a comparative study of copper and zinc ion adsorption on to activated and non-activated date-pits

319

A Comparative Study of Copper and Zinc Ion Adsorption on to Activatedand Non-activated Date-pits

Fawzi Banat*, Sameer Al-Asheh and Deaa Al-Rousan Department of Chemical Engineering, Jordan

University of Science and Technology, PO Box 3030, Irbid-22110, Jordan.

(Received 30 August 2001; accepted 7 December 2001)

ABSTRACT: Date-pits (an agricultural by-product available commercially) wereutilized, with and without activation, as an adsorbent for the removal of Zn2+ andCu2+ ions from aqueous solutions. Activated carbons were prepared from date-pitsby carbon dioxide activation at 700ºC. The effects of contact time, pH, temperatureand the adsorbent concentration on the removal of Zn2+ and Cu2+ ions were studied.The Freundlich isotherm model described the equilibrium adsorption data. Non-activated date-pits exhibited higher Zn2+ and Cu2+ ion uptake than activated date-pits.The uptake of Cu2+ ions by both activated and non-activated date-pits was higherthan the uptake of Zn2+ ions. The uptake of both metal ions increased on increasingthe pH value of the system from 3.5 to 5.0 as well as on decreasing the temperaturefrom 50ºC to 25ºC. Adsorption capacities for the non-activated date-pits towardsCu2+ and Zn2+ ions as high as 0.15 mmol/g and 0.09 mmol/g, respectively, wereobserved. This study demonstrated that date-pits without any physical or chemicalpretreatment could be used as an effective adsorbent for the treatment of waterscontaining heavy metal ions such as Zn2+ and Cu2+.

INTRODUCTION

The presence of heavy metals in water and wastewater has been of great concern to environmentalists,mainly because of their persistence, toxicity and cumulative character. Heavy metals are mainlyintroduced into surface water from industrial effluents such as those from metallurgical processes,industries involving metal plating, pigmentation and dyes, storage battery manufacture and surface-finishing industries (Kadirvelu et al. 2001). There is a considerable interest in the recovery ofheavy metals from industrial wastewaters due to stringent environmental regulations and the highvalue of many metals.

Copper and zinc are ubiquitous in the environment and are thus frequently found in surfacewater. Although copper and zinc are essential trace elements for living organisms, they becometoxic at high concentrations. Copper, in particular, combines with enzymes and other metabolicagents connected with respiration and renders them inactive. The intake of excessively large dosesof copper by man leads to hepatic and renal damage and central nervous system irritation followedby depression (WHO 1984). In addition, plants grown in a polluted environment can accumulatehigh concentrations of metals, causing a serious risk to human health when plant-based food stuffsare consumed (Wenzel and Jackwer 1999). According to the World Health Organization guide-lines (WHO 1984), the maximum acceptable level of copper and zinc in drinking water is 1 mg/land 3 mg/l, respectively.

*Author to whom all correspondence should be addressed. E-mail: [email protected].

320 Fawzi Banat et al./Adsorption Science & Technology Vol. 20 No. 4 2002

Various methods have been proposed for the removal of heavy metal ions from wastewaters,such as precipitation, solvent extraction, membrane processes, ion exchange and adsorption (Al-Asheh and Duvnjak 1997). The recovery of heavy metal ions from aqueous solutions using activatedcarbons is currently of great interest because of the large specific surface areas of such adsorbents.However, since the price of commercial activated carbon has increased continuously over the lastdecade, increasing attention has focused on the use of carbonaceous agricultural solid wastes dueto their low cost (Lua and Guo 2001a). Generally, adsorbents can be classified as low-cost materialsif they require little processing, are abundant in nature, or are by-products or waste materialsarising from another industry (Bailey et al. 1998).

