Estimation of the specific content and nature of sorption sites of iron(III) and zirconium(IV) oxyhydroxides

Abstract

The sorption of anions H 2 PO 4 -, HPO 4 2-, PO 4 3-, [Fe(CN) 6 ] 3-, and [Fe(CN) 6 ] 4-from aqueous solutions on the surface of Fe III and Zr IV oxyhydroxide hydrogels freshly precipitated at pH 4-13 was studied. The region of sorbate concentrations was from 0.00025 to 0.06 mol L -1 . The plots of the anion uptakes vs. their equilibrium concentrations are represented by iso therms of the first type, which are well described by the Langmuir equation if the quantity of the amount adsorbed is expressed as mol site g -1 . The maximum uptakes and constants of the Langmuir equation were calculated. The phosphate anions occupy the same number of sorp tion sites on the sorbents precipitated at different pH. The average specific content of sorption sites for the ferro and zirconogels in the metastability period is independent of the pH of their precipitation, being 3.1•10 -3 and 3.2•10 -3 mol site g -1 , respectively. The [Fe(CN) 6 ] 3-and [Fe(CN) 6 ] 4-anions are sorbed only on the positively charged sites of the hydrogels and occupy not more than 2•10 mol site g -1 in the studied interval of pH of precipitation.Freshly precipitated gels of metal oxyhydroxides, in particular, of Fe III and Zr IV , prepared by the fast alkaline hydrolysis of salt solutions, being washed off from the mother liquor and stored in water, exist as metastable phases. Their composition and acid base and sorption properties remain virtually unchanged for a time period from several days to several weeks. 1-5 Some published data 5 suggest that the specific surfaces of the hydrogels obtained by the alkaline hydrolysis of aqueous solutions of metal salts, such as Fe III , Zr IV , Ti IV , Cr III , and In III salts, are independent of the pH of precipitation. There fore, the interaction with protons, hydroxyl groups, and sorbate ions during the metastability period can be re duced to the recharge of the surface.In several cases, the sorption isotherms of hydro phosphate anions by the ferrogels precipitated at a con stant pH of precipitation (pH pr ) are dome shaped. 6 The sorption of phosphate anions on the ferrogels does not increase with an increase in the anion concentration, un like the sorption on goethite, 7 but somewhat decreases, and the initial regions deviate from the ordinate more strongly than those in the Langmuir isotherms. 8 This char acter of the isotherms can be explained 8,9 by different forms in which the sorbate exists in solutions.When the positive charge of the surface decreases, the uptake of several anions (sulfate, chromate, oxalate) on goethite and other crystalline oxyhydroxide sorbents 7,10 in the region of the negatively charged surface decreases to zero. For the sorption of hydrophosphate anions on the ferrogels, a change in the surface charge does not notice ably change the uptake value.It was supposed 8,9,11-13 that the character of sorption processes is also affected by the heterogeneous nature of sites on the oxyhydroxide surface.In this work, we attempted to reveal the regularities of sorption on the oxyhydroxide hyd...

Study and simulation of the adsorption of dihydrogen phosphate ions on magnetite as a function of pH

“…4) and (15), (16) after transforma tions, we obtain the following dependence of the oxide surface charge on some parameters [24]: (17) An analysis of the quantity ∂ϕ/∂pH at various back ground electrolyte concentrations [C] shows that it deviates from the Nernst equation, which is caused by the dependence of the second term in the equation given below on the background electrolyte concentra tion. Therefore, potential ϕ as a function of pH is described by the equation [24] …”

“…Using the dependence of the charge on pH, we can calculate the constants of the acid-base equilibria by Eq. (17). To this end, as noted above, it is necessary to optimize constants pK 1 -pK 3 to achieve the best coin cidence of experimental points with the calculated dependence of the charge on pH described by Eq.…”

“…To this end, as noted above, it is necessary to optimize constants pK 1 -pK 3 to achieve the best coin cidence of experimental points with the calculated dependence of the charge on pH described by Eq. (17). Table 1 gives the values of pK.…”

“…The Henry, Langmuir, Freundlich, Frumkin, and Temkin isotherms used to describe the adsorption of cations and anions cannot explain the effect of the charge, the potential, and pH on the adsorption equi librium constant and the distribution of formed com plexes over the surface [15][16][17]. Therefore, the follow ing models are being extensively used: the constant capacitance model [18], the diffusion model [19], the triplet layer model [18,20], and the surface complex formation model [5,21,22].…”