Capturing Cu²⁺ and recycling spent Cu-adsorbents as catalyst for eliminating Rhodamine B: reactivity and mechanism

The thorny problem of adsorption is the disposing of spent adsorbent. In this manuscript, the exhaust adsorbent of efficient capture Cu(II) over ZSM-5 that supported zero-valent iron (nZVI) was reused as a catalyst for eliminating Rhodamine B (RhB). Batch experiments were used to evaluate the removal performance of Cu²⁺ and RhB. The results demonstrated that the Cu²⁺ adsorption process obeyed pseudo-second-order kinetics, and the adsorption performance was dependent on solution pH. The maximum adsorption capacity at the optimal pH 4.0 was 375.9 mg/g; equilibrium was reached rapidly within 35 min. From XPS, the reduction-oxidation between Fe⁰ and Cu²⁺ was occurred in the adsorption process, and Fe²⁺, Fe³⁺, and Cu⁰ was formed. In the recycling experiments, RhB was removed by the spent Cu adsorbent, with the removal performance being dependent on the initial Cu concentration, in the order of 5 mg/L > 20 mg/L > 0 mg/L > 100 mg/L > 500 mg/L. RhB removal also improved with increasing H₂O₂ concentration. More than 99.9% of the RhB was degraded within 8 min using 1.75 mM H₂O₂, which was a large improvement over the previously used catalyst. The hydroxyl radical was found to be the main free radical responsible for RhB degradation.