Source: https://www.scribd.com/document/314911556/Absorcao-de-Cadmio-e-Zinco-CCA
Timestamp: 2019-04-23 08:11:11+00:00

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represents the adsorption equilibrium data of Cd(II) and Zn(II) ions onto RHA.
from aqueous streams include ion-exchange chromatography, reverse-osmosis, chemical precipitation, and adsorption.
E-mail address: vimalcsr@yahoo.co.in (V.C. Srivastava).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
Eng. Langmuir. Since industrial effluents can contain several metals.0 mg/l.i individual Freundlich isotherm constant of each component ((mg/g)/(l/mg)1/n ) KL constant of Langmuir isotherm (l/mg) KL. In humans zinc [Zn(II)] is found in over 20 metalloenzymes. Aspects 312 (2008) 172–184 Nomenclature ηL.i initial concentration of each component in solution (mg/l) Ki individual extended Langmuir isotherm constant of each component (l/mg) KF mono-component (non-competitive) constant of Freundlich isotherm of the single component ((mg/g)/(l/mg)1/n ) KF. respectively. (ii) study the effect of initial pH (pH0 ).cal calculated value of solid phase concentration of adsorbate at equilibrium (mg/g) qe. Excess ingestion of Zn(II) may result in acute gastrointestinal disturbances accompanied with nausea. Various isotherm equations like those of Freundlich. Srivastava et al.e. No information is available in literature for the simultaneous removal of Cd(II) and Zn(II) ions by RHA.exp experimental value of solid phase concentration of adsorbate at equilibrium (mg/g) qm maximum adsorption capacity of adsorbent (mg/g) qmax constant in extended Langmuir isotherm (mg/g) Q adsorption energy (J) R universal gas constant (8.11] have discussed the theory associated with these models. Much of the work on the adsorption of heavy metal ions by various kinds of adsorbents has focused on the uptake of single metal ions. it is important to establish the most appropriate correlation for the equilibrium sorption curves. 2.314 J/K mol) RHA rice husk ash t time (min) Greek symbols αi constant in SRS model for each component β constant of Redlich–Peterson isotherm (0 < β < 1) βi constant in SRS model for each component ηR. where it is bound by glutathione and haemoglobin in the red blood cells . Therefore. it is necessary to study the simultaneous sorption of two or more metal ions and also to quantify the interactive affect of one metal ion on the other. and (v) to examine the applicability of the multi-component adsorption isotherm equations to the competitive adsorption equilibria of the metals in binary systems. and Redlich–Peterson (R–P) have been used to describe the equilibrium characteristics of mono-component .C.i residual concentration of each component in the binary mixture at equilibrium (mg/l) C0 initial concentration of adsorbate in solution (mg/l) C0. (iii) determine the applicability of non-competitive adsorption isotherm models (i.and binary-component adsorption modelling To optimize the design of an adsorption system.V. [6. Due to toxicity of metals.and multi-component isotherm equations.i individual Langmuir isotherm constant of each component (l/mg) KR constant of Redlich–Peterson isotherm (l/g) m mass of adsorbent in the adsorbate solution (g/l) MPSD Marquardt’s percent standard deviation n mono-component (non-competitive) Freundlich heterogeneity factor of the single component ni individual Freundlich heterogeneity factor of each component nm number of measurements nP number of parameters N number of data points Ni (Q) number of sites having energy Q pH0 initial pH of the solution qe equilibrium single-component solid phase concentration (mg/g) qe. Thus. Government of India has set Minimal National Standards of 0. Instances of acute toxicity have occurred from ingestion of fruit juices that were stored in galvanized (zinc plated) steel containers . Langmuir and Redlich–Peterson (R–P)) for single component. for Cd(II) and Zn(II) for safe discharge of the effluents containing these metal ions into surface waters .2 and 5. the equilibrium adsorption data for Cd(II) and Zn(II) ions from single and binary systems onto RHA have been used to test the applicability of various mono. Consumption of rice containing high concentrations of cadmium led to a surge in the Itai-Itai disease in Japan in 1955 . The present paper aims to (i) study the feasibility of using RHA as an adsorbent for the individual and simultaneous removal of Cd(II) and Zn(II) metal ions from aqueous solutions. Srivastava et al. These isotherm equations are given in Table 1. (iv) gather experimental data on adsorption equilibrium for the binary system containing Cd(II) and Zn(II) ions. Freundlich. the study of adsorption of heavy metal ions from binary and ternary systems is very important.i θ i (Q) 173 multi-component (competitive) Langmuir adsorption constant of each component multi-component (competitive) R–P adsorption constant of each component coverage of each component at energy level Q is transported to other organs by the blood. the Ministry of Environment and Forests.i aij competition coefficients of component i by component j aR constant of Redlich–Peterson isotherm (l/mg) Ce residual concentration of the single-component at equilibrium (mg/l) Ce. Mono.i equilibrium solid phase concentration of each component in binary mixture (mg/g) qe. / Colloids and Surfaces A: Physicochem. including several that are involved in nucleic acid metabolism.
