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Humic substances (HS) are macromolecular products derived from a physical, chemical, and microbiological process called humifi-

cation. These substances play an important role in the mobility and bioavailability of nutrients and contaminants in the environment.Adsorption isotherms provide a macroscopic view of the retention phenomena. However, complementary techniques are needed in orderto study the retention mechanism. The application of the classical models and some modern ones, based on humic substances chemistry, donot accurately describe these adsorption data. The aim of this paper is to model isotherms and combine adsorption data with spectroscopyand microscopy techniques to study the Cu(II) retention on a HS. The adsorption isotherms shape varies significantly with the solution pHfrom L-type (pH 26) to S-type (pH 8). FTIR shows that, when pH is 2 the retention of Cu(II), as [Cu(H2O)6]2+, is the preferred retentionmechanism. The quantity of Cu(II) retained as [Cu(OH)(H2O)6]+ rises, as pH increases. At pH 4, Cu(II) begins to precipitate, which is thepreferred mechanism at pH 8.02. The presence of HS has a great influence on the precipitation process of Cu(II), giving rise to amorphousprecipitates. As it is shown by SEM-XRF, Cu(II) distributes heterogeneously on HS surface and accumulates on the humic phases. Thepresence of different anions (chloride and nitrate) slightly modifies the HS behavior as cation exchanger. When Cl ions are present, partof the Cu(II) form [CuCl4]2, which is stable in solution due to its negative charge; when the anion present is NO3 the formed complex,[CuNO3]+, is retained on the HS. 2003 Elsevier Inc. All rights reserved.

Keywords: Adsorption; Humic substance; Isotherms; Cu(II); FTIR; SEM

1. Introduction

The common definitions for humic substances (HS) andtheir fractions are ambiguous and, in some instances, arbi-trary [1]. However, it could be said that they are macro-molecular products derived from a physical, chemical, andmicrobiological process called humification of organicmolecules from plants, animals, microorganism tissues, andmetabolic products. The HS are the most widespread nat-ural nonliving organic materials in all terrestrial and aquaticenvironments, up to 80% of soil organic matter and up to60% of dissolved organic carbon [2]. Based upon their sol-ubility in acids and alkalis, HS can be fractioned into humicacids (HA), fulvic acids (FA) and humin. The HA can be di-vided into brown humic acids (BHA) and gray humic acids(GHA) [3]. Due to their colloidal and polyfunctional charac-

ter, these substances play an important role in the mobilityand bioavailability of nutrients and contaminants in the en-vironment [4]. One of the main features of HS in the en-vironment is their capacity to interact with metal ions toform soluble complexes, colloidal substances, and/or insolu-ble substances [3]. The interactions between HS and metalshave great interest from chemical, biological, and environ-mental perspectives, due to the capacity of HS to complexmetal ions. The study of interactions between HS and metalions requires the characterization of both. The chemical be-havior of transition metals is well known [5]. The behaviorof humic substances is scarcely known, despite their phys-ical and chemical similitudes, as a consequence of theirfractal nature [6]. Elemental analysis is probably the mostfrequent tool used in the characterization of HS. This tech-nique provides information about the distribution of majorelements and is useful to distinguish different fractions ofHS [2]. Carboxyl and phenolic groups are the main acidicJournal of Colloid and Interface

Cu(II) retention onR.A. Alvarez-Puebla,a C. Valenz

a Department of Applied Chemistry, Public Universityb Department of Inorganic Chemistry, Faculty of Ph

Received 11 February 20

Abstract* Corresponding author.E-mail address: j.garrido@unavarra.es (J.J. Garrido).

