Isolation of Humic Substances Using Waste Wood Ash Extracts: Multiparametric Optimization via Box–Behnken Design and Chemical Characterization of Products

TL;DR

This study explores using wood ash to extract humic substances from peat and lignite, finding that oak ash is a sustainable and effective alternative to traditional methods.

Contribution

The study introduces wood ash as a novel, eco-friendly extractant for isolating humic substances, optimized through a Box–Behnken design.

Findings

Oak ash extract achieved higher humic substance yields from peat compared to birch ash extract.

Optimal extraction parameters varied between 309–391 mW/cm², 116–142 min, and 67–79 °C.

Birch ash extract consistently produced more carboxylic structures in all humic substance fractions.

Abstract

This study evaluated birch and oak ash extracts as alternative extractants for isolating humic substances (HSs) from peat and lignite. The effects of ultrasound intensity, extraction time, and temperature were optimized using a Box–Behnken design and validated statistically. The highest HSs yields were obtained from peat with oak ash extract (pH 13.18), compared to birch ash extract (pH 12.09). Optimal process parameters varied by variant, falling within 309–391 mW∙cm−2, 116–142 min, and 67–79 °C. HSs extracted under optimal conditions were fractionated into humic acids (HAs) and fulvic acids (FAs), and then analyzed by elemental analysis, Fourier Transform Infrared Spectroscopy (FTIR), and Cross-Polarization Magic Angle Spinning Carbon-13 Nuclear Magnetic Resonance Spectroscopy (CP/MAS 13C NMR). The main differences in HSs quality were influenced by raw material and fraction type. However, the use of birch ash extract consistently resulted in a higher proportion of carboxylic structures across all fractions. Overall, wood ash extract, especially from oak, offers a sustainable and effective alternative to conventional extractants, particularly for HSs isolation from lignite. Notably, HSs yield from lignite with oak ash extract (29.13%) was only slightly lower than that achieved with 0.5 M NaOH (31.02%), highlighting its practical potential in environmentally friendly extraction technologies.

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