Cd(II) Sorption on Montmorillonite-Humic acid-Bacteria Composites

The characteristics of nano-micron calcite particles/common bacteria complex and its interfacial interaction

Based on the composite pollution of atmospheric microbial aerosol, this paper selects the calcite/bacteria complex as the research object which was prepared by calcite particles and two common strains of bacteria (Escherichia coli, Staphylococcus aureus) in the solution system. The morphology, particle size, surface potential, and surface groups of the complex were explored by modern analysis and testing methods, with an emphasis on the interfacial interaction between calcite and bacteria. The SEM, TEM, and CLSM results showed that the morphology of the complex could be divided into three types: bacteria adhering to the surface or edge of micro-CaCO₃, bacteria aggregating with nano-CaCO₃, and single nano-CaCO₃ wrapping bacteria. The complex's particle size was about 2.07 ~ 192.4 times larger than the original mineral particles, and the nano-CaCO₃/bacteria complex's particle size variation was caused by the fact that nano-CaCO₃ has agglomeration in solution. The surface potential of the micro-CaCO₃/bacteria complex (isoelectric point pH = 3.0) lies between micro-CaCO₃ and bacteria, while the surface potential of the nano-CaCO₃/bacteria complex (isoelectric point pH = 2.0) approaches the nano-CaCO₃. The complex's surface groups were based primarily on the infrared characteristics of calcite particles, accompanied by the infrared characteristics of bacteria, displaying the interfacial interaction from the protein, polysaccharides, and phosphodiester groups of bacteria. The interfacial action of the micro-CaCO₃/bacteria complex is mainly driven by electrostatic attraction and hydrogen bonding force, while the nano-CaCO₃/bacteria complex is guided by surface complexation and hydrogen bonding force. The increase in the β-fold/α-helix ratio of the calcite/S. aureus complex indicated that the secondary structure of bacterial surface proteins was more stable and the hydrogen bond effect was strong than the calcite/E. coli complex. The findings are expected to provide basic data for the mechanism research of atmospheric composite particles closer to the real environment.

Adsorption of cadmium on clay-organic associations in different pH solutions: The effect of amphoteric organic matter

Clay minerals are important soil components and usually coexist with organic matter, forming mineral-organic associations (MOAs), which control the speciation, mobility, and bioavailability of heavy metals. However, the adsorption mechanism of cadmium (Cd) by MOAs is still unclear, especially for the associations of amphotericorganic matter and clay minerals. In this study, 12-aminododecanoic acid (ALA) and montmorillonite (Mt) were chosen to prepare MOAs via intercalation (Mt-ALA composite) and physical mixing (Mt-ALA mixture). Batch experiments were conducted to investigate the adsorption mechanism of Cd(II) by MOAs under different pH values and initial Cd(II) concentrations. The results showed that the Cd(II) adsorption capacities followed as Mt > Mt-ALA mixture > Mt-ALA composite under acidic conditions, Mt-ALA mixture > Mt > Mt-ALA composite under neutral conditions, and Mt-ALA mixture > Mt-ALA composite > Mt under alkaline conditions, suggesting the adsorption behaviors of Cd(II) by MOAs were primarily constrained by the speciation of ALA and solution pH. Under acidic conditions, cationic HALA⁺ could intercalate into the interlayer of Mt and occupy the adsorption sites, reducing the adsorption capacity of Cd(II). As pH increased to neutral, HALA⁺ decreased and changed to a zwitterionic state, which caused ALA to release out from the interlayer of Mt-ALA composite or not easily enter into Mt-ALA mixture and promoted Cd(II) adsorption. Under alkaline conditions, the increase of anion ALA^- would cause ALA to be mainly adsorbed on the surface of Mt and chelate with Cd(II), enhancing the adsorption of Cd(II). Further analysis by Fourier transform infrared and X-ray photoelectron spectroscopy indicated that the carboxyl and amino groups of ALA both participated in the adsorption of Cd(II). These findings could extend the knowledge on the mobility and fate of Cd in clay-based soils and be used as a basis for understanding the biogeochemical behavior of Cd in the environment.

