Department of Environmental and Sustainable Engineering, University at Albany, 1400 Washington Ave, Albany, New York 12222, United States.
National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, New York 11793, United States.
Environ Sci Technol. 2024 Oct 1;58(39):17304-17312. doi: 10.1021/acs.est.4c04820. Epub 2024 Sep 18.
Calcium is commonly the most abundant element in fire residues and its speciation largely determines the geochemical properties of fire residues and their effects on postfire soil chemistry. To explore the effects of biomass composition and fire conditions on ash Ca speciation, this study characterizes the speciation of Ca in charcoal and ash samples that were derived from different plant compartments and thermal conditions, using Ca K-edge X-ray absorption near edge spectroscopy. Results showed that biomass contains abundant organic Ca complexes, which were mineralized into fairchildite and calcite after heating at 450 to 600 °C and then CaO, as temperature increased to 750 °C. Apatite could be an abundant Ca species in fire residues if the Ca/P molar ratio of the biomass is small (<2). The mineralization of organic Ca to the identified Ca minerals during burning was negligibly affected by the oxygen level. Calcium speciation in prescribed fire residues resembled that of biomass ash burned at 550 °C with similar Ca/P molar ratios. Batch experiments showed that macronutrients (Ca, Mg, K, and P) were differentially released, as a result of different solubility of minerals in ashes and reprecipitation of minerals. The aqueous solubility of Ca, Mg, and P decreased as pH increased from 5 to 9, while K showed no pH dependency and was almost completely soluble. Results from this study improve our understanding of the chemistry of fire residues and their geochemical behaviors, which can help evaluate the impact of fire on postfire soil properties and macronutrient cycling.
钙通常是火灾残留物中最丰富的元素，其形态在很大程度上决定了火灾残留物的地球化学性质及其对火灾后土壤化学的影响。为了探讨生物质组成和火灾条件对灰分中 Ca 形态的影响，本研究采用 Ca K 边 X 射线吸收近边光谱法，对源自不同植物部位和热条件的木炭和灰分样品中的 Ca 形态进行了表征。结果表明，生物质中含有丰富的有机 Ca 配合物，在 450 至 600°C 加热后会矿化为方沸石和方解石，然后随着温度升高到 750°C，Ca 会进一步矿化为 CaO。如果生物质的 Ca/P 摩尔比较小（<2），则灰分中可能存在大量的磷灰石 Ca 物种。在燃烧过程中，有机 Ca 向已鉴定的 Ca 矿物的矿化过程受氧水平的影响可忽略不计。规定火灾残留物中的 Ca 形态与在 550°C 燃烧的生物质灰分相似，具有相似的 Ca/P 摩尔比。批量实验表明，由于灰分中矿物质的不同溶解度和矿物质的再沉淀，大量营养素（Ca、Mg、K 和 P）的释放存在差异。随着 pH 值从 5 增加到 9，Ca、Mg 和 P 的水溶度降低，而 K 没有 pH 依赖性，几乎完全可溶。本研究的结果提高了我们对火灾残留物化学及其地球化学行为的认识，这有助于评估火灾对火灾后土壤性质和大量营养元素循环的影响。
