Utilization of Phosphogypsum as Sustainable Adsorbent for Removal of Crystal Violet Dye from Wastewater: Kinetics, Thermodynamics, and Applications in Textile Effluent Treatment

Abstract

This study examines the potential of phosphogypsum—a by-product of the phosphoric acid production process—as a low-cost and sustainable adsorbent for the removal of crystal violet dye from aqueous solutions. Phosphogypsum was characterized using X-ray fluorescence, X-ray diffraction, particle size distribution, and zeta potential measurements, revealing that it is primarily composed of di-hydrate calcium sulfate, with a negatively charged surface in the pH range from 1.8 to 8.2 and a mean particle size of 12.2 microns. Experiments were conducted to evaluate the effects of pH, adsorbent dose, contact time, and temperature on its adsorption ability. The results indicated that the adsorption capacity increased with the pH up to a value of 5, while higher initial dye concentrations enhanced the uptake capacity but reduced the removal efficiency. The adsorption process was well described by the Langmuir isotherm, suggesting chemisorption as the dominant mechanism, while the pseudo-second-order kinetic model indicated that adsorption primarily occurred on the exterior surface. The thermodynamic analysis revealed that the process was exothermic and spontaneous at 20 °C and 30 °C, with a decrease in favorability at higher temperatures. The adsorbent demonstrated reusability, with a removal efficiency of 71% after five regeneration cycles. Furthermore, phosphogypsum was successfully applied to treat real textile effluent, achieving significant reductions in both biochemical oxygen demand (71%) and dye content (87%). These findings highlight the potential of phosphogypsum as an effective and eco-friendly adsorbent for wastewater treatment, contributing to waste valorization and environmental sustainability.

1. Introduction

The increasing discharge of industrial effluents, particularly from the textile industry, has become a significant environmental concern due to the presence of toxic dyes and heavy metals [1]. Textile effluents contain high concentrations of synthetic dyes, which are resistant to biodegradation and can persist in water bodies for extended periods, leading to severe ecological imbalances. These pollutants are not only harmful to aquatic ecosystems, but also pose serious risks to human health due to their potential carcinogenic and mutagenic effects [2]. Among these dyes, crystal violet (CV) is widely used in textile dyeing, paper printing, and biological staining; however, its release into wastewater has raised significant environmental and health concerns [3,4]. CV is highly stable in aqueous solutions, making its removal from industrial effluents particularly challenging [5,6].

Several techniques have been investigated for the removal of CV from wastewater, including photodegradation [7,8,9], chemical oxidation [10,11], biological treatment [12,13], coagulation and flocculation [14,15], and adsorption [16,17,18,19]. Among the various treatment methods available, adsorption has gained considerable attention due to its high efficiency, ease of operation, and cost-effectiveness [5,20]. However, the high cost of conventional adsorbents—such as activated carbon—has driven the search for alternative, low-cost, and sustainable materials that can offer comparable adsorption performance while effectively addressing waste management challenges.

One such promising alternative is phosphogypsum (PG), an industrial by-product of the phosphoric acid production process that represents a promising candidate for adsorption applications. Composed primarily of calcium sulfate di-hydrate (CaSO₄·2H₂O), phosphogypsum is generated in large quantities worldwide—with approximately 300 million tons of PG produced annually—leading to significant disposal challenges and a need for effective utilization strategies [21]. Due to its unique physicochemical properties, including its porous structure, high surface area, and negative surface charge, recent studies have explored its potential as an adsorbent for the removal of heavy metals, such as cadmium, copper, and zinc, from aqueous solutions [22,23]. Additionally, phosphogypsum has been reported to remove various organic dyes from wastewater, including acid red and malachite green dyes [24,25]. These studies have highlighted its potential as an effective and sustainable adsorbent.

