Antimonate Removal from Polluted Mining Water by Calcined Layered Double Hydroxides

Abstract

Calcined layered double hydroxides (LDHs) can be used to remove Sb(V), in the Sb(OH)₆⁻ form, from aqueous solutions. Sorption batch experiments showed that the mixed MgAlFe oxides, obtained from calcined hydrotalcite-like compound (3HT-cal), removed Sb(OH)₆⁻ through the formation of a non-LDH brandholzite-like compound, whereas the mixed ZnAl oxides, resulting from calcined zaccagnaite-like compound (2ZC-cal), trapped Sb(OH)₆⁻ in the interlayer during the formation of a Sb(V)-bearing LDH (the zincalstibite-like compound). The competition effect of coexistent anions on Sb(OH)₆⁻ removal was HAsO₄²⁻ >> HCO₃⁻ ≥ SO₄²⁻ for 2ZC-cal and HAsO₄²⁻ >> HCO₃⁻ >> SO₄²⁻ for 3HT-cal. Considering the importance of assessing the practical use of calcined LDHs, batch experiments were also carried out with a slag drainage affected by serious Sb(V) pollution (Sb = 9900 μg/L) sampled at the abandoned Su Suergiu mine (Sardinia, Italy). Results showed that, due to the complex chemical composition of the slag drainage, dissolved Sb(OH)₆⁻ was removed by intercalation in the interlayer of carbonate LDHs rather than through the formation of brandholzite-like or zincalstibite-like compounds. Both 2ZC-cal and 3HT-cal efficiently removed very high percentages (up to 90–99%) of Sb(V) from the Su Suergiu mine drainage, and thus can have a potential application for real polluted waters.

1. Introduction

Antimony (Sb) is an element widely present in the environment as a result of both natural processes and anthropogenic sources [1]. Due to its potential risk for human health, the World Health Organization has set the guideline value for drinking water at 20 μg/L of Sb [2], while the European Community has established 5 μg/L [3]. The Sb concentration in uncontaminated freshwater is usually lower than drinking water limits [4,5], however considerable higher concentrations (up to mg/L) can be related to both natural sources and anthropogenic activities [1,4,6,7,8]. In natural environments, Sb is generally present in the trivalent Sb(III) and pentavalent Sb(V) oxidation states, with the Sb(III) species being ten times more toxic than the Sb(V) ones [9]. In aqueous solution, Sb(III) and Sb(V) prevail, respectively, under reducing and oxidizing conditions as antimonous acid H₃SbO₃ and antimonic acid H₃SbO₄ and their dissociation products, with the Sb(OH)₆⁻ anion being the most common and stable aqueous species in a wide range of natural pH values [1,9].

Among the techniques suitable for the abatement of Sb concentration in the solution, such as coagulation-flocculation [10], electrochemical methods [11,12] and membrane separation [13,14], the adsorption is considered a low cost and effective method [9]. Several studies reported that metal hydroxides and oxohydroxides (e.g., MnOOH, Al(OH)₃, FeOOH) are good Sb removers, however they result more efficient for Sb(III) than Sb(V) under slightly acid to acid conditions [15,16,17,18]. Also, nano-TiO₂ electroactive carbon nanotube (CNT) filter and ZrO₂-carbon nanofibers (ZNC) were tested, respectively, for Sb(III) and simultaneous Sb(III) and Sb(V) removal; in particular, it was reported that the adsorption capacity of Sb(III) is much higher than Sb(V) in ZNC [19,20]. Recent works showed that the acidic conditions are also favorable for the Sb(V) removal from solution by La and Ce-doped magnetic biochars [21,22]. With respect to the other sorbents, layered double hydroxides (LDHs) show the advantage of being able to remove hazardous anions from solution at circumneutral pH, and can therefore be potential removers for Sb(OH)₆⁻ at the circumneutral pH and oxidizing conditions usually found in the environment [23].

The LDHs are minerals with general formula [M²⁺_1-xM³⁺_x(OH)₂](Aⁿ⁻)_x/n∙mH₂O, where M²⁺ and M³⁺ are respectively bivalent and trivalent metals (Mg²⁺, Zn²⁺, Ca²⁺, Al³⁺, Fe³⁺, etc.), A^n- are anions (Cl⁻, SO₄²⁻, CO₃²⁻, etc.) and x is the M³⁺/(M²⁺ + M³⁺) molar ratio (0.20 ≤ × ≤ 0.33). The LDHs structure consists of octahedral brucite-like layers positively charged due to the partial substitution of M²⁺ by M³⁺, stacked along the c axis and intercalated with interlayer anions, which neutralize the positive charge, and variable quantity of water molecules [24,25]. The LDHs can successfully remove anionic contaminants from solution through the anion exchange with the interlayer anions, or by trapping anions in the interlayer region during the reconstruction of the layered structure by the rehydration of mixed metal oxides obtained from LDHs calcination (the so called “memory effect”) [23,26,27,28,29].

