25.1.1 Introduction
Chromium has been widely detected in groundwater and soils. To reduce human exposure, the U.S. Environmental Protection Agency (EPA) set a maximum contaminant level (MCL) of 0.1 mg/L for total chromium in drinking water.
Conventionally, Cr(VI) is removed from water through reduction of Cr(VI) to its less toxic form, Cr(III), followed by precipitation [ 1 ]. Researchers have demonstrated that Cr(VI) can be eff ectively reduced by Fe(II) according to the following generic reaction scheme, Equation 25.1 [ 2– 3 ]:
Cr(VI) + 3Fe(II) → Cr(III) + 3Fe(III) (25.1) Reduction of Cr(VI) to Cr(III) by powder or granular zero-valent iron (ZVI) particles and non-stabilized or aggregated ZVI nanoparticles has been investigated in laboratory and fi eld studies [ 4– 7 ]. The reduction was reported to follow the general pseudo-fi rst order rate law shown in Equation 25.2 [ 7 ],
v = kAs[Me] (25.2)
where v is the reaction rate, k is the rate constant (M –1 m 2 s –1 ), [ Me ] is the metal ion concentration (M), and A s is the specifi c surface area of the iron particles (m 2 /g). Equation 25.2 indicates that the reaction rate is directly proportional to the specifi c surface area of the ZVI particles. Consequently, reducing particle size is expected to greatly enhance the reaction rate expo nentially. Cao and Zhang [ 8 ] reported that the surface-area-normalized reaction rate constant of Cr(VI) reduction by non-stabilized nanoparticles was about 25 times greater than that by iron powders (100 mesh). However, agglomerated ZVI particles are
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often in the range of micron scale. As a result, they are essentially not deliverable and cannot be used for in situ applications in soils.
To control nanoparticle agglomeration, He et al. [ 9– 10 ] developed a technique for preparing stabilized ZVI nanoparticles by applying low concentrations of a starch or CMC as a stabilizer. The stabilized nanoparticles exhibited markedly improved soil mobility and greater reactivity when used for dechlorination of TCE.
25.1.2 Reduction and Removal of Cr(VI) in Water
Chromate is highly water soluble and mobile. In the subsurface environment, Cr(VI) will distribute between water and soil. Therefore, immobilization of Cr(VI) requires removal of chromate in the aqueous phase. Xu and Zhao [ 11 ] showed that at an Fe dose of 0.08 g/L (approximately 2.3 times the stoichiometric amount), about 53 percent of 34 mg/L of Cr(VI) was reduced at equilibrium, which was reached after roughly 36 hours of reaction.
It has been proposed that elemental Fe reduces Cr(VI) to Cr(III) following the stoichiometry in Equation 25.3 [ 12 ],
Fe0 + CrO2–4 + 4H2O = Cr(OH)3(s) + Fe(OH)3(s) + 2OH– (25.3) In the absence of a stabilizer, the resultant Cr(OH) 3 is a sparingly soluble precipitate ( K sp = 6.3 × 10 –31 ). Cr(III) can also be precipitated via the formation of Fe(III)–Cr(III) hydroxide according to Equation 25.4 [ 12 ],
x Cr 3+ + (1-x)Fe3+ + 3H2O ⇔ (CrxFe1-x)(OH )3(s) + 3H + (25.4) where x is equal to 0.75. The solubility of Cr x Fe 1– x (OH) 3 is lower than that of Cr(OH) 3 . Alternatively, Cr(III) may also precipitate in the form of Cr x Fe 1– x OOH [ 8 ]. In the presence of CMC, the particle agglomeration and precipitation may be somewhat inhibited. However, CMC is vulnerable to biodegradation and/or hydrolysis. Once decomposed, its particle stabilizing ability is ceased. Consequently, any residual fi ne precipitates will end up in the soil matrix through sorption and/or fi ltration eff ect.
