adsorption of lead and copper from aqueous solution

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lead was found to be 43.55% by wheat straw under optimum conditions (pH=5, ..... e. C. = equilibrium metal ion concentration (mg/L); b = Langmuir constant .... of Cd (II) from wastewater, Bioresource Technology, ... Dakiky M., Khamis M., Manassra A., Mer'eb M., (2002), ... Randall J.M., Reuter F.W., Waiss Jr. A.C., (1975),.
Environmental Engineering and Management Journal

November 2013, Vol.12, No. 11, 2117-2124

http://omicron.ch.tuiasi.ro/EEMJ/

“Gheorghe Asachi” Technical University of Iasi, Romania

ADSORPTION OF LEAD AND COPPER FROM AQUEOUS SOLUTION USING UNMODIFIED WHEAT STRAW Mehwish Anis, Sajjad Haydar, Abdul Jabbar Bari University of Engineering & Technology, Institute of Environmental Engineering & Research (IEER), Lahore, Pakistan

Abstract Potential of unmodified Wheat Straw as an adsorbent for the removal of lead and copper was studied. Effect of pH, adsorbent dose, contact time and initial metal concentration on the removal of the metal ions were also investigated. Maximum removal of lead was found to be 43.55% by wheat straw under optimum conditions (pH=5, adsorbent dose=16g/L, contact time= 240 minutes, metal concentration=100 mg/L). Unmodified Wheat straw removed 0.381 mg/g of lead. While 56.36% copper removal was achieved using unmodified wheat straw under optimum conditions (pH=5, adsorbent dose=20g/L, contact time= 120 minutes, metal concentration=100 mg/L). Unmodified Wheat Straw removed 0.587 mg/g of copper. Langmuir isotherm was found to validate the equilibrium data of adsorption while kinetics were described by pseudo second order rate equation for both the metals.Furthermore, competitive adsorption of lead and copper showed that lead had more affinity for Wheat Straw as compared to copper. It was concluded that removal efficiency of unmodified Wheat Straw is much less as compared to modified Wheat Straw. Key words: adsorption, copper, heavy metal, lead, unmodified wheat straw Received: December, 2010; Revised final: March, 2012; Accepted: March, 2012

1. Introduction Rapid industrialization and urbanization has led to the deterioration of surface as well as ground water due to discharge of municipal and industrial wastewater into water bodies. In addition to conventional pollutants, water bodies are also receiving toxic heavy metals which pose serious threats to the human health. They also accumulate in the food web through biomagnifications and pollute the entire food pyramid. According to WHO guidelines for drinking water quality, lead and copper are the heavy metals of most immediate concern (WHO, 1984). Sources of lead include discharges from battery manufacturing industries, paint industry, chemical and construction industries, domestic plumbing and road run off (HE&W, 2012). It also affects the process of brain development in children. Lead is also known to 

inhibit the anaerobic digestion if present in wastewater (WHO, 2012). Copper enters wastewater through effluent discharges from electroplating industry, paint manufacturing, printed circuit board making, wood preservatives, phosphate fertilizer production and printing process. Immediate effects of copper intake at elevated levels can result in irritation of the nose, mouth and eyes, headaches, stomachaches, dizziness, vomiting and diarrhea. Liver and kidneys can be damaged due to long term copper exposure. Wilson’s disease is also caused due to chronic copper poisoning. Copper reduces the decomposition of organic compounds by negatively affecting the activity of microorganisms (Lenntech, 2012). Conventionally, removal of heavy metals from water and wastewater is carried out by chemical precipitation, extraction, ion exchange, reverse

Author to whom all correspondence should be addressed: E-mail: [email protected]; Phone: +92429029248

