Stability of Anion Exchange Sites during Chemical Oxidation of Biochar.

Little is known about the stability of anion exchange capacity (AEC) of biochars and by what mechanisms AEC changes as biochar weathers in soil environments.  Our goal in this study was to assess the effect of oxidation treatments on AEC of biochar.  Biochars were oxidized by exposure to singlet oxygen in an alkaline (1 M NaOH) aqueous medium for 4 months to mimic what may occur in natural soil environments.  Oxidation of biochars caused AEC to decline on average by 54%.  Fourier Transform Infra-Red (FTIR) spectroscopy demonstrated that biochars produced at 700 °C (peak pyrolysis temperature) had greater levels of condensed aromatic C than biochars produced at 500 °C.  Oxidation increased the intensity of FTIR bands for carbonyl and hydroxy groups in spectra of biochars produced at 500 °C and peroxy ether groups for biochars produced at 700 °C.  Biochars produced at 700 °C exhibited a lower decline in AEC following the oxidation treatments in contrast to biochars produced at 500 °C.  The AEC of an alfalfa meal biochar produced at 700 °C did not change significantly (p = 0.19) following oxidation.  Stability of AEC in the high temperature alfalfa meal biochar is attributed to the highly condensed aromatic character of carbon in this biochar.