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Aquatic Geochemistry (v.11, #3)

Fe–Al–organic Colloids Control of Trace Elements in Peat Soil Solutions: Results of Ultrafiltration and Dialysis by O. S. Pokrovsky; B. Dupré; J. Schott (pp. 241-278).

Size fractionation of ~40 major and trace elements (TE) in peat soil solutions from the Tverskaya region (Russia) has been studied using frontal filtration and ultrafiltration through a progressively decreasing pore size (5, 2.5, 0.22 μm, 100, 10, 5, and 1 kD) and in situ dialysis through 6–8 and 1 kD membranes with subsequent analysis by ICP-MS. In (ultra) filter-passed permeates and dialysates of soil solutions, Fe, Al, and organic carbon (OC) are well correlated, indicating the presence of mixed organo-mineral colloids. All major anions and silica are present in “dissolved” forms passed through 1 kD membrane. According to their behavior during filtration and dialysis and association with mineral or organic components, three groups of elements can be distinguished: (i) species that are weakly affected by size separation operations and largely (>50–80%) present in the form of dissolved inorganic species (Ca, Mg, Li, Na, K, Sr, Ba, Rb, Cs, As, Mn) with some proportion of small (1–10 kD) organic complexes (Ca, Mg, Sr, Ba), (ii) biologically essential elements (Co, Ni, Zn, Cu, Cd) mainly present in the fraction smaller than 1 kD and known to form strong organic complexes with fulvic acids, and, (iii) elements strongly associated with aluminum, iron and OC in all ultrafiltrates and dialysates with 30–50% being concentrated in large (>10 kD) colloids (Ga, Y, REEs, Pb, Cd, V, Nb, Sn, Ti, Zr, Hf, Th, U). For most trace metals, the proportion in the colloidal fraction correlates with their first hydrolysis constant. This implies a strong control of negatively charged oxygen donors present in inorganic/organic colloids on TE distribution between aqueous solution and colloid particles. It is suggested that these colloids are formed during plant uptake of Al, Fe, and TE from mineral matrix of deep soil horizons and their subsequent release in surface horizons after litter degradation and oxygenation on redox or acid/base fronts. Dissolved organic matter stabilizes Al/Fe colloids and thus enhances trace elements transport in soil solutions.

Keywords: peat; soil; solution; colloids; ultrafiltration; dialysis; trace elements; speciation

Desert Potholes: Ephemeral Aquatic Microsystems by Marjorie A. Chan; Katrina Moser; Jim M. Davis; Gordon Southam; Kebbi Hughes; Tim Graham (pp. 279-302).

An enigma of the Colorado Plateau high desert is the “pothole”, which ranges from shallow ephemeral puddles to deeply carved pools. The existence of prokaryotic to eukaryotic organisms within these pools is largely controlled by the presence of collected rainwater. Multivariate statistical analysis of physical and chemical limnologic data variables measured from potholes indicates spatial and temporal variations, particularly in water depth, manganese, iron, nitrate and sulfate concentrations and salinity. Variation in water depth and salinity are likely related to the amount of time since the last precipitation, whereas the other variables may be related to redox potential. The spatial and temporal variations in water chemistry affect the distribution of organisms, which must adapt to daily and seasonal extremes of fluctuating temperature (0–60 °C), pH changes of as much as 5 units over 12 days, and desiccation. For example, many species become dormant when potholes dry, in order to endure intense heat, UV radiation, desiccation and freezing, only to flourish again upon rehydration. But the pothole organisms also have a profound impact on the potholes. Through photosynthesis and respiration, pothole organisms affect redox potential, and indirectly alter the water chemistry. Laboratory examination of dried biofilm from the potholes revealed that within 2 weeks of hydration, the surface of the desiccated, black biofilm became green from cyanobacterial growth, which supported significant growth in heterotrophic bacterial populations. This complex biofilm is persumably responsible for dissolving the cement between the sandstone grains, allowing the potholes to enlarge, and for sealing the potholes, enabling them to retain water longer than the surrounding sandstone. Despite the remarkable ability of life in potholes to persist, desert potholes may be extremely sensitive to anthropogenic effects. The unique limnology and ecology of Utah potholes holds great scientific value for understanding water–rock–biological interactions with possible applications to life on other planetary bodies.

Keywords: potholes; desert; Colorado Plateau; sandstone; Utah; microorganisms; redox; biofilm; cyanobacteria; limnology

Effects of Soil Composition on Zn Speciation in Drainage Waters from Agricultural Soils by Hanbin Xue; Phan Hong Nhat; Laura Sigg (pp. 303-318).

The influence of Zn speciation on Zn transport by drainage from different soils to surface water is examined in a stream catchment in an agricultural area. Drainage waters were collected from two types of soils, a mineral soil (MS) and a soil rich in organic matter (OS) by means of artificial drainage pipes. The speciation of dissolved Zn in the stream and the drainage waters was determined using ligand-exchange and voltammetry. About 50–95% of dissolved Zn is bound in strong complexes, and the free Zn²⁺ ion concentration is in the range of 1–16% of dissolved Zn. A substantial part of Zn is present in weaker organic or inorganic complexes. The simulated Zn speciation using the WHAM VI model is compared to the determined speciation. Free Zn²⁺ concentrations predicted by the WHAM VI model are generally higher than the analytically determined free Zn²⁺, but are mostly within the same order of magnitude. Effects of different soil organic matter content on Zn speciation and transport are discussed. Zn speciation in the drainage at the OS site is influenced by the distribution of organic matter between the solid and solution phase. The abundant organic Zn complexes in solution contribute to facilitate Zn transport from soil into surface waters, through the drainage at the OS site. Drainage from the OS site contributes about twice as much Zn input to the receiving water as the MS soil, as related to specific area. The mineral soil contains much lower organic matter, and a part of Zn bound with inorganic phases can hardly be released by dissolved organic ligands, leading to much higher Zn retention at the MS site.

Keywords: agricultural soil; soil drainage; surface water; Zn input; Zn speciation; Zn transport by drainage

The Rate-equation for Biogenic Silica Dissolution in Seawater – New Hypotheses by Victor W. Truesdale; Jim E. Greenwood; Andrew Rendell (pp. 319-343).

This paper investigates the kinetics of biogenic silica dissolution in seawater, through batch dissolution, where the reaction is observed as the increase in dissolved silicic acid concentration with time. It utilises new data from dissolution of the marine diatom Cyclotella cryptica, and the freshwater diatom C. meneghiniana, as well as literature results. The sum of exponentials form: $$C_{ m t}=A_{infty}(1-e^{-k_at})+B_{infty}(1-e^{-k_bt})$$ , is hypothesised as the most general rate equation, with the single exponential form occurring in a minority of cases. The consistency of this behaviour with a near-exponential decay of surface area with time, an appropriate mathematical integration, and surface heterogeneity, is discussed. (Serious errors in some existing integrations are identified.) The rate of dissolution at constant surface area is shown to decrease non-linearly as the ambient concentration of silicic acid increases. A fractional order with respect to silicic acid in the back reaction, close to 0.5, leads to a mechanism in which an intermediate is formed from the surface and an, as yet, unidentified molecule, probably water. Good preliminary fits are found between the model and literature results found using entirely different methods. A parallel treatment of hydrogen ion dependency is suggested. The likely distortion of full reaction curves from exponential behaviour imposed by the back reaction, is considered in detail.

Keywords: biogenic silica; dissolution kinetics; diatom frustules; silica cycling

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Aquatic Geochemistry (v.11, #3)

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Aquatic Geochemistry (v.11, #3)