Water Air Soil Pollut (2012) 223:3535–3544 DOI 10.1007/s11270-012-1131-7

Polydopamine Nanoparticles as a New and Highly Selective Biosorbent for the Removal of Copper (II) Ions from Aqueous Solutions Neda Farnad & Khalil Farhadi & Nicolas H. Voelcker

Received: 18 November 2011 / Accepted: 29 February 2012 / Published online: 28 March 2012 # Springer Science+Business Media B.V. 2012

Abstract The adsorption and desorption of copper (II) ions from aqueous solutions were investigated using polydopamine (PD) nanoparticles. The nanoscale PD nanoparticles with mean diameter of 75 nm as adsorbent were synthesized from alkaline solution of dopamine and confirmed using scanning electron microscopy and X-ray diffraction analysis. The effects of pH (2–6), adsorbent dosage (0.2–0.8 gL−1), temperature (298–323 K), initial concentration (20– 100 mg L−1), foreign ions (Zn2+, Ni2+, Cd2+, Fe2+, and Ag+), and contact time (0–360 min) on adsorption of copper ions were investigated through batch experiments. The isotherm adsorption data were well N. Farnad Department of Biology, Faculty of Science, Urmia University, Urmia, Iran N. Farnad Payamnour University of Urmia, Urmia, Iran K. Farhadi (*) Department of Chemistry, Faculty of Science, Urmia University, Urmia, Iran e-mail: [email protected] K. Farhadi e-mail: [email protected] N. H. Voelcker Mawson Institute, University of South Australia, GPO Box 2471, Adelaide, SA 5001, Australia

described by the Langmuir isotherm model. The maximum uptake capacity of Cu2+ ions onto PD nanoparticles was found to 34.4 mg/g. The kinetic data were fitted well to pseudo-second-order model. Moreover, the thermodynamic parameters of the adsorption (the Gibbs free energy, entropy, and enthalpy) were studied. Keywords Adsorption . Copper . Biopolymer . Polydopamine . Nanoparticles Abbreviations PD Polydopamine

1 Introduction A variety of industries release a large number of pollutants through their wastewater. Non-biodegradable metal ions are one of the toxic pollutants and can accumulate in living organism, and they have serious harmful biochemical effects on aquatic life and human beings (Shrivastava 2009; Chiron et al. 2003; An et al. 2001). Copper is one of the toxic metals. The major release of copper to wastewater is from industries like paints, metallurgical, plating, printing circuits, metal finishing processes, tannery operations, chemical manufacturing, and mining drainage. Copper is essential to the body in a very low concentration, but when it exceeds the prescribed limit, it has also a detrimental

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effect on human health. Excess copper can cause stomach and intestinal distress, liver and kidney damage, and anemia (Panayotova 2001; Gardea-Torresdey et al. 1996; Lee et al. 2006). The US Environmental Protection Agency (USEPA) and the World Health Organization (WHO) recommended the permissible levels for copper in drinking water of 1.3 and 2 mg dm−3, respectively (WHO 2004; USEPA 2002). Therefore, the concentration of this metal must be reduced to permissible levels by various methods. Different methods have been used for the removal of copper from wastewaters such as oxidation, reduction, precipitation, membrane filtration, biological process, ion exchange, and adsorption (Demirbas et al. 2009; Bailey et al. 1999; Low et al. 2000; Yang and Kocherginsky 2007). In recent years, adsorbents with high capacity, low cost, and selectivity for removal of heavy metals have been developed in adsorption technology .The nanometer materials have attracted much attention because of their unique physical and chemical properties. Most of atoms on the surface of the nanoparticles are unsaturated and can easily bind with other atoms. Nanoparticles have high adsorption capacity. Besides, the operation of adsorption process is very simple and rapid. So, there is a growing interest in the application of nanoparticles as adsorbents (Claesson and Philipse 2007; Zhang et al. 2008). Adsorbents with nitrogen-containing functional groups have been widely explored because these functional groups have been found to be one of the most effective functionalities in the adsorption or removal of heavy metal ions (Liu et al. 2008). Biopolymers and biomasses are non-toxic, selective, efficient, inexpensive, and biodegradable (Volesky 2003; Sari and Tuzen 2009; Anayurt et al. 2009; Sari et al. 2007). So, several biopolymeric adsorbents such as chitin and chitosan (Juang et al. 1999), modified starch (Zhang and Chena 2002), cellulose(Chen et al. 2009), polymer-containing algae (Leusch et al. 1995), and poly (γ-glutamic acid) (Bhattacharya et al. 1998) have been used for the removal of copper ions from aqueous solutions. This paper describes the adsorption of copper ions from aqueous solutions by polydopamine (PD) nanoparticles as a biopolymer adsorbent. Dopamine is a neurotransmitter and a biogenic catecholamine that secreted from mussels (Lee et al. 2007). PD nanoparticles were synthesized after dopamine polymerized at alkaline buffer. In this study, the effect of pH, contact time, adsorbent dose, initial concentration, and

