Monitoring Water Quality || Removal and Immobilization of Arsenic in Water and Soil Using Polysaccharide-Modified Magnetite Nanoparticles

12
Removal and Immobilizationof Arsenic in Water and Soil
Using Polysaccharide-ModifiedMagnetite Nanoparticles

Monitorin

Copyright

Qiqi Liang, Byungryul An, Dongye Zhao*
ENVIRONMENTAL ENGINEERING PROGRAM, DEPARTMENT OF CIVIL ENGINEERING,
AUBURN UNIVERSITY, AUBURN, AL, USA*CORRESPONDING AUTHOR

CHAPTER OUTLINE

12.1 Introduction ............................................................................................................................... 285

12.2 Preparation and Characterization of Polysaccharide Stabilized Magnetite and Fe–Mn
Nanoparticles ............................................................................................................................. 287
12.2.1 Synthesis of Polysaccharide-Bridged or Stabilized Magnetite Nanoparticles .......... 287

12.2.2 Effects of Starch and CMC on Stability and Surface Properties of Magnetite

g Water

� 2013

Nanoparticles ................................................................................................................. 288

12.2.3 Effects of Stabilizer on As Uptake ............................................................................... 291

12.2.4 Effect of pH on Arsenate Sorption .............................................................................. 295

12.2.5 Effects of Salinity and Organic Matter on Arsenate Sorption .................................. 295

12.3 Potential Application for in situ Immobilization of As(V) in Soil and Groundwater ......... 296

12.4 Conclusions ................................................................................................................................ 297

References........................................................................................................................................... 297

12.1 IntroductionThe presence of arsenic (As) in soil and water is widespread. The United States Envi-

ronmental Protection Agency (USEPA) estimates that approximately 2% of the US pop-

ulation receives drinking water containing >10 mg/L of As [1], and the Natural Resources

Defense Council estimates that approximately 56 million people in the United States

drink water containing As at unsafe levels.

Arsenic has been associated with various cancerous and noncancerous health effects

[2]. According to a recent report put forward by the National Academy of Science and

Quality, http://dx.doi.org/10.1016/B978-0-444-59395-5.00012-1 285Published by Elsevier B.V.

http://dx.doi.org/10.1016/B978-0-444-59395-5.00012-1

286 MONITORING WATER QUALITY

National Research Council [3], even at 3 mg/L of As, the risk of bladder and lung cancer

being caused is between 4 and 7 deaths per 10,000 people. At 10 mg/L, the risk increases to

between 12 and 23 deaths per 10,000 people [3].

Triggered by the risk concern, the USEPA announced its ruling in October 2001 to

lower the maximum contaminant level from the previous 50 mg/L (established in 1942) to

10 mg/L with effect from January 22, 2006 [2]. This ruling has a tremendous impact on

water utilities. Approximately 4100 water utilities serving approximately 13 million

people are affected by the law [4]. The compliance cost has been estimated to be

approximately $600 million per year using current treatment technologies [5].

In soil and groundwater, As predominantly exists in two oxidation states, As(V) and

As(III), with specific forms influenced by pH and redox [6] conditions. It is quite

commonly observed that the both species coexist. Consequently, the simultaneous

removal of As(V) and As(III) is often required in drinking water treatment.

Adsorption has been one of the most used technologies for the removal of trace levels

of As [7]. Numerous studies have shown that various types of iron oxides can effectively

adsorb both As(V) and As(III) [8–12]. Moreover, researchers found that reducing the size

of the adsorbent particles to the nanoscale level can substantially increase As uptake

because of the much gained specific surface area. For instance, a decrease in the particle

size from 300 to 12 nm can increase the sorption capacity for both As(III) and As(V) by

nearly 200-fold [13].

However, the nanoparticles without a stabilizer or surface modifier tend to agglom-

erate rapidly into micron-scale or larger aggregates, thereby greatly diminishing the

specific surface area and As sorption capacity.

