Fractionnement et mobilité des métaux lourds dans un sol en amont des eaux usées de tanneries Fractionation and mobility of heavy metals in soil receiving tannery waste waters

Résumé

La migration et la spéciation de Cr, de Cu et de Pb sont étudiées sur le sol d'une colonne alimentée par les eaux usées de tanneries. La localisation des métaux est évaluée à l'aide du schéma d'extractions sélectives préconisé par Legret et al. (1988). Les résultats ont montré que l'horizon superficiel renferme des teneurs en métaux très élevées. Parmi les métaux étudiés, le Cr s'avère retenu essentiellement dans la phase résiduelle mais il se présente aussi sous des formes plus mobiles. Le Cu est principalement associé avec la phase résiduelle et organique. Le Pb est lié aux phases acido-soluble et réductible. De ce fait, l'épandage des eaux usées de tanneries, sans aucun traitement préalable, peut être une source de contamination des sols et des eaux souterraines, notamment pour le Cr.

Mots-clés:

• Métaux lourds,
• sol,
• spéciation,
• migration,
• tannerie,
• eaux usées

Abstract

In Morocco, the town of Marrakesh, as in other areas of the country, is subject to multiple daily aggressions generated by industrial pollution, in particular from tanneries. The tannery waste water is discharged directly into the aquatic and terrestrial ecosystems without any preliminary processing. In order to evaluate the danger that can be generated by tannery effluents in the receiving ecosystems, and to study the ecotoxicological and environmental impact resulting from that chemical treatment of these effluents, we determined, before and after treatment, the inhibitory effect of the two most polluted effluents originating from manufactured leather: liming-deliming and chromium tanning.

The purpose of our investigation was thus to evaluate the acute toxicity of the chromium tanning and epilage-coat waste waters before and after chemical treatment. The invertebrate Daphnia pulex was used for the toxicity test and the toxicity due to these industrial waste waters was evaluated by regression analysis. The processing carried out on the chromium tanning waste water consisted of precipitation with sodium carbonate at different pH values. The treatment of liming-deliming wastewaters consisted of an elimination and recovery of the sulphide compounds without carrying out their degradation.

The chromium level was determined by atomic absorption spectrophotometry with graphite furnace atomization. The results of the toxicity test showed that the investigated chromium tanning and liming-deliming effluents presented were toxic, with respective IC50-24h values of 0.15 and 3.36 (expressed as dilution ratios). This toxicity could be explained by high levels of chromium, sulphides and organic matter in these waste waters. It is however difficult to correlate the IC50 values obtained with those of the physical-chemical parameters. The tested effluents are very complex matrices. On the other hand, we noticed a notable difference between these two effluents. The mineral tanning effluent is contaminated with chromium and organic matter, whereas the second is especially rich in organic matter and sulphide.

The total chromium concentration corresponding to the IC50-24h value of the raw chromium tanning effluent is lower than that which would be expected on the basis on the known toxicity of K₂Cr₂O₇, showing the toxicity of raw chromium waste waters is due not only to chromium, but also to other chemicals used during the manufacturing process of leather.

After processing by chemical precipitation, the IC50-24h values increased from 0.15 to 26.6 and from 3.36 to 11.1 respectively, for the chromium tanning and the epilage-coat effluents. Consequently, these treated sewages can be classified as low toxicity wastes. The comparison of the results is very difficult considering that the test conditions and the physicochemical and heavy metal characteristics of the effluents vary enormously over time, without forgetting the physicochemical composition of the experimental solution used by standard OECD (fresh water) and the standard ISO (synthetic water). From where necessity to vary tests for determining well the degree of an effluent toxicity which the effects appear differently at various levels trophic.

The diminution observed in the toxicity of the effluents is linked to a reduction of suspended matter (95 %), COD (55 %), Cr (90 %) and sulphides (50 %). Hence, this treatment presents at least two advantages. The first is environmental as just mentioned. The second is economic since chromium can be recycled for reuse in leather tanning. The results suggest that the recovered chromium is similar to commercial tanning chromium. These results also show that the fixing of chromium does not depend on the nature of the treated skin. In fact, a technical and economic analysis showed that proposed treatment can be economically beneficial (3112,5 $ per 1000 tons of skins).

Keywords:

• Tannery,
• chromium,
• toxicity,
• chemical treatment,
• water reuse,
• financial analysis,
• Daphnia test