ADSORPTION, DESORPTION AND SOLUBILITY RELATIONSHIPS OF LEAD AND CADMIUM IN SOME ALKALINE SOILS

BIJAY SINGH and G. S. SEKHON (Department of 'soi ls . Punjab Argiculrural University. Lud l i ina , India

S u m mar) 1

In some alkaline soils of Punjab (India) the adsorption, desorption and solubility relationships of lead and cadmium were investigated and the results analysed by the Langmuir equation.

Both the metals are retained in soils by adsorption o n mineral inter l ies and interaction with organic matter and calcium carbonate. At high concentrations, these probably precipitate as hydroxides. Sequential desorption of PI) and Cd nith 1 MKC1 and 0.05 MCu(CH, COO), provided a measure of their exchangeable and chelated form.

In trodt t c tio n W H I L E some information exists on the uptake of lead and cadmium by plants (Shroeder and Balassa, 1963; Motto e t al., 1970; Lagerwerff e t al. 1973; Haghiri, 1974), little is known about the chemistry of the metals in soils.

Adsorption of Pb and Cd has been studied mostly in acid soils (Lagerwerff and Brower, 1972, 1973; Nelson, 1972). although Levi- Minzi e t al. ( 1976), recently observed that adsorption of Cd by some Italian alkaline soils followed the Langmuir adsorption equation. There also exists very little information (Santillan-Medrano and Jurinak, 1975) on desorption and precipitation of Pb and Cd. The present paper reports on the adsorption, desorption and precipitation of Pb and Cd in some alkaline soils of Punjab (India).

Materials and methods Samples of surface soils were obtained from seven different locations

in the Punjab. Pertinent characteristics of the soils are reported in Table 1 .

Adsorption Triplicate 0.5 g soil samples were placed in 30 ml centrifuge tubes,

equilibrated for 24 h with 20 ml of 1 M C H , C O O N H ~ ( ~ H 7.0), centri- fuged, the supernatant liquid was discarded and the process repeated. The NHi-saturated soils were washed three times with de-ionised distilled water before equilibration for 48 h (including initial shaking for an hour) with 20 ml portions of metal ion solutions i n 0.1 MKCI (pH 7.0). Potassium chloride was added to maintain near Journal of Soil Science, 1977.28, 271-275.

212 BIJAY SINGH AND G . S. SEKHON

TABLE 1 Description of soils

Soils

Organic Calcium Sand' Clay' carbon' carbonate' CEC % % % % meqJ100g PH'

Gurdaspur sl (Haplustalf) 55.6 10.1 0.61 0.90 6.96 7.5 Jatwan cl (Haplaquept) 38.1 31.2 1.26 22.20 18.23 8.8 Jodhan sl (Ustochrept) 59.8 10.4 0.69 11.95 6.09 9.1 Kanganwal scl (Ustochrept) 55.2 24.6 0.75 13.10 12.18 9.1 Karnana I (Haplustalf) 49.2 20.2 0.76 12.05 10.44 8.6 Langrian Is (Calciorthid) 87.3 5.5 0.33 1.05 3.83 8.9 Ludhiana Is (Ustipsamment) 82.6 8.9 0.34 0.40 4.35 8.6

'Clay and sand as < 0.002 m m and > 0.02 m m respectively. 'Walkley and Black (1934). 'Puri (1949). ' In soil : water ratio of 1 : 2.

constant ionic strength in the equilibrium solution which initially contained 5, 10, 20, 50 and 100ppm of Pb o r Cd. After frequent shaking during the equilibration period, the samples were centrifuged and washed. Centrifugates were collected in 100 ml volumetric flasks and were analysed for Pb o r Cd ion concentration following colori- metric procedures (AOAC, 1970).

Desorption To desorb the exchangeable Pb o r Cd, the soil-metal systems from

adsorption studies were shaken for one hour with 20 ml 1 MKCl (pH 7.0), equilibrated for 22 h and again shaken for one hour. Centrifuga- tion, decantation and washing followed. The centrifugates were collected in 100 ml volumetric flasks and analysed for Pb and Cd.

To remove any chelated fractions, the soil-metal systems obtained after desorbing exchangeable fraction were equilibrated with 20 ml portions of 0.05 M C U ( C H , C O O ) ~ ( ~ H 5.6) for 48 h. The decanted solutions were analysed as for the exchangeable fraction.

Precipitation Soil samples after equilibration with 100 ppm Pb and Cd solutions

were suspended in 20 ml 0.1 M KCl, shaken for one hour and equilibrated for 24 h. The pH of the suspensions was determined immediately after shaking for a further hour. Centrifugation and analyses of clear extracts for Pb and Cd followed. Lead and cadmium potentials @M2+ + 2pOH -1 were computed from these data. The activity of Pb2+ and Cd2+ ions were calculated by using the extended form of the Debye-Huckel equation.

