Field characteristicsThe Duwi formation, which dates from the Campanian–Maastrichtian period27 and is part of the Abu Tartur plateau, contains the unoxidized phosphorites (black to greyish–black in color) used in this current study. Generally, the phosphorite deposits of Abu Tartur plateau are thought to be the highest grade in Egypt and have a reserve of 1 billion metric tonnes including both oxidized and non-oxidized varieties with an average of P2O5 of about 25%12,28. Duwi Formation in the Abu-Tartur area serves as a notable instance of a significant marine transgression occurrence in Egypt29,30. It consists of phosphorite accumulations, black shale and glauconite deposits (Fig. 1)12. The lower section contains phosphorite sand and thin black shale layers31. While the upper part is made up of thick brownish to dark-grey flaky shale layers with occasional presence of mudstone and marl lenses, as well as glauconite deposits from the Campanian–Maastrichtian period (middle layer)32,33. The cap on the plateau surface consists of limestone of the Kurkur formation34.Figure 1A field photograph showing a stratigraphic section of Duwi formation at the western region of the Liffyia–Maghrabi sector in Abu-Tartur mine.Characterization of prepared samplesPetrographical characteristicsTransmitted light microscope equipped with Nikon camera was utilized to examine and capture images of the pristine, non-oxidized phosphorite deposits (RB-Ph) in both optical visions, in PPL and XPL (Fig. 2a–f and Fig. 3a–c). These deposits are composed mainly of phosphatic (mudclasts and bioclasts) and non-phosphatic grains35, as demonstrated by high magnifications. Phosphatic pellets, also known as mudclasts, exhibit a range of shapes and sizes, from angular to sub-rounded. When viewed in PPL (Fig. 2a, d and Fig. 3a, c), they are typically homogeneous and lack any discernible structure, although occasional ooids may be present (Fig. 2c, d). These ooids form through the growth and crystallization of micrometric phosphate layers32,36,37. Additionally, the phosphatic grains’ colour varies from dark to light brown, which is probably related to the variations in the content of organic matter and/or iron oxide. But sometimes they appear isotropic between Crossed Nicols (Fig. 2b, c, f), aligning with previous investigations29. In contrast, fish bones and shark teeth (bioclasts) are less common than phosphatic mudclasts in the investigated samples and are naturally colourless, elongated, and angular to sub-angular in shape (Fig. 2a, b and Fig. 4a, b). Furthermore, they exhibit low birefringence and display grey interference colours of the first order, as well as lamellar twinning or undulatory extinction as was described before by several studies38,39. The non-phosphatic component is distinguished by the presence of undesirable impurities such as abundant carbonate minerals, dolomite (Fig. 2a, b), and detrital quartz (Fig. 3a, b). Also, pyrite (Fig. 2d–f) was frequently observed, along with glauconite (Fig. 2c, d), matching with previous investigations28,40,41.Figure 2Petrographical images of black phosphate (RB-Ph) in PPL (a,d,e) and XPL (b,c,f) visions: peloid grains (PL), dolomite rhombs (D), ooids (O), fish bones (B), pyrite crystals (Py) and glauconite (G).Figure 3Petrographic images of black phosphate (RB-Ph) in PPL (a,c) and XPL (b) visions: peloid grains (PL), fish bones (B), glauconite (G) and quartz (Q).Figure 4XRD patterns of the precursor black phosphate (RB-Ph) in comparison with its modified derivatives, calcination at 550 ℃/4 h (CB-Ph), 30% hydrogen peroxide (HB-Ph) and intensive grinding to nano-sizes (NB-Ph).Geochemical characteristicsThe chemical composition of the studied samples (RB-Ph, CB-Ph, HB-Ph, and NB-Ph) is depicted in Table 2. It was revealed that P2O5, CaO, SiO2, SO3 and Fe2O3 are the main components of these samples. According to the P2O5 (30.5 wt.%) content, the pristine un-oxidized phosphorite deposits (RB-Ph) can be classified as high-grade deposits42,43. However, it was noted that many of the major oxides in the treated samples (CB-Ph, HB-Ph, and NB-Ph) demonstrate distinct variations, indicating the conspicuous impact of these processes upon their chemical composition with respect to the raw one. The high reduction of CaO, P2O5, Fe2O3, F and SO3 contents of HB-Ph into 29.74, 21.6, 2.96, 1.31 and 2.27 wt.