Adsorption of copper and cadmium ions from ...

REMOVAL OF HEAVY METALS AND DYES FROM AQUEOUS SOLUTION BY DATE PITS Nasir Al-Laqtaha,*, Yousef Salamha, Gavin Walkera, Mohammad A. Al-Ghoutib, Mohammad N.M. Ahmada. School of Chemistry and Chemical Engineering, Queen’s University Belfast, Stranmillis Road, Belfast, BT9 5AG, UK a Industrial Chemistry Centre, Royal Scientific Society P.O. Box: 1438 Amman, Jordan a

Abstract: A potential usefulness of raw date pits as an inexpensive solid adsorbent for MB, Cu2+ and Cd2+ has been demonstrated in this work. This work was to provide fundamental equilibrium adsorption isotherms information and to investigate the mechanisms of adsorption of methylene blue MB, Cu2+ and Cd2+ from aqueous solution onto raw date pits. The results summarised herein are also part of an investigation conducted to evaluate the adsorption capacity by taking into consideration the experimental parameters such as pH, particle size and initial solute concentration. The experimental data obtained from the adsorption isotherms studies were tested for their applicability to two isotherm models, namely Langmuir and Freundlich, models. The Langmuir isotherm was not used to evaluate the experimental results of Cu2+ and Cd2+ since the experimental adsorption isotherm data (Ce vs. qe) were slightly linear and did not fit with typical Langmuir isotherm form. Keywords: Adsorption, Metal ions, Methylene blue, Date pits, Langmuir isotherm, Freundlich isotherm.

1. INTRODUCTION The existence of heavy metals and dyes in the aquatic system can be detrimental to a variety of living species. Many industrial processes discharge aqueous effluents containing heavy metals and dyes. Heavy metals and dyes are non-biodegradable and tend to accumulate in living organisms, causing various disorders for living organisms. Over 7x105 tons of synthetic dyes are produced annually worldwide. It is estimated that 10–15% of the dyes are lost in the dye effluent during such dyeing processes. The dyeing operations are of primary environmental concern for the textile industry for a number of reasons: dyeing is a water intensive process; large amounts of salt are often needed to improve dye fixation on the textile material; many dyes contain heavy metals (e.g. chromium and copper) as either a dye component or as a contaminant and unfixed dye releases high doses of colour to mill effluent, as well as salt and metals. Cadmium concentration in the range 0.1–100 mg/L are typical in wastewater from several industries (chemical and metal product facilities, leather and tanning processes, electricity and gas production and sanitary industries)(Vullo et al., 2008). Cadmium and copper, the metals considered in this study, are the widely used materials, where an intake of excessively large doses by man may lead to serious kidney failure and liver disease. Accordingly, improved and innovative methods of water and wastewater treatment are continuously being developed to treat water-containing metals (Al-Ghouti et al., 2003, Xue et al., 2008). Dates constitute part of a popular subsistence among the populace of the Middle East. The fruit of the date palm is composed of a fleshy pericarp and seed. Pits of date palm (seed) are a waste product of many date fruit processing plants producing pitted dates, date powders, date syrup, date juice, chocolate coated dates

*Corresponding author, Tel.: +442890974175, Email address: [email protected]