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osmosis, electrochemical treatment, and membrane techniques etc. (Volesky, 2001). These all methods have some limitations and drawbacks in one way or the other. However, according to recent research trend has shifted to the adsorptive removal of the heavy metals from water and wastewater using agro based materials as prospective adsorbents (Mahvi, 2008). Their low cost and abundant occurrence make them a potential adsorbent. A number of agro based materials, which are abundantly available, have been studied of the removal of heavy metals. These include pea nut skins (Randall et al., 1975), tea leaves (Tee and Khan, 1988), modified cellulosic materials (Shukla and Sakhardande, 1990), apple waste (Maranon and Sastre, 1991), tea waste, coffee and nut shells (Orhan and Büyükgüngör, 1993), saw dust and bark (Bin, 1995; Vazquez et al., 1994), pine bark (Al-Asheh and Duvnjak, 1998), rice hulls (Low et al., 1999), cactus leaves and charcoal (Dakiky et al., 2002), banana and orange peels (Annadurai et al., 2002), rice husk (Ajmal et al., 2003; Vazquez et al., 2002), wool, olive cake, pine needles, almond shells, modified sugar beet pulp (Reddad et al., 2002) and maize leaf (Ahalya et al., 2003). Many agricultural wastes and residues have also been investigated for their potential of removing heavy metals particularly lead and copper. About 99% removal of lead was reported by its adsorption on sawdust, maize cobs and rice husk (Ghani et al., 2007). Tea waste as an alternative adsorbent for the removal of lead from wastewater was also investigated (Sabrina and Hasmah, 2008). Adsorption capacity (mg/g) of lead was found to be 2.15 mg/g on bamboo dust (Kannan and Veemaraj, 2009). Adsorptive capacities (mg/g) of lentil, wheat shells and rice shells, for copper were found to be 8.97, 9.51 and 9.58 respectively (Aydın et al., 2008). Mercaptoacetic acid modified cassava waste was also used as an adsorbent for the elimination of copper ions from aqueous solutions (Augustine, 2007). Many studies have been conducted for the removal of lead and copper ions using wheat straw. In all these studies prior treatment was rendered to wheat straw before examining its adsorptive capacity. Removal of copper was explored by wheat straw that was pretreated by citric acid (Gong et al., 2008) and (Han et al., 2010). Agricultural products pretreated by formaldehyde, sodium hydroxide and sulfuric acid were also studied (Šæiban et al., 2008). Prior treatments may enhance the adsorption capacity of the agricultural residues. However, any pretreatment definitely adds cost to the removal process. This reduces the cost effectiveness of these materials as adsorbents over their conventional expensive alternates. Not much data on the absorptive capacity of untreated wheat straw are available in the literature for the removal of heavy metals particularly lead. Therefore, the objective of the present study was to explore the potential of raw unmodified wheat straw

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for the removal of lead along with copper as both are the metals of most immediate concern. 2. Materials and methods 2.1. Preparation of adsorbent Raw wheat straw was obtained from a local pulp and paper industry where it was used as a raw material. It was ground and sieved to a uniform size of 25 mesh. Afterwards, it was washed thoroughly with distilled water and dried in open air for 48 hours. The adsorbent was then stored in an air tight jar for further use. 2.2. FTIR of wheat straw Fourier transform infra red spectroscopy was carried out to identify the functional groups present on the surface of wheat straw responsible for the adsorption of metal ions. Wheat straw is a lignocellulosic material i.e. it contain lignin and cellulose as its constituents according to (Smook, 1992). The spectra of wheat straw confirmed the presence of carboxylic groups (2500-3300 cm-1), hydroxyl groups (3200-3700 cm-1), as shown in Fig. 1. Silicates and silica are also common constituents of wheat straw of Pakistan origin.

Fig. 1. FTIR Spectra of Wheat Straw

2.3. Preparation of stock solution Commercially available solutions of copper and lead of 1000 ppm were used as stock solutions for the experimentation. The working concentration for further experimentation was obtained by diluting these stock solutions to appropriate level with distilled water. 2.4. Batch mode removal studies Removal studies were carried out in 250 mL Erlenmeyer flasks containing 100 mL of 100 ppm lead and copper solutions separately. Fixed amount

Adsorption of lead and copper from aqueous solution using unmodified wheat straw

of wheat straw was added to these flasks. The pH of the solutions was adjusted using 0.1 N NaOH or 0.1 N H2SO4. The flasks were then agitated on an orbital shaker at 200 rpm at a temperature of 25+10C. Samples were withdrawn from respective flasks after a fixed interval of time and then filtered out before their analysis for the residual concentration of lead and copper. The concentrations of filtrates were measured using Shimadzu 6800 Atomic Absorption spectrophotometer equipped with an air-acetylene flame. The analytical wavelengths were 283.16nm and 324.57nm for lead and copper, respectively. The experiments were carried out in triplicate and average results are reported. Various quality control (QC) and quality assurance (QA) checks mentioned in “Standard Methods, 1998; Instruction Manual Shimadzu AA-6800 atomic absorption spectrophotometer were followed during copper and lead determinations. These include initial calibration, continuing calibration verification (CCV) and laboratory control sample (LCS). Analysis of the result was carried out by calculating the respective removal efficiencies of metals and the equilibrium adsorption capacities using the following formulas: Removal Efficiency (%) (Eq. 1): R.E 

C o C e  100 Co

(1)

