Adsorption of phosphate on pure and humic acid-coated ferrihydrite
Humic acid and mineral oxides are simultaneously present in soils and can form organomineral complexes. These complexes can influence the transport and fate of phosphate in the environment. The objective of this study was to investigate the adsorption of phosphate on these complexes by comparing them with phosphate adsorption on only the mineral.
Materials and methods
Phosphate adsorption on ferrihydrite (FH) and the humic acid (HA)-coated ferrihydrite (FH–cHA) complex, as a function of pH and ionic strength, was investigated through adsorption measurements of zeta potential and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy.
Results and discussion
The FH–cHA complex had a lower isoelectric point and less specific surface area than FH. A greater amount of phosphate was adsorbed on FH than on the FH–cHA complex, and phosphate adsorption increased with increasing ionic strength. The adsorption process was controlled by chemisorption. The zeta potential strongly decreased with an increase of phosphate adsorption at low pH, while it less obviously decreased at higher phosphate adsorption at high pH. The ATR-FTIR showed that the phosphate species on the FH–cHA complex was dominated by bidentate inner-sphere complexes. The addition of HA did not change the formation of the inner-sphere phosphate complexes, but it diminished the non-protonated bidentate complexes at lower pH. Also, the HA inhibited the non-protonated bidentate complexes at lower pH and generated P = O···H or P–O···H bonds by its acid groups.
Results suggested that the affinity of phosphate for the FH–cHA complex was lower than for FH, and HA also influenced the formation of the phosphate species.
KeywordsIn situ ATR-FTIR spectroscopy Ionic strength Isotherm and kinetic experiment Phosphate adsorption Surface complexation Zeta potential
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