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and date confectionery (Rahman et al., 2007). At present, pits are used mainly for animal feeds in the cattle, sheep, camel and poultry industries. A potential usefulness of raw date pits (RDP) as an inexpensive solid adsorbent for a number of metal ions and MB has been demonstrated earlier (Saad et al., 2008). Banat et al. (2002, 2003a, and 2003b) also used raw date-pits and date-pit activated carbons for the adsorption of zinc and copper ions from water. Activated date-pit carbons were also used for the adsorption of cadmium ions and other organic compounds from water. They studied the effect of contact time, pH, temperature, cadmium ion concentration, adsorbent dose, salinity, as well as the activation temperature on the removal of metal ions by date-pits. Non-activated date-pits exhibited higher Zn2+ and Cu2+ ion uptake than activated date-pits. The uptake of both metal ions increased on increasing the pH value of the system from 3.5 to 5.0 as well as on decreasing the temperature from 50 to 25◦C. Adsorption capacities for the non-activated date-pits towards Cu2+ and Zn2+ ions as high as 0.15 mmol/g and 0.09 mmol/g, respectively, were observed. Various treatment techniques available for heavy metals and dyes are reduction, ion exchange, adsorption, reverse osmosis and chemical precipitation. Most of these methods suffer from drawbacks like high capital and operational cost and there are problems in disposal of the residual metal sludge (Al-Ghouti et al., 2003). The use of activated carbons to remove organic and inorganic pollutants from waters is widely extended, because of their high surface area, microporous character and the chemical nature of their surface. However, they are expensive and their regeneration cost is also high. So there is a need for lowcost and readily available materials for the removal of toxic pollutants from water (Vullo et al., 2008). Adsorption is arguably the most important of the physicochemical processes responsible for the retention of inorganic and organic substances in the aqueous environment. Factors such as pH, nature and concentration of substrate and adsorbing ion, ionic strength, and the presence of competing and complex ions, affect the extent of adsorption. This work is also to explore the feasibility of using RDP an adsorbent for the removal of heavy metals and MB. Therefore, the aim of this work was to provide fundamental equilibrium adsorption isotherms information of MB, Cu2+ and Cd2+ from aqueous solution onto RDP. The results summarised herein are also part of an investigation conducted to evaluate the adsorption capacity by taking into consideration the experimental parameters such as pH, particle size and initial solute concentration

2. REAGENTS AND MATERIALS 2.1. Chemicals Deionized distilled water was used to prepare all solutions and suspensions. Stock solutions of metal ions were prepared from their sulphates (CuSO4·4H2O, CdSO4, analytical reagent grade, Fluka Chemie AG, Buchs, Switzerland). Methylene blue, Basic dye 9 (C.I. 52015) was used. All solute standards were prepared by dissolving appropriate amount of solute into deionized water.

2.2. The adsorbent The raw date pits were collected and washed with distilled water to be completely free from dirt and inherent pulp, dried in at 105 ◦C for 3 h and finally crushed to give a dark brown powder with different particle sizes.

2.3. Equilibrium adsorptions isotherm The equilibrium isotherms are very important in designing adsorption systems. To estimate the adsorption characteristics of an adsorbent, the isotherm adsorption of that adsorbent with a specific adsorbate is carried out. Concentration variation method is used to calculate the adsorption characteristic of adsorbent and the process. It is mainly carried out by selecting an appropriate concentration range of the adsorbate with a fixed mass of adsorbent. The ratio of adsorbent mass and the volume of solute solution was (1:1).


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Aqueous solutions of heavy metals (Cu2+ and Cd2+) and methylene blue (MB) were prepared in a final concentration of 1-50, and 50–900 mg/dm3 with 5 and 50 mg/dm3 intervals, respectively. The adsorption isotherm experiments were carried out in 100 cm3 glass bottles where 0.05 g of RDP and 50 cm3 of the appropriate concentration of the test solute solution were added. The bottle samples were subsequently capped and shaken in a shaker (Gerhardt Bonn type 655) for 72 h at 20oC (room temperature). The particle size range from 125 to 710 µm was used. The pH of the solutions was adjusted to its appropriate value by adding either 1 M HCl or 1 M NaOH (pH range from 2 to 10). After 72 h, equilibrium was reached and equilibrium concentration was estimated. The samples were filtered through a 0.45 µm cellulose nitrate membrane filter (Swinnex-25 Millipore). Duplicate samples were measured and the standard error in the readings was less than 3%. Blanks were also used. The initial and equilibrium Cu2+ and Cd2+ concentrations were determined using inductive couple plasma (ICP), IRIS Intrepid by ThermoFisher Scientific Product. The initial and equilibrium MB concentrations were determined using a Perkin–Elmer UV–Vis spectrophotometer corresponding to λ max of MB dye. The concentrations were calculated using the Beer-Lambert equation:

