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Adsorption of Zn(II) in Oxisols asaffected by selective removal of soilfractionsC.P. Jordão a , D.M. Carari a , W.L. Pereira a , R.M. Almeida a ,M.P.F. Fontes a , R.L.F. Fontes a & R.B.A. Fernandes aa Departamento de Solos , Universidade Federal de Viçosa ,36570‐000 Viçosa, Minas Gerais, BrazilPublished online: 08 Dec 2010.

To cite this article: C.P. Jordão , D.M. Carari , W.L. Pereira , R.M. Almeida , M.P.F. Fontes ,R.L.F. Fontes & R.B.A. Fernandes (2010) Adsorption of Zn(II) in Oxisols as affected by selectiveremoval of soil fractions, International Journal of Environmental Studies, 67:6, 879-897, DOI:10.1080/00207233.2010.527123

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International Journal of Environmental Studies,Vol. 67, No. 6, December 2010, 879–897

International Journal of Environmental StudiesISSN 0020-7233 print: ISSN 1029-0400 online © 2010 Taylor & Francis

http://www.tandf.co.uk/journalsDOI: 10.1080/00207233.2010.527123

Adsorption of Zn(II) in Oxisols as affected by selective removal of soil fractions

C.P. JORDÃO*, D.M. CARARI, W.L. PEREIRA, R.M. ALMEIDA, M.P.F. FONTES, R.L.F. FONTES AND R.B.A. FERNANDES

Departamento de Solos, Universidade Federal de Viçosa, 36570-000 Viçosa, Minas Gerais, BrazilTaylor and FrancisGENV_A_527123.sgm

(Received 23 September 2010)10.1080/00207233.2010.527123International Journal of Environmental Studies0020-7233 (print)/1029-0400 (online)Original Article2010Taylor & Francis0000000002010C.P.Jordãojordao@ufv.br

The relative contribution of organic matter, amorphous and crystalline Fe oxides and Al oxides to soilZn adsorption was evaluated in contaminated and uncontaminated Brazilian soils. Soil samples wereequilibrated with Zn solutions and Zn adsorption was determined using the Langmuir adsorptionisotherm. The Fe and Al oxides (non-silicated clays) and the organic matter contents of the soils werethe main contributors to the variation in Zn adsorption. The Zn maximum adsorption capacity in thesoil with the greatest sand and organic carbon contents was higher than in the higher clay content soil,which was second in organic carbon content. Related to the whole soil samples, as the soil organicmatter was removed, the Zn maximum adsorption capacity decreased in most of the observations.The removal of Fe and Al oxides decreased the soil Zn maximum adsorption capacity in some casesand increased it in others, with no clear variation in the pathway. For both whole soil and soil frac-tions, the isotherms for Zn adsorption to soil, fitted to the Langmuir equation, showed two linearportions or pathways (Part I and Part II). The bonding energy coefficient was higher in Part I (relatedto specific chemical adsorption) than in Part II (related to electrostatic interactions), which suggests ahigher affinity between Zn and soil particles in Part I as compared with Part II.

Keywords: Zinc contamination; Oxisol; Soil fractions; Langmuir adsorption isotherm

1. Introduction

The Zn concentrations in soils have increased in recent decades as a consequence of thedeposition of industrial emissions [1,2]. The risk of metal leaching through the soil profile ormetal uptake by plants depends on the concentration of the pollutants in the soil solution,which depends on the sorption–desorption equilibrium that governs their partition betweensoil solution and soil solids, especially soil colloids [3].

The influence of soil components on the adsorption of metals may be examined in twoways [4]: by studying adsorption in individual soil components such as clays [5], organicmatter [6], iron and aluminium oxides [7] and organo-mineral associations [8,9]; or by study-ing the adsorption in the soil after the removal of a specific soil component to establish itseffect on metal adsorption [10].

*Corresponding author. Tel.: +553138991071; fax: 553138992648. Email: jordao@ufv.br

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The Langmuir equation is easy to apply for studying soil adsorption and gives straightfor-ward parameters that can be interpreted in the light of soil properties. This isotherm can beused to evaluate the adsorption maxima and the bonding energies for Zn in the soil. The Lang-muir adsorption isotherm has been successfully applied to Zn adsorption on Oxisols [11–13].

The solubility of Zn and the mechanism acting in its control may vary with soil properties,such as pH, organic matter content and clay content. Zinc may be attached to Fe, Mn and Aloxides, clays, or organic matter in soils [14]. Hydrous oxides play significant roles in heavymetal retention or adsorption by soils, but little has been reported on the relative importanceof crystalline hydrous oxides on the adsorption of Zn [10].

