“Treating Waste with Waste”: Utilizing Phosphogypsum to Synthesize Porous Calcium Silicate Hydrate for Recovering of Fe²⁺ from Pickling Wastewater

Abstract

Phosphogypsum is a by-product of the wet-process phosphoric acid production, and it is rich in Ca and S. Long-term storage of Phosphogypsum can cause serious pollution to the environment; therefore, promoting the sustainable utilization of Phosphogypsum is crucial. This study proposes the use of Phosphogypsum and silicic acid in a sodium hydroxide solution for the hydrothermal synthesis of porous calcium silicate hydrate adsorbent, which is used for adsorbing Fe²⁺ from simulated hydrochloric acid pickling wastewater. Under the optimal synthesis conditions (37.5 g/L of NaOH, calcium/silicon ratio of 1.0, liquid/solid ratio of 15:1 mL/g, 110 °C, and 4 h), the conversion rate of SO₄²⁻ in Phosphogypsum is 87.41%. Porous calcium silicate hydrate exhibits excellent OH⁻ release capability in Fe²⁺-containing pickling wastewater. The adsorption process for Fe²⁺; is mainly chemical adsorption, involving ion exchange between Ca²⁺ and Fe²⁺, as well as complexation reactions of O-Si-O group, -OH group, and Si-O group with Fe²⁺. This technology aims to provide a solution for the sustainable utilization of Phosphogypsum and the recovery of Fe²⁺ from pickling wastewater, which has significant practical importance.

1. Introduction

Steel pickling wastewater is produced during the steel pickling process, which involves treating the surface of steel components to remove iron oxide scale, commonly using hydrochloric acid [1]. In recent years, China has been the world’s largest steel producer, generating millions of tons of pickling wastewater annually. This pickling wastewater is strongly acidic (pH value below 2.0). The industry typically employs acid–base neutralization methods to treat acidic wastewater. However, this results in sludge that is difficult to handle and causes significant waste of iron metals [2]. Various related treatment technologies have been reported. The spray roasting technology has a high iron recovery rate but requires strict temperature control and consumes a lot of energy [3]. The improved mineralization–neutralization technology effectively achieves acid treatment and iron recovery, but due to the complexity of the operation process, it is challenging to handle large volumes of wastewater [4]. The electrochemical technology has a significant treatment effect [5], but it is complex to operate and requires a high iron concentration. In recent years, adsorption has gained increasing attention as a simple and efficient method for treating heavy metal ions in wastewater [6,7,8]. Porous calcium silicate hydrate (PCSH) is a synthetic inorganic silicate with a high specific surface area, a well-developed pore structure, and stable chemical properties. It has good ion release and alkaline supply capabilities, and its preparation method is simple, making it a promising economic, efficient, and environmentally friendly heavy metal ion adsorbent [9,10,11]. The raw materials for preparing PCSH mostly come from solid waste, which not only achieves the resource utilization of solid waste to a certain extent, but also addresses the issue of solid waste storage [12,13,14].

Phosphogypsum (PG) is a solid waste residue produced in the wet-process phosphoric acid production and is mainly composed of CaSO₄·2H₂O. Currently, nearly 300 million tons of PG are produced globally each year, with China’s production exceeding 70 million tons [15,16]. It has been reported that approximately 5 tons of PG are generated for every ton of phosphoric acid produced. The annual accumulation of PG increases at a rate of 100 to 128 million tons, leading to severe problems such as heavy metal pollution, water eutrophication, and air pollution [17,18]. PG contains a large amount of Ca and S, and chemical purification techniques can convert it into a range of chemical products such as calcium sulfate whiskers [19], potassium sulfate [20], or ammonium sulfate [21]. Currently, the sustainable utilization of PG is a research hotspot. If the abundant calcium source in PG can be effectively utilized and converted into PCSH by reacting with a readily available silicon source (such as sodium silicate) [22], and if the conversion of important components can be enhanced during the reaction process to increase the added value of PG, this could potentially achieve the sustainable development of PG.

