The effectiveness of algal biofilms in the removal of copper, zinc, and cadmium from freshwater systems
Date
1989
Authors
Ros, Antoinette
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Abstract
Experiments were carried out in 1985, to determine whether algal biofilms were efficient in the removal of heavy metals from lake water. Acrylic plates were colonized in a field laboratory for one month by organisms from the Humpback Lake Reservoir (Vancouver Island, B.C.). These colonized plates were then subjected to a mixture of copper, cadmium, and zinc at four different concentrations in pulse-dose experiments. Mixtures of metals were at concentrations which corresponded to lx, 2x, and 5x those allowed for raw public drinking water. Depletion of metal from the solution and uptake of metal by the biofilm were measured by flame atomic absorption spectrophotometry, and the difference was determined as the daily depletion of metal from the solution. These experiments were repeated in parallel for slightly acidified conditions (0.5 pH point less).
Average daily depletion for the three mixtures ranged from 0.13, through 0.35, to 0.91 mg Cu/L and 5.26, 22.12, and 36.55 mg Zn/L under neutral conditions. Under acidified conditions, average daily depletion values were 0.21, 0.43, and 0.34 mg Cu/Land 8.16, 26.68, and 45.01 mg Zn/L for the three mixture. When these values were corrected for uptake by the exposed areas of the tanks, depletion values ranged from 0.12, 0.27, and 0.63 mg Cu/L and 6.47, 23.11 and 42.48 mg Zn/L for the three mixtures. Under acidified conditions depletion values were 0.11, 0.385, and 0.748 mg Cu/L, and 6.272, 24.670, and 36.930 mg Zn/L for the three mixtures It was not possible to determine whether there was any depletion of cadmium, because concentrations in the input solutions were below the detection levels of the methods used.
Average metal concentrations in the biomass ranged from 0.001 (control), 0.011, 0.033, and 0.053 mg Cu/mg Biomass (dry weight) and 0.003 (control), 0.018, 0.021, and 0.028 mg Zn/mg Biomass (dry weight) for the mixtures under neutral conditions. Under acidified conditions uptake values were 0.001 (control), 0.006, 0.010, and 0.014 mg Cu/mg Biomass (d.w.), and 0.003 (control), 0.005, 0.006, and 0.009 mg Zn/mg Biomass (d.w.) When compared to the exogenous concentration, copper was concentrated at factors of 2.lxl04 - 3.3x104, and zinc at factors l.lxl03 - 3. 7xl03 under neutral conditions. Under acidified conditions these factors ranged from 5. 7xl03 - l.3x104, and 3.5xl02 - 6.0xl03 for copper and zinc respectively. There was no measurable magnification of cadmium.
Copper depletion from solution values closely related to copper uptake by the algal biomass values under neutral conditions (0.18 vs. 1.26, 2.61 vs. 3.65 and 10.32 vs. 6.63 mg copper for depletion vs uptake respectively). This relationship held to a lesser extent for copper under acidified conditions. The relationship did not hold for zinc under neutral or acidified conditions. Total zinc depletion was much higher than total zinc uptake.
The algal species involved in the metal uptake ranged from a diverse mixture, with Synedra ulna, Tabellaria fenestrata, and Achnanthes minutissima being the most abundant during July - August, to a mix of Achnanthes minutissima, Bulbochaeta insignis, Mougeotia sp., and Dinobryon sp. in August - September, to an almost monoculture of Achnanthes minutissima during October - November. It was concluded that although bioconcentration factors may be high, algae may not be effective in the clean-up of water bodies unless the total algal biomass is tailored to the exogenous concentration, or the contact between the metalcontaining waters and the algae is prolonged via a long stream, for example. Furthermore, acidification by as little as 0.5 of a pH unit will severely decrease the ability of algae to incorporate metals.