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Adsorption from Brazilian soils ofCu(II) and Cd(II) using cattle manurevermicompostC.P. Jordão a , W.L. Pereira a , D.M. Carari a , R.B.A. Fernandes a ,R.M. De Almeida a & M.P.F. Fontes aa Departamento de Solos, Universidade Federal de Viçosa,36570-000, Viçosa, Minas Gerais, Brazil

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Adsorption from Brazilian soils of Cu(II) andCd(II) using cattle manure vermicompost

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

Departamento de Solos, Universidade Federal de Viçosa, 36570-000 Viçosa, Minas Gerais, Brazil

(Received 11 May 2011)

The potential of cattle manure vermicompost and Brazilian soils (whole soils and soils incubatedwith vermicompost) was assessed for adsorption of heavy metals such as Cu(II) and Cd(II) fromaqueous solutions. Experimental data have been fitted to Langmuir and Freundlich isotherms toobtain the characteristic parameters of each model, with R2 values from 0.89 to 0.99. Based on themaximum adsorption capacity obtained from the Langmuir isotherm the affinity of the studied met-als for the vermicompost and soils have been established as Cu(II) > Cd(II). The values of the sep-aration factor, RL, which has been used to predict affinity between adsorbate and adsorbent werebetween zero and 1, indicating that sorption was very favourable for Cu(II) and Cd(II) in syntheticsolution. Addition of vermicompost to soils resulted in higher distribution coefficient, Kd, as com-pared with whole soils. The thermodynamic parameter, the Gibbs energy changes, was calculatedfor each system and the negative values obtained confirm that the adsorption processes are sponta-neous. The DG� values for the substrates were between �2.630 ± 1.41 kJ mol�1 and �13.700± 1.250 kJ mol�1. Adsorption tests from multimetal systems confirm the affinity order obtained inthe individual metal tests. The adsorption capacity for Cu(II) measured in individual tests is notreduced by the presence of Cd(II). There is also desorption of Cu(II) and Cd(II) previously boundto vermicompost, whole soils and soils incubated with vermicompost by DTPA. The experimentindicates the importance of cattle manure vermicompost and oxisol amended with vermicompost inrelation to Cu(II) and Cd(II) adsorption from aqueous solution.

Keywords: Copper; Cadmium; Brazilian soil; Vermicompost; Isotherm

1. Introduction

Heavy metals cause environmental contamination through mining, metallurgical industries,agricultures and so on [1]. Heavy metals are non-biodegradable and their toxicity is slowand long-lasting [2]. Heavy metals can be accumulated in the food chain and living tis-sues, causing various diseases and disorders [3,4].

There are many methods for the removal of toxic metals from industrial effluents suchas chemical or electrochemical precipitation [5], ion exchange [6] and reverse osmosis [7].Research on various low-cost adsorbents such as activated alumina, red mud, clarifiedsludge, rice husk ash and soil has been successfully carried out to remove toxic heavymetals from aqueous solution [8–11].

*Corresponding author. Email: [email protected]

International Journal of Environmental Studies,Vol. 68, No. 5, October 2011, 719–736

International Journal of Environmental StudiesISSN 0020-7233 print: ISSN 1029-0400 online � 2011 Taylor & Francis

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The adsorption of metal ions from synthetic solution and kaolin industry wastewater oncattle manure vermicompost has been studied [12,13]. The use of vermicompost seems tobe a good alternative for reducing heavy metal contents in liquid effluents [14]. Vermicom-post can be obtained when cattle manure, together with soil, is used for an earthworm diet.The organic fraction of vermicompost represents about 50% of its weight [15].

Adsorption of metal ions to soils is significant and it affects their mobility and bioavail-ability [1]. In this study, we analysed experimental equilibrium data obtained for Cu(II)and Cd(II) adsorption on cattle manure vermicompost, whole soils and soils incubated withvermicompost. We used the Langmuir and Freundlich isotherm equations. We also con-ducted experiments including adsorption tests from multimetal systems as well as desorp-tion tests.

2. Materials and methods

2.1. Soil collection and characterisation of soil and vermicompost

The oxisols were sampled at a depth of 20–60 cm after removing the vegetation, air-driedand sieved to separate the particles smaller than 2 mm for the physical and chemical analy-ses. A clayish oxisol (TG) and a sandy oxisol (TM) were collected in the cities of Viçosaand Sete Lagoas, respectively, in Minas Gerais State, Brazil.

The soil pH was measured in deionised water (solid/solution ratio of 1:2.5) using a pHmeter. Available P and K were extracted from the soil with the Mehlich-1 extractor, whilethe organic matter content was evaluated by the Walkley-Black method [16]. ExchangeableAl, Ca and Mg were extracted with 1 mol L–1 KCl and potential acidity was extractedwith 0.5 mol L–1 pH 7.0 Ca(CH3COO)2. The total concentrations of Cu and Cd weredetermined in soil samples as described above for vermicompost. Particle size distributionin the soil was evaluated by the pipette method, using 0.1 mol L–1 NaOH solution as adispersion agent. Commercial samples of vermicompost of cattle manure, which was pro-duced in the Dom Bosco farm in the city of Viçosa (State of Minas Gerais) was used asthe adsorptive material. For the physical and chemical characterisation experiments, theraw vermicompost was air-dried for 72 h and passed through a 2 mm sieve.

