Brahaita Jan2017 [611837]

347 Carpathian Journal of Earth and Environmental Sciences, July 2017, Vol. 12, No. 2, p. 347 – 356

THE EFFICIENCY OF LIMESTONE IN NEUTRALIZING ACID MINE
DRAINAGE – A LABORATORY STUDY

Ioan -Dorian BRĂHAIȚA
1, Ioan -Cristian POP1, Călin BACIU1*, Radu
MIHĂIESC U1, Cristina MODOI1, Gabriela POPIȚA1 & Roxana- Maria TRUȚĂ1
1Babeș -Bolyai University, Faculty of Environmental Science and Engineering, Str. Fantanele 30, 400294 Cluj -Napoca,
Romania, [anonimizat] , [anonimizat] , [anonimizat], [anonimizat],
[anonimizat], [anonimizat] , [anonimizat] *Corresponding author

Abstract: The neutralization of acid mine drainage (AMD) by li mestone proved to be effective in various
laboratory tests and real -scale applications worldwide. The present contribution describes the results of an
experimental approach intended to test the limestone’s efficiency under laboratory conditions. Two open
channels have been built, with the length of 1 and 2 m respectively. In the first channel (1 m long), 2.6 kg of
limestone (granulometry 5 -10 mm) were used at two different flow rates: 25 ml/min and 50 ml/min. For the
second channel (2 m long), 5.2 kg of lim estone were used at the same flow rates. In each experiment the
samples were collected every hour for the determination of ions’ concentrations. The results of all the
experiments have shown that in the first hour, the pH value increased from 2.3 up to 4.0 4-6.26. After 5 hours
of contact, the pH value increased constantly between 3.04 and 5.82, in relation with the channel’s length
and flow rates. In regard to the flow rate, the highest efficiency in terms of neutralization was observed for
the lower flow r ate (25mL / min) associated with a pH value of 6.26 for the first hour and 4.55 after 8 hours.
Another important parameter associated with the experiments was the limestone grain size. The results
showed that larger grain size (20 -40 mm) are less effective in neutralizing acidic water compared to the case
when a smaller grain size (5 -10 mm) was used. The heavy metals (Cd, Fe) and sulphate concentration
decreased in the first couple of hours of interaction. Following this study, it can be concluded that the
contact time between water and limestone is of high importance. By adjusting the drain length and the water
flow rate, there is an increase in the contact time. Efficiency of Cd and Fe retention was observed especially
in the first hour of experiments. The same was also noticed for the ability of the limestone to remove SO 42-.

Key words : AMD neutralization, limestone, pH, heavy metals , Rosia Montana

1. INTRODUCTION

The environmental impact of mining,
especially concerning the water resources, is very
severe in many cases, even in the post -operational
phase. The acid mine drainage (AMD) continues to
represent a serious concern in Romania, although the
majority of the mines have ceased their operation.
The AMD is characterised by low pH, high
amount of dissolved solids, and the occurrence of
heavy metals and metalloids, many of them harmful
to the environment and human health (Ziemkiewicz et
al., 1997; Peppas et al., 2000) . The particular
composition of the solution depends very much on the
characterist ics of the rocks/minerals that are
dissolved, including their buffering capacity, as well as the availability of oxygen, local climate, etc. Numerous treatment technologies have been setup for the remediation of mining polluted water; however
this remains a challenging issue from an economic
and technical point of view. Nevertheless, due to high
costs and inadequate methods, a wide variety of
acidic waters still remain untreated . Previous studies
reveal that an efficient, yet inexpensive solution is represented by the usage of natural materials, which
have the potential to raise the pH of the solution, and
are able to decrease the concentration of dissolved
metals by precipitation, adsorption, etc.
The use of limestone channels is o ne of the most
common pas sive methods for acid mine water
treatment. This method favours a rise of pH and also
the removal of dissolved heavy metals through
precipitation and oxidation reactions (Hugh et al.,
2011). An advantage of this procedure is the simple

