Cd sequestration by bacteria-aluminum hydroxide composites

The competition of humic acid aggregation and adsorption on clay particles and its role in retarding heavy metal ions

Humic acid (HA) is of great importance in controlling the fate of heavy metals (HMs), however, the pivotal influence of HA aggregation within the HA-clay-HM ternary system on retarding HM mobility remains obscure. This study performed molecular dynamics simulations to delve into the consequences of HA aggregation on the environmental behavior of Cd2+ and Pb2+ (0.1-0.6 M) in the co-existence of illite particles. HA can readily aggregate into clusters, adhering to the illite surface or freely dispersing in the solution. These HA clusters significantly modulate HM mobility, contingent upon their location, arrangement, and interaction with illite. Consequently, HA exhibited a pronounced retardation effect on HM migration, stemming from the competition between HA aggregation and its adsorption on illite. Additionally, the retardation effect of HA aggregation was more obvious for Cd2+ (as compared to Pb2+), owing to its stronger interaction with the functional groups of HA. These findings contribute to the development of potential HA-based strategies for remediation of heavy metal-contaminated sites.

Efficient removal of Cr (VI) from coal gangue by indigenous bacteria-YZ1 bacteria: Adsorption mechanism and reduction characteristics of extracellular polymer

The existence of heavy metals (especially Cr (VI)) in coal gangue has brought great safety risks to the environment. The indigenous bacteria (YZ1 bacteria) were separated and applied for removing Cr (VI) from the coal gangue, in which its tolerance to Cr (VI) was explored. The removal mechanism of Cr (VI) was investigated with pyrite in coal gangue, metabolite organic acids and extracellular polymer of YZ1 bacteria. The concentration of Cr (VI) could be stabilized around 0.012 mg/L by the treatment with YZ1 bacteria. The Cr (VI) tolerance of YZ1 bacteria reached 60 mg/L, and the removal efficiency of Cr (VI) was more than 95% by using YZ1 bacteria combined with pyrite. The organic acids had a certain reducing ability to Cr (VI) (removal efficiency of less than 10%). The extracellular polymers (EPS) were protective for the YZ1 bacteria resisting to Cr (VI). The polysaccharides and Humic-like substances in the soluble extracellular polymers (S-EPS) had strong adsorption and reduction effect on Cr (VI), in which the tryptophan and tyrosine proteins in the bound extracellular polymers (LB-EPS and TB-EPS) could effectively promote the reduction of Cr (VI). YZ1 bacteria could obviously reduce the damage of Cr (VI) from coal gangue to the environment.

Multi-scale modeling of natural organic matter-heavy metal cations interactions: Aggregation and stabilization mechanisms

Interaction between natural organic matters (NOM) and heavy metal cations in aqueous environment are of great significance for maintaining stability of organic carbon and restraining transport of heavy metal contaminants in (bio)geochemical processes. We systematically explore the aggregation process and complexation between NOM and heavy metal cations (Ag⁺, Cd²⁺, Pb²⁺, Zn²⁺, Eu³⁺) under different pH condition by molecular dynamics (MD) simulations, umbrella sampling method, and quantum chemistry calculations. The character of molecular structures NOM-heavy metal complexes and association are quantified. In acidic pH condition, aggregation proceeds via H-bonding and π-π interactions between NOM fragments. In neutral condition, Ag⁺, Cd²⁺, Pb²⁺, and Eu³⁺ can form inner-sphere complexes with the surface carboxylic groups and therefore reduce intermolecular charge repulsion, eventually leading to NOM aggregation, and it shows that even without direct binding, the outer-sphere adsorbed Zn²⁺ can also result in the formation of NOM assemble through H-bonding. Consequently, these heavy metals are capable of promoting NOM aggregation regardless of the complexing ways. Complexing free energy calculations characterized the dynamic processes of cations binding to the carboxylic groups of NOM fragment and the related energy landscape. This study provides quantitative insights for understanding the environmental processes of heavy metals and cycle of C in aquatic ecosystem, and contributes to developing environment-friendly strategies for controlling heavy metal contaminants.

2D-layered Mg(OH)2 material adsorbing cellobiose via interfacial chemical coupling and its applications in handling toxic Cd2+ and UO22+ ions

The interfacial chemistry of nanocomposite materials is of overarching importance in the separation and purification science; moreover, its understanding helps to guide synthesis, clarify structure-property relationship and unearth novel applications. However, the composites feature rather complicated local structures and hydrogen bonds are often involved in the interface and the vicinity of active sites. In this regard, density functional theory first-principle calculations associated with experimental study have synergistically examined two-dimensional (2D) magnesium hydroxide material with different layers and their adsorption toward cellobiose. Hydrogen bonds are found responsible for the interfacial coupling, which make it vital to cover the dispersion correction in the calculation. The average adsorption energy ranges from -0.29 to -0.35 eV, falling well within the range of reported hydrogen-bonding strength. On the basis of calculated structural/interfacial properties and experimental findings, the 2D Mg(OH)₂ in terms of three-layer model was unraveled to substitute toxic Cd²⁺ ion and sorb radioactive UO₂²⁺ that is coordinated by water and hydroxyl groups. These reactions are thermodynamically feasible. The ion-exchanging mechanism was proposed for cadmium removal and the outer-sphere adsorption one for uranium extraction.

