Castor bean (Ricinus communis L.) presscake from biodiesel production: An efficient low cost adsorbent for removal of textile dyes

•The system is cheap and efficient for dyes removal.•Castor bean presscake has shown to be a suitable adsorbent for dyes removal.•Heat treatment increases the efficiency for tropaeolin adsorption.•Castor bean presscake has potential as low-cost natural biosorbent for dyes removal.

Castor bean (Ricinus communis L.) presscake, a byproduct derived from the biodiesel production process, was used in natura (CN) and thermally treated (CT) form as a natural and low-cost adsorbent for the removal of the dyes malachite green (MG) and tropaeolin (TP). The CN and CT were characterized by elemental analysis, infrared spectroscopy (FTIR), N2 adsorption/desorption and zeta potential. The effects of initial dye solution concentration, adsorbent mass/adsorbate volume ratio, initial pH of dye solution and contact time on MG and TP adsorption were studied using batch-type experiments a 25 °C. The equilibrium adsorption was analysed by Langmuir, Freundlich, Sips, dual site Sips (DSS) and Dubinin–Radushkevich (D–R) isotherms. Kinetics data were evaluated by pseudo-first order and pseudo-second order models. The adsorption data of MG and TP dyes onto CN and CT were best fitted to the dual site Sips model. The maximum adsorption capacities were 37.10, 33.54, 18.60 and 55.98 mg g−1 for the systems MG/CN, MG/CT, TP/CN and TP/CT, respectively. Pseudo-second order gives the better correlation to predict the kinetic process. The thermodynamic aspects were analysed.