Investigation of the usage of pumpkin wastes as an adsorbent in the removal of reactive blue 3R and reactive red P4BN textile dyes from aqueous solutions

In this study, the adsorption of Reactive Blue 3R (RB) and Reactive Red P4BN (RR) dyes with the waste consisting of the outer shell and the shell of the seeds of the pumpkin (WSSP) was studied. The effects of pH, time, dye concentration, amount of adsorbent, particle size of adsorbent and temperature on the adsorption were investigated. Characterization of WSSP was done by scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR‒ATR). The best removal was obtained at pH 3 for the RB dye and at pH 1 for the RR dye. It was determined that the adsorption process of RB dye reached equilibrium in 180 min and RR dye in 300 min. While the amount of adsorbent for the adsorption of both dyes was 1 g/L, the highest adsorption capacities were reached. In addition, it was observed that the removal percentages increased as the adsorbent particle sizes decreased. When the adsorption kinetics of RB and RR dyes were examined, it was determined that both of them were compatible with the pseudo second order kinetics. Equilibrium data of both dyes for the adsorption processes were found suitable with Langmuir isotherm. According to the temperature studies, for the adsorption of the RB dye on the WSSP, the adsorption percentage increases with the increase in temperature, and for the adsorption of the RR dye; it was observed that the adsorption percentage decreased with increasing temperature. The adsorption enthalpy (ΔH0) for the adsorption of RB dye was found to be 41.49 kJ mol−1 and it was determined that the adsorption reaction was endothermic. The entropy (ΔS0) was calculated as 139.65 J mol−1K−1. The positive value of ΔS0 indicated increased randomness at the solid-solution interface. The ΔH0 value for the adsorption of the RR dye was calculated as -38.94 kJ mol−1 and it was determined that the adsorption reaction was exothermic. The ΔS0 value was calculated as -118.49 J mol−1K−1 and this negative value showed that the adsorbate molecules at the solid-liquid interface are more ordered than those in the solution phase. For both dyes, Gibb's free energy (ΔG0) values were calculated as negative at all temperatures. Negative ΔG0 values indicated that the adsorption occurred spontaneously. As a result, it was determined that WSSP provides a good efficiency as an adsorbent in dye removal from aqueous solutions when optimum process conditions are provided.