Phosphate Extraction Research Articles

Feasibility of using phosphate wastes for enhancing high-temperature rheological characteristics of asphalt binder

The huge amount of wastes is produced annually from the phosphate extraction process which causes environmental problems in terms of energy and pollution. Using these wastes for producing asphalt mixtures can be a cost-effective and eco-friendly solution for managing them. The aim of this study is to investigate the feasibility of using phosphate wastes for fabrication of bituminous materials. Furthermore, the study objective is to discover the most effective waste to be included in asphalt mixtures. These wastes were acquired from the phosphate extraction plant, Ben Guerir city, Morocco. Phosphogypsum, fly ash, and dry phosphate sludge were the materials selected to be used as filler. Four samples were prepared, including an original asphalt binder as well as phosphogypsum, fly ash, and phosphate sludge asphalt samples. Classical characterization, viscous flow, frequency, and temperature sweep tests were carried out on these samples. The results showed that the viscous flow behavior of the asphalt binder containing phosphogypsum and fly ash wastes has been substantially more improved compared with binder with phosphate sludge filler. It was also shown that phosphogypsum waste enhances the mechanical properties of asphalt binder as well as its performance against rutting more than fly ash and phosphate sludge wastes.

Revisiting Hornos de la Peña 100 years after

The cave of Hornos de la Peña contained one of the most relevant stratigraphic sequences for the study of the Middle and Upper Paleolithic in the Cantabrian Region, northern Iberia, as well as an important group of Paleolithic rock art. The site was discovered in 1903 and excavated during 1909–1910. Those excavations did not take place in the main hall of the cave because it was already emptied for modern phosphate extraction. At that time, in the preserved deposits, the archeological excavations revealed an interesting sequence with Mousterian, Early Upper Paleolithic, Solutrean and Magdalenian cultural remains. Since then, the site has received little attention owing to the difficulties of interpreting the sequence and the biased nature of the preserved collections. In 2016 and 2017 a limited area of the preserved section was excavated to document its archeological sequence, obtain samples for dating and achieve some insights about the cultural attribution of the identified units and their correlation with Obermaier’s sequence. The results show that there is a coarse correlation between both sequences. More interestingly, this work has revealed the good preservation and interest of the Mousterian occupation, the presence of Early Upper Paleolithic, the indefinite nature of the Solutrean and the existence of a Middle Magdalenian occupation. These results provide new insights about the human presence at the site and allow to incorporate it into the new debates about the Middle and Upper Paleolithic in the Cantabrian Region.

Phosphorus, chemical base and other renewables from wastewater with three 168-L microbial electrolysis cells and other unit operations

Phosphate rock is a depleting resource and wastewater a sustainable long-term alternative for phosphorous mining. In modern wastewater treatment phosphate is concentrated 7500 times from wastewater into sludge as iron phosphate (FeP). Recently developed bioelectrochemical reactors enabled phosphate recovery from sewage sludge containing FeP. The integrated bioelectric process was found of much broader utility than initially elaborated. It refines all principle components of wastewater. The implementation is confronted to a number of challenges. Three pilot microbial electrolysis cells (MECs) of 168 L each were constructed and installed in different municipal wastewater treatment plants (WWTPs). The scale-up MECs generated renewable chemical base and co-extracted abundant species such as Na+, K+, Ca2+, Mg2+ and NH4+ from wastewater. The chemical base remobilized phosphate quantitatively from iron phosphates contained in digested sewage sludge. Phosphate extracts contained ammonia and upon magnesium (Mg2+) addition struvite crystalized. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) on heavy metals, Direct Mercury Analysis (DMA), Liquid Chromatography Mass Spectroscopy (LC-MS/MS) on organic micropollutants, metagenomics sequencing, Scanning Electron Microscopy (SEM-EDS), and X-Ray Diffraction (XRD) indicated that a highly pure struvite-fertilizer was produced. Microbial electricity co-generation was verified by electrochemical characterisation and microbiome analysis using 16S rRNA V4-V5 methodology. Geobacter, Dechloromonas, Desulfobulbus and cyanobacteria were the principal electrogens found. All in all, renewable chemical base as well as phosphate were obtained in high quantities and other renewables became accessible such as the critical material magnesium and other compounds of importance like ammonia, potassium, calcium, solid P-free sludge useful as biofuel and purified water. In general, the process recycles important compounds from waste, in a close to traceless manner while purifying wastewater.

