Sustainable Anion Exchange Materials from Peat Lignin for PFAS Remediation

Abstract
 Peat is a natural product formed largely through the inhibited decomposition of plant materials. Deposits of partially decomposed plant material accumulate in shallow bogs, occurring most abundantly in cool temperate regions of the Northern Hemisphere. Conditions that favor peat formation include the absence of oxygen due to water saturation, high humidity, acidity, and nutrient-poor environments, all of which reduce microbial activity. Peat is a slowly renewable natural resource with many applications, particularly in horticulture. Chemically, peat consists primarily of hemicellulose, cellulose, lignin, and humic and fulvic acids.

We developed two new peat-lignin-derived weak-base anion exchange materials with total anion exchange capacities of 80 and 180 mEq/100 g, respectively, by utilizing the phenolic functionality of lignin. The new materials were developed through peat granulation followed by thermal treatment to create a cross-linked natural polymer. Humic and fulvic acids and hemicellulose were then extracted by treatment with Na₂CO₃ and HCl, producing a granulated material that is stable at pH 11.0 while retaining its reactive phenolic moieties.

The surface of the peat-lignin granules was further reacted with hexamethylenetetramine under modified Duff reaction conditions. This reaction introduced amine groups rather than the expected aldehyde groups, resulting in a material with a total anion exchange capacity of 80 mEq/100 g (U.S. Patent No. 10,722,878). The addition of phenol and phenol derivatives to the hydrolyzed peat-lignin granules, followed by reaction with hexamethylenetetramine, increased the nitrogen concentration from 2.87% to 7.0% and the total anion exchange capacity from 0 to 180 mEq/100 g. The resulting capacity is approximately 2.5 times higher than that of the previously reported material. All of the reactions developed in this work are water-based and follow the principles of green chemistry. Compared with commercial anion exchange resins, our approach does not use organic solvents and relies on benign chemical reagents to introduce amino groups onto the surface of peat lignin. All of the developed reactions have been successfully scaled to 5 kg.

The weak anion exchange properties of the materials were demonstrated in column experiments by loading the material with a 0.1 N H₂SO₄ solution in water and successfully regenerating it with a 0.1 N NaOH solution in water. PFAS removal from wastewater was demonstrated through a series of batch and column experiments using treated landfill leachate, with 100% removal achieved for some PFAS compounds. The peat-lignin material was particularly effective at removing PFAS containing sulfonic acid groups from wastewater, with this performance also demonstrated at pilot scale.

The lignin-derived weak-base anion exchange material is granular, with a particle size of 0.5–0.8 mm and a bulk density of 0.44 g/ml. The material remains structurally stable at pH 11.0 and can be used in standard anion exchange tanks for wastewater treatment applications. The material can be regenerated to its original form using NaOH or Ca(OH)₂ treatment.

A reaction mechanism, a new technique for measuring anion exchange capacity, and performance data obtained using synthetic and natural waters will be presented.