Eco-Friendly Utilization of Phosphogypsum via Mechanical Activation for Sustainable Heavy Metal Removal from Wastewater

Abstract

This study examined significant changes in phosphogypsum, a byproduct of the phosphoric acid industry, induced via mechanical activation through intensive grinding using a planetary ball mill. Alterations in crystallinity, surface area, and zeta potential were monitored using X-ray diffraction, Brunauer–Emmett–Teller analysis, zeta potential measurements, X-ray photoelectron spectroscopy, and scanning electron microscopy. The severe grinding of this mining waste led to the conversion of gypsum (CaSO₄·2H₂O) to anhydrite (CaSO₄), an increase in surface area from 5.8 m²/g to 17.8 m²/g, and a decrease in pore radius from 76.6 nm to 9.3 nm. The zeta potential shifted as the isoelectric point changed from pH 8.5 to pH 4.3. These modifications enhanced the material’s potential as a cost-effective and eco-friendly adsorbent for wastewater treatment. The enhanced adsorption capabilities for Cd and Pb were evaluated, revealing a higher adsorption capacity (~40 mg/g for both) and removal efficiency (~90% for Cd and ~80% for Pb) for activated phosphogypsum. The adsorption process followed the Freundlich isotherm and pseudo-second-order kinetic model, indicating its physisorption nature and spontaneous thermodynamic characteristics, and highlighting its potential for wastewater treatment. The mechanically activated adsorbent demonstrated over 90% desorption efficiency over five cycles, ensuring effective regeneration and reusability for Cd and Pb removal. Real tannery wastewater was treated using mechanically activated phosphogypsum at pH 6 and 70 °C for 60 min, achieving a 94% Cd and 92% Pb removal efficiency, with an overall heavy metal removal efficiency of up to 83%. This study demonstrates the sustainable utilization of phosphogypsum, contributing to green wastewater management and environmental protection.

1. Introduction

The contamination of water with Cd and Pb significantly threatens aquatic ecosystems and human health owing to their toxic nature and persistence in the environment. Cd, a highly toxic metal, can accumulate in aquatic organisms through the food chain, leading to kidney, liver, and lung cancers [1,2]. Additionally, Pb, which is often present alongside Cd in industrial discharge, exhibits similar toxicological properties and can contaminate water sources, posing serious health risks even at low concentrations [3]. It is therefore necessary to develop cost-effective approaches for the removal of Cd and Pb from water to safeguard human life and ecosystems.

Several methods have been developed to remove Cd and Pb from wastewater, thereby mitigating their harmful impacts on the environment and human health, including coagulation [4,5], precipitation [6,7], bioremediation [8,9], and adsorption [10,11]. Among these approaches, adsorption techniques have attracted the attention of researchers owing to their low cost, high efficiency, and ease of operation. Several materials, such as clay [12,13], biochar [14], and activated carbon [15], have been used as adsorbents for the removal of heavy metals, including Cd and Pb, from contaminated waters. However, many of these adsorbents face limitations such as high production costs, low adsorption capacities, and environmental concerns during their synthesis and disposal [16,17].

Phosphogypsum, a byproduct of phosphate fertilizer production, presents significant environmental concerns because of its large production volume. It is estimated that producing 1 ton of phosphoric acid results in approximately 4–5 tons of phosphogypsum [18,19], while the chemical properties of phosphogypsum and its metal ion content make its direct use difficult in industrial applications [19]. Despite these challenges, efforts have been made to explore their application in construction and agriculture [18,20,21,22]. Additionally, phosphogypsum has been tested as an adsorbent for removing heavy metals, including Cd [23,24], Cu [25], As [26], Pb, and Zn [24,27], and for organic pollutant removal [28]. However, these attempts have shown that its effectiveness as an adsorbent is generally lower than that of other adsorbents. In response, some researchers have investigated chemical modifications to improve the adsorption capabilities of phosphogypsum for the removal of cadmium [29]; nickel, chromium, and zinc [30]; fluoride [31], arsenic [32]; and malachite green [33]; however, these methods can be expensive and environmentally harmful.