LEAD AND CADMIUM ADSORPTION 273

Results and discussion Adsorption data were analysed according t o the Langmuir adsorption

( 1 ) where C, =equilibrium solution concentration of Pb or Cd in meq/l, S = amount of Pb or Cd adsorbed in meq/100 g soil, Sm = adsorption maximum defined by the limit of S as C, approaches inifinity, and Ka = constant related to the bonding energy of the metal ion with soil (1 lmeq). The statistical significance of the correlation coefficient for C,/S vs. C, was the criterion by which the fit of adsorption data to Eq. 1 was tested. The fit was found t o be significant for Cd adsorption (r=O.986** to 0.999**) but for Pb, the range of correlation coefficients for different soils increased from 0.934*-0.959** to 0.973**-0.999**, when points obtained by equilibrating the soils with highest Pb concentrations were excluded. This indicates that at high concentrations, mechanisms other than adsorption are also involved in retention of Pb in alkaline soils. Lead potentials for different soils (Table 2) are less than the pKsP of Pb(OH)2, confirming that Pb is precipitated at the highest concentration used. In contrast, only cadmium potentials (Table 3) of highly calcareous and alkaline soils (Jodhan, Kanganwal, Karnana) are less than the potential associated with Cd(OH)2. Work of Santillan-Medrano and Jurinak (1975) supports these data.

Desorption of adsorbed Pb and Cd by 1 M KCl and 0.05 M CU(CH,COO)~ was studied by a Langmuir type of equation (Sinha et al. 1975) written in its linear form as

equation, which in its linear form is given by

Ce /S = 1 /K, Sm + Ce /Sm

D,/R = 1 /Kd R, + De/Rm

TABLE 2 Langmuir coefficients for adsorption of lead and its desorption by 1 M KCl and 0.05 M Cu(CH3COO), and leadpotentialsat thehighestpb concentration used (subscripts I and 2 refer to desorption by 1 M KCl and 0.05 M Cu(CH3 COO)* respectively).

Gurdaspur 1.4 1.2 0.7 4.3 46.6 29.2 14.8 Jatwan 4.6 3.7 2.2 37.4 209.3 20.2 13.2 Jodhan 2.3 1.6 1 . 1 3 .1 89.1 10.6 13.6 Kanganwal 3.3 2.4 1 .3 15.3 85.9 18.0 13.6 Karnana 2.3 2.1 1 .3 9.9 80.8 20.6 14.1 Langrian 1.9 1.2 0.6 2.5 13.5 15.6 14.7 Ludhiana 2.1 1.4 0.8 2.7 51.8 60.6 14.7

Note: pKSp of Pb(OH), = 14.9 (pure systems).

274 BIJAY SINGH A N D G . S. SEKHON

TABLE 3 Luiigmitir coc.f;ficierits for adsorptioii of cadmium and its desorption b y 1 M KCI and 0.05 M C U ( C H ~ C O O ) ~ and cadmium potentials at the highest Cd coilcentration used (subscripts I and 2 refer to desorption by 1 M KCl and 0.05 M Cu(CH3 COO)2 respectively)

Gurdaspur 1.5 I .2 0.5 1 .o 18.5 10.3 14.2 Ja t\va n 4.4 4.2 2.2 5.3 37 .7 42.9 14.6 Jodhan I .7 1.1 0.6 0.9 34.8 3 .3 11.9 Kanganwal 2.5 1.8 0.6 1.7 10.7 12.6 12.2 Karnana 1.7 1.2 0.4 1 .3 10.7 9.2 12.7 Langrian 1.3 0.9 0.3 1 . 1 12.9 17.9 15.1 Ludhiana 1.4 1.2 0.4 0.9 17.4 16.2 15.3

Note: pKSp of Cd(OH), = 13.6 (pure systems)

where R represents the amount of metal which remains adsorbed at equilibrium (meq/100 g soil), D, is the equilibrium metal ion con- centration (meqll) determined by the amount of metal desorbed, R, gives the maximum amount of metal retained and Kd is a constant related to mobility of solid phase Pb o r Cd in soil ( I /meq) . The fit of desorption data t o Eq. 3 was tested in a manner identical to Eq. 1 .

Langmuir coefficients (R, and Kd ) for desorption of exchangeable and chelated fractions are given in Tables 2 and 3 for Pb and Cd respectively. The values of R, indicate that KCI extracted relatively lower amounts of sorbed Pb and Cd than the complexing agent, copper acetate.

The effect of different soil characteristics on adsorption of Pb and Cd by soil was studied by correlating them with the adsorption maxima. The S, values of both Pb and Cd correlated significantly with CEC, clay, organic carbon and calcium carbonate content of soils. This is in accord with the work of John (1972) with cadmium The soil pH did not correlate with S, (Pb) o r S, (Cd).

Partial correlation coefficients for S, (Cd) versus CEC and organic carbon gave evidence that the retaining power of organic matter for Cd is mainly due to its CEC. Levi-Minzi etal. (1976) came to the same conclusion with some Italian alkaline soils. With Pb, various nonsig- nificant first and second order partial correlation coefficients indicated that clay, organic matter and calcium carbonate probably interact with one another in adsorbing Pb in these soils.

A ckrioivledgemerzt The first author is grateful t o the Indian Council of Agricultural

Research, New Delhi, for the award of a Senior Research Fellowship during the course of this investigation.

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(Received 2 7 Ju1.v I 9 76)