Review of the methods for determination of reactive oxygen species and suggestion for their application in advanced oxidation induced by dielectric barrier discharges.

Advanced oxidation processes (AOPs) particularly non-thermal plasmas based on electrical discharges have been widely investigated for water and wastewater treatment. Dielectric barrier discharges (DBDs) generate large amounts of selective and non-selective reactive oxygen species (ROS) such as ozone, hydrogen peroxide, atomic oxygen, superoxide molecular anions and hydroxyl radicals, having been proved to be efficient for water decontamination among various forms of electrical discharge systems. The detection and quantification methods of these oxygen species in non-thermal plasmas have been reviewed. However, their application in dielectric barrier discharge has not been well studied. It is therefore imperative to summarise the various detection and quantification methods for oxygen-based species determination in AOPs, aqueous systems and non-thermal plasma processes. Thereafter, reviewed methods are suggested for the determination of ROS in DBD configurations to understand the consumption trend of these oxidants during treatment of water effluents and to evaluate the performance of the treatment reactor configuration towards the degradation of targeted pollutants.

The leaching behaviour and geochemical fractionation of trace elements in hydraulically disposed weathered coal fly ash.

A five-step sequential extraction (SE) procedure was used to investigate the leaching behaviour and geochemical partitioning of the trace elements As, Zn, Pb, Ni, Mo, Cr and Cu in a 20-year-old fly ash (FA) dump. The weathered FA, which was hydraulically co-disposed with salt laden brine in slurry form (FA: brine ratio of 1:5), was analyzed and compared with fresh FA. The weathered FA samples were collected from three cores, drilled at a coal-fired power station in the Republic of South Africa while the fresh FA sample was collected from the hoppers in the ash collection system at the power station. The FA samples were sequentially leached using: ultrapure water; ammonium acetate buffer solution (pH 7); ammonium acetate buffer solution (pH 5); hydroxylamine hydrochloride in nitric acid (pH 2) and finally the residues were digested using a combination of HClO4: HF: HNO3 acids. Digestion of as received (unleached) FA samples was also done using a combination of HClO4: HF: HNO3 acids in order to determine the total metal content. The trace element analysis was done using ICP-OES (Varian 710-ES). The SE procedure revealed that the trace elements present in the fresh FA and the weathered FA samples obtained from the three cores could leach upon exposure to different environmental conditions. The trace elements showed continuous partitioning between five geochemical phases i.e., water soluble fraction, exchangeable fraction, carbonate fraction, Fe and Mn fraction and residual fraction. Although the highest concentration of the trace elements (ranging 65.51%-86.34%) was contained in the residual fraction, a considerable amount of each trace element (ranging 4.42%-27.43%) was released from the labile phases (water soluble, exchangeable and carbonate fractions), indicating that the trace species readily leach from the dumped FA under environmental conditions thus pose a danger to the receiving environment and to groundwater.

Chemical, mineralogical and morphological changes in weathered coal fly ash: a case study of a brine impacted wet ash dump.

The mobility of species in coal fly ash (FA), co-disposed with brine using a wet ash handling system, from a coal fired power generating utility has been investigated. The study was conducted in order to establish if the wet ash dump could act as a salt sink. The ash was dumped as a slurry with 5:1 brine/ash ratio and the dam was in operation for 20 years. Weathered FA samples were collected along three cores at a South African power station's wet ash dump by drilling and sampling the ash at 1.5 m depth intervals. A fresh FA sample was collected from the hoppers in the ash collection system at the power station. Characterization of both fresh FA and weathered FA obtained from the drilled cores S1, S2 and S3 was done using X-ray diffraction (XRD) for mineralogy, X-ray fluorescence (XRF) for chemical composition and scanning electron microscopy (SEM) for morphology. Analysis of extracted pore water and moisture content determination of the fresh FA and the weathered FA obtained from the drilled cores S1, S2 and S3 was done in order to evaluate the physico-chemical properties of the FA. The XRD analysis revealed changes in mineralogy along cores S1, S2 and S3 in comparison with the fresh FA. The SEM analysis revealed spherical particles with smooth outer surfaces for the fresh FA while the weathered ash samples obtained from cores S1, S2 and S3 consisted of agglomerated, irregular particles appearing to be encrusted, etched and corroded showing that weathering and leaching had occurred in the ash dump. The moisture content (MC) analysis carried out on the fresh FA (1.8%) and the weathered FA obtained from the drilled cores S1 (41.4-73.2%), S2 (30.3-94%) and S3 (21.7-76.2%)indicated that the ash dump was water logged hence creating favourable conditions for leaching of species. The fresh fly ash (n = 3) had a pH of 12.38 ± 0.15, EC value of 4.98 ± 0.03 mS/cm and TDS value of 2.68 ± 0.03 g/L, the pH of the drilled core S1 (n = 45) was 10.04 ± 0.50, the EC value was 1.08 ± 0.14 mS/cm and the TDS value was 0.64 ± 0.08 g/L. Core S2 (n = 105) had pH of 10.04 ± 0.23; EC was 1.08 ± 0.06 mS/cm and TDS was 0.64 ± 0.04 g/L, while core S3 (n = 66) had pH of 11.04 ± 0.09; EC was 0.99 ± 0.03 mS/cm and TDS was 0.57 ± 0.01 g/L. The changes in pH values can be attributed to the dissolution and flushing out of alkaline oxides like CaO and MgO from the dumped ash. The variations in pH values shows that the fly ash is acidifying over time and metal mobility can be expected under these conditions. The large decrease of EC in the drilled ash cores S1, S2 and S3 compared to the fresh ash indicated a major loss of ionic species over time in the ash dump. The XRF analysis showed the progressive dissolution of the major aluminosilicate ash matrix which influenced the release of minor and trace elements into the pore water enhancing their mobility as the ash dam acidified over time. Brine co-disposal on the ash may have been responsible for the slight enrichment of some species such as Na (0.27-0.56%), SO4(2-) (0.06-0.08%), Mg (0.57-0.96 %) and K (0.02-0.34%) in the disposed weathered FA. However, there was no significant accumulation of these species in the disposed FA despite continuous addition of large volumes of highly saline brine over the 20 year period that the dump existed, indicating that the ash dam was incapable of holding salts and continually released elements to the environment over the lifetime of the dam.

Comparison of CO2 capture by ex-situ accelerated carbonation and in in-situ naturally weathered coal fly ash.

Natural weathering at coal power plants ash dams occurs via processes such as carbonation, dissolution, co-precipitation and fluid transport mechanisms which are responsible for the long-term chemical, physical and geochemical changes in the ash. Very little information is available on the natural carbon capture potential of wet or dry ash dams. This study investigated the extent of carbon capture in a wet-dumped ash dam and the mineralogical changes promoting CO2 capture, comparing this natural phenomenon with accelerated ex-situ mineral carbonation of fresh fly ash (FA). Significant levels of trace elements of Sr, Ba and Zr were present in both fresh and weathered ash. However Nb, Y, Sr, Th and Ba were found to be enriched in weathered ash compared to fresh ash. Mineralogically, fresh ash is made up of quartz, mullite, hematite, magnetite and lime while weathered and carbonated ashes contained additional phases such as calcite and aragonite. Up to 6.5 wt % CO2 was captured by the fresh FA with a 60% conversion of calcium to CaCO3 via accelerated carbonation (carried out at 2 h, 4Mpa, 90 °C, bulk ash and a S/L ratio of 1). On the other hand 6.8 wt % CO2 was found to have been captured by natural carbonation over a period of 20 years of wet disposed ash. Thus natural carbonation in the ash dumps is significant and may be effective in capturing CO2.