From what information I've been able to find the bond between GFO and phosphate is more than likely a phsysical bond, weak Van der Walls forces, that are easily broken.
I think it more likely to be ionic, just as in crystalline iron phosphate. I show that without evidence here:
Iron Oxide Hydroxide (GFO) Phosphate Binders
http://reefkeeping.com/issues/2004-11/rhf/index.htm
from it:
and here are some references that discuss the binding:
Proton interaction in phosphate adsorption onto goethite. Zhong, Bin; Stanforth, Robert; Wu, Shunnian; Chen, J. Paul. Division of Environmental Science & Engineering, National University of Singapore, Singapore. Journal of Colloid and Interface Science (2007), 308(1), 40-48.
Abstract
The adsorption of phosphate on goethite is generally modeled by assuming a simple ligand exchange reaction with surface hydroxyl groups. This study investigates the binding forms of phosphate on goethite by evaluating the proton interaction and surface charge change during phosphate adsorption. It is found that OH- release stoichiometry increases with phosphate coverage, which suggests that different mechanisms predominate at different phosphate loadings. It demonstrates that surface binding changes from monodentate complexation to bidentate complexation with increasing surface phosphate coverage. The net OH- release accompanying this transformation is best interpreted with a 2pKa multisite model.
Phosphate forms monodentate complexes at the surface of goethite. Loring, John S.; Sjoberg, Staffan; Persson, Per. Department of Chemistry, Umea University, Umea, Swed. Abstracts of Papers, 232nd ACS National Meeting, San Francisco, CA, United States, Sept. 10-14, 2006 (2006), GEOC-029.
Abstract
That phosphate binds to goethite in a bridging-bidentate fashion is one of the fundamental precepts of soil chem. In this presentation, we will show that this coordination geometry is unlikely. We have measured in situ the IR spectra of phosphate adsorbed on goethite over a broad range of pH values and surface coverages using simultaneous IR and potentiometric titrns. We compare the spectra of the surface complexes with the protonation states of phosphate in soln. and two aq. metal-phosphate complexes. We have clear evidence from the OH and PO-stretching regions that phosphate binds to surface Fe(III) in a monodentate fashion, and the complexes are doubly, singly, or unprotonated depending on pH. Furthermore, our spectra indicate that phosphate is strongly hydrogen-bonded, probably to neighboring surface sites. We find no evidence of a bridging-bidentate structure, and our results challenge previous surface-complexation models, spectroscopic studies, quantum-mech. results, and theories about mineral dissoln. inhibition.
Geometry, Charge Distribution, and Surface Speciation of Phosphate on Goethite. Rahnemaie, Rasoul; Hiemstra, Tjisse; van Riemsdijk, Willem H. Department of Soil Quality, Wageningen University, Wageningen, Neth. Langmuir (2007), 23(7), 3680-3689.
Abstract
The surface speciation of phosphate was evaluated with surface complexation modeling using an interfacial charge distribution (CD) approach based on ion adsorption and ordering of interfacial H2O. In the CD model, the charge of adsorbed ions is distributed over 2 electrostatic potentials in the double-layer profile. The CD is related to the structure of the surface complex. A new approach is followed in which the CD values of the various surface complexes were calcd. theor. from the geometries of the surface complexes. MO calcns. based on d. functional theory (MO/DFT) were used to optimize the structure of hydrated surface complexes of phosphate. These theor. CD values are cor. for dipole orientation effects. Data anal. of the PO4 adsorption, applying the independently derived CD coeffs., resolves the presence of 2 dominant surface species. A nonprotonated bidentate (B) complex is dominant over a broad range of pH values at low loading (1.5 .mu.mol/m2). For low pH and high loading, a strong contribution of a singly protonated monodentate (MH or MH-Na) complex is found, which differs from earlier interpretations. For the conditions studied, the doubly protonated bidentate (BH2) and monodentate (MH2) surface complexes and the nonprotonated monodentate (M) complex are not significant contributors. These findings are discussed qual. and quant. in relation to published exptl. in-situ CIR-FTIR data and theor. MO/DFTIR information. The relative variation in the peak intensities as a function of pH and loading approx. agrees with the surface speciation calcd. with the CD model. The model correctly predicts the proton co-adsorption of phosphate binding on goethite and the shift of the IEP at low phosphate loading (1.5 .mu.mol/m2). At higher loading, it deviates.
Evidence for Surface Precipitation of Phosphate on Goethite. Ler, Adeline; Stanforth, Robert. Chemical and Environmental Engineering Department, National University of Singapore, Singapore, Singapore. Environmental Science and Technology (2003), 37(12), 2694-2700.
