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Journal of the Chilean Chemical Society

On-line version ISSN 0717-9707

J. Chil. Chem. Soc. vol.50 no.4 Concepción Dec. 2005

http://dx.doi.org/10.4067/S0717-97072005000400007 

 

J. Chil. Chem. Soc., 50, N° 4 (2005), págs: 677-684

 

CHROMIUM ADSORPTION FROM TANNERY EFFLUENTS BY ACTIVATED CARBONS PREPARED FROM COCONUT SHELLS BY CHEMICAL ACTIVATION WITH KOH AND ZNCL2

ADSORCIÓN DE CROMO DESDE EFLUENTES DE CURTIEMBRES POR CARBONES ACTIVADOS PREPARADOS DESDE CÁSCARAS DE COCO POR ACTIVACIÓN QUÍMICA CON KOH Y ZNCL2

 

S. BENDEZÚ*1, J. OYAGUE AND A. ROMERO1, R. GARCÍA2, Y. MUÑOZ2 AND N. ESCALONA2

1 Facultad de Ingeniería Química. Universidad Nacional del Centro del Perú. Real 160, Huancayo, Perú.
2 Facultad de Ciencias Químicas. Universidad de Concepción. Casilla 160-C, Concepción, Chile.


ABSTRACT

Activated carbons of high adsorbents properties for adsorption of chromium species from aqueous solution and Tannery effluents were prepared from coconut shells by means of chemical activation using as activating agents the KOH and ZnCL2, in atmosphere of N2(g) and CO2(g), respectively. It was found that the adsorption capacity of the produced activated carbons are significantly higher than that of the commercial one that was tested. Based on the results obtained from a variety of characterization methods, it has been determined the presence of functional organic groups, phenols and carbonyls, which are responsible for the activated carbons surface charge and it favored the chromium species adsorption, zeta potential measurements helped to explain the metal adsorption in terms of electrostatic attraction-repulsion behavior of the activated carbon surface and cations of Cr(III) and oxyanions of Cr(VI), respectively. It was observed that the activated carbons CK12 and CZ32 displayed the biggest adsorbents properties, it was adsorbed approximately 721.38 mg of chromium with the CK12, 534.43 mg with the CZ32, and only 274.02 mg with the commercial carbon (CC), by gram of activated carbon, respectively, at pH 4.45 from Tannery effluents.

Keywords: A. Activated carbon; B. Activation, Chemical treatment; C. Adsorption; D. Adsorption properties; E. Microporous adsorbents.

RESUMEN

Se prepararon carbones activados de elevadas propiedades adsorbentes desde cáscaras de coco mediante activación química usando como agentes activantes el KOH y ZnCL2, en atmósfera de N2(g) y CO2(g), respectivamente, para adsorber especies de cromo desde soluciones acuosas y efluentes de Curtiembres. Se halló que la capacidad de adsorción de los carbones activados obtenidos es significativamente mayor que el carbón comercial usado. En base a resultados obtenidos de los diversos métodos de caracterización, se determinó la presencia de grupos funcionales orgánicos, fenoles y carbonilos, responsables de la carga superficial de los carbones activados que favoreció la adsorción de las especies de cromo, medidas de potencial zeta ayudaron a explicar la adsorción del metal en términos de la acción electrostática atracción-repulsión de la superficie del carbón activado y los cationes Cr(III) y oxianiones Cr(VI), respectivamente. Se observó que los carbones activados CK12 y CZ32 muestran las propiedades adsorbentes más altas, se adsorbió aproximadamente 721,38 mg de cromo con el CK12, 534,43 mg con el CZ32 y solamente 274,02 mg con el carbón comercial (CC), por gramo de carbón activado, respectivamente, a pH 4,45 desde los efluentes de Curtiembres.

Palabras Claves: A. Carbón activado; B. Activación, tratamiento químico; C. Adsorción; D. Propiedades de adsorción; E. Adsorbentes microporosos.


