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Aquatic Geochemistry (v.11, #2)
Fe(II)-Na(I)-Ca(II) Cation Exchange on Montmorillonite in Chloride Medium: Evidence for Preferential Clay Adsorption of Chloride – Metal Ion Pairs in Seawater by Laurent Charlet; Christophe Tournassat (pp. 115-137).
Fe(II)–Ca(II), Fe(II)–Na(I), and Fe(II)–Ca(II)–Na(I) exchange experiments on montmorillonite were performed in chloride background. These experiments show the possible sorption of Fe2+ and FeCl+ ion pairs in exchange site positions, a result confirmed with 77 K 57Fe Mössbauer experiments. The sorption data were modeled and the cation exchange selectivity for Fe(II) were found to be nearly equal to that of Ca(II). Vanselow selectivity coefficients, for Na–Fe2+ and Na–FeCl+ reactions, were found to be equal to 0.4 (0.5 for Ca2+) and 2.3 (2.5 for CaCl+) respectively. High affinity of montmorillonite for chloride ion pairs seems to be a common mechanism as first stated by Sposito et al., (Soil Sci. Soc. Am. J. 47, 51–56, 1983a), and should have implications e.g., on the chemistry of suspended particles in seawater. Exchange selectivity coefficients derived from this study and others were used to model experimental data on river water and seawater equilibrated particles. The agreement between simulations and experimental data is very good. The simulation shows the predominance of monovalent ion (Na+ and chloride ion pairs) sorption on clay particles in seawater. This sorption of monovalent ions leads to the dispersion of particles in seawater and to the extension of a “plume” of particles spreading away from river deltas, such as that of the River Amazon.
Keywords: cation exchange; ion pair; montmorillonite; clay; iron; seawater; river
Total Dynamics of Quartz–Water System at Ambient Conditions by JIŘÍ FAIMON (pp. 139-172).
New data on the dissolution and growth of quartz in acid, nearly neutral and slightly alkaline solutions at ambient conditions are presented. During batch dissolutions, aqueous Si-concentrations increased nonlinearly towards limit values. During parallel growth experiments, however, the high Si-concentrations corresponding to 13-fold supersaturation with respect to quartz did not decreased. To interpret these results, two dynamic models (1) a reaction model and (2) a surface-complexation model, were derived. Both models were focused on (a) reversible dissolution/growth of “bulk quartz” and b) irreversible dissolution of “disturbed quartz” (weakly bounded Si-atoms) on “new surface”. Based on the reaction model, the differential rate law, $${{dn_{{
m Si(aq)}}}over{dt}}=left(p+{{r}over {a_{{
m H}^+}}}
ight)+left(u-p- {{r}over {a_{{
m H}^+}}}
ight){{n_{{
m Q_D}}}over {sumlimits_i{n_{{
m Qi}}}}}-{{1}over {V}}left(q+ {{s}over {a_{{
m H}^+}}}
ight)n_{{
m Si(aq)}},$$ was derived, where dnSi(aq)/dt is the overall Si-flux into solution [mol s−1], $$a_{{
m H}^+}$$ is H+ ion activity, nQD is the instantaneous content of disturbed quartz (weakly bounded Si-atoms onto quartz surface) [mol], $$sumlimits_ in_{{
