ADSORPTIVE REMOVAL OF Cr(VI) FROM AQUEOUS SOLUTION BY ...

AnkicaTechnology Radjenovic,and Gordana Medunic Journal of Chemical Metallurgy, 50, 1, 2015, 81-88

ADSORPTIVE REMOVAL OF Cr(VI) FROM AQUEOUS SOLUTION BY CARBON BLACK Ankica Radjenovic1, Gordana Medunic2

University of Zagreb, Faculty of Metallurgy, Aleja narodnih heroja 3, 44 000 Sisak, Croatia E-mail: [email protected] 2 University of Zagreb, Faculty of Science, Horvatovac 95, 10 000 Zagreb, Croatia E-mail: [email protected] 1

Received 16 July 2014 Accepted 05 December 2014

ABSTRACT This work is presenting data for the efficiency of carbon black for adsorption of hexavalent chromium ions from aqueous solutions. The Cr(VI) ion removal was studied by the batch adsorption technique. The paper is focused on the effect of different temperatures (293, 313 and 333 K) on the adsorption of Cr(VI) ions from aqueous solutions. Equilibrium modeling of the adsorption process was carried out and the equilibrium parameters were determined. Thermodynamic parameters were calculated using the adsorption equilibrium constant. The results suggest that the process is a typical example of exothermic adsorption. Keywords: carbon black, Cr(VI) ions, adsorption, isotherms, temperature.

INTRODUCTION With rapid industrial development, wastewaters loaded with heavy metals are directly or indirectly discharged into the environment at increasing pace, especially in developing countries. Unlike organic contaminants, heavy metals are not biodegradable and tend to accumulate in living organisms. Furthermore, many of them are well known to be toxic or carcinogenic. According to the World Health Organization (WHO), among the most toxic metals are cadmium, chromium, copper, lead, mercury and nickel [1]. Heavy metal removal from inorganic effluents can be achieved by conventional treatment processes such as chemical precipitation, ion exchange, and electrochemical removal. These processes have significant disadvantages, for instance, incomplete removal, highenergy requirements, and production of toxic sludge. It is now widely recognised that the adsorption processes

provide a feasible technique for the removal of pollutants from wastewaters. There is a continuing search for cheap, high capacity adsorbents for metal ions because of the relatively high cost of commercial sorbents, such as activated carbon [2, 3]. Chromium is an important toxic pollutant which is released into natural water from many industrial processes including electroplating, leather tanning, cement preservations, nuclear power plants, textile, steel fabrication, chromate preparation, etc. [4]. In aqueous systems the chromium can be mainly found as Cr(III) and Cr(VI). At low concentrations Cr(III) can be considered a bioelement since it plays an important role in the metabolism of plants and animals. On the contrary, Cr(VI) is hazardous due to its strong oxidising capacity and it can be adsorbed through the skin. As reported, exposure to excessive amounts of Cr(VI) may cause dermatitis, diarrhea, haemorrhaging and cancer in the digestive tract and lungs [2]. Hexavalent

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Journal of Chemical Technology and Metallurgy, 50, 1, 2015

chromium is primarily present in the form of chromate (CrO42−) and dichromate (Cr2O7 2−), posing significantly higher levels of toxicity than the other valence states. Due to this, governments apply enhanced regulation for chromium species. In Croatia, the upper limit for the discharge of Cr(VI) into inland surface waters and public drainage systems is 0.1 mg L-1 [5]. Carbon black (CB) is composed of particles that are either spherical or spheroidal with a pronounced ordering of the carbon layers. It is mainly applied as a reinforcing agent in rubber; other uses include printing inks, paints, plastics and dry electric cells. Its use as an adsorbent for different pollutants from water has been studied [6 - 9]. Adsorption ability of the CB depends on its origin, types of metal ions and experimental conditions. In the present work, we focused on the effect of temperature on the Cr(VI) removal from aqueous solutions by CB, as a non-conventional adsorbent. The principal objectives of the work include investigation of the influence of temperature on the adsorption process and modelling of the adsorption equilibrium with the least-square regression, using two-parameter isotherms. In addition, adsorption isotherms were used to describe the adsorption equilibrium and to calculate some thermodynamic parameters. EXPERIMENTAL Preparation and characterization of the adsorbent CB was produced by the oil furnace process. The oil furnace process is based on the partial combustion of highly aromatic hydrocarbon fractions. The raw materials were aromatic oil derivates produced by secondary petroleum refining processes. �� The properties of the investigated CB are presented in Table 1. The CB samples were ground and sieved to retain a particle size of 0.125 mm. The surface area properties were determined by the Brunauer-Emmett-Teller (BET) and Barrett-JoynerHalenda (BJH) methods. Microscopic analysis was performed using a scanning electron microscope (SEM). Adsorption studies The adsorption studies were conducted routinely by the batch technique. Initial concentrations of Cr(VI) ions were prepared by dissolving K2Cr2O7 in deionised water.

