Separation and Purification Technology 91 (2012) 23–29

Contents lists available at SciVerse ScienceDirect

Separation and Purification Technology

journal homepage: www.elsevier .com/ locate /seppur

The effect of humic acid binding to magnetite nanoparticleson the photogeneration of reactive oxygen species

Luciano Carlos a,⇑, Mariano Cipollone b, Delia B. Soria c, M. Sergio Moreno d, Peter R. Ogilby e,Fernando S. García Einschlag a, Daniel O. Mártire a,⇑a Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA), CCT-La Plata- CONICET, Universidad Nacional de La Plata, La Plata, Argentinab Centro de Tecnología Argentina, YPF. Ensenada, La Plata, Argentinac Centro de Química Inorgánica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, La Plata, Argentinad Centro Atómico Bariloche (CNEA), San Carlos de Bariloche, Rio Negro, Argentinae Center for Oxygen Microscopy and Imaging (COMI), Department of Chemistry, University of Aarhus, Aarhus, Denmark

a r t i c l e i n f o

Article history:Available online 27 August 2011

Keywords:Reactive oxygen speciesHumic acidsMagnetite nanoparticlesSinglet oxygen

1383-5866/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.seppur.2011.08.028

⇑ Corresponding authors. Address: Casilla de corrPlata, Argentina. Tel.: +54 221 4257430/7291; fax: +5

E-mail addresses: lcarlos@quimica.unlp.edu.ar (unlp.edu.ar (D.O. Mártire).

a b s t r a c t

The goal of this study was to investigate how the binding of humic acids (HA) to magnetite nanoparticlesaffects the ability of the HA to generate reactive oxygen species (ROS) upon irradiation. To this end, mag-netite nanoparticles coated with two different HA were prepared by the co-precipitation method. Theseparticles were characterized by several techniques. The nanoparticle-sensitized quantum yield of singletoxygen generation (UD) at kexc = 400 nm and the quantum efficiencies (kexc = 300–550 nm) of hydroxylradical and hydrogen peroxide generation were measured and compared to the corresponding quantumyields obtained from solutions of the free HA. Despite being bonded to the magnetite nanoparticles, theHA are still able to generate ROS upon irradiation. As with free HA, the yields of ROS generation by themagnetite-bonded HA depend on the particular HA involved.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Iron-based nanoparticles have unique properties that allow for awide range of applications, such as magnetic resonance imaging, fer-rofluids for audio speakers, magnetic targeted drug delivery, andmagnetic recording media [1]. The use of iron-based nanomaterialsfor environmental remediation processes has recently received con-siderable attention [2]. For example, nanoscale zerovalent iron hasbeen applied to treat chlorinated compounds, organic dyes, and heavymetals [3]. Iron oxide nanoparticles have also been used as adsorbentsto remove arsenic from water [4]. Thus, iron oxide nanomaterials arelikely to be found in engineered and natural water environments dueto their many applications. Hence, the fate of these particles in aquaticsystems is an issue of considerable concern.

Humic substances (HS) are the major organic constituents ofdissolved organic matter and therefore play an important role ininfluencing the fate and transport of colloidal particles. HS are ableto sorb onto particles changing their surface charges and affectingtheir aggregation and bioavailability [5], interfere with mineraldissolution/precipitation reactions [6], and drive redox reactions[7]. Photolysis of HS leads to the formation of several reactive

ll rights reserved.

eo 16, sucursal 4 (1900), La4 221 4254642.L. Carlos), dmartire@inifta.

species including free radicals and hydrated electrons, reactivetriplet states, hydroxyl radicals, singlet molecular oxygen, superox-ide, and hydrogen peroxide, all of which play a role in aquatic pho-tochemistry [8–16]. For this reason, the use of HS as additives inthe photochemical treatment of waters has been recently consid-ered [17–18].

Bare magnetite nanoparticles are susceptible to air oxidation[19] and are easily aggregated in aqueous systems. Thus, thestabilization of the iron oxide particles is desirable. Recent re-search indicated that humic acid (HA) has a high affinity toFe3O4 nanoparticles, and sorption of HA on the Fe3O4 nanoparti-cles enhanced the stability of nanodispersions by preventingtheir aggregation [20–22]. Liu et al. [22] employed magneticFe3O4 nanoparticles coated with HA for the removal of heavymetal ions from water. These insoluble magnetic materialshave the advantage of being easily removed with the aid of amagnet.

