Designing a process of kaolin bleaching in an oxalic acid enriched medium by Aspergillus niger cultivated on biodiesel-derived waste composed of glycerol and fatty acids

Kaolin has many industrial uses, especially in the manufacture of paper, ceramics and porcelain. The commercial value of this raw material largely depends on its brightness. However, owing to the natural impurities (primarily iron oxides), much of the kaolin is not applicable for industrial uses. In this work, the process of kaolin bleaching involved an oxalic-acid-enriched, fermented medium from the cultivation of Aspergillus niger XP, the carbon and energy source being biodiesel-derived waste composed of glycerol and fatty acids. This is a novel method of utilizing a waste by-product of the biofuel industry. Oxalic acid was produced during submerged batch culture in a bioreactor. Obtained via this route, the oxalic acid was used as the leaching agent. The central composite rotatable experimental design was used to optimize the process conditions for kaolin bleaching with the aim of achieving the highest possible brightness index and the highest possible iron oxide removal. The study had produced the following major findings: unpurified waste composed of glycerol and fatty acids from a biodiesel manufacturing plant is an effective substrate for oxalic acid biosynthesis; under the conditions optimized via the route shown above, bleaching in the oxalic-acid-enriched culture liquid improves the brightness index and consequently adds value to the kaolin.

Research Highlights
► Proposed new method for the valorization of waste glycerin from biodiesel production.
► Waste glycerin is an efficient substrate for oxalic acid biosynthesis by A. niger.
► Kaolin bleaching conditions optimized by the response surface method.
► The fermented medium was effectively used for the bleaching of kaolin.
► The bleaching process upgraded the kaolin brightness index from 74% to 79.2%.