The objective of this study was to evaluate the Zn adsorption in contaminated and uncon-taminated Brazilian soils as related to their organic matter and Fe and Al oxides contents.Soil properties affecting Zn adsorption were determined and included in the investigation.The Langmuir adsorption isotherm was used for the adsorption study.

2. Material and methods

2.1. Soil samples

Samples of Brazilian Latosols (Oxisols) from the Steel Valley, an iron-rich region in MinasGerais State, Brazil, were collected from sampling sites (1–4) located near smelters (figure1). The sites where chosen taking into account the proximity of both factories and settle-ments. Sampling sites 1–3 were situated in Piracicaba River and sampling site 4 in DoceRiver. For comparison purposes, a sampling site (identified as 5) was chosen out of the SteelValley, at the city of Viçosa, Minas Gerais State, in a non-agricultural Latosol. A set of soilsamples (0 to 15 cm depth) was air-dried, passed through a 2 mm sieve and analysed fordetermination of size distribution and pH. These samples were also used for the study of Znadsorption. Another set of soil samples (0 to 15 cm depth) was oven-dried at 60°C for thedetermination of cation exchange capacity (CEC) and organic carbon content. An additionalset of samples (0 to 15 cm depth) was air-dried, sieved to a particle size lesser than 0.18 mmand analysed for total metal concentrations determination.Figure 1. Map of the study area showing the State of Minas Gerais and the sampling sites in the Steel Valley.

2.2. Soil characterization

The soil samples were analysed for the determination of particle size distribution (pipettemethod, using 1 mol L−1 NaOH) [15]; pH in deionised water and pH in KCl (1 mol L−1 KCl),1:2.5 soil/water ratio [16]; mineralogy of the clay fraction (X-ray diffraction) [15]; organiccarbon (OC) [17]; effective cation exchange capacity (CECe) (Ca and Mg extracted with 1mol L−1 KCl, Na and K with 0.05 mol L−1 HCl) [15]; potential cation exchange capacity(CECp) calculated by the sum of the exchangeable Ca, Mg, K and Na plus the acetate buffer(pH 7) potential acidity [18]. The total metal concentrations in the soil samples were deter-mined, in triplicate, by atomic absorption spectrophotometry (AAS) after digestion with 10mL HNO3 (65% w/v), 2 mL HClO4 (70% w/v) and 10 mL HF (40% w/v).

2.3. Removal of organic matter, amorphous and crystalline iron oxides and aluminium oxides for the adsorption studies

The removal of soil fractions, individually, from soil subsamples was performed, not sequen-tially, by three procedures:

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Adsorption of Zn(II) in Oxisols 881

Figure 1. Map of the study area showing the State of Minas Gerais and the sampling sites in the Steel Valley.

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1. Removal of organic matter fraction by the sodium hypochlorite procedure [19]: fivesubsamples of each soil were shaken with sodium acetate and acetic acid buffer solution(pH 5) at 80°C for 15 min and the solid fraction was shaken with a 10% sodiumhypochlorite solution at 80°C for 15 min.

2. Removal of amorphous iron oxides: extraction of Fe (five subsamples) from each soilwith a 0.2 mol L−1 ammonium oxalate solution (pH 3) for 2 h and determination ofextracted Fe by AAS [20].

3. Removal of crystalline iron oxides by the dithionite–citrate method [20]: extraction (fivesubsamples) from each soil by using a hydrosulphite and sodium citrate solution at 5°Cfor 30 min and determination of extracted Fe by AAS.An additional extraction was used for removing both organic matter and aluminiumoxides, sequentially:

4. Extraction of Al oxides (after removal of organic matter from each soil) from the soilsolid fraction by adding a 1.25 mol L−1 NaOH solution and heating at 75°C for 1 h. TheAl concentration in the soil test extract was determined by AAS [21].The Fe concentrations in the whole soils were determined by AAS.

2.4. Langmuir adsorption isotherms for soil samples

The Zn(II) adsorption experiments were carried out by adding Zn to the soils and determiningthe Zn adsorption maximum capacity in the whole soil samples and in the samples fromwhich the soil fractions (organic matter, amorphous iron oxides, crystalline iron oxides andaluminium oxides fractions) had been removed. The Langmuir equation was chosen for theestimation of maximum adsorption capacity corresponding to soil surface saturation. Thelinear Langmuir isotherm is mathematically represented by the following equation:

where KL (L mg−1) is a constant related to the adsorption/desorption energy, qe (mg g−1) is theamount of metal ion adsorbed per gram of soil and qmax (mg g−1) is the maximum sorptionupon complete saturation of the soil surface. The experimental data were fitted to thelinearised equation by plotting Ce/qe against Ce. The Langmuir adsorption isotherms wereadjusted at constant values of pH and ionic strength.