The aim of the present work is to investigate how binding tomagnetite nanoparticles affects the ability of humic acids (HA)to generate reactive oxygen species (ROS) upon irradiation. Theresults obtained here may help to: (i) better understand the roleof HA-coated iron oxide particles in natural sunlight-containingenvironments and (ii) evaluate the applicability of HA-coatedmagnetite nanoparticles, which are easily manipulated and/orremoved, to generate reactive species and thus mediate thephotodegradation of contaminants in aqueous systems. We

24 L. Carlos et al. / Separation and Purification Technology 91 (2012) 23–29

employed the classic co-precipitation method proposed for thesynthesis of Aldrich HA-coated magnetite nanoparticles [1] toprepare magnetite nanoparticles coated with two different humicacids: Aldrich HA and Leonardite HA from the International Hu-mic Substances Society (IHSS). These particles were characterizedby several techniques and the particle-sensitized quantum yieldof singlet oxygen generation (UD) at kexc = 400 nm and the quan-tum efficiencies (kexc = 300–550 nm) of hydroxyl radical andhydrogen peroxide generation were measured and compared tothose of the free HA.

2. Experimental section

2.1. Synthesis of magnetic nanoparticles

A modification of the procedure reported in reference [22] toprepare bare and HA-coated Fe3O4 magnetic nanoparticles wasemployed. Briefly, 3.7 g FeCl3 and 3 g FeCl2 � 4H2O were dissolvedin 100 mL water and heated to 90 �C. Then, two aqueous solutionswere added rapidly and sequentially: (1) 10 mL of 25% ammoniumhydroxide and (2) 50 mL of 1.0% w/v HA. The mixture was stirredat 90 �C for 30 min and then cooled to room temperature. The blackprecipitate was purified using multiple water washes with mag-netic separation of particles from the supernatant. The black pre-cipitate obtained was stored at ambient temperature as a dryblack powder prior to use in the experiments. Commercial humicacid sodium salt (AHA) from Aldrich and Leonardite humic acid(LHA) from IHSS were used for preparation of Fe3O4/AHA andFe3O4/LHA nanoparticles.

The particles were characterized by X-ray dispersion (XRD),thermal gravimetric analysis (TGA), high-resolution transmissionelectron microscopy (HR-TEM), Fourier transform infrared spec-troscopy (FTIR), Brunauer–Emmett–Teller analysis (BET) and dy-namic light scattering (DLS). Characterization techniques andinstrumentation are described in the supplementary material.

2.2. O2(a1Dg) quantum yield (UD)

The time-resolved 1275 nm O2(a1Dg) phosphorescence signalwas recorded from suspensions of the HA-coated nanoparticlesand from solutions of free HA in D2O upon irradiation of therespective samples with the collimated output of a femtosecond la-ser (kexc = 400 nm) using an approach that has been described pre-viously [23] (Selected details of this approach are also given in thesupplementary material). These experiments were performed inD2O because the lifetime of singlet oxygen in D2O is �20 timeslonger than that in H2O which, in turn, facilitates the phosphores-cence measurement [24]. A solution of perinaphthenone (PNS) inD2O (UD

PNS = 0.97) [25] was used as the reference standard. Theabsorbances of the nanoparticle dispersion and the reference solu-tion were adjusted to 0.3 at 400 nm. For the nanoparticle disper-sion, the absorption contribution was calculated by subtractingthe scattering component from the measured absorbance accord-ing to Ruiz et al. [26]. Plots of the 1275 nm phosphorescence inten-sity against incident laser power were obtained, and the O2(a1Dg)quantum yields, UD, for Fe3O4/AHA, Fe3O4/LHA and LHA were cal-culated by comparing the resultant slopes (ms) to those obtainedfrom the PNS reference (mPNS) (Eq. (1)) [27].

UD ¼ms

mPNS �UPNSD ð1Þ

2.3. Steady-state irradiation experiments

Experiments performed to evaluate the photochemical stabilityand the quantum efficiencies of HO� and H2O2 formation from the

dispersions of coated nanoparticles were carried out in a 500 mLglass reactor equipped with a medium-pressure mercury arc lamp(Philips HPK 125 W) with a glass jacket that removed the radiationwith wavelengths shorter than 300 nm. The solution was continu-ously bubbled with analytical air and the temperature of the sys-tem was kept at (25.0 ± 0.2) �C. The incident photon rate (P0),measured using potassium ferrioxalate as actinometer [28], was3.2 � 10-6 einsteins s-1 L-1. In all the experiments, the pH of themedium was adjusted to 6.0. The concentrations of the nanoparti-cles and, independently, solutions of free HA varied over the rangesfrom 0.1 to 0.2 g L�1 and from 0.015 to 0.030 g L�1, respectively.

2.3.1. Photochemical stabilityThe stability of the coated nanoparticles in aqueous dispersion

under steady-state irradiation was followed by measuring the con-centration of both Fe3+ and Fe2+ ions and the evolution of the dis-solved organic carbon (DOC) released to the aqueous media atdifferent irradiation times. Fe3+ and Fe2+ concentration were quan-tified by colorimetric techniques through the complexes formedwith KSCN and o-phenanthroline, respectively [29], and DOC wasdetermined using a Shimadzu instrument (5000 TOC analyzer, cat-alytic oxidation on Pt at 680 �C).

