| Abstract: | The adsorption of heavy metals [cadmium (Cd 2+), cobalt (Co2+), nickel (Ni2+), zinc (Zn2+), and lead (Pb)] and calcium (Ca2+) on humic acid and silica were investigated to understand the adsorptive selectivities of heavy metals on the constituents of soil. The experiments for the adsorption of Cd and Pb were carried out in a 0.1 mol L?1 (M) sodium nitrate (NaNO3) background solution, whereas those for the other metals were done in a 0.1 M sodium chloride (NaCl) solution. The adsorptive affinities of the metal ions on the humic acid and silica were ranked by the intrinsic surface complexation constants [K m 1(int)] that were calculated approximately from the adsorptive data using a constant capacitance model. The log [K m 1(int)] values of the metals were in the order Zn2+(?2.29) > Cd2+(?2.41) > Co2+(?2.74) > Ni2+(?2.92) ?> Ca2+ (?3.33) for the humic acid and Zn2+(?4.23) > Cd2+(?4.49) > Ni2+(?4.51) ? Co2+ (?5.99) > Ca2+(?6.37) for silica. |
