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Aquatic Geochemistry (v.13, #4)
Batch Dissolution Kinetics: The Shrinking Sphere Model with Salts and Its Potential Application to Biogenic Silica by Victor W. Truesdale (pp. 267-287).
The full potential of batch dissolution experiments in geochemical and industrial applications has been hampered by the lack of an equation to describe the increase in dissolved solid concentration with time. This study provides new experimental results on the dissolution of salts and new equations, which describe dissolution according to the shrinking sphere model. Sieved salts were found to dissolve according to the shrinking sphere model while the dissolution of the parent material, raw (agglomerated) salt, fitted an exponential dissolution curve. The implications of this to the development of a systematic approach to batch dissolution, irrespective of the solid, is explored. Mathematical equations are derived for the dissolution of solids in under-saturated systems, which are much simpler than ones available, so far. In turn these provide easier comprehension of the workings of the shrinking sphere model. Finally, existing results for biogenic silica dissolution are reviewed in the light of the above-mentioned experimental and modelling advances. An earlier claim that shrinking sphere dissolution had been observed is refuted.
Keywords: Batch dissolution; Biogenic silica; Dissolution kinetics; Mineral dissolution; Salt dissolution; Silica cycling; Shrinking sphere model; Dissolution modelling
Arsenic Geochemistry of the Great Dismal Swamp, Virginia, USA: Possible Organic Matter Controls by Shama E. Haque; Jianwu Tang; William J. Bounds; David J. Burdige; Karen H. Johannesson (pp. 289-308).
Surface water samples for arsenic (As) concentration and speciation analysis were collected from organic matter-rich blackwaters of the Lake Drummond portion of the Great Dismal Swamp in southeastern Virginia, USA. Arsenic concentrations and speciation were determined by selective hydride generation, gas chromatography with photoionization detection. Surface waters from the Great Dismal Swamp are high in dissolved organic carbon (DOC) concentrations (445–9,600 μmol/kg) and of low pH (4.2–6.4). Total dissolved As concentrations [i.e., As(III) + As(V)], hereafter AsT, range from 2.2 nmol/kg to 21.4 nmol/kg. Arsenite, As(III), concentrations range from ∼1 nmol/kg to 17.7 nmol/kg, and As(V) ranges from ∼1 nmol/kg to 14.1 nmol/kg. Arsenate, As(V), is the predominant form of dissolved As in the inflow waters to the Great Dismal Swamp, whereas within the swamp proper arsenite, As(III), dominates. Arsenite accounts for 8–37% of AsT in inflow waters west of the Suffolk Scarp, and between 54% and 81% of AsT in Lake Drummond and Great Dismal Swamp waters east of the scarp. Arsenite is strongly correlated to DOC (r = 0.94) and inversely related to pH (r = −0.9), both at greater than the 99% confidence level. Arsenate is weakly related to pH and DOC (r = 0.4 and −0.37, respectively), and neither relationship is statistically significant. No statistical relationships exist between As(V) or As(III) and PO4 concentrations. The predominance of As(III) and its strong correlation with DOC in Great Dismal Swamp waters suggest that DOC may inhibit As(III) adsorption or form stable aqueous complexes with As(III) in these waters. Alternatively, phytoplankton and/or bacterially mediated reduction of As(V) may be important processes in the organic-rich blackwaters and/or sediment porewaters of the swamp, leading to the prevalence of As(III) in the water column.
Keywords: Arsenic; Chemical speciation; Organic matter; Great Dismal Swamp; Northwest River
Dissolution Kinetics of Dolomite in Water at Elevated Temperatures by Ronghua Zhang; Shumin Hu; Xuetong Zhang; Wenbin Yu (pp. 309-338).
