Preferential adsorption of extracellular polymeric substances from bacteria on clay minerals and iron oxide

The adsorption of extracellular polymeric substances (EPS) from Bacillus subtilis on montmorillonite, kaolinite and goethite was investigated as a function of pH and ionic strength using batch studies coupled with Fourier transform infrared (FTIR) spectroscopy. The adsorption isotherms of EPS on minerals conformed to the Langmuir equation. The amount of EPS-C and -N adsorbed followed the sequence of montmorillonite>goethite>kaolinite. However, EPS-P adsorption was in the order of goethite>montmorillonite>kaolinite. A marked decrease in the mass fraction of EPS adsorption on minerals was observed with the increase of final pH from 3.1 to 8.3. Calcium ion was more efficient than sodium ion in promoting EPS adsorption on minerals. At various pH values and ionic strength, the mass fraction of EPS-N was higher than those of EPS-C and -P on montmorillonite and kaolinite, while the mass fraction of EPS-P was the highest on goethite. These results suggest that proteinaceous constituents were adsorbed preferentially on montmorillonite and kaolinite, and phosphorylated macromolecules were absorbed preferentially on goethite. Adsorption of EPS on clay minerals resulted in obvious shifts of infrared absorption bands of adsorbed water molecules, showing the importance of hydrogen bonding in EPS adsorption. The highest K values in equilibrium adsorption and FTIR are consistent with ligand exchange of EPS phosphate groups for goethite surface. The information obtained is of fundamental significance for understanding interfacial reactions between microorganisms and minerals.     

Removal of trace Cr(VI) from water using chitosan-iron nanowires in porous anodic alumina

Chitosan-iron nanowires in porous anodic alumina (PAA) have been successfully prepared under ambient conditions as an adsorbent. The adsorbent was characterized by scanning electron microscopy, X-ray photoelectron spectroscopy and N₂-BET surface area. The results showed that PAA can disperse and protect Fe⁰ nanorods from oxidation. The adsorption characteristics of trace Cr(VI) onto adsorbent have been examined at different initial Cr(VI) concentrations with pH 5. Batch adsorption studies show that the removal percentage of adsorbent for the removal of trace Cr(VI) is strongly dependent on the initial Cr(VI) concentrations. Langmuir and Freundlich isotherm models were used to analyze the experiment data. The adsorption of trace Cr(VI) by adsorbent is well modeled by the Langmuir isotherm and the maximum adsorption capacity of Cr(VI) is calculated as 123.95 mg/g which is very closed to the experiment results. Intraparticle diffusion study shows that the intraparticle diffusion of adsorbent is not the sole rate-controlling step. The negative value of Gibbs free energy change, ΔG^o, indicated that the process of Cr(VI) onto adsorbent was spontaneous. This work has demonstrated that chitosan-iron nanowires in porous anodic alumina as an adsorbent has promising potential for heavy metal removal at trace level.     

Comparison of chromium speciation by CZE and ion exchange followed by AAS

The hydrogen chromate anion (HCrO4-), which is the predominant species in acidic solutions and solutions with low chromium concentration, was determined by capillary zone electrophoresis (CZE) using UV detection on-column at 200 nm. A fused-silica capillary (55 cm x 50 microm i.d.) was employed with a high negative voltage of 20 kV. Total chromium was determined after reduction by H2O2 and its complexation by EDTA. The use of H2O2 as reducing agent is advantageous, as it does not increase the conductivity of the solution. Detection limits achieved (for 200 s injection time) were 30 and 8 microg/L for Cr(VI) and Cr(III), respectively. The CZE results obtained for Cr(III) and Cr(VI) were compared with those obtained by ion exchange with subsequent AAS.     

