in situ treatment of arsenic-contaminated groundwater by air sparging

Journal of Contaminant Hydrology 159 (2014) 20–35

Contents lists available at ScienceDirect

Journal of Contaminant Hydrology

j ourna l homepage: www.e lsev ie r .com/ locate / jconhyd

In situ treatment of arsenic-contaminated groundwater byair sparging

Joseph H. Brunsting a,⁎, Edward A. McBean b

a School of Engineering, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1, Canadab School of Engineering, University of Guelph, Canada Research Chair of Water Supply Security

a r t i c l e i n f o

Abbreviations: DI, deionized; DO, dissolved oxygeoxide; HRT, hydraulic retention time; ORP, oxidationRmV, relative milli-volts.⁎ Corresponding author. Tel.: +1 519 362 6429 (ce

E-mail address: [email protected] (J.H. Bru

0169-7722/$ – see front matter © 2014 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jconhyd.2014.01.003

a b s t r a c t

Article history:Received 31 December 2012Received in revised form 22 December 2013Accepted 7 January 2014Available online 7 February 2014

Arsenic contamination of groundwater is amajor problem in some areas of theworld, particularlyinWest Bengal (India) and Bangladeshwhere it is caused by reducing conditions in the aquifer. Insitu treatment, if it can be proven as operationally feasible, has the potential to capture someadvantages over other treatment methods by being fairly simple, not using chemicals, and notnecessitating disposal of arsenic-rich wastes. In this study, the potential for in situ treatment byinjection of compressed air directly into the aquifer (i.e. air sparging) is assessed.An experimental apparatus was constructed to simulate conditions of arsenic-rich groundwaterunder anaerobic conditions, and in situ treatment by air sparging was employed. Arsenic (up to200 μg/L) was removed to a maximum of 79% (at a local point in the apparatus) using a solutionwith dissolved iron and arsenic only. A static “jar” test revealed arsenic removal by co-precipitationwith iron at a molar ratio of approximately 2 (iron/arsenic). This is encouraging since groundwaterwith relatively high amounts of dissolved iron (as compared to arsenic) therefore has a largetheoretical treatment capacity for arsenic.Iron oxidation was significantly retarded at pH values below neutral. In terms of operation,analysis of experimental results shows that periodic air sparging may be feasible.

© 2014 Elsevier B.V. All rights reserved.

Keywords:ArsenicGroundwaterIn situ treatmentAir spargingBangladesh

1. Introduction

Arsenic, widely acknowledged as biologically harmful, is acontaminant in groundwater in many areas of the world,including Cambodia, Argentina, Chile, China, Hungary, Laos,Mexico, Mongolia, Nepal, Pakistan, Taiwan, Thailand, Vietnam,and the USA (Ahmed, 2003). However, the most widespreadand serious groundwater arsenic levels are evident in WestBengal (India) and Bangladesh. The Bangladeshi arsenicproblem has been described as “the largest poisoning of apopulation in history, with millions of people exposed” (Smithet al., 2000, pg. 1093).

n; HFO, hydrous ferric–reduction potential;

ll).nsting).

ll rights reserved.

The current World Health Organization guideline for inor-ganic arsenic in drinking water is 10 μg/L (WHO, 2008).However, not all jurisdictions follow this guideline, includingBangladesh and India, where a guideline of 50 μg/L is used(Chakraborti et al., 2009; Flanagan et al., 2012; WQAA Govern-ment of India). Chronic arsenic exposure may result in severehealth effects with skin lesions, hyperkeratosis, and increasedrisk of cancers.

Although anthropogenic sources of arsenic exist (e.g.smelting operations), the most widespread problems are ofnatural geochemical origin. Groundwater arsenic concentrationsreported in the literature range from b0.5 μg/L to 5000 μg/Lunder natural conditions (Smedley and Kinniburgh, 2002).Oxides of iron, aluminum, and manganese are likely the mostimportant sources and sinks for arsenic in aquifer sediments(Stollenwerk, 2003).

Arsenic may be mobilized from soil as a result of reducingconditions in groundwater, as occurs in West Bengal andBangladesh. The reducing conditions are the result of oxidation

http://crossmark.crossref.org/dialog/?doi=10.1016/j.jconhyd.2014.01.003&domain=pdf

http://dx.doi.org/10.1016/j.jconhyd.2014.01.003

mailto:[email protected]

http://dx.doi.org/10.1016/j.jconhyd.2014.01.003

http://www.sciencedirect.com/science/journal/01697722

21J.H. Brunsting, E.A. McBean / Journal of Contaminant Hydrology 159 (2014) 20–35

of buried organicmatter in sediments. This causesmobilizationof arsenic by reduction of iron oxyhydroxides, reductivedissolution, and change in structure of iron oxide minerals(BGS and DPHE, 2001; Nickson et al., 1998; Smedley andKinniburgh, 2002).

Treatment of arsenic-impacted groundwater is feasible,and many options exist including oxidation and sedimenta-tion, coagulation and filtration, sorption techniques, andmembrane techniques. While being capable of effectivelyremoving arsenic, many treatment technologies have draw-backs due to requirements for use of chemicals, disposal ofarsenic-rich wastes, and/or technological complexity.

As an alternative option, in situ treatment (by creation ofoxidizing conditions) is a fairly simple procedure, does notrequire chemicals, and does not require disposal of arsenic-richwastes. The basis for in situ treatment of arsenic is the same asthat for in situ treatment of iron andmanganese, which has beenutilized in Europe for decades. Oxidation–reduction potential(ORP) conditions can be changed in the subsurface byintroducing dissolved oxygen (DO), causing oxidation of ferrousiron and other metals (in solution and on the soil grains). Thisprocess creates ferric iron oxyhydroxides capable of adsorbingferrous iron and other oxyanions such as arsenic (Appelo and deVet, 2003; Rott, 1985; Rott and Lamberth, 1993; van Beek, 1985;van Halem et al., 2010a). These oxidation processes areenhanced by autocatalytic effects from oxidation products(Rott and Friedle, 2000; Sung and Morgan, 1980; Tamura et al.,1980).

One method of in situ treatment for arsenic in ground-water involves injection of aerated water into the aquifer.This method has met with moderate success to date (Rottand Friedle, 1999, 2000; Sen Gupta et al., 2009; van Halem etal., 2010a, 2010b, 2010c). One possible concern regarding insitu treatment is the possibility that pore spaces in theaquifer may become clogged. However, this is not asignificant problem in reality. Subsurface treatment for ironin groundwater (by method of injection of aerated water) hasbeen used in Europe, and clogging has not been found to bean issue, even after more than a decade of operation (vanHalem et al., 2011). Iron may initially precipitate as hydrousferric oxide (HFO, of low crystallinity), but in the subsurfaceit ages and changes to thermodynamically more stable andless voluminous crystallized forms such as goethite (Mettler,

air lock

P.P.

peristaltic pump

N2 gas stainlesssteel mesh

smalldiffuser

large ponddiffuser

220 L barrel

Appa

Fig. 1. Apparatus con

2002; Rott and Friedle, 2000; Smedley and Kinniburgh, 2002;Stollenwerk, 2003), and this prevents clogging.

Another option for in situ treatment of arsenic in ground-water is direct air sparging in the saturated zone. However,besides literature regarding treatment on a deep well (Miller,2006, 2008) aswell as clean-up from a lead smelter site (Milleret al., 2002), there appears to have been limited research onthis option. Grombach (1985) suggests that introduction of airdirectly into the aquifer is the easiest method but did notundertake experimental observations.

