Application of natural zeolites for preconcentration of arsenic species in water samples

Abstract

Zeolites of the clinoptilolite type produced in Mexico and Hungary were investigated with respect to their sorption efficiency for various redox species of arsenic. Long-term experiments showed that arsenate remains stable for a long period in spiked deionised water and drinking water, as well as in the surface water of the Biela valley in Saxony, Germany. Both clinoptilolites are able to decrease the initial arsenic concentration of 200 µg l⁻¹ by more than 75% in deionised, drinking, ground and surface waters. In the case of the Mexican zeolite, both the arsenite and the arsenate concentrations (200 µg l⁻¹) can be lowered from 200 µg l⁻¹ to 10 µg l⁻¹, which is the World Health Organisation's (WHO's) recommended maximum level. It was found that the presence of cations and anions in the natural waters of Biela, Germany, and Zimapán, Mexico, did not reduce the efficiency of the selected zeolites. The Hungarian zeolite removed 75% of the arsenate in the Zimapán water and only 50% when the sample was first acidified. This zeolite totally desorbed the fixed arsenic into a water volume that was half the volume in the adsorption experiment.

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Introduction

Several drinking water sources around the world^1–5 are contaminated by toxic arsenic originating from the oxidation of arsenic-containing minerals or mining activities. Using untreated water contaminated by arsenic causes numerous diseases of the skin and internal organs.^6,7 Therefore, one of the World Health Organisation's (WHO's)⁸ most pressing aims in solving the world's drinking water problems⁹ is to develop and introduce low-cost pretreatment procedures to effectively remove various arsenic species from natural water. The use of the cation exchange properties of zeolite has received much attention over the past decade in water and industrial waste treatment.^10,11 The crystalline structure of zeolites is responsible for a number of properties which are specific and favourable for catalytic reactions. The adsorption processes observed are the result of the specific interactions with the cations in the window rings or presumably with the oxygen atoms in the framework.

Recently, we reported¹² the sorption and desorption of arsenite and arsenate from deionised water model solutions by natural zeolites. The kinetics and the influence of pH on the sorption process were also studied. Moreover, we devoted a complete study to the oxidation of arsenite by a synthetic zeolite.¹³ As part of our research on arsenic, the present paper deals with the applicability of natural zeolites for arsenic removal from natural water sources, as well as from water solutions spiked with arsenic. One report concerning the concentration of arsenate from a spiked potable water solution on iron-modified clinoptilolite and chabazite can be found in the literature.¹⁴ However, Bonnín¹⁴ did not perform speciation and the final filtrates were analysed after 24 h.

Because the dominant forms of arsenic in the typical pH range of natural waters are H₂AsO₄⁻, HAsO₄²⁻, H₃AsO₃ and, to a lesser extent, H₂AsO₃⁻, we decided to conduct a long-term monitoring study of the stability of arsenite and arsenate in different aqueous media in the absence of and in contact with zeolite in identical static experiments. The identification and quantification of the species were conducted over 4 weeks by ion chromatography separation coupled with inductively coupled plasma mass spectrometric detection (IC-ICP-MS). The zeolites ZME and ZH were selected from a variety of natural zeolites because their interaction with arsenite and arsenate, as well as with some organoarsenic compounds, had been previously studied.^12,13 ZME exhibited saturation capacities of a_m = 3 µg g⁻¹ and 25 µg g⁻¹ for arsenite and arsenate, respectively, while for ZH these values were a_m = 2 µg g⁻¹ and 6 µg g⁻¹.¹⁵

Experimental

Chemicals

Stock solutions of arsenite and arsenate with concentrations of 1 mg l⁻¹ were prepared from arsenic trioxide (Fluka) and arsenate solution (Titrisol, Merck), respectively, using deionised water. The concentration of the arsenic species is always given as the concentration of elemental arsenic. Effluent water from the Biela valley in Germany and groundwater from Pozo 5 in Zimapán, Mexico, were filtered, analysed and placed in contact with the zeolites without any further pretreatment. Biela water was used unspiked and spiked with 200 µg l⁻¹ arsenite. Both matrices were used at their original pH value and also acidified with 0.01 M HCl. Deionised water (Milli-Q, Millipore) and drinking water from Leipzig, Germany, were also spiked with both arsenate and arsenite for this study. Reference solutions (Merck) were used for the spectrometric and chromatographic quantification of cations and anions in the natural waters.

