Lignin Ammoxidation: Synthesis of Nitrogen Releasing Soil Conditioning Products from Waste Pulp Liquor and Their Pot Trial Evaluation

Abstract

PurposeLignin is a precursor of humus, and a by-product of the pulp and paper industry. This study investigated the synthesis of soil conditioning materials from industrial lignins (kraft, soda and sulphite), through ammoxidation.MethodThe industrial lignins were characterized and ammoxidized in a 1 L Parr reactor at 80 °C, 10 barg, for 4 h, with 7 wt% ammonia and 10 wt% lignin in the reaction mixture. A plant trial assessment of the products was conducted over four weeks.ResultsN-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, an oxidation step with 5% hydrogen peroxide was required (prior to ammoxidation) because the criterion for N-lignins (C/N < 20) could not be met under standard reaction conditions. 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 it had a high pH (8.81 pH) and a high salt index (63.62%) due to the pulping technique used in its isolation.ConclusionKraft and soda lignins could be successfully ammoxidized to synthesize N-lignins that are suitable for use as soil conditioning materials.Graphical Abstract