Dynamic arsenic removal on a MnO2-loaded resin

Journal of Colloid and Interface Science 280 (2004) 62–67 www.elsevier.com/locate/jcis

Dynamic arsenic removal on a MnO2 -loaded resin Véronique Lenoble, Christophe Chabroullet, Raad al Shukry, Bernard Serpaud, Véronique Deluchat, Jean-Claude Bollinger ∗ Laboratoire des Sciences de l’Eau et de l’Environnement, Faculté des Sciences, 123 avenue Albert Thomas, 87 060 Limoges, France Received 16 April 2004; accepted 27 July 2004 Available online 15 September 2004

Abstract Previous batch studies on a polystyrene matrix loaded with manganese dioxide, synthesized from an anionic commercial resin in chloride form, have proven the efficiency of this sorbent in As(V) and As(III) removal. This solid is now tested with column experiments to predict its behavior in a treatment process. An artificial water, with a composition in major ions similar to that of granitic water, often contaminated with arsenic, was prepared. This artificial water was used to simulate arsenic removal processes under near-natural conditions and with a stable composition. Furthermore, the hydride generation AAS analytical method was optimized to measure low arsenic concentrations (1 to 20 µg/L).  2004 Elsevier Inc. All rights reserved. Keywords: Arsenic removal; Artificial water; MnO2 -loaded resin

1. Introduction Arsenic is the 20th most abundant element in the earth’s crust and, thus, it is widely distributed in the environment (especially as arsenopyrite or as metal arsenates). It is introduced into water through natural and anthropogenic sources [1,2]: dissolution of mineral ores, industrial effluents and agricultural activities, and also atmospheric deposition [3]. The most common species present in water are the inorganic species: As(V), predominant in well-oxygenated waters, and As(III), predominant in groundwaters [4]. The reduced state, As(III), is much more toxic, more soluble, and more mobile than the oxidized As(V) [2]. The presence of arsenic in drinking water, even at low concentrations, is a threat to human health [5]; thus the standards for arsenic in drinking water have been greatly reduced throughout the world, for example in Europe (Directive 98/83/CE), and in the USA [6], where they were lowered to 10 µg As/L. Processes to remove excess arsenic from drinking water are therefore urgently required. Furthermore, * Corresponding author. Fax: +33-555-457-459.

E-mail address: [email protected] (J.-C. Bollinger). 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.07.034

efficient methods to measure low arsenic concentrations (especially in groundwaters) must be developed. Arsenic removal on a polystyrene resin loaded with manganese oxide, allowing the fixation of As(V) and the simultaneous oxidation and removal of As(III), was previously studied [7]. These experiments were first carried out in deionized water and only in batches, but they were aimed at being applied to natural waters, especially to granitic waters, which are often polluted with arsenic. In order to work closer to natural conditions, we found it advantageous to use a medium with the same major ionic composition as the targeted granitic waters, but with no other natural components such as trace elements or colloids. An artificial solution that emulates the major ion composition of natural granitic waters was synthesized according to a protocol already used by Gal et al. [8] to prepare an artificial saliva and by Smith et al. [9] for synthetic fresh waters. This artificial water allows working on a medium with a given composition, while day-to-day variations in As level and speciation may be important, as exemplified by Roussel et al. [10,11]: from 0.7 to 65 mg As/L in the saturated zone of tailings from a former gold mine, or from <0.01 to 0.22 mg As/L in the nearby Isle river. These variations were correlated to the succession

V. Lenoble et al. / Journal of Colloid and Interface Science 280 (2004) 62–67

of hydrological processes such as the recharge of the (contaminated) reservoirs and their discharge, induced by rainfall events. Furthermore, when As(III) and/or As(V) is added to artificial water, arsenic speciation is known, which is not always the case in natural conditions. So this artificial water, spiked with As(III) and/or As(V), was used to simulate arsenic removal in a water-treatment plant. For these experiments, the atomic absorption spectrometry hydride generation method was optimized to measure low arsenic concentrations (<10 µg/L) in this medium.