Adsorption Capacity (mg/g), (Eq. 2): q

V (C o  C ) m

(2)

where: Co = initial concentration of metal ions (mg/L); Ce = equilibrium concentration of metal ions (mg/L); m= Weight of wheat straw (g) 3. Results and discussion 3.1. Effect of initial pH of solution on removal efficiency pH of the metal solutions is one of the important variables that affect the removal through adsorption. Batch studies were conducted with 100mL of 100ppm solution of each metal agitated with 10 g/L of wheat straw at 200 rpm. It was observed that removal of lead and copper increased with increase in initial pH of metal solutions. This increase in the removal efficiency can be explained with the fact that a low pH, competition exists between hydrogen ions H+ and the metal ions for occupying the available adsorption sites. The adsorption capacity of lead was increased from 1.344 mg/g to 3.082 mg/g for a pH change of 2 to 5 as shown in Fig. 2. Increasing the pH of lead solution beyond 5 resulted in its precipitation as lead hydroxides Pb(OH)2. Increase in solution pH from 5 to 10 further increased the percentage removal and

thus q of lead from 3.082 mg/g to 6.654 mg/g. However, this increase from 3.082 mg/g to 6.654 mg/g was due to precipitation and not due to adsorption, therefore not taken into account. Similarly for copper, the removal increased from about 1.78 mg/g to 4.203 mg/g when the solution pH varied from 2 to 5. Precipitation of copper hydroxide Cu(OH)2 after pH 5 further increased the removal to 7.098 mg/g for solution pH of 10. The focus of current research was to explore the adsorptive potential of wheat straw for the removal of lead and copper, therefore, removal due to precipitation was not taken into account in the present study. Therefore, pH 5, at which maximum adsorptive removal occurred for lead and copper solutions, and after that precipitation has started in both metal solutions, was fixed for further experimentation. Similar trend of the percent removal with pH for the adsorption of copper on modified wheat straw was observed with optimum pH of 5(Han et al., 2010). 3.2. Effect of adsorbent dose on removal efficiency The effect of wheat straw quantity as an adsorbent was determined by varying its quantity from 1-25 g/L, for both lead and copper. Removal efficiency of both metals was found to increase with increase in the quantity of wheat straw per liter of aqueous solution as shown in Fig. 3. Removal efficiency of lead increased from 6.01 % to 42.76 % with an increase in the adsorbent dose from 1 g/L to 16 g/L, respectively. After 16g/L, no significant increase in the removal of lead with increase in the dose of wheat straw was observed. Therefore, 16g/L of wheat straw was considered to be the optimum value under given conditions for adsorption and hence used in further experimentation for lead removal. Similarly, for copper, removal efficiency was increased from 12.97% to 55.2% when adsorbent dose was varied from 1-20 g/L. Thereafter no appreciable increase in the removal occurred with an increase in the absorbent dose to 25 g/L. Therefore, maximum removal of copper was observed at a dose of 20 g/L and it was fixed for further analysis for copper removal. 3.3. Effect of contact time on removal efficiency Adsorption of metals is a function of the time for which the metal solution and the adsorbent remain in contact with each other. It was observed that greater the contact time of wheat straw and metal solutions, higher the metal removal till equilibrium was achieved. This may be explained by the fact that sufficient time was available for metal solutions to attain equilibrium. After a contact time of 240 minutes, maximum adsorption of 2.72 mg/g of lead was attained. No further increase in the adsorption

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capacity after 240 minutes indicated that equilibrium is attained. Similarly equilibrium adsorption capacity for copper was found to be 2.82 mg/g after contact time of 120 minutes. 3.4. Effect of initial metal ion concentration Removal of metal ions by adsorption also depends on the initial concentration of metal ions. Initial metal concentration of both lead and copper was varied from 10 mg/L to 100 mg/L. Adsorption capacity of metal was found to increase with the increase in initial concentration of metal ions. Adsorptive capacity of lead increased from 0.46 mg/g for 10 mg/L to 2.53 mg/g for 100 mg/L of initial lead concentration. Adsorptive capacity of copper was observed to increase from 0.41 mg/g to 2.76 mg/g for an increase in concentration from 10 mg/L to 100 mg/L as shown in Fig. 6. This is because of the increased driving force for mass transfer between aqueous solution of metal and the solid adsorbent. 3.5. Equilibrium studies The equilibrium data for the lead and copper uptake over a studied concentration range was fitted to most commonly employed adsorption isotherms namely Langmuir and Freundlich. Correlation regression coefficient (R2) normally dictates the quality of the isotherm fit to the experimental data. Both these models follow the experimental data reasonably well over the studied concentration range, but Langmuir isotherm best fitted the equilibrium data having greater correlation regression coefficient. Langmuir model can be expressed as follows (Eq. 3): qe

bq max C e 1  bC e

(3)

