barrier materials

ORIGINAL ARTICLE

Selection of permeable reactive barrier materials for treatingacidic groundwater in acid sulphate soil terrains basedon laboratory column tests

Alexandra N. Golab Æ Mark A. Peterson ÆBuddhima Indraratna

Received: 19 October 2008 / Accepted: 19 December 2008 / Published online: 23 January 2009

� Springer-Verlag 2009

Abstract The Shoalhaven region of NSW experiences

environmental acidification due to acid sulphate soils

(ASS). In order to trial an environmental engineering

solution to groundwater remediation involving a permeable

reactive barrier (PRB), comprehensive site characterisation

and laboratory-based batch and column tests of reactive

materials were conducted. The PRB is designed to perform

in situ remediation of the acidic groundwater (pH 3) that is

generated in ASS. Twenty-five alkaline reactive materials

have been tested for suitability for the barrier, with an

emphasis on waste materials, including waste concrete,

limestone, calcite-bearing zeolitic breccia, blast furnace

slag and oyster shells. Following three phases of batch tests,

two waste materials (waste concrete and oyster shells) were

chosen for column tests that simulate flow conditions

through the barrier and using acidic water from the field site

(pH 3). Both waste materials successfully treated with the

acidic water, for example, after 300 pore volumes, the

oyster shells still neutralised the water (pH 7).

Keywords Geochemistry � Ground water contamination

Introduction

Acid sulphate soils (ASS) impact more than 3 million ha of

coastal Australia (White et al. 1997) and as a result, acidic

groundwater is a common problem in coastal Australia.

Acid sulphate soil is the common name given to low-lying

coastal floodplain deposits containing oxidisable or partly

oxidised sulphide minerals (e.g. pyrite). In reducing and

inundated conditions the sulphide minerals are generally

inert, when exposed to atmospheric oxygen, pyrite oxida-

tion occurs and acidic products including iron are released

along with high levels of aluminium leached from the

soil (Dent 1986). The products of pyrite oxidation in the

groundwater attack concrete and steel infrastructure, clog

waterways with iron flocculates, kill fish and produce large

acid scalds that render land unusable for agriculture. Due

to rapid acid attack on infrastructure, costly sulphate-

resistant concrete and galvanised steel are needed in ASS

areas.

Although initially recognised in Australia in the 1970’s

(Walker 1972), serious research on acid sulphate soil was

not conducted until major fish kills occurred in coastal

rivers in NSW in the 1980’s. Since that time, techniques

that either prevent pyrite oxidation or remediate the resul-

tant acidic drain-water have been researched. Throughout

Australia, large-scale flood mitigation works (i.e. surface

drains and floodgates) designed to remove excess surface

water from low-lying areas have increased in situ acid

production and acid transport (White et al. 1997). The

lowering of the watertable by surface drains increases

exposure of pyrite to oxidation, thus, increasing acid

production. One-way floodgates, installed on the drains

where they discharge into the adjacent creek, maintain the

drain-water at a steady-state low tide level. Engineering

solutions such as weirs and modified floodgates are being

A. N. Golab (&)

CRC for Greenhouse Gas Technologies, Canberra,

ACT 2601, Australia

e-mail: [email protected]

M. A. Peterson

ANSTO, Lucas Heights, NSW, Australia

B. Indraratna

University of Wollongong, Wollongong,

NSW 2522, Australia

123

Environ Earth Sci (2009) 59:241–254

DOI 10.1007/s12665-009-0022-8

implemented (Indraratna et al. 2005) but these are not

feasible in very low-lying areas, because the installation of

these structures will then increase the risk of flooding

during heavy rain events. One possible solution for these

areas is the construction of a permeable reactive barrier

(PRB) that can neutralise the acidic groundwater before

entering nearby waterways.

PRBs are in situ, passive remediation tools that can be

more cost-effective than other techniques and do not dis-

rupt the existing land use. PRBs are used worldwide for the

remediation of acid mine drainage (AMD) and other con-

taminated sites (e.g. Gibert et al. 2003) through the use of a

trench filled with reactive material. The barrier intersects

the flow-path of a contaminant plume and ameliorates the

contaminated groundwater through physical, chemical and/

or biological processes, including precipitation, sorption,

and oxidation/reduction. When the acidic groundwater

comes into contact with this PRB, the acid will be neu-

tralised by the reactive materials. In this way, the PRB will

remediate acidic groundwater and decrease the amount of

aluminium and iron that reaches the drain, because both

cations are less soluble at neutral pH.

Column tests

Column tests are performed commonly to determine the

effectiveness of reactive materials prior to the installation

of the PRB. Column tests have been conducted on a wide

variety of materials to remove many different contaminants

(e.g. Gillham and O’Hannesin 1994; Orth and Gillham

1996; Mackenzie et al. 1999; Morkin et al. 2000; Abadzic

and Ryan 2001; Park et al. 2002; Waybrant et al. 2002;

Zhang et al. 2002; Kamolpornwijit et al. 2003; Su and Puls

2003; Amos et al. 2004; Gusmao et al. 2004; Komnitsas

et al. 2004; Lapointe et al. 2005; Logan et al. 2005). The

studies that are most similar to the current study are those

that have investigated the remediation of AMD because the

composition of the contaminated groundwater has many

similarities with ASS-affected groundwater. For example,

Komnitsas et al. (2004) investigated the potential use of

limestone and red mud in a PRB to remove several heavy

metal ions from the AMD mainly by precipitation, co-

precipitation and adsorption.

