J. Rene Rangel-Mendez

Immobilized redox mediators on anion exchange resins and their role on the reductive decolorization of azo dyes.

Quinoid redox mediators (RM), including 1,2-naphthoquinone-4-sulfonate (NQS) and anthraquinone-2,6-disulfonate (AQDS), were adsorbed on anion exchange resins (AER) in order to explore their catalytic effects on the reductive decolorization of azo dyes by anaerobic granular sludge. Immobilized quinones preserved their catalytic properties once adsorbed on the surface of AER. Addition of different concentrations of immobilized quinones to sludge incubations increased up to 8.8-fold the rate of decolorization of azo dyes compared to controls lacking quinones. The catalytic effects of immobilized quinones also resulted in a greater extent of decolorization in quinone-amended incubations compared to controls lacking external RM. Spectrophotometric screening did not show any detachment of either AQDS or NQS during decolorization assays confirming that the enhanced decolorization accomplished was exclusively attributed to quinones immobilized on AER. Sterile controls including the maximum concentration of immobilized quinones supplied (4.8 mM) did not show any removal of azo dyes suggesting that physical-chemical processes, such as adsorption or chemical reduction, were not responsible for the enhanced decolorization reached. To our knowledge, this is the first study demonstrating the catalytic contribution of RM immobilized on AER on the reductive (bio)transformation of azo dyes.

Spent coffee-based activated carbon: specific surface features and their importance for H2S separation process.

Activated carbons were prepared from spent ground coffee. Zinc chloride was used as an activation agent. The obtained materials were used as a media for separation of hydrogen sulfide from air at ambient conditions. The materials were characterized using adsorption of nitrogen, elemental analysis, SEM, FTIR, and thermal analysis. Surface features of the carbons depend on the amount of an activation agent used. Even though the residual inorganic matter takes part in the H(2)S retention via salt formation, the porous surface of carbons governs the separation process. The chemical activation method chosen resulted in formation of large volume of pores with sizes between 10 and 30Å, optimal for water and hydrogen sulfide adsorption. Even though the activation process can be optimized/changed, the presence of nitrogen in the precursor (caffeine) is a significant asset of that specific organic waste. Nitrogen functional groups play a catalytic role in hydrogen sulfide oxidation.

Simultaneous adsorption of Cd2+ and phenol on modified N-doped carbon nanotubes: experimental and DFT studies.

Carbon nanotubes are novel materials that have been investigated for diverse applications, but only a few studies have been focused on environmental issues. In this work, we report on the efficient adsorption of phenol and cadmium ions on N-doped carbon nanotubes (CNx), which have been modified in air at different temperatures. The pristine and modified CNx nanotubes were characterized by SEM, TGA, elemental analysis and their surface areas were also determined. The adsorption experiments of toxic pollutants were carried out in batch reactors at 25 degrees C. The characterization of modified CNx by these techniques showed an increase in oxygen content and surface area in comparison with the pristine CNx tubes. The individual adsorption capacity was 0.10 and 0.07 mmol/g for phenol and Cd(2+), respectively. The experimental data of the competitive adsorption of phenol and Cd(2+) revealed that the cadmium removal was favored as the phenol concentration increased, whereas the phenol adsorption capacity was slightly affected at any cadmium concentration. These results suggest that modified CNx tubes have a great potential in environmental applications as adsorbents of organic and inorganic compounds in aqueous phases. In addition, first-principles calculations were carried out in order to elucidate the mechanism of Cd(2+) adsorption on CNx.

Effect of phosphate on the particle size of ferric oxyhydroxides anchored onto activated carbon: As(V) removal from water.

