Single, binary and multi-component adsorption of copper and cadmium from aqueous solutions on Kraft lignin—a biosorbent

Journal of Colloid and Interface Science 297 (2006) 489–504 www.elsevier.com/locate/jcis

Single, binary and multi-component adsorption of copper and cadmium from aqueous solutions on Kraft lignin—a biosorbent Dinesh Mohan a,c,∗ , Charles U. Pittman Jr. a , Philip H. Steele b a Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA b Forest Products Department, Mississippi State University, Mississippi State, MS 39762, USA c Environmental Chemistry Division, Industrial Toxicology Research Centre, Lucknow 226 001, India

Received 20 October 2005; accepted 11 November 2005 Available online 20 December 2005

Abstract A new biosorbent for removing toxic metal ions from water/industrial wastewater has been investigated using by-product lignin from paper production. Lignin was extracted from black liquor waste, characterized and utilized for the removal of copper and cadmium from aqueous solutions in single, binary and multi-component systems. Adsorption studies were conducted at different temperatures, lignin particle sizes, pHs and solid to liquid ratios. All the studies were conducted by a batch method to determine equilibrium and kinetic parameters. The Langmuir and Freundlich isotherm models were applied. The Langmuir model fits best the equilibrium isotherm data. The maximum lignin adsorption capacities at 25 ◦ C were 87.05 mg/g (1.37 mmol/g) and 137.14 mg/g (1.22 mmol/g) for Cu(II) and Cd(II), respectively. Adsorption of Cu2+ (68.63 mg/g at 10 ◦ C and 94.68 mg/g at 40 ◦ C) and Cd2+ (59.58 mg/g at 10 ◦ C and 175.36 mg/g at 40 ◦ C) increased with an increase in temperature. Copper and cadmium adsorption followed pseudo-second order rate kinetics. From kinetic studies, various rate and thermodynamic parameters such as effective diffusion coefficients, activation energy, and activation entropy were evaluated. Adsorption occurs through a particle diffusion mechanism at temperatures 10 and 25 ◦ C while at 40 ◦ C it occurs through a film diffusion mechanism. The sorption capacity of black liquor lignin is higher than many other adsorbents/carbons/biosorbents utilized for the removal of Cu(II) and Cd(II) from water/wastewater in single and multi-component systems. © 2005 Elsevier Inc. All rights reserved. Keywords: Lignin; Black liquor; Multi-component adsorption; Copper; Cadmium; Metal ions removal

1. Introduction Heavy metals are toxic to aquatic flora and fauna even in relatively low concentrations. Some metals can be assimilated, stored and concentrated by organisms [1]. Industries, including mining and electroplating, discharge aqueous effluents containing high levels of such heavy metals as uranium, cadmium, mercury, and copper. Untreated effluents have an adverse impact on the environment [2,3]. Cadmium is very toxic, which can cause serious damage to the kidneys and bones. It is best known for its association with itai-itai disease [4]. Cadmium * Corresponding author. Fax: +1 662 325 7611.

E-mail address: [email protected] (D. Mohan). 0021-9797/$ – see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.11.023

is classified as a soft acid. Cadmium ions have little tendency to hydrolyze at pH values below 8, but above 11 all cadmium exists as its hydroxo-complex [5]. In fresh water at pH 6–8, Cd(II) predominates. CdOH+ , Cd(OH)2 , Cd(OH)2 , Cd(OH)− 3, 2− Cd(OH)4 also exist depending upon the solution pH. The chloro-complexes CdCl+ , CdCl2 , CdCl− 3 predominate in sea water and Cd(II) is present in very small amounts [5]. Free cadmium ions are highly toxic to plants and animals [6]. Cadmium accumulates in humans, causing erythrocyte destruction, nausea, salivation, diarrhea and muscular cramps, renal degradation, chronic pulmonary problems and skeletal deformity [1]. The drinking water guideline recommended by World Health Organization and American Water Works Association (AWWA) is 0.005 mg Cd/l. In many groundwaters that contain bicar-

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bonate/carbonate anions the aqueous speciation of cadmium includes several complexes with bicarbonate/carbonate. Cadmium carbonate can be a solubility control for some high alkaline environments that contain high cadmium contamination. Copper usually occurs in nature as oxides and sulfides. In acidic environments, free aqueous Cu2+ dominates. At pH 6– 8, the predominant species are Cu2+ , Cu(OH)02 , CuHCO+ 3, CuCO03 , and CuOH+ , while at pH >10 the major species are − Cu(OH)2− 4 and Cu(OH)3 [5]. Mining wastes and acid mine drainage contribute significant quantities of dissolved copper to effluent streams. Other sources are fertilizer manufacturing, petroleum refining, paints and pigments, steel-works, foundries, electroplating and electrical equipment, brass, etc. Copper is an essential element but acute doses cause metabolic disorders. Inhalation of copper produces symptoms similar to those of silicosis and allergic contact dermatitis. Chronic copper poisoning causes hemolytic anemia, neurological abnormalities and corneal opacity [7–9]. Numerous treatments have been developed for heavy metal contaminated water [1,10]. Adsorption has evolved as the front line of defense for metal ions, which cannot be removed by other techniques. The adsorption of metals by activated carbon is more complex than for organic compounds because the ionic charges affect their removal rates from solution. Metal ion adsorption by activated carbon varies with the chemical properties of adsorbate, temperature, pH, ionic strength of the liquid phase, etc. Many activated carbons are available commercially but few are selective for heavy metals and they are expensive. Due to the expense of carbon for water treatment, a search for substitutes is under way. Wastewater treatments require vast quantities of activated carbon, so improved and tailor-made adsorbents are needed for these demanding applications. Such adsorbents should be easily available, economically feasible, and readily and quantitatively regenerated chemically. Investigators have studied less expensive materials for the removal of copper and cadmium from water such as bone char [11,12], lignite [11,13], peat [14,15], sunflower stalks [16], fly ash [17], red mud [17–19], chelating cloth [20], pine bark [21], slag [22], lignin [23], bagasse carbon [1], coffee and tea [24], scrap rubber [25], Pseudomonas putida [26], coal [27], sugar beet pulp [2], cellulose graft copolymers [28], red mud [29], bagasse fly ash [30], slag [31], duolite C-433 [32]. Radovic et al. [33], Pollard et al. [34], Gupta and Ali [35] reviewed low cost adsorbents used for the remediation of toxic substances from water. The Kraft process discharges significant amounts of pulping effluent into waterways. About 90–95% of the reactive lignin biopolymer is solubilized to oligomers that contribute to the dark brown color and pollution load. Lignin oligomers released cleave to low molecular weight phenylpropanoic acids, methoxylated and/or hydroxylated aromatic acids [36]. In addition, some cellulose and hemicelluloses dissolve during alkaline pulping. Black liquors from the Kraft pulping are known to adversely impact treatment facilities and aquatic life [37]. Lignin disposal, particularly from chlorinated bleaching operations, faces severe environmental constraints. Black liquors typically consist of polyaromatic lignolytic compounds, saccharic

