Biodegradation and Structural Modification of Humic Acids in a Compost Induced by Fertilization with Steelmaking Slag under Coastal Seawater, as Detected by TMAH-py-GC/MS, EEM and HPSEC Analyses

Abstract

The compost's humic acid (HA) content decreased when it was fertilized in coastal seawater with steelmaking slag was confirmed. This study clarified the cause for this change by detailed analysis of the structural changes in HAs based on TMAH-py-GC/MS, HPSEC, and 3D-EEM spectra. An increase in the levels of pyrolysates of tannic acid with a low polymerization degree was attributed to the biodegradation of high polymerized aromatic structure. Moreover, analyses of 3D-EEM supported by HPSEC indicated that approximately 20 kDa of the fluorescent matter was generated at the protein-like peaks (Ex/Em = 220/340 and 275/350 nm) in HAs derived from a mixture of compost with steelmaking slag. It would be caused by the formation of HAs from the bacterial byproduct by a catalytic reaction of the steelmaking slag. From these findings reported herein, we conclude that bio-degradation was a major reason for the decreased HA content, and the formation of HA from a part of the degradation products would be a reason for the structural modification of HA under seawater condition.