The adsorption studies were conducted on the whole soil samples and samples from whichsoil fractions had been removed (0.1 g each). The samples were placed in 50 mL centrifugetubes followed by the addition of 15 mL of a synthetic Zn(II) solution containing 0, 2, 4, 8,16, 24, 32, 48, 64, 82, 128, 192, 256 and 300 mg L−1 Zn ( ZnCl2 in 0.02 mol L−1 KCl). ThepH of the suspension was adjusted in the range 5.8 to 6.0 (0.1 mol L−1 KCl solution or 0.1mol L−1 HCl solution) and the suspensions diluted to 25 mL with deionised water. Aftershaking for 24 h at 25 ± 2 °C, and centrifugation, the Zn(II) concentrations were determinedin the solutions. Preliminary experiments showed that after the 24 h shaking, equilibrium wasreached when the ratio between soil sample mass and the volume of Zn(II) solution was equalto 1:150.

Each experiment was replicated three times and the Zn(II) concentrations were determinedby AAS. The amount of Zn(II) adsorbed onto the soil was determined by calculating thedifference between the metal added to the soil and the metal remaining in the supernatantliquid after equilibrium.

Cq q

CqK

e

e

e

L

= +1

max max

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Adsorption of Zn(II) in Oxisols 883

2.5. Instrumentation and some relevant information

For pH determination, a TECNOW pH meter, model IRIS-7 was used. Total metal concen-trations in soil samples, iron and aluminium removed from the oxides, as well as Zn(II)concentrations from the adsorption experiments were determined with a Varian atomicabsorption spectrophotometer (model SpectrAA-200), by direct aspiration of the solutionsinto an air–acetylene flame or nitrous oxide–acetylene flame. Background correction wasused for Fe, Ca, Mg, Pb, Cd, Ni and Zn determinations.

All glassware and materials used for metal analysis were cleaned. Certified analytical-gradereagents were used throughout. Blanks were run through all experiments. The calibration blankwas checked at the beginning and at the end of the analysis for each group of samples to certifythat the instrument calibration had not drifted. The results from the concurrent analyses ofsamples of Standard Sediments (National Institute of Standards & Technology no. 2704) were:Zn = 447; Ni = 44.2; and Cu = 94.5 (in mg kg−1); Al = 6.10 and Mg = 1.22 (in %). These areall within the range of the certified values.

3. Results and discussion

3.1. Soils characteristics

The soil analysis showed important and significant differences between soil properties (table1), which can affect adsorption of Zn(II) onto the soil. Soil texture, which plays a very impor-

Table 1. Properties of soils from the Steel Valley (samples 1–4) and from the control located outside the valley (5) used for studies on Zn(II) adsorption

Physical characteristics

Soil particle distribution (%, m/m)

Soil Textural class Sand Silt Clay Mineralogy of clay fractiona

1 Sandy loam 44 20 36 k > gb > g2 Loamy sand 79 14 7 gb > k > g3 Sandy clay loam 52 14 34 k > gb > g4 Silty loam 34 57 9 gb > k > g5 Clayey 32 3 65 k > g > gb

Chemical characteristics

CEC (cmolc kg–1)

Soil pH(water) pH(KCl) Organic carbon (%, w/w) Effective Potential

1 5.3 4.2 0.78 1.74 3.842 7.4 6.1 3.58 9.56 9.563 5.6 4.8 1.48 3.79 5.294 5.3 4.9 1.13 1.39 7.095 4.6 3.7 3.51 0.31 8.41

ak = kaolinite; gb = gibbsite; g = goethite.

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tant role in the soil–metal adsorption processes, was variable as shown by the moderatelyheavy texture in soil 1, the heavy texture in soil 2, the moderately fine texture in soils 3 and 4,and the fine texture in soil 5 (table 1).

The mineralogy data (table 1) show a differential presence of the silicate clay kaoliniteand the non-silicated clays gibbsite (Al oxide) and goethite (Fe oxide) in the soils. Asless weathered soils have more kaolinite than gibbsite, soils 1, 3 and 5 are less weath-ered than soils 2 and 4. Latosols (Oxisols), commonly found in tropical regions, arehighly weathered acidic soils, with low fertility and low base content and have, predomi-nantly, kaolinite and iron/aluminium oxides in the clay fraction [22]. In the Brazilianterritory, Latosols cover about 60% of the soil surface. The ionic adsorption capacity inthe Latosols (Oxisols) is related to the surface charges of organic colloids and silicatedclays [23].

Soils 1, 3 and 4 had pH between 4.0 and 6.0, soil 2 pH above 7.0, and soil 5 pH below 5.0(table 1). Several studies have shown that the soil pH significantly affects the adsorption ofheavy metals by soils and by other mineral or organic adsorbents [24–27].

