On the Interaction of Zinc with a Ferric Oxyhydroxide

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Sorption and desorption of Zn by synthetic goethite (α-FeOOH) (surface area 95.2 m2/g, pH(PZC) 7.9) was studied in equilibrium and non-equilibrium conditions as functions of aqueous solution Zn and Mg concentrations and temperature, but at low amounts in the systems. The sorption data were analyzed with a multi—site Langinuir expression, which suggested that at pH 4.6 and 5.4 (25°C) Zn sorption could be entirely ascribed to a low capacity, high affinity site, whereas at pH 5.4 (44°C) and higher pH’s (both. 25 and 44°C) sorption could be described by two sites. At p118.5 (44°C) the data fit a model consisting of a high affinity Langmuir site coexisting with a site or mechanism following Henry’s Law. Site 1 affinity term, k1, increased with pH and temperature, but was not affected by 25 ppm Mg above pH 5.4. Log k1 was highly correlated with log K0, the thermodynamic distribution coefficient. Site 1 capacity term, b1, increased with pH up to pH 7.0 and then remained constant; it was not affected by either Mg or temperature. Site 2 affinity term, increased with temperature; at 25°C it did not vary with pH below pH 7.8, but increased four—fold at pH 8.5. Site 2 capacity term, b2, increased with both temperature and pH, and only below pH 7.0 were this parameter and decreased by 25 ppm Mg. At pH 5.4 though Mg did not affect k1, a decrease in b1 by Mg indicated some Mg ions were specifically adsorbed at site 1. However, the higher specificity of site 1 for Zn above pH 5.4 eliminated Mg effect on b1. Generally 25 ppm Mg significantly decreased Zn sorption at site 2 below pH 7.0. The sharp increase in Zn sorption with increasing pH in the pH range 5.4 -7 was attributed to the development of site 2 at pH 6.2, suggesting sorption by a second mechanism. Kurbatov plots suggested that one mole of H+ was released per mole of Zn sorbed below pH 7, and 2 moles above pH 7. A Zn (OH) 2 precipitation mechanism above pH 7.8 was emphasized by the release of 2 moles of H+ per mole of Zn sorbed in kinetic studies. Total sorption of Zn was markedly endothermic at all pH levels, the increase in sorption with temperature being most pronounced at low pH’s and high system loadings. Mean ∆G-0 values ranged from -9.1 kcal/mole at pH 5.4 to -11.2 at pH, pH 8.5, ∆H-0 from 14.5 kcal/mole at pH 5.4 to 6.5 at pH 8.5, and ∆S-0 from 76.9 cal/deg. mole at pH 5.4 to 57.7 at pH 8.5. Isosteric heats of Zn sorption, Qr , ranged from 1 to 27 kcal/mole, and were generally higher at low pH. Hysteresis was observed in desorption versus adsorption isotherms. Sorption was fast at pH 7.8 and 8.5, steady states being achieved in 2 hours. At pH’s below 7.8 a slow reaction followed an initial fast reaction. Below pH 7.8 the initial sorption reaction was nonprotolytic followed by a slow protogenic reaction.
A thesis submitted to the School of Graduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirements for the award of the Degree of Doctor of Philosophy in Soil Science, 1985