Surface complexation models: An evaluation of model parameter estimation using FITEQL and oxide mineral titration data

The ability of surface complexation models (SCMs) to fit sets of titration data as a function of changes in model parameters was evaluated using FITEQL and acid-base titration data of [alpha]-FeOOH, [alpha]-Al2O3, and TiO2. Three SCMs were evaluated: the triple-layer model (TLM), the constant capacitance model (CCM), and the diffuse-layer model (DLM). For all models evaluated, increasing the model input value for the total number of surface sites caused a decrease in the best-fit Log K values of the surface protolysis constants. In the case of the CCM, the best-fit surface protolysis constants were relatively insensitive to changes in the value of the capacitance fitting parameter, C1, particularly for values of C1 greater than 1.2 F/m2. Similarly, the best-fit values of TLM surface electrolyte binding constants were less influenced by changes in the value of C1 when C1 was greater than 1.2 F/m2. For a given C1 value, the best-fit TLM values of the electrolyte binding constants were sensitive to changes in [Delta]pKa up to [Delta]pKa values of 3. For [Delta]pKa values above 3, no changes in the best-fit electrolyte binding constants were observed. Effects of the quality and extent of titration data on the best-fit values for surface constants are discussed for each model. A method is suggested for choosing a unique set of parameter values for each of the models.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29417/1/0000493.pd

Quantification and Analysis of Micro-Level Activities Data from Children Aged 1-12 Years Old for Use in the Assessments of Exposure to Recycled Tire on Turf and Playgrounds.

BACKGROUND: There are growing health concerns about exposure to toxicants released from recycled tire rubber, which is commonly used in synthetic turf and playground mats. To better estimate children\u27s exposure and risk from recycled tire rubber used in synthetic turf and playground mats, there is a need to collect detailed accurate information on mouthing activity and dermal contact behaviors. The objective of this study was to quantify and analyze micro-level activity time series (MLATS) data from children aged 1-12 years old while playing (non-sport-related games) at turf-like locations and playgrounds. Another objective was to estimate the incidental ingestion rate of rubber crumb among children. METHODS: Hand and mouth contact frequency, hourly duration, and median contact duration with different objects were calculated for children playing on turf (i.e., parks, lawns, and gardens) (n = 56) and for children playing on playground structures (n = 24). Statistically significant differences between males and females as well as children\u27s age groups were evaluated. The daily incidental ingestion rate of rubber crumb was calculated. RESULTS: For children playing on turf, there were significant differences between younger (1-6 y) and older (7-12 y) children for the mouthing median duration with non-dietary objects and all objects. For children playing on playground structures, we found significant mouthing frequency differences between younger (1-6 y) and older children (7-12 y) with all objects, and for mouthing median duration with non-dietary objects. There were no significant differences between males and females playing on artificial turf-like surfaces or playground mats. Our estimated mean incidental ingestion rate was 0.08, 0.07, and 0.08 g rubber crumb/day for children DISCUSSION: our results suggest that age and contact duration should be considered in risk assessment models to evaluate mouthing activities when children are playing on artificial turf surfaces or playground mats

Hypochlorite degradation of crosslinked polyamide membranes I. Changes in chemical/morphological properties

When disinfecting or cleaning agents contact polymer-based membranes, hydrolysis and oxidation with the membrane change surface properties. The research reported here discusses the effect of exposure to solution of hypochloric acid (disinfecting agent) on the chemical and morphological properties of a crosslinked polyamide membrane (LFC1, Hydranautics (c)). Effects on the membrane were evaluated using AFM, XPS, FT-IR, contact angle, and streaming potential analysis. Chlorine incorporated in the membrane increased with increasing hypochlorite concentration and decreasing pH of a soaking bath. Exposed membranes were more hydrophilic and had a slightly more negative zeta potential. The chlorination broke and weakened hydrogen bonding by decreasing the number of hydrogen bonding sites. This study shows that the use of chlorine chemically changed the surface properties of the crosslinked polyamide LFC1 membrane, but there was no change in roughness and morphology.close575

Time-dependent adsorption in near coastal marine sediments: a two-step

Many important physical and chemical processes occur at phase boundaries. The role of surface phenomena is frequently underestimated or overlooked although surfaces play a significant role in many natural science disciplines. Experimental data from the literature indicate that in adsorption from solution, most of the adsorbates move to the adsorbent surface in a relatively short time period. Actual adsorption equilibrium, however, may take longer to establish. In this study time variable parametric experiments were performed with Co and a suspended marine sediment sample. Two different time dependencies were observed: a rapid step that reached equilibrium in 5-10 days, while a slower step continued for more than 100 days. Observed behavior was simulated with a time-dependent model that differentiates the slow and fast steps. The fast step was considered to be due to diffusion of the adsorbate to the external and macropore surface of the adsorbent and exchange at surface sites, while the slow step was considered as diffusion of adsorbate into the adsorbent micropore capillaries where adsorptive binding occurs. ᭧ 1998 Elsevier Science Limited. All rights reserved

Variability in Goethite Surface Site Density: Evidence from Proton and Carbonate Sorption

Goethite is a representative iron oxide in natural environments due to its abundance and thermodynamic stability and may be responsible for many surface-mediated processes, including ion retention and mobility in aqueous settings. A large variability in morphologies and specific surface areas of goethite crystals exists but little work has been done to compare surface reactivity between them. The present work offers experimental evidence for the existence of an inverse relationship between sorption capacity for protons and carbonate ions and specific surface area of goethite for three synthetic goethite preparations spanning surface area differences by a factor of 2. An explanation for this was found by assuming a variable reactive site density between preparations in direct relationship to their sorption capacity based on congruency of carbonate sorption computed on a per-site basis. Previous evidence of maximum sorption capacities supports this explanation, and site density ratios between the goethites studied here were obtained. Triple layer surface complexation modeling was successful in describing adsorption data for all goethite preparations using equal stoichiometries. A new formulation of standard state for activities of surface species based on a 1.0 mole fraction of sites on the solid allowed transformation of the conventional molar concentration-based affinity constants to values based on site occupancy. In this fashion, by applying the appropriate site density ratios, a single set of affinity constant values was found that described accurately the adsorption data for all preparations

Surface Complexation Modeling of Carbonate Effects on the Adsorption of Cr(VI), Pb(II), and U(VI) on Goethite

Dissolved carbonate species are known to affect the sorption behavior of trace species. The macroscopic description of these interactions with a thermodynamic approach has been limited by the lack of data on the binary interaction between carbonate and relevant mineral surfaces. This work follows from two detailed studies of carbonate adsorption on goethite (4, 13). It shows that independent triple-layer surface complexation modeling (TLM) of carbonate adsorption allows successful descriptions of carbonate-trace element ternary sorption on this oxide, using relatively simple and optimal stoichiometries. Carbonate adsorption was considerably enhanced in the presence of Pb(II), despite an invariant total Pb(II) sorption to equilibration with up to 1% CO2(g). Both the Pb(II)−carbonate system behavior and the anion-like pH adsorption behavior of U(VI) in the presence of CO2 were successfully modeled using binary and ternary metal-bound surface complexes. The significant reduction of Cr(VI) adsorption edges to lower pH values in the presence of CO2 was accurately simulated and explained via site competition and surface electrostatic repulsion effects on the predicted inner- and outer-sphere Cr(VI) surface complexes formed. The results of this research are highly relevant to modeling of metal transport field data and of potential soil remediation schemes using carbonate