Phosphate-Exchanged Mg–Al Layered Double Hydroxides: A New Slow Release Phosphate Fertilizer

The global phosphorus crisis provided impetus to develop fertilizers with better P use efficiency. We tested layered double hydroxides (LDHs) as slow release fertilizers with superior performance to fertilize strongly P-fixing soils. Mg–Al LDHs with varying M²⁺/M³⁺ ratios were synthesized as NO₃^– forms and were exchanged with HPO₄^2–. XRD and XANES spectroscopy confirmed the identity of the phosphate-exchanged LDH. Decreasing the M²⁺/M³⁺ ratio, i.e., increasing the anion exchange capacity, increased the selectivity of P adsorption due to the increasing charge density of the LDH layers. The fertilization efficiency of the phosphate-exchanged LDH (Mg/Al ratio of 2) was compared to that of a soluble P fertilizer in two P-deficient soils, an acid weathered soil and a calcareous soil. The P use efficiency of the P-LDH in the acid soil was up to 4.5 times higher than that of soluble P. This was likely related to a liming effect of the LDH. In the calcareous soil, the P use efficiency at low doses was only 20% above that of soluble P, whereas it was lower at high doses. These overall encouraging results warrant further studies on the boundary conditions under which P-LDHs may outperform traditional fertilizers

Polyphosphates and Fulvates Enhance Environmental Stability of PO₄‑Bearing Colloidal Iron Oxyhydroxides

Iron oxyhydroxide nanoparticles (Fe-NPs) are natural vectors of phosphate (PO₄) in the environment. Their mobility is determined by colloidal stability, which is affected by surface composition. This might be manipulated in engineered NPs for environmental or agricultural applications. Here, the stability of PO₄-Fe-NPs (HFO/goethite) was determined across contrasting environmental conditions (pH, Ca concentration) and by using fulvates (FA) and polyphosphates (poly-P’s) as coatings. The PO₄-Fe-NPs are unstable at Ca concentrations above 0.1 mM. Addition of FA and some poly-P’s significantly improved stability. Zeta potential explained colloidal stability across treatments; surface charge was calculated with surface complexation models and explained for phytic acid (PA) and hexametaphosphate (HMP) by a partial (1–4 of the 6 PO₄ units) adsorption to the surface, while the remaining PO₄ units stayed in solution. This study suggests that Ca concentration mainly affects the mobility of natural or engineered PO₄-Fe-NPs and that HMP is a promising agent for increasing colloidal stability

Internal Loading and Redox Cycling of Sediment Iron Explain Reactive Phosphorus Concentrations in Lowland Rivers

The phosphate quality standards in the lowland rivers of Flanders (northern Belgium) are exceeded in over 80% of the sampling sites. The factors affecting the molybdate reactive P (MRP) in these waters were analyzed using the data of the past decade (>200 000 observations). The average MRP concentration in summer exceeds that winter by factor 3. This seasonal trend is opposite to that of the dissolved oxygen (DO) and nitrate concentrations. The negative correlations between MRP and DO is marked (<i>r</i> = −0.89). The MRP concentrations are geographically unrelated to erosion sensitive areas, to point-source P-emissions or to riverbed sediment P concentration. Instead, MRP concentrations significantly increase with increasing sediment P/Fe concentration ratio (<i>p</i> < 0.01). Laboratory static sediment–water incubations with different DO and temperature treatments confirmed suspected mechanisms: at low DO in water (<4 mg L^–1), reductive dissolution of ferric Fe oxides was associated with mobilization of P to the water column from sediments with a molar P/Fe ratio >0.4. In contrast, no such release was found from sediments with lower P/Fe irrespective of temperature and DO treatments. This study suggests that internal loading of the legacy P in the sediments explains the MRP concentrations which are most pronounced at low DO concentrations and in regions where the P/Fe ratio in sediment is large

Body distribution of SiO₂–Fe₃O₄ core-shell nanoparticles after intravenous injection and intratracheal instillation

<p>Nano-silicon dioxide (SiO₂) is used nowadays in several biomedical applications such as drug delivery and cancer therapy, and is produced on an industrial scale as additive to paints and coatings, cosmetics and food. Data regarding the long-term biokinetics of SiO₂ engineered nanoparticles (ENPs) is lacking. In this study, the whole-body biodistribution of SiO₂ core-shell ENPs containing a paramagnetic core of Fe₃O₄ was investigated after a single exposure via intravenous injection or intratracheal instillation in mice. The distribution and accumulation in different organs was evaluated for a period of 84 days using several techniques, including magnetic resonance imaging, inductively coupled plasma mass spectrometry, X-ray fluorescence and X-ray absorption near edge structure spectroscopy. We demonstrated that intravenously administered SiO₂ ENPs mainly accumulate in the liver, and are retained in this tissue for over 84 days. After intratracheal instillation, an almost complete particle clearance from the lung was seen after 84 days with distribution to spleen and kidney. Furthermore, we have strong evidence that the ENPs retain their original core-shell structure during the whole observation period. This work gives an insight into the whole-body biodistribution of SiO₂ ENPs and will provide guidance for further toxicity studies.</p