One of the most important pollutants for soil and water is the
mining industry. Mining practices, present and past, cause environmental
problems that can damage ecosystems and human health. There may be
changes in landscapes (soil degradation and limitation of natural fresh
water resources), water composition, vegetation and animal habitats, as
well as air pollution as previously shown (Biermann et al., 2002). The
heavy metals, in particular, which are not only highly toxic but also
bio-accumulative and persistent, have long-term detrimental effects on
the environment throughout the food chain (WWF's European
Freshwater Programme, 1999).
Acid generation and metal dissolution are the primary problems
associated with pollution due to mining activities. Pyrite ([FeS.sub.2])
is responsible for starting acid generation and metal dissolution in
coal and metal mines as previously shown (Younger et al., 2002) by
Equations (1) to (4):
2[FeS.sub.2] +7[O.sub.2] + 2[H.sub.2]O [right arrow] 2[Fe.sup.+2] +
4S[O.sup.4-] +4[H.sup.+] (1)
[Fe.sup.2] + 1/2[O.sub.2] + 2[H.sup.+] [right arrow] 2[Fe.sup.3+] +
[Fe.sup.3+] + 3[H.sub.2]O [right arrow] Fe[(OH).sub.3] + [H.sup.+]
14[Fe.sup.3+] + [FeS.sub.2(s)] + [H.sub.2]O [right arrow]
2S[O.sub.4.sup.2-] + 15[Fe.sup.2+] + 16[H.sup.+] (4)
Specialized micro-organisms, e.g. Thiobacillus ferroxidans and
Ferroplasma acidarmanus can play a significant role in accelerating the
chemical reactions taking place in mine drainage and in the generation
of acidity (Macalady, 1998; Lauzon, 2002). Other sources of acidity
which to be taken into account are as follows:
[Al.sup.3+] 3[H.sub.2]O [right arrow] Al[(OH).sub.3] +3[H.sup.+]
[Fe.sup.2+] + 0.25 [O.sub.2(aq)] + 2.5 [H.sub.2]O [right arrow]
Fe[(OH).sub.3] + 2[H.sup.+] (6)
[Mn.sup.2+] + 0.25 [O.sub.2(aq)] + 2.5 [H.sub.2]O [right arrow]
Fe[(OH).sub.3] + 2[H.sup.+] (7)
At the same time, there are other metal sulphides similar to pyrite
that may release metal ions into solution but may not generate acidity
as Equation (8) shows (Younger et al., 2002).
[MeS.sub.(s)] + 2[O.sub.2(aq)] [right arrow] [Me.sup.2+] +
[CuFeS.sub.2(s)] + 4[O.sub.2(aq)] [right arrow] [Cu.sup.2+] +
[Fe.sup.2+] S[O.sub.4.sup.2-] (9)
where Me can be: Zn, Pb, Ni, Cd, Hg, Co and Cu.
The principal objective in waste water treatment is to eliminate or
reduce contaminants to permissible level that cause no adverse effects
on humans or receiving environment. Generally, the remediation of the
acid mine waste water requires pH adjustment, physical oxidation or
reduction, neutralization and heavy metal removal.
The traditional methods of removing heavy metals include
precipitation and co-precipitation of dissolved metals, foam flotation,
flocculation, sedimentation and ultra-filtration of suspended and
colloidal material, evaporation, bio-sedimentation, adsorption and ion
exchange of contaminants onto inorganic or organic solids, as previously
explained by many researches (Zamsow and Murphy, 1992; Nalco Water
Handbook, 1998; Cravotta and Trahon, 1999; Feng et al., 2000; Matlock et
Precipitation-neutralization is the major mechanism by which metals
are removed from water and deposited in sediments. Addition of alkaline
reagents as lime, limestone, ferrous salts or other compounds (e.g.
Mg[(OH).sub.2], MgC[O.sub.3], Ba[Cl.sub.2], Ca[Cl.sub.2]), is the common
method used to raise pH of acid water to alkaline values and to remove
heavy metals by precipitation (Morrison, 1998; Ott, 1998).