Many agricultural by-products have been examined for their ability to remove heavy metal ionsfrom wastewaters without activation. For example, coconut husk and palm pressed fibres havebeen used to remove chromium ions (Tan et al. 1993), beech leaves for the removal of cadmiumions (Salim et al. 1992), waste tea leaves for the removal of lead, cadmium and zinc ions (Tee andKhan 1988), sawdust for the removal of copper ions (Ajmal et al. 1998) and rice husks for theremoval of mercury ions (Khalid et al. 1999). Reports have also appeared on the preparation ofactivated carbons derived from agricultural by-products. Thus, activated carbon derived from ricehusks (Srinivasan et al. 1988), peanut hull (Periasamy and Namsivayam 1995), almond and pecanshells (Toles et al. 1997), olive and peach stones (Ferro-Garcia et al. 1988), sugarcane bagasse,rice straw pellets and soybean hull pellets (Mitchell et al. 1998) and coconut shell (Arulananthan etal. 1989) have all been employed for the removal of different types of heavy metal ions fromaqueous solutions.

Date palm is a principal fruit grown in many regions of the world and date-pits constitute ca. 6–12% of the fruit (Barreveld 1993). Date-pits are available in huge quantities when pitted dates areproduced in packing plants or in industrial date-processing plants based on juice extraction. In theUnited States, date-pits have constituted a major problem to the date industry as a solid waste.Pulverized ground date-pits are used on a small scale as a type of road base gravel on dirt roads.For these reasons, a use should be found for date-pits in order to improve the economics of theoperation as a whole and to decrease disposal problems and costs. Lua and Guo (2001a,b) examineddifferent activation techniques for the preparation of activated carbons from date-pits. They foundthat the adsorptive capacity of SO

2 by such activated carbons was comparable to that of some

commercial activated carbons. However, to our knowledge, there are no reports in the literatureconcerning the potential use of activated or non-activated date-pits for the removal of heavy metalions from aqueous solutions.

The objectives of the present research were: (a) to prepare activated carbon from date-pits; (b) toexamine the potential use of activated and non-activated date-pits in the treatment of waterscontaminated with heavy metal ions; (c) to compare the adsorptive capacity of these adsorbentswith that of coal-based commercial granular activated carbons towards Cu2+ and Zn2+ ion removalfrom aqueous solutions; and (d) to study the effect of operating parameters, such as solution pH,temperature, adsorbent concentration and adsorbate concentration, on the adsorption process.

EXPERIMENTAL

Adsorbent

Date-pits were collected from date-processing plants and dried in sunlight. The pits were thenthoroughly washed with distilled water to remove all dirt and oven-dried overnight at 105ºC. Thedried pits were crushed, milled and sieved into different particle sizes. Studies were focused on the

Comparative Study of Cu2+ and Zn2+ Ion Adsorption on to Date-pits 321

0.125–0.212 mm fraction that was used as such without any physical or chemical pretreatment, orwas converted physically to activated carbon and used in the adsorption experiments.

The process of converting date-pits to activated carbon involved two stages: carbonization andactivation. Both stages were carried out in a stainless steel reactor (300 mm length, 28 mm i.d.)placed in a vertical tube electrical furnace (Heraeus D-6450) with ca. 10 g date-pits being placedon a metal mesh in the reactor. Carbonization was carried out in nitrogen as an inert atmosphere for40 min at 500ºC. The resulting chars were activated physically using carbon dioxide for 30 min at700ºC. Subsequently, the activated date-pits were removed from the reactor, washed with 0.1 MH

2SO

4 to dissolve and remove any residual ash, and then washed thoroughly with distilled water.

Batch adsorption experiments

Copper ion solutions with concentrations in the range 10–50 mg/l and Zn2+ ion solutions in therange 20–100 mg/l were prepared from copper sulphate (CuSO

4�5H

2O) and zinc sulphate

(ZnSO4�7H

2O), respectively. Batch adsorption equilibrium experiments were undertaken by

contacting a known amount of adsorbent with 50 ml Cu2+ or Zn2+ ion solution in sealed glassbottles. The bottles were placed in a temperature-controlled shaker for a period of time sufficient toensure that equilibrium had been achieved. Kinetic experiments showed that such equilibrium wasestablished within 2–8 h. However, to ensure complete equilibrium, bottles for all such experimentswere left in the shaker for 24 h. The samples were then centrifuged and the residual concentrationof Cu2+ or Zn2+ ions in the clear supernatant solution determined by atomic absorption methods(Spectro AA10 spectrophotometer, Varian). Two replicates per sample were undertaken with theaverage value being used to calculate the metal ion uptake by the adsorbent. The difference betweenthe initial concentration, C