Previous authors reported that LDHs with different compositions are potential Sb(OH)₆⁻ removers: nitrate and chloride bearing Mg-Al LDHs, both untreated and calcined, efficiently remove Sb(OH)₆⁻ from solution through the formation of a brandholzite-like compound [30,31], and the removal capacity can be improved by doping LDHs with Fe²⁺ [32]; sulfate bearing Zn-Al and Zn-Fe(III) LDHs uptake Sb(OH)₆⁻ from solution by anion exchange [33,34] and also Fe-Mn LDHs obtained by electro-coagulation process result good removers [35]. Although interest in the use of LDHs for Sb removal from aqueous solutions has increased in recent years, to the best of our knowledge their practical use with real polluted water has not been investigated yet. Therefore, the aim of this study was to assess the Sb(OH)₆⁻ removal capacity of calcined LDHs from real water affected by serious Sb pollution.

In our previous work we showed that calcined synthetic LDHs with composition like hydrotalcite (with formula Mg₆(Al_0.5Fe_0.5)₂(CO₃)(OH)₁₆∙4H₂O) and zaccagnaite (with formula Zn₄Al₂(CO₃)(OH)₁₂∙3H₂O) remove Sb(OH)₆⁻ from solution, respectively, through the formation of a brandholzite-like phase (a non-LDH mineral with general formula Mg[Sb(OH)₆]₂·6H₂O) and a zincalstibite-like compound (an LDH mineral with general formula Zn₂Al(OH)₆[Sb(OH)₆]) [36]. In this work we used calcined hydrotalcite-like and zaccagnaite-like compounds to carry out batch experiments with coexistent anions in solution to evaluate their competition effect on Sb(OH)₆⁻ removal. We successively assessed the practical use of these sorbents with real water by sorption batch experiments performed with the drainage water flowing out from the foundry slag impoundments at the abandoned mine of Su Suergiu (Sardinia, Italy), which is affected by serious Sb pollution [6,37].

2. Materials and Methods

2.1. LDHs Synthesis and Calcination

Synthetic hydrotalcite Mg₆(Al_0.5Fe_0.5)₂(CO₃)(OH)₁₆∙4H₂O and zaccagnaite Zn₄Al₂(CO₃)(OH)₁₂∙3H₂O were prepared with a coprecipitation method at constant pH [36]. Depending on composition, a solution (0.2 M) with the desired metals was prepared by dissolving in ultrapure water (Millipore, Milli-Q©, 18.2 MΩ cm) appropriate amounts of Mg(NO₃)₂∙6H₂O, Al(NO₃)₃∙9H₂O, Fe(NO₃)₃∙9H₂O and Zn(NO₃)₂∙6H₂O. All reagents were of analytical grade (ACS-for analysis, CARLO ERBA Reagents S.r.l., Cornaredo (MI), Italy) and were used without further purification. The so-obtained metal solution was dropped into a reactor containing a Na₂CO₃ solution (0.05 M), under stirring (500 rpm), and the precipitation was induced at constant pH (ranging between 9.5 and 10.5) by adding dropwise a NaOH (0.5 M) solution. After 24 h of aging at 65 °C, the solids were recovered through filtration (30 µm pore size cellulose filter, Whatman Plc, Little Chalfont, Buckinghamshire, UK), washed with deionized water and dried at room temperature. Calcination was performed at 450 °C for 4 h. The M²⁺/M³⁺ molar ratios of synthetic hydrotalcite and zaccagnaite, and their calcined products, were close to those of the starting solutions (

Table 1. Chemical composition of synthetic untreated carbonate (-CO₃) and calcined (-cal) hydrotalcite-like (3HT) and zaccagnaite-like (2ZC) compounds.

Sample	Zn	Al	M2+/M3+	Sample	Mg	Al	Fe3+	M2+/M3+
	mmol/g	mmol/g	Molar Ratio		mmol/g	mmol/g	mmol/g	Molar Ratio
2ZC-CO3	6.8	3.2	2.1	3HT-CO3	7.4	1.3	1.4	2.8
2ZC-cal	9.0	4.1	2.2	3HT-cal	12.5	2.2	2.3	2.8

).

From now on, synthetic hydrotalcite is termed 3HT and zaccagnaite is 2ZC, the numbers before the labels indicate the M²⁺/M³⁺ molar ratio; moreover, the suffix -CO₃ will be used for the untreated carbonate LDHs and the suffix -cal for the calcined LDHs.

