Effective removal of Pb2+ and Cu2+ from highly concentrated aqueous solutions: comparative sorption study

The study explored the sorption behavior of Cu2+ and Pb2+ ions onto humates obtained from brown coal wastes. The composition and structure of soluble and insoluble humates are analyzed by methods of SEM, EDX, and FTIR. SEM and EDX showed the multiphase structure of both humate samples, which changes significantly because of the sorption of heavy metals. It is observed that copper and lead are built in different ways into the humic matrix: The analysis of FTIR spectra revealed that sorption of Cu2+ ions of both samples is assumed to occur through formation of copper dycarbonyl (Cu(CO)2) and ion-exchange mechanism, while in the case of lead sorption, phenolic and carboxyl groups are involved in the sorption process. The effect of various operational parameters such as pH, contact time, initial metal ions concentration and humates dose was investigated. The optimum pH for effective metal ion binding was found to be about 5.4–5.6 and 6.2–7.2, for Cu2+ and Pb2+ solutions, respectively. The optimum contact time was 1 h. The experimental results showed that both types of humates investigated in the work bind Pb2+ ions better than Cu2+ ones. H-B samples revealed the sorption capacity of 122 and 388 mg g–1 and H-GK samples of 58 and 350 mg g–1 for Cu2+ and Pb2+ ions, respectively. It was observed that HCl, HNO3, H2SO4 acids are good eluents for H-B and H-GK regeneration (desorption reaches 100%). The kinetic studies revealed that sorption could be described by a pseudo-second-order rate equation. Thermodynamic studies showed that the values of ΔG° were negative at all temperatures confirming the feasibility of the sorption process. The positive value of ΔS° indicated that ion exchange reactions occurred.