More recently, an ecofriendly, cost-effective drying process involving mechanical activation through severe grinding was demonstrated to lead to significant alterations in materials’ surface properties and crystallinity, positively impacting their adsorption capabilities [34,35,36,37].

While progress has been made, the use of phosphogypsum for heavy metal removal, particularly through mechanical activation, remains insufficiently studied. This study addresses this gap by demonstrating the potential of mechanically activated phosphogypsum as an efficient and sustainable adsorbent for Cd and Pb removal. The key innovation of this research is the use of mechanical activation to enhance phosphogypsum’s adsorption capacity. Unlike chemical modifications, which can be expensive and environmentally harmful, mechanical activation offers a simple, eco-friendly, and scalable alternative. This study optimizes key adsorption parameters and analyzes the underlying mechanisms, contributing to a more sustainable approach for heavy metal remediation.

The aim of this study was to employ a mechanical activation technique to boost the adsorption capacity of phosphogypsum for Cd and Pb in wastewater. Key adsorption parameters such as pH, contact time, adsorbent dosage, and temperature were optimized. Additionally, this study investigates the adsorption kinetics and thermodynamics to unravel the underlying mechanisms.

2. Materials and Methods

2.1. Materials

A representative sample of phosphogypsum was obtained from the fertilizer industry (Maaden Company, Riyadh, Saudi Arabia), KSA, characterized by its typical composition and the impurities associated with the phosphoric acid production process.

Analytical-grade Cd nitrate ([Cd (NO₃)₂]) and Pb nitrate ([Pb (NO₃)₂]) (Sigma-Aldrich (St. Louis, MO, USA) and Merck (Darmstadt, Germany), respectively) were employed as the Cd and Pb ion sources for the adsorption experiments.

2.2. Methods

2.2.1. Mechanical Activation

A Planetary Micro Mill PULVERISETTE 7 ball mill (FRITSCH, Idar-Oberstein, Germany), equipped with 250 mL Zr oxide jars and balls, was used for the mechanical activation of phosphogypsum. A predetermined amount of phosphogypsum was placed in the milling jar along with stainless steel balls of 5 mm diameter. A ball-to-powder ratio of 15:1 and a rotating velocity of 600 rpm were maintained throughout this study. Activation experiments were conducted for three milling durations (3 h, 5 h, and 7 h).

2.2.2. Characterization

An X-ray diffraction (XRD) analysis was performed using a Bruker D8-ADVANCE diffractometer (Karlsruhe, Germany) with a Cu Kα radiation source (λ = 1.54056 Å) to investigate the changes in phosphogypsum crystallinity before and after mechanical activation. X-ray photoelectron spectroscopy (XPS) was conducted using a Thermo Scientific K-Alpha spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with monochromatic Al Kα radiation to analyze the elemental composition and chemical states of the materials. Scanning electron microscopy (SEM) was performed using a Quanta 250 Field Emission Gun (FEG) (Eindhoven, The Netherlands) to examine the morphology of phosphogypsum before and after mechanical activation. The surface area and pore characteristics were determined using a Brunauer–Emmett–Teller (BET) analysis with a BELSORP-MR 6 surface area analyzer. Degassing was performed at 105 °C for 4 h. Zeta potential measurements were conducted using a zeta potential analyzer (Malvern Instrument Co., Ltd., Worcestershire, UK), and 1.0 × 10⁻³ M KCl was used as an electrolyte solution, whereas HCl and KOH were used as pH regulators to assess the surface charge of the phosphogypsum particles.

2.2.3. Adsorption Experiments

Adsorption experiments were conducted using 0.05 g of phosphogypsum and 50 mL of the desired Cd or Pb ion concentration in a round flask. The flask containing the slurry was shaken (200 rpm) for the selected interaction time. The adsorbent sample was then separated from the solution via filtration using a 0.45 μm Whatman filter paper. The concentration of the remaining ions in the synthetic solution was determined using an atomic absorption spectrophotometer (PerkinElmer, AA Analyst 200) (Waltham, MA, USA). The key adsorption parameters, including the pH, ion concentration, contact time, and temperature, were optimized.

