Title: Egg shell-activated carbon from water hyacinth for heavy metal removal from wastewater: Isotherm and kinetic studies.

The study evaluated the efficacy of eggshell-activated carbon derived from water hyacinth (Eichhornia crassipes) harvested from Lake Victoria (Jinja) for removing heavy metals (Pb, Cd, Cu, Zn, and Cr) from industrial wastewater. Wastewater samples exceeded the National Environment Management Authority (NEMA) permissible limits for discharge into water or land, with initial heavy metal concentrations ranging from 0.54 mg/L (Cr) to 93.54 mg/L (Pb). Surface activation of the biochar using phosphoric acid was confirmed via SEM and FTIR analysis. Batch adsorption experiments demonstrated that optimal heavy metal removal occurred at pH 4.5, a contact time of 90 minutes, and an adsorbent dose of 1.0 g, achieving maximum adsorption capacities between 1.04 and 31.35 mg/g. Isotherm and kinetic modeling revealed that Cd²⁺, Zn²⁺, and Cr³⁺ ions followed the Freundlich isotherm and a pseudo-second-order kinetic model, whereas Pb²⁺ and Cu²⁺ adsorption aligned more closely with a pseudo-first-order kinetic model. Ultimately, the synthesized eggshell-activated carbon proved highly efficient, achieving up to 100% heavy metal removal at lower concentrations, offering a sustainable, cost-effective solution for industrial wastewater treatment.