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Sequential extraction of copper, nickel,zinc, lead and cadmium from BrazilianOxysols: metal leaching and metaldistribution in soil fractionsS.S. Thomasia, R.B.A. Fernandesa, R.L.F. Fontesa & C.P. Jordãoa

a Departamento de Solos, Universidade Federal de Viçosa,36570-900 Viçosa, Minas Gerais, BrazilPublished online: 03 Dec 2014.

To cite this article: S.S. Thomasi, R.B.A. Fernandes, R.L.F. Fontes & C.P. Jordão (2014): Sequentialextraction of copper, nickel, zinc, lead and cadmium from Brazilian Oxysols: metal leachingand metal distribution in soil fractions, International Journal of Environmental Studies, DOI:10.1080/00207233.2014.983331

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Sequential extraction of copper, nickel, zinc, leadand cadmium from Brazilian Oxysols: metal

leaching and metal distribution in soil fractions

S.S. THOMASI, R.B.A. FERNANDES, R.L.F. FONTES AND C.P. JORDÃO*

Departamento de Solos, Universidade Federal de Viçosa, 36570-900 Viçosa, Minas Gerais, Brazil

The paper reports a study of the mobility of Cu, Ni, Zn, Pb and Cd through the profiles of tropicalsoils which received addition of those metals on the surface of soils accommodated in polyvinylchloride (PVC) columns, separated into seven rings. Leaching tests were conducted by pouringdeionized water into soil surface of the top column ring, collecting the leached effluents and ana-lysing the collected extracts for determination of metal concentrations. Soil samples from each col-umn ring were withdrawn and analysed by a sequential extraction method to identify the bindingforms of the metals retained in the soil, which correspond to six fractions of metal retained formsin soil. The metal amounts accumulated in leachates varied for each metal and depended on the soiltype studied and the horizons where the fraction was located. Compared to the amount of eachmetal applied to soil surface, only a small proportion reached the rings located at the bottom of thecolumns. In leachates, only 10% of Cu added to soil surface was found and for the other metals,percentages were even smaller. Texture and organic matter content of soils affected significantlythe mobility of heavy metals in the columns. The sequential extraction showed that solu-ble + exchangeable fraction and organic fraction corresponded to predominant forms of retention ofthe heavy metals in the soil.

Keywords: Metal; Mobility; Leaching; Extraction; Oxysol; Fractions

1. Introduction

In recent decades, there has been a continual increase in the deposit of toxic residues andeffluents, including heavy metals, through the exploitation of natural resources. This isrelated to the exponential growth of population and agricultural and industrial developmentworldwide. The dumping of urban and industrial residues and effluents in the soil has beena common practice. This has contaminated the environment. The presence of heavy metalsin the soil resulting from the application of residues, liming materials and fertilizers is amatter of concern, as the metals may be absorbed by plants and leached through the soilprofile, contaminating ground and surface waters [1–4]. Leaching tests are commonly usedin environmental impact assessment to determine whether there are risks to human healthwhen handling and reusing residues and effluents containing toxic elements. So, despitesome limitations, leaching experiments can be used to reach different objectives. Forinstance, Gomes et al. [5] performed leaching tests in order to simulate the leaching behav-iour of steel slags under landfill. In Nigeria, the analysis of the soil from a dumpsite

*Corresponding author. Email: jordão@ufv.br

© 2014 Taylor & Francis

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showed high concentrations of toxic heavy metals, which have a potential to reach toxiclevels and be transferred to the food chain [6].

The soil surface layer, which is used for agricultural production, is the main binding sur-face for heavy metal retention in the soil [7]. The risk to human health arises when thecontaminants enter the body via one of the exposure pathways, such as ingestion and inha-lation [8].

Although soil is a natural barrier of protection for underground aquifers, the factors thatcontrol the soil capacity for retention of heavy metals are very complex and the verticaland descendent movement of heavy metals through the soil profile may result in humanhealth risks. The understanding of the dynamics and behaviour of heavy metals in soils ispoor. It is not easy to assess the long-term behaviour of heavy metals in soils.

The interactions between heavy metals and soil colloids affect the mobility and retentionof metal in the soil. These interactions provide information relevant to the evaluation ofthe maximum capacity of soil for retention of heavy metal in different soil classes. Onceknown, this maximum capacity can provide guidance on alternatives and technical proce-dures to avoid exceeding the safety limit.