Abstract
Recent studies have suggested that the interaction between phosphate and goethite includes ternary adsorption/surface pptn. as well as surface complex formation. The ternary adsorption/surface pptn. process envisioned involves the dissoln. of the goethite crystal and subsequent adsorption of iron on the surface-bound phosphate. Further evidence to support the suggested process is needed. The process was investigated using two approaches. First, the sorption of iron spiked into a slurry of phosphated goethite and the effect of the iron sorption on phosphate uptake kinetics were investigated to det. whether iron would be adsorbed on the phosphated surface and whether it would enhance phosphate adsorption. Lead was also spiked into soln. for comparison. Second, changes in the .zeta.-potential of phosphated goethite were monitored with time. Adsorption of iron on the surface of phosphated goethite should increase the .zeta.-potential of the goethite. Iron spiked into a phosphated goethite slurry was adsorbed on the solid with a concurrent adsorption of phosphate. The iron adsorption did not change the slow phosphate adsorption kinetics. Lead spiked into the soln. was also sorbed but to a lesser extent than iron and with a lower apparent P
b mole ratio. Lead addn. also changed the phosphate adsorption kinetics. With time, the .zeta.-potential of phosphated goethite became more pos., returning almost to the potential of unphosphated goethite at low surface coverages. The slow increase in .zeta.-potential over time indicates that long-term reactions are occurring on the goethite surface, most likely involving the dissoln. of goethite to release iron and the subsequent reaction between the iron and surface-bound phosphate. These results provide strong support for the surface pptn. model, and are inconsistent with models envisioning the reaction between phosphate and the goethite surface as involving only monolayer surface complex formation.
Competitive adsorption of phosphate and arsenate on goethite. Zhao, Hongshao; Stanforth, Robert. Chemical and Environmental Engineering Department, National University of Singapore, Singapore. Environmental Science and Technology (2001), 35(24), 4753-4757.
Abstract
The competitive adsorption of phosphate and arsenate on goethite was investigated to better understand the bonding mechanisms for the 2 ions. The anions were added both simultaneously and sequentially. When added simultaneously, the 2 ions were adsorbed about equally, with the total surface coverage being slightly greater than for either ion alone. When added sequentially, the extent of exchange for the first ion depended on the equilibration time before the second ion was introduced-the longer the equilibration time the greater the exchange. There is a nonexchangeable fraction for both ions that is approx. equal to the initially adsorbed amt. of each ion. The results suggest a two-phase reaction on the surface, with the first phase being a rapid surface complex formation on the goethite surface, followed by the slower buildup of a surface ppt. on the adsorbed layer. The exchangeable ions are in the surface ppt. These results are incompatible with a surface complexation model (SCM) for anion adsorption on goethite and indicate that the actual reactions are more complicated than the reaction assumed in a SCM.
I think it more likely to be ionic, just as in crystalline iron phosphate. I show that without evidence here:
Iron Oxide Hydroxide (GFO) Phosphate Binders
http://reefkeeping.com/issues/2004-11/rhf/index.htm
from it:
and here are some references that discuss the binding:
Proton interaction in phosphate adsorption onto goethite. Zhong, Bin; Stanforth, Robert; Wu, Shunnian; Chen, J. Paul. Division of Environmental Science & Engineering, National University of Singapore, Singapore. Journal of Colloid and Interface Science (2007), 308(1), 40-48.
Abstract
The adsorption of phosphate on goethite is generally modeled by assuming a simple ligand exchange reaction with surface hydroxyl groups. This study investigates the binding forms of phosphate on goethite by evaluating the proton interaction and surface charge change during phosphate adsorption. It is found that OH- release stoichiometry increases with phosphate coverage, which suggests that different mechanisms predominate at different phosphate loadings. It demonstrates that surface binding changes from monodentate complexation to bidentate complexation with increasing surface phosphate coverage. The net OH- release accompanying this transformation is best interpreted with a 2pKa multisite model.
Phosphate forms monodentate complexes at the surface of goethite. Loring, John S.; Sjoberg, Staffan; Persson, Per. Department of Chemistry, Umea University, Umea, Swed. Abstracts of Papers, 232nd ACS National Meeting, San Francisco, CA, United States, Sept. 10-14, 2006 (2006), GEOC-029.
Abstract
That phosphate binds to goethite in a bridging-bidentate fashion is one of the fundamental precepts of soil chem. In this presentation, we will show that this coordination geometry is unlikely. We have measured in situ the IR spectra of phosphate adsorbed on goethite over a broad range of pH values and surface coverages using simultaneous IR and potentiometric titrns. We compare the spectra of the surface complexes with the protonation states of phosphate in soln. and two aq. metal-phosphate complexes. We have clear evidence from the OH and PO-stretching regions that phosphate binds to surface Fe(III) in a monodentate fashion, and the complexes are doubly, singly, or unprotonated depending on pH. Furthermore, our spectra indicate that phosphate is strongly hydrogen-bonded, probably to neighboring surface sites. We find no evidence of a bridging-bidentate structure, and our results challenge previous surface-complexation models, spectroscopic studies, quantum-mech. results, and theories about mineral dissoln. inhibition.