The investigation described herein is a lab-scale studyto investigate the potential of in situ treatment of arsenic-contaminated water by air sparging. As described below, theexperiments utilized a simple solution of dissolved inorganicarsenic and dissolved ferrous iron in order to demonstrate theconcept, potential, and key factors of this type of treatment, as aprecursor to possible field trials.

2. Materials and methods

The experimental apparatus is illustrated in Fig. 1.Dimensions and sampling port labels are illustrated in Fig. 2.

A large, sealable, food-grade polyethylene barrel (Fig. 1) wasused for the inlet solution, which was subjected to nitrogensparging to remove dissolved oxygen. The main apparatus wasmade of plexiglass. A small precision-flow peristaltic pump(Fisher model CON3386) was used to transport solution fromthe reservoir barrel to the inlet column. Rotameters ofappropriate size were used for measuring gas flow.

The apparatus had both an inlet and an outlet reservoir,fitted with a fine stainless steel mesh to retain the sandmedium. A uniformly graded (rounded, nominal size 0.40 mm)sandwas rinsed with deionized (DI) water and used as aquifersoil for purposes of the laboratory simulation. In chemicalcomposition, the sand was as follows: SiO2 N99.5%; TiO2

~0.10%; K2O ~0.10%; CaO ~0.03%; Fe2O3 ~0.03%; Al2O3

~0.01%; Loss on ignition ~0.12%The outlet height was used to adjust the hydraulic gradient.

Air sparging was accomplished using an aquarium aerationpump (Hagen, model Maxima-R) with tubing attached to arotameter. The outlet from the rotameter ran to a small (2.5 cm)alumina diffuser stone (Fisherbrand model ME46944C) at thesparging point in the apparatus, as shown in Fig. 1.

Water level

stainlesssteel meshair diffuser

air flow

effluent

sampling ports

Water le el

ratus Concept

ceptual design.

Water level

200

10

50

3.3head loss

40.744

22

vent

20.3

40

4

In

A1

A2

A3

B1

B2

B3

C1

C2

C3

D1

D2

D3

E1

E2

E3

F1

F2

F3

G1

G2

G3

H1

H2

H3

I1

I2

I3

Out

J1

J2

J3

Width (i.e. into the drawing) = 5 cm

Expected design head loss

Fig. 2. Apparatus dimensions (cm) and sample port labels.

22 J.H. Brunsting, E.A. McBean / Journal of Contaminant Hydrology 159 (2014) 20–35

Experiments were completed as follows:

– Experiment 1: to examine the dispersion of oxygen withinthe porous medium while sparging with air, and pumpingplain DI water through the apparatus

– Experiment 2: to examine the removal of arsenic with nodissolved iron present

– Experiment 3: to examine the removal of arsenic withdissolved iron present

– Experiment 4: a static jar test, with the same solutionconstituents as experiment 3

– Experiment 5: similar to experiment 3, but over a muchlonger duration and with a slightly higher air injectionrate.

Experiments were prepared by first flooding the drainedapparatus with nitrogen while it was empty of water.Nitrogen was injected at a flow rate of 1 L/min, through thesparging point, over a period of 24 h. Solution was thenintroduced, by pumping a solution of 1000 μg/L of arsenicsolution (50% As[III], 50% As[V] in experiment 2, and 100% As[V] in other experiments, explanation to follow in Results).This was either with or without 5.3 mg/L Fe(II), dependingon the experiment. It was found that, while pumping asolution of 200 μg/L arsenic, breakthrough of arsenic took aprohibitive amount of time. Thus, the initial solution of1000 μg/L was introduced to more quickly saturate availableadsorption sites on the sand medium. Once arsenic wasdetected at above 200 μg/L at all sampling ports, the solutionwas switched to 200 μg/L and pumped until all ports showed200 μg/L or less. There was no replacement with fresh sandbetween experiments (i.e. the same sand media was usedthroughout all experiments in this investigation).

Two hundred μg/L arsenic and 5.3 mg/L ironwere chosen asexperimental concentrations. According to calculations byRoberts et al. (2004) using data from a groundwater survey,these are mean concentrations for Bangladeshi groundwaterwith an arsenic concentration of 50 μg/L or greater. Experi-mental solutions were prepared first by lowering DO to lessthan 0.05 mg/L using nitrogen gas sparging. Constituents werethen added using granular ferrous chloride tetrahydrate,sodium arsenite solution, and sodium arsenate solution.

These specific chemicals were used to represent the chemicalspecies present in solution in a reduced groundwater environ-ment (i.e. ferrous iron, and amix of both inorganic trivalent andinorganic pentavalent arsenic).

Neither sodium nor chloride is cited as a major ionaffecting arsenic adsorption (Stollenwerk, 2003), nor are theycorrelated with arsenic in Bangladeshi groundwater (Anawaret al., 2003). Hence, the sodium compounds of arsenic andthe chloride compound of ferrous iron were selected aschemicals for use in this study, with the assumption that theeffects of the associated sodium and chloride ions would beminimal.

In Bangladeshi groundwater, the ratio of As(III)/[As(III) +As(V)] = As(III)/As(total) ranges from less than 0.1 to greaterthan 0.9, but averages around 50% to 60% (BGS and DPHE,2001; Paul et al., 2008; Rasul et al., 2002). A ratio of 50% wasused in initial experimentation, as explained in Section 3.

Measurement of iron was accomplished using a Varian220 SpectrAA flame atomic absorption spectrometer. Mea-surement of arsenic was done using either a Varian 880SpectrAA graphite furnace atomic absorption spectrometeror a Shimadzu AA-6300 graphite furnace atomic absorptionspectrometer. Samples for analysis of metals were takenusing a new BD luer-lok 60 mL syringe for each sample.Metals samples were preserved using trace-metals gradenitric acid (adjusting to pH less than 2), and refrigerated at4 °C.

Analyses were done within one week of sampling. Toensure accuracy and precision, duplicate samples were takenwith each sample set and assessed. A new calibration wasperformed with prepared standards for every sample run,and mid-point standard checks were also performed witheach sample run. Duplicate samples and mid-point standardchecks were assessed for repeatability within 10% for ironand 20% for arsenic.

Dissolved oxygen measurements were completed usingan Orion 083005MDmembrane probe attached to an Orion 4Star pH-DO portable meter. For dissolved oxygen samplesfrom the apparatus, samples were taken using a 60 mLsyringe with a 3-way stopcock. After withdrawal of thesample from the apparatus, the stopcock was set to seal the

0

1

2

3

4

5

6

7

0 5 10 15 20 25 30 35 40 45 50

Dis

solv

ed O

xyg

en (

mg

/L)

Time (hours)

A & B Column DO

B1

B2

B3

A2

0

1

2

3

4

5

6

7

0 5 10 15 20 25 30 35 40 45 50

Dis

solv

ed O

xyg

en (

mg

/L)

Time (hours)

D Column DO

D1

D2

D3

0

1

2

3

4

5

6

7

0 5 10 15 20 25 30 35 40 45 50

Dis

solv

ed O

xyg

en (

mg

/L)

Time (hours)

G Column DO

G1

G2

G3

0

1

2

3

4

5

6

7

0 5 10 15 20 25 30 35 40 45 50

Dis

solv

ed O

xyg

en (

mg

/L)

Time (hours)

J Column DO

J1

J2

J3

Fig. 3. Results of experiment 1.