348 design and the relat ively low maintenance cost s,
compared to other methods (Skousen et al., 1998, Sun
et al., 2000, Cravotta, 2003, Hammarstrom et al.,
2003).
However there are some significant
shortcomings, including a quick coverage of the
limestone grains with iron oxides , calcium sulphates
and organic matter , that will inhibit the interaction
between limestone and acid water (Akcil & Koldas ,
2006). Although it is a cheap method that can give
good results in particular cases, currently there are few
attempts to use it in R omania, in spite of the high
number of former mining sites that need environmental remediation.
Acid water treatment by using limestone,
exhibits three stages (Bobos , 2007; Sun et al. , 2000):
1. Fe
2+ chemical oxidation
Fe2+ + ¼O 2 + H+ Fe3+ + ½H 2O (1)

2. Acid water neutralization process by using
limestone:
CaCO 3 + H+ Ca2+ + HCO 3- (2)

CaCO 3 + 2 H+ Ca2+ + CO 2 + H 2O (3)

3. Precipitation of Fe3+ oxides:
Fe3+ + 3 H 2O Fe(OH) 3 + 3 H+ (4)

As shown by equations 2 and 3, one mole of
CaCO 3 can react with one to two moles of acidity
(Garrels & Christ, 1965) . As a consequence of this
treatment, the pH of the initially acid solution may
reach the circumneutral zone, (Pearson & McDonnell,
1975; Webb & Sasowsky, 1994; Br ăhaița et al., 2015).
This aspect confers the limestone a main role in acid
water treatment, both in active and passive systems.
Limestone passive treatment systems are mainly used
in post closure of mining activities (Taylor & Waters,
2003).
The objective of this laboratory experiment
was to assess the effectiveness of limestone in acid
water treatment by modifying some parameters such
as: drain length, water flow, limestone quantity, and
the type of limestone.

2. MATERIALS AND METHODS

For the laborator y experiments , limestone
samples from Sandulesti and Geomal quarr ies were
used. These rocks belong to the Upper Jurassic –
Lower Cretaceous carbonate platform developed on
the eastern edge of the Apuseni Mountains. The
limestone that has been used for the experiments
corresponds to the Upper Jurassic Stramberk facies. In
Sandulesti quarry, a 60 -m thick sequence of coarse
“reef” detrital deposits and bioconstructions is present
(Sasaran, 2006). T he limestone mainly consists of a carbonate fraction ( about 93%) formed by high
magnesio -calcite (95 -98%), and dolomite (2 -5%). A
minor siliciclastic fraction (7%), including clay
minerals ( illite, smectite, caolinite, chlorite, iron oxy –
hydroxides), and other minerals as quartz, feldspars, apatite, etc., occurs mainl y on fissures and in
dissolution voids .
Composite limestone samples with a weight of
20 kg each have been collected from the pit. The
limestone was grinded, dried at room temperature and then sieved. The grain size of the limestone used in
experiments was 5-10 mm and 20- 40 mm in the case
of Sandulesti, and 5- 10 mm for Geomal .
Two drains were built for laboratory
experiments using PVC tubes with a diameter of 10
cm. At the end of each of the drains caps fitted with
inlet and outlet valves for the water system were
installed (F ig. 1) . The length of the drains was 1 m
and 2 m, respectively .
In the 1 m long channel an amount of 2.6 kg of
limestone was used (Fig. 1a) . The established flow
rates were 50 mL/min (R2) and 25 mL /min (R3). In
the 2 m long channel (Fig. 1b), an amount of 5.2 kg
limestone was used, at the same flow rates (R5, R6).
The flow rates were set using the bucket and stop
watch method. Due to the decrease in the amount of water in the storage tank (20 L) , and consequently of
the hydrostatic pres sure, a reduction of the flow rate
was observed, therefore the flow rate had to be
checked and adjusted every hour. The water sample
used in the laboratory experiment was collected from
the Adit 714, Rosia Montana mining area.
The porosity of the limestone grain assemblage
was determined by measuring the volume of the water -filled voids and the total volume occupied by
water and limestone (5).
100VVP
tv×= (5)
P – porosity (%)
Vv – voids volume (L)
Vt – total volume (L)
The porosity was computed to be
approximately 45%. The channel’s slope was set at
0.8%. The retention time represents the time required
for the water to go through the entire length of the channel towards the exit. It was calculated using the
formula (6) (W atzlaf et al., 2000) :
QVPtL×= (6)
where
t – retention time (min)
P – porosity (%)
VL – limestone volume (L)
Q – flow rate (L/min)

