Nano-Fe₃O₄/FeCO₃ modified red soil-based biofilter for simultaneous removal of nitrate, phosphate and heavy metals: Optimization, microbial community and possible mechanism

The pollution of nitrogen, phosphorus and heavy metals in surface water is becoming more and more serious, affecting the safety of water quality. In this study, three biofilters were constructed using iron-modified red soil-based filler carriers (RSC, nano-Fe₃O₄@RSC, and FeCO₃@RSC) combined with strain Zoogloea sp. ZP7 to simultaneously remove nitrate (NO₃^--N), phosphate (PO₄^3--P), copper (Cu²⁺), and zinc (Zn²⁺). The long-term operation results showed that the three groups of biofilters could remove 85.0 %, 90.0 %, and 89.8 % of NO₃^--N, respectively. Furthermore, the addition of iron compounds enhanced the removal of PO₄^3--P and the resistance to the stress of Cu²⁺ and Zn²⁺ in the biofilter. The analysis illustrated that iron modification improved the redox activity and zeta potential of RSC surface. The secondary structure analysis of the protein showed that the microbial secreted proteins were more compact on the surface of the iron-modified RSC, which facilitated the formation of biofilm on the carrier surface. In addition, the iron-modified RSC-based biofilter also showed excellent NO₃^--N and PO₄^3--P removal efficiency in the treatment of actual surface water. The microbial community analysis results showed that Zoogloea became the dominant species in the biofilter. On the other hand, the presence of iron-reducing bacteria and the expression iron cycle-related genes may contribute to denitrification under low nutrient conditions.