Synthesis of Eskolaite (α-Cr2O3) Nanostructures by Thermal Processing of Cr2O3-Loaded Activated Carbon

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).

Highly surface activated carbon to remove Cr(VI) from aqueous solution with adsorbent recycling

To enhance the inferior removal capability of aqueous Cr(VI) by commercial activated carbon under neutral conditions. The emerging ball milling technology was employed and the removal efficiency of Cr(VI) by ball-milled highly activated carbon (HAC) increased from 68.3% to 99.0% under pH 6 and from 42.7% to 77.8% under pH 7 compared to pristine activated carbon (AC), respectively. Raman spectra and Boehm's titration results signified that the enhanced Cr(VI) removal performance of HAC under neutral conditions was associated with the enriched surface acid functional groups, in which the content of COOH groups increased from 0.31 mmol/g to 0.97 mmol/g. Two Cr(VI) removal mechanisms were proposed established on the acid and alkalic solution washed chromium-loaded HAC, involving the reduction of Cr(VI) to Cr(III) subsequently accompany with the formation of chromium hydroxides on the surface and inside the pores of HAC, and the bonding of CrO₄^2- on the surface COOH groups, as confirmed by SEM-EDX element mapping and specific surface area and porosity measurements. The Pseudo-second order model and Freundlich model fitted the adsorption kinetic and isotherm of AC and HAC well severally, suggesting that the specific interaction of Cr(VI) with the HAC surface and the Cr(VI) removal was multi-layer adsorption. Thermodynamic study exhibited the spontaneity of Cr(VI) removal on ball-milled HAC was increased. Reusability and regeneration studies of HAC denoted the potential application on Cr(VI) uptake under neutral conditions.