Chromium has been commonly applied. The use of ladle furnace slag for the removal of hexavalent chromium from an aqueous solution.

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1 Metall. Res. Technol. 113, 606 (2016) c EDP Sciences, 2016 DOI: /metal/ Metallurgical Research & Technology The use of ladle furnace slag for the removal of hexavalent chromium from an aqueous solution Ankica Radenović 1, Gordana Medunić 2 and Tahir Sofilić 1 1 University of Zagreb, Faculty of Metallurgy, Sisak, Croatia radenova@simet.hr 2 University of Zagreb, Faculty of Science, Zagreb, Croatia Key words: Ladle furnace slag; Cr(VI) adsorption; isotherms; kinetics; thermodinamics Received 27 January 2016 Accepted 30 June 2016 Abstract The ladle furnace slag is a by-product of the steel industry. Its adsorption potential for the hexavalent chromium ions from aqueous solutions was examined, and the results are presented in this paper. The equilibrium modeling of an adsorption proces was carried out and the equilibrium parameters were determined. The value of the maximum removal of the hexavalent chromium ions was mg/g at ph 4.3 and temperature of 293 K. The dynamics of an adsorption process was studied, and the values of rate constant of the adsorption, rate constant of intraparticle diffusion, and mass transfer coefficient were calculated. It was determined that the Cr(VI) adsorption mechanism is complex, while the film diffusion, and the intra-particle diffusion both contributed to the rate-determining step. Regarding the thermodynamic parameters, i.e. free energy change, enthalpy change, and entropy change, it was revealed that the Cr(VI) adsorption onto the ladle furnace slag was endothermic and spontaneous. Chromium has been commonly applied in various industrial processes. Herewith, its large quantities are being discharged into the environment. In aqueous systems, the chromium can be found mainly as Cr(III) and Cr(VI). At low concentrations, Cr(III) can be considered a bioelement due to its important role in the metabolism of plants and animals. Quite contrary, Cr(VI) is hazardous due to its strong oxidability following the absorption through the skin. As has been reported, the exposure to excessive amounts of Cr(VI) may cause dermatitis, diarrhea, haemorrhaging, and cancer in the digestive tract and lungs [1]. Due to this, state governments apply the enhanced regulation for chromium species. In Croatia, the upper limit for the discharge of Cr(VI) into the wastewater is set at 0.1 mg/l [2]. The adsorption process represents an effective and versatile method for the removal of heavy metals from water, particularly in combination with appropriate regeneration steps. This solves the waste disposal problem, whilst rendering the system more economically viable, especially if low-cost adsorbents are used. During the adsorption process development, choosing a proper adsorbent amongst various candidates is important starting point. Activated carbon is the most utilized and studied adsorbent used for the contaminant removal from an aqueous solution. However, its high cost and relatively low capacity for Cr(VI) [3]hasled to intensive search for low-cost adsorbents, such as any industrial by-product and waste material (e.g. slag, carbon anode dust, fly ash, blast furnace sludge, etc.) [4 7]. Since the metal extraction, refining and alloying processes result in large quantities of various metallurgical slags, and due to increasingly strict environmental regulations, their recycling and utilization could be an alternative solution regarding environmental pollution [8]. The main slag types generated from the iron and steelmaking industries are classified as follows: (i) blast-furnace slag (ironmaking slag), and (ii) steel-furnace slag: (a) basic-oxygen-furnace (BOF) slag, (b) electric-arc-furnace (EAF) slag, and (c) ladle Article published by EDP Sciences