To prevent particle agglomeration, various particle stabilizing techniques have been

developed in recent years [13–15]. Of many stabilizers reported, polysaccharides such

as starch and carboxymethyl cellulose (CMC, molecular weight ¼ 90,000) have been

found to be not only effective in facilitating size control of various metal and metal

oxides nanoparticles but to be also cost effective and environmentally benign. The

proper use of stabilizers can maintain the high specific surface area and high reactivity

of the nanoparticles. Further, stabilizers can facilitate manipulation of the morphology

of the nanoparticles suitable for desired environmental cleanup applications. For

example, An et al. [16] reported that the application of low concentrations of a water-

soluble starch resulted in a new class of bridged magnetite nanoparticles that maintain

the specific surface area of the nanoparticles and can settle out of the water under the

influence of gravity. These are thus highly suitable for As removal from water. Alter-

natively, Liang et al. [17] reported that fully stabilized magnetite nanoparticles can be

obtained at elevated stabilizer (starch or CMC) concentrations. The stabilized nano-

particles offer the unprecedented advantage that the particles can be delivered into the

contaminated subsurface, thereby enabling in situ immobilization of As contaminated

aquifers.

This chapter summarizes some of the latest developments in preparing stabilized or

surface modified magnetite nanoparticles and illustrates the potential uses for As

removal from water or from soils in which As is immobilized. The effects of stabilizers

Chapter 12 • Removal and Immobilization of Arsenic in Water and Soil 287

on the surface physical and chemical characteristics of the nanoparticles are also

discussed.

12.2 Preparation and Characterization ofPolysaccharide Stabilized Magnetite and Fe–MnNanoparticles

12.2.1 Synthesis of Polysaccharide-Bridged or Stabilized MagnetiteNanoparticles

For water treatment, two features of nanoparticles are desired: (1) the particles should

offer a high sorption capacity; (2) the spent particles should be amenable to easy sepa-

ration from water and must not cause any harmful effects on the treated water. To this

end, the desired form of magnetite would be a water-based suspension, in which

magnetite nanoparticles are present as interbridged flocs as described by An et al. [16].

The bridged nanoparticles offer the advantages of high As sorption capacity and easy

separation after the desired use. Alternatively, fully stabilized magnetite nanoparticles

may also be used for water treatment. However, an external magnetic field will need to be

used to separate the spent nanoparticles [18]. For in situ As immobilization, the

prerequisite is that the nanoparticles must be deliverable in the soil. Therefore, fully

stabilized nanoparticles are required.

Two main schemes exist for preparing magnetite nanoparticles, and they involve

(a) decomposition of organic iron precursors at high temperatures and (b) aqueous-

phase coprecipitation of ferrous–ferric ions at elevated pH. In the organic decomposition

scheme, for instance, Yean et al. demonstrated that at 320 �C, the reaction of FeO(OH) in

oleic acid and 1-octadecene resulted in the formation of magnetite nanoparticles with an

average size of 11.72 nm [13]. The main drawback of this approach is the use of organic

solvents and the high energy input. In contrast, the aqueous coprecipitation scheme is

rather straightforward. It is conducted at room temperature and requires only pH

adjustment in the aqueous phase, without the involvement of any organic solvents. Yet,

without a stabilizer, this method fails to control the particle size and aggregation of the

resulting nanoparticles [19].

An et al. [16] and Liang et al. [17] developed the following modified scheme for

preparing polysaccharide-bridged or fully stabilized magnetite nanoparticles:

System under N2 purging/mixing

System under N2 purging/mixing

Step 3: Grow for 24 h and then adjust pH back to neutral.

Step 2: Add 2.0 wt% NaOH until pH = 11.

Step 1: Prepare a starch or CMC (0.02~0.1 wt.%) and 0.1 g/L as Fe solution (Fe3+:Fe2+=2:1).


As rapid precipitation of Fe(OH)2 occurs after the addition of sodium hydroxide

solution to the mixture of Fe3þ–Fe2þ, the magnetite formation begins with the oxidation

of [Fe(OH)]þ (the dissolved form of Fe(OH)2 (solid) in water). Subsequently, the oxidation

gives the intermediate product [Fe2(OH)3]þ3

(aq), which can combine with another

[Fe(OH)]þ to form [Fe3O(OH)4]2þ

(aq) that has the same Fe2þ/Fe3þ ratio as magnetite has.

