Bifunctional Adsorbents Based on Jarosites for Removal of Inorganic Micropollutants from Water

Abstract

This paper presents a novel family of jarosites with the molecular formula MFe₃(SO₄)₂(OH)₆·xH₂O; M = Na, K, NH₄ that have high efficiency in the adsorption of As(V) and Pb(II) dissolved in water. The jarosites have been prepared by conventional heating at temperatures close to 95 °C for 3 h. The synthesis method was improved and optimized to reduce the time and energy consumption. The improved conventional heating method allowed for the synthesis of Na− and K−jarosites with a yield of up to 97.8 wt.% at 105 and 150 °C, respectively, in 3 h. The Na−, K−, and NH₄−jarosites were synthesized at 150 °C in 5 min via a microwave-assisted method, which yielded jarosite crystalline agglomerates with more uniform topography, shape, and size than the conventional method. Both methods allowed the selective synthesis of jarosites. Chemical decomposition of jarosites suspended in water occurred at a pH less than 2 and higher than 10 and temperatures up to 150 °C. In the solid state, the jarosites were thermally stable at least to 300 °C. The Na−jarosite presented a maximum adsorption capacity (Q_max) of 65.6 mg g⁻¹ for As(V) and 94 mg g⁻¹ for Pb(II). The jarosites are considered promising bifunctional adsorbents for the remediation of contaminated water due to their improved synthesis method, stability, and high adsorption capacity for ions of different natures.

1. Introduction

Contamination of groundwater by arsenic (As), fluoride (F⁻) and cations of heavy metals has been recognized as one of the main problems people face in arid and semi-arid regions of the world, where groundwater is the primary source of drinking water [1,2,3]. It has been found that in surface water bodies, the average values of heavy metals such as Cr, Mn, Fe, Co, Ni, As and Cd exceed the guidelines established by the World Health Organization (WHO) or the United States Environmental Protection Agency (USEPA) for drinking water. The maximum desirable contents recommended by the WHO of F⁻, As, Zn, Pb and Cd in drinking water are 1500, 10, 5000, 10, and 5 μg L⁻¹, respectively [4,5]. In addition, both groundwater and surface water can be contaminated with other metals, organic compounds, and agrochemicals [6].

Electrocoagulation, adsorption, and phytoremediation are the most efficient and cost-effective technologies for removing micropollutants from water. Adsorption is particularly noteworthy because it is a simple, fast, easily scalable, and highly efficient process for reducing the concentrations of dissolved contaminants in the water to the levels recommended by the WHO [7,8]. There are emerging adsorbents for water microcontaminants, including zeolitic imidazole frameworks (ZIFs) [9], graphene oxide and metal–organic framework (MOF)-based composite materials [10], mesoporous organosilica (MPOS) [11,12], and zeolites [13]. Zeolites are aluminosilicates with periodic cage arrangements and channels of defined shapes and sizes, making them effective cationic exchangers. They can also be functionalized to adsorb anionic species [13,14,15]. To remove F⁻, the use of carbon-based adsorbents, nano-adsorbents, bio-sorbents, inorganic materials, and composite adsorbents has been reported [16]. The proposed adsorbents to separate heavy metals dissolved in water include carbon-based materials, polymers, metals and metal compounds, minerals, industrial and agricultural waste, and functionalized mesoporous materials [17]. When contaminants of different natures are present in the water effluent, it has been recommended to use a bimetallic mixture combined with a substrate or support that can be synthetic or natural [18].

Many of the proposed adsorbents are limited to eliminating only one of the micropollutants from the water [19]. Some of them are not regenerable, so the aged adsorbents become secondary pollutants, and most of the best adsorbents reported are neither practically nor financially viable for use in underdeveloped countries [7]. Therefore, the challenge of developing adsorbent materials is to prepare multipurpose, regenerable, and technically and economically viable adsorbents to be produced and applied on a big scale in water remediation.

Jarosites are a coordination compound family with the molecular formula AFe₃(SO₄)₂OH₆·xH₂O, where A = Na, K, NH₄ and x is the number of water molecules [20]. The Fe acts as the central atom, and the sulfate (SO₄) and hydroxyl (OH) groups are the ligands. The origin of jarosites can be in natural or synthetic forms. Synthetic jarosites can be obtained via biological and chemical methods [21]. Chemical methods typically use conventional heating, low temperature (60–100 °C), long time (3–24 h), and low pH (1.1) [22]. Jarosites have been obtained for enhancing the retrieval procedure for zinc and high-value metals such as silver, gold, or copper from industrial waste in the mining and metallurgical fields [23]. The development of scalable procedures for manufacturing jarosites that can be applied in any production process has received limited attention. In Table 1 are summarized some of the typical methods reported to synthesize jarosites. The scientific community has scarcely explored the microwave-assisted synthesis of jarosites [24,25]. The crystallization parameters, such as the source of iron, reagent ratio, stirring speed, pH, temperature, and pressure, have significantly affected the morphology and the particle size of the jarosite [20]. The jarosites and their chemical and thermal decomposition products have been barely studied as anion adsorbents of metals [22,26].

