High Efficiency Mercury Sorption by Dead Biomass of Lysinibacillus Sphaericus-New Insights into the Treatment of Contaminated Water

Abstract

Mercury (Hg) is a toxic metal frequently used in illegal and artisanal extraction of gold and silver which makes it a cause of environmental poisoning. Since biosorption of other heavy metals has been reported for several Lysinibacillus sphaericus strains, this study investigates Hg removal. Three L. sphaericus strains previously reported as metal tolerant (CBAM5, Ot4b31, and III(3)7) were assessed with mercury chloride (HgCl₂). Bacteria were characterized by scanning electron microscopy coupled with energy dispersive spectroscopy (EDS-SEM). Sorption was evaluated in live and dead bacterial biomass by free and immobilized cells assays. Hg quantification was achieved through spectrophotometry at 508 nm by reaction of Hg supernatants with dithizone prepared in Triton X-114 and by graphite furnace atomic absorption spectroscopy (GF-AAS). Bacteria grew up to 60 ppm of HgCl₂. Non-immobilized dead cell mixture of strains III(3)7 and Ot4b31 showed a maximum sorption efficiency of 28.4 µg Hg/mg bacteria during the first 5 min of contact with HgCl₂, removing over 95% of Hg. This process was escalated in a semi-batch bubbling fluidized bed reactor (BFB) using rice husk as the immobilization matrix leading to a similar level of efficiency. EDS-SEM analysis showed that all strains can adsorb Hg as particles of nanometric scale that can be related to the presence of S-layer metal binding proteins as shown in previous studies. These results suggest that L. sphaericus could be used as a novel biological method of mercury removal from polluted wastewater.

Keywords: EDS-SEM; GF-AAS; Lysinibacillus sphaericus; biosorption; dead cells; dithizone; mercury.

Figure 1

Efficiency in mercury removal by L. sphaericus (strains alone and mixtures) over time.

Figure 2

Comparison of Hg quantification with ICP-OES. Data of L. sphaericus III(3)7 treatment. L: Live; D: Dead.

Figure 3

GF-AAS quantification of Hg aliquots of escalated process. (a) Hg signal of standard solution diluted 1/240. (b) Time-resolved absorbance signals for samples. (c) Hg concentration over time in escalated process with (black) and without bacteria (grey).

Figure 4

EDS-SEM analysis of L. sphaericus III(3)7 live cells after 1 h in contact with HgCl₂. (a) Image taken at 10 kV by detection of secondary electrons and backscattered electrons; (b) Data from energy dispersive spectroscopy of punctual area “Spectrum 13”.

Figure 5

EDS-SEM of dead L. sphaericus III(3)7 and Ot4b31 immobilized in RH after in contact with HgCl₂.

Figure 6

ANOM graphs of principal effects on mercury removal by L. sphaericus. (a) Effect of bacteria strain (alone or mixed); (b) Effect of bacteria treatment (dead or live).