Despite these findings, to the best of the authors’ knowledge, no studies have investigated the use of phosphogypsum for the removal of cationic dyes, particularly crystal violet, from industrial wastewater effluents. Therefore, this study aims to (i) comprehensively evaluate the adsorption capacity of phosphogypsum for CV removal under varying operational parameters, such as pH, initial dye concentration, and temperature; (ii) analyze the associated adsorption kinetics, isotherms, and thermodynamics to understand the mechanism of interaction; and (iii) assess its practical feasibility by applying it to real textile effluents. The findings of this study are expected to contribute to the development of an eco-friendly, cost-effective, and efficient wastewater treatment strategy while promoting the valorization of industrial waste materials.

2. Materials

2.1. Phosphogypsum Sample

A representative sample of phosphogypsum (as a by-product of a phosphoric acid production process), received from Ma’aden Company, Riyadh, Saudi Arabia, was characterized through XRD, XRF, and particle size analyses.

2.2. Chemicals

Crystal violet (Basic Violet-3) dye of analytical grade (99.9% purity; Sigma-Aldrich, St. Louis, MO, USA and Merck, Rahway, NJ, USA) was used for the preparation of synthetic dye solution. Its chemical structure is C₂₅N₃H₃₀Cl, with a molar mass of 408 (see Figure 1). It is classified as a cationic dye with one positive charge. Crystal violet is an important dye in the textile and paper industries. It is a component of navy blue and black inks for printing, ball-point pens, and inkjet printers. Historically, it was the most common dye used in early duplication machines. It is used to color diverse products, such as fertilizer, antifreeze, detergent, and leather. A stock solution of 1000 mg/L crystal violet dye was prepared by dissolving one gram of it in 1 L of distilled water. Effluent from a local textile factory was employed for the application experiment.

3. Methods

3.1. X-Ray Diffraction (XRD)

XRD analysis was carried out using a Rigaku Ultima IV X-ray diffractometer to investigate changes in the crystallinity of the phosphogypsum before and after mechanical activation.

3.2. Particle Size and Zeta Potential Measurements

The particle size distribution of the phosphogypsum sample was measured using a laser particle analyzer liquid feed system (BT-2001 laser particle size analyzer, Hangzhou Chincan Trading Co., Ltd., Hangzhou, China). Zeta potential measurements were conducted using a zeta potential analyzer (Malvern instrument Co., Ltd., Worcestershire, UK), where 1.0 × 10⁻³ M KCl was used to adjust the ionic strength while 0.1 M HCl and NaOH solutions were used as pH regulators to assess the surface charge of phosphogypsum.

3.3. Adsorption Experiments

An adsorption experiment was conducted by adding one gram of phosphogypsum to a dye solution in a round flask. The flask was shaken at a speed of 200 rpm for the selected contact time in a thermostatic water bath. Then, the solid phosphogypsum was separated from the dye solution via centrifuging at 2000 rpm for 5 min. The concentration of the remaining dye in the solution was determined using an UV/Vis spectrophotometer at 590 nm as the maximum absorption wavelength for the crystal violet dye. The influences of parameters, such as pH, temperature, initial dye concentration, and contact time, were systematically studied. The dye concentration was determined using a colorimeter. The order, mechanism, and thermodynamic parameters were investigated. The removal efficiency and adsorption capacity were calculated based on the initial and final dye concentrations, as follows:

$$Removal \left(\%\right)={\displaystyle \frac{C_0-C_t}{C_0}}\times 100$$

(1)

$${Adsorption capacity q}_t={\displaystyle \frac{\left(C_0-C_t\right) V}{M}}$$

(2)

where C₀ and C_t are the initial dye concentration and that at time “t” (in mg/L), respectively; V is the volume of the solution (in L), and M is the mass of the adsorbent (in g).