%, respectively, confirms the destructive impact of this acidic reagent not only upon the organic matter (i.e., reduction of organic sulfur that was expressed in the abatement of SO3 wt.%) of the pristine RB-Ph sample, but also upon its phosphatic (CaO, P2O5, and F wt.% demolition) and pyrite minerals (Fe2O3 wt.% drop)44. Conversely, the noticeable enrichment of SiO2, Al2O3, MgO, MnO, K2O and LOI after H2O2 treatment (15.90, 3.62, 1.96, 0.40, 0.15 and 19.37 wt.%) compared to the pristine sample could be ascribed to the concentration of clays, quartz, and dolomite in HB-Ph sample (Table 2). Similarly, the impact of intensive grinding to nano-size (NB-Ph) followed the same trend of reduction and enrichment in the chemical composition like the H2O2 treated sample (HB-Ph), but with little magnitude, if compared with the pristine one (Table 2). This was assured by the reduction of CaO, P2O5, Fe2O3, F and SO3 contents (34.92, 22.34, 3.36, 1.12 and 7.54 wt.%, respectively) and the enrichment of SiO2, Al2O3, MgO, MnO, K2O and LOI contents (9.1, 1.76, 1.92, 0.33, 0.18 and 16.47 wt.%, orderly) after grinding compared to RB-Ph sample (Table 2). The intensive leaching ability of the H2O2 that contributed to an increase in the solubility of CaO, P2O5, Fe2O3, F and SO3 contents in HB-Ph sample explains the invasive impact of such oxidizing agent upon the whole chemical composition of the HB-Ph sample compared to the grinding process at the NB-Ph one (Table 2)45. With respect to the calcination treatment process, approximately a slight variation in the chemical composition of the CB-Ph sample was observed in correlation with RB-Ph (Table 2). However, the adequate improvement of the SO3 content (13.74 wt.%) in the CB-Ph compared to RB-Ph (11.88 wt.%), can be ascribed to the partial oxidization of pyrite (i.e., the oxidation of sulfide into sulphate), compensating the loss of organic sulfur during the oxidation of organic matter by calcination46. On the contrary, the reduction in LO I from 9.23 wt.% in the RB-Ph to 7.75 wt.% in CB-Ph, can be ascribed to the oxidation of the organic matter and clay mineral dehydration of the pristine sample by calcination, aligning with the previously discussed petrographical investigations.
Table 2 Chemical composition of the precursor black phosphate (RB-Ph) in comparison with its modified derivatives, CB-Ph (calcinated at 550 ℃/4 h), HB-Ph (30% hydrogen peroxide treated) and NB-Ph (intensively ground to nano-sizes).Mineralogical characteristicsThe XRD patterns of the investigated phosphorites, RB-Ph, CB-Ph, HB-Ph, and NB-Ph, are depicted in Fig. 4. Concerning the XRD pattern of the pristine sample, RB-Ph, the phosphatic phase is francolite. Whereas the other associating non-phosphatic components include dolomite, pyrite, illite and quartz that were expressed by peaks of various intensities. Also, the associating organic matter were reflected by amorphous noise in the XRD pattern47. Francolite, dolomite and pyrite were reflected by intensive peaks at 2θ = 31.9°, 30.7°, and 33° with d-spacings of 2.8, 2.7, and 2.9°A, orderly. Meanwhile, the minor peaks with less intensities at 2θ = 25.7°, 26.7°, and 56.1° could be attributed to illite, detrital quartz and pyrite minerals, respectively48,49. After calcination at 550 ℃/4 h, the main peak of francolite was slightly reduced in intensity and shifted from 2θ = 31.9° to 32° in CB-Ph sample. As well, the other non-phosphatic phases, witnessed an observable reduction in their intensities (illite & pyrite), except for dolomite peak at 2θ ≈ 44.9° which witnessed an appreciable improvement in intensity with calcination. But with H2O2 treatment (HB-Ph), the phosphatic components displayed a noticeable reduction in their intensities, giving the space for dolomite to occupy the main peaks (Fig. 4). This is matching with XRF data that confirmed a remarkable increase in MgO wt.