m Qi}}$$ is the total content of Si-atoms on quartz surface [mol], V is the volume of solution [L], and nSi(aq) is the content of silicic acid in solution. The rate law parameters are $$p=k_1{A}a_{
m w}^2 $$ , q=−k2 {A}γSi, r=k3 Kw{A}a_w, s=−k4 KSi{A} γSi, and $$u = k_5{A}a_{
m w}^2$$ , where k1 and k2 are the rate constants for the dissolution and growth of bulk quartz in pure water, and k3 and k4 are the rate constants for the reaction of bulk quartz with hydroxyls. k5 is the rate constant for the dissolution of disturbed quartz. KSi and Kw are the dissociation constants of silicic acid and water, respectively. A is surface area [m2], aw is water activity, and γSi is activity coefficient of silicic acid. The values of the rate constant were determined as follows: k1=(3.0 ±1.0) × 10−14, k2 < 1.62 × 10−11, k3=(1.41 ± 0.09) × 10−7, k4=(5.26 ± 0.62) × 10−9, and k5=(1.71 ± 0.38) × 10−12, all in the units of mol m−2 s−1. In the acid, neutral, and alkaline solutions, respectively, the initial contents of disturbed quartz, n0QD, were found to be (3.42 ± 0.53) × 10−5, (2.63 ± 0.53) × 10−5, and (3.94 ± 0.50) × 10−5 mol per 1.66 mol (i.e., 100 g) of quartz samples (grain fraction of 71–150 μm in diameter). It relates to 23, 18, and 27 of total surface area (11 m2)Based on the surface-complexation model, the alternative differential rate law, $$eqalign{{{dn_{
m Si(aq)}}over {dt}} ={A}(k_{
m d(s)} a_{
m w}{{n_{
m s}}over {n}}+k_{{
m d}(0)}a_{
m w}^
u heta_{(0)}+k_{{
m d}(-)} a_{
m w}^
u heta _{(-)}+ cr quad+k_{
m d(Na)}a_{
m w}^
u heta_{({
m Na})}+ k_{{
m d}(+)}a_{
m w}^
u heta _{(+)}-sum_i{k_{{
m g(i)}}}gamma _{
m Si}{{n_{
m Si(aq)}} over V}),}$$ was derived, where A is total surface area [m2], aw is water activity, ns is the content of single bounded surface complex, –Si(OH)3 [mol], n is the total Si on quartz surface [mol], ν is a stoichiometric coefficient (2 or 3), γSi is activity coefficient of silicic acid, and V is the volume of solution [L]. kd(i) is dissolution rate constants and θ(i) is molar fraction of ith surface complex. The indexes (s), (0), (-), (Na), and (+) denote single bounded complex, –Si–(OH)3, uncharged complex, >Si–OH, negatively charged complex, >Si–O−, sodium complex, >Si–ONa, and positively charged complex, >Si–OH2+, respectively. $$sumlimits_{i} { k}_{{
m g(i)}}$$ is the sum of the rate constants for growth, kg(0) + kg(-) + kg(Na) + kg(s). The values of the rate constants were determined as follow: kd(s)=(1.79 ± 0.17) × 10−12, kd(0) < 5.8 × 10−15, kd(+)=(1.74 ± 0.06) × 10−12, kd(-)= (2.45 ± 0.09) × 10−12, kd(Na)=(7.23 ± 0.87) × 10−12 , and $$sumlimits_{i} k_{{
m g(i)}}$$ < 2.3 × 10−11, all in the unit of mol m−2 s−1. The initial contents of disturbed quartz (as a single bounded complex –Si(OH)3, n0S), were found to be (3.20 ± 0.20) × 10−5, (2.30 ± 0.20) × 10−5, and (3.65 ± 0.15) × 10−5 mol on 1.66 mol of quartz samples in the acid, neutral, and alkaline solutions, respectively.
Keywords: complex; dissolution; dynamics; flux; growth; modeling; PHREEQC; quartz; surface
Mineral Weathering in a Semiarid Mountain River: Its assessment through PHREEQC inverse modeling by Karina L. Lecomte; Andrea I. Pasquini; Pedro J. Depetris (pp. 173-194).