Geometry, Charge Distribution, and Surface Speciation of Phosphate on Goethite. Rahnemaie, Rasoul; Hiemstra, Tjisse; van Riemsdijk, Willem H. Department of Soil Quality, Wageningen University, Wageningen, Neth. Langmuir (2007), 23(7), 3680-3689.
Abstract
The surface speciation of phosphate was evaluated with surface complexation modeling using an interfacial charge distribution (CD) approach based on ion adsorption and ordering of interfacial H2O. In the CD model, the charge of adsorbed ions is distributed over 2 electrostatic potentials in the double-layer profile. The CD is related to the structure of the surface complex. A new approach is followed in which the CD values of the various surface complexes were calcd. theor. from the geometries of the surface complexes. MO calcns. based on d. functional theory (MO/DFT) were used to optimize the structure of hydrated surface complexes of phosphate. These theor. CD values are cor. for dipole orientation effects. Data anal. of the PO4 adsorption, applying the independently derived CD coeffs., resolves the presence of 2 dominant surface species. A nonprotonated bidentate (B) complex is dominant over a broad range of pH values at low loading (1.5 .mu.mol/m2). For low pH and high loading, a strong contribution of a singly protonated monodentate (MH or MH-Na) complex is found, which differs from earlier interpretations. For the conditions studied, the doubly protonated bidentate (BH2) and monodentate (MH2) surface complexes and the nonprotonated monodentate (M) complex are not significant contributors. These findings are discussed qual. and quant. in relation to published exptl. in-situ CIR-FTIR data and theor. MO/DFTIR information. The relative variation in the peak intensities as a function of pH and loading approx. agrees with the surface speciation calcd. with the CD model. The model correctly predicts the proton co-adsorption of phosphate binding on goethite and the shift of the IEP at low phosphate loading (1.5 .mu.mol/m2). At higher loading, it deviates.
Evidence for Surface Precipitation of Phosphate on Goethite. Ler, Adeline; Stanforth, Robert. Chemical and Environmental Engineering Department, National University of Singapore, Singapore, Singapore. Environmental Science and Technology (2003), 37(12), 2694-2700.
Abstract
Recent studies have suggested that the interaction between phosphate and goethite includes ternary adsorption/surface pptn. as well as surface complex formation. The ternary adsorption/surface pptn. process envisioned involves the dissoln. of the goethite crystal and subsequent adsorption of iron on the surface-bound phosphate. Further evidence to support the suggested process is needed. The process was investigated using two approaches. First, the sorption of iron spiked into a slurry of phosphated goethite and the effect of the iron sorption on phosphate uptake kinetics were investigated to det. whether iron would be adsorbed on the phosphated surface and whether it would enhance phosphate adsorption. Lead was also spiked into soln. for comparison. Second, changes in the .zeta.-potential of phosphated goethite were monitored with time. Adsorption of iron on the surface of phosphated goethite should increase the .zeta.-potential of the goethite. Iron spiked into a phosphated goethite slurry was adsorbed on the solid with a concurrent adsorption of phosphate. The iron adsorption did not change the slow phosphate adsorption kinetics. Lead spiked into the soln. was also sorbed but to a lesser extent than iron and with a lower apparent P
b mole ratio. Lead addn. also changed the phosphate adsorption kinetics. With time, the .zeta.-potential of phosphated goethite became more pos., returning almost to the potential of unphosphated goethite at low surface coverages. The slow increase in .zeta.-potential over time indicates that long-term reactions are occurring on the goethite surface, most likely involving the dissoln. of goethite to release iron and the subsequent reaction between the iron and surface-bound phosphate. These results provide strong support for the surface pptn. model, and are inconsistent with models envisioning the reaction between phosphate and the goethite surface as involving only monolayer surface complex formation. Competitive adsorption of phosphate and arsenate on goethite. Zhao, Hongshao; Stanforth, Robert. Chemical and Environmental Engineering Department, National University of Singapore, Singapore. Environmental Science and Technology (2001), 35(24), 4753-4757.
Abstract
The competitive adsorption of phosphate and arsenate on goethite was investigated to better understand the bonding mechanisms for the 2 ions. The anions were added both simultaneously and sequentially. When added simultaneously, the 2 ions were adsorbed about equally, with the total surface coverage being slightly greater than for either ion alone. When added sequentially, the extent of exchange for the first ion depended on the equilibration time before the second ion was introduced-the longer the equilibration time the greater the exchange. There is a nonexchangeable fraction for both ions that is approx. equal to the initially adsorbed amt. of each ion. The results suggest a two-phase reaction on the surface, with the first phase being a rapid surface complex formation on the goethite surface, followed by the slower buildup of a surface ppt. on the adsorbed layer. The exchangeable ions are in the surface ppt. These results are incompatible with a surface complexation model (SCM) for anion adsorption on goethite and indicate that the actual reactions are more complicated than the reaction assumed in a SCM.
