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Speciation and Mobility of Arsenic in Agricultural Lime
Author(s) -
Schmidt Gerald T.,
Lui Ka Hei,
Kersten Michael
Publication year - 2009
Publication title -
journal of environmental quality
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.888
H-Index - 171
eISSN - 1537-2537
pISSN - 0047-2425
DOI - 10.2134/jeq2008.0441
Subject(s) - lime , ferrihydrite , arsenate , x ray absorption spectroscopy , arsenite , chemistry , arsenic , genetic algorithm , extraction (chemistry) , goethite , oxide , oxyanion , titration , environmental chemistry , inorganic chemistry , adsorption , metallurgy , absorption spectroscopy , materials science , biochemistry , physics , organic chemistry , quantum mechanics , chromatography , evolutionary biology , biology , catalysis
Agricultural liming materials are used to correct soil acidity and to improve plant growth and microbial functionality. A relatively low‐grade agricultural lime was found to contain up to 125 mg kg −1 arsenic (As), which is above any fertilizing materials code threshold. The color of the milled material is brown due to ample oxide dendrites. Microprobe elemental maps confirmed that these accessory oxide mineral phases are responsible for the elevated As concentrations in the limestone. The black Mn‐bearing dendrites contain minor amounts of As, whereas the brown Fe‐bearing dendrites contain the major part of the As inventory, with an Fe/As molar ratio around 100. Because the elemental maps represent only a few sample regions of interest (ROI), the results are corroborated by a bulk five‐step sequential extraction of the lime, which suggests that a majority of the As is bound to acid‐reducible phases. Because repartitioning of the As oxyanion during extraction cannot be ruled out, X‐ray absorption spectroscopy with micrometer resolution (μ‐XAS) was used as a solid‐state speciation analysis approach. The μ‐XAS results at the Fe K‐edge for the selected ROIs revealed the brown dendrites to consist of ferrihydrite and goethite, whereas those at the As K‐edge revealed that the pentavalent As species arsenate predominates, with As‐Fe distance and coordination indicating binding as a mononuclear inner‐spheric adsorbate complex. Batch experiments with soil exposed to submerged conditions of up to 41 d revealed a negligible As release rate from the lime (approximately 40 ng kg −1 d −1 ). The results of this study corroborate regulatory codes that set the permissible As content in agricultural lime relative to the respective Fe content.