Performing homogeneous catalytic ozonation using heterogeneous Mn2+-bonded oxidized carbon nanotubes by self-driven pH variation induced reversible desorption and adsorption of Mn2+

Abstract

Heterogeneous catalysts have gained increasing attention for catalytic processes because they are easy to recover and reuse. However, it is a challenge to achieve catalytic performance as high as the level of homogeneous catalysis on heterogeneous catalysts due to the inevitable mass transfer resistance between phases. Herein, Mn²⁺-bonded oxidized carbon nanotubes (Mn²⁺-OCNTs) were investigated as a heterogeneous catalyst with homogeneous catalytic performance for catalytic ozonation. Their homogeneous catalytic performance was achieved through the reversible desorption and adsorption of Mn²⁺ on OCNT surfaces. The de-/ad-sorption reversibility of Mn²⁺ was induced by the spontaneous variations of solution pH during pollutant degradation. Benefiting from the dropped solution pH (from 6.2 to 3.5) that is caused by the generated organic acid intermediates during phenol (20 mg L⁻¹) degradation, around 92% of Mn²⁺ is rapidly desorbed from OCNT surfaces and accumulated in solution. The desorbed Mn²⁺ in the solution allowed the homogeneous catalytic ozonation for phenol mineralization, as revealed by a total organic carbon (TOC) removal efficiency of 95%, which was comparable to that of the homogeneous Mn²⁺/O₃ system. With the degradation of the generated organic acid intermediates, the solution pH was increased from 3.5 to 4.5, which contributed to the re-adsorption of Mn²⁺ on OCNT surfaces and achieved the high recovery of Mn²⁺. Moreover, the Mn²⁺-OCNT/O₃ system could not only perform homogeneous catalytic ozonation in a controlled laboratory setting, but could also be practically feasible with the assistance of oxalic acid.