2 furnace slag (LF). The LF slag is produced in the final stages of steelmaking, when the steel is desulphurized in the transport ladle, during what is generally known as the secondary metallurgy process. About 10 13% ofthesteelmakingindustryslagamounthas been generated as the secondary metallurgical slag or the ladle furnace slag [9]. The industrial secondary materials can be used for various applications, including the construction and civil engineering. The LF slag is also known as basic, refining, reducing, and as the white slag. It is a by-product of the ladle refinement, typical for all steelmaking plants. Regarding the metal removal applications from water and wastewater, utilization of the metallurgical industry slag may be preferable due to their large quantity and low cost [10 12]. Since there are no data on the use of the LF slag for the Cr(VI) removal from an aqueous solution, the present adsorption study demonstrates the possibility of its use for the relevant problem by means of an experiment on a synthetic aqueous solution. Moreover, the adsorption isotherm models and thermodynamic parameters were investigated so as to envisage the adsorption behavior. The kinetic experiments were also conducted to determine the rate of the Cr(VI) adsorption onto the LF slag. Previous work [13]wasfocusedonthe determination of the LF eco-toxicity by examining the composition of its eluate. The results showed that the LF slag does not contain any constituent which might adversely affect the environment. In order to examine adsorption potential of the LF slag, as nonhazardous industrial waste, and to solve its permanent disposal and/or recovery, bearing in mind both the volumes formed in the Croatian steel industry and experiences of developed industrial countries, a study of its adsorption ability for toxic Cr(VI) was undertaken. 1 Materials and methods 1.1 Preparation and characterization of the LF slag The testing was conducted on the LF slag generated during the production of carbon unalloyed steel produced in Croatia. For analysis, a representative sample of the LF slag was dried at 105 Cfor2handsievedto particle size of mm. The chemical composition of the LF slag was determined using Energy Dispersive Spectrometry (EDS). The mineralogical composition of the LF slag sample was determined by X-ray diffraction method using a Philips PW3710 X-ray diffractometer (Netherland) equipped with CuKα radiation and a graphite monochromator. The surface area properties were determined by the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) method using a Micromeritics ASAP 2000 (USA) adsorption instrument. 1.2 Adsorption experiments The batch mode of operation was selected in order to measure the progress of the Cr(VI) removal by the LF slag samples as adsorbents. The stock solutions were prepared by dissolving 2.8 g of K 2 Cr 2 O 7 in 1 L deionized water, thus obtaining a concentration of 1000 mg/l. All experiments were conducted in a 100 ml flask with 50 ml of Cr(VI) solution prepared from the dilution of 1000 mg/l stock solution at a certain ph (4.3), and the initial concentrations mg/l. Then a certain amount (0.2 g) of the LF slag sample was added to the solution, and the resulting suspension was shaken at the speed of 60 rpm for a predefined time. Throughout the experiment, the contact time varied from 30 to 240 min. For the adsorption isotherm study, Cr(VI) solutions of different concentrations were agitated with the known amount of adsorbent till the equilibrium was achieved (3 h). The residual Cr(VI) concentration of the solution was then determined. The effect of the solution temperature on the adsorption process was studied by determining the adsorption isotherms at three different temperatures (293, 313, and 333 K). The adsorption kinetics was determined by analyzing the adsorptive uptake of the Cr(VI) concentrations of 50 mg/l and 70 mg/l fromaqueoussolutionatdifferent time intervals (from 30 to 240 min). Following the reaction period, the suspensions were filtered through a Whatman filter paper No. 44, and the supernatant 606-page 2