i (Ce.2 Sheindorf–Rebuhn–Sheintuch  (SRS) model: (1/ni )−1 N  j=1  Modified Langmuir isotherm: qm.i = qm.i Ce.i ) qe. i.j (1/n1 )+x1 KF. ηL.j j=1 Non-modified Redlich–Peterson model: KR. Aspects 312 (2008) 172–184 Table 1 Mono.i Ce.1 1 e. and (iii) the coverage by each adsorbate molecule (or ion) at each energy level Q is given by the multi-component Langmuir isotherm equation:  θi (Q) = aij Ce.i = KF. The R–P isotherm  incorporates three parameters and can be applied either in homogenous or heterogeneous systems.j e. (2) and the competition coefficients are defined as aij = K0j /K0i and thus aji = 1/aij . to the obtain modified competitive R–P model. Fritz and Schluender  extended the mono-component Freundlich equation to give extended Freundlich isotherm for binary systems.i /ηL. Whereas in the Langmuir theory. Similarly.:   βi Q Ni (Q) = αi exp − (1) RT  where αi and βi are constants.j where  Kj = K0j exp Q RT (2)  (3) Integration of Ni (Q)θ i (Q) over energy level in the range of −∞ to +∞ yields Eq.j j=1 R.j Modified Redlich–Peterson model: KR. the basic assumption is that the sorption takes place at specific homogeneous sites within the adsorbent. which is a characteristic of each species and depends on the concentrations of the other components.2 2 e. Srivastava et al. Yang  extended the mono-component Langmuir equation to give extended Langmuir isotherm for Ki Ce.i qe. the competitive non-modified R–P model is modified.i KL. MPSD is given as   N .i 1+ Extended Langmuir isotherm: qe.i (Ce.j e.j e. and can be determined from the thermodynamic data. For that reason.i KL.1 x1 + y C z1 Ce.174 V.1 Ce. has been added in the competitive Langmuir model by Bellot and Condoret  to formulate the modified competitive Langmuir isotherm. Assuming that the surface sites are uniform.j qmax KEL. (ii) that for each component in a multi-component adsorption system. and that all the adsorbate molecules (ions) in the solution compete for the same surface sites.i Ce. The isotherm parameters of all the multi-component models can be found by minimizing Marquardt’s percent standard deviation (MPSD) . The SRS equation assumes that (i) each component individually obeys the Freundlich isotherm.2 = (1/n2 )+x2 KF.i = qe.j 1+ a C j=1 R. The competition coefficients aij in the SRS model describe the inhibition to the adsorption of component i by component j. using an interaction term ηR. there exists an exponential distribution of site adsorption energies.j R. to represent the experimental data.i . / Colloids and Surfaces A: Physicochem. Individual adsorption constants may not define exactly the multicomponent adsorption behaviour of metal ion mixtures.i Extended Freundlich isotherm: qe.j adsorption.i L.i /ηR. An interaction term.C.1 = N . Eng.e.i = N 1+ a (C /η )β. Non-modified competitive Langmuir model is the extension of the basic Langmuir model.i ) qe.i . or more likely. Various multi-component isotherm equations have been developed by various researchers. multi-component systems. from the experimental sorption data of multi-component systems. better accuracy may be achieved by using modified isotherms related to the individual isotherm parameters and the correction factors.2 Ce.i Ce.2 x 2 + y C z2 Ce.j Ce.i = N 1+ K (C /η ) j=1 L.j Ce. Sheindorf et al.  derived a Freundlich-type multi-component adsorption isotherm known as the Sheindorf–Rebuhn–Sheintuch (SRS) equation. MPSD has been used extensively [18–20] to test the adequacy and accuracy of various isotherm model to represent the experimental equilibrium sorption data.and multi-component isotherm models Reference Multi-component isotherm models 1/n Freundlich: qe = KF Ce qm KL Ce Langmuir: qe = 1 + KL Ce KR Ce Redlich–Peterson: qe = β 1 + aR Ce Multi-component isotherm models  Non-modified Langmuir model: qe.i N 1 + j=1 Kj Ce. Similarly.i  KL.i 1+ N j=1 KEL.1 qe. The Freundlich isotherm is derived by assuming a heterogeneous surface with a non-uniform distribution of heat of adsorption over the surface.i = N β.
nm the number of measurements and np is the number of parameters in the model.i )  AdTot % = 100 × C0.i − Ce. the subscripts ‘exp’ and ‘calc’ mean the experimental and calculated values.i )V mg of adsorbate (5) qe.exp i Here.i Adi % = 100 × (6) (7) where V is the volume of the adsorbate containing solution (l) and w is the mass of the adsorbent (g).i. The qe.i. .i.i − Ce.i . 2 n 1  − q q e.cal i=1 i=1 MPSD = 100 (4) N nm − np i=1 q i=1 e.i − Ce.exp e.i  (C0. individual adsorption yield (Adi %) and total adsorption yield (AdTot %) can be calculated by using the following expressions:   (C0.i = w g of adsorbent C0.i C0.
50. The samples were centrifuged using Research Centrifuge (Remi Instruments. the solubility. 3. This pH0 was found to be the optimum on the For batch desorption experiments. India) without any pretreatment for the removal of metal ions from synthetic aqueous solutions in a batch treatment process. HNO3 and CH3 COOH of known concentration were contacted with metal-loaded RHA (0. the pH0 of the adsorbate solution is the most important parameter governing sorption on different adsorbents. zinc sulphate heptahydrate (ZnSO4 ·7H2 O).5 mg/l for Cd(II) and Zn(II). since equilibrium was found to have been attained in 5 h contact time. 4. M(OH)2(S) . 20. Mumbai) maintained at 30 ◦ C. Batch adsorption studies For each experimental run. the sample was diluted with DDW to have the concentration in the range of 0.8 and 213. 3.M . The mixtures were agitated at 150 rpm for 5 h in the orbital shaker. the CT.0. and 100 mg/l). Cadmium sulphate octahydrate (3CdSO4 ·8H2 O). The contact time (t) was kept as 5 h.. the Zn(II) concentration was varied in the range of 10–100 mg/l (viz.009 and 0.6. 1. Thereafter. M(OH)+ . . Desorption studies RHA was used as obtained from a nearby paper mill (Barnala paper mill. the CT. 10. Mumbai) at 5000 rpm for 5 min and then the supernatant liquid was analyzed for residual concentration of metal ions using an atomic adsorption spectrophotometer (GBC Avanta Instrument). respectively. Cd(II) and Zn(II) ion adsorption becomes nearly constant.0. the pHf values were almost constant for 6 ≤ pH0 ≤ 7. 100 ml aqueous solution of known concentration of Cd(II). At pH ≈ 5. Detailed physico-chemical characteristics of the RHA have already been presented elsewhere . 20.5. Although the metal removal increases sharply with an increase in pH0 . At pH ≈ 10. and 100 mg/l. Therefore. With an increase in the pH value. Stock solutions having concentrations of 1 g/l of Cd(II) and Zn(II) were prepared by dissolving exact amounts of CdSO4 ·8H2 O and ZnSO4 ·7H2 O in double-distilled water (DDW). the precipitation as M(OH)2(S) plays the main role in removing the M(II) ions. The .V. The initial pH (pH0 ) of the adsorbate solution was adjusted using 1N (36. / Colloids and Surfaces A: Physicochem.4. the RHA loading was kept at 10 g/l of solution containing 100 mg/l each of Cd(II) and Zn(II) at 30 ◦ C. Adsorption isotherm experiments For single metal-ion-RHA systems. In binary metal ion mixture-RHA systems. Results and discussion 4. 