Microencapsulation of Acidithiobacillus thiooxidans by spray drying using biopolymers as wall materials: A potential alternative for its application in the mining industry

Abstract

Acidithiobacillus thiooxidans is a microorganism of great interest in bio-hydrometallurgical processes, as it is potentially capable of treating complex, low-grade (purity) sulfur minerals. However, there are concerns in the metallurgical industry regarding the microbial manipulation due to problems arising related to cellular integrity, metabolic activities, genetic stability and maintainability of a continuous generation of microbial cultures. In this work, a spray drying (SD) process (inlet temperature: 150 °C, feed flow: 1.2 L/h, and atomization rate: 27,500 rpm) was adopted to encapsulate Acidithiobacillus thiooxidans using different biopolymers as wall materials. The physicochemical properties of the powders obtained by SD reveal water activity (aw) below 0.55 with less than 10% moisture, which ensures better stability of the powders during storage with a trapping efficiency above 80%. Morphology studies by scanning electron microscopy (SEM) of the microparticles showed defined surfaces with no collapsing structures, quasi-modal size particles, low viscosity of the reconstituted powders and fast release profiles. Calorimetric analysis confirmed the viability of A. thiooxidans to withstand the SD process. By infrared analysis, the same peaks of the functional groups corresponding to the biopolymers after spray drying were observed, which evidences the microorganism encapsulation (whose signal disappeared after SD). The presence of A. thiooxidans in the microcapsules was confirmed by Transmission Electron Microscopy (TEM). Finally, viability was verified by respiratory assessment of the bacterium rendering high survival rate (1.4 × 109 Cells/mL) after the spray drying process.