Cetin Kantar

Chromium(VI) bioremoval by pseudomonas bacteria: role of microbial exudates for natural attenuation and biotreatment of Cr(VI) contamination

Laboratory batch and column experiments were conducted to investigate the role of microbial exudates, e.g., exopolymeric substance (EPS) and alginic acid, on microbial Cr(VI) reduction by two different Pseudomonas strains (P. putida P18 and P. aeuroginosa P16) as a method for treating subsurface environment contaminated with Cr(VI). Our results indicate that microbial exudates significantly enhanced microbial Cr(VI) reduction rates by forming less toxic and highly soluble organo-Cr(III) complexes despite the fact Cr(III) has a very low solubility under the experimental conditions studied (e.g., pH 7). The formation of soluble organo-Cr(III) complexes led to the protection of the cells and chromate reductases from inactivation. In systems with no organic ligands, soluble organo-Cr(III) end products were formed between Cr(III) and the EPS directly released by bacteria due to cell lysis. Our results also provide evidence that cell lysis played an important role in microbial Cr(VI) reduction by Pseudomonas bacteria due to the release of constitutive reductases that intracellularly and/or extracellularly catalyzed the reduction of Cr(VI) to Cr(III). The overall results highlight the need for incorporation of the release and formation of organo-Cr(III) complexes into reactive transport models to more accurately design and monitor in situ microbial remediation techniques for the treatment of subsurface systems contaminated with Cr(VI).

In situ stabilization of chromium(VI) in polluted soils using organic ligands: The role of galacturonic, glucuronic and alginic acids

Laboratory batch sorption and column experiments were performed to investigate the role of organic ligands such as galacturonic, glucuronic and alginic acids (main constituents of bacterial exopolymeric substances (EPS)) on Cr(VI) uptake and transport in heterogeneous subsurface media. Our batch sorption experiments demonstrate the addition of galacturonic, glucuronic and alginic acids to soils enhances Cr(VI) uptake by soil at pH values <7.7 depending on the concentration of the ligand and pH used. The enhanced Cr(VI) uptake at pH values <7.7 may be explained through either the catalytic reduction of Cr(VI) to Cr(III) by the surface-bound organic matter/Fe oxides and/or the dissolved metal ions (e.g., Fe(III)) from the soil. On the other hand, organic ligands have no or little effect on Cr(VI) uptake under highly alkaline pH conditions since the catalytic Cr(VI) reduction decreases with increasing pH. Similarly, the results from column experiments show that, depending on the concentration of organic ligands, the Cr(VI) breakthrough curves were significantly retarded relative to the organic acid-free systems at pH 7.6. A significant portion of Cr(VI) initially added to the feed solution was not readily recoverable in the effluent, indicating Cr(VI) reduction in columns, most probably catalyzed by surface-bound metal-oxides (e.g., Fe oxides) or dissolved metal ions such as Fe(II; III). The overall results suggest that EPS constituents such as glucuronic, galacturonic and alginic acids may play a significant role on Cr(VI) stabilization in subsurface systems under acidic to slightly alkaline pH conditions.

Role of microbial exopolymeric substances (EPS) on chromium sorption and transport in heterogeneous subsurface soils: I. Cr(III) complexation with EPS in aqueous solution

Chromium (III) binding by exopolymeric substances (EPS) isolated from Pseudomonas putida P18, Pseudomonas aeruginosa P16 and Pseudomonas stutzeri P40 strains were investigated by the determination of conditional stability constants and the concentration of functional groups using the ion-exchange experiments and potentiometric titrations. Spectroscopic (EXAFS) analysis was also used to obtain information on the nature of Cr(III) binding with EPS functional groups. The data from ion-exchange experiments and potentiometric titrations were evaluated using a non-electrostatic discrete ligand approach. The modeling results show that the acid/base properties of EPSs can be best characterized by invoking four different types of acid functional groups with arbitrarily assigned pK(a) values of 4, 6, 8 and 10. The analysis of ion-exchange data using the discrete ligand approach suggests that while the Cr binding by EPS from P. aeruginosa can be successfully described based on a reaction stoichiometry of 1:2 between Cr(III) and HL(2) monoprotic ligands, the accurate description of Cr binding by EPSs extracted from P. putida and P. stutzeri requires postulation of 1:1 Cr(III)-ligand complexes with HL(2) and HL(3) monoprotic ligands, respectively. These results indicate that the carboxyl and/or phosphoric acid sites contribute to Cr(III) binding by microbial EPS, as also confirmed by EXAFS analysis performed in the current study. Overall, this study highlights the need for incorporation of Cr-EPS interactions into transport and speciation models to more accurately assess microbial Cr(VI) reduction and chromium transport in subsurface systems, including microbial reactive treatment barriers.

