Synthesis of a new pyrimidine-based sorbent for indium(III) removal from aqueous solutions – Application to ore leachate

A new sorbent (TcTDG, pyrimidine derivative, with high content of N-based reactive groups and S-based sites) was successfully synthesized by the polycondensation of thiocarbazide and 2-thiobarbituric acid through reaction with formaldehyde. The material is characterized by FTIR, SEM and SEM-EDX analyses, titration, elemental analysis, BET, and TGA. After interacting with In(III), the FTIR spectrum of TcTDG shows the modification of the environment of N- and S-based reactive groups. The sorption of In(III) is first studied in synthetic solutions with TcTDG micro-particles through the evaluation of pH effect (optimum at pH 4), the uptake kinetics (equilibrium achieved in 20–30 min), the sorption isotherms and the selectivity properties from equimolar solution of different metal ions. The sorption capacity significantly decreases when increasing the temperature from 15 to 50 °C: In(III) sorption onto TcTDG is exothermic. Maximum sorption capacity at room temperature and at pH 4 reaches up to 1.87 mmol In g−1; the isotherm is equally fitted by the Langmuir and the Sips equations. Indium is fully eluted from metal-loaded sorbent using 0.3 M HCl solution (equilibrium reached in 20–30 min). The pseudo-first order rate equation fits well experimental kinetic profile; although the resistance to intraparticle diffusion plays a significant role in the control of In(III) sorption. The sorbent is highly stable at recycling: the FTIR spectrum is restored after five cycles and desorption remains complete along the cycles; after five re-uses, the sorption efficiency decreases by less than 4%. The study is extended to more complex solutions: first, with multi-component synthetic equimolar solutions before investigating the application of TcTDG to metal recovery from acidic leachate of ore sample (from Eastern Central Desert, Egypt). The sorbent shows a marked preference for In(III) at pH 4 against mono-, di- and tri-valent metal ions in synthetic solutions, the preference is also appreciable but the selectivity coefficient (SCIn/metal) is weaker for rare metals (between 3 and 6, vs. 10–35 for other metal ions). In the case of acidic leachate, the pH control to 4 leads to substantial precipitation of huge amounts of iron and the co-precipitation of other metal ions. The sorption capacity for In(III) (and Ga(III)) is significantly reduced by the complexity of the effluents; although the sorbent maintains a preference for these trivalent metal ions. The new sorbent shows promising sorption properties, although complementary investigation is required for optimizing the practical application of this material (conditioning, larger number of recycling steps, etc.).