The organic carbon contents in soils ranged from 0.78 to 3.58% with the highest in sample2 and the lowest in sample 1 (table 1). Other authors [28,29] have reported organic carboncontents closer to the values found in our study for similar soils collected from differentregions. Soil organic carbon contents are classified as low (smaller than 0.6%); medium(between 0.6 and 1.2%); and high (above 1.2%) [30].

The formation of organic–mineral complexes between the organic matter and the clay frac-tions of soil is directly related to conservation of organic matter in soil [31,32]. In general,clayish soils have higher organic matter contents as compared with sandy soils [33]. This isconfirmed for soil 5 but not for soil 2 (table 1).

The effective cation exchange capacity (CECe) of soil samples ranged from 0.31 to 9.56cmolc kg−1, while the potential cation exchange capacity (CECp) ranged from 3.84 to 9.56cmolc kg−1 (table 1). Soil samples 1 and 3 showed smaller CECe values than CECp and soilsamples 4 and 5 had CECe values much smaller than CECp.

The cation exchange capacity (CEC) is dependent on the clay minerals and organic matterof soil [32–34] and the relative contribution of these fractions to the CEC varies with pH andwith the amount and type of clay and organic matter [35]. In Oxisols, the CEC is mainly dueto kaolinite with low CEC value.

The sum of bases (Ca 2+, Mg 2+, K+ and Na+, in cmolc kg−1) of the soil is sorted into fiveclasses: very low, < 0.61; low, 0.61–1.80; medium, 1.80–3.60; good, 3.61–6.0; very good, >6.00 [36]. In our study, in general, the soil samples presented a low sum of bases, with soilsample 3 having a medium value (3.2 cmolc kg−1) and soil sample 2 an elevated value (9.3cmolc kg−1).

Table 2 shows the total metal concentrations. The Fe concentrations were higher than theworld average, 3.8% [37], although it is a common value for Brazilian Oxisols. The Alconcentrations were higher than the world average, 7.1% [37] except for soil sample 2(3.56%). The soil sample 2 had a pH 7.4 and in this condition exchangeable Al was notreleased to soil solution but precipitated in amorphous forms. It is notable that in the range ofpH 4.0–5.5 the aluminium causes damage to plants [37,38].

While Cd was not detected in the soil samples, some soils showed higher concentrations ofZn, Ni, Cr, Cu and Co as compared with the world average (table 2). The industrial activitynear the Steel Valley includes about 25 industries such as a slaughterhouse, laminationfactory, cannery and coffee toasters. But, there are no suitable data available on metalemissions from factories in the Steel Valley.

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Adsorption of Zn(II) in Oxisols 885

Elevated metal concentrations in soils could originate from pollutant emissions fromfoundries and tanneries of the Steel Valley. The loamy sand soil collected at site 2 showedunusual chemical and physical characteristics as compared with other soils of the region; forinstance, high pH (7.4), CEC (9.56 cmolc kg−1) and organic carbon (3.58%) values. This sitelies opposite a smelter along the Piracicaba River bed, and it has been used as a depot forwood, coal and other materials for many years. Besides, the high amounts of Mn, Zn, Pb andCu found in site 2 require a rigid environmental control, since soil pollution may come fromairborne industrial dust or from solid wastes dumped without regard to the risk of contamina-tion of soil and water.

The Brazilian Environmental Standards establish four classes of soil quality according tothe concentration of chemical substances [39]. The soil samples analysed in this study belongto both Class 3 and Class 4 [40]. Class 3 includes soils with at least one chemical substancewhose concentration is higher than the preventive value, while Class 4 represents soilscontaining at least one substance whose concentration is higher than the investigative value.The preventive value is defined as the limit concentration of a substance in soil capable tosustain its functions, while the investigative value is the concentration above which toxicityis considered possible to public health.

Table 2 shows that soil samples 2 and 5 belong to Class 3 since Pb, Cr and Cu concentra-tions were higher than the preventive values established by the Brazilian EnvironmentalStandards for these metals. On the other hand, soil 1 belongs to Class 4 since Ni concentrationwas higher than the investigative value for this soil. For soils of Class 3, the BrazilianEnvironmental Standards requires the control of the contamination sources as well as thequality of the soil, while soils of Class 4 require the accomplishment of procedures and actionsto control the contaminated areas.

The high concentrations of some metals present in some soils examined in this work mightaffect Zn adsorption due to competition by available sites of the samples. However, our studydid not include experiments of metal competition.

