Mullein leaf as potential biosorbent for copper(II) ions removal from synthetic solutions: optimization, kinetic and isotherm

In this study, the potential of mullein (Verbascum Thapsus) leaf as a novel biosorbent was examined for the biosorption of copper(II) ions from synthetic solutions. The effect of various factors including the initial copper(II) concentration (25-400 mg L-1), solution pH (3-7), contact time (10-120 min.), and biosorbent dosage (2-20 g L-1) was investigated. It has been observed that the biosorption of copper(II) increases with time until equilibrium was reached after 90 min. The solution pH of 4 was used as optimal because at lower pH values, functional groups of mullein can be protonated and in that form, they are not able to bind metal cations. However, at higher pH values than 4, insoluble copper(II) hydroxide can precipitate and lead to a decrease in biosorption efficiency. The maximum biosorption efficiency of 84.51% was observed at a biosorbent dosage of 2 g L-1 and an initial copper(II) concentration of 100 mg L-1. The FT-IR analysis of mullein leaf showed a presence of C-H, C-O, O-H, and COO- groups desired for copper(II) biosorption. The biosorption of copper(II) ion by mullein followed the pseudo-second-order model and the adsorption isotherm was best described by the Freundlich nonlinear model. Multilayer biosorption of copper(II) ions was determined on the heterogeneous surface of the mullein. The reuse of mullein was tested and the obtained results showed that after three desorption cycles, copper (II) biosorption efficiency was 81.4%. Hence, mullein could be an efficient and economical alternative as a biosorbent for the treatment of wastewater containing copper(II) ions. [GRAPHICS] .