Sartakov, M. P., V. A. Chumak V. A., I. V. Grekhova, and M. V. Efanov. “The Study of the Chemical Composition of Humic Acids of Initial and Heat-Treated Sapropels of Lakes of the Surgut District of the Khanty-Mansi Autonomous Okrug – Yugra”. Amazonia Investiga, vol. 7, no. 17, Dec. 2018, pp. 72-78, https://amazoniainvestiga.info/index.php/amazonia/article/view/328.