Although carbonate-carbon (C), an integral part of biochar-C, contributes to the liming properties of that material, it also interferes with the estimation of the stable organic C fraction in biochars. In this study, four methods were compared in order to quantify the carbonate-C in biochars: two direct (a titrimetric procedure and thermogravimetric analysis, TGA), and two indirect (acid treatment with separation by filtration and acid fumigation). The titrimetric method showed a high recovery of added carbonate-C (average 98.8%, range 1.5–38 mg), and the standard deviations of carbonate-C for all biochars tested were 0.3% wt) and when the isotopic signature of organic C in biochars is to be determined. The TGA method (either in N2 or a dry air atmosphere) was reliable when calcite was the main carbonate form in biochars, but was inadequate for samples containing a considerable amount of whewellite and certain carbonate-bearing minerals (e.g. magnesite) that decompose at <600°C. Because more than half of the biochar samples investigated in the literature and in this study (58% of the 117 samples) contained <0.4% carbonate-C (and 38% of these contained no detectable carbonate-C), low-cost screening methods were developed to identify the biochars needed for carbonate-C analysis. For this purpose, two methods were proposed: (i) a manometric test; and (ii) a ratio between predicted fixed C : total C (FC/TC) and measured FC/TC, where predicted FC/TC was estimated using the following relationship: (FC/TC) = –0.1081(H/C)2 – 0.1794(H/C) + 1.0097, as derived from values obtained in the literature. A decision tree, including two steps (a screening step and a titrimetric procedure) could be used to determine accurately the carbonate-C in biochars.

Determination of carbonate-C in biochars

In martian orthopyroxenite ALH 84001, pockets of feldspathic glass frequently contain carbonate masses that have been disrupted and dispersed within feldspathic shock melt as a result of impact(s). Transmission electron microscope studies of carbonate fragments embedded within feldspathic glass show that the fragments contain myriad, nanometer-sized magnetite particles with cuboid, irregular, and teardrop morphologies, frequently associated with voids. The fragments of carbonate must have been incorporated into the melt at temperatures of ~900°C, well above the upper thermal stability of siderite (FeCO3), which decomposes to produce magnetite and CO2 below ~450°C. These observations suggest that most, if not all, of the fine-grained magnetite associated with Fe-bearing carbonate in ALH 84001 could have been formed as result of the thermal decomposition of the siderite (FeCO3) component of the carbonate and is not due to biological activity.

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Magnetite in ALH 84001: An origin by shock-induced thermal decomposition of iron carbonate

Paralava and clinker products of coal combustion, Yellow River, Shanxi Province, China

Fault-related carbonate rocks and earthquake indicators: recent advances and future trends

Evaluation of ashes produced from fluidized bed combustion of residues from oranges' plantations and processing

Mrssbauer spectroscopy has been used to identify the iron-containing products that are formed during the thermal decomposition of ankerite in a CO 2 atmosphere. The decomposition takes place in three stages and evidence is produced to show that the first stage involves decomposition of ankerite to yield a periclase-wustite solid solution, (Mg,Fe)O, along with calcite and CO2, the periclase-wustite then reacting with CO 2 to produce magnesioferrite (MgFe204) and CO. In the second stage the magnesioferrite and calcite react to produce periclase and dicalcium ferrite. The third stage does not involve reaction of Fe-containing phases and corresponds to the decomposition of calcite to CaO. KEYWOR9S: ankerite, decomposition, Mrssbauer spectroscopy, CO 2 atmosphere.

A. E. Milodowski

Papers

Mössbauer Spectroscopic Study of the Decomposition Mechanism of Ankerite in CO2 Atmosphere