THE SPECIATION OF CADMIUM AND ZINC FROM PULP AND PAPER MILL EFFLUENT WITH INORGANIC LIGANDS

Abstract

The effluents from Webuye pulp and paper mill were analysed chemically using AAS to obtain the stoichiometric concentrations of the major anions and cations. However, this does not indicate how the components are distributed amongst possible species. The speciation calculation was performed using the available equilibrium constants for all the metal – ligand and proton – ligand species identified. The stoichiometric concentrations were used to construct a pH dependent model with corrected IUPAC stability constants using the SPECIES computer program. pH was measured with pH meter previously calibrated with pH buffer, alkalinity was determined by titration using phenolphthalein indicator and 0.010 M HCl, chloride was analyzed by titration using potassium chromate indicator and standardized 0.014 N mercuric (II) nitrate solution. The equilibrium distribution calculations showed that inorganic speciation behaviour was of three types; (a) Cd speciation was dominated by free Cd2+(aq) with 62.68% distribution. Bicarbonato complexes could only form to significant levels at high [HCO3-]T and high ionic strength. (b) Chloro-complexes of Cd that formed were CdCl+, CdCl2(aq) and CdCl3-. Zn formed ZnCl+, ZnCl2(aq), ZnCl3- and ZnCl42-. All these complexes were weak. (c) No hydroxo-complex of CdII formed because CdII has a comparatively low affinity for oxygen donor ligands. These results indicated that for reliable speciation calculations of CdII and ZnII in environmental systems, the accuracy of thermodynamic data is crucial. Discharge of effluent into the river was significant in speciation of CdII and ZnII.