3 was analyzed for the Cr(VI) concentration. The violet color, formed as a result of the reaction between the Cr(VI) ions and 1,2- diphenylcarbazide in acidic medium, was measuered using the Camspec M-107, Jencons (UK) spectrophotometer at 540 nm. 1.3 Data analysis The adsorption equilibrium data modelling The amount adsorbed at equilibrium, i.e. the adsorption capacity, q e (mg/g) was calculated according to the formula: q e = Δc m V (1) where q e is adsorption capacity, mg/g; Δc is quantity of adsorbed Cr(VI), mg/l (Δc = c i c e, c i initial concentration of Cr(VI), mg/l, c e equilibrium concentration of Cr(VI), mg/l; V is volume of solution, L; m is adsorbent mass, g). In order to examine the adsorption capacity of the LF slag, the experimental data points were fitted to the Langmuir, Freundlich, Temkin, and Dubinin- Radushkevich (D-R) equations applicable to the adsorption process. The Langmuir adsorption parameters were determined by transforming the Langmuir equation into the linear form [14]: c e q e = c e q m + 1 q m K L (2) where q e is adsorption capacity, mg/g, c e is the equilibrium concentration of Cr(VI), mg/l, q m is saturation adsorption capacity of the LF slag, mg/g, and K L is Langmuir constant. Linearized form of the Freundlich isotherm in the equation was represented by [15]: ln q e = ln K F + 1 n ln c e (3) where q e is adsorption capacity, mg/g, c e is the equilibrium concentrations of Cr(VI), mg/l, K F and n are the Freundlich constants. The Temkin isotherm was applied in the following form [16]: q e = B T ln K T + B T ln c e (4) where K T (L/mg) is the equilibrium binding constant; B T = RT/b is constant (R is the universalgasconstant, T is the absolute temperature, and 1/b indicates the adsorption potential of the adsorbent). The linear form of the D-R isotherm equation is as follows [17]: ln q e = ln q D K D ε 2 (5) ε = RT ln[1 + 1/c e ] (6) where q D (mg/g) is the adsorption capacity of the LF slag, K D (mol 2 /KJ 2 )isthed- R isotherm constant, and ε is the Polanyi potential Adsorption kinetics The amount of Cr(VI) ions adsorbed at any time t, q t (mg/g) was calculated according to the formula: q t = [(c i c t )/c i ] V/m (7) where c i is the initial concentration of C r(vi), mg/l; c t is the concentration of Cr(VI) at any time t, mg/l; V is the volume of solution, L; m is the adsorbent mass, g. The experimental data were analyzed using four adsorption kinetic models: the pseudo-first and -second order equations, the Elovich equation, and Weber-Morris equation. The first order rate expression of Lagergren, based on solid capacity, is generally expressed in the integrated form as follows [18]: ln(q e q t ) = ln q e k 1 t (8) where q e is the adsorption capacity, mg/g, q t is the amount of chromium ions adsorbed at any time t, mg/g; t is time, min; k 1 is the rate constant of the pseudo-first order adsorption, 1/min. A pseudo-second order adsorption kinetic rate equation in the integrated form is as follows [19]: t = 1 + t (9) q t k 2 q 2 e q e were k 2 is the rate constant of the pseudosecond order adsorption, g (1/mg.min). The linear Elovich equation is following [20]: q t = 1/βln(αβ) + 1/βlnt (10) 606-page 3

4 where α and β are Elovich kinetics constants. The intraparticle diffusion model expressed by the Weber-Morris equation is following [21]: q t= k id t 1/2 (11) where k id is the intraparticle diffusion rate constant (mg/g min 1/2 ). The diffusion coefficient for intraparticle transport, D (cm 2 s 1 ) can be described using the following equation [22]: D = 0.03 r 2 0 /t1/2 (12) where r 0 is average radius of sample grains in cm. The mass transfer analysis for the adsorption of Cr(VI) ions was determined using the following equation [22]: ln(c t /c i 1/1 + m V K) = ln(m V K/1 + m V K) {(1 + m V K/m V K)β 1 S s t} (13) where β 1 is the mass transfer coefficient; S s is the outer specific surface of the adsorbent per unit volume; m V is the mass of the adsorbent per unit volume; K is the Langmuir s constant The adsorption thermodinamics In order to study the thermodynamics of the adsorption process, the thermodynamic parameters, such as change in free energy (ΔG ), change in enthalpy (ΔH ), and change in entropy (ΔS )werecalculatedfromthe following equations: ΔG = RTlnK c (14) K c = c Ae /c e (15) log K c = ΔS /2.3R ΔH /2.3RT (16) where R is the gas constant (8.314 J/mol/K), T is the temperature (K), c Ae is the amount of Cr(VI) ions adsorbed onto the LF slag at an equilibrium (mg/l), and c e is the concentration of Cr(VI) ions in the solution at an equilibrium (mg/l). 