Comparative analysis of the efficiency of phosphate extraction from water by natural sorbents

Comparative analysis of the efficiency of phosphate extraction from water by natural sorbents

Effects of progesterone and estradiol on the inflammatory and apoptotic markers of ovariectomized rats challenged with acute septic systemic inflammation

Comparative effect of chronic administration of chloroquine phosphate and methanol extract of Landolphia owariensis on the activities of rat EnzymesIlesanmi F F, Balogun E A, Ilesanmi O S

Impact of land use/land cover change on the topsoil selenium concentration and its potential bioavailability in a karst area of southwest China

Selenium (Se) is an essential micronutrient for human health, and its abundance and potential bioavailability in the soil are of increasing concern worldwide. To date, how total soil Se and its bioavailability would respond to human disturbance or future environmental change is not yet clear, and associated controlling factors remain incompletely understood. Here, we collected soil samples (0–15 cm) from different land use/land cover types, including active cropland, grassland, shrubland, and secondary forest, in a Se-enriched area of Guangxi, southwest China. Total Se concentration and its potential bioavailability, as estimated by phosphate extractability, were investigated. Total soil Se concentration (Setotal) for all samples ranged from 220 to 1820 μg kg−1, with an arithmetic average value of 676 ± 24 μg kg−1 (Mean ± SE, the same below). The concentration of phosphate extractable Se (Sephosphate) varied between 1 and 257 μg kg−1, with an arithmetic mean value of 79 ± 5 μg kg−1, accounting for on average 13 ± 1% of the Setotal. Among the four land use/land cover types, Setotal and Sephosphate were generally more enriched in the secondary forest than those in the grassland and cropland. The content of soil organic carbon (SOC) was the overriding edaphic factor controlling the abundance and potential bioavailability of Se in topsoils. In addition, climatic variables such as mean annual precipitation and mean annual temperature were also key factors affecting the abundance and potential bioavailability of soil Se. Our results suggest that changes in land use/land cover types may deeply influence Se biogeochemistry likely via alterations in soil properties, particularly SOC content.

Mg2+ in β-TCP/Mg-Zn composite enhances the differentiation of human bone marrow stromal cells into osteoblasts through MAPK-regulated Runx2/Osx.

Inducing the osteogenic differentiation from bone marrow stromal cells (BMSCs) might be a potent strategy for treating bone loss and nonunion during fracture and improving fracture healing. Among several signaling pathways involved, mitogen-activated protein kinases (MAPKs) have been reported to play a critical role. Magnesium (Mg)-based alloys, including Mg-Zn alloy, have been used clinically as implants in the musculoskeletal field and could promote BMSC osteogenic differentiation. However, the underlying mechanisms remain unclear. In this study, we produced Mg-Zn alloy consists of Mg and low concentrations of Zn, calcium carbonate, and β-tricalcium phosphate (β-TCP; manifesting process not shown), prepared Mg, Zn, and Mg-Zn extracts, and investigated the specific effects of these extracts on human BMSC (hBMSC) osteogenic differentiation and MAPK signaling. Mg extracts and Mg-Zn extracts could significantly promote the osteogenic differentiation of hBMSCs as manifested as increased alkaline phosphatase levels, enhanced calcium nodules formation, and increased messenger RNA expression and protein levels of osteogenesis markers, including BMPs, Col-I, Runx2, and Osx; in the meantime, Mg culture medium (CM) and Mg-Zn CM both significantly enhanced the activation of MAPK signaling in hBMSCs. By adding ERK1/2 signaling, p38 signaling, or JNK signaling inhibitor to Mg-Zn CM, or conducting p38 MAPK silence in hBMSCs, we revealed that these extracts might promote hBMSC osteogenic differentiation via p38 MAPK signaling and MAPK-regulated Runx2/Osx. In conclusion, Mg2+ in β-TCP/Mg-Zn extract promotes the osteogenic differentiation of hBMSCs via MAPK-regulated Runx2/Osx interaction.