2.3.2. Quantum efficiency of H2O2 production (gH2O2)Initial rates of H2O2 production (RH2O2) were measured using

HA solutions and dispersions of HA-coated nanoparticles underthe irradiation conditions described above. H2O2 concentrationswere measured by means of an enzymatic-colorimetric methodemploying a commercial kit from Wiener for cholesterol quan-tization [30]. Aliquots of 1 mL of the reaction mixture wereadded to 1 mL of the colorimetric reagent. After 15 min of incu-bation at room temperature the absorption spectra in the wave-length range from 400 to 600 nm were recorded. Quantumefficiencies of H2O2 production (gH2O2) were determined usingEqs. (2) and (3), where Ra is the rate of light absorption(einsteins L�1 s�1), p(k) is the fraction of photons emitted bythe lamp in a small finite wavelength interval [28], a(k) is thesolution absorption coefficient (cm�1) and z is the optical pathlength (cm). For the nanoparticle dispersions, values of a(k)were calculated by subtracting the scattering component asnoted in Section 2.2. Since the emission of the medium-pres-sure mercury arc lamp is polychromatic and Ra was calculatedfrom 300 to 550 nm, our gH2O2 values are weighted averagesof all wavelengths in that range.

gH2O2¼ RH2O2

Rað2Þ

Ra ¼ P0

X550

300

pðkÞð1� 10�aðkÞzÞ ð3Þ

2.3.3. Quantum efficiency of HO� production (gHO�)Benzene was used as a probe to determine the initial rates of

HO� production (RHO�) following a literature method [31]. Underour conditions, benzene will compete with the HA for the photo-generated HO� radicals:

HAþ hv! HO� ½RHO� � ð4ÞHO� þHA! products ½kHA� ð5Þ

HO� þ benzene! phenol ½0:95� kbenzene� ð6aÞHO� þ benzene! products ½0:05� kbenzene� ð6bÞ

where kbenzene and kHA are the bimolecular rate constants for thereactions of hydroxyl radicals with benzene and HA, respectively.

L. Carlos et al. / Separation and Purification Technology 91 (2012) 23–29 25

The factor 0.95 is the efficiency of phenol formation from HO� andbenzene [31]. Invoking the steady-state assumption for hydroxylradicals, the following expression can be derived:

Rphenol ¼0:95kbenzene½benzene�0RHO

ðkbenzene½benzene�0 þ kHA½HA�0Þð7Þ

where Rphenol is the initial rate of phenol production. The methodconsists of measuring the Rphenol at different initial concentrationsof benzene, [Benzene]0. RHO� is obtained from the intercept of thelinear plot of Rphenol

�1 versus [benzene]0�1. Phenol concentrations

were determined by HPLC (see instrumental details in the Supple-mentary material). Finally, the quantum efficiencies of HO� produc-tion (gHO�) were calculated using Eq. (8).

gHO� ¼RHO�

Rað8Þ

Fig. 1. HR-TEM images of Fe3O4/LHA. Inset: Electron diffraction pattern.

3. Results and discussion

3.1. Chemical characterization

Table 1 shows the results obtained for the characterization ofboth HA-coated nanoparticles, i.e. Fe3O4/AHA and Fe3O4/LHA.

Representative HR-TEM images for the Fe3O4/LHA dispersed inaqueous medium are shown in Fig. 1. The particles appeared tobe roughly spherical in shape, with a crystalline core of Fe3O4

and an external layer of organic matter less than 1 nm in thickness,as estimated from the external halo surrounding the particles. Theelectron diffraction patterns (inset Fig. 1) allowed the assignmentof the crystalline phase present in the nanoparticles to magnetite.The average particle size was estimated by counting more than 100nanoparticles (Table 1). Aggregates larger than 200 nm in diame-ter, and with no uniform size, were also observed in the HR-TEMimages. From DLS measurements, the average hydrodynamicdiameter of Fe3O4/AHA nanoparticles was determined to be around250 nm (Table 1) in accordance with the aggregation observed inHR-TEM images.

The identity and purity of the iron oxide in Fe3O4/AHA andFe3O4/LHA were verified by XRD. As shown in Fig. 2 , XRD patternsof the Fe3O4/LHA display several relatively strong reflection peaksin the 2h region of 20–70�, which match well with the standardpattern of Fe3O4 (card number 11-0614, ICCD Database). This resultindicates that magnetite is the unique iron oxide species obtainedfrom the synthesis in accordance with the HR-TEM electron dif-fraction results. Remnant impurities of NH4Cl crystals were alsoobserved in the XRD pattern.

Fig. 3 shows the TGA curve of nanoparticles. The first slightweight loss (25–150 �C) is due to the evaporation of water,whereas that in the range 150–500 �C is assigned to the elimina-tion of the chemically adsorbed organic coating. The amounts of

Table 1Results of Fe3O4/AHA and Fe3O4/LHA characterizations.