Neutralization of acid wastes (liquors or slurries) with lime before
discharge is the current preferred method, however, the volume of the
sludge produced requires large storage volumes.
Some low-cost materials (e.g. fly ash, china clay, red mud,
zeolites, phosphate) which can remove contaminants by chemical, and/or
physical mechanisms from effluents and waste waters have been studied by
a number of researchers (Bayat, 2000a, b; Rao et al., 1999, 2000; Hequet
et al., 2001; Gupta and Sharma, 2002; Kovatcheva-Minova et al., 2002;
Komnitsas et al., 2004).
Minerals are being commonly applied in environment treatment, based
on their properties, such as surface adsorption, porous filtration,
defects of their crystal structure, high ionic exchange capacity,
dissolution, hydration, and mineralogical-biological interactions
(Reichert and Binner, 1996; Vaughan and Wogelius, 2000; Singh et al.,
Among them, a particularly effective material for the remediation
of the waste water could be calcium-phosphate minerals, frequently
referred to as apatite. Phosphates are known as permeable reactive
barriers, recognized as good adsorbents for a variety of metal ions
coming from Al, Cd, Mn, Mg, Ni, Zn, Cu, Fe, Pb. Apatite can react with
many heavy metals, metalloids and radio nuclides to form secondary
phosphate precipitates with extremely low solubility as shown by many
researchers (Jeanjean et al., 1995; Chen et al., 1997; Gomez del Rio et
al., 2004; Wright et al., 2005; Chan Choi, 2006).
According to literature data (Ma et al., 1995; Cao et al., 2004;
Wright et al., 2005) three mechanisms of metal stabilization by apatite
are recognized. First of all the metal removal is controlled by apatite
dissolution, followed by the precipitation:
2[Ca.sub.5][(P[O.sub.4]).sub.3](OH) + 14[H.sup.+] [right arrow]
10[Ca.sup.2+] + 6[H.sub.2]P[O.sup.-.sub.4] + 2[H.sub.2] O (10)
[Me.sup.2+] + 6[H.sub.2]P[O.sup.-.sub.4] + 2[H.sub.2]O [right
arrow] [Me.sub.10][(P[O.sub.4]).sub.6][(OH).sub.2] + 14[H.sup.+] (11)
Reaction (10) does not necessary lead to Reaction (11). However,
the solubilized phosphate may force the precipitation of the metal ion
as a phosphate phase as Reaction (11) shows. The solubility of the
apatite seems to be the key to the effectiveness of this mechanism. At
the same time, in closed systems (batch tests) the rate of dissolution
of the apatite is strongly affected by the contaminant concentration
because the system approaches equilibrium. Alternatively, the metal may
be adsorbed to the apatite and subsequent precipitation of the metal
phosphate may occur. The third mechanism could be cation exchange of the
metal for calcium in apatite.
At the same time, apatite acts as an excellent buffer (buffers to
pH 5.5 to 6.5) for neutralizing acidity through P[O.sup.3-.sub.4] and
OH-groups, buffering to neutral pH alone and precipitating many metal
phases as shown by Wright et al. (2005).
The treatment of the acid mine drainage from settlement pond
"Sesei Valley" of the Rosia Poieni mining district, Romania,
have assumed greater importance because of increased environmental
pollution of Aries hydrographic basin. Rosia Poieni is a copper mining
field in Apuseni Mountains, NW Romania. The drainage which flows
directly from the settlement pond "Sesei Valley" into the
Aries River represents the main cause of pollution (Forray, 2002).
Seldom, chemical treatments with calcium hydroxide and lime carried out.
Because of its low pH, mine waste water can carry large amounts of
metals in solution and suspension.
