Effects of Some Operating Parameters on Sorption and Desorption Kinetics Related to Spotted Golden Thistle Stalks and Methylene Blue; Isotherm Study at Optimal pH and Sorbent Regeneration

In this work, the sorptive removal assessment of methylene blue by spotted golden thistle stalks was enhanced. Sorption was positively impacted by increasing temperature, initial pH or ionic strength. The sorbent isoelectrical point is 2.6 according to zeta potential measurements. The Elovich and Avrami models best fitted the sorption kinetic data; and an original prediction of equilibrium time is proposed. At 25 °C and for optimal initial pH = 7, the experimental maximal sorption capacity is 75 mg g−1. The Langmuir, Redlich–Peterson, Sips and Tóth models led to good correlations of isothermal data. Desorption percentages are important in both mineral and organic acidic media; the highest levels up to 91% were recorded for 1 × 10−2 M HCl solutions. The pseudo second-order model fitted satisfactorily the desorption kinetics. An innovative concept of regeneration is proposed, which promises significant savings of eluent and operating time. The sorbent regeneration assessment showed constant uptake abilities after five cycles of use.