% gained from the dominating dolomite50. While pyrite continued demolition as a consequence of oxidation by H2O2. Also, the perceptible presence of quartz 2θ = 26.7° and 50.3° confirms that H2O2 does not affect quartz51. Therefore, the domination of non-phosphatic minerals at the expense of phosphatic ones, confirms XRF results concerning the observable reduction in P2O5 content of HB-Ph. Additionally, the intensive grinding to nano size (NB-Ph), contributed to an observable destruction, not completely, of both phosphatic and non-phosphatic phases49 (Fig. 4). This was assured by the domination of amorphous noise, except for some quartz and dolomite relics.Spectral characteristicsThe spectra of the main functional groups of the investigated phosphorites (RB-Ph, CB-Ph, HB-Ph and NB-Ph) are displayed in Fig. 5. The main functional groups of the pristine phosphorite sample (RB-Ph) emerged at the following absorption bands: 3439, 1631, 1454.3, 1428, 1043.4, 604.1, 568.8, 467.1 cm−1. Some of these bands reflect the overlapped signatures of both organic and inorganic components of the RB-Ph sample: 3439, 1631 and 604.1 cm−1, while the other bands can be correlated to the inorganic components only. The 3439 cm−1 band can be linked with the stretching vibrations of either hydroxyl (O–H) or N–H groups52,53,54,55,56. The latter group expresses the signature of the organic matter of the pristine sample. Also, the weak band at 1631 cm−1, not only was attributed to the bending mode of water “δ (H2O)”57,58, but also falls in the range of C = C (alkene) stretching vibrations59,60,61,62. The latter is typically associated with the presence of conjugated double bonds of the aromatic compounds or unsaturated moieties of the associating organic matter59,63. Similarly, it is possible to consider multiple options for the assignment at 1454.3 and 1428 cm−1 bands64. Where, both the anti-symmetrical bending vibration (ν3) of the carbonate (CO3)2− group and C-H bending vibrations of the aliphatic hydrocarbon chains as those found in alkanes, could contribute to these absorption bands in the infrared spectrum65,66,67,68,69. Furthermore, the 604.1 cm−1 band, which was attributed to the bending or rocking vibrations of the (PO4)3− group of the inorganic phosphates, may overlap with other vibrations from organic phosphate66,70. On the contrary, the 568.8 and 467.1 cm−1 bands are typically associated with the bending or rocking vibrations of the (PO4)3− group71,72, which is commonly found in inorganic phosphates rather than organic phosphates. Regarding the absorption band at 1043.4 cm−1, in the RB-Ph spectra, it could be correlated with the stretching vibrations of Si–O bond of the inorganic components only (i.e., silica, quartz or silicate minerals, illite), matching with XRD and petrographical data73. With calcination at 550 ℃ /4 h, it was observed that the spectra of the CB-Ph sample witnessed the emergence of a new absorption band at 2345 cm−1 that was correlated to the asymmetric stretching vibration of the carbon–oxygen double bond in CO274,75,76,77,78. The presence of trapped carbon dioxide within the sample pores could be ascribed to the thermal decomposition/oxidation of carbonaceous materials in OM of RB-ph during the calcination process, releasing some CO2. Also, the emergence of a new band at 678 cm−1 after calcination could be ascribed to the stretching mode of C–Cl group of the chlorinated organic residues. This was assured by the high Cl ratio (0.64 wt.%) that amplified about 16-fold compared to the pristine sample, 0.04 wt.% (Table 2). The calcination process likely led to the thermal decomposition or oxidation of carbonaceous materials in OM, resulting in the release and subsequent trapping of chlorinated compounds within the sample. Similarly, the amplification of the (PO4)3− and Si–O bands at 604, 568 and 1043 cm−1, orderly, is likely associated with the decomposition/oxidation of the associating OM in the pristine sample with calcination. It also indicates the enhanced presence of these functional groups, aligning well with XRD and XRF data (Fig. 4 & Table 2). Similarly, the chemical treatment of RB-Ph with H2O2 (30%), contributed to an improvement in the intensity of the (CO3)2− group at 1429.7 cm−1 in accordance with the oxidation of OM and some of the phosphatic components. This drove toward the domination of dolomite as displayed in XRD and XRF (LOI = 19.37 and MgO = 1.96 wt. %) (Fig. 4 & Table 2, orderly). This could be assigned to the specific reactivity of dolomite towards H2O2, which allowed it to withstand the treatment while other minerals or OM were affected. Additionally, the approximate strengthening of the (PO4)3− and Si–O bands at 603, 567, 471 and 1041.8 cm−1, orderly, in HB-Ph sample, could be confined with the OM/some phosphatic components oxidation via H2O2 as a sign of concentration of both phosphate and silicate species, aligning well with XRD data. The presence of significant amounts of SiO2 (15.9 wt.