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Colloids and Surfaces A: Physicochem. Eng. Aspects 431 (2013) 51– 59

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

jo ur nal ho me page: www.elsev ier .com/ locate /co lsur fa

emoval of citrate-coated silver nanoparticles from aqueousispersions by using activated carbon

ospodinka Gichevaa, Georgi Yordanovb,∗

Department of Chemistry, University of Mining and Geology St. Ivan Rilski, 1700 Sofia, BulgariaFaculty of Chemistry and Pharmacy, Sofia University St. Kliment Ohridski, 1164 Sofia, Bulgaria

i g h l i g h t s

Adsorption of Ag nanoparticles onactivated carbon followed Freundlichand Langmuir isotherms.Adsorption of Ag nanoparticles onactivated carbon depended on thepresence of electrolytes.Increased deposition of Ag nanopar-ticles on activated carbon wasobserved at high electrolyteconcentration.Ag nanoparticles could be completelyremoved from aqueous dispersions.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

rticle history:eceived 20 February 2013eceived in revised form 8 April 2013ccepted 12 April 2013vailable online 25 April 2013

eywords:ilver

a b s t r a c t

This article describes studies on the efficiency of activated carbon as adsorbent for water-dispersiblesilver nanoparticles. Nanoparticles of average size around 60 nm and negative zeta-potential were syn-thesized by reduction of Ag(I) ions with sodium citrate in aqueous medium. Activated carbon (Norit®

CA1) intended for water purification was found to be an efficient adsorbent for silver nanoparticles.It was found that nanoparticle adsorption on activated carbon could be described by Freundlich andLangmuir isotherms. The presence of electrolytes favored nanoparticle deposition on the surface of acti-vated carbon. When electrolytes were used above their critical coagulation concentration aggregated

anoparticlesctivated carbondsorptionitrate

sotherm

nanoparticles were settling down along with the carbon particles, where carbons were acting as filteraid. This allowed complete removal of nanoparticles from the aqueous dispersions. It is expected thatthese studies could be important for better understanding of the interactions between nanoparticlesand solid–liquid interfaces, for preparation of nanoparticle/carbon composite materials, as well as fordesigning of methods for removal of metal nanoparticles from contaminated water during treatment ofnanowastes.

© 2013 Elsevier B.V. All rights reserved.

. Introduction

The advancement of nanotechnology and development of novelanomaterials possessing new properties lead to many scientificnd technological advances [1]. Silver nanoparticles (Ag NPs), with

∗ Corresponding author. Tel.: +359 2 8161 331.E-mail address: g.g.yordanov@gmail.com (G. Yordanov).

927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.colsurfa.2013.04.039

sizes usually less than 100 nm, are the most widely used inor-ganic nanoparticles in commercial products, mainly because oftheir unique optical, catalytic and antimicrobial properties [2–6].Silver nanomaterials have found a wide range of different applica-tions in many aspects of everyday life, such as medical products,

textiles, house hold items (in fridges, air conditioning, vacuumcleaners, washing machines), paints, plastics, etc. Antibacterial andtoxic effects of nanosilver have been demonstrated on various orga-nisms and have been attributed to the nanoparticles themselves, as

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ell as to the release of Ag(I) ions as a result of oxidation [7–12].ilver nanoparticles are usually prepared by chemical reduction ofg(I) salts and can be stabilized by coating with various ligands,hich determine the surface properties and provide stabilization

ia electrostatic and/or steric repulsion between nanoparticles [3].tudies of nanoparticle stability, chemical reactivity, aggregationnd sedimentation at various conditions will help to better predicthe behavior of nanoparticles in various aqueous systems [12–16].or example, nanoparticles released into sea water may tend toggregate due to decreased electrostatic stabilization and may bencorporated into sediments thus coming into contact with ben-hic organisms. Silver nanoparticles can also undergo chemicalhanges in aqueous media, like transformation into silver sulfideAg2S), which is expected to be the most possible fate of nanosil-er, especially in aqueous environments rich in hydrogen sulfide17,18].