4. Results and Discussion

4.1. XRD and XRF Analysis

Figure 2 shows the XRD pattern of phosphogypsum, revealing its crystalline structure. The strong peaks at specific 2θ values suggest the presence of calcium sulfate dihydrate (CaSO₄·2H₂O) as the dominant phase. The high-intensity peaks around 11°, 20°, 29°, and 31° confirm the well-defined crystallinity of phosphogypsum. Additionally, the minor peaks may correspond to impurities or secondary phases, which could influence its adsorption properties. These results are essential for understanding the material’s structural characteristics and its potential applicability as an adsorbent.

The XRF results (

Table 1. XRF analysis results for phosphogypsum.

Item	CaO	SO3	SiO2	P2O5	LOI	Others	Total
%	36.54	34.95	3.34	4.65	19.51	4.35	100

) indicate that the sample was primarily composed of 91% calcium sulfate dihydrate, accompanied by 3.34% silica and 4.65% P₂O₅.

4.2. Particle Size and Zeta Potential

The particle size distribution (PSD) curve of phosphogypsum (shown in Figure 3) consists of a cumulative distribution (blue curve) and a differential distribution (red curve). The cumulative curve follows a sigmoidal shape, indicating a well-defined particle size range, with most particles being finer than 100 µm. The differential curve peaks around 10–20 µm, suggesting that the majority of the sample fell within this size range. The unimodal nature of the distribution implies a relatively uniform particle size without multiple distinct fractions.

The fine-grained nature of the phosphogypsum sample can influence its behavior in adsorption applications.

The zeta potential of the surface charge influences the interaction of phosphogypsum with its environment in the solution, including the adsorption and desorption of ions, molecules, and other particles. The point of zero charge (PZC) is generally described as the pH at which the net electrical charge of the particle surface (i.e., adsorbent’s surface) is equal to zero. In particular, the PZC of a material can help to optimize its use as an adsorbent or catalyst [26].

The zeta potential results (Figure 4) reveal the distinct electrochemical behaviors of phosphogypsum. It exhibits two isoelectric points at pH 1.8 and 8.2 (indicating a neutral surface charge at these points), with a negatively charged surface within this range and maximum negativity at pH 5. This means that, within this pH range, phosphogypsum has a strong tendency to attract and adsorb cationic species, such as metal ions and positively charged dyes.

At pH values below 1.8 or above 8.2, the surface charge becomes neutral or shifts to positive, which could enhance the adsorption of anionic species instead. The presence of two IEPs suggests that multiple surface functional groups contribute to the charge behavior, likely due to the combination of calcium sulfate and phosphate species in phosphogypsum. Thus, pH plays a crucial role in the adsorption efficiency of phosphogypsum, influencing its surface charge and its interactions with different adsorbates. The physical adsorption of cationic species is expected to occur within the pH range from 1.8 to 8.2.

4.3. Adsorption Study

4.3.1. Effect of pH

The pH of the aqueous medium is one of the most important working parameters in the sorption process, as it can cause dissociation at the adsorbent sites and alter the solution chemistry of metal ions. Figure 5 presents the effect of pH on the sorption capacity and removal efficiency of the considered cationic dye by phosphogypsum. These experiments were carried out using an initial dye concentration of 50 mg/L, an adsorbent dose of 1.25 g/L, a contact time of 60 min, and a temperature of 30 °C.

As expected, the adsorption of cationic dye onto phosphogypsum was highly dependent on the pH of the solution. The adsorption capacity and removal efficiency increased as the solution pH increased to pH 5, then gradually decreased. Basically, the presence of excess H+ ions at a low pH competes with the cationic group of the dye, causing a decrease in the amount of dye adsorbed. This behavior can be explained in terms of the zeta potential results. However, the presence of a negative charge on the surface of phosphogypsum at a pH higher than 1.8 should favor the adsorption of the cationic dye. This behavior is inconsistent with the known fact that adsorption increases with increasing pH for cationic dyes. Therefore, an electrical double layer that maintains the electrical neutrality of the system has been suggested to explain the adsorption behavior of the cationic dye onto the adsorbent surface. The maximum negativity of the phosphogypsum surface occurred at pH 5, which enhanced the electrostatic attraction forces between the positive cationic dye and the negative adsorbent, thereby improving the sorption efficiency [27,28].

