Volume 21, Issue 10

The presence of residual organic dyes in water resources results in a threat for both environment and human health due to their adverse health effects such as mutagenicity, carcinogenicity, and teratogenicity. Thus, they must be removed from industrial wastewater. Among these dyes, methylene blue (MB) is a toxic, carcinogenic, and almost non-biodegradable dye and can pose a significant threat to human health and environmental safety. Thus, it is removed from industrial effluents by a variety of methods, including adsorption, prior to discharge into the environment. This study aims to optimize the adsorption conditions of MB from an aqueous solution with nanocomposite of silver onto single-wall carbon nanotube metronidazole (Met-SWCNTs/Ag). Response Surface Methodology (RSM) based on Central Composite Design (CCD) is used to optimize and model the adsorption of MB dye (as pollutant) on Met-SWCNTs/Ag. The Met-SWCNTs/Ag is synthesized using Met-SWCNT impregnated with silver nitrate. The produced Met-SWCNT/Ag nanocomposite is characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The effect of four independent variables including nanoparticle (NP)/dye ratio, temperature, pH and contact time on MB removal on the specific surface area of SWCNT/Ag and Met-SWCNTs/Ag is evaluated. The accuracy and fit of the model for MB removal using Met-SWCNTs/Ag are estimated by ANOVA with R 2  > 0.99 and P -value < 0.0001. RSM results indicates that the NP/Dye ratio has the most significant influence on the adsorption of MB onto Met-SWCNTs/Ag. The optimal condition of the adsorption process takes place at NP/Dye ratio of 2.21, contact time of 65.57 min, and pH = 6.15 at 25.79 °C temperature leading into a 98.94 % MB removal. Isotherms and kinetic studies are performed to characterize the adsorption behavior of the adsorbent for MB removal. The adsorption behavior of the MB onto Met-SWCNTs/Ag is best described by the Langmuir isotherm model with regression coefficient R 2 of 0.9935 with the Q max of 112.42 mg/g. Adsorption kinetics of Met-SWCNT/Ag is investigated and modelled by means of the pseudo-first-order, pseudo-second-order models which is best fitted to the pseudo-second-order model. The thermodynamic study reveals that the adsorption of MB dye is spontaneous and exothermic. Experimental results suggest that the modified SWCNTs/Ag with Met achieves a higher removal efficiency of (∼98 %) when compared to SWCNTs/Ag (∼93 %).

Recovering germanium from zinc oxide dust (ZOD) produced from Pb–Zn smelter is an important pathway to extract germanium. However, the leaching efficiency of industrial germanium production is not satisfactory (usually less than 75 %). Therefore, the leaching behavior of Ge in different phases was discovered in this work. Ge in the ZOD mainly occurs in oxide, sulfide, silicate, and solid insoluble. The potential decides the oxidative dissolution of sulfide. The leaching recovery of zinc and germanium were 90 % and 80 % with oxidant, and 78 % and 80 % without oxidant, respectively. The effect sequence of oxidant type on the Zn leaching efficiency was MnO 2 > H 2 O 2 ≈ oxygen > air, but the type and addition of oxidant had no obvious effect on the leaching recovery of germanium. The final pH of leaching slurry limits the dissolution of oxide and hydrolysis-polymerization of impurity ions (such as Fe(III) and Si). Decreasing the final pH is beneficial to the leaching reaction of Zn and Ge. The germanium presented in oxide and sulfide is easy to leach, while the leaching of germanium existed in silicate and solid insoluble is relatively difficult. The structure of aluminate can be destroyed effectively using a 40 g/L HF solution. When the leaching percent of SiO 2 is 86.82 %, the leaching recovery of Ge is 96.57 %. For the ZOD with higher content of Fe and Si, germanium leaching is negatively correlated with the content of Fe and Si in the ZOD. The results provide a scientific basis for improving the leaching recovery of germanium.