%) and Al2O3 (3.62 wt.%), further confirms the increased concentration of silicate species in the HB-Ph sample (Table 2). This was also assured by the noticeable presence of Si–O–Si group in bending mode at 799.5 cm−1 of the silicate components79. On the contrary, the distinguishing functional groups of the intensively grounded NB-Ph sample were reduced in intensities compared to other treated samples. Such reduction can be correlated with the induced changes in the crystalline structure and molecular arrangement of the inorganic/organic components of the sample, leading to alterations in the vibrational modes and intensities of the functional groups observed in spectroscopic analyses, in consistent with XRD data (Fig. 4). However, some NB-Ph functional groups was not only reduced in intensity, but also shifted in frequency from ≈ 1429 to 1462 cm−1, but still ascribed to carbonate group.Figure 5FT-IR spectra of the precursor black phosphate (RB-Ph) in comparison with its modified derivatives, calcination at 550 ℃/4 h (CB-Ph), 30% hydrogen peroxide (HB-Ph) and intensive grinding to nano-sizes (NB-Ph).Microstructural characteristicsThe morphology and elemental analysis of the studied phosphorite samples (RB-Ph, CB-Ph, HB-Ph, and NB-Ph) were investigated by SEM and EDX, respectively (Fig. 6a–c and Fig. 7 a–d). The SEM micrographs revealed the inhomogeneity of the RB-Ph sample on the level of grain sizes and shapes (Fig. 6a, b). Whereas the EDX analysis confirmed the high content of C and S, as a sign upon the high OM in the RB-Ph sample (Fig. 6a). But, with calcination at 550 ℃/4 h (CB-Ph), the homogeneity in grain sizes and shapes was approximately improved with slight increase in grain sizes due to the experienced recrystallization process60,80 (Fig. 6c). Such homogeneity was interrupted with the occasional presence of some phyllosilicate grains of illite with marked flaky nature40 (Fig. 6c). Also, the calcination process contributed to an observable reduction in C and S contents in accordance with the oxidation of the associating OM of the pristine sample during the calcination process47. On the contrary, the chemical treatment by H2O2 (30%), resulted in inhomogeneous morphology in HB-Ph compared to the precursor one (Fig. 7a, b). This inhomogeneous nature could be correlated with the prevalence of curly flakes of the phyllosilicate illite to the limit of rosette morphology formation81 (Fig. 7a, b). Additionally, the EDX analysis demonstrated a noticeable decrease in C and S in accordance with the decomposition/oxidation of the OM by H2O2 (Fig. 7a). Also, the remarkable presence of Mg in the EDX analysis reflects the survived dolomite after the experienced chemical treatment by H2O2, matching with XRD data (Fig. 4). Furthermore, the impact of intensive grinding on the pristine phosphorite was demonstrated by spherical to semi-spherical grains of phosphatic and non-phosphatic components that vary in grain sizes from 200 to 10 nm (Fig. 7c, d). Some of these fine components may agglomerate together to produce larger particles in correlation with the hydrous nature of phosphatic and some of the non-phosphatic components as was assured by the high LOI content (16.47 wt.