2.2. Sorption Experiments

2.2.1. Effect of Coexistent Anions

In batch experiments, the coexistent anions were selected taking into account the chemical composition of Su Suergiu mine drainage [6,37]. The experimental solutions were prepared dissolving appropriate amounts of KSb(OH)₆ and Na₂SO₄, NaHCO₃ or Na₂HAsO₄∙7H₂O (ACS-for analysis, CARLO ERBA Reagents S.r.l., Cornaredo (MI), Italy) in ultrapure water. To perform the experiments, 0.1 g of 2ZC-cal or 3HT-cal was suspended, for 48 h under stirring, in 400 mL of solution containing equal concentrations (about 1 mmol/L) of dissolved Sb(OH)₆⁻ and one competitor at a time. Experiments with only dissolved Sb(OH)₆⁻ without competitors were also carried out to compare the results. Before the addition of the sorbents and during the experiments the pH of solutions was monitored and a portion of solution was withdrawn and acidified with HNO₃ 1% v/v for chemical analysis of Sb, As, S, Mg, Zn, Al and Fe by inductively coupled plasma optical emission spectroscopy (ICP-OES, ARL Fisons 3520, Waltham, MA, USA). At the end of the reaction time the solids were recovered through filtration (0.45 µm pore size polycarbonate filters, Whatman Plc, Little Chalfont, Buckinghamshire, UK), washed with distilled water and dried at room temperature for mineralogical characterization.

2.2.2. Sorption Experiments with Su Suergiu Mine Drainage

The real water for sorption experiments is a slag drainage sampled at the abandoned Su Suergiu mine (Sardinia, Italy), at the sampling point named SU1 (Supplementary Materials Figure S1) as reported by Cidu et al. [37]. The physical and chemical parameters were determined at the sampling site using the sampling protocol described in Cidu et al. [37]. After sampling, the slag drainage (from now on SU1) was stored in HDPE bottles at 4 °C, and batch experiments were carried out, at room temperature (25 °C), less than 24 h after the sampling.

Different amounts of 3HT-cal or 2ZC-cal, equal to 0.1, 0.25, 0.5 and 1 g, were suspended in 400 ml of SU1 under stirring for 24 h. During the experiments the pH of solutions was monitored. At the end of reaction time, the solids were separated from solution through filtration, washed with distilled water and dried at room conditions for mineralogical characterization. The solutions recovered after the experiments were stored in two different aliquots: one aliquot was unacidified for analysis of major ions by ion chromatography (IC, Dionex ICS3000, ThermoFisher SCIENTIFIC, Waltham, MA, USA); a second aliquot was acidified for trace elements analysis (Sb, As, Fe, Zn and Al) by inductively coupled plasma mass spectrometry (ICP-MS, quadrupole, PerkinElmer SCIEX ELAN DRC-e, Waltham, MA, USA) with Rh as internal standard; concentrations of Sb > 1000 μg/L were also determined by ICP-OES.

2.3. Mineralogical Characterization

Mineralogical characterization of synthetic LDHs and their calcined products before and after the experiments was performed by collecting XRD patterns in the 5–80° 2θ angular range on an automated Panalytical X’pert Pro diffractometer (PANalytical, Almelo, Netherlands), with Ni-filter Cu-Kα₁ radiation (λ = 1.54060 Å), operating at 40 kV and 40 mA, using the X’Celerator detector.

3. Results

3.1. Effect of Coexistent Anions on Sb(V) Removal

3.1.1. Sorptive Competition

The results of competition experiments showed that 2ZC-cal was slightly more effective than 3HT-cal (Figure 1). At the end of the experiments without competitors, 2ZC-cal removed 85% of Sb(OH)₆⁻ whereas 3HT-cal reached 72%. The coexistence of SO₄²⁻ and HCO₃⁻ slightly affected the Sb(OH)₆⁻ uptake by 2ZC-cal, whereas in the experiments with 3HT-cal, the percentage of Sb(OH)₆⁻ removed did not vary significantly in the presence of SO₄²⁻ but decreased up to 50% with coexistent HCO₃⁻ (Figure 1,

Table 2. Solution pH values and dissolved ions determined before and at the end of the sorption experiments performed with 2ZC-cal without competitors (Sb), and with coexistent HCO₃⁻ (Sb/HCO₃), SO₄²⁻ (Sb/SO₄) or HAsO₄²⁻ (Sb/HAsO₄). (Aⁿ⁻ = HCO₃⁻, SO₄²⁻ or HAsO₄²⁻; na = not analyzed; dl = detection limit; dl_Zn = 0.2 μmol/L; dl_Al = 7 μmol/L).

Sample	Experiment	Time	pH	Sb(OH)6−	Sb(OH)6−	An−	An−	Zn	Al
					removed		removed
		h		mmol/L	%	mmol/L	%	mmol/L
2ZC-cal	Sb	0	5.1	1.02				0	0
		48	8.5	0.15	85			<dl	<dl
	Sb/HCO3	0	8.2	1.03		1.09		0	0
		48	8.7	0.24	77	na	na	<dl	<dl
	Sb/SO4	0	5.2	1.01		1.08		0	0
		48	9.5	0.22	79	1.04	4	<dl	<dl
	Sb/HAsO4	0	8.6	1.02		1.03		0	0
		48	8.1	0.93	8.6	0.87	16	0.02	<dl

and

Table 3. Solution pH values and dissolved ions determined before and at the end of the sorption experiments performed with 3HT-cal without competitors (Sb), and with coexistent HCO₃⁻ (Sb/HCO₃), SO₄²⁻ (Sb/SO₄) or HAsO₄²⁻ (Sb/HAsO₄). (Aⁿ⁻ = HCO₃⁻, SO₄²⁻ or HAsO₄²⁻; na = not analyzed; dl = detection limit; dl_Al = 7 μmol/L; dl_Fe = 1 μmol/L).