Table 2. Total metal concentrations in soils from the Steel Valley (samples 1–4) and from the control located outside the valley (5) used for studies on Zn(II) adsorption

Mean±SDa (%, w/w) Mean±SDa (mg kg–1)

Soil Fe Al Mn Zn Pb Ni Cr Cu Co

1 11.9±0.9 8.1±1.7 408±17 73.4±2.2 8.9±1.1 73.6±5.2 141±6.0 118±5.0 21.9±0.32 19.8±1.8 3.6±0.4 4087±376 79.9±3.6 77.0±9.1 3.6±0.2 41.6±4.0 44.0±9.0 3.7±0.73 6.9±0.1 7.2±1.2 368±31 59.6±2.1 10.0±2.5 36.9±0.6 97.8±6.2 39±3.2 19.7±2.04 6.5±0.3 10.3±1.5 106±4.5 24.5±1.9 5.9±0.4 11.3±1.8 101±4.8 19±1.5 1.7±0.55 7.4±0.6 7.2±2.2 87.9±1.6 40.7±3.5 6.8±0.8 34.7±3.0 129±9 57±5.9 9.5±1.2

3.8b 7.1b 600c 50c 10c 40c 100c 30c 8c

300d 72d 30d 75d 60d 25d

450e 180e 70e 150e 200e 35e

a Mean of three replicates ± standard deviation.b Average metal concentration in soils (%, w/w) [37].c Average metal concentration in soils (mg kg–1) [37].d Preventive level of the Brazilian Environmental Standards [39].e Investigative level of the Brazilian Environmental Standards [39]

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3.2. Zn(II) adsorption by the different soil fractions

Table 3 shows correlation coefficients for Zn(II) maximum adsorption and bonding energyversus soil constituent and soil property data. The linear regression equations wereadjusted for being used in studies with soils having similar characteristics as those used inthis experiment. Although the exact mechanism of adsorption in the Langmuir isotherm isunknown, which gives rise to criticism of its use, it is reported that the Langmuir isothermadsorption maxima may be used with confidence in studies relating to adsorption in soil[40].

The significant correlation coefficients for Zn(II) maximum adsorption versus soil constit-uents and soil properties were: CECe (0.934), amorphous Fe oxides (0.898), crystalline Feoxides (0.892) and Al oxides (–0.897) (table 3). But for soil CECp, organic carbon contentand clay content there were no significant correlations, with similar results found in soilsfrom Alabama, USA [41]. In other studies, CECp and clay content have been reported assignificantly correlated with Zn(II) maximum adsorption by soils [41–44].

The studies of Gomes et al. [45] showed that zinc gave no significant correlation betweenits Kd values, which represents the sorption affinity of Zn for the solid phase, and soil chemi-cal and mineralogical characteristics. The authors found that goethite content with the corre-lation coefficient r = 0.597 of P = 0.0785 was the soil characteristic that gave the highestcorrelation, suggesting that the iron oxides may exert some influence on Zn adsorption bytropical (Brazilian) Oxisols.

The CECp and the contents of organic matter, clay and Al oxide had no significant correla-tions for Zn(II) bonding energy (table 3). In similar work, organic carbon content and claycontent had low correlations with Zn(II) bonding energy for Oxisols and Cambisols [29].

The amount of Al extracted with 1.25 mol L−1 NaOH solution was negatively correlatedwith Zn maximum adsorption (table 3). This might be due to a partial attack of the extractanton amorphous Fe oxides, crystalline Fe oxides and kaolinite (2SiO2.Al2O3.2H2O) with less

Table 3. Correlation coefficients of the constituents and properties of soil related to Zn(II) maximum adsorption capacity (A) and bonding energy (B) of the whole soils

Constituent and property of soil Correlation coefficient (r)

(A)Amorphous Fe oxides 0.898*Crystalline Fe oxides 0.892*Al oxides −0.867*CECe 0.934*CECp 0.676Organic carbon 0.604Clay −0.541

(B)Amorphous Fe oxides 0.999**Crystalline Fe oxides 0.999**Al oxides −0.673CECe 0.923*CECp 0.689Organic carbon 0.671Clay −0.485

* Significant at 5% of probability; ** Significant at 1% of probability.

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Adsorption of Zn(II) in Oxisols 887

available adsorption sites being reached. Both Fe oxides and Al oxides have OH groups thatin contact with soil solution tend to dissociate, resulting in adsorption sites for cations.

Langmuir [46] proposed the first isotherm model, which assumed monolayer coverage ofthe adsorbent surface and a finite number of identical sites. The model assumes not only ahomogeneous surface of the adsorbent but also equivalent sorption energies for each sorptionsite, and no mutual interaction between the sorbed molecules [47].