349 Table 1. Physical parameters of the experiments
Experiment Length
(m) Flow rate
(ml/min) Limestone quantity
(kg) Grain size
(mm) Type of
limestone Retention time
(min)
R2 1 50 2.6 5-10 Sandulesti 9.36
R3 1 25 2.6 5-10 Sandulesti 18.72
R5 2 50 5.2 5-10 Sandulesti 18.72
R6 2 25 5.2 5-10 Sandulesti 37.44
R7 2 25 5.2 20-40 Sandulesti 37.44
R9 2 25 5.2 5-10 Geomal 37.44

The limestone volume was calculated using the
density of limestone (ρ=2489 kg/m3). For 2.6 kg of
limestone the volume was computed to be 1.04 L, and
2.08 L for 5.2 kg of limestone.
The physical parameters of the performed
experiments are presented in t able 1.
The purpose of the first laboratory experiments
(R2, R3, R5, R6, R7) was to determine the optimum
parameters (length, limestone amount, grain size, and
water flow) in order to obtain the best treatment
efficiency . Water s amples were collected every hour
for he avy metal and major ions analys es. Samples
were filter ed on 0.45 μm filters , and stored in 50 mL
HDPE containers and refrigerated until analysis. A
WTW Multi 350i portable multimeter (Germany) was
used for measuring the physicochemical
characteristics of the water samples. The major ion
concentrations have been measured by a Dionex
ICS1500 Ion Chromatograph. For the heavy metal analyse s, the samples were acidified with HNO 3 to a
pH value bellow 2. The heavy metal concentrations
were determined on a ZEEnit 700 atomic absorption
spectrometer (AAS) Analytik Jena (Jena, Germany) ,
using t he flame AAS method. An initial analysis of
the water has been performed prior to running each
experiment, and the values were plotted on the graphs
at the initial sampling time t = 0.

3. RESULTS AND DISCUSSION

Six experiments have been conducted, by
adjusting the physical parameters of the treating
system: drain length, water flow rate, limestone
amount , and grain size. The parameters of the system
used in each experiment are presented in table 1.
The water samples were collected on an hourly
basis, and the experiments lasted for 5 hours (R2, R3 ,
and R5) , 8 hours ( experiments R6 and R7), and 12
hours (experiment R9 ). For all experiments the grain
size was 5-10 mm, except ing R7 experiment where
the limestone grain size was 20 -40 mm.
In the first experiment (R2) the channel had a
length of 1 m at a flow rate of 50 mL/min. The pH value increased from 2.73 to 3.04 after 5 hours. The
maximum efficiency of neutralization was recorded
after the first hour, when the pH value reached 4.04.
The following experiment (R3) proved to be more
efficient because the water flow rate was reduced to
25 mL/min, therefore increasing the contact time.
The pH value increased from 2.74 to 5.99 in the first
hour, reaching a value of 3.47 after five hours.
The resu lts of the experiment R5, with the
drain length of 2 m and a water flow rate of 50
mL/min, are similar to R3 (drain length – 1 m, and
flow rate – 25mL/min) , the pH after 5 hours being
3.37.
The highest efficiency of neutralization has
been observed during the experiment R6, in which a
drain of 2 m and a water flow rate of 25 ml/min
were used. In this situation, a maximum pH value of
6.26 was reached within the first hour. After 8 hours ,
the pH value was 4.55. Due to this efficiency of the
neutralization, the same drain length and flow rate
were used for the following experiments.
The 20- 40 mm grain size (experiment R7) has
(a) (b)
Figure 1. Experimental drains: (a) 1 m long ; (b) 2 m
long.