Assuming complete mixing, the initial (< 4 hours) reduction rate of Cr(VI) can be described by a pseudo-fi rst-order kinetic model in Equation 25.5 [ 4 , 7 ]:
obs
[ ]= − [ ]
d C k C
dt (25.5)
where C is the concentration of Cr(VI) in water (mg/L), t the time (h), and k obs the observed fi rst-order rate constant (h –1 ). The value of k obs was determined to be 0.08 h –1 for CMC-stabilized ZVI nanoparticles [ 11 ]. Earlier, Ponder et al. [ 7 ] reported a k obs value of 1.18 h –1 for resin-supported ZVI at an Fe-to-Cr molar ratio of 8:1, and they reported that the rate constant increases linearly
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with increasing Fe-to-Cr molar ratio [ 4 , 7 ]. In the subsurface environment, mixing is rather limited; thus, the overall reduction rate is likely controlled by mass transfer and the actual reaction rate can be much slower.
The reducing power of the ZVI nanoparticles can also be consumed by side reactions. For example, ZVI nanoparticles can also react with water via Equation 25.6 [ 13 ]:
+ −
+ → + +
0 2
( )s 2 2 aq 2( )g 2 (aq)
Fe H O Fe H OH (25.6)
Although the resultant hydrogen and Fe 2+ are fairly strong reducing agents, they are not strong enough to reduce Cr(VI) under the experimental conditions [ 11 ]. Xu and Zhao (11) observed that as the Fe dosage was increased from 0.04 g/L to 0.12 g/L, the equilibrium percentage removal of Cr(VI) increased from 24 to 90 percent. Evidently, at an Fe dosage of approximately 3.4 times the stoichiometric amount, the stabilized nanoparticles can reduce about 90 percent Cr(VI) under ambient conditions, which is 20 and 3 times, respectively, more eff ective than the resin-supported ZVI nanoparticles [ 7 ] and the non-stabilized ZVI nanoparticles [ 8 ].
25.1.3 Reduction and Immobilization of Cr(VI) Sorbed in Soil
It was demonstrated that amending Cr(VI)-laden soil with CMC-stabilized ZVI nanoparticles can substantially reduce the chromate leachability [ 11 ].
Figure 25.1 shows the transient release of total Cr or Cr(VI) when 1.5 g of a Cr(VI)-laden soil sample was mixed with 15 mL of the nanoparticle suspension containing 0.08 g/L Fe and at an initial pH of 9.0. For comparison, Cr(VI) desorption kinetic data in DI water at pH 9.0 are also superimposed. At equilibrium, roughly 36 percent of preloaded Cr(VI) was desorbed from the soil when the nanoparticles were absent. In contrast, when 0.08 g/L Fe nanoparticles were present, approximately 18 percent of the preloaded Cr(VI) was released, with no Cr(VI) detected in the aqueous phase. This observation indicates that a small dose of the stabilized nanoparticles was able to not only reduce the Cr(VI) leachability, but also completely transform all leached Cr(VI) to Cr(III).
Solution pH can aff ect both chromate speciation and the surface electric potential of soil, and thus, Cr(VI) reduction and immobilization. Xu and Zhao [ 11 ] tested Cr leachability from a sandy soil at the initial pH 9.0, 7.0, and 5.0 and in the presence or absence the Fe nanoparticles (soil = 1.5 g; solution = 15 ml). As the solution pH was decreased from 9.0 to 5.0, the DI-water desorbed Cr(VI) was reduced from 30 to 20 percent because at higher pH soil sorption sites become more negative and OH - ions compete more fi ercely with CrO 4 2–
for the binding sites. However, when ZVI nanoparticles (0.08 g/L) were present, the total leachable Cr was reduced to < 12 percent over the pH range of 5.0–9.0,
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and all desorbed Cr was detected as Cr(III). In addition, the total Cr release was much less pH dependent due to the added sorption capacity from Fe addition.
