Preparation of hydroxymethylated lignin: in a round bottom glass, 11.25 ml of aqueous sodium hydroxide (1 M, 11.25 mmol) was added to 5 g of Kraft lignin (dry basis) and the mixture was allowed to stir at room temperature. 8.75 ml of deionized water was added to the stirring mixture and the temperature was increased to either 60 or 85 °C. Then 5 ml formalin solution (37% formaldehyde, 66 mmol) was added dropwise to the lignin solution. The reaction was allowed to take place for 3 hours. Next, the product solution was quenched in cold deionized water and adjusted to pH 3 with drop by drop addition of 0.1 M hydrochloric acid. The precipitate was centrifuged and washed 3 times with deionized water to remove extra acid and reach pH 3.2.