acids derived from carbohydrate degradation, solvent extractives, including fatty acids and resin acids, and low molecular weight organic acids. Approximately 63 × 104 metric tons/yr of lignin are produced worldwide by pulping. Efforts are under way to convert black liquor into useful products, but costs have limited progress. Therefore, new uses for lignin as a low cost adsorbent might gain wide acceptance. Efforts have been made to extract the lignin from black liquor and utilize it for the removal of organic and inorganic pollutants [38–48]. Activated carbons have also been developed from lignin [49–51]. Lignin can be a good substitute for activated carbon. Lignin is the second most abundant natural polymer after cellulose, but unlike cellulose, it lacks regular repeating units. Lignin is highly cross-linked, with a complex structure as represented in Fig. 1, illustrating some typical chemical linkages present. This amorphous random polymer of substituted phenyl propane units can be processed to yield aromatics. It is the main binder for fibrous plant components, comprising from 17 to 30% of plant biomass. Lignin decomposes from 280 to 500 ◦ C. Lignin has high loading of oxygenated functions, which increase during Kraft pulping. These could aid its adsorption behavior. In summary, the basic objectives of the present investigation are: 1. To develop a cost-effective and environmentally compatible technology for preparing a lignin adsorbent from black liquor generated from Kraft pulping. 2. Optimize a process for copper and cadmium removal in single, binary and multi-component systems from water/ wastewater using treated bio-sorbent (lignin). 3. Evaluate the influence of metal ions on each other in multicomponent adsorption systems. 4. Obtain the kinetics of removal process to establish the mechanism. 5. Evaluate and compare the lignin adsorbent versus other commercially available carbons/adsorbents. 2. Materials and methods 2.1. Reagents and equipment All chemicals were AR-grade. Stock solutions of the test reagents were made by dissolving CuSO4 ·5H2 O, Cd(NO3 )2 · 4H2 O and Zn(NO3 )2 ·6H2 O in de-ionized water from a Millipore-QTM water system. The water conductivity was 11.2 M cm. The pH measurements were made using an Orion pH meter. Test solutions pHs were adjusted using H2 SO4 (0.1 N) and NaOH (0.1 N). The metal concentrations in the samples were determined by atomic absorption (Perkin–Elmer spectrophotometer) with an air–acetylene flame using Cu- and Cd-hollow cathode lamps. Infrared spectra were obtained on (2.5 wt%) lignin in KBr disks from 500–4000 cm−1 using a Perkin–Elmer spectrophotometer. XRD line profile analysis was performed using a Philips powder X-ray diffractometer. CuKα (λ = 1.54 Å) ra-

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Fig. 1. Partial structure of a hardwood lignin molecule (After Nimz 1974). 118 The phenylpropanoid units that make up lignin are not linked in a simple, repeating way. The lignin of beech contains units derived from coniferyl alcohol, sinapyl alcohol, and para-coumaryl alcohol in the approximate ratio 100:70:7 and is typical of hardwood lignin. Softwood lignin contains relatively fewer sinapyl alcohol units. Sulfonate sites are introduced and many β-O-4 linkages are cleaved during Kraft pulping. Cleavage locations are shown by line.

diation was used with a scanning step of 0.05 Å with 10◦ < 2θ < 60◦ . The scanning electron microscopy was carried out on a Phillips SEM 501 electron microscope. The lignin surface area was determined using a Quantachrome surface area analyzer using N2 -BET method. The constituents of isolated black liquor lignin were analyzed by standard chemical methods [9,52]. Linear and non-linear regression analysis was applied to each set of adsorption data. The correlation coefficients (R 2 ) and a probability value (p), representing the Freundlich and Langmuir model fits to the data, were obtained by non-linear regression using Sigma Plot V6.0. 2.2. Adsorbent development Black liquor from Eucalyptus pulping was selected. It is malodorous, deep black and its original pH was 11.2. Its specific gravity is 1.10 g/l and total solid content is 21.4%. This black liquor was acidified with aqueous HCl (3 N) until precipitation was complete. The resulting precipitate was centrifuged,

washed repeatedly with an equal volume (8 × 100 ml) of deionized water to remove chloride ions, dried at 378 K and stored in a vacuum desiccator. This lignin was further purified by dissolving in dioxane (10% w/w solution) followed by reprecipitation into stirred anhydrous ether [23,43], filtration and successive diethyl ether, toluene, and finally low boiling petroleum ether washes. The wet ether suspension was dried over anhydrous magnesium sulfate under vacuum. 2.3. Sorption procedure Batch sorption studies were performed to obtain both rate and equilibrium data due to their simplicity. Different temperatures, particle sizes, and adsorbent doses were employed to obtain equilibrium isotherms and the data required for design and operation of column reactors to treat Cu(II)- and Cd(II)-bearing wastewater. Conical flasks (100 ml) employed for isotherm studies were filled with 50-ml solutions of either Cu(II) or Cd(II). Concentrations (1 × 10−5 –5 × 10−3 M) were used based on extensive preliminary investigations and in ac-

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cord with Cu(II) and Cd(II) concentrations found in industrial effluents. Solution pH and temperature were adjusted. A known amount of purified lignin (particle size 100–150 BSS mesh) was added to each flask followed by intermittent agitation for specified times up to a maximum of 48 h. The contact time and conditions were selected on the basis of preliminary experiments, which demonstrated that equilibrium was established in 48 h. No further uptake occurred between 48 and 72 h. All equilibrium tests were conducted for 48 h. After this period solutions were filtered using Whatman no 42 filter paper and the remaining concentrations of Cu(II) and Cd(II) were determined using atomic absorption spectroscopy. Cu(II) and Cd(II) adsorption was determined over the pH range of 2–8 and at 10, 25, and 40 ◦ C to delineate the effect of temperature and to evaluate the sorption thermodynamic parameters. The Cu(II) and Cd(II) concentrations retained in the adsorbent phase were calculated by the following mass balance equation, (C0 − Ce )V (1) , W where qe is the amount (mmol/g) of metals adsorbed, C0 and Ce are the initial and equilibrium metal ion concentrations (M) in solution, V is the adsorbate volume (l) and W is the adsorbent weight (g).

