Sustainable utilization of agro-industrial waste for the production of chromium tanning agent through redox synthesis for leather industry: a circular economy approach

Almost 90% of the leather produced globally uses basic chromium sulfate (BCS) as a tanning agent for permanent stabilization of collagen protein that is produced by reducing hexavalent chromium into trivalent chromium using either organic or inorganic reducing agents. Even though organic reduction using molasses paves a greenway toward BCS manufacture, the quantum of molasses available is insufficient for BCS production to meet global needs. On the other hand, India generates approximately 500 million tonnes of agricultural waste every year, majorly comprising crop residues of rice and wheat that are burned every year to battle surplus amounts of waste. Herein, we report a new method for BCS production using polysaccharide-rich wheat bran (WB) as a reducing agent, making it a facile method to tackle molasses non-availability and burning surplus crop residues. The offer level of WB required for BCS production was optimized to match BIS norms, and the prepared product was evaluated as a tanning agent in leather production. The mechanism of chromium reduction by WB studied using IR and NMR techniques revealed polysaccharide oxidation into aldehyde and carboxylic acid following a classic redox reaction. Additionally, a method for the synthesis of metal complexes of definite morphology using agro-industrial wastes as templates is also proposed in this work. The calculated product cost also confirmed that using WB as a reducing agent in BCS production is economically feasible.