Environmental and health co-benefits for advanced phosphorus recovery

Worldwide food production is largely dependent on rock phosphate, a finite raw material used for the production of concentrated phosphorus fertilizers. With the aim to close the biogeochemical phosphorus cycle across regions and urban–rural systems, advanced phosphorus recovery applies thermochemical and precipitation techniques to transform locally available biogenic materials into concentrated phosphorus fertilizers. Due to insufficient insights into the consequential impacts of these circular processes, opportunities to align advanced phosphorus recovery with agricultural sustainability are still widely unknown. Here we show that environmental and health life cycle impacts are often lower for phosphorus fertilizers sourced from secondary raw materials than for rock phosphate-derived products, especially in areas of high livestock and population density. Including externalities from rock phosphate extraction and avoided current-day management of biogenic materials in the comparative product life cycle severely alters the cost assessment relative to an analysis that considers only internal costs from manufacturers’ production processes. Societal costs incurred for circular products derived from sewage sludge, manure and meat and bone meal are up to 81%, 50% and 10% lower than for rock-derived superphosphate, respectively. Even without accounting for rock phosphate depletion risks, short-term and local environmental and health co-benefits might underlie the societal cost effectiveness of advanced phosphorus recovery. Agriculture depends critically on phosphorus fertilizer from rock phosphate. This study finds environmental and health benefits from instead recovering phosphorus from secondary sources, such as sewage and manure.

Improvement in flow-sheet of extraction chromatography for trivalent minor actinides recovery

Extraction chromatography flow-sheet employing octyl(phenyl)-N,N-diisobutylcarbonoylmethylphosphine oxide and bis(2-ethylhexyl) hydrogen phosphate extractants for trivalent minor actinide recovery was modified to improve column separation performance. Excellent trivalent minor actinides recovery performance was obtained by column separation experiments on nitric acid solution containing the trivalent minor actinides and representative fission product elements, i.e. recovery yields > 93% with sufficient decontamination factors against the fission products. Those are the best performance which we have ever obtained by experiments inside hot cell.

Role of sulphate in affecting soil availability of exogenous selenate (SeO<sub>4</sub><sup>2-</sup>) under different statuses of soil microbial activity

We investigated sulphate application, different statuses of soil microbial activity and their joint effects as variables associated with changes in potentially plant-available selenium (Se<sub>ppa</sub>) and soil Se fractionation during the course of an incubation study. The time-resolved behaviour of added selenate (400 µg Se/kg as Na<sub>2</sub>SeO<sub>4</sub>) in two agricultural soils was elucidated by means of single extraction (50 mmol/L NH<sub>4</sub>H<sub>2</sub>PO<sub>4</sub>), sequential extraction procedure (SEP) and chemical speciation analysis in phosphate extracts. The decrease in phosphate-extractable Se, a consequence of soil aging, was inhibited by sulphate (by 34% and 29% in Chernozem and Cambisol, respectively) and by gamma-irradiation (by 46% and 20% in Chernozem and Cambisol, respectively) after 72 days of incubation as compared to the control treatments. Glucose amendment dramatically decreased Se<sub>ppa</sub> only in the Chernozem. After 1 year, the initial soil treatment with respect to inhibited or stimulated microbially-mediated processes substantially controlled the distribution pattern of exogenous Se as observed using the SEP. Application of sulphur fertilisers and sources of labile organic matter is thus an essential agronomic practice to correct unfavourable amounts of Se<sub>ppa</sub>.

