Adsorption, Desorption And Precipitation Of Several Soil Organic Phosphates On Iron And Aluminum(Oxyhydr)Oxides

Phosphorus(P) is an essential nutrient for plant growth and a key element involved in mediating the connection between biotic and abiotic reactions in the biosphere. Organic phosphates(OPs), especially inositol hexakisphosphates, abundant in most soils and sediments, are important components of P pool in the environment. Adsorption, desorption and precipitation reactions on iron and aluminum(hydr)oxides surface affect and determine the speciation, transport and cycling of organic phosphate in terrestrial and aquatic environments. In this research, the adsorption, desorption and precipitation of organic phosphates(glycerophosphate, GP; glucose-6-phosphate, G6P; adenosine triphosphate, ATP; and myo-inositol hexakisphosphate, IHP) on a several iron(Fe) and aluminum(Al)(oxyhydr)oxides, such as hematite(α-Fe2O3), goethite(α-Fe OOH), γ-Al2O3, α-Al2O3, boehmite(γ-Al OOH) and amorphous aluminum hydroxide(Al(OH)3, AAH), and the underlying mechanisms were studied by combining batch ad-desorption techniques, Zeta(δ) potential measurements, powder X-ray diffraction(XRD), solid-state 31P/27 Al nuclear magnetic resonance spectroscopy(NMR), and in situ attenuated total reflectance Fourier transform infrared spectroscopy(ATR-FTIR). The results are as follows.1. Desorption of P by citrate is mainly achieved through ligand exchange, and desorption increases with decreasing p H. Desorption percentages of IHP/Pi by citrate are much higher than those by H2 O and 0.02 M KCl. Desorption by H2 O is slightly greater than that by 0.02 M KCl because the electrostatic repulsion between the P molecules is larger in H2 O. Due to the multi-P bonds between IHP and the goethite surface and higher affinity of IHP for goethite than that of Pi, maximum desorption percentages of IHP are lower than those of Pi. Desorption curves(desorption concentration in solution vs. sorption density) of IHP or Pi on goethite by KCl or H2 O can be well fitted by exponential equation, while those by citrate are well fitted by linear equation. Desorption amounts of P in the first cycle account for more than 58% of the total desorption, followed by substantial decreases in the second and third cycles. There is a re-sorption of Pi from solution in the late stage of desorption by KCl and H2 O, resulting in a sharp decrease in desorption percentage. Re-sorption of IHP does not occur probably resulting from its poor diffusion into goethite causing by the large molecule, and its low desorption as well. The initial desorption rate of Pi by KCl and H2 O decreases with increasing pre-sorption time whereas that of IHP is barely affected by pre-sorption time.2. The kinetics results show that adsorption of both IHP and Pi has an initial rapid uptake followed by a slow adsorption process. Re-adsorption occurs in the later stage of Pi desorption by H2 O. The adsorption of IHP and Pi on hematite decreases with increasing p H and the adsorption amounts are 0.67 and 1.78 μmol m-2 at p H 5, respectively. At p H 3 to 10, the surface of IHP adsorbed hematite is highly negative charged, leading to increasing IHP adsorption with ion strength. The desorption of IHP/Pi decreases sharply with the increasing desorption cycle and decreasing pre-adsorption amount. In combination with adsorption-desorption characteristics, δ potential measurements and ATR-FTIR analysis, it is suggested that each adsorbed IHP molecule binds on hematite in form of inner-sphere surface complexes via 2 phosphate groups.3. The maximum sorption amounts of OPs and Pi increase with decreasing crystallinity of the minerals on a per mass basis: α-Al2O3 < γ-Al OOH < AAH. With an exception of IHP sorption on AAH, the maximum surface area-based sorption densities increase with decreasing molecular weight(MW) of OPs and Pi: IHP < ATP < G6 P < GP < Pi. Despite having the largest MW, IHP has greater sorption amounts on AAH than the other OPs because of the transformation of surface complexes to surface precipitates. Sorption kinetics of OPs are first a rapid sorption followed by a long and slow sorption process. Of the three Al(oxyhydr)oxides, AAH has the greatest first rapid sorption density and initial sorption rate of OPs within five minutes, both factors decreasing with increasing MW of OPs. The initial desorption percentages of OPs by KCl generally increase with decreasing MW of OPs, whereas the maximum desorption percentages of OPs by citrate are 4–5 times those achieved