1. INTRODUCTION

The Tanneries are identified as the most polluting industries, because the great majority of them throw their effluents containing high chromium concentrations to the rivers and lakes contaminating and destroying the ecosystem. The chromium adsorption from the effluents by means of activated carbons is profiled like an excellent option to correct the environmental problem, in view of its remarkable adsorbents properties [1-3].

The activated carbons occupy an important position in various adsorption processes in both liquid and gas phases as adsorbent material. They can be prepared starting from such carbonaceous materials as the coal, petroleum peat, polymers, wood [4,5], coconut shells, palm seeds [6], cork waste [7] among others. The activated carbon can be gotten ready for chemical activation [8]; chemical (with ZnCL2) and physical activation in a simultaneous way (with CO2 or vapor of water) [9,10]. This investigation displays the chromium adsorption from aqueous solutions and Tannery effluents by means of natural adsorbents obtained from coconut shells, product of the agricultural industry waste, activated chemically with KOH or ZnCL2, in atmosphere of N2(g) or CO2(g), respectively. Different preparation parameters like the ratio activating agent/precursor, time and activation atmosphere were examined to evaluate the influence of the activation conditions on the obtained activated carbons adsorbent properties; also, the influence of aqueous solutions pH and the chromium initial concentration on the process of adsorption was studied.

2. EXPERIMENTAL

2.1. Preparation

It was prepared activated carbons from coconut shells by chemical activation [11] using as activating agents the KOH and ZnCL2 (Merck), in an inert nitrogen atmosphere and CO2 (g), respectively. The carbonaceous precursor selected for the present study was crushed and sieved to obtain a size of uniform particle, 7.0 mm, and dried in an oven at 110ºC for 8 hours.

The dry precursor was impregnated with a concentrated solution of KOH (or ZnCL2) in a rotavapor at 25ºC, during 3 hours, using an impregnation ratio [weight of activating agent (g) / Weight of precursor (g)] from 0.25 up to 3.0 (the quantity of KOH or ZnCL2 retained by the precursor, are expressed as Potassium grams (or Zinc) retained by dry precursor grams, XK or XZ, as it is the case), followed by heating up to 80ºC and it completes evaporation, up to vacuum, then, drying at 110ºC for 24 hours. The impregnated precursor (20 g) was placed in a horizontal reactor of stainless steel; the system is mounted in a furnace Labor LR-202, under a flow (100 cm3/min.) N2 the activation process is carried out at 800°C during one hour, to a heating speed of 10°C/min.; in the same way an activation was developed in atmosphere of CO2(g) at the same conditions, in different periods of time. The activated products were then cooled in N2 atmosphere until reaching the room temperature; after cooling, the obtained carbons were washed with a solution of HCl 0.1N and abundant distilled water until complete ions removal. Leaching was carried out several times until the pH of the water-carbon mixture was near 7. The resulting activated carbons were dried overnight in an oven at 110ºC.

The obtained activated carbons nomenclature includes the coconut shells (C) identification, the activating agents (K and Z), followed by a number that indicates the ratio activating agent/precursor. For example, the sample CK1 got ready using the impregnation method using as activating agent the KOH with a ratio XK = 0.25; the sample CK11, got ready to a reason XK = 0.25 and activated in atmosphere of CO2(g) during 1 hour.

The preparation conditions and the identification codes of the activated carbons are listed in the Table 1.


Table 1. Activated carbons.  Activation conditions and yield.