Techniques Parameters calculated Fe3O4/AHA

Fe3O4/LHA

TEM Average diameter (nm) 7.0 6.7DLS Average hydrodynamic diameter (nm) 254.2 134.0BET SBET (m2 g�1) 65.1 72.2TGA HA adsorbed to Fe3O4 (%) 15.7 18.4Acid-base

TitrationTotal acidity (meq g�1)* 14.5 11.6COOH group (%) 15.5 13.9OH group (%) 84.5 86.1

* g refers to the HA mass in grams. HA contents of 15% and 18% adsorbed to Fe3O4/AHA and Fe3O4/LHA, respectively were used to evaluate the total acidity.

the sorbed AHA or LHA on the surface of the Fe3O4 nanoparticleswere calculated from these results (Table 1).

Carboxylic (–COOH) and phenolic (–OH) functional group con-tents measured by titration [32] are shown in Table 1. The ratio–COOH/–OH published for AHA and LHA is, for both acids, about3.2 [33–34]. Although both carboxylic and phenolic groups areresponsible for metal complexation [35], the smaller –COOH/–OHratio found for Fe3O4/AHA and Fe3O4/LHA (about 0.2) suggests thata significant fraction of the carboxyl groups of these HA may bebonded to the magnetite surface. As such, fewer carboxylic acidgroups would be available for titration in the suspensions of theHA-coated particles than in the HA solutions with the same organicmatter content.

The FTIR spectra of the LHA and Fe3O4/LHA nanoparticles areshown in Fig. 4. Magnetite was confirmed in the Fe3O4/LHA sampleby a strong absorption at �588 and 630 cm�1, which correspondsto Fe–O stretching vibration [19]. Significant differences in thebands assigned to C=O stretches (�1400 and 1620 cm�1) are ob-served. These differences are consistent with the carboxylate an-ions interacting with the FeO surface [36] and suggest thatcarboxylate groups indeed play an important role in the bondingof the HA to the magnetite surface. This result is in agreement withother authors [22] and with the observed change in the –COOH/–OH ratio upon binding of the HA groups to the Fe3O4 nanoparticles(see above).

The comparison of the UV–Vis spectra of aqueous dispersions ofthe coated nanoparticles to those of HA solutions with the samecarbon content, as calculated from the TGA analysis, is shown inFig. 5. The UV–Vis spectra show significant differences, further

2Θ (degrees)

20 30 40 50 60 70 80

Rel

ativ

e in

tens

ity

0.0

0.2

0.4

0.6

0.8

1.0

1.2

(220)

(311)

(511)

(422)(400)

NH4Cl

NH4Cl

Fig. 2. XRD pattern obtained for Fe3O4/LHA.

Temperature (°C)

0 100 200 300 400 500 600 700 800 900

Rem

aini

g sa

mpl

e w

eigh

t (%

)

75

80

85

90

95

100Water loss

Organic matter loss

Fig. 3. TGA curve obtained for Fe3O4/LHA.

wavenumber (cm-1)

5001000150020002500300035004000

Tra

nsm

issi

on

LHA

Fe3O4/LHA

Fig. 4. FTIR spectra of LHA and Fe3O4/LHA.

λ (nm)200 300 400 500 600 700

Abso

rban

ce

0.0

0.2

0.4

0.6

0.8

1.0

LHAFe3O4/LHA

Fig. 5. UV–Vis spectra of LHA solutions (0.03 g L�1), Fe3O4/LHA aqueous dispersion(0.2 g L�1). The pH in both samples was adjusted to 6. The scattering component ofthe Fe3O4/LHA samples was subtracted according to Ruiz et al. [26].

26 L. Carlos et al. / Separation and Purification Technology 91 (2012) 23–29

confirming the binding of humic acids to Fe3O4. Interestingly, a sig-nificant decrease in the absorbance at wavelengths lower than350 nm is observed for the nanoparticle dispersions.

3.2. Photochemical stability

Aqueous dispersions of the Fe3O4/AHA and Fe3O4/LHA(0.1 g L�1) coated nanoparticles were irradiated for 4 h using a

polychromatic source as indicated in Section 2. The analysis forFe3+ and Fe2+ in the samples showed that, prior to irradiation, theconcentrations of Fe3+ and Fe2+ were lower than 10�6 and 10�7 M,respectively. This indicates that, in the purification step, all the labileiron ions were removed. Upon irradiation, no appreciable changes in[Fe3+] or [Fe2+] were observed indicating that the magnetite core ofthe nanoparticles does not release iron ions to the aqueous medium.Furthermore, the nanoparticles were removed from the medium bya simple magnetic separation and DOC analysis was performed onthe separated aqueous phase. No DOC increase was detected after4 h irradiation, suggesting that under these conditions the organicmatter remains attached to the magnetite particle.

3.3. Generation of ROS

The quantum yields of singlet oxygen generation, UD, in air-sat-urated HA solutions and dispersions of the coated nanoparticleswere determined using the intensity of the time-resolved1275 nm singlet oxygen phosphorescence as a probe (Fig. 6). Toaccurately assess the amount of singlet oxygen produced, the valueof the phosphorescence intensity at time = 0 was used in the anal-ysis. Absolute values of UD were obtained using the sensitizer PNSas a reference standard (Table 2). The data clearly show that, uponbinding of the HA to the magnetite nanoparticle, the yield ofphotosensitized singlet oxygen production decreases.

