Zero valent iron (ZVI) was produced by using wasted acid and iron oxide that are by-products of a pickling line at a steel work. The reaction activity of the produced ZVI was evaluated through decomposition experiments of Orange II aqueous solution. The ZVI particles produced directly from wasted acid (A-ZVI) were not easy to handle because they were very small (10-200 nm) and were easily oxidized in the air. The size of ZVI particles produced from the iron oxide recovered from the regeneration process of wasted acid (O-ZVI) increased with reduction temperature due to coalescence. Correspondingly, the specific surface area of O-ZVI decreased with increasing reduction temperature. The decomposition efficiency of synthesized ZVI particles was higher at a lower pH. In particular, no significant decomposition reaction was observed at pH of 4 or higher with O-ZVI. The decomposition efficiency of A-ZVI was higher than that of O-ZVI or commercially purchased ZVI, but it is not easy to handle. On the other hand, O-ZVI is easier to handle, but has the drawback of low reaction activity at high pH. Further elaboration is required for practical applications of these synthesized ZVIs.