Analytical procedures

The IC-ICP-MS system for the determination of the arsenic species consisted of an LC 250 binary pump (Perkin-Elmer) coupled with an Elan 5000 ICP-MS (Perkin-Elmer) via a cross-flow nebuliser. A volume of 200 µl was always injected. The guard and separation columns used were IonPac AG7 and IonPac AS7 (Dionex), respectively. The mobile phase was a nitric acid solution with a flow rate of 1 ml min⁻¹. Elution was performed using the following gradient elution:¹⁶ 100% E1 for 1 min, linear gradient 0–100% E2 in 1 min, 100% E2 for 6 min, linear gradient 100–50% E2 in 2 min, 50% E2 for 5 min, linear gradient 50–0% E2 in 1 min, 100% E1 for 5 min, where eluent E1 was 0.4 mM HNO₃ and eluent E2 50 mM HNO₃. The cation concentration of the natural waters and the total amount of arsenic released after desorption were determined by atomic emission spectrometry (ICP-AES) using a Spectroflame P/M (Spectro A.I.) with pneumatic nebulisation (cross-flow nebuliser). The cations analysed were: K⁺, Mg²⁺, Ca²⁺, Al³⁺, Mn²⁺, Fe³⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺. The anion concentration was determined by ion chromatography using a DX 500 Dionex chromatograph with conductivity detection, an IonPac AS11 analytical column and an AG11 guard column. Elution was performed at a flow rate of 2 ml min⁻¹ by means of the Elugen EGC-KOH cartridge for the hydroxide gradient: 0.5 mM KOH for 2.5 min, 0.5–5.0 mM KOH in 3.5 min, 5.0–38.3 mM KOH in 12 min, 0.5 mM NaOH for 7 min. The anions analysed were: F⁻, Cl⁻, Br⁻, NO₂⁻, NO₃⁻ and SO₄²⁻.

Zeolites

The zeolites used in this work were natural Mexican and Hungarian clinoptilolites with a mesh size of 60 and are described in Table 1. They were characterised by X-ray diffraction, fluorescence spectroscopy and IR spectroscopy. Their activation was described in a recent paper.¹² Synthetic mordenite was used as a powder, while the natural zeolites ZME and ZH were studied as grain and as powdered samples.

Table 1 Studied zeolite types and their composition

Zeolite	Type	Si/Al		Fe2O3 (%)		Origin
		na^a	a^b	na	a
a na, Raw zeolitic material.b a, Activated zeolite.
ZH	Clinoptilolite	6.25	6.34	1.07	0.60	Hungary, natural
ZME	Clinoptilolite	5.18	5.54	1.76	1.56	Mexico, natural
ZS	Mordenite	6.97	7.38	0.18	0.20	Synthetic

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Procedures

A given volume of arsenic solution was equilibrated with a weighed amount of zeolite for a definite time. It was initially shaken for 3 min in an ultrasonic bath and then for 60 min in a horizontal shaker. Our initial experiments on the application of zeolites for arsenic removal¹² demonstrated that, during shaking, the temperature of the solution rose by 3 °C. This heating had no effect on the equilibrium state of the arsenic redox pair when the arsenic species were determined in the equilibrium solution immediately after immersing the zeolite into the solution. Due to the acid properties of the zeolites, static experiments prepared at acidic pH values maintain a very stable acidity. In the initial trials, the solid to solution ratio was varied and then maintained at 1 ∶ 5. The experiments were carried out at 23 °C using glass containers as batch reactors. Every week, 2 ml of the supernatant was removed for analysis. For the post-concentration trials by means of desorption, the zeolites in the batch solutions were filtered after adsorption. They were dried on filter paper and then at 100 °C for 3 h. A weighed amount (15 g) of the zeolite containing arsenic was put in contact with HCl 1 M, 2 M and 3 M, HNO₃ 1 M and H₃PO₄ 1 M. For the adsorption experiments, 75 ml of the arsenic solution was used, with 35 ml for desorption. This enabled the arsenic to be recovered into a smaller solution volume. After 7 days the supernatant solution was analysed by ICP-AES.

Calculations

The IC-ICP-MS chromatograms on mass ⁷⁵As were quantitatively evaluated by integrating the peaks corresponding to As(III) and As(V), applying the Peak Fitting Module of the software Microcal Origin 5.0. The relative species content was calculated as 100(C_a/C₀), where C_a is the adsorption equilibrium concentration of the solution in contact with the zeolite and C₀ is the initial concentration of the species in the solution in the absence of zeolite. The desorption ratio d was evaluated by means of the relation d = 100[C_d/(C₀ − C_a)], where C_d is the concentration of the resulting solution in contact with zeolite after desorption.