4.3.2. Effect of Initial Concentration

The impact of the initial dye concentration on the adsorption capacity is presented in Figure 6. The concentration of the adsorbate species significantly affects adsorption in practice [27]. The results showed that the adsorption capacity increased with increasing initial dye concentrations, while it only slightly increased with increasing initial dye concentrations higher than 200 mg/L and up to 400 mg/L.

Increasing the initial concentration increased the uptake capacity but decreased the removal efficiency. The observed decrease in the removal efficiency is due to the increasing amount of remaining dye species when increasing the initial concentration. A higher concentration means that more dye molecules are accessible and, hence, more molecules are adsorbed under a fixed amount of sorbent [28]. In this way, the driving forces to transfer molecules through the medium to the sorbent solid surface increase; thus, the sorbent is exposed to a greater amount of dye species, which steadily load the sites until they are saturated [29]. Adsorption isotherms is considered the optimal analysis method to describe sorption behaviors [30].

4.3.3. Analysis of Isotherm Models

In order to discuss the adsorbent–adsorbate interaction, three isotherm models were used to describe the equilibrium relationships between the adsorbent and adsorbate, consequently determining the ratio between the amount adsorbed and that remaining in the solution after a constant contact time of 60 min at room temperature. The equilibrium adsorption data were analyzed using the Langmuir, Freundlich, and Temkin isotherms.

The Langmuir isotherm [31] can be expressed as follows:

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

(3)

where C_f (mg/L) is the final dye concentration, q_t (mg/g) is the adsorbed dye amount at time t, q_max (mg/g) (maximum sorption) denotes the monolayer adsorption capacity, and b (L/mg) is a binding constant related to the free energy of adsorption. The Langmuir model adopts the idea that sorption occurs at a precise homogeneous surface of the adsorbent, where the dye species flow through the pores and the apertures of the lattice to replenish the substitutable dye species of the sorbent.

The Freundlich isotherm model [32] equation can be written as follows:

$${ln q}_t=ln k+{\displaystyle \frac{1}{n}}ln C_f$$

(4)

where Cf is the final dye concentration (mg/L), qt is the adsorbed amount of dye (mg/g), and K and n are constants related to the adsorption capacity and adsorption intensity, respectively. K is indicative of the extent of the sorption and n is the sorption intensity, whereas 1/n represents the sorption capability [33,34]. If n = 1, the barrier between the two phases is unaffected by concentration; if it is less than 1, it indicates typical adsorption; and, if it lies between 1 and 10, a favorable sorption process is indicated [35].

The Temkin isotherm model can be expressed as follows:

$$q_e=Bln{A}_T+Bln{C}_e$$

(5)

where B = RT/b is the heat of sorption constant (J/mol), b is the Temkin constant, A_T is the binding constant (L/g) obtained from the plot of qt against ln C_f, R is the universal gas constant (8.314 J/mol/K), and T is the temperature (in K). According to Temkin’s isotherm, adsorption is defined by an equal binding energy distribution up to a maximum binding energy, and the adsorption heat steadily decreases as the adsorbate molecules cover the adsorbent’s surface [36].

The sorption isotherms are presented in Figure 7 and a comparison of the isotherm constants in combination with the regression coefficients (R²) is provided in

Table 2. Parameters of Langmuir, Freundlich, and Temkin isotherm models.