0021-9797/$ see front matter 2003 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2003.08.068nce 270 (2004) 4755www.elsevier.com/locate/jcis

humic substance

-Calahorro,b and J.J. Garrido a,

varra, Campus Arrosada, E-31006 Pamplona, Spainy, University of Granada, E-18071 Granada, Spaincepted 26 August 2003groups. The determination of such groups in HS is closelyconnected with the retention of metal ions.

lloid48 R.A. Alvarez-Puebla et al. / Journal of Co

Adsorption isotherms provide helpful information aboutthe retention capacity and the strength with which the ad-sorbate is held on the adsorbent [7]. To date, only a fewreports on using adsorption isotherms in the study of sometransition metal ions on HS have been made [810]. Theretention process has been studied using the classical Lang-muir, Freundlich, or Tth models and other more refinedand modern ones that use the double diffuse layer theory,such as Model V by Tipping and Hurley [11], NICA andNICADonnan by Kinniburgh et al. [12], Stockholm byGustafsson [13], or the Liu and Gonzalez model [9]. Theclassical models do not adequately fit the experimental data,whereas modern ones need the knowledge of a great numberof variables, some of which are difficult to determine exper-imentally. These models are usually based on the HS surfacechemistry.

The adsorption isotherms analysis is useful to studythe retention process [7], because it provides a macro-scopic view of the retention phenomena. Thus, comple-mentary techniques are needed to study the different reten-tion process [14]. In line with that, many techniques havebeen employed to study interactions between HS and metalions, as reviewed by Senesi [15] and Schulten and Leinwe-ber [16]. Infrared spectroscopy has also been widely used tostudy the retention of some metal ions on several HS [17,18].

The aim of this work is to model isotherms and combineadsorption data with spectroscopy and microscopy tech-niques in order to study the Cu(II) retention on a HS. Thisglobal aim may be outlined as follows: (a) measurement ofCu(II) adsorption isotherms on HS; (b) development of a the-oretical model to fit in to the experimental data, consideringthe chemistry of Cu(II) in aqueous solution and the globaladsorption as the sum of several single processes; and (c)characterization of the retention process using FTIR, XRD,XRF and SEM.

2. Experimental methods

2.1. Materials

The adsorbent used in this study was a commercialHS (Fluka Chemical Co). HS characterization included (a)humic substance partition as a function of pH using themethod proposed by the International Humic SubstancesSociety [19]; (b) chemical composition using a Carlo ErbaEA 1108 elemental analyzer and a Link Analytical PentalerEXL-10; (c) mineralogical composition of the inorganicfraction by X-ray diffractometry (Siemens D500); (d) scan-ning electronic microscopy (JEOL JSM 6400 mod. 6210);and (e) porous texture by N2 (77 K) and CO2 (273 K) ad-sorption measured with a Micromeritics 2010 by static vol-umetry [20]. The strong acidic groups and the total acidity ofHS were determined by calcium acetate and barium hydrox-

ide methods [3], respectively. The weak acidic groups werecalculated as the difference between the total acidity and thatand Interface Science 270 (2004) 4755

Table 1Physical and chemical characteristics of the humic substance

Fraction percentageFA 2.8BHA 27.1GHA 60.9Humin 9.2

Main minerals Qa, Mb, Kc

Elemental analysis in percentage by weightCd 47.9Hd 4.91Nd 0.67Sd 1.18Sie 2.15Ale 0.80Fee 0.86Ke 0.24Cae 2.99

Surface areaAs N2 (77 K)/m2 g1 (DRK)

lloidR.A. Alvarez-Puebla et al. / Journal of Co

Table 2X-ray fluorescence microanalysis from several spots on HSCu system sur-face

% (w/w) Initial concentration of Cu(II) in mmol kg10 320 640 1280

Spot 1 S a a 0.69 0.57Si 33.7 34.6 27.6 25.9Fe a a 0.68 aAl a a 0.61 aK a a a aCa a a 0.93 0.30Cu a a 0.32 0.51

Spot 2 S 0.97 1.13 0.97 1.48Si 1.67 1.69 1.67 1.91Fe 1.09 1.82 1.72 2.29Al 1.08 1.11 1.08 1.01K 0.32 0.35 0.32 aCa 2.87 2.34 1.87 1.95Cu a 0.63 1.21 1.62

Spot 3 S 1.19 0.91 0.86 1.18Si 0.75 0.81 0.91 0.75Fe 0.90 0.90 0.92 1.06Al 0.73 0.77 0.84 0.73K a a a aCa 0.88 1.04 0.68 0.88Cu a 1.12 1.81 2.23

a Not detected.