Results and discussion

Stability of arsenite and arsenate solutions

The complete water–zeolite systems studied are listed in Table 2. A detailed analysis of the natural Biela and Zimapán waters is presented in Table 3. Note the relatively high arsenate concentration and the absence of heavy metals, at least in concentrations above the ICP-AES detection limits in water samples, which are quoted in parentheses: Cu²⁺ (60 µg l⁻¹), Zn²⁺ (60 µg l⁻¹), Cd²⁺ (100 µg l⁻¹) and Pb²⁺ (270 µg l⁻¹). The most abundant anion in the Biela water was sulfate and in the Zimapán water chloride, since municipal water supplies are chlorinated by adding chlorine gas to water. Chlorine's powerful oxidising action acts as a germicide to safeguard public health. Both natural waters contained a high concentration of calcium compared to the levels of potassium, magnesium, aluminium, manganese, iron and nickel. Examination of Fig. 1 reveals that the species arsenite is stable for a long period in spiked (200 µg As l⁻¹) deionised water (Fig. 1a) as well as in the Biela water (Fig. 1c). By contrast, arsenite from a 1 mg l⁻¹ stock solution in deionised water is immediately transformed into arsenate (Fig. 1b) when it is diluted in drinking water to give a 200 µg l⁻¹ arsenate solution. Arsenate is stable in both spiked (200 µg As l⁻¹) deionised (Fig. 1d) and drinking (Fig. 1e) water. The stability of different arsenic solutions was first monitored without zeolites. Hence any future observation in the systems containing zeolites was related to the action of the zeolite. Each point in the figures of this study was calculated from a duplicate analysis by IC-ICP-MS. Comparable chromatograms presenting species separation can be consulted in other studies.^12,15,16

Fig. 1 Monitoring curves of the arsenic concentration variation in different acidified solutions: (a) deionised water spiked with As(III); (b) drinking water spiked with As(III); (c) Biela water spiked with As(III); (d) deionised water spiked with As(V); (e) drinking water spiked with As(V). Spiking levels, 200 µg l⁻¹. Arsenite, filled symbols; arsenate, open symbols.

Table 2 Monitored aqueous systems in contact with zeolites

Sample	Zeolite^a	As removed after 30 days (%)
		By ZME	By ZH
a For zeolite names, abbreviations and description, see Table 1.
Deionised water spiked with 600 µg l^−1 As(III) and acidified	ZH
	ZME
	ZS
Deionised water spiked with 200 µg l^−1 As(III) and acidified	ZH	95	74
Drinking water spiked with 200 µg l^−1 As(III) and acidified	ZME	97	97
Deionised water spiked with 200 µg l^−1 As(V) and acidified		98	95
Drinking water spiked with 200 µg l^−1 As(V) and acidified		97	95
Biela water spiked with 200 µg l^−1 As(III) and acidified		98	75
Biela water		98
Zimapán water acidified		95	51
Zimapán water		95	76

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Table 3 Origin and composition of the studied natural water samples

Sample	Origin	Anions/mg l^−1					Cations/mg l^−1			Arsenite/µg l^−1	Arsenate/µg l^−1
		F^−	Cl^−	Br^−	NO3^−	SO4^2−	Mg^2+	K^+	Ca^2+
Biela	Saxony, Germany	9.8	13.7	0.1	2.1	64.5	5.3	9.1	29.6	0	177
Zimapán	Hidalgo, Mexico	1.0	331	0.4	0.6	64.8	13.4		50.9	0	445

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Zeolite–arsenic solution systems

Different procedures for the activation of zeolites can have a significant impact on their behaviour. We performed acid washing, which removes the non-zeolitic minerals from the active crystalline zeolite. We studied the effect of activation on the behaviour of the synthetic ZS and the natural ZME zeolite (see Table 1) in contact with arsenic solutions. As shown in Fig. 2, the powdered and unactivated synthetic mordenite (ZS-pna) induced the oxidation of arsenite into arsenate. Hence we found 45% arsenate and 35% arsenite in the solution after contact with this zeolite. The difference (20%) can be considered as the sorbed amount of arsenite. By contrast, only arsenite was found in the solution after contact with the activated¹² synthetic mordenite (ZS-pa). The cause of arsenate formation has been ascertained previously.¹³ Arsenate sorption (13%) by ZME was the same when the zeolite samples were taken before activation in powder (ZME-pna) or grain (ZME-gna) form. However, after the activation of ZME, the removal of arsenate from the solution increased drastically. The reason for the different arsenic sorption capacities of zeolites is probably the different chemical compositions of these materials. Speciation in this case was useful since it revealed the transformation of arsenite into arsenate, an effect that could be used to lower the toxicity of waters polluted with arsenite. This result could not be detected by spectroscopic methods, which quantify the total arsenic content.¹⁴ Although activation of the synthetic mordenite did not improve the efficiency of arsenite removal, oxidation into arsenate was inhibited. The natural clinoptilolite ZME showed an increase in efficiency after activation from 15 to 90% (see Fig. 2), with no influence of the solid material particle size being observed. In the following studies, granular natural zeolites were used.

