Proceedings of the 13 th International Conference of Environmental Science and Technology Athens, Greece, 5-7 September 2013 REMOVAL OF METAL IONS FROM ACIDIC SOLUTIONS USING PEAT A LOW COST SORBENT MARIAN HOLUB 1, MAGDALENA BALINTOVA 1 and PETRA PAVLIKOVA 1 1 Technical University of Kosice, The Faculty of Civil Engineering, Institute of Environmental Engineering, Vysokoskolska 4, 042 00 Kosice, Slovak Republic e-mail: marian.holub@tuke.sk EXTENDED ABSTRACT Acid mine drainage is one of the most significant environmental problem resulting from the oxidation of pyrite in presence of water, air and microorganisms, providing an acidic solution that contains heavy metal ions. Sorption techniques are widely used to remove heavy metal ions from different water solutions, because they are highly effective and low cost. The main objective of this study was to remove metal ions from acidic solutions by using natural non modified peat, a low cost sorbent. Sorption of copper and zinc in singlecomponent aqueous systems was investigated. In order to simulate real condition of acid mine drainage, acidic solutions (ph approx. 4) were used. The removal of metals was carried out in batch experiments as a function of initial concentration and contact time. The sorption isotherm data from experiments were fitted to the relevant model. The results indicate that there is significant potential for peat as an adsorbent material for heavy metals removal under acidic conditions. Keywords: peat, sorption, acidic solutions 1. INTRODUCTION Acid mine drainage (AMD) is a serious current environmental problem. The occurrence of AMD is usually associated with mining activities, particularly after closure and abandonment of mining works. This situation also occurred in Slovak Republic, where the mining activities had a long tradition and now there are some localities with existing AMD generation conditions. The main characteristic of such waters is their low ph and high content of heavy metals as a result of weathering of remnant sulfide minerals, e.g. pyrite, chalcopyrite, tetrahedrite, tennantite or gersdorffite (Samesova et al., 2009). These heavy metals are non biodegradable, tend to accumulate in living organisms bodies and causing various disorders (Zheng et al., 2008). Penetration into internal organs is caused by their high dissociation and easy solubility, which allows simple transition through cell membranes (Andras et al., 2012). There are different ways of the removal of toxic metals from AMD, including chemical precipitation (Matlock et al. and Hammarstrom et al., 2002; 2003), neutralization (Doya et al., 2003), ion exchange (Feng et al., 2000), reverse osmosis, hydrolysis (Chartrand et al., 2003) and sorption (Mohan et al., 2006). Among above listed various water treatment techniques, sorption is generally preferred due its high efficiency, easy handling, availability of various sorbents and low cost. The sorption of pollutants from aqueous solutions plays an important role also in the treatment of other wastewaters or in drinking water treatment. In recent years there has been considerable interest in the use of lowcost sorbents such as peat (Ho et al., 2000), lignite (Mohan et al., 2006) or olive stone (Aziz et al., 2009). In this work, the study of using of natural peat as a sorbent for copper and zinc removal from model acidic solutions is presented. The experiments were carried out for the purpose of the follow parameters investigation: equilibrium time establishment, isotherm
modeling, adsorption capacity of peat in dependence on initial concentration, ph change and removal efficiency. To simulate AMD conditions experiments were realized with acidic solutions (ph 4). 2. MATERIAL AND METHODS 2.1. Adsorbent As a sorbent was used commercial peat PEATSORB (REO AMOS Slovakia). In experiments was used finer fraction which was prepared by sieving through a 2 mm sieve. After sieving, the separated fraction of peat was dried at 105 C for 2 h and then allowed to cool in the desiccators. 2.2. Apparatus and instrumentation A Colorimeter DR890 (HACH LANGE, Germany) with appropriate reagents was used to determine concentration of dissolved Cu(II). Determination of Zn(II) concentration was carried out by flame AAS method (SpectrAA-30, Varian Australia) ph were determined by ph meter inolab ph 730 (WTW, Germany) which was standardized using buffer solutions of different ph values (4.01, 7.00). 2.3. Adsorbate solutions preparation Concentrated stock solutions of heavy metals were prepared by dissolution of copper sulfate pentahydrate and zinc sulfate heptahydrate in deionized water. Working solutions were prepared by dilution to the desired initial concentration of appropriate heavy metal. The initial ph of each solution was adjusted to the required value (ph=4.1 4.2) by adding 0.001M H 2SO 4. It should be noted that sulphate anions are not forming precipitates or complexes with copper and zinc cations at the test conditions and are considered to be inert. 