Effect of competitive ions on the arsenic removal by mesoporous hydrous zirconium oxide from drinking water

Effect of competitive ions on the arsenic removal by mesoporous hydrous zirconium oxide from drinking water

Materials Research Bulletin 45 (2010) 1628–1634 Contents lists available at ScienceDirect Materials Research Bulletin journal homepage: www.elsevier...

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Materials Research Bulletin 45 (2010) 1628–1634

Contents lists available at ScienceDirect

Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu

Effect of competitive ions on the arsenic removal by mesoporous hydrous zirconium oxide from drinking water Anatoly Bortun a, Mila Bortun a, James Pardini a, Sergei A. Khainakov b, Jose´ R. Garcı´a b,* a b

MEL Chemicals Inc., 500 Barbertown Point Breeze Road, Flemington, NJ 08822, USA Departamento de Quı´mica Orga´nica e Inorga´nica, Universidad de Oviedo, C/ Julia´n Claverı´a, 8, 33006 Oviedo, Spain

A R T I C L E I N F O

A B S T R A C T

Article history: Received 13 January 2010 Received in revised form 5 July 2010 Accepted 16 July 2010 Available online 23 July 2010

Adsorption properties of 302-type commercially available hydrous zirconium oxide (302-HZO) towards arsenic and some competitive anions and cations have been studied under batch and column conditions. Due to amphoteric properties, anion exchange performance of hydrous zirconium oxide is pH dependent. Media exhibits high affinity towards arsenic in a broad pH range, with high adsorption capacity at pH < 8. It was shown that silicate and phosphate ions are arsenic’s main competitors affecting media adsorption capacity. Presence of transition metal cations in <1 ppm does not affect 302HZO capacity on arsenic, whereas alkaline-earth cations improve arsenic removal. The possibility for significant increase of 302-HZO adsorption capacity on arsenic at pH > 8 by using ‘‘solid acidifier’’ technique is discussed. Results of 302-HZO field trials are presented. ß 2010 Elsevier Ltd. All rights reserved.

Keywords: A. Inorganic compounds A. Oxides D. Phase equilibria

1. Introduction Arsenic compounds are rather widely distributed in nature and they can be often found in groundwater and drinking water, mainly due to erosion and weathering of As-containing soils and minerals, as well as from industrial effluents and via atmospheric deposition (burning of fossil fuels). Considering the high toxicity of arsenic to humans (potential carcinogenic effect), its removal from water sources used for drinking purposes is very important. There are known different methods of drinking water treatment targeting large municipal-scale or small water treatment utilities (homes, wells). Among them are coagulation/filtration, reverse osmosis, electrodialysis, adsorption and ion exchange, etc. [1–4]. Ion exchange treatment with the use of synthetic inorganic adsorbents has recently attracted attention mainly because of their much higher selectivity towards arsenic than that of organic resins. According to the literature data, the most promising inorganic adsorbents for selective arsenic removal are those based on alumina, ferric oxides/hydroxides, and titanium or zirconium hydroxides [5–7]. Activated aluminas and ferric hydroxides have received the most attention and arsenic adsorption on them has been studied in detail. It was found that due to amphoteric properties of these oxides, arsenate adsorption is pH dependent and media capacity decreases with increasing pH and that arsenate is selectively/specifically adsorbed by forming inner-sphere

* Corresponding author. Tel.: +34 985 103030; fax: +34 985 103446. E-mail address: [email protected] (J.R. Garcı´a). 0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.07.011

surface complexes [8]. However, both activated alumina and ferric hydroxides have several major drawbacks, namely, low chemical stability in the presence of acids and bases that could result in leaching of adsorbed arsenic from the spent media during disposal, poor kinetics of exchange and low adsorption capacity of activated alumina. Hydrous zirconium oxide, however, shows high resistance to acids, alkalis, oxidants and reductants and exhibits also a remarkable affinity towards the arsenate ion [9]. Unfortunately, arsenic adsorption on zirconium oxides has not been studied in as much detail as on aluminas and ferric oxides. This can be related to the higher price of zirconium-based adsorbents and difficulties in making them in granular form suitable for column type applications. MEL Chemicals has successfully solved the latter problem by developing a process for making a variety of zirconium-based adsorbents, including hydrous zirconium oxides [10], in the form of 30–60 mm micro-spheres. Such adsorbents can be used in thin bed applications (bed depth 5–20 cm) without creating significant back pressure, but enabling quick kinetics of adsorption superior to any resin (residence time 5–20 s). Currently MEL Chemicals produces two grades of hydrous zirconium oxides (HZO’s) with narrow 20–30 mm particle size distribution on commercial scale: microporous 301-type and mesoporous 302-type oxide. The results of physical and chemical characterization of 301-HZO and 302-HZO type adsorbents, with main stress on the effect of temperature on particle size, porosity change and efficiency of arsenic sorption from aqueous solutions was recently reported [11], showing that 302-HZO is a mesoporous material that differs significantly from other commercially available zirconium hydroxide products: it is more basic, exhibits higher affinity towards