Coagulation removal of Sb(V) from textile wastewater matrix with enhanced strategy: Comparison study and mechanism analysis

The wide use of antimony in textile industry has posed threat to ecological health and attracted increased attention. The objective of this work was to develop enhanced coagulation strategies including PFS/FeSO4 and aerated PFS/FeSO4 for efficient antimony elimination from textile wastewater matrix. With a dosage of 0.75 mM Fe, aerated PFS/FeSO4 coagulation could achieve 82.6% removal of 500 μg L−1 Sb(V) from simulated textile wastewater, which was better than PFS (77.6%) and PFS/FeSO4 coagulation (79.9%). Compared with PFS and PFS/FeSO4 coagulation, aerated PFS/FeSO4 strategy could meet the indirect discharge standard (<100 μg L−1), without any other additional treatment. pH ranged from 5 to 6 could reach 93.8% Sb(V) removal, by affecting coagulant hydrolysis and charges on flocs. Phosphate ion with a level more than 0.03 mM could compete with Sb(V) species and thus reduced its removal. Temperature of 35 °C could lead to enhanced Sb(V) removal by accelerating coagulant hydrolysis. Flocs of aerated PFS/FeSO4 had smaller average size than that of PFS and PFS/FeSO4 coagulation. FeOOH was the hydrolysis product of aerated PFS/FeSO4 strategy. Adsorption, rather than direct and co-precipitation was predominant in the coagulation mechanism. From the phosphate extraction test, 64% of the Sb could form inner-sphere surface complex during aerated PFS/FeSO4 coagulation removal.

Partial Fluxes of Phosphoric Acid Anions through Anion-Exchange Membranes in the Course of NaH2PO4 Solution Electrodialysis.

Electrodialysis (ED) with ion-exchange membranes is a promising method for the extraction of phosphates from municipal and other wastewater in order to obtain cheap mineral fertilizers. Phosphorus is transported through an anion-exchange membrane (AEM) by anions of phosphoric acid. However, which phosphoric acid anions carry the phosphorus in the membrane and the boundary solution, that is, the mechanism of phosphorus transport, is not yet clear. Some authors report an unexpectedly low current efficiency of this process and high energy consumption. In this paper, we report the partial currents of H2PO4−, HPO42−, and PO43− through Neosepta AMX and Fujifilm AEM Type X membranes, as well as the partial currents of H2PO4− and H+ ions through a depleted diffusion layer of a 0.02 M NaH2PO4 feed solution measured as functions of the applied potential difference across the membrane under study. It was shown that the fraction of the current transported by anions through AEMs depend on the total current density/potential difference. This was due to the fact that the pH of the internal solution in the membrane increases with the growing current due to the increasing concentration polarization (a lower electrolyte concentration at the membrane surface leads to higher pH shift in the membrane). The HPO42− ions contributed to the charge transfer even when a low current passed through the membrane; with an increasing current, the contribution of the HPO42− ions grew, and when the current was about 2.5 ilimLev (ilimLev was the theoretical limiting current density), the PO43− ions started to carry the charge through the membrane. However, in the feed solution, the pH was 4.6 and only H2PO4− ions were present. When H2PO4− ions entered the membrane, a part of them transformed into doubly and triply charged anions; the H+ ions were released in this transformation and returned to the depleted diffusion layer. Thus, the phosphorus total flux, jP (equal to the sum of the fluxes of all phosphorus-bearing species) was limited by the H2PO4− transport from the bulk of feed solution to the membrane surface. The value of jP was close to ilimLev/F (F is the Faraday constant). A slight excess of jP over ilimLev/F was observed, which is due to the electroconvection and exaltation effects. The visualization showed that electroconvection in the studied systems was essentially weaker than in systems with strong electrolytes, such as NaCl.

Randomized Controlled Study to Evaluate Microbial Ecological Effects of CPP-ACP and Cranberry on Dental Plaque.