3. Results and Discussion

3.1. Physico-Chemical Characterizations

3.1.1. XRD Analysis

The XRD pattern of the pristine phosphogypsum is shown in Figure 1. The sharp peaks at approximately 11.6°, 20.8°, 23.2°, and 29.2° indicated well-crystallized gypsum (JCPDS 01-070-0982). These peaks correspond to the (020), (021), (040), and (041) planes, respectively. The presence of only gypsum peaks in the pattern suggests a relatively pure phosphogypsum sample. However, XRF analysis (

Table 1. XRF analysis of phosphogypsum analysis.

Item	CaO	Na2O	MgO	Al2O3	SiO2	P2O5	SO2	LOI
%	36.5	0.21	0.12	0.63	3.3	4.2	34.85	16.5

) shows that silica and phosphorous are the main impurities in phosphogypsum.

Figure 2 compares the XRD patterns of the raw phosphogypsum samples and the mechanically activated samples after 3 h, 5 h, and 7 h of grinding. As shown in the figure, the appearance of anhydrite peaks even after 3 h of grinding suggests that the mechanical stress, and possibly the heat generated during severe grinding, were sufficient to initiate the conversion of gypsum into anhydrite. This transformation may be due to the dehydration of gypsum under mechanical force, which removes crystalline water molecules from its structure. With an increase in the grinding time, the peaks in anhydrite became more pronounced, indicating a further conversion of gypsum to anhydrite. This suggests that it is possible to achieve phase transformations in phosphogypsum via mechanical activation, potentially enhancing its reactivity and suitability for various applications.

3.1.2. XPS Analysis

The high-resolution XPS Ca2p, S2p, and O1s spectra of mechanically activated phosphogypsum and the original phosphogypsum (Raw) are displayed in Figure 3. As shown in the figure, the Ca2p spectrum of raw sample exhibits peaks at 348.0 eV and 351.0 eV, typically attributed to the Ca2p3/2 and Ca2p1/2 spin–orbit coupling components of calcium in CaSO₄·2H₂O (gypsum) [38,39]. Peaks were observed at the same binding energies, indicating that the oxidation state of Ca remained unchanged as Ca²⁺ even after mechanical activation. The notable increase in peak intensity might be attributed to the surface becoming more enriched in Ca due to the conversion of gypsum CaSO₄·2H₂O to anhydrite CaSO₄, which has a higher relative content of Ca compared to hydrous gypsum.

The peaks at 168.0 eV are characteristic of S2p in sulfate groups (SO₄²⁻) in gypsum and the mechanically activated sample, indicating that the sulfate groups remain chemically unchanged; however, the slight broadening of the peaks might suggest some changes in the sulfate environment due to the dehydration process, consistent with the XRD results.

The O1s spectrum of the raw sample shows a broad peak at approximately 533 eV, which typically corresponds to O in sulfate groups. The peaks located at 531.6 eV and 533.9 eV are attributed to SO₄²⁻ of gypsum [38,40] and the adsorbed water molecules, respectively [41]. The higher intensity of the 533 eV peak in the mechanically activated sample suggests enhanced surface adsorption of water molecules, likely due to increased surface reactivity after mechanical activation.

These XPS findings corroborate the XRD results, confirming that mechanical activation primarily induces physical changes (e.g., water removal) rather than chemical alterations, leading to the transformation of gypsum into anhydrite.

These XPS findings corroborate the XRD results showing that mechanical activation leads to the dehydration of gypsum into anhydrite, with the primary change being the physical (water removal) rather than chemical alteration of the constituent ions.

3.1.3. Microstructure Analysis

The surface morphologies of the raw and mechanically activated phosphogypsum were examined via field-emission scanning electron microscopy (SEM), as shown in Figure 4. The raw phosphogypsum particles appeared as clumped, relatively large aggregates with a layered structure (Figure 4A). A closer view of the raw phosphogypsum particles shows that the particle shapes were more defined, exhibiting sharp edges and smooth surfaces, which are characteristic of crystalline structures (Figure 4B). Conversely, the particles of the mechanically activated phosphogypsum were notably finer and more dispersed compared to the raw material with high surface roughness (Figure 4C,D). This surface morphology alteration results from the severe bombardment of the particles by the grinding media. Additionally, these dramatic changes are expected to improve the adsorption capability of the phosphogypsum particles.