50.0. respectively. M(OH)2 0 . Punjab. Chemicals All the chemicals used in the study were of analytical reagent (AR) grade.2.5 for C0 = 100 mg/l. Test solutions were prepared by diluting the stock solutions with DDW to have the component concentration in the range of 10–100 mg/l. The samples were withdrawn from the flasks at different time intervals to check whether equilibrium has been attained. Analysis of cadmium(II) and zinc(II) The concentration of Cd(II) and Zn(II) in the sample was determined by flame atomic absorption spectrophotometer (GBC Avanta. respectively. RHA 3.5 g/l) HCl or 1N (40 g/l) NaOH aqueous solution without any further adjustment during the sorption process.D. Effect of initial pH (pH0 ) The effect of pH0 on the sorption was studied by adjusting the pH0 in the range of 2–8. 30. however. It is obvious that the adsorption of M(II) must be higher in alkaline solution. Mumbai. a series of 250 ml Erlenmeyer flasks containing 50 ml of DDW or aqueous solution of HCl. all the experiments were conducted at pH0 ≤ 8. The influence of the pH0 of metal ion solution on the extent of adsorption of Cd(II) and Zn(II) ions onto RHA is shown in Fig. Australia) with the detection limit of 0. the increase in adsorption is gradual. Zn(II) or binary mixture of these components was taken in a 250 ml conical flask containing fixed amount of RHA. In these experiments. the pH of the solution was found to rise sharply. for Cd(II) and Zn(II). Zn(II)] are present in deionized water in the forms of M2+ . the mixture was centrifuged and the supernatant was analyzed for metal ions released into the solvent.5 g) at 30 ± 1 ◦ C. H2 SO4 . Eng. In all cases.9 nm. Thereafter. These flasks were agitated at a constant shaking rate of 150 rpm in a temperature controlled orbital shaker (Remi Instruments.8 and 0. of the M(OH)2(S) is very high. Metal ion concentrations were determined with reference to appropriate standard metal ion solutions. In addition to the speciation of metal ions. 3.. But at higher pH. At higher pH0 (≥6. etc. NaOH and HCl were obtained from S. for each initial concentration of Cd(II) solution: viz.1. The pHf values are higher than the pH0 values for pH0 < 7. initial metal ion concentration was varied from 10 to 100 mg/l.M of M(OH)2(S) decreases. the pH of the solution remained invariant with time. 2 shows the final pH values (pHf ) as a function of pH0 in Fig. During the initial stage of the sorption process up to 30 min. 3. The adsorption of metal ions increases with an increase in pH0 .1.7. by using air–acetylene flame. 3. CT. Fig. The system pH changes during the adsorption process. 30.4–1. Up to pH0 4. Srivastava et al. increases drastically at pH0 > 4. 3. Effect of initial pH (pH0 ) It is known that metal species [M(II) = Cd(II). Experimental 175 basis of batch tests conducted to determine the effect of pH0 on adsorption capacity of RHA for metal ions. Before the analysis.008 mg/l at the wavelength of 228.0.5.3. so the M2+ is the main species present. 10.2–1.C. Fine Chemicals.0). which. Aspects 312 (2008) 172–184 3. the pH0 of the solution was maintained at 6.M of M(OH)2(S) is much smaller and the main species in the solution is M(OH)2(S) . 2.
Adsorption of cations is favoured at pH > pHPZC . It is also clear that the amount of adsorbed Zn(II) is larger than that of Cd(II). This preference/affinity of Zn(II) is because of the chemical characteristics of the ions and the ionic radius. a significant electrostatic repulsion exists between the positively charged surface of the RHA and the metal ions.C. Cd and Cu onto activated carbon  and onto a selected mixture of mosses . However. while Fig. 2. and the hydrolysis of adsorbates to release basic cations in solution. and therefore. C0 = 100 mg/l. Also. respectively. for Cd(II) and Zn(II).0) is lower than that at higher pH0 (≥6. Fig.39 g/mol) of the Cd(II) and Zn(II) ions were found to be 5. and RHA dosage = 10 g/l. To understand the adsorption mechanism. The pHPZC for RHA is 8. T = 30 ◦ C. The variation in equilibrium system pHf with pH0 during the sorption of Cd(II) and Zn(II) onto RHA. 3.70 and 4. pHf values are. / Colloids and Surfaces A: Physicochem.83 and 0. whereas the specific adsorption of anions shifts pHPZC towards higher values. and C0 = 100 mg/l. As the pH0 of the system increases. It means that the amount of H+ adsorption is less in Zn-RHA system leading to a slight increase in the system pH. the rise in pH for Zn-RHA system is lower than that of Cd-RHA system. Effect of pH0 on the removal of cadmium(II) and zinc(II) ions for mono-component adsorbate aqueous solution by RHA. Cd(II) and Zn(II) ion adsorption at lower pH0 (pH0 ≤ 6. it may be concluded that the H+ in the solution competes with Cd(II) and Zn(II) for the adsorption sites of RHA at pH0 6. the number of positively charged sites decreases and the number of negatively charged sites increases.6 and 7. The degree of pH variation was small in higher pH0 solutions because of their higher buffering capacities  (Fig. Therefore.83 A) ˚ . for the uptake of Zn.74 A) 65. However. it was found that with pH0 6. t = 5 h. Aspects 312 (2008) 172–184 Fig. resulting in the reduced uptake of metal ions. Eng. the amount of Zn(II) ions adsorption is more than that of Cd(II) ions.0. the adsorption of anions is favoured at pH < pHPZC . Srivastava et al. The theoretical monolayer surface coverage ratio as calculated from the specific surface area of RHA (36. The increase in the solution pH during sorption process appears to be the combined result of the simultaneous and. the . Effect of RHA dosage on the removal of Cd(II) and Zn(II).51 and ionic size (0. Besides.0). t = 5 h. it is necessary to determine the point of zero charge (pHPZC ) of the adsorbents. This is because the surface charge developed at low pH0 is not favourable to adsorption.0.3 . For pH0 below 6. a higher concentration of H+ in the solution competes with Cd(II) and Zn(II) for the adsorption sites. considered to indicate the equilibrium pH values .74 A) Ma and Tobin  also reported higher sorption of Zn(II) than that of Cd(II) onto peat.17 mg/g for cadmium and zinc. This may be due to the smaller ionic size ˚ than that of cadmium ion (0. of zinc ion (0.44 m2 /g) and the ˚ and molecular weight (112. the pH of the solution rises sharply and stablizes at pH 7. 1. T = 303 K. therefore. The specific adsorption of cations shifts pHPZC towards lower values.0. respectively. Similar results have been reported by other researchers.176 V. perhaps competitive adsorption of metal ions and H+ ions onto adsorbents . 2). A negatively charged surface site on the RHA favours the adsorption of metal ions due to electrostatic attraction.
