Influence of seawater ions on phosphate adsorption at the surface of hydrous ferric oxide (HFO)

• Published in: The Science of the total environment 2020-06, Vol.721, p.137826-137826, Article 137826
• Main Authors: Zhang, Hailong, Elskens, Marc, Chen, Guoxing, Snoeck, Christophe, Chou, Lei
• Format: Article
• Language: English
• Subjects: CD-MUSIC; EDS; FTIR; HFO; Phosphate; Visual MINTEQ
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2020.137826

The adsorption of phosphate on hydrated ferric oxide (HFO) was studied in solutions containing major seawater ions (Na+, Mg2+, Cl−, SO42−, Ca2+, K+) at pHs 6.5, 7.5 and 8.5. The presence of these ions promotes phosphate adsorption and the process is electrostatic in nature. Despite this electrostatic force, the precipitation of hydroxyapatite in the presence of Ca2+ at pH 8.5 also plays an important role in the removal of phosphates from the dissolved phase. Energy dispersive X-ray spectroscopy (EDS) and Fourier transform infrared (FTIR) spectra support that phosphate adsorption on HFO surface can be attributed to inner sphere complexes with the formation of bidentate complexes (FeO)2PO2 in the presence of main seawater ions at pH = 8.5. The results of EDS clearly indicated that Fe-P-Ca complexes, Fe-P-Mg, or other phosphate-bridged ternary complexes were not formed during adsorption in the presence of NaCl, KCl, CaCl2, Na2SO4 and MgCl2. This observation differs somewhat from that the typical explanation used to describe the phosphate adsorption mechanism on HFO. The CD-MUSIC model makes it possible to describe this adsorption mechanism of phosphate on HFO in the presence of 0.7 M NaCl, and these outcomes are coherent with the experimental FTIR and EDS results. [Display omitted] •Phosphate adsorption on HFO is promoted by seawater ions through electrostatic interactions.•Inner-sphere complexation occurs leading to the formation of (FeO)2PO2.•No phosphate-bridged ternary complexes such as Fe-P-Ca or Fe-P-Mg are formed.