This research hypothesized that the cation A and the ligands SO₄ and OH of the jarosite family can be exchanged for heavy metal cations such as Pb(II), arsenate (AsO₄³⁻) and fluoride, respectively. The primary goal of this research was to develop scalable methods to obtain multifunctional adsorbents that can efficiently remove inorganic water microcontaminants. Therefore, the Na−, K− and NH₄−jarosites were prepared using faster hydrothermal methods than those reported in the literature. Well-known techniques were used to characterize and determine the jarosites’ adsorption potential for removing inorganic micropollutants from water [9,15,27]. Particular emphasis was placed on using the Langmuir model, considering that the adsorption of micropollutants is carried out in a monolayer on the homogeneous surface of the jarosite with a finite number of metallic ligands A (Na, K, NH₄) and sulfate groups (SO₄). Theoretically, these ligands are substituted for Pb(II) and the arsenate group, respectively, and the resulting lead arsenojarosites would have negligible interactions with the Pb(II) ions and AsO₄³⁻ present in the solution.

Table 1. Reported methods for the synthesis of A-jarosites; A = Na, K, Tl, Pb, Cu and Zn.

M−Jarosites	Reagents	Conditions	Reference
K−jarosite	1.66 g Fe2SO4; 0.2 g HNO3; 40 mL H2O	100 °C; 12 h	[28]
Tl−jarosite	9.9 g Fe2SO4; 1.5 g Tl2 (SO4); 1000 mL H2O	94 °C; 24 h	[29]
Na−jarosite	17.2 g Fe2SO4; 5.6 g NaOH; 100 mL H2O	95 °C; 4 h	[28]
Pb−jarosite	17.2 g Fe2SO4; 0.9 g Pb (NO3)2; 100 mL H2O
As−jarosite	17.2 g Fe2SO4; 14 g AsO4; 100 mL H2O
As−jarosite	1.5 g Fe2SO4; 0.14 g FeAsO3; 100 mL H2O	100–120 °C; 3–10 h	[30]
K−jarosite	12.5 g Fe2SO4; 39 g K2SO4; 500 mL H2O	95 °C; 3 h	[31]
Na−jarosite	6.25 g Fe2SO4; 13 g Na2SO4; 200 mL H2O; 0.25 mL H2SO4
K−jarosite	17.2 g Fe2SO4*5H2O; 5.6 g KOH; 100 mL H2O	95 °C; 4 h	[32]
K−jarosite	4.10 g Fe2(SO4)3; 0.27 g K2SO4	95 °C; 4h	[33]
Na−jarosite	4.10 g Fe2(SO4)3; 0.27 g Na2SO4
Na−jarosite	12.2 g Fe2(SO4)3; 4.0 g NaOH; 100 mL H2O	60–95 °C; 24 h	[33]
K−jarosite	52 g Fe2SO4; 11.2 g KOH; 1000 mL H2O	95 °C; 4 h.	[34]
PbAs−jarosite	0.054 M Fe2(SO4)35 H2O; 0.00946 M H3AsO4; 1000 mL H2O	95 °C; 4 h	[35]
PbCu−jarosite	0.054 M Fe2(SO4)35 H2O; 0.315 M CuSO45H2O; 1000 mL H2O
PbZn−jarosite	0.054 M Fe2(SO4)35 H2O; 0.306 M ZnSO4; 1000 mL H2O
KAs−jarosite	0.34 M Fe2(SO4)3; 0.2 M K2SO4; 0.2 M KH2AsO3; 0.11 M H2SO4	94 °C; 24 h.	[36]
NaV−jarosite	0.3 M VCl3; 0.6 M Na2SO4; 35 mL H2O	150 °C; 5 h	[37]
K−jarosite *	0.4 M Fe 2(SO4); 0.6 M Na2SO4; 35 mL H2O	225 °C; 30 min	[36]

* For the synthesis of these jarosites, the microwave-assisted method was used.

Table 1. Reported methods for the synthesis of A-jarosites; A = Na, K, Tl, Pb, Cu and Zn.

M−Jarosites	Reagents	Conditions	Reference
K−jarosite	1.66 g Fe2SO4; 0.2 g HNO3; 40 mL H2O	100 °C; 12 h	[28]
Tl−jarosite	9.9 g Fe2SO4; 1.5 g Tl2 (SO4); 1000 mL H2O	94 °C; 24 h	[29]
Na−jarosite	17.2 g Fe2SO4; 5.6 g NaOH; 100 mL H2O	95 °C; 4 h	[28]
Pb−jarosite	17.2 g Fe2SO4; 0.9 g Pb (NO3)2; 100 mL H2O
As−jarosite	17.2 g Fe2SO4; 14 g AsO4; 100 mL H2O
As−jarosite	1.5 g Fe2SO4; 0.14 g FeAsO3; 100 mL H2O	100–120 °C; 3–10 h	[30]
K−jarosite	12.5 g Fe2SO4; 39 g K2SO4; 500 mL H2O	95 °C; 3 h	[31]
Na−jarosite	6.25 g Fe2SO4; 13 g Na2SO4; 200 mL H2O; 0.25 mL H2SO4
K−jarosite	17.2 g Fe2SO4*5H2O; 5.6 g KOH; 100 mL H2O	95 °C; 4 h	[32]
K−jarosite	4.10 g Fe2(SO4)3; 0.27 g K2SO4	95 °C; 4h	[33]
Na−jarosite	4.10 g Fe2(SO4)3; 0.27 g Na2SO4
Na−jarosite	12.2 g Fe2(SO4)3; 4.0 g NaOH; 100 mL H2O	60–95 °C; 24 h	[33]
K−jarosite	52 g Fe2SO4; 11.2 g KOH; 1000 mL H2O	95 °C; 4 h.	[34]
PbAs−jarosite	0.054 M Fe2(SO4)35 H2O; 0.00946 M H3AsO4; 1000 mL H2O	95 °C; 4 h	[35]
PbCu−jarosite	0.054 M Fe2(SO4)35 H2O; 0.315 M CuSO45H2O; 1000 mL H2O
PbZn−jarosite	0.054 M Fe2(SO4)35 H2O; 0.306 M ZnSO4; 1000 mL H2O
KAs−jarosite	0.34 M Fe2(SO4)3; 0.2 M K2SO4; 0.2 M KH2AsO3; 0.11 M H2SO4	94 °C; 24 h.	[36]
NaV−jarosite	0.3 M VCl3; 0.6 M Na2SO4; 35 mL H2O	150 °C; 5 h	[37]
K−jarosite *	0.4 M Fe 2(SO4); 0.6 M Na2SO4; 35 mL H2O	225 °C; 30 min	[36]