Interaction of Pseudomonas putida with kaolinite and montmorillonite: a combination study by equilibrium adsorption, ITC, SEM and FTIR

Equilibrium adsorption along with isothermal titration calorimetry (ITC), Fourier transform infrared spectra (FTIR) and scanning electron microscopy (SEM) techniques were employed to investigate the adsorption of Pseudomonas putida on kaolinite and montmorillonite. A higher affinity as well as larger amounts of adsorption of P. putida was found on kaolinite. The majority of sorbed bacterial cells (88.7%) could be released by water from montmorillonite, while only a small proportion (9.3%) of bacteria desorbed from kaolinite surface. More bacterial cells were observed to form aggregates with kaolinite, while fewer cells were within the larger bacteria–montmorillonite particles. The sorption of bacteria on kaolinite was enthalpically more favorable than that on montmorillonite. Based on our findings, it is proposed that the non-electrostatic forces other than electrostatic force play a more important role in bacterial adsorption by kaolinite and montmorillonite. Adsorption of bacteria on clay minerals resulted in obvious shifts of infrared absorption bands of water molecules, showing the importance of hydrogen bonding in bacteria–clay mineral adsorption. The enthalpies of −4.1 ± 2.1 × 10⁻⁸ and −2.5 ± 1.4 × 10⁻⁸ mJ cell⁻¹ for the adsorption of bacteria on kaolinite and montmorillonite, respectively, at 25 °C and pH 7.0 were firstly reported in this paper. The enthalpy of bacteria–mineral adsorption was higher than that reported previously for bacteria–biomolecule interaction but lower than that of bacterial coaggregation. The bacteria–mineral adsorption enthalpies increased at higher temperature, suggesting that the enthalpy–entropy compensation mechanism could be involved in the adsorption of P. putida on clay minerals. Data obtained in this study would provide valuable information for a better understanding of the mechanisms of mineral–microorganism interactions in soil and associated environments.     

Analytical speciation of chromium in in-vitro cultures of chromate-resistant filamentous fungi

In this work, different analytical speciation schemes have been used to study the reduction of Cr(VI) by a chromate-resistant strain of filamentous fungi Ed8 (Aspergillus sp), indigenous to contaminated industrial wastes. As demonstrated previously, this strain has the capability to reduce chromate present in the growth medium without its accumulation in the biomass, yet the reduced chromium end-products have not been characterized. Liquid growth medium, initially containing 50 mg L(-1) Cr(VI), was analyzed for Cr(III)/Cr(VI) and for total Cr at different time intervals (0-24 h) after inoculation with fungi. Three hyphenated procedures, based on the Cr(III)-EDTA formation and species separation by anion-exchange or ion-pairing reversed-phase chromatography with ICP-MS or DAD detection were used. The results obtained for Cr(VI) in each case were consistent, demonstrating efficient reduction of chromate during 24 h of Ed8 growth. However, pre-column complexation with EDTA did not ensure complete recovery of the reduced forms of chromium in the above procedures. An alternative speciation scheme, based on extraction of Cr(VI)-benzyltributylammonium bromide (BTAB) ion pairs into chloroform and subsequent determination of residual chromium by ICP-MS has provided evidence on the effective conversion of chromate into reduced chromium species in the growth medium. The results indicate the feasibility of using Ed8 strain for chromate bioremediation purposes. Analytically it can be concluded that speciation of chromium in biological systems should not be limited to its two most common oxidation states, because the actual reduced chromium species are not converted quantitatively to Cr(III)-EDTA.     

Preconcentration of Cr(VI) ion on melamine formaldehyde resin

Summary Trace amounts of Cr(VI) have been enriched from aqueous solutions by exchange of Cr(VI) on melamine-formaldehyde resin. The exchange capacity increased at lower pH-values. The material was used to preconcentrate Cr(VI) very efficiently from 0.1 g ml^–1 solutions of chromate. After the Cr(VI) adsorbed on the column is eluted, it is analysed by atomic absorption spectroscopy and almost 100% recovery was achieved in every instance. The effect of other anions was examined and it was observed that the adsorption of Cr(VI) was not significantly affected by the presence of other anions.I dedicate this study to my supervisor, Prof. Dr. Huseyin Afsar     