Photocatalysis-based technologies have been proposed for the treatment of wastewater containing atrazine (ATZ), a persistent and recalcitrant pollutant. This study aims to evaluate and compare the efficiency of TiO 2 P25 and TiO 2 modified with carbon (C–TiO 2 Kronos vlp 7000) in the photocatalytic degradation of ATZ in aqueous systems. The experiments were performed in a tubular photochemical reactor equipped with a compound parabolic collector (CPC) irradiated by simulated solar light. The materials were characterized by X-ray diffraction, infrared spectroscopy, BET specific surface area, and diffuse reflectance spectroscopy. For TiO 2 P25, ATZ removals varied in the range 86–100 % after 120 min of irradiation, although the total organic carbon (TOC) analyses indicated that no significant ATZ mineralization occurred (<20 %). C–TiO 2 Kronos vlp 7000, on the other hand, was not able to completely remove ATZ after 120 min of irradiation. In this case, pesticide removals were 37–45 % over 120 min, while C–TiO 2 performed better with regard to ATZ mineralization, with 38 % TOC removal. Given the low mineralization of atrazine, the intermediate compounds formed were identified for each photocatalytic material.

Supercritical hydrothermal combustion, as a quick homogeneous oxidizing process, offers a promising treatment option for industrial wastewater. This paper established a computational fluid dynamics model of a water-cooled hydrothermal combustion burner to investigate the thermal flame characteristics. The effects of the fuel mass flow rate, fuel concentration, initial reactor temperature, reaction pressure, and oxidant temperature on the thermal combustion ignition were revealed. The results indicate that the fuel concentration (from 10 wt% to 60 wt%) and initial reactor temperature (from 623 to 773 K) had less effect on the ignition temperature. In contrast, the ignition temperature increases by 398 K with increasing fuel mass flow rate (from 24 kg h −1 to 1080 kg h −1 ). As the oxygen temperature increases (from 273 to 673 K), the ignition temperature gradually decreases to 573 K and then increases. An increase in reaction pressure can facilitate a decrease in ignition temperature to a certain extent, and the optimal reaction pressure is 25 MPa. This study provides a vital reference for a hydrothermal burner’s scale-up design and ignition operation.

A massive use of nitrogen based fertilizers in agriculture is worldwide one of the main causes for nitrate contamination of groundwater. Methods for removing nitrate from aquatic environment through physical and/or chemical processes often turn out to be not applicable because of unaffordable financial resource as well as essential infrastructure lack. On the other hand, biological processes seem to have potentiality to overcome these limitations since they are less expensive and easier to be performed. Accordingly, in the present work, a moving bed biofilm reactor (MBBR) filled with Kaldnes K1 as carrier media was used to remove nitrate from a synthetic groundwater at bench scale. Acetate was used as organic source. Different operational conditions were tested: influent nitrate concentrations of 30, 40, 50 and 60 mg/L; hydraulic retention times of 24, 18, 12 and 8 h; and COD/NO 3 -N mass ratios of 3.00 and 2.98. Experimental results showed that NO 3 -N = 60 mg L −1 , HRT = 8 h and COD/NO 3 -N ratio = 2.98 were the optimal operating conditions that allowed achieving a NO 3 -N removal by 99 % and a COD removal by almost 100 %. Moreover, almost no NO 2 − -N accumulation and null COD concentration were observed at the optimal operating conditions. An activated carbon filter was placed downstream to remove residual organic compounds prior to disinfection unit, thus avoiding the potential formation of harmful disinfection by-products (e.g. trihalomethanes (THMs)). The MBBR was able to show a rapid recovery whenever the operating conditions were defined as more severe, thus proving that the operating conditions can vary over a wider range. Furthermore, the results showed that the MBBR system can be used effectively as a biological process to remove nitrate from groundwater.

The CeO 2 and CeCoO composite oxide catalysts with bimodal mesopore structures were prepared by hard-template method and used for ethanol complete oxidation in air. The physicochemicalphysicochemical properties of the prepared catalysts were characterized by XRD, BET, TEM, XPS, H 2 -TPR, and O 2 -TPD. The Co species can be dissolved into CeO 2 lattice to form Ce–O–Co solid solution, which promotes reactive oxygen species to be formed on the prepared CeCoO oxide catalysts surface. The bimodal mesopore structures can be obtained by the used hard-template method, and the pore structures of the prepared CeCoO oxide catalysts can be affected by the introduction of Co species. The synergistic effects from bimodal mesopore structures and reactive oxygen species can effectively boost ethanol complete oxidation to final product CO 2 . The CeCo2 catalyst with Ce/Co mole ratio of 2.0 exhibites superior ethanol complete oxidation activity and service stability, the ethanol oxidation conversion and final oxidation product CO 2 selectivity reached 99.8 % and 99.2 % at 200 °C, respectively. This work indicates that the bimodal mesoporous CeCoO solid solution composite oxide catalyst is a promising candidate for OVOCs oxidation elimination from air.

