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Equilibrium and Kinetics of Borate Adsorption‐Desorption on Pyrophyllite in Aqueous Suspensions
Author(s) -
Keren R.,
Grossl P. R.,
Sparks D. L.
Publication year - 1994
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1994.03615995005800040016x
Subject(s) - adsorption , desorption , chemistry , aqueous solution , ionic strength , reaction rate constant , analytical chemistry (journal) , ion , equilibrium constant , relaxation (psychology) , boron , inorganic chemistry , kinetics , chromatography , organic chemistry , social psychology , psychology , physics , quantum mechanics
This study was conducted to elucidate the dynamic aspects of the adsorption‐desorption of borate ions on edge surfaces of 2:1 clay minerals. A pressurve‐jump relaxation method was used to evaluate the elementary processes involved in the adsorption‐desorption of borate ions by pyrophyllite in aqueous media at pH 9 and ionic strength of 0.01 (NaNO 3 ). This clay was selected because of the small deviation from the ideal structural formula of the dioctahedral 2:1 clay minerals. At pH 9, 37% of the total B in solution is in the B(OH) − 4 form, whereas of the total adsorbed B, the fraction of the adsorbed B(OH) − 4 is assumed to be ≈ 0.99 at all levels of adsorbed B studied. This high fraction is probably due to the absence of repulsive forces associated with the planar surfaces. A linear correlation ( R 2 = 0.94) between the reciprocal value of the relaxation time, τ −1 , and the sum of concentrations of the free adsorption sites and borate ions in solution at equilibrium was determined. The forward rate constant, k 1 , for the adsorption was 10 4.26 L mol −1 and the backward rate constant, k ‐1 , for the desorption was 10 1.11 s −1 . The desorption rate constant was three orders of magnitudes smaller than the adsorption rate constant. The intrinsic equilibrium constant obtained from the kinetic measurements (log 10 K kinetic = 3.15) agreed relatively well with that calculated from the static studies (log 10 K static = 3.51). The suggested reaction scheme for the B‐pyrophyllite interaction is: S( OH ) 2 + B( OH ) 4 ⇋ SO 2 − B( OH ) 2 + 2 H 2 Oor S( OH ) 2 + B( OH ) 4 ⇋ SO 2 H − B( OH ) 3 + H 2 O