Selective scavenging of copper, zinc, lead, and arsenic by iron and manganese oxyhydroxide coatings on plankton in lakes polluted with mine and smelter wastes: results of energy dispersive X-ray micro-analysis

Abstract Energy dispersive X-ray micro-analyses of the remains of individual organisms in plankton samples, together with more conventional analyses of sediments and water, were performed for the purpose of investigating the accumulation of heavy metals by plankton in three Canadian Shield lakes polluted with Cu, Zn, Cd, Pb, As, and SO2−4 from a base metal mine and smelter. The results showed that appreciable quantities of Cu and Zn were bound to crustacean exoskeletons, loricae of the rotifer Keratella, and loricae of the euglenophytcs Trachelomonas and Strombomonas; euglenophyte loricae also contained As and Pb, but an associated chrysophyte cyst, though richer in Cu, Zn, and Pb, had no detectable As. In all specimens except the chrysophyte cyst, the trace elements were associated with Fe and Mn, implying that Fe and Mn oxyhydroxides deposited on the organisms' armour had scavenged them from the water. Euglenophyte loricae were richest in Fe and Mn and associated trace elements. Moreover, the inferred FeOOH and MnOOH phases discriminated between different elements, Zn being selectively bound by MnOOH whereas Cu and As were bound by FeOOH. Adsorbed Cu and Zn were associated with S to a significant degree only where the sediments were poorest in sulfide, suggesting that the metals were complexed by thiol compounds secreted by organisms as a defense against metal toxicity wherever environmental sultide levels were too low to provide protection. The Fe, Mn, Zn, and Cu concentrations and Mn Fe and Zn Cu ratios of plankton hard parts decreased with the Eh of the sediments. Dissolved Fe and Mn released into the water column from anoxic but sulfide-poor sediments probably precipitated as FeOOH and MnOOH on surfaces of organisms wherever the water hud sufficient dissolved O2, the MnOOH FeOOH ratio and hence the Zn Cu ratio increasing with O2 concentration; but strongly reducing conditions and H2S production in sediments interfered with the formation of both FeOOH and MnOOH. Fe precipitating as FeS in the sediments whilst Mn remained in solution. Sulfides inhibited bio-accumulation of Cu and Zn by hindering the formation of FeOOH and MnOOH coatings and by suppressing the release of dissolved Cu and Zn (Cu in particular) from the sediments. The Cu, Zn, Fe. and Mn concentrations and Zn Cu and Mn Fe ratios of plankton hard parts and the concentrations of bio-available (DTPA-extractable) Cu and Zn in the sediments were lowest in a lake whose sediments had anomalously low Eh and high sultide levels caused by sewage effluents, algal blooms, and SO2−4, even though these sediments had the highest total Cu and Zn content. Thus, bio-accumulation of metals depended on the bio-availability, not the total supply, of the metals. The results of the research are consistent with the following generalisations: (1) FeOOH and MnOOH are commonly deposited on exposed surfaces of a wide range of plankton hard parts in different freshwater environments: (2) the FeOOH and MnOOH coatings scavenge heavy metals and metalloids from the water and are largely responsible for the passive accumulation of these elements by plankton: (3) FeOOH and MnOOH have different trace element affinities, thus affecting the proportions as well as the quantities of the elements bound to the plankton surfaces: (4) the accumulation of metals by plankton hard parts is controlled by environmental variables such as Eh and sulfide and O2 levels, as these factors regulate the precipitation of FeOOH and MnOOH surface films, determine the MnOOH FeOOH ratio, and control the speciation, solubility and bio-availability of metals stored in bottom sediments, whereas total metal concentrations in sediments may have no appreciable effect on the bio-accumulation of the metals; and (6) heavy metal bio-availability is limited mainly by sulfides generated in sediments: where sulfides are lacking, thiol compounds are released into the water by the plankton to prevent toxic effects of the metals. These conclusions have significant implications for the cycling and ecological effects of toxic and nutritional trace elements and for the monitoring, abatement, and prevention of water pollution due to mining, smelting, and other industrial activities.