Metal Sequestration in Biochar Suspensions.

Impounded metal mine waste (tailings) are C-depleted, and contain high concentrations of metal contaminants.  Biochars may improve the physical and chemical properties, biological activity and diversity of tailings.  The objective of this study is to characterize the sequestration of 7 metals in suspensions of biochars adjusted to pH 4.5.  Mono-metallic solutions of the oxyanions As and Se and the cations Cd, Cu, Pb, Ni, and Zn were added at a single rate of 3 mmol kg^-1 biochar.  Biochars were produced from softwood, hardwood, grass, and poultry litter under different conditions using large-scale pyrolysis units.  In this study, the amount of metal sequestered by the biochar suspensions depended on the type of biochar and metal.  For example, the greatest decrease in Cd and Ni solution concentrations was observed with biochars obtained from fast pyrolysis of willow at 450 ^oC.  For the majority of biochars, a greater proportion of Cu and Pb were removed from solution compared to other metals.  The following properties of these biochars were also characterized: organic C, inorganic C, H/organic C ratio; % volatile matter, % fixed matter, % ash using proximate analysis; total specific surface and micropore  areas; total elemental composition following acid digestion and dissolved elements in the biochar suspension, both analyzed using inductively coupled plasma-atomic emission spectroscopy or mass spectroscopy; and functional group composition using Diffuse Reflectance Infrared Fourier Transform Spectroscopy. The relationship between these properties and decreased soluble metal concentrations will be examined to determine whether specific biochar properties govern metal sequestration.  The influence of both the organic and inorganic components will be considered, since sorption to specific functional groups and precipitation with inorganic components are viable mechanisms for metal sequestration in the presence of biochars.  This study will help determine whether biochar addition to tailings has the added benefit of metal sequestration.