Fig. 2 Effect of zeolite activation on the removed and formed species in deionised water solutions (200 µg l⁻¹ arsenite) and non-acidified Biela water (177 µg l⁻¹ arsenate) after 7 days contact with powdered zeolite ZS [unactivated (ZS-pna) and activated (ZS-pa)], unactivated zeolite ZME [as powder (ZME-pna) and as grain (ZME-gna)] and activated ZME as grain (ZME-ga).

The first step in setting up the batch experiments was to select the ratio between the zeolite mass and the solution volume. For this purpose, two zeolites of the clinoptilolite type (see Table 1) and synthetic mordenite were selected. Arsenite and arsenate species were studied individually in deionised water containing 600 µg l⁻¹ arsenic, a value exceeding the concentration in the real water samples from Biela and Zimapán. From our experience with a larger set of natural zeolites,¹² the mass to volume ratio (m/V) interval analysed was 1 ∶ 100 to 1 ∶ 4, which corresponds with the fractions 0.01 and 0.25 on the x-axis of the respective plot (Fig. 3). The lowest removal efficiency for arsenite was achieved after 7 days contact with the mordenite ZS, as can be seen in Fig. 3A, while the highest removal of both arsenite (Fig. 3A) and arsenate (Fig. 3B) was accomplished by the zeolite ZME. Our previous kinetic studies¹² demonstrated that arsenate removal strongly depends on the contact time. For two different contact periods, Fig. 3B shows the dependence of arsenate removal on the mass to volume ratio after contact for 1 day and 7 days in the case of ZME. Both curves exhibit a decay profile, with the rate of change being appreciably smoothed out by 0.18. Therefore, the value m/V = 0.2 was chosen for subsequent experiments.

Fig. 3 Variation of the arsenite (A) and arsenate (B) concentrations as a function of the relation of zeolite mass to solution volume for the zeolites ZME, ZH and ZS after 7 days contact (A) and for zeolite ZME (B) after 1 day and 7 days contact. Solution initial concentration, 600 µg l⁻¹.

Zeolite–natural water systems

Based on this result, 15 g of the clinoptilolites ZME and ZH were immersed in 75 ml of the aqueous media systems specified in Table 2. The main idea was to apply zeolites by simply adding them to polluted natural water sources under static treatment, instead of filling cartridges with these materials, a procedure which would lead to a separate dynamic filtration system with defined energetic and maintenance requirements. For arsenite, the results presented in Fig. 4B indicate that, in the worst case, 75% is removed from a 200 µg l⁻¹ aqueous solution, as well as from the Biela water spiked with 200 µg l⁻¹ arsenite, after 25 days contact with the zeolite ZH. The resulting residual arsenite concentration would comply with the Mexican limit of 50 µg l⁻¹ total arsenic. In the case of the zeolite ZME (Fig. 4A and 4C), utilised for both arsenite and arsenate removal in all the aqueous media studied, the data are more satisfactory since 95% removal would signify a residual amount of 10 µg l⁻¹, which meets the German and American limits of 10 and 25 µg l⁻¹ arsenic, respectively. The amount of arsenite and arsenate remaining in the solution (10 µg As l⁻¹) was within the detection limit of the IC-ICP-MS method employed in this work for these species and the reported contents correspond to the mean value ± 5%. Long-term monitoring of the investigated solutions, together with speciation analysis, confirmed the removal of a given arsenic species from solution by zeolites ZME and ZH. After 5 days contact with ZME and ZH, 75% arsenate removal can be achieved in all systems (Fig. 4C) except in Zimapán water (Fig. 4D). After a longer contact period (20 days), 95% arsenate removal was reached. It was found that the presence of alkali cations and anions (see Table 3) in the natural matrices of the Biela and Zimapán waters did not affect the efficiency of the selected zeolites. The concentrations of iron, nickel, copper, zinc, cadmium and lead, if present, were below the ICP-AES analysis detection limit. An explanation for the generally better performance of ZME in removing arsenite and arsenate in comparison to ZH may be due to the almost twofold higher content of iron (see Table 1) in the zeolite ZME, which can form insoluble compounds containing arsenic, such as FeAsO₄.¹²

Fig. 4 Effect of contact time on the arsenite (A, B) and arsenate (C, D) concentration diminution of different acid solutions in contact with zeolites ZME (A, C) and ZH (B, D). (a) Deionised water spiked with As(III); (b) Biela water spiked with As(III); (c) Zimapán water; (d) deionised water spiked with As(V); (e) drinking water spiked with As(V); (f) drinking water spiked with As(III).

