# Organic Acids for Lignin and Hemicellulose Extraction from Black Liquor: A Comparative Study in Structure Analysis and Heavy Metal Adsorption Potential

## TL;DR

This paper compares organic acids to sulfuric acid for extracting lignin and hemicellulose from black liquor, finding that citric acid produces higher purity lignin with better heavy metal adsorption potential.

## Contribution

The study introduces citric acid as an effective and environmentally friendly alternative to sulfuric acid for lignin extraction with improved adsorption properties.

## Key findings

- Lignin precipitated with citric acid showed higher purity and better adsorption of heavy metals like Cu2+ and Cr3+.
- Organic acids, especially citric acid, produced lignin yields comparable to sulfuric acid while reducing sulfur content.
- The method aligns with circular economy principles by reusing paper industry by-products.

## Abstract

This study presents a method for extracting lignin and hemicellulose from black liquor using organic acids (citric, malic, and acetic) in comparison to the traditional sulfuric acid method. We investigated and compared the influence of the acid type on the structural properties of the resulting precipitates in the context of their potential applications. The lignin fractions were characterized for their chemical structure (ATR-FTIR, NMR), thermal stability (TGA), morphology and surface elemental composition (SEM-EDS), bulk elemental composition (C, H, N, S), and molecular weight distribution (GPC). The hemicellulose fractions were analyzed for their molecular weight (GPC), surface elemental composition (EDS), and chemical structure (ATR-FTIR). These analyses revealed subtle differences in the properties of the individual materials depending on the extraction method. We showed that organic acids, particularly citric acid, can effectively precipitate lignin with yields comparable to the sulfuric acid method (47–60 g/dm3 vs. 50 g/dm3). Simultaneously, this method produces lignin with higher purity (regarding sulfur content) and an increased content of carboxyl groups. This latter aspect is of particular interest due to the enhanced potential of lignin’s adsorption functions towards metal ions. AAS analysis confirmed that lignin precipitated with citric acid showed better adsorption efficiency towards heavy metals compared to lignin precipitated with sulfuric acid, especially for Cu2+ ions (80% vs. 20%) and Cr3+ ions (46% vs. 2%). This enhanced adsorption efficiency of the isolated lignins, combined with the environmental benefits of using organic acids, opens a promising perspective for their application in water treatment and environmental remediation. Furthermore, the presented research on the valorization and reuse of paper industry by-products fully aligns with the fundamental principles of the Circular Economy.