In order to better understand the influence and mechanism of soil-derived humic acid (SHA) on adsorption of P onto particles in soils, the amounts of PO4adsorbed by synthetic goethite (α-FeOOH) were determined at different concentrations of SHA, pH, ionic strength and order of addition of adsorbents. Addition of SHA can significantly reduce the amount of PO4adsorption as much as 27.8%. Both generated electrostatic field and competition for adsorption sites were responsible for the mechanism by which SHA inhibited adsorption of PO4by goethite. This conclusion was supported by measurement of total organic C (TOC), infrared spectral features and Zeta potential. Adsorption of PO4onto goethite was inversely proportional to pH. Order of addition of PO4and SHA can influence adsorption of PO4as follows: prior addition of PO4≥simultaneous addition prior addition of SHA. Iron and SHA apparently form complexes due to prior addition of SHA. Observations made during this study emphasized that PO4forms different types of complexes on the surface of goethite at different pH, which dominated the interaction of SHA and PO4adsorption on goethite. Based on these observations, the possible modes of SHA inhibition of PO4adsorption on goethite were proposed. © 2013 Elsevier Ltd.
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