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temperature on the removal of copper ions from aqueous solution were investigated. The kinetic and thermodynamic parameters were calculated from the experimental data. Moreover, the adsorption process of copper ions was examined using Longmuir and Freundlich isotherm methods.

2 Experimental 2.1 Chemicals Dopamine hydrochloride and (hydroxymethyl)aminomethane (Tris) were supplied by Fluka (Steinheim, Germany) and used to prepare PD nanoparticles. All of the metal nitrates and chloride salts with the highest purity available were purchased from E. Merck (Darmstadt, Germany) and used without further purification. All solutions were prepared with deionized water. 2.2 Apparatus All pH measurements were made at 25±1°C with a Metrohm instrument Model 744 (Switzerland) using a combined glass electrode. Centrifugations were performed with a Hettich centrifuge model MIKRO 22R (Germany). A Shimadzu AA-670 atomic absorption spectrophotometer (Makati City, Japan) was used in the determination of heavy metal ions concentration. Morphological features of nanoparticles were obtained with a Philips XL30 scanning electron microscopy (Eindhoven, the Netherlands). X-ray diffraction (XRD) measurements were carried out on a X’Pert PW 3040 diffractometer (Lelyweg, Almelo, the Netherlands) with CuKα radiation (l00.154056 nm) at V055 kV and I050 mA. 2.3 Preparation of PD Nanoparticles PD nanoparticles were synthesized from dopamine solution (5 mg mL−1) at alkaline pH (10 mM Tris– HCl, pH 8.5). After adjusting the pH of dopamine solution by Tris buffer, the color of solution changed from transparent to light brown because pH08.5 induced oxidation of dopamine (Lee et al. 2007). The obtained dark-brown solution (during 20 h) was freeze-dried to prepare very light PD nanoparticles. PD nanoparticles were rinsed three times with Tris–HCl buffer (pH 8.5), centrifuged for 15 min at 8,000 rpm, and

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then dark-brown supernatant was separated and dried at room temperature.

2.4 Batch Adsorption Procedure Adsorption experiments were performed triplicate in a tube containing 5 mL of test solution. An adsorbate stock solution of copper (II) (1,000 mg L−1) was prepared by dissolving appropriate amount of CuSO4.5H2O in deionized water. This solution was diluted to the required concentration. The pH of solution was adjusted using 0.1 mol L−1 HCl or 0.1 mol L−1 NaOH solutions. The effects of pH (2–6), initial metal ion concentration (20–100 mg L−1), adsorbent dosage (0.2–0.8 g L−1), and temperature (298–323 K) on the removal of Cu2+ ions were studied in a batch mode of operation for a specific period of contact time (0–360 min). The suspensions were shaken for a predetermined contact time in an electrical shaker. After attaining equilibrium, the nanoparticles were separated by centrifuging at 8,000 rpm for 15 min, and the residue amount of metal ions was determined in aqueous phase using atomic absorption spectrophotometer. The amount of the adsorbed cation (milligrams per gram) was calculated from Eq. 1: qe ðmg=gÞ ¼

ðC0  Ce ÞV 1; 000W

2.7 Study of Foreign Ion Effect In order to determine the amount of the copper ions adsorbed by adsorbent in the presences of foreign ions, 5 mL of 40 mg L−1 of cations (Cu2+, Zn2+, Ni2+, Cd2+, Fe2+, and Ag+) were shaken for 270 min. Then solid/liquid phases were separated by centrifuging and the removal percentage of each metal was calculated by Eq. 2: %Removal ¼

Co  Ce  100 Co

ð2Þ

where C0 and Ce are the initial and the final concentration of metal ions in solution phase, respectively. 2.8 Desorption Studies Adsorption studies were performed by mixing 4 mg PD nanoparticles and 5 mL solution of 20 mg L−1 copper (II) solutions (pH05). The copper (II) loaded PD nanoparticles was separated by centrifuging at 8,000 rpm for 15 min. Desorption studies were performed on loaded nanoparticles using 5 mL of 0.1 mol L−1 HNO3 solution. All the adsorption experiments were carried out at room temperature.

