Quasi-homogeneous adsorption behavior of a magnetic iron-based hydrated material for Congo red

Congo red (CR) and its degradation products are hazardous to water environment. A hydrated floccule material was prepared with iron salts and ammonia, and CR could be efficiently removed via adsorption. The experimental adsorption capacity reached 32,622 mg/g floccule, tenfold to hundredfold the state-of-the-art values. Fe^2 + and hydrated structure were vital for its removal ability. FTIR, UV-Vis, Raman, XPS, and Zeta potential results confirmed that the SO₃^– groups in CR were strongly attracted by Fe²⁺ ions in the adsorbent via electrostatic and coordination interactions. The adsorption process followed pseudo-second-order and intraparticle diffusion kinetics in dilute and concentrated CR solutions, respectively. UPS, XPS, EDS, and ICP-OES data proved that the adsorbent surface was covered by a monolayer of CR, which could diffuse into the bulk of adsorbent with resistance. This floccule material exhibited quasi-homogeneous adsorption behavior for CR, consistent with a Langmuir isotherm model. The adsorbent could be reused 15 times with removal rates ≥ 99 % (pristine CR content = 1000 ppm), and CR could be recovered through desorption with alkali solutions. Merely 0.2 % of iron was wasted due to leaching during reuse.