Pillared graphene oxide composite as an adsorbent of soluble hydrocarbons in water: pH and organic matter effects.

Graphene oxide (GO) is a single-atom-thick sheet of carbon with oxygen-containing functional groups decorating its basal plane and edge sites. Most of its high surface area can be lost due to restacking of individual layers during the synthesis and drying of GO-based bulk sorbents. There is great interest to increase the specific surface area of graphene-based sorbents by introducing organic molecules as "pillaring agents" between GO sheets to hinder the stacking process and create sorbents with elevated surface area. This work synthesizes pillared GO by introducing chitosan (CS), a linear polysaccharide with various molecular weights. A composite of low molecular weight CS at a CS/GO ratio of 0.1 is shown to have the highest specific surface area (up to 70.5 m2/g) in comparison to the medium and high CS molecular weight, pristine GO, and the CS/GO composite materials. The affinity of the optimized GO/CS composites towards benzene, toluene, and naphthalene was evaluated at 19.3 mg/L of organic matter content while altering pH. Sips and Langmuir adsorption isotherm models well described the adsorption behavior, and benzene adsorption performance was reduced at low pH. Related to the presence of dissolved organic matter (DOM) in solution, lower diffusivity constants (k1) in hydrocarbon systems were recorded. Our results demonstrate the feasibility of CS as a potential pillaring agent in CS/GO composites to increase specific surface area and enhance the capture of soluble hydrocarbons from aqueous solutions.

Improved reductive transformation of iopromide by magnetite containing reduced graphene oxide nanosacks as electron shuttles.

The novel application of magnetite containing reduced graphene oxide nanosacks (MrGO-N) as electron shuttles to improve the reductive degradation of pharmaceutical pollutant, iopromide (IOP), was evaluated. The MrGO-N were synthesized by ultrasonicated nebulization process, and their physicochemical characterization was performed by potentiometric titrations, zeta potential, high resolution transmission electron microscopy (HR-TEM), X-ray diffraction, as well as by Raman and Fourier transform infrared spectroscopies. Results demonstrated the thermal reduction of precursor graphene oxide sheets, the removal of different oxygenated groups, and the successful assembly of magnetite nanoparticles (MNP) in the graphene sacks. Also, reduction experiments revealed 72 % of IOP removal efficiency and up to 2.5-fold faster degradation of this pollutant performed with MrGO-N as redox catalysts in batch assays and with sulfide as electron donor. Chemical transformation pathway of IOP provides evidence of complete dehalogenation and further transformation of aromatic ring substituents. Greater redox-mediating ability of MrGO-N was observed, which was reflected in the catalytic activity of these nanomaterials during the reductive degradation of IOP. Transformation byproducts with simpler chemical structure were identified, which could lead to complete degradation by conventional methodologies in a complementary treatment process. Redox-mediating activity of MrGO-N could potentially be applied in wastewater treatment systems in order to facilitate the biodegradation of priority contaminants.

Biofilm microbial composition changes due to different surface chemical modifications of activated carbon cloths in the biotransformation of 4-nitrophenol.

Activated carbon cloths (ACCs) were used as biofilms supports in the anaerobic biotransformation of 4-nitrophenol (4NP). As received ACC material (AW) was oxidized with HNO3 (OX) and then functionalized with anthraquinone-2,6-disulfonate (AQ). The three ACCs were packed in hybrid UASB reactors and seeded with anaerobic granular sludge for biotransformation experiments. The results indicated that ACC-packed bioreactors improved the biotransformation of 4NP by twofold as compared to the control reactor without support materials. However, the biotransformation effciency of AW, OX and AQ was very similar (59%), indicating the role of ACC as biofilm support and not as redox mediator. After 4NP biotransformation several physicochemical and biological changes were observed like (1) the point of zero charge (pHPZC) shift from acidic values (AW = 5.0, OX = 3.4, AQ = 3.1) to neutral values (pHPZC = 7.6 on average), (2) increase in the concentration of acidic and basic surface functional groups over ACC materials and the amount of supported biomass on ACCs due to biofilm formation, and (3) enrichment of exoelectrogenic microorganisms belonging to the genera Geobacter over carbonyl-rich ACC surface as revealed by 16S rRNA amplicon sequencing. Overall, the results suggest that chemical modifications of ACCs changed the microbial composition of the biofilm, but the higher concentration of carbonyl groups on ACC did not affect the biotransformation of 4NP.

Synthesis, structural characterization and Cu(ii) adsorption behavior of manganite (γ-MnOOH) nanorods.

New alternatives for the removal of transition metal ions that present an environmental risk are required. The chemical adsorption of these ions on surfaces with chemisorbent properties represents a promising area of research. In this work, manganite (γ-MnOOH) nanorods were synthesized, with a surface area of 20.22 m2 g-1, pore size of 32.18 nm and pore volume of 0.1627 cm3 g-1. After chemical and structural characterization of the manganite sample, it was evaluated as an adsorbent of Cu(ii) from aqueous solution. The equilibrium adsorption data were well fitted by the Langmuir isotherm, and the results indicated that the maximum adsorption capacity of Cu(ii) was 11.926 mg g-1. Cu(ii) ion adsorption on the manganite surface is a spontaneous and exothermic process (ΔG°< 0 and ΔH°< 0). The negative value of ΔS° suggests the stability of the adsorption process without structural change at the manganite-aqueous solution interface. A scheme for chemisorption of Cu(ii) ions on the hydroxylated surface of manganite is proposed.