As(V)

As(III)

Fig. 4. Range of ORP and pH of experiments, plotted on an Eh–pH diagram foraqueous arsenic species in the system As–O2–H2O at 25 °C and 1 bar totalpressure (Smedley and Kinniburgh, 2002, pg. 521).

Table 1Relationship between DO and ORP in DI water with 5.3 mg/L iron and200 μg/L arsenic.

DO (mg/L) ORP (RmV)a

0.41 2667.67 340

a RmV refers to “relative milli-volts”, the milli-volt reading relative to thestandard hydrogen electrode.


tip, and then the plunger was removed using gentle forceagainst the vacuum. The probe was then used to gently stirthe sample in the syringe while taking a reading.

pH measurements were taken using an Orion 8102BNUWPprobe attached to an Orion 4 Star pH-ISE Benchtop meter. ForORP readings, an Orion 9179BNMD epoxy body gel-filled ORPtriode was used, attached to an Orion 4 Star pH-ISE Benchtopmeter (since this pH meter is also capable of ORP readings).

Speciation for As(V) and As(III) was done using a fieldmethod developed by Clifford et al. (2004). This relies onlowering of pH using acetic acid and complexing of iron withethylenediaminetetraacetic acid (EDTA) to remove interfer-ence of iron, and capturing As(V) with a chloride-formresin in glass mini-columns, hence allowing As(III) to passthrough.

In terms of iron speciation, iron can be considered to be inFe(II) (ferrous) dissolved form until oxidation and precipita-tion, at which point it converts to Fe(III) (ferric) form asiron(III) oxide-hydroxide. This appears as rusty brownprecipitate.

3. Results

3.1. Experiment 1 — to examine the dispersion of DO

In experiment 1, solution flow rate was 16 mL/min. Basedon measured porosity of the sand of 0.35, hydraulic retentiontime (HRT) was calculated as 16 h (assuming uniform flow).The airflow sparging rate was 16 ± 5 mL/min.

Results are shown in Fig. 3, organized by column ofsampling ports.

Port A2 is located up-gradient from sparging, and repre-sents inlet conditions. As shown in Fig. 3, the DO concentrationnear the inlet remained below 0.6 mg/L throughout theexperiment. Above the sparging point, the DO concentrationrose to between 4 and 5 mg/L, with the lowest sampling port(B3) directly above the sparging point showing the quickestrise. The air traveled in bubbles, creating airflow channels. Overthe extent of vertical rise, these airflow channels migratedapproximately 20 cm both up-gradient and down-gradient ofthe sparging point, forming a “V” pattern.

At the “D” column, DO concentrations appeared to riseslightly faster at D3, but effective transfer of oxygen to portsD1 and D2 was observed as the DO levels were 5 mg/L by theexperiment's end. Concentrations of DO rose to approxi-mately 5 mg/L along the bottom at sampling ports G3 and J3.However, there was not effective transfer of oxygen tomigrated water sampled at ports G1, G2, J1, or J2. Thisindicates the existence of a region of lower velocity flow inthe apparatus, as flow velocity was insufficient to transferwater from the sparging point to sampling ports G1, G2, J1, orJ2 by the end of the experiment.

Non-uniform grading of sand in the apparatus likelycaused the zone of low-velocity flow referred to above,approaching ports G1, G2, J1, and J2. During experimentalset-up, sand was first rinsed in a bucket, and then wasdeposited in the apparatus under submersion in DI waterwith a continuous flow of water from the inlet towards theoutlet. Some residual dust was observed as cloudiness in thewater during deposition of the sand. This dust and fine sandparticles were likely deposited in the upper part of the

apparatus near the outlet end because of the water flowmaintained during application of sand in the apparatus.

3.2. Experiment 2 — to assess arsenic removal with no iron

Characteristics and operating conditions of Experiment 2were as follows:

– Total water flow volume of 42.5 L over 48.8 h, so averageflow of 14.3 mL/min

– HRT calculated as 18 h– Initial pH 7.2 at the inlet and 5.7 at the outlet. This drop in

pH through the apparatus was unexpected. It is possiblethat oxidation of As(III) under unstable conditions (seeFig. 4) caused release of H+, resulting in a mildly acidicenvironment in an unbuffered solution (i.e. DI water).

– Air sparging flow rate at 16 ± 5 mL/min– Inlet solution containing 100 μg/L As(III) and 100 μg/L

As(V) to examine behavior of arsenic species.

Air sparging commenced immediately at the start of theexperiment. In the inlet sampling (for the first two samplings at0 h and 9 h), the As(III) was measured at 95 and 107 μg/L,respectively, and the As(V) was measured as 109 and 135 μg/L,respectively. This shows a balance between As(III) and As(V)

Table 2Relationship between DO and ORP during air sparging trials in the apparatus(using only DI water).

Sampling port DO (mg/L) ORP (RmV)

Barrel 0.07 332Inlet column 1.05 450Inlet 2.28 460Outlet 4.16 545


(approximately half each), with a total arsenic value ofapproximately 200 μg/L being achieved. However, speciationtests for samples taken from within the sand matrix and at theoutlet (i.e. all sampling ports except the inlet) showed the largemajority at zero As(III), with a few scattered tests showing up to50 μg/L As(III).

It was not expected that As(III) would be so readilyoxidized, in particular because the DO was low in the systemat the start of the experiment. It is apparent that As(III) wasnot stable under the conditions achieved in this experiment.During pre-experiment trials, and during experiment 1, ORPreadings were taken under various conditions as per Tables 1and 2.

ORP is affected by DO levels, but is also affected by manyadditional parameters such as pH, temperature, and smallreaction currents. Based on these results, one limitation of thisexperimental setupwas that reducing conditions (encounteredin Bangladeshi groundwater high in arsenic) could not beachieved by just deaerating DI water. As such, the approachwas to control DO concentrations in the solution and apparatusas closely as possible. The approximate pH and ORP rangeencountered during all experiments in this investigation isplotted in Fig. 4.

As per Fig. 4 under conditions of these experiments, As(III)was unstable. As(III) is typically very slow to oxidize in a simplesolution of water with DO, on the order of months (Cherry etal., 1979), but the rate of transformation of As(III) to As(V)increases rapidly with manganese- and iron-oxides/-hydrox-ides (Rott and Friedle, 2000). Metal oxides are present in thesand matrix, and this catalyst effect may explain why As(III)

0

1

2

3

4

5

In A1 A2 A3 B1 B2 B3 E1 E2

DO

(m

g/L

)

Sampling Port

DO Levels in the Apparatus Dur

Fig. 5. DO levels in the apparat

was apparently found to be present in the inlet but not atsample points within the sandmatrix. The rest of the summaryof experiment 2 deals only with As(total), taken as As(V).

The DO levels at each port are shown in Fig. 5, showingconcentrations at the beginning and end of air sparging.

As shown, there was distinct oxygen transfer to ports A1,B1, B2, B3, E1, E2, E3, J3, and the outlet. It appears that theairflow channel (from sparging) migrated sufficiently totransfer some oxygen to port A1. In this experiment, DO actsas a tracer, indicating which areas of the apparatus receivedwater flow from the sparging area during the experiment. Assuch, it is clear that J1 and J2 did not receive water from thesparging area by the end of the experiment, due to the low flowzone described previously. In addition, measurements takenprior to the start of sparging indicated that the concentrationsof arsenic at J1 and J2were lower than at other ports at the startof sparging (i.e. this area had a lower “saturation” of availableadsorption sites), and as such may have exhibited biasedresults. J1 and J2 are therefore omitted in analyses to follow.