The vermicompost pH was measured in deionised water (solid/solution ratio of 1:2.5)using a pH meter. Moisture content of the vermicompost was determined by the percentageloss in weight after drying the sample at 60�C for 24 h; organic matter content was mea-sured by ignition in a furnace at 550�C for 24 h and ash content after heating at 800�C for2 h [17]. The C and H contents were measured with an infrared detector and the N contentwas measured with a thermal conductivity detector.

The total concentrations of Cu and Cd were determined in vermicompost using 0.5 gportions of air-dried samples. They were digested individually at 200�C with 5 mL of65% (w/v) HNO3. A 5 mL aliquot of concentrated HClO4 (70% w/v) as well as a 5 mLaliquot of HF (40% w/v) were added and the mixtures re-evaporated to near dryness.Finally, a 5 mL aliquot of 65% (w/v) HNO3 was added. The mixture was re-evaporated tonear dryness and diluted with deionised water to 25 mL [15]. The total metal concentra-tions in the resultant solutions were measured by an inductively coupled plasma opticalemission spectrometer (ICP-OES).

The extraction of humic substances was performed as follows. The vermicompost sam-ple (1 g) was poured into a 300 mL glass-stoppered conical flask. The extractant mixture(10 mL) consisting of 0.1 mol L–1 NaOH was added to the flask. The flask was shaken by

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hand for a few minutes and then nitrogen gas was passed through the solution for 24 h, inorder to produce a stable, non-oxidised inert atmosphere. The mixture obtained was centri-fuged at 2500 rpm steady of 2500g for 8 min. The procedure was repeated three times andthe residue obtained was used for determination of carbon (humin). To the extract contain-ing humic acids and fulvic acids was added concentrated HCl sufficient to reduce the pHto less than 1. The suspension was placed in an oven at 70�C for 10 min and allowed tosettle for 12 h, and centrifuged at 3000 rpm steady of 3000g for 5 min. The carbon con-tent in the insoluble humic acid as well as in the fulvic acid fraction was determined bythe Yeomans and Bremner method [18].

Infrared spectroscopy of the vermicompost, previously dried at 60�C for 4 h, was donefrom a 100 mg sample in KBr.

2.2. Metal adsorption studies

For the incubation experiments, the soils TG and TM and the vermicompost, namelyVC, were air-dried and passed through a 2 mm sieve. The mixture of the vermicom-post with the soils TG and TM was handmade in order to obtain better homogeneity.The dose of vermicompost used in the experiment was 17.5 grammes per cubed deci-metre (35 t ha–1). The moisture content was kept close to the field capacity and thematerial maintained in incubation during 15 days before the Cu(II) and Cd(II) adsorp-tion on the substrate. After the incubation step, the mixtures obtained were air-dried forseven days.

The soil TG-amended vermicompost (TG+VC) and soil TM-amended vermicompost(TM+VC) obtained from the incubation experiments, as well as vermicompost VC, werepassed through a 0.25 mm sieve. The Langmuir and Freundlich adsorption isotherms forCu(II) and Cd(II) were obtained at constant ionic strength and at the natural pH values ofthe substrate including the vermicompost and whole soils or soils incubated with vermi-compost. For the adsorption of Cu(II) and Cd(II) individually, the experiment was con-ducted in 50 mL centrifuge tubes in which were added 1 g of the substrate and 10 mL ofa synthetic Cu(II) solution or a synthetic Cd(II) solution. Copper concentrations of 0, 2,50, 100, 150, 200, 250, 300, 330, 360 or 400 mg L�1 as CuCl2 in 0.1 mol L�1 KCl or Cdconcentrations of 0.5, 3, 6, 9, 12, 15, 17, 20, 22 or 24 mg L�1 as CdCl2 in 0.1 mol L�1

KCl were used in the adsorption experiment. The suspensions were shaken for 4 h at 25± 2�C and the pH was measured (3.90–6.34). Then, the suspensions were centrifuged andthe Cu and Cd concentrations determined in the solutions by atomic absorption spectro-photometry (AAS).

For the competitive adsorption of Cu and Cd on the substrates, when adsorption testswere performed for multimetal systems, each initial solution was equimolar for both met-als, 0.0889, 0.1333 and 0.1782 (in mol L�1). These metal concentrations were also usedin the non-competitive adsorption experiments. Adsorption of equimolar individual solu-tions as well as equimolar mixed solutions of Cu(II) and Cd(II) was evaluated like individ-ual adsorptions as described above.

The desorption of Cu and Cd previously bound to the vermicompost was conductedwith 0.005 mol L�1 DTPA (diethylenetriaminepentaacetic acid) in 0.01 mol L�1 CaCl2,buffered at pH 7.3 with 0.1 mol L�1 TEA (triethanolamine) at a soil/solution or compost/solution ratio of 1:2. The extractions were performed at room temperature with continuousagitation for 2 h [19].

