Spectroscopic investigation of Cu2+, Pb2+ and Cd2+ adsorption behaviors by chitosan-coated argillaceous limestone: Competition and mechanisms

In the present study, the competitive adsorption of Cu 2+ , Pb 2+ , and Cd 2+ by a novel natural adsorbent (i.e., argillaceous limestone) modified with chitosan (C-AL) was investigated. The results demonstrated that both intraparticle diffusion and chemisorption marked significant contributions to the Cu 2+ adsorption process by both raw argillaceous limestone (R-AL) and C-AL in mono-metal adsorption systems. Antagonism was found to be the predominant competitive effect for Cu 2+ , Pb 2+ and Cd 2+ adsorptions by C-AL in the multi-metal adsorption system. The three-dimensional simulation and FTIR analysis revealed that the presence of Cu 2+ suppressed Pb 2+ and Cd 2+ adsorptions, while the effect of Cd 2+ on Cu 2+ and Pb 2+ adsorptions was insignificant. The spectroscopic analyses evidenced that amide groups in C-AL played a crucial role in metal adsorption. The preferential adsorptions of Pb 2+  > Cu 2+  > Cd 2+ were likely due to the different affinities of the metals to the lone pair of electrons on the N atom from the amide groups and/or the O atoms from the –OH and -COO - groups on C-AL. The interactions between C-AL and metal ions and between various metal species influenced their competitive adsorption behaviors. C-AL exhibited a superior metal adsorption capacity in comparison with that the capacities of other natural adsorbents reported during the last decade, suggesting its potential practical applications.