Competitive interaction between soil-derived humic acid and phosphate on goethite

• Addition of soil humic acids (SHA) can significantly reduce adsorption of phosphate.
• Mechanism includes generated electrostatic field and competition for adsorption site.
• Order of addition of phosphate and SHA can influence adsorption of phosphate.
• Phosphate forms different types of complexes dominated the interaction between phosphate and SHA.

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 PO4 adsorbed 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 PO4 adsorption 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 PO4 by goethite. This conclusion was supported by measurement of total organic C (TOC), infrared spectral features and Zeta potential. Adsorption of PO4 onto goethite was inversely proportional to pH. Order of addition of PO4 and SHA can influence adsorption of PO4 as 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 PO4 forms different types of complexes on the surface of goethite at different pH, which dominated the interaction of SHA and PO4 adsorption on goethite. Based on these observations, the possible modes of SHA inhibition of PO4 adsorption on goethite were proposed.