qe =

2.4. Kinetic studies Successful application of adsorption processes demands the development of cheap, nontoxic, available adsorbents of known kinetic parameters and sorption characteristics. Foreknowledge of optimal conditions would enable a better process design and modeling. Thus, the effect of contact time, amount, adsorbent particle size and adsorbate concentration were studied. At desired temperatures, predetermined amounts of lignin were added to stoppered flasks (100 ml) containing 50 ml solutions of Cu(II) and Cd(II) in a thermostatic shaking assembly. The solutions were agitated. At predetermined intervals lignin was separated and analyzed for Cu(II) and Cd(II) uptake by atomic absorption analysis of the solution. The Cu(II) and Cd(II) adsorbed was calculated by Eq. (1). 2.5. Quality assurance/quality control All batch isotherm tests were replicated twice and the experimental blanks were run in parallel to establish accuracy, reliability and reproducibility. All glassware was presoaked in a 5% HNO3 solution for 3 days, rinsed with de-ionized water and oven-dried. Blanks were run and corrections applied if necessary. All the observations were recorded in triplicate and average values are reported. 3. Results and discussion 3.1. Characterization The elemental composition of the isolated lignin shows the presence of carbon 63.2%, hydrogen 5.11%, nitrogen 1.72%,

Fig. 2. Scanning electron micrograph of the lignin sample, 150–200 British standard sieve (BSS).

sulfur 1.44%, sodium 1.21%, iron 0.03%, ash 3.2%. The surface area of lignin is 1260 m2 /g. Lignin (1 g) samples were stirred with de-ionized water (100 ml) at pH 6.8 for 2 h and left for 24 h in an airtightstoppered flask. A slight pH increase was observed. This lignin is stable in water, salt solutions, dilute acids, dilute bases and organic solvents. X-ray spectra (not provided) exhibited no peaks, indicating its amorphous nature. SEM observations of the lignin (Fig. 2) revealed its complex and porous surface texture and porosity. Pores and internal surface are requisite for an efficient adsorbent. Infrared spectra (not provided) of lignin exhibited stretching absorptions at 3410 (–O–H) and 2943 (sp3 C–H) cm−1 . Peaks at 1602 and 1509 cm−1 were assigned to aromatic skeletal (ring-breathing) vibrations. The complex amorphous nature of lignin and the random linking of units give complex and unique IR spectra of each lignin type and method of isolation. The IR spectra confirmed these Eucalyptus black liquor lignins are of the guaiacyl type. 3.2. Sorption studies Experiments were conducted to determine the optimum pH for the Cu(II) and Cd(II) uptake by lignin (Fig. 3). The greatest adsorption occurred at high pH values as expected [48]. All experiments were conducted at pH values below the onset of metal hydrolysis and precipitation, estimated as pH > 6.3 for Cu(OH)2 and pH 8.2 for Cd(OH)2 [53,54]. The adsorption of Cu(II) and Cd(II) by surface functional groups was strongly pH dependent. Cu(II) and Cd(II) adsorption is very low at pHin  2, and then increases to 90–95% within the next 3 pH units. At pHin  8.0, metal removal takes place by adsorption and by precipitation caused when OH− ions formed complexes with Cu(II) and Cd(II). The final pH increases with increasing initial pH, in acidic pHs. Thus, neutralization and sorption are parallel processes. After an initial pH of 6.0 is reached, the final pH values become almost constant. Further studies were performed at an initial pH of 4.5 for Cu(II) and Cd(II) in order to correlate

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(mmol/g) and b is the constant related to free energy or net enthalpy of adsorption (b ∝ e−H /RT ). Freundlich isotherm The Freundlich model does not indicate a finite sorbent uptake capacity and can only be applied in the low to intermediate concentration range. The Freundlich equation can be written as Eqs. (4) and (5). 1/n

qe = KF Ce

Fig. 3. Effect of pH on the adsorption of Cu(II) and Cd(II) on lignin.

metal removal with the adsorption process. The smaller adsorption values observed at low pH are attributed to competition between the protons and Na, Mg, Ca, etc. cations released by lignin into the solution. Protonation of negative oxygen functions on lignin reduces their ability to coordinate with Cu(II) or Cd(II), so at low pH metal ions are released. At high pH values, metal removal from solution is enhanced by metal oxide precipitation. A pH of 4.5 was chosen for Cu(II) and Cd(II) adsorption in both single and multi-component systems. The Cu(II) and Cd(II) adsorption isotherms at their optimum adsorption pH for the lignin are shown in Fig. 4 at three temperatures. The adsorption plots were constructed on a mmol basis for direct comparison. The atomic weights of Cu (63.5) versus Cd (112.4) can cause a misinterpretation when plots are constructed on a weight basis. The isotherms are positive, regular and concave to the concentration axis. The metal ion uptake increased with an increase in temperature, indicating an endothermic process. Cu(II) and Cd(II) uptake by lignin is almost 100% at low adsorbate concentrations and decreases at higher concentrations. Modeling the equilibrium data allows comparison of different biomaterials under different operating conditions. Thus, adsorption studies were carried out at 10, 25 and 40 ◦ C to determine the adsorption isotherms. The isotherm parameters were then evaluated using non-linear Langmuir and Freundlich models. Langmuir isotherm The Langmuir adsorption isotherm sheds no light on the mechanistic aspects of adsorption. It provides information on uptake capabilities and also reflects the usual equilibrium process behavior. The Langmuir equation is: qe =

Q0 bCe 1 + bCe

or Ce = qe



1 Q0 b



(non-linear form) 

1 + Q0

(2)

 · Ce

(linear form),

(3)

where qe is the amount of solute adsorbed per unit weight of adsorbent (mmol/g), Ce is the equilibrium concentration of solute bulk solution (mol/l), Q0 is the monolayer adsorption capacity