0, and the equilibrium concentration, C

e, was used to calculate the uptake,

qe, using the following equation:

(C0 – C

e)V

qe

= –––––––– (1) M

where V is the volume of solution and M is the amount of adsorbent employed.The effect of solution pH on the sorption of Cu2+ and Zn2+ ions was examined at four pH levels:

3.5, 4.0, 4.5 and 5.0, with pH adjustments being made using 0.1 M HCl solution. Experimentswere conducted at 25ºC, 40ºC and 50ºC, respectively, to determine the effect of temperature on theadsorption of Cu2+ or Zn2+ ions.

RESULTS AND DISCUSSION

Kinetics of the sorption process

The kinetics of the adsorption of Cu2+ and Zn2+ ions by activated and non-activated date-pits wereinvestigated using batch experiments employing different initial metal ion concentrations. Thevariation of metal uptake in solution with time for both Zn2+ and Cu2+ ion sorption are presented inFigures 1 and 2, respectively. As shown in the figures, metal ion uptake increased with increasinginital metal ion concentration for both non-activated and activated date-pits.

For both Zn2+ and Cu2+ ion removal by non-activated date-pits, the sorption process was so fastthat most of the removal occurred during the first 2 h, with no appreciable change in metal ion


Figure 1. Effect of Zn2+ ion concentration on its uptake by (a) non-activated date-pits and (b) activated date-pits. Sorbentconcentration = 5 mg/ml; initial metal ion concentration (mg/l): , 25; , 45; D, 55; Ñ, 85; {, 125.


Figure 2. Effect of Cu2+ ion concentration on its uptake by (a) non-activated date-pits and (b) activated date-pits. Sorbentconcentration = 5 mg/ml; initial metal ion concentration (mg/l): , 10; , 20; D, 30; Ñ, 40; {, 50.


uptake being observed after that time. However, the time necessary to reach equilibrium for bothCu2+ and Zn2+ ion removal by activated date-pits was dependent on the initial metal ion concentrationand was longer than that required in the case of non-activated date-pits.

For both Zn2+ and Cu2+ ion removal by activated date-pits, a relatively fast uptake occurredduring the first hour followed by a gradual removal at a lower rate over the next 7 h. This suggeststhat surface diffusion flux was more important than pore diffusion flux during the first hour butless important over the next 7 h. The appearance of a slow rate removal region when activated date-pits were used may be explained by the fact that activation usually increases the porosity of asorbent and as a result magnifies the effect of pore diffusion which is slower than surface diffusion.

However, this increase in porosity did not lead to an increase in the Zn2+ and Cu2+ ion uptake byactivated date-pits relative to the non-activated variety as shown by the data recorded in Figure 3.For this reason, it is believed that some surface functional groups capable of participating in thesorption process were destroyed during thermal activation so that no improvement was effected inthe adsorptive capacity of the date-pits. Lua and Guo (2001a) have reported that the thermal activationof date-pits had a significant effect on the surface organic functional groups.

In general, the uptake of Cu2+ ions by activated or non-activated date-pits was higher than forZn2+ ions. This is in agreement with explanations based on ionic radii found in the literature. Forexample, Ferro-Garcia et al. (1988) found that the amount of Cu2+ ions adsorbed on carbons derivedfrom agricultural by-products such as almond shells, olive stones and peach stones was higher thanthat of Zn2+ ions under similar operating conditions. They attributed this behaviour to the fact thatCu2+ ions have a smaller ionic radius than Zn2+ ions so that Cu2+ ions will be capable of greateraccessibility to the surface of certain pores than Zn2+ ions.