In this study, PG is used as the reactive material, by adding silicic acid and conducting hydrothermal synthesis of PCSH in a sodium hydroxide solution system. We perform a thermodynamic feasibility analysis of the synthesis process. The study mainly investigates the effects of hydrothermal conditions on the conversion rate of SO₄²⁻ in PG. Subsequently, PCSH prepared under optimal conditions is used to adsorb Fe²⁺ from simulated hydrochloric acid pickling wastewater. Kinetic models of the adsorption process are established, and the adsorption mechanism is analyzed. This study provides theoretical references for the industrial technological feasibility of “preparing PCSH from PG and removing Fe²⁺; from pickling wastewater”. The goal is to promote the sustainable resource utilization of PG and purify and recover iron-containing pickling wastewater, thereby achieving “waste treatment using waste”.

2. Materials and Methods

2.1. Materials

PG used in this study was sourced from a phosphate chemical plant in Guizhou Province. Prior to use, the washing process for PG involves placing it into a beaker containing deionized water, subjecting it to ultrasonic treatment for 10 min, and then filtering it. This operation is repeated twice, dried at 105 °C for 48 h, and then crushed, ground, and sieved through a 200-mesh screen. The silica acid (H₂SiO₃) and sodium hydroxide (NaOH) used were of analytical grade (Aladdin Biochemical Technology Company, Shanghai, China). Solutions of sodium hydroxide at various concentrations were prepared by diluting with deionized water (18.25 MΩ·cm, 25 °C) and then standardizing. The main chemical components of the washed PG were determined using X-ray fluorescence (XRF) spectroscopy, and the fluorine content was measured using an ion-selective electrode method, with results shown in

Table 1. Main chemical composition of PG (wt%).

Composition	SO3	CaO	P2O5	SiO2	Fe2O3	Al2O3	Na2O	K2O	F
Content	51.58	39.61	5.11	2.25	0.61	0.38	0.16	0.15	0.17

. It can be observed that the primary oxides in PG are sulfur trioxide and calcium oxide, with SO₃ accounting for 51.58 wt% and CaO for 39.61 wt%. This indicates that PG is rich in S and Ca elements. Additionally, P₂O₅ accounts for 5.11 wt%, and SiO₂ for 2.25 wt%. Furthermore, PG contains trace amounts of metal impurities, and F content is 0.17 wt%.

The XRD and SEM-EDS results of PG are demonstrated in Figure 1a–c, indicating that the main phases are calcium sulfate and calcium sulfate hemihydrate, along with minor amounts of calcium hydrogen phosphate and quartz. The inner surface of PG contains abundant Ca and S elements, with a generally consistent surface structure, and the distribution of each element is basically uniform.

2.2. Methods

2.2.1. Preparation of PCSH

The experimental process, as illustrated in Figure 1, primarily involves two processes: hydrothermal reaction and solid–liquid separation, conducted in a single reactor equipped with a mechanical stirring system. Initially, solid silica acid is mixed with the prepared sodium hydroxide solution and stirred for 15 min, PG is then added, and the mixture is stirred for an additional 5 min. The mixture is transferred to the single reactor and heated to a constant temperature until the reaction is complete. After the reaction, the pH of the slurry liquid is measured at room temperature. The slurry is subjected to ultrasonic treatment for 5 min before solid–liquid separation, which is repeated three times. The collected solid product is dried in an oven at 60 °C for 24 h and then crushed, ground, and sieved through a 200-mesh screen to obtain the PCSH product. The calcium/silicon ratio indicates the CaO/SiO₂ molar ratio of PG to silicic acid, and the liquid/solid ratio indicates the volume/mass ratio (mL/g) of sodium hydroxide solution to PG. The specific condition parameters are shown in Table S1.

