![]() Though the introduction of melamine into MOFs was very meaningful, melamine as single absorbent had not been extensively studied by many researchers. found that melamine modified MOFs as absorbance had high adsorption capacity (205 mg g −1) and showed great adsorption performance on Pb (II) 34. Recent studies indicated that melamine derived nitrogen doped magnetic carbon demonstrated an excellent removal of Cr (VI) 32, and the modified melamine-Cr-MOF had a 1.5-fold increase in adsorptive removal of artificial sweeteners from water 33. Melamine was usually used in manufacture of plastics, adhesives, cleaners, and yellow dye 30, 31, showed great adsorption activity when it was modified with metal-organic frameworks. Solvent extraction 12, 26 and ion exchange 27, 28, 29 were also used to recover vanadium or chromium from the solution. In this process, vanadium was hydrolyzed and the retained vanadium was removed with Fe (II) and Fe (III), then the chromium was reduced to chromium (III) and it was precipitated as Cr 2O 3 In other research, hydrolysis and neutralization were used to separate and recover vanadium and chromium from vanadium (V)-containing chromate (VI) solution 25. Then hydrolysis precipitation was used to separate vanadium (V) and chromium (III). The vanadium (III) and chromium (III) were leached out via acid leaching, and then vanadium (III) was oxidized to vanadium (V) with MnO 2 while chromium was still existed as chromium (III). ![]() A process consisted of acid leaching, selective oxidation hydrolysis precipitation were investigated to separate and recover chromium from vanadium-chromium bearing reducing slag 24. Some experiments differed from the difference of the vanadium-chromium source had been conducted. Vanadium and chromium could be efficiently leached out through above technologies, but efficiently separated vanadium and chromium was hard to achieve as the high similarity physicochemical properties of vanadium and chromium. The common methods for leaching out vanadium and chromium were hydrometallurgical technologies, like sodium salt roasting-water leaching 16, calcium roasting-acid leaching 17, 18, 19, molten roasting and sub-molten technology 20, 21, and also oxidation leaching technologies 22, 23. A considerable number of investigations had been performed to obtain optimum technology for recovery of vanadium and chromium. To date, the main raw materials for vanadium recovery were converter vanadium slag 7, 8, 9, stone coal 10, 11, 12, steel slag 13 and waste catalyst materials 14, 15. In nature, vanadium and chromium are generally well-dispersed. Vanadium, chromium and their compounds are important national strategy resources and widely used in petrochemical, catalyst, iron steel industries, due to their excellent physicochemical properties 1, 2, 3, 4, 5, 6. While the adsorption of chromium (III) was followed the pseudo-first-order model and the adsorption isotherm was conformed to the Freundlich model as the adsorption efficiency was 98.63% under optimal conditions. ![]() 99.89% of vanadium (V) was adsorbed by melamine at the optimal conditions, the adsorption kinetic was followed the pseudo-second-order model and the adsorption isotherm conformed to the Langmuir model. The results showed that melamine was a good sorbent for adsorption of vanadium (V) and chromium (III). The effects of experimental parameters including dosage of melamine, reaction temperature and reaction time on the adsorption process were investigated. In this paper, separation and recovery of vanadium (V) and chromium (VI) from a leaching solution was investigated by adsorption of vanadium (V) with melamine, followed by electro-reduction of chromium (VI) and adsorption of chromium (III) with melamine, respectively. The vanadium (V) and chromium (VI) was hard to separate directly due to the similar nature.
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