The mechanisms related to the soil availability of heavy metals for plants as well as thebioavailability of heavy metals for human beings in the urban environments are poorlyunderstood. The metal availability for plants depends on the mobility of the metals in thesoil and their retention on the soil particles. There is a need for research into the behaviourof metals in the soil in different environmental conditions and for extended periods. Intropical areas, highly weathered soils are predominant, and weather conditions of high tem-peratures and rainy seasons contribute to a higher metal mobility through the soil, leadingto more metal leaching. Tropical soils are those with hyperthermic and isohyperthermictemperature regimes, with a mean annual temperature of 22 °C or higher [9] covering 38%of the earth’s surface [10].

The interactions between metals and soil are complex; there are adsorption anddesorption (sorption), chelation, exchange with the solid phase, dissolution, precipitationand oxidation–reduction reactions. In most tropical soils, the sorption is attributed to thehigh concentrations of Fe and Al oxides in the soil mineral fraction [11,12]. These oxidesplay an important role in the heavy metal immobilization in the soil [13–15] and their rela-tively high sorption capacity contributes to lower the mobility of the heavy metals in thesoil, restraining their bioavailability for plant absorption. Additionally, the soil ionicstrength and soil pH affect the surface charges of the colloids, playing a significant role inthe soil heavy metal immobilization, mainly in oxidic and kaolinitic soils. These character-istics regulate to a great extent the soil adsorption [16]. Similarly, the organic matter ofmineral soils influences the retention and mobility of heavy metals in such soils [17], sincethe formation of organic–heavy metals complexes via chelation is one of the most impor-tant bonding mechanisms in the superficial soil horizon. It is known that the formation ofinsoluble chelates leads to a lesser heavy metal availability to plants, while soluble chelatesallow a greater heavy metal bioavailability in the soil profile [18], and the high number ofcarboxylic groups in the humic and fulvic acids are responsible for the organic mattersorption capacity [1].

The distribution of the metal species in the soil profile and the metal retention andmobility in the soil depend on the soil properties. Understanding the mechanisms that con-trol these relationships is a challenge because the soil system is complex. This study exam-ined the mobility of Cu, Ni, Zn, Pb and Cd through the soil profile of two tropical soils(Oxisols) by combining leaching and sequential chemical extraction techniques, after the

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addition of the metals to the soil surface of the soil columns in polyvinyl chloride (PVC)tubes divided into six rings.

2. Materials and methods

2.1. Soil collection, pre-treatment and characterization of soil

Samples of Brazilian Latosols were collected from the cities of Três Marias (TM) andViçosa, Minas Gerais State. The samples, collected at the depths of 0–20 cm (horizon A)and 40–70 cm (horizon B), were air-dried, passed through a 2 mm sieve and analysed forthe determination of the physical and chemical characteristics of the soils. The soil charac-teristics were determined as follows: particle size distribution [19]; pH in water, 1:2.5 soil/water ratio [20]; organic carbon by Walkley-Black [21]; organic matter content (1.7 × %organic carbon); exchangeable Al (1 mol L−1 KCl extraction); H + Al (0.5 mol L−1 Ca(OAc)2 at pH 7.0 extraction); effective exchangeable capacity (CEC)e (Ca and Mgextracted with 1 mol L−1 KCl, Na and K with 0.05 mol L−1 HCl) [19]; potential exchange-able capacity (CEC)p calculated by the sum of the exchangeable Ca, Mg, K and Na plusthe acetate buffer (pH 7) potential acidity [22]. Table 1 summarizes the soil characteristics.

2.2. Leaching experiments

The metal leaching experiments were carried out in PVC columns separated into sevenrings (6.6 cm diameter and 7 cm length each), which were superimposed, walled with sili-con glue at the junctions and externally fastened with adhesive ribbon. The six lower ringswere loaded with soil, which was accommodated over a thin bed of glass wool betweentwo sheets of Whatman No. 42 filter paper placed at the bottom of the leaching column.The soil surface was covered with a glass wool layer to avoid disturbances on the soil infil-tration part. The tops of the columns were covered with styrofoam to minimize evaporation.Each of the five metals evaluated was applied in four leaching columns loaded with A andB horizons from two Brazilian Oxysols in three replications, a total of 60 soil columns.

