Investigation on partial acidulation of rock phosphate using acetic acid and acidic calcium phosphate solutions

This study explores an innovative method for the partial conversion of sparingly soluble rock phosphate (RP) into a biphasic, highly soluble calcium phosphate mixed with crystalline apatite. The RP was reacted with acetic acid and monocalcium phosphate (MCP) (a commercial triple super phosphate) solutions. The formation of low-order crystalline phosphate phases and associated chemical environments were analyzed using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The Rietveld refinement of XRD data obtained from progressive acidulation of RP at different time intervals showed the formation of anhydrous forms of calcium-deficient hydroxyapatite (CDHA), dicalcium phosphate (DCPA), monocalcium phosphate (MCPA), and octacalcium phosphate (OCP), with these more soluble mineral phases constituting up to 27%. FTIR analysis confirmed the formation of protonated phosphate (HPO 4 2-) enriched calcium phosphate, the presence of which was indirectly corroborated by the increase in solution pH and the formation of β-tricalcium phosphate upon calcination. SEM images showed that both acetic acid and MCP treatments led to comparable morphological changes, including irregular surface features, fuzzy boundaries, and localized agglomeration likely from new, more soluble mineral phases. Collectively, our results suggest the formation of protonated calcium phosphate and more soluble minerals during acidification. These phases exhibit high solubility, thereby effectively transforming the original RP into biphasic form containing highly soluble and sparingly soluble original minerals. These findings highlight the potential to optimize acidulation of RP to achieve the desired solubility for target applications.

Phosphorus (P) is an essential macronutrient for crop plants, yet its availability in global agriculture depends largely on the mining and processing of natural rock phosphate (RP) (Steiner et al., 2015). Economic RP deposits are primarily composed of apatite-group minerals, with fluorapatite being the most common. These minerals exhibit a hexagonal crystal structure with PO 4 3− tetrahedra coordinated to Ca 2+ ions in two different orientations (Broderick et al., 2021). A series of anions can partially substitute PO 4 3−, including F −, OH −, and CO 3 2−. Natural RP minerals have very low solubility in neutral to alkaline soils and are thus ineffective to supply P for plants except in more acidic environments.

The mined RP is commonly acidulated on an industrial scale using concentrated sulfuric acid to enhance its solubility. This process converts fluorapatite mineral in RP into phosphoric acid (H 3 PO 4), which is then modified into monoammonium phosphate (MAP) and diammonium phosphate (DAP) for commercial usage. The MAP and DAP salts have very high solubility (>350 g/L) and thus serve as the primary form of P fertilizers in modern agriculture (Rajan et al., 1996). Even though the acidulation of rock phosphate is a more than a century-old practice—originally patented by Sir John Bennet Lawes in 1843— consistently used globally, it does present several drawbacks due to the use of strong acid in production and the rapid release of P in soils. For instance, localized acidification around fertilizer grains/particles affects seed growth and development of plant roots, and can disrupt microbial communities. The higher solubility of these synthetic fertilizers results in premature dissolution and potential loss from soils, which could contaminate water bodies. In light of these concerns, along with the finite nature of phosphate mineral resources, there has been a growing interest in exploring alternative approaches for developing response-release P fertilizers (Ivanchenko et al., 2025; Sakhno et al., 2022a) or improving the reactivity of RP (Sagoe et al., 1997) to minimize the downstream negative consequences, as mentioned above.

Strategies to improve the solubility of RP have been explored, utilizing mechanical, chemical, and physicochemical methods of treatment designed to destabilize the apatite lattice. For example, mechanical activation through high-energy grinding breaks down particles, disrupts long-range crystalline order, and increases the surface area available for dissolution. This treatment is also known to induce partial amorphization of the apatite mineral, thereby increasing the solubility (Lim et al., 2003; Tõnsuaadu et al., 2011). Moreover, alternative methodologies, such as controlled precipitation of nano-structured calcium phosphates (Bekah et al., 2025; Tosun et al., 2021) have been investigated for their ability to improve the extent and kinetics of dissolution by altering Ca/P ratio, cation and anion chemistry, hydration state, and structural defects (Sakhno et al., 2022a; Sakhno et al., 2022b). This is because phosphate compounds formed with group 1 elements (e.g., H +, Na +, and K +) and NH 4 + have high solubility. While these studies provide valuable insights into the mechanism of dissolution, their direct application to RP materials remains limited due to the variations in mineralogy and surface chemistry.

