The interaction of contaminants, such as lead (Pb), with minerals and other aquifer materials controls the movement of these contaminants in groundwater. Adsorption/ precipitation are assumed to control the solubility of inorganic compounds. The stronger these interactions, the lesser will be the downwards movement of contaminant. Adsorption/ precipitation are often controlled by the nature of the cations, the nature of the adsorbent, and the specific geochemical conditions. In soil, Pb is thought to be mostly bound to the solid phase and, therefore, insoluble. Similar to other heavy metals, Pb reaching the soil may interact with various soil constituents, and eventually affect its solubility and mobility. The present study deals with the adsorption of Pb²⁺, in an acidic soil, representing Typic Dystrudepts, of Himachal Pradesh, North-west India.  

A fine loamy mixed dystrudepts soil of Palampur, Himachal Pradesh, north west India was collected to study the Pb adsorption by it.  The soil has pH 6.1 and EC 0.08 dS m^-1 (1:2 Soil:Water), cation exchange capacity 23.3 cmol (+) Kg^-1 and organic carbon 9.1 g Kg^-1. The soil contained 65% sand, 13% silt and 22% clay. To study Pb adsorption, 2g of soil (in duplicate) was shaken with 20 ml of 0.01 M Ca(NO₃)₂ solution containing seven different lead concentrations (40 µg ml^-1, 80 µg ml^-1, 100 µg ml^-1, 120 µg ml^-1, 140 µg ml^-1, 160 µg ml^-1 and 200 µg ml^-1) for two hours followed by equilibration for 24 hours at 298 ⁰K. The samples were then centrifuged; the supernatant was decanted and analyzed for Pb on GBC flame atomic absorption spectrophotometer (AAS). Lead adsorption was computed from the difference in initial and final concentrations. Lead adsorption was also studied at 318 ⁰K.  Langmuir adsorption isotherms were derived from Pb adsorption data and adsorption parameters adsorption maxima ‘b' and bonding energy coefficient ‘k' were calculated. The value of thermodynamic equilibrium constant, K^o for adsorption reaction can be defined as K^o = a_s/a_e = g_s C_s/ g_e C_e, where a_s denotes activity of adsorbed metal, a_e activity of metal in equilibrium solution, C_s milligrams of metal adsorbed per litre of solution in contact with the adsorbent surfaces, C_e milligrams of solute per litre of solution in equilibrium solution, g_s is the activity coefficient of the sorbed metals and  g_e represents the activity coefficient of metal in equilibrium solution.  Since at lower concentration the activity coefficient approaches unity, the equation gets reduced to K^o = C_s/ C_e. The values of K^o were obtained by plotting ln(C_s/ C_e ) vs. C_s and extrapolating to zero C_s . The standard enthalpy (DH⁰) was obtained from integrated form of Vant Hoff equation:  

ln K₂^o/K₁^o= DH^o / R .[1/T₁ -1/T₂].

The Pb sorption data at 298 ⁰K and 318 ⁰K were well described by Langmuir adsorption isotherm (Figure 1). The conformity of the lead adsorption data to the linear form of the Langmuir equation was indicated by the highly significant correlation coefficients (r² = 0.97 to 0.99) derived from data pairs of C_e/(x/m) and C_e.  With the increase in temperature from 298 ⁰K to 318 ⁰K, the Langmuir adsorption maxima (b) remained unchanged, whereas, the bonding energy value ‘k' increased from 0.15 to 0.50 cm³ µg^-1. The heat released or absorbed during adsorption of Pb is differential molar heat of adsorption or Enthalpy of adsorption, ΔH_ads. Heat of adsorption provides stability of adsorbed species, strength of adsorbate-substrate bonds and thermodynamic driving forces for surface reactions. The values of ΔH (between 298 and 318 ⁰K) computed using Vant Hoff equation  varied from 5.09 to 1.09 kcal mole^-1 for a constant surface, Ø, of 650 to 1800 µg Pb g^-1 soil. Figure 2 shows a plot of ΔH values as a function of of Pb adsorption. The positive values of differential isosteric heat of adsorption (ΔH) on soil between 298 and 318 ⁰K elucidate that adsorption of Pb is an endothermic process which leads to chemisorption or precipitation of Pb compounds on the surface of soil matrix. It is conclusively evident from this investigation that the surface properties of soils help in retarding the downwards movement of Pb in soil profile and thus preventing the contamination of groundwater. Visual minteq programme was used to compute aqueous species of Pb  in soil solution suspension of 0.01 M Ca(NO₃)₂ after equilibration for 24 hours containing 0.114 mM total Pb (II), 0.15 mM CO₃^2- and 0.50 mM SO₄^2-. .Mole fraction diagram for the dominant aqueous species of Pb was drawn (Figure 3).  Figure shows that, in the pH range of 4-7, the dominant aqueous Pb species include Pb²⁺(aq), PbNO₃⁺ and PbSO₄⁰(aq). Where as in neutral to alkaline pH range, the dominant aqueous species of Pb were PbOH⁺, PbCO₃⁰ , Pb₃OH₄⁺² and Pb(OH)₂. At its original pH 6.1, the per cent distribution of different lead species in fine Loamy mixed typic dystrudepts soil solution suspension was Pb²⁺ 80.3%, PbNO₃⁺ 12.2%, PbSO₄ (aq) 3.3%, PbOH⁺ 1.6%, Pb(NO₃)₂⁰ (aq) 0.29%, PbCO₃⁰ 0.40% and PbHCO₃⁺ 1.9%. It can be concluded that depending upon the soil properties, the charged species of Pb could interact with the surface constituents while the ion pairs having zero charge could contaminate the groundwater because of their free downward infiltration.