HA are not only sources of singlet oxygen, but also act as effec-tive quenchers of singlet oxygen [37]. The time-resolved 1275 nmphosphorescence signals from air-saturated LHA solutions and thenanoparticle dispersions showed a single exponential decay withsimilar lifetimes (sD) (Table 2). Since the lifetime of O2(a1Dg) inneat D2O is �67 ls [38], the lower sD values obtained in AHA,LHA, Fe3O4/LHA and Fe3O4/AHA systems reflect the involvementof HA in the O2(a1Dg) deactivation. It is noteworthy that the abilityof HA to deactivate O2(a1Dg) does not appreciably change when HAis bonded to the magnetite nanoparticles, although a slight system-atic binding-dependent decrease in sD is observed (Table 2). Theseobservations might reflect the fact that, upon binding to the Fe3O4

nanoparticles, (1) the effective local concentration of HA increasesslightly and (2) the more efficient deactivating HA functionalgroups remain available to interact with O2(a1Dg).

In any event, the smaller values of UD measured for the nano-particle dispersions compared to those of the solutions of HAclearly indicate that, upon binding to the Fe3O4 particle, HAbecomes a poorer O2(a1Dg) sensitizer. This effect could be due,

Laser power (mW)0 1 2 3 4 5 6 7

I 0 (co

unts

)

0

50

100

150

200

250LHAFe3O4/LHA

Fig. 6. Laser power dependence of the time-resolved singlet oxygen phosphores-cence signal extrapolated to time = 0, I0, for air-saturated LHA solutions and theFe3O4/LHA aqueous dispersion.

Table 2O2(a1Dg) lifetime (sD) and quantum yields (UD).

Samples sD (ls) UD

AHA 52.2 1.0 � 10�3 (*)

Fe3O4/AHA 47.6 (7.0 ± 0.3) � 10�4

LHA 51.9 (4.0 ± 0.2) � 10�3

Fe3O4/LHA 47.3 (6.6 ± 0.8) � 10�4

(*) reported by Aguer et al. [44].

time (min)0 10 20 30 40 50

[H2O

2] (m

M)

0.000

0.002

0.004

0.006

0.008

LHAFe3O4/LHA

Fig. 7. H2O2 formation from the irradiation of LHA solutions (0.03 g L�1) and Fe3O4/LHA dispersions (0.2 g L�1).

1/[benzene]0 (M-1)

0 1000 2000 3000 4000 5000 6000 7000

1/R

phen

ol (x

10-8

s M

-1)

0.0

0.5

1.0

1.5

2.0LHAFe3O4/LHA

Fig. 8. RHO� determination for LHA and Fe3O4/LHA.

L. Carlos et al. / Separation and Purification Technology 91 (2012) 23–29 27

for example, to quenching of the triplet states of HA, which are theprecursors of O2(a1Dg), by iron ions [39].

Initial rates of HO� (RHO�) and H2O2 (RH2O2) production from HAsolutions and dispersions of HA-coated magnetite nanoparticleswere determined under steady irradiation (k > 300 nm). To com-pare results obtained from samples with the same amount of or-ganic matter, dispersions containing 0.2 g L�1 Fe3O4/AHA orFe3O4/LHA and 0.03 g L�1 solutions of AHA or LHA were employed(see TGA results in Table 1). Fig. 7 shows the time evolution of theconcentration of H2O2 upon irradiation of both LHA and Fe3O4/LHAsamples. A decrease in the H2O2 formation rate, along with thegH2O2, is observed when the HA is bonded to the magnetic nano-particles (Table 3). This effect is more pronounced for LHA thanfor AHA.

It is accepted that the photogeneration of H2O2 mainly arisesfrom the population of charge-transfer states of the HA, whichserve as precursors of H2O2 (Eqs. (9) and (10)) [9].

HA�þ=�� þ O2 ! HA�þ þ O��2 ð9Þ

2O��2 þ 2Hþ ! H2O2 þ O2 ð10Þ

HA�+/�� represents an intramolecular charge-transfer complexwith diradical character, and HA�+ stands for the semi-oxidizedform of HA. Charge-transfer states may be populated by intramo-lecular electron transfer involving excited singlet and triplet states

Table 3H2O2 and HO� initial formation rates (R) and quantum efficiencies (g).

Samples RH2O2 (M s�1) gH2O2

AHA (1.2 ± 0.1) � 10�9 (4.9 ± 0.4) � 10Fe3O4/AHA (3.8 ± 0.3) � 10�10 (3.0 ± 0.1) � 10LHA (3.1 ± 0.1) � 10�9 (10.1 ± 0.5) � 1Fe3O4/LHA (2.3 ± 0.5) � 10�10 (8.2 ± 1.7) � 10

[9]. The ability of metal ions to efficiently deactivate excited statesof HA [39], could explain the observed decrease in the quantumefficiencies of H2O2 generation by the particles compared to thoseof the free HA.

