James J. Morgan

Chemical Composition of Acid Fog

Fog water collected at three sites in Los Angeles and Bakersfield, California, was found to have higher acidity and higher concentrations of sulfate, nitrate, and ammonium than previously observed in atmospheric water droplets. The pH of the fog water was in the range of 2.2 to 4.0. The dominant processes controlling the fog water chemistry appear to be the condensation and evaporation of water vapor on preexisting aerosol and the scavenging of gas-phase nitric acid.

Reduction and dissolution of manganese(III) and manganese(IV) oxides by organics: 2. Survey of the reactivity of organics

Reduction and dissolution of manganese(III,IV) oxide suspensions by 27 aromatic and nonaromatic compounds resembling natural organics were examined in order to understand the solubilization reaction in nature. At pH 7.2 10/sup -3/ M formate, fumarate, glycerol, lactate, malonate, phthalate, propanol, propionaldehyde, propionate, and sorbitol did not dissolve appreciable amounts of oxide after 3 h of reaction. The following organics did dissolve manganese oxides under these conditions and are listed in order of decreasing reactivity: 3-methoxycatechol approx. catechol approx. 3,4-dihydroxybenzoic acidapprox. ascorbate > 4-nitro-catechol > thiosalicylate > hydroquinone > 2,5-dihydroxybenzoic acid > syringic acid > o-methoxyphenol > vanillic acidapprox. orcinol approx. 3,5-dihydroxybenzoic acid > resorcinol > oxalite approx. pyruvate approx. salicylate. Relative reactivities of organic substrates are discussed in terms of surface complex formation prior to electron transfer. Dissolution of manganese oxides by marine fulvic acid was enhanced by illumination, verifying that the reaction is photocatalyzed.

Reactions at oxide surfaces. 1. Oxidation of as(III) by synthetic birnessite

The rates and mechanisms of the reactions between aqueous As(III) and synthetic birnessite (δ-MnO_2) particles were studied. The experimental results at pH 4 indicate that the depletion of As(III) from solution is rapid, with a time scale of minutes. The oxidation product As(V) is released almost as quickly, while the release of the reduction product Mn(II) is slightly slower. The results also show that the concentration of dissolved oxygen has no effect on the rate of reaction. These observations suggestthat (i) birnessite directly oxidizes As(III) through a surface mechanism, (ii) the adsorption of As(III) is the slowest step in the production of As(V), and (iii) the reaction products As- (V) and Mn(II) are released via different mechanisms. The time-dependent behavior of the aqueous reactants and products over a pH range from 4 to 8.2, and a temperature range from 15 to 35 °C is also discussed. The rates of As(III) oxidation by inorganic redox reactions with manganese dioxides are compared to observed As(III) oxidation rates in natural aquatic systems.

Reduction and dissolution of manganese(III) and manganese(IV) oxides by organics. 1. Reaction with hydroquinone.

The chemical processes by which manganese oxides are solubilized by reduction in anoxic waters are poorly understood. A study of the reduction and dissolution of manganese oxide suspensions by hydroquinone was undertaken to determine the rate and mechanism of the solubilization reaction. Dissolution of the manganese (III,IV) oxide suspension by hydroquinone in the pH range 6.5 , pH < 8.5 is initially described by the following empirical rate law: d(Mn/sup 2 +/)/dt = k/sub 1/(H/sup +/)sup 0.46(QH/sub 2/)/sup 1.0/((MnO/sub x/)/sub 0/ - (Mn/sup 2 +/)) where (Mn/sup 2 +/) is the dissolved manganese concentration, (QH/sub 2/) is the hydroquinone concentration, and (MnO/sub x/)/sub 0/ is the amount of manganese oxide added. The apparent activation energy was found to be at +37 kJ/mol. Calcium and phosphate inhibited the reaction, by adsorbing on the oxide surface. A model is proposed for the observed rate dependence, according to which complex formation between hydroquinone and manganese oxide surface sites occurs prior to electron transfer.

