Arsenic contamination has raised great concern around the world as the cause of many severe health issues, including internal and external cancers. Discharging arsenic waste into the environment before proper handling is strictly forbidden. In order to treat the arsenic-bearing wastes in a safe way, we used the oxidation alkali leaching to dissolve arsenic in the low-risk state arsenate (AsO43-) firstly, followed by recycling of the alkali solution after arsenate removal. In this paper, we synthesized high-concentrated ferric oxyhydroxide gels (HFGs) with different supersaturation in a hydrothermal system to adsorb arsenate anions at high alkali without consuming the hydroxyls. The arsenate adsorption onto HFGs followed the pseudo second-order and intra-particle diffusion kinetics and fitted Langmuir isotherms model well. Through characterization of the synthesized HFGs particles by XRD, Raman spectra, SEM and TGA, it was confirmed that the surface hydroxyl sites on the ferric oxyhydroxide reacted with arsenates to form the bidentate surface complexation. In addition, the synthesized HFG at lower supersaturation had lower crystallinity and more surface hydroxyl sites, resulting in higher adsorption capacity for arsenate. Using HFG as the adsorbent, the arsenic (V) concentration in the copper slag alkali leaching solution can be reduced from 2084 mg/L to 71.8 mg/L, which can realize stabilization of the high-concentrated arsenate at high alkali and alkali solution recycling.