Arsenic, adsorption kinetics

Pigna M, Colombo C, Violante A (2003) Competitive sorption of arsenate and phosphate on synthetic hematites (in Italian). Proceedings XXI Congress of Societa Italiana Chimica Agraria SICA (Ancona), pp 70-76 Quirk JP (1955) Significance of surface area calculated from water vapour sorption isotherms by use of the B. E. T. equation. Soil Sci 80 423-430 Rancourt DG, Fortin D, Pichler T, Lamarche G (2001) Mineralogical characterization of a natural As-rich hydrous ferric oxide coprecipitate formed by mining hydrothermal fluids and seawater. Am Mineral 86 834-851 Raven K, Jain A, Loeppert, RH (1998) Arsenite and arsenate adsorption on ferrihydrite kinetics, equilibrium, and adsorption envelopes. Environ Sci Technol 32 344-349... 

Violante A, Krishnamurti GSR, Pigna M (2008) Mobility of trace elements in soil environments. In Violante A, Huang PM and Gadd G (eds) Wiley-JUPAC series on biophysico-chemical processes of metals and metalloids in soil environments. John Wiley Sons, Hoboken, USA Waltham AC, Eick MJ (2002) Kinetic of arsenic adsorption on goethite in the presence of sorbed silicic acid. Soil Sci Soc Am J 66 818-825 Waychunas GA, Fuller CC, Rea BA, Davis J (1996) Wide angle X-ray scattering (WAXS) study of two-line ferrihydrite structure Effect of arsenate sorption and counterion variation and comparison with EXAFS results. Geochim Cos-mochim Acta 60 1765-1781... 

Rau, M. Rieck, D. Evans, J.W. (1987) Investigation of iron oxide reduction by TEM. Metallurgical Transactions 188 257-278 Raven, K.P. Jain, A. Loeppert, R.H. (1998) Ar-senite and arsenate adsorption on ferrihy-drite Kinetics, equilibrium, and adsorption envelopes. Environ. Sci. Techn. 32 344-349 Rea, B.A. Davis, J.A. Waychunas, G.A. (1994) Studies of the reactivity of the ferrihydrite surface by iron isotopic exchange and Moss-bauer spectroscopy. Clays Clay Min. 42 23-34... 

Luengo, C., Brigante, M. and Avena, M. (2007) Adsorption kinetics of phosphate and arsenate on goethite. A comparative study. Journal of Colloid and Interface Science, 311(2), 354-60. 

Raven, K.P., Jain, A. and Loeppert, R.H. (1998) Arsenite and arsenate adsorption on ferrihydrite kinetics, equilibrium, and adsorption envelopes. Environmental Science and Technology, 32(3), 344-49. 

Fuller, C. C., J. A. Davis, and G. A. Waychunas. 1993. Surface chemistry of ferrihydrite Part 2. Kinetics of arsenate adsorption and coprecipitation. Geochim. Cosmochim. Acta 57 2271-2282. 

Manning, B. A. and D. L. Suarez. 2000. Modeling arsenic adsorption and heterogeneous oxidation kinetics in soils. Soil Sci. Am. J. 64 128-137. 

Kinetics of Arsenic Adsorption. Arsenite adsorption on hematite is rapid, attaining equilibrium within 50 minutes and follows a first-order adsorption rate expression (42). The removal of arsenite is partially diffusion controlled and... 

Arsenate adsorption on ferrihydrite consisted of a period of rapid uptake followed by slow adsorption for at least 8 days 43). The rate of the slow adsorption reaction is considered to be limited by diffusion into the ferrihydrite aggregates. Slow adsorption kinetics similar to those for phosphate are expected for arsenate because of the similar chemistry of these two anions. Arsenate adsorption data adhere to the Elovich kinetic model indicating a diffusion limited reaction. Arsenate desorption rates were much slower than arsenate adsorption rates, also consistent with a diffusion limited process. A model was developed that assumes that 63% of adsorbing sites are located at the exteriors of aggregates and reach arsenate equilibrium rapidly, while 37% of adsorbing sites are located in the interiors of aggregates with access being diffusion limited. 

H. and Ochieng, A. (2003) Adsorption kinetics of arsenic removal from groundwater by iron-modified zeolite. Journal of Chemical Engineering of Japan, 36,1515-22,... 

Lin, T.-F. and Wu, J.-K. (2001) Adsorption of arsenite and arsenate within activated alumina grains equilibrium and kinetics. Water Research, 35(8), 2049-57. 