Adsorption from Brazilian soils of Cu(II) and Cd(II) using cattle manure vermicompost 721

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Each adsorption or desorption experiment was replicated three times and the adsorptionor desorption of Cu(II) and Cd(II) was calculated as the difference between that added andthat in the supernatant.

2.3. Instrumentation and some relevant information

For pH determination, a DIGIMED pH meter, model DM 21 was used. The X-ray diffractionmeasurement was performed in a Panalytical instrument, model X‘Pert Pro. The IR spectros-copy was conducted in a PerkinElmer spectrophotometer, model Spectrum 1000.

Total metal concentrations in the original vermicompost and soil samples weredetermined by inductively coupled plasma optical emission spectrometry (ICP-OES),using a PerkinElmer model Optima 3300 DV instrument. The Cu(II) and Cd(II) con-centrations in the supernatants from the adsorption experiments, as well as from thesupernatants from the competition and desorption tests were determined using a Varianatomic absorption spectrophotometer (model SpectrAA-200), by direct aspiration of thesolutions into an air–acetylene flame. Background correction was used for Cd determi-nation.

The vermicompost and the original soil samples were subjected to X-ray diffractionanalysis to evaluate the mineral components. This was done at angles of 2h, from 5 to45�, using radiation tube Co-Ka (k = 1.78897 Å), at 40 kV and 20 mA.

All glassware and materials were cleaned for metal analysis. Certified analytical-gradereagents were used throughout. Blanks were run through all experiments. The calibrationblank was checked at the beginning and at the end of the analysis for each group of sam-ples to certify that the instrument calibration had not drifted. The concurrent analyses ofsamples of Standard Sediments (National Institute of Standard & Technology no. 2704)gave the following values within the range of certified values: Zn = 447; Ni = 44.2; andCu = 94.5 (in mg kg�1); Al = 6.10 and Mg = 1.22 (in %).

3. Results and discussion

3.1. Vermicompost and soil characterisation

Table 1 shows the selected physicochemical properties of the vemicompost and the soilsstudied. The X-ray diffraction spectrum of the vermicompost and soils TG and TMrevealed three major components: kaolinite, goethite and quartz. Gibbsite has been onlydetected in soil TM. Kaolinite and quartz are commonly present in tropical soils and havelow chemical activities. Thus, they do not exert a major influence on the cation retentioncapacity of the vermicompost.

The IR spectrum of the vermicompost sample presented broad bands due to superim-posed absorption signals (figure 1). The broad band centred at 3450 cm�1 can be assignedto the stretching vibration of O–H with hydrogen bonding. The intense absorption bandsat 1007 and 1101 cm�1 are characteristic of the C–O stretching in polysaccharide or poly-saccharide-like substances or even Si–O of silicate present in the sample. Low-intensityabsorption bands at 1458, 1558, and 1652 cm�1 are due to the aromatic C=C stretchingvibrations [20]. The absence of characteristic bands of COOH stretching at 1720 and 1250cm�1 could be due to its low content in the sample. The bands in the area below 900cm�1 are due to contamination. Interactions between metals and humic acids of soil takeplace with the displacement, decrease or disappearance of some bands [21].


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Table 1. Selected physicochemical properties of vermicompost and soils studied

Soil and vermicompost characteristic Soil TM Soil TG Vermicompost VC

pH in H2O (1:2.5) 4.9 4.1 6.4Moisture (%, w/w) – 40.97Organic carbon (%, w/w) 0.99 1.8 21.87Total organic matter (%, w/w) 1.20 3.11 31.18Ash (%, w/w) – – 36.49Carbon (%, w/w) – – 15.25Hydrogen (%, w/w) – – 1.13Nitrogen (%, w/w) – – 2.60Humic acid (%, w/w) – – 1.37Fulvic acid (%, w/w) – – 3.42Available P (mg dm�3) 0.20 3.0 –Available K (mg dm�3) 29 36 –Exchangeable Al (cmolc dm

�3) – 2.2 –Exchangeable Ca (cmolc dm

�3) 0.78 0.43 –Exchangeable Mg (cmolc dm

�3) 0.10 0.07 –H + Al (cmolc dm

�3) 2.5 11.3 –CECe (cmolc dm

�3) 0.95 2.8 –Total metal (in mg kg�1) – – –Cu 15.0 17.4 37.9Cd 0.30 0.40 0.57Sand (%, w/w) 77 31 –Silt (%, w/w) 4 3 –Clay (%, w/w) 19 66 –Texture sandy clayish –

Figure 1. Infrared adsorption spectrum of the vermicompost sample.


