Mechanistic insights into remediation of low concentration of Cr(VI) using algal biochar-derived activated carbon

Chromium (Cr (VI)) is one of the highly persistent, carcinogenic, non-biodegradable heavy metals with excessive mobility. Efficient and sustainable methods for Cr (VI) removal from water and wastewater bodies are being developed and in great demand. However, effluents with lower concentrations of Cr (VI) such as < 10 µg/mL which are still toxicologically hazardous required urgent solutions. In this study, we have successfully synthesized biochar (BC) as an adsorbent from Chlorophyceae algae and activated it with KOH (KOH-AC) to treat low levels of Cr (VI). The study examined the crucial influencing factors, such as the starting concentrations of Cr (VI) ranging from 1 to 10 ppm, and the contact time varying from 10 to 120 min. Results showed that BC and KOH-AC achieved Cr (VI) removal efficiencies of 97.37% and 99.44%, respectively, at an initial concentration of 1.0 ppm. Freundlich adsorption isotherm and pseudo-second-order kinetic model (R 2 = 0.999) well fitted to describe maximum Cr (VI) uptake onto the adsorbent material and the equilibrium time. The Langmuir adsorption capacity of BC was analyzed to be 25.32 mg/g and that for KOH-AC as 7.24 mg/g. Findings provide evidence that the rate of adsorption is influenced by the adsorption capacity rather than the concentration of the adsorbate. These results set basis for the development of algae-derived AC for the efficient removal of low-concentrated Cr (VI) effluents.