(non-linear form), (4) 1 log qe = log KF + log Ce (linear form) (5) n where qe is the amount of solute adsorbed per unit weight of adsorbent (mmol/g), Ce is the equilibrium concentration of solute in solution (mol/l), KF is the relative adsorption capacity constant of the adsorbent (mmol/g) and 1/n is the intensity of the adsorption constant. The Freundlich (dashed lines) and Langmuir (solid lines) isotherms for Cu(II) and Cd(II) adsorption by the Eucalyptus black liquor lignin at different temperatures are presented in Fig. 4, over a wide concentration range. The corresponding Freundlich and Langmuir parameters and correlation coefficients are given in Table 1. The correlation coefficients demonstrate that both Langmuir and Freundlich models adequately fitted the data. The monolayer adsorption capacity (Q0 ) for Cu(II) was higher than that of Cd(II) as calculated from non-linear Langmuir isotherms. In summary, no true statistical difference was found. Solute–surface interactions complicate adsorption in multicomponent systems [55]. Multi-ion systems have received less attention than single-ion systems. Therefore, the adsorption of Cu(II) and Cd(II) was determined in binary and ternary systems. Adsorption isotherms were obtained at pH 4.5 and 25 ◦ C over a concentration range of 1 × 10−5 –5 × 10−3 M using a Cu(II) to Cd(II) ratio of 1:1. The Freundlich and Langmuir adsorption isotherms for Cu(II) and Cd(II) in binary and ternary systems are presented in Fig. 5. Other metal ions clearly compete with Cu(II) and Cd(II) ions. Zn(II) is least interfering among Cu(II), Cd(II), and Zn(II) ions in binary systems. Both Langmuir and Freundlich constants (Q0 , b, KF , n) are presented in Table 2. The adsorption capacities of Cu(II) and Cd(II) in the presence and absence of other metal ions are also presented in Table 2. Analysis of the regression coefficients showed that Langmuir and Freundlich models adequately described the adsorption data, but the Langmuir isotherm fits the single and in multi-component system (see Fig. 6) data better for all the metal ions. Freundlich and Langmuir constants, KF and Q0 , have different meanings, but their values lead to the same conclusion about correlating the experimental data with the sorption model. KF and Q0 fundamentally differ. The Langmuir isotherm assumes that the adsorption free energy is independent of both surface coverage and monolayer formation. In contrast, while the surface reaches saturation, the Freundlich isotherm does not predict surface saturation by the adsorbate. Therefore, the surface covering is mathematically unlimited. In conclusion, Q0 is the monolayer adsorption capacity while KF is the relative adsorption capacity.

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Fig. 4. Cu(II) and Cd(II) adsorption on lignin at 10, 25 and 40 ◦ C and pH 4.5. (- - -) Lines represent data fit by Freundlich isotherms. (—) Lines are data fit by Langmuir isotherms. Table 1 Freundlich and Langmuir isotherm constants for the adsorption of Cu(II)and Cd(II) on Eucalyptus black liquor lignin Temperature (◦ C)

10 25 40

Freundlich constants

Langmuir constants

Cu(II)

Cd(II)

Cu(II)

Cd(II)

KF (mmol/g)

n

R2

KF (mmol/g)

n

R2

Q0 (mmol/g)

b × 10−3

R2

Q0 (mmol/g)

b × 10−3

R2

24.90 29.20 25.60

0.53 0.41 0.35

0.9928 0.9740 0.9738

15.44 44.70 55.47

0.69 0.75 0.62

0.9668 0.9912 0.9948

1.08 1.37 1.49

1.67 10.18 24.67

0.9940 0.9687 0.9514

0.53 1.22 1.56

0.36 0.28 0.98

0.9875 0.9959 0.9923

The effect of ionic interactions [1,56] on sorption may be represented by the ratio of the sorption capacity for one metal ion in presence of the other metal ions, Qmix , to the sorption capacity for the same metal when it is present alone in the solution, Q0 . When (Qmix /Q0 ) > 1 sorption is promoted by the presence of other metal ions; (Qmix /Q0 ) = 1 there is no net interaction; (Qmix /Q0 ) < 1 sorption is suppressed by other metal ions. The values of Qmix /Q0 are far less than 1 (Table 2), so sorption of both Cu(II) and Cd(II) is suppressed by other metal ions. This agrees with the sorption isotherms obtained for

Cu(II) and Cd(II) in absence and presence of various metal ions. Qmix decreased in the order Cu(II) < Cu–Cd < Cu–Zn < Cu– Cd–Zn for Cu(II) and Cd(II) < Cd–Zn < Cd–Cu < Cd–Cu–Zn for Cd(II) in multi-component systems. Clearly, both the metals follow almost the same trend. Values of the KFmix /KF ratio were determined to find if a correlation exists between KFmix /KF and Qmix /Q0 . Table 2 indicates that Langmuir (Qmix /Q0 ) and Freundlich (KFmix /KF ) constant values approximately follow the same trend. Overall, lignin’s adsorption capacity for both Cu(II) and Cd(II) decreases more in ternary versus binary systems.

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Fig. 5. Single, binary and ternary adsorption of Cu(II) and Cd(II) adsorption on lignin at 10, 25 and 40 ◦ C and pH 4.5. (- - -) Lines represent data fit by Freundlich isotherms. (—) Lines are data fit by Langmuir isotherms. Table 2 Freundlich and Langmuir isotherm constants for single and multi-component adsorption of Cu(II) and Cd(II) on Eucalyptus black liquor lignin Metal ions

System

KF

n

R2

KFmix /KF

Q0

Cu Cu Cu Cu Cd Cd Cd Cd

Cu alone Cu + Zn Cu + Cd Cu + Cd + Zn Cd alone Cd + Zn Cd + Cu Cd + Cu + Zn

29.21 15.43 17.66 3.11 44.70 10.67 7.41 6.18

0.41 0.51 0.63 0.54 0.75 0.73 0.72 0.79

0.9740 0.9842 0.9878 0.9709 0.9913 0.9870 0.9683 0.9668

– 0.53 0.60 0.11 – 0.24 0.17 0.14

1.37 0.689 0.609 0.16 1.22 0.33 0.23 0.14

The essential characteristic of a Langmuir isotherm can be expressed as a dimensionless constant, defined by Weber and Chackravorti [57] as the separation factor, RL , given in Eq. (6). RL =

