Phosphorus as a key to unlock the potential of hydrogen-oxidizing bacteria for ammonium recovery from mining wastewater into microbial protein.

Rare earth wastewater with high concentrations of NH4+-N poses serious environmental challenges. Conventional treatment converts NH4+-N into N2, leading to nitrogen resources loss. This study proposes a hydrogen-oxidizing bacteria (HOB)-based strategy to convert NH4+-N in real wastewater into microbial protein (MP). Based on the finding that HOB growth was inhibited in real wastewater, the effects of excess (S, Ca) and deficient (Mg, K, P) elements were investigated. P deficiency, rather than excess S/Ca or Mg/K deficiency, was identified as the primary factor limiting HOB performance. P addition increased cell dry weight and MP yield by 66 % and 71 %, respectively, and enhanced CO2 fixation and NH4+-N conversion rates. Microbial community and functional enzyme analyses revealed that P addition selectively enriched Paracoccus, which may be associated with the predicted increase in electron transport-related enzymes (EC: 1.9.3.1, 3.6.3.34), promoting H2 oxidation and nitrogen assimilation and driving microbial growth. Mg, K, and P synergistically further improved the NH4+-N recovery and carbon utilization rate. Moreover, HOB biomass showed high affinity for rare earth elements (REEs), with recovery ratio more than 88 % for Y, Dy and Er. This work establishes a process for NH4+-N recovery, MP synthesis, and REE recovery. We highlight the role of nutrient balancing, particularly P availability, in optimizing this bio-recovery platform for high-value utilization of mining wastewater.