Role of microbial exopolymeric substances (EPS) on chromium sorption and transport in heterogeneous subsurface soils: II. Binding of Cr(III) in EPS/soil system

Laboratory batch sorption and column experiments were performed to investigate the effects of microbial EPSs isolated from Pseudomonas putida P18, Pseudomonas aeruginosa P16 and Pseudomonas stutzeri P40 on Cr(III) mobility in heterogeneous subsurface soils. Our batch and column results indicate that microbial EPS may have a pronounced effect on Cr(III) sorption and transport behavior depending on system conditions (e.g., pH, type of EPS). While EPS had no effect on Cr(III) sorption at pH<5, it led to a significant decrease in Cr(III) sorption under slightly acidic to alkaline pH range. Column experiments performed at pH 7.9 suggest that, in the presence of EPS, chromium(III) was significantly mobilized relative to non-EPS containing system due to the formation less sorbing and highly soluble Cr-EPS complexes and competition of EPS against Cr for surface sites. A two-site non-electrostatic surface chemical model incorporating a discrete ligand approach for the description of Cr-EPS interactions accurately predicted Cr(III) sorption and transport behavior in the presence of EPS under variable chemical conditions. Our simulations show that an accurate description of Cr(III) transport in the presence of EPS requires incorporation of proton and Cr(III) binding by EPS, EPS binding by soil minerals, Cr(III) binding by soil minerals, and ternary Cr(III)-EPS surface complexes into the transport equations. Although this approach may not accurately describe the actual mechanisms at the molecular level, it can improve our ability to accurately describe the effects of EPS on Cr(III) mobility in subsurface environment relative to the use of distribution coefficients (K(d)).

Interactions Between Uronic Acids and Chromium(III)

Laboratory ion-exchange experiments were performed to investigate the complexation behavior of Cr(III) with uronic acids, such as galacturonic, glucuronic, and alginic acid (main constituents of bacterial exopolymeric substances). The experimental data were analyzed with a chemical equilibrium model in FITEQL to determine the reaction stoichiometries and stability constants for the formation of Cr-ligand complexes. Analysis of ion-exchange data with a chemical model indicates that the accurate description of Cr(III) complexation with both glucuronic and galacturonic acids requires postulation of a mixture of 1:1/1:2 complexes between Cr(III) and ligands under the experimental conditions studied (e.g., pH 4), but that the Cr-alginic acid binding can be modeled based on a reaction stoichiometry of 1:1 Cr-alginic acid complexes. Because of the complex nature of alginic acid, a nonelectrostatic, discrete ligand approach was used to determine proton and Cr binding with the functional groups of alginic acid. In this approach, alginic acid was conceptualized as being composed of a suite of two monoprotic acids (HL1 and HL2) with arbitrarily assigned pKa values of two and four, respectively. The results indicate that Cr binding with uronic acids mainly occurs through carboxylic groups under acidic to slightly alkaline pH conditions (e.g., pH < 8). The overall results of the present study indicate that the formation of such Cr-ligand complexes may have a pronounced effect on Cr(III) transport, solubility and bioavailability in natural systems.

Modeling Cd(II) adsorption to heterogeneous subsurface soils in the presence of citric acid using a semi-empirical surface complexation approach

Laboratory batch sorption experiments were conducted to understand the effect of citrate on cadmium sorption to heterogeneous subsurface soils. Our results indicate that citrate may have a pronounced effect on Cd(II) sorption depending on system conditions (e.g. pH, ligand concentration). While the presence of citrate had no effect on Cd(II) sorption at pH < 4, it led to a decrease in Cd sorption under slightly acidic to alkaline pH range depending on the concentration of citrate used. Maximum effect of citrate on Cd(II) sorption was observed at pHs between 5 and 7. This coincides with the observed range of maximum citrate adsorption and the formation of Cd-citrate complexes. A two-site non-electrostatic surface chemical model (SCM) based on the Generalized Composite (GC) approach was able to describe the experimental data well over a wide range of conditions, with only six different surface reactions including two ternary (Cd/citrate/soil) surface complexes. Although the semi-empirical surface model used in the simulations does not accurately represent the actual mechanisms at the molecular level, it is relatively simple, and can be effectively used in transport calculations as an alternative to the K(d) approach.

Binding of Pu(IV) to galacturonic acid and extracellular polymeric substances (EPS) from Shewanella putrefaciens, Clostridium sp. and Pseudomonas fluorescens