The Langmuir constants, along with the correlation coefficients have been calculated fromthe corresponding plots (figures 2–6) for adsorption of Zn(II) on soil. Table 4 presents the results.While figures 2–6 show the linear plots of Ce versus qe, table 3 shows the qmax and b values.The parameter qe reflects the metal affinity for the organic matter binding sites of the soil.Figure 2. Langmuir isotherms for Zn(II) sorption by whole soils: (a) sample 1; (b) sample 2; (c) sample 3; (d) sample 4; (e) sample 5.Figure 3. Langmuir isotherms for Zn(II) sorption by the O.M.-removed: (a) sample 1; (b) sample 2; (c) sample 3; (d) sample 4; (e) sample 5.Figure 4. Langmuir isotherms for Zn(II) sorption by the amorphous Fe oxides-removed: (a) sample 1; (b) sample 2; (c) sample 3; (d) sample 4; (e) sample 5.Figure 5. Langmuir isotherms for Zn(II) sorption by the crystalline Fe oxides-removed: (a) sample 1; (b) sample 2; (c) sample 3; (d) sample 4; (e) sample 5.Figure 6. Langmuir isotherms for Zn(II) sorption by the Al oxides-removed: (a) sample 1; (b) sample 2; (c) sample 3; (d) sample 4; (e) sample 5.Comparisons of Zn(II) adsorption among soils without fractionation have shown the lowermaximum adsorption capacity to be in soil 1 (table 3). This soil was relatively low in organiccarbon and clay contents (table 1). Humic substances and clay minerals were scarce in thissoil, resulting in low Zn(II) adsorption. However, clay content was the soil characteristic thatbest correlated to Zn adsorption for six Brazilian Oxisols [12]. The predominant presence inthis soil of kaolinite, a mineral that does not adsorb Zn except in suspensions with pHbetween 7.6 and 8.8 [48] certainly contributed to these results. Lindsay [49] reported that Znadsorption by soils is highly affected by the pH.

The removal of the organic matter fraction from the whole soil 1 resulted in a decrease ofalmost 50% of Zn(II) adsorption (table 4). The decrease was, certainly, influenced by thedecrease of adsorption sites for Zn (II) in the humic materials containing carboxyl groups andphenolic hydroxyl groups. Although organic matter occurs in lower amounts than inorganicmatter in soils, the former is responsible for a great part of the ionic exchange reactions. From25 to 90% of the total CEC of the top layer of mineral soils is believed to be caused byorganic matter [33].

The organic matter plays an important role in the bonding energy and high organic mattercontent soils have enhanced bonding energy [50,51]. Table 4 shows that the removal oforganic matter from the whole soil 1 increased the bonding energy coefficient. It seems thatthe removal of the organic matter fraction from the whole soil 1 resulted in an exposure of theadsorptive surfaces of clays. As these surfaces (which were covered due to the clay–humatecomplex formation) were removed there was more contact with the silicated clay sites.Therefore, the Zn (II) adsorption to these sites resulted in the increase of the bonding energy.

Shuman [52] reported that amorphous hydrous oxides had a high capacity for heavy metalretention while Hinz and Selim [53] showed that the removal of organic matter and iron oxidein Windsor and Mahan soil, treated with peroxide and peroxide/dithionite, resulted in doublingand quadrupling Zn retention, respectively, compared with the untreated Windsor soil.

The removal of fractions of amorphous Fe oxides and crystalline Fe oxides from the wholesoil 1 resulted in the increase of Zn(II) adsorption and in the decrease of the bonding energy(table 3). The removal of the Fe oxide soil component by dithionite–citrate–bicarbonate(DCB) generally increases heavy metal adsorption by soils [54]. The increment of the qmaxvalues, representing adsorptive capacity, seems to be due to the increase in the exposure ofthe adsorption sites of clays that could be forming complexes with oxides.

Iron oxides have a strong affinity to kaolinite surfaces, which restrain Zn(II) adsorption[55]. If amorphous Fe oxides and crystalline Fe oxides are formed in the colloidal state theyare immediately adsorbed on the surfaces of silicated clays [56]. The decrease of the bondingenergy due to the removal of Fe oxides from the whole soil 1 was due to the reduction ofZn(II) selectivity by the available sites. Enhanced bonding energy values suggest that Zn(II)is adsorbed with larger affinity by the sites [57].

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Figure 2. Langmuir isotherms for Zn(II) sorption by whole soils: (a) sample 1; (b) sample 2; (c) sample 3; (d)sample 4; (e) sample 5.

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Adsorption of Zn(II) in Oxisols 889

Figure 3. Langmuir isotherms for Zn(II) sorption by the O.M.-removed: (a) sample 1; (b) sample 2; (c) sample 3;(d) sample 4; (e) sample 5.

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Figure 4. Langmuir isotherms for Zn(II) sorption by the amorphous Fe oxides-removed: (a) sample 1; (b) sample2; (c) sample 3; (d) sample 4; (e) sample 5.