Activated
Carbons
Activating agent (g)/
Precursor (g)
aXi     
Activation
Atmosphere
Activation 
Temperature
(ºC) 
Activation
Time
(h)   
Yiel
b(%)

CK1
CK2
CK3
CK4
CK5
CK6
CK7
CK8
CK11
CK12
0,25
0,50
0,75
1,00
1,50
2,00
2,50
3,00
0,25
0,25
N2(g)
N2(g)
N2(g)
N2(g)
N2(g)
N2(g)
CO2(g)
CO2(g)
800
800
800
800
800
800
800
800
800
800
1
1
1
1
1
1
1
1
1
2
22,50 
19,00
15,50
13,50
11,00
7,50
5,00
3,00
11,05
5,50
           
CZ1
CZ2
CZ3
CZ4
CZ5
CZ6
CZ7
CZ8
CZ31
CZ32
0,25
0,50
0,75
1,00
1,50
2,00
2,50
3,00
0,75
0,75
N2(g)
N2(g)
N2(g)
N2(g)
N2(g)
N2(g)
N2(g)
N2(g)
CO2(g)
CO2(g)
800
800
800
800
800
800
800
800
800
800
1
1
1
1
1
1
1
1
1
2
57,00
51,00
43,00
35,50
31,50
27,00
19,00
8,50
25,90
12,25

a  Xi  : XK o XZn , as it is the case.
b In base of dry Coconut shells.

2.2. Characterization

The surface areas and microporous volume of the activated carbons that displayed a bigger efficiency in the chromium adsorption were determined by adsorption of N2 with an automated adsorption instrument (Micromeritics Gemini 2370) at -196°C. The surface areas and the activated carbons microporous volume were determined from BET and Dubinin-Radushkevich (D-R) equations [12], respectively, at relative pressure (P/P°) of 0.005-0.200. The amount of N2 at relative pressures near unity corresponds to the total amount adsorbed in both the microporous a mesoporous (determined at P/Po = 0.95 in this case).

Information on the surface chemistry of the samples was provided by Fourier transform infrared spectroscopy (FTIR). A FTIR Spectrometer Shimadzu 7800 coupled with a system D.R.S. was used in recording the spectra from 4000 to 400 cm-1 with 250 scans. The samples were prepared by first thoroughly mixing 1 mg of carbon and 100 mg of KBr in an agate mortar.

The surface charge on the activated carbons was measured using a Zeta meter Inc. model 3.0 [13].

2.3. Chromium adsorption

The chromium adsorption capacity of all the activated carbons was measured from aqueous solutions. In sealed flasks of 125 ml, 200 mg of activated carbon (previously milled and sieved by mesh 200 and drying at 110ºC overnight) is contacted with 100 ml of a solution of CrCl3.6H2O or K2CrO4 (Merck) containing 2000 ppm of Cr(III) or Cr(VI), respectively, at 25ºC. The solutions were shaken at constant speed for 5 hours. The initial and final chromium content of the solution was determined in an Atomic Absorption Shimadzu AA-6800 Spectrophotometer, using a lamp of chromium hollow cathode and a wave longitude of 357.90 nm and an UV-Vis Shimadzu UV-1203 Spectrophotometer, using diphenylcarbazide sachetts and a wave longitude of 542.00 nm, respectively.

Stock solutions of Cr(III) and Cr(VI) of varied concentrations were prepared: 200, 1000 and 2000 ppm, respectively; it was studied the effect of pH (ranging from pH 2 to 8) in the chromium species adsorption from aqueous solutions of the activated carbons that showed the best properties. The pHs of the solutions were adjusted to the appropriate pH using 0.1N HCl or NaOH, as it is the case, before beginning the experiments. Finally the adsorbents properties of the best selected activated carbons and an commercial activated (Riedel-of Haën) were measured in the chromium adsorption from Tannery effluents to the same conditions described previously.

3. RESULTS AND DISCUSSION

3.1. Effect of the ratio activating agent/precursor

Fig. 1 and 2 display the influence of the ratio activating agent/precursor in the adsorbent activated carbons capacity prepared from coconut shells, expressed in mg of adsorbed Chromium per gram of dry activated carbon.