2 Results and discussion 2.1 Characterization of the LF slag Regarding the major mineral phases in the LF slag, calcium, silicon, magnesium, and Table 1. Chemical composition of the LF slag. Slag component (wt %) CaO SiO FeO 1.54 Al 2 O MgO Na 2 O 0.43 K 2 O 0.36 TiO P 2 O Cr 2 O aluminium oxides represented >92% of the total mass (Table 1). The obtained results are consistent with literature data [13, 23]. The minerals detected in the LF slag could be attributed to mayenite (12CaO 7Al 2 O 3, Ca 12 Al 14 O 33, C 12 A 7 ), periclase (MgO), gehlenite (2CaO Al 2 O 3 SiO 2, Ca 2 Al 2 SiO 7 ), larnite (β-2cao SiO 2, β-ca 2 SiO 4 ), shanonite (γ-2cao SiO 2, γ-ca 2 SiO 4 ), and tricalcium aluminate (3CaO Al 2 O 3, Ca 3 Al 2 O 6, C 3 A). X-ray diffraction analysis confirmed that the LF slag consisted mainly of metal oxide in various oxide, silicate and aluminate form (Fig. 1). Calcium silicates under their various allotropic forms are the major mineral phases in the LF slags. These results are comparable with the results published previously [24, 25]. The average pore diameter of the LF sample was d = 3.21 nm, the total pore volume (1,7 300 nm) V p = cm 3 g 1,and BET surface area S BET = 3.04 m 2 g 1.According to the IUPAC, the pores of porous material are classified in three groups: micropores (width d < 2 nm), mesopores (2 nm < d < 50 nm), and macropores (d > 50 nm). Based on the results, the LF slag may be considered a mesoporous material [26]. 2.2 Adsorption isotherms An equilibrium analysis is the most important fundamental information required to evaluate the affinity or capacity of an adsorbent. Adsorption isotherms describe how an adsorbate interacts with adsorbents, and therefore they are critical in optimizing the use of adsorbents. The equilibrium adsorption isotherm of Cr(VI) ions on the LF slag at 293 K is shown in Figure page 4

5 Fig. 1. XRD pattern of the LF slag. Fig. 2. Adsorption isotherm of Cr(VI) onto the LF slag at 293 K. The values of isotherm constants and correlation coefficients were determined (Table 2). The statistical significance of the correlation coefficient (R 2 ) was the criterion by which the fitting of the data isotherm was tested. The Langmuir isotherm which is applicable for the monomolecular layer adsorption was successfully applied so as to obtain the maximum adsorption capacity as well as no transmigration of the adsorbate molecules on the surface plane [12]. The values of K L and q max can be determined from the slope and intercept of the linear plot of c e /q e versus c e as shown in Figure 3. As seen from Table 2, the adsorption capacity, q m was mg/g. The Freundlich adsorption isotherm gives an expression encompassing the surface heterogeneity and the exponential distribution of active sites and their energies. Constants, K F and n are indicators of the adsorption capacity and adsorption intensity, respectively. The values of K F and n were calculated from the intercept and slope of the plot of lnq e versus lnc e, as shown in Figure 4. The Freundlich constants were calculated for the chromium ions, and they were found to be K F = , and n = , respectively (Table 2). 606-page 5