1 . 1 + bC0

(6)

b × 10−3 10.18 2.14 0.69 1.07 0.28 0.29 0.31 0.23

R2

Qmix /Q0

0.9687 0.9871 0.9958 0.9860 0.9956 0.9939 0.9846 0.9703

– 0.53 0.60 0.10 – 0.24 0.17 0.14

Here, b is the Langmuir constant; C0 is the initial concentration and RL indicates the shape of the isotherm (RL > 1 unfavorable; RL = 1 linear; 0 < RL < 1 favorable; RL < 0 irreversible) (see Scheme 1, supplementary information). RL was determined at different temperatures, over a broad concentration range. RL values were less than 1 and greater than 0 in

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Fig. 6. Comparative evaluation of Langmuir and Freundlich regression coefficients for single- and multi-component adsorption of Cu(II) and Cd(II) on lignin. Table 3 Important ionic properties of Cu(II) and Cd(II) Ion

Coordination number

Cd(II) 6 Cu(II) 6

Ionic Hydrated Ionic Pauling Standard reduction radius ionic poten- electropotential vs NHE (Å) radius (Å) tial negativity M2 + 2e → M (V) 0.95 0.73

8.52 8.38

2.8 1.9

1.69 1.90

−0.403 0.341

each case, indicating favorable adsorption of Cu(II) and Cd(II) on lignin in both single and multi-component systems. 3.3. Sorption mechanism No single mechanism can explain the process of metal removal by this lignin adsorbent. Several factors contribute to metal ions sorption. Acid-insoluble Kraft lignin contains abundant polyhydric phenols, other oxygenated functions and sulfur-containing groups at its surface. These are involved in the adsorbate uptake. Ion exchange may also dominate where heavier and higher valence metal ions replace H+ , Na+ and other ions on lignin surface functions. Traces of metals present in the as-obtained lignin can enter into reactions with metal ions lower in the electrochemical series, causing heavy metals to deposit on the surface. Copper ions have smaller hydrated radii (8.38 Å) than cadmium (8.52 Å). Therefore, they can enter into smaller pores and have greater access to the surface of lignin. The larger hydrated radius of Cd(II) 8.52 Å versus that of Cu(II) 8.38 Å induces a quick saturation of adsorption sites, because of steric crowding. In addition, the crystal field stabilization energy (CFSE) of a Cu(II) complex with a d 9 complexes is equal to zero. Thus, the complexation of Cu(II) is expected to be more stable than that of Cd(II), so Eucalyptus black liquor lignin prefers Cu(II) to Cd(II) ions. The adsorption surface for Cu(II) ions is then larger than that available for Cd(II) ions. Several ionic properties are presented in Table 3. More electronegative ions will be more strongly attracted to the surface. Cu(II) has the highest

adsorption capacity and highest electronegativity [14]. Electronegativity and standard reduction potential show a trend with sorption capacity. Further, Cd(II) possesses the greater ionic potential (2.8), and stronger attraction to the sorbent than Cu(II) with an ionic potential of 1.9. However, the carbon-Cd(II) interaction forces are weaker, so cadmium ions are retained only on those surface centers with higher negative charge density [10– 12,14,15]. Multi-component systems have additional features versus those of single components. Interaction effects are possible between different species in solution and potential interactions on the surface. Surface interactions depend on both the sorption mechanism and reversibility. Competition between the different metal ions for surface sites occurs and depends on the ion’s characteristics. The sorption capacity for Cu(II) in the three binary and single ternary systems is always significantly greater than the other metal ions, in agreement with the single component data. Thus, the relative ionic property orders are good quantitative indicators of the relative sorption capacities of single and multi-component metal ion systems. 3.4. Kinetic studies The adsorbent concentration effects the uptake rate of Cu(II) and Cd(II) (see Fig. 7). The uptake increases as the amount of adsorbent increases. There is a substantial increase in adsorption when lignin dosage increases from 2 to 4 g/l, while the increase on adding another 4 g/l of lignin is not as significant. Therefore, the amount of lignin was kept at 4 g/l in all the subsequent kinetic studies. Moreover, the half-life of the process (t50 ) decreases upon increasing amount of adsorbent, confirming that the adsorption rate is dependent on the amount of lignin. The rate of Cu(II) and Cd(II) uptake onto lignin is rapid. Typically, 35–50% of the ultimate adsorption occurs within the first hour. This fast initial adsorption subsequently gives way to a slow approach to equilibrium. Steady sate adsorption is reached in 48 h.

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Fig. 7. Effect of contact time on the uptake rate of Cu2+ (- - -) and Cd2+ (—) by lignin at different adsorbent concentrations at pH 4.5, 25 ◦ C and an adsorbate concentration of 2 × 10−3 M.

Fig. 8. Lagergren plots (first order) at 10, 25, and 40 ◦ C for the adsorption of Cu2+ and Cd2+ by lignin at pH 4.5, using 4 g/l of adsorbent and adsorbate concentration of 2 × 10−3 M.

To determine the order of the adsorption kinetics, first-order and second-order kinetic models were tested to fit the experimental metal removal data and to assist water treatment process design. 3.4.1. Pseudo-first order kinetic model One kinetic model describing adsorption is Lagergen’s [58] pseudo-first order equation (7), further cited by Ho et al. [59], dqt = k1 (qe − qt ). dt

(7)

The term k1 (h−1 ) is the first order adsorption rate constant, qe is the amount of metal adsorbed at equilibrium and qt is the amount adsorbed at time ‘t’.