After separation, the filtrate is diluted and made up to a 500 mL volume in a volumetric flask to measure the SO₄²⁻ content. The determination method for SO₄²⁻ employs “Barium Chromate Spectrophotometry” (HJ/T 342-2007) [23], and the details are shown in Text S1. The conversion rate of SO₄²⁻ in PG is calculated as follows (Equation (1)):

$$\mathsf{\phi }={\displaystyle \frac{{\mathrm{m}\left({\mathrm{S}\mathrm{O}}_4^{2-}\right)}_{\mathrm{E}\mathrm{x}\mathrm{p}}}{\mathrm{m}{\left({\mathrm{S}\mathrm{O}}_4^{2-}\right)}_{\mathrm{T}\mathrm{h}\mathrm{e}}}}\times 100$$

(1)

where φ refers to the mass percentage (%) of SO₄²⁻ in the filtrate to the total SO₄²⁻, m(SO₄²⁻)_Exp is the mass of SO₄²⁻ in the filtrate, and m(SO₄²⁻)_The is the total mass of sulphate in the PG.

2.2.2. Adsorption Experiment

The simulated pickling wastewater studied in the experiment primarily utilized iron powder and hydrochloric acid to prepare 1000 mg/L Fe²⁺; standard solution (the pH value of simulated pickling wastewater is approximately 2). The adsorption experiments and the dilution and volumetric processes use oxygen-free deionized water. Several centrifuge tubes were taken, 50 mL of a certain concentration of Fe²⁺ solution was added, and the solution pH was stabilized using hydrochloric acid/sodium hydroxide. A certain amount of PCSH was added into the centrifuge tubes, and they were quickly placed in a constant temperature shaker at 25 °C with a speed of 200 rpm to react for 12 h. After the reaction was complete, the stable pH value of the solution was measured, the supernatant was passed through a 0.22 μm organic filter membrane, and the Fe²⁺ concentration in the solution was measured. The determination method for Fe²⁺ uses the “O-phenanthroline Spectro-photometric Method” (HJ/T 345-2007 [24]) that measures absorbance at a wavelength of 510 nm.

2.3. Characterization and Analytical Methods

The pH of the slurry liquid or solution was determined using a pH meter (pH-3e, REX, Suzhou, China). The chemical composition of the samples was determined by X-ray fluorescence spectrometry (XRF); the samples were analyzed in their physical phase by X-ray diffraction spectroscopy (XRD) (continuous scanning: 5°/min), the morphology of the samples was observed by electron scanning electron microscopy (SEM), and the elemental distribution and content were determined using energy-dispersive X-ray spectroscopy (EDS). The specific surface area and average pore size of the synthesized products were tested using N₂ adsorption–desorption (BET). Fourier-transform infrared spectroscopy (FTIR) was used to analysis the types of functional groups on the surface of the product before and after adsorption. X-ray photoelectron spectroscopy (XPS) was used to analyze the surface elemental composition and valence states of the product before and after adsorption (Figure 2).

3. Results and Discussion

3.1. Synthetic Thermodynamic Analysis

Using the HSC Chemistry 10.3.4 software, the thermodynamic analysis of the hydrothermal synthesis of porous calcium silicate was conducted, incorporating all possible reactions from Equations (2)–(11):

H₂SiO₃(s) + OH⁻(aq) = H₃SiO₄⁻ (aq)

(2)

H₃SiO₄⁻(aq) + OH⁻(aq) = H₂SiO₄²⁻(aq) + H₂O(l)

(3)

H₂SiO₄²⁻(aq) + OH⁻(aq) = HSiO₄³⁻(aq) + H₂O(l)

(4)

HSiO₄³⁻(aq) + OH⁻ (aq) = SiO₄⁴⁻(aq) + H₂O(l)

(5)

2CaSO₄·0.5H₂O(s) + 4OH⁻(aq) = 2Ca(OH)₂(s) + 2SO₄²⁻ + H₂O(l)

(6)

CaSO₄(s) + 2OH⁻ (aq) = Ca(OH)₂(s) + SO₄²⁻(aq)

(7)

Ca(OH)₂(s) + H₃SiO₄⁻(aq) = CaSiO₃(s) + OH⁻(aq) + 2H₂O(l)

(8)

Ca(OH)₂(s) + H₂SiO₄²⁻(aq) = CaSiO₃(s) + 2OH⁻(aq) + H₂O(l)