Manganese(II) oxidation kinetics on metal oxide surfaces

Abstract The rates of oxidation of Mn(II) by oxygen in the presence of α-FeOOH, γ-FeOOH, silica, and δ-Al2O3 were studied experimentally. These solids enhanced the rate of Mn(II) oxidation over the initial homogeneous solution rate, the order being γ-FeOOH > α-FeOOH > SiO2 > δ-Al2O3. The kinetic data can be partially accounted for by a rate expression of the form − d[ Mn(II) ] dt = k ∗ {〉 SOH }[ Mn 2+ ] [ H + ] 2 ·A·p O 2 , where {〉SOH} is the surface concentration of hydroxyl sites and A is the mass concentration of solids in suspension. The reaction on α-FeOOH and γ-FeOOH is strongly temperature dependent (apparent activation energy ∼100 kJ mole−1) and is not affected by normal laboratory light levels. The oxidation rates on α-FeOOH are lower at higher ionic strengths, and are appreciably influenced by individual ions; Mg2+, Ca2+, silicate, salicylate, phosphate, chloride, and sulfate inhibit the overall oxidation reaction. A surface complex formation model was used to describe the adsorption of Mn2+ to the oxide surfaces. In terms of the surface species (>SO)2Mn, a proposed rate law which partially accounts for observations is − d[ Mn(II) ] dt = k ″ {(〉 SO ) 2 Mn }·A·p O 2 . An alternative formulation which accounts for the observed rates postulates the surface species >SOMnOH.

Adsorption of aquatic humic substances on colloidal-size aluminum oxide particles: Influence of solution chemistry

The adsorption of Suwannee River humic substances (HS) on colloidal-size aluminum oxide particles was examined as a function of solution chemistry. The amount of humic acid (HA) or fulvic acid (FA) adsorbed decreased with increasing pH for all solutions of constant ionic strength. In NaCl solutions at fixed pH values, the adsorption of HA and FA increased with increasing ionic strength. The presence of Ca2+ enhanced the adsorption of HA but had little effect on FA. For identical solution conditions, the amount (by mass) of HA adsorbed to alumina was always greater than FA. Adsorption densities for both HA and FA showed good agreement with the Langmuir equation, and interpretations of adsorption processes were made from the model parameters. For FA, ligand exchange appears to be the dominant adsorption reaction for the conditions studied here. Ligand exchange is also a major adsorption reaction for HA; however, other reactions contribute to adsorption for some solution compositions. At high pH, cation and water bridging become increasingly important for HA adsorption with increasing amounts of Na+ and Ca2+, respectively. At low to neutral pH values, increases in these same two cations make hydrophobic bonding more effective. Calculations of HS carboxyl group densities in the adsorbed layer support the proposed adsorption reactions. From the adsorption data it appears that fewer than 3.3 HS-COO− groups per nm2 can be bound directly as inner-sphere complexes by the alumina surface. We propose that the influence of aqueous chemistry on HS adsorption reactions, and therefore on the types of HS surface complexes formed, affects the formation and nature of organic coatings on mineral surfaces.

A physicochemical model for colloid exchange between a stream and a sand streambed with bed forms

Fine sediment exchange between a stream and the surrounding subsurface influences downstream contaminant transport and stream ecology. Fundamental models for this exchange were developed on the basis of (1) the hydraulics of bed form-driven advective pore water flow and (2) subsurface colloid transport processes. First, a model was developed to predict the advective flow induced in a sand bed by stream flow over bedforms. The resulting “pumping” exchange rate was calculated based on the streamflow conditions, bed form geometry, and bed depth. The pumping exchange of suspended sediment was then calculated by superimposing advective transport and particle settling in the bed and including the effect of physicochemical filtration by bed sediment. The filtration coefficient approach was used to predict the reduction in the concentration of transported particles. Both settling and filtration cause colloids to be trapped in stream beds, producing a higher net exchange rate relative to conservative solutes. When transported particles are completely trapped in a single pass through the bed, the exchange calculation is simplified because only the particle flux to the bed must be considered. In this case, the net exchange rate may be adequately represented by an effective piston velocity (flux/concentration) or loss rate to the bed in the advection-dispersion equation for the stream. Solute and colloid exchanges are predicted by the models without the use of fitting coefficients; only measurable hydraulic and particle parameters were used as model inputs. Simulations are presented which show the effect of stream parameters, settling, and filtration on net particle exchange. This fundamental approach to modeling stream-subsurface exchange potentially has great utility for understanding and predicting the transport and fate of reactive substances in streams.