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For the determination of the adsorption isotherm, 0.25 g of the CB adsorbent was added into 50 mL glass flasks containing solutions of known chromium concentration (50 - 250 mg L-1). The time required for reaching the equilibrium condition, was estimated at regular intervals of time untill an equilibrium was reached (75 min). After the reaction period, the suspension was filtered through a Whatman filter paper No. 44 and the supernatant was analysed for the Cr(VI) concentration. The violet color that forms as a results of the reaction between Cr(VI) ions and 1,2-diphenylcarbazide in acidic medium, was measured spectrophotometrically at 540 nm. The uptake values were determined as a difference between the initial Cr(VI) concentration, and the one in the supernatant. The optimal pH solution (2.4) was determined during preliminary experiments. The adsorption process was carried at 293, 313, and 333 K. All experiments were duplicated and the average values were used for further calculations. RESULTS AND DISCUSSION Characterization of the carbon black The characteristic values of the CB surface area properties are: the BET surface area is SBET = 107.29 m2 g-1, the total pore volume (1.7 - 300 nm) is Vp = 743∙10-3 cm2 g-1, and the average pore diameter is d = 16.99 nm (Table 1). This surface area value could be is explained by the finely grained particle size of the CB and its porous nature. The pore size distribution of the CB is shown in Fig.1. The fraction of the pores between 10 nm and 100 nm constitute the greatest proportion. According to the IUPAC, the pores of a porous material are classified in three groups: micropores (width d < 2 nm), mesopores (2 nm < d < 50 nm), and macropores (d > 50 nm). On the basis of the obtained results, the CB may be considered a mesoporous material [10]. Fig. 2a shows the SEM image of the CB sample surface prior the adsorption with clearly visible mostly round particles of different sizes. Agglomeration of individual particles was by Van der Waals forces resulting into a structure previously described in literature [6]. The changes were caused by Cr(VI) ion adsorption. Accumulations and deposites are shown in Fig. 2b.

Ankica Radjenovic, Gordana Medunic

Fig. 1. The CB pore size distribution.

Fig. 3. Dependence of the Cr(VI) distribution coefficient on the adsorbent dose (ci = 200 mg L-1).

Fig. 2. SEM image (3000x) of the CB surface: a) before, and b) after the Cr(VI) adsorption; ci = 200 mg L-1

Fig. 4. Equilibrium adsorption isotherm of the Cr(VI) ions onto the CB at 293 K.

Relationship between the distribution coefficient and an adsorbent dose The distribution coefficient Kd reflects the binding ability of the surface for an element. The distribution coefficient values for Cr(VI) were calculated by equation [11]: Kd =cs / cw (1) where cs is the concentration of chromium in the solid particles (mg kg-1), and cw is its concentration in the solution (mg L-1) at the equilibrium state. Fig. 3 shows that the Kd values (for initial chromium concentration, ci = 200 mg L-1) change with the adsorbent dose, thus indicating the heterogeneous surface of the adsorbent. The maximum Kd was given at 5 g L-1 and all experiments were performed at this adsorbent concentration.

and the affinity of an adsorbent. The most simple method to determine isotherm constants for the two parameter isotherms is to transform the isotherm variables so that the equation is converted to a linear form and then linear regression is applied [12]. The amount adsorbed at equilibrium, i.e. the adsorption capacity, qe (mg g-1) was calculated according to the formula:

Modelling of the adsorption equilibrium Equilibrium isotherm equations are used to describe the experimental adsorption data. The equation parameters and the underlying thermodynamic assumptions for these equilibrium models often provide an insight in the adsorption mechanism as well as the surface properties

qe =

∆c ⋅V m

(2)

where qe is the equilibrium adsorption capacity, mg g-1; Δc is the quantity of an adsorbed adsorbate, mg L-1 (Δc = ci - ce; ci - initial concentration of the adsorbate, mg L-1; ce - equilibrium concentration of the adsorbate, mg L-1); V is the volume of the solution, L; m is the adsorbent mass, g. The equilibrium adsorption isotherm of the Cr(VI) ions onto the CB at 293 K is shown in Fig. 4. This relationship showed that the adsorption capacity increased with the equilibrium concentration of the Cr(VI) ions in solution, progressively reaching the saturation of the

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Table 1. Properties of the used CB.

