Effect of amorphous silica and silica sand on removal of chromium(VI) by zero-valent iron

Chemosphere 66 (2007) 858–865 www.elsevier.com/locate/chemosphere

Effect of amorphous silica and silica sand on removal of chromium(VI) by zero-valent iron Young Ju Oh a, Hocheol Song b, Won Sik Shin c, Sang June Choi c, Young-Hun Kim d,* a Korea Water Resources Corporation, Daejeon, Republic of Korea School of the Environment, Clemson University, Clemson, SC 29634, USA Department of Environmental Engineering, Kyungpook National University, Taegu, Republic of Korea d Department of Environmental Engineering, Andong National University, 388 Song-Chun, Andong, Kyung-Pook 760-749, Republic of Korea b

c

Received 18 January 2006; received in revised form 9 June 2006; accepted 13 June 2006 Available online 26 July 2006

Abstract The effect of two surfaces (amorphous silica and silica sand) on the reduction of chromium(VI) by zero-valent iron (Fe(0)) was investigated using batch reactors. The amendment of both surfaces significantly increased the rate and extent of Cr(VI) removal. The rate enhancement by amended surfaces is presumed to result from scavenging of Fe(0)–Cr(VI) reaction products by the provided surfaces, which minimized surface deactivation of Fe(0). The rate enhancing effect was greater for silica compared to sand, and the difference is attributed to silica’s higher surface area, greater affinity for reaction products and pH buffering effect. For a given mass of Fe(0), the reactivity and longevity of Fe(0) to treat Cr(VI) increased with increasing dose of silica. Elemental analyses of the reacted iron and silica revealed that chromium removed from the solution was associated with both surfaces, with its mass distribution being approximately 1:1 per mass of iron and silica. The overall result suggests reductive precipitation was a predominant Cr(VI) removal pathway, which involves initial reduction of Cr(VI) to Cr(III), followed by formation of Cr(III)/Fe(III) hydroxides precipitates. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Reductive precipitation; Chromium adsorption; Iron passivation; Dissolved silica

1. Introduction Ranked the 18th in the CERCLA Priority List of Hazardous Substances in 2005 (ATSDR, 2005), chromium is one of the important pollutants found in soils, waste sites, groundwaters, and surface waters in the US. Although chromium is a naturally occurring substance, the majority of chromium raising environmental concerns is from anthropogenic sources. In recent years, a great deal of research efforts has gone to find better strategies to treat chromium contamination. In particular, a number of studies have demonstrated that zero-valent iron (Fe(0)) is an effective reductant for Cr(VI) (Powell et al., 1995; Puls *

Corresponding author. Tel.: +82 54 820 5818; fax: +82 54 820 6187. E-mail address: [email protected] (Y.-H. Kim).

0045-6535/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2006.06.034

et al., 1999; Melitas et al., 2001; Gandhi et al., 2002; Singh and Singh, 2003). For example, Powell et al. (1995) reported that scrap iron filings and cast iron effectively removed Cr(VI), with its removal efficiency varying with the surface area and composition of iron. They also observed that the presence of aluminosilicate aquifer materials significantly enhanced the removal of Cr(VI) by iron, and attributed the enhancement to the material’s ability to generate protons. Protons are generated by dissolution of aluminosilicate minerals (e.g. montmorillonite and kaolinite) along with silica, and by subsequent reaction of silica with iron oxides (Powell et al., 1995). The primary effect of the generated protons in a Fe(0) treatment system is to maintain solution pH and thus reduce surface passivation of iron. A similar buffering effect was also reported for quartz grains in Fe(0)–Cr(VI) reaction (Blowes et al., 1997).