Sample	Experiment	Time	pH	Sb(OH)6−	Sb(OH)6−	An−	An−	Mg	Al	Fe
					removed		removed
				mmol/L	%	mmol/L	%	mmol/L
3HT-cal	Sb	0	5.2	1.03				0	0	0
		48	9.3	0.29	72			0.20	<dl	<dl
	Sb/HCO3	0	8.3	1.02		1.05		0	0	0
		48	9.5	0.51	50	na	na	0.22	<dl	<dl
	Sb/SO4	0	5.1	1.03		1.08		0	0	0
		48	9.1	0.28	73	1.06	2	0.33	<dl	<dl
	Sb/HAsO4	0	8.6	1.03		1.02		0	0	0
		48	9.8	0.99	3.7	0.43	57	0.19	<dl	<dl

).

The HAsO₄²⁻ anion was the strongest competitor in the experiments with both 2ZC-cal and 3HT-cal, with percentages of Sb(OH)₆⁻ removed lower than 10%. Moreover, the HAsO₄²⁻ concentration at the end of the experiment with 3HT-cal markedly decreased by about 60% (Table 3), showing a strong affinity of HAsO₄²⁻ for the interlayer region of 3HT.

The effect of coexistent anions on Sb(OH)₆⁻ uptake resulted to be HAsO₄²⁻ >> HCO₃⁻ ≥ SO₄²⁻ for 2ZC-cal and, in partial agreement with previous work [30], HAsO₄²⁻ >> HCO₃⁻ >> SO₄²⁻ for 3HT-cal.

At the end of the experiments with 3HT-cal, slight concentrations of Mg were determined, whereas the concentrations of Al and Fe were always below the corresponding detection limits (Table 3).

3.1.2. Kinetics

In the experiments with both 2ZC-cal and 3HT-cal the solution pH values increased sharply (up to about 11) after the addition of sorbents (Figure 2a,b), and decreased after 48 h in the range of 8.1–9.8 (Table 2 and Table 3; Figure 2a,b). Most of the Sb(OH)₆⁻ was removed within the first six hours (Figure 2c,d) indicating that Sb(OH)₆⁻ uptake occurred mainly during the reconstruction of the lamellar structure of LDHs [36] as schematized in reaction (1):

$$\begin{array}{c}{M}_{1-x}^{2+}{M}_x^{3+}{\left(OH\right)}_2{(C{O}_3)}_{\frac{x}{2}}\stackrel{calcination}{\to }{M}_{1-x}^{2+}{M}_x^{3+}{O}_{1+\left(\frac{x}{2}\right)}+xSb{\left(OH\right)}_6^{-}\\ \stackrel{reconstruction}{\to }{M}_{1-x}^{2+}{M}_x^{3+}{\left(OH\right)}_2{(Sb{\left(OH\right)}_6)}_x+xO{H}^{-}\end{array}$$

(1)

The Sb(OH)₆⁻ removal as a function of time was studied through the pseudo-first order [38] and the pseudo-second order kinetic models [39].

The Sb(OH)₆⁻ sorption capacity has been calculated through the Formula (2):

q_t = (C₀ − C_t) ∙ V/W

(2)

where the sorption capacity (q_t) is the amount of Sb(OH)₆⁻ sorbed per unit of sorbent (mmol/g) at the reaction time t (h), C₀ and C_t are the Sb(OH)₆⁻ concentrations in solution (mmol/L) before the addition of the sorbent and at the reaction time t, V is the volume of solution (L) and W the weight of sorbent (g).

The pseudo-first order and the pseudo-second order equations are expressed as follows:

$$pseudo-firstorderkineticmodel{k}_1=\frac{2.303}{t}·log\frac{q_e}{q_e-q_t}$$

(3)

$$pseudo-secondorderkineticmodel{k}_2=\frac{1}{t}·\frac{q_t}{q_e(q_e-q_t)}$$

(4)

where q_e is the Sb(OH)₆⁻ sorption capacity at equilibrium (mmol/g), k₁ (1/h) and k₂ (g/mmol h) are the rate constant of sorption. These equations expressed in the linear form result as follows:

$$pseudo-firstorderkineticmodel\frac{d{q}_t}{dt}=k_1\left(q_e-q_t\right)$$

(5)

$$pseudo-secondorderkineticmodel\frac{d{q}_t}{dt}=k_2{(q_e-q_t)}^2$$

(6)

To verify the applicability of kinetic models at the sorption system, the experimental data were plotted as log(q_e − q_t) vs. time for the pseudo-first order (Supplementary Material Figure S2) and t/q_t vs. time (Supplementary Material Figure S3) for the pseudo-second order kinetic model. If the plots give a linear correlation, then the theoretical sorption capacity at equilibrium and the rate constants can be calculated from the slope and the intercept of the straight lines. The good fit of the data, the r² values close to the unit and the good agreement between the experimental sorption capacity at equilibrium (q_e) and the theoretical sorption capacity (q_calc) indicated that the sorption system is better described by the pseudo-second order kinetic model (

Table 4. The Sb(OH)₆⁻ sorption capacity of 2ZC-cal and 3HT-cal determined at equilibrium from the experimental data (q_e) and calculated from the kinetic models (q_calc).