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3.2. Metal adsorption studies

The equilibrium sorption isotherm is fundamentally important in the design of sorption iso-therms. The Langmuir sorption isotherm has been successfully applied to the sorption pro-cesses of many pollutants and has been the most widely used sorption isotherm forsorption of a solute from a liquid solution. The test of data fit to the Langmuir equationby linearity of the ‘Langmuir plot’, viz. Ceq/q plotted as a function of Ceq, is a poor test offit because the plotting of Ceq against itself substantially decreases the data variability andalways results in a statistically significant correlation coefficient [22]. In our study, datawere fitted to the non-linear Langmuir and Freundlich adsorption isotherms using the soft-ware STATISTICA� (StatSoft Inc.) to estimate adsorption constants that indicate theadsorption capacity and affinity of the adsorbents.

The following equation mathematically represents the non-linear Langmuir isotherm:

qe ¼ ðqmax � b � CeÞ=½1þ ðqmaxCeÞ�

where qe is the amount of metal adsorbed per g of vermicompost (milligrams per gramme),b is a constant related to adsorption energy (litres per milligram), qmax is the maximummetal adsorption capacity on vermicompost (milligrammes per gramme) and Ce is themetal concentration at equilibrium (milligrammes per litre).

In our study, we plotted the adsorption isotherms between the amounts of heavy metalspecies removed from chloride solution per gram of the substrate (milligrammes pergramme) as a function of the equilibrium free ion concentration in the solution (milli-grammes per litre). The Langmuir constants, along with the correlation coefficients havebeen calculated from the corresponding plots (figure 2 and figure 3) for adsorption of Cu(II) and Cd(II) on soils TG and TM, soils incubated with vermicompost TG+VC and TM+VC, as well as vermicompost VC. Table 2 presents the results.

While figure 2 and figure 3 show the non-linear plot of Ce versus qe, table 2 shows theqmax and b values. The parameter qe reflects the metal affinity for the vermicompost bind-ing sites. The values of constants indicate favourable conditions for adsorption. Differentsubstrates show different capacities to adsorb heavy metals. The highest adsorption maxi-mum capacities of Cu(II) and Cd(II) calculated by the Langmuir equation using the non-linear form at 25�C for soils and incubated were found in the incubated soil TM+VC, 1.47mg g�1 and 0.41 mg g�1, respectively. This was certainly due to the addition of organicmatter (vermicompost) to the soil.

The studies of Linhares et al. [23] reported qmax values for Cd(II) in Brazilian highlyweathered tropical soils ranging from 0.136 to 1.6 mg g�1, while Wan Zuhairi [9] foundvalues between 2.36 and 25.64 mg g�1 for 29 soil samples collected from landfill sites inSouth Wales, United Kingdom.

Small b values indicated that Cu2+ ions and Cd2+ ions were bonded strongly to vermi-compost. According to the Langmuir model, adsorption occurs uniformly on active sites ofthe adsorbent and once an adsorbate occupies a site, no further adsorption can take placethere [24].

The shapes of Cu(II) and Cd(II) isotherms were of ‘L2’ type, according to Giles’s classi-fication for isotherms [24] indicating that the data have reached a maximum value, result-ing in the presence of the plateau. The L-isotherm type (or Langmuir isotherm type) is


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usually associated with ionic substrates (e.g. metal cations) sorption with weak competitionfrom the solvent molecules [25].

The Langmuir parameters can also be used to predict affinity between adsorbate andadsorbent using the dimensionless separation factor RL, which has been defined by Hallet al. [26] as:

RL ¼ 1

1þ bC0

where RL is the dimensionless separation factor, C0 the initial concentration (milligramsper litre) and b is the Langmuir constant (litres per milligram). The value of RL can beused to predict whether a sorption system is ‘favourable’ or ‘unfavourable’ in accordancewith the following criteria:

6420C

e (mg L-1)

Ce (mg L-1) Ce (mg L-1)

0

1

3

4q

e (m

g g

-1)

qe

(mg

g-1

)

3020100Ce (mg L-1)

0,0

0,4

0,8

1,2

qe

(mg

g-1

)q

e (m

g g

-1)

2416800,0

0,3

0,5

0,8

2401608000,0

0,6

1,2

1,8

(a) (b)

(c) (d)

Ce (mg L-1)

qe

(mg

g-1

)

2416800,0

0,4

0,8

1,2(e)

Figure 2. Adsorption isotherm of Langmuir (-) and Freundlich (—) for copper: (a) vermicompost VC; (b) soilTG; (c) soil TM; (d) soil amended with vermicompost TG+VC; (e) soil amended with vermicompost TM+VC; (�)experimental data.


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Separation factor, RL Type of isotherm

RL > 1 UnfavourableRL = 1 Linear0 < RL < 1 FavourableRL = 0 Irreversible

The RL values of Cu(II) and Cd(II) adsorption on vermicompost, whole soils and soilsincubated with vermicompost are shown in figure 4 and figure 5. The RL values werebetween 0 and 1 and indicated that sorption was very favourable for both metals. Figure 4

3020100

Ce (mg L-1)

-1Ce (mg L )

Ce (mg L-1)

Ce (mg L-1)

0,00

0,32

0,64

0,96

qe

(mg

g-1

)q

e (m

g g

-1)

(b) (a)

12840

0,00

0,06

0,12

0,18

(c)

48321600,00

0,06

0,12

0,18

qe

(mg

g-1

)

qe

(mg

g-1

)

qe

(mg

g-1

)

(d)

64200,00

0,06

0,12

0,18

(e)

181260Ce (mg L-1 )

0,00

0,15

0,30

0,45

Figure 3. Adsorption isotherm of Langmuir (-) and Freundlich (—) for cadmium: (a) vermicompost VC; (b) soilTG; (c) soil TM; (d) soil amended with vermicompost TG+VC; (e) soil amended with vermicompost TM+VC; (�)experimental data.