The surface area of iron oxyhydroxides is a key factor when removing As from water. However, research related to this matter shows that this issue has not been explored in detail. The use of capping agents is a viable method to synthesize ferric oxyhydroxide nanoparticles; however, this method to our knowledge has not been applied for the anchorage of iron oxyhydroxide nanoparticles on activated carbon (AC). In the present work, the addition of PO(4) (as a capping agent) in forced hydrolysis of FeCl(3) in AC was investigated. Results revealed that the surface area of modified materials reached a maximum of about 900 m(2)/g with a molar ratio PO(4)/Fe of 0.1. Moreover, microscopy studies indicate a size range of iron nanoparticles from 2 to 300 nm, where the smallest particles are attained with the highest concentration of PO(4). The surface charge distribution of modified samples became less positive; however, the As removal increased, indicating that electrostatic interaction is not the controlling sorption mechanism. Modified samples showed a 40% increase on As(V) adsorption capacity when using a molar ratio PO(4)/Fe of 1.5. The proposed method allowed anchoring of iron oxyhydroxides nanoparticles on AC, which have a high As(V) adsorption capacity (5 mg/g).

Reactive adsorption of SO2 on activated carbons with deposited iron nanoparticles.

The effect of iron particle size anchored on the surface of commercial activated carbon on the removal of SO(2) from a gas phase was studied. Nanosize iron particles were deposited using forced hydrolysis of FeCl(3) with or without H(3)PO(4) as a capping agent. Dynamic adsorption experiments were carried out on either dry or pre-humidified materials and the adsorption capacities were calculated. The surface of the initial and exhausted materials was extensively characterized by microscopic, porosity, thermogravimetric and surface chemistry. The results indicate that the SO(2) adsorption capacity increased two and half times after the prehumidification process owing to the formation of H(2)SO(4) in the porous system. Iron species enhance the SO(2) adsorption capacity only when very small nanoparticles are deposited on the pore walls as a thin layer. Large iron nanoparticles block the ultramicropores decreasing the accessibility of the active sites and consuming oxygen that rest adsorption centers for SO(2) molecules. Iron nanoparticles of about 3-4 nm provide highly dispersed adsorption sites for SO(2) molecules and thus increase the adsorption capacity of about 80%. Fe(2)(SO(4))(3) was detected on the surface of exhausted samples.

Enhanced microbial decolorization of methyl red with oxidized carbon fiber as redox mediator.

The anaerobic degradation of azo dyes under anaerobic conditions is possible but at a slow rate. Redox mediators (quinones, activated carbon) are used to improve the reduction rate. The aim of this work was to use activated carbon fiber (ACF) as a redox mediator for the anaerobic reduction of the azo dye methyl red. ACF was chemically modified with 8M HNO₃ to increase its redox-mediating capacity and used in chemical and anaerobic biological batch assays for the reduction of methyl red. ACF increased its redox-mediating capacity up to 3-fold in chemical assays; in biological assays ACF increased the reduction rate up to 8-fold compared to controls without ACF. However, since the ACF served as support for biomass, a biofilm formed on the fiber significantly reduced its redox-mediating capacity; substrate consumption suggested that the electron transport from ACF to methyl red was the rate-limiting step in the process. These results are the first evidence of the role of ACF as a redox mediator in the reductive decolorization of methyl red, in addition to the effect of biofilm attached to ACF on methyl red reduction. Due to the versatile characteristics of ACF and its redox-mediating capacity, carbon fibers could be used in biological wastewater treatment systems to accelerate the reductive transformation of pollutants commonly found in industrial effluents.

Graphene oxide as electron shuttle for increased redox conversion of contaminants under methanogenic and sulfate-reducing conditions.

Graphene oxide (GO) is reported for the first time as electron shuttle to increase the redox conversion of the azo compound, reactive red 2 (RR2, 0.5mM), and the nitroaromatic, 3-chloronitrobenzene (3CNB, 0.5mM). GO (5mgL(-1)) increased 10-fold and 7.6-fold the reduction rate of RR2 and 3CNB, respectively, in abiotic incubations with sulfide (2.6mM) as electron donor. GO also increased by 2-fold and 3.6-fold, the microbial reduction rate of RR2 by anaerobic sludge under methanogenic and sulfate-reducing conditions, respectively. Deep characterization of GO showed that it has a proper size distribution (predominantly between 450 and 700nm) and redox potential (+50.8mV) to promote the reduction of RR2 and 3CNB. Further analysis revealed that biogenic sulfide plays a major role on the GO-mediated reduction of RR2. GO is proposed as an electron shuttle to accelerate the redox conversion of recalcitrant pollutants, such as nitro-benzenes and azo dyes.

