Synthesis of eco-friendly ZnO-based heterophotocatalysts with enhanced properties under visible light in the degradation of organic pollutants

Abstract Heterogeneous photocatalysts have been widely used for the removal of various organic pollutants from wastewater. The main challenge so far resides in the sustainability of the process, with regard to the synthesis and the application under visible light. In this study the precipitated materials from the Moringa oleifera seed (MO), groundnut shells (GS) and apatite (A) agrowastes were functionalized with zinc oxide (ZnO) and silver (Ag) solution, to produce a novel bioheterophotocatalysts. Various analytical techniques such as scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), photoluminescence (PL) and X-ray diffraction (XRD) were used for the characterization of the novel photocatalysts. It was proven that agrowastes can also enhance the photocatalytic activity of a ZnO-based photocatalyst as pure metals. The combination of MO/GS/A/ZnO/Ag in a 1:1:1 ratio resulted in a lower band gap of 1.59 eV, as compared to the band gap of 2.96 eV for ZnO/Ag. These photocatalysts' efficiency was also tested on the photodegradation of polycyclic aromatic hydrocarbon (PAHs) derived from coal leaching in various water sources such as acidic mine drainage, alkaline mine drainage and sewage wastewater. From MO/GS/A/ZnO/Ag, the removal efficiency was found to be 69.59%, 61.07% and 61.68%, compared to 52.62%, 37.96 and 44.30% using ZnO/Ag in acidic mine drainage, alkaline mine drainage and sewage wastewater for 60 min under solar irradiation

An understanding of lump coal physical property behaviour (density and particle size effects) impacting on a commercial-scale Sasol-Lurgi FBDB gasifier

Thermal processes which utilize coarse coal, such as fixed-bed gasification and chain grate stoker boilers, are dependant on a stable particle size for stable operation. During coarse coal utilization, thermal fragmentation of lump coal (upon heating) produces hydrodynamic effects (pressure drop fluctuations) manifesting itself in a variety of ways, and include: channel-burning and solids elutriation. Primary thermal fragmentation occurring in the drying zone of a fixed-bed reactor is primarily a function of moisture content release with ensuing particle size reduction. Large particles tend to fragment more than finer particles, thus leading to hydrodynamic problems. From fragmentation studies it was elucidated that a thermal “stable size” is reached through the process of thermal fragmentation for optimum heat transfer and utilization during the drying and pyrolysis zone regions of the coarse coal utilization process. In this paper, the Sasol-Lurgi MK IV FBDB gasifier turn-out physical property profiles (bulk density and particle size distribution) results will be discussed. It was found that these profiles provided significant insight into the complex heterogeneous nature of the coal transformation processes occurring within the fixed-bed reactor. In the case of the bulk density profile, a shrinking core and flaking mechanism was proposed to explain the increase in density occurring in the bottom half of the gasifier. The +25 mm size fraction distribution profile was found to clearly show the fragmentation effects occurring within the reactor. Primary fragmentation was inferred as the mechanism responsible for causing breakage of this size fraction down to a remaining ca. 15% +25 mm fraction. The significant breakage of the coarse +25 mm fraction is expected to influence unstable gasifier conditions in the top part of the gasifier, due to pressure drop fluctuations caused by void packing. A good correlation was obtained for the relationship between bulk density versus the −25 mm + 6.3 mm size fraction content, indicating that the bed-packing density is highly dependent on the relative abundance of this intermediate size fraction. The −6.3 mm size fraction distribution profile was found to not be significantly different between the four reaction zones identified in the gasifier. Breakage of the coarser +6.3 mm sizes occurred continuously, and could possibly be related to breakage caused by the ash-grate when sampling. The Ergun Index was successfully used to profile the fragmentation zones identified and to show areas within the gasifier where pressure drop and resultant instability occurs. This is the first-ever identification of this phenomenon occurring within a fixed-bed gasifier and is expected to lead to significant optimization challenges to ensure better stabilit

The South African industry use of Mössbauer spectroscopy to solve operational problems

