Huihui Du

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

Soil components (e.g., clays, bacteria and humic substances) are known to produce mineral-organic composites in natural systems. Herein, batch sorption isotherms, isothermal titration calorimetry (ITC), and Cd K-edge EXAFS spectroscopy were applied to investigate the binding characteristics of Cd on montmorillonite(Mont)-humic acid(HA)-bacteria composites. Additive sorption and non-additive Cd(II) sorption behaviour is observed for the binary Mont-bacteria and ternary Mont-HA-bacteria composite, respectively. Specifically, in the ternary composite, the coexistence of HA and bacteria inhibits Cd adsorption, suggesting a “blocking effect” between humic acid and bacterial cells. Large positive entropies (68.1 ~ 114.4 J/mol/K), and linear combination fitting of the EXAFS spectra for Cd adsorbed onto Mont-bacteria and Mont-HA-bacteria composites, demonstrate that Cd is mostly bound to bacterial surface functional groups by forming inner-sphere complexes. All our results together support the assertion that there is a degree of site masking in the ternary clay mineral-humic acid-bacteria composite. Because of this, in the ternary composite, Cd preferentially binds to the higher affinity components-i.e., the bacteria.

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.

Copper adsorption on composites of goethite, cells of Pseudomonas putida and humic acid

Summary In the soil environment, iron oxides co‐occur commonly with different types of organic constituents to produce iron oxide–organic composites that play an important role in the biogeochemical cycling of trace metals. We investigated copper (C u) adsorption on synthetic goethite, P seudomonas putida (CCTCC M209319) bacterial cells, humic acid (HA) and their binary and ternary composites with batch adsorption experiments coupled with isothermal titration calorimetry (ITC). Morphological characterizations show that the three components can form closely combined and heterogeneous aggregates with one another. The kinetics of sorption of C u to these composite materials conforms to the pseudo‐second‐order model, whereas the C u sorptivities deviate from linear additivity. Specifically, C u sorptivities on the binary goethite– P. putida and P. putida – HA and ternary goethite– P. putida – HA composites are less than expected assuming additivity, whereas the opposite is seen for the binary goethite– HA composite. There is considerable masking of adsorption sites in the ternary goethite– P. putida – HA system, but the binding sites on bacteria are not completely covered, as shown by the adsorption enthalpy and entropy (19.60  kJ  mol−1 and 120.9 J mol−1 K−1, respectively) for the ternary composite. We conclude that the binding affinity of C u for the binary goethite–humic acid composite is larger than that for the bacterial composites with goethite or humic acid, or both. This research indicates that bacteria, iron oxide and humic substances exhibit different behaviour in the sequestration of heavy metals when they form various binary and ternary complexes in natural environments. HighlightsHow does mineral–organic interaction affect the binding behaviour of trace elements?First investigation of C u sorption to goethite–bacteria–humic acid composites.Goethite– HA shows enhanced C u adsorption, whereas the opposite is true for bacterial complexes.Various bacteria–iron oxide–humic substance composites behave differently in metal sequestration.

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.

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-LIII 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.