Direct and Quinone-Mediated Palladium Reduction by Geobacter sulfurreducens: Mechanisms and Modeling

Palladium(II) reduction to Pd(0) nanoparticles by Geobacter sulfurreducens was explored under conditions of neutral pH, 30 °C and concentrations of 25, 50, and 100 mg of Pd(II)/L aiming to investigate the effect of solid species of palladium on their microbial reduction. The influence of anthraquinone-2,6-disulfonate was reported to enhance the palladium reaction rate in an average of 1.7-fold and its addition is determining to achieve the reduction of solid species of palladium. Based on the obtained results two mechanisms are proposed: (1) direct, which is fully described considering interactions of amide, sulfur, and phosphoryl groups associated to proteins from bacteria on palladium reduction reaction, and (2) quinone-mediated, which implies multiheme c-type cytochromes participation. Speciation analysis and kinetic results were considered and integrated into a model to fit the experimental data that explain both mechanisms. This work provides elements for a better understanding of direct and quinone-mediated palladium reduction by G. sulfurreducens, which could facilitate metal recovery with concomitant formation of valuable palladium nanoparticles in industrial processes.

Effects of surface-engineered nanoparticle-based dispersants for marine oil spills on the model organism Artemia franciscana.

Fine particles are under active consideration as alternatives to chemical dispersants for large-scale petroleum spills. Fine carbon particles with engineered surface chemistry have been shown to stabilize oil-in-water emulsions, but the environmental impacts of large-scale particle introduction to the marine environment are unknown. Here we study the impact of surface-engineered carbon-black materials on brine shrimp (Artemia franciscana) as a model marine microcrustacean. Mortality was characterized at 50–1000 mg/L, and levels of heat shock protein 70 (hsp70) were characterized at sublethal particle concentrations (25–50 mg/L). Functionalized carbon black (CB) nanoparticles were found to be nontoxic at all concentrations, while hydrophobic (annealed) and as-produced CB induced adverse effects at high concentrations. CB was also shown to adsorb benzene, a model hydrocarbon representing the more soluble and toxic low-molecular weight aromatic fraction of petroleum, but the extent of adsorption was insufficient to mitigate benzene toxicity to Artemia in coexposure experiments. At lower benzene concentrations (25–75 mg/L), coexposure with annealed and as-produced CB increased hsp70 protein levels. This study suggests that surface functionalization for increased hydrophilicity can not only improve the performance of CB-based dispersants but also reduce their adverse environmental impacts on marine organisms.

Immobilization of metal–humic acid complexes in anaerobic granular sludge for their application as solid-phase redox mediators in the biotransformation of iopromide in UASB reactors

Metal-humic acid complexes were synthesized and immobilized by a granulation process in anaerobic sludge for their application as solid-phase redox mediators (RM) in the biotransformation of iopromide. Characterization of Ca- and Fe-humic acid complexes revealed electron accepting capacities of 0.472 and 0.556milli-equivalentsg(-1), respectively. Once immobilized, metal-humic acid complexes significantly increased the biotransformation of iopromide in upflow anaerobic sludge blanket (UASB) reactors. Control UASB reactor (without humic material) achieved 31.6% of iopromide removal, while 80% was removed in UASB reactors supplied with each metal-humic acid complex. Further analyses indicated multiple transformation reactions taking place in iopromide including deiodination, N-dealkylation, decarboxylation and deacetylation. This is the first successful application of immobilized RM, which does not require a supporting material to maintain the solid-phase RM in long term operation of bioreactors. The proposed redox catalyst could be suitable for enhancing the redox conversion of different recalcitrant pollutants present in industrial effluents.