The aim of the present work was to show the utilization usefulness
of apatite in the treatment of the acid mine waste water and to
establish the macro-kinetic mechanism of the process. The experimental
result in this study goes through the following stages: a) reduction of
heavy metal concentration using apatite as permeable reactant; b)
determination of the macro-kinetic mechanism in the neutralization
process of the acid mine waste water with apatite and c) mathematical
description of the process based on the kinetic models and the
determination of the kinetic parameters.
Materials and Methods
The samples of acid mine waste water analyzed in this work comes
from the settlement pond Sesei Valley. The flow of the waste water
ranges from 0.150 to 0.370 [m.sup.3] [s.sup.-1] during the year.
Quantities and concentrations of the drainage source are usually
characterized by low pH (1.6 - 2.8), acid concentration ranges from 3 to
5 kg [m.sup.-3] and high content of heavy metals (e.g. Cu = 14 - 21 g
[m.sup.-3], [Fe.sub.tot] = 49 - 176 g [m.sup.-3], Zn = 6 - 9 g
[m.sup.-3], Mn = 5 - 8 g [m.sup.-3]).
The composition of the used apatite (mineral apatite from Russia)
shows 39.1% [P.sub.2][O.sub.5], 56.0% CaO, 1.8% F, 2.6% Cl, 40%
[H.sub.2]O, 0.1% C[O.sub.2] and 12% residuum after the screening on the
0.15 mm screen.
The reaction between waste water and solid phase (apatite) has been
studied in batch experiments using a "ceramic ball mill"
reactor. The "ball mill" reactor shows the advantage to
realize the grinding of the apatite to increase the active area of the
solid thus increasing the efficiency of the neutralization process. The
ceramic ball mill has the internal diameter D = 0.19 m and the lenght L
= 0.12 m. The use of thermostated water, which flows through the jacket
of the ball mill, offers the possibility to control the temperature
within [+ or -] 0.5[degrees]C.
Removal of Heavy Metals
For the reduction of the heavy metal concentration, 5 samples with
a constant solid/liquid mass ratio of 1:41 and
ratios ranging from 1:2 to 1:6 were prepared. Each sample was mixed in
the "ball mill" reactor at a rotation speed n = 70 rpm for 60
min, contact time which was required to reach the optimal conversion
based on our kinetic experiments. This process was repeated three times
for each sample. After 60 min the collected samples from the reactor
were separated from the solid phase by filtration and the concentrations
of the metals were measured by a Varian atomic absorption
spectrophotometer SpectrAA-880 type with deuterium background
A filter cloth of polyamide PA 6.6 with a mean mesh size of 8
[micro]m was used for the filtration.
The pH values were measured by means of a digital pH-meter.
For the second stage, the kinetic experimental measurements were
carried out at three different temperatures near to the ambient
temperature, 291 K, 296 K and 298 K, with a constant solid/liquid ratio
of 1:41 and a constant ratio of [Ca.sub.5](P[O.sub.4])3F/
[Ca.sub.5][(P[O.sub.4]).sub.3](OH) of 1:6. The process evolution was
followed by the conversion of free sulphuric acid, determined by the
titration with NaOH solution 0.05 kmol [m.sup.-3]. Periodic 10
[cm.sup.3] suspension was collected ranging from 1 min to 60 min and
filtered from solid phase with a discontinuous filter apparatus. Three
replicate runs were carried out for each set of experimental conditions.
RESULTS AND DISCUSSION
The influence of the apatite
ratios on the neutralization efficiency and heavy metal removal was
based on the analysis of the metal concentration in the initial
untreated waste water sample [P.sub.0] and resulted treatment samples
([P.sub.1]-[P.sub.5]). The results are presented in Table 1.
The analysis of the initial sample P0 has shown that the
concentration of Cu is 190 times higher than MAC, 5.9 times higher for
Mn, 13.9 times higher for Zn and 16.5 higher for Fe.