Cadmium transfer in contaminated soil-rice systems: Insights from solid-state speciation analysis and stable isotope fractionation

Initial Cadmium (Cd) isotope fractionation studies in cereals ascribed the retention of Cd and its light isotopes to the binding of Cd to sulfur (S). To better understand the relation of Cd binding to S and Cd isotope fractionation in soils and plants, we combined isotope and XAS speciation analyses in soil-rice systems that were rich in Cd and S. The systems included distinct water management (flooded vs. non-flooded) and rice accessions with (excluder) and without (non-excluder) functional membrane transporter OsHMA3 that transports Cd into root vacuoles. Initially, 13% of Cd in the soil was bound to S. Through soil flooding, the proportion of Cd bound to S increased to 100%. Soil flooding enriched the rice plants towards heavy isotopes (δ^114/110Cd = -0.37 to -0.39%) compared to the plants that grew on non-flooded soils (δ^114/110Cd = -0.45 to -0.56%) suggesting that preferentially light Cd isotopes precipitated into Cd sulfides. Isotope compositions in CaCl₂ root extracts indicated that the root surface contributed to the isotope shift between soil and plant during soil flooding. In rice roots, Cd was fully bound to S in all treatments. The roots in the excluder rice strongly retained Cd and its lights isotopes while heavy isotopes were transported to the shoots (Δ^114/110Cd_shoot-root 0.16-0.19‰). The non-excluder rice accumulated Cd in shoots and the apparent difference in isotope composition between roots and shoots was smaller than that of the excluder rice (Δ^114/110Cd_shoot-root -0.02 to 0.08‰). We ascribe the retention of light Cd isotopes in the roots of the excluder rice to the membrane transport of Cd by OsHMA3 and/or chelating Cd-S complexes in the vacuole. Cd-S was the major binding form in flooded soils and rice roots and partly contributed to the immobilization of Cd and its light isotopes in soil-rice systems.

Microbial Cd(II) and Cr(VI) resistance mechanisms and application in bioremediation

The heavy metals cadmium (Cd) and chromium (Cr) are extensively used in industry and result in water and soil contamination. The highly toxic Cd(II) and Cr(VI) are the most common soluble forms of Cd and Cr, respectively. They enter the human body through the food chain and drinking water and then cause serious illnesses. Microorganisms can adsorb metals or transform Cd(II) and Cr(VI) into insoluble or less bioavailable forms, and such strategies are applicable in Cd and Cr bioremediation. This review focuses on the highlighting of novel achievements on microbial Cd(II) and Cr(VI) resistance mechanisms and their bioremediation applications. In addition, the knowledge gaps and research perspectives are also discussed in order to build a bridge between the theoretical breakthrough and the resolution of Cd(II) and Cr(VI) contamination problems.

Cadmium Immobilization in the Rhizosphere and Plant Cellular Detoxification: Role of Plant-Growth-Promoting Rhizobacteria as a Sustainable Solution

Food is the major cadmium (Cd)-exposure pathway from agricultural soils to humans and other living entities and must be reduced in an effective way. A plant can select beneficial microbes, like plant-growth-promoting rhizobacteria (PGPR), depending upon the nature of root exudates in the rhizosphere, for its own benefits, such as plant growth promotion as well as protection from metal toxicity. This review intends to seek out information on the rhizo-immobilization of Cd in polluted soils using the PGPR along with plant nutrient fertilizers. This review suggests that the rhizo-immobilization of Cd by a combination of PGPR and nanohybrid-based plant nutrient fertilizers would be a potential and sustainable technology for phytoavailable Cd immobilization in the rhizosphere and plant cellular detoxification, by keeping the plant nutrition flow and green dynamics of plant nutrition and boosting the plant growth and development under Cd stress.

Remediation of Cd, Pb and as Co-contaminated Paddy Soil by Applying Different Amendments

The current study investigated the efficiency of sepiolite (SE), sodium humate (HS), microbial fertilizer (JF) and SE combined with JF/HS in a ratio of 2:1 (w/w) (JF-2SE and HS-2SE) on Cd, Pb and As bioavailability in field trials with rice (Oryza sativa L.). The results showed that all the amendments remarkably decreased (p < 0.05) the contents of available Cd and available Pb in soil. Only JF-2SE treatment reduced available As concentration in soil. All the amendments were found to effectively reduce (p < 0.05) the contents of As in brown rice. Both JF-2SE and HS-2SE co-applications reduced the concentrations of Cd in brown rice to 0.108 and 0.135 mg kg^-1, and that of Pb reduced to 0.2 and 0.175 mg kg^-1, which met the national standard limit of China. Thus, the co-application of JF/HS-2SE can be a promising remediation strategy in Cd, Pb and As co-contaminated paddy soil.

