Biochar Reduces Bioavailable Heavy Metals Concentrations in Mine Land Soils.

Biochar used in production agricultural settings may be cost prohibitive.  Thus, application of this material for novel uses is paramount for sustaining biochars future utility.  Biochar mediated heavy metal sorption for reclamation purposes in mine-land settings may fill this niche.  Mining-affected lands are a global issue; in the USA alone there are an estimated 500,000 abandoned mines encompassing hundreds of thousands of hectares.  In a proof-of-concept study, we added lodgepole pine, tamarisk, and switchgrass biochar (0, 5, 10, 15% by weight) to five different western US mine land soils affected by various heavy metals (Cd, Cu, Mn, Pb, Zn).  Extraction with 0.01M CaCl₂ showed that increasing biochar application rate significantly decreased ‘bioaccessible’ metals in almost all instances.  A concomitant increase in solution pH was observed, suggesting that metals may be rendered bio-inaccessible through precipitation as carbonate or (hydr)oxide phases, or sorbed onto mineral surfaces.  Following the 0.01M CaCl₂ extraction, biochar-soil mixtures were air-dried and metals were further extracted using a four-step sequential removal procedure.  Results from selective extraction suggest that, as compared to the controls, most metals in the biochar-amended mine land soils were associated with exchange sites, carbonate, and oxide phases.  Biochar may play a pivotal role as a soil amendment in the future of mine land reclamation, although elevated pH levels should be maintained to prolong sequestration while lessening the possibility of metal resolubilization.