The effect of the experimental conditions on the hydrocracking (HCK) of a hydrotreated light cycle oil (HDT LCO) was studied in this work. The catalyst tested was a 50/50 weight mixture of nickel-molybdenum-phosphorous on alumina (NiMo/Al 2 O 3 ) and a commercial ZSM5 zeolite (HCK 50/50). The experimental conditions tested were 340, 350, 360, and 370 °C; 7.5 MPa; 0.9, 1.2, 1.5, and 1.8 h −1 LHSV, and H 2 /HC of 752 m 3 /m 3 . Two phases: gas and liquid, were obtained as HDK products. The gas phase consisted mostly of C 1 –C 5 paraffins, iso-paraffins, and olefins. The liquid phase was characterized by GC-PIONA and was distributed in lumps as follows: NAPA by C 11 to C 13 -naphthalenes; TET by C 11 to C 13 -tetralins; IND by C 9 to C 13 -indanes and indenes; AKB by C 9 to C 13 -alkylbenzenes; BTEX by benzene, toluene, ethylbenzene, and xylenes; NAPE by C 9 to C 13 -naphthenes; and PIP by C 3 to C 14 paraffin, iso-paraffin, and olefin type hydrocarbons. Using this classification, the results showed that increments in temperature and decrements in LHSV produced increments in the formation of gases, PIP, BTEX, and NAPE. At the same conditions, AKB, TET, NAPA, and IND decreased sharply. TET and NAPA derivatives were no longer present at high temperatures (360–370 °C). It seemed to be a limit of the BTEX formation directly related to the TET and IND presence, and it did not seem to depend on the transalkylation process of AKB hydrocarbons. Instead, AKB hydrocarbons were directly correlated to NAPE hydrocarbon formation by hydrogenation. A kinetic model was prepared. The model presented correlation coefficients higher than 98 %. The kinetic model that was made predicted that neither increasing the temperature nor lowering the LHSV would improve the BTEX formation when departing from this feedstock.

The hydrodynamics of airlift bioreactors, which offer an interesting alternative to conventional stirred-tank bioreactors, has generally been evaluated using experimental approaches, requiring time, energy, and reagents. However, computational fluid dynamics (CFD) has emerged as an important and valuable tool for the analysis and design of these devices, saving time and experimental effort, while providing a large amount of information. In this study, four geometries of a square cross-section 10-L split airlift bioreactor operating with distilled water were simulated using CFD, and the hydrodynamics variables gas hold-up and liquid velocity were evaluated. CFD satisfactorily predicted the hydrodynamic parameters, when compared to experimental data, allowing adequate prediction of the shear rate distribution in airlift bioreactors. The results indicated that different shear rate distributions were obtained by geometric modifications in the bioreactor, showing that its design should be considered to satisfy different specific bioprocess requirements.

The numerical simulation length of the cassette model is determined by studying the effects of the cassette frame rotated alone and the rotated together with the agitator on mixing under different cassette lengths. The effects of different rotating speed, filling ratio of silicon particles, shape and number of agitators on the mixing characteristics of silicon particles in the cassette are studied when the outer frame of the cassette rotated alone and the cassette rotated together with the agitator. The mixing degree is quantitatively analyzed by particle trajectory, mixing index and rate, and the optimum working conditions of each influencing parameter are obtained. The results show that considering the computation time and the influence of the two motion modes on the mixing index and rate, the numerical simulation length of the cassette model is 115 mm. In order to obtain the best mixing effect and mixing speed, it is recommended that the rotating speed of the cassette is 8.91 rad/s, the filling rate is 23.60 %, and the shape of the agitator is rectangular and the number is 6. When the cassette and agitator rotate together, the filling rate has little effect on particle mixing. Considering the production efficiency, mixing effect and rate, as well as the stability of mixing, the recommended speed is 4.08 rad/s, the filling rate is 41.40 %, and the rectangular shape and number of six agitators are ideal choices.