Structural studies of Na-montmorillonite exchanged with Fe2+, Cr3+, and Ti4+ by N2 adsorption and EXAFS

The structures of Fe(2+)-, Cr(3+)-, and Ti(4+)-modified montmorillonite prepared from ion exchange of the Na-clay with Fe(2+), Cr(3+), and Ti(4+) were investigated. Conventional BET surface area and spectroscopic analysis by extended adsorption fine structure (EXAFS) were applied. It was shown that the BET surface area of Na-clay was similar to that of Fe-clay, but somewhat different from those of Cr- and Ti-clay; it decreased in the order Na- > Fe- > Ti- > Cr-montmorillonite. This sequence appeared to be consistent with the ion size Na(+) (0.95 nm)>Fe(2+) (0.65 nm)>Cr(3+) (0.62 nm), except for Ti(4+) (0.69 nm). EXAFS data showed that some Si atoms within montmorillonite were replaced by Ti atoms and that a neostructure of titanium oxide was formed.     

The Adsorption of Herbicides and Pesticides on Clay Minerals and Soils. Part 2. Atrazine

The adsorption of atrazine and two model compounds,2-chloropyrimidine and 3-chloropyridine on clay minerals(bentonite, montmorillonite and kaolinite), organic matter (humic acid) and soil (with and without organic matter) has beenstudied using FT-infrared spectroscopy (IR), thermogravimetric analysis (TGA), high pressureliquid chromatography (HPLC) and X-ray diffraction (XRD).3-Chloropyridine, 2-chloropyrimidine and atrazine were adsorbedthrough hydrogen bonding on bentonite, montmorillonite, humic acid and soil. In addition tohydrogen bonding, protonation of 3-chloropyridine and atrazine was also observed.In the adsorption of 2-chloropyrimidine on bentonite and montmorillonitean ion exchange mechanism also occurred. No adsorption of 3-chloropyridine or 2-chloropyrimidine wasobserved on the kaolinite clay mineral.Both the clay minerals and organic matter of soil contribute tothe adsorption of organic compounds on soil but the clay minerals bentonite and montmorilloniteplay a major role in their adsorption on soil.     

Preparation of reduced graphene oxide/montmorillonite composite hydrogel and its applications for chromium(VI) and organic compounds adsorption

A novel 3D porous reduced graphene oxide/montmorillonite composite hydrogel (rGO–MMT) was prepared by solvent method, where the MMT nanosheets were homogenously dispersed in 3D rGO hydrogel. The porous 3D structure and the high dispersion of MMT nanosheets can promote the adsorption capacity. The effects of MMT content (wt%), the initial concentration of Cr(VI) solution (C0), pH value (pH0), the adsorption dose and temperatures on the adsorption capacity of rGO–MMT for Cr(VI) ions have been investigated. The optimum pH value for Cr(VI) adsorption is 2, and the adsorption capacity increases with MMT content and adsorption temperature. The rGO–MMT composite hydrogel displays the excellent adsorption property for both the heavy metal and organic pollutants. The adsorption capacity of rGO–MMT composite hydrogel is obviously higher than those of single rGO hydrogel and MMT due to the synergistic adsorption of rGO hydrogel and MMT. The adsorption of Cr(VI) ions on the rGO–MMT composite hydrogel follows linear pseudo-second-order kinetics, and the Langmuir model describes the adsorption process much better. Thermodynamic parameters indicate that adsorption is spontaneous, favorable and endothermic in nature.

Graphic abstract

     

Adsorption of Pesticides on Soil Components

The adsorption of two pesticides (2,4-phenoxy acetic acid (2,4D) and thiabendazol) on silica, alumina, kaolin and montmorillonite is studied from adsorption isotherms and enthalpies. 2,4D is not adsorbed by silica, kaolinite and montmorillonite even in the presence of 0.01 mol l?1 divalent cations. On alumina, the energy of adsorption is comparable with that of the formation of an acid-base complex. Thiabendazol can be adsorbed on silica and clays from an ethanol solution. Most adsorption isotherms are of the Langmuir type and correspond to roughly constant adsorption enthalpies as a function of coverage except for kaolin where adsorption on both lateral and basal faces can be involved. Adsorption after introducing humic acids to the system was also studied for kaolin.     