Most recently, the formation of amorphous forms of phosphate phases, particularly amorphous calcium phosphate (ACP), has attracted interest as a potential slow-release fertilizer (Ghribi et al., 2025; Ivanchenko et al., 2025; Sakhno et al., 2025). ACP is characterized by short-range atomic order, high hydration, and a Ca/P ratio lower than that of stoichiometric apatite (Dorozhkin, 2010; Dorozhkin, 2021). These structural features give ACP significantly higher solubility, but this could contribute to greater losses from soils (Sakhno et al., 2025). An alternative approach involves the formation of an amorphous or poorly crystalline Ca-P on the surface of RP grains. Such Ca-P layer accelerates P release under both chemical and microbially catalyzed environments (Zhang et al., 2021). Empirical studies have demonstrated that RP, when subjected to storage or maturation in aqueous media, can develop hydrated amorphous layers, which are evidenced from vibrational features in the ν 4 PO 4 and ν 2 CO 3 regions in FTIR spectra (Joksa et al., 2023; Rey et al., 2007). These amorphous surface films provide a more readily available P pool compared to the underlying crystalline RP lattice.

The dissolution kinetics of ACP vary with pH, ionic composition, temperature, and the presence of structural protons (Sakhno et al., 2023). However, the majority of existing research predominantly investigates RP dissolution under conditions of strong inorganic or organic acids, while the impacts of mild acidification remain inadequately explored. The latter is more important because RP undergoes partial structural modification, including carbonate removal and the formation of the ACP phase on the surfaces. Elucidating these preliminary transformations is crucial for a comprehensive understanding of how acidic environments can influence RP reactivity. Further, some organic acids, such as citric, oxalic, and malic acid, can promote selective Ca 2+ complexation, resulting in partial destabilization of the apatite surface (Wei et al., 2011). These ligands disrupt local Ca-P phases, protonate PO 4 3- groups, and generate HPO 4 2− rich environments with disordered surface domains that can reorganize into ACP phases, especially on low-crystallinity apatite (Wang et al., 2016). This finding is consistent with previous studies, indicating that acid-driven dissolution of apatite typically results in calcium-depleted, structurally disordered surface layers (Dorozhkin, 2012).

The objective of this study is to compare the effects of weak acids on modifying the structure of RP and facilitating the formation of ACPs. The partial amorphization of a Moroccan rock phosphate was achieved through two mild acid treatments: one utilizing a weak organic acid (acetic acid) and the other employing an inorganic acidic solution derived from monocalcium phosphate (MCP) solution. Structural evolution was tracked primarily through PXRD, with a focus on changes in crystallinity, phase composition, and peak broadening associated with structural disorders. Fourier-transform infrared (FTIR) spectroscopy was subsequently employed to confirm phosphate phase changes. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) provided complementary insights into surface morphology and elemental distribution.

Material: rock phosphate

The research presented in this communication used an untreated rock phosphate (RP). This crystalline RP is composed primarily of apatite with a non-stoichiometric general formula of Ca 10 −xM x (PO 4) 6 −x(CO 3)y(OH) 2 −y (where 0 ≤ x ≤ 1). While other naturally occurring minerals associated with RP are present in very low amounts, quartz is the most prevalent mineral. The received RP was first finely ground into a powder, using a laboratory-scale planetary ball mill under dry milling conditions at a rotational speed of 300 rpm for 1 h, with a ball-to-powder mass ratio of 10:1. The powder was then formed into granules using the granulometry method, achieving the target size of ≤250 µm by sieving. All treatment and analyses are conducated using these granules.