ð1Þ

where C0 (milligrams per liter) and Ce (milligrams per liter) are the metal concentrations before and after adsorption, respectively; V (milliliters) is the volume of the solute; and W (grams) is the weight of the adsorbent.

2.5 Kinetic Studies Procedure Adsorption kinetics were conducted at 298 K by agitating 40 mg L−1 of copper (II) solutions at pH 5 with 4 mg of PD nanoparticles to different time intervals (30, 90, 165, 270, 330, and 360 min).

2.6 Thermodynamic and Isotherm Studies Procedure Thermodynamic and isotherm studies were done at 298, 313, and 323 K by agitating copper (II) solutions (20–100 mg L−1) at pH 5 with 4 mg of PD nanoparticles for 270 min.

3 Results and Discussion 3.1 Characterization of Adsorbent The surface of adsorbent was characterized by scanning electron microscopy (SEM). Figure 1a, b shows the images of PD nanoparticles used as an adsorbent in our studies. As can be seen, the synthesized PD particles appeared as grains with sizes ranging from 63 to 87 nm. X-ray diffraction pattern for nanoparticles presented an intense peak at 25.8°. The measured particles size from SEM images was confirmed by XRD data which full width at half max (FWHM) is taken from X-ray peak for 2θ025.8° of sample. The determination of particle size has been carried out using well known Scherer’s formula (Cullity and Stock 2001) to be about 60 nm as Eq. 3: D¼

0:9l B cos θ

ð3Þ

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Fig. 1 SEM of PD nanoparticles: a 63 nm and b 87 nm

where D is the thickness of crystallite (nanometers), l is X-ray wavelength (0.154 nm), B is FWHM or integral breadth, and θ is the diffraction angle.

structure of adsorbent was completely destroyed, so pH05 was selected for further studies.

3.3 Effect of Adsorbent Dose 3.2 Effect of pH In this study, the control of solution pH was important because low pH influences the structure of adsorbent and high pH affects the copper speciation. At high pH, copper ions form precipitates or complexes with hydroxide ions (Lee et al. 2006). So, the effect of pH on the adsorption of copper (II) ions was studied in the range 2–6. Figure 2 shows the effect of pH on the adsorption of copper ions by PD nanoparticles. As can be seen, the adsorption amount of copper ions is increased at pH 2–5. For pH>5, the adsorption ability of copper was decreased probably due to the formation of copper hydroxide adducts resulting in diminished activity of copper ions in solution. At pH <2, the

It is obvious that the amount of removed compound directly depends on the amount of adsorbent and removal of target compound increases with an increase in adsorbent amount (Wang et al. 2008; Cicek et al. 2007; Sari et al. 2007). For this purpose, the effect of 0.2–1.0 gL−1 of PD nanoparticles was evaluated on the removal of copper ions from a 40 mg L−1 solution at 293 K (Fig. 3). The obtained results showed that 0.8 gL−1 of adsorbent removes copper ions up to 70%, and no significant efficiency in the removal percentage values is observed with increasing the amount of PD nanoparticles. So we selected 0.8 gL−1 as optimum adsorbent amount for further experiments.

Fig. 2 Effect of pH on adsorption copper (II) ions. Conditions: adsorbent amount, 0.8 g L −1 ; copper (II) concentration, 40 mg L−1; contact time, 270 min

Fig. 3 Effect of adsorbent dosage on adsorption of copper (II) ions. Condition: pH05; initial copper (II) ions concentration, 40 mg L−1; contact time, 270 min

