Exogenous stimulants enhance the efficacy of remediation of Cr(Ⅵ)-contaminated water by Bacillus sp.
Microbial remediation of hexavalent chromium Cr(Ⅵ)contamination in soil and water has been extensively studied to date in both laboratory and field settings.Challenges arise due to competition among indigenous microorganisms and the nutrient-deficient conditions of the sites,adversely affecting the colonization of exogenous chromium(Cr)-reducing microbes.To address this issue,this study embarked on screening low-cost and non-secondary pollution nitrogen/carbon/mineral/ion-based microbial stimulants.We investigated the impact of various stimulant application methods(non-embedded and embedded)on the efficacy of reduction of Cr(Ⅵ)by Bacillus sp.and we characterized the Bacillus sp.surface using scanning electron microscopy-energy dispersive spectroscopy(SEM-EDS).The results indicate that separate addition of sugarcane powder(carbon source),soybean meal(nitrogen source),calcium ion Ca(Ⅱ)(ion type),and montmorillonite(mineral type)significantly increased the efficiency of Bacillus sp.to reduce Cr(Ⅵ).Further single-factor multi-level experiments determined their optimal dosages as 10 g∙L-1,10 g∙L-1,10 mmol∙L-1,and 8 g∙L-1,respectively.On the 8th day,compared to the bare bacteria,the addition of these four stimulants increased the reduction of Cr(Ⅵ)by 17.48,15.56,11.70,and 18.84 percent points,respectively.Building on the single-factor experiments,sugarcane powder and montmorillonite,the two most effective stimulants,were chosen for a response surface methodology to establish the optimal dosage of a combined stimulant(sugarcane powder+montmorillonite)with the bacteria.The results revealed an optimal dosage of 8 g∙L-1 for sugarcane powder,6 g∙L-1 for montmorillonite,and 0.3%(V/V)for the bacterial agent.After 8 days of cultivation,compared to the bare bacteria,the addition of the combined stimulant increased the reduction of Cr(Ⅵ)by 32.53 percent points,which was significantly more than the reduction by the single stimulant groups.Considering that direct addition of stimulants into the environment can be utilized by indigenous microorganisms,resulting in poor long-term effectiveness for stimulating exogenous microbial chromium reduction,this study further explored the long-term stimulatory effect of embedded slow-release stimulants on the reduction of Cr(Ⅵ)by Bacillus sp.We found that with the assistance of embedded stimulants,the ability of Bacillus sp.to reduce Cr(Ⅵ)increased continuously over time,with Cr(Ⅵ)reduction levels rising from 86.08%(two days)to 97.09%(28 days).In contrast,for bacteria treated with non-embedded combined stimulants,the reduction levels decreased from 86.92%(two days)to 75.93%(28 days).SEM observations of bacteria after 28 days of remediation showed significant shrinkage,partial rupture,and incomplete morphological structure in bare bacteria without any stimulant.Bacteria treated with non-embedded combined stimulants exhibited slight shrinkage,punctate ruptures,and relatively complete morphological structures.In contrast,bacteria treated with embedded stimulants appeared smooth,with intact morphological structures and a fuzzy surface.EDS analysis revealed a higher content of O and Cr in the fuzzy structures compared with other elements,indicating that the embedded stimulants enhanced the fixation of Cr on bacterial cell surfaces,thus reducing the toxicity of Cr toward bacterial cells.In summary,this study shows that the reduction effect of mixed stimulant is better than that of single unmixed stimulant groups,and the optimal dosages are 8 g∙L-1 for sugarcane powder,6 g∙L-1 for montmorillonite,and 0.3%for bactericide.The use of non-embedded stimulants is conducive to rapid improvement of the reduction of Cr(Ⅵ)by bacteria in the short term,while embedded stimulants provide bacteria with a more long-term capacity to reduce Cr(Ⅵ)and thus effectively reduce the toxicity of Cr(Ⅵ).
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