The obtained results presented in Table 1 have shown that the use
of apatite in the treatment of acid mine waste water leads to the
decrease of metal concentration. The analysis of the treated samples has
shown an advanced purification, the concentration of the heavy metals
after the treatment of the waste water with apatite being 25 to 1000
times less than the Maximum Admitted Concentration by European
Regulations (NTPA 001/2005). For example, the analysis of the sample
[P.sub.5] has shown a decrease of the heavy metal concentration 735
times for Cu, 16.3 times for Mn, 581 times for Zn and 828 times for the
total Fe in comparison with the concentration of the initial sample.
Similar results were obtained using Apatite II as shown by Wright
et al. (2005). Apatite II was operated successfully, reducing for
example the concentrations of Pb 1657 times, Cu 164 times, Mn 427 times,
and Zn 750 times.
At the same time, it is obvious that apatite should have minimal
fluorine substitution in the hydroxyl position for metal reduction.
In order to establish the macro-kinetic mechanism of the global
neutralization process the average experimental kinetic results obtained
in "ball-mill" reactor for a solid/liquid ratio of 1:41 and
of 1:6 were plotted as shown in Figure 1. These ratios have provided
very good efficiency of the heavy metal removal and good sedimentation
As Figure 1 shows the conversion of [H.sub.2]S[O.sub.4] increases
rapidly in the first 10 min, up to the conversion [[eta].sub.H2SO4] =
0.25 - 0.30. After this period, the increase of the conversion is
slower. The change of the curvature after 10 min suggests the change of
the mechanism in the neutralization process.
[FIGURE 1 OMITTED]
The influence of the temperature on the neutralization rate has
been studied in the range 291 - 298 K. The kinetic measurements have
shown the positive effect of the temperature on the conversion.
To establish the mechanism of the neutralization process the
kinetic data presented in Figure 1 were preceded using the horizontal
section method (Calistru and Ifrim, 1987). In accordance with this
method for a constant conversion, the relative constant rate of the
process [bar.k] can be written as Equation (12):
[bar.k] = 1/[[tau].sub.i] (12)
Considering the influence of the temperature on the rate constant
[bar.k] (Arrhenius equation), by plotting In k vs. 1/T (Figure 2), a
linear relationship is obtained and one can determine the activation
energy Ea from the slope (-Ea/R).
k = [k.sub.0] x [e.sup.-] Ea/RT (13)
Using regression analysis a standard deviation of the obtained
activation energy between 2.54 and 10.45 kJ [mol.sup.-1] and a
correlation coefficient between 0.986 and 0.998 were found.
By the values of the activation energy the mechanism of the process
was estimated. The obtained values Ea >> 42 kJ [mol.sup.-1] (e.g.
Ea = 115.50 [+ or -] 7.50 kJ [mol.sup.-1] for a conversion of sulphuric
acid [[eta].sub.H2SO4] = 0.05, Ea = 60.90 [+ or -] 9.50 kJ [mol.sup.-1]
for [[eta].sub.H2SO4] = 0.10 and Ea = 55.75 [+ or -] 10.45 kJ
[mol.sup.-1] for [[eta].sub.H2SO4] = 0.15) suggest that up to a
conversion [[eta].sub.H2SO4] = 0.15 the determining process is the
transformation process, adsorption followed by reaction, which means
surface--controlled reactions as previously shown by Sparks (1995).
At a conversion of sulphuric acid [[eta].sub.H2SO4] > 0.15, the
obtained values of activation energy are Ea < 42 kJ/[mol.sup.-1] (Ea
= 37.55 [+ or -] 4.05 kJ [mol.sup.-1] for conversion [[eta].sub.H2SO4] =
0.2, Ea = 37.54 [+ or -] 2.54 kJ [mol.sup.-1] for [[eta].sub.H2SO4] =
0.3 and Ea = 37.44 [+ or -] 2.90 kJ [mol.sup.-1] for [[eta].sub.H2SO4] =
0.4) indicating diffusion - controlled processes. This means a combined
process model, which involves the transfer in the liquid phase followed
by the chemical reaction at the surface of the solid as previously shown
by Sparks (1989, 1995).