Sample	Experiment		Pseudo-First Order Kinetic Model			Pseudo-Second Order Kinetic Model
		qe	qcalc	k1	r2	qcalc	k2	r2
		mmol/g	mmol/g	1/h		mmol/g	g/mmol h
2ZC-cal	Sb	3.47	1.86	0.093	0.848	3.52	0.235	0.999
	Sb/HCO3	3.18	1.59	0.052	0.631	3.19	0.196	0.992
	Sb/SO4	3.17	1.86	0.080	0.856	3.23	0.186	0.997
	Sb/HAsO4	0.35	4.78	0.068	0.754	0.36	1.568	0.992
3HT-cal	Sb	2.94	1.51	0.143	0.761	3.02	0.272	0.999
	Sb/HCO3	2.04	1.13	0.074	0.641	2.07	0.303	0.997
	Sb/SO4	2.99	1.58	0.137	0.738	3.08	0.231	0.999
	Sb/HAsO4	0.15				0.16	12.9	0.995

), suggesting that the Sb(OH)₆⁻ uptake by both 2ZC-cal and 3HT-cal might principally occur by chemisorption.

3.1.3. Characterization of Sorbents

The XRD pattern of 2ZC-CO₃ (Figure 3a) showed the characteristic basal reflections (003) and (006) attributable to a zaccagnaite-like compound [40].

After calcination, in the XRD pattern of 2ZC-cal, the absence of LDH basal reflections and the presence of broad peaks at 32.1° and 36.4° 2θ, ascribable to ZnO, indicated the collapse of the lamellar LDH structure and the formation of a disordered ZnO, with Al probably dispersed in its structure [41]. Peaks of undesired phases were not detected. The XRD patterns of solids recovered after the sorption experiments showed two different LDH phases: the peaks at angular position 11.6° and 23.5° 2θ corresponded to the basal reflections (003) and (006) of 2ZC-CO₃; instead, the peaks at 5° and 18° 2θ were attributable, respectively, to the (001) and (002) reflections of a zincalstibite-like compound [36,42].

The characteristic hydrotalcite-like compound basal reflections (003) and (006), visible in the XRD patterns of 3HT-CO₃ (Figure 3b), were no longer detectable in the calcined phase (3HT-cal) which showed peaks at about 42° and 62° 2θ ascribable to a disordered MgO with the trivalent metals Fe and Al probably dispersed in its structure [26]. After the sorption experiments, the solids showed the characteristic peak of a brandholzite-like compound at 19.2° 2θ and a further brandholzite peak at 33.6° 2θ [36,43], except for the experiment with coexistent HAsO₄²⁻ where the characteristic brandholzite peak was barely visible and additional peaks at low angle compatible with the basal reflections of 3HT-CO₃ were clearly recognizable.

3.2. Sorption Experiments with Su Suergiu Mine Drainage (SU1)

3.2.1. Solutions

For convenience in this section the concentrations of ions in solution will be expressed as mg/L and μg/L.

The results of the chemical analysis of SU1 slag drainage showed a Ca-SO₄ dominant chemical composition and high concentrations of Sb (9900 µg/L) and arsenic (As = 3390 µg/L) (

Table 5. The pH values, electrical conductivity (EC) and chemical composition of SU1 slag drainage before (SU1 column) and after the sorption experiments performed with different amounts (0.1, 0.25, 0.5, 1g) of 2ZC-cal and 3HT-cal (SU1 + 2ZC-cal and SU1 + 3HT-cal columns).

		SU1	SU1 + 2ZC-cal				SU1 + 3HT-cal
			Weight of Sorbent (g)				Weight of Sorbent (g)
			0.1	0.25	0.5	1	0.1	0.25	0.5	1
EC	mS/cm	2.40	2.33	2.24	2.14	2.04	2.24	2.12	1.99	1.75
pH		8.2	8.1	8.1	8.0	7.9	8.4	9.2	9.4	9.7
SO4	mg/L	1000	1080	1050	1000	960	1070	1050	960	770
HCO3	mg/L	485	252	169	125	80	127	48	29	18
Cl	mg/L	59	64	65	63	63	64	64	63	63
NO3	mg/L	0.6	4.4	0.7	1.3	0.6	1.4	1.9	3.5	6.3
F	mg/L	1.7	1.6	1.1	0.5	0.3	0.8	0.4	0.1	0.1
Ca	mg/L	362	278	264	234	244	131	68	80	129
Mg	mg/L	63	62	56	45	25	120	140	109	59
Na	mg/L	166	168	161	167	163	166	166	166	162
K	mg/L	4.7	4.4	4.4	4.8	4.3	4.7	4.6	4.8	4.6
Zn	μg/L	30	1490	174	260	242	<20	<20	<20	<20
Al	μg/L	<30	78	105	67	<30	700	<30	<30	<30
Fe	μg/L	<20	<20	<20	<20	<20	<20	<20	<20	<20
Sb	μg/L	9900	6190	78	34	20	9830	170	430	1000
As	μg/L	3390	92	35	8.3	4.5	150	25	6.5	<0.5

).

