Investigating the Evolution of Structural Characteristics of Humic Acid Generated during the Continuous Anaerobic Digestion and Its Potential for Chromium Adsorption and Reduction

Abstract

Humic acid (HA), as an important by-product, has been demonstrated to affect anaerobic digestion performance and subsequent land application of digestate via the batch anaerobic digestion process. However, the knowledge about the evolution of structure and function of HA during continuous anaerobic digestion (AD) is still unclear. Therefore, the current study examined the structural changes in HA produced during the continuous AD process and its metal-adsorption-reduction abilities. The results of three-dimensional fluorescence spectroscopy showed a general upsurge in humic-like components’ abundance (70–77%), with an increase in humification index (2.56–3.43). Likewise, the content of HA increased from 4.8 g L−1 to 6.9 g L−1 in the continuous AD process. The evolution of C-H, O-H, C=O, C=C, and C-O functional groups of HA was observed via the 2D COS FTIR analysis. Moreover, the concurrent dynamics of functional groups contributed to the higher adsorption (255.2 mg g−1) of Cr (VI) and reduction (60.3 mg g−1) of Cr (VI) to Cr (III) after 168 days of the continuous AD process. The findings of the current study not only advanced understanding of the evolution of HA during continuous anaerobic digestion and its metal remediation potential but also support further research toward developing an eco-friendly and innovative strategy for the remediation of heavy metals contaminated soils employing anaerobic digestate as an auxiliary agent.