Roles of cation efflux pump in biomineralization of cadmium into quantum dots in Escherichia coli

Cadmium (Cd) is a typical and widely present toxic heavy metals in environments. Biomineralization of Cd ions could alleviate the toxicity and produce valuable products in certain waste streams containing selenite. However, the impact of the intrinsic Cd(II) efflux system on the biotransformation process remains unrevealed. In this work, the significance of the efflux system on Cd biomineralization was evaluated by constructing engineered Escherichia coli strains, including ΔzntA with suppressed Cd(II) efflux system and pYYDT-zntA with strengthened Cd(II) efflux system. Compared to the wild type (WT), 20% more Cd ions were accumulated in ΔzntA and 17% less were observed in pYYDT-zntA in the presence of selenite as determined by inductively coupled plasma atomic emission spectrometer. Through combination with X-ray absorption fine structure analysis, it was discovered that 50% higher production of CdS_xSe_1-x quantum dots (QDs) was achieved in the ΔzntA cells than that in the WT cells. Moreover, the ΔzntA cells exhibited the same viability as the WT cells and the pYYDT-zntA cells because accumulated Cd ions were transformed into biocompatible QDs. In addition, the biosynthesized QDs had a uniform particle size (3.82 ± 0.53 nm) and a long fluorescence lifetime (45.6 ns), which could potentially be utilized for bio-imaging. These results not only elucidate the significance of Cd(II) efflux system in the biotransformation of Cd ions and selenite, but also provide a promising way to recover Cd and Se as valuable products in certain waste streams.

Binding of Cd(II) by Amorphous Aluminum Hydroxide-Organophosphorus Coprecipitates: From Macroscopic to Microscopic Investigation

The mobility of Cd(II) in soils, sediments, and aquatic systems is strongly dependent on adsorption behaviors occurring at the mineral-water interface, and this process may be influenced by the presence of organic phosphorus (OP). In this study, we investigate Cd(II) adsorption onto amorphous aluminum hydroxide (AAH), both in the presence and absence of OP, represented by the widely abundant myo-inositol hexakisphosphate (IHP). Isothermal adsorption experiment coupled with attenuated total reflection Fourier transform infrared (ATR-FTIR) and 1H solid-state NMR spectra were employed. Physiochemical characterization shows that IHP can increase the surface negative charge and the number of surface sites. Isothermal results show that high IHP loading enhances Cd(II) adsorption while no obvious increase is observed at low IHP loading. The overall effect of IHP on Cd(II) sorption depends on the extent of two positive processes, i.e., (1) IHP can form ternary complexes with adsorbed Cd(II) on AAH and (2) IHP can increase the negative surface charge of AAH, and a negative process, i.e., AAH competes with Cd(II) for AAH surface sites. ATR-FTIR results confirm the possible formation of three structurally distinct ternary complexes, i.e., the AAH-IHP-Cd, AAH-Cd-IHP, and AAH-Cd-IHP-Cd. The analysis of 1H solid-state NMR demonstrates that IHP only increases the number of surface OH groups rather than changes their chemical environment and speciation. Cd does not bind to the AAH surface but mainly binds with the OH groups of IHP. All findings of this work suggest that the presence of high dose of OP promotes the retention of Cd(II) in soils, thereby decreasing their bioavailability to biota.

Molecular Mechanisms of Lead Binding to Ferrihydrite-Bacteria Composites: ITC, XAFS, and μ-XRF Investigations