2. Materials and methods All solutions were prepared with high-purity deionized water (resistivity 18.2 M
cm) obtained with a Milli-Q water purification system (Millipore Milli-Q and Elgastat Prima 1-3). All glassware was cleaned by soaking in 10% HNO3 and rinsed three times with deionized water. 2.1. Artificial water Based on an original compilation of 23 groundwater compositions from granitic areas, used for the production of drinking water but with 10
As (µg/L)
100, the determination of average major ion concentrations and physicochemical parameters was carried out (Table 1). The artificial water was then prepared according to Smith et al. [9], but this protocol does not include silica, an important component of granitic waters; therefore we had to modify the preparation by adding a silica solution to the mixture. The final pH was above the required value, as the silica solution is strongly basic, but this solution could not be acidified because complex polymers can be formed at pH < 11.5, whereas in natural systems silica occurs as monomers [12]. Table 1 Characterization of groundwaters (compilation of 23 groundwaters’ compositions from granitic areas) used to produce drinking water, with arsenic concentration between 10 and 100 µg/L, and ion chromatography analysis of the prepared artificial water: results in mg/L Groundwater analysis

Artificial water analysis

Lower value

Average

Higher value

Average

Standard deviation

Calcium Magnesium Sodium Potassium Sulfate Chloride Nitrate Silica

1 0.3 1.81 0.6 0.5 1.5 0.9 7.52

3.6 1.6 6.5 1.1 2.5 6.0 7.7 16.8

6.5 3.2 8.6 2.3 4.3 7.8 23.7 28

3.6 1.5 7.0 0.9 4.0 8.6 10.9 n.a.

0.2 0.1 0.5 0.1 0.4 0.6 0.5 n.a.

pH Alkalinity (mg CaCO3 )

5.4 8

5.9 14

6.1 25

6.0 9

0.1 2

Note. N.a.: not analyzed.

63

Our artificial water was prepared according to the following optimized steps (Table 2): — Initial concentrated stock solutions were prepared by weighing the amount of each salt necessary to reach the required concentration of each major ion. The following salts and reagents were used to provide the targeted concentrations: Ca(NO3 )2 ·4H2 O (Fluka, >99.5%), MgCl2 ·6H2 O (Prolabo, Normapur), KHCO3 (Acros, Pour Analyse), and Na2 SiO3 ·5H2 O (Fluka, Pour Analyse). — These solutions were then arranged into groups to prevent incongruent solubility, thus producing two concentrated secondary stock solutions, named S1 and S2. — An S3 solution was also prepared, whose concentration was a compromise between the required dissolved silica concentration and its sodium contribution. — These three stock solutions (S1, S2, and S3) were mixed together to produce the final solution. — The decrease in pH and the balance of hydrogenocarbonate were supplied by bubbling air through this solution for 12 h. — The balance of Cl− required in the solution was then supplied by adding 10 µL of 37% HCl (Prolabo, Normapur). — The balance of SO2− 4 required in the solution was then supplied by adding 100 µL of 18 M H2 SO4 (Prolabo, Normapur). The composition of this water and the reproducibility of the protocol were checked by a Dionex DX120 ion chromatograph, with an anionic AS9.HC column (Na2 CO3 as mobile phase) and a cationic CS12.A column (CH3 SO3 H as mobile phase) (n > 5 for each preparation). It appeared (Table 1) that pH and alkalinity values (measured according to the ISO 9963-1 standard [13]) were near the average range (respectively 6.0 ± 0.1 and 9 ± 2 mg CaCO3 ). Ionic balance was respected, as silica was present only as uncharged silica. 2.2. Arsenic and manganese analysis During dynamic experiments, arsenic and manganese determinations (for concentrations >20 µg/L) were carried out using a Varian SpectrAA 800 graphite furnace atomic absorption spectrometer (GFAAS), with Zeeman background correction. All measurements were based on integrated absorbance and carried out using hollow-cathode lamps (Varian) at 193.7 and 279.5 nm for arsenic and manganese, respectively. For arsenic, the modifier used was a palladium– magnesium mixture (20 µL of MgNO3 solution at 10 g/L in 200 µL of PdNO3 solution at 1 g/L), pretreatment temperature was 1400 ◦ C, and atomization temperature was 2500 ◦ C. The calibration range was 20–100 µg As/L, the accuracy was 5%, and RSD was ±7% (repeatability tests, n > 100). For manganese, the modifier used was a magnesium solu-

64

V. Lenoble et al. / Journal of Colloid and Interface Science 280 (2004) 62–67

Table 2 Detailed preparation of 1 L of artificial water Initial stock solutions Salt Purity Required molar (%) mass (g) weight taking purity into account

Secondary stock solutions Initial stock solution volume (L)

