Uptake of Cd(II) Ions by Cadmium Sulphide from Aqueous Solution

Cadmium sulphide was prepared and characterized in the laboratory. The average particle diameter was found to be 42.6 μm and the surface area was determined to be 190±5 m²g^-1. The uptake of Cd(II) onto cadmium sulphide was studied as a function of pH (1-10), concentration (10^-4-10^-1 M) of hydrochloric, nitric, perchloric and sulphuric acids, shaking time (1-30 min), concentration of cadmium (3.02 × 10^-4-1.77 × 10^-3 M), amount of solid cadmium sulphide (10-300 mg) and of temperature (293-323 K). Maximum sorption (R_d = 3667 cm³ g^-1, 97.8%) of cadmium (2.16×10^-4M) was observed from 0.01 M sulphuric acid solution, after equilibrating sorbate with the sorbent for five minutes. The sorption data were evaluated and followed by Langmuir, Freundlich and D-R isotherms with correlation factors ≥ 0.99. Langmuir isotherm yields the value of Q = 0.15±0.03 mmole g^-1 and of b = (7.95±0.8)× 10⁴ dm³ mole^-1. The values of C_m = 0.58±0.05 mmole g^-1 and of β= -0.002792±0.000105 kJ² mole^-2 and of E = 13.8±0.26 kJ mole^-1 have been computed from Dubinin-Radushkevich(D-R) isotherm. Freundlich isotherm gives the value of maximum sorption capacity of 4.2±0.94 mmole g^-1 and of 1/n = 0.36±0.02. The variation of equilibrium constant, K_c, with temperature has shown the value of ΔH = -39.9±5.9 kJ mole^-1, ΔS = -93±19 J mole^-1 and of ΔG = -12.7 kJ mole^-1 at 293 K with a correlation factor (r) of 0.9593. All the cations and anions tested, viz. Tl(I), Cu(II), Zn(II), Fe(III), EDTA, thiourea, Hg(II), oxalate and nitrite, tend to reduce the uptake significantly (≤44%). The negative value of ΔH indicates exothermic nature of sorption. The selectivity quotients of different metal ions investigated gave higher values for trivalent Dy, Sm, Pr, Er, Tm, Eu, Fe and divalent Zn with respect to Cd(II) indicating their possible separation from Cd(II), Ag(I) and As(III) which have shown higher affinity for the sorbent.