We have used biochar (to be ...

We have used biochar (to be clear, elemental carbon) in anoxic biochemical reactors  filled with static media for treating acidic and neutral mining influenced water.  The biochar is not consumed or degraded, but functions as an electron shuttle that facilitates redox reactions.  Xu et al. reported in Environmental Science and Technology in 2016 (Biochar-Facilitated Microbial Reduction of Hematite) that biochar accelerated the rate of hematite (Fe2O3) reduction by two-fold.  In bench scale testing, we found that including nominal amounts of biochar (18% by wt.) to biochemical reactors filled predominantly with wood chips reduced nitrate and sulfate at about the same kinetic rate as BCRs filled predominantly with more-easily digested straw mixed with only 7% biochar.  The recommendations below to conduct bench and pilot testing are good, but you might want to start with an "Edisonian" approach of multiple jar tests or proof of concept mini-reactors of 250 mL size to identify the most promising mixes before advancing to bench testing. Good luck.