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A commercial grade of mesoporous hydrous zirconium oxide 302-type with narrow particle size distribution (d50  25–30 mm) has been used in all adsorption experiments. The 302-HZO used has the following properties: packed density = 0.95 g/cm3; Si < 500 ppm; SO4 < 200 ppm; Cl < 500 ppm; surface area > 300 m2/g; pore volume > 0.20 cm3/g; average pore diameter > 2.5–3.0 nm. 302-HZO is resistant to mineral acids (except HF), bases, oxidants, and organic solvents. The material is non-toxic and is NSF Standard 61 certified.

water was used having the following composition: [Ca2+] = 40 mg/ L; [F] = 1 mg/L; [Mg2+] = 12 mg/L; [SiO2] = 20 mg/L; [SO42] = 50 mg/L; [As(V)] = 0.05 mg/L; [N-NO3] = 2 mg/L; pH 8.5; [NSF/ ANSI 53 – 2002, Addendum 1.0 – 2002]. The challenge water has been passed through the adsorbent bed with a constant flow rate of 360 mL/h. pH of challenge water has been adjusted before experiment to required value (from 6.5 to 9.0) by addition of NaOH or HCl. In some cases concentrations of Ca2+ and silicate ions has been deliberately varied in a broad range. Water samples purified by 302-HZO have been collected (every 200 bed volumes). Standard 10 in. IsoluxTM cartridges (0.5 L) have been used for media testing on a pilot plant unit. NSF53 challenge water has been passed through the cartridge at a fixed flow rate 2.85 L/min. Water has been passed through the media until exhaustion in cycles of 30 min run/30 min idle. Arsenic concentration in all test solutions has been analyzed using Varian 220 AAS with graphite furnace. Concentrations of all other ions of interest have been analyzed by ICP method on Varian ICP-OES.

2.2. Adsorption properties

3. Results and discussion

302-HZO adsorption capacity for arsenic as a function of pH has been determined under batch conditions. 0.1 g of 302-HZO has been contacted with 100 mL of 1.0 mM solution NaH2AsO4 under constant shaking for 24 h. pH of solutions have been adjusted to target value by addition of 0.1 M NaOH or 0.1 M HCl. Adsorption capacity has been calculated according to the formula

Hydrous zirconium oxide 302-HZO has an iso-electric point (IEP) of pH  7.5, which is in agreement with literature data [10]. At the IEP hydrous zirconium oxide has equal amounts of positively and negatively charged hydroxyl groups. Below the IEP positively charged hydroxyl groups predominate, and anion adsorption increases. At pHs higher than the IEP, negatively charged groups predominate and cation adsorption increases at the expense of anion adsorption. Arsenic adsorption on 302-HZO follows this rule (Fig. 1). Arsenic uptake is high in acid media (160 mg As/g at pH 2–3) and decreases gradually with pH increase reaching <5 mg As/g at pH 11. The ability of 302-HZO to adsorb arsenic at pHs higher than IEP, which could be a result of media surface non-homogeneity, makes it possible to use hydrous zirconium oxide for arsenic removal in a broad pH range covering pH of drinking water (pH 6.5–8.5) and arsenic contaminated ground water (pH 4.5–10.5). Iso-electric points for several other adsorption materials are: 7.7–9.3 (Al2O3); 6.0–7.2 (CeO2); 7.2–9.2 (Fe2O3); 4–7 (TiO2) [12]. Arsenic adsorption for all materials will follow the pattern shown in Fig. 1 except that the curve will be shifted left or right depending in the relative value of the IEP. Drinking water and ground water are complex multi-component systems containing different cations and anions in amounts often [(Fig._1)TD$IG]exceeding arsenic concentration by a thousand times. Considering

arsenic (V), show higher kinetics of ion exchange and improved thermal stability that allows retention of adsorption properties even after treatment at 500–550 8C. In this paper, the effect of some competitive anions and cations on the arsenic removal from drinking water by 302-HZO hydrous zirconium oxide have been studied under batch and column conditions. 2. Experimental 2.1. Materials