%) in the XRF data (Table 2). Also, the disappearance of flaky gains of the clay mineral (illite) affirms the complete structure destruction of these grains with intensive grinding. Moreover, the EDX analysis also illustrates a decline of both C and S contents in NB-Ph compared to the pristine RB-Ph. This could be correlated to the decomposition of the associating OM with grinding to nano-sizes (Fig. 7c). However, the presence of both Mg and Si confirms the presence of dolomite and silicates in NB-Ph sample, matching with XRD and XRF data (Fig. 4 & Table 2).Figure 6SEM images and EDX of the precursor black phosphate (RB-Ph) (a,b) and its modified derivative, calcination at 550 ℃/4 h (CB-Ph), (c).Figure 7SEM images and EDX of HB-Ph (30% hydrogen peroxide) (a,b) and NB-Ph (intensive grinding to nano-sizes) samples (c,d).Organic and inorganic componentsThe total organic carbon (TOC), total inorganic carbon (TIC) and total sulfur (TS) of the pristine phosphorite sample (RB-Ph) before and after calcination (CB-Ph), H2O2 (30%) treatment (CB-Ph) and intensive ball-mill grinding (NB-Ph) were investigated and complied in Table 3. It was revealed that the calcination of the raw sample at 550 ℃/4 h resulted in a remarkable reduction of the TOC and TIC from 0.543 to 0.097% and 1.627 to 1.052%, orderly. This reduction was correlated with the decomposition of the associated OM47 and some of the carbonate minerals (dolomite), respectively. This decomposition results in the release of volatile gases such as carbon dioxide (CO2) and water vapor from these organic and inorganic components. Whereas the slight reduction of the TS by calcination from 5.286% in the RB-Ph to 4.768% in CB-Ph sample (Table 3), could be correlated with the decomposition/oxidation of pyrite82, matching with XRD results (Fig. 4). It’s important to note that the observed reduction in TS may not solely be attributed to pyrite oxidation, but, also due to the decomposition of organic compounds containing sulfur, such as sulfides or organic sulfates, and their conversion into gaseous sulfur compounds (e.g., sulfur dioxide, SO2), which escape from the CB-Ph matrix83. Also, the depletion of the TOC (0.209%) in the HB-Ph sample compared to RB-Ph, could be correlated with the decomposition of the associated OM with H2O2 (30%) treatment, releasing carbon dioxide (CO2) and water vapor84,85. Conversely, the TIC (5.065%) in the HB-Ph sample displayed a noticeable increase compared to RB-Ph (Table 3). This could be assigned to the prevalence of dolomite that survived from the H2O2 attacks due to its high chemical resistance to chemical alteration, in agreement with XRD data. Such dolomite, the in organic carbon source, prevalence in concentration was also enforced by the combined effects of OM oxidation and carbon dioxide release. Also, the intensive abatement of TS (0.643%) with H2O2 treatment probably was correlated not only with pyrite decomposition, but also due to the oxidation of organic compounds containing sulfur by such strong oxidizing agent, releasing SO2 from the HB-Ph86. Finally, with intensive grinding of RB-Ph to nano-sizes (NB-Ph sample), both TOC (0.410%) and TS (2.669%) were slightly and moderately reduced, orderly, in comparison with RB-Ph (Table 3). The breakdown or fragmentation of the incorporated OM during the experienced mechanical grinding and its accompanying heat of friction, resulted in an increase of surface exposure to air. This triggered the oxidation or decomposition of these matters and the evaporation of their volatile components, leading to such reduction in TOC value in the NB-Ph. Similarly, the moderate reduction of TS could be correlated with the destruction of pyrite structure and OM oxidation/decomposition during the milling process, aligning with XRD data. These collaborative processes facilitated the release of sulfur in form of SO2 and hence the reduction of TS with grinding in NB-Ph. On the contrary, the TIC content (2.936%) was surprisingly enhanced with grinding compared to the pristine sample (Table 3). During the grinding process, the mechanical forces that caused the breakdown/decomposition of organic matter and dolomite of the pristine sample lead to the release of both carbon dioxide (CO2) and water vapor (H2O) into the semi-closed grinding system that was marked with high vapor pressure. These released gases probably reacted with each other, leading to the formation of carbonic acid (H2CO3), which can further dissociate into bicarbonate ions (HCO3−). These bicarbonates probably combined with other cations released from the decomposition of other minerals such as potassium (K+) from illite, to form amorphous bicarbonate salts that lack the long-range order required to be detected in the XRD pattern. The presence of these bicarbonate salts was confirmed in both FT-IR spectra (carbonate group at 1462 cm−1) and LOI (16.46 wt.%) of the XRF data.
