Saprolite-soil transformations in the Piedmont and Mountains of North Carolina
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Keywords:
Saprolite
Gibbsite
Halloysite
Abstract Background Glyphosate (GLP) is a widely used herbicide with possible adverse effects on human health and the environment. In soils, GLP strongly adsorbs on clay‐sized minerals, depending on pH, the amount of organic carbon, as well as the contents and properties of Al and Fe oxyhydroxides and clay minerals. Many clay‐sized minerals have already been investigated regarding GLP adsorption behavior, but information on minerals commonly found in volcanic soils is still lacking. Aim The aim of this study was to investigate for the first time the pH‐dependent adsorption of GLP on allophane and halloysite, typical minerals found in volcanic soils. Methods GLP adsorption was studied in batch experiments at three pH values (5, 6, and 7). Synthetic allophanes with two different initial Al:Si ratios (1.4 and 1.8) and a halloysite were used as adsorbents. Results The adsorption capacity (AC) increased with rising Al:Si ratio and decreasing pH. The AC of allophane was significantly higher than that of halloysite. GLP adsorption on allophane was larger than that reported for other clay minerals and Al and Fe oxyhydroxides, especially at low pH. The AC of halloysite was higher than reported for most other clay minerals. Conclusion Different mineral formation pathways in volcanic soils, notably the formation of halloysite versus allophanes, strongly affect the soils’ retention capacity for GLP. The high AC of allophanes may induce the low mobility of GLP in allophane‐containing soils. Long‐term use of GLP may accumulate the herbicide in these soils with potential effects on biodiversity and ecosystem services.
Halloysite
Imogolite
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Saprolite
Laterite
Parent rock
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Abstract Mineralogical analyses of clays from two Alotenango soil horizons were made by utilizing transmission and scanning electron microscopy, infrared, X‐ray diffraction, and chemical methods. This report of spheroidal halloysite demonstrates the presence of this mineral form in an area of important volcanic ash‐derived soils where it has not been previously identified. The presence of spheroidal halloysite in Central American volcanic soil and its presence in volcanic soils of Hawaii, Japan, and New Zealand indicate its wide geographic occurrence in soils of volcanic origin. Oblate halloysite spheroids tend to settle preferentially, offering an end‐view along the axis of some particles and correspondingly, reduced 00 l X‐ray reflections from portions of layers not parallel to the plane of the powder mount. Halloysite spheroids appear to be composed of roughly concentric layers with occasional interlayer separations. The interior structure suggests inclusion of amorphous cores and interlayers which appear to be removed by 0.5 N NaOH. The exterior layers often retain a shell‐like appearance after NaOH treatment. The apparent association of spheroidal halloysite and allophane interlayers suggests that the crystalline component forms alternately with the amorphous one. The crystalline exterior may protect amorphous interior material from rapid alteration and cause allophane to persist in a soil where minerals representing more advanced stages of weathering have accumulated.
Halloysite
Volcanic glass
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