Carboxyl-functionalized bis(carbamoylcarboxylic) acid ligands as a novel alternative for hazardous metal ions removal by coagulation-flocculation

In this work, a new series of carboxyl-functionalized bis(carbamoylcarboxylic) acid ligands (bCCAs) was synthesized to evaluate their performance as coagulant-flocculant agents. The interaction of these ligands with metal ions was evaluated in solution by UV-Vis spectroscopy in organic media finding 1:2 and 1:1 (metal:ligand) complexes. Zeta potential (pZ) analysis demonstrate the negative charge of these ligands at low pH, as well as the charge neutrality adding metal ions reaching the isoelectric point (IEP) due to the complexation in aqueous media. The Log K_a values (2.517 to 3.887) indicate a strong metal-ligand interaction in water. These new bCCAs demonstrate outstanding performance in the simultaneous removal of metal ions such as Ca²⁺, Cd²⁺, Cu²⁺, Ni²⁺, Pb²⁺ and Zn²⁺, acting as both coagulant and flocculants in a coagulation-flocculation process, and showing excellent removal capacities (308 to 533 mg Mⁿ⁺/g of L or 1.94 to 3.68 mol Mⁿ⁺/mol of L), clarification kinetics (48.6 to 136.8 T%/h) and sedimentation ratios (96 to 156 mm/h). Comparing with the bCCAs type ligands previously studied, these carboxyl-functionalized analogues perform the fastest coagulation-flocculation process. These results indicate a high potential for effectively removing metal ions from industrial wastewater.