Results for arsenic are summarized by “inlet and outlet” andby row of sampling ports (i.e. top row 1, middle row 2, andbottom row 3) in Fig. 6. Samples were taken at these points inorder to examine spatial heterogeneity both upstream anddownstream of air sparging.

Statistical paired t-tests were completed, the results ofwhich indicate greater than 90% certainty that arsenicconcentrations decreased across the apparatus at all sam-pling ports.

Although iron was not present in solution in this experi-ment, somemetal oxideswere nonetheless present in the sand.These could be responsible for some adsorption of arsenic,withsubsequent regeneration of adsorption sites upon oxidation byDO. Summarized results for ports downstream of sparging areshown in Table 3.

From examination of Table 3, it appears that the ports alongthe bottom flowpath (i.e. E3 and J3) show lower removal valuesfor arsenic than the ports at E1 and E2. Ports E1 and E2 werelikely along a slower flow path (approaching the lower-flowzone) and as such thewater samples at these ports had a highercontact time with the sand than corresponding samples alongthe bottom flow path. This suggests that removal of arsenic is

E3 J1 J2 J3 Out

ing Experiment 2

Start

49 hours end

us during experiment 2.

0

50

100

150

200

250

0 5 10 15 20 25 30 35 40 45 50

As

(µg

/L)

Time (hours)

Inlet and Outlet - As(total)

In

Out

0

50

100

150

200

250

0 5 10 15 20 25 30 35 40 45 50

As

(µg

/L)

Time (hours)

Top Sample Ports - As(total)

A1

B1

E1

0

50

100

150

200

250

0 5 10 15 20 25 30 35 40 45 50

As

(µg

/L)

Time (hours)

Middle Sample Ports - As(total)

A2

B2

E2

0

50

100

150

200

250

0 5 10 15 20 25 30 35 40 45 50

As

(µg

/L)

Time (hours)

Bottom Sample Ports - As(total)

A3

B3

E3

J3

Fig. 6. Results of experiment 2 for arsenic.


Table 3Calculated percent removals of arsenic for ports downstream of air spargingthat exhibited heightened DO by the end of experiment 2.

Mean inlet arsenic conc.(μg/L)

Port End arsenic conc.a

(μg/L)Removalb

(%)

E1 155 31226 E2 165 27

E3 187 17J3 190 16Out 152 34

a End of air sparging.b Calculated for the average inlet concentration compared to the

concentrations at sampling ports at the end of air sparging.


dependent on contact time under these conditions, withremoval at E1 and E2 in the range of 30%. Measurements atthe outlet show the highest removal; however, the result herewas likely affected by solution flow from J1 and J2, which wereomitted from analysis for reasons stated previously.

3.3. Experiment 3 — removal of arsenic with iron present

Characteristics and operating conditions of experiment 3were as follows:

– Mean flow of 19.3 mL/min of water– HRT calculated as 13 h– Initial pH of 5.8 at the inlet and 5.1 at the outlet. The initial

low pH was due to hydrolysis of the ferrous chloride upondissolution in water, and the pH dropped further acrossthe apparatus as in experiment 2.

– Initial DO readings at the inlet and outlet of 0.32 mg/L and0.53 mg/L respectively

– Air sparging flow rate at 16 ± 5 mL/min.– Experiment run for 48 h with air sparging, followed by

24 h without air sparging– Inflow conditions with approximately 200 μg/L As(V)

(used because As[III] was found to be quickly oxidized)and approximately 5 mg/L iron as Fe(II).

Experimental results for iron are summarized in Fig. 7. Ahighlighted data point on each graph marks the end of airsparging.

Fig. 7 shows that the concentration of iron variedsomewhat at the inlet for the first 30 h of the experiment(between 4 and 5 mg/L) but the outlet concentration of ironwas, with the exception of one measurement point, belowthe inlet concentration. It is unknown why one measurementperiod differs from the dominant trend; however, a pairedt-test was done to consider all measurements as whole (i.e.all sample times at the inlet and outlet). This showed 97.5%certainty that there was removal of iron between the inletand outlet a level greater than zero. Examination of ports A2,B2, and E2 demonstrate the effect of air sparging, sincefollowing the start of air sparging, the concentrations at B2and E2 decreased slightly from levels at A2.

The removal of iron is low, particularly since iron istypically easily oxidizable (Mettler, 2002), but the low pH(between 5.1 and 5.8 from the inlet to the outlet) shows thatthe oxidation rate of Fe(II) is highly dependent on pH. Theoxidation rate of ferrous iron is very significantly slowed atpH values below 7 (Morgan and Lahav, 2007), and hence, the

iron removal in experiment 3 would be greatly increased ifthe pH had been closer to neutral. Buffering with sodiumhydroxide was attempted, but this caused precipitation ofiron and as such was not used in experimentation.

Although the rate of iron oxidation was quite slow, thisslow rate did allow observation of the relationship betweenremoval of iron and arsenic in small increments. Examinationof this is experimentally valuable in predicting removal ofarsenic where greater precipitation of iron is expected. Theconcentrations of arsenic in the apparatus throughoutexperiment 3 are shown in Fig. 8. The highlighted datapoints mark the end of air sparging.

As illustrated in Fig. 8, the arsenic concentration at theoutlet was considerably lower than that of the inlet. Theaverage difference in iron concentrations between the inlet andoutlet over the first two readings (i.e. before the end of the firstHRT, as an estimate of the background treatment effect)indicates a removal of approximately 0.4 mg/L. It is apparentthat even a fairly small drop in iron concentration (i.e. 9%,0.4 mg/L drop from an inlet concentration of 4.3 mg/L,averaged over the first two data points) leads to a relativelylarge removal effect on arsenic, in this case a drop of 76 μg/L(i.e. 42% drop from an inlet concentration of 180 μg/L, averagedover the first two data points).

From Fig. 8, it follows that arsenic was in steady decline atthe outlet throughout the entire duration of air sparging inExperiment 3, from an initial value of 109 μg/L down to thelowest value of 41 μg/L at the end of air sparging. Thisdownward trend is also seen for ports B1, E1, E2, E3, and J3.The removal percentages for various sampling ports exposed toextra DO through air sparging, based on the end-of-spargingarsenic concentrations as compared to the average inlet arsenicconcentration over the course of air sparging (i.e. 176 μg/L), areshown in Table 4.

The higher removal values shown here, as compared tothose from experiment 2, show the importance of thepresence of dissolved iron for the removal of arsenic. At theoutlet, estimated removal was improved from 34% to 77%from experiment 2 to experiment 3.

Recovery of arsenic concentrations post-sparging is difficultto assess in this experiment, due to the apparent spike inarsenic at ports A1, A2, and A3 after air sparging ended. Thereason for this spike is unknown, but it is apparent that limitingrecovery to one day was not nearly sufficient to see arsenicrecover to pre-sparging levels. This is illustrated in Fig. 8 asports E3 and J3 only recovered to 67 μg/L and 25 μg/Lrespectively after a 24-hour recovery period, as compared to114 μg/L and 98 μg/L respectively at the beginning of theexperiment. This “lag effect” for recovery suggests that applyingsparging intermittently may be feasible.