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3.4 Effect of Initial Concentration Figure 4 shows the effect of initial concentration on the removal of Cu2+ ions. Adsorption process was carried out with changing initial copper ion concentration from 20 to 100 mg L−1 using 0.8 gL−1 of adsorbent dosage at pH 5 for 270 min. The increase in initial concentration of metal ions causes the increase in the amount of the metal adsorbed ions per unite weight of adsorbent. This phenomenon may cause the increase in the driving force of the concentration at the adsorbent–adsorbate interface, thus increases the uptake capacity of heavy metal ion on the adsorbent (Liu et al. 2009). 3.5 Effect of Contact Time The effect of shaking time on adsorption process for a period of time was studied. After regular intervals of time, suitable aliquots were analyzed for copper concentration. Figure 5 indicates that the time required for equilibrium is 270 min. 3.6 Effect of Temperature Adsorption experiments were conducted at different temperatures (298, 313, and 323 K) by agitating 40 mg L−1 of copper (II) solution at pH 5 with 0.8 g L−1 of PD nanoparticle for 270 min. The results showed that maximum adsorption capacity of Cu (II) decreases from 36 mg g−1 at 298 to 18 mg g−1 at 323. This indicates that the adsorption of copper ions on the adsorbent is an exothermic process (Fig. 6). The decrease in the adsorption of copper (II) ions caused by increasing temperature may be due to either the damage

Fig. 4 Effect of initial concentration on adsorption of copper (II) ions. Conditions: pH05; adsorbent dosage, 0.8 gL−1; contact time, 270 min

Fig. 5 Effect of contact time on adsorption copper (II) ions. Condition: pH05; initial copper (II) ions concentration, 40 mg L−1; adsorbent dosage, 0.8 gL−1

of active binding sites in the biopolymer or increasing the tendency to desorb metal ions from the interface to the solution (Sari and Tuzen 2008). 3.7 Adsorption Kinetic Model Adsorption experiments were performed to determine the removal rate of copper ions by PD nanoparticles. The amounts of metal adsorbed (milligrams per gram) at time t, qt, was calculated as follows (Eq. 4): qt ¼

ðC0  Ct ÞV W

ð4Þ

where C0 and Ct are the metal concentration in liquid phase at the initial and any time t (milligrams per liter),

Fig. 6 Effect of temperature on adsorption of copper (II) ions. Condition: pH05; initial copper (II) ions concentration, 40 mg L−1; adsorbent dosage, 0.8 gL−1; contact time, 270 min

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Linearized form of the above equation is Eq. 8: t 1 1 ¼ þ t qt k2 q2e qe

Fig. 7 Adsorption kinetics of copper (II) ions on PD nanoparticles (fitting of the pseudo-first-order kinetic model)

respectively. V is the volume of the solution (milliliters), and W is the weight of the adsorbent (grams). In this study, in order to analyze the kinetic adsorption, two common pseudo-first-order and pseudosecond-order models proposed by Lagergren and Svenska (1898) and Ho and McKay (1998), respectively, were tested. The Lagergren first-order kinetic is given as (Lagergren and Svenska 1898): dq ¼ K 1 ð qe  q t Þ dt

ð5Þ

Linearized form of the above equation as follows (Eq. 6): logðqe  qt Þ ¼ log qe 

k1 t 2=303

ð6Þ

where qe and qt are the amounts of solute adsorbed per unit mass of the adsorbent (milligrams per gram) at equilibrium time and time t (minutes), respectively, and k1 is the rate constant (per minute). The plots of log (qe −qt) versus t were used to determine the rate constant, k1 (Fig. 7). The calculated correlation coefficients and rate constant for pseudofirst-order were summarized in Table 1. The Ho and McKay (1998) pseudo-second order equation is given as: dq ¼ k 2 ð qe  qt Þ 2 dt

ð7Þ

ð8Þ

where k2 is the rate constant of second-order adsorption (grams per milligram per minute). The equilibrium adsorption amount (qe) and the pseudo-second-order rate parameters (k2) can be calculated from the slope and intercept of plot of t/qt versus t (Fig. 8). The values of constants are shown in Table 1. The experimental data for the adsorption of the copper (II) ions with kinetic models confirmed that the higher correlation coefficients (R2) values of the adsorption data were well represented by pseudo-second-order kinetics for the entire adsorption period which supported the assumption for the model that the adsorption is due to chemisorption (Kumar et al. 2005).