Ecological approaches to dental caries prevention play a key role in attaining long-term control over the disease and maintaining a symbiotic oral microbiome. This study aimed to investigate the microbial ecological effects of 2 interventional dentifrices: a casein phosphopeptide-amorphous calcium phosphate (CPP-ACP) dentifrice and the same dentifrice supplemented with a polyphenol-rich cranberry extract. The interventional toothpastes were compared with each other and with an active control fluoride dentifrice in a double-blinded randomized controlled trial. Real-time quantitative polymerase chain reaction (qPCR) analysis was used to determine changes in the bacterial loads of 14 key bacterial species (8 caries associated and 6 health associated) in the dental plaque of trial participants after they used the dentifrices for 5 to 6 wk. From the baseline to the recall visit, significant differences were observed between the treatment groups in the bacterial loads of 2 caries-associated bacterial species (Streptococcus mutans [P < 0.001] and Veillonella parvula [P < 0.001]) and 3 health-associated bacterial species (Corynebacterium durum [P = 0.008], Neisseria flavescens [P = 0.005], and Streptococcus sanguinis [P < 0.001]). Compared to the fluoride control dentifrice, the CPP-ACP dentifrice demonstrated significant differences for S. mutans (P = 0.032), C. durum (P = 0.007), and S. sanguinis (P < 0.001), while combination CPP-ACP-cranberry dentifrice showed significant differences for S. mutans (P < 0.001), V. parvula (P < 0.001), N. flavescens (P = 0.003), and S. sanguinis (P < 0.001). However, no significant differences were observed in the bacterial load comparisons between the CPP-ACP and combination dentifrices for any of the targeted bacterial species (P > 0.05). Overall, the results indicate that dentifrices containing CPP-ACP and polyphenol-rich cranberry extracts can influence a species-level shift in the ecology of the oral microbiome, resulting in a microbial community less associated with dental caries (Australian New Zealand Clinical Trial Registry ANZCTR 12618000095268). The results of this randomized controlled trial indicate that dentifrices containing casein phosphopeptide-amorphous calcium phosphate (CPP-ACP) and polyphenol-rich cranberry extracts were able to beneficially modulate the microbial ecology of dental plaque in a group of high caries-risk patients. This could contribute toward lowering the risk of developing new caries lesions, an important goal sought by patients, clinicians, and policy makers.

Phosphate-limited ocean regions select for bacterial populations enriched in the carbon-phosphorus lyase pathway for phosphonate degradation.

SummaryIn tropical and subtropical oceanic surface waters phosphate scarcity can limit microbial productivity. However, these environments also have bioavailable forms of phosphorus incorporated into dissolved organic matter (DOM) that microbes with the necessary transport and hydrolysis metabolic pathways can access to supplement their phosphorus requirements. In this study we evaluated how the environment shapes the abundance and taxonomic distribution of the bacterial carbon–phosphorus (C–P) lyase pathway, an enzyme complex evolved to extract phosphate from phosphonates. Phosphonates are organophosphorus compounds characterized by a highly stable C–P bond and are enriched in marine DOM. Similar to other known bacterial adaptions to low phosphate environments, C–P lyase was found to become more prevalent as phosphate concentrations decreased. C–P lyase was particularly enriched in the Mediterranean Sea and North Atlantic Ocean, two regions that feature sustained periods of phosphate depletion. In these regions, C–P lyase was prevalent in several lineages of Alphaproteobacteria (Pelagibacter, SAR116, Roseobacter and Rhodospirillales), Gammaproteobacteria, and Actinobacteria. The global scope of this analysis supports previous studies that infer phosphonate catabolism via C–P lyase is an important adaptive strategy implemented by bacteria to alleviate phosphate limitation and expands the known geographic extent and taxonomic affiliation of this metabolic pathway in the ocean.