Figure 25.2 shows the chromium elution histories during two separate column runs when 0.06 g/L ZVI nanoparticle suspension at pH 5.60 or DI-water was pumped through a Cr-loaded sandy soil bed under otherwise identical conditions. As shown in Fig. 25.2(a), the elution of total Cr with DI water displayed a much higher and broader peak and a longer tailing than with the ZVI nanoparticle suspension. Mass balance calculation revealed that DI water eluted a total of approximately 12 percent of the pre-sorbed Cr(VI), whereas the Fe suspension leached only about 4.9 percent, a 59 percent reduction.
When plotted as Cr(VI) (Fig. 25.2(b)), all of the leached Cr(VI) was converted to Cr(III) during the treatment. In both cases, the effl uent pH was in the range of 5.2 to 5.7. In the presence of the ZVI Fe nanoparticles, the peak concentration dropped abruptly to < 0.28 mg/L at 1 BV and to < 0.007 mg/L after 5 BV of the nanoparticle suspension was passed. This observation indicates that the transformed Cr(III) is not only much less toxic but also much less mobile than Cr(VI). The transformed Cr(III), either in the form of fi ne precipitates or associated with the oxidized Fe nanoparticles, was subject to natural fi ltration eff ect as it travels through the soil [ 11 ].
The nanoparticle treatment also reduces the TCLP and the California Waste Extraction Test (WET) extractability of Cr(VI). Xu and Zhao [ 11 ] observed that the equilibrium Cr concentration in the TCLP extractant was 0.4 mg/L for the untreated soil, compared to only 0.04 mg/L when the same soil was treated with approximately 5.7 BV of the ZVI nanoparticle suspension at pH 5.60, and all TCLP-leached Cr for the treated soil was present in the form of
Time (h)
0 20 40 60 80
Cr (àg/L)
0 1000 2000 3000 4000 5000 6000
Total Cr in Fe suspension Cr(VI) in Fe suspension Cr (VI) in DI water
Figure 25.1 Leaching of Cr or Cr(VI) from a contaminated sandy loam soil with nanoparticle suspension (Fe = 0.08 g/L) or DI water. (Solution volume = 15 ml;
Soil = 1.5 g; Initial Cr in soil = 83 mg/kg).
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Cr(III). Upon the same brief nanoparticle treatment, the WET-leached Cr concentration was reduced from 1.2 mg/L to 0.28 mg/L.
For in situ remediation uses, the nanoparticles must be highly mobile in soil to ensure delivery of the nanoparticles to the targeted locations. The soil transportability of the CMC-stabilized nanoparticles was tested by measuring the breakthrough curve of the Fe nanoparticles when a ZVI suspension (Fe = 0.06 g/L) was passed through a packed sandy soil bed [ 11 ]. More than 81 percent of ZVI introduced broke through rapidly in less than 1 BV, indicating that the nanoparticles were highly mobile through the soil bed. Approximately 19 percent of Fe introduced was retained in the soil bed after 3 BV of the
BV
0 1 2 3 4 5 6
0 2000 4000 6000 8000 10000 12000 14000 16000
1 2 3 4 5 6
0 200 400 600 800 1000 1200
Fe at pH 5.60 DIW at pH 5.60
Cr (μg/L)
BV
0 1 2 3 4 5 6
Cr (VI) (μg/L)
0 2000 4000 6000 8000 10000 12000 14000 16000
1 2 3 4 5 6
0 200 400 600 800 1000 1200
Fe at pH 5.60 DIW at pH 5.60 (a)
(b)
Figure 25.2 Cr elution histories during two separate column runs using nanoparticle suspension (Fe = 0.06 g/L) or DIW at an infl uent pH 5.6: (a) Total Cr; (b) Cr(VI);
Insets: close-up of Cr elution histories after 1.9 BV.
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suspension was passed and more Fe was retained in the soil thereafter. Under the subsurface conditions, the stabilized ZVI nanoparticles will be converted to iron minerals in weeks, and the minerals will eventually be incorporated in the ambient geo-media.