Clay minerals as a source of cadmium to estuaries

Given the high surface reactivity of clay minerals, it is assumed that flocculation will lead to metal accumulation in marginal marine settings. However, the degree of metal sorption to clays is impacted by solution pH and ionic strength, and it remains unknown whether riverine clays indeed serve as a metal sink once they encounter seawater where pH and ionic strength markedly increase. Here, we conducted cadmium (Cd) adsorption experiments to three types of common clay minerals – kaolinite, illite and montmorillonite. We found that 20–30% of Cd from illite and montmorillonite surfaces were desorbed when transitioning from freshwater to seawater pH and ionic strength conditions, while kaolinite showed no discernible differences. Synchrotron X-ray adsorption spectroscopy confirmed that Cd release corresponded to a change in bonding from outer- to inner-sphere complexes when clays encountered seawater pH and ionic strength conditions. If other trace nutrients (such as Cu, Zn, Co) adsorbed onto riverine clay minerals behave in a similar manner to Cd, we speculate that their desorption in marginal marine settings should exert a significant impact on the productivity of the biosphere.

Conduction Band of Hematite Can Mediate Cytochrome Reduction by Fe(II) under Dark and Anoxic Conditions

While it was recently reported that the conduction band of iron minerals can mediate electron transfer between Fe(II) and different Fe(III) lattice sites during Fe(II)-catalyzed mineral transformation, it is unclear whether such a conduction band mediation pathway occurs in the microbial Fe(II) oxidation system under dark and anoxic subsurface conditions. Here, using cytochrome c (c-Cyts) as a model protein of microbial Fe(II) oxidation, the in vitro kinetics and thermodynamics of c-Cyts reduction by Fe(II) were studied. The results showed that the rates of c-Cyts reduction were greatly enhanced in the presence of the semiconducting mineral hematite (Hem, α-Fe₂O₃). The electrochemical experiments separating Fe(II) and c-Cyts demonstrated that electrons from Fe(II) to the electrode or from the electrode to c-Cyts could directly penetrate hematite, resulting in enhanced current. Independent photochemical and photoluminescence experiments indicated that c-Cyts could be directly reduced by the conduction band electrons of hematite which were generated under light illumination. In c-Cyts+Fe(II)+Hem, the redox potential of Fe(II)-Hem was shifted from -0.15 to -0.18 V and that of c-Cyts+Hem changed slightly from -0.05 to -0.04 V. For the bulk hematite, Mott-Schottky plots illustrated that the flat band was shifted negatively and positively in the presence of Fe(II) and oxidized c-Cyts, respectively, and the surface electron/charge density was higher in the presence of Fe(II)/c-Cyts. As a consequence, the redox gradients from adsorbed Fe(II) to adsorbed c-Cyts allow electron transfer across the conduction band of hematite and facilitate c-Cyts reduction. This mechanistic study on conduction band-mediating electron transfer could help interpret the role of semiconducting minerals in the microbial Fe(II) oxidation process under dark anoxic conditions.

Co-effect of minerals and Cd(II) promoted the formation of bacterial biofilm and consequently enhanced the sorption of Cd(II)

Heavy metal pollution is very common in soils. Soils are complex systems including minerals, bacteria, and various other substances. In Cd(II) contaminated soil, the combined effects of clay minerals and heavy metals on bacterial biofilm and Cd(II) adsorption are unappreciated. Our study showed that the combination of clay minerals (goethite, kaolinite, and montmorillonite) and heavy metals promoted Serratia marcescens S14 biofilm development significantly more than clay minerals or Cd(II) alone. The amount of biofilm after binary treatment with clay minerals and Cd(II) was 2.3-7.3 times than that in control. Mineral-induced cell death and the expression of the fimA, bsmA, and eps were key players in biofilm formation. Binary treatment with montmorillonite and Cd(II) significantly enhanced biofilm development and consequently increased the adsorption of Cd(II). Cd(II) removal is the result of co-adsorption of bacteria and minerals. Bacterial biofilm played an important role in Cd(II) adsorption. FTIR spectroscopy showed the components of biofilm were not affected by minerals and revealed the functional groups -OH, -NH, -CH₂, -SH, -COO participated in Cd(II) immobilization. Our findings are of fundamental significance for understanding how minerals and Cd(II) affect biofilms and thereby enhance Cd(II) adsorption and predicting the mobility and fate of heavy metals in heavy metal-contaminated soil.

