Phosphate Adsorption On Iron Oxides And Iron Oxides-Humic Acid Complexes

Phosphorus is an essential element for plant growth and is also one of the important elements for water eutrophication problem. Phosphorus rarely exists in free, yet often combines with organic matter, mineral and so on in terrestrial and aquatic ecosystems. Minerals and organic matter are ubiquitous in environments. They are often simultaneously present and tend to form complexes that are cemented together by various interactions in soils and sediments. The complexes formed are the center unit in soils. Humic acid(HA) is the important component of organic matter and exists very widely. There are a lot of the researches on the interactions between phosphate and iron oxides or between HA and iron oxides, yet the information about the adsorption of phosphate on iron oxides-HA complex is still little. The aim of this study was investigate to the adsorption of phosphate on iron oxides-HA complexes and iron oxides isotherm, kinetic and the change of Zeta potential. The effects of aged time, ionic strength and p H were also determined. The ATR-FTIR spectroscopy was also to investigate the mechanisms of phosphate adsorption at iron oxide surfaces and complexes in both binary and ternary systems under in situ condition and to elucidate the chemical characteristics of phosphorus in solid-liquid interface and provide the scientific basic for phosphorus management in soils. Main results are below:(1) Humic acid was coated on ferrihydrite(FH), goethite and hematite by chemical adsorption to form the HA-coated iron oxide complexes and they are named with FH-c HA, GH and HH, respectively. The amount of HA on ferrihydrite was highest. The HA-coated iron oxide complexes caused a decrease in the specific surface area(SSA) and the isoelectric point(IEP) of oxides. The HA coated enhanced the buffer ability for p H and also afforded more surface sites of hydroxyl on GH and HH complexes, while the HA reduced the surface sites of ferrihydrite.(2) The complexes were obtained by co-precipitating ferric ions with low(FH-HA1) and relatively high(FH-HA2) amounts of HA. They were assigned to two-line ferrihydrite(FH) and had lower crystalinity with an increase of HA added. The IEP and SSA values of the mineral decreased, which corresponds to an increase in the amount of HA. The surface sites also reduced in the process of coprecipitation.(3) The isotherm adsorption on pure and coated complexes fitted well by Langmuir and Freundlich equations. The presence of HA caused a decrease in the maximum phosphate adsorption and the adsorption affinity of phosphate on the complexes. The pseudo-second-order model was applied to fit the kinetic experiments best and it suggested that the adsorption process was controlled by chemical interaction. The HA coated also decreased the initial and average adsorption rate and slowed the process of phosphate adsorption on complexes.(4) Phosphate adsorption on goethite or hematite was insensitive to changes of ionic strength. While it increased with increasing ionic strength and on HA-coated iron oxides complexes, and even first increased and then reduced at high ionic strength. For FH and FH-c HA complexes, the phosphate adsorption increased with an increase of ionic strength and HA weakened the negative effect of p H on phosphate adsorption. In the system of FH-HA complexes, the sensitivity of phosphate adsorption to the change in the p H was greater for FH than for FH-HA complexes. The increase in the ionic strength promoted the adsorption of phosphate onto FH and FH-HA1 at a higher p H. However, for the FH-HA2 complex, the increase in the ionic strength inhibited the adsorption of phosphate onto FH-HA2 at a lower p H but increased the phosphate adsorption at a higher p H.(5) When phosphate adsorption on goethite and hematite, the Zeta potential of phosphate-bound iron oxides was linearly reduced with the increment of phosphate surface coverage, while the Zeta potential of complexes with adsorbed phosphate stayed at the same level and was lower. For FH-c HA complex, the Zeta potential decreased with an increase of the surface phosphate coverage at low p H and the values on the complexes was lower, while the Zeta potential changed less with higher surface coverage at p H8.5. There was a break in the slope of the curves at lower p H. For FH-HA complexes, the Zeta potential decreased with increasing phosphate adsorption and there were negative correlation between Zeta potential and phosphate adsorption.(6) From ATR-FTIR spectroscopy, the intensity of the feature peaks reduced with an increase of the carbon content and it suggested that the phosphate adsorption became less. The inner-sphere complexes of phosphate-metal complexation were formed on the surfaces of iron oxides. When phosphate was adsorbed on GH and HH complexes, the presence of HA promoted the monoprotonated phosphate complexes from nonprotonated complexes, especially at p H 8.5, may influenced phosphate species by H bond. And HA also promoted the monoprotonated complexes. For FH-c HA complex, phosphate species was dominant by bidentate binuclear complexes. The addition of HA did not change the conformation of the inner-sphere phosphate complexes, but it inhibited the nonprotonated bidentate complexes at lower p H on FH-c HA complex.When phosphate adsorption on FH-HA coprecipitated complexes(FH-HA1 and FH-HA2), phosphate species were dominated by protonated and nonprotonated bidentate inner sphere complexes on FH-HA1 complexes at p H4.5 and the nonprotonated bidentate complexes were dominated at p H8.5. These results were consistent with FH and suggested that the low amounts of HA had less effect on phosphate species on FH-HA1 complexes. When phosphate was adsorbed on FH-HA2 complexes, the kinds of phosphate species were similar as that on FH and FH-HA1 complexes. The presence of HA promoted the formation of the protonated complexes at p H8.5 and the HA molecular combined with the nonprotonated complexes by H bond or influenced the molecular symmetry of the predominant nonprotonated bidentate binuclear complexes. These results resumed that the formation of ternary metal-phosphate-humic acid ternary complexes at the mineral surfaces.