Equations (12.1)–(12.3) depict the key chemical reactions involved [20]:

FeðOHÞ2ðsolidÞ4½FeðOHÞ�þðaqÞ þOH� (12.1)

þ 1 þ3 �
2½FeðOHÞ�ðaqÞ þ 2O2 þH2O/½Fe2ðOHÞ3�ðaqÞ þOH (12.2)
½Fe ðOHÞ �þ3 þ ½FeðOHÞ�þ þ 2OH�/½Fe OðOHÞ �2þ þH O (12.3)
2 3 ðaqÞ ðaqÞ 3 4 ðaqÞ 2
At highly oxidizing environments or low pHs, [Fe3O(OH)4]2þ

(aq) will further oxidize to

goethite or ferric hydroxides. However, at low amounts of dissolved oxygen and high pH,

[Fe3O(OH)4]2þ

(aq) will nucleate and/or crystallize to form magnetite particles:

Fe3OðOHÞ42þðaqÞ þ 2OH�/Fe3O4ðsolidÞ þ 3H2O (12.4)

In the absence of a stabilizer, the magnetite particles will agglomerate into micron-

scale or larger aggregates that will settle out of the aqueous phase under the influence of

gravity. However, in the presence of an effective stabilizer such as starch or CMC, the

growth rate and extent, and thus, the size, of the nanoparticles can be controlled

[14,21,22]. Researchers at the Auburn University [16,17,22] also demonstrated that the

particle size and surface chemistry can be manipulated by means of starch and/or CMC

stabilizers at various concentrations and of various molecular weights and degrees of

substitution.

Figure 12-1 shows the transmission electron microscopy (TEM) images of bridged and

stabilized magnetite nanoparticles. Figure 12-1a shows that the bridged particles are

interconnected, yet they remain identifiable individual nanoparticles, that is, the particles

did not aggregate into larger solid particles because of the coating of starch on the

nanoparticles. Figure 12-2 illustrates how stabilizers affect the stability and morphology

of the resulting nanoparticles, and demonstrates that starch can serve as a bridging or

flocculating agent at lower concentrations and can also act as a stabilizer at elevated

concentrations.

12.2.2 Effects of Starch and CMC on Stability and Surface Propertiesof Magnetite Nanoparticles

Polysaccharides (starch and cellulose) are the most abundant biopolymers and represent

the greatest components of biomass on the planet. The starch and CMC stabilizers used

in the preparation of the magnetite nanoparticles are slightly modified polysaccharides,

of which starch is a linear, neutral, and gelling polymer, whereas CMC is a linear, anionic,

and nongelling polymer with a pKa value of 4.3 [16]. Because of the difference in the

FIGURE 12-1 (a) TEM images of starch-bridged magnetite nanoparticles (26.6 � 4.8 nm, mean � standard deviation)prepared at a concentration of 0.057 g/L of Fe in the presence of 0.049 wt% starch and (b) TEM images of starch-stabilized magnetite nanoparticles (75 � 17 nm) prepared at a concentration of 0.1 g/L of Fe with 0.1 wt% starch[15,16].

(a) (b) (c)

FIGURE 12-2 A conceptualized illustration of the stabilizer effect on particle interactions in aqueous solution: (a)Particles aggregate without a stabilizer; (b) particles are bridged and form loosely packed flocculates in the presenceof appropriate concentrations of a polymer stabilizer; and (c) particles remain stable when the surface-attachedstabilizer concentration is high enough [15]. (For color version of this figure, the reader is referred to the onlineversion of this book.)


functionality of the two polysaccharides, their stabilizing effectiveness and the surface

properties of the resultant nanoparticles are expected to differ.

CMC is a polysaccharide that is modified from cellulose by replacing the native

CH2OH group in the glucose unit with a negatively charged carboxymethyl group. The

pH

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Zeta P

oten

tial (m

V)

-160

-140

-120

-100

-80

-60

-40

-20

0

20

40

CMC-stabilized Fe3O4

Non-stabilized Fe3O4

Starch-stabilized Fe3O4

FIGURE 12-3 The z potential for bare and 0.1 wt% starch- or CMC-stabilized magnetite nanoparticles at various pHlevels of the solution. All particle suspensions were prepared at a concentration of 0.1 g/L (as Fe) [16]. (For color versionof this figure, the reader is referred to the online version of this book.)