Acidulation of rock phosphate

Experimental setup for acidulation

Two types of partial acidulation media were used to acidify the natural RP: acetic acid and monocalcium phosphate (MCP) [commercially known as triple super-phosphate (TSP)] solutions. The 0.1 M acetic acid solution was prepared by dissolving 4.791 g of sodium acetate (ACS grade reagent, Fisher Scientific) and 2.498 g of glacial acetic acid (99.99% purity) in 800 mL of distilled water. Then, the pH of the solution was adjusted to 4.8 by adding 1 or 0.1 M NaOH or HCl, and the total volume was adjusted to 1 L. The monocalcium phosphate (MCP) solution was prepared by dissolving 2 g of commercial triple super-phosphate (TSP) in 100 mL of distilled water. Since MCP solutions are distinctly acidic, with a pH range of 2.4–3.5 due to proton release during hydrolysis, the final pH was raised to 4.8 using 1 or 0.1 M NaOH solution.

For both partial acidulation experiments, a constant ratio of PR to reagent (1.4:1) was maintained, using 700 mg of RP per 500 mL of reagent solution. The mixture was kept under constant agitation using a magnetic stirrer for the entire duration of the experiment. The pH of the suspension was monitored daily to account for its evolution during the reaction. For this, 1.5 mL of the suspension was collected, centrifuged for 30 min at 1200 RPM, and the pH of the supernatant was measured using a micro-pH electrode. At selected time points (2 h and 5, 8, and 12 days), 15 mL of suspension was pipetted, transferred to 50 mL Falcon tubes, and centrifuged at 1,200 rpm for 30 min. After centrifugation, the supernatant was separated and stored for analysis. To halt the hydrolysis reaction in RP pellets, 3 mL of ethanol was added and vortexed to uniformly disperse the suspension before centrifuging for 5 min to separate RP. This procedure was repeated twice to ensure the reaction was significantly inhibited. The partially acidulated RP samples were then dried overnight in a freeze dryer before characterization.

All collected samples during acidification reactions were analyzed using Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), X-ray fluorescence (XRF), and electron scanning microscope (SEM) coupled with Energy-Dispersive X-ray Spectroscopy (EDS). Additionally, the treated RPs were also calcined in the furnace at 1,200 °C for 20 min and analyzed using FTIR to characterize the transformation of amorphous materials formed during acidulation, such as the formation of β -TCP (tricalcium phosphate) and association of the amorphous layer at the surface of the rock phosphate grains.

Characterization of rock phosphate

XRF analyses

The Rigaku Supermini 200 WDXRF was used to analyze the RP by X-ray fluorescence and determine its elemental composition. The 2 g of RP samples were calcined at 1,000 °C for 2 h to determine the loss on ignition (LOI). The calcined samples were then homogenized and pelletized. The pellets were carefully placed in the sample holder and securely positioned in the XRF spectrometer. The XRF was configured with a voltage of 30 kV and a current of 2 mA for data acquisition. The LOI value obtained was entered into the spectrometer program. The element content data from XRF is represented in weight percentage (wt%).

PXRD analyses

The PXRD patterns of original and acidulated RP were recorded using a Bruker D8 powder diffractometer equipped with a high-speed Xcelerator detector, employing Cu Kα radiation (λ = 1.542 Å) at 40 kV and 40 mA. The device was configured with receiving slits and a 1/2° divergence. A flat amorphous glass plate was used as a sample holder. The 2θ ranged from 5° to 55° with an increment of 0.06, a counting time of 1 s, and a total number of steps of 668. To increase the signal-to-noise ratio, a point detector with SOLLER slits was employed, which operates in OD mode and enables the measurement of one angle at a time.