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Table2.

Sum

maryof

theisotherm

constantsandthecorrelationcoefficientsforLangm

uirandFreundlichisotherm

s

Substrate

(a)

Langm

uir

Freundlich

Cop

per

Cadmium

Cop

per

Cadmium

q max

bR2

q max

bR2

q max

bR2

q max

bR2

(mgg�

1)

(Lmg�

1)

(mgg�

1)

(Lmg�

1)

(mgg�

1)

(Lmg�

1)

(mgg�

1)

(Lmg�

1)

VC

4.24

±0.22

0.99

±0.01

0.99

0.47

±0.06

0.86

±0,01

0.98

1.97

±0.21

2.85

±0.03

0.98

0.18

±0.02

3.20

±0,02

0.97

TG

1.14

±0.16

0.14

±0.01

0.98

0.17

±0.01

0.37

±0.01

0.99

0.27

±0.03

2.42

±0,05

0.98

0.05

±0.00

2.14

±0.14

0.99

TM

1.10

±0.11

0.06

±0.00

0.99

0.19

±0.02

0.11

±0.01

0.97

0.10

±0.01

1.65

±0.12

0.98

0.04

±0.00

2.82

±0.38

0.97

TG+VC

1.39

±0.12

0.50

±0.01

0.86

0.18

±0.02

2.16

±0.02

0.96

0.77

±0.09

8.70

±0.23

0.89

0.11

±0.01

3.35

±0.10

0.99

TM+VC

1.47

±0.19

0.14

±0.01

0.96

0.41

±0.01

0.41

±0.01

0.96

0.34

±0.04

2.54

±0.28

0.97

0.14

±0.03

2.71

±0.18

0.99

a VC,verm

icom

post;TG,soilTG;TM,soilTM;TG+VC,soilTG-amendedverm

icom

post;TM+VC,soilTM-amendedverm

icom

post.


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and figure 5 also show that sorption was more favourable for the higher initial metal ionconcentration than for the lower one.

Adsorption of Cu(II) and Cd(II) to particles is presented in the form of a conditionalparticle–water distribution coefficient, Kd, mL g�1:

Kd ¼ amount of metal adsorbed at equilibriumðmgg�1Þconcentration of metal in the solution equilibriumðmgL�1Þ

(a)

0 120 240 3600.00

0 .05

0 .10

RL

RL

RL

C0 (mg L-1)

C0 (mg L-1) C0 (mg L-1)

C0 (mg L-1)

(b)

45030015000.00

0.15

0.30

(c)

30020010000.0

0.1

0.2

0.3

RL

(d)

45030015000.0

0.1

0.2

0.3

C0 (mg L-1)

RL

(e)

36024012000.00

0.08

0.16

0.24

Figure 4. Values of separation factor, RL: for the adsorption of copper by the substrate: (a) vermicompost VC;(b) soil TG; (c) soil TM; (d) soil amended with vermicompost TG+VC; (e) soil amended with vermicompost TM+VC; (j) experimental data.


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A high value of distribution coefficient is the characteristic of a good adsorbent [27].Table 3 shows the Kd values found in this study.

The distribution coefficient Kd is a suitable index for comparing sorptive capacity of dif-ferent soils or materials for any particular metal ion when measured under similar experi-mental conditions [28]. In the experiments conducted in vermicompost, the Kd value forCu(II) was 999 L g�1 and for Cd(II) it was 49 L g�1. For the soils studied, the distributioncoefficient of Cu(II) ranged from 3.82 L g�1 to 31.0 L g�1, while for Cd(II) the valuesranged from 0.34 L g�1 to 25.1 L g�1. For the soils amended with vermicompost, the Kd

values obtained varied between 0.59–66.3 L g�1 for Cu(II) and 1.54–59 L g�1 for Cd(II).

(a)

60402000.00

0.37

0.74

(b)

2416800.00

0.45

0.90

(c)

60402000.0

0.3

0.6

0.9

(d)

2416800.0

0.2

0.4

(e)

60402000.00

0.37

0.74

RL R

LR

LRL

RL

C0 (mg L-1) C0 (mg L-1)

C0 (mg L-1)

C0 (mg L-1)

C0 (mg L-1)

Figure 5. Values of separation factor, RL: for the adsorption of cadmium by the substrate: (a) vermicompost VC;(b) soil TG; (c) soil TM; (d) soil amended with vermicompost TG+VC; (e) soil amended with vermicompost TM+VC; (j) experimental data.


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Table3.

