Wettability analysis method for assessing the effect of chemical pretreatment on brown coal biosolubilization by Gordonia alkanivorans S7

•An alternative method of contact angle measurement was developed using OCT.•A relationship between the wettability and the pre-treatment of coal was observed.•Gordonia alkanivorans can liquefy coal with higher contact angle more effective.•HNO3 treatment introduce O2 and NO3 groups into a coal, that increase liquefaction. Coal wettability is of considerable importance for determining the efficiency of the solubilization process. In principle, hydrophobic coal structure should be modified for effective contact with microorganisms in the coal biosolubilization process. The paper considers the image analysis method for determining the contact angle of chemically pretreated coal, where the measurement surface is prepared by pressing coal powder into tablets. Due to sessile drop asymmetry resulting from the heterogeneity of the substrate, liquid-solid contact angles are evaluated along a three-phase contact line. The source of a three-dimensional drop image is a scan obtained with an infrared light beam from an optical coherence tomography device. The effect of coal pretreatment on biosolubilization by Gordonia alkanivorans S7 was investigated with the use of the method. To emphasize the relationship between coal wettability (hydrophilicity or hydrophobicity) and the possibility of coal liquefaction, Fourier Transform Infrared (FT-IR) spectroscopic analyses were performed. Data obtained from infrared spectra showed changes in coal structure and confirmed a positive influence of nitric acid on coal biosolubilization. Due to a high liquefaction degree, it is possible to implement the process of lignite biosolubilization by G. alkanivorans S7 on a larger scale. The contact angle measurement method can be successfully applied in the selection of methods for the chemical and biological conversion of coal in various technological processes.