Mass Transfer, Kinetic, Equilibrium, and Thermodynamic Study on Removal of Divalent Lead from Aqueous Solutions Using Agrowaste Biomaterials, Musa acuminata, Casuarina equisetifolia L., and Sorghum bicolor

Three distinct agricultural waste materials, viz., casuarina fruit powder (CFP), sorghum stem powder (SSP), and banana stem powder (BSP) were used as low-cost adsorbents for the removal of toxic lead(II) from aqueous solutions. Acid treated adsorbents were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and Fourier transform infrared spectroscopy (FTIR). The effects of parameters like adsorbent dose, pH, temperature, initial metal ion concentration, and time of adsorption on the removal of Pb(II) were analyzed for each adsorbent individually and the efficiency order was BSP > SSP > CFP. Based on the extent of compatibility to Freundlich/Langmuir/Dubinin-Radushkevich/Temkin adsorption isotherms and different models (pseudo-first and second order, Boyd, Weber's, and Elovich), chemisorption primarily involved in the case of BSP and SSP, whereas simultaneous occurrence of chemisorption and physisorption was proposed in the case of CFP which was correlating with the thermodynamic study results conducted at different temperatures. Based on the observations, it was proposed that three kinetic stages involve in the adsorption process, viz., diffusion of sorbate to sorbent, intra particle diffusion, and then establishment of equilibrium. These adsorbents have a promising role towards the removal of Pb(II) from industrial wastewater to contribute environmental protection.