Valuesof

distributio

ncoefficient(K

d)fortheadsorptio

nof

Cu(II)andCd(II)by

verm

icom

post,soil,

andsoilam

endedverm

icom

postin

aqueoussolutio

n

Copper

VC

a

50b

999±

75c

TG

27.33

±0.6

TM

23.82

±0.3

TG+VC

508.04

±0.2

TM+VC

100

28.2

±0.1

100

302±

1950

31.0

±1.7

5015

.2±

0.1

100

66.3

±3.5

150

18.4

±0.2

150

249±

2510

018

.4±

1.5

100

14.0

±0.1

150

9.29

±0.1

200

15.6

±0.1

200

178±

1515

016

.0±

1.4

150

12.9

±1.1

250

1.38

±0.1

250

14.8

±0.9

250

202±

9.0

200

14.4

±0.9

200

13.0

±1.3

330

0.63

±0.01

300

15.7

±0.2

300

156±

1325

013

.5±

1.3

250

12.4

±1.1

360

0.65

±0.01

330

14.2

±1.1

330

52.3

±3.0

300

12.6

±0.6

300

11.7

±1.2

400

0.59

±0.01

360

14.3

±1.2

360

62.2

±2.5

330

12.6

±0.7

––

––

–40

060

.4±

4.6

360

12.2

±1.0

––

––

––

–40

012

.2±

1.2

––

––

––

–44

012

.1±

0.9

––

––

–

Cadmium

VC

0.5

49.0

±0.5

TG

0.5

2.57

±0.18

TM

325

.1±

1.2

TG+VC

632

.3±

0.3

TM+VC

0.5

13.3

±0.13

329

.0±

2.1

37.22

±0.7

91.36

±0.1

916

.8±

0.2

659

.0±

5.4

628

.3±

1.9

64.33

±0.3

121.12

±0.2

1215

.4±

0.9

1229

.8±

1.2

933

.0±

2.0

93.92

±0.2

151.37

±0.1

156.23

±0.5

1516

.0±

1.6

1227

.6±

2.1

122.61

±0.2

171.07

±0.1

175.31

±0.3

1712

.2±

1.3

1528

.1±

2.1

152.17

±0.1

200.98

±0.01

204.56

±0.3

209.28

±1.0

1726

.0±

1.5

171.82

±0.1

220.90

±0.02

223.75

±0.2

227.21

±0.5

2023

.4±

1.9

201.48

±0.1

240.91

±0.02

243.49

±0.2

246.43

±0.3

2223

.9±

2.4

221.40

±0.2

300.65

±0.05

––

304.48

±0.1

2424

.0±

1.2

241.38

±0.1

500.51

±0.02

––

403.05

±0.3

344.75

±0.3

––

600.34

±0.00

––

502.62

±0.1

441.59

±0.1

––

––

––

––

602.19

±0.2

540.81

±0.1

––

––

––

––

––

a Substrate:a V

C,verm

icom

post;TG,soilTG;TM,soilTM;TG+VC,soilTG-amendedverm

icom

post;TM+VC,soilTM-amendedverm

icom

post.1

bAdded

solutio

n,mgL�1;c K

d,Lg�

1.


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A state of equilibrium is established between the amount of cations adsorbed and theirconcentration in equilibrium solution. If the metal content of the solution is increased, theamount of adsorbed cation also increases until the maximum adsorption capacity has beenreached. In our study, 50% of the samples examined had Kd values decreased with theincrement of metal concentration in solution. This occurred because a rapid metal maxi-mum adsorption capacity was reached, yet still there was metal in solution. The other halfof the samples exhibited also a sequential decrease in Kd values when more concentratedmetal solutions were added to the substrate. In this case, however, the smallest appliedmetal concentration was not enough to saturate the adsorption sites and the Kd values wereincreased in the first moment. A further saturation of the solid phase with the increment ofthe metal concentration results in reduced Kd values and a greater amount of metal in solu-tion.

The Kd values for Cu(II) and Cd(II) adsorption on vermicompost were greater than thosefor whole soils and soils incubated with vermicompost. In general, addition of vermicom-post to soils resulted in higher Kd values as compared with whole soils. Shaheen et al.[29] determined (Cu) Kd values of different soils to evaluate their ability to retain it andcontrol its mobility and therefore availability. The distribution coefficients differed signifi-cantly between soils and ranged from 0.080 L g�1 for an acid Greek alfisol to 7.502 Lg�1 for Egyptian lacustrine entisol. Agbenin and Olojo [30] reported (Cu) Kd values from0.122 to 0.970 L g�1 for a savannah alfisol and showed that removal of organic matterfrom soil reduced the Kd values 40 times as compared with the natural soil.

The adsorption behaviour of Cu(II) and Cd(II) on the substrates can also be describedby the Freundlich adsorption isotherm [31]. The form of the Freundlich isotherm equationused in this study was:

qe ¼ Kf C1=ne

where qe is the adsorbed heavy metal per unit weight of adsorbent (milligrammes pergramme), Ce is the concentration of solute in the solution at equilibrium (milligrammes perlitre), Kf is the sorption coefficient (milligrammes per gramme) and 1/n is the sorption inten-sity. The applicability of the Freundlich isotherm was also analysed by plotting Ce versusqe. The calculated constants for the Freundlich adsorption isotherm are presented in table 2and the data were in good agreement with those for the Langmuir isotherm. The values ofthe correlation coefficients indicate that there was a strong positive relationship for the dataand that the metal/substrate sorption data follow the Langmuir and Freundlich isotherms.