Equilibrium isotherms

Relationships between the equilibrium concentrations in solution and the corresponding metal ionuptakes by activated and non-activated date-pits were obtained and are presented in Figure 4 in theform of the linearized Freundlich isotherm model which may be written as:

ln qe

= ln kF

+ l/n ln Ce

(2)

where qe (mmol/g) corresponds to the adsorption capacity in equilibrium with a metal ion

concentration Ce (mmol/l) in solution and k

F and n are the Freundlich coefficients. The value of k

F

is related to the adsorption capacity while the l/n value is related to the adsorption intensity. Thesecoefficients can be obtained from the intercept and slope of the linear plot of ln q

e versus ln C

e. It

will be seen from Figure 4 that the linearized Freundlich isotherm model fitted the experimentaldata well, a fact supported by the value of R2, the goodness-of-fit, listed in Table 1.

Table 1 also lists the Freundlich isotherm constants obtained. Since the value of kF is an indication

of the sorption capacity of the sorbent, it should be noted that the sorption capacity of non-activateddate-pits towards both Zn2+ and Cu2+ ions was higher than that of activated date-pits, and that thesorption capacity towards Cu2+ ions by the two adsorbents was higher than that towards Zn2+ ions.The change in the k

F value was greater in Cu2+ ion sorption than in Zn2+ ion sorption by both

activated and non-activated data-pits, indicating that activation had a more pronounced effect onCu2+ ion sorption than on Zn2+ ion sorption. The values of l/n listed in Table 1 demonstrate that thesorption intensity of the Cu2+ ion by the two adsorbents was greater than that of the Zn2+ ion andthat the sorption intensities of both Zn2+ and Cu2+ ions were decreased by activation. Briefly, theseresults show that Zn2+ and Cu2+ ion uptake by activated date-pits was less than that by non-activatedpits.


Figure 3. Comparison between non-activated and activated date-pits for the removal of (a) Zn2+ ions and (b) Cu2+ ions.Sorbent concentration = 5 mg/ml.


Figure 4. Freundlich isotherms for the sorption of (a) Zn2+ ions and (b) Cu2+ ions by: , non-activated date-pits; , activateddate-pits. Sorbent concentration = 5 mg/ml.


TABLE 1. Freundlich Constants for the Sorption of Zinc and Copper Ions by Activated andNon-activated Date-pitsa

System kF

l/n R2

ì Zn2+ ions 0.078121 0.32684 0.99739Non-activated date-pits í

î Cu2+ ions 2.565337 1.29798 0.9939

ì Zn2+ ions 0.068935 0.41162 0.99213Activated date-pits í

î Cu2+ ions 1.79614 1.37741 0.99918

aSorbent concentration = 5 mg/ml.

Effect of initial pH

The pH value of an aqueous solution is an important controlling parameter in an adsorption process(Benefield et al. 1982). The effect of pH on Zn2+ and Cu2+ ion adsorption was studied over the pHrange 3.5–5.0 and the corresponding experimental data are depicted in the linearized form of theFreundlich equation in Figures 5 and 6, with the numerical values of the Freundlich constants beinglisted in Table 2. As illustrated in Figures 5 and 6, uptake of metal ions by the non-activated andactivated date-pits was pH-dependent with an increase in pH leading to an increase in the metal ionuptake. This was partly due to hydrogen ions competing with the metal cations for the exchangeablesites on the sorbent surface where they were strongly adsorbed at lower pH and partly due to theinfluence of pH on the surface charge of the sorbent. These results are compatible with the findingsof other researchers. For example, Ajmal et al. (1998) found that Cu2+ ion uptake by sawdustdecreased at lower pH values and Tee and Khan (1988) found that Zn2+ ion uptake by waste tea leavesdecreased at lower solution pH. These results confirm previous findings concerning the levels ofuptake by activated and non-activated date-pits, since at any solution pH the uptake of Zn2+ and Cu2+

ions by non-activated date-pits remained higher than that by activated date-pits (Figure 7).

