Efficient adsorption of Cr(VI) on sunflower seed hull derived porous carbon

• Highly porous carbons were produced from sunflower seed hull by ZnCl2 activation.
• The SSH-derived porous carbons exhibited high yield and controllable pore size.
• The SSH-derived porous carbons were efficient Cr(VI) adsorbents.
• The optimum pH for Cr(VI) adsorption varied with initial Cr(VI) concentration.

Highly porous activated carbon prepared from sunflower seed hull (SSH), an abundant agriculture byproduct, was demonstrated as low-cost and efficient adsorbent for Cr(VI) removal. The porous carbon can be micropore or mesopore dominant depending on the impregnation ratio of the ZnCl2 activating reagent and SSH. The micropore dominant samples exhibited superior Cr(VI) adsorption capacity as compared to the samples with higher percentage of mesopore volume. It was interesting to note that the optimum pH for Cr(VI) removal in higher concentration solutions was lower as more H+ ions were required to produce an appropriate amount of the easily absorbable HCrO4− ions. However, hexavalent chromium was reduced to trivalent chromium in solution at pH ≤ 2.0, which reduced the overall removal rate of the total chromium. The adsorption behaviors of Cr(VI) on SSH activated carbons can be well described by Langmuir isotherm model and pseudo-second-order kinetic model, implying that the adsorption occurred through monolayer formation on the surface of adsorbents via chemical interactions.