Kinetics of ion-pair formation on variable-charge minerals using the frequency domain method
Xiufu Shuai and Russell S. Yost
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 ultravioletvisible light and pH detectors, respectively. A mathematical model was derived to describe the diffusion process of a counterion from aqueous solution to plane, the recombinationdissociation reaction between a charged surface site and the counterion, and the coupled transport process described by the convectiondispersion 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.