Roles of different humin and heavy-metal resistant bacteria from composting on heavy metal removal

This study aims to assess the roles of different humin and heavy-metal resistant bacterial community from composting on heavy metal removal. The results showed that the concentration of Cu²⁺, Zn²⁺, Ni²⁺, Pb²⁺, Cr³⁺ and Cd²⁺ decreased with adding the compost-derived humin, but the removal rates were relatively low (<30% on average). The heavy metal resistant bacteria from composting have better metal binding capacities than humin, and the combined addition of humin and bacteria could further stimulate the biosorption of heavy metals with 60-80% removal of metals and improve the diversity and biomass of bacterial community. There was obviously increased synergy between the humin from maturity phase and bacteria for metal biosorption ("1 + 1 > 2"). Structural equation modeling showed that microbial biomass and humin humification are the key factors for the biosorption of heavy metals. Combining humin from maturity phase with heavy-metal resistant bacteria was suggested to control heavy metal pollution in composts.

Adsorption behaviors of Cu2+, Zn2+ and Cd2+ onto proteins, humic acid, and polysaccharides extracted from sludge EPS: Sorption properties and mechanisms

To clarify the adsorption behaviors of typical heavy metals onto sludge extracellular polymeric substances (EPS), the adsorption capacities and mechanisms, as well as the contributions of the different EPS components (proteins, humic acids and polysaccharides), to the adsorption of Zn²⁺, Cu²⁺ and Cd²⁺ were separately explored. Overall, proteins exhibited a relatively high adsorption capacity for the three metals ions, followed by humic acid, whereas least for polysaccharides. The adsorption of Cu²⁺ and Cd²⁺ onto proteins, humic acid and polysaccharides fit well to the Freundlich isotherm, whereas Langmuir model was the best fit for Zn²⁺ bindings onto polysaccharides/humic acid. The binding of Cu²⁺, Zn²⁺ and Cd²⁺ onto the three EPS components was exothermically favorable, and significant electrostatic interactions were observed for the heavy metals sorption onto humic acid and proteins. In addition, the effect of metal ions sorption on the spectrum of the proteins, polysaccharides and humic acid was also explored.

Effect of humic acid on the stabilization of cadmium in soil by coprecipitating with ferrihydrite

Coprecipitation of humic acid (HA) with ferrihydrite (Fh) has been proposed to reduce the activity of heavy metals in aqueous solutions. The effect of the amount of HA added to the coprecipitates on the stabilization of Cd in soil is unclear. In this research, five different Fh-HA coprecipitates were synthesized to study the impact of different HA additions on the fractionation of Cd in the soil and the optimal addition ratio of C/Fe. Characterization technique as Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), specific surface area analyzer, and scanning electron microscopy (SEM) was used in order to test and analyze of the microstructure and physicochemical property of the coprecipitates. The results showed that the Fh-HA coprecipitate is mainly combined by the coordination exchange of -OH on the surface of the Fh with the carboxyl group of the HA. Adding HA could stabilize Fh and increase its surface roughness. Changes in the fractionation of the Cd were used to evaluate the stabilization effect of the coprecipitate. Before treatment, Cd in different contaminated soils was existed only a small amount of residual fraction. After the addition of the Fh-HA coprecipitate, the proportion of residual Cd in each contaminated soil increased. When the C/Fe ratio was 1.5, the maximum residual fraction were 62.94%, 55.67%, and 52.99% respectively. Residual Cd could remain relatively stable indicating that the Fh-HA coprecipitate is a promising amendment for repairing Cd-contaminated soil. The addition of HA has strengthened the active role of Fh on stabilizing heavy metals.

Efficient Demulsification of Acidic Oil-In-Water Emulsions with Silane-Coupled Modified TiO2 Pillared Montmorillonite

Emulsified pickling waste liquid, derived from cleaning oily hardware, cause serious environmental and ecological issues. In this work, a series of grafted (3-aminopropyl)triethoxysilane (APTES) TiO2 pillared montmorillonite (Mt), Ti-Mt-APTES, are prepared and characterized for their assessment in demulsification of acidic oil-in-water emulsion. After titanium hydrate is introduced through ion exchange, montmorillonite is modified by hydrophobic groups coming from APTES. The Ti-Mt-APTES in acidic oil-in-water emulsion demulsification performance and mechanism are studied. Results show that the prepared Ti-Mt-APTES has favorable demulsification performance. The Ti-Mt-APTES demulsification efficiency (ED) increased to an upper limit value when the mass ratio of APTES to the prepared TiO2 pillared montmorillonite (Ti-Mt) (RA/M) was 0.10 g/g, and the 5 h is the optimal continuous stirring time for breaking the acidic oil-in-water emulsion by Ti-Mt-APTES. The ED increased to 94.8% when 2.5 g/L of Ti-Mt-APTES is added into the acidic oil-in-water emulsion after 5 h. An examination of the demulsification mechanism revealed that amphiphilicity and electrostatic interaction both played vital roles in oil-in-water separation. It is demonstrated that Ti-Mt-APTES is a promising, economical demulsifier for the efficient treatment of acidic oil-in-water emulsions.