Fig. 1. Variation of the humic substance DOM percentage as a function ofpH.

mined by atomic absorption spectrophotometry (PerkinElmer, Mod. 2100) and the amount of Cu(II) retained wascalculated from the difference between the initial amountand the equilibrium concentration. The residue was vacuum-dried at 338 K.

To test the influence of ionic strength, a new isotherm atpH 2 was obtained under similar conditions except that Cuwas added from 10,000-ppm solution of Cu(NO3)2 (Merck,analytical grade); and ionic strength and pH were adjustedwith 0.25 M NaNO3 and dilute HNO3 solutions.

The isotherm at pH 2 was replicated five times, to test

the reproducibility of adsorption data. In all cases, the errorpercentage was less than 1.7%.and Interface Science 270 (2004) 4755 49

2.3. FTIR spectroscopy and SEM

Selected samples of the isotherms (doped with 0, 80, and1280 mmol of Cu(II) per kg of HS), washed three times withethanol during 5 min, vacuum-filtered, and dried at 333 Kunder vacuum for 24 h, were characterized with transmissionFTIR (Nicolet, Avatar 360) on pressed KBr pellets (150 mgKBr and 1 mg of sample). The transmission FTIR cell wasflushed with N2 gas for 10 min before scanning to removeatmospheric water vapor and CO2 from the spectrophotome-ter. The spectral resolution was set to 1 cm1 and 150 scanswere collected in each spectrum. XRD (Siemens, D500) wasrecorded in the samples where FTIR showed the existence ofa copper inorganic phase. Micrographs of the HACu sys-tem were obtained by SEM on pressed pellets using a JEOLJSM 6400 Mod. 6210 microscope equipped with an EDAXsystem (EXL-10) from Link Analytical Pentaler.

3. Theoretical

The following equilibrium equations for copper as a func-tion of pH have been assumed (in all the cases it has beenassumed that Cu2+ has a coordination number of 6 and itsstructure has the tetragonal distortion due to the JahnTellereffect) [5]:

(1)[Cu(H2O)6]2+ +OH [Cu(OH)(H2O)5

]+ +H2O,

(2)

[Cu(OH)(H2O)5

]+ 12[(H2O)4Cu(OH)2Cu(H2O)4

]2+

+H2O,12[(H2O)4Cu(OH)2Cu(H2O)4

]2+ +OH

(3) [Cu(OH)2(H2O)4],

(4)

[Cu(OH)2(H2O)4

]+OH [Cu(OH)3(H2O)3]

+H2O,

(5)

[Cu(OH)3(H2O)3

] +OH [Cu(OH)4(H2O)2]2

+H2O.In this paper [(H2O)4Cu(HO)2Cu(H2O)4]2+ and

[Cu(OH)(H2O)5]+ have been considered equivalent species.In the same way, [Cu(OH)2(H2O)4] corresponds to the hy-droxide Cu(OH)2, which will be found mainly as a precipi-tated solid [23].

The retention of a metal ion on a HS could be de-scribed with a model that assumes different binding mech-anisms [24,25]. In the experimental conditions describedabove, one, two, and even three different simple retentionprocesses can take place:

Process 1. Adsorption of the cation [Cu(H2O)6]2+

(6)2 S-H+ [Cu(H2O)6]2+

[S2Cu(H2O)4

]+ 2H3O+.Process 2. Adsorption of the cation [Cu(OH)(H2O)5]+(7)S-H+ [Cu(OH)(H2O)5

]+[SCu(OH)(H2O)5

]+H3O+.

lloid50 R.A. Alvarez-Puebla et al. / Journal of Co

Process 3. Precipitation of Cu(OH)2 on the solid surface

(8)[Cu(H2O)6]2+ + 2OH Cu(OH)2.

The adsorption processes usually fit into kinetic laws suchas

(9)dCdt

= kaCn(1 ) kd,where ka and kd are the adsorption and desorption rate coef-ficients, respectively; is the coverage surface fraction; andn is the partial order of the process [26,27].

