Abstract
<title>Abstract</title> <p>The discharge of heavy metal-laden effluents from paint manufacturing operations is a significant and growing environmental hazard in industrialising economies, yet few biosorption studies test authentic multi-metal industrial effluent rather than synthetic single-metal solutions. This study investigates methanol/hydrochloric acid esterified banana peel waste (E-BPW) as a zero-cost biosorbent for removing chromium, nickel, cobalt, and lead from real paint manufacturing wastewater sourced from Brighto Paints Limited, Lahore, Pakistan. E-BPW was prepared by collection, drying, Tyler-standard sieve fractionation (850–1180 µm), and Fischer esterification with methanol and hydrochloric acid at 60°C for 48 h. Fourier-transform infrared spectroscopy confirmed carboxyl, hydroxyl, and ester carbonyl groups as the principal metal-binding moieties. Batch adsorption experiments evaluated the effects of initial metal concentration, solution pH, contact time, and adsorbent dosage, with metal concentrations quantified by flame atomic absorption spectroscopy. Under optimised conditions (pH 6–8, 80 min, 5 g·L⁻¹, 25°C), E-BPW achieved removal efficiencies of 80.3% (Cr), 68.9% (Co), 63.1% (Pb), and 56.0% (Ni). Regeneration with 2 mol·L⁻¹ HNO₃ recovered 99.3% of adsorbed metals, indicating strong potential for circular reuse. These findings demonstrate that waste-to-resource valorisation of agricultural byproducts can deliver technically effective, low-cost heavy metal removal for small-to-medium industrial applications, supporting Sustainable Development Goal 6 and circular-economy principles. The absence of isotherm, kinetic, and surface-characterisation data is acknowledged as a limitation, and a research agenda toward pilot-scale validation is proposed.</p>