Property Heating loss, % Iodine number, mg g-1 Density, kg m-3 Ash, % Sulfur, % Carbon, % BET surface area, m2 g-1 Total pore volume (1.7-300 nm) cm2 g-1 Average pore diameter, nm adsorbent. In order to determine the CB adsorption capacity, the experimental data points were fitted to the Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) equations, applicable to the adsorption process. For each isotherm, the initial Cr(VI) concentrations were varied while the adsorbent mass in each sample was constant. The Langmuir isotherm describes quantitatively the formation of a monolayer adsorbate on the outer surface of the adsorbent, and following that stage no further adsorption takes place. The model assumes uniform energies of the adsorption onto the surface without transmigration of adsorbate in the plane of the surface. Based on these assumptions, the Langmuir isotherm in its linear form was represented by the following equation [13]:

Value 0.57 119 340 0.27 1.0 98.0 107.29 743∙10-3 16.99 qm is saturation adsorption capacity of the CB, mg g-1; KL is the Langmuir constant. The values qm and KL were computed from the slope and intercept of the Langmuir plot of ce/qe versus ce, as shown in Fig. 5. The Langmuir isotherm parameters and correlation coefficient are shown in Table 2. Arelatively low correlation coefficient (R2 = 0.8641) was obtained. The linearised form of the Freundlich isotherm (Fig. 6) is presented as [14]:

1 = ln qe ln K F + ln ce n

(4)

where qe is the CB adsorption capacity, mg g-1; ce is the equilibrium concentration of the chromium ions, mg L-1;

where qe is the equilibrium adsorption capacity, mg g-1; ce is the equilibrium concentrations of the chromium ions, mg L-1; KF and n are the Freundlich constants. KF and n are empirical constants of the Freundlich isotherm measuring the adsorption capacity and intensity of the adsorption, respectively. Our result for n (2.1929) is in a good agreement with the mathematically evaluated values of n for a number of mass transfer operations

Fig. 5. Linearised Langmuir isotherm for the Cr(VI) adsorption onto the CB.

Fig. 6. Linearised Freundlich isotherm for the Cr(VI) adsorption onto the CB.

ce c 1 = e + qe q m qm ⋅ K L

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(3)


Fig. 7. Linearised Dubinin-Radushkevich isotherm for the Cr(VI) adsorption onto the CB.

Fig. 8. Linearised Freundlich isotherm for the Cr(VI) adsorption onto the CB at various temperatures.

of systems (values of n between 1 and 10 would represent beneficial adsorption) [15]. As can be seen from Table 2, the R2 value (0.9835) is the highest compared to other applied isotherms. The Dubinin-Radushkevich (D-R) isotherm was applied in order to deduce the heterogeneity of the apparent energy of adsorption on the adsorbent surface. The linear form of the D-R isotherm equation is [16]:

forces and that these forces are more important than ion-exchange and particle diffusion.

ln = qe ln qD − K Dε 2

(5)

= ε RT ln [1 + 1/ ce ]

(6)

where qD (mg g-1) is the adsorption capacity of the adsorbent, KD (mol2 kJ-2) is the D-R isotherm constant related to the adsorption energy, ε is the Polanyi potential, R is the gas constant (J molK-1) and T (K) is the absolute temperature. The mean adsorption energy E (kJ mol-1) can be obtained from the value of KD [13,17] by using the equation: E = 1/[ 2KD ]1/2 (7) The isotherm constants qD and KD were calculated from the slope and intercept of the plot of lnqe vs. Ɛ 2 (Fig. 7) and listed in Table 2. The KD value was calculated as 189.69 mol2 kJ-2. The magnitude of E is useful for estimating the type of the adsorption process. If E is between 8 and 16 kJ mol-1, the adsorption process proceeds followed by ion exchange, and if E < 8 kJ mol-1, the adsorption is physical in nature. As shown in Table 2, the E value is 0.0513 kJ mol-1 for the Cr(VI) adsorption onto the CB. This energy value falls in the range for the physisorption mechanism, suggesting that the adsorption process was dominated by physical

Comparison of the Cr(VI) removal with different adsorbents reported in the literature The adsorption capacity varies and it depends on the characteristics of the individual adsorbent, pH value, the initial concentration of the adsorbate and temperature [18]. The presented values are related to the adsorption processes described by the Freundlich isotherm. The adsorption capacities of the CB for the removal of Cr(VI) have been compared with those of other adsorbents reported in the literature (Table 3).

Table 2. Isotherm parameters for the Cr(VI) adsorption onto the CB at 293 K.

Isotherm parameters

Value

Freundlich isotherm KF, (mg g-1)∙(L mg-1) 1/n n R2

1.0740 2.1929 0.9835

Langmuir isotherm qmax, mg g-1 KL, L mg-1 R2

12.71 0.0211 0.8641

Dubinin-Radushkevich isotherm KD, mol2 (kJ2)-1 qmax, mg g-1 R2 E, kJ mol-1

189.69 10.31 0.9250 0.0513 85


Table 3. Comparison of the Cr(VI) adsorption data described by the Freundlich isotherm model with those for various adsorbents [18].