The high value of EC (electrical conductivity) is related to the high contents of Ca²⁺ and SO₄²⁻, with SO₄²⁻ mainly deriving from the oxidation of sulfides; moreover, the high concentration of HCO₃⁻ avoids the decrease of pH, which resulted in slightly alkaline values (Table 5). The high concentration of both Sb and As is the consequence of the water interaction with the foundry slags [6,37]. It has been reported that in the Su Suergiu mine water, the Sb(III), when detected, results <2% of total dissolved Sb (water fraction < 0.45 μm) and that all the Sb(V) occurs as Sb(OH)₆⁻ [6,44]. The concentration of Sb(III) determined in the SU1 sampled for sorption experiments with 3HT-cal and 2ZC-cal resulted to be 147 μg/L, therefore, in the present work, the Sb(OH)₆⁻ is considered the only Sb form involved in the Sb removal processes. Excluding the experiments performed with 0.1 g of sorbents, at the end of reaction time up to 90–99% of Sb was removed from the solution, with 2ZC-cal slightly more effective than 3HT-cal (Figure 4a), whereas As was effectively removed in all experiments (Figure 4b). In almost all experiments, the Sb and As concentrations decreased close to, or below, the limits established for drinking water (Figure 4c,d).

3.2.2. Sorbents

The XRD patterns of solids recovered after all experiments showed peaks at low angles compatible with the carbonate bearing LDHs, indicating the reconstruction of the typical lamellar LDH structure (Figure 5 and Figure 6). In the range of pH values of the experiments, Sb and As prevail, respectively, as Sb(OH)₆⁻ and HAsO₄²⁻, but peaks attributable to a brandholzite-like compound were not visible in the solids recovered after the experiments performed with 3HT-cal (Figure 5), and only after the experiment with 0.1 g of 2ZC-cal the characteristic peak of a zincalstibite-like compound was clearly recognizable (Figure 6). Therefore, as a consequence of the complexity of the chemical composition of SU1, the Sb(OH)₆⁻ removal did not occur through the formation of Sb(OH)₆⁻ bearing phases, but rather, it is reasonable to suppose that Sb(OH)₆⁻ was incorporated in the interlayer region together with other anions, i.e., CO₃²⁻ and HAsO₄²⁻. It is also noticeable that all samples contained additional well defined peaks at about 30° 2θ ascribable to calcite (Figure 5 and Figure 6), and after the experiment with 0.1 g of 2ZC-cal, the peaks attributable to monohydrocalcite were also present (Figure 6).

4. Discussion

4.1. Effect of Coexistent Anions

The results of mineralogical characterizations and chemical analysis, in agreement with previous works, showed that 2ZC-cal removed Sb(OH)₆⁻ from solution through the reconstruction of a zincalstibite-like compound [36], and the 3HT-cal removed Sb(OH)₆⁻ through the formation of a low ordered brandholzite-like compound [30,36]. The zincalstibite is an Sb(OH)₆⁻ bearing LDH (with general formula Zn₂Al(OH)₆[Sb(OH)₆] [25,42], whereas the brandholzite is a non-LDH phase with general formula Mg[Sb(OH)₆]₂·6H₂O, whose the layered structure is characterized by the presence of two layers, {[Sb(OH)₆]₉}⁹⁻ and {[Sb(OH)₆]₃[Mg(H₂O)₆]₆}⁹⁺, alternatively stacked along the c axis [43].

In agreement with the results of chemical analysis, the diffraction peaks of the Sb(OH)₆⁻ bearing phases were more intense and well defined in the XRD patterns of solids recovered after the experiments wherein the greatest amounts of Sb(OH)₆⁻ were removed from solution. In the experiments performed with 3HT-cal and HCO₃⁻ and HAsO₄²⁻ as coexistent anions, the decrease in Sb(OH)₆⁻ removal was linked to the appearance of the (003) and (006) basal reflections of LDHs (Figure 3b). In particular, in the experiment with coexistent HCO₃⁻ the competition effect was improved by the increase of solution pH values, up to 10.6 (Figure 2b), that favors the prevalence of CO₃²⁻ in solution that has a high affinity for the LDH interlayer [23,30]. Also HAsO₄²⁻ has a high affinity for hydrotalcite-like compounds [27,45], but previous authors have observed that the removal capacity of calcined MgAl-LDHs is higher for Sb(OH)₆⁻ than for HAsO₄²⁻, suggesting that the uptake of Sb(OH)₆⁻ through the selective crystallization of a brandholzite-like compound is more favorable than the sequestration from solution by the intercalation in the interlayer [46]. The results of our work showed that, when coexisting in solution, HAsO₄²⁻ strongly competes with Sb(OH)₆⁻ and is preferentially removed, probably due to its higher specific ionic charge.

