Thermodynamic description of the process of vanadium adsorption in carbon sorbent
DOI:
https://doi.org/10.14529/met160202Keywords:
sorption extraction of vanadium, modified carbon sorbents, thermodynamics of sorption of vanadiumAbstract
The paper describes a method of vanadium adsorption from acidic solutions on a highly-developed surface of the activated charcoal, modified by cationic surfactants. It is estimated that in an acidic media vanadium polyanions are mainly sorbed. It was proved that ions of copper, nickel, iron, calcium, magnesium, sodium and potassium do not interfere the sorption extraction of vanadium from the aqueous solution. The best values of vanadium sorption on the modified charcoal sorbent are obtained in the pH range from 2 to 4, i.e. in the acidic medium. In the acidic medium the hydroxides of other metals do not form the precipitate, thus enabling to extract the pure product. The reduction of the vanadium sorption with the increase of pH is connected with the fact that vanadium polyanions to which polyacids Н2V4O11, Н6V10O28, H4V6O17 correspond, are decomposed into anions to which acids Н4V2O7, HVO3, H3VO4 correspond. Therefore, one sorption center captures an anion containing a fewer number of vanadium atoms.
Filling the sorbent surface with vanadium compounds is similar to the Langmuir type sorption enabling to make thermodynamic description of this process and to determine main energy components.
Thermodynamic studies showed that the extraction of vanadium corresponds to physical adsorption of polyanions on a positively charged surface of charcoal (the degree of vanadium extraction is about 84%). Physical adsorption is confirmed by the fact that the DG of the reaction decreases with the increasing temperature, and the heat effect of the reaction does not exceed 13 kJ/mol. Additionally saturated sorbents were studied using X-ray diffraction. Phases of vanadium oxide V2O5 and a composite oxide of vanadium and manganese MnO∙V2O5. were detected on the X-ray photograph. Thus, the end product of calcination of saturated sorbent is vanadium pentoxide doped with manganese. Calcination of saturated sorbent together with a reducing agent enable to obtain metallic vanadium, the final product purity being about 99%.
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