Integrating Eq. (7) from qt = 0 at t = 0, gives   q e − qt ln = −k1 t or qe k1 log(qe − qt ) = log qe − t (linear form), 2.303   (non-linear form). qt = qe 1 − e−k1 t

(8) (9) (10)

The first order equation did not apply throughout all the contact times in this work. It was applicable over the initial 30–40 min sorption period. Plots of log(qe − qt ) versus “time” at different temperatures (Fig. 8) and adsorbate concentrations (Fig. 1 in the supplementary information) deviated considerably from the data after a short period. The calculated slopes and intercepts from the plots were used to determine the rate constant k1 and equilibrium capacity (qe ). The calculated qe values are

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Table 4 Pseudo-first order kinetic parameters for Cu(II) and Cd(II) adsorption on Eucalyptus black liquor lignin Initial concentration (M)

Temperature (◦ C)

2 × 10−3 2 × 10−3 2 × 10−3 1 × 10−3 2 × 10−3 3 × 10−3

10 25 40 25 25 25

Cu(II)

Cd(II)

k1 (h−1 )

qe a (mmol/g)

(mmol/g)

0.12 0.19 0.22 0.15 0.18 0.21

0.33 0.45 0.28 0.30 0.45 0.63

0.37 0.49 0.49 0.25 0.49 0.72

qe b

R2

k1 (h−1 )

qe a (mmol/g)

qe b (mmol/g)

R2

0.9803 0.9832 0.9700 0.9789 0.9832 0.9924

0.11 0.19 0.10 0.23 0.19 0.13

0.157 0.266 0.200 0.146 0.265 0.372

0.175 0.253 0.337 0.135 0.253 0.374

0.9740 0.9984 0.9854 0.9830 0.9984 0.9969

a As calculated by first order equation. b As calculated by kinetic plots.

Fig. 9. Pseudo-second order kinetic plots for the adsorption of Cu2+ and Cd2+ by Eucalyptus lignin at pH 4.5, using 4 g/l adsorbent at 10, 25 and 40 ◦ C.

lower than the experimental value. The values of k1 , qe and regression coefficients provided in Table 4, demonstrate that Cu(II) and Cd(II) adsorptions by purified black liquor lignin are not first-order. 3.4.2. Pseudo-second order kinetic model The simplest way to describe the kinetics of metal removal, without stoichiometric data, is by using equation k

L + M  LM, k 

where L is the number of active lignin sites occupied, M is the free metal solution concentration and LM is the concentration of metal bound to lignin. The terms k  and k  are the adsorption and desorption rate constants, respectively. The pseudo-second order reaction is greatly influenced by the amount of metal on the adsorbent’s surface and the amount of metal adsorbed at equilibrium [60]. The rate is directly proportional to the number of active surface sites. The pseudosecond order reaction rate expression can be written as 2  d(L)t = k (L)0 − (L)t , dt

(11)

where(L)0 and (L)t are the number of active sites on the adsorbent at times t = 0 and “t ”, respectively. This can be written in terms of the amount adsorbed through equation [61,62] dqt (12) = k2 (qe − qt )2 , dt where k2 (g/(mmol h)) is the pseudo-second order adsorption rate constant, qe is the amount adsorbed at equilibrium and qt is the amount of metal adsorbed at time “t ”. Separating the variables and integrating, where q0 = 0 when t = 0 and qt = qt when t = t , gives: 1 1 = + k2 t. (qe − qt )2 qe Rearranging Eq. (13) gives: 1 t t = + . qt k2 qe2 qe

(13)

(14)

The product k2 qe2 is the initial sorption rate (Eq. (15)) Rate = k2 qe2 .

(15)

Plotting t/q t against “t ” at different temperatures (Fig. 9) and adsorbate concentrations (Fig. 2 in the supplementary information) provided second order sorption rate constants (k2 ) and qe

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499

Table 5 Pseudo-second order kinetic parameters for Cu(II) and Cd(II) adsorption on Eucalyptus black liquor lignin Initial concentration (M)

Temperature (◦ C)

2 × 10−3 2 × 10−3 2 × 10−3 1 × 10−3 2 × 10−3 3 × 10−3

10 25 40 25 25 25

Cu(II) k2 × 10−2

Cd(II) qe b

g/(mol h)

qe a (mmol/g)

0.47 0.66 1.99 0.27 0.78 0.55

0.42 0.52 0.51 0.33 0.52 0.77

0.37 0.49 0.49 0.25 0.49 0.72

R2

k2 × 10−2 g/(mol h)

qe a (mmol/g)

qe b (mmol/g)

R2

0.9946 0.9960 0.9996 0.9540 0.9989 0.9970

1.88 1.22 0.46 0.95 1.22 1.22

0.15 0.27 0.41 0.19 0.27 0.35

0.18 0.25 0.34 0.13 0.25 0.37

0.9903 0.996 0.9965 0.9969 0.9960 0.9987

(mmol/g)

a As calculated by second order equation. b As calculated by kinetic plots.

Fig. 10. Comparative evaluation of pseudo-first order and pseudo-second order regression coefficients, R 2 , for the adsorption of Cu(II) and Cd(II) on lignin.

values from the slopes and intercepts (see Table 5). The correlation coefficients (R 2 ) for these plots are superior (in most cases 0.99) (Fig. 10). The experimental qe values were compared to qe values determined by pseudo-first and second order rate kinetic models. The qe values calculated from the pseudosecond order kinetic model exhibit excellent agreement with the experimental qe values (Fig. 11). Thus, the sorption process is pseudo-second order. The pseudo-second order model is based on the assumption that the rate-limiting step is a chemical sorption between the adsorbate and adsorbent. This provides the best correlation of the data. These two models do not provide a definite mechanism; therefore, another simplified model was also tested. The mathematical treatment of Boyd et al. [63] and Reichenberg [64] distinguishes between diffusion in the particle, film diffusion and a mass action-controlled exchange mechanisms. This treatment laid the foundations of sorption/ion exchange kinetics. Three steps which occur in the adsorption of an adsorbate by a porous adsorbent are: (i) transport of the adsorbate to the external surface of the adsorbent (film diffusion);

(ii) transport of the adsorbate within the pores of the adsorbent (intraparticle diffusion); and (iii) adsorption of the adsorbate on the exterior surface of the adsorbent. Process (iii) is rapid and does not represent the rate-determining step in the uptake of adsorbate (1). Three distinct cases occur for the remaining two steps in the overall transport: case I: external transport > internal transport; case II: external transport < internal transport; case III: external transport ≈ internal transport. In cases I and II, the rate is governed by diffusion in the film and in the particle, respectively. In case III, the transport rate of adsorbate to the boundary is not significant. This leads to liquid film formation with a concentration gradient surrounding the sorbent particles. External transport is usually rate limiting in systems, which have (a) poor mixing, (b) dilute adsorbate concentration, (c) small particle size, and (d) high adsorbate affinity for adsorbent. In contrast, intra-particle transport limits the overall transfer for those systems that have (a) high adsorbate concentration,

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Fig. 11. Correlation between experimental qe , and calculated qe using first and pseudo-second order kinetic models.