The affinity of the adsorbent for a metal can be measured by the parameter Kf. The Fre-undlich isotherm constants Kf and n incorporate all factors affecting the adsorption processsuch as adsorption capacity and intensity of adsorption. For Cu(II) adsorption, values of Kf

(in milligrammes per gramme) reported in the literature [22,32,33] for different adsorbentsinclude 3.24 (tree fern), 0.029–0.06770 (soil) and 6.17 (soil). In the case of Cd(II) adsorp-tion, values of Kf of 1.02 and 0.0029–0.06770 for bagasse fly ash [34] and soil [23],respectively, have been reported. In this study, values of Kf for Cu(II) and Cd(II) adsorp-tion in soil ranged from 0.10 to 0.77 and from 0.04 to 0.14, respectively. Values of nbetween 1 and 10 (i.e. 1/n less than 1) represent a favourable adsorption [35].

Vermicompost and soil exhibited good adsorption capacity and the acquired adsorptiondata were reasonably fitted to the Freundlich model as well as to the Langmuir model.While Langmuir isotherms presume to reach an adsorption plateau where no further


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adsorption can occur, Freundlich isotherms presume that adsorption does not reach a maxi-mum, but remains increasing slightly.

Isotherms have been used to determine thermodynamic parameters of the process, forinstance, free energy change [34]. The Gibbs energy changes (DG�) for the adsorption pro-cess was obtained at 25�C using the following equation:

�G0 ¼ �RT lnK

where, R is the ideal gas constant (8.314 J mol�1 K�1) and K is the equilibrium constantat temperature T in K obtained from the Langmuir isotherms. The equilibrium constant canbe represented as follows:

K ¼ Cs=Ce

where Cs is the metal concentration on the adsorbent at equilibrium in milligrammes per litreand Ce is the equilibrium concentration of metal in solution in milligrammes per litre [36].

The DG� values were –13.700 ± 1.250 kJ mol�1, –6.670 ± 1.72, –6.130 ± 2.71, –6.250± 1.95 and –7.660 ± 2.83 kJ mol�1 for Cu(II) adsorption on VC, TG, TM, TG+VC and TM+VC samples, respectively, while the DG� values for Cd(II) adsorption on these substrateswere –7.780 ± 1.55, –2.630 ± 1.41, –2.820 ± 1.50, –5.470 ± 1.53 and –6.500 ± 1.40 kJ mol�1,respectively. The Gibbs energy changes indicate the degree of spontaneity of the adsorptionprocess, where more negative values reflect a more energetically favourable adsorption pro-cess. It is noted that DG� values up to –20 kJ mol�1 are consistent with electrostatic interac-tion between sorption sites and the metal ion (physical adsorption), while DG� values morenegative than –40 kJ mol�1 involve charge sharing or transfer from the adsorbent surface tothe metal ion to form a coordinate bond (chemical adsorption) [37].

The DG� values obtained in this study for vermicompost confirm the feasibility of thisadsorbent and spontaneity of the adsorption and show that physical adsorption is the pre-dominant mechanism in the sorption process for all substrates studied. As for the DG� val-ues obtained in this study and in view of favourable characteristics of vermicompost toadsorb heavy metals [38], it is obvious that vermicompost was a good adsorbent forremoving Cu(II) and Cd(II) from synthetic solutions.

As shown in table 4 there is a clear predominance of Cu(II) over Cd(II) for the adsorp-tion on soil binding sites. Table 4 also shows that the affinity of soil for Cu(II) and Cd(II)adsorption is evident, and a general trend. There was an increase in metal adsorption by soilwhen individual metal tests were used, compared with the treatment of simultaneous metaltests, certainly due to the greater number of available sites in the case of individual tests.

The adsorption values were very similar in individual tests and in competitive tests atthe low metal concentrations. These results were due to a great availability of reactivegroups at low metal concentrations. But there was competition between Cu(II) and Cd(II)for the available sites of the substrate at higher metal concentrations. In general, Cu(II)was adsorbed preferentially since the concentration of Cu(II) adsorbed was very similar tothe initial metal concentration. On the contrary, adsorption of Cu(II) and Cd(II) on vermi-compost was not severely depressed by increasing their concentrations in the solutions.This was attributed to the great number of available sites in the vermicompost.

Table 4 shows that the soils incubated with vermicompost TG+VC and TM+VC had aslight increase in Cu(II) and Cd(II) adsorption as compared with the whole soils TG and


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Table4.

