Iron-mediated protein–humic acid interactions under aerobic and anaerobic conditions: Implications for protein hydrolysis and wastewater treatment

Proteins and carbohydrates are both major biodegradable fractions in wastewater. Complexation with coexisting compounds, such as iron (Fe) and humic acids (HA), which are both commonly present in wastewater, could influence the different degradation rates of proteins and carbohydrates. Depending on the redox conditions, Fe exists as Fe(II) or Fe(III), with differing binding affinities and chemical behaviour. This research aims to systematically assess the complex interaction between Fe, protein, and HA compounds under aerobic and anaerobic conditions. The results showed that the addition of Fe(III) and HA to a protein solution inhibited its hydrolysis rate by more than 90 % under aerobic conditions. In contrast, interactions between the same compounds and carbohydrates were much weaker and had a minimal effect on hydrolysis rates. Complexation with Fe, proteins, and HA was indicated by increased molecular sizes and reduced concentrations of free iron, protein, and HA. FTIR results showed that Fe(III) formed complexes with proteins and HA through electrostatic and coordination bonds involving various functional groups. Anaerobic reduction of Fe(III) to Fe(II) by hydrazine resulted in weaker binding and the formation of smaller, less stable protein-humic acid complexes. These findings suggested that modulating Fe complexation under alternating aerobic and anaerobic conditions, such as those found in redox-cycling wastewater treatment, can be a promising strategy to enhance protein degradation.