The formation of HO� radicals was followed by analyzing theproduction of phenol from solutions containing HA and benzene(see Section 2.3.3.). Fig. 8 compares the plots of 1/Rphenol vs 1/[ben-zene]0 for LHA and Fe3O4/LHA. The values of RHO� and gHO� areshown in Table 3. A decrease in the gHO� values is observed whenthe HA is bonded to the nanoparticle surface. As also observed forgH2O2, a more pronounced nanoparticle-dependent decrease ingHO� is obtained for Fe3O4/LHA than for Fe3O4/AHA.

Although the mechanism of O2(a1Dg) photoproduction throughenergy transfer from triplet excited states of HA to molecular oxy-gen O2ðX3P

g� Þ is well known, there is no such clarity in the mech-anism of HO� photogeneration. There is evidence for severalpathways, including oxygen-dependent, oxygen-independent,

RHO� (M s�1) gHO�

�4 (1.3 ± 0.1) � 10�9 (5.3 ± 0.4) � 10�4

�4 (9.1 ± 1.7) � 10�10 (3.9 ± 0.6) � 10�4

0�4 (4.7 ± 1.1) � 10�9 (1.6 ± 0.4) � 10�3

�5 (2.5 ± 0.8) � 10�10 (8.7 ± 2.0) � 10�5

28 L. Carlos et al. / Separation and Purification Technology 91 (2012) 23–29

and hydrogen peroxide-dependent paths, but the mechanism re-mains unclear [40]. Our results do not yield information on themechanism of HO� generation from HA, but it is important to notethat the gHO� values obtained from HA and HA bonded to magne-tite are similar to those found for gH2O2.

4. Conclusions

It is well established that, through the photogeneration of trip-let excited states and reactive oxygen species (ROS), humic acidsact as sensitizers able to promote the degradation of contaminantsin natural waters. On this basis, the use of humic acids as additivesin the photochemical treatment of waters has been recently con-sidered [17,41].

The comparison of quantum efficiencies for the production ofROS obtained in this work showed that: (i) both for free humic acidsolutions and for the humic acid-coated nanoparticle dispersions,singlet oxygen is the reactive oxygen species formed with the high-est quantum yield, and (ii) the quantum efficiencies of H2O2 andhydroxyl radical production are similar. Moreover, binding of thehumic acids to magnetite nanoparticles resulted in lower yieldsof ROS generation by the two different humic acids tested. The dataalso indicate that the production of ROS both by the free andbonded humic acids depends on the particular humic acid involved(i.e., LHA vs AHA).

Despite the lower production of ROS when the humic acids arebonded to the nanoparticles, these materials could still be usefulfor contaminant degradation in aqueous systems. More impor-tantly, the use of metal-containing particles provides a convenientapproach for separating, removing and recycling the humic acidsby applying external magnetic fields. Thus the humic acid-coatedmagnetite nanoparticles are potential candidates for treating con-taminants susceptible to HA-photosensitized degradation (e.g.,amoxicillin and clofibric acid [42–43]).

Acknowledgments

This research was supported by ANPCyT, Argentina (PICT 2007# 00308 and PICT 2008 # 00686), and CICPBA, Argentina. L.C.,D.B.S., and F.S.G.E. are research members of CONICET. D.O.M. is aresearch member of CICPBA. The work in Denmark was supportedby a grant from the Danish National Research Foundation. Theauthors thank Drs. Virginia Martín Torres and Inmaculada GarcíaMoreno from CSIC (Madrid, Spain) for the DLS measurements.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.seppur.2011.08.028.

References

[1] S. Laurent, D. Forge, M. Port, A. Roch, C. Robic, L. Vander Elst, R.N. Muller,Magnetic iron oxide nanoparticles: synthesis, stabilization, vectorization,physicochemical characterizations and biological applications, Chem. Rev.108 (2008) 2064–2110.

[2] M. Auffan, H.J. Shipley, S. Yean, A.T. Kan, M. Tomson, J. Rose, J.Y. Bottero,Nanomaterials as adsobents, in: M.R. Wiesner, J.Y. Bottero (Eds.),Environmental Nanotechnology: Applications and Impacts of Nanomaterials,McGraw-Hill, New York, 2007, pp. 371–392.

[3] C. Noubactep, The fundamental mechanism of aqueous contaminant removalby metallic iron, Water SA 36 (2010) 663–670.

[4] C.T. Yavuz, J.T. Mayo, W.W. Yu, A. Prakash, J.C. Falkner, S. Yean, L. Cong, H.J.Shipley, A. Kan, M. Tomson, D. Natelson, V.L. Colvin, Low-field magneticseparation of monodisperse Fe3O4 nanocrystals, Science 314 (2006) 964–967.