Initial stock Dilution Initial solution factor stock concentrasolution sample tion (mmol/L) (mL)

Stock solution volume (L)

Artificial water Secondary Dilution Secondary stock factor stock solution solution concentrasample tion (mL) (mmol/L)

Artificial water volume (L) (S1 + S2 + S3)

Concentration in final artificial water (mmol/L)

98 98

3.8474 2.2185

0.1 0.1

159.665 106.943

100 100

5 5

S1 0.5 0.5

1.597 1.069

20

50

1

0.0798 0.0535

KHCO3

100.12 100

1.0293

0.1

102.297

250

2

S2 0.5

0.409

20

50

1

0.0205

Na2 SiO3 ·5H2 O

212.14

1.6403

0.5

100

10

1

0.15

Ca(NO3 )2 ·4H2 O 236.15 MgCl2 ·2H2 O 203.30

97

15

tion, pretreatment temperature was 1200 ◦ C, and atomization temperature was 2400 ◦ C. The calibration range was 3–12 µg Mn/L, the accuracy was 3%, and RSD was ±5% (repeatability tests, n > 20). A colorimetric method, based on similar properties of phosphate and arsenate, allows the measurement of As(V) concentrations through the formation of a molybdate-blue complex [14]. 2.3. Hydride generation For arsenic concentrations lower than 20 µg As/L, hydride generation atomic absorption spectrometry (HGAAS) was used, with a calibration range from 1 to 20 µg/L. The principle is to volatilize arsenic from As(III) to arsine AsH3(g) , due to a reaction of nascent hydrogen on arsenic. This nascent hydrogen comes from the decomposition of borohydride ions (BH− 4 ) by concentrated HCl [15]. Arsine obtained from As(III) gives the best signal; therefore, all As(V) in the sample should be previously reduced to As(III) by adding KI and HCl. All experiments were thus carried out with our optimized protocol: 1 M HCl and 2% KI were added to the sample, and a waiting time of 3 h allowed complete reduction of As(V); then the whole was mixed with 5 M HCl and 0.3% NaBH4 . The products used were KI (Merck, >99.5%, As < 0.00001%), HCl (Merck, 32%, As < 0.000001%), and NaBH4 (Fluka, >96%, As < 0.000005%). Sodium borohydride solutions were prepared daily by dissolution in 0.5% NaOH w/v (Merck, >99%, As < 0.0001%) to prevent BH− 4 decomposition and to improve dissolution. The apparatus was a VGA-77 (Varian) connected to a SpectrAA 220 flame AAS (Varian). The analysis parameters were as follows: slit width 0.5 nm, sample flow 7 mL/min, HCl and NaBH4 solution flows 1 mL/min, delay before measurement 45 s, and air and acetylene flows 3.5 and 1.5 mL/min respectively. The resulting value is the average of three measurements for each standard or sample. Detec-

S3 undiluted

tion was performed using an arsenic hollow-cathode lamp (Varian) at 193.7 nm. Experiments were performed with deionized water or artificial water spiked with As(III) and/or As(V). When regularly used, this optimized protocol easily allowed the measurement of arsenic concentrations below 10 µg/L. The standard deviation was below 3% and 5% for measurements in deionized water and in artificial water, respectively. The detection limit was 0.1 µg/L (for both As(III) and As(V)) and the calibration range was 1–20 µg/L. Arsenic impurities in the components of the artificial water can explain the higher standard deviation observed when using it. 2.4. Removal experiments The solid sorbent used in this study is a labmade polystyrene resin covered with manganese dioxide. An anionexchange resin, based on a microporous styrene/divinylbenzene copolymer with a trimethylammonium cation-active functional group, was used to prepare our media, according to the slightly modified Prigent et al. [16] protocol: 1 g of Dowex 1 × 8–400 dry resin in the chloride form (Sigma, particle size 80 µm) was shaken for 5 h with 100 mL of 0.05 M KMnO4 solution. After filtration, the excess permanganate ions and nonretained oxides were rinsed away with deionized water. The thin black solid was then air-dried (at T
40 ◦ C) and sheltered from light in closed containers. This sorbent is thereafter named R-MnO2 . Removal experiments were carried out in batch reactors and columns, with deionized water and artificial water, both spiked with As(III) and/or As(V). For batch retention experiments, the solid suspension was 1.6 g/L, contact time was 2 h and pH measurements were realized before and after experiments (optimized parameters in pure water [7]). The column removal experiments were carried out in a glass column, diameter = 1 mm, bed height = 5 cm, sample flow = 5 mL/h, void volume = 4 mL. The solution was injected upward in order to avoid preferential pathways.