Surfactant immobilized interlayer species bonded to montmorillonite as recyclable adsorbent for lead ions

Removal of lead ( Pb2+ ) ions from aqueous solution by adsorption onto surfactant-immobilized interlayer species bonded to montmorillonite clay (SIIS-clay) was investigated. Surfactant-immobilized interlayer chromate bound to clay (SIICr-clay) was prepared by treatment of montmorillonite clay with hexadecyltrimethylammonium (HDTMA) bromide followed by chromate adsorption at the intergallery framework of the clay. Experiments were carried out as a function of solution pH, solute concentration, and temperature (5-45 degrees C). The Dubinin-Kaganer-Radushkevich (DKR) model was adopted to describe the single-solute adsorption isotherms. Adsorption energy for lead ions on SIICr-clay computed from the DKR equation shows that a precipitation mechanism was operative. The thermodynamic parameters ( Delta G 0, Delta H 0, and Delta S 0) for lead ion adsorption on SIICr-clay were also determined from the temperature dependence. The kinetics of metal ion adsorption was examined and the first-order rate constant was finally evaluated. Adsorbed lead ions could be recovered completely on leaching with the disodium salt of ethylenediamine tetraacetic acid (EDTA) solution and the adsorbent was found to readsorb lead ions repeatedly after leaching. Thus, surfactant-modified smectite clays offer an effective method for designing a recyclable adsorbent for lead ions.     

Determination of hexavalent chromium in exhaled breath condensate and environmental air among chrome plating workers

Chromium speciation has attracted attention because of the different toxicity of Cr(III), which is considered relatively non-toxic, and Cr(VI), which can cross cell membranes mainly as a chromate anion and has been classified as a class I human carcinogen. The aims of the present study were to measure soluble Cr(VI) levels in environmental samples, to develop a simple method of quantifying Cr(VI) in exhaled breath condensate (EBC), and to follow the kinetics of EBC Cr(VI) in chrome plating workers.Personal air samples were collected from 10 chrome platers; EBC was collected from the same workers immediately after the work shift on Tuesday and before the work shift on the following Wednesday. Environmental and EBC Cr(VI) levels were determined by means of colorimetry and electrothermal absorption atomic spectrometry, respectively.The method of detecting Cr(VI) in environmental air was based on the extraction of the Cr(VI)-diphenylcarbazide (Cr(VI)-DPC) complex in 1-butanol, whereas EBC Cr(VI) was determined using a solvent extraction of Cr(VI) as an ion pair with tetrabutylammonium ion, and subsequent direct determination of the complex (Cr(VI)-DPC) in EBC.Kinetic data showed that airborne Cr(VI) was reduced by 50% in airway lining fluid sampled at the end of exposure and that there was a further 50% reduction after about 15 h. The persistence of Cr(VI) in EBC supports the use of EBC in assessing target tissue levels of Cr(VI).     

Comparison of chromium speciation by CZE and ion exchange followed by AAS

The hydrogen chromate anion (HCrO₄ ^–), which is the predominant species in acidic solutions and solutions with low chromium concentration, was determined by capillary zone electrophoresis (CZE) using UV detection on-column at 200 nm. A fused-silica capillary (55 cm × 50 μm i.d.) was employed with a high negative voltage of 20 kV. Total chromium was determined after reduction by H₂O₂ and its complexation by EDTA. The use of H₂O₂ as reducing agent is advantageous, as it does not increase the conductivity of the solution. Detection limits achieved (for 200 s injection time) were 30 and 8 μg/L for Cr(VI) and Cr(III), respectively. The CZE results obtained for Cr(III) and Cr(VI) were compared with those obtained by ion exchange with subsequent AAS.     