Clogging

The biggest limitation for PRBs is not the exhaustion of the

reactive material but clogging of the pore spaces through

mineral precipitation and accumulation (Mackenzie et al.

1999; Phillips et al. 2000; Kamolpornwijit et al. 2003).

Column tests provide an ideal controlled system to study

clogging. Several authors have reported that clogging

occurs near the column inlet and not homogeneously

through the pore spaces, indicating that clogging may occur

rapidly (e.g. Kamolpornwijit et al. 2003; Bilek 2006). For

example, Kamolpornwijit et al. (2003) found that the

porosity at the column inlet reduced by up to 45.3% over

72 days of operation under accelerated flow conditions.

The significant negative effect of clogging is that it may

cause groundwater to bypass the barrier, rendering

it ineffective for treating contaminated groundwater

(Kamolpornwijit et al. 2003).

It is important to understand the precipitates that will

form in a PRB. Precipitates that have been encountered in

field installations of PRBs and column tests include fer-

rous/ferric (oxyhydr)oxides (e.g. goethite, akaganeite,

lepidocrocite, maghemite, magnetite and amorphous iron

oxyhydroxides), iron sulphides (e.g. mackinawite and

amorphous ferrous sulphide), and iron and calcium car-

bonates (e.g. aragonite, calcite and siderite) (Mackenzie

et al. 1999; Puls et al. 1999; Vogan et al. 1999; Phillips

et al. 2000; Roh et al. 2000). Variation in the nature of

precipitates occurs from site to site and some barriers form

many of these precipitates, while others form none. The

precipitates in Eqs. 1–3 are likely to form due to the

interaction of iron with water and carbonates and the sat-

uration of calcium carbonate in the groundwater with rising

pH (Mackenzie et al. 1999).

Fe2þ þ 2OH� $ Fe OHð Þ2 sð Þ KFe OHð Þ2 ¼ 8� 10�6 ð1Þ

Fe2þ þ CO2�3 $ FeCO3 sð Þ KFeCO3

¼ 3:2� 10�11 ð2Þ

Ca2þ þ CO2�3 $ CaCO3 sð Þ KCaCO3

¼ 2:8� 10�9 ð3Þ

Mineral precipitation within the reactive media zone is

complex and appears to be controlled by more than simple

mineral equilibrium considerations based on solution pH

(Mackenzie et al. 1999). According to Liang et al. (2003),

chemical equilibrium modelling can correctly predict the

amounts of precipitates that will form in a PRB, based on

the thermodynamic properties of the reactive material and

the groundwater constituents.

In addition to chemical clogging, in the case of the

oyster shells, the issue of biological clogging may be

pertinent. Logan et al. (2005) utilised limestone and sul-

phate reducing bacteria (SRB) in a column and found that

substantial alkalinity was generated; they assume that the

alkalinity is due to both bicarbonate dissolution and

microbial activity. SRB are greatly limited by substrate

availability and factors such as nutrient availability, sul-

phate availability, metal toxicity and pH are far less

important (Logan et al. 2005). Some PRBs utilise SRB

because the reduction of sulphate is a sink for protons and

therefore decreases the acidity of the groundwater and soil

solution (Kuyucak and St-Germain 1994; Loy et al. 2004).

The oyster shells may host SRB, which will enhance the

242 Environ Earth Sci (2009) 59:241–254

123

neutralising capacity of the oyster shells but also may

increase the risk of biological clogging.

The reduction of sulphate by organic carbon can be

represented by Eq. 4:

2CH2Oþ SO2�4 ! 2HCO�3 þ H2S ð4Þ

where CH2O represents short-chain organic carbon mole-

cules that are capable of being oxidised by SRB.

Methods

The groundwater at the field site is acidic (pH as low as 3)

with high Al (up to 40 mg/L) and Fe (up to 530 mg/L)

levels. The purpose of the column tests is to test the suit-

ability of the materials or mixture of materials to neutralise

the acidity and remove Al and Fe from the groundwater.

Reactive material selection

The work of Golab et al. (2006) involved the batch testing

of 13 alkaline materials for use in the PRB, with an

emphasis on waste materials, including concrete, lime-

stone, calcite-bearing zeolitic breccia and blast furnace

slag. For the current study, the batch tests were extended to

cover another 12 alkaline materials, including oyster shells,

recycled concrete and dredged shelly material using the

methodology of Golab et al. (2006). Following the batch

tests two materials were selected for column testing—these

are recycled concrete and oyster shells. Both of these

materials are waste materials, have good neutralising

abilities and can be crushed to suitable grain sizes for use in

the clay soil at the field site. Similarly, Ahn et al. (2003)

and Perez-Lopez et al. (2007) tested waste materials but for

the remediation of mine leachate.

Three clear plastic columns with a 1.5 L capacity were

used in a vertical position with the water pumping from

bottom to top. The columns have nine sampling ports, one

before the entrance of the column to sample the influent,

and others at 7.5, 10.5, 15, 20, 30, 40 and 60 cm along the

column and one at the outlet to sample the effluent. The

design of the columns was loosely based on those of

Gillham and O’Hannesin (1994) and Lapointe et al. (2005).