ðC in  C eq ÞV 1000M

where G is the ion exchange capacity (mmol/g), Cin the initial As concentration in solution (mM), Ceq the As concentration in solution after reaching equilibrium (mM), M the weight of adsorbent (g), and V is the volume of solution (mL). 302-HZO affinity towards arsenic and the effect of some competitive anions has also been studied under batch conditions. 5.00 g of adsorbent has been contacted with 1.0 L of test solution for 18 h under constant mixing. After that supernatant liquor has been analyzed for anions of interest and equilibrium pH was recorded. Three types of test solutions have been used. In the first type only one anion (AsO43, PO43, SiO32, VO3, BO33, CrO42 or SeO32) was present and solution concentration was 1.0 mM. Under such conditions maximum theoretical uptake is 0.2 mmol/g, which is less that 5% of media total capacity. Second type of test solutions contained equimolar (1.0 mM) amounts of two anions: AsO43 + PO43, AsO43 + SiO32, AsO43 + VO3 or AsO43 + BO33. The third type of solution contained equimolar (1.0 mM) amounts of five anions: AsO43 + PO43 + SiO32 + VO3 + BO33. pH in test solutions was adjusted to target value with 5% NaOH or 5% HCl. Media adsorption efficiency in removal of anions was presented in two ways: as percentage of uptake or distribution coefficients (Kd) versus pH. Distribution coefficients have been calculated according to the formula: Kd ¼

ðC in  C eq ÞV MC eq

where Kd is the distribution coefficient (mL/g), Cin the initial concentration in solution (mM), Ceq the concentration in solution after reaching equilibrium (mM), M the weight of adsorbent (g), and V is the volume of solution (mL). High values of distribution coefficients indicate high selectivity toward the respective ions. The following conditions have been used for column experiments: 1.00 g of 302-HZO hydrous zirconium oxide has been put into glass columns with an 8 mm inner diameter. NSF53 challenge

Fig. 1. 302-HZO capacity for As(V) removal as a function of pH.

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this potential, the effects of their competition on arsenic removal by 302-HZO has been studied under batch and column conditions. 3.1. Competitive effect of anions on arsenic adsorption under batch conditions Adsorption of the anions most common in drinking water and groundwater, on 302-HZO is shown in Fig. 2. As seen all tested ions, with the exception of BO33, are removed almost quantitatively (>99.9%) from individual solutions at pH 8–9. The situation changes in basic solutions where differences in media affinity become clear. CrO42, BO33 and SeO32 uptake decreases with pH increase and becomes close to zero at pH 10–11, which is in good agreement with the electrostatic model of adsorption. According this model, the amount of positively charged functional groups is negligible at pH = IEP + 3. Adsorption of a second group of anions, AsO43, PO43 and VO3, is less sensitive to pH and their uptake remains high (>80– 90%) up to pH 11.5–12.0. A decrease in 302-HZO efficiency of removal for AsO43, PO43 and VO3 occurs only at pH > 11.5– 12.0, reaching a 50% level at pH 13. Silicate adsorption on hydrous zirconium oxide is even less sensitive to pH increase and remains almost quantitative up to pH 13, at which point no positively charged groups on the surface can exist. It is worth noting that silicate adsorption on HZO is a complex process and there is certain pH range 8–10 at which media has the highest affinity (log Kd > 5.5) towards SiO32 (Fig. 2b). 302-HZO affinity towards silicate decreases at higher and lower pH.

[(Fig._2)TD$IG]

Fig. 2. 302-HZO data for several anions as a function of pH: (a) adsorption from 1 mM solutions and (b) distribution coefficients.