The linear form of the Langmuir isotherm can be given by Eq. (4):

Ce 1 C   e qe qmaxb qmax

(4)

q where: e = equilibrium adsorption capacity (mg/g); Ce = equilibrium metal ion concentration (mg/L); b = Langmuir constant related to energy of q max = maximum adsorption adsorption (mg/L); capacity corresponding to the complete monolayer coverage (mg/g). The Langmuir isotherms fitted to the adsorption of lead on wheat straw is shown in Fig. 7. Adsorption of copper on wheat straw also followed the Langmuir isotherm as shown in Fig. 8. Langmuir constants obtained from slope and intercept of the straight line fitted to the adsorption,

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along with correlation coefficient (R2) are given in Table 1. The adsorption capacity of modified wheat straw for lead and copper is presented and compared with raw unmodified wheat straw in Table 2. The adsorption capacities shown in the Table 2 show that prior treatment of adsorbents, result in enhanced adsorption capacities for metal ions.

3.6. Adsorption kinetics Study of the adsorption kinetics is important in describing the metal uptake rate during adsorption process. Kinetic data of lead and copper was analyzed by fitting the data to two well known kinetic models. These include (1) pseudo first-order and (2) pseudo second-order kinetic models (Lagergren, 1898) and (Ho and McKay, 2000). Regression coefficient (R2) is the parameter for determining the best fit model. Higher the value of regression coefficient better will be the fit of data to the given equation. Linearized for of pseudo first order rate equation after integration and application of boundary conditions can be given as (Eq. 5): log( q e  q t )  log q e 

K ad t 2 . 303

(5)

where: qe = amount of metal adsorbed per unit mass of adsorbent at equilibrium (mg/g); qt = amount of metal adsorbed per unit mass of adsorbent at time t (mg/g); Kad = rate constant (min-1) Similarly the linear form of pseudo second order rate equation can be given as (Eq. 6): 1 t  q t kq e

2



1 t q e

(6)

where: qe = amount of metal adsorbed per unit mass of adsorbent at equilibrium (mg/g); qt = amount of metal adsorbed per unit mass of adsorbent at time t (mg/g); k = pseudo 2nd order rate constant (g/mg.min). For the best fit model, the kinetic data should yield a straight line according to equations with high value of regression coefficient. Fig. 9 demonstrated that adsorption of lead on wheat straw followed pseudo 2nd order rate equation. This implies that a single metal ion can occupy two adsorbent sites by a chemical reaction which is a rate controlling step (Khalid et al., 2000). Adsorption of copper on wheat straw also followed pseudo 2nd order rate kinetics. The constants of pseudo 1st order and pseudo 2nd order rate equations are given in Table 3.

3.7. Competitive adsorption Presence of multiple metal ions in aqueous solution was incorporated in the present research by studying the competitive adsorption of lead and copper on wheat straw.

Adsorption of lead and copper from aqueous solution using unmodified wheat straw

Different concentrations of lead and copper were mixed and analyzed for their adsorption on wheat straw. The pH of the mixture was fixed to be 5 and total volume of the solution taken was 100 mL. The mixtures remained in contact with 20 g/L of

wheat straw for 300 minutes to ensure equilibrium conditions. Percentage removal of metal ions at their different mixture concentrations is illustrated in Fig. 10.

Table 1. Constants for adsorption of Lead and Copper on wheat straw Metals Lead Copper

R2 0.96 0.99

Langmuir Isotherm Constants qmax(mg/g) b(mg/L) 0.38 1.01 0.58 0.77

R2 0.94 0.95

Freundlich Isotherm Constants Kf 2.62 5.52 1.71 5.35

Fig. 2. Effect of pH on the adsorption capacity of Pb+2 and Cu+2 on Wheat Straw

Fig. 3. Effect of Adsorbent Dose on the removal of Pb+2 and Cu+2 by Wheat Straw

Fig. 4. Effect of Adsorbent Dose on the adsorption capacity of Pb+2 and Cu+2 by Wheat Straw

Fig. 5. Effect of contact time on adsorption capacity of Pb+2 and Cu+2 on Wheat Straw

Fig. 6. Effect of initial metal ion concentration on adsorption capacity on Wheat Straw