* For the synthesis of these jarosites, the microwave-assisted method was used.

With the use of jarosites as an adsorbent, the secondary contamination by iron ions of the water resulting from the separation of the micropollutants is negligible because the solubility in water of the jarosite is limited to a relatively narrow range of strongly acidic or basic conditions. As the pH increases or decreases, the jarosite is transformed into ferric oxyhydroxide insoluble in water. Small jarosite solubility product values (log Ksp) have been reported, indicating they are poorly soluble. For example, log Ksp values = −8.4 ± 0.7 have been reported at a pH between 1 and 3 and a temperature of 4–35 °C [33]. The significantly low solubility of the jarosite at a pH between 3 and 9 makes it possible to use it in the potabilization of underground water, for example. Experimental adsorption trials using jarosites demonstrated they are highly efficient bifunctional adsorbents for separating anionic species such as As(V) and cationic species such as Pb(II) metallic species from water.

2. Materials and Methods

For the synthesis of the jarosites, iron salts were used or hydroxides of the A cation; A = Na, K or NH₄ were used. KOH with a purity of 88 wt.% from J.T. Baker™ (Phillipsburg, NJ, USA), NaOH with a purity >99 wt.% supplied by Aldrich™ (Burlington, MA, USA), (NH₄)₂SO₄ and Na₂SO₄ at 99 wt.% purity from Jalmek™ (San Nicolas de los Garza, Mexico), and FeCl₃ and Fe₂(SO₄)₃ with 73 wt.% purity from Jalmek™ brand (San Nicolas de los Garza, Mexico) were used. Additional chemicals from Sigma Aldrich™ were used as purchased. Deionized water was used in all the experiments. The jarosites were synthesized using alternative raw materials via the conventional hydrothermal crystallization method (jarosites 1) or microwave-assisted crystallization (jarosites 2). Figure 1 depicts a schematic illustration of the ongoing research.

The solid A-jarosites were analyzed using various techniques, including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electronic microscopy (SEM), thermogravimetric analysis (TG), particle size analysis, and finally, the capacity to adsorb As(V) and Pb(II) was determined.

2.1. Conventional Hydrothermal Synthesis of Na− and K−Jarosites 1

For the synthesis of Na−jarosite 1, 12.32 g of 73 wt.% Fe₂(SO₄)₃ was dissolved in 35 mL water. To the resulting solution, a second solution was slowly added and with gentle stirring, which was prepared by dissolving 3.96 g of NaOH in 35 mL of water. The resulting suspension was transferred to the Teflon™ vessel of a 100 mL stainless steel Parr™ reactor, which was hermetically sealed and placed in an oven previously heated to 105 °C and left to react for 3 h. Upon completion of the specified duration, the mixture was permitted to cool. Next, the solid was filtrated and subjected to three consecutive washings with 75 mL of water. Drying occurred for 24 h at 110 °C, and the resulting product was labeled Na−jarosite 1. For the K−jarosite 1 preparation, the methodology employed entailed employment of 5.77 g of 98 wt.% KOH instead of the Na precursor, along with raising the crystallization temperature to 150 °C for 3 h. The Na−jarosite 1 and K−jarosite 1 were obtained with a yield of 97.8 and 99.3 wt.%, respectively. NH₄−jarosite 1 was not studied from the reaction system analogous to that used to synthesize Na− and K−jarosite 1, Fe₂(SO₄)₃: AOH: H₂O; M=Na, K, NH₄, because under the studied reaction conditions, the NH₄OH decomposes into H₂O and NH₃.

2.2. Microwave-Assisted Synthesis of Na−, K− and NH₄−Jarosites 2

For the microwave-assisted synthesis of jarosites, FeCl₃ and the equivalent amount of Na₂SO₄, K₂SO₄, or (NH₄)₂SO₄ were used, respectively. To prepare Na−jarosite 2, 8.1 g of FeCl₃ with 73 wt.% purity and 8.52 g of Na₂SO₄ with 99.9 wt.% purity were separately dissolved in 35 mL of water, respectively. Both solutions were slowly mixed and with gentle stirring. The resulting suspension was transferred to a 100 mL Teflon™ reactor, and an additional 5 mL of water was added. The reactor was sealed and placed in a MARS™ brand model 3100 microwave reactor. The reaction system was heated to 150 °C at a rate of 10 °C min⁻¹, and this temperature was maintained for 5 min with 800 Watts of power microwave radiation. Finally, the reactor was cooled, and the obtained solid was recovered by filtration and washed three times with 75 mL of water at 85 °C. The solid was dried at 110 °C for 24 h. K−jarosite 2 and NH₄−jarosite 2 were prepared in the same way. Finally, Na−, K− and NH₄−jarosites 2 were obtained in yields of 2.74 g (35 wt.%), 3.13g (40.07 wt.%) and 6.66 g (87.6 wt.%), respectively.