Speciation of aqueous chromium(VI) solutions with the aid of Q-mode factor analysis followed by oblique projection

The identities of the species of chromium(VI) that are present in aqueous solution, their spectra and their equilibria, continue to be a subject of discussion in the literature. In this paper, the composition of the Cr(VI) equilibria was estimated from the UV-vis spectra of dilute potassium dichromate solutions, without any prior knowledge of the quantities of pure components, with the aid of Imbrie Q-mode factor analysis (Q-mode FA) followed by Varimax rotation and Imbrie oblique projection. Combining these results with the k-matrix method, it was possible to obtain the spectra of the individual Cr(VI) species. Sets of 3.3x10(-4) and 3.3x10(-5) mol l(-1) Cr(VI) solutions were studied. In the pH range from 1 to 12, two factors were identified, which were related to the two species, chromate ion (CrO(4)(2-)) and bichromate ion (HCrO(4)(-)). When the analysis was extended to concentrated acid media, another factor appeared, which was related to chromic acid (H(2)CrO(4)). No evidence for the dichromate ion (Cr(2)O(7)(2-)) was seen at the Cr(VI) concentrations used. The spectra of the pure components were obtained and pK values for the first and second chromic acid dissociations were estimated as -0.54 and 5.8, respectively.     

Study of chromium(VI) adsorption onto modified activated carbons with respect to analytical application

Two different types of modification of activated carbon, by treatment with concentrated solution of HNO₃ and outgassing treatment at high temperature, were studied in order to obtain the most effective adsorption of chromium(VI) ions from water solution. The basic parameters affecting the adsorption capacity of Cr(VI) ions on modified activated carbons were studied in details and the effect of modifications of activated carbons has been determined by studying the initial runs of adsorption isotherms. The obtained Cr(VI) adsorption isotherms were well fitted in the Freundlich equation. The reduction of Cr(VI) to Cr(III) and further ion exchange mechanism of adsorption onto oxidizing activated carbon and surface precipitation to Cr(OH)₃ in case of outgassing activated carbon were found as the main adsorption mechanisms of Cr(VI) ions onto modified activated carbons. Presence of chlorides and nitrates in studied adsorption system strongly decreased the adsorption ability of Cr(VI) onto outgassing activated carbon and mechanism of this behavior is proposed.     

Bidentate reagents form cyclic organic-Cr(VI) molecules for aiding in the removal of Cr(VI) from water: density functional theory and experimental results

Hexavalent chromium, Cr(VI), in the form of chromate (CrO₄ ^2?) or dichromate (Cr₂O₇ ^2?) is a well-described carcinogen found in the drinking water in many parts of the country at levels deemed unsafe by the U.S. Environmental Protection Agency and the World Health Organization. We report on the ability of bidentate organic molecules containing diols or diamines to capture chromate ions from aqueous sources by forming cyclic organic-Cr(VI) carbonates or ureas. After their formation, the cyclic organic-Cr(VI) molecules are readily absorbed onto granulated activated charcoal to facilitate Cr(VI) removal. Using density functional theory, E ₀ values for the reactions of diols and diamines with chromate were calculated and correlated with the experimental findings of Cr(VI) removal.     

The Adsorption of Herbicides and Pesticides on Clay Minerals and Soils. Part 1. Isoproturon