6 Fig. 3. Langmuir isotherm for the Cr(VI) adsorption onto the LF slag. Fig. 4. Freundlich isotherm for the Cr(VI) adsorption onto the LF slag. The Temkin and Pyzhev take into account the indirect adsorbent adsorbate interactions on the adsorption isotherms. It is based on the assumption that due to the adsorbate adsorbate repulsions, the heat of the adsorption of all molecules in the layer decreases linearly with the coverage of molecules, whilst the adsorption of adsorbate is uniformly distributed [27]. Both, K T ( L/mg) and B T (7.3552) were determined from the plot q e versus lnc e (Fig. 5). The experimental data were fitted to the Dubinin-Radushkevich isotherm model in order to determine the adsorption type. The isotherm constants q D and K D were calculated from the slope and intercept of the plot of ln q e vs. ε 2 (Fig. 6, Table2). The value of K D was calculated to be mol 2 /kj 2.The mean adsorption energy E (kj/mol) can be obtained from the value of K D by using the equation [27]: E = 1/[2K D ] 1/2 (17) The magnitude of E is useful for the estimation of the adsorption process type. If E is between8 and 16kJ/mol, the adsorption process proceeds followed by an ion exchange, but if E < 8kJ/mol, the adsorption is physical in nature. As shown in Table 2, thee value was kj/mol for the Cr(VI) adsorption onto the LF slag. This energy value accounts for the physisorption mechanism. 606-page 6

7 Fig. 5. Temkin isotherm for the Cr(VI) adsorption onto the LF slag. Fig. 6. Dubinin-Radushkevich isotherm for the Cr(VI) adsorption onto the LF slag. According to the correlation coefficient, experimental data were fitted equally well to all isotherm models (R 2 = ), except for the Freundlich isotherm (R 2 = ). 2.3 The adsorption kinetics modelling The adsorption kinetics is useful for the description of the solute uptake rate which controls the residence time of the adsorbate uptake at the solid-solution interface. The adsorption on an adsorbent from the aqueous phase involves three steps: 1/ the transport of the adsorbate from the bulk phase to the exterior surface of the adsorbent (film diffusion); 2/ the transport into the adsorbent by either pore diffusion and/or surface diffusion (intraparticular diffusion); and 3/ the adsorption on the surface of the adsorbent. Figure 7 shows the results of the adsorption capacity of Cr(VI) ions on the LF slag versus time at 293 K. Evidently, the removal of chromium ions takes place in two steps: a relatively fast phase continuing up to 60 min, followed by a slow progress until the state of equilibrium (3 h). The high initial uptake rate is due to the availability of a large number of adsorption sites at the onset of the process. The sticking probability is also high on the bare surface accounting for the high 606-page 7

8 Fig. 7. Results of the Cr(VI) adsorption capacity onto the LF slag vs. time at 293 K. Table 2. Isotherm model parameters for the Cr(VI) adsorption onto the LF slag. Isotherm parameter Value Freundlich isotherm K F n R Langmuir isotherm q max,mg/g K L,L/mg R Temkin isotherm K T,L/mg B T R Dubinin-Radushkevich K D,mol 2 /kj q max,mg/g R E,kJ/mol adsorption rate. Furthermore, the LF slag is mesoporous, and therefore the diffusion of solute into the pores appears to be easier [25]. The applicability of suitable kinetic model was evaluated by the magnitude of the correlation coefficients R 2 shown in Table 3. The pseudo-first order kinetic model did not produce a good linearity (figure not shown). The obtained data (Table 3) suggest that the adsorption of the Cr(VI) onto the LF slag would not follow the pseudo-first order rate model. According to the obtained results (Table 3), the pseudo-second order kinetic model not provides an accurate description of adsorption kinetic for the Cr(VI) ions onto LF slag. Although the Elovich model does not predict any definite mechanism, it was useful in describing the predominantly chemical adsorption on highly heterogeneous adsorbents. By plotting the linear dependence of q t versus lnt (Fig. 8), the constant β was determined from the slope of the straight line, whilst the constant α was obtained from the intercept. The constant α is related to the rate of chemisorption, while β is related to the surface coverage [26]. The Elovich model did not fit well to the experimental data (Table 3). 