addition of carboxymethyl groups greatly enhances the water solubility and interaction

withmetals such as Fe onmagnetite surfaces. In contrast, starch is a neutral polymer, and

the solubility and interaction with the nanoparticles are expected to be weaker than that

for CMC. Liang et al. [17] showed that CMC is a much more powerful stabilizer than

starch is. He and Zhao [21,22] indicated that CMC stabilizes nanoparticles through

electrosteric repulsions, whereas starch works only through steric hindrance arising from

the osmotic force when the individual starch molecular domains overlap as the starch-

coated nanoparticles collide. Liang et al. [17] showed that the minimum starch

concentration needed to fully stabilize 0.1 g/L as Fe of magnetite nanoparticles is

0.04 wt% of starch because complete particle stabilization was realized at a starch

concentration of 0.04 wt%, compared to only 0.005 wt% when CMC is used.

Figure 12-3 compares the zeta (z) potential of bare and starch or CMC-stabilized

magnetite nanoparticles. The z potential is an important parameter that governs the

interparticle electrostatic interactions and stability of nanoparticles in water. It can also

strongly affect the adsorption characteristics of ionic species such as arsenate. Generally,

a high z potential (the absolute value of z is >60 mV) indicates strong electrostatic

stabilization [23]. Figure 12-3 shows that due to the starch coating, a nearly neutral

surface was evident over a pH range of 2–9 for starch-stabilized particles. In contrast,

CMC-stabilized nanoparticles displayed a much more negative surface with a z value

ranging from �120 to �150 mV at pH above the pKa value of CMC. The pH of the point of

zero charge (PZC) is estimated to be <pH 2 for CMC-stabilized particles. For bare

magnetite particles, the z potential shifted from þ15 mV at pH 3 to �31 mV at pH 11,

Ce

(mg/L)

0 1 2 3 4 5 6

q (

mg

/g

)

0

10

20

30

40

50

60

70




Commercial Fe3O4

FIGURE 12-4 Arsenate sorption isotherms for various types of magnetite particles. Magnetite dosage ¼ 0.1 g/L as Fe,starch, or CMC ¼ 0.1 wt% for stabilized particles; initial As ¼ 0.03–8.24 mg/L; final pH ¼ 6.8 � 0.4; equilibrationtime ¼ 144 h. Data are given as the mean of duplicates, and errors refer to deviation from the mean [16]. (For colorversion of this figure, the reader is referred to the online version of this book.)


resulting in a pHpzc value of 6.1. The PZC value for the stabilized nanoparticles differed

significantly from that for bare magnetite particles, for example, 7.9–8.0 obtained by Illes

and Tombacz [24] or 6.3–6.8 reported by Yean et al. [13] and Marmier et al. [25]. Based on

Figure 12-3, it is expected that the nearly neutral starch-stabilized nanoparticles would

offer a more favorable surface condition for arsenate uptake than the highly negatively

charged CMC-stabilized counterparts.

12.2.3 Effects of Stabilizer on As Uptake

The simple surface modification can substantially enhance arsenate adsorption capacity.

Figure 12-4 compares arsenate sorption isotherms at an equilibrium pH of 6.8 � 0.4 for

bare and stabilized nanoparticles. The classic Langmuir isotherm model was used to

interpret the equilibrium data:

q ¼ b �Q � Ce

1þ b � Ce(12.5)

where b is the Langmuir affinity coefficient related to sorption energy (L/mg), Ce is the

aqueous-phase concentration of As (mg/L), q is the solid-phase concentration of As (mg/

g), and Q (mg/g) is the Langmuir capacity.

The starch-stabilized nanoparticles displayed a very favorable, nearly rectangular

isotherm with a Q value of 62.1 mg/g, which is 72% greater than that for the CMC-

stabilized counterparts. Both stabilized nanoparticles offered a much greater sorption

capacity than did the bare magnetite (Q ¼ 26.8 mg/g). The commercial magnetite

FIGURE 12-5 FTIR spectra of bare and starch-bridged magnetite nanoparticles with or without arsenate [16].


powder offered an even lower As sorption capacity than the laboratory-prepared bare

magnetite.

