AbstractNitrogen fixation of wheat straw ammonium sulfite pulping spent liquor and the chelating property of nitrogen-fixed ammonium lignosulfonate were studied. Results showed that free ammonium nitrogen in spent liquor could be fixed by formaldehyde. When the amount of formaldehyde was 10% based on the dry weight of lignosulfonate, 30% of inorganic nitrogen was converted into organic nitrogen, of which 87.4% was ammonium lignosulfonate and 12.6% was urotropine. The proper chelating condition of nitrogen-fixed ammonium lignosulfonate was as follows: pH:3, hydrogen peroxide:10%, FeSO4: 40.9%, and 50 oC for 30 min. Under this reaction condition, the chelating ratio of Fe2+ was measured as 15.1%. Chelation did not result in Fe(OH)3 precipitation under alkaline conditions. Effects of H2O2 dosage on the structure of ammonium lignosulfonate were also studied. The content of carboxyl, phenolic hydroxyl, and conjugated carbonyl groups in lignosulfonate that could be chelated with metal ions increased after ammonium lignosulfonate was oxidized. Average molecular weight and distribution were also determined with GPC. Results showed that the proportion of higher molecular weight components increased after oxidation of ammonium lignosulfonate, indicating that oxidative degradation and condensation reaction proceeded during oxidative treatment and condensation was the main reaction. The increase of molecular weight could improve the chelating ability of ammonium lignosulfonate.