the rate of metal ion removal decreases.03 – – 30. m (g/l) 10 10 10 10 10 10 25 25 25 25 25 25 20 20 20 2.27 1.5–200 1–10 1–10 a b c Tannic acid. the incremental metal ions removal becomes very low.03 0. the minimum ionic sizes in two-dimensions need to be considered.82 38.29 Present study Present study                    .  used fly ash for Cd(II) and Zn(II) removal with a mopt value of 10 g/l at pH0 6.68 12. After lapse of some time. a large number of vacant surface sites are available for adsorption.27 0.67 1.43 2.36 0.55 0. 4.71 0.88 6. the metal ions removal increases due to increased metal ions uptake by the increased amount of adsorbent.33 0.12 1.and meso-pore size distribution of the RHA and the shape of the pores coupled with the ionic size of the two metal ions play important roles in their sorption uptake by RHA.5 5. Srivastava et al.87 5.19 7.V. the minimum ionic size of the metal ion in one-dimension is critical for an adsorbent with slit shaped pores.19 0.30 0.47 0.5–200 12. and thereafter. Rao et al. Maximum removal of metal ions at C0 = 100 mg/l was found to be 62.89 0. Activated carbon.31 0. At m > 7 g/l.5 – – 2 2 t (h) 5 5 5 5 – – 1. the results cannot be interpreted based on monolayer surface coverage ratio.42 0.56 0. the adsorbent surface becomes saturated with metal ions and the residual metal ion concentration in the solution is large.43 0. i.8% for Cd(II).5 2 2 2 12 12 2 24 1 1 KF ((mg/g)/(mg/l)1/n ) 1/n qm (mg/g) KL (l/mg) Reference 0.87 6. 4 shows the effect of t on the uptake of Cd(II) and Zn(II) ions from aqueous metal ion solutions.75 1. in the present case.17 3.5 6.C.34 71. Cho et al.47 0. Therefore.45 0.3. Zn(II) > Cd(II).13 1.09 0.35 0.2 5.04 5.6 6 6 5.59 28.55 5.96 1.26 0.7 5.80 0. after 5 h contact time.45 0.7 57.27 29.16 – – 2.10 1.36 0.09 0.17 0.31 35.30 0. With an increase in m.19 0.0 and C0 = 100 mg/l. the adsorption is chemisorptive in nature.92 8.  found mopt = 25 g/l with zeolites and bentonite while Shukla and Pai  found mopt = 20 g/l with unmodified dye loaded and oxidized Jute. An increase in the adsorption with the adsorbent dosage can be attributed to the larger availability of greater surface area and more adsorption sites. This figure reveals that the removal of metal ions increases with an increase in the adsorbent dosage from 1 to 10 g/l.45 0.46 1.5 1. Eng. In the case of an adsorbent having cylindrical pores with circular cross-section.91 0. 3.61 0.87 5.76 0.36 3. / Colloids and Surfaces A: Physicochem.42 0.5 1. The removal remains unchanged above 10 g/l of RHA dosage. 4.5 1. The rate of metal ion removal is found to be very rapid during the initial 15 min.43 0.23 0.6 30 30 30 30 25 25 30 30 30 30 30 30 35 35 35 25 25 25 25 RTc RT 10–100 10–100 10–100 10–100 10–400 10–400 25–100 25–100 25–100 25–100 25–100 25–100 38–212 38–212 38–212 112–1120 65–650 12.14 0. Accordingly all the batch experiments were conducted with a contact time of 5 h under vigorous shaking conditions.75 1. At m < 7 g/l. However.95 8.5 1.85 1. a steady state approximation was assumed and a quasi-equilibrium situation was accepted. Table 2 summarizes the optimum experimental conditions and the results for the adsorptive removal of Cd(II) and Zn(II) by various adsorbents.5 2.22 0.19 0.5% for Zn(II) and 29. Different investigators have reported different values of optimum m (mopt ) for the sorptive removal of metallic ions by different adsorbents.5 6.50 0. The residual concentrations at 5 h contact time were found to be higher by a maximum of ∼1% than those obtained after 24 h contact time. Room temperature.29 1. The monolayer surface adsorption occurs when the adsorption is physical in nature. Fig. Effect of adsorbent dosage (m) The effect of m on the uptake of Cd(II) and Zn(II) ions onto RHA was studied and is shown in Fig. During the initial stage of sorption.01 1. Hence.66 – – 0.5 1.e. While considering adsorption characteristics. No significant change in metal ion removal is observed after about 120 min. Aspects 312 (2008) 172–184 adsorption order is found to be in the order of increasing molecular weight and ionic radius. the remaining vacant surface sites are difficult to be occupied due to repulsive forces Table 2 Optimum experimental parameters and isotherm constants for adsorption of cadmium and zinc ions on various adsorbents as reported in literature Adsorbent Adsorbate pH T (◦ C) C0 (mg/l) Rice husk ash Rice husk ash Bagasse fly ash Bagasse fly ash Fly ash Fly ash Zeolite 4A Zeolite 13X Bentonite Zeolite 4A Zeolite 13X Bentonite Jute unmodified Jute dye loaded Jute oxidized Vermiculite Vermiculite Bentonite Na-enriched bentonite TAa immobilised ACb TAa immobilised ACb Cd Zn Cd Zn Zn Cd Cd Cd Cd Zn Zn Zn Zn Zn Zn Cd Zn Zn Zn Cd Zn 6 6 6 6 6 6 6 6 6 6. the removal efficiency becomes almost 177 constant.2. The micro.02 0.47 42.02 63. and at about m = 10 g/l.18 – – 0.38 0.89 30. Effect of contact time (t) Aqueous metal ion solutions with C0 = 100 mg/l were kept in contact with the RHA for 24 h.04 0.