(9)

Ca(OH)₂(s) + HSiO₄³⁻(aq) = CaSiO₃(s) + 3OH⁻ (aq)

(10)

Ca(OH)₂(s) + SiO₄⁴⁻(aq) + H₂O(l) = CaSiO₃(s) + 4OH⁻(aq)

(11)

The relationship between △rG/△rH and temperature is shown in Figure 3. In alkaline solutions, silicic acid easily reacts with OH− to form various free forms of silicate, which are the main sources of calcium silicate formed with calcium hydroxide (Equations (2)–(5)). Most reactions can proceed spontaneously at lower temperatures without additional energy consumption. From Equations (6) and (7) and △rG/△rH−T relationship curves, calcium sulfate or calcium sulfate hemihydrate minerals combine with OH⁻ and Na⁺ to form CaOH₂(s) and Na₂SO₄, and finally, Ca(OH)₂(s) combines with different forms of silicate to form a stable precipitate of CaSiO₃ (Equations (8)–(11) (Figure 4)).

3.2. Effect of Different Condition Parameters on SO₄²⁻ Conversion Rate in PG

Figure 5a and Figure 6a, respectively, show the SO₄²⁻ conversion rate in PG and the XRD patterns of the products at different NaOH concentrations. The results indicate that the SO₄²⁻ conversion rate increases with the increase in NaOH concentration, stabilizing at 37.5 g/L with a conversion rate of 87.41%. The pH changes significantly within the 35.0 g/L to 37.5 g/L range, providing the product with good alkalinity conducive to raising the pH of acidic wastewater. Therefore, 37.5 g/L is selected as the optimal NaOH concentration for hydrothermal synthesis. Higher alkaline concentrations mainly promote the conversion of calcium sulfate mineral phases to hydrated calcium silicate mineral phases (characteristic peaks at 29.355°, 32.053°, and 50.077°, PDF#33-0306), and a small amount of phosphorus converts to hydroxyapatite mineral phases (PDF#76-0694) and dicalcium phosphate mineral phases (PDF#26-0306), all of which are beneficial for adsorbing heavy metal ions [25,26].

Figure 5b and Figure 6b, respectively, show the SO₄²⁻ conversion rate in PG and the XRD patterns of the products at different calcium/silicon ratios. The results indicate that an increase in the calcium/silicon ratio enhances the SO₄²⁻ conversion rate. When the calcium/silicon ratio exceeds 1.0, the increase in the SO₄²⁻ conversion rate is minor, with the slurry pH value and SO₄²⁻ conversion rate showing a consistent trend. Therefore, a calcium/silicon ratio of 1.0 is chosen as the optimal ratio for hydrothermal synthesis. A lower calcium/silicon ratio results in unreacted calcium sulfate mineral phases, while at a calcium/silicon ratio of 1.2, the higher diffraction peaks indicate higher product crystallinity.

Figure 5c and Figure 6c, respectively, show the SO₄²⁻ conversion rate in PG and the XRD patterns of the products at different liquid/solid ratios. The results indicate that increasing the liquid/solid ratio from 10:1 to 15:1 significantly affects the SO₄²⁻ conversion rate, after which changes are minor. The slurry pH value and SO₄²⁻ conversion rate exhibit a consistent trend, making 15:1 the optimal liquid/solid ratio for hydrothermal synthesis. Corresponding XRD patterns show incomplete PG conversion at a 10:1 mg/L liquid/solid ratio.

Figure 5d and Figure 6d, respectively, show the SO₄²⁻ conversion rate in PG and the XRD patterns of the products at different reaction temperatures. The results indicate that the SO₄²⁻ conversion rate peaks at 110 °C due to higher temperatures, causing particle encapsulation on the material surface, thereby hindering the reaction [27,28]. Thus, 110 °C is selected as the optimal temperature for hydrothermal synthesis, with the slurry pH value and SO₄²⁻ conversion rate showing a consistent trend. At 180 °C, a tobermorite (PDF#40-0513) mineral phase appears, and due to its higher crystallinity, it is not conducive to providing adsorption active sites.

