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The complexation of aluminum and cadmium with sulphate and malonic acid: Results of potentiometric and solubility studies
Abstract
Potentiometric and solubility experiments were performed to determine the speciation and molal formation quotients for aluminum-sulphate, aluminum-malonate and cadmium-malonate complexes. Measurements of the complexation of Al$\sp{3+}$ and Cd$\sp{2+}$ with malonate were performed at temperatures ranging from 5 to 75$\sp\circ$C, whereas the Al-sulphate experiments were performed at temperatures from 5 to 125$\sp\circ$C. The ionic strength of the experimental solutions was varied between 0.1 and 1.0 molal, with NaCl and sodium-trifluoromethanesulfonate (NaTr) being used as the supporting electrolyte for the Al and Cd complexation experiments, respectively. In addition to measuring the association reactions, it was necessary to measure the dissociation of malonic acid in NaTr media, as these values were required to interpret the Cd-malonate association data. From the complexation studies, two Al-malonate and Al-sulphate species were identified, whereas a single Cd-malonate species was identified. The formation quotients for these species were modelled by empirical equations to describe their temperature and ionic strength dependence. The equilibrium constants obtained for the formation of each malonate species at infinite dilution and 25$\sp\circ$C are: $\rm Al(CH\sb2C\sb2O\sb4)\sp+ = 10\sp{7.49};\ Al(CH\sb2C\sb2O\sb4)\sbsp{2}{-} = 10\sp{12.62}$; and $\rm CdCH\sb2C\sb2O\sb4 = 10\sp{3.45}$, and for each sulphate species at 50$\sp\circ$C are: $\rm Al(SO\sb4)\sp+ = 10\sp{3.72}$; and $\rm Al(SO\sb4)\sbsp{2}{-}$ = 10$\sp{5.57}.$ Differentiation of the empirical equations with respect to temperature, provided thermodynamic quantities of $\rm\Delta H,\ \Delta S,$ and $\rm\Delta C\sb{p}$ for each complex. The modelled formation quotients for Al-sulphate at 50$\sp\circ$C were confirmed by reacting powdered gibbsite (Al(OH)$\sb3$) with aqueous $\rm H\sb2SO\sb4$-NaCl (10$\sp{-3.5}$ and 10$\sp{-4}$ molal H$\sp+$) solutions at 0.1 and 1.0 molal ionic strength. This study clearly indicated that the solubility of gibbsite is enhanced by the formation of Al-sulphate complexes. In addition, gibbsite solubility experiments were performed at 5$\sp\circ$C in $\rm H\sb2SO\sb4$-NaCl and HCl-NaCl (0.01 molal H$\sp+$) solutions. The dissolution rate of gibbsite in the sulphate solution was significantly faster than in the chloride solution.
Subject Area
Geochemistry|Chemistry
Recommended Citation
Ridley, Moira Kenina, "The complexation of aluminum and cadmium with sulphate and malonic acid: Results of potentiometric and solubility studies" (1996). ETD collection for University of Nebraska-Lincoln. AAI9734637.
https://digitalcommons.unl.edu/dissertations/AAI9734637