Function Of Solution Chemistry Research Articles

Chemical interactions between dissolved organic matter and low-molecular weight organic compounds: Impacts on membrane separation

This work considers the hypothesis that the association of trace-level hydrophobic organic compounds with dissolved organic matter (DOM), similar to macromolecular materials found in natural water systems, may influence the contaminant's ability to permeate across synthetic membranes. A batch dialysis system using semipermeable membranes in conjunction with a model lipid phase was used to explore the impact of systematic changes in solution-chemistry on the permeability of four low-molecular weight organic compounds (LMWOCs) of environmental concern: cyclonite, atrazine, naphthalene, and 2,4-dichlorophenol. Similar to previous studies, no correlation could be drawn between molecular weight and contaminant permeability across the membrane for the four LMWOCs. However, contaminant transport was observed to depend on the polarity and hydrophobicity of the LMWOCs. Moreover, the interactions between the organic compounds and DOM varied as a function of solution chemistry (i.e., pH and divalent electrolyte concentration). These results demonstrate considerable variability in the importance and the underlying mechanisms of interactions that may occur between LMWOCs and natural organic matter during membrane separations.

Natural organic matter removal by nanofiltration: effects of solution chemistry on retention of low molar mass acids versus bulk organic matter

The main emphasis of this study is the difference in behaviour between low molar mass (LMM) acids and bulk natural organics such as humic substances and natural organic matter. Stirred cell experiments were used to investigate charge and size effects in the rejection behaviour of bulk natural organics as well as LMM acids by nanofiltration membranes. To distinguish between size and charge effects thorough characterisation of membrane, natural organics and ionic environment was carried out. Membrane zeta potential was determined as a function of the ionic environment and the intrinsic membrane rejection was measured with dextran, ionic solutions and synthetic surface water. The behaviour of three thin film composite (TFC), and one cellulose acetate (CA) membranes was studied as a function of solution chemistry (pH, ionic composition and strength, calcium concentration, and organic type). Rejections of DOC, UV 254 nm , as well as the cations calcium and sodium were measured. Natural organics in selected feed and permeate samples were also characterised using liquid chromatography organic carbon detection (LC-OCD) which allows the characterisation of the organic fractions in the low concentration permeates and hence give insight into the rejection of low molar mass (LMM) organics. The retention of such LMM compounds is important due to their impact on the microbiological regrowth potential of the product water. The results emphasised that charge and size are both important for cations and low molar mass acids. While the bulk of the natural organics are retained due to size exclusion that is independent on solution chemistry for the membranes investigated, the retention of LMM acids follows different mechanisms. The analysis of organic fraction in the permeates showed that the rejection of low molar mass acids is strongly pH, and thus charge, dependent while this effect of solution chemistry on the LMM acids was masked when only bulk organic rejection was measured.

Uranium(VI) Sorption Complexes on Montmorillonite as a Function of Solution Chemistry

We have investigated the effect of changes in solution chemistry on the nature of uranyl sorption complexes on montmorillonite (SAz-1) at different surface coverages (1.43–53.6 μmol/g). Uranyl uptake onto SAz-1 between pH 3 and 7 was determined in both titration and batch-mode experiments. These pH values result in solutions that contain a range of monomeric and oligomeric aqueous uranyl species. Continuous-wave and time-resolved emission spectroscopies were used to investigate the nature of U(VI) sorbed to SAz-1. A discrete set of uranyl surface complexes has been identified over a wide range of pH values at these low to moderate coverages. For all samples, two surface complexes are detected with spectral characteristics commensurate with an inner-sphere complex and an exchange-site complex; the relative abundance of these two species is similar over these pH values at low coverage (1.43–2.00 μmol/g). In addition, surface species having spectra consistent with polymeric hydroxide-like sorption complexes form at the moderate coverages (∼34–54 μmol/g), increasing in abundance as the capacity of the amphoteric surface sites is exceeded. Furthermore, a species with spectral characteristics anticipated for an outer-sphere surface complex is observed for wet paste samples at low pH (3.7–4.4) and both low (∼2 μmol/g) and moderate (∼40 μmol/g) coverage. There are only subtle differences in the nature of sorption complexes formed at different pH values but similar coverages, despite markedly different uranyl speciation in solution. These results indicate that the speciation in the solution has minimal influence on the nature of the sorption complex under these experimental conditions. The primary control on the nature and abundance of the different uranyl sorption complexes appears to be the relative abundance and reactivity of the different sorption sites.