Figure 5e and Figure 6d, respectively, show the SO₄²⁻ conversion rate in PG and the XRD patterns of the products at different reaction times. The results indicate that the PG conversion rate maximizes at 4 h and then slowly decreases. Therefore, 4 h is chosen as the optimal reaction time for hydrothermal synthesis. The XRD diffraction peaks at different times are similar, with the presence of a dicalcium phosphate mineral phase at 4 h.

3.3. SEM and BET Characterization of PCSH

Figure 7a–o show SEM images of PCSH under different condition parameters. Figure 7a–c show that, at an NaOH concentration of 32.5–35.0 g/L, many unreacted materials are attached to the product surface, with no noticeable porous structure. As the NaOH concentration increases to 37.5 g/L, the product adopts an overall loose porous structure. Figure 7d–f show that, at lower calcium/silicon molar ratios, the surface is covered with many impurities. When the calcium/silicon ratio increases to 1.2, the product surface hardness increases, reducing the number of adsorption active sites. Figure 7g–i show that, at lower liquid/solid ratios, the reaction is incomplete, and many unreacted solid particles are attached to the product surface. As the liquid/solid ratio increases to 25:1, the fine particles on the surface gradually decrease, but the pore structure is not prominent. Figure 7j–l show that products synthesized at low temperatures exhibit a dense gel state. As the temperature increases, the surface porosity of the product becomes more apparent. When the temperature reaches 180 °C, the product surface structure hardens, mainly due to the formation of a highly crystalline tobermorite mineral phase, consistent with the XRD analysis results. Figure 7m–o show that, with an extended reaction time, the pore structure on the product surface becomes more apparent, which is beneficial for providing more adsorption active sites.

Figure 8a and

Table 2. Pore structure parameters of PCSH.

Material	Specific Surface Area (m2/g)	Average Pore Size (nm)	Pore Volume (cm3/g)
PCSH	93.5066	20.5658	0.4819

, respectively, show N₂ adsorption–desorption curves and pore structure parameters of PCSH under optimal hydrothermal conditions. The nitrogen adsorption–desorption curve of PCSH forms a good closed loop, indicating good desorption performance. The specific surface area of PCSH reaches 93.5066 m²/g, with an average pore diameter of 20.5658 nm, classifying it as a mesoporous structure. Figure 8b,c present the SEM-EDS images of PCSH. It can be observed that PCSH has well-developed pore structures, and the surface mainly contains Ca and Si and a small amount of P, with calcium content accounting for 26.86 wt% and silicon content accounting for 20.66 wt%, indicating a widespread distribution of porous calcium silicate. Subsequently, PCSH synthesized under optimal conditions will be used as an adsorbent for removing Fe²⁺ from simulated pickling wastewater.

3.4. PCSH Adsorbs Fe²⁺ from Simulated Pickling Wastewater

Figure 9a shows the effects of different initial concentrations on Fe²⁺ adsorption capacity. Adsorption conditions are 25 °C, pH = 4, and PCSH additive amount of 0.8 g/L, and as the concentration of Fe²⁺ increases, the adsorption capacity of PCSH for Fe²⁺ increases rapidly. When the Fe²⁺ concentration reaches 250 mg/L, the adsorption capacity is 309.54 mg/g. The subsequent increase in adsorption capacity gradually decreases. Therefore, 250 mg/L is considered the optimal initial concentration for subsequent experiments. Figure 9b shows the effect of different PCSH additive amounts and initial pH conditions on the removal rate of Fe²⁺. As the initial pH or PCSH additive amount increases, the removal rate of Fe²⁺ is promoted. At pH of 4 and PCSH additive amount of 0.8 g/L, the Fe²⁺ removal rate reaches a maximum of 99.05%. The equilibrium pH of the solution after adsorption significantly increases, with the pH value exceeding 6.0, indicating that PCSH has a good OH⁻ releasing capability [29], as shown in Figure 9c.