3.1.4. Zeta Potential

The zeta potentials of the raw and mechanically activated phosphogypsum samples are shown in Figure 5. Pristine gypsum consistently exhibited a low negatively charged surface at various pH levels, except for a positive zeta potential at pH 10. Two isoelectric points (IEPs) were observed: one below pH 2 and the other at pH 9. These results agree with those of previous studies [42]. In contrast, the mechanically activated gypsum exhibited a more dynamic and pH-dependent pattern. At an acidic pH, the zeta potential became highly positive, signifying a shift towards a positively charged surface compared with that of pristine gypsum. However, at an alkaline pH, the zeta potential reverted to a negative value, and the IEP was observed at pH 4.3. The significant shift towards more negative values, particularly at higher pH levels, can be attributed to the higher dissolution rates of Ca and P ions in the mechanically activated sample solution. When leached into the solution, these ions leave behind a more negatively charged surface.

The zeta potential data complement the findings from the XRD, XPS, and SEM analyses, showing that mechanical activation significantly enhances the ionic reactivity and surface charge properties of phosphogypsum. These changes are expected to improve the adsorption performance of the mechanically activated phosphogypsum in wastewater treatment, where charge interactions are crucial for the adsorption and removal of contaminants.

3.1.5. Surface Area

The N₂ adsorption–desorption isotherms of raw and mechanically activated phosphogypsum are shown in Figure 6. The inset graphs show a plot of the pore size distribution, and the BET surface area, average pore diameter, and total pore volume are presented in Table S1. The isotherm of raw phosphogypsum showed a low adsorption capacity, which remained flat over a wide range of relative pressures, indicating few accessible pores. The shape of the isotherm suggests a material with low porosity, which is typical of dense non-porous materials.

In contrast, the isotherm of the mechanically activated sample exhibits significantly higher adsorption, particularly at higher relative pressures, indicating the creation of more accessible pore structures. The shape of this isotherm is typical of mesoporous materials (type B of DeBoer’s classification). The average pore radius decreased significantly, from 76.62 nm in the raw sample to 9.34 nm in the mechanically activated sample, indicating the creation of finer pores as larger aggregates were broken down. The total pore volume more than doubled, from 0.0258 cm³/g to 0.0574 cm³/g, indicating smaller and more numerous pores in the mechanically activated sample. Consequently, a significant increase in surface area, from 5.81 m²/g in the raw sample to 17.86 m²/g in the mechanically activated gypsum, was observed. These observations confirm that mechanical activation causes a dramatic destruction of the particle structure, rendering it with a more complicated surface morphology and an altered pore structure, and in turn improving its surface reactivity, which aligns with the improved adsorption performance observed in the study.

3.2. Adsorption Study

3.2.1. Effect of pH

pH is one of the most important working parameters in the adsorption process because it can cause dissociation at the adsorbent sites and alter the solution chemistry of metal ions. Figure 7 shows the effects of pH on the adsorption capacity and removal efficiency of Cd and Pb using raw and mechanically activated phosphogypsum. These experiments were conducted with a metal ion concentration of 50 mg/L, an adsorbent dose of 1 g/L, a contact time of 60 min, and a temperature of 30 °C. As shown in the figure, the adsorption capacity and removal efficiencies increase as the solution pH transitions from acidic to alkaline regions for the two samples. Moreover, the mechanically activated phosphogypsum sample exhibited a higher adsorption efficiency than the raw sample. This behavior can be explained by the zeta potential results (Figure 5), where the shift towards alkaline regions resulted in the particles of mechanically activated phosphogypsum becoming more negatively charged, thus enhancing the electrostatic attraction forces between the metal ions (Cd or Pb) and the adsorbent, thereby improving their adsorption efficiency. Furthermore, the high surface roughness, increased surface area, and smaller pore size of mechanically activated phosphogypsum contributed to its higher removal efficiency compared to the raw phosphogypsum sample, even in the acidic and neutral pH regions, where both samples exhibited positive charges.

