Dual effects of humic substances on microbially induced vivianite crystallization: A trade-off between iron reduction and crystallization process

Recovering phosphorus (P) from wastewater is crucial for sustainable resource management, and microbially induced vivianite crystallization represents a promising approach for P recovery. However, different forms of humic substances (HS) exert complex and distinct influences on this process through their interactions with both Fe reduction and crystallization. This study systematically investigated the dual roles of three representative HS types (fulvic acid (FA), humic acid (HA), and humin (HM)) in regulating microbially induced vivianite formation. All three HS types promoted Fe(III) reduction, with the promoting effect decreasing in the order FA > HA > HM. In contrast, FA and HA inhibited vivianite crystallization, while HM exhibited a slight promoting effect. Owing to differences in their biodegradability, the duration of these promoting or inhibitory effects also varied substantially. Consequently, HA achieved the most favorable balance between sustained Fe reduction and minimal crystallization inhibition. Quantitative XRD analysis confirmed that the HA group produced the highest vivianite purity (86.1%), significantly exceeding the FA (77.0%), control (78.2%), and HM (74.5%) groups. Electrochemical, spectroscopic, and microbial community analyses further elucidated the trade-off mechanism between Fe reduction and crystal growth. Overall, HA was identified as the most promising HS fraction for optimizing vivianite-based P recovery. These findings also offer implications for understanding P fixation in natural sediments and mitigating eutrophication.