Comparison of equilibrium adsorption isotherms of zinc(II) ion at varying concentrations of cadmium(II) ion.88 mg/g for Cd(II). 5 and 6. da Fonseca et al. 5. an increase in initial concentration of metal ions enhances the adsorption uptake of the Cd(II) and Zn(II) ions.0 are shown in Figs. an increase in Zn(II) ion concentration decreases the individual adsorption yield of Cd(II) and total adsorption yield for each experimental run. between the solute molecules adsorbed on the solid surface and the bulk phase.91 to 2. and 100 mg/l. The initial concentration provides the necessary driving force to overcome the resistances to the mass transfer of Cd(II) and Zn(II) ions between the aqueous and the solid phases. 30. and m = 4 g/l. The non-linear adsorption isotherms of Cd(II) ions in the absence and presence of increasing concentrations of Zn(II) ions are shown in Fig. 50. T = 30 ◦ C.0.37 mg/g for Zn(II). Effect of contact time on the adsorption of Cd(II) and Zn(II) by RHA. When the initial ion concentration increases from 10 to 100 mg/l. The simultaneous adsorption of Cd(II) and Zn(II) ions from binary mixtures was also investigated at pH0 6. Aspects 312 (2008) 172–184 Fig. The increase in initial concentration also enhances the interaction between the metal ions in the aqueous phase and the RHA . Therefore. tion of 0. an increase in the initial metal concentration up to 100 mg/l increases the equilibrium uptake and decreases the adsorption yield of both the components. It is seen that the equilibrium Cd(II) uptake increases with an increase in the initial Cd(II) concentration up to 100 mg/l at all Zn(II) ion concentrations. 20. The equilibrium uptake of Cd(II) decreases continuously with increasing Zn(II) ion concentration. the metal ions have to traverse farther and deeper into the pores encountering much larger resistance. . Srivastava et al.178 V. / Colloids and Surfaces A: Physicochem. the metal ions are adsorbed into the meso-pores that get almost saturated with metal ions during the initial stage of adsorption. 10.0. 5. 6.  have reported 12 h equilibrium contact time for the removal of Cd(II) and Zn(II) by vermiculite at 25 ◦ C. T = 303 K. T = 30 ◦ C. This results in the slowing down of the adsorption during the later period of adsorption. C0 = 100 mg/l. Single and binary adsorption of cadmium(II) and zinc(II) ions The equilibrium uptakes and the adsorption yields obtained for single component (Cd(II) and Zn(II)) solution at pH0 6. Also. Thereafter. In the first stage of adsorption studies. 4. 4. C0 [Zn(II)] = 10–100 mg/l. In general. The results also show that the equi- Fig. t = 5 h. while initial Cd(II) concentration was changed from 0 to 100 mg/l.0. and RHA dosage = 10 g/l. The individual and total adsorption equilibrium uptakes and yields of Cd(II) and Zn(II) ions on RHA as obtained at different Cd(II) concentrations in the absence of Zn(II) or the presence of Zn(II) ions with increasing concentrations are also listed in Table 3. However.C.4. and from 0. Besides. pH0 6. pH0 6. and RHA dosage = 10 g/l. Eng. As seen from the figures and the table. t = 5 h.95 to 5. C0 [Cd(II)] = 10–100 mg/l. it is observed that the adsorption capacity of RHA for Zn(II) is greater than that for Cd(II). and reported in Table 3. at each initial Zn(II) ion concentra- Fig. the loading capacity of RHA increases from 0. Comparison of the equilibrium adsorption isotherms of cadmium(II) ion at varying concentrations of zinc(II) ion. a contact time of only 2 h was required to attain the equilibrium adsorption of Zn(II) ion onto bentonite .
99 11.84 43.10 25.34 40.90 26.58 56.30 0 1.37 mg/g at 100 mg/l initial Zn(II) ion concentration.25 46.81 74.97 73.33 2.84 4.78 38.70 32.V.10 16.37 0 0.51 0.97 mg/g.84 18.13 61.59 1.10 19. antagonism (the effect of the mixture is less than that of each of the components in the mixture) and non-interaction (the mixture has no effect on the adsorption of each of the adsorbates in the mixture) .00 53.19 1.00 0.91 1.33 48.90 0.20 49.29 77.70 mg/l Zn(II) ion)/200 mg/l initial total concentration].30 21.05% [AdTot % = 23.79 2.87 2.37 67.and binary component systems were also compared.39 76. 6 depicts the variations in the uptake of Zn(II) at equilibrium with increasing initial Zn(II) concentrations (from 0 to 100 mg/l) at a constant initial Cd(II) concentration (10–100 mg/l) at pH0 6.09 8.76 mg/l Cd(II) + 53. equilibrium uptake of Zn(II) is found to be 5.22 43. multi-component adsorbates–adsorbents generally exhibit three possible types of behaviour: synergism (the effect of the mixture is greater than that the single components in the mixture).92 39.70 57.83 1.63 35.54 73.78 0.84 29.96 47.97 44. At 100 mg/l initial Cd(II) concentration.97 2.70 67.52 2.70 17. However.70 0 85.12 1.22 2.90 34.08 70.78 2.01 46.93 56.50 0 5.64 mg/g.76 29.08 6.47 9.30 62. Eng.74 2.67 56.41 2.Cd C0.23 3.48 0.60 16.54 37. With no Cd(II) present in the solution.33 44.10 89.04 44.10 13.75 31.68 1.50 0 47.29 61.68 23.50 0 4.82 8.97 73.10 4.35 0 0. adsorbed Cd(II) quantities at equilibrium are found to be 2.55 71.Cd qe.30 32.49 1.24 18.15 24.16 