1. Twardowska, I. and Kyziol, J., Environ. Int., 2003, vol. 28, pp. 783–791.
2. Varshal, G.M. and Buachidze, N.S., Zh. Anal. Khim., 1983, vol. 38, pp. 2156–2167.
3. Varshal, G.M., Velyukhanova, T.K., and Koshcheeva, I.Ya., Guminovye veshchestva v biosfere (Humic Substances in the Biosphere), Moscow: Nauka, 1993, pp. 97–117.
4. Orlov, D.S., Gumusovye kisloty pochv i obshchaya teoriya gumifikatsii (Humus Acids of Soils and General Humification Theory), Moscow: Moskovskii Gos. Univ., 1990, p. 325.
5. Bergelin, A., The acid–base properties and aggregation of humic materials, Dissertation, Stockholm (Sweden): Int. Royal Inst. of Technology, 2001.