Role of colloids in the physical speciation of metals in the dispersion plume of a major municipal effluent
The physical speciation of metals contained in municipal effluents is significantly influenced by the physical and chemical conditions of the receiving water. Waters in the immediate area of an effluent outfall are rich in organic matter; metals are largely complexed and the abundance of colloids can modify metal bioavailability and bioaccumulation. The distributions of aluminum (Al), cadmium (Cd), copper (Cu), iron (Fe), manganese (Mn), silver (Ag) and zinc (Zn) were determined in the dissolved (< 0.45 μm), colloidal (< 0.45 μm and > 10 kDa) and permeable fractions (ultrafiltered < 10 kDa) at different points within the dispersion plume of the major effluent from the Montreal treatment plant. Concentrations of colloidal and permeable metals were typically high near the effluent outfall. Because of the treatment process used by the City of Montreal, this effluent is a major source of Fe and more than 70% of dissolved Fe is present in colloidal form. Of the metals studied, Ag was most associated with colloids near the outfall, followed by Cu. Colloids found in the so-called dissolved or filterable phase can influence the transport and fate of discharged metals in different ways. The colloidal proportion, however, declines rapidly during the mixing of wastewater with receiving water, which seems to suggest that this fraction, by its lower abundance, could have limited influence on the long-range transport of metals released by the effluent. Our results provide information on the role of colloids in determining the geochemical fate of metals contained in wastewater once they are released to the receiving environment. The study highlights the importance of physical speciation in the assessment of the environmental impact of urban effluents on discharge areas.