Lignin ammoxidation: Synthesis of nitrogen releasing soil conditioning products from waste pulp liquor and their pot trial evaluation

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Abstract Purpose This study investigated the ammoxidation of three industrial lignins of which two were respectively precipitated from kraft and soda liquors, and the third one, a filtered sodium lignosulfonate for the synthesis of soil conditioning materials.Method The industrial lignins were characterized for structural properties and ammoxidized in a 1 L Parr reactor at 80 oC, 10 barg, for 4 hours, with 7 wt. % ammonia and 10 wt. % lignin in the reaction mixture. A plant trial assessment of the products was conducted over four weeks.Results N-lignins with C/N ratios of 12.30, 15.81, and 14.64 were obtained from kraft, soda and sodium lignosulfonate against a standard requirement of a C/N < 20. However, soda lignin and sodium lignosulfonate could not meet the criteria for N-lignins (C/N < 20) under standard reaction conditions, requiring an additional pre-oxidation with 5% hydrogen peroxide prior to ammoxidation. In the plant trial, N-modified kraft (3.50 t/ha) and soda lignin (3.21 t/ha) recorded crop yields that were 71% and 57% higher than the control (2.04 t/ha), respectively. The sodium lignosphonate, while it met the requirements for use as a soil fertilizing material in terms of nitrogen content, resulted in complete crop failure. Further characterization showed that due to its high pH (8.81 pH) and a high salt index (63.62%) due to the pulping technique used in its isolation, was unsuitable as a raw material for soil fertilizing materials.Conclusion Kraft and soda lignins could be successfully ammoxidized to synthesize N-lignins that are suitable for use as soil conditioning materials.

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