Green Adsorbent from Sorghum Husk Ash for Efficient Heavy Metal Removal: Adsorption Mechanisms and Kinetic Insights

Abstract

Heavy metal contamination in water poses a serious environmental and health hazard, necessitating the development of sustainable, low-cost adsorbents. Agricultural waste-based materials have gained attention, yet sorghum husk ash remains underexplored for value-added applications. This study has addressed this gap by synthesizing zeolite from sorghum fly ash and evaluating its efficiency as a bio-sorbent for the removal of Pb(II), Ni(II), and Cd(II) from synthetic aqueous solutions. Zeolite was prepared using ultrasound-assisted alkali fusion followed by a hydrothermal method. Characterization techniques, including FT-IR, XRD, XPS, BET isotherms, and SEM, analysed the functional groups and surface morphology of zeolite. Batch experiments showed maximum removal efficiency of 98.3% for Pb(II), 86.3% for Cd(II), and 60.5% for Ni(II), with equilibrium reached in 2 h. Adsorption efficiency increased significantly from pH 3 to 7 for all three metal ions, reaching a maximum at neutral pH. Beyond pH 7, a slight to moderate decline in removal efficiency was observed, particularly for Cd(II) and Ni(II), likely due to the formation of metal hydroxide precipitates at alkaline conditions. Isotherm studies indicated alignment with the Langmuir model, and kinetics followed a pseudo-second-order model. The bio-sorbent demonstrated effective regeneration and reuse for up to five cycles, highlighting its potential as an efficient material for heavy metal removal.