Table 3 Total carbon (organic & inorganic) and total sulfur of the precursor black phosphate (RB-Ph) in comparison with its modified derivatives, CB-Ph (calcinated at 550 ℃/4 h), HB-Ph (30% hydrogen peroxide treated) and NB-Ph (intensively ground to nano-sizes).Geometrical characteristicsThe N2 adsorption/desorption isotherms of RB-Ph and its modified derivatives (CB-Ph, HB-Ph, and NB-Ph) can be classified as Type III isotherms (Fig. 8)87, with typical H3 hysteresis loops due to capillary condensation on the surface of mesoporous (2–50 nm)88,89. These isotherms indicate monolayer followed by multi-layer adsorption; the approximately flat regions at low and medium P/P0 in all isotherms is correlated with N2 monolayer adsorption90,91. Whereas at high P/P0, the N2 adsorption was converted into multi-layer by the investigated samples. The progressive tightening of the hysteresis loops of the modified derivatives with countable preference with NB-Ph, demonstrates the supremacy of the mesopores. However, the incapacity to attain the equilibrium state of N2 adsorption by any of the addressed samples, infers the broad variability in their pore diameters. Aligning with N2 isotherm outcomes, the geometrical parameters of the studied samples (Table 4) displayed that the intensive grinding and calcination processes resulted in a deep reduction of SBET and total pore volume (Vt) of the NB-Ph (7.60 m2/g and 0.03 cm3/g) and CB-Ph (12.40 m2/g and 0.07 cm3/g) samples. In opposition, the H2O2 treatment resulted in a minor reduction in SBET (24.30 m2/g) of HB-Ph compared to RB-Ph (26.20 m2/g). Regarding NB-Ph, the deviation from the fact stating that “the lower the particle sizes, the greater the surface area” might be ascribed to the higher rate of agglomeration with intensive grinding to nano-size as a consequence of the high LOI content of this sample that exceeded 16 wt.%89,92,93,94. Similarly, with calcination at 550 ℃/4 h, the decomposition of OM by oxidation and the escaping of H2O vapor, CO2 and other volatiles probably led to pores collapse and hence the noticeable reduction in both SBET and Vt of CB-Ph compared to RB-Ph95. Additionally, the increase in grain sizes that accompanied the experienced recrystallization process with calcination could also justify such SBET and Vt reduction, aligning with the SEM results (Fig. 6c). Conversely, the decomposition of associating OM with H2O2 contributed to the evacuation of the blocked and semi-blocked pores with these matters by oxidation that approximately preserved the SBET (24.3 m2/g) close to the pristine sample. However, the collapse of the walls between neighboring mesopores pores resulted in the retrogradation of the overall pore volume of HB-Ph via their partial re-blocking with prevailing dolomite rhombs in agreement with XRD data. As well, the penetrative ability of H2O2 as a powerful activating agent to produce new micro-pores, especially upon the surface of the flaky particles, side by side with the original meso-pores, could rationalize the recorded average pore diameter, Dp (11.03 nm) that represent the lowest value among all the investigated samples (Table 4).Figure 8N2 adsorption–desorption isotherms of the precursor black phosphate (RB-Ph) in comparison with those of its modified derivatives, calcination at 550 ℃/4 h (CB-Ph), 30% hydrogen peroxide (HB-Ph) and intensive grinding to nano-sizes (NB-Ph).Table 4 Textural parameters of the precursor black phosphate (RB-Ph) sample obtained from N2 adsorption–desorption isotherms in comparison with its modified derivatives, CB-Ph (calcinated at 550 ℃/4 h), HB-Ph (30% hydrogen peroxide treated) and NB-Ph (intensively ground to nano-sizes).Solubility studiesImpact of acetic acid concentrationThe effect of acetic acid (AA) concentration upon P dissolution from the prepared phosphorite samples, PS (RB-Ph, CB-Ph, HB-Ph and NB-Ph) at various liquid/solid ratios (AA/PS, w/w) ranging from 0.5:1 to 4:1, was investigated. These investigations were conducted at a fixed PS dose of 1.5 g, shaking speed of 200 rpm/2 h and 25 ml of DW at room temperature (Fig. 