The removal capacity, iron and phosphorus forms, and microbial distribution of low concentration phosphorus by...

An iron anode electrolytic biochar biofilter with distributed effluents was constructed to explore the roles of iron anode electrolysis and biochar substrate in phosphorus (P) removal. The results showed that iron anode electrolysis is the primary factor driving P removal. Subsequently, the released iron ions were effectively adsorbed by biochar, which is another pathway for enhanced P removal. At a voltage as low as 5 V, the average removal rate of PO₄^3--P was 98.37 ± 1.01 %. Under power-off conditions, the average removal rate of PO₄^3--P by iron-loaded biochar was as high as 95.92 ± 2.61 %. Biochar substrate can effectively adsorb Fe(II) or Fe(III) produced by the iron anode, with an iron loading of 4.3 ± 0.62 mg/g biochar. X-ray powder diffractometer (XRD) analysis found that the principal crystalline iron oxides loaded on biochar were FeOOH and Fe₃O₄. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) showed that iron-loaded biochar exhibits strong electron-exchangeability and effectively reduces charge-transfer resistance. 16S rRNA analysis showed that biochar can serve as a carrier for microorganisms, and iron electrolysis enriched the microbial community structure of the biochar substrate, as the Fe(II)/Fe(III) electron pairs generated by iron anode electrolysis enriched the electron donors or acceptors of microbes. Due to its redox functional groups and graphite-like structure, biochar can shuttle electrons. It facilitates the extracellular electron transfer of iron-reducing bacteria and indirectly promotes the Fe(II)/Fe(III) cycle, thereby influencing the iron-binding and adsorption capacity of PO₄^3--P. The phosphorus adsorbed by iron-loaded biochar was mainly composed of NaOH-P and BD-P, indicating that biochar-loaded iron enhances phosphate fixation and improves P resource utilization during adsorption.