2.3. Adsorption Potential of Inorganic Water Micropollutants Presented by Jarosites

To assess the potential of jarosites as bifunctional adsorbents capable of removing both metalloid anions (e.g., As(V)) and heavy metal cations (e.g., Pb(II)), their adsorption behaviors were examined via kinetic and isotherm studies using typical Mexican groundwater quality data [38]. Na₂HAsO₄ 7H₂O (spectrum, 98.0–100 wt.%) was used to prepare an As(V) solution with a concentration of 750 µg L⁻¹, a level commonly observed across groundwaters worldwide. A 100 ppm solution of Pb(II) was prepared for the cation adsorption study. Next, 0.1631 g Pb(NO₃)₂ with 99.99 wt.% purity were dissolved in 150 mL of water. The resulting solution was transferred to a volumetric flask and made up to 1000 mL with water.

To determine the adsorption at equilibrium (Q_t) and the percentage of removal, Equations (1) and (2) were used:

Q_t = (C₀ − C_t) V/m

(1)

Percentage removal, % = ((C₀ − C_t)/C_t) × 100

(2)

C₀ and C_t are the initial and final concentrations of adsorbate at the time t (mg L⁻¹), respectively, V is the volume of solution (L) and m is the amount of adsorbent (g). Langmuir’s mathematical model, as represented by Equation (3), was applied to determine the maximum adsorption capacity for the adsorbate (Q_max) presented by the jarosites.

1/q_e = 1/q_m + 1/q_m KLC_e

(3)

In Equation (3), q_e is the quantity of adsorbate adsorbed at equilibrium (mg g⁻¹), q_m is the maximum theoretical amount of adsorbate adsorbed at equilibrium, C_e is the concentration at equilibrium in the liquid phase (mg L⁻¹) and KL is the adsorption equilibrium constant (L mg⁻¹). In a 1/C_e vs. 1/q_m graph, the intercept point on the ordinate axis corresponds to the inverse of the maximum adsorption capacity of the adsorbent under study (1/q_m) [39].

2.3.1. As(V) Adsorption Kinetics on Jarosites

In a 0.5 L Erlenmeyer flask, 0.25 L of aqueous As(V) solution with a concentration of 750 μg L⁻¹ was placed, and the pH was adjusted to 7 using 0.1 M NaOH and 0.1 M H₂SO₄. Then, 0.25 g of the Na−, K−, or NH₄−jarosite was added to the solution. The resulting suspension was kept in an orbital incubator for 24 h with gentle shaking at 150 rpm and 25 °C. Samples of 10 mL were taken more frequently during the first 6 h to monitor the arsenic content. Next, the solid was separated using a 0.45 μm pore-size membrane filter, and the solution was analyzed by means of the inductively coupled plasma (ICP).

2.3.2. Arsenate Adsorption at Equilibrium on Jarosites

To ensure the equilibrium state and obtain the maximum adsorption capacity of As(V) presented by the jarosites, solutions with As(V) concentrations of 0.5, 1, 2, 5, 10, 20, 50, 100, and 200 mg L⁻¹ were made and the experimental jarosites were tested. The general procedure consisted of adjusting the pH of 0.020 L of each solution, adding 0.02 g (m) of the corresponding jarosite, and homogenizing. The suspension was kept at 25 °C for 24 h with stirring equivalent to 150 rpm. Finally, the liquid was filtered off and analyzed using an ICP spectrometer (Perkin-Elmer model OPTIMA 8300, Shelton, CT, USA).

2.3.3. Pb(II) Adsorption Kinetics on Jarosites

To determine the Pb(II) adsorption capacity, 0.25 g of the adsorbent and 250 mL of the solution with 100 mg L⁻¹ of adsorbate were placed in a 500 mL Erlenmeyer flask. The pH of the initial suspension was adjusted to 3 with 0.1 M NaOH and 0.1 M H₂SO₄ solutions. The adsorbent–adsorbate system was placed in an orbital incubator. Stirring equivalent to 150 rpm and a temperature of 25 °C were set. Aliquots of 15 mL were taken at the following times: 1, 2, 3, 5, 10, 15, 20, 60, 240, 360 and 1440 min. The recovered aliquots were filtered by gravity in the first stage and through a membrane filter with a pore size of 0.45 µm in the second stage. Finally, the filtrate obtained from each sample was labeled and analyzed using an atomic absorption spectroscopy (AAS) spectrometer (Thermo Scientific, model ICE-3300, Waltham, MA, USA).

2.3.4. Effect of pH and Coexistent Ions on As(V) Removal by Na−Jarosite

To evaluate the effects of anions commonly found in groundwater on the removal capacity of As(V) presented by Na−jarosite 1, 20 mL of a solution with 1 mg L⁻¹ of As(V) was used, and the amount of the corresponding salt was sufficient to have a concentration of 50 mg L⁻¹ of sulfates (SO₄²⁻), carbonates (CO₃²⁻) and nitrates (NO₃¹⁻). To the resulting solution, 0.02 g of Na−jarosite 1 was added. The sorbent–adsorbate system was placed in an orbital incubator and left shaking at 150 equivalent rpm for 24 h at 25 °C. Finally, the liquid was separated via filtration and the As(V) content was analyzed via ICP.