The adsorption of isoproturon and two model compounds, N,N-dimethylurea and4-isopropylaniline, on clay minerals (bentonite,montmorillonite and kaolinite), organic matter (humic acid) and soil (with and without organic matter) has been studied using FT-infrared spectroscopy (IR), thermogravimetric analysis (TGA), high pressure liquid chromatography (HPLC) and X-ray diffraction (XRD).N,N-dimethylurea interacted with bentonite and montmorillonite by the coordination of the carbonyl group, directly or indirectly through water molecules, with exchangeable cations. Adsorption on humic acid was due to hydrogen bonding with the active sites of the adsorbent. The amino group ofN,N-dimethylurea appears tobe relatively inactive during adsorption. The mechanisms involved in the adsorption of 4-isopropylaniline were hydrogen bonding and protonation. No adsorption of 4-isopropylaniline was observed on kaolinite. The investigation of isoproturon suggested that both the carbonyl and amino groups of isoproturon were involved in interactions with the active sites of the adsorbents. Both the clay minerals and organic matter of soil contribute to the adsorption of organic compounds on soil but the clay minerals bentonite and montmorillonite play a major role in their adsorption on soil.     

The use of solution microcalorimetry to evaluate chemically modified fish scales as a viable adsorbent for heavy metals

Here, we report for the first time the direct and simultaneous determination of kinetic and energetic parameters of Cr(VI) sorption on chemically modified fish scales (GA-scale) using solution microcalorimetry. Characterization has suggested that electrostatic interactions between scales collagen positive charges and chromate negative charges constitute the majority of the interactions. The microcalorimetric kinetic data of Cr(VI) sorption on GA-scale were successful adjusted to a three-parameter exponential function. The enthalpies of Cr(VI) sorption on GA-scale are highly exothermic (from −226.43 to −183.79 kJ mol⁻¹), and Cr(VI) interaction energies decrease as initial Cr(VI) in solution increases. The kinetic and thermodynamic from solution microcalorimetry results suggest that the interactions GA-scale/Cr(VI) occur mainly by surface reactions. The maximum adsorption capacity of GA-scale for Cr(VI) was found to be comparable with some commercial adsorbent samples.     

Speciation of Cr (III) and Cr (VI) via Reversed Phase HPLC with Inductively Coupled Plasma Emission Spectroscopic Detection (HPLC-ICP)24

Abstract

Chromium ions, viz., chromic (Cr^±3=III) and chromate (Cr^±6 = VI), can be reliably, conveniently, reproducibly, and quickly separated and detected by the use of conventional paired-ion, reversed phase (RP) high performance liquid chromatography (HPLC) together with refractive index (RI) and/or inductively coupled plasma emission spectroscopic (ICP) detection. A number of novel paired-ion approaches have now been developed, using PIC A (tetrabutylammonium hydroxide) or PIC B (sodium n-alkyl sulfonate) separately in the mobile phase. This allows for the retention of each Cr species depending on the particular ion pairing reagent being used, while the remaining Cr ion elutes in the solvent front. Changing the ion pairing reagent reverses the overall situation. The total time for each HPLC analysis is about 10 mins. ICP detection provides for a complete, overall method of speciation for both Cr (III) and Cr (VI) via two separate injections, together with quantitation for both species. This method of using paired-ion RP-HPLC can easily be applied to other mixtures of inorganic cations and anions, presumably with equally successful results. Minimum limits of detection are computed for chromate via direct-ICP, using at least two basic methods for such computations. It is suggested that virtually all chromatographic detection limits can be significantly improved by the application of newer, spectroscopic based methods of automated computation of detection limits.     

Potentiometric studies of the complexes of chromium(VI), molybdenum(VI) and tungsten(VI) with some Azoxine S dyes

The equilibrium constants of the complexation reactions of Cr(VI), Mo(VI) and W(VI) with 8-hydroxyquinoline-5-sulphonic acid (OXS), 7-phenylazo-8-hydroxyquinoline-5-sulphonic acid (PAZOXS), 7-(4-sulphophenylazo)-8-hydroxyquinoline-5-sulphonic acid (SPAZOXS) and 7-(4-sulphonaphthylazo)-8-hydroxyquinoline-5-sulphonic acid (SNAZOXS) have been determined by potentiometric pH titration. The values in the case of chromate are different from those for molybdate and tungstate. The order of stabilities is OXS > PAZOXS > SPAZOXS > SNAZOXS.