Absorbance = ε.Cs .l

(1)

where ε is the molar absorptivity, Cs the concentration of sample, and l the thickness of the absorbing medium (1 cm). Percentage solute removal was then calculated as follows:

removal (%)

Ci Ce 100% Ci

(2)

where Ci and Ce are the initial and equilibrium solute concentrations, respectively. 3. RESULTS AND DISCUSSION 3.1. Adsorption isotherms Effect of pH value: Preliminary investigations showed that the equilibrium is attained in 72 hours. In these experiments, the initial pH of solute containing solution was adjusted to the desired value using either 1 M HCl or 1 M NaOH solution. To the pH adjusted solute solution, RDP were added. The effect of pH on the percentage of solute removal from aqueous solution by RDP is illustrated in Figure 1. The Figure shows the maximum percentage removal of the MB from aqueous solution taking place at pH (pH 4–10), and the removal rate decreases with the decrease in pH. Lower adsorption of MB at low pH is probably due to the presence of excess H+ ions competing with the cation groups on the dye for the adsorption sites. The percentage removal of the Cd2+ decreased with further increase in pH (above pH 10), and the slightly constant removal rate was achieved under acidic and basic conditions (pH 2-10). Variation in pH can affect the surface charge of the adsorbent and the degree of ionisation and speciation of the metal adsorbate. As the surface charge density decreases with an increase in the solution pH, the electrostatic repulsion between positively charged dye (MB) and the surface of RDP is lowered, which may result in an increase in the rate of adsorption (Chen and Lin, 2001). An optimum pH range usually between pH 4.0 and pH 6.0 leaves the binding sites unprotonated and metal binding is maximised. At pH’s above this optimum range, most metals tend to precipitate out of solution in the hydroxide form (O’Connell et al., 2008). RDP is acidic in nature (pHsolution = 4.6) due to the presence of various functional groups that include alcohols, phenolic hydroxides, and ethers. Copper or cadmium ion may form complex with surface functional groups of RDP such as cellulose-OH, and phenolic –OH through ion-exchange reactions. At lower pH (that is under acidic condition), the functionality of these groups is not changed. At higher pH, this group begins to neutralize changing their activity, binding properties. A slightly constant behaviour for Cu2+ and Cd2+ was observed between pH values 2-10.


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MB

120

Cu Cd

100

% Removal

80

60

40

20

0 0

2

4

6

8

10

12

14

pH value

Figure 1: pH dependency of MB, Cu2+ and Cd2+ onto RDP. Experimental conditions: mass of adsorbent = 0.05 g, volume of solute solution = 50 cm3, equilibrium time = 72 hours, shaking speed = 80 rpm, particle size 125-300 μm, temperature = 22oC, initial MB concentration = 100 mg/dm3, and initial Cu2+ and Cd2+ concentration = 50 mg/dm3. Effect of Particle Sizes: Understanding the mechanism of heavy metals and dyes adsorption on RDP surface is essential for an effective removal of these solutes from wastewaters. The breaking up of the large particle to form smaller ones probably serves to open the sealed channels in the adsorbent, which then becomes available for adsorption. The size of the particle investigated in this work has little effect on solute adsorption onto RDP at low initial solute concentrations as expressed by the total surface area when the particle size changed from (125-300) to (500-710) μm. However, this effect was noted for MB, Cu2+, and Cd2+ at equilibrium concentrations higher than 100, and 2 mg/dm3, respectively. This behaviour indicates that the mechanism of adsorption did not depend on the particle size alone (i.e. surface area or channels) (Al-Ghouti et al., 2003). The adsorption isotherm data were further analysed using the Langmuir and Freundlich’s models, which are the most frequently applied models (Al-Ghouti et al., 2003). The experimental data were fitted to the Langmuir, and Freundlich equations (Eqs. (4) and (5), respectively) to describe the adsorption of MB, Cu2+, and Cd2+.

