Highly selective and efficient removal and extraction of heavy metals by layered double hydroxides intercalated with the diphenylamine-4-sulfonate: A comparative study

Six new NiCr layered double hydroxides (LDHs) intercalated with different inter-layer anions, including diphenylamine-4-sulfonate (DPA), dodecyl sulfonate (DS), pentansulfonate (PS), terephthalate (TA), fumarate (FA) and 2-ethylhexyl hydrogenphosphate (EHP), were synthesized and used to removal of Cd2+, Pb2+, Cu2+, and Zn2+ ions from water samples. The maximum adsorption capacities (qm) for the mentioned LDHs based on their inter-layer anions were DPA > DS > PS > TA > FA > DEHP. This observation highlights the key role of the structure of the inter-layer anions in the adsorption behavior of LDHs. The sulfonate-based LDHs showed better adsorption efficiency than carboxylate-based LDHs, and phosphonate-based LDHs showed the lowest adsorption efficiency. Also, at both high and low concentrations of the metal ions, the length of R– groups and the type of functional groups of inter-layer anions play a key role in the adsorption process. Subsequently, DPA-LDH, due to having the highest qm value among intercalated-LDHs, was chosen as an efficient sorbent for the removal of the metal ions from aqueous samples and its selectivity for adsorption of metal ions was investigated. DPA-LDH displayed a selectivity order of Zn2+ < Cu2+ <<< Cd2+ < Pb2+ for their adsorption, providing good separation of much more toxic metal ions of Cd2+ and Pb2+ from Zn2+ and Cu2+. For the highly toxic Pb2+ and Cd2+, enormous qm values as large as 479 and 282 mg g−1 can be obtained, respectively; and these are among the highest qm values reported for Pb2+ and Cd2+ sorbents. The very high distribution coefficient (Kd) values for Pb2+ (1.99 × 108 mL g−1) and Cd2+ (1.99 × 107 mL g−1), obtained using a V/m ratio of 2000 mL g−1, place the DPA-LDH at the top of materials known for such removal.