4.52 3.30 0 2.12 3.10 89.05 librium uptake of Cd(II) ion decreases with increasing initial Zn(II) ion concentration.42 53.58 0.92 0 0. without Zn(II) ions and in the presence of 100 mg/l Zn(II) ion concentration.81 13.50 56.20 70.58 65.40 0 95.85 33.23 4.00 51.85 1. The combined effect of the binary mixture of Cd(II) and Zn(II) seems to be antagonistic.20 14.01 11.32 52.24 72.49 2.21 1. When the Cd(II) concentration is kept at 100 mg/l at the same initial Zn(II) ion concentration.90 77.54 1.91 74.84 41.45 3.30 65.10 22.23 = 100 × [(28.90 28.C.20 83.54 60.67 0 0.68 0.40 mg/l Cd(II) + 29.21 5. Fig.00 46.97 0 0 0 0 0 0 91.50 0 59.90 80. To analyze the antagonistic sorption behaviour of the two components.34 45.10 59.86 1.88 and 1. In general. it was expected that the total adsorption yield must be equal to 41.55 65. the adsorption yields of single.25 3.40 60. the equilibrium Zn(II) uptake decreases to 2.64 0 0.73 1.04 1.32 1. For instance.60 50.50 53.10 59.63 62.23 4.48 1.50 36.33 1.45 2.39 26.88 1.Zn qe.56 15.81 43.95 1.Zn Ce.70 mg/l Zn(II) ion)/200 mg/l initial total concentration].90 83.78 54.40 24.60 75.58 68.93 56.50 28.70 0 73. Similar adsorption patterns are observed both in the individual-Zn(II) ion and binary Ni(II)–Zn(II) ion systems. / Colloids and Surfaces A: Physicochem.22 11.23% for the total metal concentration of 200 mg/l containing equal (100 mg/l) concentrations of Cd(II) and Zn(II) in the mixture [AdTot % = 41.80 0 4.63 28.88 2.00 95.69 6.43 38.50 2.20 0 55.16 46. Srivastava et al.59 46.40 40.70 91.96 41.91 0.55 1.70 50.11 32.10 68.82 9.82 1.60 0 0.37 1. from Table 3.69 2.34 10.81 39. Thus it becomes clear that the binary metal ion solution . Increase in Cd(II) concentration decreases the equilibrium uptake of Zn(II) ion.26 27.71 27.01 27.Zn AdCd % AdZn % AdTot % 0 0 0 0 0 0 10 10 10 10 10 10 20 20 20 20 20 20 30 30 30 30 30 30 50 50 50 50 50 50 100 100 100 100 100 100 0 10 20 30 50 100 0 10 20 30 50 100 0 10 20 30 50 100 0 10 20 30 50 100 0 10 20 30 50 100 0 10 20 30 50 100 0 0 0 0 0 0 0.11 2.19 1. the experimental total adsorption yield was found to be only 23.70 0.22 1.68 5.45 43.80 29.20 83.90 50.42 13.76 27.74 11.90 55.30 0 0 0 0 0 0 0.50 53.81 19.63 1.03 1.90 5.76 8.75 35.46 4. The results given in Table 3 indicate that the presence of Cd(II) retards the equilibrium uptake of Zn(II) ions.Cd Ce.0.69 46.45 0 0.29 81.59 44. Aspects 312 (2008) 172–184 179 Table 3 Comparison of individual and total adsorption equilibrium uptake and yields found at different cadmium(II) concentrations at varying concentrations of zinc(II) ions onto rice husk ash C0. Zn(II) ion equilibrium uptake increases with an increase in the initial Zn(II) ion concentration up to 100 mg/l. respectively.05 = 100 × [(16.
The data in Table 4 also indicate that the amount of Zn(II) ions per unit weight of RHA for the complete monolayer surface coverage was higher than that of Cd(II). the R–P and Freundlich models show better fit to the experimental adsorption data than the Langmuir model.43 1. Freundlich and R–P models for Cd(II) and Zn(II) onto RHA at pH0 6.9630 1.70 5.21 5.94 0.3095 0. indicate the adsorption capacity and adsorption intensity.51 . The R2 values are closer to unity for the R–P and the Freundlich models in comparison to that for the Langmuir model.28 29.07 2.24 0.46 2. indicating favourable adsorption. structure.87 5.25 46. etc. ionic strength.50 1.37 0. etc. ionic size. Aspects 312 (2008) 172–184 Table 4 Isotherm parameters values for the removal of cadmium(II) and zinc(II) by rice husk ash Adsorbate KL (l/mg) qm (mg/g) R2 Langmuir constants Cadmium(II) Zinc(II) 0. However.7613 0.3577 0.95 1. A large value of KL also implies the strong affinity of Zn(II) ions to RHA.43 1. necessary to be cautious while using these isotherm parameter values in the design of adsorption systems. etc.53 2. It is. it is difficult to identify a common denominator from the physical and chemical properties of Cd(II) and Zn(II) ions which explains the interactive mechanism and the increase in the selectivity for sorption of an adsorbate from the binary mixtures.6785 0. in particular. ionic nature or standard redox potential. respectively.8809 0.68 5.9999 0.0.30 0.88 0. Several authors have reported Freundlich and Langmuir constants for adsorption of Cd(II) and Zn(II) on various adsorbents under different environmental conditions. Therefore. There are possible interaction effects between different species in the solution and.).3196 9. functional groups. ionic charge. 