RESUMEN
Humic substances comprise up to 70% of the total organic matter in soils, between 50 and 80% of the dissolved organic matter in water, and about 25% of dissolved organic matter in groundwater. Elucidation of the complex structure and properties of humic substances requires advanced analytical tools; however, they are of fundamental importance in medicine, agriculture, technology, and the environment, at large. Although they are naturally occurring, significant efforts are now being directed into their extraction owing to their relevance in improving soil properties and other environmental applications. In the present review, the different fractions of humic substances were elucidated, underlying the mechanisms by which they function in soils. Furthermore, the extraction processes of humic substances from various feedstock were illustrated, with the alkali extraction technique being the most widely used. In addition, the functional group and elemental composition of humic substances were discussed. The similarities and/or variations in the properties of humic substances as influenced by the source and origin of feedstock were highlighted. Finally, the environmental impacts of humic substances were discussed while highlighting prospects of humic acid production. This review offers enormous potential in identifying these knowledge gaps while recommending the need for inter- and multidisciplinary studies in making extensive efforts toward the sustainable production of humic substances.
Asunto(s)
Materia Orgánica Disuelta , Sustancias Húmicas , Sustancias Húmicas/análisis , Ambiente , Suelo/química , AgriculturaRESUMEN
In the present study, the pyrolysis behavior of Nigerian oil sands was investigated using thermogravimetric analysis. This was done with the aim of deriving kinetic models that can be relevant in the development of the natural resource. The effects of different heating rates (10, 20, and 30 °C/min) on oil sand pyrolysis were studied. The results of the study indicated that three regions comprising low-temperature oxidation, devolatilization, and high-temperature oxidation were obtained at all heating rates. The peak temperatures were observed to rise with an increasing heating rate, a phenomenon described as thermal hysteresis. Mineralogical analysis showed the presence of diffraction peaks corresponding to chlorite, quartz, aragonite, dolomite, calcite, and montmorillonite minerals and the notable absence of expandable clay minerals which are known to pose problems during tailing management and the aqueous bitumen extraction process. The kinetic analysis showed that the activation energy increased with the degree of conversion, with the highest activation energy of 14.682 kJ mol-1. The Coats-Redfern kinetic model gave the best model fit for oil sand pyrolysis.
RESUMEN
BACKGROUND: Soil contamination resulting from the use and handling of petrochemicals and other petroleum products during power generation activities is an increasing global concern due to its adverse impact on the ecosystem. OBJECTIVES: The present study was carried out to determine the concentrations and speciation of potentially toxic metals in oil-contaminated soils around transformer installation areas in Ile-Ife, Nigeria, and to confirm soil pollution levels with hazard quotient and hazard index analysis. METHODS: Soils from the transformer oil-contaminated and uncontaminated (control) areas were collected at 0-15 cm and 15-30 cm depths and analyzed for heavy metal concentrations using atomic absorption spectrometry. The metals were fractionated and their hazard evaluated to confirm the pollution level of the contaminated soils. RESULTS: The concentrations of cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), lead (Pb) and zinc (Zn) in the two sets of oil-impacted soils were higher than in the control soils. The metals also had relatively moderate bioavailability and mobility potential with more of the proportion retained in the residual fraction. Chronic daily intake (CDI) of the metals increased in the order of: Cd < Cr < Pb < Ni < Mn < Cu < Zn < Fe, while chronic daily intake risk exposure pathway followed the order of: CDIinhalation < CDIdermal < CDIingestion. CONCLUSIONS: The study concluded that the concentrations of the metals were within permissible limits, but the chronic daily dosage was significant and may pose a health hazard to humans with long term exposure to these heavy metal contaminants. COMPETING INTERESTS: The authors declare no competing financial interests.