In the equilibrium, Eq. (9) easily leads to

(10)ns

nsm= = KC

n

1+KCn .

When n= 1 (Langmuir equation conditions),

(11)ns = KnsmC

1+KC ,where ns are the moles of Cu(II) adsorbed per mass unit ofHS; nsm is the retention capacity of HS for Cu(II); and K isthe kinetic equilibrium constant.

When the single experimental adsorption isotherms havea final segment of high slope it could be attributed to a simplemultilayer process or, as in this case, to precipitation on thesurfaces of low solubility species. The kinetic law can beexpressed using the equation

(12)dCdt

= k1Cb k2ns

which, at equilibrium, leads to

(13)ns = (k1/k2)Cb =ACb.In the most general case, the adsorption global process

consists of the three single processes indicated by Eqs. (6),(7), and (8). The experimental isotherms will fit into the nextgeneral equilibrium equation,

ns = ns1 + ns2 + ns3(14)= K1n

sm(1)C

1+K1C +K2n

sm(2)C

1+K2C +ACb,

where ns1 and ns2 are the moles of Cu(II) adsorbed per gram

of HS as [Cu(H2O)6]2+, [Cu(OH)(H2O)5]+, respectively,and ns3 the moles of Cu(II) precipitated as Cu(OH)2 on theHS surface.

4. Results and discussion

4.1. Characterization of the commercial humic acid

Table 1 shows the results for the humic substance parti-tioning, elemental analysis, surface area, and acidic groups.

The main fractions are humic acids (88%), corresponding toBHA (27.1%) and GHA (60.9%). Fulvic acids and huminand Interface Science 270 (2004) 4755

are minor components, 2.8% and 9.2%, respectively. Ele-mental analysis also shows the presence of inorganic ele-ments (Si, Al, Fe, Ca, and K), indicative of the presenceof mineral phases. The mineral phases were identified byXRD and FTIR spectroscopy on the humin fraction be-cause it is the fraction richest in inorganic compounds.The X-ray diffractogram showed -quartz (2 = 26.7,20.9, 50.2) to be the main component of the crystallinephase, whereas kaolinite-Md (2 = 12.5, 25.0, 38.7) andmuscovite-M1 (2 = 8.8, 26.5, 19.8) were minor com-ponents. On the other hand, FTIR spectra [28] exhibited thepresence of (a) -quartz, doublet band at 797 and 778 cm1;(b) kaolinite-Md, two sharp peaks at 3695 and 3619 cm1,assigned to two of the four OH characteristic tensionsof this clay, and a small weak band at 694 cm1; and(c) muscovite-M1, characterized by a shoulder at 915 cm1and a weak peak at 469 cm1. The bands between 1165 and1033 cm1 are assigned to SiO tensions due to both quartzand clays, whereas the band at 533 cm1 was the result ofthe typical SiOAl deformation in clays.

The HS porous texture has been studied using adsorp-tion isotherms of N2 (77 K) and CO2 (273 K) (Table 1).The molecular size of both adsorbates is similar (0.30 and0.33 nm, respectively); however, the sample adsorbed CO2but no N2 was retained, which can be explained by the highlynarrow microporosity of this kind of material. Whereas CO2can penetrate into the micropores, N2 cannot, due to dif-fusion restrictions because of its lower kinetic energy at77 K [29,30].

The HS titration curve had two equivalence points (Ta-ble 1) corresponding to two acidic group families: car-boxylic (pKA = 3.92) and phenolic (pKB = 8.22) acidicgroups. The concentration of carboxylic acidic group wasgreater (4.02 mol kg1) than that of phenolic acidic group(2.95 mol kg1). These values were in agreement with thebibliography for HS [2].

The dissolved organic matter (DOM) percentage (Fig. 1)was practically constant in the pH range 24, first increasingslightly from pH 4 to 6 and then abruptly from pH 6 upward.Since the sample is a HS, composed of different fractions(FA, BHA, GHA, and humin), the DOM percentage in thefirst segment is the result of the FA dissolution, which is sol-uble at any pH value [3]. As the pH increases, so does thenegative charge, due to the continuum ionization of acidicgroups and hence the solubility.