Effects of Solution Chemistry on the Oxidative Transformation of 1-Naphthol and Its Complexation with Humic Acid

The extent of 1-naphthol (1-hydroxynaphthalene) complexation with humic acid (HA) was studied as a function of solution chemistry [pH (4−11), ionic strength, I (0.001 and 0.1 M LiCl), and dissolved O2 (DO) concentration (0 and 8 mg L-1)] using spectroscopic (fluorescence, UV absorbance) and macroscopic (equilibrium dialysis) techniques. 1-Naphthol is transformed by DO in aqueous solutions; oxidation increases with pH and I, producing (hydroxy)naphthoquinones and coupled reaction products. Quenching of 1-naphthol fluorescence by HA increased with equilibration time from 1 to 7 d. This time-dependent relationship was found to result from (a) weak complexation of 1-naphthol by HA and (b) oxidative transformation of 1-naphthol (slow reaction) resulting in the formation of secondary products that are more reactive with HA. Sorption of 14C-labeled compound (as measured by equilibrium dialysis) followed a pH-dependent trend with minimal removal below pH 7.0, a sharp increase over a narrow pH range, and maximum r...

Effects of Solution Chemistry on the Partitioning of Phenanthrene to Sorbed Surfactants

Solution chemistry effects on surfactant micelle formation, surfactant sorption on kaolinite, and phenanthrene partitioning to surfactant micelles and sorbed surfactants were studied. For the anionic surfactant sodium dodecyl sulfate (SDS), critical micelle concentration (cmc) values decreased with increasing ionic strength but were unaffected by pH changes. For the nonionic surfactant Tween 80, the cmc was unaffected by pH and ionic strength changes. SDS sorption on kaolinite showed strong pH and ionic strength dependency: sorption increased as pH decreased and ionic strength increased, respectively. For Tween 80, only pH changes affected its sorption on kaolinite. Micellar phenanthrene partition coefficients (Kmic) increased with increasing ionic strength for SDS but were not affected by changes in pH. For Tween 80 Kmic values, no pH or ionic strength effect was observed. Phenanthrene distribution coefficients (KD) between the solid and aqueous phases varied as a function of solution chemistry and were primarily dependent on the concentration of surfactant that was sorbed. Organic carbon-normalized partition coefficients (Koc) of the sorbed surfactants showed a much greater affinity for phenanthrene than that predicted for natural organic matter. For sorbed SDS aggregates, Koc values fell into two general regions depending on whether the pH was above or below the point of zero charge (PZC) of kaolinite. Plausible structural differences for sorbed SDS aggregates are discussed to explain these regional Koc values.

Pyrite formation by reactions of iron monosulfides with dissolved inorganic and organic sulfur species

Pyrite formation has been investigated at 70°C and pH 6–8 by aging precipitated, disordered mackinawite, Fe9S8, and greigite, Fe3S4, in solutions containing aqueous H2S, HS−, Sx2−, S2O32−, SO32−, colloidal elemental sulfur, and the organic sulfur species thiol, disulfide, and sulfonate. Pyrite formed in all experiments where unoxidized iron monosulfides were aged with species containing zero-valent sulfur, i.e., polysulfides and colloidal elemental sulfur, but not with hydrogen sulfide (or bisulfide), the sulfoxy anions, or the organic sulfur species. Pyrite formation also occurred in experiments where the starting monosulfides were air-exposed prior to aging in sulfide solutions, or when air was bubbled through a reaction vessel containing iron monosulfides suspended in a sulfid sulfide solution. The experiments indicate the rate of conversion from iron monosulfides to pyrite is not only a function of solution chemistry (i.e., pH and aqueous speciation), but also depends on the surface oxidation state of the precursor iron monosulfides.Measurements of δ34S of reactants and products from pyrite-forming experiments suggest that the conversion from iron monosulfides to pyrite may proceed via loss of ferrous iron from, rather than via addition of zero-valent sulfur to, the precursor monosulfides. The sulfur isotopic composition of pyrite in sedimentary environments should reflect the sulfur isotopic composition of the precursor iron monosulfide plus sulfur sources incorporated during surface-controlled growth processes.Pyrite forms produced in this study ranged from poorly developed octahedral grains, in experiments where initial pyritization rates were the slowest, to framboidal aggregates in experiments where initial pyritization rates were the fastest. Although greigite formation occurred in experiments that produced framboids, not all experiments that produced greigite led to framboid formation. The formation of pyrite with framboidal texture is apparently favored when iron monosulfides rapidly convert to pyrite.