Table 1. Selected physical and chemical properties of the oxisols.

Characteristics

Soil from Três Marias Soil from Viçosa

Horizon A Horizon B Horizon A Horizon B

pH in H2O (1:2.5) 4.54 4.89 4.99 4.13Organic matter content (g kg−1) 1.8 1.7 7.7 3.1Extractable Ca2+ (cmolc kg

−1) 0.45 0.78 0.46 0.43Extractable Mg2+ (cmolc kg

−1) 0.03 0.10 0.02 0.07Extractable Al3+ (cmolc kg

−1) 0.39 – 0.77 2.22Extractable K+ (mg kg−1) 18 29 26 36Extractable P (mg kg−1) 0.2 0.2 0.6 3.0H+Al (cmolc kg

−1) 3.0 2.5 4.1 11.3CECe (cmolc kg

−1) 0.92 0.95 1.32 2.81CECp (cmolc kg

−1) 3.53 3.45 4.65 11.89Sand (g kg−1) 76 79 28 31Silt (g kg−1) 5 4 7 3Clay (g kg−1) 19 19 65 66Textural class Sandy loam Clayish

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Analytical grade nitrate salts of metals (Cu, Ni, Zn, Pb and Cd) were added individuallyto the top of the columns at concentrations equivalent to the ‘intervention value for agri-culture soils’, established by Brazilian guidelines [23]. The value of intervention of eachmetal is the concentrations in soil above which there is a potential risk to human health.The doses were applied only in the upper rings on top of the soil columns. The doses were(mg kg−1): Cu = 200; Ni = 70; Zn = 450; Pb = 180; and Cd = 3.0. Table 2 shows theamounts of each metal, in mg, applied to the soils.

After the addition of the solid metal nitrates, deionized water (260 mL each) in 16 appli-cations was poured twice a week into each column at a flow of 5 mL min−1 to obtain awater addition of 1194 mm, equivalent to the mean annual rainfall of the studied region(1200 mm). The water flow was controlled by a valve coupled to a capillary tube whichwas connected to a plastic bottle placed above each column.

Leachates were collected in polyethylene terephthalate bottles and the concentrations ofCu, Ni, Zn, Pb and Cd determined by an inductively coupled plasma optical emissionspectrophotometer (ICP-OES, Perkin Elmer, model Optima 7300 DV). All plastic bottleswere cleaned for the leaching experiments.

2.3. Sequential extractions of heavy metals in the soil from the leaching columns

Sequential extraction is widely used to estimate the amounts of the metals and their distri-bution in soil fractions and to help the modelling of predicted metal bioavailability andmetal leaching. After the leaching procedures in the soil columns, soil subsamples werecollected from each PVC ring of the columns, oven-dried at 110 °C and submitted to man-ual shaking in order to obtain better homogeneity. Extractions were conducted by mechani-cal shaking in 1 g of the dried soil subsample placed in a 50 mL polycarbonate centrifugetube. The suspensions were centrifuged at 1483 × g for 8 min, and when necessary, werealso filtered through Whatman 541 filter papers. The extraction method [24] establishes sixfractions’ names to identify the forms of the metals extracted from each soil layer in thePVC rings, except for the top ring where the metal nitrate salts were added:

� Exchangeable/solution heavy metal fraction (Exc-HM): extraction with 25 mL of1 mol L−1 KNO3 for 30 min at room temperature.

� Specifically adsorbed on iron oxides and aluminium oxides heavy metal fraction(FeAl-HM): extraction of the metals from the remaining residue (Exc-HM frac-tion) with 25 mL of 0.167 mol L−1 NaH2PO4/0.03 mol L−1 NaF/0.0083 mol L−1

EDTA at pH 3.65 for 30 min in a water bath at 70 °C.� Organically complexed heavy metal fraction (OM-HM): extraction of the metals

from the remaining residue (FeAl-HM fraction) with 25 mL of 0.7 mol L−1

NaClO at pH 8.5 for 20 min in a water bath at 70 °C.

Table 2. Amounts of heavy metals added onto the top of the oxisols column.

SoilAmount of metal added (mg)

Cu Ni Zn Pb Cd

Sandyloam

402 141 905 362 6

Clayish 345 121 776 311 5

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� Al oxides occluded or highly adsorbed heavy metals fraction (AlOx-HM): extrac-tion of the metals from the remaining residue (OM-HM fraction) with 25 mL of1 mol L−1 NaOH/0.05 mol L−1 NaF/0.025 mol L−1 EDTA for 20 min in a waterbath at 80 °C.