Adsorbent

pH

Temperature (°C)

q (mg g-1)

Acetic acid of N. crassa (ATCC 12526) Activated carbon, CZ-105 Algae, C. Vulgaris Carbon black from the wheat straw Cation-exchange resin, IRN77 Maghemite nanoparticles Native biomass of N. crassa (ATCC 12526) Raw rice bran Sodium hydroxide of N. crassa (ATCC 12526) Uncalcined hydrotalcite Carbon black (this study)

1.0 3.0 2.0 1.0 3.5 2.5 1.0 5.0 1.0 2.0 2.4

25

15.90 40.40 6.0 21.34 35.40 9.20 0.40 0.07 7.40 4.60 12.71

Effect of temperature and calculation of thermodynamic parameters Temperature affects the adsorption capacity by alternating the molecular interaction and solubility [19]. The thermodynamic parameter values for the adsorption processes actually indicate its practical application since both energy and entropy considerations must be taken into account so as to determine the process which will occur spontaneously [20]. Since the Freundlich constant, KF is essentially the equilibrium constant for the investigated adsorbent/adsorbate system, the variation of KF with temperature can be used to estimate the thermodynamic parameters accompanying the adsorption. Plots of linearised Freundlich equations (Eq. 4) for Cr(VI) at various temperatures are shown in Fig. 8. The constants relating to the Freundlich isotherm model are calculated and presented in Table 4. The changes of the standard enthalpy (ΔH°) and entropy (ΔS°) were determined [20] using the van’t Hoff equation (Eq. 8), respectively, from the slope and intercept of the plot of ln KF vs. 1/T (Fig. 9): ln KF = ΔS° / R – ΔH°/ R T (8) where KF is the Freundlich constant, and R is the gas constant (J mol-1 K-1). Both the enthalpy and entropy factors must be

25 30 25 22.5 25 25 25 25 20

Fig. 9. The Freundlich isotherm constant for the Cr(VI) adsorption onto the CB as a function of temperature.

considered in order to determine the Gibbs free energy (ΔG°) of the process. Reactions occur spontaneously at a given temperature if ΔG° has a negative value. The Gibbs free energy (ΔG°) associated with the adsorption can be calculated from the Gibbs-Helmholtz equation: ΔG° = ΔH° – T ΔS° (9) The values of the thermodynamic parameters calculated from equations 8 and 9 for the adsorption of Cr(VI) ions onto the CB, are given in Table 5. The negative values of the standard enthalpy change, ΔH° = – 3.20 kJ mol−1, imply that the interaction between the hexavalent chromium ions and the CB is exothermic

Table 4. Parameters of the Freundlich isotherm at various temperatures.

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T (K)

KF

293 313 333

1.0704 0.9139 0.9176

n

R2

2.1929 2.2472 2.2727

0.9835 0.9725 0.9831


Table 5. Thermodynamic parameters for the Cr(VI) adsorption onto the CB.

T (K) 293 313 333

∆Go (kJ mol-1) - 6.29 - 6.50 - 6.71

in nature. If the heat of the adsorption process is < 40 kJ mol−1, it is a physical process [20, 21]. The negative values of free energy changes ΔG° confirmed the spontaneous nature of the process and feasibility of the adsorption of Cr(VI) onto the CB (Table 5). The adsorption processes with ΔG° values in the – 6.29 to – 6.71 kJ mol-1 range correspond to spontaneous physical processes [21]. As indicated in Table 5, ΔS° values for the adsorption process are negative. These values are expected for ΔS°, since during the physical adsorption the degrees of internal freedom of the system are decreased [22]. CONCLUSIONS The removal of toxic hexavalent chromium ions from an aqueous solution by their adsorption on the CB was investigated at three different temperatures. The adsorption was relatively rapid and equilibrium was attained within 75 minutes at the studied conditions. The equilibrium data were fitted to the Langmuir, Freundlich, and Dubinin–Radushkevich isotherm models. The Freundlich model best fitted the equilibrium data over the entire concentration range studied. The obtained adsorption capacity for the Cr(VI) ions was a good indicator of the CB potential for its use in this adsorption system. The adsorption process was spontaneous and a negative value of the standard enthalpy change showed the exothermic and physical nature of the adsorption. In general, the experiments revealed that CB, as a mesoporous material, might be a quite effective low-cost adsorbent of Cr(VI) ions during purification of its water solutions. REFERENCES 1. WHO, Guidelines for drinking-water quality, 1st Addedum. In: Chemical Fact Sheets, World Health Organization, Geneva, 2006. 2. F. Fu, Q. Wang, Removal of heavy metal ions from wastewaters: A review, J. Environ. Manage., 92,

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