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 declined removal ability as pH increased 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 powder 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 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 being washed by the acid solution which can expand the longevity of AC and recover Cr(III). HIGHLIGHT In this work, we scrutinized the mechanism of poor removal capacity of commercial activated carbon on toxic heavy metal Cr(VI) under neutral pH conditions. Differing from the most accepted view that electrostatic repulsion is the main consideration, our study suggested that the relatively more Cr2O3 precipitate on the surface of activated carbon under higher pH led to the low Cr(VI) sequestration capability.