A peristaltic pump was used to pump acidic water from the

field site through the columns at a known flow rate (16 mL/

min). An accelerated flow-rate was selected compared to

that achievable in the field in an attempt to test the lon-

gevity of the reactive materials over a relatively short time

period in the laboratory. As a result, the faster flow-rate

may cause different reactions to occur than would actually

occur in the field. One column contained crushed oyster

shells (Column A), another contained crushed recycled

concrete (Column B) and a third contained half of each,

with the concrete on the influent end (Column C). Once the

columns were operational samples were collected from

each port every hour for the first half day, then once a day

for the first week and then once a week for several months,

in a similar way to Bertocchi et al. (2006). The samples

were collected slowly to avoid disturbing the flow and were

analysed immediately for pH, electrical conductivity (EC)

and oxidation and reduction potential (ORP). Samples were

also collected for analysis by ICPAES after 1 h, 48 h,

1 week, 4 weeks and 7 weeks. The samples were filtered

under pressure through a 0.45 lm membrane and refrig-

erated in high density polyethylene (HDPE) bottles until

analysis for major anions and cations.

Native acidic water was collected regularly from the

field site and used to run in the column tests, as was done

by Kamolpornwijit et al. (2003) and Bilek (2006). In the

current study, the removal of the water from its native

temperature and pressure conditions and use at laboratory

temperature (15–21�C) and atmospheric pressure may

mean that the laboratory column tests do not replicate

field conditions. It is worth noting that the field site has a

temperate climate and the natural range of average

groundwater temperature is 10.5�C in winter to 16.6�C in

summer. Also, the groundwater is very shallow (average

1.2 m over a two-year study period) and in an unconfined

aquifer that is not pressurised. The water was stored in a

closed container prior to pumping through the column but

this would not have prevented air from the headspace in the

container from interacting with the water. As a result, the

ORP of the water may be higher than that naturally

occurring in the field, which may cause changes to the

redox sensitive groundwater content, e.g. Fe2?/Fe3?. The

natural range of ORP measured in the field, however, over

a two-year period was -177 to 478 mV, while the ORP

range of the water used for the column tests ranged from

221 to 592 mV (Fig. 2). Following the advice of Su and

Puls (2003), the columns were covered with dark plastic to

exclude light to simulate the subsurface environment and

encourage the potential growth of reducing bacteria. All of

the column tests were run at room temperature, as was

done by Mackenzie et al. (1999).

Once the column tests were complete, Columns A and B

were disassembled and solid samples (consisting of reac-

tive materials and precipitates) were examined by SEM-

EDS to study the nature and properties of the precipitates

formed and to help identify the geochemical reactions that

took place in the columns. Similarly, several authors have

removed sub-samples of the reactive material after the

completion of column tests and tested them by either XRD

or SEM-EDS or a combination of both (e.g. Waybrant et al.

2002; Komnitsas et al. 2004; Lapointe et al. 2005). The use

of SEM-EDS to identify the precipitates that form on the

surface of reactive materials is a recognised technique that

Environ Earth Sci (2009) 59:241–254 243

123

has been utilised by other researchers (e.g. Mackenzie et al.

1999; Abadzic and Ryan 2001; Waybrant et al. 2002).

Several authors have also utilised XRD but amorphous,

poorly crystallised or fine-grained minerals are difficult to

identify by this technique (e.g. Furukawa et al. 2002;

Kamolpornwijit et al. 2004).

The material extracted from the columns were prepared

using a similar method to that of Phillips et al. (2000). Prior

to extraction of the samples from the influent end, the

column was drained slowly and the water from the column

was collected. The end-cap of the column was unscrewed

and reactive material with precipitates was extracted from

the column. The extracted material was washed immedi-

ately with acetone then filtered under pressure on a

Buchner funnel. The reactive material and precipitates

were collected on the filter paper. Some of the precipitates

easily dislodged from the reactive materials and rinsed onto

the filter paper. The reactive material was separated from

the precipitates using tweezers, collected in a vial and

sealed to prevent oxidation. The filter paper with precipi-

tates attached to it was placed in a separate sealed

container. Small amounts of each were secured to Al stubs

and a sample of each fresh reactive material was also

secured to a stub. These samples were carbon coated with a

carbon sputter coater and stored in a vacuum desiccator

until examination. The samples were examined with a

scanning electron microscope equipped with an energy

dispersive X-ray analyser (EDS). Minerals on the surfaces

of the reactive materials were analysed using EDS to

determine the elements present.

Geochemical speciation mass-transfer modelling with

PHREEQC (version 2.12.04); (Parkhurst and Appelo 1999)

was used to determine, which minerals were saturated

along the length of the columns. PHREEQC performs

numeric algorithms and simultaneously processes the

coupled chemical reactions defining protonation/deproto-

nation, ion pair, and surface complexation, precipitation/

dissolution, and oxidation/reductions, which are needed to

predict the secondary mineral precipitation in a PRB

(Liang et al. 2000). The llnl.dat thermodynamic database

was used (provided with the PHREEQC code) in this

article to obtain consistent results. Modelling was per-

formed on each sample that was collected from each

sampling port over the life of the columns.

Results

Column A—oyster shells

The oyster shells successfully maintained a pH above 6.8

even after 300 pore volumes of acidic water had passed

through the column (Fig. 1). Based on the effluent pH, the

oyster shells did not show any sign of diminished neu-

tralising ability despite the vast quantity of acidic water

that passed through the column. With time, the lower half

of the column became less effective (Fig. 1). Air was

pumped through the column after the 208th pore volume of

acidic water to replicate unsaturated conditions that may

occur during a severe drought. Until that point in time, the

lower half of the column was no longer neutralising the

acidity, whereas the upper half was continuing to do so.

