Abstract: The ubiquity of ammonium nitrogen (\mathrmNH_4^+ -N) in water bodies intensifies the risk of eutrophication, making the development of efficient and low-consumption removal technologies a critical challenge in current environmental remediation. Constructing remediation systems by loading nano zero-valent iron (nZVI) onto widely available and structurally unique biomass carriers presents a promising solution. A nano zero-valent iron/loofah fiber composite (nZVI-LF) was prepared in situ through a liquid-phase reduction method using natural loofah fiber (LF) as a biomass carrier. Response surface methodology (RSM) was employed to optimize and determine the two optimal process parameters—nZVI concentration and dispersant polyethylene glycol (PEG) dosage during the preparation. Furthermore, the removal characteristics and underlying mechanisms of \mathrmNH_4^+ -N by nZVI-LF were systematically explored through characterization analysis alongside kinetic and thermodynamic experiments. The results indicated that: (1) The RSM optimization identified that the optimal preparation parameters for nZVI-LF were nZVI concentration 0.544 g/L and PEG dosage 1.055 g. Under these conditions, the average particle size of nZVI was approximately 34.1 nm, with crystalline nZVI uniformly and firmly anchored on the LF surface. The in-situ generated Fe—O/Fe—OH active shell synergistically enhanced the supply of reaction sites with the carrier pores. (2) The adsorption of \mathrmNH_4^+ -N onto nZVI-LF conformed to pseudo-second-order kinetic and Langmuir models, representing a predominantly monolayer chemisorption process controlled by both film diffusion and intra-particle diffusion. This process was an entropy-driven, spontaneous, and endothermic reaction, with a theoretical maximum adsorption capacity of 10.49 mg/g. (3) Mechanistic insights revealed that the removal of \mathrmNH_4^+ -N by nZVI-LF followed a composite pathway of "electrostatic enrichment—surface complexation/ion exchange fixation—hydrogen bonding coordination", in which the ion exchange at the hydroxylated sites on the nZVI surface provided the dominant contribution. (4) nZVI-LF exhibited excellent resistance to co-existing ionic interference (inhibitory strength: \mathrmPO_4^3- > \mathrmSO_4^2- > Cl⁻) and good cyclic stability, maintaining a capacity retention rate of over 76% after five cycles. The nZVI-LF composite prepared in this study demonstrates significant potential for engineering applications and provides a theoretical foundation for the design of advanced ammonium adsorbents.