� Fe oxides crystalline occluded or highly adsorbed heavy metals fraction (FeOx-HM): extraction of the metals from the remaining residue (AlOx-HM fraction)with 25 mL of 4.2 mol L−1 HCl/0.0375 mol L−1 ascorbic acid/0.03 mol L−1

sodium citrate for 30 min at 70 °C.� Residual heavy metal fraction (Res-HM): extraction of the metals from the

remaining residue (FeOx-HM fraction) with concentrated HNO3/HCLO4/HF for20 min at 70 °C.

The dosage of the metal concentrations in the sequentially obtained soil extracts wasperformed by inductively coupled plasma optical emission spectrometry (ICP/OES, PerkinElmer, model Optima 7300 DV).

2.4. Data analysis

For data evaluation, each metal was analysed individually within each soil and horizon forall variables recorded. Results of pH and metal concentration and quantities were com-pared during all 16 leaching procedures. Data obtained for each metal in six fractions fromsequential extraction were contrasted, comparing soils and horizons.

3. Results and discussion

3.1. Physical and chemical characteristics of the soil samples

Table 1 presents the physical and chemical parameters of soil samples. The pH determina-tion of soil is an important task, as excessive acidity or alkalinity can be detrimental. ThepH values of the soil samples were acidic and varied from 4.13 to 4.99. The sandy loamtexture soil from TM was chemically poor with low CEC value and low organic mattercontent, while the clayish soil from Viçosa (TG) showed higher CEC with the organicmatter varying from medium to high in content. Some slight variations in the characteristicbetween the horizons of the sandy loam soil were also observed and the low fertility ofthe two soils was attributed to low concentrations of phosphorus and exchangeable bases.

3.2. Heavy metal concentrations in the leachates

Figure 1 shows the amounts (in mg) of Cu, Ni, Zn, Pb and Cd accumulated in the leach-ates for each soil sample. Small amounts of metals were leached from the soils (table 3).Dijkstra et al. [25] reported the leaching of heavy metals (Cu, Ni, Zn, Pb and Cd) fromeight contaminated soils over a wide range of pH (0.4–12). They found that the leachedconcentrations of the metals were generally much lower than the total concentrations andshowed also a strong pH dependency.

The results indicated that about 10% of Cu added to the sandy loam soil was found inthe leachates, while the other metals were present at smaller percentages (table 3). Thepresence of coarse particles and low contents of soil organic matter certainly cause the rel-atively high percentage of Cu in the leachates of TM. As can also be seen from table 3,Cu was adsorbed more strongly by the clayish soil than Ni and Zn. This result can be

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associated with the greatest amounts of organic matter (table 1). Copper adsorption wasconsistent with its higher affinity for the soil organic and inorganic components as a resultof its chemical nature.

Differences in metal retention by soil can be explained in terms of electronegativity,which is higher for Cu (2.0) than for Zn (1.6) [26]. Adsorption of metals by soil isaffected not only by the chemical properties of the metal, but also by the nature of the soilcolloids [27]. The increased affinity for organic matter of Cu relative to Zn has beenreported by several workers [28–30].

The clayish soil accounted for the higher Zn percentage in the leachates in contrast toCu (table 3). This certainly was caused by the higher amounts of Zn added to the soil ascompared to the other metals.

Figure 1. Amounts of heavy metals leached from the study soils.

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Metal amounts accumulated in the leachates during the experiments varied for eachmetal and were dependent on the soil type and horizons studied (figure 1). With regard tothe leached metals, a common characteristic for most of them was the initial increment ofmetal amounts in the leachates until, at least, the third leaching step, followed by lowmetal increments. Cadmium showed small increments during the leaching procedure.

Although Cd was added in smaller amounts than Cu, Ni, Pb or Zn, it is the metal mostlikely to be transferred from soil to other environmental compartments. Cd, Cu, Ni and Pbwere potentially mostly mobile from the superficial sandy loam soil to the water environ-ment, while Zn was more strongly retained in the soil columns even after the passage of16 effluents.

