Mass Transfer from Metallic Contaminants during a Three-Week Thermophilic Composting Experiment

ABSTRACT

The trace metal content of municipal solid waste (MSW) compost represents a potential environmental concern. In this research, the effect of the composting environment on the corrosion of typical metal contaminant types found in the organic fraction of MSW was investigated. Selection of contaminants was based upon their trace metal content and their prevalence in MSW. For instance, galvanized nails and Zn-plated screws both contain the trace metal zinc on a steel base, stainless steel flat washers contain chromium, copper wires and brass screws have a high copper content, and light bulb tails contain tin and lead content in the foot contact. A synthetic feedstock created to have a low trace metal content was used. Selected contaminants were exposed to a three-week thermophilic composting process, under either low (4.5. to 5.5) or high (8 to 9) pH conditions. The weight loss of each contaminant was determined using a corrosion weight loss method. Characterization of corrosion conditions and products was accomplished using a scanning electron microscope with an energy dispersive x-ray spectrometer attachment (SEM-EDX), an x-ray diffractometer, a gas chromatograph, and an ion chromatograph. The metal composition of each contaminant was determined through SEM-EDX analysis. All contaminants showed higher weight losses in the low pH environment, which suggests that the corrosion rate was accelerated under acidic composting conditions. A high mass transfer rate of zinc from the galvanized nails and Zn-plated screws was observed, which suggests that these might be important contributors to the zinc content in MSW composts. Copper wires and brass screws exhibited moderate mass transfer rates of copper and zinc. Low amounts of trace metals were transferred from the stainless steel flat washers and light bulb tail contacts.

ACKNOWLEDGMENTS

This project was financially supported by the Natural Sciences and Engineering Research Council of Canada and the Edmonton Waste Management Centre of Excellence. The authors wish to thank Kelvin Lien and Jela Burkus for analytical assistance and Ryan Hinton for assistance with the development of the low pH feedstock.