X m

qe

K L Ce 1 a L Ce

(3)

where qe is the equilibrium solid phase solute concentration. It is usually expressed as the amount of solute adsorbed per unit mass of adsorbent (mg/g), X the amount of solute adsorbed (mg), m the mass of adsorbent (g), KL the adsorption constant (dm3/g), Ce the equilibrium liquid phase concentration (mg/dm3), aL the Langmuir isotherm constant (dm3/g); it is related to KL by Qmon.aL = KL and Qmon is the Langmuir monolayer capacity.

qe

K F Ce

1/ n

(4)

where KF is the Freundlich isotherm constant (mg/g (mg/dm3)n) which is an indicator of the adsorption capacity and n refers to adsorption tendency. Linearised plot of log qe vs. log Ce is obtained from the model and KF and 1/n can be determined from the slope and intercept. The value of 1/n is indicative of the relative energy distribution on the adsorbent surface.


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The experimental data obtained from the adsorption isotherm studies were tested for their applicability to two isotherm models, namely Langmuir and Freundlich, models and the results are shown in Figure 3. For Langmuir model, Ceq/qe versus Ce plots were constructed from the experimental data of MB. These plots gave non-linear plots. In other words, it indicates that there was a heterogeneous adsorption. Moreover, the experimental data was well fitted with the Freundlich model. The Langmuir isotherm was not used to evaluate the experimental results of Cu2+ and Cd2+ since the experimental adsorption isotherm data (Ce vs. qe) were slightly linear and did not fit with typical Langmuir isotherm form. However, the linear isotherm is appropriate for adsorption relationships in which the energetics of adsorption is uniform with increasing concentration and the loading of the adsorbent is low (Benjamin and Leckie, 1981). When the data of MB adsorption onto RDP were plotted according to linearised form of the Langmuir isotherm model, two lines were obtained at low and high concentrations. This indicated the existence of two different types of adsorption sites with a wide spectrum of binding energies on the surface of the adsorbent (Khraisheh et al., 2004). The surface sites with highest energy were occupied first. This behaviour could be explained as follows: (i) the columbic attraction between the adsorbent and the adsorbate decreases as the MB molecules adsorb, because the surface charge becomes more positive, (ii) there are unfavourable chemical interactions between adjust adsorbed species and (iii) variety of site types on the adsorbent; as a result a variety for the adsorbate (Benjamin and Leckie, 1981). Allen et al. (1989) explained the deviation from linearity as follows: when all of the available monolayer sites are taken up then some fresh internal surface can be created. The creation of the additional surface arises from the pressure of adsorbate molecules forcing into the macropore and micropore structures. Moreover, the adsorbate molecules can be linked to molecular wedges creating access to new surfaces and effectively clearing blocked pores. Thus, this process tends to further increase the adsorptive capacity of the adsorbent. However, this occurrence suggests a change in the rate and extent of adsorption as new sites. The adsorption isotherms of MB, after applying the linear form of the Langmuir model at different particle sizes are shown in Figure 3. A comparison of literature value of adsorption capacity of different adsorbents with the value reported herein is considered. Table 1 shows the adsorption capacity of MB onto various adsorbents. Although a direct comparison between the examined adsorbents with those obtained in literature is difficult, due to the varying experimental conditions employed in those studies. However, the adsorbents used in the present study show reasonably high adsorption capacity as compared with other adsorbents. Figure 4 shows that the behaviour of adsorption processes of Cu2+ and Cd2+ onto RDP is almost linear and consequently, the Langmuir adsorption isotherm was not successfully applied. Langmuir isotherm was not utilized to evaluate the results, since the obtained adsorption isotherms did not present the typical Langmuirian form. The experimental evidence indicates that an isotherm plateau was not reached. The isotherms exhibited the Freundlich behaviour, R2 > 0.97, which indicates a heterogeneous surface binding (Khraisheh et al., 2004). The Freundlich adsorption isotherm equation predicts that the solute concentrations on the adsorbent will increase so long as there is an increase in the dye concentration in the aqueous solution. However, the values of qe were predicted by introducing corresponding values of KF and 1/n as well as the final Cu2+ and Cd2+ concentration (Co= 50 mg/ dm3) and the results are shown in Table 4. Therefore, it is clear from the experimental results that the adsorption capacity corresponding to the finial equilibrium concentration was increased in order: Cu2+ > Cd2+ at particle sizes higher than 125-300µm. This behaviour could be explained by the capability if Cu2+ ions to form a stable complex with RDP are higher at big particle sizes and may also according to a different mechanism. The 1/n value from the Freundlich equation indicates that the relative distribution of energy sites depends on the nature and strength of the adsorption process. For example, the value of 1/n of adsorption Cu2+ onto RDP surface is 0.815, in fact this value refers to 82% of the active sites that have equal energy where adsorption took place. Moreover, the closer the n value is to 1 indicates homogenous surface.