To determine the effect of the pH on the removal capacity of As(V) presented by Na−jarosite 1, the pH of 20 mL samples of the solution containing 1 mg L⁻¹ of As(V) was adjusted to 3, 4, 5, 6, 7, 8, 9 and 10 using NaOH and 0.1 M HCl. Next, 200 mg of Na−jarosite 1 was added to each of the resulting solutions. The suspension was placed in an orbital incubator and stirred at 150 rpm at 25 °C for 6 h. Then, the solution was filtered, and the As(V) content was determined in the filtrate by means of the ICP.

3. Results and Discussion

3.1. Jarosite Characterization

The Na−jarosite 1 and K−jarosite 1 synthesized via the improved conventional method developed in this research were analyzed using an XRD Xpert-Philips PW3040 (PHILIPS/PANALYTICAL, Almelo, Netherlands) diffractometer under the same conditions. Thus, the Na−jarosite 2, K−jarosite 2 and NH₄−jarosite 2 obtained via microwave-assisted hydrothermal crystallization were also analyzed. The results obtained are shown in Figure 2. All the jarosites present the XRD pattern established in the crystallographic charts corresponding to each jarosite and are consistent with the XRD patterns already reported [22]. A slight shift in some of the XRD peaks distinguishes the Na−, K− and NH₄−jarosites.

The intensity of the diffraction peaks presented by Na−jarosite 2 is higher than that shown by the diffraction peaks of Na−jarosite 1. The same behavior is observed in the case of K−jarosite 2 and K−jarosite 1. As the intensity of the diffraction peaks is directly proportional to the crystallinity of the jarosite obtained, it is possible to conclude that microwave-assisted crystallization allows more crystalline jarosites than those obtained via conventional heating.

The FTIR analysis (Figure 3a) confirmed that the Na−, K− and NH₄−jarosites obtained through both conventional and microwave-assisted methods were unique crystalline products. The broad band of weak intensity observed at 3359.8 cm⁻¹ in the spectrum of the Na−jarosite was attributed to the movement of tension of the O–H bond, corresponding to both the water of crystallization and the O–H bond in the Fe–O–H fragment. On the other hand, the three bands of low, medium, and high intensity at 993.6, 1071.9 and 1174.2 cm⁻¹ were attributed to different vibration modes (symmetric stress and asymmetric and bending stress) of the S–O bond in the SO₄²⁻ ion. The results agree with the data described in the literature [33,40]. The FTIR spectrum of NH₄−jarosite 2 (Figure 3b) shows the NH₄−jarosite 2 characteristic bands at 1420.6, 1116.3, 1072.7, 952.9 and 625.6 cm⁻¹. The band at 1420.6 cm⁻¹ corresponds to the vibration of the N–H bond of the ammonium group. The slight shift in some of the bands of the different jarosites can be attributed to the nature of the cation that compensates for the charge of the jarosite structure.

The obtained jarosites were prepared in static systems. Na−jarosite 1 and K−jarosite 1, which were prepared via conventional heating, presented a bimodal particle size distribution (PSD) in which 90% of the particles had a size smaller than 83.8 and 65.6 µm, respectively, and their average particle size (APS) was 13.8 µm and 12.9 µm, respectively. In contrast, the Na−, K− and NH₄−jarosites 2 synthesized via the microwave-assisted process had a monomodal PSD with an APS of 18.3, 10.5 and 7.1 μm, respectively. In Figure 4 and

Table 2. Particle size distribution of Na−, K−, and NH₄−jarosites synthesized via different processes *.

Synthesis Method	Typical Heating		Microwaves-Assisted Crystallization
	Na−Jarosite 1	K−Jarosite 1	Na−Jarosite 2	K−Jarosite 2	NH4−Jarosite 2
D90, %	65.6	83.8	48.3	36.5	16.2
D50, %	12.9	13.8	18.3	10.5	7.1
D10, %	3.9	1.3	10.3	5.9	3.2
D90/D10	16.82	64.46	4.69	6.18	5.06

* Typical heating (jarosites 1) and microwave-assisted crystallization (jarosites 2) methods.

, it can be observed that the jarosites prepared via conventional heating (jarosites 1) presented larger PSD than the jarosites prepared via microwave-assisted crystallization (jarosites 2), which had a D90/D10 ratio near 1. These differences can be attributed to the crystallization conditions under which the jarosites were obtained since the production method could influence its ultimate appearance. This trend may persist amongst other materials too [41]. While the Na− and K−jarosites 1 were obtained at 105 °C and a reaction time of 3 h, the Na−, K− and NH₄−jarosites 2 were obtained at 150 °C in 5 min.

The crystallization of the Na−, K− and NH₄−jarosites was also monitored by means of SEM with a Philips model JEOL 7800 prime microscope (Akishima, Japan). Figure 5 includes the micrographs of the jarosites synthesized under different crystallization conditions. Figure 5a corresponds to the Na−jarosite (JAR) obtained as a by-product of the industrial process for producing electrolytic zinc. Figure 5b,c correspond to Na−jarosite 1 and K−jarosite 1, which were obtained via conventional heating. Finally, Figure 5d–f correspond to the Na−, K− and NH₄−jarosites obtained via microwave-assisted crystallization. The Na−jarosites obtained via the conventional crystallization method (JAR and Na−jarosite 1) were obtained as crystalline agglomerates of irregular shape and size. K−jarosite 1 was obtained as a pseudo-spherical agglomerate of a regular size of less than one micrometer.

