Tailoring Carbon Nanotubes to Enhance their Efficiency as Electron Shuttle on the Biological Removal of Acid Orange 10 Under Anaerobic Conditions.

Ana Rita Silva,M Madalena Alves,O Salomé G.P Soares,Luciana Pereira,M Fernando R Pereira

doi:10.3390/nano10122496

Ana Rita Silva, M Madalena Alves + Show 3 more

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https://doi.org/10.3390/nano10122496

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Abstract

Anaerobic treatments have been described for the biodegradation of pollutants. However, the reactions proceed slowly due to the recalcitrant nature of these compounds. Carbon nanomaterials (CNM) intermediate in, and favor, the electron transfer, accelerating the anaerobic reduction of pollutants, which act as final electron acceptors. In the present work, different carbon nanotubes (CNT) with modified surface chemistry, namely CNT oxidized with HNO3 (CNT_HNO3) and CNT doped with nitrogen in a ball milling process (CNT_N_MB) were prepared using commercial CNT as a starting material. The new CNM were tested as redox mediators (RM), 0.1 g L−1, in the biological reduction of the azo dye, Acid Orange 10 (AO10), with an anaerobic granular sludge, over 48 h of reaction. Methane production was also assessed to verify the microorganism’s activity and the CNM’s effect on the methanogenic activity. An improvement in the biological removal of AO10 occurred with all CNM (above 90%), when compared with the control without CNM (only 32.4 ± 0.3%). The best results were obtained with CNT_N_MB, which achieved 98.2 ± 0.1% biological AO10 removal, and an 11-fold reduction rate increase. In order to confer magnetic properties to the CNM, tailored CNT were impregnated with 2% of iron-samples: CNT@2%Fe, CNT@2%Fe_N_MB, and CNT@2%Fe_HNO3. The better performance of the CNT doped with nitrogen was confirmed with CNT@2%Fe_N_MB, and the magnetic character facilitated its recovery after treatment, and did not affect its good catalytic properties. No dye removal was observed in the abiotic assays, so the removal was not due to adsorption on the CNM. Furthermore, the microorganism’s viability was maintained during the assay and methane production was not affected by the presence of the CNM. Despite the toxic character of the aromatic amines formed, detoxification was observed after the biological process with thermally treated CNT.

Highlights

The growth in industrialization, associated with ineffective wastewater treatments, leads to the contamination of water resources with different pollutants, including azo dyes
The increase of SBET observed for the N-doped sample may have been due to the reduction of the entanglement of the carbon nanotubes (CNT) during the ball milling, which leads to shorter CNT by breaking up the tubes, without affecting the tube diameters; the presence of N-surface groups may have an attractive effect between the tubes, leading to a higher agglomeration of the sample [23,30]
In the blank assays with carbon nanomaterials (CNM) and Granular sludge (GS), but without substrate, removals of around 40% where obtained with the CNM. These results suggest that the CNM can stimulate the microorganisms, and provide faster electron transfer compared with the assay without CNM, even without substrate addition, probably because of the presence of some residual substrate that was not consumed during pre-incubation

Summary

Introduction

The growth in industrialization, associated with ineffective wastewater treatments, leads to the contamination of water resources with different pollutants, including azo dyes. It was shown that the rates of reduction of dyes and of aromatic amines were greatly improved, in batch and in continuous bioreactors, by adding small amounts (0.1 g L−1) of different CNM and, in some cases, no reduction occurred in their absence [4,9,10,13]. These insoluble nanomaterials can be retained in bioreactors, avoiding the need for being continuously added during the process, and are easier to remove after the process [14,15,16,17]

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