Summarizing the data it is possible to conclude that AsO43, PO43, VO3 and especially SiO32 adsorption on 302-HZO takes place predominantly by a chemisorption mechanism with formation of new compounds having strong covalent bonds. Selectivity series for 302-HZO at pH > 10 suggest possible strong competition for arsenic from silicate, phosphate and vanadate anions: SiO32 > AsO43  VO3  PO43  SeO32  BO33 > CrO42. At a pH of 7.5–8.0, a range typical of drinking water applications, the selectivity series changes slightly: SiO32  AsO43  VO3 PO43  SeO32 > BO33 > CrO42. To check this, arsenic adsorption from binary and multicomponent systems has been studied under batch conditions. As seen in Fig. 3a, the presence of equimolar amounts of competitive ions in binary systems has a minor impact (decrease 10%) on arsenic removal at pH > 11 and no effect at pH < 9–10, the range most common for drinking water. This suggests that 302-HZO has a high affinity towards arsenic and that the amount of active adsorption sites (both for chemisorption and electrostatic adsorption) is high enough for uptake of both species. Fig. 3b shows 302-HZO affinity towards arsenic and silicate ions when they are present together. Adsorption curves for both ions have a shape similar to that found in the case of their adsorption from individual solutions. Arsenic in the presence of SiO32 is removed quantitatively at pH < 8, whereas at higher pH arsenic Kd values decrease monotonously with pH increase. Silicate ions are adsorbed preferentially to arsenic in alkaline solutions (pH > 9) with maximum affinity in the pH range 8.5–10.5. However, arsenic becomes the preferentially adsorbed ion at pH < 8, [(Fig._3)TD$IG]

Fig. 3. 302-HZO data: (a) the influence of competing anions in the As(V) removal process (equimolar 1 mM binary solutions), and (b) distribution coefficients for AsO43 and SiO32 as a function of pH (1 mM solutions).

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whereas 302-HZO affinity towards silicate decreases with pH decrease. Arsenic adsorption from multi-component systems in the presence of other anions (SiO32, PO43, VO3 and BO33) is shown as one of the curves in Fig. 3a. Competitive ions have a substantial effect on arsenic uptake. In basic media (pH > 10) arsenic removal drops 15–20% in comparison with uptake from individual or binary solutions. Almost quantitative arsenic removal occurs only at pH < 7, instead of pH 8–9 for previous cases. In general, hydrous zirconium oxide selectivity towards anions from multi-component systems remains similar to that found for individual solutions, SiO32 > PO43  AsO43  VO3  BO33, with the main difference of lower values of uptake. The latter could be a result of significantly higher total loading (1.0 mmol/g) achieved in the case of multi-component system in comparison with individual solutions (0.2 mmol) and, hence, more strong competition for active adsorption sites. The data generated by batch experiments clearly show that arsenic adsorption on hydrous zirconium oxide is pH dependent. 302-HZO is an efficient adsorbent for arsenic removal from acidic media or at pH < 8.5. It was shown that hydrous zirconium oxide is able to adsorb amounts of arsenic even at pH much higher than the IEP. Phosphate, vanadate and, especially, silicate anions are strong competitors to arsenic for hydrous zirconium oxide adsorption sites. Considering that silicate is a common component of groundwater and drinking water and that its content typically hundreds times higher than that of arsenic, it is possible to expect that silica presence could be as critical as pH for efficient arsenic removal from aqueous streams.

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technique has been proposed [13,14]. A solid acidifier can be any cation exchanger in H-form. Cation exchangers do not adsorb arsenic, but positioned before hydrous zirconium oxide they can remove cations (Na+, Ca2+, Mg2+) from the water stream and release back an equivalent amount of protons (R-H + Na+ ! RNa + H+), which results in water acidification and lowering its pH to targeted level optimal for 302-HZO performance. Depending on the type and quantity of solid acidifier used and the flow rate and water composition, pH of treated water can be lowered to pH 1.5– 3.0 in the case of strong cation exchange resins or to pH 3–7 in the case of medium-strong or weak cation exchange resins, inorganic cation exchangers, etc. Acidified water (pH < 6) after passing through a cartridge with 302-HZO typically returns into standard drinking water pH range due to the pronounced buffering ability of hydrous zirconia (Zr–OH + HAn ! Zr–An + H2O [An = Cl, HSO4, HCO3, etc.]). Solid acidifier decreases water pH until saturation with cations. After that spent media can be regenerated by treatment with mineral acids and re-used in multiple cycles. Data on 302-HZO performance in ‘‘alkaline’’ solutions without presence and with presence of solid acidifier are shown in Fig. 5. Two types of challenge water have been used in these experiments. In the first case NSF53 challenge water with pH  8.5 and arsenic content 50 ppb was used. In the second case NSF53 challenge