After the aeration, the pH in the lower half of the column

increased again. The increase in pH coincided with a

temporary rise in ORP in the lower half of the column

(Fig. 2). This indicates that the conditions may have

become hostile for reducing bacteria. The fluctuation in

ORP of the source water towards the end of the study

caused large fluctuations in the ORP along the column but

the magnitude of the fluctuations diminished with distance

along the column. The source water was collected from

the field and as such was susceptible to natural changes

in redox conditions caused by intermittent rainfall events

in an overall drought period. The inverse correlation

between pH and ORP is very strong in the lower half of

the column (Table 1). No correlation exists in the upper

half of the column, possibly due to the action of reducing

bacteria causing the ORP to stay low and the pH to stay

neutral.

Along the length of the column the concentration of

calcium liberated from the oyster shells commonly

increased slightly with distance along the column (Fig. 3).

A clear trend is evident with increasing pore volume of

acidic influent, in that the [Ca2?] initially sharply increased

in concentration, then plateaued after 30 pore volumes

before decreasing again after 100–150 pore volumes. The

initial increase is likely due to the rapid dissolution of the

CaCO3 of the shell by the acidity. The concentration of

sulphate did not show a trend of decreasing with distance

along the column (Fig. 4). The oyster shells removed the

dissolved iron from solution (in column effluent after

7 weeks, [Fe] = 0.05 mg/L, compared to 4 mg/L in the

influent; Fig. 5).

Column B—recycled concrete

The recycled concrete maintained a pH above 10.5 even

after 90 pore volumes of acidic water was passed through

the column and did not show any sign of diminished

neutralising ability (Fig. 1). The ORP at the column outlet

was stable despite large variations in the source water

(Fig. 2). Column B displayed high initial [Ca2?] and

[SO42–] and both of these diminished with increasing pore

volumes of acidic influent (Figs. 3, 4, respectively). Simi-

larly, Column B successfully removed Fe and Al from

solution (Figs. 5, 6, respectively).


123

Column C—recycled concrete/oyster shell

The column containing half concrete, half oyster shells

produced alkaline effluent (pH 9.8; Fig. 1). The ORP at the

column outlet did not display large variations despite large

variations in the source water (Fig. 2). Initially, the [Ca2?]

in Column C was high (although less than half that of

Column B) and it rapidly decreased with pore volumes of

acidic influent. This trend in [Ca2?] is similar to that of

Column B and is in contrast to Column A.

Discussion

Acidic water was regularly collected from the field to run

through the column (labelled source in Figs. 1, 2) and as a

result varied slightly in its composition over time. The

oyster shells and concrete contain different alkaline com-

ponents, i.e. CaCO3 and Ca(OH)2, respectively. The pH

achieved by each reactive material was controlled by the

reaction kinetics of the dominant alkaline mineral. The

concrete achieved a pH that is consistent with the disso-

lution of lime (pH 10–12; Fig. 1). The oyster shells

achieved a pH consistent with the dissolution of calcite

(pH *7.4; Fig. 1) (Golab et al. 2006).

Column A—oyster shells

In the column tests of Komnitsas et al. (2004) involving

limestone remediating synthetic AMD, the pH at a sampling

port 20 cm along the column dropped dramatically to pH 3

after 20 pore volumes, indicating either complete exhaus-

tion of the neutralising capacity of the limestone or more

likely, severe armouring of the reactive sites on the lime-

stone. Subsequently, a similar drop occurred at the sampling

port 60 cm along the column after 70 pore volumes

(Komnitsas et al. 2004). In contrast to the findings of

Komnitsas et al. (2004), in the current study, the pH of the

water in Column A did not dramatically drop at any sam-

pling port, indicating that either the neutralising capacity of

the oyster shells was not exhausted when the column was

decommissioned after more than 300 pore volumes had

passed through it or the reactive sites were not severely

armoured. The difference between the two column tests is

likely largely due to the difference in the structure and

surface area of the reactive materials. While both limestone

0 50 100 150 200 250 300

24

68

Pore Volume

pH

Source 7.5 cm 30 cm Outlet

0 20 40 60

34

56

7

Distance (cm)

pH

17 PV 105 PV 192 PV

0 20 40 60 80

24

68

10

Pore Volume

pH


0 20 40 60

46

810

Distance (cm)

pH

13 PV 64 PV 91 PV

0 10 20 30 40

24

68

10

Pore Volume

pH


0 20 40 60

46

810

Distance (cm)

pH6 PV 23 PV 48 PV

a b

c d

e f

Fig. 1 Performance of reactive

materials, as indicated by pH

versus pore volume and distance

along the column. a, b oyster

shells in Column A, c, drecycled concrete in Column B

and, e, f half concrete, half

oyster shells in Column C


123

and oyster shells are composed largely of CaCO3, the oyster

shells have a fragile multi-layered structure that allows the

outer layers of the shell to be corroded by the acid, exposing

inner fresh layers of shell as reactive sites (Fig. 7). It is also

probable that the much higher concentrations of metals and

lower pH of the synthetic AMD used by Komnitsas et al.

(2004) was an important factor.