The increasing number of different types and quantities ofommercial products containing nanosilver products will certainlyreate a new type of nanowastes. Nanocomposite materials con-aining nanosilver may release nanoparticles and Ag(I) ions to thenvironment. The Ag-containing nanowastes can be released alsonto the environment from factories during the production of theommercial nano-products [19]. Therefore, methods for manage-ent of Ag-nanowastes need to be developed. Since the research

n removing nanoparticles from wastewaters is in its early stages,here is insufficient data to make general conclusions about the effi-iency of current wastewater treatment systems [20,21]. Removalf Ag NPs in simulated wastewater treatment processes has beenreviously done via aeration and utilization of sequence batch reac-ors [22], as well as by sorption of the nanoparticles into biomass19]. The removal of nanoparticles by the latter method proved toe successful, however these findings suggest that the high con-ent of silver in the resulting biomass may limit its utilization forgricultural applications (because Ag NPs from the utilized biomassan inhibit the growth of useful microbial populations in the soil)20].

Although, adsorption-based methods are largely utilized inater purification systems, studies on adsorption of nanoparti-

les to support adsorbent surfaces are less known (usually theanoparticles are utilized as adsorbents but are not considereds adsorbates [23,24]). The adsorption of ligand-free and citrate-oated Ag NPs on various microparticle supports (TiO2, Ca3(PO4)2,aSO4) has been previously investigated as efficient synthesis routeo microparticle-supported nanoparticles [25]. Deposition of metalanoparticles on the surface of various solid supports usually aimsevelopment of materials for heterogeneous catalysis [26–28]. It

s worth noting that such studies may be also useful for design-ng wastewater treatment process for removal of nanoparticlesrom contaminated water. It should be taken into account thathe use of surfactants and ligands to ensure colloidal stability ofg NPs may influence their adsorption on solid–liquid interfaces

25,29]. Although, various types of activated carbon are widelysed as adsorbents in water treatment processes, systemic stud-

es of nanoparticle removal by activated carbon are missing in thecientific literature.

This article reports studies on the efficiency of activatedarbon as adsorbent for water-dispersible silver nanoparticles.ilver nanoparticles were synthesized by reduction of Ag(I)ons with sodium citrate in aqueous medium. Activated car-on (Norit® CA1) intended for water purification was used as

model sorbent for investigation of the adsorption of silveranoparticles. The effect of electrolytes on nanoparticle stabil-

ty and deposition on activated carbon were investigated. Basedn these observations, a simple method for complete removal ofitrate-coated silver nanoparticles from aqueous dispersions wasroposed.

Physicochem. Eng. Aspects 431 (2013) 51– 59

2. Experimental

2.1. Materials and reagents

Silver nitrate (AgNO3, 98%), sodium citrate (Na3C6H5O7·2H2O,p.a.), sodium nitrate (NaNO3, >99.8%), potassium chloride (KCl,>99.5%), potassium nitrate (KNO3, >99.5%) and sodium hydroxide(NaOH, >98%) were from Merck. Sodium chloride (NaCl, >99.8%)was from Fluka. Powdered activated carbon (Norit® CA1) intendedfor water treatment was purchased from Sigma–Aldrich and wasused as received. Dialysis tubing cellulose membranes (with molec-ular weight cut-off size of 12,400 Da) were from Sigma. Beforeuse, the dialysis membranes were washed with hot distilled water(60 ◦C) for 20 min. All other reagents were of analytical grade (>98%purity). Distilled water was used in all experiments.

2.2. Synthesis of silver nanoparticles (Ag NPs)

Silver nanoparticles were prepared by citrate reduction of Ag(I)ions in aqueous medium according to previously described proce-dure [30,31]. Briefly, aqueous solution of AgNO3 (60 ml; 1.0 mM)was heated to boil under reverse condenser and then aqueoussolution of sodium citrate (3 ml; 2%) was added upon intensivemagnetic stirring. Brown dispersion of pale green opalescence con-taining Ag NPs was formed within 15 min of boiling. The obtainedsuspension was stirred under reflux for additional 30 min to com-plete the reaction. Finally, a green-gray Ag colloid was obtained,which was stable upon storage at room temperature for at leastfew months (Ag NPs may sediment but can be redispersed easilyby mechanical shaking).

2.3. Characterization of Ag NPs

The Ag NPs were characterized by transmission electronmicroscopy (TEM) with electron diffraction, dynamic light scat-tering (DLS) and UV–vis absorbance spectroscopy. Samples of AgNPs were imaged and electron diffraction analysis was performedby using JEM-2100 microscope (JEOL) at acceleration voltage of200 kV. The nanoparticle size analysis and measurements of zeta(�) potential were performed in aqueous medium by using DLS sys-tem Zetasizer Nano ZS (Malvern Instruments, UK). The z-averagenanoparticle sizes were calculated based on five measurements.Light scattering experiments have been carried out at scatteringangle 90◦ at 25 ◦C. UV–vis absorbance spectra were measured inquartz cuvettes by using a double-beam spectrophotometer Shi-madzu UV-190.