 
Fig. 1. Adsorption of Cr(III) and Cr(VI) from aqueous solutions by the activated carbons of the series CK. It influences of the impregnation ratio (KOH/precursor). Condition: initial concentration = 2000 ppm Cr(III) or Cr(VI), temperature 25ºC.   Fig. 2. Adsorption of Cr(III) and Cr(VI) from aqueous solutions for the activated carbons of the series CZ. It influences of the impregnation ratio (ZnCL2/precursor). Condition: initial concentration = 2000 ppm Cr(III) or Cr(VI), temperature 25ºC.

It is observed in the series CK, Fig.1, to ratio KOH/precursor 0.25, the activated carbon CK1 adsorption capacity is the maximum with regard to the other ones, adsorbs approximately 468.01 mg Cr(III) and 484.87 mg Cr(VI) per gram of activated carbon, respectively, that represents 58.48% of Cr(III) and 60.61% of Cr(VI) of the total content of chromium in the aqueous solution; nevertheless, as the activating agent, KOH, is increased until XK = 3.00, a significant decrease of the activated carbon adsorbents properties is observed (CK2 - CK8), until reaching a minimum adsorption of 149.57 mg of Cr(III)/g (18.69%) and 194.48 mg of Cr(VI)/g (24.31%) with the CK8.

In the series CZ, Fig. 2, an adsorption of 224.63 mg Cr(III) (28.08%) and 262.01 mg Cr(VI) (32.75%) per gram of activated carbon is observed in the CZ1; when increasing the ratio ZnCL2 / precursor until XZn = 0.75, a maximum adsorption capacity is achieved with CZ3, 371.11 mg Cr(III)/g (46.39%) and 386.73 mg Cr(VI)/g (48.34%) are adsorbed; an additional increase of the ratio XZn, up to 3.00 causes a very significant decrease of the activated carbons (CZ4 - CZ8) adsorption capacity.

The activated carbons CK1 and CZ3 displayed the best adsorbents properties for adsorbing chromium species from aqueous solutions.

3.2. Effect of the activation atmosphere

The activation was carried out at 800°C and a constant heating speed (10°C/min), and it was avoided the sintering of the chemical reagent that affects in a negative way in the porosity generation that is associated with a low capacity of chromium adsorption [14]. The reaction happens in the whole internal surface of the carbon achieving it to increase the size of the pore. The control of the temperature is critical, since to temperatures under the 800°C the reaction speed is very slow, while, the reaction is corrosive at 1000°C; this is observed because it decreases the size of each particle with an evident inactive interior [15]. During the activation with KOH some oxygenated complexes are formed in the surface which is responsible for the additional gasification of the carbon liberating gaseous products as CO2, CO, etc. [16], besides, an excessive emission of combustion gases as the ratio KOH/precursor increases. It is evidenced, also, during the activation of the impregnated precursors with ZnCL2, little emission of combustion gases, even with the increase of the ratio ZnCL2/precursor, facilitating the generation of the porous, well-developed porosity than the previous case.

With regard to the activation atmosphere, the use of the nitrogen gas, inert atmosphere, allowed to obtain bigger surface area and microporosity than that activated carbon obtained in air atmosphere [11].

The activation in atmosphere of CO2(g), it improved notably the textural characteristics of the activated carbons CK12 and CZ32 with regard to CK1 and CZ3, respectively, in 2 hours of activation; different activation methods on the same precursor can produce completely different characteristics of the activated carbons [17].

3.3. Surface area and volume of pores

Fig. 3 displays the isotherms of nitrogen adsorption at -196°C for the activated carbons of the series CK1 and CZ3 (obtained at 800°C in different times and activation atmospheres) and one commercial activated (CC). All the isotherms, Fig. 3, except for the case of the commercial activated carbon, were found to be typical of microporous adsorbents (type I) [18]. The activated commercial (CC) exhibit steep type IV isotherms, indicative of highly microporous and mesoporous adsorbents. The slope at the end of the isotherm was due to a limited adsorption of nitrogen to higher relative pressures, indicating that the condensation of capillarity takes place in the mesoporous.


Fig. 3. Adsorption isotherms of N2 at -196°C on the activated carbons of the series CK1, CZ3 and one commercial carbon (CC).