Langmuir Isotherm Parameters
R2	0.9963
qmax (Calculated)	65.14
qmax (Experimental)	62.96
b	0.0189
Freundlich Isotherm Parameters
R2	0.8421
n	1.3268
KF	0.7507
Temkin Isotherm Parameters
R2	0.9307
B	52.14
AT	0.316

. The sorption was found to be best described by the Langmuir isotherm model, which assumes that sorption occurs at a specific homogeneous surface of the sorbent; that is, during the sorption process, dye molecules have to move through the pores and the channels of the lattice in order to replace the exchangeable cations of the sorbent. The regression coefficient (R²) of Langmuir model linear fitting was 0.9963 and, so, it also closely predicts the maximum sorption capacity with respect to the experimental results. On the other hand, the Freundlich model assumes a heterogeneous surface with a non-uniform distribution of heat of adsorption over the surface, and adsorption might occur as multiple-layer process. The value of n between 1 and 10 in the Freundlich model indicated that the dye was favorably adsorbed onto phosphogypsum at all initial dye concentrations. Furthermore, the low correlation coefficient (R²) of the Freundlich model’s linear fit was 0.8421, which suggests that although the adsorption process involves some degree of surface heterogeneity, the primary adsorption mechanism aligns more with monolayer adsorption, as described by the Langmuir model. As the sorption results obtained with the Langmuir model fit well, the sorption process is considered to be chemical sorption [28]. However, it is important to note that the surface composition of phosphogypsum may influence its adsorption efficiency, particularly if soluble components, such as phosphate ions, are released into the solution, potentially affecting the dye uptake behavior.

The Temkin isotherm model lies between the Freundlich and Langmuir models and, so, the Temkin isotherm could not be used to describe this adsorption process. Although the Temkin model is not suitable to describe the process considered, the value of the heat of sorption constant (B = 52 kcal/mol) indicated that chemical sorption occurred. Moreover, its positive value indicates that the sorption process is exothermic, which aligns with the observed decrease in adsorption efficiency at higher temperatures.

4.3.4. Effect of Contact Time

The effect of contact time at room temperature and up to 120 min was studied at a pH of 5. It can be deduced, from Figure 8, that the adsorption process comprised two phases: a primary rapid phase and a second slow phase. The first rapid phase lasted approximately 5 min, which was due to the readily available adsorbate cations (cationic dye) and a large number of vacant sorption sites on the surface of the adsorbent. However, from 5 to 60 min, the sorption rate gradually decreased as the available active sites became increasingly occupied, with the extent of sorption measured according to the number of cations transferred from the solution to these active sites. Consequently, adsorption continued to increase with time until saturation of the active sites was reached, at which point (i.e., at approximately 60 min) a plateau value was observed.

The higher initial rate suggests that adsorption initially occurs on the external surface, followed by penetration into the internal pores. Additionally, the larger sorption volumes observed in the initial period indicate more significant sorption on the external surface compared to within the pores. It was found that increasing the time from 60 to 120 min only increased the absorbed amount from 42 to 43 mg/g, while the removal efficiency increased from 52.8% to 54.5% [28].

4.3.5. Sorption Kinetics

The extent of the sorption of the dye by phosphogypsum was examined via the Lagergren pseudo-first-order (PFO) and pseudo-second-order (PSO) models. These models are shown in Figure 9 and were calculated using the following equations [37,38]:

$$\mathrm{Pseudo}\text{-}\mathrm{first}\text{-}\mathrm{order} \mathrm{equation}: \mathit{ln} (qe-qt)=\mathit{ln}qe -{ K}_{1}t$$

(6)

$$\mathrm{Pseudo}\text{-}\mathrm{first}\text{-}\mathrm{second} \mathrm{equation}: {\displaystyle \frac{t}{\mathit{qt}}} ={\displaystyle \frac{1}{k2q{e}^2}} + {\displaystyle \frac{1}{qe}} t$$

(7)

where q_t (mg/g) and q_e (mg/g) denote the amount of dye 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.