Fig. 7. Langmuir isotherm for adsorption of lead on Wheat Straw

n

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Fig. 8. Langmuir isotherm for adsorption of copper on Wheat Straw

Fig. 9. Plot of pseudo 2nd order rate equation for adsorption of lead on Wheat Straw

Table 2. Adsorption capacities of wheat straw for Lead and Copper Metal

Adsorbent Modified wheat straw Raw wheat straw Modified wheat straw Modified wheat straw Raw wheat straw

Lead Lead Copper Copper Copper

q (mg/g) 8.48 0.381 78.13 39.17 0.587

Reference Rios et al. (1999) Present Study Gong et al. (2008) Han et al. (2010) Present Study

Table 3. Kinetic constants for adsorption of Lead and Copper on Wheat Straw Metals R2 Lead Copper

Pseudo 2nd order constants

Pseudo 1st order constants

0.572 0.055

k (mg/gmin) 0.138 0.115

qe (mg/g) 2.77 1.66

Experimental qe(mg/g) 2.72 2.82

It is clear from Fig. 10, that lead has greater removal efficiency as compared to copper in a competitive environment. It was also observed that removal efficiency of 50 mg/L copper in a mixture with 50 mg/L concentration of lead was less i.e. 44.03 % as compared to removal of 50 mg/L of lead solution alone i.e. 65.33 % under similar experimental conditions.

R2 0.673 0.720

k (mg/gmin) 4.77 x 10-3 0.011

qe (mg/g) 2.39 2.10

Experimental qe(mg/g) 2.72 2.82

The competition of cations in a mixture of solution is due to a limited number of adsorption sites at a given dose of adsorbent. Metals having more affinity towards adsorbent displace the metals which have low affinity. The sorption affinity depends on many parameters like equilibrium between adsorbent sites and metal cations, stability of bond between metal ions and adsorbent and types of metal and adsorbent, interaction and distribution of reaction groups on the adsorbent.

3.8. Effect of BOD on removal efficiency

Fig. 10. Competitive adsorption of copper and lead on wheat straw (Wheat Straw Dose= 20 g/L; pH = 5; Contact Time = 300 minutes)

This clearly shows that the presence of other metals may affect the removal of a specific metal, from the situation, when the metal is present alone.

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Industrial discharges, in addition to heavy metals, also contain organic content which can be determined in terms of BOD of the wastewater. To study the effect of BOD on the removal of metals, metal solutions of known BOD were prepared according to the procedure given in the book of Standard Methods as 5210 B . Known concentration of heavy metals and optimum doses of wheat straw were added to the solution of known BOD. The results of varying solution BOD on the percent removal of lead and copper by wheat straw is presented in Table 4. Effect of solution BOD on the removal of metal ions is shown in Fig. 11. Adsorption of lead first increased with increase in solution BOD and after reaching a

Adsorption of lead and copper from aqueous solution using unmodified wheat straw

maximum value of 40.98% at BOD of 191 mg/L. Thereafter it dropped to 35.53% at 286 mg/L of BOD. For copper the observation was different than lead. Removal of copper decreased linearly with increase in the solution BOD. Table 4. Removal of copper and lead by wheat straw at different BOD of the aqueous solution BOD of Blank (mg/L)

BOD of Copper Sol. with WS (mg/L)

% Removal of Copper

135 169 226

164 197 293

37.29 36.37 33.74

BOD of Pb+2 Sol. with WS (mg/L) 158 191 286

% Removal of Pb+2 33.77 40.98 35.53

WS= Wheat Straw

Fig. 11. Effect of BOD of aqueous solution on the removal of metal ions by wheat straw

The removal of copper decreased from 37.29% to 33.74% when the BOD increased from 164 mg/L to 293 mg/L, respectively. It was also observed that the BOD of the solutions having wheat straw was high as compared to blank (without having wheat straw) samples. This may be due to the fact that wheat straw is itself an organic matter therefore it may increase the overall oxygen demand of the aqueous solution. 4. Conclusions Following conclusions can be drawn on the basis of the present study: 1. Adsorption capacity of lead and copper on raw unmodified wheat straw is 0.381 mg/g and 0.587 mg/g. 2. Langmuir isotherm was found to fit the equilibrium data for adsorption of lead and copper on raw unmodified wheat straw. 3. Adsorption kinetics of metals was well represented by pseudo second order rate kinetics. 4. Lead showed more affinity towards raw unmodified wheat straw as compared to copper when present together in solution. 5. Adsorption of copper on raw unmodified wheat straw decreased with increase in BOD of the

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