9a & Table 1). It was demonstrated that P dissolution increases as the acid concentration rises to a particular level; the dissolution rate was decelerated beyond 2:1 ratio for all addressed samples, i.e., the dissolution rate became insignificant (Fig. 9a). This observation aligns with the findings of other reported data that revealed that the P dissolution rates increase as pH decreases, i.e., increasing the acidity of the medium96,97. In general, the proton (H+) and ligand (carboxylic group) of organic acid-promote the dissolution processes of P. But in monodentate coordinated organic ligands (such as acetic acid), the carboxylic group effect on the P dissolution is minimal to nonexistent98. The protonation of P–O group regulates such dissolution process99,100. With the inspection of Fig. 9a, it was revealed that the P dissolution rate from the RB-Ph sample was the highest compared to the other treated samples with calcination and H2O2. Conversely, the particles diameter (i.e., nano-size) had a conspicuous role in the dissolution process of P as the highest values were recorded in NB-Ph sample, aligning with other reported data101.Figure 9Effects of acetic acid (AA) concentration (a) and retention time (b) on phosphorus (P) dissolution of the precursor black phosphate (RB-Ph) in comparison with its modified derivatives, calcination at 550 ℃/4 h (CB-Ph), 30% hydrogen peroxide (HB-Ph) and intensive grinding to nano-sizes (NB-Ph).Although the remarkable P release pattern from thermally treated phosphates that was observed and discussed in several previous investigations102, the environmental trade-offs of this high-energy process that provide a critical perspective on its practical application were less emphasized in these studies. Investigations of these drawbacks, such as high energy requirements and emissions, although they were not within the scope of the current study but are important considerations for practical application of this processing technique. Similarly, the enhanced P release via chemical treatment with H2O2as a viable method for increasing phosphorus availability, was also reported by several studies103. However, the environmental considerations of residual H2O2 in soil and its impact on soil microbiota, are areas that were less covered before, and warrant further research to develop environmentally safe protocols. On the other hand, aligning with the current study, mechanical grinding investigations reported improved nutrient release from finely ground mineral phosphates104. But the current study further revealed that this method also mitigates the negative environmental impacts associated with excessive use of synthetic fertilizers, a consideration not fully explored in previous works. However, grinding can sometimes lead to issues such as dust generation and increased energy consumption.Unlike several reported data105,106, it was revealed that OM had a deep and conspicuous impact on the solubility of phosphorus (P); more labile or easily decomposable OM may have transient effects on P solubility. This probably was accomplished through mechanisms such as complexation or the release of organic acids that accompanied OM decomposition by the applied acetic acid, increasing the availability of P for dissolution. Similarly, the OM decomposition most likely allows the release of the adsorbed P by the binding sites on the surface of OM to the solution107. On the contrary, the severe reduction of OM, either by calcination or H2O2 treatments, i.e., the driving factor that was enhancing P solubility was approximately eliminated, led to a decrease in P solubility. However, the drop in P2O5 wt.% content in the HB-Ph (21.06 wt.%) after H2O2 treatment cannot be neglected as another intervening factor for the reduction of soluble P during the AA leaching process (Fig. 9a). Conversely, the slight reduction in TOC of the NB-Ph sample (0.410%) coupled with the nano- particle size can collectively justify the remarkable dissolution rate of phosphorous that exceeded 700 ppm for all applied acid ratios although the previously mentioned slight reduction in the P2O5 wt.% content (22.34 wt.