Elías Razo-Flores

Biodegradation of selected azo dyes under methanogenic conditions

Biological treatment of wastewaters discharged by the textile industry could potentially be problematic due to the high toxicity and recalcitrance of the commonly-used azo dye compounds. In the present report, the fate of two azo dyes under methanogenic conditions was studied. Mordant Orange 1 (MO1) and Azodisalicylate (ADS) were completely reduced and decolorised in continuous UASB reactors in the presence of cosubstrates. In the MO1 reactor, both 5-aminosalicylic acid (5-ASA) and 1,4-phenylenediamine were identified as products of azo cleavage. After long adaptation periods, 5-ASA was detected at trace levels, indicating further mineralization. ADS, a pharmaceutical azo dye constructed from two 5-ASA units, was completely mineralized even in the absence of cosubstrate, indicating that the metabolism of 5-ASA could provide the reducing equivalents needed for the azo reduction. Batch experiments confirmed the ADS mineralization. These results demonstrate that some azo dyes could serve as a carbon, energy, and nitrogen source for anaerobic bacteria.

Continuous detoxification, transformation, and degradation of nitrophenols in upflow anaerobic sludge blanket (UASB) reactors.

The anaerobic transformation and degradation of nitrophenols by granular sludge was investigated in upflow anaerobic sludge blanket (UASB) reactors continuously fed with a volatile fatty acid (VFA) mixture as the primary substrate. During the start‐up, subtoxic concentrations of 2‐nitrophenol (2‐NP), 4‐nitrophenol (4‐NP), and 2, 4‐dinitrophenol (2, 4‐DNP) were utilized. 4‐NP and 2, 4‐DNP were readily converted to the corresponding aromatic amine; whereas 2‐NP was converted to nonaromatic products via intermediate formation of 2‐aminophenol (2‐AP). These conversions led to a dramatic detoxification of the mononitrophenols because the reactors treated the nitrophenolics at the concentrations which were over 25 times higher than those that caused severe inhibition. VFA removal efficiencies greater than 99% were achieved in both reactors at loading rates greater than 11.4 g COD per liter of reactor volume per day even at volumetric loading of mononitrophenols up to 910 mg/L · d.

Detoxification and partial mineralization of the azo dye mordant orange 1 in a continuous upflow anaerobic sludge-blanket reactor

Abstract In batch toxicity assays, azo dye compounds were found to be many times more toxic than their cleavage products (aromatic amines) towards methanogenic activity in anaerobic granular sludge. Considering the ability of anaerobic microorganisms to reduce azo groups, detoxification of azo compounds towards methanogens can be expected to occur during anaerobic wastewater treatment. In order to test this hypothesis, the anaerobic degradation of one azo dye compound, Mordant orange 1 (MO1), by granular sludge was investigated in three separate continuous upflow anaerobic sludge-blanket reactors. One reactor, receiving no cosubstrate, failed after 50 days presumably because of a lack of reducing equivalents. However, the two reactors receiving either glucose or a volatile fatty acids (acetate, propionate, butyrate) mixture, could eliminate the dye during operation for 217 days. The azo dye was reductively cleaved to less toxic aromatic amines (1,4-phenylenediamine and 5-aminosalicylic acid) making the treatment of MO1 feasible at influent concentrations that were over 25 times higher than their 50% inhibitory concentrations. In the reactor receiving glucose as cosubstrate, 5-aminosalicylic acid could only be detected at trace levels in the effluent after day 189 of operation. Batch biodegradability assays with the sludge sampled from this reactor confirmed the mineralization of 5-aminosalicylic acid to methane.

Biodegradability of N-substituted aromatics and alkylphenols under methanogenic conditions using granular sludge.

The biodegradability of seventeen N-substituted aromatic and six alkylphenol compounds were evaluated under methanogenic conditions. Biodegradation was assessed in batch assays inoculated with unacclimated and predigested anaerobic granular sludge at 30°C under agitated conditions over a 150 day period. The compounds were supplied at sub-toxic concentrations in the assays in order to prevent inhibition to the methanogens. The biodegradability test was performed by the measurement of the methane composition in the headspace of the serum flasks. The methanogenic consortia completely mineralized 2-, 3-aminobenzoate, 2-aminophenol and 4-cresol; whereas, 4-aminobenzoate was only partially degraded. The other N-substituted compounds and the alkylphenols tested were not biodegradable under the experimental conditions employed. An additional biodegradability assay was conducted with sludge from an upward-flow anaerobic sludge bed reactor adapted to the degradation of 2-nitrophenol. This sludge mineralized 2-aminophenol without any lag phase while the unadapted sludge required 110 days of acclimation. The three aminobenzoate isomers were fully mineralized by the adapted sludge after similar lag periods observed in the unadapted sludge. The 2-nitrophenol adapted sludge cross-acclimatized to the mineralization of 5-aminosalicylate and 4-aminophenol. This constitutes the first report demonstrating the anaerobic mineralization of 5-aminosalicylate, which indicates that at least some azo dye cleavage products can be degraded in methanogenic consortia.

Precipitation and recovery of metal sulfides from metal containing acidic wastewater in a sulfidogenic down‐flow fluidized bed reactor

This study reports the feasibility of recovering metal precipitates from a synthetic acidic wastewater containing ethanol, Fe, Zn, and Cd at an organic loading rate of 2.5 g COD/L‐day and a COD to sulfate ratio of 0.8 in a sulfate reducing down‐flow fluidized bed reactor. The metals were added at increasing loading rates: Fe from 104 to 320 mg/L‐day, Zn from 20 to 220 mg/L‐day, and Cd from 5 to 20 mg/L‐day. The maximum COD and sulfate removals attained were 54% and 41%, respectively. The biofilm reactor was operated at pH as low as 5.0 with stable performance, and no adverse effect over COD consumption or sulfide production was observed. The metals precipitation efficiencies obtained for Fe, Zn, and Cd exceeded 99.7%, 99.3%, and 99.4%, respectively. The total recovered precipitate was estimated to be 90% of the theoretical mass expected as metal sulfides. The precipitate was mainly recovered from the bottom of the reactor and the equalizer. The analysis of the precipitates showed the presence of pyrite (FeS2), sphalerite (ZnS) and greenockite (CdS); no metal hydroxides or carbonates in crystalline phases were identified. This study is the first in reporting the feasibility to recover metal sulfides separated from the biomass in a sulfate reducing process in one stage. Biotechnol. Bioeng. 2009;102: 91–99.