Yean et al. [13] synthesized and tested a class of magnetite nanoparticles (11.72 nm)

and observed a comparable Langmuir Q of 64 mg-As/g-Fe at pH ¼ 8 at As(V) concen-

trations <3.7 mg/L. However, compared to the particle preparation method, starch-

stabilized magnetite nanoparticles are not only much more easy to prepare but are also

likely to be more cost effective and more environmentally friendly as the technique

eliminates the need for organic solvents and heating.

An et al. [16] investigated the nature of the chemical bonding between magnetite,

starch, and arsenate through Fourier transform infrared (FTIR) spectroscopy. Figure 12-5

shows the FTIR spectra for bare magnetite particles and starch-bridged samples before

and after sorption of arsenate. The absorption band at 571 per cm was obtained for all

three types of magnetite samples tested, and it can be assigned to the Fe–O stretching

vibration for the Fe3O4 particles [26]. The presence of starch coating reduced the

absorption of the radiation by the core magnetite and, thus, weakened the peak intensity

for the two starch-coated samples. On comparing the spectra of the starch-bridged

magnetite before and after arsenate sorption, we clearly observed a new broad band

between 750 and 850 per cm for the arsenate-laden magnetite, which was due to the Fe–

O–As complexes. This is consistent with the results obtained by Pena et al. [27] who

observed an FTIR band at 808 per cm for As(V) sorbed on TiO2 and ascribed the band to

the Ti–O–As groups. Pena et al. [27] compared FTIR bands for sorbed and dissolved

arsenate species, and they noted a marked shift in the band positions of arsenate on

sorption. For dissolved H2AsO4�, two peaks were observed at 878 and 909 per cm corre-

sponding to the symmetric and asymmetric stretching vibrations of As–O bonds. On

adsorption, the peaks were shifted to 808 and 830 per cm, respectively. The shifts in band

Starch concentration (wt%)

0 5e-3 0.02 0.04 0.06 0.08 0.1

q

(m

g/g

)

0

10

20

30

40

50

60

70(a)

0 2e-3 5e-3 0.01 0.02 0.04 0.06 0.1

q (

mg

/g

)

0

10

20

30

40

50

60

70

CMC concentration (wt%)

(b)

FIGURE 12-6 Effect of starch (a) or CMC (b) concentration on As(V) uptake (q). The nanoparticle dosage¼ 0.1 g/L as Fe;initial As(V) ¼ 8.24 mg/L; equilibrium pH ¼ 6.8 � 0.4; equilibration time ¼ 144 h. Data are given as the mean ofduplicates, and errors refer to the standard deviation from the mean [16].


positions were attributed to symmetry reduction resulting from inner-sphere complex

formation (i.e. the formation Fe–O–As complexes) [27,28]. Goldberg and Johnston [28]

also reported that for arsenate sorbed on amorphous iron oxide, there existed two distinct

bands corresponding to surface complexed and non-surface-complexed As–O groups,

respectively. For the case of starch-bridged nanoparticles, only one single band was

observed, and the wave number was shifted to approximately 800 per cm, indicating that

surface complexation was the predominant mechanism for arsenate sorption to

the starch-bridged nanoparticles. In addition, Zhang et al. [29] reported that the rise in

pH

1 2 3 4 5 6 7 8 9 10 11 12 13 14

q (m

g/g

)

0

10

20

30

40

50

60

70

80




FIGURE 12-7 Equilibrium arsenate uptake as a function of the pH of the final solution. Experimental conditions: initialAs ¼ 8.0 mg/L; nanoparticles ¼ 0.1 g/L (as Fe); CMC or starch (for stabilized nanoparticles) ¼ 0.1 wt%; equilibrationtime¼ 144 h. Data given as themean of duplicates, and errors refer to deviation from themean [16]. (For color versionof this figure, the reader is referred to the online version of this book.)


M–As–O (M: metal) peak was coupled with weakening of the M–OH peak when As(V) was

adsorbed onto Fe–Ce oxide, and the researchers ascribed this phenomenon to the ion

exchange reaction between –OH and arsenate anions. However, no M–OH bond was

evident for the starch-bridged magnetite, which suggests that the sorption of As(V) did

not necessarily conform to the standard ion exchange stoichiometry.