South Africa is a country that is very rich in mineral resources but the use of Mössbauer spectroscopy to solve operational industrial problems is however very limited. In the Bushveld Igneous Complex the main minerals extracted from the ore are the platinum group metals and chromium, but secondary recovery of base metals such as nickel, copper and cobalt forms an integral part of the process. Losses of nickel in the slag can amount to about 4%and subsequent a slag cleaning furnace is used to reduce the loss to less than 0.5% nickel oxide. The Fe2+/Fe3+ ratio and mineralogy was used to determine the partial oxygen pressure in the furnaces and also the efficiency of the nickel recovery. From the Mössbauer results, augmented with XRD, SEM, EMP-WDX and MLA analyses, optimum conditions were determined to ensure minimum metal losses. The use of Mössbauer spectroscopy in the coal industry, to investigate mineral changes that occur during its use, is also of importance. The main minerals present in coal were determined with the aid of various techniques, such as Mössbauer, XRD, SEM and HR-TEM, with the major iron minerals found to be pyrite, illite, ankerite and jarosite. A large quantity of coal is used to produce syngas via gasification plants for the production of synthetic fuels. The change of the mineral matter during gasification was studied and the changes occurring during the gasification process were followed. The syngas produced, is further treated by means of the Fischer–Tropsch process where an iron catalyst is incorporated in the process. The usefulness and fouling of the catalyst is being studied with the aid of Mössbauer spectroscopy

A multi-analytical study on the sulphur components in some high sulphur Indian Tertiary coals

The main source of industrial energy in the world is coal. To better understand the distribution of sulphur containing components in high sulphur Tertiary coals, a multi-analytical analysis was carried out on four industrially important high sulphur northeast region (NER) Indian coals. Some of the relevant information on the distribution and speciation of sulphur functionalities in these Tertiary coals were obtained by using chemical analysis, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and Mössbauer spectroscopy techniques. The study revealed the presence of various sulphur compounds such as pyrite (FeS2), disulphides (S–S), sulphone (−SO2–) and aryl thioether (R1–S–R2) in the coals, which are essential to be removed before the coal could be utilised. Sulphur is present in high amounts in MEG (4.54 %) and NG (4.56 %) coal samples. The lowest amount of sulphur was found in TP coal. In the present study, it is shown that the NER Tertiary coals contain higher amounts (>50 %) of organic sulphur, which may be difficult to remove by conventional methods. X-ray photoelectron spectroscopy (XPS) analysis of these Tertiary coals revealed that sulphur is present in two forms namely inorganic and organic. TPR also provide information on sulphur association in coals as iron pyrite, sulphides, thiophene, thiols, etc. Fe components in NER coals contain typically pyrite and Mössbauer spectroscopy where pyrite, illite, marcasite and hematite were observed, which is in agreement with the results obtained with other technique

Reactivity study of fine discard coal agglomerates

In this laboratory study thermo-gravimetric analyses were conducted to determine the influence of the addition of an alkaline metal catalyst (K2CO3 incorporated into a fine discard coal agglomerate mixture by physical mixing) on the CO2 gasification char reactivity of 10 mm coal pellets subjected at high temperatures (900–1000 °C). It was found that catalyst addition significantly increased the reaction rate of the carbon conversion. The reaction rate was doubled for the 5% catalyst addition runs compared to the base case (0% catalyst at 900 °C), 120% faster at 950 °C and, 93% faster at 1000 °C. The temperature influenced the reaction rate, thereby decreasing the reaction time from 6.4 h at 900 °C to 3.4 h at 950 °C and, 1.9 h at 1000 °C for the 5% catalyst addition. The fastest time needed for conversion was with the use of 5% catalyst addition at 1000 °C, which required only 13% of the total time needed in comparison with the base case (14 h). The conversion plots were further analyzed, and it was concluded that the lines followed the homogeneous model (REF) up till a conversion of 50% for the pelletized particles and 20% for the raw coal.The observed activation energy was also found to be lower in the coal pellet catalyzed system when compared to raw run of mine (R.O.M.) coal of the same dimensions as for the coal agglomerates (199 kJ/mol) and was calculated to be 195 kJ/mol, 184 kJ/mol, and 156 kJ/mol for the 1%, 3% and 5% catalyst addition runs, respectively. These results show that there is an opportunity to possibly improve on the fluidised-bed/large particle gasifier throughput by speeding up the time needed for the rate limiting CO2 gasification reaction in a catalysed system comprising of fine discard coal agglomerates containing an alkaline additiveSouth African Research Chairs Initiative (SARChI) of the Department of Science and Technology and National Research Foundation of South Africa (Coal Research Chair Grant No. 86880