2.4. Characterization of adsorbent and observation of adsorbedNPs

The morphology of adsorbent (activated carbon) and visual-ization of adsorbed Ag NPs were performed by using a scanningelectron microscope (SEM) JSM 6390 (JEOL) equipped with energydispersive spectroscopy (EDS) microprobe (Oxford INCA Analyzer)at acceleration voltage of 20 kV. Secondary electron imaging (SEI)and back-scattered electron (BSE) imaging were performed. EDS-spectra were collected (for 100 s) from selected sample areas.Specific surface area of the adsorbent was determined by themethod of Klyachko-Gurvich [32].

2.5. Stability of Ag NPs

The stability of Ag NPs in aqueous dispersions was tested atvarious concentrations of trisodium citrate (Na3C6H5O7), sodiumchloride (NaCl) and sodium nitrate (NaNO3) in order to determinethe critical concentrations of the respective electrolytes that cause

es A: Physicochem. Eng. Aspects 431 (2013) 51– 59 53

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anoparticle coagulation. Nanoparticle destabilization and aggre-ation were observed by changes in the UV–vis absorbance spectra.amples of nanoparticles were dialysed in attempt to remove resid-al citrate. Dialysed samples of Ag NPs were prepared by placing0 ml of the as-obtained dispersion into hydrated tubing mem-ranes that were dialysed against 2 L of distilled water for 12 h;he water was changed with pure distilled water each 3 h.

.6. Adsorption of Ag NPs on activated carbon

Preliminary experiments with different types of activated car-ons were performed. The activated carbon Norit® CA1 was foundo be highly efficient and therefore it was chosen for the experi-

ents reported here. Two series of experiments were performedy diluting the initial nanoparticle stock dispersion (with concen-ration of Ag NPs of 105 �g/ml) with (i) 3 mM Na3C6H5O7; andii) with aqueous solution of NaCl (to obtain 10 mM NaCl in thenal dispersion). Experiments at higher salinity (30 mM NaCl) werelso performed. Constant amount of adsorbent with respect to theotal volume of the aqueous medium was used in all experiments0.5 mg/ml). Ag NPs (used at initial concentrations up to 50 �g/ml)ispersions were mixed with the adsorbent in closed vials (of 4 ml)or a time period of 12 h at constant temperature (20 ◦C) (althoughew hours were usually needed for equilibration) and then thedsorbent was filtered off with common filter paper (with aver-ge pore size 11 �m; no detectable retention of nanoparticles wasbserved). The concentration of NPs in the aqueous dispersions wasetermined by spectrophotometric measurements at a wavelengthf 430 nm (the plasmon resonance peak absorbance). Calibrationurve was built by combining spectrophotometric measurementsat 430 nm) and gravimetric determination of nanoparticle concen-ration in the dispersions. Gravimetric analysis was performed byentrifugation of concentrated nanoparticle dispersion in preciselyre-weighted (±10 �g) Eppendorf tubes (for 60 min at 15,000 × g;ach time, 1 ml of nanoparticle dispersion was centrifuged and thelear supernatant was replaced with another 1 ml of the NPs dis-ersion and so on, until NPs from at least 5 ml of the NPs dispersionere collected; this procedure was performed in order to collect

larger amount of NPs from the relatively diluted dispersion andherefore to decrease the relative weighting error), washing withistilled water (centrifugation and washing was repeated at leastriplicate) and drying in vacuum, until constant mass was reached.ach experiment was performed at least triplicate.

. Results and discussion

.1. Synthesis and characterization of Ag NPs

The citrate-based reduction method for preparation of metalanoparticles has been originally developed by Turkevich for theynthesis of colloidal gold [33,34]. Later, it has been found thathe citrate ions can play multiple roles in the synthesis of sil-er nanoparticles, including a reducing agent, a stabilizer, and aomplexing agent [35,36]. It has been supposed that citrate ionsould be oxidized by Ag(I) to acetone-1,3-dicarboxylate and otherroducts [33,37], although the detailed chemical mechanism of theeaction may need further investigations. Variations of the exper-mental parameters, such as pH, reagent concentrations, molaratios, temperature, etc., may result in the formation of nanoparti-les of various shape, size and optical properties.