As the stabilizer concentration can greatly affect the particle stability and morphology,

Liang et al. [17] also observed strong effects of the stabilizer concentration on the arsenate

uptake capacity, especially when starch was used. Figure 12-6a shows the equilibrium

uptake of As(V) with magnetite particles prepared in the presence of various starch

concentrations.Overall, theAsuptake increasedwith increasing stabilizer concentrationsor

with decreasing particle sizes. In accord with the observed physical stability, when the

nanoparticles are fully stabilized (starch ¼ 0.04 wt% or higher), a sorption plateau was

evident. Compared to bare magnetite particles, the 0.04 wt% starch-stabilized particles

offered a 2.2 times greater As(V) uptake. However, when the stabilizer concentration was

further increased from 0.04 to 0.1 wt%, the capacity gain was only 14%. The capacity

enhancement for stabilized magnetite particles can be attributed to the smaller size and,

thus, to a greater specific surface area for more stable particles. However, a denser layer of

starch results in elevatedmass transfer resistance and increases the energy barrier for some

of the sorption sites, and thermodynamically, the presorbed starchmolecules diminish the

effective sorption sites of the particles.

Figure 12-6b shows that CMC stabilization does not offer as much capacity

enhancement for As(V) removal. At the experimental pH of 6.8, both H2AsO4� and HAsO4

2�


are predominant arsenate species. As CMC-stabilized magnetite particles carry a highly

negative surface, sorption of these anions would have to overcome a substantial energy

barrier due to the electrostatic repulsion between like charges. Consequently, even

though CMC stabilization gives rise to a smaller particle size and greater specific surface

area, the arsenate uptake by CMC-stabilized magnetite was much lower than that by the

starch-stabilized counterparts.

12.2.4 Effect of pH on Arsenate Sorption

The solution pH can affect the surface potential of the nanoparticles (as shown in

Figure 12-3) and the speciation of arsenate in the aqueous phase, both of which can affect

arsenate sorption. Figure 12-7 shows arsenate uptake as a function of solution pH for the

bare, starch-, and CMC-stabilized magnetite particles. Starch-stabilized nanoparticles

displayed amuch greater As sorption capacity throughout the pH range tested. In general,

the lower the pH, the greater the sorption capacity. This is attributed to the fact that the

surface of the magnetite particles is less negatively charged at lower pHs, and the surface

is positively charged at pH <pHPZC (Figure 12-3). Although the z potential for starch-

stabilized particles seems to be nearly the same at <pH 7 (Figure 12-3) due to the starch

coating, the more positive core particle surface becomes more favorable for arsenate

anions at lower pH values. At an extremely low pH of 2, however, the As uptake was

lowered primarily because of partial dissolution of the nanoparticles and partial

decomposition of starch [17]. Arsenate uptake diminished with increasing pH due to

elevated electrostatic repulsion and more fierce competition of hydroxyl anions.

12.2.5 Effects of Salinity and Organic Matter on Arsenate Sorption

One of the potential uses of the magnetite nanoparticles is to remove As from spent ion

exchange brine [16]. To this end, the nanoparticles should be able to tolerate high

salinity and competition from high concentrations of competing anions [16]. The

presence of brine at concentrations as high as 10% did not show any appreciable

suppression in arsenate sorption capacity. This observation violates the principle of

selectivity reversal commonly cited in standard ion exchange regeneration [30], but this

can be attributed to strong specific interactions such as strong inner-sphere complex-

ation between As and the nanoparticles. To quantify the relative affinity, the arsenate

and chloride binary separation factor aAs/Cl is calculated based on the equilibrium

sorption data [30]:

aAs=Cl ¼qAsCCl

CAsqCl(12.6)

where qAs and qCl are the concentrations of arsenate and chloride, respectively, in the

solid phase, whereas CAs and CCl are the concentrations of arsenate and sulfate in the

solution, respectively. As a rule, a separation factor value of >1 indicates greater selec-

tivity for arsenate than for chloride. The calculated aAs/Cl values were 102, 139, 147, and


290 at 2%, 4%, 6%, and 10% of NaCl, respectively, confirming the nanoparticles’ strong

affinity for arsenate over chloride.

