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Adsorption of lead and copper ions from aqueous effluents on rice husk ash in a dynamic system

Abstract

This study evaluated the kinetic adsorption of Pb and Cu ions using rice husk ash as adsorbent in a fixed bed. The maximum adsorption capacities obtained for lead and copper ions in the fixed bed were 0.0561 and 0.0682 mmol/g (at 20 ºC), respectively. The thermodynamic studies indicated that the lead adsorption process was exothermic and spontaneous, while the copper adsorption process was endothermic and spontaneous. Characterization results indicated the presence of several functional groups, amorphous silica and a fibrous and longitudinal structure of rice husks. Rice husk ash (RHA) from northern Brazil can be used as a bioadsorbent for the individual removal of Pb(II) and Cu(II) ions from metal-containing effluents.

Adsorption; Heavy metal removal; Rice husk ash


SEPARATION PROCESSES

Adsorption of lead and copper ions from aqueous effluents on rice husk ash in a dynamic system

M. G. A. VieiraI,* * To whom correspondence should be addressed. E-mail: melissagav@feq.unicamp.br, E-mail: ambrosio@feq.unicamp.br ; A. F. de Almeida NetoI; M. G. C. da SilvaI; C. N. CarneiroII; A. A. Melo FilhoII

IDepartment of Products and Processes Design, Phone: + (55) 19 35213895, Fax: + (55) 19 35213965, University of Campinas, UNICAMP, 13083-852, Campinas - SP, Brazil. meuris@feq.unicamp.br

IIDepartment of Chemistry, Federal University of Roraima, Boa Vista - RR, Brazil. E-mail: candicenobrega@gmail.com; antonioalvesufrr@gmail.com

ABSTRACT

This study evaluated the kinetic adsorption of Pb and Cu ions using rice husk ash as adsorbent in a fixed bed. The maximum adsorption capacities obtained for lead and copper ions in the fixed bed were 0.0561 and 0.0682 mmol/g (at 20 ºC), respectively. The thermodynamic studies indicated that the lead adsorption process was exothermic and spontaneous, while the copper adsorption process was endothermic and spontaneous. Characterization results indicated the presence of several functional groups, amorphous silica and a fibrous and longitudinal structure of rice husks. Rice husk ash (RHA) from northern Brazil can be used as a bioadsorbent for the individual removal of Pb(II) and Cu(II) ions from metal-containing effluents.

Keywords: Adsorption; Heavy metal removal; Rice husk ash.

INTRODUCTION

Agricultural residues, especially rice husk, are byproduct of the rice milling industry, being produced in large quantities as a waste, creating environmental problems.

Rice husk is mostly used as a fuel in the boiler furnaces of various industries to produce energy. The ash generated after burning the rice husk in the boiler is called rice husk ash. The rice husk ash was collected from the particulate collection equipment attached upstream to the stack of rice-fired boilers. The ash generated represents a severe disposal problem (Naiya et al., 2009; Srivastava et al., 2006).

Rice husk consists mainly of crude protein (3%), ash (including 17% silica), lignin (20%), hemicellulose (25%), and cellulose (35%), rendering it suitable for metallic cation fixation (Krishnani et al., 2008).

Today, heavy metals are the most serious pollutants, becoming an extreme public health problem. Processes such as chemical precipitation, solvent extraction, ion exchange, reverse osmosis or adsorption are commonly carried out with the aim of removing heavy metals, metalloid species, and their compounds from aqueous solution. Among these processes, the adsorption process is a simple and effective technique for the removal of heavy metals from wastewater (Ahmaruzzaman, 2010).