In actual pickling wastewater, the stored wastewater easily comes into contact with air or dissolved oxygen in water, causing Fe²⁺ to oxidize to Fe³⁺. As the pH increases, Fe³⁺ undergoes chemical precipitation to form Fe(OH)₃. Figure 9d shows the adsorption effects of PCSH on Fe²⁺ under different Fe³⁺; contents. Under the conditions of a solution pH of 4 and a PCSH dosage of 0.4 g/L, the removal effect of Fe²⁺ significantly improves with the increase in Fe³⁺ content. This is attributed to the adsorption performance of Fe(OH)₃, whose presence aids in the removal of Fe²⁺ [30].

3.5. Adsorption Kinetics

Figure 10a shows the relationship between Q_t of PCSH for Fe²⁺ and the time of adsorption. As the adsorption time increases, the adsorption capacity Q_t continuously increases, but the rate of increase gradually slows down after 60 min and basically reaches the maximum Q_t at 240 min. The experimental scatter plots were fitted using pseudo-first-order, pseudo-second-order, and Elovich kinetic models, with the corresponding equations given in Equations (12)–(14) [31,32,33].

$${\mathrm{Q}}_{\mathrm{t}}={\mathrm{Q}}_{\mathrm{e}}(1-{\mathrm{e}}^{(-{\mathrm{K}}_1\mathrm{t})})$$

(12)

$${\mathrm{Q}}_{\mathrm{t}}={\mathrm{K}}_2{\mathrm{Q}}_{\mathrm{e}}^2\mathrm{t}/(1+{\mathrm{K}}_2{\mathrm{Q}}_{\mathrm{e}}\mathrm{t})$$

(13)

$${\mathrm{Q}}_{\mathrm{t}}=\mathrm{a}+\mathrm{b}\mathrm{l}\mathrm{n}\mathrm{t}$$

(14)

Figure 10b–d and

Table 3. PCSH adsorption kinetics parameters for Fe²⁺.

Model	Concentration (mg/L)	R2	Qe (mg/g)	K1 (min−1)	K2	a	b
Pseudo- first-order	200	0.9198	233.71	0.0643	—	—	—
	250	0.8844	293.11	0.0953	—	—	—
	300	0.8386	328.45	0.1193	—	—	—
Pseudo- second-order	200	0.9819	256.77	—	0.00035	—	—
	250	0.9840	314.84	—	0.00048	—	—
	300	0.9769	349.69	—	0.00058	—	—
Elovich	200	0.9767	—	—	—	83.34	0.0235
	250	0.9569	—	—	—	411.02	0.0237
	300	0.9426	—	—	—	1391.66	0.0248

, respectively, show the fitted curves and kinetic parameters for the three kinetic models. The results indicate that the R² values of the fitted curves for the pseudo-second-order kinetic model are higher than those for the pseudo-first-order model and Elovich kinetic model, ranging from approximately 0.9769 to 0.9840, with a smaller fluctuation range and closer proximity to 1, which is markedly better than those of the pseudo-first-order and Elovich models. Therefore, the adsorption process of Fe²⁺ by PCSH is more consistent with the pseudo-second-order kinetic model, indicating that the adsorption process of PCSH for Fe²⁺ is mainly chemical adsorption [34].