By means of PHREEQC inverse modeling, we have simulated the weathering reactions in Los Reartes River, a mountainous (2400–670 m a.s.l.) drainage basin from the Sierras Pampeanas of Córdoba, Argentina, analyzing the effect of lithology, relief, and climate. The steep upper half of the basin (slopes > 20) is occupied by exposed granite; the remaining area is mostly metamorphic, with cropping out gneisses and progressively decreasing slopes (< 6). Climate is semihumid to semiarid; rainfall mainly occurs in summer and decreases with decreasing height. PHREEQC inverse models developed using water chemical data showed that (a) oligoclase was the major supplier of solutes, while the main precipitated phase was kaolinite in the granite domain; (b) muscovite is the chief supplier of solutes and illite is the main precipitated phase in the gneissic realm; (c) the steeper portions of the metamorphic reach are less crucial in supplying solutes than the lower ones, thus highlighting the importance of the water residence time in the kinetics of dissolution; (d) in the driest time of the year (winter, 20 mm/month) we registered the highest production of dissolved and precipitated phases; fluxes (mmol/month), however, are higher at the end of the rainy season; (e) CO2 consumption is important all along the Los Reartes drainage basin and, in terms of mmol/kg H2O, the lowermost portion of the basin is the most significant supplier; (f) CO2 accounts for over 50 of all the species involved in the weathering reactions occurring at the Los Reartes drainage basin.
Keywords: Argentina; hydrochemistry; inverse modeling; mountain rivers; PHREEQC; weathering
Partitioning and Speciation of Trace Metal Diagenesis in Differing Depositional Environments in the Sediments of the Oman Margin by Rengasamy Alagarsamy; George A. Wolff; Roy Chester (pp. 195-213).
Organic-rich sediment samples collected from a transect within, and below, the Oman Margin oxygen minimum zone (OMZ) were analysed using a sequential leaching technique to characterise the diagenetic behaviour and speciation of Mn and Fe in operationally defined sediment host fractions. Trace metals showed distinct diagenetic behaviour in the two contrasting environments that were sampled. The absence of non-detrital Mn in the cores below the OMZ site is attributed to the lack of easily reducible oxides in surficial sediments and to the reduction and export of any moderately reducible aged oxides. The reactive form of solid phase Mn showed a “classic” feature of enrichment in the upper layer of the sediments at the abyssal site, reflecting the presence of an oxidising sedimentary layer which acts as a Mn trap during its recycling. The diagenetic Mn enrichment was inferred from typical downcore colour changes and an upward-increasing Mn content in the upper core sections. An easily reducible Fe oxide layer was observed in the abyssal sediments at an identical depth to the Mn enrichment suggesting that Fe associated with Mn oxides also has undergone sub-oxic diagenesis. However, the association of Fe with organic matter did not indicate diagenetic modification; i.e., the binding strength of the metal with organic materials appears to be sufficiently strong to preserve the trace metal. The speciation signature of non-detrital Fe differed from that of Mn. The association of Fe with organic matter suggests that this metal does not undergo diagenetic modification and is preserved in abyssal sediments. The contrasting behaviour of Mn and Fe observed between cores within the OMZ were particularly interesting. Another interesting observation was that, for cores below the OMZ, the iron oxides were associated with the Mn-oxide peak, rather than deeper in the sediments as observed by earlier studies in the Atlantic [Froelich et al. (1979). Geochim. et Cosmochim Acta 43, 1075–1090].
Keywords: abyssal site; iron; manganese; Oman Margin; oxygen minimum zone; speciation
Significance of Landscape Age, Uplift, and Weathering Rates to Ecosystem Development by Anne E. Carey; W. Berry Lyons; Jeffrey S. Owen (pp. 215-239).
The combined roles of chemical and physical weathering have profound effects on the development of terrestrial ecosystems. Landscapes which are tectonically active are rapidly denudated and continually produce nutrient solutes from fresh bedrock. The chemical weathering yields are related to climate, but also to geologic factors such as uplift rates. Long-term nutrient production and the phase of ecological development are closely related to geological setting. Until recently, this relationship has not been seriously considered by ecologists. Our analysis of existing information suggests, however, not only that the nature and type of geologic processes provide significant insights into landscape, but also to ecological development. In this paper, we describe the impact of geologic uplift on the long term generation of soluble nutrients from bedrock by evaluating long-term data from a number of forested sites and comparing it to newly collected data from Taiwan, an area undergoing rapid tectonic uplift.
Keywords: nutrients; weathering; streams; tectonic uplift; Taiwan; Puerto Rico; Oregon; North Carolina; New Hampshire