Phosphorus speciation in soils with low to high degree of saturation due to swine slurry application.

Soils with continuous application of swine slurry (SS) may present high phosphorus (P) content and high risk for environmental pollution. The aim of this study was to characterize the forms of phosphorus accumulation in 15 fields with increasing degrees of P saturation (DPS) in a watershed with a high density of swine farming. Soil samples collected from 0 to 10 cm were chemically characterized for water soluble phosphorus (WSP), DPS, Hedley chemical fractionation, and chemical speciation by P K-edge XANES. WSP increased linearly to a value of 137% of DPS, with subsequent stabilization at 2.7 mg kg-1. Only the inorganic fractions of the chemical fractionation changed with increasing DPS. Phosphorus forms considered labile increased up to 144% of DPS, with subsequent stabilization. The moderately labile fraction 0.1 M NaOH and non-labile 1 M HCl increased exponentially. Phosphorus K-edge XANES analysis demonstrated that P associated to apatite, amorphous aluminum minerals, and goethite were the main forms of P found and only the latter had a correlation with DPS (-0.57*). With increasing DPS, there are changes in the dynamics of P in the soil, with a reduction in forms associated to Fe and an increase in forms linked to Al. The forms linked to Al buffer the WSP and are recovered in the first inorganic fractions of the Hedley chemical fractionation.

Formation of Zn-Al layered double hydroxides (LDH) during the interaction of ZnO nanoparticles (NPs) with γ-Al2O3.

Zinc and aluminum layered double hydroxides (Zn-Al LDH) are a common group of major Zn species in various Zn-contaminated soil/sediment environments, yet their formation pathways and underlying mechanisms under varied conditions are not well understood. This study investigated the formation of Zn-Al LDHs through the direct interaction of two solid substrates, ZnO nanoparticles (NPs) and a representative Al oxide, γ-Al2O3. Batch experiments and complementary microscopic and spectroscopic analyses were conducted to elucidate the reaction kinetics and mechanisms, as well as the morphologic and structural evolution of the products. Dissolved Zn and Al concentrations decreased significantly in a dual solid system compared to a single solid system. A bulk Zn-Al LDH phase was found to form under a wide pH range (6.5-9.5). Aside from Zn-Al LDH, γ-Al2O3 was the main remaining solid phase at pH 6.5, whereas ZnO NPs were the main residual solid phases at pH 9.5. Formation of amorphous Zn(OH)2 was also observed at both pH values, likely due to Zn2+ release at low pH and Al(OH)4- adsorption at high pH. It is proposed that the formation of Zn-Al LDH occurs via a dissolution-sorption-coprecipitation process, where the solubility of ZnO NPs or γ-Al2O3 solid phases determines the reaction pathways and kinetics under varied pH conditions. The results from this work revealed the transformation mechanisms for ZnO NPs under conditions from weakly acidic to alkaline pH with highly available Al particles and shed light on the environmental fate of ZnO NPs in Zn or ZnO NP contaminated environments.

Residence time and pH effects on the bonding configuration of orthophosphate surface complexes at the goethite/water interface as examined by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy.

Identifying the mechanisms by which P is bound to soils and soil constituents is ultimately important as they provide information on the stability of bound species and their reactivity in the environment. EXAFS studies were carried out to provide information on how the local chemical environment of sorbed P changes as an effect of pH and time. Goethite was reacted with orthophosphate at a P concentration of 0.8mmolL(-1) P at pH 3.0, 4.5 and 6.0. The residence time effect on the mechanisms of P sorption on goethite was also evaluated for two different reaction times, 5 and 18days, on goethite suspensions reacted at pH 4.5. The objective of this study was to understand how P sorption mechanisms change over a wide pH range when subjected to P concentrations above the P saturation ratio of goethite. Phosphorus K-edge EXAFS spectra were collected at 2150eV in fluorescence mode and the structural parameters were obtained through the fits of sorption data using Artemis. The monodentate surface complex was shown to be the predominant mechanism by which P sorbs at the goethite surface under the experimental conditions. The lack of a discrete Fe-P shell and the presence of highly disordered structures, particularly, at R-space ⩾3.5 suggested the formation of P surface precipitates at the goethite/water interface.

Surface loading effects on orthophosphate surface complexation at the goethite/water interface as examined by extended X-ray Absorption Fine Structure (EXAFS) spectroscopy.

To investigate the effect of P surface loading on the structure of surface complexes formed at the goethite/water interface, goethite was reacted with orthophosphate at P concentrations of 0.1, 0.2, and 0.8 mmol L(-1) at pH 4.5 for 5 days. The P concentrations were chosen to ensure that P loadings at the surface would allow one to follow the transition between adsorption and surface precipitation. Extended X-ray Absorption Fine Structure (EXAFS) spectra were collected in fluorescence mode at the P K-edge at 2150 eV. The structural parameters were obtained through the fits of the sorption data to single and multiple scattering paths using Artemis. EXAFS analysis revealed a continuum among the different surface complexes, with bidentate mononuclear ((2)E), bidentate binuclear ((2)C) and monodentate mononuclear ((1)V) surface complexes forming at the goethite/water interface under the studied conditions. The distances for P-O (1.51-1.53Å) and P-Fe (3.2-3.3Å for bidentate binuclear and around 3.6Å for mononuclear surface complexes) shells observed in our study were consistent with distances obtained via other spectroscopic techniques. The shortest P-Fe distance of 2.83-2.87Å was indicative of a bidentate mononuclear bonding configuration. The coexistence of different surface complexes or the predominance of one sorption mechanism over others was directly related to surface loading.

Long-term manure application effects on phosphorus speciation, kinetics and distribution in highly weathered agricultural soils.

Phosphorus (P) K-edge XANES and Fe K-edge EXAFS spectroscopies along with sequential P chemical fractionation and desorption kinetics experiments, were employed to provide micro- and macro-scale information on the long-term fate of manure application on the solid-state speciation, kinetics and distribution of P in highly weathered agricultural soils of southern Brazil. Soil test P values ranged from 7.3 up to 16.5 times as much higher than the reference soil. A sharp increase in amorphous Fe and Al amounts were observed as an effect of the consecutive application of manures. Whereas our results showed that the P sorption capacity of some manured soils was not significantly affected, P risk assessment indices indicated that P losses should be expected, likely due to the excessive manure rates applied to the soils. The much higher contents of amorphous Fe and Al (hydr)oxides (55% and 80% increase with respect to the reference soil, respectively) in manured soils seem to have counterbalanced the inhibiting effect of soil organic matter on P sorption by creating additional P sorption sites. Accordingly, the newly created P sorbing surfaces were important to prevent an even larger P loss potential. Phosphorus K-edge XANES lent complimentary hints on the loss of crystallinity and transformation of originally present Fe-P minerals into poorly crystalline ones as an effect of manuring, whereas Fe K-edge EXAFS provided insights into the structural changes underwent in the soils upon manure application and soil management.