Method for preparing a lignosulfonate polymer

A method for preparing a water-insoluble lignosulfonate polymer from a first solution comprising brown liquor that contains lignosulfonate precursors, the method comprising:

• a) separating lignosulfonate precursors from the first solution to provide a second solution with at least 5 wt % of lignosulfonate precursors, wherein the separating comprises filtering out components with a predefined particle diameter of less than 1 μm from the first solution and removing low-molecular weight components with a molecular weight of less than 500 Da from the first solution,
• b) adding an enzyme and gas to the second solution to form polymerizable lignosulfonate radicals, wherein the enzyme is adapted to catalyze the formation of oxygen radicals at phenolic hydroxyl groups, wherein the enzyme comprises a laccase, a peroxidase, or a mixture of these enzymes, and wherein the gas comprises oxygen added as gas with an admixture of not more than 10 vol % of other gases, and
• c) polymerizing lignosulfonate precursors of the resultant solution of b) to form a water-insoluble lignosulfonate polymer.