In the current study, at the beginning of the column tests

pH increased rapidly due to dissolution of CaCO3 in the

oyster shells. The pH profiles show that although the lower

section of the reactive material gradually lost its efficiency

to buffer pH, continuous dissolution of CaCO3 at the upper

parts of the columns still added alkalinity to the system

over a relatively long period of time (Fig. 1); a similar

trend was reported by Komnitsas et al. (2004).

In the current study, a black film developed in circles on

the wall of Column A at the base after 20 pore volumes and

the film appeared in fresh areas up the column with time, so

that the black circles coated the column wall up to 40 cm

along the column after 61 pore volumes and continued to

appear along the entire length of the column. The black

film appeared to be deposited on the surface of the column

wall, as was reported by Christensen et al. (1996) but when

the column was dismantled, it was discovered that the

black film not only coated the column wall but was ubiq-

uitous throughout and covered the oyster shells as well.

The black film is taken to indicate the activity of sulphate

or iron (III) reducing bacteria (Christensen et al. 1996).

Other points of evidence indicate the presence of reducing

bacteria in Column A, including the decrease in ORP in the

0 50 100 150 200 250 300

020

040

060

0

Pore Volume

OR

P


0 20 40 60

010

030

050

0

Distance (cm)

OR

P

17 PV 105 PV 192 PV

0 20 40 6080

010

030

050

0

Pore Volume

OR

P


0 20 40 60

100

200

300

400

500

Distance (cm)

OR

P

13 PV 64 PV 91 PV

0 10 20 30 40

−10

00

100

300

500

Pore Volume

OR

P


0 20 40 60

010

020

030

040

050

060

0

Distance (cm)

OR

P6 PV 23 PV 48 PV

a

c

e

b

d

f


materials, as indicated by

oxidation–reduction potential

(ORP) versus pore volume

and distance along the column.

a, b oyster shells in Column A,

c, d recycled concrete in

Column B and e, f half concrete,

half oyster shells in Column C

Table 1 Correlation between pH and ORP in Column A, B and C,

respectively

Sampling location Column A Column B Column C

r N r N r N

7.5 cm 20.87 24 20.87 12 -0.28 8

10.5 cm 20.89 24 20.90 12 -0.64 8

15 cm 20.90 24 20.75 12 -0.64 8

20 cm 20.87 24 20.67 12 20.86 8

30 cm 20.87 24 -0.52 12 20.90 8

40 cm 20.43 24 -0.49 12 20.77 8

60 cm -0.30 24 -0.37 12 -0.41 8

Outlet -0.25 24 20.60 12 -0.51 8

Source 20.83 18 20.64 13 20.77 10

Significant r-values at the 95% confidence interval are bolded


123

lower half of the column (Fig. 2) and the detection of H2S

gas (by its smell). It is assumed that the reducing bacteria

were already living on the oyster shells in the estuary and

that the organic material on the surface of the oyster shells

provided the food source required for their continued

growth. The oyster shells were collected fresh from a

nearby oyster farm and the exterior of the shells was coated

in algae and in some cases the unopened shells contained

oysters within. Alternatively, the source water may have

introduced the reducing bacteria. Once the population of

reducing bacteria was established, the abundant iron and a

small proportion of sulphate in the groundwater were

reduced by the bacteria in the columns, leading to a low-

ering of the ORP along the length of the column (Fig. 2), a

decrease in the levels of Fe and to a small extent SO4 and

the neutralisation of the acidity (Fig. 1). The formation

of alkalinity was greatly enhanced by the dissolution of

the oyster shells (Fig. 7), which buffered the incoming

acidity and probably protected the bacteria (Kuyucak and

St-Germain 1994).

Figure 8 displays the variation in ORP with distance

along the column at four different time intervals. The

gradient of the curves between each sampling point

varies with distance along the column. The final steep

section of the curve changed with time, for example,

after 12 pore volumes the final steep section of the curve

is between the inlet and the first (7.5 cm) sampling port,

after 30 pore volumes the final steep section of the curve

is between the first and second (10.5 cm) sampling ports,

after 105 pore volumes it lies between the fourth and fifth

(30.5 cm) sampling ports, and after 112 pore volumes it

lies between the fifth and sixth (40.5 cm) sampling ports.

The final steep section of the curve in a plot of ORP

versus distance along the column is taken to be the

frontier zone of active reduction. The frontier zone of

active reduction coincided with the zone of most rapid

neutralisation (c.f. Figs. 1, 8) and the visible growth of

reducing bacteria within the columns. Extensive precip-

itation occurred on the surface of the oyster shells at the

base of the column and systematically spread upwards

with time due to bacterial reduction and chemical pro-

cesses. The spread of precipitates was visibly obvious

through the column wall and the precipitates were seen in

the SEM images after the column was decommissioned

(Fig. 9). Concurrently, however, the fragile oyster shells

were rapidly consumed by the acidity, causing the

10 20 30 40 50 60 70

050

100

150

200

250

Distance (cm)

[Ca]

(m

g/L)

0.75PV 12.4PV 30PV 105PV 156PV 278PV

0 50 100 150 200 250

050

100

150

200

250

Pore Volume

[Ca]

(m

g/L)

7.5cm 30cm Outlet

0 20 40 60

010

020

030

040

0

Distance (cm)

[Ca]

(m

g/L)

0.75PV 11.6PV 22PV 53PV

0 10 20 30 40 50

050

100

150

200

250

Pore Volume

[Ca]

