Use of Industrial Food Waste for Chromium Removal from Tannery Wastewater

Tannery wastewater requires an adequate treatment for chromium removal before discharge, to prevent environmental pollution. Chromium sulphate, in fact, is the most used compound to stabilise animal hides, and therefore high concentrations of Cr(III) are found in spent tanning baths. To front the problem precipitation processes, based on lime addition, are frequently adopted, because of their low cost. Nonetheless, due to the colloidal form of precipitates, these processes have to be carried out with great attention to reach required performances. An interesting alternative to precipitation processes is represented by adsorption of chromium on low cost-materials, especially if chromium can be recovered from produced sludges, regenerating the adsorbent, and reducing sludge toxicity. Among others, chitosan, a carbohydrate obtained from deacetilation of chitin, the second polysaccharide for diffusion in the environment, seems to be very attractive. Previous studies, in fact, have shown that it has a good sequestrating ability towards many heavy metals, including chromium, and can be added during treatment as component of seafood shells, instead of being dosed as pure chemical product. Starting from these considerations it is presented an experimental study, whose aim is to better understand the advantages of chitosan use for tannery wastewater treatment. The study includes a preliminary investigation on the main mechanisms responsible for chromium removal in presence of chitosan, discerning between adsorption on polymeric groups of this polysaccharide, and formation of insoluble chromium hydroxide, caused by pH increase induced by chitosan addition. Process efficiency is also quantified, and treatment optimisation is proposed. This part of the study, carried out with synthetic wastewater to exclude any effects related to compounds other than chromium, allows to recognize adsorption as the main process causing chromium removal, and to assess the good performances related to chitosan as sequestrating agent. Successive investigations, carried out using real wastewater, concern, instead, the possibility of chromium recovery from produced sludge, and the possibility of process development using seafood shells instead of commercial chitosan. Obtained results, illustrated and discussed in the paper, show that chromium recovery from sludge is possible at very low pH values, and that shrimp shells use increases chromium removal efficiency, due to the development of precipitation processes together with adsorption ones