3.8 Thermodynamic Studies Thermodynamic parameters such as the change in free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) were calculated using the following equations (Kobya 2004; Sayan 2006): $G0 ¼ RT 1n Kb 1n Ke ¼

ð9Þ

$S  $H  1  R R T

ð10Þ

where Kb (liters per mole) is the product of Langmuir constant b (liters per milligram) and molar weight of copper, R is the gas constant (8.314 JK−1 mol−1), and T is the absolute temperature (kelvin), respectively. ΔH° and ΔS° can be calculated from the slope and intercept of the Van’t Hoff plot of lnKe versus (1/T) from Eq. 10. Negative value of ΔH° (296 kJ/mol) indicates that the adsorption process is exothermic. The negative values of ΔG° (29.8, 31.3, and 32.3 kJ/ mol at 298, 313, and 323 K, respectively) revealed that a spontaneous process of adsorption of Cu+2 by adsorbent has occurred, and the negative value of ΔS°

Table 1 The pseudo-first and pseudo-second-order kinetic model constants T (°K)

298

First order k1 (min−1) 2.37

Kinetic qe (mg g−1) 1.00

Model R2

Second-order k2 (g mg−1 min−1)

0.928

0.003880

Kinetic qe (mg g−1)

Model R2

37.03

0.999

Water Air Soil Pollut (2012) 223:3535–3544

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Fig. 8 Adsorption kinetics of copper (II) ions on PD nanoparticles (fitting of the pseudo-second-order kinetic model)

(9.65 J/mol K) revealed the decreased randomness at the solid/solution interface (Inbaraj et al. 2009).

3.9 Isotherm Studies Adsorption isotherm represents the relationship between the mass of adsorbate adsorbed at constant temperature per unit mass of the adsorbent and the liquid phase adsorbate concentration. Calculating maximum adsorption capacity (qe) and sorption mechanism is usually done by fitting experimental data to an isotherm model. The experimental data were fitted to Langmuir (1918) and Freundlich (1906) adsorption isotherm models. The Langmuir isotherm derived from simple mass action kinetics is based on the assumptions that molecules are adsorbed as a saturated monolayer of one molecule thickness with no transmigration in the plane of the surface, and the interaction between adsorbed molecules is negligible with energy of adsorption remaining constant, which can be expressed as Eq. 11: qe ¼

qm bce 1 þ bCe

Table 2 Isotherm models and their linear forms

ð11Þ

Equation 11 can be linearized to five different linear forms as shown in Table 2. The adsorption data of copper ions onto PD nanoparticles were analyzed according to Langmuir equations (Table 2). The separation factor can be determined from Langmuir equation. The separation factor is related to the adsorption system. In the literature, it is regarded as: RL >1, unfavorable; RL 01, linear; 0
1 1 þ bC0

ð12Þ

where b is Langmuir constant and Co is first concentration of adsorbate. The Freundlich isotherm is an empirical equation used for non-ideal adsorption on heterogeneous surfaces as well as multilayer adsorption and is derived by assuming an exponentially decaying adsorption site energy distribution, which can be expressed as Eq. 13: qe ¼ KF Ce1=n

ð13Þ

The linear form of the Freundlich isotherm is Eq. 14: 1 1n qe ¼ 1n Kf þ 1n Ce n

where Kf is a constant indicative of the relative adsorption capacity of the adsorbent (mg1−(1/n) L 1/n g−1) and n is a constant indicative of the intensity of the adsorption. The correlation coefficients and other parameters obtained are shown in Tables 3 and 4. According to the results, Langmuir isotherm provided good fitting. The values of the coefficient of correlation obtained from Langmuir-2 expression indicate that there is strong positive evidence that the adsorption of copper ions on

Isotherm

Linear form

Langmuir-1

1 qe Ce qe

Langmuir-2 Langmuir-3

qe ¼

qm bce 1þbCe

Langmuir-4 Langmuir-5 Freundlich

ð14Þ

qe ¼ KF Ce1=n

¼ bq1m

þ q1m

¼ bqe þ bqm

1 1 qe vs: Ce Ce qe vs: Ce qe vs: Cqee qe Ce vs: qe

¼ bqm q1e  b

1 Ce

¼ q1m Ce þ qm1 b

qe ¼  1b qe Ce 1 Ce

1 Ce

Plot

qe Ce

þ qm

ln qe ¼ ln Kf þ 1n ln Ce

vs: q1e

ln qe vs: ln Ce

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Table 3 Parameters of the Langmuir isotherm for the adsorption of Cu2+ ions onto PD nanoparticles Isotherm

b (L mg−1)

qm (mg g−1)