A number of materials have been widely investigated as adsorbent in water pollution control. Some of the important ones include zeolites (Wang and Peng, 2009; Okolo et al., 2000), activated carbon (Wahby et al., 2011; Moreno-Piraján et al., 2011; Attia et al., 2010; Giraldo and Moreno-Piraján, 2008), rubber ash (Mousavi et al., 2010), clay (Galindo et al., 2013; Almeida Neto et al., 2012; Vieira et al., 2010a; Vieira et al., 2010b), expanded perlite (Torab-Mostaedi et al., 2010), vermiculite (Nishikawa et al., 2012), algae (Vieira et al., 2008; Vijayaranghavan et al., 2005; Silva et al., 2003), bacteria (Yilmaz et al., 2010), coir pith (Parab et al., 2006), sugarcane bagasse (Lv et al., 2008; Gupta and Ali, 2004), olive stone (Calero et al., 2009) and rice husks (Vieira et al., 2012; Vieira et al., 2011; Senthil Kumar et al., 2010; Naiya et al., 2009; Srivastava et al., 2006; Ye et al., 2010; Nakbanpote et al., 2007; Tarley and Arruda, 2004; Tarley et al., 2004).

In this context, adsorption of copper and lead from aqueous solution were evaluated using this potential material, through kinetic and thermodynamic studies in batch and fixed bed. Mathematical models were used to investigate the adsorption kinetics in batch and fixed bed. Langmuir and Freundlich's isotherm equations were applied to the experimental data. The adsorbent was characterized before and after the lead and cooper adsorption process.

MATERIALS AND METHODS

Adsorption

Samples of Oryza sativa L. rice husks from the North region of Brazil were used for adsorption. The rice husks were triturated in a food processor and calcined at 500 ºC in a muffle furnace for one hour.

Metal Adsorbate and Chemical Speciation

The adsorption experiments were conducted using an aqueous solutions of Cu(NO3)2.3H2O and Pb(NO3)2 at a fixed concentration. The Cu(II) and Pb(II) solution pH was maintained at a level below minimal precipitation in order to assure the exclusive occurrence of the adsorption process and no chemical precipitation of copper and lead ions in the oxide and hydroxide forms (CuO and Pb(OH)2), respectively. The effect of pH was estimated with HYDRA (Puigdomenech, 2004). The chemical precipitation of copper and lead in aqueous solution as oxide and hydroxide (CuO and Pb(OH)2), respectively, occurs in the pH range of 5.0-12.0. Thus, the pH of the adsorbate solution was kept to 4.5 to ensure minimal precipitation on the adsorbent surface, thus making the adsorption of charged metal ions more favorable. The pH of the metal adsorbate solution was measured with a pH-meter with automatic temperature compensation, and maintained it at the proper value using nitric acid and ammonium hydroxide.

Column Sorption Procedure

Copper and lead adsorption runs in the dynamic system were performed in a glass column with internal diameter of 1.4 cm and 14.0 cm in height. The bed height used in the experiments was 14.0 cm. Before the runs, the rice husk ashes were deposited inside the column and put in contact with deionized water for 2 hours.

The solutions containing the metal species were fed at the base of the column through a peristaltic pump (Masterflex) at a constant flow rate defined by preliminary tests in which the mass transfer zone (MTZ) for lead was obtained. Column effluent samples were collected at time intervals pre-set by a FC203 fraction collector (Gilson).

The amounts of metal retained in the bed from the point of rupture (qu) until saturation (q) were obtained by mass balance using column saturation data from the breakthrough curves. The area under the curve (1-C/C0) to the breaking point is proportional to qu and to exhaustion of the bed is proportional to q. The amounts retained were calculated from Eqs. (1 - 2), respectively.

The MTZ can then be calculated based on the qu/q ratio according to Eq. (3) (Geankoplis, 1993):

The MTZ has a maximum value which corresponds to the bed height (Ht). As the efficacy of mass transfer increases, this value decreases until reaching the ideal condition where the MTZ is zero and the breakthrough curve is a step function.

The percentage of total removal (%Rem) during adsorption was obtained by considering the metal fraction in solution retained in the adsorbent solid, from the total effluent used in the adsorption process until bed saturation. The amount of adsorbed metal was calculated by considering the curve area (1-C/C0) versus t. The integral of the metal adsorption curves was determined with the software Origin® version 6.0.

