Abstract
ABSTRACTThe objective was to evaluate phosphate desorption characteristics of synthetic hematite, goethite, and allophane and commercial alumina after loading at maximum adsorbed phosphate levels to determine their potential to release phosphate at a constant, low level to sustain plant growth in soilless media and reduce phosphate leaching. Desorption isotherms were measured at pH 6.4 ± 0.1 using a continuously stirred-flow reactor. The time period during which dissolved phosphate was maintained within the range of 5–0.2 mg·L−1 phosphate-P decreased in the order: allophane (12.4 d) > alumina (4.6 d) > goethite (3.6 d) > hematite (1.9 d). Allophane released the most phosphate during the desorption process (40% of maximum adsorbed phosphate; 12.7 mg∙g−1) followed by alumina and goethite (19–20%; ≈2.5 mg∙g−1) and lastly hematite (5%; 0.1 mg∙g−1). Allophane demonstrated the greatest potential as a phosphate-charged source for soilless root media, in amount and duration of phosphate release.
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