The citrate synthesis, reported here, was performed at ini-

ial concentrations of Ag(I) and citrate ions, 1 mM and 4 mM,espectively. The synthesis resulted in the formation of Ag NPsf polyhedral shape and sizes of individual particles 40–100 nm,s seen from the TEM observation (Fig. 1). The analysis with DLS

Fig. 1. A representative TEM image of Ag nanoparticles prepared by reduction ofAg(I) ions with sodium citrate. Electron diffraction pattern and image of the aqueousdispersion are given as insets.

showed z-average size of 58 nm (PDI was 0.394). Measurementsof the zeta-potential showed that the as obtained NPs were nega-tively charged in the medium after the synthesis (conductivity ofthe medium was 1.19 ± 0.05 mS/cm) with electrophoretic mobility−2.8 ± 0.1 �m cm/Vs and zeta potential −35.9 ± 1.6 mV. The neg-ative zeta-potential could be attributed to surface adsorption ofcitrate ions. It should be taken into account that hydroxylation ofoxidized metal ions on nanoparticle surface could also contributeto negative zeta-potential [25]. It is generally accepted that citrateions are weakly-bound ligands and their relatively small size isnot enough to provide for steric stabilization of nanoparticles [33].Therefore, the colloidal stability of the as obtained Ag NPs wasmainly attributed to electrostatic repulsion between nanoparticles(electrostatic repulsion is usually considered to be important forstabilization of colloids with absolute value of the zeta-potential>30 mV). The colloidal dispersion obtained after the synthesis con-tained 105 ± 5 �g/ml Ag NPs, which indicated practically completereduction of the Ag(I) ions. Indeed, previous investigations haveshown that boiling time of about 20–40 min was necessary forcomplete reduction of Ag(I) by citrate (1–5 mM) ions [36].

The UV–vis absorbance spectra of the obtained Ag NPs showedthe characteristic surface plasmon resonance absorbance bandwith a maximum located at a wavelength of 430 nm (Fig. 2). Theplasmon absorbance has been explained as a result of the interac-tion of incident light with electrons from the nanoparticle surface[38,39]. The plasmon absorbance and the light scattering producedby the nanoparticles determined the observed brown-greenishcolor of the Ag NPs dispersions. The spectrum followed a simi-lar profile at various nanoparticle concentrations and the opticalabsorbance of dispersions linearly depended on the particle con-centration following the Bouguer–Lambert–Beer law (Fig. 2). Thisobservation allowed the spectrophotometric determination of theAg NPs concentration that was used for evaluation of the nanopar-ticle adsorption on activated carbon (Section 3.3).

3.2. Colloidal stability of Ag NPs

The obtained Ag NPs could be stored for at least few monthsat room temperature after their preparation without observableaggregation (sedimentation of NPs was observed, but NPs could beeasily redispersed by simple mechanical shaking). Our experiments

indicated that increase of ionic strength of the medium resultedin nanoparticle destabilization and aggregation. Increasing of theconcentration of monovalent electrolyte (NaCl, KCl, NaNO3, KNO3)above a critical coagulation concentration (>30 mM) resulted in

54 G. Gicheva, G. Yordanov / Colloids and Surfaces A:

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ig. 2. UV–vis absorbance spectra of Ag nanoparticle aqueous dispersions of variousoncentrations. A calibration curve at a wavelength of 430 nm is given as inset, evi-ent for the validity of the Beer–Lambert–Bouguer law in the case of nanoparticles.

rapid decrease of the plasmon absorbance and appearance ofbsorbance at longer wavelengths, which indicated a scattered lightrom large aggregates (Fig. 3). The color of the dispersion changedrom yellow to gray and after few hours visible sedimentation ofanoparticles aggregates was observed. Similar behavior of citrate-oated Ag NPs has been previously observed in ecotoxicology media15]. The basic mechanism of colloidal stabilization of citrate-oated Ag NPs in aqueous dispersions was therefore supposedo be the electrostatic repulsion between the negatively chargedarticles. The increase of ionic strength resulted in compressionf the electrical double layer and decrease of the zeta-potentialalthough the exact absolute value of zeta-potential <30 mV wasifficult to be measured accurately because of particle instability).hus at higher electrolyte concentrations the van der Waals attrac-