3.2.2. Effect of Initial Concentration

The effects of the initial concentrations of Cd and Pb ions on the adsorption capacity and removal efficiency are shown in Figure 8. The maximum adsorption capacities achieved at an initial metal ion concentration of 250 mg/L were 215 mg/g for Cd and 170 mg/g for Pb using mechanically activated phosphogypsum, and 138 mg/g for Cd and 171 mg/g for Pb using raw phosphogypsum. The increased adsorption capacity of the mechanically activated phosphogypsum compared to that of the raw sample was due to its unique surface features, created via mechanical activation.

Figure 8 shows that increasing the initial ion concentration enhanced the uptake capacity but reduced the removal efficiency. The decline in the removal efficiency resulted from the increase in residual metal ions as the initial ion concentration increased. At higher concentrations, more ions are available, thereby increasing the amount absorbed by the sorbent. The driving force required to overcome the barrier for ion migration through the medium to the solid surface of the sorbent increases, thereby progressively loading the sites until saturation is reached [43].

3.2.3. Isotherm Models Study

The sorption process was analyzed using the Langmuir and Freundlich isotherms to describe the behavior [44]. According to [45], the Langmuir isotherm is described by the following equation:

$${\displaystyle \frac{C_f}{q_t}}={\displaystyle \frac{C_f}{q_{max}}}+{\displaystyle \frac{1}{{bq}_{max}}}$$

where C_f (mg/L) is the final ion concentration, q_t (mg/g) is the number of ions sorbed at time t, q_max (mg/g) is the maximum sorption for the monolayer adsorption capacity, and b (L/mg) is the binding constant related to the adsorption free energy. The Langmuir model suggests that sorption occurs on a uniform adsorbent surface, where ions move through the pores and openings of the structure to replace the ions of the adsorbent. As shown in Figure S1 and

Table 2. Parameters of the Langmuir and Freundlich isotherm models.

Isotherm	Parameter	Cadmium		Lead
		Raw	MA	Raw	MA
Langmuir	R2	0.8545	0.8955	0.8479	0.9043
	qmax (Cal.)	36	5	100	44
	qmax (Exp.)	171	214	138	169
	b	6.5 × 10−3		4.5 × 10−3
Freundlich	R2	0.9966	0.9940	0.9986	0.9965
	n	1.35	1.34	1.32	1.27
	KF	6.91	15.77	3.91	5.78

, the R² values from the linear fits of the Langmuir model are 0.84 and 0.90. In contrast, the Freundlich isotherm model presented by [46] is determined using the following equation:

$${lnq}_t=lnk+{\displaystyle \frac{1}{n}}ln{C}_f$$

where q_t (mg/g) is the sorbed amount, C_f (mg/L) is the final concentration, K is the extent of the sorption, and n is the sorption intensity. The function 1/n represents sorption capability [47]. If n = 1, the interface between the two phases is unaffected by the concentration changes. An n value of <1 indicates typical adsorption, whereas values between 1 and 10 suggest a favorable sorption process [48]. From Table 2, the n values are 1.27 and 1.35, and the R² values exceed 0.99 for the two samples, respectively, indicating a favorable and predominantly physical sorption process [49].

The Freundlich model provides a better fit (R² > 0.99) compared to the Langmuir model, suggesting that adsorption occurs on a heterogeneous surface with non-uniform energy distribution. This is consistent with the observed changes in surface morphology and pore structure after mechanical activation, as discussed in the SEM and BET analyses.

3.2.4. Effect of Contact Time

Figure 9 shows the effects of contact time on the adsorption capacity and removal efficiency of Cd and Pb using raw and mechanically activated samples. The equilibrium time was 60 min. A higher adsorption rate was observed within the first 20 min, which was attributed to readily available adsorbate ions and vacant adsorption sites on the surface of the adsorbent. However, from 20 to 60 min, the adsorption rate gradually decreased as the available active sites became increasingly occupied. The extent of adsorption was measured based on the number of ions transferred from the solution to the active sites. Consequently, the adsorption continued to increase with time until saturation was reached [50]. Moreover, the ions required additional time to penetrate smaller pores.