Y.J. Oh et al. / Chemosphere 66 (2007) 858–865

Spectroscopic analyses of Fe(0) surfaces after reaction with Cr(VI) suggested the removal pathway involves initial reduction of Cr(VI) to Cr(III), followed by adsorption of Cr(III) onto the surfaces, or by precipitation of Cr(III)/ Fe(III) hydroxides (Powell et al., 1995; Blowes et al., 1997; Pratt et al., 1997; Astrup et al., 2000; Wilkin et al., 2005). The surfaces for Cr(III) adsorption may include a range of secondary minerals including goethite, lepodocrocite, maghemite, and hematite, most of which are derived from reactions between inorganic constituents in water and Fe(0). The type of the secondary minerals is primarily dependent on the solution chemistry of water being treated. For example, Wilkin et al. (2005) recently obtained 8-year monitoring results of a permeable reactive barrier (PRB) placed in sulfate-rich groundwater in Elizabeth City, NC, and reported that the majority of chromium removal occurred in regions where the secondary mineral formation was most significant and the removed chromium was associated with iron sulfide minerals. Blowes et al. (1997) and Pratt et al. (1997) reported that Cr(III) precipitated on the surface of Fe(0) in the form of Cr(III)-goethite mixtures in a calcite-saturated water. The removal of Cr(III) via precipitation is consistent with the fact that the Cr(III)/Fe(III) hydroxides, CrxFe1x(OH)3, has lower solubility than Crhydroxides (Sass and Rai, 1987; Eary and Rai, 1988). Despite the importance of precipitates formation in Cr(VI) removal process, less is known about the effects of such reaction precipitates on the performance of Fe(0). In general, the buildup of such minerals is assumed to have negative impacts on the performance of Fe(0) since the development of mineral phases on iron surface can lead to surface deactivation by acting as a physical barrier that limits the access of contaminant to iron surface. Recently, investigations performed in conditions close to ideal field systems revealed that the precipitation of corrosion products reduces the long-term performance of Fe(0) (Klausen et al., 2003; Vikesland et al., 2003; Kohn et al., 2005). Therefore, in this study, we investigated the possibility of using surfaces (silica and sand) for adsorbing Fe(0)-chromium reaction precipitates in an effort to enhance the chromium removal, as well as to improve the longevity of Fe(0). Kinetic experiments were conducted with varying amount of the surfaces and Fe(0). It was hypothesized that amendment of those surfaces enhances the Fe(0) reaction with Cr(VI) by providing surfaces to preferentially adsorb reaction precipitates so that performance of Fe(0) is increased to merit its practical applications. 2. Materials and methods In field-scale applications of PRBs, sand or aquifer material is generally mixed with the reactive iron metal and backfilled in subsurface trenches. In this study, sand and silica were used in conjunction with Fe(0) since the former is a widely used material, and the latter is a simple analogue to aquifer mineral surfaces. The electrolytic elemental iron was obtained from Fisher chemicals. The