ion between nanoparticles exceeds electrostatic repulsion and theispersion is destabilized (particles are “salted out”). Nanoparti-le aggregation was irreversible and NPs could not be redispersed

ig. 3. Changes in the optical absorbance of Ag nanoparticle (13 �g/ml) aqueousispersions before and after addition of 30 mM NaCl. Photographs of the respectiveispersions are given as insets. The same changes in spectra were observed in 30 mMolutions of other salts (NaNO3, KCl, KNO3, trisodium citrate), as well as after 24 hf dialysis. The intensity of the plasmon absorbance dramatically decreased as aesult of nanoparticle destabilization and aggregation. In 10 mM NaCl the spectrumemained unchanged and nanoparticles were stable for at least few days.

Physicochem. Eng. Aspects 431 (2013) 51– 59

by ultrasonication. It should be noted that aggregation was alsoobserved when Ag NPs were dispersed in 10 mM phosphate buffers(at various pH from 3 to 9), which compromised studies of theeffect of pH in buffer systems on the adsorption of nanoparticleson activated carbon.

The effect of trisodium citrate concentration on the colloidalstability was also investigated. We found that trisodium citrateat concentrations above 10 mM resulted in the same changesin the optical properties of the dispersion (like the monovalentelectrolytes) and promoted irreversible nanoparticle aggregation.Although, the citrate was considered as a stabilizing agent, highercitrate concentrations resulted in increased ionic strength thatdecreased the electrostatic stabilization of the colloid. On the otherhand, some minimum amount of citrate was required for stabiliza-tion of Ag NPs. Indeed, our attempts to remove the citrate ligandby dialysis resulted in destabilization and aggregation of the dia-lyzed NPs. Although, we could not determine the lower limit ofcitrate concentration required for nanoparticle stabilization, pre-vious studies have demonstrated that at least 1 mM citrate wasneeded to stabilize Ag NPs [36].

3.3. Adsorbent characteristics

Preliminary tests on the adsorption of Ag NPs on various typesof activated carbons were performed. Experiments were carriedout with chemically treated (such as the Norit® CA1, which wastreated with phosphoric acid according to the description providedby the manufacturer) and non-treated carbons (Norit® GAC andother activated carbons), which indicated Norit® CA1 as the mostsuitable adsorbent for Ag NPs (data not shown). The active sur-face area, as measured by the method of Klyachko-Gurvich, forthe carbons Norit® CA1 and Norit® GAC were 980 and 1200 m2/g,respectively (for the other tested carbons the active surface areawas ∼500 m2/g). The carbon Norit® GAC however proved to bemuch less efficient for adsorption of Ag NPs than Norit® CA1. Thegranulated form of Norit® GAC may limit the diffusion of nanopar-ticles to the surface inside the granules, especially the diffusion oflarger aggregated form of nanoparticles which are formed in pres-ence of higher concentration of electrolyte. Therefore, the activatedcarbon Norit® CA1, which was in a powdered form and exhibitedlarge surface area and nanoparticle adsorption efficiency, was cho-sen to perform the experiments reported in the next paragraphs.The SEM observations showed that samples of Norit® CA1 car-bon had particle sizes from 20 to 80 �m, and individual particlesdisplaying various kinds of porosity. The SEM resolution in ourexperiments was not high enough to allow observation of the struc-ture of pores smaller than c.a. 100 nm. However, such pores mostprobably exist in Norit® CA1, because according to the datasheet,this carbon is especially effective for adsorption of high molecu-lar weight structures, suggesting highly mesoporous structure. Itis quite possible that nanoparticles are adsorbed in pores of sizescorresponding to the particle size. Therefore, the pore size couldbe more important for adsorption of nanoparticles than the totalsurface area if the activated carbon acts as a mesoporous molec-ular sieve. According to the datasheet provided by manufacturer,Norit® CA1 has molasses number (EUR) 180; BET surface area of1400 m2/g; methylene blue adsorption min. 25 g/100 g; apparentdensity 0.37 g/cm3.

