Abstract
The use of bacterial strains as agents in bioremediation processes could reduce the harmfulness of potential toxic elements (PTEs) from water and soil with low or even no impact on the natural ecosystems. In this study, two new metal resistant-bacterial strains (Q3 and Q5) of Bacillus sp. were isolated from a sulfurous spring and their potential (as pure cultures or mixed) to remove Pb(II) and Cd(II) from an aqueous matrix was evaluated and optimized using response surface methodology (RSM). The optimal conditions for Cd(II) removal from all tested strains combinations were observed at an initial pH 5, a temperature of 38 °C, and an initial Cd(II) concentration of 50 mg L−1, while the performance of bacterial strains on Pb(II) removal was strongly correlated to initial pH and temperature conditions. Moreover, the efficiency of bacterial strains in removing both PTEs, Pb(II) and Cd(II), from an aqueous matrix was considerably higher when they were used as a mixed culture rather than pure. According to field emission SEM (FESEM) and EDS analysis, the two bacterial strains showed different mechanisms in removing Cd(II): Bacillus sp. Q5 bio-accumulated Cd(II) in its periplasmic space, whereas Bacillus sp. Q3 bio-accumulated Cd(II) on its cell surface. On the other hand, Pb(II) is removed by chemical precipitation (lead sulfide) induced by both Bacillus sp. Q3 and Q5. This study discloses new aspects of Pb(II) and Cd(II) bioremediation mechanisms in Bacillus species that can be extremely useful for designing and operating novel PTEs bioremediation processes.
Highlights
Potential toxic elements (PTEs) such as lead (Pb(II)), cadmium (Cd(II)), copper (Cu(II)), and chromium (Cr(VI)) can generate harmful effects on ecosystems and human health [1,2,3]
Isolation and identification of the new bacterial strains; Study of the critical growth conditions for the selected bacterial strains in an environment contaminated by Pb(II); Optimization of the operating conditions for the bioremediation process conducted by the selected bacterial strains; 4
According to growth rate and metal bioremediation results, the new isolated bacterial strains Q3 and Q5 showed a different behavior with variations of initial pH: the growth rate is higher at pH 8, whereas Pb(II) and Cd (II) removal are better performed at a pH ranging between 5 and 6
Summary
Potential toxic elements (PTEs) such as lead (Pb(II)), cadmium (Cd(II)), copper (Cu(II)), and chromium (Cr(VI)) can generate harmful effects on ecosystems and human health [1,2,3]. Pb(II), as a hazardous metal ion it damages DNA and proteins activity; it is a teratogenic and mutagenic element and causes cancer, renal failure, neurodegenerative impairment and injures the reproductive system [4,5]. Pb(II) detrimentally affects food production and its safety because it causes oxidative stress to plants and impairs their growth [6,7]. Cd(II) is one of the most toxic elements of the Earth; it undermines plant growth, reduces food safety, and threatens human health [2]. Different conventional methods are used to eliminate and reduce the toxicity of PTEs from contaminated soils and waters, including oxidation-reduction, ion exchange, neutralization, membrane separation, and electrochemical treatment methods [9,10,11,12], such methods are expensive and their application at large scale is not consolidated [6,13].
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