TABLE 2. Freundlich Constants for the Influence of pH on the Sorption of Zinc and Copper Ions byActivated and Non-activated Date-pitsa

Adsorbent pH Zinc ions Copper ions

kF l/n R2 kF l/n R2

ì 3.5 0.0332 1.3021 0.9943 0.2569 1.5020 0.9938ï 4.0 0.0554 1.0894 0.9874 0.6652 1.8494 0.9949Non-activated date-pits í

ï 4.5 0.0747 0.5940 0.9919 0.3934 0.9967 0.9703

î 5.0 0.0929 0.5562 0.9918 0.8458 1.0085 0.9983

ì 3.5 0.0094 0.6656 0.9797 0.5653 1.6340 0.9843ï 4.0 0.0257 0.6475 0.9913 0.7517 1.2965 0.9726Activated date-pits íï 4.5 0.0392 0.4813 0.9683 0.3313 0.6149 0.9733

î 5.0 0.0596 0.3612 0.9396 0.2393 0.3491 0.9897

aSorbent concentration = 5 mg/ml.


Figure 5. Freundlich isotherms for Zn2+ ion removal by (a) non-activated date-pits and (b) activated date-pits at different pHvalues: , –––, 3.5; , -----, 4.0; D, ....., 4.5; Ñ, –.–.–, 5.0. Sorbent concentration = 5 mg/ml.


Figure 6. Freundlich isotherms for Cu2+ ion removal by (a) non-activated date-pits and (b) activated date-pits at different pHvalues: , –––, 3.5; , -----, 4.0; D, ....., 4.5; Ñ, –.–.–, 5.0. Sorbent concentration = 5 mg/ml.


Figure 7. Comparison between the effect of the initial pH value on the Zn2+ and Cu2+ ion uptake by activated and non-activated date-pits. Sorbent concentration = 5 mg/ml; initial Zn2+ and Cu2+ ion concentrations = 40 mg/l.


TABLE 3. Freundlich Constants for the Influence of Temperature on the Sorption of Zinc and Copper Ionsby Activated and Non-activated Date-pitsa

Adsorbent Temperature Zinc ions Copper ions(ºC)

kF l/n R2 kF l/n R2

ì 25 0.0781 0.3268 0.9974 2.5653 1.2980 0.9939Non-activated date-pitsí 40 0.0677 0.4223 0.9951 1.3295 1.2229 0.9989

î 50 0.0618 0.4420 0.9972 1.4591 1.4636 0.9918

ì 25 0.0689 0.4116 0.9921 1.7946 1.3774 0.9992Activated date-pits í 40 0.0619 0.4597 0.9984 0.9686 1.2694 0.9949

î 50 0.0529 0.4841 0.9982 0.7559 1.2648 0.9997

Sorbent concentration = 5 mg/ml.

Effect of temperature

The effect of temperature on the sorption capacity of the sorbents investigated towards Zn2+ andCu2+ ions was investigated at three different temperatures. The adsorption data obtained are presentedin the linear form of the Freundlich model in Figures 8 and 9. It will be seen that the adsorptiondata for the two metal ions fitted the Freundlich model well as supported by the values of theregression coefficient R2 listed in Table 3. The results show that the uptake of metal ions diminishedwith increasing temperature thereby indicating that adsorption of Zn2+ and Cu2+ ions by the twotypes of date-pits studied was exothermic in nature. An increase in temperature could causedissociation of some active sites responsible for metal ion sorption on the adsorbent surface andthus decrease the number of available sites for metal ion sorption.

As for any equilibrium system, the thermodynamic parameters that characterize the equilibriumsystem are the enthalpy change (DH), the Gibbs free energy change (DG) and the entropy change(DS). These thermodynamic parameters were determined via the following equations (Lopez-Delgado et al. 1996):

FK

c= –––– (3)

1 – F

DG = –RT ln Kc

(4)

–DH DSln K

c= –––– + ––– (5)

RT R

where F is the fraction of metal ions adsorbed at equilibrium. The values of DH and DS may becalculated from the slope and intercept, respectively, of the plot of equation (4). The values ofthese thermodynamic parameters are listed in Table 4.

