Understanding the origin of feii emission is important because it is crucial to construct the main sequence of Active Galactic Nuclei (AGNs). Despite several decades of observational and theoretical effort, the location of the optical iron emitting region and the mechanism responsible for the positive correlation between the feii strength and the black hole accretion rate remain open questions as yet. In this letter, we report the optical feii response to the central outburst in PS1-10adi, a candidate tidal disruption event (TDE) taking place in an AGN at $z = 0.203$ that has aroused extensive attention. For the first time, we observe that the feii response in the rising phase of its central luminosity is significantly more prominent than that in the decline phase, showing a hysteresis effect. We interpret this hysteresis effect as a consequence of the gradual sublimation of the dust grains situating at the inner surface of the torus into gas when the luminosity of the central engine increases. It is the iron element released from the sublimated dust that contributes evidently to the observed feii emission. This interpretation, together with the weak response of the hb emission as we observe, naturally explains the applicability of relative feii strength as a tracer of the Eddington ratio. In addition, optical iron emission of this origin renders the feii time lag a potential standard candle with cosmological implications.
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