Experiments were also carried outwith sparging rates of 75and 250 mL/min, and actually resulted in less effective transferof oxygen to solution. The higher flow rate likely resulted in alessened air/water contact time (as compared to a lower flowrate of air), resulting in a lower DO concentration in solution.

3.4. Experiment 4 — static test for conditions of experiment 3:removal of arsenic with iron present

Experiment 4 was carried out to examine characteristics ofiron and arsenic co-precipitation in a simplified environment,

0

1

2

3

4

5

6

0 10 20 30 40 50 60 70 80

Fe

(mg

/L)

Fe

(mg

/L)

Time (hours)

Inlet and Outlet - Fe

In

Out

0

1

2

3

4

5

6

0 10 20 30 40 50 60 70 80

Time (hours)

Top Sample Ports - Fe

A1

B1

E1

0

1

2

3

4

5

6

0 10 20 30 40 50 60 70 80

Time (hours)

Middle Sample Ports - Fe

A2

B2

E2

0

1

2

3

4

5

6

0 10 20 30 40 50 60 70 80

Time (hours)

Bottom Sample Ports - Fe

A3

B3

E3

J3

Fe

(mg

/L)

Fe

(mg

/L)

Fig. 7. Experiment 3 results for iron.



in free solution in a static test. The purposewas to demonstratereaction kinetics under the experimental conditions created inthis investigation. A 750 mL anaerobic solution was preparedin a 1 L glass beaker (by nitrogen sparging followed by additionof iron and arsenic solutes), samples were taken, and thenvigorous aeration was carried out for 10 min. Sampling wascontinued over successive days to examine the time needed forsettling of iron flocs with arsenic adsorption. Samples weretaken from 1 cmbelow the surface of thewater to allow effectsof flocculation and settling to be easily observed. At the end ofthe static test, the solution was stirred vigorously and sampledto test for re-suspension of metals.

Experimental observations and results were as follows:

– pH (initial, after nitrogen sparging): 7.3– DO (initial, after nitrogen sparging): 0.10 mg/L– pH (after iron and arsenic added, before aeration): 5.5– DO (after aeration): 7.75 mg/L.

To test if adsorption to the side of the container made anydifference to arsenic concentrations, the experiment wascarried out again, but just with DI water and 200 μg/L As(V)in solution. All arsenic samples tested to within ±8%, with noclear positive or negative trend. As such, adsorption ofarsenic to glass was considered to be negligible.

As shown in Table 5, there was a small drop in ironconcentration over the course of the experiment, with theending (five-day) concentration of 5.05 mg/L being 0.24 mg/L lower than the initial concentration of 5.29 mg/L (i.e. 5%removal). However, this drop in iron concentration made avery large difference to the arsenic concentration, with theending (five-day) concentration of 47 μg/L being 157 μg/Llower than the initial concentration of 204 μg/L (i.e. 77%removal). This explains how a relatively small drop in ironconcentrations (as in previous experiments) has a large effecton arsenic removal. The mass ratio of removal of iron toarsenic (as calculated from total removal over the course ofthe experiment) is:

240 μg=L ironð Þ= 157 μg=L arsenicð Þ ¼ 1:5 giron=garsenic:

This translates to a molar ratio of:

1:5 giron=garsenicð Þ 1 moliron=55:85 gironð Þ� 74:92 garsenic=1 molarsenicð Þ¼ 2 moliron=molarsenic:

The 77% removal of arsenic in this experiment comparesfavorably with calculated removals at the outlet at the end ofair sparging in experiment 3 (also 77%). Brownish precipitatewas observed on the bottom of the beaker at theexperiment's end. Due to this finding, and that vigorousstirring re-suspended iron and arsenic, co-precipitation isconfirmed as the removal mechanism. The results ofexperiment 4 also verify that the oxidation and settlingprocess for iron is slow under these experimental conditions,as a five-day reaction time only produced a 0.24 mg/L drop iniron concentration.

According to Morgan and Lahav (2007), reaction aroundneutral pH would achieve a precipitous drop in iron in only1 h, due to rapid oxidation. However, in this experiment, ahigh degree of removal of arsenic is shown to be possible

with sufficient time, when iron is over an order of magnitudehigher in concentration (i.e. when the concentration of iron ismuch higher than that of arsenic, even a small amount of ironremoval, relative to its total concentration, may in this casecause removal of a large portion of arsenic present at an iron/arsenic molar removal ratio of 2.0).

3.5. Experiment 5 — examining removal of arsenic with ironpresent, but over an extended period

This experiment was similar to experiment 3 but over anextended period (six days of air sparging followed by ninedays of flow with no aeration). The purpose was to examinethe effect of allowing longer times for air sparging andcontact time of solution within the medium. The recovery ofthe system was also examined (i.e. the rise of arsenicconcentration post-sparging following its drop in concentra-tion during sparging).

Characteristics and operating conditions for experiment 5were as follows:

– Average flow of 19.1 mL/min– HRT of 13 h– Initial pH of 6.0 at the inlet and 5.4 at the outlet– Initial DO readings at the inlet and outlet of 0.27 mg/L and

0.57 mg/L, respectively– Air sparging flow rate of 30 ± 5 mL/min (since greater

oxygen transfer was not achieved at higher flow ratestested, and a flow rate of 30 mL was more easily andconsistently kept steady than the 16 mL/min flow rateused previously).

For the beginning and end of the six-day air spargingperiod, DO levels in the apparatus are shown in Fig. 9. Noterminal DO measurement for the outlet was taken and assuch is not shown.

As shown in Fig. 9, there was effective oxygen transfer tothe middle and top sampling ports downstream of sparging,but not nearly asmuch along the bottom. It is considered likelythat some disturbance of the sand above the sparging pointoccurred after trials with a higher airflow rate (250 mL/min inan experiment mentioned at the end of Section 3.3). With adisturbed sand medium above the sparging point, air bubblesmay have had faster travel through the lower depth of theapparatus, and hence had less transfer of oxygen to solution inthis section. In addition, there was little oxygen transfer tosolution above the sparging point at column B. It is consideredlikely that there was greater lateral migration of airflowchannels in the downstream direction during this experiment.

It was noted during this experiment that there was anincreasing amount of rusty brown iron precipitate accumu-lating on the peristaltic tubing and on the plexiglass in theinlet column as well. This illustrates the potential for ironprecipitation at fairly low DO levels as experienced at theinlet (i.e. at 0.5 mg/L or lower), given sufficient time. Resultsfor iron removal are shown in Fig. 10, with a highlighted datapoint on each graph marking the end of air sparging.

It is apparent that the outlet concentration is generallybelow that of the inlet, with the exception of one point at 73 hat which the concentrations are equivalent. On average, theoutlet is below the inlet by 0.25 mg/L over the course ofthe experiment. This pattern matches well with previous

0

50

100

150

200

250

0 10 20 30 40 50 60 70 80

As

(µg

/L)

Time (hours)

Inlet and Outlet - As(V)

In

Out

0

50

100

150

200

250

0 10 20 30 40 50 60 70 80

As

(µg

/L)

Time (hours)

Top Sample Ports - As(V)

A1

B1

E1

0

50

100

150

200

250

0 10 20 30 40 50 60 70 80

As

(µg

/L)

Time (hours)

Middle Sample Ports - As(V)

A2

B2

E2

0

50

100

150

200

250

0 10 20 30 40 50 60 70 80

As

(µg

/L)

Time (hours)

Bottom Sample Ports - As(V)

A3

B3

E3

J3

Fig. 8. Experiment 3 results for arsenic.