The isotherms in Fig. 3 and the Table 2 can be used to follow the activating agents effect and the activation atmosphere on the textural characteristic of activated carbon. The activated carbons CK1 and CZ3 obtained in an inert atmosphere, N2(g), at 800ºC during 1 hour show structures mainly constituted by narrow microporous (the isotherm is type I and the adsorbed quantity outside of the microporous is very small, this is shown by the plateau of the almost horizontal isotherm). The BET surface areas and total volume of pores are of the same order of magnitude, in both activated carbons; the CK1, was obtained using a ratio KOH/precursor, XK = 0.25, while this last carbon the CZ3, XZn = 0.75 developed a porous system of the same order of the CK1 using three times the activating agent content, ZnCL2, which demonstrates that the KOH is a very reagent activating agent.


Table 2.  Textural characteristics of the activated carbon samples 

Activated
Carbons
aXi SBET
(m2/g)  
Vp
(cm3/g)
Vo
(cm3/g)
Vm
(cm3/g)
Bulk density
(cm3/g)

CK1 0.25 771 0.360 0.350 0.010 0.506
CK11 0.25 868 0.432 0.409 0.023 0.359
CK12 0.25 1266 0.636 0.463 0.173 0.297
             
CZ3 0.75 763 0.381 0.352 0.029 0.714
CZ31 0.75 989 0.494 0.456 0.038 0.667
CZ32 0.75 1041 0.520 0.480 0.040 0.588
             
bCC   - 628 0.427 0.287 0.140 -

 aXi:   Ratio  Activating agent (g) / Precursor (g).
SBET: BET surface area;   Vp: total volume;  Vo: micropore volume;  Vm: mesopore volume.
bCC: Commercial carbon.

In the Table 2, the evolution of the BET surface areas, pores volume and bulk densities of the activated carbons (CK11, CK12, CZ31 and CZ32) obtained in atmosphere of CO2(g) from coconut shells impregnated with KOH (XK = 0.25) and ZnCL2 (XZn = 0.75) are shown, respectively. All the carbons follow the same pattern, with a continuous decrease in the bulk density due to a mainly internal gasification of the particles and a continuous increase of the surface area; however, although the decrease in the density is more lineal, the maximum development of the surface area takes place, the CK12 shows an area of 1266 m2 and the CZ32 1041 m2 per gram of activated carbon. On the other hand, for a low carbonization there is a decrease in the surface area and an increase in the bulk density, this is observed in that activation in inert atmosphere, N2(g); the CK1 shows an area of approximately 771 m2 and the CZ3 of 763 m2 per gram of activated carbon, respectively.

The observed difference, Table 2, of the BET surface area and bulk density suggests that the pore size distribution is being modified during the activation. An important development of the activated carbons total porosity is observed in atmosphere of CO2(g) with regard to CK1 and CZ3, respectively. This is the action type found in the activation with CO2(g) during the activated carbon preparation [19]. The total pore volume of a gram of activated carbon is increased continually up to about 0.636 cm3/g in the CK12 and 0.520 cm3/g in the CZ32. The porosity evolution indicates that the narrow microporosity of the activated carbon is being destroyed by widening to larger pores. This porosity evolution explains the increase in the surface area, producing activated carbons with a high-adsorption capacity, near to 0.636 cm3/g in the CK12, of which 0.463 cm3/g corresponds to the total microporosity.

These values are much bigger than those obtained by CK1 (Vp = 0.360 cm3/g and V0 = 0.350 cm3/g) in an inert atmosphere, N2(g), and this is basically due to the much smaller pore volume compared with the activated CK12. Consequently, the activation in CO2(g) atmosphere produces a very high elimination of internal atoms of carbon, with the result of a higher porosity.