[(Fig._5)TD$IG]

3.2. Evaluation of hydrous zirconium oxide performance under column conditions 3.2.1. Effect of pH Effect of water pH on arsenic capacity is shown in Fig. 4. Hydrous zirconium oxide has high affinity towards As at pH 6.5 and purifies 70,000 bed volumes of challenge water to arsenic level below 5 ppb. Under these conditions, a thousand times excess of sulfate and carbonate ions, as well as 400 times excess of silicate, does not affect media performance. Increase in pH of challenge water results in a gradual decrease of media purification ability, which is in an agreement with batch experimental data. At pH 8.5 arsenic capacity is almost 7 times lower than that at pH 6.5 and becomes extremely low at pH 9.5. Such a significant dependence of media performance on pH is a general problem for all polyvalent metal based hydroxides, which creates a negative impact on economics of water purification. Considering this, in order to improve 302-HZO performance under ‘‘alkaline’’ conditions pH adjustment of incoming water by using a so called ‘‘solid acidifier’’ [(Fig._4)TD$IG]

Fig. 4. 302-HZO performance on arsenic removal from NSF53-type challenge water as a function of pH.

Fig. 5. As(V) breakthrough curves on standard 10 in. POU with 302-HZO from: (a) NSF53 challenge water (pH 8.5, As = 50 ppb, 2.85 L/min) without and in the presence of solid acidifier (SA – IRC50), and (b) ‘‘soft’’ NSF53 challenge water (pH 8.9, without Ca2+ and Mg2+ ions, As = 50 ppb, 2.85 L/min) without (blue) and in the presence of solid acidifier (SA – IRC50) (red). Green line show As(V) removal curve in the case when spent solid acidifier cartridge have been replaced after 4000 L with a fresh one. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.).

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water did not contain calcium and magnesium and as a result had a higher pH  8.9 (‘‘soft’’ water). Arsenic containing water has been treated by 302-HZO only or by combination of solid acidifier and 302-HZO. Solid acidifier (weak acid cation exchange resin IRC50) has been packed into a standard 10 in. cartridge (volume 1.0 L). 302-HZO has been packed into a radial-flow 10 in. Isolux cartridge (volume 0.5 L). Challenge water has been passed through the bed of 302-HZO or beds of resin and 302-HZO with flow rate 0.75 GPM at 15 psi excess pressure. As seen, under test conditions 302-HZO is able to purify 4000 L of NSF53 challenge water and only 800 L of ‘‘soft’’ water. Use of the solid acidifier allows hydrous zirconium oxide to purify 10,000 L of NSF53 challenge water with pH  8.5 and 4000 L of ‘‘soft’’ water. This is a 2.5 and 5 time increase in capacity, respectively. Moreover, in the case of ‘‘soft’’ water treatment spent solid acidifier has been replaced after 4000 L for a fresh one. As a result of this operation 302-HZO continued to remove arsenic to a non-detectable level and purified an additional 2500 L of water giving total 6500 L. Repeated replacement of spent SA media with fresh one several times allows purification 11,000 L of ‘‘soft’’ water. A similar result can be achieved by using initially higher ratio SA:HZO or using solid acidifier lowering pH to level lower than IRC50 (pH  6–7). 3.2.2. Effect of competitive ions As was found in batch tests, some anions can compete strongly with arsenic for 302-HZO adsorption sites thus reducing media adsorption capacity for arsenic. Among them are silicate anions that are typically present in huge excess to arsenic in most types of drinking water and groundwater. 302-HZO is able to purify 10,500 BV of NSF53 challenge water (pH 8.5, 50 ppb As(V)) containing 20 ppm of SiO2 (see supplementary data, Fig. S1). An increase in SiO2 content in the challenge water to higher levels of 40 ppm and 80 ppm results in a significant decrease in media capacity for As(V)-purified volume drops to 3700 BV and 1800 BV, respectively. At the same time hydrous zirconium oxide capacity on As(V) significantly increases (up to 25,000 BV) when treated water does not contain SiO2. Such a strong negative effect of silica on arsenic adsorption can be a result of two factors often working simultaneously. First, the silica content in challenge water is hundreds of times higher than that of As(V) and considering the high 302-HZO affinity towards silicate its adsorption is the same order of magnitude higher than that of arsenic. As a result silicate can compete for and occupy most active adsorption sites. Second factor is a blocking of 302-HZO surface by silica. It is well known fact that soluble silica can exist in form of meta- (SiO32) or ortho-silicates (SiO44), and that both forms have a strong tendency for polymerization in a broad range of pH and concentration. According to literature data soluble silica at ppm levels of concentration exist in drinking water predominantly as monomer species. However, silica adsorption on the surface of hydrous oxides results in an increase in its concentration (up to 500–1000 times), which can trigger polymerization processes resulting in formation of gelatinous coatings around media particles. Such a coating can significantly reduce the accessibility of the surface adsorption sites for exchangeable ions by preventing their diffusion to the surface. Development of such a scenario is most probable in soft waters with high pH and, theoretically, should be less probable in ‘‘hard’’ water. The latter relates to the fact that calcium (Ca2+) and magnesium (Mg2+) ions, adsorbed on the media surface due to 302-HZO amphoteric properties, can behave as silica polymerization inhibitors by forming insoluble silicates. This hypothesis is in good agreement with experimental data on arsenic removal from NSF53 challenge water (pH 8.5, As = 50 ppm, SiO2 = 20 ppm) containing different amounts of Ca (from 0 ppm to 120 ppm) (see Fig. S1). 302-HZO efficiency on