Table 4Percent removal of arsenic at the end of air sparging for various ports inexperiment 3.

Mean inlet arsenic conc.(μg/L)

Port End arsenic conc. (μg/L)

Removal(%)

B1 49 72176 B2 84 52

B3 104 41E1 36 80E2 27 85E3 41 77J3 20 89Out 41 77


experiments. However, a noted difference in this experimentcompared to previous experiments is that there was sufficienttime for flow to reach the sampling ports at J1 and J2 (asdemonstrated by the elevated DO levels). Since J1 and J2 are inthe low-flow area, it is likely that thewater at these points had acontact time in the apparatus much greater than that of theHRT. The effect of this on iron concentration is shown at ports J1and J2 in Fig. 10 (i.e. a visibly lower iron concentration than atE1 and E2, respectively, from approximately 200 to 250 h).

Interpreting these findings in concert with those fromexperiment 4, the lower iron concentration is due to longerreaction time once water reaches ports J1 and J2. Results forarsenic are shown in Fig. 11.

All three rows of sampling points (top, bottom, and middle)show a decline in arsenic concentrations at ports downstreamof air sparging after the start of air sparging, but this trend ismuch more distinct at points E1, E2, J1, and J2. This higherremoval of arsenic at ports E1, E2, J1, and J2 is likely from acombination of two factors. Firstly, DO is transferred veryeffectively to E1, E2, J1, and J2 as all had DO concentrations at4.0 mg/L or greater at the end of air sparging, whereas E3 and J3had concentrations at 1.3 mg/L or lower. This is likelyresponsible for the higher removal of arsenic at ports E1 andE2. Over the course of the experiment the iron removal at E1and E2 is greater than that at E3 (with mean concentrationdifferences of 90 and 70 μg/L comparing E3–E1 and E3–E2,respectively).

A second reason for the higher removal of arsenic is that J1and J2 lie in a lower-flowzone. As previouslymentioned, iron isremoved at J1 and J2 to a greater extent than at other ports,because these lie in a lower-flow region and reaction timeswere longer, sufficient to allow more iron to precipitate.

The recovery of the system is also evident as the arsenicconcentrations at downstream ports eventually exhibit anupward trend after the end of air sparging, and this is shown

Table 5Results of experiment 4.

Sample time (days) Fe (mg/L) As(V) (μg/L)

Start 5.29 2041 5.23 1772 5.21 1643 5.19 1214 5.08 735 5.05 47End mix 5.28 122

much more quickly at ports E3 and J3, with ports J1 and J2having a muchmore gradual recovery. Even after nine days ofrecovery time post-sparging, the arsenic concentrations at J1and J2 have not returned to near original levels. Theestimated percent removal of arsenic for each port down-stream of sparging is shown in Table 6 (based on theconcentration of arsenic at the end of air sparging).

When compared to the removal values for arsenic inexperiment 3 (shown in Table 4), the removal values inTable 6 tend to be smaller. It was noted that, when comparinginitial conditions, there was less initial breakthrough of arsenicin experiment 3 than in subsequent experiments (i.e. at initialconditions at the start of air sparging, there was a largerdifference between inlet and outlet concentrations in experi-ment 3 than in experiments 4 and 5). This contributed to ahigher measured performance in terms of arsenic removal.However, comparing removal values for different ports withinexperiment 5 reveals that the removal of arsenic is distinctlyhigher at J1 and J2, from a greater amount of co-precipitationwith iron. This further demonstrates the distinct removal ofarsenic with a relatively small change in iron concentrations,when iron exists in solution at much higher concentrations.

4. Discussion

It is clear that iron oxidation is slow (on the order of days)at pH values of 5.5 to 6.0. Judging by the rate kinetics shownin Morgan and Lahav (2007), a neutral pH would likely causea further marked increase in iron removal. The pH-neutralconditions typical for Bangladeshi groundwater are thereforeconducive to rapid iron oxidation as compared to theseexperiments. It is important to note that, while arsenic didnot reach drinking water standards (i.e. 10 μg/L) in theseexperiments, performance may be improved under neutralconditions with more rapid iron oxidation. This requiresfurther assessment.

An important design aspect to consider is the effect of theconcentration of DO on the removal of iron and subsequentremoval of arsenic.While higher DO did not result in obviouslyhigher iron removal (e.g. comparing ports E1 and E2 with portE3 in experiment 5), this did result in higher arsenic removal,suggesting that therewas some slightly greater removal of ironresulting inmore pronounced co-precipitation. However, giventhe sensitivity of iron oxidation kinetics to pH, this behavior(showing the benefit of greater DO concentrations) maychange at concentrations closer to neutral pH. This requiresfurther assessment in conditions more comparable to those inthe field, since treatment zone volumemay bemore importantthan DO concentration according to van Halem et al. (2010a).

In regard to the effect of contact time on arsenic removal,results from experiment 5 suggest that contact time (underthose specific experimental conditions) was important, asallowing sufficient time in that experiment for lowered ironlevels (i.e. at ports J1 and J2 in a low-flow zone) resulted in adramatically lower level of arsenic. However, it is importantto note that this behavior partly resulted from below-neutralpH causing a depressed reaction rate for the oxidation of iron,and may not be completely applicable to neutral conditions.In regard to the effect of time on the adsorption of arsenic toiron minerals, some experiments indicate a fairly rapidprocess (Banerjee et al., 2008; Pierce and Moore, 1982)

0

1

2

3

4

5

6

7

8

In A1 A2 A3 B1 B2 B3 E1 E2 E3 J1 J2 J3 Out

DO

(m

g/L

)

Sampling Port

DO Levels in the Apparatus During Experiment 5

Start

6 days end

Fig. 9. DO levels in the apparatus during experiment 5.


with over 93% of adsorption of arsenate occurring in 24 h atpH 6 in one study (O'Reilly et al., 2001).

Performance under these experimental conditions hasshown maximum performance in the range of 79% removal ofarsenic (considering experiment 5, not experiment 3 in whichthere was a smaller amount of initial breakthrough of arsenic).Measured between the inlet and outlet, removal of 59% wasobserved. These experiments also illustrate the high potential foreffective treatment in solutions in which iron is much higherthan arsenic. However, it is important to consider performance ina more complex field environment when assessing futurepotential of this method. A neutral pH will result in greaterperformance in regard to removal of iron (and subsequentco-precipitation of arsenic), but there is also likely to besignificant hindrances because of competing ions, such asphosphate, sulfate, carbonate, and silica. Phosphate is a particularconcern, since phosphate binds to HFO almost identically asarsenate (Waychunas et al., 1993), is a significant competitor forarsenic adsorption (Stollenwerk, 2003), and may hinder in situtreatment (Brennan and McBean, 2011; Brunsting, 2012;Brunsting and McBean, 2014; van Halem et al., 2010a).

Although below-neutral pH did cause suppression of ironoxidation kinetics in these experiments, this did provide abenefit in allowing examination of the relationship of iron toarsenic in co-precipitation at small increments of iron removal.Experiment 4 showed iron and arsenic to be removed byco-precipitation at a molar ratio in the range of two (iron/arsenic) and, as such, when the concentration of iron is manytimes that of arsenic, a relatively small drop in iron concentra-tion may cause a large impact on arsenic removal. This isdemonstrated in apparatus experiments (particularly in exper-iment 5, see Figs. 10 and 11 for ports J1 and J2). Groundwater inBangladeshwith high arsenic (N50 μg/L) is typically high in ironas well, with an average of 5300 μg/L (Roberts et al., 2004), andtherefore may be conducive to in situ treatment for arsenic.