V. Lenoble et al. / Journal of Colloid and Interface Science 280 (2004) 62–67

(a)

(b) Fig. 1. Retention isotherms in deionized water (1) and artificial water (!) for As(III) (a) and As(V) (b) on R-MnO2 : batch experiments. Symbol size corresponds to uncertainty.

3. Results and discussion 3.1. Batch experiments It was previously shown that the MnO2 sites scattered on the resin beads’ surface oxidize As(III) and that the R-MnO2 sorbent retains As(V) [7]. When As(III) and As(V) retention during batch experiments is studied, maximal retention capacities are the same with deionized or artificial water (Figs. 1a and 1b): 53 mg As(III)/g resin (0.7 mmol/g) and 22 mg As(V)/g resin (0.3 mmol/g). The major ions therefore do not have any influence on As(III) fixation on R-MnO2 under our operating conditions. As(III) removal was higher than As(V) removal, whereas the same amount was expected: thermodynamics showed that the formation of a manganese(II) arsenate precipitate, Mn3 (AsO4 )2(s) , can occur [7]. The retention capacity of this resin is 0.7 mmol As(III)/L and 0.3 mmol As(V)/L [7], which is above other common sorbent capacities. However, sorbents loaded with Fe(III) present high capacities too, and both Fe(III)- and MnO2 loaded sorbents can remove both arsenic(III) and arsenic(V) and oxidize the remaining As(III) into the less toxic As(V) form [17]. 3.2. Column experiments Dynamic experiments were necessary to predict R-MnO2 behavior in a treatment process and its ability to remove arsenic either to a drinking level or, at least, to the concentration required for an industrial water to be discharged

65

into natural waters. This study was necessary to complete the batch experiments. In column experiments, particle size is a decisive parameter: the smaller the size, the higher the effective treatment capacity of the bed [18]. Such parameters were taken into account in the experimental part, as the Dowex resin used has small beads (80 µm in diameter). During dynamic studies, due to the elution of manganese released, no precipitation of Mn3 (AsO4 )2(s) was possible in situ. This phenomenon led to the fact that, in column experiments, the maximal retention capacities were the same toward As(III) and As(V), i.e., 4 mg/g R-MnO2 (Figs. 2a and 2b). However, it should be noted that these values were below the maximal retention capacities obtained in batch experiments. This can be partly explained by the flow rate, which may not provide sufficient contact time between RMnO2 and arsenic. In batch experiments, when the resin is oversaturated with arsenic, As(III) was still oxidized and the residual arsenic was only present as As(V). In dynamic experiments, As(V) measurement, carried out using a colorimetric method as defined in the experimental section, proved that residual arsenic(III) was not completely oxidized. This may be due to the fact that the oxidizing sites were probably less accessible in column experiments than in batch experiments. No pH variation could be observed during dynamic experiments, whereas a decrease in pH of 0.3 for As(V) and of 1.5 for As(III) was noticed in batch experiments; this confirms the absence of Mn3 (AsO4 )2(s) formation, as our previous study has shown that proton release was linked to precipitate formation [7]. As can be seen in Figs. 2a and 2b, manganese release during arsenic retention in dynamic experiments was above 0.05 mg Mn/L (or 0.91 µmol/L), which is the maximal value allowed by the European directive concerning manganese release during drinking water treatment. So R-MnO2 can be used in the case of drinking water treatment, provided a further stage removing manganese is included in the process. This removal as a precipitate can be done by acting on pH or oxygen content. It should be noted that for in situ application, the flow rate should be adapted, as the value used in these experiments is quite below typical flow rates in water treatment. It can be seen from the same figures that manganese and arsenate concentrations are below the maximum contamination levels (MCL) allowed by the European Directive concerning manganese and arsenate concentration in an industrial water to be discharged into natural waters: 1 mg Mn/L (or 18 µmol Mn/L) and 0.05 mg As/L (or 0.7 µmol As/L). R-MnO2 can then be used to treat such effluents in a one-step process. Therefore, dynamic experiments allowed testing R-MnO2 use in a treatment process, either for drinking water production, providing a two-stage process is set up, or for industrial water to be discharged into natural waters within a one-step process.

