Anion Exchange Capacity of Biochar.

Soil biochar applications have been shown to reduce anion leaching, possibly by increasing the anion exchange capacity (AEC) of soils.  Little, however, is known about the AEC of biochar.  We hypothesize that N heterocycles formed during pyrolysis may contribute anion exchange sites to biochar.   While pyrrolic N will not contribute to AEC, pyridinic N may be a good source of AEC at pHs below the pKa of the nitrogenous functional group.  In this study, we are investigating the chemistry of AEC in biochars and the processing conditions that optimize AEC formation in biochar.

A suite of chars were prepared through slow pyrolysis of alfalfa meal, corn stover, and cellulose at 500^◦C and 700^◦C.  AEC was measured using bromide as an index anion after equilibrating the biochars at pH 4, 6, and 8, in a complete factorial experimental design.   The results were analyzed using the GLIMMIX procedure in SAS.  AEC was found to increase significantly with decreasing pH.  AEC of alfalfa and corn stover chars was significantly greater than that of cellulose (pr(t) < 0.05).  Chars prepared at 700^◦C exhibit significantly greater AEC than those prepared at 500^◦C (p < 0.0001, Tukey-Kramer).  BET-N₂ surface area was significantly higher for the 700^◦C chars than the 500^◦C chars.  Alfalfa chars were found to have the lowest surface areas among the three feedstocks. 

Future studies will focus on characterizing the chemistry pertaining to AEC in biochars.  Functional groups will be identified using FTIR spectroscopy in chars demonstrating significant AEC.  SEM-EDX spectroscopy with elemental mapping will be used to visually correlate the spatial distribution of adsorbed Br and structural N on a bromide saturated char.