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iron was sieved into different fractions and the fraction within 100–270 mesh (53–150 lm) was used. Iron was acid-washed using 0.05 N HCl before use. The sand (297– 590 lm) obtained from Joomoonjin Silica Sand Co., Korea was a purified silica sand predominantly composed of quartz-like material. The silica (63–200 lm) was a high purity amorphous silica (>99.8%) obtained from Aldrich. Both solids were pre-washed with distilled deionized water (DDW) and baked at 550 °C for 2 h to remove residual organics. BET surface areas determined by a surface area analyzer (Micromeritics ASAP 2010) were 0.09, 0.15, and 472 m2 g1 for acid-washed Fe(0), pre-washed sand, and silica, respectively. The Cr(VI) stock solutions were prepared by dissolving reagent-grade K2CrO4 (Fisher) into DDW (18 MX cm1) and used within 24 h. The DDW used for Cr(VI) solution preparation was deoxygenated by purging with high purity N2 for at least 1 h. The kinetics experiments were performed by adding Cr(VI) solutions to 40 ml HCl-cleaned glass vials containing 1 g of Fe(0) and surfaces in a N2-filled glovebag (Cole-Parmer). Suspensions of 30 ml, typically consisting of 10 mg Cr(VI) l1, variable amounts of Fe(0), and surfaces, were mixed on a shaking incubator (Vision 8480SF) at 25 ± 0.2 °C and 180 rpm. Vials were sacrificed at each scheduled sampling time. The sampling was carried out by first allowing the solids to settle for 1 min and then removing 20 ml of aliquot with a 10 ml gas-tight syringe (Hamilton). The aliquot was filtered immediately through pre-cleaned 0.20 lm nylon filters (Millipore) and diluted for Cr(VI) analysis. For the reaction with 2 g Fe(0) and 1 g silica, the concentrations of Cr, Fe, and Si precipitated on Fe(0) and silica were measured along with their aqueous phase concentrations. In order to measure surface-bound reaction products (Cr and Si from Fe(0), and Cr and Fe from silica), Fe(0) and silica after the reaction were retrieved and treated using the following procedure. The reaction vials were centrifuged at 3000 rpm for 10 min. The supernatant was decanted and the remaining solids were washed twice with DDW. The DDW wash solutions were combined with the original supernatant before the solution was filtered through 0.20 lm nylon filters and acidified for measurements of aqueous phase concentrations. The solids were dried at 110 °C in an over for 2 h. The dried solids were separated into Fe(0) and silica using a magnet, and 0.5 g of each solid was transferred to separate 40 ml polypropylene bottles. To each bottle, a 15 ml of 30% high purity HNO3 (ICP grade, Merck) was added and the vial was placed on the shaking incubator at 25 °C and 180 rpm for 2 h to release metal species from the solids. The extracted solution was then taken out from the vial and diluted for analysis. This dissolution procedure was repeated twice for both Fe(0) and silica to ensure complete dissociation of precipitated product during Cr(VI) reduction. After the acid treatments, yellowish-brown color developed on the silica during the reaction disappeared. There was no visually noticeable color changes on Fe(0)

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3. Results and discussion

(a) 1.0

12

0.8 10

pH

0.6 8

Fe0 1 g + sand 3 g Fe0 only sand only pH of Fe0 1 g / sand 3 g system C0 = 10 mg l-1

0.4

0.2

6

0.0

4 0

20

40

60

80

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140 10

(b) 1.0

3.1. Effect of sand and silica

8

Fig. 1 compares the rates of Cr(VI) reduction by Fe(0) alone with those by Fe(0) amended with sand or silica. The results indicate the rates of Cr(VI) removal increased in the presence of both surfaces. Controls (without Fe(0)) showed only a fraction of Cr(VI) was removed via adsorption onto sand (<5%), and virtually no adsorption of Cr(VI) on silica occurred. The amendment of sand to Fe(0) dramatically increased Cr(VI) reduction such that, in the reaction involving 1 g Fe(0) and 3 g sand, Cr(VI) concentration dropped below detection limit after 120 min (Fig. 1a). In contrast, Cr(VI) removal by Fe(0) alone was less than 15% of its initial concentration during the same time frame. The solution pH of Fe(0)/sand system initially increased to above 9 and reached a plateau with an increased reaction time, which was similar to that of Fe(0) alone system. This feature of the pH changes is commonly observed in the leading edge of PRBs due to abrupt depletion of protons, and is generally considered to be unfavorable for contaminant reduction as passivating layers develop on the surface of Fe(0) at high pH conditions. However, in this study, Cr(VI) removal significantly increased in the presence of sand despite the high pH condition. Considering the fact that sand was neither an effective adsorbent for Cr(VI) nor an effective buffering agent, the role of sand in the enhanced reaction may be found in its interaction with reaction precipitates. In other words, sand served as a scavenger for reaction precipitates and as a result, buildup of the precipitates on Fe(0) was minimized. Furthermore, several studies have demonstrated that iron-oxides coated on sand are effective adsorbents for many metal contaminants including chromium (Bailey et al., 1992; Anderson et al., 1994), arsenic (Vaishya and Gupta, 2003), cadmium (Benjamin et al., 1996; Lai et al., 2001), and lead (Lai et al., 2001). Solution pH maintained above 9 for the most of the reaction period may have provided favorable conditions for formation of such corrosion products and their subsequent

C/C0

0.8

Fe0 1 g + silica 1 g Fe0 1 g + silica 0.5 g Fe0 1 g + silica 0.2 g Fe0 only silica only pH of Fe0 1 g / silica 1 g

