Simultaneously Sequestrating and Reducing Bichromate by the Built-in Ethylenediamine Group inside Polystyrene Adsorbent

Simultaneous sequestration and reduction of bichromate [Cr(VI)] poses an attractive and practical fashion for treating Cr(VI)-contaminated wastewater, for it not only immobilizes the pollutant but also reduces its toxicity. To this end, an EDA-functionalized adsorbent (EDA@CMPS) was developed with a simple one-step method by integrating ethylenediamine (EDA, a reversible redox unit) onto the skeleton of chloromethylated polystyrene adsorbent (CMPS), and it can sequestrate Cr(VI) and in situ reduce Cr(VI) to Cr(III), which was then immediately locked inside the adsorbent phase. Batch experiments confirmed that EDA@CMPS had a high removal capacity toward Cr(VI) (419 mg/g, at 25 °C), a large proportion of which was reduced to Cr(III) without leaking into the aqueous phase at pH ≥ 2. Ubiquitous anions (Cl^–, NO₃^–, SO₄^2–, and H₂PO₄^–) and cations (Mg²⁺ and Ca²⁺) have a certain effect on the removal efficiencies, but EDA@CMPS still demonstrated satisfied Cr(VI) removal even with the competing ions at 100-times higher concentration. FT-IR and XPS analyses revealed that the amine group of EDA was responsible for sequestrating and reducing Cr(VI), where the amine group was oxidized into an imine group, which then played a key role in immobilizing the generated Cr(III) via chelation. Attractively, even under neutral conditions, EDA@CMPS also demonstrated decent removal and reduction performance toward Cr(VI). Furthermore, the yielded imine group can be perfectly restored to an amine group by simple alkali-acid treatment. Repeated removal–regeneration cycles verified the reusability of EDA@CMPS in treating Cr(VI) solution. Fixed-bed column experiments and treatment of a real electroplating wastewater further validated the potential of EDA@CMPS in treating Cr(VI) wastewater for practical application.