350 been less efficient in neutralizing the acid water , as
the final pH value was 3.36. In the last experiment
(R9) Geomal limestone was used and the
neutralization efficiency was similar to experiment
R6, in which Sandulesti limestone was used under
analogous conditions .
Regarding the removal of Fe (III), the highest
efficiency in all experiments was noticed within the
first hour. In the exper iment s R6 and R9, with 2 m
drain length, and 25 mL/min water flow, the iron removal efficiency was more than 99% (Table 3). It
has been noticed that after 5 hours , the efficiency of
limestone in neutralizing the acidic water was below
8%, in the experiment R5. In the situation when
Geomal limestone was used (experiment R9) ,
efficiency dropped to 33% after 12 hours. In this
case the length of the drain and the water flow rate
was 2 m, and 25 mL/min respectively .
The efficiency of removing the Cd (II) was
the highest during the first hour. This reached the
value of 75% for the experiment R9, with the drain having a length of 2 m and flow rate of 25 ml/min,
using Geomal limestone.
In the experiment R 6 (drain length – 2m,
water flow rate – 25 mL/min ), the SO
42- removal
efficiency was higher than 50.8%. In the situation when a 1 m long drain and 50 mL/min flow rate was
used (experiment R2) , the efficiency of removing
SO
42- was the lowest, approximately 18% after the
first hour.
Open l imestone channels are favouri ng the
oxidation of Fe2+ to Fe3+ which precipitates as iron
hydroxide on the surface of the limestone.
Additionally, aluminium hydroxides and gypsum
precipitate, decreasing limestone ability to neutraliz e
water (Ziemkiewicz et al., 1997; Hammarstrom et
al., 2003). Once the limestone granules are covered
with precipitate, a reduction of the limestone neutralization efficiency was noticed after the first
hour. Hammarstorm et al ., have noticed in their
experiment that the pH value increased from 2.9 to 7, but after 48 hours, due to the covering of the
limestone with precipitate, it decreased below 4. In
their experiment, it was used a reactor with
limestone and water from a coal mine.
The t ables 2 to 3 and f igures 2 to 5 show the
results of the physicochemical measurements , metal
(Cd and Fe) , and ion ( SO
42- and Ca2+) analyses. All
graphs illustrate the positive effect of the limestone
channel over the acidic waters. The pH values of the
water samples increased to a ci rcumneutral value
within the first hour, followed by a descending trend
due to the armouring of limestone (Fig. 2). This
phenomenon can also be observed for the SO 42-
anion (Fig. 5). Concerning the iron, the decrease of concentration (Fig. 4) is more obvious compared to
the cadmium concentration (Fig. 3).
Examining the Figure 2 (a), it appears that the
pH value increases to wards a circum neutral value
when a lower granulation of the limestone is used.
This is due to an increase in the specific surface area
of the grains . Moreover, the concentration of Cd, Fe
and SO 42- decreased when a 5 -10 mm granulation
was used (Fig. 3a, 4a, 5a).
During all experiments, the concentration of
Ca2+ has increased from a minimum value of 359.33
mg / L to a maximum value of 1042.01 mg / L in the
experiment R9. Dissolution o f limestone due to
acidic water leads to the release of Ca2+ in solution.
The Ca2+ ion promotes gypsum, iron
oxyhydroxysulphates, or iron oxyhydroxychlorides
precipitation (Soler et al., 2008). The study of Soler
et al., (2008) showed that using small grai ns (1- 2
mm) the ability of the limestone to remove iron is
much higher compared to the experiment that used
larger grains (2 -5 mm). In the experiment suggested
by Soler et al ., (2008) a column with limestone and a
solution (HCl and H 2SO 4, pH=2) were used, with a
concentration of Fe (III) between 250 and 15,000
mg/L at a constant flow rate.
There are two presumed reasons for the high
iron removal efficiency of the limestone. The first is
represented by the rough surfaces of the limestone,
that favour the so rption of metal ions, and the
second is given by the presence of dissolved calcium
carbonate which increases the value of the pH,
therefore leading to the precipitation of iron and
other metals as oxides, hydroxides, or carbonates
(Aziz et al., 2008).
Alcolea et al., (2012) used a drain with a total
length of 1986 m, and limestone blocks measuring
60-150 cm in length on the bottom of the channel,
and between 30- 40 cm on the drain walls. In the
study of Alcolea et al. , (2012) a significant growth in
the concentration of Ca2+ was observed towards the
outlet of the channel. The value of the pH increased
at the same time as the Ca2+ concentration.
The high value of the electrical conductivity,
as well as the high concentration of sulphates, indicate the presenc e of a large amount of ions.
During the early hours of our experiments, there was a raise in the electrical conductivity, followed by a
downward trend below the initial value. A similar
trend was also observed in the aforementioned study (Alcolea et al., 2 012). In most of our experiments,
the total dissolved solids ( TDS ) value increases
during the first hour, presumably due to the
dissolution of limestone, followed later on by a
decrease, as the grains surface is covered with
precipitates.