The low amounts of Mg determined at the end of experiments with 3HT-cal indicated a slight dissolution of sorbent (6–15%) (Table 3). Because the concentrations of dissolved Fe and Al were always below the corresponding detection limits and no Al and/or Fe secondary phases were observed in the XRD patterns of solids recovered at the end of experiments (Figure 3b), the low amount of Al and Fe released in the solution might precipitate as amorphous solids. The Fe and Al of undissolved phase that removed the Sb(OH)₆⁻ through the formation of brandholzite-like compound, probably remained in undetermined sites of the low ordered brandholzite structure.

The XRD pattern of the experiment performed with 2ZC-cal and HAsO₄²⁻ as coexistent anion showed well defined basal reflections compatible with the original 2ZC-CO₃ LDH, while the characteristic zincalstibite-like compound peaks were scarcely visible (Figure 3a). Moreover, two broad undefined humps in the angular ranges 30–33° and 35–37° 2θ indicated that, at the end of the experiment, part of the 2ZC-cal did not react and explained the low Sb(OH)₆⁻ removal that, unlike the experiment performed with 3HT-cal, cannot be attributable to the preferential uptake of HAsO₄²⁻ (Table 2). Previous authors reported the high affinity of As(V) for sulfate bearing ZnAl-LDHs [47,48] but, at the best of our knowledge, experiments on the As(V) removal by calcined ZnAl-LDHs is lacking and needs further study.

4.2. Sorption Experiments with Su Suergiu Mine Drainage (SU1)

The results suggested that both 2ZC-cal and 3HT-cal are suitable for the Sb (and also As) removal from SU1; however, to assess their practical use, the overall quality of treated water must also be considered. The decrease of EC value after the experiments can be attributable to the decrease of Ca²⁺ and HCO₃⁻. The contents of Na⁺, K⁺ and Cl⁻ did not vary significantly, instead sensible variations of SO₄²⁻, NO₃⁻ and F⁻ were observed in a few cases. The partial dissolution of sorbents explained the irregular increase of dissolved Zn or Mg and Al. The limit of Al in drinking water is established at 0.2 mg/L by both the WHO and EU, whereas the concentration of Zn in drinking water is not regulated by the WHO and EU, but rather the Italian Legislation set 2 mg/L [49]; therefore, the concentration of Al exceeded the limit only in one experiment and the Zn limit was never reached (Table 5).

In the experiment performed with 0.1 g of 3HT-cal the amount of Sb removed was dramatically lower with respect to the other experiments with higher amounts of 3HT-cal. In this case it was possible to observe that the Sb removal did not increase with the weight of 3HT-cal used. The Ca²⁺ and HCO₃⁻ decreases did not show correlation (Supplementary material Figure S4). In particular, the Ca²⁺ concentration suggested a lower CaCO₃ precipitation at the end of experiments with 0.1 and 1 g of 3HT-cal with respect to the other ones. In the first case (i.e., 0.1 g of 3HT-cal) the CaCO₃ precipitation should be limited by the low increase of pH (8.2–8.4), while in the experiment with 1 g of 3HT-cal, where the pH values increase up to 10.3 (Figure 5) and CO₃²⁻ prevails in solution, the CaCO₃ precipitation should be hindered by the uptake of CO₃²⁻ in the interlayer during the 3HT-cal rehydration (Figure 5). This could also explain the low Sb removal in spite of the high amount of 3HT-cal. In fact, at high pH the Sb uptake was hindered by CO₃²⁻ that strongly competes for the entry in the interlayer region of the reconstructing lamellar LDH structure. The experiments with 0.5 and 0.25 g of 3HT-cal seemed the best compromise to reach the most favorable conditions for the highest Sb and As removal from 400 mL of SU1.

As observed above, at the end of the experiments with 0.25, 0.5 and 1 g of 2ZC-cal, the amount of Sb removed from solution was markedly higher with respect to 0.1 g of sorbent. Moreover, it is possible to observe that the dissolved concentrations of Sb and As slightly decreased as the amount of 2ZC-cal used increased. The decrease of Ca²⁺ and HCO₃⁻ observed at the end of the experiments did not show clear correlation (Supplementary material Figure S4). The sequestration of Ca²⁺ is attributable to the precipitation of calcium carbonates, which can also explain the slight decrease of Mg²⁺ in some cases (Table 5). The slight differences in residual Ca²⁺ concentration indicated the precipitation of nearly equivalent amounts of calcium carbonates in the different experiments (Table 5). Differently, the HCO₃⁻ concentration decreased as the amount of 2ZC-cal increased because its uptake from the solution occurred by both the precipitation of calcium carbonates and the entry in the interlayer during the LDHs reconstruction (Supplementary material Figure S4). It is possible to note that only in the experiments with 0.5 and 1 g of 2ZC-cal the pH of solution slightly increased after the addition of sorbents (up to 8.7) and successively decreased to 7.9–8.0, whereas in the experiments with 0.1 and 0.25 g, the variation of pH values was negligible and remained in the range of 8.1–8.4 (Figure 6). These limited pH variations among the experiments with 2ZC-cal can explain the precipitation of nearly equivalent amounts of calcium carbonates. The relatively low pH values can also explain the limited precipitation of calcium carbonates, deducible from the concentrations of Ca removed from solution (Supplementary material Figure S4), with respect to that observed in the experiments performed with 3HT-cal. At these pH values around 8, HCO₃⁻ prevails among the dissolved carbonate species, therefore it is possible that the reconstruction of LDH by rehydration of 2ZC-cal occurred via the intercalation of HCO₃⁻, as well as CO₃²⁻.