2.4 The adsorption mechanism The rate of the adsorption process is controlled by the slowest step, which could be either diffusion or intraparticle diffusion. It is known that a pore and surface area play important role during the adsorption process. Since the LF slag is the mesoporous material, the effect of the intra-particle diffusion on the adsorption process should be taken into account. Intra-particle diffusion varies with the square root of time. If the Weber Morris plot of q t versus t 1/2 is linear and passes through the origin, the adsorption process is controlled only by intra-particle diffusion [25]. It can be seen from Figure 9 that the relationship is not linear for the entire range of reaction time. Namely, the plot of q t vs. t 1/2 shows two parts. The first segment with the sharper 606-page 8

9 Table 3. Kinetic model parameters for the Cr(VI) adsorption onto the LF slag. Isotherm parameter Initial concentration Parameter Value 50 mg/l k 1,g/mgmin Pseudo- first order R mg/l k 1,g/mgmin R mg/l k 2,g/mgmin Pseudo- second order R mg/l k 2,g/mgmin R mg/l α,mg/gmin Elovich β, g/mg R mg/l α,mg/gmin β, g/mg R mg/l k id,g/mgmin 1/ Weber-Morris D,cm 2 /s R mg/l k id,g/mgmin 1/ D,cm 2 /s R Fig. 8. Elović model for adsorption of Cr(VI) onto the LF slag. slope can be attributed to the diffusion of the Cr(VI) ions through the bulk solution. The second segment reflects the further adsorption stage, which could be characteristic for the intraparticle diffusion into the LF slag pores. The values of k id were calculated from the slopes of the graph (Fig. 9,Table3), which was further supported by the calculation of the intraparticle diffusion coefficient using the Equation (12). The best value of the kinetics parameters for the Cr(VI) adsorption onto the LF slag were obtained by applying the Weber-Morris model (R 2 = and , Table 3). According to [22], D value of the order of the cm 2 s 1 is indicative of the intraparticle difusion as the rate limiting factor. In this study, the obtained values of D (Table 3) wereoftheorderof10 9 cm 2 s 1, which was more than two orders of magnitude higher, indicating that the intraparticle diffusion was not the only rate controlling step. It was concluded that both, the film (boundary layer) diffusion as well as the intraparticle diffusion might have been involved in the adsorption process. β 1 The values of mass transfer coefficient were determined graphically from the 606-page 9

10 Fig. 9. Intraparticle diffusion plot for adsorption of Cr(VI) onto the LF slag. slopes and the plots of ln (c t /c i 1/1 + m V K) vs. t (Fig. 10). The values of mass transfer coefficients β 1 were cm s 1 (c i = 50 mg L 1 ) and cm s 1 (c i = 70 mg L 1 ), respectively, with a high value of the correlation coefficient. The values obtained from the study indicated that therateofthecr(vi)transportfromthebulk, solution phase to the solid phase was quite rapid. 2.5 The adsorption thermodynamic study The thermodynamic parameters, such as change in free energy (ΔG ), change in enthalpy (ΔH, and change in entropy (ΔS ) were estimated from the Equations (14), (15) and (16), and the slope and intercept of the plot of log K c vs. 1/T (Fig. 11). The values of the thermodynamic parameters are listed in Table 4. The positive value of ΔH reveals that the process is endothermic in nature, i.e., the adsorption is accompanied by the absorption of the heat. The value of ΔS was found to be positive. This infers an increase of randomness at solid solution interface during the fixation of Cr(VI) species on the active sites of the adsorbent. Moreover, the positive value of ΔS reflects the affinity of the adsorbent for the chromium species. The spontaneous nature of the adsorption process is evident from the negative values of ΔG.ThedecreaseinΔG value with increasing temperature reveals that the adsorption of Cr(VI) onto the LF slag becomes more favourable at higher temperatures. Generally, the value of ΔG for the physisorption is in the range 20 to 0 kj/mol, but the chemisorption is between 400 and 80 kj/mol [27]. The calculated ΔG values based in Equation (14) were from 6.14 to 7.82 kj/mol (for c i = 50 mg Cr(VI)/L), and from 5.39 to 6.81 kj/mol (for c i = 70 mg Cr(VI)/L), respectively, corresponding to temperatures (Table 4). Therefore, the values of ΔH (<84 kj/mol) and ΔG suggest that the adsorption of Cr(VI) ions onto the LF slag was the physisorption process. 