The SEM micrograph of HS (Fig. 2a) shows a heteroge-neous surface where white particles are immersed into a grayphase that produces distinct color tones. The X-ray fluores-cence microanalysis performed on the micrograph markedspots (Fig. 2a; Table 2) is in agreement with the heteroge-neous distribution of the mineral phases. Silicon is the onlyelement detected in the white particles (spot 1). The per-centage of Si, Al, and Fe is slightly greater in the light grayphases (spot 2) than in the dark gray ones (spot 3) suggest-

ing that while quartz is free, not bonded to HS, the clays areimmersed in the organic matrix, bonded to HS.

lloidFig. 2. SEM micrographs of humic substance. SEM micrographs of the residues of selected points, (a) 0, (b) 320, (c) 640, and (d) 1280 mmol kg1, fromthe sorption isotherms at pH 2 of Cu(II) on a humic substance (I = 0.05 M NaCl; 298 K). (The results of the XRF analysis on the marked spots are listed inTable 2.)

4.2. Copper adsorption isotherms on HS

The shape of the isotherms and the amount of Cu(II)retained varied with pH (Fig. 3). According to the Giles clas-sification [31], the isotherms can be included in the L-type,except for the isotherm at pH 8.02, which was S-type. Atthis pH, the slope of the isotherm was zero for equilibriumconcentrations less than 1 mM. From pH 2.06 to 3.2 the ad-sorption isotherms can be classified as L-1 type, and at 4.11and 6.13, the isotherms were L-3 type.

The lineal model shows R2 > 0.95, but due to the narrowapplication range (040 mmol kg1), describes only a partof the retention process. The Freundlich model fits well too;nevertheless, the fit of a set of data to a power law is too gen-eral, so it gives little information about the retention process.The adsorption isotherms (Fig. 3) did not fit adequately tothe Langmuir equation, which suggests that the experimentalsystem did not follow the reversibility and kinetic conditionsof the Langmuir equation, or that the global process is com-posed of more than one single process, in agreement withthe stability diagrams for the different species of Cu(II) inaqueous solution as a function of pH and Cu(II) concentra-tion [23].

The global process that occurs in the experimental

Fig. 3. Adsorption isotherms at pH constant of Cu(II) on a humic substance(I = 0.05 M NaCl; 298 K).

At 2.06 pH 3.2, two simultaneous single processestake place, adsorption of [Cu(H2O)6]2+ and [Cu(H2O)5(OH)]+:

(15)ns = ns1 + ns2 =K1n

sm(1)C

1+K1C +K2n

sm(2)C

1+K2C .R.A. Alvarez-Puebla et al. / Journal of Coisotherms seems to be formed by two single processes, soEq. (14) reduces to the following:and Interface Science 270 (2004) 4755 51At pH 4.11 and 6.13, two simultaneous processes takeplace, adsorption of [Cu(H2O)5(OH)]+ and precipitation of

Fig. 4. Surface speciation of Cu retention on HS at pH of (a) 2.06, (b) 2.51, (c) 3.2obtained applying Eqs. (15), (16), and (17).52 R.A. Alvarez-Puebla et al. / Journal of Colloid and Interface Science 270 (2004) 4755

Cu(OH)2 on HS surface:

(16)ns = ns2 + ns3 =K2n

sm(2)C

1+K2C +ACb.

At pH = 8.02, a single precipitation process of Cu(OH)2takes places:

(17)ns = ns3 =ACb.

The experimental data (ns vs C) fit very well to Eqs. (15),(16), or (17), to give the results shown in Table 3, as indi-cated by R2 values (continuous lines in Fig. 3). This modelreveals a great selectivity of the HS to adsorb the differentCu(II) species. The selectivity depends on pH and adsorp-tive concentration.

