Potential of Compost and Spent Coffee Grounds as Sources of Humic-Like Substances: Extraction Modeling and Optimization by Fractional Factorial Design

Humic substances (HSs) play a crucial role in soil health and have potential applications beyond agriculture. This study evaluates the renewable raw materials such as compost from urban green waste (GWC) and spent coffee grounds (SCG) as sources of humic-like substances (HLSs), which may be an alternative to traditional resources such as peat, lignite, and leonardite. The quantitative analysis focused on modeling and optimizing the efficiency of ultrasound-assisted extraction of humic-like acids (HLAs) as a function of time, ultrasound intensity, extractant concentration (NaOH), and temperature. Experimental points were determined based on the experimental matrix according to fractional factorial design (FFD), and the obtained polynomial models were evaluated using the Fisher test, lack of fit assessment, and determination coefficients. Experimental verification with optimal values of the evaluated parameters demonstrated the possibility of extracting HLAs from GWC and SCG at levels of 27.08 and 19.11%, respectively. The qualitative analysis included humic-like and fulvic-like acids and their comparison with reference samples. Spectroscopic methods (UV–vis, ATR-FTIR, CP/MAS, and ¹³C NMR) as well as elemental analysis (CHNSO) were used in this part of the study. The results obtained showed the presence of functional groups characteristic of humic substances (polysaccharides, phenols, alcohols, and amides) in the molecular structure of the products. Additionally, they exhibited a higher content of nitrogen and sulfur compared to commercial samples, as well as lower hydrophobicity, which may indicate their biological activity.