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Heavy metal removal and neutralization of acid mine waste water--kinetic study.
Abstract:
The influence of the apatite on the efficiency of neutralization and on heavy metal removal of acid mine waste water has been studied. The analysis of the treated waste water samples with apatite 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 Concentration Limits admitted by European Norms (NTPA 001/2005). In order to establish the macro-kinetic mechanism in the neutralization process, the activation energy, Ea, and the kinetic parameters, rate coefficient of reaction, [k.sub.r], and [k.sub.t] were determined from the experimental results obtained in "ceramic ball-mill" reactor. The obtained values of the activation energy 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 of [H.sub.2]S[O.sub.4] equal 0.15 the global process is controlled by the transformation process, adsorption followed by reaction, which means surface--controlled reactions. At a conversion of sulphuric acid [[eta].sub.H2SO4] > 0.15, the obtained values of activation energy Ea < 42 kJ [mol.sup.-1] (e.g. Ea = 37.55 [+ or -] 4.05 kJ [mol.sup.-1] for [[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) indicate 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. Kinetic parameters as 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]) and transfer coefficient, [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.

On a etudie l'influence de l'apatite sur l'efficacite de neutralisation et le retrait de metaux lourds de l'eau usee de mines acides. L'analyse des echantillons d'eaux usees traitees par l'apatite montre une purification avancee, la concentration de metaux lourds avec le traitement de l'eau usee par l'apatite etant de 25 a 1000 fois moindre que les limites de concentration maximales admises par les normes europeennes (NTPA 001/2005). Afin d'etablir le mecanisme macro-cinetique dans le procede de neutralisation, l'energie d'activation, Ea, le coefficient de vitesse de reaction des parametres cinetiques, [k.sub.r], et le coefficient de transfert, [k.sub.t], ont ete determines a partir des resultats experimentaux obtenus dans un reacteur << broyeurs a boulets en ceramique >>. Les valeurs obtenues pour l'energie d'activation Ea > > 42 kJ [mol.sup.-1] (p. ex., Ea = 115,50 [+ or -] 7,50 kJ/[mol.sup.-1] pour la conversion de [[eta].sub.H2SO4] 0,05, Ea = 60,90 [+ or -] 9,50 kJ/[mol.sup.-1] pour la conversion de [[eta].sub.H2SO4] 0,10 et Ea 55,75 [+ or -] 10,45 kJ [mol.sup.-1] pour las conversion de [[eta].sub.H2SO4] 0,15), suggerent que jusqu'a une conversion de [[eta].sub.H2SO4] egale a 0,15, le developpement de procede important est le procede de transformation, l'adsorption, suivi par la reaction, qui correspond a des reactions controlees a la surface. A une conversion de [H.sub.2]S[O.sub.4] > 0,15, les valeurs obtenues pour l'energie d'activation Ea < 42 kJ/[mol.sup.-1] (p. ex, Ea = 37,55 [+ or -] 4,05 kJ/[mol.sup.-1] pour la conversion de [[eta].sub.H2SO4] 0,2, Ea = 37,54 [+ or -] 2,54 kJ/[mol.sup.-1] pour la conversion de [[eta].sub.H2SO4] 0,3 et Ea = 37,44 [+ or -] 2,90 kJ/[mol.sup.-1] pour la conversion de [[eta].sub.H2SO4] 0,4) ce qui correspond a des procedes controles par la diffusion. Il s'agit donc d'un modele de procedes combines, qui implique le transfert dans la phase liquide suivi d'une reaction chimique a la surface du solide. Les parametres cinetiques comme le coefficient de vitesse de reaction ([k.sub.r]) avec des valeurs comprises entre (5,02 [+ or -] 1,62) [10.sup.-4] et (8,00 [+ or -] 1,55) [10.sup.-4] ([s.sup.-1]) et le coefficient de transfert ([k.sub.t]) compris entre (8,40 [+ or -] 0,50)[10.sup.-5] et (10,42 [+ or -] 0,65) [10.sup.-5] (m/[s.sup.-1]), ont ete determines.

Keywords: apatite, heavy metal removal, neutralization, acid mine waste water, and macro-kinetic mechanism

Authors:
Ghirisan, Adina L.
Dragan, Simion
Pop, Alexandru
Simihaian, Marinela
Miclaus, Vasile
Pub Date:
12/01/2007
Publication:
Name: Canadian Journal of Chemical Engineering Publisher: Chemical Institute of Canada Audience: Academic Format: Magazine/Journal Subject: Engineering and manufacturing industries Copyright: COPYRIGHT 2007 Chemical Institute of Canada ISSN: 0008-4034
Issue:
Date: Dec, 2007 Source Volume: 85 Source Issue: 6
Accession Number:
192102159
Full Text:
INTRODUCTION

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+] + [H.sub.2]O (2)

[Fe.sup.3+] + 3[H.sub.2]O [right arrow] Fe[(OH).sub.3] + [H.sup.+] (3)

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.+] (5)

[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+] + S[O.sub.4.sup.2-] (8)

[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 al., 2002).

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., 2001).

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.

EXPERIMENTAL

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 [Ca.sub.5][(P[O.sub.4]).sub.3] F/[Ca.sub.5][(P[O.sub.4]).sub.3](OH) 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 correlation.

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.

Kinetic Measurements

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 [Ca.sub.5][(P[O.sub.4]).sub.3]F/[Ca.sub.5][(P[O.sub.4]).sub.3](OH) 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 [Ca.sub.5][(P[O.sub.4]).sub.3]F/[Ca.sub.5][(P[O.sub.4]).sub.3](OH) ratio 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 rate.

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 particle.

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]

CONCLUSIONS

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.

NOMENCLATURE

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 Cluj-Napoca, Romania

* Author to whom correspondence may be addressed.

E-mail address: ghirisan@chem.ubbcluj.ro
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])

Greek Symbols

[v.sub.ap]           stoichiometric coefficient of apatite (-),
[v.sub.H2SO4]        stoichiometric coefficient of [H.sub.2]
                     S[O.sub.4] (-),
[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])

                    F:OH
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
MAC                                         1.000

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
(NTPA 001/2005).
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