Fig. 4 shows the distribution of the amount of copperspecies adsorbed as a function of C0, for processes (15),

Table 3Fitted parameters for Eqs. (15), (16), and (17)pH ns

m(1) K1 nsm(2) K2 A b R

2

(mmol kg1) (mmol kg1)2.06 171 2.97 490 0.08 0.99932.51 160 3.30 439 0.21 0.99973.20 171 3.42 1635 0.22 0.99944.11 1.91 103 0.30 2.10 11.3 0.99976.13 5.56 103 4.70 102 2.33 8.03 0.99928.02 20.4 1.77 0.99480, (d) 4.11, (e) 6.13, and (f) 8.02, as a function of initial metal concentration,

lloidR.A. Alvarez-Puebla et al. / Journal of Co

(16), and (17), calculated using the parameters shown in Ta-ble 3. Between pH 2.51 and 3.20 (Figs. 4a4c) there is anintersection point in the curves of the amount of species ad-sorbed. This intersection decreases as pH increases due tothe increase in the concentration of OH ions. When C0was below this point, Cu(II) was preferably adsorbed asCu(H2O)6]2+.

At pH 4.11 (Fig. 4d) most of the strong carboxylic acidicgroups are ionized, increasing the surface charge and theamount of adsorbed metal at low C0. The increase of theOH concentration determines that Cu adsorption is higheras [Cu(H2O)5(OH)]+ than [Cu(H2O)6]2+. The adsorbed[Cu(H2O)5(OH)]+ probably promotes the precipitation ofCu in unsaturated conditions (pKsp Cu(OH)2 = 19.1), whichbehaves as precipitation micronuclei for Cu(OH)2 growth, inaccordance with the surface precipitation model, where theinterface is a mixing zone for the ions of the new solid phaseand those of the substrate [32].

From pH 6.13 the DOM percentage (Fig. 1) rises, caus-ing a competition between the bulk HS and the dissolved HSfor the Cu ions. The net effect is the shift of the precipitationedge to higher C0 with pH. At pH 6 (Fig. 4e), the adsorp-tion [Cu(H2O)5(OH)]+ was higher than the precipitation ofCu(OH)2 for C0 < 1100 mmol kg1. At pH 8 (Fig. 4f), thepreferred retention mechanism was the precipitation. The in-hibition of the adsorption of aquacomplex ions by bulk HSand the shift of the precipitation edge show that the dissolvedHS forms very stable complexes with Cu(II).

The FTIR spectra at several selected points (0, 80, and1280 mmol kg1 Cu) of the adsorption isotherms at pH 2,4, 6, and 8 are presented in Fig. 5. They give reasonablesupport to the results obtained in the application of the model(Eq. (14)). All the spectra exhibit bands from the mineralphases (quartz, muscovite and kaolinite), which enlarge theirintensity as pH increases due to the HS dissolution.

At pH 2 (Fig. 5a), the Cu(II) is adsorbed as [Cu(H2O)6]2+, as shown by the increased intensity of thebands: (a) 3420 cm1, due to OH stretching (OH); (b)1600 cm1, due to HOH deformation (OH); and (c)525 cm1, due to CuO stretching [33]. The OH stretch-ing (3420 cm1) and the asymmetric and symmetricCOO stretchings (1610 and 1380 cm1) increase withmetal concentration, while the OH stretching of H-bondedCOOH (2500 cm1) [10], the C=O stretching of COOH(1710 cm1), and the CO stretching and OH defor-mation decrease (1240 cm1). So, at pH 2 (Fig. 5a),[Cu(H2O)6]2+ is the predominant adsorbed Cu(II) species;the formation of Cu(II) mixed complexes with water and HSfunctional groups is suggested by the reduction of the inten-sity in the broad shoulder of OH stretching of H-bonded,C=O stretching and CO stretching and OH deformationof COOH, and the increase of asymmetric and symmetricCOO stretchings as metal concentration increased [10,17].

