Impact of HNO3 Solution Treatment of South Brazil Coal Matrices on Their Chemical Composition and Humic Acids Yield

Abstract

In this study, changes on the chemical composition and on the humic acids yield of coal matrices promoted by 25% HNO 3 treatment were investigated, by means of elemental, thermogravimetric analyses, and Fourier transform infrared spectroscopy (FTIR). The "run of mine" (ROM) matrix C-MTR (ROM coal) showed the highest C content (41%), highest C/N ratio (76) and lowest ash content (54%) of the studied samples. In contrast, coal tailings presented a higher degree of aromaticity. Treatment with HNO 3 solution increased N content, but not that of C. Therefore, the C/N ratio of all samples decreased to values between 11 and 20. The insertion of N occurred in the aromatic structures, while the oxidation occurred in both the aromatic and aliphatic chains. HNO 3 treatment increased the humic substances content and the largest increase being observed in the coal tailings (16 to 31% increment in comparison to the untreated sample). The generated humic acids showed in general a chemical composition similar to that of soil humic acids, but a greater aromaticity degree.Keywords: humic substances, coal oxidation, FTIR spectroscopy, TGA IntroductionCoal deposits in South Brazil represent about 90% of the national coal resources, but its use as a fuel source or in the metallurgical industry is highly restricted due to its low rank and high ash content.1 In Brazil, about 75% of the mined material is discarded after beneficiation process. The great amount of produced coal tailing is usually stored in open piles and a strong acidification of the surrounding environment is observed due to the formation of acid mine drainage. 2,3A sustainable fate for this coal residue is the production of humic substances (HS) that, besides being a source of C and N to microorganisms and plants, plays an important role in chemical and physical properties of the soil. 4 Environmental benefits of HS are mainly related to their high complexing capacity for metal ions that results from the oxygen-containing functional groups present in their structure associated to their high specific surface. 5,6 The use of oxidative chemical methods for increasing the yield of humic acids (HA) of coal and similar carbonaceous samples is well known and it is widely reported in the scientific literature since 1930.7-10 However, systematic studies that investigate the effects of these methods in the chemical composition of both the bulk coal sample and of their HS are less frequent. Usually, spectroscopic and chemical characterization is focused on the coal HA since the purification of this fraction is easily attainable.10-12 The difficulty in analyzing the chemical composition of the organic fraction of coal matrices arises from the presence of inorganic components. To overcome the interference of the inorganic matrix in the spectroscopic, thermic, pyrolitic and chemical analyses, the treatment with diluted hydrofluoric acid (HF) solution has been used in soil samples in order to concentrate carbon and remove paramagnetic compounds.13,14 According to Rumpel et al.,14 no ...

Carbones de Bajo Rango como Recurso para Enmiendas Húmicas mediante Transformación Microbiana

“…La obtención de SH a partir de carbón generalmente se realiza a través de los mismos métodos básicos de extracción química de SH a partir del suelo, compost y otros restos de materiales orgánicos estabilizados (Fong et al, 2007;Dick et al, 2017;Pájaro et al, 2017), dentro de los cuales el más común y económico es la extracción con álcali. Como efectos agronómicos de su utilización, se ha demostrado que las SH obtenidas de diferentes clases de carbón desencadenan efectos positivos en la germinación de semillas, contenido y balance nutricional en los tejidos vegetales, absorción de macro y micronutrientes, longitud y crecimiento radical, llenado de frutos y semillas, y calidad en una amplia variedad de plantas de cultivo (Sharif et al, 2002;García et al, 2012;Rose et al, 2014;Olaetxea et al, 2017;Olk et al, 2018).…”