0.6

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6

4

C0 = 10 mg l-1

0.2

pH

particles but some loss of bulk Fe(0) occurred. The concentrations of surface-bound metals obtained in the second treatment were less than 3% of those in the first treatment. The concentrations of Cr(VI) were determined colorimetrically to a detection limit of 10 ppb with a UV/VIS spectrophotometer (DR 2500/HACH) using 1,5-diphenylcarbazide as a complexing agent. The total dissolved Cr, Fe and Si were analyzed with a Inductively coupled plasma–optical emission spectrometer (ICP–OES, OptimaTM 4300DV, Perkin Elmer). Microscopic images of selected Fe(0) and silica samples were obtained with a Hitachi S-4200 scanning electron microscopy (SEM), and their surface elemental compositions were obtained with energy-dispersive X-ray spectrometer (EDS). The solution pH in batch experiments was measured using a pH meter (Denver model 15).

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860

0.0

2 0

1

2

3

4

5

6

Time (min) Fig. 1. Cr(VI) reduction by Fe(0) in the presence of (a) sand and (b) silica.

deposition on sand surface. The chromium removed from the solution through this pathway is likely to be adsorbed onto the surface either as Cr(VI) or Cr(III) since Fe-bearing minerals are not only capable of directly adsorbing Cr(VI) but also reducing Cr(VI) to Cr(III) (Eary and Rai, 1988; Bailey et al., 1992; Anderson et al., 1994). Although the possibility that chromium was removed by interaction with the iron-oxides deposited on Fe(0) surface cannot be ruled out, it appears that iron-oxides on Fe(0) has much less affinity for chromium as indicated by insignificant Cr(VI) removal by Fe(0) alone. Removal of Cr(VI) occurred more rapidly when silica was used. Fig. 1b indicates 10 mg l1 Cr(VI) was completely removed within 5 min at all silica doses, with the rate of reaction increased with increasing silica dose. Fig. 2 shows a schematic diagram of the possible pathways of Cr(VI) reduction in the presence of silica. The greater enhancement of reaction rate by silica compared to sand may be attributed to its higher surface area, greater affinity for reaction products and pH buffering effect. The surface area of the silica is approximately three orders of magnitude

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Table 1 Cr, Fe, and Si contents in the reacted solids and solution Cr

Fe

mg Fe(0) Silica Solution Total

1.41 0.65 0.39 2.08

% (±0.16) (±0.02) (±0.11) (±0.23)

46.9 21.7 13.1 69.2

(±5.4) (±0.6) (±4.0) (±7.4)

Si

mg

mg

– 2.31 (±0.23) 0.47 (±0.06) –

0.77 (±0.08) – 1.17 (±0.04) –

Solution volume: 30 ml, Fe(0) dose: 2 g, silica dose: 1 g, number of spiking: 10, theoretical total amount of Cr: 3 mg.

greater than the sand. Silica surface has been shown to strongly bind Fe(III) and Cr(III) via surface complexation (Fendorf et al., 1994; Flogeac et al., 2005). Therefore, the silica used in this study may have provided sites for adsorption of Fe(III), Cr(III), and possibly other reaction precipitates such as Fe(III)-hydroxides and Cr(III)/Fe(III) hydroxides. Further, solution pH maintained constant at around seven independent of the amount of silica added (only pH of 1 g Fe(0)/1 g silica is shown in Fig. 1b) may also have contributed to the enhanced Cr(VI) removal. The effect of pH on Cr(VI) removal observed in this study is consistent with Alowitz and Scherer (2002), who reported fast removal of Cr(VI) at pH 7 and negligible removal at pH 9 in the reaction with Fe(0). However, considering a similar solution pH in the reactors containing different amount of silica, the increase in Cr(VI) removal rate with increas-

Fe0 0.25 g + silica 1 g pH of Fe0 0.25 g + silica 1 g

C0 = 10 mg l-1

BSiOH $ BSiO þ Hþ

(b) 1.0

6

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6

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2 400

(c) 1.0

600

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Fe0 1 g + silica 1 g pH of Fe0 2 g + silica 1 g