The jarosites obtained via microwave-assisted synthesis (150 °C, 5 min) depicted in Figure 5d–f have a larger particle size than those obtained via conventional heating at 105 °C for 3h. As shown in Figure 5a–c, the Na−, K− and NH₄−jarosites 2 consist of large crystalline agglomerates of different shapes, sizes and topographies. Na−jarosite 2 has rounded vertices, while K−jarosite 2 has well-defined angular vertices. Another notable finding is that the NH₄⁺ counterion also influences the arrangement of molecules to form monocrystals. Thus, NH₄−jarosite 2 comprises single crystals larger than one micrometer, as shown in Figure 5d.

According to the results described here, it is possible to affirm that the physical and chemical characteristics with which jarosites are obtained depend both on the crystallization conditions and the chemical composition of the reaction mixture. This is an advantage for this family of materials because it is possible to design products such as adsorbents and ion exchangers according to the needs of a given process.

To apply jarosites as an adsorbent for water micropollutants, they must be chemically stable over a wide pH range. Although the adsorption process occurs at around a pH equal to 7 units, the regeneration of saturated adsorbents with micropollutants is carried out with solutions of acids or bases. The chemical decomposition of jarosites occurs at a pH less than 2 and higher than 10 [20,42,43], and then jarosites are stable in the pH range of 2–10.

The synthesis, pelletization, and regeneration processes of jarosites involve drying and calcination operations, so it is essential to assess their thermal stability. Therefore, the weight losses were monitored as a function of the increase in temperature via thermogravimetric analysis (TG). The weight loss derivative as a temperature function was graphed to obtain additional information. As shown in Figure 6a,b, the thermal decomposition processes of Na−jarosite 2 and NH₄−jarosite 2 were similar.

The weight change observed between 200 and 300 °C is attributed to a loss of water during crystallization. From 300 to 500 °C, jarosites undergo a dehydroxylation process that decomposes them into metal oxides, sulfates, and water. Finally, between 500 and 700 °C, metal sulfates are transformed into metal and sulfur oxides. The multiple thermal processes that NH₄−jarosite 2 undergoes in the interval between 300 and 500 °C, as shown in Figure 6b, can be attributed to the transformation of NH₄−jarosite into H−jarosite, which implies the loss of ammonia (NH₃) and the dehydroxylation that leads the loss of H₂O molecules.

The thermal stability of the experimentally obtained jarosites agreed with the stability reported in the literature for this type of material [44]. In conclusion, jarosites can be thermally treated at least at 300 °C without undergoing irreversible changes. Therefore, these materials remain viable candidates for use cases such as treating polluted waters.

3.2. Potential for Adsorption of Micropollutants Presented by Jarosites

The kinetics of As(V) adsorption presented by the powders of Na−jarosite 1 and K−jarosite 1, and NH₄−jarosite 2 were determined in a batch process. The initial concentration (C₀) of As(V) of around 750 µg L⁻¹ was selected because it is the content of this micropollutant found in the groundwater of the most contaminated wells from the region of “La Laguna” in northern Mexico.

Table 3. Adsorption kinetics of As(V) on Na−, K− and NH₄ jarosites *.

	Na−Jarosite 1			K−Jarosite 1			NH4−Jarosite 2
Time min	As(V) µg L−1	Removal wt.%	qt, mg g−1	As(V) µg L−1	Removal wt.%	qt, mg g−1	As(V) µg L−1	Removal wt.%	qt, mg g−1
0	754	0.00	0.00	752	0.00	0.00	746	0.00	0
1	80	89.39	0.67	226	69.95	0.53	743	0.40	0.003
2	50	93.37	0.70	186	75.27	0.57	739	0.94	0.007
3	20	97.35	0.73	88	88.30	0.66	735	1.47	0.011
5	20	97.35	0.73	50	93.35	0.70	710	4.83	0.036
10	20	97.35	0.73	20	97.34	0.73	710	4.83	0.036
20	20	97.35	0.73	20	97.34	0.73	617	17.29	0.129
60	20	97.35	0.73	20	97.34	0.73	490	34.32	0.256
120	20	97.35	0.73	20	97.34	0.73	363	51.34	0.383
240	20	97.35	0.73	20	97.34	0.73	77	89.68	0.669
360	20	97.35	0.73	20	97.34	0.73	43	94.24	0.703
1440	20	97.35	0.73	20	97.34	0.73	20	97.32	0.726

* Jarosites 1 were obtained via the conventional method; NH₄−jarosite 2 was obtained via the microwave-assisted method.

shows the content of As(V) in the solution and the percentage removal and the removal capacity (qt) of As(V) calculated as a function of the contact time of the contaminated water with the jarosites under study. Based on the micropollutant removal percentage and the time required to remove the maximum amount of As(V) contained in a 754 µg L⁻¹ solution, it can be concluded that the order of effectiveness of the jarosites studied is as follows:

Na−jarosite 1 > K−jarosite 1 > NH₄−jarosite 2

Na−jarosite 1 removes close to 100% of the AsO₄³⁻ contained in water in less than 2 min. On the other hand, K−jarosite 1 requires between 5 and 10 min to remove 97.34% of the total As(V) in the solution. This makes these jarosites particularity attractive for the formulation of As(V) adsorbents present in water. Even though, at equilibrium, NH₄−jarosite 2 manages to remove a percentage close to that reached by Na−jarosite 1 and K−jarosite 1, the time required to do so is over 240 min. Due to the time it takes to remove all the As(V) dissolved in the solution, it is possible to conclude that NH₄−jarosite 2 is not a promising adsorbent for As(V) under the evaluated conditions. Figure 7 shows the adsorption kinetics of AsO₄³⁻ on the Na−, K− and NH₄−jarosites. The X and Y axes were shifted to negative values to better distinguish the adsorption curves from the axes.

