A case study of microwave processing of metal hydroxide sediment sludge from printed circuit board manufacturing wash water

The large quantity of wash water used in the electroplating and etching pro cess in the manufacturing of printed circuit boards (PCBs) contains a high level of heavy metal ions (Cu (++), Zn (++), Ni (++), Cr (+++), Pb (++)). T hese potentially toxic ions are removed from the wash water effluent throug h a polyelectrolyte flocculation and hydroxide precipitation process during which a hydroxide sediment sludge rich in metal ions and polymers is gener ated. This sediment sludge possesses some unique characteristics and proper ties in terms of composition, fine particle size distribution, high specifi c surface area; and a tendency to agglomerate after drying. Direct disposal of this classified "special waste" (Department of Environment of Northern Ireland, The Special Waste Regulations, Northern Ireland, 1998) at landfill sites may cause serious soil and underground water pollution through a gra dual ionic leaching process. This gaper describes an experimental investiga tion, exploratory in nature, which employs microwave radiation for detoxifi cation of the sediment sludge through microwave heating, drying and metal i on immobilization within the sediment solids. The effectiveness of microwav e assisted binding and immobilization of the metal ions within the sediment solids was studied in conjunction with an evaluation of microwave energy e fficiency in comparison to the more conventional convective healing and dry ing processes. Given a sufficient amount of microwave radiation, leaching o f Cu2+ and Pb2+ was reduced by 2700% and 1080%, respectively, over a period of 12 weeks, and further leaching was not delectable within six months at simulated local landfill aqueous conditions. This paper also attempts, thro ugh experimental observation, to add to the very limited understanding of t he complex interactions and binding of free metal ions with the polymeric m aterials and metal hydroxides under the influence of an electromagnetic fie ld. The high specific surface of the sediment solids and their adsorption p roperties were further explored and characterized in a study of adsorption of reactive dyes by the microwave processed solids. (C) 2000 Elsevier Scien ce Ltd. All rights reserved.