3.4. Adsorption isotherms of Ag NPs on activated carbon

Mixing of activated carbon (Norit® CA1) with aqueous disper-

sion of citrate-coated Ag NPs resulted in noticeable nanoparticleadsorption on the carbon surface (quantitatively determined bythe change in absorption spectrum of the nanoparticle disper-sion). Nanoparticles could be observed on the carbon surface with

G. Gicheva, G. Yordanov / Colloids and Surfaces A: Physicochem. Eng. Aspects 431 (2013) 51– 59 55

Fig. 4. Deposition of Ag nanoparticles on activated carbon: (a) vials with Ag NPsba

s(bnHdssAtvaiabstEFfusw

ia

Fig. 5. EDX spectra of: (a) pure activated carbon; (b) activated carbon with deposited

efore (left) and after (right) addition of activated carbon; (b) SEM-BSE image ofctivated carbon with deposited Ag nanoparticle aggregates.

canning electron microscopy by detecting backscattered electronsBSE), because heavy elements (with high atomic number, like Ag)ackscatter electrons stronger than light elements (low atomicumbers), and therefore appear brighter in the SEM-BSE images.owever, the BSE imaging mode of the SEM in our experimentsid not have high enough resolution to observe single particlesmaller than 100 nm and therefore we could not clearly observemall single adsorbed particles at low concentrations of electrolyte.t higher electrolyte concentrations (especially close and higher

han the critical coagulation concentration) the amount of sil-er deposited on the carbon visibly increased (Fig. 4a), mainly asggregated nanoparticles, which allowed their observation by BSEmaging (Fig. 4b). Probably, at high electrolyte concentrations suchggregates are first formed and then settled down along with car-on particles. Elemental analysis by energy-dispersive X-ray (EDX)pectroscopy confirmed that the brighter particles that appear inhe BSE images are actually composed of silver. Fig. 5a shows theDX data for carbon before adsorption and the EDX spectrum inig. 5b shows the presence of silver. The pure carbon adsorbent wasound to contain phosphorus and oxygen that could be from resid-al phosphates from the chemical activation of carbon; the oxygenignals in the EDX spectrum could also be attributed to adsorbedater and oxygen molecules.

All adsorption experiments for determination of adsorptionsotherms were performed with the same batches of Ag NPs andctivated carbon in order to avoid any batch-to-batch variations

silver nanoparticles.

in nanoparticle characteristics that may influence the adsorptionprofiles. Two series of adsorption experiments were carried outin order to determine the adsorption isotherms at various elec-trolyte composition of the dispersion medium. The first batch ofexperiments was performed by dilution of the initial Ag NPs stockdispersion with 3 mM trisodium citrate. It was supposed that AgNPs would remain quite stable in this medium, considering thefact that the citrate was a stabilizing ligand and its concentrationwas not high enough to reduce the electrostatic stabilization of theparticles. The second batch of experiments was performed in thepresence of 10 mM NaCl in order to evaluate its effect on the adsorp-tion profile. This concentration of salt was not high enough toinduce irreversible coagulation of the particles; neither to cause anysignificant changes in their optical properties. The adsorption pro-files observed in the presence of 3 mM citrate and 10 mM NaCl areshown in Fig. 6. Significantly increased adsorption of nanoparticlesand higher adsorption efficiency were observed in the presence of10 mM NaCl. Further increase of NaCl concentration above 30 mMresulted in increased deposition (mostly in the form of aggregates)and complete removal of Ag NPs from the dispersion (see Section3.5). The data from the first two sets of experiments was used toobtain Langmuir and Freundlich adsorption isotherms by using Eqs.(1) and (2), respectively.

1Q

= 1Qmax

+ 1QmaxKLc

(1)

56 G. Gicheva, G. Yordanov / Colloids and Surfaces A: Physicochem. Eng. Aspects 431 (2013) 51– 59

Fig. 6. (a) Adsorption isotherms; and (b) adsorption efficiency of Ag NPs on activatedc

l

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Fig. 7. Freundlich adsorption isotherms for adsorption of Ag NPs on activated carbon(Norit® CA1): temperature = 20 ◦C; adsorbent = 0.5 mg/ml; time = 12 h.

before salt addition), Ag NPs were almost instantly and com-

TF

arbon.

n(Q ) = ln(KF) + 1n

ln(c) (2)

here, Q is the amount of adsorbate, Ag NPs (per given mass ofdsorbent), Qmax is the maximum amount of adsorbate as the equi-ibrium concentration of Ag NPs in the dispersion increases, KL is theangmuir equilibrium constant, KF and n are constants for a givendsorbate and adsorbent at a particular temperature, and c is thequilibrium concentration of adsorbate (Ag NPs) in the dispersion.