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y = 0.0033x + 0.3578 R2 = 0.9804

2.5

Ce/qe (g/dm 3)

2

The first concentrations

1.5

1

The last four concentrations

y = 0.0079x + 0.0464 R2 = 0.9923

The w hole concentation range Linear (The first concentrations) Linear (The last four concentrations)

y = 0.0036x + 0.2224 R2 = 0.9762

0.5

Linear (The w hole concentation range)

0 0

100

200

300

400

500

600

700

Ce (mg/dm 3)

Figure 3: Adsorption isotherm of MB onto RDP after linerising the Langmuir isotherm model. Experimental conditions: mass of RDP = 0.05 g, volume of solution = 50 cm3, equilibrium time = 72 hours, temperature = 20oC and shaking speed = 80 rpm, particle size: 125-300 µm and pH of solute solution = 4.0. Table 1 The maximum adsorption capacity (mg/g), Qmax, of MB on various adsorbents Adsorbent Qmon (mg/g) Reference RDP 277 Present work Activated carbon (FS-400) 388 Al-Ghouti et al., 2003 Diatomite 198 Al-Ghouti et al., 2003 Manganese oxides modified diatomite 320 Khraisheh et al., 2004 (MOMD) Calcined diatomite 137 Al-Ghouti et al., 2005 Microemulsion modified calcined diatomite 277 Khraisheh et al., 2005 (μE-CD) Banana and orange peels 20.8 and 18.6 Annadurai et al., 2002 Wood 84 Poots and McKay, 1976 Cotton waste 24 Poots and McKay, 1976 Bentonite 150 Poots and McKay, 1976 Particle size = 125-300 um

1.8

Particle size = 300-500 um Particle size = 500-710 um

1.6

Linear (Particle size = 125-300 um)

1.4

Linear (Particle size = 300-500 um) Linear (Particle size = 500-710 um)

1.2

Log qe

1 0.8 0.6 0.4 0.2 0 -1.5

-1

-0.5

0

0.5

1

1.5

-0.2 Log Ce

Figure 4: Adsorption isotherm of Cd2+ onto RDP after linerising the Freundlich isotherm model. Experimental conditions: mass of RDP = 0.05 g, volume of solution = 50 cm3, equilibrium time = 72 hours, temperature = 20oC and shaking speed = 80 rpm, and pH of solute solution = 4.0.


