We characterized the lignin fragments in the alkaline oxygen delignification-stage waste liquor from a pulp and paper mill as a soil-conditioning agent. Chemical reactions of this lignin fragment should be very similar to those of alkaline-oxygen treatment of kraft lignin. Alkaline oxygen delignification-stage waste liquor was subjected to gel filtration chromatography and four different molecular weight fractions (F1, F2, F3, and F4) were obtained. These four fractions were used in plant growth experiments. A very clear positive effect in removing the aluminum (Al) toxicity to root growth of radish was found for all four fractions. When aluminum concentration in the nutrient solution was as low as 0.5ppm, root elongation was practically inhibited, but if 12.5ppm of lignin fragment was added, the toxic effect was practically removed and root elongation better than that in the control was observed. However, each fraction showed different performance in removing aluminum toxicity, and the highest molecular weight fraction, which contained some aromatic structures, showed better performance in removing Al toxicity. The reason for their different behaviors on Al toxicity was also examined.
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