Treatment of tannery unhairing wastewater using carbon dioxide and zinc cations for greenhouse gas capture, pollution removal and water recycling

The aim of this study is to treat the tannery unhairing wastewaters (WW) issued from an important leather tanning plant, which is a large environmental threat and a severe industrial issue. High levels of pH, chemical oxygen demand (COD) mainly as alkaline proteins, conductivity, suspended solids and sulfide are the main characteristics of these highly polluted wastewaters. This study is based on the coupling of CO2 neutralization with Zn2+ precipitation. The use of CO2 leads to the neutralization of the WW alkalinities, and the precipitation of Ca2+ and COD to CaCO3 and solid proteins respectively, while Zn2+ leads to the precipitation of sulfide as ZnS avoiding any H2S gas evolution. The originality of this paper comes from the simultaneous capture of greenhouse gas, the removal of WW pollution, and the recycling of treated water. The pollutants removal efficiency is evident and can be seen through the decrease of pH from 13 to 6.5, a decrease in the conductivity from 8000 to 2000 mu S, and in COD from 20000 to 80000 to 3000 mg O-2 L-1, as well as obtaining pristine final water for recycling. In addition to the simplicity and cost-effectiveness of these results, the kinetics of the CO2 and Zn2+ reactions is instantaneous, contrary to the conventional biological slow process and the catalytic sulfide oxidation reaction. CO2 and Zn2+ have never been studied as a treatment method for tannery WW. This will open promising perspectives for industrial applications.