with KCl. Overall, strong specific sorption of OPs occurs on the surface of Al(oxyhydr)oxides, and molecular structure and size of OPs, as well as crystallinity and crystal structure of the minerals, are the key factors affecting the interfacial reactions and environmental behavior of OPs.4. The AAH(16.01 μmol m-2) exhibits much higher sorption density than boehmite(0.73 μmol m-2) and α-Al2O3(1.13 μmol m-2). Kinetics of IHP sorption and accompanying OH- release, as well as δ potential measurements, indicate that IHP is initially adsorbed on AAH through inner-sphere complexation via ligand exchange, followed by AAH dissolution and ternary complex formation; lastly, the ternary complexes rapidly transform to surface precipitates and bulk phase analogous to aluminum phytate(Al-IHP). The p H level, reaction time, and initial IHP loading evidently affect the interaction of IHP on AAH. In situ ATR-FTIR and solid-state 31 P and 27Al NMR spectra further demonstrate that IHP sorbs on AAH and transforms to surface precipitates analogous to Al-IHP, consistent with the results of XRD analysis. This study indicates that active metal oxides such as AAH strongly mediate the speciation and behavior of IHP via rapid surface complexation-precipitation reactions, thus affecting the mobility and bioavailability of IHP in the environment.5. The results show that the maximum sorption densities(μmol m-2) for IHP and Pi increase with decreasing γ-Al2O3 particle size. The sorption affinity of γ-Al2O3 for IHP and Pi generally increases with decreasing particle size, and the sorption affinity for IHP is approximately one order of magnitude greater than that for Pi. In our experimental time scale(48 h), surface complexation is the main mechanism for IHP and Pi sorption on large size γ-Al2O3. While an additional surface precipitation mechanism, indicated by solid-state 31 P and 27 Al NMR data, is partly responsible for the greater sorption density on very small size γ-Al2O3. Compared with Pi, the effect of particle size on the sorption of IHP is more pronounced. The results suggest a size-dependent surface reactivity of Al2O3 nanoparticles with Pi/IHP. The underlying mechanism will also be relevant for other small nanosize(hydr)oxide particles and is important for understanding of the role of nano oxides in controlling the mobility and fate of organic and inorganic phosphates in the environment.6. The XRD analysis indicates that without the post-sorption of IHP on γ-Al2O3, the formation of Zn-Al layered double hydroxide(LDH) precipitates occurs at Zn(II) concentration ≥ 0.72 m M, and with the post-sorption of IHP(0.89 μmol m-2), Zn-Al LDH precipitates are formed at Zn(II) concentration ≥ 1.50 m M. Therefore, the post-sorption of IHP restrains the formation of LDH. The ATR-FTIR and solid-state NMR spectra further suggest that after the post-sorption of IHP, γ-Al2O3-IHP-Zn ternary surface complexes are formed with the addition of low concentration of Zn(II)(< 0.72 m M), zinc phytate(Zn-IHP) precipitates are developed with the addition of moderate concentration of Zn(II)(0.72-1.50 m M), and Zn-IHP precipitates and LDH are formed when the Zn(II) concentration is higher(> 1.50 m M). Additionally, the sequence of additional IHP and Zn can influence the sorption mechanism in the ternary systems. At p H 7, Zn-IHP precipitates form and no LDH precipitates form if Zn is added prior to IHP.7. The sorption experiments show that the presence of IHP promotes Zn(II) adsorption, and vice versa, the presence of Zn(II) facilitates IHP adsorption. The sorption extent and mechanisms of organic phosphate and metal ions on goethite in the ternary systems are different from those in the binary IHP/goethite and Zn/goethite systems. The ATR-FTIR studies suggest that in the presence of Zn(II), Gt-IHP-Zn ternary complexes are formed at the goethite surface probably, in which IHP bridges the mineral surface and Zn. These results imply that while divalent metals and IHP are simultaneously present, the dissolution and speciation of these compounds are significantly influenced by the interaction in the ternary systems.This study systematically ascertains the adsorption, desorption and precipitation reaction characteristics and mechanisms of organic phosphate on Fe and Al oxides at the molecular level, which will improve the mechanistic understanding of environmental behavior of OPs and recycling of P. It is also of great theoretical and practical significance to promote effective regulation and scientific management of P fertilizer.