The crystalline content in the calcium phosphate materials was estimated based on the published formula (Aina et al., 2012), as shown in Equation 1: where I 300 is the intensity of (300) hkl reflection in the XRD pattern, V 112/300 is the background intensity of the “valley” between (112) and (300) hkl reflections.

FTIR spectroscopic analyses

The infrared spectra were collected using a Nicolet 380 spectrometer (resolution 4 cm −1, 64 scans, detector: DTGS) equipped with an attenuated total reflectance (ATR) cell at room temperature. The dynamometric limiter of the ATR cell was used to ensure that the pressure applied to the diamond crystal via each sample remained constant during each measurement, allowing intensity to be referenced at the same optical density. This enabled correlating the relative amounts of functional groups and species in the powders and comparing the intensities of infrared absorption bands.

SEM and EDS analyses

SEM image analysis was performed to evaluate surface morphology and microstructural changes in RP following treatment. RPs acidulated at different time points were mounted on SEM stubs using carbon adhesive tape. A thin carbon film was deposited using a sputter coater to improve conductivity and reduce beam-induced charging, ensuring optimal imaging. We used TESCAN CLARA FE-SEM with an Everhart-Thornley detector for high-resolution imaging. The operating parameters included an acceleration voltage of 15 kV, a vacuum pressure of 0.0079 Pa, a working distance of 10.56 mm, and a dwell time of 3.2 µs in an ultra-high-resolution mode. The EDS analyses were performed to assess the elemental composition primarily of P and Ca, and other trace elements, and the relative change after acidification.

XRF results on RP composition

The XRF analysis of the elemental composition of the RP shows Ca and P as most dominant elements with CaO content of 49.5%, P 2 O 5 31.8%, and SiO 2 content 5.8% (Supplementary Figure S1). The volatile elements constituted 9.9%, while all others were present as trace elements, with following content: Al 2 O 3 (0.9%), Fe 2 O 3 (0.2%), SO 3 (2.1%), Na 2 O (0.8%), K 2 O (0.1%), MgO (1.3%), TiO 2 (<0.1%), Cr 2 O 3 (<0.1%), ZnO (<0.1%), SrO (0.2%), and Mn 2 O 3 (<0.1%). Light elements, particularly fluoride, cannot be detected by XRF even though the RP is a fluorapatite, but its presence is shown in the XRD peak.

The elemental composition of our reference RP is similar to that reported for other sedimentary phosphate rocks originating from North Africa and comparable deposits. Phosphate rocks from the Metlaoui-Gafsa Basin in Tunisia contain CaO and P 2 O 5 as the dominant oxides, with CaO values close to 46–50 wt% and P 2 O 5 around 29–31 wt% (Messai et al., 2023), reflecting the predominance of carbonate-fluorapatite as the main mineral phase. Similarly, XRF analyses of phosphate rocks from the Sokoto Basin in Nigeria are composed of CaO contents between 45 and 51 wt% and SiO 2 contents typically ranging from 5 to 8 wt% (Tikpangi and Olonkwoh, 2015), which are consistent with apatite-rich phosphorites containing quartz as a non-phosphate mineral phase. Furthermore, comparable major oxide distributions have also been reported for Mazıdağı phosphate rock in Turkey, with CaO and P 2 O 5 contents of around 50 wt% and 21 wt%, respectively (Özer et al., 2000). These components dominate the bulk composition and are associated with fluorapatite, carbonate-fluorapatite, and calcite. The low contents of Al 2 O 3 and Fe 2 O 3 observed in our RP are characteristic of sedimentary phosphorites dominated by apatite mineral phases.

