Long-term behavior of a portland cement-electroplating sludge waste form in presence of copper nitrate

Abstract

The microchemistry and microstructure of a waste form containing a synthetic electroplating sludge in portland cement (OPC) in the presence of various amounts (2%, 5%, and 8%) of copper nitrate were studied over a period of eight years. The electroplating sludge, dewatered to 25% solids, contained 86.2 mg g −1 of Ni, 84.1 mg g −1 of Cr, 18.8 mg g −1 of Cd, and 0.137 mg g −1 of Hg. The OPC to sludge ratio was 0.3:1. A waste form without copper nitrate was prepared as control. The microstructure and microchemistry were studied by scanning electron microscopy, energy dispersive X-ray spectrometry, X-ray diffractometry, thermal analysis and Fourier-transform infrared spectroscopy. Increasing amounts of copper nitrate effected significant changes in crystallinity, porosity and phase chemistry. The principal copper-bearing phase in the presence of copper nitrate was CuO · 3H 2O, whose appearance was both time- and concentration-dependent. Some amount of copper was also trapped in the newly formed crystalline calcium silicates. At the same copper nitrate level, subtle changes in the microchemistry occurred over time, but the microstructure remained qualitatively unchanged. Some of the sludge heavy metal phases disappeared with increasing copper nitrate concentration but others were not affected.