The present work reports on the potential of using a modified activated carbon as an eco-friendly adsorbent (second stage waste), obtained from a bio-diesel waste production plant, for the removal of Cr(III) ions from aqueous solutions. Chemical activation of the green energy waste (GEW), using sulfuric acid as a dehydrating agent, was adapted in this study The effects of pH, contact time, dosage, and initial concentration were evaluated and optimized in a batch processing mode. The modified activated carbon was fully characterized to observe morphological changes using SEM, XRD, and FT-IR techniques. SEM images however, showed significant changes in the carbon morphology before and after the adsorption of Cr(III) ions. The adsorption results indicated that the equilibrium data were in accordance with the Langmuir isotherm, yielding a maximum monolayer adsorption capacity of 171.0 mg g − 1 at 29° C. Moreover, the kinetic studies indicated that the adsorption process followed a pseudo-second-order model. Assessment of our results revealed that GEW-AC was considered as a prospective adsorbent which could be used as a cost-effective substitute for marketable activated carbons for the removal of Cr(III) ions from wastewater systems.