A new insight into the restriction of Cr(VI) removal performance of activated carbon under neutral pH condition

Activated carbon has been widely used to remove hazardous Cr(VI); however, the impact of Cr₂O₃ precipitate on gradually declining removal ability as pH increases has received little attention. Herein, to investigate the effect of Cr₂O₃, SEM-EDX (scanning electron microscope-energy dispersive X-ray analysis) coupling elements mapping of chromium-loaded powdered activated carbon (PAC) revealed that a chromium layer was formed on the PAC exterior after being treated with Cr(VI) at pH 7. XPS (X-ray photoelectron spectroscopy) study confirmed that 69.93% and 39.91% Cr₂O₃ precipitated on the PAC surface at pH 7 and pH 3, respectively, corresponding to 17.77 mg/g and 20 mg/g removal capacity. Exhausted PAC had a removal efficiency of 92.43% after Cr₂O₃ being washed by H₂SO₄ solution, which was much higher than the removal efficiency of 51.27 % after NaOH washing. This further verified that the intrinsically developed Cr₂O₃ precipitate on PAC under neutral conditions limited the durability of PAC as an adsorbent. Consecutive elution assessments confirmed that adsorption and reduction ability both declined as pH increased. Raman spectroscopy and C 1s spectra of materials demonstrated two distinct Cr(VI) removal mechanisms under pH 3 and pH 7. In conclusion, the exhausted AC after Cr(VI) adsorption can be rejuvenated after the surface coated Cr₂O₃ is washed by the acid solution, which can expand the longevity of AC and recover Cr(III).