arsenic adsorption increases almost linearly with increase in water hardness. As a result media capacity in ‘‘hard’’ water (33,000 BV) is three times higher than that in standard NSF53 water (10,500 BV). Interestingly, other cations present in drinking water do not show such a drastic effect on hydrous zirconium oxide adsorption efficiency for arsenic removal. This can be related to the fact of extremely low media affinity towards sodium (which is a major component in water systems) and much lower concentration of other ions, like Fe3+ or Mn2+. It was found that 302-HZO capacity for arsenic remains unaffected in the range of Fe(III) concentration up to 1 ppm and Mn(II) up to 0.5 ppm. 3.2.3. Field tests 302-HZO performance on arsenic removal has been tested in the field on numerous occasions. Results of three representative field trials are shown below. Well #1 was an 18-month trial of a whole-house residential system at a home in Hunterdon County, New Jersey. Well #2 was a pilot-scale trial conducted at a site in New Mexico. Well #3 was a pilot-scale trial conducted at a municipal well site in Hunterdon County, New Jersey. A summary of water chemistry from three wells is presented in the Table 1 and arsenic breakthrough curves are shown in Fig. 6. As seen, water in wells #1 and #2 has a relatively high pH  8.0–8.2, similar silica content (25 ppm), but different amounts of arsenic. Based on laboratory and pilot unit test results on arsenic adsorption from NSF53-type challenge water we could expect purification of 30,000 BV of water before exceeding a 10 ppb limit. Exactly the same result has been achieved by treatment of water from well #2. In the case of well #1 302-HZO purified more than 35,000 BV to non-detectable level of arsenic (<1.5 ppb). Better media performance in water from well #1 again is in agreement with lab data – presence of calcium and magnesium ions improves 302-HZO adsorption capacity for arsenic when all other conditions are similar. Well #3 shows the effect of lower pH and an even higher level of calcium and magnesium ions. For this well, the pH of 7.5 is significantly lower than that of either of the other wells. Combined calcium and magnesium of 112 ppm is significantly higher. In this case, in excess of 112,000 bed volumes were purified. Unfortunately, no silica data exist for this trial. However, based on numerous other well tests in the Hunterdon County area, a silica level of 20–30 ppm would be expected. 3.3. Media regeneration Considering the high cost of hydrous zirconium oxide in comparison to other adsorption materials, it was logical to check the possibility of spent media regeneration and re-use in adsorption cycles. In contrast to activated alumina or ferric hydroxides, 302-HZO is chemically stable in alkaline media which makes it possible to use caustic for regeneration. It was found that

Table 1 Water composition and media performance during field tests. Raw water

Well #1

Well #2

Well #3

pH Total As (ppb) Ca (ppm) Mg (ppm) SiO2 (ppm) Fe (ppb) Mn (ppb) Purified volume (BV)

8.2 20.9 50.2 26.7 25.8 15 <10 >35,000

8.0–8.1 42.0 17.5 7.5 25.0 43 <10 30,000

7.5 16.0 112 (see note) (See note) No data <30 10 >112,000

Note: For well #3, 112 ppm is the combined total for calcium and magnesium calculated from a total hardness figure of 280 ppm as CaCO3.