C0 = 10 mg l-1

0.8 0.6

2 0

1000 10

8

6

100

200

300

400 10

(d) 1.0

Fe0 2 g + silica 1 g pH of Fe0 2 g + silica 1g

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0.8

C/C0

200

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0

4

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8

6

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4

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10

8

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Fe0 0.5 g + silica 1 g pH of Fe0 0.5 g + silica 1 g

C0 = 10 mg l -1

8

0.6

Log K ¼ 6:8

In Fe(0) systems, oxidation of iron metal consumes protons. At high pH, the silanol groups dissociate to compensate the depletion of protons. The measured solution pH, ranged between 6.5 and 7.5, is in good agreement with expected value where surface silanol groups exert its maximum buffering capacity (pKa  7). In addition, negatively charged surface sites rendered by release of proton from silanol groups may have served as collectors for Fe(II) or Fe(III), increasing the probability that reaction products precipitate on silica. In this study, it is apparent that silica dissolution occurred during the reaction as indicated by the detection of Si both in the solution and acid extracted solution of

C/C0

C/C0

0.8

10

pH

(a) 1.0

ing silica dose is more likely due to the increase in adsorbing surface for reaction products, rather than effect of pH. The buffering effect of silica largely arises from acid/base reactions of the surface-bound silanol group („SiOH) (Iler, 1979).

pH

Fig. 2. A schematic diagram of the main features of the Cr(VI) reduction by Fe(0) in the presence of silica.

0

100

200

300

Time (min)

2 400 500 1050

4 0.2 0.0 0

100

200

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400

2 660

Time (min)

Fig. 3. Reaction capacity of Fe(0)/silica mixtures with varying mass of Fe(0) in the multiple reduction of Cr(VI): (a) 0.5 g, (b) 1 g, (c) 2 g, and (d) 3 g Fe(0).

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3.2. Cr(VI) removal capacity of Fe(0)/surface mixtures Although many studies have demonstrated the effectiveness of Fe(0) system to treat Cr(VI) and noted the importance of reduction rates, relatively little attention has given to reduction capacity or longevity of Fe(0). In field applications of Fe(0), system longevity is equally important as reaction rates because it governs the lifespan of treatment systems. In this study, the reaction capacity of Fe(0) in the reduction of Cr(VI) was investigated by repeated spiking of Cr(VI) into reactors containing varying mass of Fe(0). Fig. 3 shows results of four sets of experiments that were performed with different iron loading ranging from 0.25 to 2 g in the presence of a fixed amount of silica (1 g). Fig. 3a indicates that 10 mg l1 of Cr(VI) was completely removed after 2 h of reaction with 0.25 g Fe(0). After the first reduction cycle, however, the reaction rate became markedly slower. For 0.5 g Fe(0)/1 g silica mixture, the reactivity of Fe(0) was maintained up to the second reduction cycle before the reduction slowed down after the third spike of Cr(VI) (Fig. 3b). Similar retardation of Cr(VI) reduction occurred after seventh and 13th spike for systems containing 1 g Fe(0)/ 1 g silica and 2 g Fe(0)/1 g silica, respectively (Fig. 3c and d). In cases where Fe(0) to silica mass ratio was less than 1 (Fig. 3a–c), the solution pH remained between 6.5 and 7.5 throughout the experiments. For the 2 g Fe(0)/1 g silica mixture, however, the solution pH started to rise after seventh spiking of Cr(VI) and continued to increase up to 9. This suggests that the proton generation rendered

by dissolution of silica was exceeded by the consumption by Fe(0). However, the fact that all the reactions involving lower doses of Fe(0) eventually slowed down at some points without apparent pH increase suggests solution pH may not be an universal indicator of deactivation process occurring on Fe(0) surface. To establish a quantitative relationship between mass of Fe(0) and Cr(VI) reduced, the reduction capacity was determined by calculating the mass of Cr(VI) removed over first 400 min for a given mass of Fe(0). The 400 min of reaction time was chosen since most of the reactions became markedly slower after 400 min. Fig. 4a shows the amount of Cr(VI) removed as a function of Fe(0) mass. The result indicates a linear dependence of Cr(VI) removal on mass of Fe(0), suggesting the extent of Cr(VI) removal is proportional to iron surface area. Similar experiments were carried out with varying amount of silica (0.5–3 g) and a fixed amount of Fe(0)