The higher initial rate suggests that adsorption initially occurred on the external surface, followed by penetration into the internal pores [51]. Additionally, the larger adsorption volumes observed in the initial period indicated more significant adsorption on the external surface than within the pores [52]. The maximum removal efficiencies achieved after a 60 min contact period were 86% for Cd and 68% for Pb with mechanically activated phosphogypsum, and 68.5% for Cd and 55% for Pb with raw phosphogypsum.

3.2.5. The Adsorption Kinetics

The extent of the sorption of Cd and Pb ions by raw and mechanically activated phosphogypsum was examined using the Lagergren pseudo-first-order (PFO) and pseudo-second-order (PSO) models. The Lagergren values for the PFO and PSO models are denoted by the following equations:

The PFO equation [53]:

$$\ln q_e-q_t=\ln q_e-k_{1}t$$

The PSO equation [54]:

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k_2{q}_e^2}}-{\displaystyle \frac{t}{q_e}}$$

where q_t (mg/g) and q_e (mg/g) are the number of ions adsorbed by the adsorbent at time t (min) and at equilibrium, respectively, and k₁ (min⁻¹) and k₂ (g·mg⁻¹·min⁻¹) are the equilibrium rate constants.

In addition, their equivalent limits are listed in

Table 3. The parameters of pseudo-first-order and pseudo-second-order models.

Item	Cadmium Ions		Lead Ions
	Raw	MA	Raw	MA
	The pseudo-first-order model
R2	0.9770	0.9760	0.9746	0.9798
K1	−5.12 × 10−2	−5.36 × 10−2	−4.02 × 10−2	−5.18 × 10−2
Calculated qe	218	286	216	239
Experimental qe	176	219	150	174
	The pseudo-second-order model
R2	0.9929	0.9959	0.9948	0.9944
K2	2.98 × 10−4	2.69 × 10−4	2.33 × 10−4	3.18 × 10−4
Calculated qe	188	250	172	191
Experimental qe	176	219	150	174

. Although the PFO and PSO models are the most widely applied models that forecast closer values of the maximum sorption capacity, the best fit was found using PSO linear retrogression, based on the R² closest value to unity. The kinetic fitting quality changes in the following order: PSO > PFO.

3.2.6. Effect of Temperature

The effects of temperature on the removal efficiency and sorption capacity of Cd and Pb ions by raw and mechanically activated phosphogypsum samples are illustrated in Figure 10. In these experiments, the solution pH was maintained at 6, the initial ion concentration was 250 mg/L, and the adsorbent dose was 1 g/L. As depicted in the figure, the removal efficiency and adsorption capacity are directly proportional to temperature, with the maximum adsorption capacity being reached at 70 °C. This indicates that the adsorption is endothermic. Most adsorption studies have suggested that increasing the temperature enhances the sorption process [55]. At higher temperatures, the uptake is typically greater, owing to an increase in the energetic sites of the sorbent material. Increased temperatures also energize the system, promoting the attachment of ions to mineral surfaces [56]; moreover, the ion movement accelerates as the viscosity of the solution decreases [57], resulting in higher removal efficiencies [58].

3.2.7. Thermodynamic Study

Changes in thermodynamics parameters, such as Gibb’s free energy (ΔG°), entropy (ΔS°), and enthalpy (ΔH°) [55], were determined. ΔH° and ΔS° were computed from the Van’t Hoff equation [59]:

$$\ln K_c={\displaystyle \frac{{∆S}^{°}}{R}}-{\displaystyle \frac{{∆H}^{°}}{RT}}$$

where kc = F/(1 − F) and F = (Co − Ce)/Co, R is the universal gas constant, and T is the temperature in K. The relationship of lnkc versus 1/T (Figure S3) provides a straight line with a slope of −ΔH°/R and an intercept equal to ΔS°/R. The positive values of ΔH° in Table S2 specify the endothermic sorption process. Additionally, the ΔH° values are approximately ≥40 kJ.mol⁻¹, which verifies the physisorption process, as shown by the Freundlich isotherm [60].

Additionally, ΔS° exhibited positive values, suggesting a degree of unpredictability at the interface between the sorbent and adsorbate, inferring that sorption is less advantageous at lower temperatures. The positive ΔS value shows that the stochastic affinity of this process, which suggests adsorption, is a favorable spontaneous reaction.