Changes in mineralogy during acidulation of rock phosphate with acetic acid and MCP solutions

The phase identification of grounded RP by PXRD pattern revealed primarily as an apatite-based phase (Figure 1A). The generally flat background and sharp, well-defined peaks indicate a significant degree of long-range ordering, with ca 85% crystallinity in RP (Table 1). Rietveld refinement shows apatite-rich RP as mostly composed of fluorapatite (FAP) and carbonated apatite (CAP) with respective ratios of 29% and 49%. Other phases, including anhydrous forms of calcium-deficient hydroxyapatite (CDHA), dicalcium phosphate (DCPA), monocalcium phosphate (MCPA), and octacalcium phosphate (OCP), contribute to a combined total of 13% of crystalline phase, which is common for natural minerals. This phase identification was supported by the combined interpretation of PXRD with Rietveld refinement and FTIR vibrational features, together with relative surface Ca/P trends obtained from EDS as complementary methods. Since FAP and HAP exhibit overlapping diffraction patterns, their relative proportions were interpreted along with spectroscopic and compositional trends, and should be regarded as semi-quantitative.

Treatment with acetic acid (Figure 1B) or MCP solutions (Figure 1C) resulted in the partial conversion of the original RP constituents, leading to an increase in the abundance of acidic phases. The CDHA phase is known to have a Ca/P ratio below 1.67 and, to maintain charge balance, contains HPO 4 2− ions. The content of this phase increased approximately tenfold from 2 h to 5 days during treatment in acetic buffer (Table 1). This observation is supported by FTIR data, which show an increase in vibrational bands associated with HPO 4 2− species.

A similar trend was observed for DCPA, whose content increased approximately twofold by day 8 in acetic buffer. In contrast, MCP in the same buffer exhibited a rapid increase (approximately fifteenfold) within 2 h, followed by a gradual decrease to near its original level by day 12. Overall, all three acidic phases exhibit a common trend: their content increases with contact time, reaches a maximum at a specific exposure duration, and decreases upon prolonged treatment. This phase evolution is influenced by the acidic environment, leading to the formation of amorphous phase variations during the contact period.

Partial acidulation of RP powder with acetic acid and MCP solutions led to a moderate decrease in the content of less soluble mineral phases compared to the untreated RP material (Table 1). Based on these results, the ratio of less soluble to more soluble minerals in the untreated RP material is approximately 85:15. The mechanical and thermal activation of RP is well known. It can cause a reasonable extent of amorphization and formation of more soluble minerals from originally crystalline materials (Fang et al., 2019). Although Rietveld refinement did not reveal a consistent trend of decreasing less soluble mineral content, the overall trend indicates a notable decrease and a corresponding increase in more soluble phases. For instance, CAP shows a general decreasing trend, whereas FAP content appears to increase slightly following acetic acid treatment. The contribution of HAP fluctuates over time, along with the variable behavior of FAP, maintaining a combined FAP + HAP content of approximately 65%–70%. This variability may be attributed to the similarity of the diffraction patterns of these two minerals and limitations in the fitting algorithms. The content of more soluble phases does not exhibit a well-defined time-dependent trend as well. The overall effect of the treatment results in a greater formation of more soluble phases throughout the time it was evaluated. The highest total content of more soluble phases was observed after 8 days of treatment (around 27%).

Both MCP and acetic acids are highly acidic (ranging in pH from 3.5 to 4.2). The starting pH of the acidulation reaction solution was 4.5, close to the pH values of the reagents. As the reaction progressed, the pH gradually increased, reaching about 6.0–6.2 after 12 days. The extended pH measurements taken over 30 days showed a progressively increasing pH value, eventually stabilizing within a narrow range of 7.0–7.4. The gradual increase and near neutralization of pH reflect the buffering of dissolved ions. Phosphate released from apatite dissolution consumes protons, leading to a progressive rise in solution pH as the reaction continued (Margolis and Moreno, 1992). The extended time required to reach a stable pH highlights the slow kinetics of the dissolution reaction under mild acidification conditions. These findings underscore the importance of long-term pH monitoring to accurately capture ongoing RP dissolution.