Based on the shrinking core model and on the values of the
activation energy previously determined, the mathematical model
appropriate to the macro-kinetic transformation process, as well as to
the combined process (transfer-transformation) can been written as
Equations (14) and (15):
[FIGURE 2 OMITTED]
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (14)
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (15)
From the plot F([eta]) = [1-[(1-0.
56[[eta].sub.H2SO4]).sup.1/3]]/(1/[[eta].sub.H2SO4]) vs. time the
kinetic parameters of the process, rate coefficient of reaction
[k.sub.r] and transfer coefficient [k.sub.t], were determined (Figure
3). It is necessary to mention that transfer refers to movement of
sulphuric acid to the external surface of apatite and not the
diffusivity of the liquid reactant along the wall surfaces in a
The linear dependence obtained in both cases confirms the
"transformation" model at first of the neutralization process,
and the "transfer-transformation" model after 10 min for all
experimental temperatures. A good linearity was obtained in both cases,
a correlation coefficient between 0.977 and 0.996 was found.
From the slope of the plot the rate coefficient of reaction
[k.sub.r] with values ranging from (5.02 [+ or -] 1.62) [10.sup.-4] to
(8.00 [+ or -] 1.55) [10.sup.-4] ([s.sup.-1]) was obtained. It is
obvious that within the first 10 min the global process already changes
its mechanism. The new macro-kinetic mechanism is the
"transfer-transformation", with the mathematical Equation
(15). The transfer coefficient in liquid phase [k.sub.t] ranging from
(8.40 [+ or -] 0.50) [10.sup.-5] to (10.42 [+ or -] 0.65) [10.sup.-5] (m
[s.sup.-1]) was determined from the slope of the linear dependence
obtained for the experimental measurement after 10 min.
[FIGURE 3 OMITTED]
It was shown that the treatment of the acid mine waste water with
apatite is effective for enhanced removal of metals. Using apatite in
the neutralization and heavy metal removal of the acid mine waste water
a significant decrease of the metal concentration can be obtained, under
the permissible level acknowledged by the European Norms (NTPA 001/2005)
(Table 1). The rapid decrease of the heavy metal concentration in the
acid mine waste water is determined by the presence of phosphoric acid
resulted in the reaction between apatite and sulphuric acid (Reaction
(10)). The precipitation Reaction (11) instantaneous takes into account
that it is an ionic reaction. This means that the rate determining step
in the progress of the global process is the reaction between apatite
and sulphuric acid.
By the values of the activation energy the mechanism of the process
was estimated. The obtained values of the activation energy Ea > >
42 kJ [mol.sup.-1] indicate surface--controlled reaction, which
corresponds to the transformation model. The obtained values below Ea
< 42 kJ [mol.sup.-1] indicate diffusion in the liquid
phase--controlled process, which corresponds to a combined model.
According to the experimental data the mechanism of the process was
estimated. Kinetic parameters [k.sub.r] with values ranging from (5.02
[+ or -] 1.62) [10.sup.-4] to (8.00 [+ or -] 1.55) [10.sup.-4]
([s.sup.-1]) and [k.sub.t] ranging from (8.40 [+ or -] 0.50) [10.sup.-5]
to (10.42 [+ or -] 0.65) [10.sup.-5] (m [s.sup.-1]) were determined.
The use of apatite in the neutralization of the acid mine waste
water leads to total elimination of the sterile storehouse and the
resulted precipitate rich in phosphate could be used as fertilizer with
microelements (Fe, Mn, Mg, Cu, etc.). At the same time the waters
subjected to treatment with apatite can be recycled.
Manuscript received May 9, 2006; revised manuscript received
November 20, 2006; accepted for publication March 1, 2007.
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Adina L. Ghirisan (1) *, Simion Dragan (1), Alexandru Pop (1),
Marinela Simihaian (2) and Vasile Miclaus (1)
(1.) Faculty of Chemistry and Chemical Engineering, Babes-Bolyai
University, 11 Arany Janos, 400028 Cluj-Napoca, Romania
(2.) Environmental Protection Agency, 99 Calea Dorobantilor, 400609
* Author to whom correspondence may be addressed.
