Abstract <p>Lignin sludge accumulated at the dumpsites of paper mills causes a negative environmental impact. To a great extent, this is due to the lack of efficient remediation technologies, which could convert lignin into the soil-like substrate. The goal of this study was to evaluate the potential of in situ application of advanced oxidation processes (AOP) to initiate oxidative polymerization of phenols, which is the basis of humification. We suggested that in case of success, the treated lignin sludge could serve as a soil-like substrate stimulating plant growth. Lignin sludge sampled at the dumpsite of the Baikal Pulp and Paper Mill was studied. The AOP treatment was carried out under mild conditions, i.e., at room temperature, using sodium percarbonate, potassium persulfate, calcium peroxide, and hydrogen peroxide with different activators. In addition, different water-absorbing materials were used, including cellulose, activated charcoal, and vermiculite. Two different techniques of reagent application were tested: (a) homogeneous mixing with the lignin sample and (b)&#xa0;model injection into wells. Humification index of the treated lignin sludge was found directly as the ratio of the contents of humic and fulvic acid fractions in alkaline extracts. In addition, it was characterized via ultraviolet-visible spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry. It was shown that in case of percarbonate and persulfate, the AOP treatment increased the content of oxidized high-molecular weight aromatic components in the lignin, which was proved by the growth of humification index and optical parameters. We concluded that under the mild conditions applied in this study, oxidative polymerization predominates over destruction of lignin polyphenolic complex, which leads to transformation of lignin sludge into soil-like substrate supporting plant growth.</p>