(a) 4

Chromate removal (mg)

the reacted iron (Table 1). Some researchers have argued that dissolved silica, mainly silicic acid (H4SiO4), promotes Fe(0) reaction by releasing protons to the solution through interaction with iron oxyhydroxide surface (Powell et al., 1995; Powell and Puls, 1997). In contrast, dissolved silica has been reported to inhibit oxidative corrosion of Fe(0), thereby exhibiting detrimental effect on Fe(0) performance (Klausen et al., 2001, 2003; Kohn et al., 2003, 2005). The inhibition of Fe(0) reaction by silica has been postulated to be due to the adsorption of silica onto iron surface to form a protective layer over iron. The adsorption of dissolved silica is likely to be manifested at high pH as solubility of silica increases with pH. However, in this study, it appears that dissolved silica did not have any negative impacts on Cr(VI) reduction since the addition of solid silica not only dramatically increased the reaction rates but also effectively buffered solution pH. In fact, one recent study indicated that dissolved silica, as high as 50 mg l1 as SiO2, had no adverse effect on iron reaction at pH 7.5 (Kohn et al., 2003). The measured aqueous concentration of silica in this study (83 mg l1 as SiO2) at 2 g silica dose was less than the solubility limit of amorphous silica (120 mg l1 as SiO2 at pH 7) after 10 reduction cycles of Cr(VI). This indicates that the kinetics of silica dissolution was not fast enough to reach equilibrium under pH  7 condition.

3

2

1

0

0.0

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Fe0 dose (g)

(b)

1.0

0.8

Chromate removal (mg)

862

0.6

0.4

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2 silica dose (g)

3

Fig. 4. (a) Effect of Fe(0) loading on the removal of Cr(VI) (silica dose: 1 g, solid line is a linear regression with R2 = 0.998). (b) Effect of silica loading on the removal of Cr(VI) (Fe(0) dose: 0.5 g, dashed line is a connection of individual points).

Y.J. Oh et al. / Chemosphere 66 (2007) 858–865

8

6 Fe0 0.5 g + silica 0.5 g pH of Fe0 0.5 g + silica 0.5 g

0.2

C 0 = 10 mg l-1

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0

Fe 0.5 g + silica 1 g 0 pH of Fe 0.5 g + silica 1 g

0.8

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0.8

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(b)1.0

pH

10

(a)1.0

863

0.4 4

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4 0.2

0.0

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100

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300 900

Time (min)

0.0

2 0

50

100

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1050

Time (min)

Fig. 5. Reaction capacity of Fe(0)/silica mixtures with varying mass of silica in the multiple reduction of Cr(VI): (a) 0.5 g, (b) 1 g, (c) 2 g, and (d) 3 g silica.

(0.5 g). As Fig. 5 reveals, the extent of the reaction of Cr(VI) with Fe(0) increased with increasing silica loading. In the system containing 0.5 g silica, the retardation in reaction rate was apparent after the first cycle of reduction (Fig. 5a). Similar retardation was observed for 1 g silica system at third reduction cycle (Fig. 5b). The reactions with 2 and 3 g of silica remained active until third reduction cycle where complete removal of Cr(VI) was achieved in 280 and 160 min, respectively. The Cr(VI) removal capacity of these reactions was similarly calculated from the total amount of Cr(VI) removed within 400 min reaction (Fig. 4b). The removal capacity lineally increased up to 1 g of silica and flattened out at silica loading greater than 2 g. This suggests that at lower silica dose, the availability of adsorbing surface sites for reaction products is an important factor in determining Cr(VI) removal capacity, while at higher silica dose, Cr(VI) removal is not influenced by the site availability. Therefore, it appears that Cr(VI) removal enhancement by the benefit of using silica has its limitation and there exists an optimum dose of silica for a given mass of Fe(0). Further, the possibility that inhibition of iron reaction occurs in the presence of excessive silica cannot be ruled out since iron passivation by silica may increase at higher silica loading. The optimum mass ratio of Fe(0) to silica is approximately 1:2 in this study. 3.3. Fate of reduced Cr(VI) To investigate the fate of reduced Cr(VI), selected Fe(0) and silica samples were analyzed with SEM and EDS, and extracted with acid for ICP–OES analyses after the reac-

