Wastewater toxicity of tannin- versus chromium-based leather tanneries in Marrakesh, Morocco

The toxicity of leather tanning wastewater from a traditional tannery (TT), based on vegetable tannin (VT) was compared to wastewater from a tannery combining the use of chromium-based tanning (CT) with VT-based tanning operations. Wastewater samples were collected in Marrakesh, Morocco, from a TT and a CT plant, and from some sampling points in Marrakesh urban sewer, and the VT content was measured in wastewater samples. A set of bioassays were utilized testing wastewater toxicity in sea urchin (Paracentrotus lividus) embryos and sperm, Daphnia magna, and marine microalgae (Dunaliella tertiolecta). Toxicity endpoints included: a) developmental defects, sperm fertilization success and offspring damage in sea urchins; b) D. magna immobilization, and c) algal growth rate. Toxicity tests on TT and CT effluents (TTE and CTE) were run at dilutions ranging 0.1 to 2% (sea urchins and algae) or up to 12% in D. magna. Parallel bioassays were run on VT extract (VTE) at nominal tannin concentrations ranging 0.1 to 10 mg/L. The results consistently showed a higher toxicity of CTE compared to TTE. The effects in sea urchin and algal bioassays showed monotonic concentration-related trends for CTE toxicity, whereas TTE displayed a trend suggesting a hormetic effect at TTE dilutions of 0.1 to 0.3%, followed by toxic effects at TTE levels 1%. The same trend was observed for VTE, consistent with a prevailing role of tannin in TTE-associated effects. The relatively low wastewater toxicity of VT-based tanneries may prompt a renewed interest in utilizing this leather tanning process.