Coupled reaction/transport modeling of a chemical barrier for controlling uranium(VI) contamination in groundwater

Adsorption of uranium onto amorphous ferric oxyhydroxide can be used for in situ containment of uranium(VI) present in mill tailings and for prevention of potential groundwater contamination. Adsorption of uranium(VI) is strongly influenced by groundwater chemistry, especially pH and total dissolved carbonate concentration. To evaluate the effectiveness of adsorption onto amorphous ferric oxyhydroxide as a containment barrier, the extent of uranium adsorption was quantified as a function of solution chemistry and other adsorption parameters. The adsorption model was used within a coupled hydrogeochemical transport model to evaluate the conditions under which amorphous ferric oxyhydroxide can be effective in preventing groundwater contamination. The modeling results depicted spatial and temporal evolution of the tailings-groundwater system, and permitted delineation of the uranium(VI) plume in groundwater under different scenarios. The analysis showed that adsorption of uranium(VI) onto amorphous ferric oxyhydroxide can be an effective mechanism for preventing release of uranium(VI) into groundwater especially for neutral-pH mill tailings. It was also found that essentially no retardation occurred in tailings with alkaline pore fluids. An economic analysis showed that the amorphous ferric oxyhydroxide treatment can be applied to neutral-pH mill tailings in a cost-effective manner. The results obtained in the study support the conceptual foundation of geochemical engineering in waste management.

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 Ca 2+ 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 Ca 2+, 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 nm 2 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.

Clay mineral formation and transformation in rocks and soils

Three mechanisms for clay mineral formation (inheritance, neoformation, and transformation) operating in three geological environments (weathering, sedimentary, and diagenetic-hydrothermal) yield nine possibilities for die origin of clay minerals in nature. Several of these possibilities are discussed in terms of the rock cycle. The mineralogy of clays neoformed in the weathering environment is a function of solution chemistry, with the most dilute solutions favouring formation of the least soluble clays. After erosion and transportation, these clays may be deposited on the ocean floor in a lateral sequence that depends on floccule size. Clays undergo little reaction in the ocean, except for ion exchange and the neoformation of smectite; therefore, most clays found on the ocean floor are inherited from adjacent continents. Upon burial and heating, however, dioctahedral smectite reacts in the diagenetic environment to yield mixed-layer illite-smectite, and finally illite. With uplift and weathering, the cycle begins again.

Corrosion fatigue crack growth of titanium alloys in aqueous environments

The rate of fatigue crack propagation for Ti-6Al-6V-2Sn and Ti-6 A1-4V in aqueous environments has been measured as a function of solution chemistry, frequency, and stress wave form. Depending on the specific encironment, three types of fatigue crack growth rate behavior have been observed as a function of frequency. Crack growth rates increase with decreasing frequency in distilled water, while addition of Na2SO4 produces frequency-independent behavior. In solutions containing chloride or bromide ions, a reversal in frequency-dependence takes place at ΔKscc. Below this transition ΔK level, crack growth rates decrease with decreasing frequency due to passive film formation at the crack tip. Above ΔKscc corrosion fatigue crack growth is due to SCC under cyclic loading. The ΔK transition in fatigue is lower than the static stress corrosion threshold because of repeated rupture of the passive film at the crack tip, approaching KIsco only for very slow cycling frequencies.