In our previous work we performed sorption experiments, carried out with ultrapure water containing only Sb(OH)₆⁻, to determine the maximum theoretical Sb(OH)₆⁻ sorption capacity (q_max) of 2ZC-cal and 3HT-cal through the Langmuir isotherm [36]. The values of q_max were 4.37 mmol/g (i.e., 532 mg/g) for 3HT-cal and 4.54 mmol/g (i.e., 553 mg/g) for 2ZC-cal [36]. In the present work we have observed that the coexistence of other anions in solution can affect the Sb(OH)₆⁻ removal capacity of sorbents tested. In order to construct the isotherm, being the starting Sb concentration of sorption experiments that of SU1, the solid/liquid ratio has been changed. However, the sorption data did not fit for the calculation of the isotherm because, due to the complexity of the SU1 chemical composition, also other processes, like the precipitation of calcium carbonates, occurred during the interaction between water and calcined LDHs having an effect on the Sb removal.

It is worth mentioning that the LDHs exhibit the possibility of being reused by regeneration, operated through calcination or anion exchange, for consecutive sorption–regeneration–sorption cycles [50,51,52,53,54]. This is a very important characteristic for their practical use in water treatment because it can reduce the amount of post-treatment waste materials. In this regard, as far as we know, there is no data about the regeneration of ZnAl-LDH after Sb(V) adsorption; whereas it has been reported that, because brandholzite is a non-LDH mineral, the brandholzite formation connected with the Sb(V) removal by calcined MgAl-LDHs can negatively affect the MgAl-LDH regeneration capacity [46,55]. In this work we have observed that in the experiments performed with SU1, the Sb(V) removal by 3HT-cal occurred by intercalation in the LDH interlayer. It is reasonable to suppose that this removal mechanism may positively influence the effectiveness of LDHs regeneration. Therefore, further studies should be performed in order to assess the potential use of both 3HT-cal and 2ZC-cal regenerated after Sb(V) removal from real polluted water.

5. Conclusions

In this work the Sb(V) (in the Sb(OH)₆⁻ form) removal capacity of calcined hydrotalcite-like (3HT-cal) and zaccagnaite-like (2ZC-cal) compounds has been studied in order to assess their potential for the practical use with real Sb(V) polluted water. For this purpose, first the effect of other anions on the Sb(OH)₆⁻ removal capacity of 2ZC-cal and 3HT-cal were tested through batch sorption experiments with coexistent anions in solution. Successively batch sorption experiments were carried out with the slag drainage (SU1) sampled at the abandoned Su Suergiu mine (Sardinia, Italy) affected by relevant Sb pollution.

In agreement with previous studies, the results of our experiments with coexistent anions showed that 3HT-cal and 2ZC-cal removed Sb(OH)₆⁻ through the formation of brandholzite-like and zincalstibite-like compounds, respectively. Among the anions tested, the competition effect on Sb(OH)₆⁻ removal resulted to be HAsO₄²⁻ >> HCO₃⁻ ≥ SO₄²⁻ for 2ZC-cal, and HAsO₄²⁻ >> HCO₃⁻ >> SO₄²⁻ for 3HT-cal.

The results of the sorption experiments showed that both 3HT-cal and 2ZC-cal effectively removed Sb and As from SU1 and, thus, these phases might have potential use for practical application with real polluted water. In the case under study, due the high concentration of carbonate species in SU1, Sb was mainly removed by intercalation in the interlayer of carbonate bearing LDHs rather than by the formation of zincalstibite-like and brandholzite-like compounds, as instead observed in the competition experiments performed with synthetic solutions. The results also indicated that the precipitation of calcium carbonates (i.e., calcite and monohydrocalcite) may favor the Sb removal, subtracting CO₃²⁻ from the solution as a possible strong competitor of Sb for the interlayer region of LDHs. Therefore, especially in the experiments with 3HT-cal, the Sb removal capacity was markedly influenced by the liquid/solid ratio that determines the solution pH, the correlated precipitation of calcium carbonates and the competition effect of carbonate species in solution. The Sb removal by intercalation in the interlayer of carbonate bearing LDHs rather than the formation of antimonate bearing phases may represent an advantage for the LDHs regeneration, therefore further studies should be addressed in that direction.