351 Table 2. Variation of the concentration of Cd, Fe , Ca, and SO 42- during the experiments a. 1 m long channel, 50 mL /min flow rate and 5 -10 mm grain size ; b. 1 m long
channel, 25 mL /min flow rate and 5 -10 mm grain size; c. 2 m long channel, 50 mL /min flow rate and 5 -10 mm grain size .
Time
(min) a. Cd
(mg/L) Fe
(mg/L) SO 42-
(mg/L) Ca2+
(mg/L) b. Cd
(mg/L) Fe
(mg/L) SO 42
(mg/L) Ca2+
(mg/L) c. Cd
(mg/L) Fe
(mg/L) SO 42
(mg/L) Ca2+
(mg/L)
0 R2-i 0.236 1353.68 5410.2 460.19 R3-i 0.210 1366.33 5583.7 461.5 R5-i 0.206 1351.69 5500 .8 444.7
60 R2-1 0.247 530.16 4425.0 659.36 R3-1 0.174 823.52 5230.4 612.71 R5-1 0.175 589.50 2839.1 606.83
120 R2-2 0.234 1078.96 5070.7 707.18 R3-2 0.209 856.11 2882.6 579.07 R5-2 0.192 482.80 4156.3 557.17
180 R2-3 0.234 1179.40 6725.2 576.35 R3-3 0.220 930.62 3237.4 666.96 R5-3 0.199 6.43 3120.0 656.94
240 R2-4 0.249 1217.98 5705.2 637.6 R3-4 0.204 806.22 3905.1 604.67 R5-4 0.234 894.03 5189.9 699.71
300 R2-5 0.232 1246.58 5885.0 523.73 R3-5 0.196 887.38 4984.3 741.66 R5-5 0.231 1127.52 5677.5 1523 .45

Table 3. The concentration of Cd, Fe and SO 42- a. 2 m long channel, 25 mL /min flow rate and 5 -10 mm grain size; b. 2 m long channel, 25 mL /min flow rate and 20- 40
mm grain size; c. 2 m long channel, 25 mL /min flow rate and 5 -10 mm grain size .
Time
(min) a. Cd
(mg/L) Fe
(mg/L) SO 42
(mg/L) Ca2+
(mg/L) b. Cd
(mg/L) Fe
(mg/L) SO 42
(mg/L) Ca2+
(mg/L) c. Cd
(mg/L) Fe
(mg/L) SO 42
(mg/L) Ca2+
(mg/L)
0 R6-i 0.214 1381.63 5772.4 390.72 R7-i 0.241 1360.31 5271.8 452.28 R9-i 0.220 1359.68 5025.5 359.33
60 R6-1* 0.076 3.98 2942.2 697.58 R7-1 0.152 517.53 3030.2 487.13 R9-1** 0.053 3.48 3406.3 863.43
120 R6-2 0.138 462.18 2837.0 543.63 R7-2 0.221 686.49 4304.2 654.63 R9-2 0.123 505.82 3376.3 857.93
180 R6-3 0.227 619.43 2866.9 513.45 R7-3 0.197 781.61 4454.7 715.91 R9-3 0.178 764.98 3524.9 937.73
240 R6-4 0.211 623.43 2886.5 476.49 R7-4 0.211 931.95 4756.1 702.06 R9-4 0.196 769.64 2650.5 603.87
300 R6-5 0.213 675.18 3028.5 682.08 R7-5 0.235 1099.58 4830.7 709.04 R9-5 0.206 750.35 3447.3 678.21
360 R6-6 0.205 678.50 3166.9 608.08 R7-6 0.215 1059.66 4558.5 658.95 R9-6 0.216 755.67 3699.4 877.36
420 R6-7 0.209 729.06 3318.1 625.33 R7-7 0.221 1086.94 4942.0 628.81 R9-7 0.242 579.26 3660.3 956.87
480 R6-8 0.222 530.16 3716.5 675.92 R7-8 0.215 1066.98 4925.1 627.81 R9-8 0.223 697.86 4031.9 1042.01
540 R9-9 0.188 722.41 3606.5 618.48
600 R9-10 0.180 680.50 4248.4 962.35
660 R9-11 0.190 716.42 4098.9 958.26
720 R9-12 0.186 909.33 4638.7 864.64
* First occurrence time w as 71 min
**First occurrence time was 86 min