Kinetic experiments of dolomite dissolution in water over a temperature range from 25 to 250°C were performed using a flow through packed bed reactor. Authors chose three different size fractions of dolomite samples: 18–35 mesh, 35–60 mesh, and 60–80 mesh. The dissolution rates of the three particle size samples of dolomite were measured. The dissolution rate values are changed with the variation of grain size of the sample. For the sample through 20–40 mesh, both the release rate of Ca and the release rate of Mg increase with increasing temperature until 200°C, then decrease with continued increasing temperature. Its maximum dissolution rate occurs at 200°C. The maximum dissolution rates for the sample through 40–60 mesh and 60–80 mesh happen at 100°C. Experimental results indicate that the dissolution of dolomite is incongruent in most cases. Dissolution of fresh dolomite was non-stoichiometric, the Ca/Mg ratio released to solution was greater than in the bulk solid, and the ratio increases with rising temperatures from 25 to 250°C. Observations on dolomite dissolution in water are presented as three parallel reactions, and each reaction occurs in consecutive steps as $$ hbox{CaMg}({{
m CO}_{3}})_{2} ({
m s})={{
m MgCO}_{3}}({
m s})+{{
m Ca}^{2+}}+{
m CO}_{3}^{2-} $$ $${
m MgCO_{3}}({
m s})={{
m Mg}^{2+}}+{{
m CO}_{3}}^{2-} $$ where the second part is a slow reaction, and also the reaction could occur as follows: $$ {{
m CaMg}({{
m CO}_{3}})_{2}}({
m s})+{{
m Mg}^{2+}}={{
m Ca}^{2+}}+{2{
m MgCO}_{3}}({
m s}) $$ The following rate equation was used to describe dolomite dissolution kinetics $$ {
m Rate}= Upsigma r_{ij}=Upsigma k_{ij}(a_{i})^{n} $$ where $$Upsigma{r}_{ij}$$ refers to one of each reaction among the above reactions; k ij is the rate constant for ith species in the jth reaction, a i stands for activity of ith aqueous species, n is the stoichimetric coefficience of ith species in the jth reaction, and define $$n=n_{ij}$$ . The experiments prove that dissolved Ca is a strong inhibitor for dolomite dissolution (release of Ca) in most cases. Dissolved Mg was found to be an inhibitor for dolomite dissolution at low temperatures. But dissolution rates of dolomite increase with increasing the concentration of dissolved Mg in the temperature range of 200–250°C for 20–40 mesh sample, and in the temperature range of 100–250°C for 40–80 mesh sample, whereas the Mg2+ ion adsorption on dolomite surface becomes progressively the step controlling reaction. The following rate equation is suitable to dolomite dissolutions at high temperatures from 200 to 250°C. $$ {-r_{
m Ca^{2+}}}= k (m_{
m Ca^{2+}})^{n}+k_{
m ad} ({{K}_{
m Mg^{2+}}} m_{
m Mg^{2+}}) /(1+ {K}_{
m Mg^{2+}} m_{
m Mg^{2+}} ) $$ where $$-{r}_{
m Ca^{2+}}$$ refers to dissolution rate (release of Ca), $$m_{
m Ca^{2+}}$$ and $$m_{
m Mg^{2+}}$$ are molar concentrations of dissolved Ca and Mg, k ad stands for adsorption reaction rate constant, K Mg refers to adsorption equilibrium constant.At 200°C for 40–60 mesh sample, the release rate of Ca can be described as: $$ {-r ({
m mol},{{
m m}^{-2}},{{
m s}^{-1}})}={0.55 imes10^{-4}}(m_{
m Ca})^{-0.36}+0.135 imes10^{-4}(m_{
m Mg})/(1+ 0.14,m_{
m Mg}) $$
Keywords: Dolomite; High temperature; Dissolution kinetics; Release rate; Adsorption reaction
Use of Pitzer Equations to Examine the Formation of Mercury(II) Hydroxide and Chloride Complexes in NaClO4 Media by Melchor González-Dávila; J. Magdalena Santana-Casiano; Frank J. Millero (pp. 339-355).
The thermodynamic stability constants for the hydrolysis and formation of mercury (Hg2+) chloride complexes $$ hbox{Hg}^{2+} + nhbox{H}_{2}hbox{O} = hbox{Hg(OH)}_n^{(2-n)+} + nhbox{H}^{+ } quadquad eta_n $$ $$ hbox{Hg}^{2+} + nhbox{Cl}^{-} = hbox{HgCl}_n^{(2-n)+ } quadquad eta_i $$ have been used to calculate the activity coefficients for Hg(OH) n (2–n)+ and HgCl n (2–n)+ complexes using the Pitzer specific interaction model. These values have been used to determine the Pitzer parameters for the hydroxide and chloro complexes $$(eta_{
m ML}^{(0)}, ,eta_{
m ML}^{(1)}$$ and C ML). The values of $$lambda_{ij}$$ and $$zeta_{ijk}$$ have been determined for the neutral complexes (Hg(OH)2 and HgCl2). The resultant parameters yield calculated values for the measured values of log $$eta^{ast}$$ to ±0.01 from I = 0.1 to 3 m at 25°C. Since the activity coefficients of $$hbox{Hg(OH)}_n^{(2-n)+}$$ and $$hbox{HgCl}_n^{(2-n)+}$$ are in reasonable agreement with the values for Pb(II), we have estimated the effect of temperature on the chloride constants for Hg(II) from 0 to 300°C and I = 0–6 m using the Pitzer parameters for $$hbox{PbCl}_n^{(2-n)+}$$ complexes. The resulting parameters can be used to examine the speciation of Hg(II) with Cl− in natural waters over a wide range of conditions.
Keywords: Mercury(II) hydroxide; Chloride complexes; NaClO4 ; Speciation; Activity coefficients; Pitzer equations