Monitoring solutions in contact with clinoptilolites indicates that zeolite performance is acceptable for the removal of arsenite and arsenate in both deionised and natural water. To elucidate whether or not the addition of HCl to the Biela and Zimapán water affects the removal of arsenate, long-term analysis of these systems was performed and compared with the acidified systems (see Fig. 5). The acidified medium has practically no effect on the activity of the zeolite ZME (Fig. 5A) in contact with both water matrices (95% arsenic removal), whereas the zeolite ZH (Fig. 5B) removed 75% of arsenate when the Zimapán water was not previously acidified and only 50% when the sample was acidified. Slight differences in the results obtained for the two natural water matrices could be attributed to their ionic composition. The removal results are summarised in Table 2.

Fig. 5 Effect of the acidification of the batch solution on the removal of arsenate in Biela water and Zimapán water in contact with zeolites ZME (A) and ZH (B). Full lines, without HCl addition; broken lines, with HCl addition. (a) Biela water without zeolite; (b) Biela water; (c) Zimapán water.

Zeolite regeneration by desorption

Natural zeolites transformed into contaminated waste can be stored or regenerated. Their storage after arsenic preconcentration requires the strong fixation of arsenic to the solid to avoid pollutant transfer from water to soil. When fixation is weak, desorption can be induced in order to regenerate the zeolites and to produce the recovery of arsenic. In order to manage adequate solution volumes during desorption, smaller solution volumes after desorption can be produced in comparison with those taken during adsorption. A diminution in the solution volume handled is then described by quotient values V_d/V_a < 1 in Table 4, where V_d and V_a are the volumes in the desorption and adsorption steps, respectively. In order to achieve arsenic desorption from the zeolites, the zeolites enriched with arsenic were treated with different acids. Table 4 shows the desorption ratio [d (%)] in the different trials and demonstrates the viability of the procedure when 1 M HCl was used. In general, almost the total amount of sorbed arsenic at the zeolite ZH can be released (92 < d < 100%) with the ratio V_d/V_a = 0.5. In the natural Zimapán water, the increase in hydrochloric acid concentration from 1 to 3 M had a small effect on desorption, whereas nitric and phosphoric acid did not bring about the recovery of the adsorbed arsenic in the Biela water. Another factor influencing the desorption ratio d is the mass to desorption volume relation m/V_d, as was demonstrated by us above for adsorption. The variation of m/V_d from 0.4 to 0.2 led to an increase in the desorbed ratio from 48 to 79% for the Biela water in contact with zeolite ZME.

Table 4 Desorption of arsenic by leaching the zeolites for 7 days in different media^a

Sample	Zeolite	dtotal As (%)	Leaching solution		m/Vd	Vd/Va
			Acid	Molarity
a d, Desorption ratio; m, zeolite mass (in g); Vd, volume in the desorption step (in ml); Va, volume in the adsorption step (in ml).
Deionised water spiked with 200 µg l^−1 As(III) and acidified	ZME	95	HCl	1 M	0.4	0.5
	ZH	100
Drinking water spiked with 200 µg l^−1 As(III) and acidified	ZME	57
	ZH	100
Deionised water spiked with 200 µg l^−1 As(V) and acidified	ZME	71
	ZH	100
Drinking water spiked with 200 µg l^−1 As(V) and acidified	ZME	66
	ZH	100
Biela water spiked with 200 µg l^−1 As(III) and acidified	ZME	48
	ZH	92
Zimapán water	ZH	92	HCl	1 M
		100		2 M
		100		3 M
Biela water	ZME	48	HCl	1 M
		0	HNO3, H3PO4
		79	HCl		0.2	0.3

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Conclusions

These results demonstrate that natural zeolites remove arsenic from contaminated natural waters. Additionally, the materials studied can be regenerated by thorough leaching in acid solutions to induce desorption of the previously fixed arsenic species.

Acknowledgements

MPEG is grateful to the Alexander von Humboldt Foundation, Germany, for providing a Georg Forster fellowship to carry out this research work. We are thankful to F. Pérez-Moreno for kindly providing the Zimapán water samples.

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Footnote

† Presented at the Whistler 2000 Speciation Symposium, Whistler Resort, BC, Canada, June 25–July 1, 2000.

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