The standard Gibbs free energy change ΔG° is estimated using the following equation [60]:

$${∆G°=-RTlnK}_c$$

Table S2 shows that the sorption is unprompted because the ΔG° has negative values. Notably, the negative values of ΔG° increased with an increasing temperature, suggesting that the sorption is more favorable at higher temperatures [60].

These thermodynamic findings align with the Freundlich isotherm results, confirming that the adsorption process is predominantly physical, spontaneous, and temperature-dependent. The endothermic nature of the process and the increased spontaneity at higher temperatures further explain the enhanced adsorption performance observed using phosphogypsum.

3.2.8. Regeneration and Reusability of Adsorbent

The regeneration and reusability of the mechanically activated sample for cadmium and lead ion removal were examined under the adsorption conditions of an initial ion concentration of 250 mg/L, pH 6, temperature 70 °C, and a contact time of 60 min. The regeneration process was conducted using 0.1 M HNO₃ with a solid/liquid ratio of 1:10, a contact time of 15 min, and a temperature of 65 °C. As shown in Figure 11, the adsorbed ions were successfully desorbed with an efficiency exceeding 90% for the first five cycles. These results indicate that the mechanically activated adsorbent can be easily separated and reused multiple times.

3.2.9. Comparison Studies

To evaluate the performance of phosphogypsum adsorbent, its adsorption capacity for lead and cadmium ions was compared with that of other reported adsorbents, as summarized in

Table 4. Comparison of phosphogypsum with other reported adsorbents for Pb2+ and Cd2+ removal.

Adsorbent	Ads. Capacity (mg/g)		pH	Reference
	Pb2+	Cd2+
Alkali-modified almond shells	9	7	5-6	[61]
Magnetic biochar	40	16	5.0	[62]
Modified reed biochar	17	3	7.0	[63]
Bone composite	29	42	4.0	[64]
Modified dairy manure biochar	175	68	7.0	[65]
Activated carbon	9.3	9.2	6.3	[66]
Zirconium organic frameworks	176	195	6.0	[67]
Ageratum conyzoid biomass	30	36	6.0	[68]
Phosphogypsum	230	234	6.0	This work
Activated phosphogypsum	235	243	6.0	This work

. The results demonstrate that phosphogypsum, both in its raw and activated forms, exhibits superior performance in removing lead and cadmium ions compared to other materials. Notably, many of these alternative adsorbents were synthesized using high-cost chemical methods and advanced techniques. This comparison emphasizes the significant improvements achieved through mechanical activation and highlights the competitive advantage of our approach, which relies on a low-cost and sustainable material.

3.2.10. Application in Real Wastewater

Real wastewater from a tannery workshop was used for the application. The treatment was performed by adding a mechanically activated sample to the wastewater after adjusting the pH to 6 and maintaining a temperature of 70 °C for 60 min. As shown in Table S3, cadmium and lead ions were removed with an efficiency of 94% and 92%, respectively, while other heavy metal ions were also successfully removed, with an overall removal efficiency of up to 83%.

4. Conclusions

In this study, the mechanical activation of phosphogypsum through intensive grinding significantly enhanced its physicochemical properties, improving its effectiveness as an eco-friendly adsorbent for Cd and Pb removal from synthetic wastewater. The transformation from gypsum (CaSO₄·2H₂O) to anhydrite (CaSO₄) increased the surface area and reduced the pore radius, enhancing the adsorption capacity.

The optimization of adsorption parameters (pH, contact time, adsorbent dosage, and temperature) confirmed that mechanically activated phosphogypsum outperformed its raw form. The adsorption process followed the Freundlich isotherm (physisorption) and PSO kinetic model, highlighting the importance of surface interactions.

The adsorbent achieved 94% Cd and 92% Pb removal efficiency from real tannery wastewater at pH 6 and 70 °C, with over 90% desorption efficiency over five cycles, thus demonstrating excellent regeneration and reusability. This study highlights the innovative use of mechanical activation to transform phosphogypsum into an effective, eco-friendly adsorbent, offering a sustainable solution for heavy metal remediation and waste valorization.