RESUMEN
Oxometalates of vanadium(V), chromium(VI), and manganese(VII) have negative impacts on water resources due to their toxicity. To remove them, the kinetics of 0.04 mM oxometalates in natural and synthetic water were studied using graphene nanoplatelets (GNP). The GNP were dispersible in water and formed aggregates >15 µm that could be easily separated. Within 30 min, the GNP were covered with ~0.4 mg/g vanadium and ~1.0 mg/g chromium as Cr(OH)3. The reaction of 0.04 mM permanganate with 50 mg of GNP resulted in a coverage of 10 mg/g in 5 min, while the maximum value was 300 mg/g manganese as Mn2O3/MnO. TEM showed a random metal distribution on the surfaces; no clusters or nanoparticles were detected. The rate of disappearance in aerated water followed a pseudo second-order adsorption kinetics (PSO) for V(V), a pseudo second-order reaction for Cr(VI), and a pseudo first-order reaction for Mn(VII). For Cr(VI) and Mn(VII), the rate constants were found to depend on the GNP mass. Oxygen sorption occurred with PSO kinetics as a parallel slow process upon contact of GNP with air-saturated water. For thermally regenerated GNP, the rate constant decreased for V(V) but increased for Cr(VI), while no effect was observed for Mn(VII). GNP capacity was enhanced through regeneration for V(V) and Cr(VI); no effect was observed for Mn(VII). The reactions are well-suited for use in water purification processes and the reaction products, GNP, decorated with single metal atoms, are of great interest for the construction of sensors, electronic devices, and for application in single-atom catalysis (SAC).
RESUMEN
A new type of functional lignin-based spherical particles (L-CTAB) prepared with the use of hexadecyltrimethylammonium bromide (CTAB) was applied as an effective biosorbent for removing vanadium(V) ions. The porous structure, characteristic functional groups, electrokinetic stability, morphology and size of the L-CTAB particles were examined. The conditions of removal were also investigated, including pH (2-12), sorbent mass (0.1-0.5 g), concentration (10-100 mg/dm3), phase contact time (1-240 min) and temperature (293-333 K). At pH 5.0 the maximum sorption percentage (%S) of V(V) was 45%, while at pH 2.0 it was 32%. The maximum sorption capacity of V(V) for L-CTAB was found to be 10.79 mg/g. The kinetic data indicate that the sorption followed the pseudo-second-order and film diffusion models. Sorption equilibrium for V(V) ions removal by L-CTAB was reached after 60 min at the initial concentrations 10 and 50 mg/dm3. It has been shown that the adsorption of V(V) ions on the surface of L-CTAB is a heterogeneous, endothermic and spontaneous reaction, as evidenced by the calculated values of thermodynamic parameters - free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) - for the tested systems at different temperatures. HCl solutions, used as an L-CTAB regeneration agent, quantitatively eluted V(V) ions.