Fluoride-related health hazards are associated with the use of fluoride-contaminated water for drinking and cooking. This corresponds only to 2-4 L per capita per day. Fluoride removal in rural areas in LDCs, where centralized water treatment and distribution facilities are unavailable, should consequently be carried out at a household level and the system applied should be simple and affordable. In this regard, tea bag POU system becomes handy. Although this kind of system has not been specifically reported for water defluoridation, it has been tested for arsenic [37,107], It is therefore a short-term potential technique worth considering. In this technique, adsorption medium is placed in a tea bag-like packet, which is subsequently placed in a bucket of water to be treated. To ensure faster defluoridation kinetics, the bag should be swirled inside the water. It therefore operates like a batch reactor and hence requires a relatively longer adsorption time to achieve the permissible levels. Since the swirling motion is supposed to be human-powered, the technique would require a material with very fast kinetics or very fine adsorption media. 

It is the purpose of this paper to describe some of the major mechanisms that control arsenic in aquatic systems. Particularly, this paper addresses the problem of arsenic speciation and compartmentalization in sediments. To this end, results obtained from speciation, compartmentalization, kinetic, and adsorption studies using both field and laboratory samples will be interfaced in a descriptive model for arsenic in heterogeneous systems. The model has particular significance... 

Kinetics and Adsorption. If microorganisms only take up dissolved arsenic species, then adsorption can affect rates of species transformations by lowering concentrations of reactants. In this section calculations of changes In concentrations of reactants and products of arsenic species transformations In sediments are presented. Rates of transformation are assumed to be proportional to species concentrations and the rate of another reaction, V, coupled to the transformation. That Is, the transformation reaction Is not assumed to be a source of energy or structural material for the microorganisms. Thus,... 

Assuming first order kinetics, these conversion rates translate to half lives of 5.8 to 34.7 months. Thus, Wool son s model ecosystems were probably at equilibrium at all times with respect to adsorption and desorption of arsenic species. 

Examples of the Influence of Adsorption on Kinetics. Three cases of arsenic species transformations influenced by adsorption are considered in this section. The three cases considered are 1) reactant not adsorbed, product adsorbed (e.g. demethylation of cacodylic acid to arsenate), 2) reactant adsorbed, product not adsorbed (e.g. methylation of arsenate to cacodylic acid),... 

Sorption of monomethyl arsonic acid (MMAA), dimethyl arsinic acid (DMAA), and arsenate on anaerobic bottom sediments from the Menominee River, Wisconsin are described by Langmuir Isotherms. These results were Incorporated Into a kinetic model of arsenic species transforamtlon which takes sorption Into account. Model predictions were found to be sensitive to the sediment water content and r, the adsorptive capacity of the sediment. Demethylatlon of MMAA and DMAA was observed In sediment Incubation experiments. The predictions of the sorption/kinetic model were In good agreement with the results of the Incubation experiments. 

Typically, the rate of simple (outer-sphere) electron-transfer reactions, such as Fe(CN)e + e - Fe(CN)6 , is much slower at titanium dioxide than at metallic electrodes . This is consistent both with the flat shape of the voltammetric peaks in Fig. 2 and their shift to more negative potentials with increasing the sweep rate. The kinetics of the cathodic reactions at Ti02 appear to be markedly affected by the co-adsorption of some anions from the supporting electrolyte. The phosphate ions are not unique to cause such effects arsenate, fluoride and certainly other anions are expected to act in a similar way. 

Theoretical studies of the properties of the individual components of nanocat-alytic systems (including metal nanoclusters, finite or extended supporting substrates, and molecular reactants and products), and of their assemblies (that is, a metal cluster anchored to the surface of a solid support material with molecular reactants adsorbed on either the cluster, the support surface, or both), employ an arsenal of diverse theoretical methodologies and techniques for a recent perspective article about computations in materials science and condensed matter studies [254], These theoretical tools include quantum mechanical electronic structure calculations coupled with structural optimizations (that is, determination of equilibrium, ground state nuclear configurations), searches for reaction pathways and microscopic reaction mechanisms, ab initio investigations of the dynamics of adsorption and reactive processes, statistical mechanical techniques (quantum, semiclassical, and classical) for determination of reaction rates, and evaluation of probabilities for reactive encounters between adsorbed reactants using kinetic equation for multiparticle adsorption, surface diffusion, and collisions between mobile adsorbed species, as well as explorations of spatiotemporal distributions of reactants and products. 

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