{lcub}rm MoOsb{lcub}x{rcub}Ssb{lcub}4-x{rcub}{rcub}sp{lcub}2-{rcub}rbrack/dt = {lcub}Sigma ksb{lcub}rm HA{rcub}lbrack{lcub}rm HA{rcub}rbrack{rcub}lbrack{lcub}rm MoOsb{lcub}x{rcub}Ssb{lcub}4-x{rcub}{rcub}sp{lcub}2-{rcub}rbracklbrack{lcub}rm Hsb2S{rcub}rbrack{dollar} - {dollar}{lcub}Sigma krmsb{lcub}HA{rcub}lbrack HArbrack/Ksb{lcub}eq{rcub}{rcub}lbrack MoOsb{lcub}x-1{rcub}Ssb{lcub}5-x{rcub}sp{lcub}2-{rcub}rbrack,{dollar} where {dollar}(Sigma ksb{lcub}rm HA{rcub}lbrack{lcub}rm HA{rcub}rbrack = ksb{lcub}rm Hsb2O{rcub}+ksb{lcub}rm H+{rcub}lbrack{lcub}rm H{rcub}sp+rbrack +krmsb{lcub}NH4+{rcub}lbrack NHsb4sp+rbrack +...).{dollar}; The concentration of Mo in seawater is {dollar}{lcub}sim{rcub}10sp{lcub}-7{rcub}{dollar} M, large enough that thiomolybdates are potentially important ligands for transition metals. The solubilities of FeS (pyrrhotite) and two Cu-S mineral assemblages, chalcocite-djurleite and anilite-covellite, were measured as a function of {dollar}rmSigma Ssp{lcub}2-{rcub}, Sigma Mo,{dollar} and pH. Copper and iron concentrations in solutions containing thiomolybdates are enhanced by many orders of magnitude relative to similar solutions containing no Mo. The following constants were obtained for I = 0.7 M:{dollar}{dollar}eqalign{lcub}&rm 2FeS (s){lcub}+{rcub}2MoOsb2Ssb2sp{lcub}2-{rcub}sbsp{lcub}spgets{rcub}{lcub}sbto{rcub}lbrack (MoOsb2Ssb2)sb2Fesb2Ssb2rbracksp{lcub}4-{rcub} K = (1.81pm0.65){lcub}times{rcub}10sp{lcub}-3{rcub}cr&rm 0.5Cusb2S+MoSsb4sp{lcub}2-{rcub}+ 0.5Hsp+sbsp{lcub}spgets{rcub}{lcub}sbto{rcub}CuMoSsb4sp- + 0.5HSsp- K = 295pm201cr&rm NHsb4CuMoSsb4 (s)sbsp{lcub}spgets{rcub}{lcub}sbto{rcub}NHsb4sp+ + CuMoSsb4sp- Ksb{lcub}sp{rcub} = (3.45pm0.69)times10sp{lcub}-7{rcub}cr{rcub}{dollar}{dollar}Rate constants for the formation of thiomolybdates and stability constants for their complexes with Fe and Cu are used to predict the speciation of Mo under a variety of naturally occurring conditions. |