4.08 11. Freundlich and Redlich–Peterson models for the individual adsorption of cadmium(II) and zinc(II) to rice husk ash C0 (mg/l) Ce (mg/l) qe.C. the equilibrium adsorption data of Cd(II) and Zn(II)-RHA can be represented more appropriately by the R–P and the Freundlich models in the studied concentration range.81 4.48 2. It is noted that R–P constant.76 2.91 1.00 1. Table 5 Comparison of the experimental and calculated qe values evaluated from the mono-component Langmuir. Eng.0392 5. The experimental equilibrium sorption data obtained for the single component and the binary systems indicate that the adsorption capacity of RHA for Cd(II) is.81 13. concentration.9942 Adsorbate KF ((mg/g)/(mg/l)1/n ) n R2 Freundlich constants Cadmium(II) Zinc(II) 0.91 1.51 1. Since n < 1.9988 0.5.9998 of Zn(II) and Cd(II) exhibits inhibitory (antagonistic) effects on adsorption resulting in lower sorption yield. The factors that affect the sorption preference of an adsorbent for different kinds of adsorbates may be related to the characteristics of the binding sites (e.6785 0. therefore.88 0.33 2.96 2.85 2.0 are presented in Table 5 along with the MPSD values.91 1.52 3.9971 Adsorbate KR (l/g) aR (l/mg) β R2 Redlich–Peterson constants Cadmium(II) Zinc(II) 9.71 2.1903 3.93 1.) and the solution chemistry (e.39 3.53 5. the single-component Freundlich constants.9963 0. both the Cd(II) and Zn(II) ions are favourably adsorbed by RHA.g. Higher the value of n.83 2. pH.8638 7. the properties of the adsorbates (e.47 0.95 1.83 2.58 5. In view of the lower MPSD values.49 2.42 26. β normally lies between 0 and 1.3836 0. It may be seen that the isotherm parameters differ widely in their values for different adsorbents.exp (mg/g) qe.33 Zinc(II) 10 20 30 50 100 MPSD 0.16 4. the higher will be the affinity and the heterogeneity of the adsorbent surface. potential interactions on the surface depending on the adsorption mechanism. surface properties. molecular structure. less than that of Zn(II).2631 0.49 1.49 3.). The comparison of the single component experimental equilibrium adsorption uptake and the predicted uptake (qe ) from the Langmuir.180 V. The magnitude of KF also showed the higher uptake of Zn(II) than that of Cd(II) ions by RHA at pH0 6. / Colloids and Surfaces A: Physicochem.86 2. ionic weight.71 71.83 31. R2 .29 2.0 obtained from the fitting of experimental data are listed in Table 4 alongwith the regression coefficients.g. as given in Table 2.1870 0. in general.calc (mg/g) Langmuir Freundlich R–P Cadmium(II) 10 20 30 50 100 MPSD 0.48 1. Srivastava et al.29 2. KF and n.78 2. Freundlich and R–P adsorption isotherm parameters for Cd(II) and Zn(II) at pH0 6.g. Single-component adsorption isotherm models The individual Langmuir. It is found from Table 4 that the RHA shows greater heterogeneity for Zn(II) than that for Cd(II) ions.
i ) estimated were much greater than 1.080 l/mg for Cd(II) and 0. 21. 8. reflecting the affinity between RHA and the metals in the binary system are: 0.708 0. The comparisons of the experimental and calculated qe values of Cd(II) and Zn(II) ion in mixtures are also presented in the parity plots (Figs. Fig. for modified R–P model (MPSD = 26. ηR.080 0.120 4. The parametric values of all the multi-component adsorption models are given in Table 6. The use of the extended Langmuir model (MPSD = 30.i 1.324 mg/g.233 Extended Freundlich model SRS model xi yi zi aij aij 0. in the modified Langmuir model (MPSD = 24. .862 16.i . This value is considerably lower than the sum of the maximum Fig.465 1. / Colloids and Surfaces A: Physicochem. viz.846 24. Multi-component adsorption isotherm models The simultaneous adsorption data of Cd(II) and Zn(II) on the RHA have been fitted to the multi-component isotherm models.891 Modified Langmuir model Extended Langmuir model Modified R–P model ηL.645 1 1.744 0. Comparison of the experimental and calculated qe values of zinc(II) ions in a binary mixture of cadmium(II) and zinc(II) ions. All the modified Langmuir coefficients (ηL. The overall total metal ions uptake (qmax ) by RHA is 4.9).6). The Ki values.0 indicating that non-modified multi-component Langmuir model related to the individual isotherm parameters could not be used to predict the binary-system adsorption.233) also does not improve the fit to the binary adsorption data of metal ions onto RHA.324 1.613 0.128 26. Aspects 312 (2008) 172–184 181 Table 6 Multi-component isotherm parameter values for the simultaneous removal of cadmium(II) and zinc(II) by rice husk ash MPSD Adsorbate Cadmium(II) Zinc(II) MPSD Adsorbate Cadmium(II) Zinc(II) MPSD Non-modified Langmuir model Non-modified R–P model 37. 7 and 8).V..744 0.6. modified and extended Langmuir models. the use of the interaction term.7). ηL.649 53.820 0.603 0. this indicates that all the multicomponent isotherm models could represent the experimental adsorption data for the binary systems with varying degree of fit. 7. similarly. Srivastava et al. improved the fit of the non-modified R–P model (MPSD = 53.195 0. The MPSD values for the model fit of the experimental data set of Cd(II) and Zn(II) are also given in Table 6. Eng. non-modified.7) clearly increased the fit of non-modified Langmuir model. The use of interaction term.i Ki qmax ηRP.820 1 4.120 l/mg for Zn(II).C. Comparison of the experimental and calculated qe values of cadmium(II) ions in a binary mixture of cadmium(II) and zinc(II) ions.559 0.i . However.404 The multi-component non-modified Langmuir model shows a poor fit to the experimental data (MPSD = 37.689 30. the extended Freundlich and the SRS models and non-modified and modified R–P models. Since most of the data points are distributed around the 45◦ line.
