Utilization of Alkaline-oxygen Delignification Stage Waste Liquor for the Removal of Aluminum Toxicity to Plant Growth

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Quite a large area on the earth is covered by the so-called damaged soil, particularly by acid soil, on which plant growth is generally limited. This is partly due the low pH of soil water in those area, but is mainly due to the high aluminum ion concentration in the soil water. We have been working to develop the new soil conditioning agents from industrial lignins by a partial oxidation and have reported that the partially oxidized lignin is quite effective to remove aluminum toxicity to plant. In this study, chemical characteristics of lignin fragments in the alkaline-oxygen stage waste liquor and their functions to aluminum toxicity were carefully investigated. It was found that the lignin fragments as a whole and its high molecular weight fraction showed the positive effect to move aluminum toxicity, but the lower molecular weight fractions showed different behaviors dependent on the molecular weight. A fraction containing a lot of oxalic acid showed positive effect in the case of a lower dosage, but with an increased dosage it showed a clear negative effect for the removal of aluminum toxicity. This behavior can be explained by the complex formation between oxalic acid and aluminum ion which was confirmed by 27AL-NMR spectroscopy. Free oxalic acid itself is toxic to the plant growth.