%) with intensive milling process (Table 2). Also, the amorphous nature of the phosphatic-components that lack a well-defined crystalline structure and often more reactive than crystalline counterparts, aligning with XRD data, explain the displayed P dissolution rate of NB-Ph through a greater exposure to the acid solution. This facilitated faster dissolution and higher phosphorus concentrations in the solution. In other words, the lack of a well-defined crystalline structure also means that there are fewer constraints on the dissolution process, allowing easier access to the phosphorus components and hence their subsequent dissolution108,109.Considering the obtained adequate results and for economic/environmental reasons, the 2:1 ratio was nominated for the conduction of the subsequent experiments.Impact of applied retention timeThe impact of different retention times (0.5, 1, 2, 4 and 6 h) on P dissolution from the investigated PS (RB-Ph, CB-Ph, HB-Ph and NB-Ph), using a fixed concentration of acetic acid, AA was invasively investigated (Fig. 9b). These leaching experiments were conducted at AA/PS ratio of 2:1, w/w, shaking speed of 200 rpm, and 25 ml DW at room temperature (Table 1). It was displayed that raising the contact time from 0.5 to 2 h was accompanied by an observable increase in the dissolution rate of P for all the investigated samples (Fig. 9b). However, beyond 2 h of retention time, the P dissolution rate was insignificant, signifying that equilibrium state was attained. This was attributed to the consumption of hydrogen ions in solution with time110,111,112,113. Furthermore, the P dissolution rate followed the following trend for the studied samples over the investigated range of contact time: NB-Ph (730–980 ppm) > RB-Ph (470–710 ppm) > CB-Ph (410–650 ppm) > HB-Ph (130–580 ppm). This remarkable dissolution rate of NB-Ph then RB-Ph in the second rank, reflects the very important role played not only by the particle size of the PS/amorphous nature but also their OM contents, with some preference of the formers. Moreover, the drop in both P2O5 wt.% content (21.06 wt.%) and TOC (0.209%), as well as the crystalline nature of the HB-Ph with H2O2 treatment, can be accounted as intervening factors for the reduction of soluble P during the AA leaching process.Regeneration studiesIn order to evaluate the potential amount of dissolved P through the leaching processes out of RB-Ph, CB-Ph, HB-Ph, and NB-Ph, separately, several leaching cycles were conducted, using a fixed concentration of AA that achieves a ratio of AA/PS, 2:1 w/w as depicted in Fig. 10, Table 1. Unlike NB-Ph, the dissolution rate of P at the first leaching cycle of RB-Ph, CB-Ph and HB-Ph samples in comparison with the results of the equivalent leaching experiments of the previously investigated parameters (acid concentration and retention time), was obviously reduced (Fig. 10). This could be attributed to the decline of the solid–liquid interface in accordance with agglomeration/stacking of these PS particles within the AA solution, preventing the hydrogen ions of the acetic acid from having the opportunity to contact the surfaces of the unreacted particles114. The immunity of the NB-Ph against P dissolution declines at the 1st leaching cycle could be justified by the high diffusion of its particles within the AA solution. This led to an increase in the solid–liquid interface and hence the P dissolution rate was improved compared to the obtained results of the previously mentioned equivalent experiments (Fig. 10). Moreover, a gradual decrease in P dissolution for all the addressed PS was observed with each applied leaching cycle (Fig. 10). However, the dissolution persistency till the 5th leaching cycle with a preference of NB-Ph sample over the others, confirms the continuity of P solubility in accordance with particle size reduction during the successive leaching cycles115,116.Figure 10Regeneration studies elaborate the magnitude of phosphorus (P) dissolution rate along five leaching cycles of the precursor black phosphate (RB-Ph) in comparison with its modified derivatives, calcination at 550 ℃/4 h (CB-Ph), 30% hydrogen peroxide (HB-Ph) and intensive grinding to nano-sizes (NB-Ph) samples.
Comprehensive insights into phosphorus solubility and organic matter’s impact on black phosphate leaching