1.29

33.1

0.9011

298

0.111

44.0

0.8857

313

0.664

13.7

0.1116

323

0.403

34.6

0.9989

298

2.38

33.3

0.9762

313

0.9578

323

Langmuir-1

Langmuir-2

0.103 Langmuir-3

Langmuir-4

9.92 32.7

0.8809

298

0.155

40.1

0.4164

313

0.816

14.5

0.1042

323

0.673

33.2

0.8709

298

0.0647

54.7

0.4164

313

0.1042

323

5.41

0.684

33.7

0.9011

298

0.0914

47.6

0.8857

313

0.1164

323

0.123

8.59

PD nanoparticles follows the Langmuir isotherm. Although Freundlich isotherm has good correlation coefficients, the values of RL (Table 5) and higher correlation coefficients in Table 3 confirm that Langmuir isotherm is the best model for adsorption mechanism. In the next experiments, the copper (II) adsorption capacity of nanoscale PD was found as 33.4 mg Cu (II)/g of adsorbent. The maximum copper (II) adsorption capacity of proposed biosorbent was compared with some of the other reported copper (II) biosorbents in literature (Table 6). As can be seen, PD nanoparticles show a larger or similar capacity in comparison with some reported biosorbents. It must be noted that although the adsorption capacity of PD nanoparticles for the removal of copper (II) ions is not comparable with some reported biosorbents, its high selectivity for copper (II) ions as an unique characteristic compensates this fault.

Table 4 Parameters of the Freundlich isotherm for the adsorption of Cu2+ ions onto PD nanoparticles Isotherm

1/n

KF (mg1−(1/n) L1/n g−1)

R2

T (K)

298

313

323

RL

0.0584

0.0103

0.195

T (K)

0.763

0.0851 Langmuir-5

R2

Table 5 Separation factor in different temperature

T (°K)

3.10 Desorption Studies The results showed that adsorbed copper ions were completely desorbed from adsorbent using nitric acid (0.1 mol L−1) after 270 min. So, it may be stated that PD nanoparticles can be utilized as a new solid phase for preconcentration and determination of Cu2+ from aqueous solutions. The surface generation of used PD nanoparticles was not possible, and the structure of PD nanoparticles was destroyed in acidic solutions. 3.11 Effect of Foreign Ions The adsorption of Cu (II) on PD nanoparticles is influenced by the cation ions in the suspension. The obtained results showed that adsorption yield of Cu (II) ions do not change in the presence of cations such as Zn2+, Ni2+, Cd2+, and Fe2+, whereas Ag+ ions reduce the removal of Cu2+ ions and this could be probably attributed to the reduction of the Ag+ ions on the sorbent surface.

4 Conclusions In this study, a simple and fast method was used for the synthesis of PD nanoparticles as adsorbent. Nanostructure of PD was confirmed by SEM and Table 6 Comparison of maximum adsorption capacity of PD nanoparticles with some different reported biosorbents in the removal of Cu (II) ions from water samples Biosorbent

0.1339

19.3

0.9808

298

0.2574

12.8

0.5512

313

0.1368

24.0

0.3248

323

Reference

Spirulina platensis

96.8

Ulva fasciata sp.

26.88

Kumar et al. (2006)

Hazelnut shell-activated carbon

58.27

Demirbas et al. (2009)

Micrococcus sp.

36.5

Wong et al. (2001)

Elaeis guineensis kernel Freundlich

qm (mg g−1)

Powdered waste sludge PD nanoparticles

3.92 156 34.4

Solisio et al. (2006)

Tumin et al. (2008) Pamukoglu and Kargi (2006) This work

Water Air Soil Pollut (2012) 223:3535–3544

XRD analysis. PD nanoparticles are biodegradable and nontoxic, and the results of adsorption showed that PD nanoparticles can be effectively used as a high capacitance sorbent for the removal of copper ions from aqueous solutions. The maximum percentage of metal adsorption by nanostructured adsorbent occurs at pH 5 during 270 min. All kinetic results confirmed a pseudo-second-order model adsorption. The experimental results were analyzed using Langmuir and Freundlich adsorption isotherms. The results obtained were well fitted in the linear forms of Freundlich and Longmuir adsorption isotherms, but according to the correlation coefficients (R2) and separation factor (RL), Longmuir II isotherm was better than Freundlich isotherm. The maximum uptake capacity of Cu2+ ions onto PD nanoparticles was found to 34.4 mg g−1. Moreover, the results demonstrate that the adsorption process is spontaneous and exothermic under conventional conditions. At pH05, the adsorption percentages of Cu (II) on PD nanoparticles are influenced by silver ions. Acknowledgments The authors appreciate Mrs. Sima Farhadi for her efforts with the English edition of the text.

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