[(Fig._6)TD$IG]

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gives quantitative desorption of arsenic and silica. Typical desorption curves for regeneration of spent HZO with 5% NaOH solution are shown as supplementary data (see Fig. S2). Caustic solution flow rate or contact time during regeneration has significant impact on efficiency of arsenic desorption. Increase in contact time/decrease in flow rate from 25 to 2.5 BV/h makes desorption peak sharp and allows quantitative removal of arsenic with 6–8 BV of regeneration solution. After washing from excess of sodium and pH adjustment with mineral acid to 6–7 hydrous zirconium oxide can be used for arsenic adsorption again. This is illustrated by data presented in Fig. 7. 302-HZO has been used for arsenic removal from NSF53 ‘‘soft’’ challenge water. Arsenic breakthrough occurred after treating 300 BV of water. Total volume of solution treated in the first run was 1900 L. Spent media was regenerated with 10 BV of 5% NaOH at flow rate 2 BV/h without removal from the cartridge. After that media has been washed with 20 BV of water and its pH was adjusted to 6 with 3% HCl. This adsorption–desorption cycle has been repeated two times. In both cases 302-HZO was able to purify the same amount of water (1100 L) as in the first cycle, which suggests no deterioration in media performance. 4. Conclusions

Fig. 6. Arsenic breakthrough curves for three field trials: (a) well #1 and well #2, and (b) well #3.

even a low concentration of NaOH (0.5–1%) allows almost quantitative removal of adsorbed sulfate, carbonate and borate from spent media, but only partially removes arsenic (50–70%) and silicate (70–90%). Increase in NaOH concentration to >5%

[(Fig._7)TD$IG]

Adsorption properties of commercially available hydrous zirconium oxide of 302-type towards arsenic and some competitive anions and cations have been studied under batch and column conditions. Due to amphoteric properties, the exchange performance of hydrous zirconium oxide is pH dependent. Media exhibits high affinity towards arsenic in a broad pH range with high adsorption capacity at pH < 8. The following selectivity series were found for 302-HZO in alkaline media: SiO32 > AsO43  VO3  PO43  SeO32 > BO33 > CrO42 > SO42  Cl  NO3. It was shown that silicate (and phosphate) ions are the main competitors affecting media performance for arsenic removal from drinking water or groundwater. The presence of small quantities of transition metals in water (<1 ppm) does not affect 302-HZO capacity on arsenic, whereas alkaline-earth cations improve arsenic removal. The possibility for significant increase of 302-HZO adsorption capacity on arsenic at pH > 8 by using ‘‘solid acidifier’’ technique was discussed. Results of 302-HZO field trials show high efficiency of hydrous zirconium oxide for treatment of arsenic contaminated water. Comparable published performance data is not available for other adsorption media such as activated alumina or ferric oxides/ hydroxides. However theory suggests their qualitative performance will be similar to 302-HZO. However quantitatively there will be differences. Given the complex multi-component nature of ground water and its enormous variability from site to site, complete analysis and pilot-scale testing for individual sites is critical to predicting performance of any media at any site. Acknowledgments We express our thanks for financial support from the Spanish Ministerio de Educacio´n y Ciencia (MAT2006-01997 and Factorı´a de Cristalizacio´n–Consolider Ingenio 2010) and the Gobierno del Principado de Asturias (PCTI 2006-2009).

Appendix A. Supplementary data Fig. 7. Arsenic breakthrough curves for fresh 302-HZO (#1), and after one (#2) or two (#3) regenerations with 5% NaOH. NSF53 ‘‘soft’’ challenge water (pH 8.8, As = 50 ppb).

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.materresbull.2010.07.011.

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