Adsorption Thermodynamics

The adsorption experiments were performed using aqueous solutions of Cu(NO3)2 and Pb(NO3)2 of fixed concentrations in batch mode, at room temperature and under constant stirring of 225 rpm. At equilibrium times of 1000 and 600 min for removal of lead and copper, respectively (Vieira et al., 2012), 4 mL-aliquots of the metal ion solutions were removed and centrifuged. The supernatant liquid was diluted and its concentration was determined by atomic absorption spectrometry (Perkin Elmer AA Analyst 100 with air-acetylene oxidizing flame). Removal capacity in the solid phase (q) at each time was obtained by Eq. (4):

The pH of the solutions defined by metallic speciation was 4.0 for Pb and 4.5 for Cu. The pH of the solutions was adjusted using 0.2 M HNO3 or 0.25 M NH4OH. Thermodynamic parameters for the adsorption process (ΔH (kJ/mol), ΔS (J/mol.K) and ΔG (kJ/mol)) were evaluated using the thermodynamic Eqs. (5) - (6).

The graph of ln(Kd) versus 1/T must be linear with slope (-ΔH/R) and intercept on the y axis of (ΔS/R), providing the ΔH and ΔS values. The Gibbs Free Energy variation (ΔG) is the fundamental criterion of process spontaneity. A given process occurs spontaneously at a given temperature if ΔG < 0.

Mathematical Model of the Adsorption Column

Bohart and Adams (1920) developed one of the simplest models to represent the breakthrough curve. This model assumes that the adsorption rate is proportional to the residual capacity for adsorption and concentration of the adsorbed species, and does not consider it to be important to stress the axial dispersion. In this case the intraparticle diffusion is negligible. The mass transfer rates satisfy Eqs. (7) and (8):

where z is the height of the bed, v the flow velocity, ε is the porosity of the bed, ρL is the density of the bed, t is the process time and the parameters k, represent the constant removal rate.

The choice of this model is due to the fact that it assumes that the removal capacity is constant and that the isotherms obtained for the adsorbents show irreversible behavior. Thus, for a better representation of this study we used the sorption capacity of the adsorbent as the amount of metal removed when the system is in equilibrium. The initial boundary conditions are represented by Eqs. (9) and (10):

The analytical solution of the model of Adams and Bohart is given by Eq. (11), as shown by Ruthven (1984):

where:

and q0 represents the quasichemical concentration of metal in the solid state at time zero of the elution.

Adsorbent Characterization

The physical-chemical characterization of rice husk included morphological analysis by scanning electron microscopy (SEM) with EDX and Fouriertransform infrared spectroscopy (FTIR). The above mentioned analyses were performed for samples of rice husks in natura, calcined and after metal ion adsorption. Table 1 shows the analyses and their respective equipment.

RESULTS AND DISCUSSION

Determination of the Operating Flow Rate for Fixed Bed Adsorption

The determination of the operating flow rate was based on the Mass Transfer Zone (MTZ). The concentration of adsorbate metal solution was kept constant at 0.48 mmol/L and 1.57 mmol/L for lead and copper, respectively, while the feed flow rate varied from 2, 3, 4 and 5 mL/min. Figure 1 shows the breakthrough curves at different flow rates in the adsorption of lead and copper. The experimental data were fit by the quasichemical solution function.


From Figure 1, it appears that the breakthrough curves present different behaviors, indicating the influence of diffusional resistances. The adsorption process has a strong resistance to the saturation of the bed in the entire flow range investigated, as evidenced by the more elongated breakthrough curves and the large areas of the mass transfer zone.

According to Geankoplis (1993), the mass transfer zone (MTZ) represented by the curve delineates a rupture length of the bed in which the concentration is given from the breaking point until the point of exhaustion. The shorter the length of the MTZ, the closer the system is to ideality, indicating a low diffusional resistance, and hence a more favorable adsorption process.

In the fixed bed adsorption process, when the flow rate is increased, the resistance to mass transfer in the liquid film outside is reduced; consequently, the mass transfer zone (MTZ) is reduced, as observed by Vijayaranghavan et al. (2005). However, using higher flow rates, the MTZ is increased, because the fluid does not have a residence time sufficient for the adsorption to occur. Table 2 shows the values of MTZ, qu, q and percentage removal (%Rem) of lead and copper in RHA.