Biotransformation and Biodegradation of Selected Nitroaromatics under Anaerobic Conditions

The fate of four nitroaromatic compounds (5‐nitrosalicylate, 5NSA; 4‐nitrobenzoate, 4NBc; 2,4‐dinitrotoluene, 2,4DNT; nitrobenzene, NB) was studied in 160 mL laboratory‐scale upward‐flow anaerobic sludge bed reactors supplied with a mixture of volatile fatty acids and/or glucose as electron donors. All the nitroaromatics were transformed stoichiometrically to their corresponding aromatic amines. After prolonged reactor operation, 5NSA and 4NBc were completely mineralized to CH4 and CO2, whereas 2,4DNT was partially transformed to a nonidentified and nondegradable metabolite. Batch nitro‐reduction experiments indicated that the position of the nitro group in relation to the other substituents in the aromatic ring plays a key role in the rate of the nitro‐group reduction. The results obtained indicate that certain nitroaromatic compounds can be completely mineralized and serve as a carbon and energy source for anaerobic bacteria.

Chemical and enzymatic sequential pretreatment of oat straw for methane production

Oat straw was subjected to sequential pretreatment: acid/alkaline/enzymatic, to convert the lignocellulosic material in soluble sugars. The hydrolysates from acid pretreatment (2% HCl, 90 °C) and enzymatic pretreatment (cellulase, pH 4.5, 45 °C) were used as substrates in two lab-scale UASB reactors for methane production. The acid and enzymatic hydrolysates contained 25.6 and 35.3g/L of total sugars, respectively, which corresponded to a COD of 23.6 and 30.5 g/L, respectively. The UASB reactor fed with acid hydrolysate achieved a maximum methane yield of 0.34 L CH(4)/g COD at an organic loading rate (OLR) of 2.5 g COD/L-d. In the reactor fed with enzymatic hydrolysate the methane yield was 0.36 LCH(4)/g COD at OLR higher than 8.8 g COD/L-d. The anaerobic digestion of both hydrolysates was feasible without the need of a detoxification step. The sequential pretreatment of oat straw allowed to solubilize 96.8% of hemicellulose, 77.2% of cellulose and 42.2% of lignin.

Fermentation of lactose and its constituent sugars by Escherichia coli WDHL: Impact on hydrogen production

Fermentations of lactose, glucose and galactose using Escherichia coli WDHL, a hydrogen over producer strain, were performed. With glucose as substrate pyruvate was mainly routed to the lactate pathway, resulting in hydrogen production and yield of 1037 mL and 0.30 mol H(2)/mol of glucose, respectively. When galactose was the substrate, the pyruvate formate lyase pathway was the main route for pyruvate and a fermentation yield of 1.12 mol H(2)/mol of galactose and a hydrogen production of 2080 mL were obtained. The fermentation of lactose or glucose plus galactose showed a similar yield of 1.02 mol H(2)/mol of hexose consumed. This work clearly demonstrated that the kinetics of hydrogen and metabolites production as well as the hydrogen yield were affected by the type of sugar used as substrate as reflected by the deviations from the metabolic hydrogen-production pathway.

Effect of initial sulfide concentration on sulfide and phenol oxidation under denitrifying conditions.

The objective of this work was to evaluate the effect of the initial sulfide concentration on the kinetics and metabolism of phenol and sulfide in batch bioassays using nitrate as electron acceptor. Complete oxidation of sulfide (20 mg L(-1) of S(2-)) and phenol (19.6 mg L(-1)) was linked to nitrate reduction when nitrate was supplemented at stoichiometric concentrations. At 32 mg L(-1) of sulfide, oxidation of sulfide and phenol by the organo-lithoautotrophic microbial culture was sequential; first sulfide was rapidly oxidized to elemental sulfur and afterwards to sulfate; phenol oxidation started once sulfate production reached a maximum. When the initial sulfide concentration was increased from 20 to 26 and finally to 32 mg L(-1), sulfide oxidation was inhibited. In contrast phenol consumption by the denitrifying culture was not affected. These results indicated that sulfide affected strongly the sulfide oxidation rate and nitrate reduction.

Arsenic mobility controlled by solid calcium arsenates: A case study in Mexico showcasing a potentially widespread environmental problem

An As-contaminated perched aquifer under an urban area affected by mining was studied over a year to determine the contamination source species and the mechanism of As mobilization. Results show that the dissolution of calcium arsenates in residues disposed on an inactive smelter has caused high levels of As pollution in the adjoining downgradient 6-km perched aquifer, reaching up to 158 mg/L of dissolved As, and releasing a total of ca. 7.5 tons of As in a year. Furthermore, free calcium ion availability was found to control As mobility in the aquifer through the diagenetic precipitation of calcium arsenates (Ca5H2(AsO4)4·cH2O) preventing further mobilization of As. Results shown here represent a model for understanding a highly underreported mechanism of retention of arsenate species likely to dominate in calcium-rich environments, such as those in calcareous sediments and soils, where the commonly reported mechanism of adsorption to iron(III) oxyhydroxides is not the dominant process.