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4. CONCLUSIONS Knowledge of adsorption parameters is essential for understanding the adsorption mechanisms involved. The maximum adsorption capacity for MB, Cu2+ and Cd2+ onto RDP was reached after 72 h. The pH of the solute solution is a very important parameter since it affects the solute adsorption capacity on RDP. For MB the adsorption capacity increased with the increase in pH. The opposite effect is observed for Cu2+ and Cd2+. There is a slight effect of particle size of RDP on adsorption of MB, Cu2+ and Cd2+ at low initial solute concentrations but at higher concentrations an effect was observed. The results shows that the behaviour of adsorption processes of Cu2+ and Cd2+ onto RDP is almost linear and consequently, the Langmuir adsorption isotherm was not successfully applied. The experimental evidence indicates that an isotherm plateau was not reached. The isotherms exhibited the Freundlich behaviour, R2 > 0.97, which indicates a heterogeneous surface binding. 5. REFERENCES Al-Ghouti, M.A., M. A. M. Khraisheh, S. J. Allen and M. N. Ahmad (2003). The removal of dyes from textile wastewater: a study of the physical characteristics and adsorption mechanisms of diatomaceous earth. Journal of Environmental Management, 69(3), 229-238. Allen, S. J. G. MaKay, and K.Y.H. Khader (1989). Equilibrium adsorption isotherms for basic dyes onto lignite. J. Chem. Tech. Biotechnology, 45, 291-302. Annadurai, G., R.S. Juang, and D.J. Lee (2002). Use of cellulose-based wastes for adsorption of dyes from aqueous solutions. Journal of Hazardous Materials, 92(3), 263-274. Banat, F., S. Al-Asheh, and D. Al-Rousan (2002). A comparative study of copper and zinc ion adsorption on to activated and non-activated date-pits, Adsorpt. Sci. Technol. 20 (4) (2002) 319–335. Banat, F., S. Al-Asheh, and L. Al-Makhadmeh (2003a) Kinetics and equilibrium study of cadmium ion sorption onto date pits: an agricultural waste, Adsorpt. Sci. Technol. 21(3), 245–260. Banat, F., S. Al-Asheh, and L. Al-Makhadmeh (2003b). Preparation and examination of activated carbons from date pits impregnated with potassium hydroxide for the removal of methylene blue from aqueous solutions, Adsorpt. Sci. Technol. 21(6), 597–606. Barreveld. W.H. (1993). Date palm products. FAO Agricultural Services Bulletin, (101), Food and Agriculture Organization of the United Nations, Rome, Italy. Benjamin, M.M., and J.O. Leckie (1981). Multiple-site adsorption of Cd, Cu, Zn, and Pb on amorphous iron oxyhydroxide. Journal of Colloid and Interface Science, 79(1), 209-221.. Chen, J.P., and M. Lin (2001). Equilibrium and kinetic of metal ion adsorption onto a commercial H-type granular activated carbon: experimental and modelling studies. Water Res. 35(10), 2385–2394. Khraisheh, M. A. M. M.A. Al-Ghouti, S.J. Allen, and M.N.M. Ahmad (2004). The effect of pH, temperature and molecular size on the removal of dyes from textile effluent using manganese oxides modified diatomite. Water Environmental Research. 79(6), 51-59. Khraisheh M.A.M. and M.A. Al-Ghouti (2005a). Enhanced Dye Adsorption by Microemulsion-Modified Calcined Diatomite (μE-CD). Adsorption, 11, 547–559. Khraisheh M.A.M, M.A. Al-Ghouti, S.J. Allen and M.N.M. Ahmad (2005). Effect of OH and Silanol groups in the removal of dyes from aqueous solution using diatomite. Water Research, 39, 922-932. O’Connell, D.W., C. Birkinshaw, and T.F. O’Dwyer (2008). Heavy metal adsorbents prepared from the modification of cellulose: A review. Bioresource Technology 99 (2008) 6709–6724. Poots, V.J.P., G. McKay and J.J. Healy (1976). The removal of acid dye from effluent using natural adsorbents—II Wood. Water Research, 10(12), 1067-1070. .Rahman, M.S., S. Kasapis, N.S.Z. Al-Kharusi, I.M. Al-Marhubi, and A.J. Khan (2007). Composition characterisation and thermal transition of date pits powders. Journal of Food Engineering, 80,1–10. Saad, E.M., R.A. Mansour, A. El-Asmy, M.S. El-Shahawi (2008). Sorption profile and chromatographic separation of uranium (VI) ions from aqueous solutions onto date pits solid sorbent. TALANTA, in press. Streat, M., J.W. Patrick and M.J. Camporro Perez (1995). Sorption of phenol and para-chlorophenol from water using conventional and novel activated carbons, Water Res. 29 (2), 467–472. Vullo, D.L., H.M. Ceretti, M.A. Daniel, S.A.M. Ramı´rez, and A. Zalts (2008). Cadmium, zinc and copper biosorption mediated by Pseudomonas veronii 2E. Bioresource Technology, 99, 5574–5581. Xue, Y., H. Hou, and S. Zhu (2008). Competitive adsorption of copper(II), cadmium(II), lead(II)and zinc(II) onto basic oxygen furnace slag. Journal of Hazardous Materials, In Press.


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