The observed minimal salt effect contrasts with the results obtained with bare

magnetite nanoparticles (20 nm) reported by Shipley et al. [31]. For bare particles, in the

presence of high concentrations of cations, the electrostatic double layer is compressed,

increasing particle aggregation and decreasing the available sorption sites. For starch-

bridged magnetite particles, the particles were bridged, yet they were held apart due to

the interparticle steric repulsion arising from the osmotic force of the starch on the

particle surface, and increasing ionic strength did not affect the surface charge as the

ionic strength exerts little effect on the steric repulsion.

Natural Organic Matter can affect particle stabilization and may compete with arse-

nate. Liang et al. [17] compared arsenate sorption isotherms for starch-stabilized

magnetite nanoparticles in the presence of 0, 10 and 20 mg/L as TOC (Total Organic

Carbon) of DOM (Dissolved Organic Matter). They observed that the presence of high

concentrations of DOM can inhibit arsenate sorption capacity. Similar results were

reported by others, who studied As uptake by amorphous iron oxides [31–34].

12.3 Potential Application for in situ Immobilizationof As(V) in Soil and Groundwater

Although As has been detected widely in soil and groundwater, effective technologies

have been lacking for in situ remediation of As contaminated soil and groundwater. It is

even more challenging to remove As from groundwater in deep aquifers. Traditionally,

As contaminated soils have been treated by capping, soil replacement, and solidifica-

tion/stabilization and acid washing [35]. These methods are costly and only tempo-

rarily solve the contamination problem. The stabilized nanoparticles hold promise to

develop an innovative in situ immobilization technology that may revolutionize

conventional remediation practices. For example, the stabilized nanoparticles may be

injected into deep aquifers to facilitate in situ adsorption and immobilization of As in

groundwater. To this end, the nanoparticles must satisfy some of the key attributes,

including the following: (1) The particles must be deliverable, that is, mobile under

pressure so that they can be delivered into the contaminated source zones; (2) they

must have a high As sorption capacity; and (3) the delivered nanoparticles will be

immobile under the natural groundwater conditions, that is, once the external injection

pressure is removed, the nanoparticles remain in position and serve as a permanent

sink for As.

Liang et al. [36] studied breakthrough behaviors of starch-stabilized magnetite

nanoparticles through 1-D column experiments. The results demonstrates that the water

leachable As(V) from As(V)-laden sandy soil is greatly reduced on the nanoparticle

treatment compared to simulated groundwater. Under various pore flow velocities

ranging from 10�3 to 10�7 cm/s, the faster flow simulates the injecting flow condition,


whereas the lower end velocity simulates the slow groundwater flow. The results indicate

that the mobility of nanoparticles in soil could be manipulated by controlling the

injection velocity. For example, at a magnitude of 10�3 cm/s, the nanoparticles are readily

deliverable with the full breakthrough Ce/C0 level remaining at >0.90. When we halved

the pore velocity to 1.2� 10�3 cm/s, the full breakthrough Ce/C0 level was lowered to 0.80.

When the flow rate was reduced to 10�7 cm/s, there was no breakthrough of the nano-

particles. Based on the breakthrough curves and by applying the classical filtrationmodel,

Liang et al. claimed that the maximum distance can be predicted as a function of pore

flow velocity.

12.4 ConclusionsStabilized magnetite nanoparticles have great potential for the enhanced removal of As

from contaminated water and for soil remediation. Both starch and CMC can act as

effective stabilizers in the preparation of highly stable magnetite nanoparticles of a much

greater As sorption capacity than conventional iron oxide particles. The particle stability,

degree of aggregation, and size may be controlled by manipulating the type and

concentration of the stabilizer. The starch coating renders the z potential of particles

nearly neutral over a broad pH range, whereas the use of CMC results in a highly negative

surface. Consequently, CMC acts as a more effective stabilizer than starch does, whereas

starch-stabilized magnetite particles offer a much greater arsenate uptake capacity.

Acidic pH favors As(V) uptake, whereas As(III) sorption was observed over a wide pH

range. Stabilization broadens the pH range of maximum sorption. Preliminary results

indicated that stabilized magnetite nanoparticles can be used for in situ injection into

contaminated soil, thereby facilitating the effective immobilization of As. Column

breakthrough tests revealed that stabilized nanoparticles are highly transportable

through a sandy soil under external pressure. Once the external pressure is removed, the

nanoparticles remain immobile under natural conditions.

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