In soil columns contaminated with Cu, the sandy loam soil from TM allowed a largermovement of Cu than the clayish soil from Viçosa for both studied horizons. The amountsof Cu were practically constant in the leachates after the passage of the second effluent. Inthe horizon A of the sandy loam soil from TM, Ni had higher mobility as compared to theclayish soil from Viçosa, although for the horizon B, the higher mobility was found in theclayish soil. Zinc was relatively more strongly eluted in the clayish soil columns than inthe sandy loam soil columns. It seems that the organic matter content of the superficialclayish soil retained a higher amount of Zn than did the horizon B.

3.3. Sequential extraction of heavy metals

The sequential extraction of heavy metals from soils has long been of great interest to soilscience because groundwater can be contaminated through metal leaching and metal bio-availability. Sequential extraction is an important tool in the study of the distribution ofheavy metals in soil, since it allows identifying the relationships between the chemicalforms and the various soil compartments. Total metal concentration has been used to char-acterize contaminated sites, but it does not really reflect metal bioavailability. The knowl-edge of the heavy metal distribution in specific fractions of soil can contribute to theprognosis and simulation of future events at certain locations.

Mobility and bioavailability of heavy metals in soils and sediments depend on the physi-cochemical forms in which the metal is associated with the solid. The results indicated thatmost of the heavy metals added to the columns were retained by both soils (table 3).Among the soil fractions examined in this work, the exchangeable/solution heavy metalfraction is of larger environmental and ecological concern because this fraction is mostmobile and bioavailable. While the residual fraction is not available for plant uptake, the

Table 3. Percentage of total metal amount (mg) leached from the soils in relation to the total metal amount (mg)applied in the column.

MetalSandy loam soil Clayish soil

Horizon A Horizon B Horizon A Horizon B

Cu 8.8 10.1 0.8 0.7Ni 6.7 0.5 1.9 6.9Zn 2.3 5.4 10.6 15.7Pb 4.0 0.2 0.1 NDa

Cd 0.03 0.02 0.04 0.02

aNot determined.

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other fractions examined are potentially available because a change in pH, oxidation–reduction potential or salinity of the environment can lead to liberation of the heavy metalsfrom soil [31]. Figures 2–4 show the results of the sequential chemical extraction of heavymetals from soils.

3.3.1. Copper

Copper was most mobile in the horizon A of the sandy loam soil with the organic matterand exchangeable/soluble fraction being the dominant fractions, but Cu concentrations inresidual fractions of this soil increased from the first to the last PCV ring (figure 2). Forthe horizon B, the dominant fractions were the same as the horizon A, mainly in the initialPVC rings. The residual fraction consists of minerals which hold metal within their crystalstructure and which are not expected to release the metal in solution under the conditionsnormally encountered in nature. The mobility of Cu in the horizon B of the sandy loamsoil was smaller than that of the superficial horizon, probably through the contribution ofthe higher pH value of the latter (table 1).

In the clayish soil, Cu mostly accumulated in the first and in the second PVC rings,where organic matter, Al and Fe oxides/kaolinite, and residual fractions are the predomi-nant soil fractions. The organic phase present in soil is the most important component con-trolling Cu mobility because Cu is easily adsorbed into organic matter. The strong bondingof copper in soil may be caused by the formation of stable complexes with humifiedorganic matter [32]. The possibility of such complexes forming has been investigated inseveral studies [33,34]. Copper shows a strong affinity for soil organic matter so that theorganic fraction Cu was high compared to that for other metals even though the absoluteamounts were low [35]. Humic materials cause the well-known close association of Cuand organic matter. The preference of Cu for the organic phase, as well as the importanceof the residual phase, is often observed in soils [36,37] and also in aquatic sediments [38].

The mineralogical constitution in the horizon B of the clayish soil might explain thepreference of Cu for the Fe and Al oxides/kaolinite fraction. In the case of the horizon A,the dominant phase was the organic fraction because of the higher organic matter contentin this soil.

The relatively higher Cu levels found in the exchangeable/solution fraction of the sandyloam soil as compared with the clayish soil was entirely expected since the former is com-posed of coarse particle size. The soil type and composition play an important role for heavymetal retention. In general, coarse-grained soils exhibit a lower tendency for heavy metaladsorption than fine-grained soils. The fine-grained soil fraction contains soil particles withlarge surface reactivities and large surface areas, such as clay minerals, iron and manganeseoxyhydroxides, humic acids, and others and displays enhanced adsorption properties [39].

