Competitive adsorption of hydrogen, calcium, potassium, magnesium, and aluminum on banana roots: Experimental data and modeling

Aluminum (Al) adsorption on root exchange sites is one of the mechanisms that can be responsible for Al toxicity. Experimental measurements of calcium (Ca), magnesium (Mg), potassium (K), and Al adsorption on the main root axes of 5 banana cuttivars (Grande Naine, Agbagba, Obino l'Ewaï, Kayinja, and Igitsiri) were carried out at pH 3.75 and 4.25. The data obtained were used to calibrate a mathematical model describing ion exchange as arising from Donnan effects and specific surface-ion interactions. Adsorption of Ca, K, Mg, and Al ions increased with their concentration in solution and with increasing pH. The depletion rate of Ca, Mg, and K in the presence of the low Al concentration was greater at pH 4.25 than at pH 3.75. Therefore, any model based on constant exchange selectivity would fail to describe such complex equilibria. The calibration of the model involves to fix values for the intrinsic dissociation constants (K_R-X) of cations (X) with root exchange sites (R^-). Values from the literature were used for H-surface (K_R-H = 10^-3.2) and Ca-surface (K_R2-Ca = 10^-1.1) complexes. No specific ion-surface interaction was assumed for K (K_R-K = 10⁶) and a value of K_R2-Mg = 1 was found adequate for Mg-surface complexes. Non-linear regression led to K_R3-Al = 10^{(-3.78±0.024)}. Introducing possible R-Al²⁺, R₂-Al⁺, R-Al(OH)₂, and R₂-Al(OH) complexes did not improve the fitting. Using one single set of parameters for all experiments on the 5 banana cultivars resulted in good fitting of Al and Mg adsorption, and slight underestimation of Ca adsorption, although good estimate was obtained with pure Ca solutions. We conclude that Al is very efficient in competing with other cations for root exchange sites despite the low Al concentrations usually occurring in soil solutions. Magnesium was confirmed to form only little association with fixed charged groups, so that Al loading on roots occurs mainly to the detriment of Mg.