Étude de l'absorption des phosphates, azote, chrome et cadmium par trois algues vertes isolées à partir d'effluents urbains

La capacité de déphosphatation et de dénitrification ainsi que la sensibilité et l'accumulation de deux métaux lourds (Cr et Cd) de Scenedesmus quadricauda, Chlorella pyrenoidosa et Chlamydomonas reinhardtii isolées des eaux usées de la ville de Fès, ont été étudiées dans les conditions de laboratoire. Les résultats ont montré que S. quadricauda présente les meilleurs taux de réduction des phosphates (83%), et d'ammonium (75%). L'étude de la sensibilité vis à vis du chrome et du cadmium des trois espèces, a montré qu'au delà d'une concentration de 1 ppm, la croissance se trouve réduite et que S. quadricauda présente une sensibilité plus faible que les deux autres espèces. Les résultats des expériences d'absorption conduites avec du chrome VI marqué au51 Cr et du cadmium marqué au109 Cd ont montré que les quantités de chrome et du cadmium prélevées par S. quadricauda dépassent celles des deux autres espèces. L'utilisation potentielle des trois espèces dans le traitement des effluents urbains est discutée à la lumière des résultats de ces expériences.Wastewater discharge is an environmental problem, especially in developing countries. Biological wastewater treatment seems to be an alternative for these countries due to its low cost. Algae are used in such wastewater treatment systems in many parts of the world. In addition to preventing environmental damage, the reuse of treated wastewater can be an alternative water supply in arid regions and during drought periods.In this paper, three algal (Chlamydomonas reinhardtii, Chlorella pyrenoïdosa and Scenedesmus quadricauda) species were isolated from an urban effluent collected from a Mediterranean city (Fez, Morocco, situated between 4° to 7° W and 33° to 35° N) and their ability to remove phosphate and nitrogen was tested under laboratory conditions. The toxicity and uptake of chromium and cadmium were also studied.Fez city wastewater was sampled monthly during one year (2000) at three main stations located along the Boukhrareb stream which crosses the city. The following parameters were measured according to methods described by RODIER J. (1996): Chemical oxygen demand, COD; Biological oxygen demand, BOD; NO3 -; NH4 +; PO43 -; electrical conductivity (EC) and organic matter. Heavy metals (Cr, Fe and Zn) were measured by INAA (Instrumental Thermal Neutron Activation Analysis) in wastewater samples dried on Teflon sheets at 70°C.The collected samples were incubated under controlled conditions and species of alga were identified, isolated, and then transferred to fresh culture medium. Chlamydomonas reinhardtii, Chlorella pyrenoïdosa and Scenedesmus quadricauda were chosen for these experiments because of their abundance in wastewater over the year and for their capacities for biomass production. The removal of nutrients from aqueous media was studied by using monospecific cultures. Algae were in the exponential growth phase when the experiments were started and BG11 culture medium was used under standard growth conditions. Initial concentrations of phosphate and nitrogen were 2.24 mg×L-1 (PO43 -), 12.84 mg×L-1 (NO3 -) and 0.26 mg×L-1 (N-NH4 +). The concentrations decreased during the experimental period, and degrees of removal ranged from 83 to 75% for phosphate, from 85 to 77% for nitrogen and from 52 to 75% for ammonium.To evaluate the sensitivity of the algal cells to heavy metals, the algal strains were exposed for one week to chromium as K2Cr2O7 and cadmium as CdCl2 at concentrations of 0.1, 0.5, 1, 1.5 and 2 ppm (mg×L-1) of Cr or Cd. Culture growth was monitored by microscopic counting of algal cells to determine quantitative biomass changes. Results demonstrated that growth inhibition occurred at 1 ppm Cr and Cd, whereas levels of 1.5 and 2 ppm of both metals were lethal to the cultures, but no effect on growth was seen in algae exposed to 0.1 ppm of Cd or 0.1 ppm of Cr. Scenedesmus quadricauda was more resistant than Chlamydomonas reinhardtii and Chlorella pyrenoïdosa to chromium and cadmium.Radiotracers (51Cr and 109Cd) were used to trace the biosorption of chromium and cadmium by the three algal strains. Cr(VI) as K2 Cr2O7 and Cd as CdCl2 were added to the algal suspension culture to give concentrations of 1, 1.5 and 2 ppm Cd and Cr respectively. At various stages during growth of the unialgal cultures, samples were removed, a pulse of51 Cr or109 Cd was added, and the uptake rates of Cr(VI) and Cd were determined. To evaluate the amount of heavy metals removed by the algal cells, microscopic cell counting was carried out. All the experiments were conducted under the above-mentioned conditions for each concentration studied, the measurements were repeated three times and the mean values from the three replicates were calculated. The results showed that chromium and cadmium removal by algae was dependent on the metal concentration in the culture medium. The quantity of metal removed also varied with algal species, where 20 µg, 23 µg and 35 µg of Cd were accumulated by 106 cells of Chlorella, Chlamydomonas and Scenedesmus respectively at a metal concentration of 2 ppm. About 25 µg of Cr were accumulated by 106 cells of Chlorella and Chlamydomonas whereas, 106 cells of Scenedesmus accumulated 32 µg of Cr at a concentration of 2 ppm.The experimental data presented here demonstrated that there was not a great difference in the uptake capabilities between the three algal strains, and reaffirmed the potential of algae in the purification of metal-contaminated wastewater. This wastewater treatment system has many advantages and it is an efficient way to significantly reduce the release of nutrients (e.g., phosphorus, nitrogen, ammonia) and heavy metals (e.g., chromium and cadmium)