Amorphous calcium phosphates are known to exhibit higher solubility than crystalline apatite phases (hydroxyapatite, carbonated apatite, fluorapatite) (Sakhno et al., 2024; Ivanchenko et al., 2025). Therefore, partial amorphization of RP into other calcium phosphate phases, such as CDHA, DCPA, MCPA, and OCP, increases overall solubility. Additionally, it is important to examine chemical changes in RP resulting from interactions with organic and inorganic acids to assess the formation of more soluble Ca–P phases. In summary, both treatments lead to varying degrees of partial transformation of RP minerals into more soluble Ca–P phases.

FTIR results on compositional evolution during partial acidulation of rock phosphate

The FTIR spectra of the different Ca–P materials are presented in Figure 2. A weak, broad absorption band in the 3,500–2,500 cm −1 region is observed and is attributed to the asymmetric and symmetric stretching vibrations (ν asym/ ν sym) of hydroxyl (OH) groups. This feature is primarily detected in samples before calcination. The consistent presence of this band in untreated RPs suggests that OH vibrations are inherent in the material. In the 800–1,200 cm −1 region, the spectra exhibit absorption bands corresponding to the fundamental vibrational modes of phosphate (PO 4 3−) groups. Specifically, the ν 1 P − O vibration appears at 964 cm −1, while the ν 3 asymmetric stretching modes ν 3 aP − O are observed at 1,093 cm −1 and ν 3 cP − O at 1,027 cm −1. The ν 1 P − O is sensitive to changes in the chemical environments outside the P6 3 /m space group, as shown by a slight shift toward lower frequencies with increasing contact time in acidic solutions. This behavior is characteristic of Ca–P crystalline apatite-based materials and involves the formation of amorphous nucleation precursors within the metastable zone during the acidification process. This is typical for non-classical nucleation pathways, as reported previously (Ivanchenko et al., 2025). Additionally, slight peak broadening in PXRD patterns, along with FTIR band broadening and the formation of β -tricalcium phosphate after calcination (Bohner et al., 2020), suggest partial structural disorder beyond fully crystalline phases. Such poorly ordered Ca–P domains cannot be directly quantified by XRD analysis. However, the variation in the amorphous phase as the fuction of reaction time induced by acidification is supported by PXRD data.

Although fundamental vibrational modes partially overlap, resulting in complex spectral profiles, the emergence of an absorption band with a maximum at 920 cm -1 is clearly observed in calcined samples (Figures 2B,D). The intensity of this band increases with increasing exposure time of RP to both organic and inorganic acids. This feature has previously been associated with HPO 4 2− species (Sakhno et al., 2025). Similarly, absorptions at 1,167 and 1,147 cm −1 are known to emerge for acidified phosphate HPO 4 2− species in P6 3 /m space group. Their relative intensities increase progressively with the duration of RP exposure to acidic solutions, following the trend from untreated RP to RP after 5 days of treatment. Additionally, absorption bands corresponding to carbonate groups are observed at 1,456, 1,427, and 866 cm −1 before calcination (Figures 2A,C). The gradual decrease in the intensity of these bands after exposure to organic and inorganic acids suggests that RP material absorbed protons from acidic solution, lowering the surface pH of RP. This, in turn, facilitated the formation of an acidified Ca-P amorphous phase. The carbonate content in this amorphous phase is pH-dependent and decreases at lower pH (Sakhno et al., 2022b). After calcination at 1,200 °C, these bands disappear in both acidified RP. This difference in carbonate content between the original calcined RP and acid-treated calcined RPs provides indirect confirmation that protons from the acidic solutions are partially incorporated into the RP solids, which subsequently contributed to the decomposition of CO 3 ions to CO 2.

Changes in surface morphology during acidulation

Scanning electron microscopy images enabled assessing morphological changes in RP grain surfaces induced by partial acidulation with acetic acid and MCP solutions, while EDS provided complementary elemental information. The untreated RP exhibited a more crystalline form with clear grain boundaries and smooth surfaces (Figure 3a), which is consistent with the predominance of crystalline apatite phases determined by PXRD. After partial acidulation, RP treated with either acetic acid or MCP solution displayed noticeable alterations. In both cases, particle surfaces become somewhat irregular and appear fuzzy, indicating etch pits and other localized dissolution features (Figures 3b,c). In some instances, finer particles emerged from the original aggregates, likely representing new phases of more soluble Ca-P minerals formed during acidulation. These observations are consistent with the preferential dissolution of Ca-rich surface domains, as reported in weak acid promoted acidification of rock phosphates (Dorozhkin, 2012).