During the chemical activation, the activating agent, KOH (or ZnCL2) it minimizes the formation of tar and some other liquids that could block the pores and to inhibit the development of the porous structure in the activated carbon; the molecules of the KOH break down forming potassium oxides and molecules of water, these begin the gasification process of the structure from the precursor to high temperatures [16]; in a second stage an additional activation is developed in the activating agent presence, CO2(g), the presence of this increased so much in the surface area as in the total activated carbon pore volume; this phenomenon would be due to that activating agent CO2(g) that would propitiate an additional gasification of the activated carbon structure, to obtain activated carbons of high-adsorbent properties.

3.4. Surface chemistry

The Fourier transformed infrared (FTIR) spectra of the coconut shells and activated carbons of the series CK and CZ are shown in Fig. 4. The spectrum of the coconut shells displayed the following bands that are attributed to the stretching vibrations: 3458 cm-1, O-H in alcohols; 2356 and 1552 cm-1, C=O in ketones; 1633 cm-1, C=O in quinones; 1483 and 3020 cm-1, C=C and =C-H, respectively, in aromatic rings; 1190 cm-1, C-O in ethers; 792 and 750 cm-1, C-H out of plane bending in benzene derivatives. After activation at different activation atmosphere and time, the ether and ketonic groups were absent in the activated carbons of the series CK and CZ due to their thermal instabilities; besides, the spectrum of commercial carbon (CC) displayed the same vibration bands observed in the activated carbons prepared from coconut shells.


  Fig. 4. Fourier transformed infrared spectras of the Coconut shells and activated carbons of the series CK and CZ, and the commercial carbon (CC).

According to Corapsioglu et al. [20], the properties of the activated carbons can vary depending on the raw material nature, as well as, activating agent and of activation conditions. The activated carbon surface can contain, protonated (C-OH2+), neutral (C-OH) or ionized (C-O-) groups, these results are in agreement with functional groups observed in the series of activated carbons CK and CZ, those can be loaded positive or negatively, depending on the pH range in that one works. The main oxygenated groups, present in the obtained activated carbons surface are carbonyl groups (ketone and quinone) and phenols, respectively.

3.5. The effect of solution pH and chromium initial concentration

The effect of aqueous solutions pH and the chromium initial concentration about adsorption capacity of the activated carbon CK1 are observed in Fig. 5, where a significant growth of the adsorption of Cr(III) is shown as the solution pH which is increased from 2 to 5, and is showed the highest adsorption capacity of approximately 98.78% of the total content of Cr(III) for a solution of 200 ppm; when increasing the initial concentration of the solution from 200 to 2000 ppm of Cr(III) a notorious decrease of adsorption capacity is observed. For all the concentrations the maximum of the species adsorption of Cr(III) from aqueous solutions took place at pH 5; at pH bigger than 5 began to precipitate the Cr(III) as Cr(OH)3 of greenish gray color that was not favorable to the present study.


Fig. 5. The effect of aqueous solutions pH and Cr(III) and Cr(VI) initial concentration in the solution about adsorption capacity of the activated carbon CK1.

On the other hand, at pH 3, the maximum adsorption of Cr(VI) of approximately 99.53% was reached for a solution of 200 ppm; at pH bigger than 3 decreases the activated carbon CK1 adsorption capacity; when increasing the Cr(VI) initial concentration from 200 to 2000 ppm, was observed a similar trend shown in the adsorption of the Cr(III) from the aqueous solutions, to bigger Cr(VI) initial concentration which is smaller than the adsorption capacity of CK1.

Nevertheless, in the activated carbon CZ3, Fig. 6, the maximum adsorption of Cr(III) of approximately 78.87% was reached when the initial concentration was 200 ppm and pH 5. Similarly to that observed in the CK1, a smaller capacity of adsorption of Cr(III) to bigger initial concentrations (1000 and 2000 ppm) and at pH values smaller than 5.


Fig. 6. The effect of aqueous solutions pH and the Cr(III) and Cr(VI) initial concentration in the solution about adsorption capacity of the active carbon CZ3.