For pH 4.11 (Fig. 5b), the increase of asymmetric andsymmetric COO stretchings and the reduction of the OH

stretching of H-bonded, C=O stretching and CO stretchingand Interface Science 270 (2004) 4755 53

Fig. 5. FTIR spectra of the residues of selected points (C0 = 0, 80, and1280 mmol kg1) from the sorption isotherms at constant pH of Cu(II) ona humic substance (I = 0.05 M NaCl, 298 K). (a) pH 2.06, (b) pH 4.11, (c)pH 6.13, and (d) pH 8.02. The spectra of [Cu(H2O)6]Cl2 and Cu(OH)2 arealso included.

and OH deformation in COOH suggest that the forma-tion of complexes with carboxylic acid groups is the pre-ferred adsorption mechanism. The increase in the amountadsorbed compared to that at pH 2 is due to the smallercompetition between Cu(II) and H+ for the adsorptionsites in HS [34] and to the presence of the highly reac-tive [Cu(OH)(H2O)5]+. However, some authors have sug-gested that the adsorption phenomena at low pH can also bedue to the formation of complexes between Cu(II) and car-bonyl [35] or nitrogen-containing groups from the HS [36].The complexation with carbonyl groups cannot be confirmedby FTIR because their bands overlap with the bands dueto the C=O group of COOH (1710 cm1), asymmet-ric COO stretching (1610 cm1), and (OH) of waterin [Cu(H2O)6]2+. Complexation with N-containing groups

could not be confirmed either. We did not find bands aris-ing from these groups or metalN-containing groups, in

lloid54 R.A. Alvarez-Puebla et al. / Journal of Co

agreement with Evangelou et al. [37], probably becausethe signals NH vibrations, 1920 cm1, and amide bands,1644 cm1, overlapped other absorption bands, so that, theirexistence cannot be disregarded.

At pH 6 (Fig. 5c), Cu(II) forms a hydroxide, as re-vealed by the three strong sharp peaks (3445, 3357, and3316 cm1), particularly that at 3360 cm1, assigned toOH stretching from copper hydroxides such as Cu(OH)2,and a medium band (1614 cm1), appearing as a sharp peakat 16301635 cm1, assigned to OH deformation in hy-droxyl groups [38]. In the CuHS system, the OH stretch-ing rises slightly when C0 = 80 mmol kg1 and stronglywhen C0 = 1280 mmol kg1. At C0 = 1280 mmol kg1,this band shows a shoulder (3363 cm1) that could be theOH stretching corresponding to the Cu(OH)2 precipitate.The bands assigned to asymmetric and symmetric COOstretchings (1610 and 1380 cm1) increase with metalconcentration. In the signal at 1610 cm1 a weak sharppeak at 1630 cm1 that rises with metal concentration can beseen. This band seems to be due to OH deformation for thehydroxyl groups of the hydroxide. The C=O stretching ofCOOH (1710 cm1) and the CO stretching and OH de-formation (1240 cm1) do not show a great variation, un-like the OH stretching of H-bonded COOH (2500 cm1),which fades to disappearance as metal concentration rises.These changes suggest a combination of mechanisms inthe retention process. At C0 = 80 mmol kg1, Cu(II) re-tention forming complexes with carboxylic acid groups isthe preferred adsorption mechanism, according to Alcacioet al. [39]; however, at C0 = 80 mmol kg1, the spectra ex-hibit bands due to copper hydroxide (shoulder at 3363, anda sharp peak at 1631 cm1), suggesting that, at this metalconcentration, the precipitation of copper is an important re-tention mechanism.

At pH 8 (Fig. 5d), at high copper concentrations the bandsdue to copper hydroxide (3363 and 1633 cm1) and OHstretching clearly rose, suggesting the formation of an amor-phous copper hydroxide, although Cu(OH)2 crystal phaseswere not detected by XRD, because the crystals were not ofsufficient size.

The SEM micrographs obtained for samples correspond-ing to C0 = 320, 640, and 1280 mmol kg1, from theisotherm at pH 2, are presented in Figs. 2b and 2c. Ta-ble 2 shows the XRF results for the marked spots in themicrographs. XRF shows different distribution of adsorbedCu(II) on the HS surface. In the white phase (spot 1), mainlyformed by quartz, Cu(II) adsorption is only detected at highmetal concentration. Since this phase has a low surface areaand due to the size and penetration capacity of the X-raybeam, it is likely that the small amount of copper detectedmight be a consequence of the complexation to HS that isenclosing the mineral. The light gray phase (spot 2), formedby HS and clays, adsorbs copper in all the range of concen-tration but to a lower extent than the dark gray phase (spot 3),

which is formed only by HS. Despite HS having many sur-face functional groups, the medium retention in the humic-and Interface Science 270 (2004) 4755

clay system (spot 2) can be explained because some of themare occupied cleaving the HS to the clay surface [40].