Binding of Pb(II) to ferrihydrite-Bacillus subtilis composites formed in the presence of bacterial cells were investigated through macroscopic and microscopic techniques. Diffuse layer model (DLM) fitting and isothermal titration calorimetry (ITC) analysis indicated that the hydroxyl group played a key role in Pb(II) sorption onto composites by masking reactive sites, such as carboxyl and phosphoryl groups of bacterial cells. Negative enthalpy (from -39.29 to -57.87 kJ mol^-1) and positive entropy (from 135.61 to 193.47 kJ mol^-1) of Pb(II) sorption onto composites revealed that inner-sphere complexes formed through exothermic reactions and was driven by both entropy and enthalpy. Spatial distribution of these inner-sphere species at varied Pb(II) loading demonstrated that interactions between Pb(II) and bacterial cells preceded that of mineral components in composites, using microfocus X-ray fluorescence spectroscopy (μ-XRF) maps and microfocus X-ray absorption near edge structure (μ-XANES) spectra. Combined with bulk Pb L_III-edge X-ray absorption fine structure (XAFS) spectrum, we inferred that mononuclear bidentate edge-sharing hydroxyl-Pb complexes, monodentate mononuclear carboxyl-Pb and phosphoryl-Pb complexes predominantly contributed to Pb(II) inner-sphere binding with mineral and bacterial fractions in composites, respectively. The molecular binding mechanisms obtained in this study provide further insight into the sequestration and migration of toxic metals in natural environments.

Heavy metal behaviour at mineral-organo interfaces: Mechanisms, modelling and influence factors

The mineral-organo composites control the speciation, mobility and bioavailability of heavy metals in soils and sediments by surface adsorption and precipitation. The dynamic changes of soil mineral, organic matter and their associations under redox, aging and microbial activities further complicate the fate of heavy metals. Over the past decades, the wide application of advanced instrumental techniques and modelling has largely extended our understanding on heavy metal behavior within mineral-organo assemblages. In this review, we provide a comprehensive summary of recent progress on heavy metal immobilization by mineral-humic and mineral-microbial composites, with a special focus on the interfacial reaction mechanisms of heavy metal adsorption. The impacts of redox and aging conditions on heavy metal speciations and associations with mineral-organo complexes are discussed. The modelling of heavy metals adsorption and desorption onto synthetic mineral-organo composites and natural soils and sediments are also critically reviewed. Future challenges and prospects in the mineral-organo interface are outlined. More in-depth investigations are warranted, especially on the function and contribution of microorganisms in the immobilization of heavy metals at the complex mineral-organo interface. It has become imperative to use the state-of-the-art methodologies to characterize the interface and develop in situ analytical techniques in future studies.

Effects of Pb(II) and Cr(VI) Stress on Phosphate-Solubilizing Bacteria (Bacillus sp. Strain MRP-3): Oxidative Stress and Bioaccumulation Potential

The aim of this work was to ascertain the effects of Pb(II) and Cr(VI) on bacterial growth, generation of reactive oxygen species (ROS), activities of superoxide dismutase (SOD), and catalase (CAT), as well as the localization of bioaccumulated heavy metals in a phosphate-solubilizing bacterium. The results showed that the ROS increased from 1.4-fold to 1.8-fold of control under Pb(II) stress and decreased from 1.6-fold to 1.1-fold of control under Cr(VI) stress corresponding to metal concentrations (0.5–5 mmol·L⁻¹). The SOD activities were ROS dependent; however, the CAT activities increased under both Pb(II) and Cr(VI) stress, from 11.4 to 21.8 U·mg⁻¹ and 11.4 to 32.9 U·mg⁻¹, respectively. Intra/extracellular accumulation were investigated by scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM-EDS) and it was calculated that extracellular accumulated Pb accounted for 61.7–95.9% of the total accumulation, while extracellular accumulated Cr only accounted for up to 3.6% of the total accumulation. Attenuated total reflection/Fourier-transform infrared spectroscopy (ATR-FTIR) analysis confirmed that the functional groups involved in those extracellular accumulation were not located in the loosely bound extracellular polysaccharides substances.

Competitive binding of Cd, Ni and Cu on goethite organo-mineral composites made with soil bacteria

Soil is a heterogeneous porous media that is comprised of a variety of organo-mineral aggregates. Sorption of heavy metals onto these composite solids is a key process that controls heavy metal mobility and fate in the natural environment. Pollution from a combination of heavy metals is common in soil, therefore, understanding the competitive binding behavior of metal ions to organo-mineral composites is important in order to predict metal mobility and fate. In this study, batch experiments were paired with spectroscopic studies to probe the sorption characteristics of ternary CdNiCu sorbates to a binary organo-goethite composite made with Bacillus cereus cells. Scanning electron microscopy shows that goethite nano-sized crystals are closely associated with the bacterial surfaces. Sorption experiments show a larger adsorptivity and affinity for Cu than Cd/Ni on goethite and B. cereus, and the goethite-B. cereus composite. X-ray photoelectron spectroscopy reveals that carboxylate and phosphate functional moieties present on the bacterial cell walls are primarily responsible for metal sorption to the goethite-B. cereus composite. Synchrotron-based X-ray fluorescence shows that Cu and Ni are predominately associated with the bacterial fraction of the goethite-B. cereus composite, whereas Cd is mainly associated with the goethite fraction. The findings of this research have important implications for predicting the mobility and fate of heavy metals in soil multi-component systems.