[5] C.L. Tiller, C.R. O’Melia, Natural organic matter and colloidal stability: modelsand measurements, Colloids Surf. A 73 (1993) 89–102.

[6] A.J. Slowey, Rate of formation and dissolution of mercury sulfidenanoparticles: the dual role of natural organic matter, Geochim. Cosmochim.Acta 74 (2010) 4693–4708.

[7] T. Borch, R. Kretzschmar, A. Skappler, P. Van Cappellen, M. Ginder-Vogel, A.Voegelin, K. Campbell, Biogeochemical redox processes and their impact oncontaminant dynamics, Environ. Sci. Technol. 44 (2010) 15–23.

[8] A. Pajares, M. Bregliani, J. Natera, S. Criado, S. Miskoski, J.P. Escalada, N.A.García, Mechanism of the photosensitizing action of a mixture humic acid-riboflavin in the degradation of water-contaminants, J. Photochem. Photobiol.A 219 (2011) 84–89.

[9] R.M. Dalrymple, A.K. Carfagno, C.M. Sharpless, Correlations between dissolvedorganic matter optical properties and quantum yields of singlet oxygen andhydrogen peroxide, Environ. Sci. Technol. 44 (2010) 5824–5829.

[10] W.J. Cooper, Sunlight induced photochemistry of humic substances in naturalwaters: major reactive species, Adv. Chem. Ser. 219 (1989) 332–362.

[11] P.M. David Gara, G.N. Bosio, V.B. Arce, L. Poulsen, P.R. Ogilby, R. Giudici, M.C.Gonzalez, D.O. Mártire, Photoinduced degradation of the herbicide clomazonemodel reactions for natural and technical systems, Photochem. Photobiol. 85(2009) 686–692.

[12] F.H. Frimmel, H. Bauer, J. Putzien, P. Murasecco, A.M. Braun, Laser flashphotolysis of dissolved aquatic humic material and the sensitized productionof singlet oxygen, Environ. Sci. Technol. 21 (1987) 541–545.

[13] R.G. Zepp, A.M. Braun, J. Hoigne, J.A. Leenheer, Photoproduction of hydratedelectrons from natural organic solutes in aquatic environments, Environ. Sci.Technol. 21 (1987) 485–490.

[14] S. Canonica, Oxidation of aquatic organic contaminants induced by excitedtriplet states, Chimia 61 (2007) 641–644.

[15] P.P. Vaughan, N.V. Blough, Photochemical formation of hydroxyl radical byconstituents of natural waters, Environ. Sci. Technol. 32 (1998) 2947–2953.

[16] L. Carlos, B.W. Pedersen, P.R. Ogilby, D.O. Martire, The role of humic acidaggregation on the kinetics of photosensitized singlet oxygen production anddecay, Photochem. Photobiol. Sci. 10 (2011) 1080–1086.

[17] M. Sturini, A. Speltini, F. Maraschi, A. Profumo, L. Pretali, E. Fasani, A. Albini,Photochemical degradation of marbofloxacin and enrofloxacin in naturalwaters, Environ. Sci. Technol. 44 (2010) 4564–4569.

[18] A. Bianco Prevot, D. Fabbri, E. Pramauro, C. Baiocchi, C. Medana, E. Montoneri,V. Boffa, Sensitizing effect of bio-based chemicals from urban wastes on thephotodegradation of azo-dyes, J. Photochem. Photobiol. A 209 (2010) 224–231.

[19] D. Maity, D.C. Agrawal, Synthesis of iron oxide nanoparticles under oxidizingenvironment and their stabilization in aqueous and non-aqueous media, J.Magn. Magn. Mater. 308 (2007) 46–55.

[20] E. Illés, E. Tombácz, The role of variable surface charge and surfacecomplexation in the adsorption of humic acid on magnetite, Colloids Surf. A230 (2003) 99–109.

[21] E. Illés, E. Tombácz, The effect of humic acid adsorption on pH-dependentsurface charging and aggregation of magnetite nanoparticles, J. ColloidInterface Sci. 295 (2006) 115–123.

[22] J.F. Liu, Z.S. Zhao, G.B. Jiang, Coating Fe3O4 magnetic nanoparticles with humicacid for high efficient removal of heavy metals in water, Environ. Sci. Technol.42 (2008) 6949–6954.

[23] P. Salice, J. Arnbjerg, B.W. Pedersen, R. Toftegaard, L. Beverina, G.A. Pagani, P.R.Ogilby, Photophysics of squaraine dyes: role of charge-transfer insinglet oxygen production and removal, J. Phys. Chem. A 114 (2010)2518–2525.

[24] P.R. Ogilby, Singlet oxygen: there is indeed something new under the sun,Chem. Soc. Rev. 39 (2010) 3181–3209.

[25] R. Schmidt, C. Tanielian, R. Dunsbach, C. Wolff, Phenalenone, a universalreference compound for the determination of quantum yields of singletoxygen O2(1Dg) sensitization, J. Photochem. Photobiol. A 79 (1994) 11–17.