Adsorption(lmol

g�1)forindividual

andcompetitiveexperimentsof

equimolar

solutio

nsof

copper

andcadm

ium

forthedifferentsamples

Amou

ntof

metal

addedby

thesubstrate,

lmol

g�1

Amou

ntof

adsorbed

metal

bythesubstrate,

lmol

g�1

VC

TG

TM

TG+VC

TM+VC

Individual

adsorptio

nCu

0.88

90.889

±0.000

0.88

7±

0.00

10.794±

0.019

0.88

7±

0.003

0.888±

0.005

1.33

31.323

±0.002

1.31

4±

0.02

41.048±

0.008

1.32

5±

0.005

1.329

±0.011

1.78

21.770

±0.005

1.73

7±

0.01

31.261±

0.015

1.77

3±

0.012

1.771±

0.008

Cd

0.88

90.889

±0.000

0.67

8±

0.00

50.815±

0.008

0.85

2±

0.003

0.837±

0.013

1.33

31.332

±0.001

0.95

3±

0.00

20.840±

0.025

1.26

5±

0.022

1.237±

0.051

1.78

21.773

±0.003

1.118±

0.01

60.848±

0.012

1.46

7±

0.095

1.247±

0.019

Com

petitiveadsorptio

nCu

0.88

90.890

±0.003

0.88

9±

0.00

50.806±

0.030

0.88

6±

0.009

0.887±

0.004

1.33

31.323

±0.027

1.30

9±

0.00

11.005±

0.028

1.32

6±

0.015

1.319±

0.051

1.78

21.768

±0.008

1.69

0±

0.01

61.223±

0.007

1.77

2±

0.004

1.759±

0.022

Cd

0.88

90.889

±0.002

0.74

1±

0.01

50.810±

0.019

0.81

5±

0.023

0.815±

0.039

1.33

31.333

±0.001

0.90

7±

0.05

20.907±

0.021

1.17

6±

0.006

0.996±

0.016

1.78

21.770

±0.020

0.59

8±

0.01

20.933±

0.014

1.26

6±

0.017

1.149±

0.008


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TM, respectively. This was certainly due to the enrichment of the soils with organic matterpresent in the vermicompost.

The preferential adsorption of Cu(II) with respect to Cd(II) by the substrates wasexpected. Copper has greater specific binding affinity than cadmium due to the differencesbetween these metals as regards their physical and chemical characteristics such as capac-ity for hydroxylation, ionic potential and electronegativity [39].

3.3. Extraction of Cu(II) and Cd(II) previously bound to vermicompost, whole soils andsoils incubated with vermicompost

The percentages of Cu extracted from vermicompost and the previously Cu-loaded soilswere 57.06, 29.80, 39.94, 60.57 and 73.97 for VC, TG, TM, TG+VC and TM+VC sam-ples, respectively, while for Cd(II) extraction, the values were 74.06, 64.61, 79.66, 89.00and 85.35, respectively. Thus, the Cu(II) extraction was smaller than for Cd(II) and theaddition of vermicompost to soil increased the amount of extractable metal. These resultsare in keeping with observation of preferential adsorption of Cu with respect to Cd[10,39].

Conclusions

In this study, addition of vermicompost to soil increased adsorption of Cu(II) and Cd(II).Copper was more strongly adsorbed than Cd in vermicompost, whole soils and soils incu-bated with vermicompost by effect of both the physicochemical properties of the two met-als and the increased affinity of Cu(II) for organic matter. The Cu(II) and Cd(II)adsorption isotherms corresponding to vermicompost, whole soils and soils incubated withvermicompost are L2 type. The clayish soil TG had higher maximum capacities for Cu(II)and Cd(II) than the sandy soil TM. The addition of vermicompost to soils resulted inhigher Kd values as compared with whole soils.

The values of the correlation coefficients (from 0.89 to 0.99) indicated that there was astrong positive relationship for the data and that the metal/substrate sorption data followthe Langmuir and Freundlich adsorption isotherms. The equilibrium isotherms were usedfor assessing the maximum adsorption capacity of the metals onto vermicompost and soilsand the values obtained varied between 1.10–4.24 mg g�1 for Cu(II) and 0.17–0.47 mgg�1 for Cd(II). The DG� values obtained in this study for vermicompost and soils confirmthe feasibility of these adsorbents and the spontaneity of the adsorption. The physicaladsorption was the predominant mechanism in the sorption process.

The distribution coefficient (Kd) values for Cu(II) and Cd(II) adsorption on vermicom-post were greater than those for whole soils and soils incubated with vermicompost.

Competition of Cu(II) and Cd(II) for the available sites of the substrates occurred athigher metal concentrations, while the adsorption values were very similar in individualtests and in competitive tests at low metal concentrations. Extraction of Cu(II) occurredmuch less readily than extraction of Cd(II) and the addition of vermicompost to soilincreased the amount of extractable metal.

The data obtained do not give a complete picture of using vermicompost and oxisolamended with vermicompost in the treatment of contaminated water. Nevertheless, suchvalues as are determined do show that vermicompost can be suitable to remove Cu(II) andCd(II) from aqueous solution.


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Acknowledgements

We thank the Brazilian National Research Council (CNPq, Brazil) for financial support.

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