(m

g/L)

Source 7.5cm 30cm Outlet

0 20 40 60

050

100

150

Distance (cm)

[Ca]

(m

g/L)

0.75PV 5.6PV 17PV 23PV

0 5 10 15 20

050

100

150

Pore Volume

[Ca]

(m

g/L)


a b

c d

e f



calcium concentration versus

pore volume and distance along

the column. a, b oyster shells in

Column A, c, d recycled

concrete in Column B and

e, f half concrete, half oyster

shells in Column C


123

development of large voids and consequently the column

did not become totally clogged. The consistent neutrali-

sation of acidity by Column A throughout the duration

of the tests may indicate that as the outer layers of the

shells were consumed by acidity, the inner layers were

exposed and thereby the surface area to volume ratio was

increased (Fig. 7).

Column B—recycled concrete

At the beginning of the column tests, pH increased rapidly

due to dissolution of Ca(OH)2 in the recycled concrete then

rapidly dropped at the port 30 cm along the column, fol-

lowed by a stabilisation and gentle rise in pH (Fig. 1). The

initially high pH followed by a rapid drop is taken to

indicate that free Ca(OH)2 was initially available in the

crushed concrete and once this had fully reacted the pH

plateaued. This theory is supported by the initially high

[Ca2?] which diminished with time (Fig. 3). The pH

achieved at the column outlet was consistently high, indi-

cating that continuous dissolution of Ca(OH)2 in the upper

parts of the column added alkalinity to the system over a

relatively long period of time and a similar trend was

reported by Komnitsas et al. (2004). The spread of pre-

cipitates was visibly obvious through the column wall and

the precipitates were seen in the SEM images after the

column was decommissioned (Fig. 10).

Column C

The ORP of Column C in the lower half of the column

(concrete) is similar to that of Column B, in that it drops

rapidly with distance along the column but in the upper half

of the column (oyster shells) it mirrors the behaviour of

Column A in that it plateaus. In terms of the amount of

Ca2? in solution, Column C (half concrete) behaved in a

similar way to Column B (pure concrete) but with less than

half the [Ca2?]. This indicates that the concrete in Column

C also contributed free Ca(OH)2 into solution but due to

the lesser amount of concrete in the column the amount of

freely available Ca2? was also less. In the lower half of

Column C (concrete) the [Ca2?] rose rapidly (as for Col-

umn B) but the upper half (oyster shells) plateaued (as for

Column A). The trends in ORP and [Ca2?] in Column C

reflect the combination of effects from both the oyster

shells and recycled concrete.

10 20 30 40 50 60 70

020

040

060

080

0

Distance (cm)

[SO

4] (

mg/

L)

0.75PV 12.4PV 30PV 105PV 156PV 278PV

0 50 100 150 200 250

020

040

060

080

0

Pore Volume

[SO

4] (

mg/

L)

7.5cm 30cm Outlet

0 20 40 60

020

040

060

080

0

Distance (cm)

[SO

4] (

mg/

L)

0.75PV 11.6PV 22PV 53PV

0 10 20 30 40 50

020

040

060

080

0

Pore Volume

[SO

4] (

mg/

L)


0 20 40 60

010

020

030

040

050

0

Distance (cm)

[SO

4] (

mg/

L)

0.75PV 5.6PV 17PV 23PV

0 5 10 15 20

010

020

030

040

050

0

Pore Volume

[SO

4] (

mg/

L)


a b

c d

e f



sulphate concentration versus






shells in Column C


123

Formation of precipitates

When the acidic solution comes into contact with the oyster

shells and concrete, Ca2? ions are released (Fig. 3), alka-

linity is added into the system and pH increases. Komnitsas

et al. (2004) reported that when the column solution pH

exceeded 3.75 gypsum (CaSO4 � 2H2O) formed and pre-

cipitated, as evidenced by a lowering of sulphate

concentration at the initial stages of the column test in their

study. In the current study, however, this did not occur in

any of the three columns (Fig. 4). Similarly in the case of

ten different anoxic limestone drains (ALDs), Watzlaf et al.

(2000) report that sulphate levels were unaffected by the

ALDs. In the current study, the [SO42-] of the influent

varied with time because, it was collected from the field

and is therefore subject to natural variations. The lack of

trend in [SO42-] indicates that while some precipitation of

gypsum may have occurred it was not the dominant pre-

cipitate. This is confirmed by the SEM-EDS results of the

precipitates formed on the reactive materials, containing on

average only 6.9% S on the oyster shells and 1.7% S on the

recycled concrete (Table 2).

Bright orange precipitates visibly spread upwards in

each of the columns with time, for example, even after just

15 pore volumes, precipitates visibly extended more than

20 cm up Column C. In Column B after 56 pore volumes

precipitates were clearly visible beyond the 40 cm mark

and were very densely clustered up to 20 cm. In Column A,

the orange precipitates were densely clustered up to 35 cm

and clearly visible throughout the length of the column

after 105 pore volumes. The bright orange precipitate is

thought to be amorphous Fe(OH)3. The dramatic decrease

in [Fe] along the length of each column supports this

hypothesis (Fig. 5), as do the results of the SEM-EDS

analyses on the precipitates formed on the reactive mate-

rials (Table 2), containing on average 11% on the oyster

shells and 15.5% Fe on the recycled concrete. With

increasing pore volumes of acidic influent and increasing

appearance of precipitates, the neutralising ability of the

lower section of each column was less than that of the

upper regions of the column, indicating that the reactivity

of the oyster shells and concrete was decreased due to

partial coating by precipitates. Similarly, Komnitsas et al.