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.

Soil mineral alters the effect of Cd on the alkaline phosphatase activity

The toxicity of heavy metals (HMs) to soil enzymes is directly influenced by the status of the enzyme (free vs. immobilized on minerals) and the duration of exposure. However, little information is available on the interaction effect of HMs, mineral, and exposure time on soil enzyme activities. We investigated the interaction mechanism of alkaline phosphatase (ALP) with minerals (montmorillonite and goethite) and the response of free and immobilized ALP to cadmium (Cd) toxicity under different exposure times. The adsorption isotherms of ALP on both minerals were L-type. The maximum adsorption capacity of goethite for ALP was 3.96 times than montmorillonite, although both had similar adsorption constant (K). Goethite showed a greater inhibitory effect on ALP activity than montmorillonite. The toxicity of Cd to free- and goethite-ALP was enhanced with increasing exposure time, indicating a time-dependent inhibition. However, Cd toxicity to montmorillonite-ALP was not affected by the exposure time. The inhibition of Cd to soil enzyme activity is influenced by the properties of mineral complexes and the duration of exposure. A further understanding of the time pattern of HMs toxicity is helpful for accurately assessing the hazards of HMs to soil enzyme activity.

Binding of Cd by ferrihydrite organo-mineral composites: Implications for Cd mobility and fate in natural and contaminated environments

Adsorption and coprecipitation of organic matter with iron (hydr)oxides can alter iron (hydr)oxide surface properties and their reactivity towards nutrient elements and heavy metals. Organo-mineral composites were synthesized using humic acid (HA) and iron oxide, during coprecipitation with ferrihydrite (Fh) and adsorption to pre-formed Fh with two C loadings. The Fh-HA coprecipitated composites have a higher C content and smaller surface area compared to the equivalent adsorbed composites. NanoSIMS shows there is a high degree of spatial correlation between Fe and C for both composites, but C distribution is more uniform in the coprecipitated composites. The C 1s NEXAFS reveals a similar C composition between the Fh-HA coprecipitated and adsorbed composites. However composites at high carbon loading are more enriched in aromatic C, likely due to preferential binding of carboxyl functional groups on aromatic rings in the HA. The amount of Cd sorbed is independent of the composite type, either coprecipitated or adsorbed, but is a function of the C loading. Composites with low C loading show Cd sorption that is almost identical to pure Fh, while composites with high C loading show Cd sorption that is intermediate between pure Fh and pure HA, with sorption significantly enhanced over pure Fh at pH < 6.5. A bidentate edge-sharing binding was identified for Cd on pure Fh and Cd-carboxyl binding on pure HA. These findings have significant implications not only for the sequestration of Cd in contaminated environments but also the coupled biogeochemical cycling of Cd, Fe and C in the critical zone.

Impacts of a Compound Amendment on Cd Immobilization, Enzyme Activities and Crop Uptake in Acidic Cd-Contaminated Paddy Soils

This study assessed the effectiveness of limestone-montmorillonite-rapeseed residue-Si fertilizer compound amendment on the bioavailability and crop uptake of cadmium (Cd) and enzyme activities in acidic paddy soils. Applying the compound amendment at ratios of 1%-3% increased soil pH by 0.1-1.9 units, decreased leaching ratios of soil Cd 4.0%-22%, and decreased exchangeable and carbonated Cd 42%-55% and 27%-49%, respectively. Organic matter-bound Cd increased 47%-62% (p < 0.05). Cadmium concentrations decreased in the roots, culms, leaves, and grains of rice grown in the Cd-contaminated soils by 37%-81%, 18%-73%, 29%-64% and 27%-63%, respectively, (p < 0.05). Catalase and urease activities increased 2.5%-63% and 3.9%-36%, (p < 0.05), respectively. Applying this compound amendment may significantly mitigate soil acidification and decrease the bioavailability and crop uptake of Cd in acidic Cd-contaminated paddy soils.