The Freundlich and Langmuir constants of Ag NPs adsorptionn activated carbon were calculated from the respective fits andre given in Table 1. The obtained linear fits of the experimentalata (Figs. 7 and 8) and the relative high values of the correlationoefficients (R2 ∼ 0.99) indicated that both the Freundlich and theangmuir isotherms could describe quite well the adsorption of AgPs on the carbon surface. It should be noted that previous investi-ations have shown that the adsorption of citrate-coated Ag NPs onicroparticle supports (such as BaSO4, which was quite different

n nature in comparison with the activated carbon that was usedere) was according to Freundlich model [25].

able 1reundlich and Langmuir constants of Ag NPs adsorption on activated carbon. Correlation

Medium Freundlich constants

KF n R2

3 mM citrate 1.8 ± 0.2 1.34 ± 0.08 0.990

10 mM NaCl 6.5 ± 0.5 1.56 ± 0.08 0.994

Fig. 8. Langmuir adsorption isotherms for adsorption of Ag NPs on activated carbon(Norit® CA1): temperature = 20 ◦C; adsorbent = 0.5 mg/ml; time = 12 h.

3.5. Deposition of Ag NPs on activated carbon in the presence ofelectrolytes

As described in Section 3.2 monovalent electrolytes at con-centrations >30 mM caused irreversible aggregation of Ag NPs inaqueous medium. This was attributed to electrostatic destabiliza-tion of the colloid (decreased repulsive forces), which favorednanoparticle aggregation mediated by van der Waals attractiveforces. When the electrolyte concentration was increased abovethe critical coagulation concentration in the presence of activatedcarbon (the carbon was dispersed within the Ag NPs dispersion

pletely deposited on the carbon surface, mostly as aggregates. Theadsorbed nanoparticles could be observed as slight irregularities

coefficients (R2) for the fits with the respective equations are also given.

Langmuir constants

Qmax (�g/mg) KL (ml/�g) R2

46 ± 16 0.032 ± 0.010 0.98865 ± 15 0.096 ± 0.023 0.989

G. Gicheva, G. Yordanov / Colloids and Surfaces A: Physicochem. Eng. Aspects 431 (2013) 51– 59 57

Fig. 9. SEM-SEI (top) and SEM-BSE (bottom) images of activated carbon before (a) and after (b) deposition of Ag NPs on the carbon surface in the presence of 40 mM NaCl.Aggregates of coagulated nanoparticles are seen as brighter dots on the BSE image of (b).

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n the carbon surface in the SEI mode. However, in the BSE modeeavy elements (like silver) appear brighter on the image. SEM-SE observation of carbon surface before nanoparticle depositionhowed no bright areas (Fig. 9a), while nanoparticle deposited onhe adsorbent surface could be observed as bright dots (Fig. 9b).robably, at higher electrolyte concentrations, nanoparticle aggre-ates can be settled down along with the carbon particles. Theormation of aggregates on the carbon surface could probably bexplained also by a phenomenon known as heterogeneous coag-lation – coagulation that takes place on interfaces with higherate than the homogeneous coagulation that takes place in the bulkf the dispersion. The highly lipophilic surface of activated carbonay serve for the deposition of destabilized nanoparticles. What-

ver the explanation is, this observation may serve as a methodor efficient quantitative removal of nanoparticles from aqueousispersions. For example, by using aqueous dispersion, containing05 �g/ml Ag NPs and 0.5 mg/ml Norit® CA1, it was possible tochieve fast and complete removal of Ag NPs (by quantitative depo-ition on the carbon surface) by simply adding 40 mM NaCl. Otheralts, such as KNO3, KCl, NaNO3 and trisodium citrate could alsonduce deposition/aggregation of Ag NPs on activated carbon whensed at concentrations higher than the critical coagulation con-entration. However, sodium chloride (NaCl) in comparison withther salts, such as nitrates, is a cheaper and non-toxic substancehat is known to be suitable for industrial application as a salt-ng out agent. Indeed, NaCl is often used in various applications toncrease the ionic strength of water in attempting to remove a dis-ersed product from the aqueous phase. The observed effect coulde applied as a simple, fast, cost-effective and efficient method foremoval of Ag NPs from aqueous media. For such applications, pow-

ered carbon can be prepared in the block form or filled in suitableolumns to also act as a mechanical filter for aggregated nanopar-icles (at salt concentrations higher than the critical coagulationoncentration).