E-mail address: email@example.com
Ea activation energy (kJ [mol.sup.-1])
R gas constant (= 8.314 J [mol.sup.-1] [K.sup.-1])
[k.sub.r] rate coefficient of reaction ([s.sup.-1])
[k.sub.t] transfer coefficient (m [s.sup.-1])
[C.sup.0.sub.H2SO4] acid concentration (kmol [m.sup.-3])
r particle radius (m)
[S.sub.gr] particle surface ([m.sup.2])
[S.sub.r] reaction surface ([m.sup.2])
[v.sub.ap] stoichiometric coefficient of apatite (-),
[v.sub.H2SO4] stoichiometric coefficient of [H.sub.2]
[rho]* molar density (kmol [m.sup.-3]),
[[eta].sub.H2SO4] conversion of sulphuric acid (-),
[tau] time (s)
Table 1. Concentration of the heavy metals
Concentration (g [m.sup.-3])
Sample apatite [Cr.sub.tot]
[P.sub.0] -- 0.071 [+ or -] 0.014
[P.sub.1] 1:2 0.062 [+ or -] 0.015
[P.sub.2] 1:3 0.040 [+ or -] 0.008
[P.sub.3] 1:4 0.010 [+ or -] 0.004
[P.sub.4] 1:5 0.014 [+ or -] 0.007
[P.sub.5] 1:6 0.016 [+ or -] 0.007
Sample Pb Cd
[P.sub.0] 0.024 [+ or -] 0.003 0.111 [+ or -] 0.012
[P.sub.1] < 0.005 0.011 [+ or -] 0.005
[P.sub.2] < 0.005 0.011 [+ or -] 0.005
[P.sub.3] < 0.005 0.014 [+ or -] 0.005
[P.sub.4] < 0.005 0.003 [+ or -] 0.001
[P.sub.5] < 0.005 0.007 [+ or -] 0.003
MAC 0.200 0.200
Sample Cu Mn
[P.sub.0] 19.13 [+ or -] 0.85 5.951 [+ or -] 0.150
[P.sub.1] 4.143 [+ or -] 0.630 0.240 [+ or -] 0.040
[P.sub.2] 0.080 [+ or -] 0.005 0.407 [+ or -] 0.075
[P.sub.3] 0.013 [+ or -] 0.002 0.316 [+ or -] 0.055
[P.sub.4] 0.032 [+ or -] 0.003 0.421 [+ or -] 0.073
[P.sub.5] 0.026 [+ or -] 0.005 0.365 [+ or -] 0.025
MAC 0.100 0.100
Sample Ni Zn
[P.sub.0] 0.220 [+ or -] 0.015 6.971 [+ or -] 0.175
[P.sub.1] 0.188 [+ or -] 0.013 0.798 [+ or -] 0.040
[P.sub.2] 0.006 [+ or -] 0.001 0.005 [+ or -] 0.001
[P.sub.3] < 0.005 0.002 [+ or -] 0.001
[P.sub.4] < 0.005 0.002 [+ or -] 0.001
[P.sub.5] < 0.005 0.012 [+ or -] 0.001
MAC 0.500 0.500
Sample [Fe.sub.tot] Co
[P.sub.0] 82.85 [+ or -] 0.85 0.761 [+ or -] 0.155
[P.sub.1] 4.905 [+ or -] 0.098 0.003 [+ or -] 0.001
[P.sub.2] 0.218 [+ or -] 0.007 0.008 [+ or -] 0.003
[P.sub.3] 0.069 [+ or -] 0.005 0.044 [+ or -] 0.007
[P.sub.4] 0.197 [+ or -] 0.030 0.005 [+ or -] 0.001
[P.sub.5] 0.010 [+ or -] 0.004 0.003 [+ or -] 0.001
MAC 0.500 0.100
MAC is the Maximum Admitted Concentration by European Regulations