Pb sorption on montmorillonite-bacteria composites: A combination study by XAFS, ITC and SCM

Though abundant studies have targeted the characterization of heavy metal adsorption by either clay minerals or bacteria, to date, minimal literature exists which specifically assesses bacteria-clay mineral interactions in the context of metal immobilization. The adsorption of Pb onto montmorillonite, Pseudomonas putida, and their 1:1, 2:1, 6:1 and 12:1 mass ratio composites were investigated by using a combination of atomic force microscope (AFM), X-ray diffraction (XRD), surface complexation modeling (SCM), Pb-L_III edge extended X-ray absorption fine structure (EXAFS) spectroscopy and isothermal titration calorimetry (ITC). The SCM and EXAFS demonstrated that Pb ions coordinate with phosphoryl and carboxyl functional groups on bacteria at low and high concentrations, respectively. The ITC analysis found adverse enthalpy values for Pb adsorption to permanent (-2.91 kJ/mol) and variable charge sites (6.93 kJ/mol) on montmorillonite. The ternary bridging model, EXAFS and ITC provide molecular and thermodynamic evidences for the formation of enthalpy driven (-4.74 kJ/mol) ternary complex (>AlO-Pb-PO₄) in the composites. The proportion for the bridging structures increased at pH > 5 and high bacterial mass ratios. The formation of ternary complex did not result in the enhanced adsorption of Pb on the composites, but promoted the allocation of Pb on the mineral fraction. The results obtained from SCM, EXAFS and ITC may provide an essential assumption for predicting the speciation and fate of Pb in soils and associated environments.

Cd sequestration by bacteria-aluminum hydroxide composites

Microbe-associated aluminum (Al) hydroxides occur naturally in aquatic and geologic environments and they might play a crucial role in the sequestration of trace metals because these composite solids comprise both reactive mineral and organic surface, but how they do it still remains unknown. Here we replicate Al hydroxide organo-mineral composite formation in soil and sediments by synthesising composites using Pseudomonas putida cells, during coprecipitation with Al hydroxide. Morphological and ATR-FTIR analysis show closely attached nano-sized Al hydroxides on the bacterial surface. For composites dominated by either bacteria or Al hydroxide, an enhanced metal adsorption is observed on the composites than on pure Al hydroxide at pH < 6. Cd uptake by the mainly Al mineral composite is approximately additive, i.e., the sum of the end-member metal adsorptivities, whereas that on the mainly bacteria composite is non-additive. This non-additive sorption is not only due to the blockage of surface reactive sorption sites, but more importantly the changes of surface charge when bacteria and Al mineral are interacted. EXAFS results show that Cd is predominately sorbed as a bidentate corner-sharing complex on the amorphous Al hydroxide surface and a carboxyl-binding on the bacterial surface. This study has important implications for understanding both Al and trace metal cycling in microbe-rich geologic environments.

Distribution and mobility of exogenous copper as influenced by aging and components interactions in three Chinese soils

Copper contamination of soils is a global environmental problem. Soil components (organic matter, clay minerals, and microorganisms) and retention time can govern the adsorption, fixation, and distribution of copper. This study evaluated the interaction effects of soil components and aging on the distribution of exogenous copper. Three typical Chinese soils (Ultisol, Alfisol, and Histosol) were collected from Hunan, Henan, and Heilongjiang Provinces. Soils were incubated with rice straw (RS) and engineered bacteria (Pseudomonas putida X4/pIME) in the presence of exogenous copper for 12 months. Sequential extraction was employed to obtain the distribution of Cu species in soils, and the mobility factors of Cu were calculated. The relationships between soil properties and Cu fractions were analyzed with stepwise multiple linear regression. The results show that organic carbon plays a more important role in shaping the distribution of relatively mobile Cu, and iron oxides can be more critical in stabilizing Cu species in soils. Our results suggest that organic matter is the most important factor influencing copper partitioning in Ultisols, while iron oxides are more significant in Alfisols. The mobility of exogenous Cu in soils depends largely on organic carbon, amorphous Fe, and aging. The introduction of both rice straw and rice straw + engineered bacteria enhanced the stabilization of Cu in all the three soils during aging process. The introduction of bacteria could reduce copper mobility, which was indicated by the lowest mobility factors of Cu for the treatment with bacteria in Black, Red, and Cinnamon soils at the first 4, 8, and 8 months, respectively. Different measures should be taken into account regarding the content of organic matter and iron oxides depending on soil types for the risk assessment and remediation of Cu-contaminated soils.

