Characterizing
the inorganic phase of biochar, beyond determining
element concentration, is needed for appropriate application of these
materials because mineral forms also influence element availability
and behavior. Inorganics in 13 biochars (produced from Poultry litter,
switchgrass, and different types of wood) were characterized by proximate
analysis, chemical analysis, powder X-ray diffraction (XRD), and scanning
electron microscopy with energy-dispersive X-ray (SEM-EDX) spectroscopy.
Principal component analysis (PCA) was used to compare biochars and
characterize associations between elements. The biochars were produced
using commercial-scale reactors and represent materials with properties
relevant to field application. Bulk inorganic concentration and composition
were responsible for differentiating biochars after PCA of chemical
data. In comparison, differentiation based on PCA of diffractogram
fingerprints was more nuanced. Here, contributions from cellulose
and turbostratic crystalline C influenced separation between samples.
It was also sensitive to mineral forms of Ca (whewellite and calcite).
Differences in crystalline C and Ca minerals separated two biochars
generated from the same willow feedstock using the same pyrolysis
conditions at different temperatures. PCA of 606 SEM-EDX point scans
revealed that inorganics belong to four main clusters containing Ca,
Fe, [Al, Si], and [Cl, K, Mg, Na, P, S] consistent with XRD identification
of calcite, magnetic Fe-oxide, silicates, and sylvite. It further
suggested that amorphous P-containing minerals associated with Ca
(not identified through XRD) were constituents of willow and poultry
litter-derived biochars. However, unlike PCA of XRD, it was not able
to differentiate the two biochars derived from willow. The three analysis
methods provided different perspectives on the properties of the biochar
inorganic phase. Combining information from multiple methods is needed
to better understand the inorganic composition of biochars.
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.
Indoor exposures to metals arise from a wide variety of indoor and outdoor sources. This study investigates the impact of humid indoor conditions on the bioaccessibility of Zn in dust, and the transformation of Zn species during weathering. House dust samples were subjected to an oxygenated, highly humid atmosphere in a closed chamber for 4 to 5 months. Zinc bioaccessibility before and after the experiment was determined using a simulated gastric acid extraction. Bulk and micro X-ray absorption structure (XAS) spectroscopy was used to speciate Zn in dust. Exposure to humid conditions led to a significant increase in Zn bioaccessibility in all samples, which was due to a redistribution of Zn from inorganic forms toward the organic pools such as Zn adsorbed on humates. ZnO readily dissolved under humid conditions, whereas ZnS persisted in the dust. Elevated humidity in indoor microenvironments may sustain higher Zn bioaccessibility in settled dust compared to drier conditions, and part of this change may be related to fungal growth in humid dust. These results help to explain the greater bioaccessibility of certain metals in house dust compared to soils.
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