Nature-Inspired Coral-like Layered [Co0.79Al0.21(OH)2(CO3)0.11]· m H2O for Fast Selective ppb Level Capture of Cr(VI) from Contaminated Water

Abstract

Rapid industrialization has led to the release of hexavalent chromium (Cr(VI)), a "Class A" human carcinogen, mutagen, and teratogen in biological systems. Current adsorbents like anionic exchange resins and metal-organic frameworks can remove harmful heavy metal oxyanions from water but are not stable in a broad pH range, suffer from selectivity, and cannot capture them from trace values below the tolerance limits given by the U.S. EPA (100 ppb) and WHO (50 ppb). Herein, we have synthesized nature-inspired coral-like three-dimensional hierarchical structures of [Co_0.79Al_0.21(OH)₂(CO₃)_0.11]·mH₂O (CoAl-LDH) that sets a new benchmark for sequestering oxyanions of Cr(VI). CoAl-LDH shows a broad pH working range (1.93-12.22), high selectivity toward saturated water samples containing monovalent (Cl^-, F^-, Br^-, and NO₃^-) and divalent (SO₄^2-) anions with fast kinetics (reaches equilibrium within a minute), high capacity (93.4 ± 7.8 mg g^-1), and high distribution coefficient of 1.09 × 10⁶ mL g^-1. Unlike other materials, it can decrease Cr(VI) concentration up to 0.012 ppb. This high selectivity for Cr(VI) is linked to the weak bonding interaction between Cr₂O₇^2- and brucite-like layers, as revealed from thermogravimetric and infrared spectroscopy. With these remarkable features coupled with low cost and an environmentally friendly nature, we have also designed an anion exchange column that can remove >99% Cr(VI) with just 1 wt % of CoAl-LDH and 99 wt % of sand and is a prominent candidate for the elimination of Cr(VI) from industrial effluents.