2.4. Batch adsorption experiments Batch experiments were carried out at room temperature (23±0.2 C) in beakers by adding of a constant mass of peat (1.0 g) in 100mL of metal solution (Cu(II) or Zn(II)). In order to define the time required to establish equilibrium, peat was mixed at 500 rpm with 100 ml of model acidic solution with 50 mg/l concentration of appropriate metal for different contact time (1, 3, 5, 7, 10, 30 and 60 minutes). After that, the solid was removed by filtration through a laboratory filter paper for qualitative analysis. In order to obtain data for isotherms study, the various solutions with initial Cu(II) and Zn(II) concentrations ranging from 10 to 90 mg/l (10, 20, 30, 50, 70 and 90) were prepared. Adsorbates were mixed with 1 g of adsorbent for the time required to establish equilibrium. After that adsorbents were removed by filtration and equilibrium concentrations were determined. In all experiments, measurement of ph at the beginning and at the end of process was also done. The copper and zinc adsorption capacities were calculated by the following equation: ( c ce q 0 ) V, (1) m where q, adsorption capacity per unit mass of peat (mg/g), c 0 is the initial concentration of metal ions (mg/l); c e, equilibrium concentration of metal ions (mg/l); V, the volume of the aqueous phase (L) and m, mass of peat (g). The % heavy metal removal was calculated using the following equation: ( c0 ce ) % metal ion removal 100% (2) c 0
3. THEORETICAL BACKGROUNG 3.1. Adsorption isotherm Two of the most commonly used isotherm theories have been adopted in this paper, namely, the Freundlich and Langmuir equilibrium isotherm theories. The Freundlich equation has been widely used for isothermal adsorption. This is a special case for heterogeneous surface and her form can be represented by the following equations (3, 4): q e kc e (3) or in the linear form: 1 lg qe lg k lg c e, 4) n where k represents the sorption capacity when metal equilibrium concentration equals to 1; c e, the equilibrium concentration of remaining metal in the solution (mg/l); q e, the amount of adsorbed per unit weight (mg/g) and n, Freundlich constants related to adsorption intensity. Langmuir s isotherm model is valid for monolayer adsorption onto a surface containing a finite number of identical sites. This model is based on the assumption that a maximum adsorption corresponds to a saturated monolayer of solute molecules on the adsorbent surface, that the energy of adsorption is constant, and that there is no transmigration of adsorbate in the plane of the surface (Zheng et al., 2008). This model is represented as follows (Langmuir, 1918): Qmax bce qe (5) 1 bce or in the linear form: 1 1 1, (6) qe Qmax bce Qmax where q e is the amount of a metal adsorbed per mass unit of sorbent at equilibrium (mg/g); Q max, the amount of adsorbate at complete monolayer (mg/g) and b (L/mg) is the Freundlich constant that relates to the heat of adsorption. 4. RESULTS AND DISCUSSION 4.1. ph change ph is one of the most important parameters controlling uptake of heavy metals from aqueous solutions. Correct ph adjustment can improve efficiency of sorption. On the other hand ph increase can lead to precipitation of metal, which was the main reason of the ph measurements in filtrates after sorption. Resulting from Figure 1, using of peat in conditions of higher concentrations of copper or zinc cations is connected with ph decrease of the tested samples. This fact is due to releasing of hydrogen ions from turf brush into solution (Ho et al., 2000). The higher the initial concentration, the stronger is ph decrease, because the ion exchange is more intensive. At lower concentrations the ion exchange isn't so dominant and can't cause decreasing of the ph value. In this range increasing of ph is caused by alkaline effect of peat, which was confirmed by control measurements with deionized water. 1 n
Figure 1. The ph changes of the filtrates after sorption with different initial concentration. 4.2. Study of adsorption kinetics The approach to equilibrium for the batch mode sorption of copper and zinc onto peat is shown in Figure. 2. The most intensive metal uptake occurred within first 10 minutes. The first intensive concentration decrease is followed by further slow drop in the first hour. The equilibrium is established for 4 hours. Resulting from the fact that copper is better adsorbable than zinc, the establishment of equilibrium is in the case of copper reached earlier. Figure 2. Comparison of batch removal rate of copper and zinc cations onto peat. Initial ph=4; c 0=50 mg/l; c t/c 0 current and initial concentrations, respectively. 4.3. Study of adsorption isotherms The sorption data were analyzed using programs DataFit 9.0.59 and Origin Pro 9.0. Results of calculations are shown in the Table 1. The equilibrium adsorption data better correlated with Langmuir isotherm model for both tested metal cations. From the values in the table and also from the Figures 3. and 4. is clearly evident, that the sorption can be better described with Langmuir model. The fit for copper sorption was obtained with regression coefficient r 2 =0.9923, for zinc sorption is r 2 =0.9654.
Table 1. Freundlich and Langmuir isotherm parameters. Metals Freundlich isotherm constants Langmuir isotherm constants k 1/n r 2 Qmax b r 2 Cu 2+ 0.841 0.529 0.9587 7.534 0,0688 0.9923 Zn2+ 0.868 0.313 0.9548 3.495 0.119 0.9654 Figure 3. Comparison of adsorption isotherms of copper. Figure 4. Comparison of adsorption isotherm of zinc Using the Langmuir model, the maximum sorption capacity for the metals can be estimated as: Cu (7.534 mg/g)>zn (3.495 mg/g). The k values from Freundlich model, reported in Table 2., can be used to indicate the relative sorption capacity of peat. These values show the same trend as that Q max for the metals studied. Based on the obtained parameter 1/n, can be concluded that both isotherms have non-linear course (1/n=1 is the value for linear isotherm). 5. CONCLUSIONS This study showed the possibility of peat utilization for Cu and Zn removal from model acidic solutions. From kinetic data was found, that the equilibrium was reached for 4
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