82). The competition coefficients aij and aji were estimated from the competitive adsorption data of Cd(II) and Zn(II) ions by using MS EXCEL 2002 program. The SRS model fitted to the binary adsorption data of Cd(II) and Zn(II) onto RHA reasonably well (MPSD = 21. aij . 9. The SRS model. Eng. (a) Cadmium(II) uptake and (b) zinc(II) uptake. describe the inhibition to the adsorption of component i by component j. Binary adsorption isotherms cadmium(II)–zinc(II) onto RHA. the binary adsorption of metal ions onto RHA can be represented satisfactorily and adequately by the extended Freundlich model. 4. For the desorption experiments. / Colloids and Surfaces A: Physicochem. This is expected as RHA has a heterogeneous surface and the adsorption of the single metal ions have also been well represented by the Freundlich isotherm equation. A comparison of the competition coefficients shows that the uptake of the more favourably adsorbed Zn(II) was strongly affected by the presence of Cd(II) (a21 = 1. which have to be determined experimentally. Batch desorption experiments were carried out and the desorption efficiencies are compared in Fig. aij .i . HCl. It may also imply that there may be a variety of binding sites on RHA showing partial specificity to the individual metal ions.182 V.36]. RHA shows different capacities. A comparison of MPSD values for different isotherm models shows that the extended Freundlich model best-fits the experimental adsorption data of Cd(II) and Zn(II) ions from binary systems onto RHA. For that reason. For the SRS model. Therefore. several solvents (acids.C. The isotherm coefficients can be determined from the mono-component isotherm except for the adsorption competition coefficients. As can be seen.7. Freundlich isotherm equations. 10. mineral acids. KF.e. 9. Srivastava et al. for Cd(II) and Zn(II) and competition coefficients during their coexistence. The multi-component SRS model applies to those systems where each component individually obeys the single-component Freundlich isotherm. Acetic acid showed the maximum desorption efficiency of 10. and HNO3 showed almost equal but higher recovery efficiency . while the inhibition exerted in the reverse situation was less (a12 = 0. It is evident that the modification of the Freundlich equation as given by extended Freundlich model takes into account the interactive effects of individual metal ions between and among themselves and also the adsorbent RHA reasonably well. H2 SO4 . This suggests that the surface sites of the RHA are heterogeneous. Three-dimensional (3D) adsorption isotherm surfaces are used to evaluate the performance of the binary metal ions adsorption system [35. the predictions are found to be satisfactory. The surfaces are predicted by the extended-Freundlich model and the symbols are experimental data. i. Desorption study and disposal of spent RHA The regeneration of the adsorbent and/or disposal of the adsorbate-loaded adsorbent (or spent adsorbent) is very important. The competition coefficients seem to prove that the sorption of Cd(II) and Zn(II) ions onto RHA was inhibited by the presence of either one. The use of deionized water resulted in only a limited amount of metal ion desorption (<5%). On the other hand.4). the entire adsorbent surface is homogeneous and that there is no lateral interaction between the adsorbate molecules. The two components individually were found to obey the singlecomponent Freundlich model. also fitted the equilibrium binary metal adsorption data reasonably well. which is also based on Freundlich model. The competition coefficients. A 3D graphical representation of the sorption isotherm plot for the binary metal adsorption system is given in Fig.6% for Cd(II) and 8. bases and water) have been used. Aspects 312 (2008) 172–184 total capacities of Cd(II) and Zn(II) ions resulting from the single component adsorption. In this plot.8% for Zn(II). The information obtained from the maximum capacities seems to violate the basic assumptions of the Langmuir model. and thus the affinity of each binding site for the adsorbate molecules should be uniform. the experimental data points are shown along with the predicted isotherms using the extended Fig.74). and some of the sites may be specific to certain metals . the adsorption sites of Cd(II) and Zn(II) in the binary system onto RHA may likely be partially overlapped.
Process Biochem. Gong. Chem. Purif. 278 (1–3) (2006) 175. S. Schluender. Jenkins. Rebhum. MA. 32 (11) (1998) 3289.D. Appl.  O. 11 (1963) 431. Hazard. Dimitrova. Springer. 13 (4) (2000) 391. Satyavenia. A.  J. Ucer. N.  V.A. Mishra. any of the mineral acids can be selected as the optimal eluting agent for the system studied. Comparative studies for selection of technologies for arsenic removal from drinking water. J. C0 (solvent concentration) = 0. Murthyb.R. 5. It can be utilised for making blended fuel briquettes that could be used as a fuel in the furnaces.S. Mishra. Duvnjak. F.I. Sep.  V. Choudary.  S. Mall. Mishra. Oren. and m = 10 g/l. J. W.C.  MINAS. Further studies on the disposal of metal loaded RHA is in progress in our laboratory. Rao. J.M. Boston.A. Sheindorf. Gas Separation by Adsorption Processes. B. Colloid Surf.M.C. Tiwari.C.M. de Oliveira.P. Based on MPSD error function.  M.C.  V.H. New Delhi. for both single component and the binary solutions under similar experimental conditions. I. McKay. J. Srivastava. Y. / Colloids and Surfaces A: Physicochem.M. Srivastava. Germany. Tobin.26].R. Mohankrishnan. including diffusion of trace metals within oxide particles or into micro-pores [37–39]. Hydrogen ions released from the acids replace metal cations on the RHA. of India. R.T. Gordillo. Mall.  D.S.D.T. Marquardt. Yun. Mishra. Q. Overall. Chem.  R. 127 (2005) 187. Oh.N. Bellot. Hazard. Chem.  S. Biophys. V. D. J. notification issued there under central pollution control Board. C. Nordbert. Snoeyink. Sci. I. A. I. however. Saha.  V.S. J.M.  J.D. Environ. Srivastava. Carbon 26 (3) (1988) 363. A.R. For the present study. New York.O. 1 (2) (1997) 194. The affinity of RHA for Zn(II) ions was greater than that  Q. Appl. N. Mishra.S. and re-adsorption . Aygun. Isotopes 48 (7) (1997) 877. B 134 (2006) 257.M. Several explanations have been proposed for such observations. Sugita. 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Math. K. Ramesh.  S. Biochim. Eng. P. 278 (2004) 1. Srivastava. (≈26%) for Cd(II). Conclusion The present study shows that the rice husk ash (RHA) is an effective adsorbent for the removal of Cd(II) and Zn(II) metal ions from aqueous solution. Dikshit. K.M. Chemosphere 60 (10) (2005) 1416. Garcia. Desorption of metal ions from metal-loaded RHA by solvents. M. Mehandgiev. A: Physicochem. Peterson. rules. Asp.  C. Aspects 312 (2008) 172–184 183 for Cd(II). R.  G. Govt. Ma. Utrilla. C. It may be concluded that the RHA may be used for the individual and simultaneous removal of Cd(II) and Zn(II) ions from metal-containing effluents. Mishra. 79 (1981) 136. Srivastava et al.M.
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