The differences in morphology and size of the crystalline agglomerates of the Na−, K− and NH₄−jarosites (Figure 5) may be associated with the different As(V) adsorption capacity they have. For example, Na−jarosite 1, which is predominantly made up of irregular particles smaller than 0.5 µm in size, exhibits the highest efficiency in relation to As(V). On the other hand, K−jarosite 1, which comprises pseudospherical crystalline agglomerates with a size of around 0.5 µm, exhibits an As(V) adsorption capacity similar to that of Na−jarosite 1. In contrast, NH₄−jarosite 2 consists of single crystals with an average size of around 7.1 µm and requires the longest time to reach equilibrium in the As(V) adsorption process.

Table 4. Equilibrium adsorption of As(V) on the Na−, K− and NH₄−jarosites *.

	Na−Jarosite 1			K−Jarosite 1			NH4−Jarosite 2
C0, mg L−1	1/Ce	Removal wt.%	1/qe	1/Ce	Removal wt.%	1/qe	1/Ce	Removal wt.%	1/qe
0.5 0.75 1 3 5 7 8 10 30 50 100 150 200	52.632 34.483 25.000 14.925 5.000 3.333 1.490 5.000 - 0.040 0.018 0.010 0.006	96.20 96.13 96.00 97.77 96.00 95.71 91.61 98.00 - 49.40 44.00 35.64 16.26	2.079 1.387 1.042 0.341 0.208 0.149 0.136 0.102 - 0.040 0.023 0.019 0.031	50.000 33.333 29.412 5.000 0.990 0.280 0.228 0.150 0.045 0.024 0.010 - 0.005	96.00 96.00 96.60 93.33 79.80 48.90 45.25 33.50 26.27 18.08 4.25 - 7.33	2.083 1.389 1.035 0.357 0.251 0.292 0.276 0.299 0.127 0.111 0.235 - 0.068	50.000 - 20.000 - 5.000 2.941 2.273 0.916 0.113 0.025 0.011 - 0.005	96.00 - 95.00 - 96.00 95.14 94.50 89.08 70.43 20.10 5.21 - 6.76	2.083 1.333 1.053 - 0.208 0.150 0.132 0.112 0.047 0.100 0.192 - 0.074

* Jarosites 1 were obtained via the conventional method; jarosites 2 were obtained via the microwave-assisted method.

shows the adsorption parameters in the As(V) equilibrium reached after 24 h of adsorption on the Na−, K− and NH₄−jarosites. Na−jarosite 1 and NH₄−jarosite 2 remove more than 89 wt.% of the As(V) present in solutions containing 10 mg L⁻¹. On the contrary, K−jarosite 2 barely removes 80 wt.% of the As(V) contained in solutions with 5 mg L⁻¹. This behavior can be an argument in support of the hypothesis that the counterion that neutralizes the jarosite structure has an important effect on the nature of the interaction between the jarosite and As(V).

To estimate the maximum adsorption capacity of As(V) (Q_max) presented by the jarosites, the well-known Langmuir’s linear mathematical models were applied. The calculation of the Q_max was made based on the trend line of the results obtained by plotting 1/Ce vs. 1/qe. By analogy between the linear representation of the Langmuir isotherm and the equations of the trend lines obtained experimentally, the values of the maximum theoretical adsorption capacity of As(V) by the jarosites were obtained.

Due to the magnitude of the coefficient of determination (R²) being greater than 0.97, it can be concluded that Langmuir’s nonlinear mathematical model better describes the phenomenon of As(V) adsorption on the jarosites. The Q_max presented by Na−jarosite 1, K−jarosite 1 and NH₄−jarosite 2 was 65.55, 8.11 and 9.01, mg g⁻¹, respectively. The Q_max presented by Na−jarosite 1 is higher than those reported in the recent literature. Our research group reported that merlinoite chemically modified with iron (Fe–merlinoite) and iron–zirconium (FeZr–merlinoite) had the highest capacity to adsorb As(V) reported for zeolites (27 and 42.31 mg g⁻¹, respectively) [13]. Here, it is reported that Na−jarosite 1 has a higher Q_max than chemically modified zeolites. Na−jarosite 1, besides having a higher Q_max than chemically modified zeolites, is synthesized quickly and does not require any functionalization.

The adsorption results demonstrate that jarosites are highly efficient bifunctional adsorbents for separating anionic species such as As(V) and cationic species such as Pb(II) from water. To explain the adsorption processes of these water microcontaminants on the surface of jarosites, it is recommended to use isotherm models such as the Freundlich, Tempkin, and Dubinin–Radushkevich models. It is also advisable to determine the changes in the Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) to assess the spontaneity and feasibility of the adsorption processes of As(V) and Pb(II) on jarosites [9].