Chrysomya megacephala larvae feeding favourably influences manure microbiome, heavy metal stability and greenhouse gas emissions

Chrysomya megacephala is a saprophagous fly whose larvae can compost manure and yield biomass and bio‐fertilizer simultaneously. However, there are concerns for the safety of the composting system, that is risk of diseases spread by way of manure pathogens, residue of harmful metals and emission of greenhouse gases. Microbiota analysis and heavy metal speciation by European Communities Bureau of Reference were evaluated in raw, C. megacephala‐composted and natural stacked swine manure to survey pathogenic bacterial changes and mobility of lead and cadmium in manure after C. megacephala feeding; the emission rate of CH₄ and N₂O from manure during C. megacephala composting and natural stacking was also measured. C. megacephala composting altered manure microbiota, reduced the risk of pathogenic bacteria and maintained the stability, and microbiota changes might be associated with heavy metal fractions, especially in Pseudomonas and Prevotella. In addition, C. megacephala‐composting significantly reduced the emission rate of CH₄ and N₂O in comparing with natural stacking situation and the first two days should be the crucial period for CH₄ and N₂O emission measurement for manure treatment by C. megacephala. Moreover, OTU26 and Betaproteobacteria were changed after C. megacephala composting which might play a role in emission of CH₄ and N₂O, respectively.

Surface complexation modeling of Cu(II) sorption to montmorillonite-bacteria composites

Surface complexation modeling, isothermal titration calorimetry, and batch adsorption were employed to characterize the adsorption of Cu onto montmorillonite, Pseudomonas putida X4, and their composites at mass ratios of 2:1, 6:1, and 12:1, respectively. Different enthalpy values were found for Cu adsorption to permanent (-6.43kJ/mol) and variable charge sites (8.51kJ/mol) on montmorillonite. The component additivity (CA) method was used to predict the adsorption of Cu on the composites by combining end member models for montmorillonite and P. putida. A reduced adsorption was observed at pH<5.5 due to physical blocking between montmorillonite and P. putida. By contrast, the enhanced binding at high pH levels was ascribed to the formation of bridging structures among the clay mineral-Cu-bacteria complexes. These deviations in the CA method were corrected by adding reactions of >RCOOH…XNa and >RCOOCuOHXNa. The increased adsorption of the composites confirmed the decrease of permanent negative charge sites and the formation of ternary complexes. The results help elucidate the effect of Cu adsorption onto clay minerals-bacteria composites. The newly developed "CA-site masking-bridging" model can be used to predict Cu speciation in systems in which the active interaction of bacteria and clay minerals occurs.

Molecular investigation on the binding of Cd(II) by the binary mixtures of montmorillonite with two bacterial species

Bacteria-phyllosilicate complexes are commonly found in natural environments and are capable of immobilizing trace metals. However, the molecular binding mechanisms of heavy metals to these complex aggregates still remain poorly understood. This study investigated Cd adsorption on Gram-positive Bacillus subtilis, Gram-negative Pseudomonas putida and their binary mixtures with montmorillonite using surface complexation model, Cd K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and isothermal titration calorimetry (ITC). We have shown that larger amounts of Cd are adsorbed by B. subtilis than by P. putida at pH<∼6, and Cd sorption that binding to phosphate groups plays a more important role in P. putida than in B. subtilis. This remind us that we should consider the microbe species when predict the biochemical behavior of trace metals in microbe-bearing environments. The observed Cd adsorption on the binary bacteria-clay composites was more than that predicted based on the component additivity approach. When taking bacteria-clay (1:1 mass ratio) as a representative example, an approximately 68%:32% metal distribution between the bacterial and mineral fraction was found. Both the EXAFS and ITC fits showed that the binding stoichiometry for Cd-carboxyl/phosphate was smaller in the binary mixtures than that in pure bacteria. We proposed that the significant deviations were possibly due to the physical-chemical interaction between the composite fractions that might reduce the agglomeration of the clay grains, increase the negative surface charges, and provide additional bridging of metals ions between bacterial cells and clays.

Mobilization of cadmium from Festuca ovina roots and its simultaneous immobilization by soil in a root-soil-extractant system (in vitro test)

Mobilization of cadmium accumulated in Festuca ovina L. roots and simultaneous immobilization of this metal by soil were studied in three chambers connected into one system containing: (1) roots in an extractant solution, (2) soil in an extractant solution, and (3) extractant solution alone. Six extractants sterilized by filtration were used: 0.1 M NaNO₃ (NA), 0.1 mM desferrioxamine B (NA + DFOB) and 1 mM citric acid (NA + CA) in NA, and a water extract of soil (SE) supplemented with the same compounds. SE mobilized 53% of the Cd introduced to the system with roots. The addition of DFOB or CA to SE increased Cd extraction from roots by 17%, while the same compounds introduced to NA did not change mobilization of Cd (60% efficiency). Regardless of the extractant used, mobilization of Cd from roots was about 25% lower when extraction was done in a control system without soil. The metal released from roots was gradually immobilized by the soils loaded into all systems during a 4-day incubation. Sequential extractions of Cd from the soils showed that the metal released from roots with NA was stabilized mainly by soil Mn and Fe oxides, while that released with SE was stabilized by soil organic matter.