3.6. Interaction forces between Ag NPs and activated carbon

The observed adsorption of Ag NPs on activated carbon raised aninteresting question about the nature of interaction forces betweenthe adsorbed nanoparticles and the surface of activated carbon.The increased adsorption of Ag NPs at increased salt concentra-tion indicated that nanoparticle adsorption was favored whenthe electrostatic stabilization of particles was decreased. Previ-ous investigations on the adsorption of citrate-coated Ag NPs (andother metal NPs, like Au, Pt, Fe) on inorganic (calcium phos-phate, titanium dioxide, barium sulfate) microparticle supportshave demonstrated that adsorption efficiency was very sensitiveto ligand concentration [25]. In scientific literature, it is usuallyconsidered that the adsorption process is initiated by electrostaticinteractions between nanoparticles and solid surfaces. For exam-ple, electrostatic attraction has been considered as the major forcethat caused adsorption of positively charged colloids on negativelycharged solid surfaces [40]. However, using a hydrophobic adsor-bent, like the activated carbon described here, is a little bit different.Our findings indicated that steric repulsion by ligand shell wasnot sufficient to avoid nanoparticle adsorption on the carbon sur-face at high ionic strength, when electrostatic repulsion betweenparticles was actually decreased. It is generally assumed that thehydrophobic activated carbon physically binds various materials byvan der Waals forces or London dispersion forces. It is rather possi-ble that such forces may be involved in the interaction betweenAg NPs and the carbon surface. If so, the forces of interactionbetween a nanoparticle and a surface are expected to depend alsoon the adsorbent surface topography. Roughness on the adsorbentsurface may result in a greater total area of contact between a

nanoparticle and a surface, which increases the force of attraction,as well as the tendency of mechanical interlocking. Indeed, previ-ous investigations have shown clearly that porosity is responsiblefor an increase in the dispersive adsorption energies and that the

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8 G. Gicheva, G. Yordanov / Colloids and Surfa

on-specific hydrophobic component was highly important [41]. Itould be speculated also that the phosphoric acid treatment maye responsible for the formation of mesoporous structure of NoritA1, which makes it more suitable for adsorption of nanoparti-les (∼50 nm) than other carbons. Some insight into the naturef metal–carbon interactions could be obtained from structuraltudies of composite materials containing metal nanoparticles andarbon nanotubes. For example, previous studies have shown thatold (Au) nanoparticles interacted with carbon nanotubes throughan der Waals forces [42]. Other studies have indicated that plat-num (Pt) nanoparticles interact with carbon nanotubes throughynergic bonding involving charge redistribution between C 2p-erived states and Pt 5d bands [43]. Whether any of these cases isimilar to the adsorption of Ag NPs on activated carbon is currentlynclear and remains to be investigated.

. Conclusions

This article described studies on the removal of silver nanopar-icles from aqueous dispersions by means of adsorption ontoctivated carbon. Nanoparticles of average size around 60 nmnd negative zeta-potential were synthesized by reduction ofg(I) ions with sodium citrate in aqueous medium and coulde adsorbed by mesoporous activated carbon (Norit® CA1) fol-

owing Freundlich and Langmuir isotherm models. The presencef electrolytes favored nanoparticle deposition on the surface ofctivated carbon. It was shown that nanoparticles could be com-letely removed from the aqueous dispersions by aggregation onhe carbon surface when electrolytes were used above their criti-al coagulation concentration. The results reported in the currentrticle could be important for various research areas, such as foretter understanding of interactions between nanoparticles andolid–liquid interfaces, preparation of metal nanoparticle/carbonaterials that may be of interest in heterogeneous catalysis, and

or highly efficient removal of metal nanoparticle pollutants fromontaminated aqueous media.

cknowledgements

This research was financially supported by the Bulgarian Sci-nce Fund (project DMU-03/86). The authors are thankful to CMSTOST Action CM1101. The technical support from Dr. Danielaarashanova (IOMT-BAS) and Dr. Yasen Atanasov (Sofia Univer-ity) with TEM observation and DLS measurements, respectively,s greatly acknowledged. The SEM/EDX observation and specificurface measurements were performed at the Institute of Physicalhemistry (BAS).

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