At pH 3 and Cr(VI) initial concentration of 200 ppm, the maximum adsorption around 79.17%, was reached, Fig. 6, which decreased significantly as the solution pH is increased up to 8 and, a similar trend was shown when increasing the initial solution concentration of 200 up to 2000 ppm Cr(VI), respectively.

3.6. Surface charge

The surface charge of the activated carbons, CK1 and CZ3, play an important role in the chromium species adsorption (anions and cations) from aqueous solutions, for it was investigated the zeta potential as a function of pH, Fig. 7, in the case of the activated carbon CK1, around pH 3.5 (and pH 3.2 in the CZ3), a lightly negative zeta potential, an increase in the solution pH up to 6, it led to a more negative zeta potential, therefore, more negative surface charges in the activated carbon.


Fig. 7. Effect of pH on the Zeta potential of activated carbons of the series CK1 (CK11, CK12) and CZ3 (CZ31, CZ32).

In the literature [1,21] is reported oxyanions formation such as Cr2O72- and CrO42- in Cr(VI) aqueous solutions at around pH values 2 and 7.5, respectively; while the presence of Cr(III) in cationic form around pH 3. The repulsion between the negatively charged surface of the activated carbons and the chromium oxyanions in the solution could be responsible for the decrease of the chromium adsorption when increasing the solution pH, consequently, an relatively high adsorption of Cr(VI) is obtained to low pH (at pH 2) and at pH 6 are favored notably the adsorption of Cr(III). The data of chromium adsorption (III and VI) displayed in the Fig. 5 and 6 are in agreement with the action of zeta potential versus the pH of the activated carbons CK1 and CZ3, respectively. The surface functional groups containing oxygen in both activated carbons are the responsible ones for the negative zeta potential [20]. The decrease in the oxyanions adsorption especially to high pH at 2 could be attributed to the interionic repulsion of the different types of oxyanions of Cr(VI) and the negatively charged functional groups. It is observed that the activated carbon CK1 has a bigger negative surface charge in the whole pH range than CZ3, however, the CK1 showed a bigger Cr(III) and Cr(VI) adsorption capacity than the activated carbon CZ3, this would be finally attributed to the biggest internal surface and microporosity.

The effect of pH on the activated carbons zeta potential obtained in atmosphere of CO2(g) is shown in Fig. 7; the surface charge of activated carbons CZ31 and CZ32 as much to low pH (< 3) as high pH (> 5) suffers a slight change with regard to CZ3; while in the activated CK11 and CK12 the surface charge suffers a very significant change to next pH 3, where the carbons show a positive zeta potential, therefore, a positive surface charge and pH around 6, the population of the negative surface charge decreases with regard to that observed in the activated CK1; the presence of positive surface charges at pH 3 favored the adsorption of the oxyanions of CrO42- notably, where a bigger interionic attraction prevails, and to high pH at 5 is very favored the adsorption of cationics species like the Cr(III) from aqueous solutions. This is in agreement with the results shown in the Fig. 8, at pH 3 the activated carbon CZ3 adsorbed at around 59.04% Cr(VI) and at pH 5 53,23% Cr(III), of the chromium species total content from aqueous solutions, while, with the CZ32 at pH 3 73.10% Cr(VI) and at pH 5 70.03% Cr(III), respectively, a significant improvement is displayed in the adsorption due to the activation in atmosphere of CO2(g), however, in those activated ones of the series CK, a very significant increase is observed in the chromium adsorption, for example, with the CK1 at pH 3 it was adsorbed 69.55% Cr(VI) and 66.78% Cr(III) at pH 5, nevertheless, in those activated in CO2(g) these values were increased notably to the same conditions, 87.55% of Cr(VI) and 84.64% of Cr(III) were adsorbed, from aqueous solutions, with the activated carbon CK12; comparing the activated carbons adsorbent properties CK12 and CZ32 with the commercial carbon (CC), this latter it showed only an adsorption capacity of the order of 35% of the total content of the chromium species from aqueous solutions.