tion. The reacted Fe(0) and silica were retrieved from the reactor that initially contained 2 g Fe(0) and 1 g silica and was run for 10 Cr(VI) reduction cycles. Microscopic analysis of the Fe(0) indicates that, prior to reaction with Cr(VI), the surfaces are relatively free from surface impurities, but after the reaction, nano-sized clusters of precipitates formed on the surface (Fig. 6a and b). EDS of the reacted Fe(0) showed small inclusions of O, Cr and Si (Fig. 6c). However, EDS of the reacted silica did not detect Fe and Cr (data not shown), although yellowish-brown color was developed on silica during the reaction. The lack of Fe and Cr peaks in the EDS spectrum is probably because Fe and Cr were precipitated on a region higher than the probing depth of EDS (2 lm), or the X-ray beam was mislocated on the sample. Contrary to the EDS analysis, ICP–OES analysis of acid extracted solution of the reacted silica indicated that both Fe and Cr were associated with silica (Table 1). Fe, Cr, and Si contents in the reacted Fe(0) and silica, and aqueous solution are listed in Table 1. The mass recovery Cr (aqueous + solid phase) was 69% of the total Cr(VI) added. The 31% loss of Cr(VI) cannot be clearly explained at the moment. The possible sinks for unaccounted Cr(VI) may include loss of solids in the retrieval process, incomplete extraction of Cr(VI) from solids, sorption of Cr(VI) onto the wall of the reaction vial, or other unknown experimental artifacts. The Cr distribution result shows that a relatively large fraction of Cr was retained on the Fe(0) (47%) compared to that adsorbed on silica (22%). Per mass of Fe(0) and silica, however, the amounts of Cr recovered from both

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Fig. 6. SEM images of Fe(0) (a) before and (b) after the reaction, and (c) EDS of the reacted Fe(0).

surfaces shows approximately 1:1 distribution. Assuming this distribution of Cr holds, it is expected that the amount Cr on silica will increase with increasing silica dose up to the point where Cr(VI) removal is not influenced by the amount of silica. 4. Conclusions In this study, silica and sand were amended in the reduction of Cr(VI) by Fe(0). The expected role of those materials was to provide surfaces to preferentially adsorb reaction products including Cr(III), Fe(III) and other reaction precipitates such as Fe(III)-hydroxides and Cr–Fe hydroxides. The results of kinetic experiments provide evidence that both sand and silica are effective in enhancing Fe(0) reactivity for Cr(VI) reduction. The rate enhancing effect was far greater for silica, presumably due to its higher surface area, greater affinity to reaction products and pH buffering by surface silanol groups. Cr(VI) removal capacity of a

given mass of Fe(0) increased with increasing silica dose up to a certain point but flattened out at higher silica loading. The optimum mass ratio of Fe(0) to silica found in this study was approximately 1:2. Elemental analyses of the reacted iron and silica revealed that the reduced chromium was adsorbed onto both surfaces, with its mass distribution being approximately 1:1 per mass of iron and silica. The overall results obtained in this study suggest reductive precipitation was a predominant Cr(VI) removal pathway, and highlight the fact that silica and sand are good rate enhancing reagents for Cr(VI) reaction with Fe(0) reaction, which may offer practical advantages in field application of Fe(0) based treatment systems since those materials are common constituents of aquifer materials. Acknowledgement This work was supported by the Korea Research Foundation (R05-2004-000-11925-0).

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