Removal of hexavalent chromium from water using hydrochar obtained with different types of feedstock

Abstract

Hydrochar (HC) was obtained through the hydrothermal digestion of beetroot, carrot, or spent coffee grounds. The yield (i.e., mass of HC produced per mass of feedstock used) was ≈80%, ≈20%, and ≈50% with spent coffee, carrot, and beetroot, respectively. The surface area determined with the Brunauer–Emmett–Teller method was 3.06 m2·g–1 for HC from beetroot, 1.19 m2·g–1 for HC from carrot, 2.84 m2·g–1 for HC from dark roast coffee, and 3.22 m2·g–1 for HC from original roast coffee. Fourier transform infrared spectroscopy experiments showed that amine groups had a significantly higher density in HC produced with spent coffee grounds than that produced using either beetroot or carrot. HC could sorb hexavalent chromium (Cr(VI)) from water. The concentration of Cr(VI) in water decreased from 0.3 ppm to ≈0.07 ppm and to ≈0.1 ppm within 90 min using 20 g·L–1 of HC made from spent coffee grounds and from beetroot and carrot, respectively.