66

V. Lenoble et al. / Journal of Colloid and Interface Science 280 (2004) 62–67

(a)

(b) Fig. 2. Column experiments with As-spiked artificial water: study of arsenic residual concentration (E), manganese release (2), and pH values (Q) during As(III) retention (a) and As(V) retention (b). MCL for discharge of industrial waters are indicated for both Mn and As.

4. Conclusion In this study, the optimization of an artificial water preparation protocol allowed a sample of water representative of that encountered in granitic areas, often contaminated with arsenic, to be prepared. The composition of this artificial water is a result of the compilation of physicochemical properties of groundwaters used to produce drinking water. This composition and the reproducibility of the protocol were checked by ion chromatography measurements. A polystyrene resin loaded with manganese oxide, previously considered in batch experiments, was now studied for its retention capacities in dynamic experiments and using artificial water spiked with arsenic. This allowed the identification of various important details. The retention capacities

were the same for As(III) and As(V), and these values were below the maximal retention capacities obtained in batch experiments; residual arsenic(III) was not completely oxidized and no pH variation occurred during column experiments, whereas a decrease was noticed in batch experiments. These results are due to manganese release during the column process. These dynamic experiments, under near-natural conditions, allowed us to validate our arsenic removal process with a polystyrene resin loaded with manganese oxide. With due adaptation of both the flow rate and the bed volume, this MnO2 -loaded resin can be applied to drinking water production with a two-stage process. Furthermore, this sorbent can also be interesting in the case of industrial waste waters or acid mine drainage treatment.

V. Lenoble et al. / Journal of Colloid and Interface Science 280 (2004) 62–67

Acknowledgments The authors thank Mr. Jaouen, Mr. Lajarthe, Mr. Conchard, and Mr. Duchez (local employees of the French Health and Human Services Administration) for providing us with the data on groundwaters. This work was made possible by grants from the Contrat de Plan Etat-Région Limousin and the Conseil Régional du Limousin. References [1] [2] [3] [4] [5] [6]

M.J. Kim, J. Nriagu, S. Haack, Chemosphere 52 (2003) 623. P.L. Smedley, D.G. Kinniburgh, Appl. Geochem. 17 (2002) 517. J.F. Ferguson, J. Gavis, Water Res. 16 (1972) 1259. J. Matschullat, Sci. Total Environ. 249 (2000) 297. B.K. Mandal, K.T. Suzuki, Talanta 58 (2002) 201. A. Brandstetter, E. Lombi, W.W. Wenzel, D.C. Adriano, in: D.L. Wise, D.J. Trantolo, E.J. Cichon, H.I. Inyang, U. Stottmeister (Eds.), Remediation Engineering of Contaminated Soils, Dekker, New York, 2000, Chap. 33, p. 715.

67

[7] V. Lenoble, C. Laclautre, B. Serpaud, V. Deluchat, J.-C. Bollinger, Sci. Total Environ. 326 (2004) 197. [8] J.Y. Gal, Y. Fovet, M. Adib-Yadzi, Talanta 53 (2001) 1103. [9] E.J. Smith, W. Davison, J. Hamilton-Taylor, Water Res. 36 (2002) 1286. [10] C. Roussel, H. Bril, A. Fernandez, Hydrogeologie (1998) 3. [11] C. Roussel, H. Bril, A. Fernandez, J. Environ. Qual. 29 (2000) 182. [12] C.C. Davis, W.R. Knocke, M. Edwards, Environ. Sci. Technol. 35 (2001) 3158. [13] Afnor Qualité des Eaux, vol. 2, ISO 9963-1 standard, Afnor édition, Paris, 1997, pp. 115–125. [14] V. Lenoble, V. Deluchat, B. Serpaud, J.-C. Bollinger, Talanta 61 (2003) 267. [15] P. Carrero, A. Malavé, J.L. Burguera, M. Burguera, C. Rondòn, Anal. Chim. Acta 438 (2001) 195. [16] S. Prigent, F. Clanet, M. Rousseau, Cah. Assoc. Sci. Eur. Eau Santé 4 (1999) 35. [17] E. Lombi, W.W. Wenzel, R.S. Sletten, J. Plant Nutr. Soil Sci. 162 (1999) 451. [18] S. Ouvrard, M.O. Simonnot, P. de Donato, M. Sardin, Ind. Eng. Chem. Res. 41 (2002) 6194.