Sorption of Molybdenum onto Titanium Hydroxide

Abstract

Three types of ionic groups with different pKi values were revealed for T-5 sorbent by potentiometric titration: for acid groups, pK1 = 7.5 ± 0.5, pK2 = 10.2 ± 0.5, and pK3 = 11.0 ± 0.2; for base groups, pK4 = 6.5 ± 0.2, pK5 = 4.5 ± 0.2, and pK6 = 2.7 ± 0.2. Thus, titanium hydroxide is an amphoteric ion exchanger. Each exchange group of T-5 sorbent, characterized by its definite pKi value, was assigned to a definite hydroxy group. T-5 and T-52 sorbents exhibit high specificity to Mo in acid and neutral solutions. All the isotherms logCS–logCL are described by the Langmuir equation for the polyfunctional sorbent for at least two exchange sites in both nitric and sulfuric acid solutions, irrespective of the Mo speciation. Two groups of values were obtained and confirmed for T-5 and T-52 sorbents depending on the Mo concentration: kd ≈ 104 mL g–1 at concentrations lower than 10–5 M and kd = 1000–2000 mL g–1 at a concentration of 10–5 M. The exchangeable hydroxy groups in T-5 sorbent exhibit high heat resistance, and even the sorbent calcined at 900°C preserves certain reliably measurable capacity for Mo. For T-5, the capacity for Mo only slightly depends on the solution acidity and composition and is about ~1 mmol g–1.