Interaction processes of ciprofloxacin with graphene oxide and reduced graphene oxide in the presence of montmorillonite in simulated gastrointestinal fluids

This study investigated the interaction processes of ciprofloxacin (CIP) with graphene oxide (GO) and reduced GO (rGO) in presence of montmorillonite (Mont) in simulated gastrointestinal fluids. The order of CIP adsorption affinity was rGO+Mont > GO+Mont > rGO+Mont+pepsin > rGO > GO+Mont+pepsin > Mont > Mont+pepsin > GO > rGO+pepsin > GO+pepsin in simulated gastric fluid. Mont enhanced the adsorption of CIP on GO and rGO due to hydrated Si species coating on GO and rGO in the simulated gastric fluid. Meanwhile, π–π interaction between CIP and graphene caused the great shift of two cyclopropyl CH₂ and one cyclopropyl in CIP molecules. And GO, rGO, and Mont interacted mainly with CIP by COOH groups. CIP and pepsin molecules could intercalate and increase the basal spacing of Mont as well. After the various interaction systems of adsorbent-adsorbate transferring to the simulated intestinal fluid, CIP was continuously adsorbed by GO and rGO. In addition, adsorbed CIP was released from Mont into the solution through electrostatic repulsion. The decrease ratio of CIP was the lowest in the GO/rGO+Mont+pepsin systems. Therefore, the mixture of Mont and GO/rGO decreased the CIP concentration in gastrointestinal fluid to weaken further antibiotic activity of CIP.

Feasibility study on the application of coal gangue as landfill liner material

Coal gangue is one of the largest industrial solid waste all over the world, and many methods have been proposed for the recycling of coal gangue. In the present study, the feasibility of using coal gangue as landfill liner material is studied through a series of laboratory tests in terms of hydraulic conductivity, sorption characteristics and leaching behavior. The results indicated that the hydraulic conductivity of coal gangue could be smaller than the regulatory requirement 1×10^-7cm/s with a void ratio less than 0.60. The batch sorption experiments performed on Pb²⁺ and Zn²⁺ illustrated that the coal gangue showed remarkable sorption capacity for the two heavy metals, and the sorption capacity for Pb²⁺ was larger than that for Zn²⁺. Both the pseudo first-order and pseudo second-order models fitted well with the sorption kinetics data of the Pb²⁺ and Zn²⁺ on the coal gangue, and the Langmuir model was found to best-fit the sorption isotherms. The sorption capacity decreased in presence of multiple heavy metals, both for Pb²⁺ and Zn²⁺. Concentrations of heavy metals leached from the coal gangue were all below the regulatory limits from China MEP and U.S. EPA. These desirable characteristics indicated that the coal gangue has potential to be used as landfill liner materials.

Competitive adsorption of Pb and Cd on bacteria-montmorillonite composite

The characteristics and mechanisms of competitive adsorption of trace metals on bacteria-associated clay mineral composites have never been studied, despite their being among the most common organic-mineral complexes in geological systems. Herein, competitive adsorption of Pb and Cd on Pseudomonas putida-montmorillonite composite was investigated through adsorption-desorption experiment, isothermal titration calorimetry (ITC), and synchrotron micro X-ray fluorescence (μ-XRF). From the experiment, stronger competition was observed on clay mineral than on bacteria-clay composite because more non-specific sites accounted for heavy metal adsorption on clay mineral surface at the studied pH 5. Both competing heavy metals tended to react with bacterial fractions in the composite, which was verified by the higher correlation of Cd (and Pb) with Zn (R² = 0.41) elemental distribution than with Si (R² = 0.10). ITC results showed that competitive adsorption exhibited a lower entropy change (ΔS) at the metal-sorbent interfaces compared with single-metal adsorption, revealing that Cd and Pb are bound to the same types of adsorption sites on the sorbent. The competitive effect on bacteria-clay composite was found to be helpful for a better understanding on the fixation, remobilization and subsequent migration of heavy metals in multi-metal contaminated environments.