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Adsorption of Zn(II) in Oxisols 891

Figure 5. Langmuir isotherms for Zn(II) sorption by the crystalline Fe oxides-removed: (a) sample 1; (b) sample 2;(c) sample 3; (d) sample 4; (e) sample 5.

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Figure 6. Langmuir isotherms for Zn(II) sorption by the Al oxides-removed: (a) sample 1; (b) sample 2; (c)sample 3; (d) sample 4; (e) sample 5.

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The increase of Zn(II) adsorption as well as the bonding energy due to the removal of Aloxides from the whole soil 1 was larger than 100%. Aluminium oxides affect the mobility ofmetallic ions present in soil solution due to their metal adsorption capacity, which is highlydependent of the acidity. Up to pH 6, the adsorption could occur through permanent negativecharges because the acidity favours the emergence of those negative charges [58]. In ourexperiment, NaOH 1.25 mol L−1 solution was used for the extraction of the Al oxide fractionand for the decrease of the pH to 5.8–6 HCl was used. The qmax value might be enhanced due

Table 4. Maximum adsorption capacity (qmax), bonding energy coefficient (KL) and coefficient of determination (r2) for Zn(II) adsorption in two linear portions (Part I and Part II) fitted to the Langmuir isotherm, on whole soil

sample and on the soil with fractions removed. Soils from the Steel Valley (samples 1–4) and from the control located outside the valley (5)

Part I Part II

r2qmax

(mg g–1)KL

(L mg–1) r2qmax

(mg g–1)KL

(L mg–1)

Whole soil1 0.802 1.19±0.01 0.236±0.003 0.932 5.08±0.41 0.019±0.0022 0.584 10.57±0.48 0.323±0.018 0.997 24.34±0.10 0.085±0.0143 0.896 2.20±0.29 0.406±0.012 0.963 17.17±0.73 0.012±0.0024 0.854 3.81±0.05 0.032±0.003 0.906 12.59±0.44 0.008±0.0015 0.914 1.25±0.24 0.337±0.001 0.943 12.28±0.60 0.009±0.001

O.M.-removed1 0.825 0.59±0.04 0.436±0.013 0.935 2.43±0.21 0.024±0.0022 0.888 1.86±0.10 1.501±0.144 0.979 21.38±0.35 0.032±0.0033 0.882 0.350±0.022 0.77±0.05 0.822 4.13±0.052 0.010±0.0014 0.937 3.418±0.029 0.18±0.04 0929 2.07±0.08 0.023±0.0015 0.848 1.18±0.003 0.566±0.009 0.909 14.08±0.40 0.010±0.001

Amorphous Fe oxides-removed1 0.926 1.93±0.047 0.352±0.012 0.920 6.66±0.10 0.013±0.0012 0.932 3.60±0.071 0.886±0.011 0.946 13.89±0.18 0.027±0.0023 – – – 0.952 3.31±0.081 0.016±0.0014 0.697 5.56±0.23 0.034±0.006 0.939 16.04±0.74 0.008±0.0015 0.947 0.28±0.01 0.686±0.069 0.873 12.27±0.55 0.006 ± 0.001

Crystalline Fe oxides-removed1 0.673 1.99±0.18 0.092±0.003 0.867 7.42±0.73 0.011±0.0012 0.985 3.58±0.17 1.874±0.052 0.981 8.20±0.56 0.072±0.0013 0.783 1.39±0.02 0.460±0.003 0.875 13.37±0.76 0.012±0.0014 0.413 7.50±0.30 0.020±0.003 0.868 21.48±1.42 0.006±0.0015 0.826 2.61±0.19 0.184±0.013 0.874 6.65±0.62 0.030±0.002

Al oxides-removed1 0.963 2.44±0.12 2.882±0.001 0.985 14.40±0.23 0.062±0.0042 – – – 0.996 13.02±0.49 0.037±0.0013 0.991 1.40±0.10 1.388±0.08 0.984 8.84±0.50 0.023±0.0014 0.744 1.21±0.11 0.341±0.011 0.914 10.33±0.75 0.012±0.0015 0.806 4.13±0.09 0.289±0.091 0.983 12.76±0.64 0.051±0.001

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to the activation of the surfaces of silicated clays and oxides, both treated with NaOHsolution.

The whole soil 2 showed the largest maximum adsorption capacity among the soils (table3). This was probably related to its high CEC (table 1) which reflects a high number of adsorp-tion sites. Since this soil had the highest organic carbon content and the lower clay content, thesoil organic fraction seems to be the main features responsible for the Zn(II) adsorption.