Their equivalent limits are listed in

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

Pseudo-First-Order Model Parameters		Pseudo-Second-Order Model Parameters
R2	0.8913	R2	0.9944
K1	−0.0432	K2	0.0009
Calculated qe	31.7	Calculated qe	46.4
Experimental qe	43.6	Experimental qe	43.6

. The PFO and PSO models have been widely applied to forecast values of the maximum sorption capacity, and the best fit was found when using pseudo-second-order model linear retrogression, based on its R² value being closer to unity. The kinetic fitting quality was observed to be in the subsequent order: PSO > PFO. Furthermore, the larger adsorption volume in the first period demonstrated higher sorption on the exterior surface, rather than in the pores [39].

4.3.6. Effect of Temperature

The solution pH was maintained at 5, the initial dye concentration was set at 100 mg/L, and the adsorbent dose was 1.25 g/L. As depicted in Figure 10, the removal efficiency and adsorption capacity are inversely proportional to temperature. This may be due to the increased mobility of the dye molecules and their escape from the solid phase (i.e., phosphogypsum) to the liquid phase. This behavior might arise from the weaker van der Waals and dipole forces, accompanied with the low heat of sorption. The decrease in the sorption capacity with rising temperatures may be due to the damage of active sorption sites or an increased tendency to desorb dye species from the interface to the solution [40]. The minimum sorption capacity occurred at 60 °C, indicates that the adsorption is an exothermic process.

4.3.7. Thermodynamic Study

In order to evaluate the spontaneity of the sorption process, thermodynamic parameters, such as enthalpy (ΔH°), entropy (ΔS°), and Gibb’s free energy (ΔG°), for the sorption processes were computed using the Van ’t Hoff equation [40,41]:

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

(8)

where kc = F/(1 − F) and F = (C_o − C_e)/C_o [42], R is the universal gas constant, and T is the temperature (in K).

By plotting 1/T against ln kc (Figure 11), the values of ΔH° and ΔS° were computed from the slope and the intercept of the plot of ln kc vs. 1/T, which yielded a straight line with an acceptable coefficient of determination (here, R = 8.314 J/mol K).

Table 4. Thermodynamic parameters.

Temp., °C	ΔH°, KJ·mol−1	ΔS°, J·k−1	ΔG°, J·k−1·mol−1
20	−72.7	−126	−796
30			−282
40			218
50			778
60			1355

shows that the value of ΔH° was negative, suggesting that the reaction was exothermic and its value was 72.7 kJ/mol; this indicates a chemically controlled process. Generally, the enthalpy change due to chemical sorption takes values between 40 and 120 kJ/mol. A negative value of ΔS° indicates the decrease in randomness at the solid/liquid interface during adsorption. Thus, the adsorption process is not favorable at higher temperatures.

The values of the standard Gibbs free energy change were estimated using the following equation [43]:

$$∆G°=-RT lnKc$$

(9)

The free energy change values (ΔG°) were negative at 20 °C and 30 °C, which indicates that this adsorption process is spontaneous in nature and no energy input from outside the system is required. On the other hand, the values of ΔG° were positive at 40 °C to 60 °C, indicating that this sorption process is not spontaneous in nature and energy input from outside of the system is required. Overall, the increase in the ΔG° value with increasing temperatures indicates that the adsorption process becomes less favorable at high temperatures.

4.3.8. Regeneration

The regeneration ability of an adsorbent is important for lowering the cost of the process and for the possible recovery of the pollutant extracted from wastewater. Desorption of the adsorbed dye (at 1.25 g/L of 100 mg/L dye solution, pH = 5, and 20 °C) was studied using 0.1 M HNO₃ solution for 15 min (1:10 solid/liquid ratio). The regeneration efficiency was reduced to 71% after the fifth reuse cycle (Figure 12).

4.3.9. Application to Real Sample

To evaluate the practical applicability of the developed adsorption process, real wastewater from a local textile dyeing factory was utilized. The wastewater was treated at the optimum conditions of 2.5 g/L adsorbent dosage, an initial dye concentration of 100 mg/L, pH 5, and a temperature of 2 °C for a contact time of one hour. The natural pH of the wastewater was 8.3; as such, it was adjusted to pH 5 using 0.1 M HCl solution prior to treatment. The textile wastewater contained a mixture of dyes, iron, and zinc, along with organic contaminants, such as biochemical oxygen demand (BOD)-related compounds.