Table 2 shows that the lowest values of MTZ and satisfactory values of useful (qu) and total amount of metal adsorbed (q) and percentage of total removal (%Rem) were obtained at a flow rate of 4 mL/min for both ions. The values of q0 and k are presented in Table 3. The parameter q0 was only little influenced by changes in the flow rate. This can be attributed to the constant number of sites used by lead or copper during the adsorption process. The parameter k varies with the increase of the operating flow rate. For each operating flow rate, a new breakthrough curve is established.

Thermodynamics of Adsorption

Thermodynamic data were obtained through the static method in a thermostated bath with constant stirring (225 rpm) at various temperatures (24 - 74 ºC). An increase in temperature increases the adsorption capacity, which means that the increase of energy favours adsorption on the RHA surface. Pb/RHA presents a negative enthalpy variation during adsorption (exothermic process). Naiya et al. (2009) obtained results that indicated that the Pb removal degree increases with an increase in temperature for an initial concentration of 50 ppm, pH 5.0, and contact time of 1 hour, using RHA as adsorbent: for 30 ºC, they obtained 94.85%; for 40 ºC, 95.30%; and for 50 ºC, 96.02% of metal removal.

Figure 2 shows adsorption isotherms for 1g RHA/ 100 mL of adsorbate solution. The equilibrium data were adjusted by Langmuir and Freundlich's models. The initial concentration of Pb varied from 5 to 800 ppm for the four temperature levels and the initial concentrations of Cu varied from 5 to 500 ppm for the three temperature levels.


The parameters obtained from the Langmuir and Freundlich models and Henry's constants are presented in Table 4. Figure 2 and Table 4 show that Langmuir's model better represented the experimental data for the adsorption isotherms.

The Langmuir-Freundlich is a versatile isotherm expression that can represent both Langmuir and Freundlich behaviours. A general form of the Langmuir-Freundlich isotherm equation for copper and lead adsorption on RHA can be written as Eq. (14):

where, q is the amount of metal adsorbed on the ash at equilibrium (mmol/g), Qm is the adsorption capacity of the system (mmol of adsorbate/g adsorbent), Ceq is the aqueous phase concentration at equilibrium (mmol/ L), Ka is the affinity constant for adsorption (L/g) and n is the index of heterogeneity

The Langmuir-Freundlich isotherm was fitted to all isotherm datasets as shown in Figure 3. The fitted values of Qm, Ka and n are summarized in Table 5. The parameter estimation process was robust and the Origin solver converged to these parameter values for a wide range of initial conditions. The only restriction used in the solver was a non-negativity constraint for the Ka values. The R2 values of the fitting were greater than 0.92. These results indicate that the Langmuir-Freundlich isotherm can be used to describe temperature effects on lead and copper adsorption.


The thermodynamic parameters obtained are presented in Table 6. Pb/RHA interactions occurred spontaneously (ΔG<0) and the adsorption process is exothermic. Spontaneity increases with the rise in temperature, varying from -17.243 to -18.156 kJ/mol for the temperature range of 24 to 74 ºC. The Cu/RHA interactions also occurred spontaneously (ΔG<0), but the adsorption process is endothermic. Similar results were obtained by different authors, who also verified an endothermic behaviour for adsorption of copper ions on several clays (Bhattacharyya and Gupta, 2008; Eren and Afsin, 2008; Bhattacharyya and Gupta, 2007; Weng et al., 2007). Thus, the adsorption of Cu(II) onto clays has to overcome a small activation energy barrier and an increase in energy supply makes it easier for Cu(II) to adsorb onto the clay surface. Such activated adsorption following an endothermic path has also been reported earlier (Futalan et al., 2011).

Characterization

FTIR Analysis

The FTIR technique is an important tool to identify characteristic functional groups. In Figure 4(a) and (b), note the presence of OH groups on the sample surface (Kamath and Proctor, 1998). This OH stretching is due to silanol groups (Si-OH). Besides, a methyl group has been identified. Stretches due to the presence of lignin (Tarley et al., 2004), aldehydes and ketones, aromatic rings (Tarley and Arruda, 2004) and siloxane (Si-O-Si) (Nakbanpote et al., 2007; Tarley and Arruda, 2004; Tarley et al., 2004) groups in the samples are also present. Table 7 shows the functional groups and their respective wavelengths identified in the spectra.


