Insight into the roles of soluble, loosely bound and tightly bound extracellular polymeric substances produced by Enterobacter sp. in the Cd2+ and Pb2+ biosorption process: Characterization and mechanism

Heavy metals (HMs) are introduced into the environment through industrial activities such as electroplating, steel production, alloy manufacturing and mining, and this adversely impacts the environment and arouses intense concern [1], [2]. Among HMs, cadmium (Cd) and lead (Pb) are not essential elements for plants, animals or humans, are classified as strong carcinogens and can enter animal and human bodies along the food chain [3]. Among conventional methods used for HM remediation, adsorption has great prospects due to its high efficiency, simplicity and relatively low cost [4]. Recently, biosorbents, including fungi, bacteria, and algae, which are considered renewable resources, have attracted increasing attention as adsorbents for HMs [2], [3], [5]. As an important bridge between microbes and the external environment, extracellular polymeric substances (EPSs) have large pores on their surfaces and increase the number and types of functional groups on the surfaces of microbes [6], [7]. EPSs are macromolecular polymers formed by metabolic secretion, shedding of surface substances, and dissolution and release upon death of the microorganism (bacteria, fungi or algae); EPSs have physicochemical properties such as surface electronegativity, adsorptivity, flocculation, hydrophilicity and hydrophobicity [8]. EPSs adhere to or cover microbial cell surfaces, forming a coated and embedded colloidal matrix that acts as both a binder for microbial aggregation and a barrier that protects the microbes from the harsh external environment [7]. Therefore, EPSs have great potential from removing HM ions from aquatic solutions.

Material characterizations have always been employed in the analyses of adsorption mechanisms, which might provide better understanding of the microscopic processes and key functional groups involved in adsorption; these can be divided into three categories, including microscopic morphology observations, physical structure determinations and compositional analyses. Generally, the characterization methods used in determinations of EPS mainly included scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrometry (FT-IR), X-ray diffraction (XRD), and three-dimensional excitation emission matrix fluorescence spectroscopy (3D-EEM) [9], [10], [11], [12], [13]. The reactions between EPSs and HM ions occur via various mechanisms, including complexation, ion exchange, redox and surface precipitation [14], [15], [16], [17]. Zhang et al. [18] noted that Burkholderia cepacia GYP1 exhibited substantial Cd2+ accumulation capability, mainly because Cd2+ was first adsorbed on EPSs, complexed with carboxyl groups (-COOH) and amino groups (-NH2) in the EPSs, and then transported into bacterial cells. Moreover, H+, Na+, K+ and Ca2+ present on the surfaces of the EPSs can be replaced by HM ions (such as Cd2+, Pb2+ and Cu2+) that have higher affinities or binding capacities through ion exchange, resulting in adsorption of the HM ions [19]. In addition, EPSs secreted by microbes have certain redox properties that effectively catalyze changes in the metal oxidation states, thereby reducing the solubilities, mobilities and bioavailabilities of the metal. Li et al. [14] reported that the EPSs secreted by P. aeruginosa and their functional groups played vital roles in reduction of Cr6+ by reducing Cr6+ to Cr3+ and converting it to Cr(OH)3 or Fex-Cr(1-x)(OH)3 precipitates. Thus, the adsorption processes involving EPSs and HM ions may proceed via multiple mechanisms, resulting in complex compositional and structural changes.

Generally, EPSs can be classified into two types: soluble EPSs (S-EPSs) and bound EPSs (B-EPSs). S-EPSs mainly exist as soluble macromolecules, mucus and colloids, which are usually wrapped on the outer cell walls of microbes [20]. In addition, B-EPSs mainly exist as capsule-like and condensed colloids that are tightly wrapped around cells and can be further divided into loosely bound EPSs (LB-EPSs) and tightly bound EPSs (TB-EPSs) [21]. Previous studies indicated that different layers of EPSs exhibited different methods for resisting HM toxicity [22]. Li et al. [23] confirmed that S-EPSs secreted by Bacillus vallismortis sp. exhibited the highest capacity for adsorption of Cu2+, and this exhibited a close relationship with the sulfhydryl protein content. Hou et al. [24] indicated that LB-EPSs secreted by Sinorhizobium meliloti played more important roles than TB-EPSs and S-EPSs in Cu2+ immobilization. In addition, Lu et al. [25] noted that LB-EPSs constituted the first reaction barrier for immobilization of HM ions such as Pb2+, Cu2+ and Zn2+. In addition, adsorption kinetic and adsorption isotherm models were always employed to understand the adsorption process. Adsorption kinetics provide information on the adsorption rate, the properties of the adsorbent used and the mass transfer mechanism [26], and the models typically include the pseudo first-order, pseudo second-order, intraparticle diffusion and Elovich models [27], [28]. While the adsorption isotherm model can be used to describe the relationships between two variables, the amount of material adsorbed and the concentration of adsorbate in solution at equilibrium at a constant temperature, the Freundlich and Langmuir models are the two most common isotherm models[13], [29], [30]. Moreover, the adsorption thermodynamic parameters allow us to determine whether the adsorption process can proceed spontaneously and the thermodynamics of the adsorption process that proceeds spontaneously [30]. Thus, model fitting provides strong support in exploring the mechanism of the adsorption process.

In our previous study, we investigated the interactions between Pb2+ and different layers of EPSs secreted by Enterobacter sp. FM-1 (FM-1), and we found that the LB-EPSs played a vital role in Pb2+ adsorption [31]. Therefore, different layers of EPSs might present various contributions and mechanisms in adsorption of other HM ions. However, most investigations have been focused on the roles or mechanisms of EPSs in adsorption of single HM ions. Hence, it is indispensable to have a deep understanding of the behaviors and mechanisms of multiple-HM ion adsorption by different layers of EPSs. In this study, the contributions of different layers of EPSs (S-EPSs, LB-EPSs and TB-EPSs) secreted by Enterobacter sp. to adsorption of Cd2+ and Pb2+ were analyzed. FT-IR, 3D-EEM, XPS, XRD and SEM were used to characterize the adsorption process. Therefore, the current study was designed to (1) compare the capacities of EPSs for adsorption of Cd2+ and Pb2+, (2) explore the contributions of EPSs to Cd2+ and Pb2+ adsorption, and (3) clarify the mechanisms used by EPSs for detoxification and protection from Cd2+ and Pb2+.

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