Rapid degradation of azo dye methyl orange using hollow cobalt nanoparticles.

A rapid and efficient method for methyl orange degradation using hollow cobalt (Co) nanoparticles is reported. Hollow Co nanoparticles were fabricated by a galvanic replacement reaction using aluminum (Al) nanoparticles as the template material. The methyl orange degradation characteristics were investigated by measuring the time dependent UV-Vis absorption of the dye solution, which showed a very fast degradation rate under acidic conditions. At an initial methyl orange concentration of 100 mg/L (pH = 2.5) and Co nanoparticle dosage of 0.5 g/L, the azo dye degradation efficiency reached up to 99% within 4 min, and the degradation constant rate was up to 2.444 min(-1), which is the highest value among other studies. A comparison of the decolorization rates at similar conditions with several other azo dyes, including Congo red, Amaranth, and Orange G, showed that the dye with a simpler structure and lower molecular mass decolorized considerably faster than the ones having a more complicated structure (higher molecular mass). The methyl orange degradation was also conducted using hollow nickel (Ni) nanoparticles and commercially available solid spherical Co and Ni nanoparticles. The results showed that Co-based nanoparticles outperformed Ni-based nanoparticles, with the hollow Co nanoparticles exhibiting the fastest degradation rate. Using the hollow Co nanoparticles is a very promising approach for the remediation of methyl orange dye containing wastewater due to the fast degradation rate and high degradation efficiency. In addition, these hollow Co nanoparticles are easily recycled because of their magnetic property.

Toxicity of carbon nanotubes to the activated sludge process.

The discharge of carbon nanotubes (CNTs) from industrial waste or disposal of such materials from commercial and/or domestic use will inevitably occur with increasing production and enter into wastewater treatment facilities with unknown consequences. Therefore, a better knowledge of the toxicity of CNTs to biological processes in wastewater treatment will be critical. This study examined the toxicity of multi-walled carbon nanotubes (MWCNTs) on the microbial communities in activated sludge. A comparative study using the activated sludge respiration inhibition test was performed on both unsheared mixed liquor and sheared mixed liquor to demonstrate the potential toxicity posed by MWCNTs and to illustrate the extent of extracellular polymeric substances (EPS) in protecting the microorganisms from the toxicity of CNTs. Respiration inhibition was observed for both unsheared and sheared mixed liquor when MWCNTs were present, however, greater respiration inhibition was observed for the sheared mixed liquor. The toxicity observed by the respiration inhibition test was determined to be dose-dependent; the highest concentration of MWCNTs exhibited the highest respiration inhibition. Scanning Electron Microscopy (SEM) images demonstrated direct physical contact between MWCNTs and activated sludge flocs.

Examination of purified single-walled carbon nanotubes on activated sludge process using batch reactors.

This paper evaluated the impact of over 90% purified single-walled carbon nanotubes (SWCNTs) on the activated sludge wastewater treatment process through a batch-scale study. We found that SWCNTs adsorbed 16.9% of the soluble COD (sCOD, soluble chemical oxygen demand) without activated sludge present. Statistical analysis of the experimental data demonstrated that only four experimental parameters, i.e., surface charge, sCOD, mixed liquor suspended solids, and sludge volume index were significantly impacted by the addition of SWCNTs; other parameters such as pH, dissolved oxygen, specific resistance to filtration, and relative hydrophobicity were not significantly impacted. Further examination of the four affected parameters illustrated that SWCNTs improved sludge settleability and made the surface of the activated sludge flocs less negatively charged. The addition of SWCNTs appeared to improve sCOD removal due mostly to absorption. This research also found that the impacts observed were from the SWCNTs, not the impurities within the SWCNTs tested.