The negative values of DG for the Cu2+ ion and the corresponding positive values for the Zn2+ ionindicate that the sorption of the Cu2+ ion was more favourable and spontaneous than that of the Zn2+

ion. The negative values of DH confirm the exothermic nature of the adsorption process while thenegative values of DS indicate the possibility of a favourable adsorption process. Viraraghavan


Figure 8. Freundlich isotherms for Zn2+ ion removal by (a) non-activated date-pits and (b) thermally activated date-pits atdifferent temperatures. Temperature (ºC): , –––, 25; , -----, 40; D, ....., 50. Sorbent concentration = 5 mg/ml.


Figure 9. Freundlich isotherms for Cu2+ ion removal by (a) non-activated date-pits and (b) thermally activated date-pits atdifferent temperatures.Temperature (ºC): , –––, 25; , -----, 40; D, ....., 50. Sorbent concentration = 5 mg/ml.


TABLE 4. Thermodynamic Parameters for the Adsorption of Zinc and Copper Ionsa

Adsorbent Metal DG (kJ/mol) DH DS R2

ion25ºC 40ºC 50ºC

(kJ/mol) (kJ K/mol)

Non-activated date-pitsì Zn2+ 0.891 1.745 2.090 –13.661 –0.049 0.991í

Cu2+ –4.816 –3.741 –3.003 –26.410 –0.073 1.000î

Activated date-pitsì Zn2+ 1.565 2.097 2.535 –9.899 –0.038 0.998í

Cu2+ –3.487 –2.750 –2.412 –16.476 –0.044 0.997î

aInitial metal ion concentration = 40 mg/ml.

and Dronamraju (1993) found that the values of DG, DH and DS were all negative for the adsorptionof Cu2+, Ni2+ and Zn2+ ions from wastewater by fly ash. According to these authors, association,fixation or immobilization of metal ions due to adsorption may be the reason behind the decreasein entropy for the adsorption process.

Mitchell et al. (1998) examined the effect of different physical and chemical oxidation treatmentson the adsorptive capacity towards Cu2+ ions of granular activated carbons (GACs) made fromseveral agricultural by-products. They found that Cu2+ ion uptake lay approximately in the range0.01–0.8 mmol Cu2+ ion/g GAC. As commercial GACs usually adsorb between 0.2 and 0.3 mmolCu2+ion/g GAC, they concluded that carbons made from some agricultural by-products such assoybean hulls possessed metal adsorption capacities three- or four-times greater than anycommercially available GAC. The adsorption capacity towards Cu2+ ions of activated and non-activated date-pits was dependent on the operating conditions and lay in the range 0.02–0.15mmol/g. Thus, the adsorptive capacity towards Cu2+ ions of non-activated date-pits is almost one-half that of commercial GACs.

CONCLUSIONS

Date-pits that arise as a solid waste from date-processing plants were examined, both with andwithout activation, for the treatment of Zn2+ or Cu2+ ion-bearing water solutions. Under similaroperating conditions, the uptake capacities towards Zn2+ and Cu2+ ions by the non-activated materialwere higher than those for the CO

2-activated material. The uptake of both metal ions was dependent

on the solution pH, temperature and initial metal ion concentration. Such uptake increased withincreasing initial metal ion concentration and with increasing solution pH. However, increasingtemperature led to a decrease in the metal ion uptake. The Freundlich isotherm model was applicableto the adsorption data for Zn2+ and Cu2+ ions. The thermodynamic parameters obtained confirmedthe exothermic nature of the adsorption process. The uptake capacity for Cu2+ ions exhibited byactivated and non-activated date-pits was greater than that towards Zn2+ ions, the uptake capacitytowards Cu2+ ions by non-activated date-pits being about one-half that of commercial GACs towardsthe same ion. Nonetheless, the fact that date-pits can be utilized as an adsorbent without any pre-treatment makes it a good candidate for the removal of metal ion pollutants such as copper andzinc, which are ubiquitous in aqueous solution.


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a comparative study of copper and zinc ion adsorption on to activated and non-activated date-pits

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