3.3.2. Nickel

The contribution of the residual phase to the total extract of Ni in the horizon A of thesandy loam soil was significant (figure 2). Probably, some of the Ni released by the NaClOoxidation step (organic fraction) was readsorbed by refractory organic matter consisting ofparaffin-like material and resistant structural (non-humified) residues. Only the completedestruction of the matrix by a strong treatment (HNO3/HClO4/HF) would release Ni fordetermination.

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Column ring, horizon B sandy loam soil

Residual

Occluded into Fe oxides

Occluded into Al oxides

Organic matter

Fe and Ad oxides/kaolinite

Exchangeable/soluble

Figure 2. Copper and nickel concentrations in fractions of the study soils at the end of leaching experiment.

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In regard to the sandy loam soil, the largest Ni portion found in the residual fraction ofthe horizon A probably reflects the tendency for contaminant metals to become associatedwith this fraction, thus reducing the significance of the non-residual phases in metal mobil-ity and bioavailability. For the horizon B of the sandy loam soil containing Ni, larger con-centrations were found from the first to the last PVC ring. This indicated a lesser mobilityof Ni as compared with the horizon A, even though the exchangeable/soluble fraction wasof primary importance in metal bioavailability.

Nickel mobility in the horizon B of the sandy loam soil was less than that of the super-ficial horizon’s because of the contribution of the higher pH value of the latter. The mostimportant factor in Ni mobility in soil would seem to be pH, while factors such as the claycontent and the amount of hydrous Fe and Mn oxides in the soil are of secondary impor-tance [40]. Nickel mobility in the depth layer of the clayish soil was more significant thanin the surface layer because of the greater pH of the former.

Only a little Ni mobility was observed in the clayish soil, especially for horizon A. Inhorizon B, the Ni concentrations of each geochemical fraction in the different PVC ringswere practically constant. Besides the residual fraction, the larger Ni concentrations wereassociated with the exchangeable/soluble fraction in both horizons of the clayish soil. Forthe horizon A, the organic phase made a great contribution to the retention of Ni mainlyfor the first PVC rings.

3.3.3. Zinc

There was not great Zn mobility along the column rings in the sandy loam soil(figure 3). In most column rings, Zn association with this soil was dominated by theexchangeable/soluble fraction. A great portion of organic matter and clay particles werepresent in the soil from Viçosa, while the soil from TM contained relatively high sandcontent (table 1). Clays are known for their ability to remove heavy metals effectivelyby specific adsorption and cation exchange as well as metal oxyhydroxides. Soil organicmatter exhibits a large number and variety of functional groups and high CEC values,which results in enhanced heavy metal retention ability mostly by surface complexation,ion exchange and surface precipitation [39,41,42]. Zn is commonly associated with theseveral geochemical fractions of soils, but organic fraction has been most effective in Znretention [31].

In contrast to the mobility of Zn in the two horizons of the sandy loam soil, the behav-iour of Zn in the clayish soil showed that there was a greater mobility in horizon B whencompared to Zn mobility in the horizon A. Nevertheless, as in the case of the sandy loamsoil, a preference of exchangeable/soluble phase was observed. For the horizon B of theclayish soil, the distribution pattern of Zn showed that the organic phase as well the Aland Fe oxides/kaolinite phase were of primary importance in the Zn scavenging efficiency.

3.3.4. Lead

Lead was most mobile in the surface layer of the sandy loam texture soil and it was princi-pally associated with the residual fraction, followed by the exchangeable/solution fraction(figure 3). These fractions also represented a significant proportion of Pb associated withthe horizon B. The horizon B had a smaller mobility as compared to that of horizon Aand Pb practically did not reach the end of the column.

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0

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Zn c

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Column ring, horizon A very clayish soil

0

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Column ring, horizon B very clayish soil

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Column ring, horizon A sandy loam soil

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Column ring, horizon B sandy loam soil

4 5

Residual

Occluded into Fe oxides

Occluded into Al oxides

Organic matter

Fe and Ad oxides/kaolinite

Exchangeable/soluble

Figure 3. Zinc and lead concentrations in fractions of the study soils at the end of the experiment.

