Novel application of (NH₄)₂Mo₃S₁₃ for vanadium adsorption: Experiments, characterization, kinetic and equilibrium modeling

The adsorption of vanadium on (NH4)(2)Mo3S13 was analyzed via experimental and theoretical approaches. Kinetic and equilibrium adsorption data were quantified at 293-323 K and pH 3.2. Vanadium kinetics followed a pseudo-first order equation with adsorption rate constants ranging from 0.006 to 0.032 min(-1), while the maximum experimental vanadium adsorption capacities ranged from 9.9 to 362 mg/g under tested operating conditions. Equilibrium data were fitted by a monolayer model that considered that the vanadium ions were removed via two different active sites of (NH4)(2)Mo3S13 surface. These adsorption sites were related to the Mo moieties of this material. The calculated adsorption capacities of these active sites followed the next trend: Q(1) (293 K) > Q(2) (293 K), Q(2) (303 K) > Q(1) (303 K) and Q(2) (313 K) > Q(1) (313 K). In particular, the calculated saturation vanadium adsorption capacities were 124 - 376 mg/g. The impact of adsorption temperature on the calculated number of vanadium ions that were adsorbed by both active sites was also studied. It was concluded that the temperature facilitated the adsorption of vanadium ions via the first active site of (NH4)(2)Mo3S13 surface, while an opposite trend was identified regarding the second type of active site. The interaction energies involved in the adsorption of both active sites were also calculated. These results indicated that both sites contributed in different degree to reduce the vanadium concentrations in the aqueous solutions for depollution purposes. Overall, this study provides useful insights on the vanadium adsorption mechanism for an alternative and competitive adsorbent, which is promising to be implemented in large-scale water purification systems.