An alternative for the removal of hexavalent chromium under natural conditions using biowastes

This study aims to assess the capacity of six biomaterials to limit Cr(VI) concentrations under the physicochemical conditions of watercourses. To this end, various biomasses available within a short supply chain in an insular environment (passion fruit shells PFS, banana peels BP, coffee beans CB, pine sawdust PS and bark PB, and coconut fiber CF) were studied at 5 g/L in contact with 0.15 mg/L of Cr(VI). Cr(VI) was mainly trapped by all biomasses (56.25-77.98%), with simultaneous release of variable amounts of Cr(III) (1.43-30.91%) and Cr(VI) (0.78-42.31%) into solution. These results suggest a biosorption-coupled reduction mechanism after 7 days at a pH of 7.5. PFS and BP significantly limited the concentration of toxic Cr(VI) with 0.78% remaining in solution. The amounts of Cr(III) released into the solution were 30.91% and 21.85% for passion fruit shells and banana peels, respectively. Natural cations and anions have little influence on the efficiency of PFS, BP and PB for trapping Cr(VI). PFS and BP have highest reduction capacities, at 17.90 and 5.80 mmol/g, respectively, but BP exhibits faster kinetics according to Park's model. All acid-treated biomasses significantly lower Cr(VI), with PFS, BP and CB leaving 0.82, 0.82, and 0.98% remaining, respectively. The Cr(VI) adsorption fitting for biomasses follows the pseudo-second-order (PSO) kinetic model, the non-linear Langmuir model, and the Freundlich isotherm model. The addition of carboxylic groups on the sawdust surface limited the Cr(VI) concentration by up to 4.22%. This study suggests passion fruit shells and banana peels as promising options for removing Cr(VI) naturally.