[26] A.E. Ruiz, P. Caregnato, V.B. Arce, M.D.L.M. Schiavoni, V.C. Mora, M.C. Gonzalez,P.E. Allegretti, D.O. Mártire, Synthesis and characterization of butoxylatedsilica nanoparticles. reaction with benzophenone triplet states, J. Phys. Chem.111 (2007) 7623–7628.

[27] F. Wilkinson, W.P. Helman, A.B. Ross, Quantum yields for the photosensitizedformation of the lowest electronically excited singlet state of molecularoxygen in solution, J. Phys. Chem. Ref. Data 22 (1993) 113–262.

[28] F.S. García Einschlag, L. Carlos, A.L. Capparelli, A.M. Braun, E. Oliveros,Degradation of nitroaromatic compounds by the UV–H2O2 processusing polychromatic radiation sources, Photochem. Photobiol. Sci. 1 (2002)520–525.

[29] M. Rios-Enriquez, N. Shahin, C. Duran-De-Bazua, J. Lang, E. Oliveros, S.H.Bossmann, A.M. Braun, Optimization of the heterogeneous Fenton-oxidation ofthe model pollutant 2, 4-xylidine using the optimal experimental designmethodology, Sol. Energy 77 (2004) 491–501.

[30] C.C. Allain, L.S. Poon, C.S.G. Chan, W. Richmond, P.C. Fu, Enzymaticdetermination of total serum cholesterol, E. Clin. Chem. 20 (1974) 470–475.

[31] D. Vione, M. Ponzo, D. Bagnus, V. Maurino, C. Minero, M.E. Carlotti, Comparisonof different probe molecules for the quantification of hydroxyl radicals inaqueous solution, Environ. Chem. Lett. 8 (2010) 95–100.

[32] M. Schnitzer, U.C. Gupta, Determination of acidity in soil organic matter, SoilSci. Soc. Am. Proc. 29 (1965) 274–277.

[33] J.D. Ritchie, E. Michael Perdue, Proton-binding study of standard and referencefulvic acids, humic acids, and natural organic matter, Geochim. Cosmochim.Acta 67 (2003) 85–93.

[34] K. Gao, J. Pearce, J. Jones, C. Taylor, Interaction between peat, humic acid andaqueous metal ions, Environ. Geochem. Health 21 (1999) 13–26.

L. Carlos et al. / Separation and Purification Technology 91 (2012) 23–29 29

[35] D.S. Smith, R.A. Bell, J.R. Kramer, Metal speciation in natural waters withemphasis on reduced sulfur groups as strong metal binding sites, Comp.Biochem. Physiol. Part C: Toxicol. Pharmacol. 133 (2002) 65–74.

[36] W. Yantasee, C.L. Warner, T. Sangvanich, R.S. Addleman, T.G. Carter, R.J.Wiacek, G.E. Fryxell, C. Timchalk, M.G. Warner, Removal of heavy metals fromaqueous systems with thiol functionalized superparamagnetic nanoparticles,Environ. Sci. Technol. 41 (2007) 5114–5119.

[37] D.P. Hessler, F.H. Frimmel, E. Oliveros, A.M. Braun, Quenching of singlet oxygen(1D(g),) by humic substances, J. Photochem. Photobiol. B 36 (1996) 55–60.

[38] P.R. Ogilby, C.S. Foote, Chemistry of singlet oxygen. 36. Singlet molecularoxygen (1Dg) luminescence in solution following pulsed laser excitation.Solvent deuterium isotope effects on the lifetime of singlet oxygen, J. Am.Chem. Soc. 104 (1982) 2069–2070.

[39] K. Lang, D.M. Wagnerová, S. Klementová, P. Kubát, Humic substances – excitedstates, quenching by metal ions, and photosensitized degradation ofchlorophenols, Collect. Czech. Chem. Commun. 62 (1997) 1159–1168.

[40] S.E. Page, W.A. Arnold, K. McNeill, Assessing the contribution of free hydroxylradical in organic matter-sensitized photohydroxylation reactions, Environ.Sci. Technol. 45 (2011) 2818–2825.

[41] A. Georgi, A. Schierz, U. Trommler, C.P. Horwitz, T.J. Collins, F.D. Kopinke,Humic acid modified Fenton reagent for enhancement of the working pHrange, Appl. Cat. B: Environ. 72 (2007) 26–36.

[42] R. Andreozzi, M. Raffaele, P. Nicklas, Pharmaceuticals in STP effluents and theirsolar photodegradation in aquatic environment, Chemosphere 50 (2003)1319–1330.

[43] H. Xu, W.J. Cooper, J. Jung, W. Song, Photosensitized degradation of amoxicillinin natural organic matter isolate solutions, Water Res. 45 (2011) 632–638.

[44] J.P. Aguer, C. Richard, F. Andreux, Effect of light on humic substances, Analusis27 (1999) 387–390.