Abstract
This study investigated and compared several improvement strategies to increase the yield and quality of exopolymeric substances (EPS) from Bacillus cereus. This includes co-culturing of B. cereus with Trichoderma asperellum, cultivation in media with metal (Zn) stress and supplementation with molasses. EPS is subsequently extracted from these different cultures and subjected to characterization and metal removal tests in single-metal systems (Cu, Pb, Zn, Cd, Cr). Results indicate that co-cultivation of B. cereus and T. asperellum produced EPS which have attributes differing from single cultivation. These changes were detected via functional group changes using Fourier-Transform Infrared Spectroscopy, as well as the increase in carbohydrate and protein content. However, the interaction of these two microbes were merely additive and did not result in improved EPS yield nor the subsequent metal removal efficacy in comparison to single cultivation (control). By contrast, supplementation of Zn (metal stress with 50 mg L− 1 Zn) improved EPS quality and metal removal, but decreased EPS yield. The application of 1% molasses was the only strategy demonstrating high yield and efficient metal removal. EPS quality and yield (0.45 mg mL− 1) and metal removal efficacy (Cu: 58%, Pb: 98%, Zn: 83%, Cd: 73%, Cr: 96%) were improved significantly. This study showed that among the three improvement strategies (co-cultivation, metal stress, molasses), supplementation with molasses was the most effective as it improved both yield and quality of EPS significantly, suggesting that this approach may be adopted for future production of bulk EPS for up-scaling of wastewater treatment.
Highlights
Metals are commonly found in effluents from various industries and their rampant discharge into waterways pollute the environment [1, 2]
3.1 Characterisation of exopolymeric substances (EPS) produced from co-cultivation, metal stress and molasses 3.1.1 Co-cultivation The EPS collected from single cultures of B. cereus and T. asperellum, and from the co-culture were all white in colour
The EPS yield from co-cultivation were significantly higher compared to the yield derived from single culture of B. cereus (0.21 mg mL− 1)
Summary
Metals are commonly found in effluents from various industries and their rampant discharge into waterways pollute the environment [1, 2]. Metals do not biodegrade but accumulate over time, resulting in metal poisoning of aquatic plants, animals and humans [3,4,5]. Metal poisoning in humans causes interference to protein folding and enzyme regulation [6], which leads to vascular diseases, kidney damage, and cancer [7, 8]. Removal of metals from the environment is critical. Several approaches have been adopted with physicochemical techniques such as ion exchange and chemical precipitation as the most typical techniques applied. They are costly and produce toxic wastes [7]. Bio-based techniques are preferred as they are cheaper and more environmentally friendly
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