As shown in experiment 5, there is some lag time inconcentrations of arsenic returning to pre-sparging levels aftercessation of air sparging. This is caused by dropping levels of DO,subsequent cessation of oxidation of iron, and exhaustion ofadsorption sites on iron oxyhydroxide (van Halem et al., 2010a).

More laboratory research may be beneficial, in examiningmore complex solutions at conditions more closely simulating

those in the natural aquifer, with other constituents such asbicarbonate, calcium, magnesium, phosphate, and other ions.Examination of the effect of arsenic speciation on treatmenteffectiveness may also be beneficial, in examining treatmentunder conditions in which both As(III) and As(V) are presentand fairly stable.

An additional consideration for further research, especial-ly in applying this technique in the field, is the temporaryeffect of treatment (i.e. oxidizing conditions diminishingonce air injection ceases). Depending on the rate of recovery,sparging may not be needed continuously during treatment,and may be applied periodically. Examination of the effect ofintermittent versus continuous injection of air may bebeneficial in further laboratory and field study, to investigatehow this may dictate proper operational conditions to ensureeffectiveness.

5. Conclusions

The main conclusions are as follows:

– Maximum removal of arsenic was 79% under conditions ofthese experiments, at a local point in the apparatus (downto 33 μg/L from 158 μg/L, see experiment 5). Measuredbetween the inlet and the outlet, removal of 59% wasobserved.

– Iron and arsenic were removed by co-precipitation (in asimple solution where only these two solutes are present) ata molar ratio of approximately 2 (iron/arsenic). This showsthat the theoretical natural treatment capacity for arsenic isvery high in waters with iron at substantially higheramounts, such as in many Bangladeshi groundwaters.

– There is a lag effect post-sparging for iron and arsenicconcentrations returning to pre-treatment levels. Furtherexperiments could exploit and study this by applyingperiodic air sparging and investigating effects.

– Iron oxidation is significantly retarded at pH values belowneutral. If neutral conditions were achieved in an experi-mental solution, and experiments of this investigation wererepeated, it is likely that there would be even greaterremoval of arsenic because of increased oxidation andprecipitation of dissolved iron.

0

1

2

3

4

5

6

0 50 100 150 200 250 300 350

Fe

(mg

/L)

Time (hours)

Inlet and Outlet - Fe

In

Out

0

1

2

3

4

5

6

0 50 100 150 200 250 300 350

Fe

(mg

/L)

Time (hours)

Top Sample Ports - Fe

A1

B1

E1

J1

0

1

2

3

4

5

6

0 50 100 150 200 250 300 350

Fe

(mg

/L)

Time (hours)

Middle Sample Ports - Fe

A2

B2

E2

J2

0

1

2

3

4

5

6

0 50 100 150 200 250 300 350

Fe

(mg

/L)

Time (hours)

Bottom Sample Ports - Fe

A3

B3

E3

J3

Fig. 10. Experiment 5 results for iron.


0

20

40

60

80

100

120

140

160

180

200

0 50 100 150 200 250 300 350

As

(µg

/L)

Time (hours)

Inlet and Outlet - As(V)

In Out

0

20

40

60

80

100

120

140

160

180

200

0 50 100 150 200 250 300 350

As

(µg

/L)

Time (hours)

Top Sample Ports - As(V)

A1

B1

E1

J1

0

20

40

60

80

100

120

140

160

180

200

0 50 100 150 200 250 300 350

As

(µg

/L)

Time (hours)

Middle Sample Ports - As(V)

A2

B2

E2

J2

0

20

40

60

80

100

120

140

160

180

200

0 50 100 150 200 250 300 350

As

(µg

/L)

Time (hours)

Bottom Sample Ports - As(V)

A3

B3

E3

J3

Fig. 11. Experiment 5 results for arsenic.


Table 6Percent removal of arsenic at various ports during experiment 5.

Mean inlet arsenicconc. (μg/L)

Port End arsenic conc.(μg/L)

Removal(%)

E1 57 64158 E2 50 68

E3 96 39J1 37 77J2 33 79J3 89 44Out 64 59


Acknowledgments

The authors would like to thank the Natural Sciences andEngineering Research Council (NSERC) and the Ontario Gradu-ate Scholarship program for scholarshipmonies provided duringthe course of this research. TheOntario Research Foundation andthe Canada Research Chair program also provided funding forexperimental work.

References

Ahmed, M.F., 2003. Arsenic contamination: regional and global scenario. In:Ahmed, M.F. (Ed.), Arsenic Contamination: Bangladesh Perspective. ITN,Bangladesh Dhaka, pp. 1–20.

Anawar, H.M., Akai, J., Komaki, K., Terao, H., Yoshioka, T., Ishizuka, T., Safiullah, S.,Kato, K., 2003. Geochemical occurrence of arsenic in groundwater ofBangladesh: sources and mobilization processes. J. Geochem. Explor. 7,109–131.

Appelo, C.A.J., de Vet, W.W.J.M., 2003. Modeling in situ iron removal fromgroundwater with trace elements such as As. In: Welch, A.H.,Stollenwerk, K.G. (Eds.), Arsenic in Groundwater. Kluwer AcademicPublishers, Dordrecht, pp. 381–401.

Banerjee, K., Amy, G.L., Prevost, M., Nour, S., Jekel, M., Gallagher, P.M.,Blumenschein, C.D., 2008. Kinetic and thermodynamic aspects ofadsorption of arsenic onto granular ferric hydroxide (GHG). Water Res.42 (13), 3371–3378.

BGS, DPHE, 2001. Arsenic contamination of groundwater in Bangladesh. In:Kinniburgh, D.G., Smedley, P.L. (Eds.), British Geological Survey (TechnicalReport, WC/00/19. 4 Volumes). British Geological Survey, Keyworth.

Brennan, R., McBean, E., 2011. The role of orthophosphate and dissolvedoxygen in the performance of arsenic–iron removal plants in Bangladesh.J. Environ. Sci. Health, Part A: Tox. Hazard. Subst. Environ. Eng. 46 (4),426–435.

Brunsting, J., 2012. A Laboratory-Scale Investigation of In Situ Treatment ofArsenic-Contaminated Groundwater by Air Sparging. Masters disserta-tion University of Guelph.

Brunsting, J., McBean, E., 2014. Phosphate interference during in situ treatmentfor arsenic in groundwater. J. Environ. Sci. Health A 49 (6), 671–678.

Chakraborti, D., Das, B., Rahman, M.M., Chowdhury, U.K., Biswas, B.,Goswami, A.B., Nayak, B., Pal, A., Sengupta, M.K., Ahamed, S., Hossain,A., Basu, G., Roychowdhury, T., Das, D., 2009. Status of groundwaterarsenic contamination in the state of West Bengal, India: a 20-year studyreport. Mol. Nutr. Food Res. 53 (5), 542–551.

Cherry, J.A., Shaikh, A.U., Tallman, D.E., Nicholson, R.V., 1979. Arsenic speciesas an indicator of redox conditions in groundwater. J. Hydrol. 43 (1–4),373–392.