Rapid analytical method of 90Sr in urine sample: Rapid separation of Sr by phosphate co-precipitation and extraction chromatography, followed by determination by triple quadrupole inductively coupled plasma mass spectrometry (ICP-MS/MS)

A rapid analytical method for determining 90Sr in urine samples (1–2 L) was developed to assess the internal exposure of workers in a radiological emergency. Strontium in a urine sample was rapidly separated by phosphate co-precipitation, followed by extraction chromatography, and the 90Sr activity was determined by triple quadrupole inductively coupled plasma mass spectrometry (ICP-MS/MS). Measurement in the MS/MS mode with an O2 reaction gas flow rate 1 mL min−1 showed no tailing of 88Sr at m/z = 90 up to 50 mg L−1 Sr. The interferences of Ge, Se and Zr at m/z = 90 were successfully removed by phosphate co-precipitation, followed by extraction chromatography with a tandem column of Pre-filter, TRU and Sr resin. This analytical method was validated by the results of the analyses of synthetic urine samples (1.2–1.6 L) containing a known amount of 90Sr along with 1 mg of each of Ge, Se, Sr and Zr. The turnaround time for Sr purification from the urine sample and the 90Sr measurement by ICP-MS/MS was about 10 h. The detection limit of 90Sr was approximately 1 Bq per urine sample, which was lower than 15 Bq per urine after a day of intake giving 5 mSv of unplanned exposure of worker limited by Nuclear Regulation Authority of Japan.

Anoxic storage regenerates reactive Fe(II) in reduced nontronite with short-term oxidation

Fe(II)-bearing clay minerals represent a large electron source under redox-dynamic systems. Although redox reactions between Fe(II)-bearing clay minerals and oxidants (such as O2) have been widely investigated in redox-dynamic systems, one important process that remains unexplored is the response of Fe(II) reactivity in clay minerals to long-term anoxic storage following short-term oxidation. Here we used reduced nontronite NAu-2 as a model Fe-bearing clay mineral, exposed its suspension to air for oxidation in a short time (1, 3 and 5 h), and then stored it under anoxic conditions without any exogenous oxidants for a long time (0–120 h). After different time of anoxic storage, we evaluated the reactivity of Fe(II) by air oxidation, measured the Fe(II) fractions by phosphate extraction and characterized the variation of Fe(II) by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Mössbauer. We found that anoxic storage increased prominently the rate of Fe(II) oxidation by O2 and the resultant production of hydroxyl radicals, and the increase was more obvious for the NAu-2 with a longer time of anoxic storage. Both phosphate extraction and XPS results revealed that the reactive Fe(II) associated with edge sites was regenerated and the regeneration increased with the increase in anoxic storage time. FTIR and Mössbauer spectra identified the rearrangements of NAu-2 structure during anoxic storage. The mechanism of reactive Fe(II) regeneration by anoxic storage was attributed to the increase in reactive Fe(II) associated with edge sites, which was most likely induced by the electron transfer from Fe(II) at the interior sites to Fe(III) at the edge sites in the octahedral sheets of nontronite. The driving force for electron transfer was presumed to be the redox potential gradient between edge and interior sites because of different Fe(II)/Fe(III) ratios. Our findings improve the understanding of Fe(II)/Fe(III) interconversion as well as the resultant reducing impact on substance transformation induced by Fe(II)-bearing clay minerals under redox-dynamic systems.

Application of SAFT-VRE in the Flowsheet Simulation of an Advanced PUREX Process

SAFT-VRE is an extension of the statistical associating fluid theory for potentials of variable range (SAFT-VR) that can be used to describe the thermodynamic properties of strong-electrolyte solutions. Here the SAFT-VRE method is used in a flowsheet simulation code to calculate the densities of uranyl nitrate/nitric acid aqueous solutions and the activities of species that are needed for the calculation of the distribution coefficients of nitric acid and uranium in the nitric acid–30% tributyl phosphate (TBP) extraction system that is used in an advanced PUREX process for the reprocessing of spent nuclear fuels. Simulation results of both single-stage extraction experiments and of a multistage flowsheet test showed that the SAFT-VRE method can be used in flowsheet simulations with reasonable accuracy, thus demonstrating for the first time that thermodynamically-based mesoscale SAFT-VRE models can be linked to the macro-scale model of the PUREX solvent extraction process.