Fig. 8. Adsorption of chromium species from aqueous solutions with the activated carbons of the series CZ3, CK1 and one commercial carbon (CC). Condition: initial concentration = 2000 ppm Cr(III) or Cr(VI), temperature 25ºC.

The adsorbent properties shown by the activated carbons in the chromium adsorption presented the following order: CK12 > CZ32 >> CC.

The capacity of relatively high chromium adsorption displayed by the activated carbon CK12 with regard to the CZ32 and commercial carbon (CC) it can be related, with several aspects, such as BET surface area and microporous volume in the activated CK12 (1266 m2/g; 0.463 cm3/g), regarding to CZ32 (1041 m2/g; 0.480 cm3/g) and activated commercial (628 m2/g; 0,287 cm3/g), what would favor a great dispersion of the surface organic functional groups (phenols and carbonyls), therefore, a bigger interaction of the chromium species with the functional groups.

3.7. Chromium adsorption from Tannery Effluents

The tannery effluents used in this investigation grouped the following characteristic: pH 4.45 and a chromium total content of 2572 ppm, once filtered to take away the solid residues, the chromium adsorption process was carried out following the same procedure, previously described, without varying original water characteristics; for this final study the best obtained activated carbons CK12, CZ32 and commercial one (CC) were used.

It is displayed in the Fig. 9 the adsorbent samples adsorption capacity, the commercial carbon (CC) adsorbed 34.24% (274.02 mg Cr/g), CZ32 66.78% (534.43 mg Cr/g) and the CK12 90.14% (721.38 mg Cr/g), respectively, of the total content of chromium from the effluents. The activated carbon CK12, again, is reached the maximum chromium adsorption, this virtue is attributed to the biggest adsorbent property shown during the whole carried out study which is associated to the biggest BET surface area, microporous volume and population of the functional organic groups located in their external and internal surface, respectively.


Fig.9. Chromium adsorption from tannery effluents. Conditions: initial concentration = 2572 ppm, pH = 4.45, temperature 25ºC.

CONCLUSIONS

Activated carbons of high-adsorbent properties, surface area and microporosity were obtained from coconut shells by means of chemical activation using as activating agents the KOH and ZnCL2, in atmosphere of N2(g) and CO2(g), respectively. The activated carbon adsorbent properties are dependent of the activation parameters. The presence of functional organic groups, phenols and carbonyls was determined by Fourier transform infrared spectroscopy, which is responsible for the surface charge of the activated carbons and it favored the chromium species adsorption from aqueous solutions and tannery effluents. The chromium adsorption depend on the pH solution and chromium initial concentration. For a solution of 2000 ppm of Cr(III) or Cr(VI), it was adsorbed at around 84.64% (635.42 mg Cr(III)/g) at pH 5 and 87.55% (657.27 mg Cr(VI)/g) at pH 3 with the activated carbon CK12, at 25ºC; 70.03% (560.44 mg Cr(III) /g) and 73.10% (585.01 mg Cr(VI) /g) with the CZ32; 35.03% (280.34 mg Cr(III) /g) and 33.95% (271.70 mg Cr(VI) /g) with the commercial carbon (CC), to the same pH and temperature conditions, of the chromium total content in the aqueous solutions.

The tannery effluents were treated with the biggest obtained activated carbons and a commercial one, it was adsorbed approximately 90.14% (721.38 mg Cr/g) of the chromium total content with the activated carbon CK12, 66.78% (534.43 mg Cr/g) with the CZ32, and only the 34.24% (274.02 mg Cr/g) with the CC, respectively, at pH 4.45.

ACKNOWLEDGMENTS

The authors thank Dra. Ruby Cid Araneda and Dr. Renán Arriagada Acuña from Research Laboratory ZEOCAR of the Facultad de Ciencias Químicas of Universidad de Concepción, Concepción, Chile, for helpful comments.

 

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*E-mail address: s_bendezu@hotmail.com (S. Bendezú).

 

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