Abstract
Lignin modifying enzymes from fungi and bacteria are potential biocatalysts for sustainable mitigation of different potentially toxic pollutants in wastewater. Notably, the paper and pulp industry generates enormous amounts of wastewater containing high amounts of complex lignin-derived chlorinated phenolics and sulfonated pollutants. The presence of these compounds in wastewater is a critical issue from environmental and toxicological perspectives. Some chloro-phenols are harmful to the environment and human health, as they exert carcinogenic, mutagenic, cytotoxic, and endocrine-disrupting effects. In order to address these most urgent concerns, the use of oxidative lignin modifying enzymes for bioremediation has come into focus. These enzymes catalyze modification of phenolic and non-phenolic lignin-derived substances, and include laccase and a range of peroxidases, specifically lignin peroxidase (LiP), manganese peroxidase (MnP), versatile peroxidase (VP), and dye-decolorizing peroxidase (DyP). In this review, we explore the key pollutant-generating steps in paper and pulp processing, summarize the most recently reported toxicological effects of industrial lignin-derived phenolic compounds, especially chlorinated phenolic pollutants, and outline bioremediation approaches for pollutant mitigation in wastewater from this industry, emphasizing the oxidative catalytic potential of oxidative lignin modifying enzymes in this regard. We highlight other emerging biotechnical approaches, including phytobioremediation, bioaugmentation, Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-based technology, protein engineering, and degradation pathways prediction, that are currently gathering momentum for the mitigation of wastewater pollutants. Finally, we address current research needs and options for maximizing sustainable biobased and biocatalytic degradation of toxic industrial wastewater pollutants.
Highlights
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We explore the key pollutant-generating steps in paper and pulp processing, summarize the most recently reported toxicological effects of industrial lignin-derived phenolic compounds, especially chlorinated phenolic pollutants, and outline bioremediation approaches for pollutant mitigation in wastewater from this industry, emphasizing the oxidative catalytic potential of oxidative lignin modifying enzymes in this regard
Laccases and the lignin modifying peroxidases have been reported to catalyze the oxidation of lignin model compounds and to have potential as catalysts for bioremediation of phenolic, including oligophenolic and chlorophenolic compounds, and even non-phenolic compounds, some of which are considered as environmental contaminants and endocrine-disrupting compounds (EDCs) (Falade et al, 2018; Grelska and Noszczyńska, 2020; Kalyani et al, 2016; Kamimura et al, 2019; Paz et al, 2020; Verma et al, 2020)
Summary
Lignin consists of three specific phenylpropanyl units that are biopolymerized in the plant cell wall to function as a three-dimensional amorphous polymer: guaiacyl alcohol (G unit), p-coumaryl alcohol (H unit), syringyl alcohol (S unit). Due to it being a polyaromatic, ether-linked hydrophobic polymer, lignin is “waterproof” and quite resistant to biological degradation. For this reason, harsh chemicals and high temperatures are used in paper and pulp processing, in turn resulting in the generation of harmful paper mill effluents (Kamimura et al, 2019; Pu et al, 2015). The pulping process relies on chemical additives, followed by bleaching that involves a series of chemical and oxidative treatment steps to obtain white paper (Bajpai, 2018)
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