Scanning Electron Microscopy

Figure 5 presents SEM micrographs of the rice husks and rice husk ash before and after adsorption. SEM micrographs of the rice husk ash (5b) indicated that the surface was highly irregular and porous in nature (Naiya et al., 2009). The cob-shaped cellulose skeleton is visible (Della et al., 2001). According to Della et al. (2001), the inner epidermis presents a pore structure resulting from removal of lignin and cellulose during the burning process; cellulose is the major organic constituent of rice husks. Based on the mapping of metals in the micrographs performed by EDX it is possible to observe a uniform distribution of the metallic ions lead (Figure 5c) and copper (Figure 5d) on the RHA surface after adsorption.


 




According to Tarley and Arruda (2004), the morphology of the adsorbent can facilitate adsorption of metals in different parts of the material. Therefore, based on morphology and on the fact that a higher concentration of silica is present in the outer epidermis of the rice hulls, one can conclude that this material presents a morphological profile with the capability to retain metal ions.

The semi-quantitative chemical composition obtained by EDX (coupled to SEM) of the compounds in RHA is shown in Table 8. Ion exchange can be an additional process for removal of copper and lead by RHA, being verified by the results of chemical composition presented in Table 8, where the percentage of exchangeable potassium in the RHA drastically decreased after adsorption. The mechanism of ion exchange can be written as follows:

2K+ - RHA + Cu2+→ Cu2+ - RHA + 2K+

2K+ - RHA + Pb2+→ Pb2+ - RHA + 2K+

Similar results were obtained by Galindo et al. (2013) in a study of the removal of cadmium and lead using sodic clay. The authors reported that ion exchange can be an additional process for removal of cadmium and lead on sodic clay and that the amount of exchangeable cations Na+ was reduced after metal adsorption.

CONCLUSIONS

The study of mass transfer parameters, as well as the breakthrough curves, indicated that the most suitable operating flow rate, i.e., which minimizes the diffusional resistances in the bed for removal of lead and copper by RHA was 4 mL/min. When the flow rate was increased, the breaking and the saturation points, as well as the useful (qu) and the total (q) amount of metal adsorbed decreased. RHA from northern Brazil can be used as a bioadsorbent for the individual removal of Pb(II) and Cu(II) ions from metal-containing effluents and the maximum sorption found for both ions was 0.0561 and 0.0682 mmol metal/g of RHA, respectively. The thermodynamic study indicated that the Pb adsorption process was exothermic and spontaneous, while Cu adsorption was endothermic and spontaneous. Sorption isotherms were well-described by Langmuir's model. The physical characterization of rice husks in nature, rice husk ash before and after adsorption of metal ions pointed to properties such as the presence of functional groups (carboxyl, silanol, hydroxyl, etc.), of amorphous silica and of a fibrous and longitudinal structure of this material.

NOMENCLATURE

a, n

Freundlich coefficients

b, qm

Langmuir coefficients representing the equilibrium constant for the adsorbate-adsorbent equilibrium and the maximum adsorbed amount on the monolayer

C metal concentration in a solution in the column outlet

ppm

C0

initial metal concentration in the liquid

ppm K Henry constant Kd adsorbate distribution coefficient =qeq/Ceq L/g Q amount of adsorbed metal per unit of adsorbent mass mg of metal/g of adsorbent qu amount of adsorbed metal per unit of adsorbent mass up to the breakthrough point

mg of metal/g of adsorbent

R universal gas constant 8.314×10-3 kJ/(K mol) T temperature K tb time until breakthrough point min V volumetric outflow of the metal solution cm3/min ΔG Gibbs free energy variation

kJ/mol

ΔH enthalpy variation kJ/mol ΔS entropy variation J/(K mol)

(Submitted: July 10, 2012; Revised: July 5, 2013; Accepted: August 8, 2013)

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  • *
    To whom correspondence should be addressed. E-mail:
  • Publication Dates

    • Publication in this collection
      07 July 2014
    • Date of issue
      June 2014

    History

    • Accepted
      08 Aug 2013
    • Reviewed
      05 July 2013
    • Received
      10 July 2012
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