Adsorption of Pb2+ and methylene blue by Al-incorporated magadiite

Abstract

The coexistence of heavy metals and organics in industrial wastewater is prevalent, and the removal of these toxic pollutants concurrently is of one of the greatest concerns facing wastewater treatment. In this study, a series of Al-incorporated magadiite (AlMagx, x represents nominal Si/Al ratio) were synthesized hydrothermally and investigated for their removal of Pb2+ and methylene blue (MB) in single- and binary-component solutions. The AlMagx featured a higher density of surface hydroxyl groups (6.4–12.7 sites·nm−2) and a greater negative surface than did the Al-free Magadiite (Mag). For single-component adsorption, the adsorption of both pollutants was pH-dependent. Under an optimal pH (= 3.00), the adsorption isotherms for Pb2+ and MB were consistent with the Langmuir and Freundlich models, respectively. AlMag240, having the highest surface hydroxyl density, displayed a maximum adsorption capacity of 113.5 and 220.2 mg·g−1 for Pb2+ and MB, respectively, exceeding that of Mag or other clay minerals. For Pb2+–MB binary-component adsorption, the removal efficiencies of Pb2+ declined by ∼4%–34% with higher MB concentrations, while MB's adsorption was slightly promoted by co-occurring Pb2+. Examination of adsorption mechanisms revealed that MB was adsorbed onto the external surface of AlMagx mainly via hydrogen bonding and electrostatic attraction, while Pb2+ was removed by ion exchange and subsequent hydroxyl-facilitated precipitation. The declined adsorption of Pb2+ was due to the rapid occupation of negative sites and hydroxyl sites by MB, and the slight promotion of MB adsorption with coexisting Pb2+ is likely owing to the increased negative charge of the adsorbents due to the increase in the solution ionic strength. These results demonstrated that the solution component is crucial to the adsorption process and that the AlMagx was a new adsorbent for the simultaneous removal of Pb2+ and MB.