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Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

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Record number 398703
Title Diffusion of neutral and ionic species in charged membranes: Boric acid, arsenite, and water
Author(s) Goli, E.; Hiemstra, T.; Riemsdijk, W.H. van; Rahnemaie, R.; Malakouti, M.J.
Source Analytical Chemistry 82 (2010)20. - ISSN 0003-2700 - p. 8438 - 8445.
Department(s) Soil Chemistry and Chemical Soil Quality
Sub-department of Soil Quality
Publication type Refereed Article in a scientific journal
Publication year 2010
Keyword(s) self-diffusion - humic-acid - atomistic simulation - dynamic speciation - nafion membranes - organic-matter - coefficients - boron - equilibrium - transport
Abstract Dynamic ion speciation using DMT (Donnan membrane technique) requires insight into the physicochemical characteristics of diffusion in charged membranes (tortuosity, local diffusion coefficients) as well as ion accumulation. The latter can be precluded by studying the diffusion of neutral species, such as boric acid, B(OH)30(aq), arsenite, As(OH)30(aq), or water. In this study, the diffusion rate of B(OH)30 has been evaluated as a function of the concentration, pH, and ionic strength. The rate is linearly dependent on the concentration of solely the neutral species, without a significant contribution of negatively charged species such as B(OH)4-, present at high pH. A striking finding is the very strong effect (factor of 10) of the type of cation (K+, Na+, Ca2+, Mg2+, Al3+, and H+) on the diffusion coefficient of B(OH)30 and also As(OH)30. The decrease of the diffusion coefficient can be rationalized as an enhancement of the mean viscosity of the confined solution in the membrane. The diffusion coefficients can be described by a semiempirical relationship, linking the mean viscosity of the confined solute of the membrane to the viscosity of the free solution. In proton-saturated membranes, as used in fuel cells, viscosity is relatively more enhanced; i.e., a stronger water network is formed. Extraordinarily, our B(OH)3-calibrated model (in HNO3) correctly predicts the reported diffusion coefficient of water (DH2O), measured with 1H NMR and quasi-elastic neutron scattering in H+-Nafion membranes. Upon drying these membranes, the local hydronium, H(H2O)n+, concentration and corresponding viscosity increase, resulting in a severe reduction of the diffusion coefficient (DH2O ˜ 5-50 times), in agreement with the model. The present study has a second goal, i.e., development of the methodology for measuring the free concentration of neutral species in solution. Our data suggest that the free concentration can be measured with DMT in natural systems if one accounts for the variation in the cation composition of the membrane and corresponding viscosity/diffusion coefficient.
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