Biotic and abiotic catalysts for enhanced humification in composting: A comprehensive review

Humification is an environmentally important process that converts organic matter and/or pollutants into structurally refractory substances through various catalytic mechanisms. Humification is a major carbon sequestration strategy that can be enhanced using many biotic and abiotic catalysts. With the application of catalysts during composting, the content of humic substances can increase by more than 100% compared to control samples. Despite growing interest, there is a lack of comprehensive reviews of the different humification pathways driven by adding various types of catalysts. In this review, we consulted recent literature to compare catalysts with respect to their humification potentials, pathways, products, and operating conditions. We critically evaluated the advantages and limitations of individual catalysts to elucidate gaps in current knowledge and future research directions. In general, biotic catalysts show higher performance compared to metal oxides, whereas abiotic catalysts can be produced at a lower cost. Among the biotic catalysts, laccase is particularly attractive due to its eco-friendliness and high performance. Among abiotic catalysts, MnO2 had the highest humification activity compared to other metal oxides, such as Fe2O3 or Al2O3, whereas clays with a 2:1-layered structure demonstrated higher potential than did those with a 1:1 structure. More research is recommended using advanced techniques to provide a more comprehensive comparison of catalyst performance. Finally, we have discussed the potential of novel catalysts to enhance humification.