3.6. Adsorption Mechanism Analysis

Figure 11a shows the XRD patterns of PCSH before and after Fe²⁺ adsorption. The adsorption products mainly include hedenbergite, iron phosphate, PCSH, and quartz, indicating that, besides PCSH participating in the Fe²⁺ adsorption process, hydroxyapatite and calcium hydrogen phosphate also contribute to Fe²⁺ adsorption. Figure 11b presents the FTIR spectra of PCSH before and after Fe²⁺ adsorption. After Fe²⁺ adsorption, the absorption peaks corresponding to the characteristic peaks of CaO at 1450–1490 cm⁻¹ significantly decrease [35], and the stretching vibration of the O-Si-O group at 875.78 cm⁻¹ is almost undetectable [36]. This suggests that, during the adsorption process, there is ion exchange between Ca²⁺ and Fe²⁺, and there are complexation reactions between O-Si-O groups and Fe²⁺. The antisymmetric stretching vibration of the adsorbed water molecule -OH group at 3448.55 cm⁻¹ and the antisymmetric stretching vibration of the Si-O group at 1048.94 cm⁻¹ show a significant shift [37,38], possibly due to the complexation reactions between Fe²⁺ and -OH groups and Si-O groups. Figure 11c,d are the XPS full spectrum and fine spectrum of PCSH before and after Fe²⁺ adsorption, respectively. PCSH mainly contains elements like Ca, Si, and O, and the presence of C is due to contamination by CO₂ during sample drying. In the XPS spectra, the energy peak of Ca2p significantly decreases after Fe²⁺ adsorption, and an Fe2p energy peak appears at a binding energy of 978.08 eV, indicating that ion exchange between Ca²⁺ and Fe²⁺ primarily occurs during the adsorption process, and this is consistent with the XRD and FTIR analyses. After adsorption, the O1s spectrum shows peaks at binding energies of 533.08 eV for Si-O-H groups, 532.08 eV for Si-O-Si groups, and 531.08 eV and 530.08 eV for Si-O-Fe groups, further verifying the complexation reactions between Si-O groups and -OH groups with Fe²⁺. Figure 11e shows the SEM images of PCSH after Fe²⁺ adsorption. It is evident that Fe²⁺ is significantly adsorbed onto the surface of PCSH. The morphology of the adsorption products still retains a certain surface area and porous structure, indicating a relatively stable structure.

Based on the above analysis, the mechanism of Fe²⁺ adsorption from simulated hydrochloric acid pickling wastewater by PCSH is proposed, as shown in Figure 12. PCSH itself contains abundant Ca-OH groups, releasing OH⁻ in solution: a part of Ca²⁺ undergoes ion exchange with Fe²⁺, and the released OH⁻ raises the pH of the solution. Due to the inevitable oxidation of Fe²⁺ to Fe³⁺ by traces of dissolved oxygen in the solution, the increase in pH promotes precipitation reactions between Fe³⁺ and OH⁻ to form Fe(OH)₃. The porous structure of PCSH can provide more adsorption sites, facilitating the substantial removal of Fe²⁺. The adsorption mechanism is primarily characterized by chemisorption, involving not only ion exchange between metals, but also complexation reactions between Fe²⁺ with -OH groups, O-Si-O groups, and Si-O groups. After Fe²⁺ adsorption by PCSH, Fe²⁺ mainly exists in the adsorption product in the form of hedenbergite.

4. Conclusions

In this study, a multifunctional adsorbent PCSH was synthesized via hydrothermal synthesis using PG, and it was utilized to remove Fe²⁺ from simulated hydrochloric acid pickling wastewater from the steel industry. By controlling the main reaction parameters in the “synthesis–adsorption” process, the transformation of important components during synthesis and the efficient recovery of Fe²⁺ were achieved. The main conclusions are as follows:

The hydrothermal reaction at a sodium hydroxide concentration of 37.5 g/L, calcium/silicon ratio of 1.0, liquid/solid ratio of 15:1, reaction temperature of 110 °C, and reaction time of 4 h resulted in conversion rate of 87.41% for SO₄²⁻ in PG. Under optimal synthesis conditions, the specific surface area of the product PCSH is 93.51 m²/g and an average pore diameter of 20.57 nm. At an initial Fe²⁺ concentration of 250 mg/L, an initial pH of 4, PCSH dosage of 0.8 g/L, and reaction time of 240 min, the maximum adsorption capacity of PCSH for Fe²⁺ was 309.54 mg/g, with a removal rate of 99.05%. The adsorption process for Fe²⁺ is mainly chemical adsorption, involving ion exchange between Fe²⁺ and Ca²⁺, as well as complexation reactions between Fe²⁺ and O-Si-O, Si-O, and -OH groups. The pH of the pickling wastewater significantly increased after adsorption, indicating that PCSH has an excellent OH⁻ releasing capability.