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  CdIIformed  because  CdIIhas  a  comparatively  low  affinity  for  oxygen  donor  ligands. These results indicated that for reliable speciation calculations of CdIIand ZnIIin environmental systems, the accuracy of thermodynamic data is crucial. Discharge of effluent into the river was significant in speciation of CdIIand ZnII. 

Key words: Model, inorganic, speciation, ligand, complex