Abstract The conditional stability constants for trace-level concentrations of Pu(IV) complexing with galacturonic acid and EPS, isolated from axenic Clostridium sp., P. fluorescens and Shewanella putrefaciens CN32 cultures, were determined at pH 4 and an ionic strength of 0.1 M NaCl using an ion-exchange technique. The analysis of ion-exchange data with Schubert´s technique indicates that the Pu binding by galacturonic acid and EPS from Clostridium sp. and S. putrefaciens can be described based on the formation of 1:1 Pu(IV)-ligand complexes. However, the accurate description of Pu binding by EPS from P. fluorescens requires postulation of a mixture of 1:1/1:2 complexes between Pu(IV) and ligands under the experimental conditions studied. The results from the ion-exchange experiments were also modeled based on a non-electrostatic, discrete ligand approach in which bacterial EPS is conceptualized as being composed of a suite of monoprotic acids, HLi, of arbitrarily-assigned pKa (i) values ( e.g. , 4, 6 and 8). The examination of ion-exchange data in a chemical model suggested that only the pKa 4 (L1) and 6 (L2) ligands are sufficient to accurately simulate the Pu(IV)/EPS binding, implying that carboxylic groups in EPS are the primary binding sites for complexing with Pu(IV) under the experimental conditions examined. The affinity of EPS for complexing Pu(IV) decreases in the order of Clostridiumsp.>S. putrefaciens>P. fluorescens although the concentrations of carboxylic groups in EPS decrease in the order of P. fluorescens >S. putrefaciens>Clostridiumsp. This discrepancy may be due to differences in binding affinities between Na+ ion in solution and EPS ligands. At I=0.1 M, models demonstrated that the EPS from P. fluorescens exhibits a much stronger affinity for the Na+ ion compared to ligands from other EPS; therefore, the deprotonated carboxylic sites of EPS from P. fluorescens are hypothesized to be mostly bound by Na+ in solution.

Movement and adsorption of methamidophos in clay loam and sandy loam soils

Laboratory batch and column experiments were performed to better understand the sorption and transport behaviour of commercial-grade methamidophos (Tamaron SL 600) in clay loam (CL) and sandy loam (SL) soils. The batch sorption experiments show that the soil texture and methamidophos concentration play a major role in the sorption and migration behaviour of methamidophos. At low surface coverage (q < 0.6 mg g−1), methamidophos sorbs onto the CL soil more strongly than onto the SL soil. However, for q > 0.6 mg g−1, the SL soil exhibits a much higher sorption affinity for methamidophos than the CL soil. The equilibrium isotherms for the sorption of methamidophos onto the SL and CL soils were non-linear, and were best described by the Freundlich equation. The results of column experiments indicate that the recovery of methamidophos during desorption was incomplete due to either partially irreversible sorption to high-energy surface sites or strongly rate-limited desorption. Methamidophos was more readily leached out from the SL soil column as consistent with the batch isotherm data.

Role of low molecular weight organic acids on pyrite dissolution in aqueous systems: implications for catalytic chromium (VI) treatment

A systematic study combining batch experiments with spectroscopic analyses was carried out to better understand the effects of various organic acids on pyrite dissolution and subsequent Cr(VI) removal in aqueous systems. Our results suggest that organic acids had no effect on total Fe dissolution from pyrite relative to systems containing no acid. However, while nearly 100% of total Fe dissolved from pyrite was in Fe(II) form in the absence of ligands, the addition of organic acids led to significant oxidation of Fe(II) species to Fe(III). The degree and extent of Fe(II) oxidation increased in the order: tartrate < salicylate < oxalate ≈ citrate < EDTA. Except for salicylate (an aromatic compound), this stimulatory effect observed in Fe(II) oxidation was well correlated with the strength of Fe-ligand complexes. In systems containing Cr(VI), the amount of Fe dissolved increased significantly relative to non-Cr(VI) containing system, and the ligands enhanced the dissolution of surface oxidation products from pyrite. Overall, it is clear that the dissolution of pyrite with organic acids had very little effect on solution phase Cr(VI) removal, but significantly stimulated surface phase Cr(VI) reduction by removing surface oxidation products, and thus creating new surface sites for extended Cr(VI) removal.

Role of Major Groundwater Ions on Reductive Cr(VI) Immobilization in Subsurface Systems with Pyrite

Laboratory batch and column experiments were performed to better understand the effects of Ca2+, Mg2+, and HCO3− on Cr(VI) removal from aqueous systems with pyrite. Batch results show that increasing HCO3− concentration led to an increase in Cr(VI) removal by pyrite due to pH buffering capacity of HCO3−. However, while Ca2+ and Mg2+ individually had no effect on Cr(VI) removal at pH 4, the addition of Ca2+ or Mg2+ to systems containing HCO3− resulted in a significant decrease in Cr(VI) removal at pH 8 relative to the systems containing HCO3− alone. The XPS data proved that while Ca2+ precipitated as CaCO3(S) onto pyrite surface, Mg2+ sorbed and/or accumulated as Mg(OH)2(S) onto oxidized pyrite surface. The formation of surface precipitates (e.g., CaCO3) inhibited further Cr(VI) reduction by blocking electron transfer between Cr(VI) and pyritic surface sites. While the precipitation of Ca2+ as CaCO3 led to a significant decrease in effluent pH, the decrease in effluent pH was very low in systems containing Mg2+, most probably due to much higher solubility of Mg2+ at pH 8. Zeta potential measurements provided further evidence that while Ca2+ or Mg2+ had no effect on zeta potential of pyrite particles under acidic conditions (e.g., pH < 7), the addition of Ca2+ or Mg2+ to systems containing Cr(VI) reversed the pyrite surface potential from negative to positive under alkaline pH conditions (e.g., pH > 8) relative to system containing only Cr(VI), suggesting the sorption and/or accumulation of surface precipitates on pyrite surface.