Nature-Inspired Coral-like Layered [Co_0.79Al_0.21(OH)₂(CO₃)_0.11]·mH₂O 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 corallike three-dimensional hierarchical structures of [Co 0.79 Al 0.21 (OH) 2 (CO 3 ) 0.11 ]•mH 2 O (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 NO 3 − ) and divalent (SO 4 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 × 10 6 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 Cr 2 O 7 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.

Removal of CrO42–, a Nonradioactive Surrogate of 99TcO4–, Using LDH–Mo3S13 Nanosheets

“…At neutral pH, from 1000 ppb of a Cr 6+ (CrO 4 2– ) spiked deionized water (DIW) solution, LDH–Mo 3 S 13 can capture over 99.99% of Cr 6+ leaving the final concentration below 1 ppb with a K d value of ∼1.0 × 10 7 mL/g in 24 h (Tables S2 and S3). This concentration is well below the tolerance limits provided by the U.S. EPA (100 ppb) and WHO (50 ppb) for drinking water. , Also, it should be noted that the value of K d is higher to or comparable to other high performing materials known in the literature. , At pH ∼ 11, LDH–Mo 3 S 13 can sequester over 98.7% of Cr 6+ decreasing the final concentration to 13 ppb with the K d ∼ 7.7 × 10 4 mL/g in 48 h. Conversely, at pH ∼ 2, LDH–Mo 3 S 13 can sequester over 74% of Cr 6+ ions. The lower effectiveness at this pH may be attributed to the slow decomposition of LDH–Mo 3 S 13 .…”

“…Our experiment determines that the maximum adsorption of Cr 6+ is about 225 mg/g. This value of sorption capacity is superior to other high performing materials, namely LDH–MoS 4 ∼ 130 mg/g and CoAl–LDH ∼ 93.5 mg/g, , cationic aluminum oxyhydroxides ∼ 105.4 mg/g, and anion exchange resin (IRN78 ∼ 63.5 mg/g), and comparable to metal organic frameworks (MOR-1-HA ∼ 242) and non-LDH cationic layered material (TJU-1 ∼ 279 mg/g) . The experimentally obtained Cr 6+ sorption isotherm data are fitted with the Langmuir model which yielded R 2 ∼ 0.98 (Figure D).…”

“…With a reduction potential of −0.22 V (for the S 2– /SO 4 2– couple), sulfides are capable of reducing the highly soluble TcO 4 – (TcO 4 – /Tc 4+ is E ° ∼ +0.74 V) and CrO 4 2– (CrO 4 2– /Cr 3+ is E ° ∼ +1.23 V) ions to insoluble Tc­(IV) and Cr­(III) species. ,− Because of the known chemistry of sulfides and other reductants for the reductive precipitation of highly redox-active Tc 7+ and Cr 6+ , we introduced (Cr 6+ O 4 ) 2– as a nonradioactive surrogate for a reductive precipitation driven separation. ,,, In addition to being a suitable surrogate for TcO 4 – , CrO 4 2– itself is highly toxic and also persists within the radioactive waste of the Hanford nuclear waste streams. − Because of the leakage in the Hanford underground tanks, chromate has been dispersing in the environment along with various radionuclides. − Moreover, during the vitrification process of the LAW, CrO 4 2– is transformed into crystalline spinel which in turn jeopardizes the integrity of the glass waste form. , Therefore, in addition to pertechnetate, it is crucial to remediate chromate from water and legacy nuclear wastes.…”

“…Numerous metal sulfides, such as pyrrhotite (Fe 1– x S) and pyrite (FeS 2 ), , chalcocite (Cu 2 S), , and stibnite (Sb 2 S 3 ), and layered metal sulfides are known to immobilize TcO 4 – by reductive precipitation. , Layered double hydroxides (LDHs) are anionic clays which can also remove TcO 4 – /CrO 4 2– but with ion-exchange and surface sorption mechanisms. ,,, Considering the efficiency of LDH and metal sulfides for TcO 4 – /CrO 4 2– remediation, we hypothesized that a hybrid structure of metal sulfide intercalated LDH will boost the removal of pertechnetate from water and legacy nuclear waste. Altogether, it becomes important to investigate LDH-metal sulfide based materials, evaluate the remediation of TcO 4 – , and delineate the sorption mechanisms involved in this process.…”

“…6,15−21 Because of the known chemistry of sulfides and other reductants for the reductive precipitation of highly redoxactive Tc 7+ and Cr 6+ , we introduced (Cr 6+ O 4 ) 2− as a nonradioactive surrogate for a reductive precipitation driven separation. 1,9,22,23 In addition to being a suitable surrogate for TcO 4 − , CrO 4 2− itself is highly toxic 24 and also persists within the radioactive waste of the Hanford nuclear waste streams. 25−27 Because of the leakage in the Hanford underground tanks, chromate has been dispersing in the environment along with various radionuclides.…”