Localized agglomeration of fine particles is observed in both acid-treated RPs, particularly at intermediate contact times. Such features have been reported during partial dissolution and surface reorganization of calcium phosphate materials and are consistent with the formation of disordered or poorly crystalline CaP surface domains (Combes and Rey, 2010). The presence of these regions corresponds with the increase in the calcium deficient and acidic phosphate phases identified by PXRD and is further supported by the emergence of HPO 4 2− -related vibrational bands observed in FTIR spectra (Figure 2). This indicates morphological changes due to structural surface modifications. Comparable alterations in RPs have been reported under protonation, which involves surface roughening and partial dissolution without complete transformation (Maharana and Dhal, 2022).

The EDS spectra taken from selective SEM images show the changes in elemental composition during acidulation, particularly the variations in P and Ca intensities (Figure 4). The acid treatments induced notable shifts in Ca and P intensities compared to RP, with the 8 days product exhibiting the most intense peaks among the samples. Interestingly, this peak intensity, however, is higher than MCP (TSP, a commercial reference). Since the intensity among different samples could be compounded by other factors, a relative ion ratio generally serves as a good measure for reflecting changes during phosphate solubilization. The Ca/P ratio of untreated RP was 1.54, which decreased to 1.35 in 2 h and reached a minimum of 1.31 after 12 days in acetic acid treatment. Conversely, the lowest Ca/P ratio observed in MCP treatment was 1.03, followed by a partial recovery to ratios between 1.21 and 1.26. These trends indicate preferential enrichment of P relative to Ca on the surfaces, consistent with the development of calcium deficient apatite, particularly at intermediate time points. The Ca deficient surface compositions inferred from EDS align with the Rietveld refinement of XRD data (Table 1), indicating the development of calcium-deficient phases. Furthermore, FTIR spectra showing the emergence of HPO 4 2- bands and a depletion of carbonates support the surface-confined modification of RPs under mild acidic conditions. These results are in agreement with past studies (Mavropoulos et al., 2003).

Partial acidulation of Moroccan rock phosphate studied under mild acidic conditions induced a degree of structural reorganization but without complete transformation of apatite. These changes were identified through PXRD and Rietveld refinement, supplemented by FTIR and SEM-EDS. Partial acidulation with acetic acid and MCP solutions resulted in a moderate decrease in carbonate content in carbonated fluorapatite and an increase in calcium-deficient hydroxyapatite and the emergence of several short-range order calcium phosphate phases (DCPA, MCPA, and OCP). These phases generally increased with prolonged contact time; however, some appeared to form during the initial granulation of rock phosphate. FTIR spectroscopy highlighted the emergence of HPO 4 2- -related bands, shifts in ν 1 (P-O) vibrations, and progressive carbonate depletion. Formation of HPO 4 2- rich calcium phosphate is a key indicator of enhanced solubility. Therefore, the dissolution kinetics of these materials are expected to be much higher, as evidenced by past studies (Sakhno et al., 2022b). Calcination of the reaction products further confirmed the presence of structurally disordered Ca-P environments and formation of β-tricalcium phosphate, which are normally formed from low Ca/P ratio materials. SEM images showed that both acetic acid and MCP treatments produced comparable morphological changes, including irregular surface features and fuzzy boundaries, and localized agglomeration of potentially new products. These results collectively suggested the formation of protonated calcium phosphate and more soluble mineral phases during acidification, contributing to the transformation of original rock phosphate into a biphasic mixture of more soluble and sparingly soluble mineral forms.