351

352

Figure 2. The pH value depending on the time, and (a) grain size (R6 -R7; (b) type of limestone (R6- R9); (c, d) water
flow rate (R2 -R3, R5 -R6); (e, f) length of the channel (R2 -R5, R3 -R6).

The SO 42- concentration decreased in the early
hours, followed by an upward trend. This increase is
due to the fact that CaCO 3 began to be less reactive,
therefore the gypsum that precipitated previously was dissolved. These findin gs were noticed in the
study of Offeduu et al., (2015) that used a limestone
column. In this study, two synthetic acidic solutions (H
2SO 4) were used, one with a higher concentration
of Fe (III) at pH=2 and the other with a higher concentration of Al at pH= 2 and 3.

By comparing the performance of the system
at two flow rates, it can be observed that the 25 mL/min is more efficient than the 50 mL/min flow
rate. On the other hand, with a longer drain, a greater
residence time, and an increased efficiency can b e
noticed ( McDonald et al., 2001; Sdiri et al., 2012 ). In
terms of channel length, analysing the f igures 2, 3, 4,
and 5 (c and d), it can be observed that the 2 m long
channel is more efficient than the 1 m channel (Figs.
2 to 5 (e and f) ). As a result of these experiments, it
has been shown that the 25 mL/min flow rate and the 2 m length are more suitable, and this regime
was used in the experiments R6, R7, and R9.
0 60 120 180 240 300 360 420 480 540 600 660 720
Time (min)2.02.53.03.54.04.55.05.56.06.57.0pHSandulesti
Geomal(b)
2 m long
25 mL/min
0 60 120 180 240 300 360 420 480
Time (min)2.02.53.03.54.04.55.05.56.06.5pH 5-10 mm
20-40 mm(a)
2 m long
25 mL/min
0 60 120 180 240 300
Time (min)2.53.03.54.04.55.05.56.06.5pH50 mL/min
25 mL/min(d)
1 m long
0 60 120 180 240 300 360 420 480
Time (min)2.53.03.54.04.55.05.56.06.5pH50 mL/min
25 mL/min(c)
2 m long
0 60 120 180 240 300 360 420 480
Time (min)2.53.03.54.04.55.05.56.06.5pH1 m
2 m(e)
25 mL/min
0 60 120 180 240 300
Time (min)2.53.03.54.04.55.05.56.06.5pH1 m
2 m(f)
50 mL/min

353 4. CONCLUSIONS

The limestone channel usage is a method
frequently applied in mining areas to reduce heavy
metal concentrations and to increase the pH value of
acid water. Taking into account the large number of
such areas in Romania (e.g. Apuseni Mountains,
Eastern Carpathians, etc. ), the results of this study
represent a basis for the developm ent of future, more
complex research in this field. The main objective of
this study was to determine the optimum parameters,
such as: drain length, flow rate, and limestone quantity in the treatment process of acid water from Rosia Montana.
As a result of the experiments it was observed
that the efficiency of water neutralization by using
limestone increases proportionally with the growth
of the drain length. Moreover, heavy metals (Fe, Cd)
and sulphate (SO
42-) removal is also increased.
Nevertheless, in addition to the length of the drain it
was noted that the flow rate is also important. Thus,
a low flow determines a higher efficiency of acidic
water treatment.