The removal of fractions of amorphous Fe oxides and crystalline Fe oxides from the wholesoil 2 resulted in the decrease of Zn(II) adsorption (table 3) which may be related to the pres-ence of significant amounts of microaggregates of oxidic clays. Similar results have beenreported for soils of Georgia, USA [55]. However, it has also been reported that the adsorptionsites of Fe oxides fraction were at a lower number as compared with those available when thisfraction was removed [6]. This fact indicates that Fe oxides affect Zn(II) adsorption, as hasbeen reported by other workers [5,59,60].

The decrease of Zn(II) adsorption in the whole soil 2 due to the treatment for eliminationof Al oxides was, probably, due to the solubilisation of soil minerals, as it was shown byresults found in another work [21]. Results of adsorption of Zn(II) in Oxisol after Al oxidesremoval from whole soil are very scarce and comparison is difficult. However, soils withoutAl oxides can retain much lower amounts of sulphate than soils without treatment [61].

The concentration values of organic carbon in samples 3 and 4 were closer while in thesample 5 the organic carbon concentration was higher (table 1). The textural classes of samples3 and 4 were very different and while there was a predominance of sand in the former, silt wasthe main constituent of the latter. In the case of sample 5, clay was the main constituent andthe CECp and organic carbon were both also elevated. Sample 5 did not show enhanced maxi-mum adsorption capacity for Zn(II) as compared with samples 3 and 4 (table 3). Probably, clayand organic matter in sample 5 were less active than those present in samples 3 and 4.

In the organic matter-removed samples 3 and 4 (table 3), the maximum adsorption capacitiesof Zn(II) were reduced. Sample 3 also presented a maximum adsorption capacity decrease afterremoving other fractions examined in our study but the bonding energy values varied.

Figures 2–6 shows the Langmuir isotherms for whole soils and soils with various fractionsremoved. In most cases, two linear portions of the curve were found (table 4). The two linearportions of the curve were considered separately because the lower points (hereafter referredto as Part I) in most instances, had an obviously different slope from the upper points (hereafterreferred to as Part II).

Langmuir [46] did not restrict the application of his isotherms to simple cases, but alsoapplied them to more complex surfaces including different adsorption energies. Thus, severalstudies have shown the adjustment of curves for zinc adsorption to consist of two parts[41,50,61]. The individualized analysis of each part takes into account that the parts implydifferent types of sites [29] with different bonding energies [50].

The Zn–soil adsorption study showed a larger bonding energy and a lower maximumadsorption capacity in Part I of the curves when compared with Part II (table 4). Because inPart I the Zn(II) concentrations at the equilibrium were lower than in Part II it might besuggested that at low Zn concentrations there is higher affinity in the adsorption of Zn to theavailable sites. This higher Zn(II) affinity in Part I of the curve could be due to the occurrenceof an additional interaction such as a specific chemical adsorption that is stronger than theelectrostatic interaction. It is notable that in Part II of the curve Zn(II) was retained by elec-trostatic interaction. Similar results were reported by other workers [41,50] and the existenceof two linear portions in the adjusted isotherm, as reported in our study, could be due to theheterogeneity of the surface energy [57].

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Adsorption of Zn(II) in Oxisols 895

The adsorption isotherm in sample 2, where Al oxides were removed (figure 6b), did notshow Part II of the curve since the treatment with 1.25 mol L−1 NaOH solution destroyedmost of the adsorption sites. This sample had only Part I of the curve, as similarly observedby Taylor et al. [41]. In sample 3, the removal of amorphous Fe oxides diminished theadsorption sites, which resulted in the absence of Part I of the curve (figure 4c).

Conclusions

The organic matter content and clay content are the main contributors to the variation in Znsorption in the Oxisols studied.

Zinc adsorption is more markedly reduced in the absence of the organic matter in the soilsand the removal of Fe oxides increases Zn(II) adsorption more than the organic matter removal.

A good fit of the adsorption isotherm for Zn(II) may be achieved with the Langmuir modeland the Zn(II) adsorption on soils is a function of initial Zn concentration.

The higher soil clay and organic carbon contents may not reflect enhanced maximumadsorption capacity for Zn(II) in the soil.

In Latosols (Oxisols), two linear portions for soil Zn adsorption are shown: a Part I withlower Zn(II) concentrations at the equilibrium (higher affinity–specific chemical adsorption)and a Part II with higher Zn concentrations (lower affinity–electrostatic interaction).

It is possible that there are elevated concentrations of Zn, Ni, Cr, Cu and Co in some soils,generated from the Steel Valley smelters pollutant emissions. This is a matter of environmentalconcern. Related to Zn, the environmental significance of the role of the soil in its retentionshould be evaluated. For instance, the use of some soils from the Steel Valley region foragriculture purposes would not be indicated and the impact of heavy metals on groundwaterquality should also be evaluated.

Acknowledgements

We thank the Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) andthe Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brazil, forfinancial support.

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