Toxic heavy metal ions have a broad range of sources, especially in water resources [1
]. Municipal wastes, mining and smelting of metalliferous ores, fertilizers, burning of fossil fuels, agriculture runoff and domestic effluent are the main sources of heavy metal contamination, which is difficult to remove and has detrimental effects on ecological systems and human health [2
]. Among the various heavy metal ions presenting in wastewater, Pb(II) is one of the most prevalent. Sources of lead include tetraethyl lead-added gasoline, electrical storage batteries, mining, plating, lead smelting, photographic materials, explosive manufacturing, ammunitions, printing pigments, ceramic glass industries, etc. [4
]. Lead has various harmful effects on human health [5
]. Some of the known deleterious effects are with brain, kidney, reproductive system, nervous system, bone and heme synthesis [6
]. It is, therefore, essential to remove Pb(II) from wastewater before disposal.
Antimony (Sb) is another well-known and hazardous toxic heavy metal with a particular ability to dissolve precious metals, such as gold. It was also used to purify gold from copper and silver until the 18th century. It is common to find compounds of antimony in silver, copper and lead containing ores. Anti-friction alloys, batteries, ammunitions like small arm and tracer bullets, type-metal and cable sheathing are main Sb-containing products since the use of Sb increases the hardness and the mechanical strength greatly. Sb also be used in brake linings, lead storage batteries, flame retardants, semiconductor components, as a catalyst in plastics, and an additive in glassware and ceramics. Consequently, it is introduced in the environment by anthropogenic pathways [7
]. Sb(III) and Sb(V) ions, the main forms of Sb under environmental, biological and geochemical conditions, hydrolyze easily in aqueous solutions, with Sb(OH)3
being the dominant chemical species in the aqueous environment [8
]. Sb(III), however, is considered ten times more toxic than Sb(V) [9
]. Due to the neutral character over a wide pH range of Sb(III), it is more strongly absorbed by its natural sorbents, making it more stable in the neutral environment [10
]. Sb is classified as a toxic pollutant, potentially carcinogenic to humans and therefore as a pollutant of priority interest by entities such as the Environmental Protection Agency of the United States (USPEA) [11
]. The USEPA’s maximum contaminant levels for Pb and Sb in drinking water is 15 μg/L and 6 μg/L, respectively [12
]. Sb(III) is toxic for both the environment and human and it is a part of pollution resources the same as Pb(II). Sb and Pb, most of which was generated and then also consumed by the battery industry, and often coexist in industrial wastewaters or other Pb-Sb consumption source water systems. Therefore, attention should be paid to find out an efficient method to remove or isolate these two toxic heavy metal ions.
Ion exchange, reverse osmosis, membrane filtration, chemical precipitation, and evaporation are widely used methods for removing heavy metals from wastewater [14
], but these technologies are either expensive for the treatment of secondary byproducts or ineffective when the concentration of metal ions is low. Sometimes, complex synthesis of exchangers is essential [15
]. By contrast, biosorption is better because it is cheap and ecofriendly, especially when the biosorbents are original bacteria in soil. For biosorption, various biological materials have been used for removal, preconcentration and extraction of pollutants from aqueous solutions. For example, algae (e.g., Sargassum nayans
, Sargassum wightii
), fungi (e.g., Aspergillus niger
, Rhizopus arrhizus
, Mucor Penicillium
spp.), bacteria (e.g., Azotobacter
spp., Bacillus megaterium
, Bacillus subtillis
spp., Pseudomonas marginalis
) and yeast (e.g., Saccharomyces cerevisae
) have already been used to remove heavy metal ions, due to their excellent recovery effect, eco-friendly, and economic features [21
]. It has been found that bacteria are more efficient and cost-effective biosorption materials to remove toxic metals, especially for low concentrations of heavy metals in solution [25
]. Bacteria, ubiquitous in soil and water, are a major group of unicellular living organisms belonging to the prokaryotes and are suitable as adsorbent materials due to their small size and large specific surface area. Most importantly chemical compounds presenting in a bacterial cell wall are capable of passively sequestering metals [27
]. These chemical groups are comprised of carbonyl, hydroxyl, sulfhydryl, carboxyl, sulfonate, thioether, amine, amide, imine, phosphonate, imidazole, and phosphodiester groups. Factors such as accessibility of sites, affinity between sites and metals (i.e., binding strength), number of sites in the biosorbent materials and the chemical state of these sites (i.e., availability) all have influence on the importance of any given group for biosorption of a certain metal by a certain biomass.
Microbial populations living in metal polluted environments adapt to the toxic concentrations of heavy metals and become metal resistant [28
]. In view of the interest in the use of microorganisms for the reclamation of contaminated sites, it is important to identify their response towards toxic heavy metals. In addition, studies of microbial populations in heavy metal polluted regions (especially in soil and water) are necessary since the microbes have adapted to survive in high heavy metal concentrations. The development of biosorbents isolated from polluted areas could be used to aid the advancement of alternative and cost-effective adsorbents. Recent studies have shown that some resistant bacterial microorganisms isolated from metal polluted sites are capable of absorbing Pb or Sb [29
]. Although using these bacteria to remove Pb or Sb has been reported, it has been rarely studied using heavy metal resistant bacterial as biosorbent for biosorption and removal of Pb and Sb.
Bacillus subtilis (B. subtilis)
, a rod-morphology bacterium, is a Gram positive aerobic spore, with a greater ability to bind metals than Gram-negative ones due to their different cell wall structures [38
]. Teichoic acids and acids associated with the cell wall, whose phosphate groups are key components for the uptake of metals, are specific contents of Gram-positive cells. The main agents in the uptake of heavy metals are carboxyl groups, the sources of which are the teichoic acids associated with the peptidoglycan layers of the cell wall [40
are inexpensive and easily available in the upper layers of the soil. Although there have been reports in the literature on the biosorption of Pb(II) by B. subtilis
], there is no report on the biosorption of Pb(II) and Sb(III) together. Compared to the live biomass, dead bacterial biomass has a number of advantages, such as easy storage, the ability to treat large volumes of wastewater with low metal concentrations, short operation time, available with no harmful byproducts, and the absence of restrictions on enzymatic activities caused by metal adsorption [43
]. In addition, the dead biomass does not require a continuous supply of nutrients, it is not affected by toxic wastes, and it can be regenerated and reused for many cycles. As a result, the usage of these dead microbial cells for adsorption is more advantageous for water treatment [44
Therefore, the aim of this study is to explore and optimize the biosorption conditions of Pb(II) and Sb(III) in aqueous solution by a Pb-resistant bacterium, B. subtilis, in a batch system. The biosorption capacity obtained from the batch system is useful in providing fundamentals for industrial application of biosorption. The objective of the present work is to investigate: (1) thermodynamics and kinetics of biosorption of Sb(III) and Pb(II) ions onto B. subtilis biomass, (2) optimum biosorption parameters including pH, biomass dosage, contact time and temperature, (3) kinetic mechanism fitting to the Langmuir, Freundlich, Temkin and Dubinin-Radushkevich(D-R) models, (4) potential application strategy for using B. subtilis to remove Sb(III) and Pb(II) from water as well as its application prospectives for phytoremediation of Pb and Sb from polluted soils.