3 Conclusion The present study shows that the LF slag, a by-product of the steel industry, is an effective adsorbent for the removal of toxic Cr(VI) ions from a synthetic effluent. The research conducted on the adsorption process of the Cr(VI) onto the LF slag was based on the batch experimental data. The equilibrium between the bulk liquid phase Cr(VI) concentration and the adsorbent surface concentration was practically achieved in 3 h. The adsorption process efficiently followed the Langmuir, Temkin, and Dubinin- Radushkevich isotherm models. The removal of Cr(VI) was rapid in the initial stages, and became slower afterwards; that 606-page 10

11 Fig. 10. Mass transfer plot for the adsorption of Cr(VI) onto the LF slag. Table 4. Thermodynamic parameters of the Cr(VI) adsorption onto the LF slag. Concentration (mg/l) Temperature (K) ΔG (kj/mol) ΔH (kj/mol) ΔS (J/Kmol) Fig. 11. Adsorption thermodynamics for the removal of Cr(VI) by LF slag. 606-page 11

12 was also confirmed by the mass transfer studies. The double nature of the curves was attributed to the fact that the adsorption in the initial stages was due to the boundary layer diffusion, whereas in the later stages the adsorption was due to the intraparticle diffusion. The thermodynamic analyses suggested that the adsorption process was endothermic and spontaneous, whilst the adsorption was mainly via the physisorption. The results might open future perspectives regarding the valorization of the LF slag utilization as well as the economical treatment of wastewaters of various origins. References [1] F. Fu, Q. Wang, J. Environ. Manag. 92 (2011) [2] Official Gazette No 80/2013, Ordinance on the limitation of emissions of waste water (in Croatian), 2013 [3] M.Bhaumik,A.Maityb,V.V.Srinivasu,M.S. Onyangoa, Chem.Eng.J (2012) [4] T.A. Kurniawan, G.Y.S. Chan, W.H. Lo, S. Babel, Sci. Total Environ. 366 (2006) [5] A. Ra denović, J. Malina, A. Štrkalj, Desalination Water Treatment 21 (2010) [6] A.Štrkalj,A.Ra denović, J. Malina, J. Mining Metallurgy Section B - Metallurgy 46 (2010) [7] J. Malina, A. Ra denović, Chem. Biochemical Eng. Quarterly 28 (2014) [8] M. Reuter, Y. Xiao, U. Boin, Recycling and environmental issues of metallurgical slags and salt fluxes, in: VII International Conference on Molten Slags Fluxes and Salts, The South African Institute of Mining and Metallurgy, 2004, pp. 349 [9] V. Zalar Serjun, B. Mirtić, A. Mladenović, Mater. Technol. 47 (2013) [10] D.H. Kim, M.C. Shin, H.D. Choi, C.I. Seo, K. Baek, Desalination 223 (2008) [11] Y. Xue, H. Hou, S. Zhu, J. Hazardous Mater. 162 (2009) [12] S.Chen,Q.Yue,B.Gao,Q.Li,X.Xu,Chem. Eng. J. 168 (2011) [13] T. Sofilić, A. Mladenović, V. Orešèanin, D. Barišić, Characterization of ladle furnace slag from the carbon steel production, in Proceedings Book of the 13th International Foundrymen Conference - Innovative Foundry Processes and Materials, edited by Z. Glavaš, Z. Brodarac, N. Dolić, Faculty of Metallurgy, Sisak, 2013, p. 354 [14] I. Langmuir, J. Am. Chem. Soc. 40 (1918) [15] H.M. Freundlich, J. Phys. Chem. 57 (1906) [16] M.I. Temkin, V. Pyzhev, Acta Phys. Chim. USSR 12 (1940) [17] M.M. Dubinin, Chem. Rev. 60 (1960) [18] S. Lagergren, Kungliga Svenska Vetenskapsakademiens Handlingar 24 (1898) 1-39 [19] G. McKay, Y.S. Ho, Process Biochemistry 34 (1999) [20] F.-C. Wu, R.-L. Tseng, R.-S. Juang, Chem. Eng. J. 150 (2009) [21] W.J.Jr. Weber, J.C. Morris, J. Sanitary Eng. Division 89 (1963) [22] J. Setién, D. Hernández, J.J. González, Const. Build. Mater. 23 (2009) [23] M. Thommes, Chemie Ingenieur Technik 82 (2010) [24] S. Gupta, B.V. Babu, Chem.Eng.J.150 (2009) [25] D. Borah, S. Satokawa, S. Kato, T. Kojima, J. Hazardous Mater. 162 (2009) [26] M.K. Aroua, S.P.P. Leong, L.Y. Teo, C.Y. Yin, W.M.A.W. Daud, Bioresource Technol. 99 (2008) [27] S.Chen,Q.Yue,B.Gao,X.Xu,J. Colloid Interface Sci. 349 (2010) page 12