A Bibliometric Analysis of the Capabilities of Modified Lignin for Heavy Metal Adsorption

Water is essential for human health and socioeconomic development. However, human activities pollute water sources with different heavy metals, and traditional water treatment methods often fail to remove these contaminants. Adsorption, mainly using agroindustrial byproducts like lignin, has attracted attention due to its wide availability, cost-effectiveness, and high potential to eliminate heavy metals. Chemical lignin modifications, such as amination, phenolate, oxidation, and carboxylation, have been explored to improve reactivity, selectivity, and robustness in different pH conditions. This work aims to revise the most proficient papers in this field of research. The PRISMA methodology uses lignin, heavy metals, and adsorption as keywords. The results show that China and Thailand lead the research on heavy metal adsorption with lignin. Institutions like Khon Kaen University and Åbo Akademi University, and authors such as D.C. Tsang stand out for their high visibility and contributions in first-quartile (Q1) journals. The publishers Elsevier, Springer, MDPI, and Royal Society of Chemistry presented an impact on citations without significant differences, while Bioresource Technology and Chemical Engineering Journal led the citations. The revised indicates that optimal conditions for adsorption include pH between 5 and 7, temperatures between 298.15 K and 323.15 K, and average contact times of 3 to 5 hours. Furthermore, the Langmuir model predominates in more than 85% of the studies. Pb²⁺, Cr⁶⁺, Cu²⁺, and Cd²⁺ are the most studied metals with maximum adsorption capacities spanning 1264.8 mg/g for Cr⁶⁺ and 349.7 mg/g for Pb²⁺ with modified lignin.