Adsorptive potential of coconut fruit shell biochar as low-cost adsorbent for sequestration of rhodamine B dye: kinetics, thermodynamics and phytotoxicity studies

The present study aims to use coconut fruit shell biochar (CFSB), an efficient sorbent for Rhodamine B dye (RhB) elimination from aqueous solution. The availability of functional groups and morphology of CFSB surface were analyzed by applying Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Batch experiments were executed to test impact of pH, contact time, concentration of dye, biochar amount, agitation speed, temperature on RhB dye elimination from aqueous solution by CFSB. Maximum 96% RhB dye removal was reported at pH 2 by CFSB with RhB dye (250 mg/L concentration). Experimental results were found with best agreement with pseudo second order kinetic model. Equilibrium data were explained with Langmuir, Freundlich and Temkin models but results demonstrated best fit with Langmuir model with a higher correlation coefficient (R2 = 0.9818) and 8.1 mg/g monolayer RhB dye uptake potential. Thermodynamic assessment depicted Gibb's free energy change (Delta G degrees) had negative values, confirming spontaneity and feasibility of sorption process. Enthalpy (Delta H degrees) and entropy (Delta S degrees) changes were 32.039 kJ mol-1 and 92.802 J K-1), respectively. More importantly, even after five adsorption and desorption cycles, CFSB reflected 43% uptake capacity for RhB dye, suggesting that CFSB has a good practical application prospects. Phytotoxicity investigation revealed that CFSB treated solution of RhB dye increased growth of mung bean seedling and biochemical constituents. Therefore, coconut fruit shell biochar can be applied as economic green material for removal of RhB dye from contaminated water and transform industrial effluents into reusable asset.