Clifford, D.A., Karori, S., Ghurye, G., Samanta, S., 2004. Field SpeciationMethod for Arsenic Inorganic Species. American Water Works Associ-ation Research Foundation, Denver.

Flanagan, S.V., Johnston, R.B., Zheng, Y., 2012. Arsenic in tube well water inBangladesh: health and economic impacts and implications for arsenicmitigation. Bull. World Health Organ. 90, 839–846.

Grombach, P., 1985. Groundwater treatment in situ in the aquifer. WaterSupply 3 (1), 13–18.

Mettler, S., 2002. In Situ Removal of Iron fromGroundWater: Fe(II) Oxygenation,and Precipitation Products in a Calcareous Aquifer. Doctoral dissertationSwiss Federal Institute of Technology, Zurich (Retrieved fromETHBibliothekKnowledge Portal).

Miller, G.P., 2006. Subsurface Treatment for Arsenic Removal. WERC andAWWA Research Foundation, USA.

Miller, G.P., 2008. Subsurface Treatment for Arsenic Removal Phase IIInvestigation. WERC and AWWA Research Foundation, USA.

Miller, R.J., Thompson, W., Walker, M., 2002. Control of a groundwaterarsenic plume by air sparging. In: Gavaskar, A.R., Chen, A.S.C. (Eds.),Remediation of Chlorinated and Recalcitrant Compounds, Proceedings ofthe Third International Conference on Remediation of Chlorinated andRecalcitrant Compounds. Monterey CA, May 2002. Battelle Press,Columbus, OH, pp. 2085–2094.

Morgan, B., Lahav, O., 2007. The effect of pH on the kinetics of spontaneousFe(II) oxidation by O2 in aqueous solution — basic principles and asimple heuristic description. Chemosphere 68 (11), 2080–2084.

Nickson, R., McArthur, J.M., Burgess, W.G., Ahmed, K.M., Ravenscroft, P., Rahman,M., 1998. Arsenic poisoning of Bangladesh groundwater. Nature 395, 338.

O'Reilly, S.E., Strawn, D.G., Sparks, D.L., 2001. Residence time effects onarsenate adsorption/desorption mechanisms on goethite. Soil Sci. Soc.Am. J. 65, 67–77.

Paul, S.C., Rahman, M.A., Siddique, N., Shafiqul Alam, A.M., 2008. Voltammetricstudy of arsenic (III) and arsenic (V) in ground water of Hajigonj andKalkini in Bangladesh. Pak. J. Anal. Chem. 9 (1), 49–55.

Pierce, M.L., Moore, C.B., 1982. Adsorption of arsenite and arsenate onamorphous iron hydroxide. Water Res. 16 (7), 1247–1253.

Rasul, S.B., Munir, A.K.M., Hossain, Z.A., Khan, A.H., Alauddin, M., Hussam, A.,2002. Electrochemical measurement and speciation of inorganic arsenicin groundwater of Bangladesh. Talanta 58, 33–43.

Roberts, L.C., Hug, S.J., Reuttimann, T., Billah, M.D.M., Khan, A.W., Rahman, M.T.,2004. Arsenic removal with iron(II) and iron(III) in waters with high silicateand phosphate concentrations. Environ. Sci. Technol. 38 (1), 307–315.

Rott, U., 1985. Physical, chemical and biological aspects of the removal ofiron and manganese underground. Water Supply 3 (2), 143–150.

Rott, U., Friedle, M., 1999. Subterranean removal of arsenic from groundwa-ter. In: Chappell, W.R., Abernathy, C.O., Calderon, R.L. (Eds.), ArsenicExposure and Health Effects. Elsevier Science Ltd., Oxford, pp. 389–396.

Rott, U., Friedle, M., 2000. Eco-friendly and cost-efficient removal of arsenic,iron and manganese by means of subterranean ground-water treatment.Water Supply 18, 632–636.

Rott, U., Lamberth, B., 1993. Groundwater clean up by in-situ treatment ofnitrate, iron and manganese. Water Supply 11 (3–4), 143–156.

Sen Gupta, B., Chatterjee, S., Rott, U., Kauffman, H., Bandopadhyay, A., DeGroot,W., Nag, N.K., Carbonell-Barrachina, A.A., Mukherjee, S., 2009. A simplechemical free arsenic removal method for community water supply — acase study fromWest Bengal, India. Environ. Pollut. 157 (12), 3351–3353.

Smedley, P.L., Kinniburgh, D.G., 2002. A review of the source, behaviour anddistribution of arsenic in natural waters. Appl. Geochem. 17 (5), 517–568.

Smith, A.H., Lingas, E.O., Rahman, M., 2000. Contamination of drinking-waterby arsenic in Bangladesh: a public health emergency. Bull. World HealthOrgan. 78 (9), 1093–1103.

Stollenwerk, K.G., 2003. Geochemical processes controlling transport ofarsenic in groundwater: a review of adsorption. In: Welch, A.H.,Stollenwerk, K.G. (Eds.), Arsenic in Ground Water: Geochemistry andOccurrence. Kluwer Academic Publishers, Dordrecht, pp. 67–100.

Sung, S., Morgan, J.J., 1980. Kinetics and product of ferrous iron oxygenationin aqueous systems. Environ. Sci. Technol. 14 (5), 561–568.

Tamura, H., Kawamura, S., Hagayama, M., 1980. Acceleration of the oxidationof Fe2+ ions by Fe(III)-oxyhydroxides. Corros. Sci. 20 (8–9), 963–971.

van Beek, C.G.E.M., 1985. Experiences with underground water treatment inthe Netherlands. Water Supply 3 (2), 1–11.

van Halem, D., Olivero, S., de Vet, W.W.J.M., Verberk, J.Q.J.C., Amy, G.L., vanDijk, J.C., 2010a. Subsurface iron and arsenic removal for shallow tubewell drinking water supply in rural Bangladesh. Water Res. 44 (19),5761–5769.

van Halem, D., Heijman, S.G.J., Amy, G.L., van Dijk, J.C., 2010b. Subsurface arsenicremoval for small-scale application in developing countries. Desalination251, 241–248.

van Halem, D., Heijman, S.G.J., Johnston, R., Huq, I.M., Ghosh, S.K., Verberk, J.Q.J.C.,Amy, G.L., van Dijk, J.C., 2010c. Subsurface iron and arsenic removal: low-costtechnology for community-based water supply in Bangladesh. Water Sci.Technol. 62 (11), 2702–2709.

van Halem, D., de Vet, W., Verberk, J., Amy, G., van Dijk, H., 2011.Characterization of accumulated precipitates during subsurface ironremoval. Appl. Geochem. 26, 116–124.

Waychunas, G.A., Rea, B.A., Fuller, C.C., Davis, J.A., 1993. Surface chemistry offerrihydrite: Part 1. EXAFS studies of the geometry of coprecipitated andadsorbed arsenate. Geochim. Cosmochim. Acta 57 (10), 2251–2269.

WHO, 2008. Guidelines for Drinking-waterQuality: Third Edition Incorporatingthe First and Second Addenda. World Health Organization, Geneva.

WQAA, no date. Water Quality Standards for Various Uses of Water: DrinkingWater Standards of BIS (IS: 10500: 1991). Water Quality AssessmentAuthority, Government of India. wqaa.gov.in/Content/wqs.aspx.

http://refhub.elsevier.com/S0169-7722(14)00011-4/rf0005




















































































































in situ treatment of arsenic-contaminated groundwater by air sparging

Documents