Metal Leaching in Mine Tailings: Short-Term Impact of Biochar and Wood Ash Amendments

Abstract

Biochar is perceived as a promising amendment to reclaim degraded, metal-contaminated lands. The objective of this study was to compare the potential of biochar and wood ash amendments to reduce metal(loid) leaching in mine tailings. A 2-mo leaching experiment was conducted in duplicate on acidic and alkaline tailings, each mixed with 5 wt.% of one of the following amendments: three wood-derived, fast-pyrolysis biochars (OC > 57 wt.%) and two wood ash materials (organic carbon [OC] ≤ 16 wt.%); a control test with no carbon input was also added. The columns were leached with water after 1, 2, 4, 8, 16, 32, and 64 d, and the leachates were monitored for dissolved metals, OC, and pH. For the acidic and alkaline tailings, the most significant impact on metal mobility was observed with wood ash materials due to their greater neutralization potential (>15% CaCO eq.) compared with biochar (≤3.3% CaCO eq.). An increase of 1 pH unit in the wood ash-treated alkaline tailings led to an undesirable mobilization of As and Se. The addition of biochar did not significantly reduce the leaching of the main contaminants (Cu and Ni in the acidic tailings and As in the alkaline tailings) over 2 mo. The Se attenuation noted in some biochar-treated acid tailings may be mainly due to a slight alkaline effect rather than Se removal by biochar, given the low capacity for the fresh biochars to retain Se under acidic conditions (pH 4.5). The increased loss of dissolved OC in the biochar-amended systems was of short duration and was not associated with metal(loid) mobilization.

Differentiating Inorganics in Biochars Produced at Commercial Scale Using Principal Component Analysis

“…Total IC refers to total inorganic C. Data points correspond to the following: 1 = Airex 1; 2 = Airex 2; 3 = Pyrovac; 4 = BassWd 400; 5 = BassWd 450; 6 = Willow 450; 7 = Willow 500; 8 = BasquesC; 9 = BasquesF; 10 = SGrass 400; 11 = SGrass 500; 12 = Poultry litter 450; 13 = Poultry litter 500; 14 = WdAsh A; 15 = WdAsh B. For samples digested using two different methods, A are samples acid-digested using aqua regia without hydrofluoric acid (HF) and B are samples acid-digested with HF, as published by Beauchemin et al 7 Loadings plots are shown in the Supporting Information (Figure S1).…”

“…Organic C functional groups in Airex 2, BasquesF, Poultry litter 450, Pyrovac, SGrass 450, and Willow 450 were previously characterized. 11 Airex 1, BassWd 400, BassWd 450, WdAsh A, and WdAsh B were used in a study by Beauchemin et al on the impact of biochar on metal leaching from mine tailings; 7 Airex 2, BasquesF, Poultry litter 450, Pyrovac, SGrass 450, and Willow 450 were used in a study by Clemente et al on metal removal from solution. 11 The feedstock for Willow, SGrass, Poultry, Airex 2, and Pyrovac were chemically characterized to evaluate differences in element composition between feedstock and the resulting biochars.…”

“…5 To this end, powder X-ray diffraction (XRD) is used to identify minerals associated with pyrolysis residues. 6,7,10,13,16 Combining chemical and mineralogical data has allowed investigators to describe nutrient release and interactions between biochars and environmental contaminants. 7,10,13 Scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDX) provides spatially resolved element composition of amorphous and crystalline phases and often complements chemical and XRD data.…”

Towards a Soil Remediation Strategy Using Biochar: Effects on Soil Chemical Properties and Bioavailability of Potentially Toxic Elements

“…The earlier studies did not focus on the quantity of biochar applied but more on the effects of various biochars on a wide range of soils and establishing the mechanisms of interaction [ 133 , 177 , 183 , 184 ]. Subsequently, researchers investigated the effects of biochar on Zn bioavailability in combination with plant species grown in contaminated soils.…”

Municipal Biosolids Capillary Barrier Covers for Reactive Mine Tailings

“…Of considerable interest is the profile for iron, which shows significant concentrations of iron in the pre-oxidized tailings, but with very low concentrations in the pore water of the unoxidized tailings. Previous studies analyzing acid digestion of the tailings from Q and P areas show that the pre-oxdized tailings are typically ~20-25% iron by weight (Beauchemin, Clemente, et al, 2014), Arsenic concentrations are shown to be high in initial saturation extraction from the unoxidized tailings, however, arsenic concentrations within all other measured tailings are significantly lower.…”