Enhancing methylene blue sorption on spotted golden thistle stalks by soft chemical pretreatments

Abstract

Soft chemical treatments of dried stalks of the spotted golden thistle yielded novel sorbents; both the stiff nature and mass of the raw material were preserved. For minimal environment impact, the solution concentrations of the tested reagents were optimized at ambient temperature. The sorbents obtained were tested for the removal of methylene blue. The approach used kinetics and equilibrium studies. NaOH (0.1 mol L−1) treatment was efficient and afforded two new materials with seriously improved sorption features: the equilibrium time was reduced from 17–19 h to 10–12 h whilst, maximal sorption capacity increases from 65 to 100 and 112 mg g−1 i.e., enhancements of 54 and 72%. The Avrami model followed by the pseudo second-order model, best fitted the kinetic data. The isotherms were of Hmx type and were satisfactorily fitted to the Redlich-Peterson model. The Langmuir, Tóth, and Sips models were suitable to describe the H type isotherms relating to limited data ranges. Microanalysis, scanning electron microscopy (SEM), SEM coupled to energy dispersive X-ray, and attenuated total reflectance infrared spectroscopy showed an increase of the lignocellulosic content and the strengthening of the interactions between the dye and the sorbent. However, zeta potential measurements showed that the charge of the surface became almost inexistent after the pretreatment.