(b) good mixing, (c) large adsorbent particle size, and (d) low adsorbate/adsorbent affinity. Kinetic data obtained in this work were analyzed by applying the Reichenberg [64] and Helfferich [65] mathematical models using Eqs. (16)–(19).

∞ 6  1 −Di tπ 2 n2 F =1− 2 (16) exp π n2 r02 n=1 or F =1−

∞  2  6  1 exp −n Bt , π2 n2

(17)

n=1

where F is the fractional equilibrium that is reached at time ‘t ’, obtained by the expression F=

Qt , Q0

(18)

where Qt is the amount of adsorbate taken up at time ‘t ’. Q0 is the maximum equilibrium uptake. B is given by B=

π 2 Di = time constant, r02

(19)

where Di is the effective diffusion coefficient of ions in the adsorbent phase and r0 is the radius of spherical adsorbent particles. The term n is an integer that defines the infinite series solution. B · t values (the product of multiplying B by time t ) were obtained for each observed value of F from Reichenberg’s table [59] at different temperatures. The linearity test of B · t vs time plots was employed to distinguish between adsorption controlled by film versus particle diffusion. If a B · t vs time plot (slope = B) is linear and passes through the origin, then the adsorption rate is governed by diffusion in the particle. Otherwise, it is governed by film diffusion. The Cu(II) and Cd(II) plots at different temperatures are given in Fig. 12. The B · t vs. time plots are nearly linear and pass through the origin

at temperatures 10 and 25 ◦ C, indicating the mechanism’s ratecontrolling step is diffusion in the particle. However at 40 ◦ C, film diffusion is rate-controlling. The effective diffusion coefficients at different temperatures were estimated from the slopes of the B · t plots. The effective diffusion coefficients (Di ) are 2.35 × 10−14 m2 /s (10 ◦ C), 4.25 × 10−14 m2 /s (25 ◦ C) 5.98 × 10−14 m2 /s (40 ◦ C) for Cu(II) and 2.30 × 10−14 m2 /s (10 ◦ C), 4.56 × 10−14 m2 /s (25 ◦ C), 2.30 × 10−14 m2 /s (40 ◦ C) for Cd(II), respectively. Diffusion coefficient values for the two metal ions follow the same trend in which these get adsorbed on lignin. The increase in adsorbate mobility and a decrease in retarding forces acting on the diffusing adsorbate ions cause Di to increase with temperature. Although diffusion coefficient values are low, they are high enough to cause adsorbate transport from bulk to solid phase. Similar values were reported previously [1,56,66]. The activation energy, Ea , the activation entropy, S ‡ , and pre-exponential factor, D0 (analogous to the Arrhenius frequency factor), were evaluated using equations (Fig. 3 in the supplementary information),

Ea , Di = D0 exp − (20) RT

kT S # exp . D0 = 2.72d 2 (21) h R Here, k = Boltzmann constant; h = Planck constant; R = gas constant; d = distance between two active adsorbent sites. The value of d is conventionally 5 Å in lignin. The pre-exponential factors (D0 ) are 3.77 × 10−10 and 4.35 × 10−4 m2 /s for Cu(II) and Cd(II), respectively. The energy of activation (Ea ) values are 9.84 [Cu(II)] and 24.33 [(Cd(II)] kJ/mol while the entropy of activation (S ‡ ) values are 5.68 and 9.62 J/(K/mol) for Cu(II) ad Cd(II), respectively. The formation of the “activated species” through which the adsorption proceeds is endothermic. The positive values of (S ‡ ) indicate that an increase in degree of freedom occurs during adsorption. This suggests that adsorption of Cu(II) and Cd(II) from solution occurs with a partial

D. Mohan et al. / Journal of Colloid and Interface Science 297 (2006) 489–504

501

Fig. 12. B · t vs time plots for the adsorption of (a) Cu(II) and (b) Cd(II) at 10, 25 and 40 ◦ C.

desolvation of both these ions and the active sites at the solid surface [22,66–68]. 4. Conclusions This work demonstrates that Kraft lignin derived from Eucalyptus black liquor can be used as a metal ion adsorbent for treating water/wastewater contaminated with Cu(II) and/or Cd(II) in both single and multi-component systems. Kraft process black liquors are produced in huge amounts and represent a serious pollution problem. The use of Kraft lignins as a value-added adsorbent (over its fuel value), while simultaneously reducing the quantity of this waste, should now be considered. Large quantities of low cost adsorbents will increasingly be needed for water treatment. The sorption capacity of the Eucalyptus black liquor is higher than most adsorbents/carbons/biosorbents now utilized for the removal of Cu(II) and Cd(II) from water/wastewater (see Table 6).

Lignins are inexpensive; most are nontoxic and available in large quantities as a waste. For the first time, the sorption capacities of Cu(II) and Cd(II) were compared in binary, ternary and multi-component systems (Table 7). The capacities are promising in both single and multi-component systems. Eucalyptus lignin, a biosorbent isolated from the black liquor wastewater from pulping, was fruitfully applied to Cu(II) and Cd(II) remediation from water/wastewater in both single and multiTable 6 Adsorption capacities for copper cadmium on various adsorbents Adsorbents

Cu(II) (mg/g)

Cd(II) (mg/g)

Eucalyptus black liquor lignin

137.03

87.06

Bagasse carbon Sugar beet pulp Bone char

24.39 100

Ref.

This study 31.11 [1] 21.16 [2] – [10] (continued on next page)

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Table 6 (continued)

Table 6 (continued) Adsorbents

Cu(II) (mg/g)

Cd(II) (mg/g)

Ref.

Adsorbents

Lignite Peat Norit carbon, PK1-3 Lignite Peat Peat Sunflower stalks Fly ash Red mud Rud mud/blast-furnace sludge Chelating cloth Pine bark Slag Lignin Coffee Coarse tea Tea Green tea A.C. Zeolite Yuzu Chitosan Aloe Scrapp rubber Pseudomonas putida Coal (leonardite) Blast furnace slag Seafood processing waste sludge Tea industry waste Oxidized anthracite Rud mud/blast-furnace sludge Microbeads Carbon prepared from apricot stones Carbon prepared from coconut shells Carbon prepared from lignite coal Carbon prepared from peanut hulls (PHC) Commercial activated carbon, India Montmorillonite Kaolinite Hematite Chitosan Iron oxide Bentonite Almond shell carbon Olive stone carbon Peach stone carbon Kaolinites Pine bark Sphagnum peat, SFO6 Pseudomonas aeruginosa, PU21 Darco HDB Nuchar SN Activated carbons after (1) ash discharging (2) followed by oxidation by 13% H2 O2 (3) followed by outgassing at 1400 K in argon atmosphere and contacting with an atmosphere at room temperature Sargassum vulgare Sargassum filipendula Sargassum fluitans Ascophyllum nodosum Padina sp. Sargassum sp.