(2004) report a drop in the pH profile and concentration of

10 20 30 40 50 60 70

010

2030

Distance (cm)

[Fe]

(m

g/L)

0.75PV 12.4PV 30PV 105PV 156PV 278PV

0 50 100 150 200 250

010

2030

Pore Volume

[Fe]

(m

g/L)

7.5cm 30cm Outlet

0 20 40 60

05

10

Distance (cm)

[Fe]

(m

g/L)

0.75PV 11.6PV 22PV 53PV

0 10 20 30 40 50

05

10

Pore Volume

[Fe]

(m

g/L)


0 20 40 60

−2

02

46

8

Distance (cm)

[Fe]

(m

g/L)

0.75PV 5.6PV 17PV 23PV

0 5 10 15 20

−2

02

46

8

Pore Volume

[Fe]

(m

g/L)


a b

c d

e f


materials, as indicated by iron

concentration versus pore

volume and distance along the

column. a, b oyster shells in




shells in Column C


123

Fe and takes this to indicate the formation of Fe(OH)3 in

their column tests. Furukawa et al. (2002) identified fer-

rihydrite as a precipitate in their PRB. Identification of

ferrihydrite by XRD is difficult because it is poorly crys-

tallised and fine-grained. Ferrihydrite naturally occurs in

iron-rich soils that experience oscillating redox environ-

ments, especially if dissolved silica, phosphate or other

ions are sorbed on the ferrihydrite surfaces to inhibit con-

version to more crystalline assemblages. In the study of

Komnitsas et al. (2004) into the potential use of limestone

and red mud in a PRB to remove several heavy metal ions

from the AMD mainly by precipitation, co-precipitation

and adsorption, the authors reported that Fe precipitated

mainly as goethite and ferrihydrite, and Al as boehmite and

gibbsite.

The concentration of Al also dropped rapidly along the

length of each column. The results of the SEM-EDS

analyses on the precipitates that formed on the reactive

materials show an accumulation of Al (Table 2), with on

average 31% on the oyster shells and 28% on the recycled

concrete. It is suggested that the Al precipitated out of

solution as Al hydroxide, oxyhydroxides and to a lesser

extent hydroxysulphates. The formation of Al hydroxides

and oxyhydroxides is due to the rise in pH, because the

solubility of these minerals decreases with alkaline condi-

tions. Aluminium sulphate hydroxide hydrate (Al3(SO4)2

(OH)5 � 9H2O) was identified in the precipitates collected

by Golab et al. (2006) from batch tests of recycled con-

crete. The XRD traces in the tests by Furukawa et al.

(2002), Kamolpornwijit et al. (2004) and Golab et al.

(2006) were difficult to interpret due to ‘humps’, indicating

the amorphous nature of the precipitates, which were

believed to be iron oxide, ferrihydrite, Al(OH)3 and AlO-

HSO4. Hence, the findings from other researchers support

our hypothesis that amorphous Fe and Al oxides and

hydroxides formed on the surfaces of the reactive materials

in the column tests. The results of the geochemical speci-

ation/mass transfer modelling that were completed using

PHREEQC also indicated that an array of carbonates,

aluminium oxides, and iron oxides are saturated at different

distances along Column A (Fig. 11). Only basic modelling

was performed in this project and more thorough modelling

is being performed in conjunction with additional column

tests that will more closely replicate field conditions. As a

10 20 30 40 50 60 70

01

23

4

Distance (cm)

[Al]

(mg/

L)

0.75PV 12.4PV 30PV 105PV 156PV 278PV

0 50 100 150 200 250

01

23

4

Pore Volume

[Al]

(mg/

L)

7.5cm 30cm Outlet

0 20 40 60

010

2030

40

Distance (cm)

[Al]

(mg/

L)

0.75PV 11.6PV 22PV 53PV

0 10 20 30 40 50

010

2030

40

Pore Volume

[Al]

(mg/

L)


0 20 40 60

05

1015

20

Distance (cm)

[Al]

(mg/

L)

0.75PV 5.6PV 17PV 23PV

0 5 10 15 20

05

1015

20

Pore Volume

[Al]

(mg/

L)


a b

c d

e f



aluminium concentration versus






shells in Column C


123

result, the results of the modelling from the current project

are not used to draw conclusions about the nature of the

precipitates.

Fig. 7 Comparison of SEM images of fragments of oyster shell. acorroded and pitted due to attack by acid, which was collected after

Column A was decommissioned, b fresh oyster shell that was not

used in the column

-50

50

150

250

350

450

550

0 10 20 30 40 50 60 70

Distance along column (cm)

OR

P (

mV

)

12 PV

30 PV

105 PV

112 PV

Reducing Front

Fig. 8 Change in ORP along the length of column A (oyster shells) at

different time intervals (denoted by the number of pore volumes that

had passed through the column)

