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In this work, the adsorption behavior of Sr onto a synthetic iron(III) oxide (hematite with traces of goethite) has been studied. This solid, which might be considered a representative of Fe3+ solid phases (iron corrosion products), was characterized by X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS), and its specific surface area was determined. Both XRD and XPS data are consistent with a mixed solid containing more than 90% hematite and 10% goethite. The solid was further characterized by fast acid-base titrations at different NaCl concentrations (from 0.1 to 5 M). Subsequently, for each background NaCl concentration used for the acid-base titrations, Sr-uptake experiments were carried out involving two different levels of Sr concentration (1 × 10−5 and 5 × 10−5 M, respectively) at constant solid concentration (7.3 g/L) as a function of −log([H+]/M). A Surface Complexation Model (SCM) was fitted to the experimental data, following a coupled Pitzer/surface complexation approach. The Pitzer model was applied to aqueous species. A Basic Stern Model was used for interfacial electrostatics of the system, which includes ion-specific effects via ion-specific pair-formation constants, whereas the Pitzer-approach involves ion-interaction parameters that enter the model through activity coefficients for aqueous species. A simple 1-pK model was applied (generic surface species, denoted as >XOH−1/2). Parameter fitting was carried out using the general parameter estimation software UCODE, coupled to a modified version of FITEQL2. The combined approach describes the full set of data reasonably well and involves two Sr-surface complexes, one of them including chloride. Monodentate and bidentate models were tested and were found to perform equally well. The SCM is particularly able to account for the incomplete uptake of Sr at higher salt levels, supporting the idea that adsorption models conventionally used in salt concentrations below 1 M are applicable to high salt concentrations if the correct activity corrections for the aqueous species are applied. This generates a self-consistent model framework involving a practical approach for semi-mechanistic SCMs. The model framework of coupling conventional electrostatic double layer models for the surface with a Pitzer approach for the bulk solution earlier tested with strongly adsorbing solutes is here shown to be successful for more weakly adsorbing solutes. [ABSTRACT FROM AUTHOR]

The adsorption of Zn 2+ ions onto biogenic elemental selenium nanoparticles (BioSeNPs) was investigated. BioSeNPs were produced by reduction of selenite (SeO 3 2− ) in the presence of anaerobic granules from a full scale upflow anaerobic sludge blanket (UASB) reactor treating paper mill wastewater. The BioSeNPs have an iso-electric point at pH 3.8 at 5 mM background electrolyte concentration. X-ray photoelectron spectroscopy showed the presence of a layer of extracellular polymeric substances on the surface of BioSeNPs providing colloidal stability. Batch adsorption experiments showed that the uptake of Zn 2+ ions by BioSeNPs was fast and occurred at a pH as low as 3.9. The maximum adsorption capacity observed was 60 mg of zinc adsorbed per g of BioSeNPs. The Zn 2+ ions adsorption on the BioSeNPs was largely unaffected by the presence of Na + and Mg 2+ , but was impacted by the presence of Ca 2+ and Fe 2+ ions. The colloidal stability of BioSeNPs decreased with the increasing Zn 2+ ions loading on BioSeNPs (increase in mg of zinc adsorbed per g of BioSeNPs), corresponding to the neutralization of the negative surface charge of the BioSeNPs, suggesting gravity settling as a technique for solid–liquid separation after adsorption. This study proposes a novel technology for removal of divalent cationic heavy metals by their adsorption on the BioSeNPs present in the effluent of an UASB reactor treating selenium oxyanions containing wastewaters. [ABSTRACT FROM AUTHOR]

The mechanism of selenium(IV) uptake by maghemite was investigated on both the macroscopic and the molecular level. Maghemite nanoparticles exhibited fast adsorption kinetics toward selenium(IV). Batch experiments showed a decreased sorption with increasing pH (3.5-11). Ionic strength variations (0.01 to 0.1 M NaCl) had no significant influence on selenium(IV) uptake. Electrophoretic mobility measurements revealed a significant shift toward lower values of the isoelectric point of maghemite upon selenium(IV) uptake, suggesting the formation of inner-sphere surface complexes. At the molecular level, using X-ray Absorption Fine-Structure Spectroscopy (EXAFS), the formation of both bidentate binuclear corner-sharing (2C) and bidentate mononuclear edge-sharing (1E) inner-sphere surface complexes was observed, with a trend toward solely 1E complexes at high pH. The absence of a tridentate surface complex as observed for arsenic(III) and antimonite(III) might be due to the relatively small size of the SeIVO3 unit. These new spectroscopic results can be implemented in reactive transport models to improve the prediction of selenium migration behavior in the environment as well as its monitoring through its interaction with maghemite or maghemite layers at the surface of magnetite. Due to its chemical stability even at low pH and its magnetization properties allowing magnetic separation, maghemite is a promising sorbing phase for the treatment of Se polluted waters. [ABSTRACT FROM AUTHOR]

Deposition of latex colloids on a structured siliconsurface wasinvestigated. The surface with well-defined roughness and topographypattern served as an analogue for rough mineral surfaces with half-poresin the submicrometer size. The silicon topography consists of a regularpit pattern (pit diameter = 400 nm, pit spacing = 400 nm, pit depth= 100 nm). Effects of hydrodynamics and colloidal interactions intransport and deposition dynamics of a colloidal suspension were investigatedin a parallel plate flow chamber. The experiments were conducted atpH ∼ 5.5 under both favorable and unfavorable adsorption conditionsusing carboxylate functionalized colloids to study the impact of surfacetopography on particle retention. Vertical scanning interferometry(VSI) was applied for both surface topography characterization andthe quantification of colloidal retention over large fields of view.The influence of particle diameter variation (d=0.3–2 μm) on retention of monodisperse as well as polydispersesuspensions was studied as a function of flow velocity. Despite electrostaticallyunfavorable conditions, at all flow velocities, an increased retentionof colloids was observed at the rough surface compared to a smoothsurface without surface pattern. The impact of surface roughness onretention was found to be more significant for smaller colloids (d= 0.3, 0.43 vs. 1, 2 μm). From smooth to rough surfaces,the deposition rate of 0.3 and 0.43 μm colloids increased bya factor of ∼2.7 compared to a factor of 1.2 or 1.8 for 1 and2 μm colloids, respectively. For a substrate herein, with constantsurface topography, the ratio between substrate roughness and radiusof colloid, Rq/rc, determinedthe deposition efficiency. As Rq/rcincreased, particle–substrate overall DLVO interactionenergy decreased. Larger colloids (1 and 2 μm) beyond a criticalvelocity (7 × 10–5and 3 × 10–6m/s) (when drag force exceeds adhesion force) tend to detach fromthe surface irrespective of the impact of roughness. For polydispersesolutions, an increase in the polydispersity and flow velocity resultedin a reduction of colloid deposition efficiency due to the resultingenhanced double-layer repulsion. Quantification of surface topographyvariations of two endmembers of natural grain surfaces showed thathalf-pore depths and roughness of sedimentary quartz grains are mainlyin the micrometer range. Grains with diagenetically formed quartzovergrowths, however, show surface roughness mainly in the submicrometerrange. Thus, surface topography features applied in the here presentedanalogue study and resulting variation in particle retention can serveas quantitative analogue for particle reactions in diageneticallyaltered quartz sands and sandstones. The reported impact of particlepolydispersity can have an important application for quantitativeprediction of retention of varying types of minerals, such as differentclay minerals in the environment under prevailing unfavorable conditions. [ABSTRACT FROM AUTHOR]