barrier materials
TRANSCRIPT
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).
244 Environ Earth Sci (2009) 59:241–254
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
Source 7.5 cm 30 cm Outlet
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
Source 7.5 cm 30 cm Outlet
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
Environ Earth Sci (2009) 59:241–254 245
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
Source 7.5 cm 30 cm Outlet
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
Source 7.5 cm 30 cm Outlet
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
Source 7.5 cm 30 cm Outlet
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
Fig. 2 Performance of reactive
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
246 Environ Earth Sci (2009) 59:241–254
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)
Source 7.5cm 30cm Outlet
a b
c d
e f
Fig. 3 Performance of reactive
materials, as indicated by
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
Environ Earth Sci (2009) 59:241–254 247
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)
Source 7.5cm 30cm Outlet
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)
Source 7.5cm 30cm Outlet
a b
c d
e f
Fig. 4 Performance of reactive
materials, as indicated by
sulphate 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
248 Environ Earth Sci (2009) 59:241–254
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)
Source 7.5cm 30cm Outlet
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)
Source 7.5cm 30cm Outlet
a b
c d
e f
Fig. 5 Performance of reactive
materials, as indicated by iron
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
Environ Earth Sci (2009) 59:241–254 249
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)
Source 7.5cm 30cm Outlet
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)
Source 7.5cm 30cm Outlet
a b
c d
e f
Fig. 6 Performance of reactive
materials, as indicated by
aluminium 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
250 Environ Earth Sci (2009) 59:241–254
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
Environ Earth Sci (2009) 59:241–254 251
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
Distance Along Column (cm)
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
Distance Along Column (cm)
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
252 Environ Earth Sci (2009) 59:241–254
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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