Fig. 9 SEM image of a fragment of oyster shell coated in precipitates

after Column A was decommissioned. Note that the surface of the

shell is corroded and pitted due to attack by acid but precipitates have

then coated it

Fig. 10 SEM image of a fragment of recycled concrete that is coated

with precipitates, which was collected after Column B was

decommissioned

Table 2 Results of SEM-EDS analysis of precipitates formed on the

surface of oyster shells and recycled concrete extracted from Columns

A and B, respectively when the columns were decommissioned

Element Oyster shell Concrete

1 2 3 Mean 1 2 3 4 Mean

Al 28.8 32.8 31.4 31.0 29.6 28.2 27.8 26.8 28.1

Fe 8.4 13.1 10.7 14.6 16.4 14.9 16.0 15.5

S 3.0 3.8 14.0 6.9 1.8 1.3 2.3 1.6 1.7


123

In the case of Column A, where black circles formed on

the surfaces of the oyster shells and on the walls of the

column, it is possible that under the reducing conditions

caused by reducing bacteria ferrous monosulphide or

mackinawite formed. In support of this hypothesis

Waybrant et al. (2002) observed small (2–10 lm) spheres

on the surfaces of wood particles within their reactive

mixture. Energy-dispersion X-ray analysis indicated that

these spheres were composed primarily of Fe and S with

minor amounts of Ca, Si, Mg and O and were interpreted to

be precipitates of ferrous monosulphide or mackinawite

(Waybrant et al. 2002).

In the current study, clogging within Column A appears

to follow the sequence (Kamolpornwijit et al. 2003; Bilek

2006):

1. mineral precipitation occurred at the influent interface;

Fe and Al hydroxides and oxyhydroxides;

2. clogging forced the water to flow along preferential

flow-paths rather than homogeneously through the

pore spaces, thus accelerating the rate of clogging; and

3. microbial growth in Column A favoured the slow flow-

paths, thereby exacerbating the preferential flow-path

development and generating reducing conditions,

leading to the precipitation of ferrous monosulphides,

thus causing further clogging.

Conclusions

The overall buffering capacity of the reactive material

within the column system is controlled by several factors,

including mineral dissolution, iron and aluminium hydro-

lysis and subsequent formation of precipitates (Komnitsas

et al. 2004).

The column tests have shown that both reactive mate-

rials (recycled concrete and oyster shells) are successful in

remediating acidic groundwater. A combination of the two

materials is the most desirable PRB fill material for the

following reasons: (1) the recycled concrete efficiently

neutralises the acidity and removes Al from solution; (2)

-5

-2.5

0

2.5

5

0 20 40 60 80

Distance Along Column (cm)

Sat

urat

ion

Inde

x

Dolomite (CaMg(CO3)2) Dawsonite (NaAlCO3(OH)2)

Calcite (CaCO3) Magnesite (MgCO3)

Aragonite (CaCO3) Monohydrocalcite (CaCO3:H2O)

-5

-4

-3

-2

-1

0

1

2

3

0 20 40 60 80


Sat

urat

ion

Inde

x

Dawsonite (NaAlCO3(OH)2) Diaspore (AlOOH)

Boehmite (AlOOH) Gibbsite (Al(OH)3)

Corundum (Al2O3) Alunite (KAl3(OH)6(SO4)2)

-10

-5

0

5

10

0 20 40 60 80


Sat

urat

ion

Inde

x

Hematite (Fe2O3) Magnetite (Fe3O4)

Goethite (FeOOH) Ferrite-Mg (MgFe2O4)

a

c

bFig. 11 Variation in saturation

index of minerals, modelled

using PHREEQC, with distance

along column A. a carbonates,

b aluminium oxides and c iron

oxides


123

the oyster shells enhance the growth of reducing bacteria,

which in turn lead to the precipitation of Fe and SO4 out of

solution; and (3) when oyster shells were used in the

absence of concrete they were consumed faster than the

concrete and were more severely clogged with precipitates.

As such, the combination of recycled concrete and oyster

shells should lead to the most efficient removal of the

contaminants from the groundwater. Together, this indi-

cates that a layered PRB is the best option, with a small

amount of waste concrete at the influent side to neutralise

the acidity before the groundwater reaches the oyster

shells.

Only one other PRB is reported to have been installed

into ASS and it contained limestone and was under oxi-

dising conditions (Waite et al. 2002). The PRB was rapidly

clogged with precipitates due to the oxidising conditions

and this severely affected the neutralising ability of the

PRB. The PRB under design for the current project will be

under reducing conditions to more efficiently remove Fe

and Al and to minimise the risk of clogging of the pores

and pacification of the reactive materials. The proposed

PRB will also utilise waste materials rather than using

quarried limestone.

Alkaline waste materials are potential options for use in

PRBs for the treatment of acidic groundwater. Both oyster

shells and recycled concrete continued to neutralise acidity

even after an extended period of time. The site character-

isation and reactive material selection process have shown

that it may be possible to use a PRB to treat low-lying ASS

affected areas without compromising the principles of

PRBs. However, the column tests were not conducted

under temperature, pressure, or geochemical conditions, or

at a flow-rate experienced in the field and therefore may not

be indicative of a full-scale operation. The PRB differs

from that of Waite et al. (2002) in that it will be under

reducing conditions instead of oxidising conditions and

will utilise waste materials. The research reported here has

applications for the remediation of other acidic ground-

water sources, for example AMD.

Acknowledgments This research was funded by an Australian

Research Council grant in collaboration with the Manildra Group and

Shoalhaven City Council. We gratefully acknowledge the assistance

of Glenys Lugg, Warwick Papworth, Bob Rowlan and Stephen Hay.

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