Figure 3. The concentration of Fe depending on time, and (a) grain size (R6 -R7); (b) type of limestone (R6 -R9); (c, d)
water flow rate (R2 -R3, R5 -R6); (e, f) length of the channel (R2 -R5, R3 -R6). 0 60 120 180 240 300 360 420 480
Time (min)-20002004006008001000120014001600Fe (mg/L) 5-10 mm
20-40 mm(a)
2 m long
25 mL/min
0 60 120 180 240 300 360 420 480 540 600 660 720
Time (min)-20002004006008001000120014001600Fe (mg/L) Sandulesti
Geomal(b)
2 m long
25 mL/min
0 60 120 180 240 300
Time (min)40050060070080090010001100120013001400Fe (mg/L) 50 mL/min
25 mL/min(c)
1 m long
0 60 120 180 240 300 360 420 480
Time (min)-20002004006008001000120014001600Fe (mg/L) 50 mL/min
25 mL/min(d)
2 m long
0 60 120 180 240 300
Time (min)-20002004006008001000120014001600Fe (mg/L) 1 m
2 m(e)
50 mL/min
0 60 120 180 240 300 360 420 480
Time (min)-20002004006008001000120014001600Fe (mg/L) 1 m
2 m(f)
25 mL/min

354

Figure 4. The concentration of Cd depending on time, and (a) grain size (R6 -R7); (b) type of limestone (R6 -R9); (c, d)
water flow rate (R2 -R3, R5 -R6); (e, f) length of the channel (R2 -R5, R3 -R6).

It can be concluded that the most important
factor in neutralizing the acid water is the contact
time between water and limestone. This time of
contact can be increased by using a longer channel, a
lower water flow rate, and a larger amount of
limestone. The limestone has the potential to
immobilize heavy metals like Cd and Fe in the first
couple of hours of interaction.
A fine r granulation (5- 10 mm) of the
limestone is more effective compared to a coarser granulation (20 -40mm), both for acidic water
neutralization, and for heavy metals or sulphate
removal from acidic water. This is due to the larger
specific surface area of the smaller grains.
The consumption of limestone can be
correlated with the decrease of SO 42-, also observed
in the laboratory experiments. The two types of
limestone used demonstrate similar behaviour. 0 60 120 180 240 300 360 420 480
Time (min)0.060.080.100.120.140.160.180.200.220.240.26Cd (mg/L) 5-10 mm
20-40 mm(a)
2 m long
25 mL/min
0 60 120 180 240 300 360 420 480 540 600 660 720
Time (min)0.040.060.080.100.120.140.160.180.200.220.240.26Cd (mg/L) Sandulesti
Geomal(b)
2 m long
25 mL/min
0 60 120 180 240 300
Time (min)0.170.180.190.200.210.220.230.240.250.26Cd (mg/L) 50 mL/min
25 mL/min(c)
1 m long
0 60 120 180 240 300 360 420 480
Time (min)0.060.080.100.120.140.160.180.200.220.240.26Cd (mg/L) 50 mL/min
25 mL/min(d)
2 m long
0 60 120 180 240 300
Time (min)0.170.180.190.200.210.220.230.240.250.26Cd (mg/L) 1 m
2 m(e)
50 mL/min
0 60 120 180 240 300 360 420 480
Time (min)0.060.080.100.120.140.160.180.200.220.24Cd (mg/L) 1 m
2 m(f)
25 mL/min

355

Figure 5. The concentration of SO 42- depending on time and (a) grain size (R6 -R7); (b) type of limestone (R6 -R9); (c, d)
water flow rate (R2 -R3, R5 -R6);; (e, f) length of channel (R2 -R5, R3 -R6).

Acknowledgements

The present contribution was financially supported by a
grant of the Romanian National Authority for Scientific
Research, CCCDI – UEFISCDI, project 3 -005 Tools for
sustainable gold mining in EU (SUSMIN). The authors
thank Holcim Romania for providing the average
mineralogical composition of limestone, and fo r the help
offered during the sample s collection .

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Received at: 28. 06. 2016
Revised at: 15. 12. 2016
Accepted for publication at: 29. 12. 2016
Published online at: 11. 01. 2017