Table 5. Evaluation of textile effluent before and after treatment.

Item	Unit	Effluent	Treated	Removal, %
pH	0–14	8.1	--	--
TDS	mg/L	2397	431	82.0
TSS	mg/L	185	28	84.9
COD	mg/L	614	129	79.0
BOD	mg/L	93	27	70.9
Fe	mg/L	4.1	0.65	84.1
Zn	mg/L	1.9	0.17	91.0
Color	%	37	5	86.5
Turbidity	NTU	246	49	80.1

details the values obtained for the evaluation of the treated wastewater. The lowest removal efficiency of about 71% was achieved for BOD (Bio-Oxygen Demand), while the highest removal efficiency of 91% was achieved for zinc ions. The coloring materials (dyes) were reduced to 13%.

Importantly, no noticeable leaching of hazardous components from phosphogypsum was observed in the treated effluent, further supporting its potential as a safe and effective adsorbent. However, additional stability studies are recommended to confirm its long-term environmental safety and assess any potential release of trace elements under different wastewater conditions.

4.3.10. Comparison Studies

To evaluate the performance of our adsorbent, we compared its adsorption capacity with other reported adsorbents for the removal of crystal violet dye, as detailed in

Table 6. The adsorption capacities of phosphogypsum and other reported adsorbents.

Adsorbent	Max Adsorption	Dye	pH	Reference
Eragrostis Plana Nees	76	Crystal Violet	8	[44]
Peanut Husk	21		2	[17]
Charred Rice Husk	62		10	[45]
Xanthated Rice Husk	90		10	[45]
Coconut Husk	54		5	[46]
Naturel Polysaccharide	32		10	[16]
Phosphogypsum	63		5	This work

. The natural phosphogypsum showed comparable adsorption capacity to the other adsorbents, most of which were prepared via chemical methods using expensive techniques.

5. Conclusions

This study successfully demonstrated the potential of phosphogypsum—a by-product of phosphoric acid production—as an effective adsorbent for crystal violet (CV) as an exemplary cationic dye. The phosphogypsum was found to consist of 91% calcium sulfate di-hydrate with 3.34% silica and 4.65% P₂O₅. It exhibited a negatively charged surface at a pH ranging from 1.8 to 8.2, with the maximum negativity at a pH of 5. The adsorption capacity increased with increasing pH up to 5 and higher initial dye concentrations, although its removal efficiency decreased at higher concentrations. The adsorption process was identified to follow the Langmuir isotherm model, indicating a monolayer chemical adsorption mechanism. Equilibrium was reached within 60 min, and the adsorption kinetics aligned well with the pseudo-second-order model, suggesting that adsorption primarily occurred on the external surface. The process was exothermic, as indicated by the inverse relationship between the adsorption capacity and temperature.

Thermodynamic analysis confirmed the spontaneity of adsorption at 20 °C and 30 °C, with negative ΔH° (exothermic nature), negative ΔS° (decreased randomness at the solid–liquid interface), and negative ΔG° values at lower temperatures. However, adsorption became less favorable at higher temperatures.

The reusability study demonstrated that the phosphogypsum could be regenerated and reused, with a 71% removal efficiency after five cycles. When applied to real textile effluent, phosphogypsum reduced the dye content by 87%, confirming its viability for use in industrial wastewater treatment.

Overall, these findings confirm that phosphogypsum is a cost-effective, efficient, and reusable adsorbent for the removal of cationic dyes, achieving this study’s objective of evaluating its adsorption performance for wastewater treatment applications. Future work can explore its long-term operational feasibility and use in large-scale implementations.