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This article in SSSAJ

  1. Vol. 74 No. 5, p. 1568-1576
     
    Received: Apr 28, 2009
    Published: Sept, 2010


    * Corresponding author(s): xiufu@hawaii.edu
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doi:10.2136/sssaj2009.0161

Kinetics of Ion-Pair Formation on Variable-Charge Minerals Using the Frequency Domain Method

  1. Xiufu Shuai *a and
  2. Russell S. Yostb
  1. a Water Resources Research Center, Univ. of Hawaii at Manoa, Honolulu, HI 96822
    b Dep. of Tropical Plant and Soil Sciences, Univ. of Hawaii at Manoa, Honolulu, HI 96822

Abstract

Predicting nutrient behavior is ever more critical to understanding and management of the environment, particularly in highly weathered and tropical environments. The fate of nutrients in the environment seems heavily influenced by the kinetics of ion-pair formation on the surfaces of variable-charge minerals coupled with transport processes. The objective of this study was to generate the coupled processes in column experiments and estimate the reaction rates using the frequency domain method. Columns were packed with ground natural minerals (gibbsite, goethite, and hematite). The input signals were designed as a sinusoidal change in NaNO3 concentration within 0.1 and 0.2 mmol L−1 and constant pH 4.0, and the highest frequency of the input signals was 0.714 min−1 The input and output signals of NO3 and H+ concentrations were monitored by ultraviolet–visible light and pH detectors, respectively. A mathematical model was derived to describe the diffusion process of a counterion from aqueous solution to β plane, the recombination–dissociation reaction between a charged surface site and the counterion, and the coupled transport process described by the convection–dispersion equation. Results showed that the output signals were dominated by the designed frequency and thus the coupled processes were linear. The aqueous H+ concentration changed linearly with that of the aqueous NO3 concentration. The mathematical model fit the measured transfer function of the processes. The estimated rates of recombination of ion pairs were 52.0, 30.5, and 8.0 L mol−1 min−1, and the estimated rates of dissociation of the ion pair were 0.189, 0.256, and 0.285 min−1 for the natural gibbsite, goethite, and hematite, respectively.

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