40 60 40 40.25 21.13 11.18 42.18 198.2 106.44 10.57

– – –

[10] [10] [10] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [24] [24] [24] [24] [24] [24] [24] [24] [25] [26] [27] [31] [53] [69] [70] [71] [72] [73] [73] [73] [74] [74] [75] [75] [76] [77] [78] [79] [80] [80] [80] [81] [82] [83] [84] [85] [85]

Ulva sp. Gracillaria Natural dolomite Thermally activated dolomite Activated carbon from sugar beet pulp (300 ◦ C) Activated carbon from sugar beet pulp (400 ◦ C) Activated carbon from sugar beet pulp (500 ◦ C) Fe(III)/Cr(III) Arundo donax stems (ADS) Brazil nut shells (BNS) Coral line algae (CLA) P. ruscifolia (PRS) Sugar cane bagasse (SCB) Bone char C. vulgaris L. taylorii L. tayloriiphos Fly ash Vermicullite Carbon prepared from soybean, SH(1:3)-C Carbon prepared from soybean, SH(1:2)-S Apple waste Phosphated apple waste Canola meal Moss Bark Picea bark Pseudotsuga bark Pinus bark Larix bark Tectona bark Afzelia bark Sphagnum peat Sphagnum peat of Britain and Ireland Heurteauville peat Oligotrophic peat Chitosan Chitosan + tartarate Chitosan + citrate Chitosan + sluconate Chitosan + EDTA Calgon F-400 Norit Ro 3515 Blast furnace slag Activated carbon developed from fertilizer waste Poly(EGDMA-HEMA) beads Eradiata (marine algae) S. rimosus S. cerevisiae P. chrysogenum F. vesiculosus A. nodosum Tea leaves Peat Laminaria japonica Fucus vesiculosus Orgabosolv lignin Peat Acid treated peat Coal

28 11.23 6.7–7.7 6.47 7.36 11.35 6.37 6.21 5.20 3.00 35.52 104.3 130 – – 15.73 11.29 28.1 10.57 18.3 12.01 11.10 9.80 89.29 2.74 40.46 17.98 0.244 56.0 98 4–70 2.5 6.0 3.27 0.3–2.13 14.16 61.27 36–75 28.9 7.67

2.69 114.94 12.60 29.3 207.3 66.67 16.07 71.2 – – – – – – – – – – – – 120 67–87 20.97 38 20.97 8.64 98.48 16.07 2.9 47.62 32.30 25.64 – – – – – – – – 8.32 9.21 7.44 0.2–0.88 – 24.4–42.6 38.12 – –

5.0 9.6 15.3

88.80 74.19 78.68 214.70 84.30 85.43

[86] [86] [86]

59.09 56.66 50.83 72.44 62.91

[87] [87] [87] [88] [89] [89]

Cu(II) (mg/g)

Cd(II) (mg/g)

Ref.

65.19 33.72 1.02 64.93 68.03

47.65 37.49 – –

[89] [89] [90] [90] [91]

71.99

–

[91]

79.99

–

[91]

39 5.7 19.4 29.7 7.4 10.7 53.62 33.72 41.59 283.27 – – – – – – – – – – – – – – – – – – – – – – – – – – – –

– – – – – 45.05 – – – 1.38 0.72 50.83 46.38 10.8 36.2 17.6 11.2 9.6 17.4 14.7 12.0 12.9 31.3 25.8 1.56 9.18 19.57 6.354 2.75 4.29 2.60 2.48 0.59 6.35 6.35 127.09 0.0529

[92] [93] [93] [93] [93] [93] [94] [95] [95] [95] [96] [97] [98] [98] [99] [99] [100] [100] [100] [101] [101] [101] [101] [101] [101] [102] [103] [104] [104] [105] [105] [105] [105] [105] [106] [106] [107] [108]

– – – – – – – – – – – – – – –

4.6 70.52 9.1 4.0–5.0 8.62 5.4–7.4 4.98 27.0 12.66 101.03 74.98 1.10 >52.4 >77.5 1.62

[109] [110] [111] [111] [111] [111] [111] [112] [113] [114] [114] [115] [116] [116] [117]

D. Mohan et al. / Journal of Colloid and Interface Science 297 (2006) 489–504

Table 7 Adsorption capacities for copper and cadmium in multi-component systems on various adsorbents at 25 ◦ C Me- System tal

Adsorbent

Adsorption capacity (mg/g)

Reference

Cu Cu Cu Cd Cd Cd Cd Cd Cd Cu Cu Cu Cd Cd Cu Cu Cd

Eucalyptus black liquor lignin Eucalyptus black liquor lignin Eucalyptus black liquor lignin Eucalyptus black liquor lignin Eucalyptus black liquor lignin Eucalyptus black liquor lignin Bagasse carbon Bagasse carbon Bagasse carbon Peat Peat Peat Peat Peat Coal (leonardite) Tea industry waste Tea industry waste

38.71 43.81 9.95 25.71 36.93 16.12 33.11 30.02 29.11 10.54 14.29 6.54 10.23 5.64 19.06 6.65 2.59

This study This study This study This study This study This study [1] [1] [1] [14] [14] [14] [14] [14] [27] [69] [69]

Cu(Cu–Cd) Cu(Cu–Zn) Cu(Cd–Cu–Zn) Cd(Cd–Cu) Cd(Cd–Zn) Cd(Cd–Cu–Zn) Cd(Cd–Cu) Cd(Cd–Zn) Cd(Cd–Cu–Zn) Cu(Cu–Cd) Cu(Cu–Zn) Cu(Cu–Cd–Zn) Cd(Cd–Cu) Cd(Cd–Cu–Zn) Cu(Cu–Ni) Cu(Cu–Cd) Cd(Cu–Cd)

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