3.3. Effect of pH and the Presence of Coexisting Ions on the Removal of As(V)

Based on the Q_max and As(V) adsorption rate, Na−jarosite 1 was selected as the most efficient jarosite as an As(V) adsorbent. To have more evidence of the potential of Na−jarosite as an adsorbent, the effect of the pH and the coexistence of SO₄²⁻, HCO₃⁻ and NO₃⁻ ions on the removal of As(V) contained in water was determined. Figure 8a shows the As(V) removal study results as a function of the solution’s pH.

From Figure 8a, it is evident that at a pH between 5 and 10, Na−jarosite 1 maintains As(V) removal close to 98 wt.%, although at pH 3, Na−jarosite 1 has a removal greater than 92 wt.%. Regarding the presence of coexisting ions, it is evident too that the presence of SO₄²⁻, HCO₃⁻ and NO₃⁻ ions does not significantly affect the removal of As(V) presented by Na−jarosite (Figure 8b). The fact that Na−jarosite 1 has a high Q_max, that it quickly adsorbs As(V), and that its removal capacity is not affected in a wide pH range and the presence of coexisting ions mean that Na−jarosite 1 is a good alternative for the remediation of water contaminated with arsenic.

3.4. Potential of Na−Jarosite 1 to Adsorb Pb(II) Ions Dissolved in Water

To assess the removal capacity of heavy metal cations presented by the jarosites, the evaluation of Pb(II) adsorption on the Na−jarosite 1 surface was monitored concerning time. The adsorption process was carried out at 25 °C using 0.25 g of Na−jarosite 1 and 250 mL of a Pb(II) solution with 100 mg L⁻¹. The results obtained are shown in Figure 9.

Although the jarosite instantly removed Pb(II), the removal of Pb(II) contained in 250 mL of the solution reached around 75 wt.%. This behavior can be attributed to the fact that the amount of Pb(II) in the test solution was higher than the maximum adsorption capacity that Na−jarosites can have and that the jarosite was completely saturated. With these data, it was possible to calculate that Na−jarosite 1 can adsorb 93.7 mg of Pb(II) per gram.

Although it is necessary to carry out additional experiments to determine the maximum Pb(II) adsorption capacity of the jarosites, it is possible to state that this adsorption capacity is comparable to that of complex materials based on carbon, metals, and magnetic compounds [17], such as biochar-coated magnetite nanoparticles, which are prepared from the root of C. odorata and functionalized with 3-aminopropyltriethoxysilane (APTES), that have a maximum adsorption capacity for Pb(II) of 64.9 mg g⁻¹ [45].

The proposed mechanism for separating inorganic micropollutants of different chemical natures from water involves the substitution of the A cation as well as the SO₄ and OH ligands of the jarosite family represented by the formula AFe₃(SO₄)₂(OH)₆·xH₂O, where A = Na, K, NH₄, by Pb(II), AsO₄³⁻ and F⁻ ions, respectively. However, additional studies using the XRD, FTIR, and X-ray photoelectron spectroscopy (XPS) techniques on jarosites saturated with As(V) and Pb(II) must be conducted to support this proposal.

4. Conclusions

The challenge in designing adsorbents for water purification and aqueous effluent treatment is to prepare multipurpose, regenerable adsorbents that are technically and economically viable to be produced and applied on a large scale. The results of the research reported here show that: (a) jarosites can be obtained as single crystalline species with higher yields (up to 87.6 wt.%) and shorter times (up to 5 min) than those reported for many of the materials reported in the literature; (b) the shapes and particle sizes of jarosites depend on the synthesis conditions under which they are obtained and the type of counterion involved; and (c) due to their thermal (300 °C) and chemical stability in the pH range of 2–10, jarosites can be applied in water treatment to remove inorganic contaminants.

Following this evidence, it was found that the morphology, particle size and type of counterion that neutralizes the charge of the jarosites determines the rate for adsorbing As(V) in the jarosites. Na−jarosite 1 was the most effective adsorbent to remove As(V). It reached a Q_max of 65.5 mg g⁻¹, which was greater than that of the other As(V) adsorbents that are obtained through complex processes and less viable to be scaled up to a higher level, such as the recently reported chemically modified zeolites (FeZr–merlinoite; Q_max 42.31 mg g⁻¹).

Na−jarosite 1 exhibits a high As(V) removal efficiency of over 97 wt.% in the pH range of 5 to 10, even in the presence of ions such as SO₄²⁻, HCO₃⁻ or NO₃⁻ ions. The As(V) adsorption capacity is maintained at a pH around 7. One of the advantages of using jarosites as adsorbents over complex materials such as modified zeolites reported in the literature is that jarosites can be prepared directly and rapidly using mild conditions and without requiring chemical functionalization. Conversely, synthesizing and functionalizing zeolitic adsorbents requires complex operations and harsh conditions.

Na−jarosite 1 was also found to be effective in removing Pb(II) ions from the solution within the first few minutes of contact. Based on the results, it is estimated that this jarosite can adsorb 93.7 mg g⁻¹ of Pb(II), which is higher than the Pb(II) adsorption Q_max reported for adsorbents that require complex manufacturing methods, such as biochar-coated magnetite nanoparticles with a Pb(II) Q_max of 64.5 mg g⁻¹.

Regarding the results obtained, it can be summarized that jarosites exhibit suitable characteristics for the efficient removal of both anionic species (As(V)) and cationic species (Pb(II)) present in water. Additionally, due to the number of interchangeable OH ligands in their molecules, jarosites could potentially remove fluoride ions as well and become the first trifunctional adsorbent that allows the removal of inorganic micropollutants from water.