4.3. Copper adsorption on HS as a function ofionic strength

Fig. 6a shows the adsorption isotherms (298 K) for cop-per at pH 2 and ionic strength 0.05 M in NaCl and NaNO3.Both isotherms have similar shapes and, according to Giles,can be classified as L-1. At low concentrations, copper ad-sorption is similar. However, from C0 = 220 mmol kg1 up-ward the retention in the isotherm with NaNO3 as electrolyteis larger than that with NaCl. This fact can be explainedbecause copper has a greater tendency to form complexeswith chloride than with nitrate [5]. The stability of the com-plexes formed with chloride, [CuCl4]2, partially inhibitsthe retention on HS due to their negative charge. Compar-ison between FTIR spectra for samples corresponding toC0 = 320, 640, and 1280 mmol kg1 from isotherm withnitrate (Fig. 6b) with those obtained for the isotherm withchloride (Fig. 5a) show that the nitrateHS system FTIR ex-hibits a strong sharp peak at 1384 cm1, assigned to theasymmetric stretching (E) of the D3h nitrate anion [33].As copper retention increases the symmetry species E losesits degeneration, giving rise to another two active IR peaks,

Fig. 6. (a) Adsorption isotherms (298 K) at pH 2 of Cu(II) on a hu-mic substance with 0.05 M different nature ionic strength, NaCl andNaNO3. (b) FTIR spectra of the residues at selected points (0, 320, and11280 mmol kg ) from the adsorption isotherms at pH 2 of Cu with ionicstrength of NaNO3.

R.A. Alvarez-Puebla et al. / Journal of Colloid and Interface Science 270 (2004) 4755 551423 cm1 (B2) and 1355 cm1 (A1), suggesting the loseof symmetry from the D3h to the C2v point group [41]. Thestrong and sharp peak due to asymmetric nitrate stretching(E) suggests that NO3 is electrostaticaly adsorbed onto HSin a process similar to pyrophosphate retention [42]. Thedecrease of the peak of 1348 cm1 as metal concentrationrises, giving rise to 1423 cm1 (B2) and 1355 cm1 (A1),suggests the formation of [CuNO3]+ retained onto HS. Thiscomplex had been described in water/acetone mixtures byCastro and Jagodzinski [41,43].

5. Conclusions

The HS surface is heterogeneous, with highly narrowmicroporosity. The shape of the isotherms at constant pHchanged with pH from L-type (pH 26) to S-type (pH 8) dueto the rise in the DOM percentage, which forms very stablecomplexes with copper. The isotherm data do fit adequatelyto a several-single-adsorption-processes-based model. WhenpH is 2 the retention of Cu(II), as [Cu(H2O)6]2+, is the pre-ferred retention mechanism. The retained quantity of Cu(II)as [Cu(OH)(H2O)6]+ increases with pH. Starting from pH 4the Cu(II) begins its precipitation and is the preferred reten-tion mechanism at pH 8.02. Starting from pH 6, the Cu(II)forms very stable complexes with DOM. The presence ofHS has a great influence on the precipitation process ofCu(II), giving rise to amorphous precipitates instead of crys-tal phases. Metals distribute heterogeneously on HS surfaceand accumulate on the humic phases not associated withclays. When Cl ions are present, part of the Cu(II) form[CuCl4]2, which due to its negative charge is stable in so-lution; when the anion present is NO3 the complex formed,[CuNO3]+, is retained onto the HS.

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Cu(II) retention on a humic substanceIntroductionExperimental methodsMaterialsAdsorption isothermsFTIR spectroscopy and SEM

TheoreticalResults and discussionCharacterization of the commercial humic acidCopper adsorption isotherms on HSCopper adsorption on HS as a function of ionic strength

ConclusionsReferences