Chemical aspects of iron oxide coagulation in water: laboratory studies and implications for natural systems.

Initial coagulation rates of colloidal hematite (α-Fe2O3) particles (diameter less than 0.1 µm) were measured experimentally in well-defined laboratory systems at constant temperature. The relative stability ratio,W, was obtained at various ionic strengths in NaCl medium and at pH values in the range from 3 to 12. ExperimentalW values ranged from 1 to 104 in various systems. The results delineate the roles ofspecific andgeneralized coagulation mechanisms for iron oxides. Among the specifically-interacting species (ΔGads0 >ΔGcoul0) studied were phosphate, monomeric organic acids of various structures, and polymeric organic acids. The critical coagulation-restabilization concentrations of specifically-interacting anions (from 10−7 to 10−4 molar) can be compared with the general effects of non-specific electrolyte coagulants (10−3 to 10−1 molar). The laboratory results are interpreted with the help of a Surface Complex Formation/Diffuse Layer Model (SCF/DLM) which describes variations of interfacial charge and potential resulting from variations of coagulating species in solution. Comparison of these laboratory experiments with observations on iron behavior in estuarine and lake waters aids in understanding iron removal mechanisms and coagulation time scales in natural systems.

Oxidative removal of Mn(II) from solution catalysed by the γ-FeOOH (lepidocrocite) surface

Abstract A laboratory study was undertaken to ascertain the role of surface catalysis in Mn(II) oxidative removal. γ-FeOOH, a ferric oxyhydroxide formed by O 2 oxidation of ferrous iron in solution, was studied in the following ways: surface charge characteristics by acid base titration, adsorption of Mn(II) and surface oxidation of Mn(II). A rate law was formulated to account for the effects of pH and the amount of surface on the surface oxidation rate of Mn(II). The presence of milli-molar levels of γ-FeOOH was shown to reduce significantly the half-life of Mn(II) in 0.7 M NaCl from hundreds of hours to hours. The numerical values of the surface rate constants for the γ-FeOOH and that reported for colloidal MnO 2 are comparable in order of magnitude.

Kaolinite exchange between a stream and streambed: Laboratory experiments and validation of a colloid transport model

Experiments were conducted in a recirculating flume to elucidate the fundamental physical and chemical processes which control the stream-subsurface exchange of colloids. Results are presented on the rate of exchange of colloids (kaolinite clay) and a conservative solute (lithium) from a stream to a sand streambed covered by stationary bed forms (dunes, ripples). Kaolinite and lithium were added to the recirculating stream, and their exchange with the bed was observed over time. Kaolinite was observed to be much more extensively trapped in the streambed than lithium owing to nonconservative processes. By the end of most experiments, essentially all added kaolinite was taken up by the streambed. The observed exchange rates can be explained by analyzing the solute and particle fluxes through the stream-subsurface interface and the physicochemical interactions between transported kaolinite and the bed sediment. The colloid pumping model predicts particle exchange based on pumping hydraulics, particle settling in the bed, and filtration by the bed sediments. Observed colloid and solute exchanges were successfully predicted by the process-based models without the use of fitting coefficients. Hydraulic parameters measured in the flume and particle parameters measured in separate experiments were used as model inputs. The successful prediction of experimental results validates the modeling approach of combining a fundamental hydraulic exchange model with a physicochemical model for colloid transport and filtration in the streambed. Further, because colloid transport behavior was interpreted in terms of basic exchange and trapping processes, the results of this study are expected to be directly applicable to the analysis of fine sediment dynamics in natural streams.