Activated carbon with excellent chromium(VI) adsorption performance prepared by acid-base surface modification

In the present work, activated carbon (AC) with excellent Cr(VI) adsorption performance especially at low concentrations was prepared by an acid-base surface modification method. Raw activated carbon (AC0) was first oxidized in boiling HNO3 (AC1), then treated with a mixture of NaOH and NaCl (AC2). Batch equilibrium and continuous column adsorption were conducted to evaluate the adsorption performance. Boehm titration, elemental analysis, and N2/77 K adsorption isotherm methods were used to characterize the surface properties and pore structure of modified ACs. The results revealed that the modified AC exhibited excellent Cr(VI) adsorption performance in terms of adsorption capacity and adsorption rate: AC2 > AC1 > AC0. Modification caused SBET to decrease and the total number of surface oxygen acidic groups to increase. HNO3 oxidization produced positive acid groups, and subsequently NaOH treatment replaced H+ of surface acid groups by Na+, and the acidity of AC decreased. The main cause of higher Cr(VI) adsorption capacity and rate for AC2 was the presence of more oxygen surface acidic groups and suitable surface acidity. HNO3-NaOH modification shows potential for the preparation of high quality AC for the effective removal of low concentrations of Cr(VI).