Reducing the risks of water and territory pollution by toxic metal-containing effluents of machine-building industry

The article is devoted to the baromembrane method for cleaning washing waters of degreasing sites and for the bright nickel plating of galvanic productions. The authors analyzed specific features of technologies for part metallization in terms of the formation and separation of washing wastewater flows for their separate cleaning. Studies were carried out on the membrane extraction of nickel sulfate and sodium phosphate compounds on two types of membranes: nanofiltration OPMN-P and reverse osmosis Hydranautics ESPA-1. High selectivity (98-99,9%) and specific productivity of OPMN-P membranes (25 l/m2 per hour) were proved during the extraction of nickel sulfate and sodium phosphate at concentration ranging from 50 to 200 mg/l specific to washing water of the processes of degreasing and bright nickel plating. The advantage of using nanofiltration membranes OPMN-P over reverse osmosis Hydranautics ESPA-1 membranes was substantiated. A technological scheme to create a closed water supply system at these sites was developed.

Sensitivity of Three Phosphate Extraction Methods to the Application of Phosphate Species Differing in Immediate Plant Availability

Extractive tests for determining the plant-availability of soil phosphorus (P) give varying results due to the inherently different characteristics of the extraction solution. Generally, classical soil P tests such as the Olsen or calcium acetate/lactate (CAL) method do not give an indication on the total amount of plant available P, but merely give an indication of the equilibrium between soil and extraction solution. It is also not entirely clear which fractions of P are directly determined through the various methods of extraction, i.e., determined P must not be immediately plant available, as is the case for rock phosphate. It is therefore possible that extraction methods either over or under estimate the amount of P available for plant consumption. In this research, we compared three methods of soil P determination (CAL, Olsen and diffusive gradients in thin films (DGT)) with regards to their ability to determine P species (Ca(H2PO4)2, CaHPO4, Ca3(PO4)2 and Inositol-6-hexakisphosphate) added to soils of high sorption capacity, immediately after as well as two weeks after application. For each of the methods, it could be shown that sorption processes in the soil immediately (0 days incubation) fix P to a point where it is not extractable through any of the described methods. These sorption processes continue over time, leading to a further decrease of determined P. The acidic CAL extraction method gives higher results of extractable P compared to the Olsen method. Due to the extraction of Ca3(PO4)2, the CAL method may overestimate immediately plant-available P. The most suitable methods for the determination of immediately plant available P may therefore be the Olsen and DGT methods. Organic IP6 is not determined by any of the extraction methods. At low concentrations of soil P, the DGT method may fail to give results.

Resource sovereignty and accumulation in the blue economy: the case of seabed mining in Namibia

&lt;p&gt;Following its global emergence, the blue economy agenda is now touted as a mechanism through which the Republic of Namibia can achieve long-term sustainable and equitable growth. In (re)defining the ocean, seabed mining has been central to these discussions. Drawing on fieldwork and semi-structured interviews undertaken with key actors in Namibia and South Africa, between 2016 and 2017, as well as recent policy debates and discourse surrounding the potential extraction of marine phosphate in Namibia this article critically examines the framing of the marine environment as an extractive space. The blue economy presents opportunities for new forms of capitalist accumulation and this has resulted in struggles over who can accumulate in the marine sphere. This article therefore analyses the emerging and competing claims to sovereignty over this "new" resource frontier, including by state and non-state actors, and identifies which actors have been included or excluded from the blue economy agenda. In discussing sovereignty over this frontier and resources therein, it undertakes a rigorous analysis of the complications created by the ocean as a three-dimensional, voluminous, "borderless" space.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Key Words: &lt;/strong&gt;Namibia, seabed mining, sovereignty, frontier, blue economy, EEZ&lt;/p&gt;