Treatment of tannery wastewater is problematic due to high and variable concentrations of complex pollutants often combined with high salinity levels. Two series of horizontal subsurface flow constructed wetlands (CWs) planted with Arundo donax and Sarcocornia fruticosa were set up after a conventional biological treatment system operating at a tannery site. The aim of the CWs was polishing organics and nitrogen from the high salinity effluent (2.2-6.6 g Cl(-) L(-1)). Both plant species established and grew well in the CW. Arundo, however, had more vigorous growth and a higher capacity to take up nutrients. The CWs were efficient in removing COD and BOD(5) with removal efficiencies varying between 51 and 80% for COD (inlet: 68-425 mg L(-1)) and between 53 and 90% for BOD(5) (inlet: 16-220 mg L(-1)). Mass removal rates were up to 615 kg COD ha(-1) d(-1) and 363 BOD(5) kg ha(-1) d(-1). Removal efficiencies were 40-93% for total P, 31-89% for NH(4)(+) and 41-90% for Total Kjeldahl Nitrogen. CW systems planted with salt tolerant plant species are a promising solution for polishing saline secondary effluent from the tannery industry to levels fulfilling the discharge standards.
The fundamental understanding of the barrier layer (δ b ) growth in TiO 2 nanotubes (NTs) is here established and compared with the classical metal oxidation theory from Mott and Cabrera. The role of δ b in the anodization of TiO 2 NTs under different applied potentials and times was analyzed using scanning transmission electron microscopy (STEM). Contrary to the well-known case of anodic aluminum oxide, we found that δ b of TiO 2 NTs progressively grows over time due to the nonsteady anodization regime. We then establish a relation between the phenomenological growth of the barrier layer with time and applied voltage, δ b (V,t) using the high-field Mott and Cabrera conduction theory.The developed model was found to be in excellent agreement with the experimental data from both STEM and anodization curves. On the basis of these results, the relationship between δ b and the anodization time and potential can now be quantitatively understood.Theory of the oxidation of metals goes back to the late 1940s, when Mott and Cabrera discussed the growth of oxide thin films formed by anodic oxidation under an applied electric field. 1 In the Mott−Cabrera picture, the oxide growth of Ti and other valve metals (Al, Hf, Ta, W, etc.) is governed by the high-field conduction mechanism. 1−3 Under higher fields, the entry of a cation across the metal/oxide interface into the oxide is the oxide growth ratedetermining step. Thus, during oxidation, both the rate of oxidation and the rate-limiting process depend on the thickness of the oxide. 1 According to the underlying theory, the growth kinetics of the passive film is described by the relation between current density (j) and the electric field strength (E = V/δ b , where V is the applied potential and δ b the oxide thickness)Under this approach, the electrochemical oxidation of metals can lead to (i) stable continuous oxide 2 films, if the oxide is insoluble to the electrolyte, or (ii) nanoporous oxide films if the oxide is fairly soluble in the presence of an acidic electrolyte. 4 Indeed, in past decades, Al and Ti electrochemical anodization together with other valve metals (Hf, Ta, W, etc.) has been widely studied because highly regular hexagonal arrangements of pores or nanotubes can be obtained. Both anodic aluminum oxide (AAO) nanoporous and anodic TiO 2 nanotubes (NTs) have stimulated considerable scientific and technological interest with extensive use in practical nanostructures. 5−9 In particular, the distinct properties of anodic TiO 2 NTs make it highly attractable for a wide range of applications, mainly in renewal energy sources such as H 2 generation by water photoelectrolysis and dye-sensitized solar cells (DSCs). 6,7Because Zwilling et al. first introduced the anodic oxidation of Ti using fluoride-based electrolytes, 10 anodization parameters such as electrolyte composition, applied potential, time, temperature, and Ti surface roughness were found to significantly influence the growth and morphology of TiO 2 NTs. This influence is seen in the crucial geo...
Hematite is getting great attention as an environmentally friendly material for photoelectrochemical water splitting, due to its narrow band gap (1.9−2.2 eV), nontoxicity, low cost, high stability and wide availability. However, hematite shortcomings such as its low absorption coefficient, short hole diffusion length, or poor electrical conductivity lead to multiple electron−hole recombinations and efficiency losses. This work describes the preparation of nanostructured hematite photoelectrodes by a hydrothermal method followed by thermal annealing under different conditions. A large spectrum of materials science characterization techniques were used to unify the broad and underlying physical-chemical processes by which a material's structure and properties influence the performance of these photoelectrodes. In particular, Sn diffusion into hematite via a high-temperature annealing scheme is fairly analyzed by Rutherford backscattering spectrometry to assess the in-depth Sn distribution profiles and by extended X-ray absorption fine structure analysis for structural order analysis. The increase of photocurrent with annealing temperature and time, besides being related with percent Sn diffusion along the hematite photoelectrode, is also correlated with nanowires morphology, porosity features, and structural crystalline order enhancement. This study shows that an accurate combination of the semiconducting photoelectrode intrinsic properties, such as percent Sn profile content, one-dimensional nanowire diameter, porosity, and structural crystalline order, naturally leads to photoelectrodes with improved conductivity to photogenerated carriers and reduced band gap.
The toxicity of high salinity tannery wastewater produced after an activated sludge secondary treatment on the germination and seedling growth of Trifolium pratense, a species used as indicator in toxicity tests, was evaluated. Growth was inhibited by wastewater concentrations >25% and undiluted effluent caused a complete germination inhibition. Constructed wetlands (CWs) with Arundo donax or Sarcocornia fruticosa were envisaged to further polish this wastewater. Selection of plant species to use in CWs for industrial wastewater treatment is an important issue, since for a successful establishment they have to tolerate the often harsh wastewater composition. For that, the effects of this wastewater on the growth of Arundo and Sarcocornia were assessed in pot assays. Plants were subject to different wastewater contents (0/50/100%), and both were resilient to the imposed conditions. Arundo had higher growth rates and biomass than Sarcocornia and may therefore be the preferred species for use in CWs treating tannery wastewater. CWs planted with the above mentioned plants significantly decreased the toxicity of the wastewater, as effluent from the CWs outlet stimulated the growth of Trifolium at concentrations <50%, and seed germination and growth even occurred in undiluted effluent.
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