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Lead mobility in the clayish soil was low for both studied horizons. The largest Pbconcentrations were associated with organic ligands and Al and Fe oxides/kaolinite. Thepresence of soil organic matter plays a key role in Pb adsorption. Soil organic matter mayimmobilize Pb via specific adsorption reactions, while mobilization of Pb can also be facil-itated by its complexion with dissolved organic matter or fulvic acids [43].

The contribution of the exchangeable/soluble phase to the total extract of Pb in both soilhorizons of the clayish soil was significant in the superior rings. The residual phase alsoappears to be of importance in Pb retention in the first rings. As can be seen in figure 3,Pb was also held in lattice positions of Fe oxides in horizon A of the clayish soil. Thiswas not observed for the other rings examined as well for the Cu, Ni, Zn and Cd experi-ments. The low Pb mobility in soil has been described in the literature [44].

The occluded Al oxide fraction also contributed to Pb retention in the clayish soil. The Pbadsorption onto Al2O3 has been found to involve several mechanisms. In general, the adsorp-tion kinetic of Pb exhibits biphasic behaviour. An initial fast reaction is followed by a slowerreaction. The slow adsorption reaction is not caused by surface precipitation of Pb but per-haps by diffusion to internal sites, adsorption onto sites that have slower reaction ratesthrough low affinity and probably formation of additional adsorption sites through the slowtransformation of Al2O3 into the less-reactive solid phase. The initial fast reaction is mostlikely caused by chemical reactions on readily accessible surface sites [45].

3.3.5. Cadmium

The exchangeable/soluble and residual fractions were the predominant phases in the sandyloam soil, certainly because of the medium texture of the soil as well to the low Cd affinityfor the solid phase (figure 4). In both horizons of this soil, Cd mobility was significant but

0.00

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Cd

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Column ring, horizon A very clayish soil

0.00

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0.00

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Cd

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rtion

(m

g kg

-1)

Column ring, horizon B very clayish soil

Residual

Occluded into Fe oxides

Occluded into Al oxides

Organic matter

Fe and Ad oxides/kaolinite

Exchangeable/soluble

Figure 4. Cadmium concentrations in fractions of the study soils at the end of leaching experiment.

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only small Cd concentrations were found in the last two rings of the column. In the clayishsoil, the behaviour of Cd between the two horizons was completely different. In the horizonA, the exchangeable/soluble form was dominant and Cd mobility was observed until thefifth ring of the column. In horizon B, besides the exchangeable/soluble form, Cd was alsoassociated with the residual, occluded Al oxide, organic matter and Fe, and Al oxides/kao-linite fractions in the first ring of the column. This observation shows that Cd and Ni areserious contaminants and existed manly as exchangeable/soluble fraction.

4. Conclusions

The transference of heavy metals from solid residues and effluents to the environment is amatter of concern, since many of these materials frequently enter the soil. Knowing thedynamics of the transport of these metals through the soil profile may help to establish cri-teria for the proper disposal of residues and effluents in the soil surface. Tropical soils con-stitute a large proportion of land used for crop and food production. The high migration ofthe heavy metals in soils pointed out the danger of dumping industrial and urban wasteson soil. The aim of the present study with column leaching experiments was to evaluatethe potential risk of groundwater contamination by the excessive leaching of heavy metalsfrom tropical soils. Thus, the interactions of heavy metals with the several component min-erals and organic content of the soils was examined by the distribution and mobility ofheavy metals in different depths of the soil profiles.

The leaching experiment provided information on the mobility of Cu, Ni, Zn, Pb andCd in soils, which in turn relates to their potential bioavailability to plant uptake andhuman health risk. For the studied soils, the total amount of metals eluted during the entireexperiment (16 additions of solution in the soil profiles) was generally small, varied foreach metal examined and depended on the soil type and horizons studied. The establisheddifference of retention capacity and mobility of heavy metals for the two soil types wasalso obviously related to the difference in their granulometry and composition.

The soluble + exchangeable fraction and the organic fraction were the predominant sinksfor the heavy metals studied. The organic phase presented in soil was the most importantcomponent controlling Cu mobility because Cu is easily adsorbed into organic matter. Ofthe five studied metals, Cd was the one least retained by the soil column and it was themost likely to be transferred from soil to other environmental compartments.

Acknowledgements

We thank the Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG,CRA 1902/06) and the Conselho Nacional de desenvolvimento Científico e Tecnológico(CNPq), Brazil, for financial support.

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