Novel application of magnetic nano-carbon composite as redox mediator in the reductive biodegradation of iopromide in anaerobic continuous systems.

The redox-mediating capacity of magnetic reduced graphene oxide nanosacks (MNS) to promote the reductive biodegradation of the halogenated pollutant, iopromide (IOP), was tested. Experiments were performed using glucose as electron donor in an upflow anaerobic sludge blanket (UASB) reactor under methanogenic conditions. Higher removal efficiency of IOP in the UASB reactor supplied with MNS as redox mediator was observed as compared with the control reactor lacking MNS. Results showed 82% of IOP removal efficiency under steady state conditions in the UASB reactor enriched with MNS, while the reactor control showed IOP removal efficiency of 51%. The precise microbial transformation pathway of IOP was elucidated by high-performance liquid chromatography coupled to mass spectroscopy (HPLC-MS) analysis. Biotransformation by-products with lower molecular weight than IOP molecule were identified in the reactor supplied with MNS, which were not detected in the reactor control, indicating the contribution of these magnetic nano-carbon composites in the redox conversion of this halogenated pollutant. Reductive reactions of IOP favored by MNS led to complete dehalogenation of the benzene ring and partial rupture of side chains of this pollutant, which is the first step towards its complete biodegradation. Possible reductive mechanisms that took place in the biodegradation of IOP were stated. Finally, the novel and successful application of magnetic graphene composites in a continuous bioreactor to enhance the microbial transformation of IOP was demonstrated.

Immobilized redox mediator on metal-oxides nanoparticles and its catalytic effect in a reductive decolorization process.

Different metal-oxides nanoparticles (MONP) including α-Al(2)O(3), ZnO and Al(OH)(3), were utilized as adsorbents to immobilize anthraquinone-2,6-disulfonate (AQDS). Immobilized AQDS was subsequently tested as a solid-phase redox mediator (RMs) for the reductive decolorization of the azo dye, reactive red 2 (RR2), by anaerobic sludge. The highest adsorption capacity of AQDS was achieved on Al(OH)(3) nanoparticles, which was ∼0.16 mmol g(-1) at pH 4. Immobilized AQDS increased up to 7.5-fold the rate of decolorization of RR2 by anaerobic sludge as compared with sludge incubations lacking AQDS. Sterile controls including immobilized AQDS did not show significant (<3.5%) RR2 decolorization, suggesting that physical-chemical processes (e.g. adsorption or chemical reduction) were not responsible for the enhanced decolorization achieved. Immobilization of AQDS on MONP was very stable under the applied experimental conditions and spectrophotometric screening did not detect any detachment of AQDS during the reductive decolorization of RR2, confirming that immobilized AQDS served as an effective RMs. The present study constitutes the first demonstration that immobilized quinones on MONP can serve as effective RMs in the reductive decolorization of an azo dye. The immobilizing technique developed could be applied in anaerobic wastewater treatment systems to accelerate the redox biotransformation of recalcitrant pollutants.

Chitosan selectivity for removing cadmium (II), copper (II), and lead (II) from aqueous phase: pH and organic matter effect.

The aim of this study was to investigate the selectivity of chitosan for cadmium, copper and lead in the presence and absence of natural organic matter (NOM) in different pH solutions. Adsorption isotherms of one and three adsorbates at initial concentration of 5-100mg/L were carried out in batch reactors at pH 4, 5, or 7 and 25 degrees C in reactive and clarified water. The chitosan employed had a MW of 107.8 x 10(3)g/mol and degree of acetylation (DA) of 33.7%. The chitosan adsorption capacity at pH 4 in reactive water was 0.036, 0.016, 0.010mmol/g for Pb(2+), Cd(2+), and Cu(2+), respectively, and it decreased for Pb(2+) and Cd(2+) in clarified water. Conversely, experiments carried out in clarified water showed that the cadmium adsorption capacity of chitosan was enhanced about three times by the presence of NOM at pH 7: an adsorption mechanism was proposed. Furthermore, it was found that the biosorbent selectivity, in both reactive and clarified water at pH 4, was as follows Cu(2+)>Cd(2+)>Pb(2+). Finally, the preliminary desorption experiments of Cd(2+) conducted at pH 2 and 3 reported 68 and 44.8% of metal desorbed, which indicated that the adsorption mechanism occurred by electrostatic interactions and covalent bonds.

Adsorption of cadmium by activated carbon cloth: influence of surface oxidation and solution pH.

The surface of activated carbon cloth (ACC), based on polyacrylonitrile fibre as a precursor, was oxidised using nitric acid, ozone and electrochemical oxidation to enhance cadmium ion exchange capacity. Modified adsorbents were physically and chemically characterised by pH titration, direct titration, X-ray photoelectron spectroscopy, elemental analysis, surface area and porosimetry, and scanning electron microscopy. BET surface area decreased after oxidation, however, the total ion exchange capacity increased by a factor of approximately 3.5 compared to the commercial as-received ACC. A very significant increase in cadmium uptake, by a factor of 13, was observed for the electrochemically oxidised ACC. Equilibrium sorption isotherms were determined at pH 4, 5 and 6 and these showed that cadmium uptake increased with increasing pH. There was clear evidence of physical damage to ozone-oxidised fibre, however, acid and electrochemically oxidised samples were completely stable.