Publications
How magnetism-based fractional spinels contribute to the bioavailability of geogenic chromium in serpentine soils of Taiwan
Authors: Yang,C.Y., W.H. Lee, S.L. Wang, Z.Y. Hseu
Geoderma Regional • 2024
Serpentine soils are highly rich in geogenic chromium (Cr), typically associated with spinel minerals. The high resistance of these minerals to weathering has raised concerns regarding their contribution to Cr bioavailability in serpentine soils. This study collected soil horizon samples from two pedons (Entisol and Ultisol) in eastern Taiwan and applied a two-step magnetic separation method to divide the bulk soils into strongly magnetic (SM), weakly magnetic (WM), and nonmagnetic (NM) fractions. The basic characteristics of the bulk soils were examined. To characterize the mineralogy and geochemistry of the fractions and determine their quantitative contribution of bioavailable Cr in serpentine soils, we analyzed their mineral composition, magnetic properties, and elemental composition and valence by using various spectrometric techniques, such as X-ray diffraction, electron probe microanalysis, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy, as well as vibrating sample magnetometry. The results indicated the main Cr-bearing minerals were magnetite and chromite in the SM and WM fractions, with minor occurrences in layer silicates, such as serpentine and chlorite, in the NM fraction. The total Cr content decreased in the following order: SM > WM > NM. The SM fraction had the lowest weight proportion, and this proportion was lower for the Ultisol than the Entisol. This observation indicated that the weathering of Cr spinels is associated with the substantial cation substitution of Al, Ca, Mn, and Ni for Fe and Cr in broken mineral grains and increases in the Fe(III) and Cr(VI) concentrations in magnetite. The SM fraction, mainly consists of magnetite and chromite, exhibited the highest concentration of bioavailable Cr extracted by 0.1 M HCl. However, the Cr-bearing layer silicates represented the largest potential pool of bioavailable Cr in the bulk soils because the weight proportion of the NM fraction was higher than those of the other fractions and increased during pedogenesis.
Increases in Ca/Mg ratios caused the increases in the mobile fractions of Cr and Ni in serpentinite-derived soils in humid Asia
Authors: Yang,C.Y., D.Q. Nguyen, H.T.T. Ngo, I.A. Navarrete, A. Nakao, S.T. Huang, Z.Y., Hseu
Catena • 2022
Serpentinite-derived soils are characterized by low Ca/Mg ratio and abnormally high Cr and Ni concentrations due to the geogenic sources. The breakdown of parent materials during pedogenesis may lead to the changes in both Ca/Mg ratios and fractions of Cr and Ni. This study hypothesized that total Ca/Mg (Cat/Mgt) and exchangeable Ca/Mg (Cae/Mge) ratios increase with soil development, associated with the increases in potentially mobile fractions (PMFs) of Cr and Ni. Thirty-five soil horizon samples of eight pedons from Japan, Taiwan, Philippines, and Vietnam were collected, and the exchangeable concentrations of Ca and Mg were measured. The fractions of Cr and Ni were further determined by the BCR sequential extraction, and the sum of the acid soluble, reducible, and oxidizable fractions was regarded as the PMFs of Cr and Ni. Based on the elemental and mineral analyses, the studied samples from different countries varied in the parent materials. The elemental mappings with electron probe microanalysis elucidated that the sources of Cr and Mg were spinels and silicates, such as chromite, serpentine, and chlorite, while those of Ni were the silicates. The Cae/Mge ratio was a potential index of weathering degree of the soils. The PMF of Cr mainly associated with the organic matter and Fe/Mn oxides increased during pedogenesis, corresponding to the significant (P < 0.05) increase in Cae/Mge ratio. Furthermore, the PMF of Cr was predicted by a function of Cat/Mgt and Cae/Mge ratios, clay, and organic carbon using multivariate linear regression analysis. Although the PMF of Ni was poorly correlated with the Cae/Mge ratio, it was predicted by Cat/Mgt and Cae/Mge ratios, total Ni, and dithionite-citrate-bicarbonate extractable Fe. These results supported that the breakdown of serpentinitic framework led to loss of Mg, increase of Ca/Mg ratios, and increase of Cr and Ni in PMFs during pedogenesis.
Kinetics of Chromium Reduction Associated with Varying Characteristics of Agricultural Soils
Authors: Yang,C.Y., Y.L. Tseng, Z.Y. Hseu
Water • 2022
Chromium (Cr)(VI) is carcinogenic; thus, the excessive presence of Cr(VI) in soils can pose potential risks to water quality, food safety, and human health. The kinetics of Cr(VI) reduction in soils are important for assessing the fate of Cr in the environment. The present study tested physio-chemical and microbial properties in twenty-eight agricultural soils collected in Taiwan to evaluate the relationship between the reduction rate of Cr(VI) and soil properties, using 49-day incubation at 25 °C. At the beginning of incubation, 100 mg Cr(VI) kg−1 was spiked into the soils. The reduction of Cr(VI) was described by first-order kinetics at a significant level (p < 0.05) for the tested soils. The rate constant (k) of Cr(VI) reduction ranged from 0.01 to 4.21 day−1. In addition, the k value significantly increased with organic carbon (OC) and cation exchange capacity, but significantly decreased with increasing pH and dithionite-citrate-bicarbonate extractable Mn (Mnd). However, a predictive model using stepwise regression analysis indicated that the k value of the kinetics was controlled by OC, dissolved organic carbon (DOC), and Mnd, thereby identifying the complex interactions between Cr(VI) reduction and soil factors in the humid tropics.