Effect Of Carbonate And Phosphate Ratios On P Speciation And Their Interaction Mechanism In Loess

Binding reactions occurring at mineral/water interfaces are of fundamental importance in controlling the sequestration and bioavailability of nutrients in aqueous environments. Phosphorus (P) is an essential nutrient for plant growth and it is also defined as’the disappearing nutrient’due, in part, to the potential shortage of phosphate rock resources and a fast growth in demand for phosphate-based fertilizers. In this study, the binding mechanisms between phosphate and calcite was separated by both experiment and speciation model (ECOSAT-FIT). To study the effect of carbonate and phosphate ratios on the transformation of calcium-orthophosphates X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray (SEM/EDX), fourier transform infrared spectroscopy (FTIR) and magic-angle spinning nuclear magnetic resonance (MAS-NMR) were performed. To study the effect of carbonate on binding amounts of phosphate with loess containing carbonate and without carbonate and their phosphorus speciation environmental scanning electron microscopy with energy dispersive X-ray (ESEM) and X-ray photoelectron spectroscopy (XPS) were performed. The main results are showed in the following:1. Adsorption and precipitation of phosphate on calcitePhosphate can be adsorbed or precipitated on calcite which could be phosphate super-binder in alkaline environment. The quick binding mainly occurs within low amounts of8.2-32.8μM P and followed by a slow step. The binding amount of P at pH8.0-9.0after fitting with Langmuir qmax are28.5,27.5and21μM P/g calcite. The data from equilibrium calculation of speciation and transport (ECOSAT) software show that adsorption mainly occurs within low amounts of8.2-24.6μM P and followed by gradually decreases of adsorption occurred. At pH8.0the major mechanism of mineral/water interfaces was adsorption, while, at pH9.0during high amount of49.3and57.5μM P completely precipitation occurred. The pH is a very important parameter during ion exchange process. The mechanism of mineral/water interfaces was adsorption followed by precipitation with the compound mineral formation of hydroxyl-apatite.2. Effect of carbonate and phosphate ratios on the calcium orthophosphate transformationThe synthesized product at the low (0.15) and high (1.8) molar ratio of PO43-/CO32-is calcium phosphate hydrate at pH9.0, and hydroxyl-apatite (HAp) at pH8.0, respectively. Fourier transform infrared spectroscopy of product at the high ratio (1.8) of PO43-/CO32-shows that the CO32-peaks disappear, and the strong peaks at1412and1460cm-1are assigned to the vibrations of PO43-in HAp.31P nuclear magnetic resonance spectra of products at the low (0.15-0.6) to high (1.2-1.8) ratios of PO43-/CO32-are obtained at2.9and2.7ppm, respectively. These spectra could be related to calcium-phosphate phases which are located in apatite or hydroxyl-apatite layers. The structure of apatite could also have an effect on the chemical shift of31P MAS-NMR spectra. The types, morphologies and chemical components obtained from XRD and SEM/EDX were in alignment with FTIR and NMR measurements. Molar ratios of PO43-/CO32-are effective on the reduction of carbonate activity during the formation and infiltration events of calcium-phosphate surface precipitates, and are subsequently enclosed during HAp formation.3. Phosphate binding with components of loessThe loess containing carbonate and/or without carbonate was well fitted by Langmuir model (R2=0.997and0.985, respectively). The binding capacities of loess containing carbonate (qmax=15.9μM P/g) and for loess without carbonate (qmax=10.3μM P/g). The variation between loess containing carbonate and without carbonate was5.6μM P/g. Loess containing carbonate (15.9μM P) with17%carbonate and binding amount of loess without carbonate (10.3μM P) was calculated. The data show43.2μM P can bind with1g calcium carbonate and is4.2times higher than that loess without carbonate. The phosphorus fractionation in sample137.5μM P bound with loess containing carbonate was in the order:Caio-P (440mg/kg)> Ca2-P (48mg/kg)> Fe-P (12mg/kg)> O-P (11.6mg/kg)> Cag-P (9.0mg/kg)>Al-P (3.2mg/kg) with84%,9.0%,2.0%,2.0%,1.7%and0.6%. X-ray photoelectron spectroscopy results show the atomic ratio of Ca/P (44,2.37), C/P (141,6.4) and O/P (652,40) in sample39.3and118μM P bound with loess containing carbonate. This variation between sample39.3and118uM P bound with loess containing carbonate shows a variety of active chemical reactions existed. During39.3and118μM P bound with loess containing carbonate the synthesized product was tri-calcium-phosphate and fluorapatite, respectively. Environmental scanning electron microscopy with energy dispersed X-ray in sample118μM P bound with loess containing carbonate shows an elemental distribution of Ca/P1.89which is related to apatite-groups. Amounts of phosphate to loess containing carbonate are effective on the reduction of calcium activity during the ion exchange and formation of calcium-phosphate precipitates.