Efficient, stable and selective adsorption of heavy metals by thio-functionalized layered double hydroxide in diverse types of water

The designing of efficient, selective and stable adsorbents for heavy metals removal remained a challenging task, as most adsorbents did not exhibit consistent performance in diverse types of water. Unlike traditional adsorbents based on surface oxygen groups, thio-functionalized layered double hydroxide (LDH) as Mn-MoS₄ was designed by inserting MoS₄²⁻ anions into lamellar layers in LDH, and was applied as a highly stable, efficient and selective adsorbent for heavy metals removal. The record high distribution coefficient (K_d ∼ 10⁷–10⁸ mL/g), fast kinetics and enormous saturated uptake capacities (594, 564 & 357 mg/g) for Hg²⁺, Ag⁺ and Pb²⁺ placed Mn-MoS₄ as the most efficient adsorbent for the removal of such metals so far. Further, Mn-MoS₄ can completely tolerate the impact of huge concentration of background electrolytes (Ca²⁺, Mg²⁺, Na⁺, Cl⁻, NO₃⁻ & SO₄²⁻), and effectively control the concentrations of Hg²⁺, Ag⁺ and Pb²⁺ ions from tap water, lake water or industrial wastewater far below the limitation in drinking water. Similarly, Mn-MoS₄ is a potential filter due to its consistent performance over a broad pH range (from 2.0 to 11.0), changing ionic strength (up to 100 mM) or in the presence of organic matters. These remarkable features originate from the dominant chemical adsorption mechanism through inner-sphere coordination between thio groups as soft Lewis acid and Hg²⁺, Ag⁺ and Pb²⁺ as soft Lewis bases. More interestingly, LDH layers offer protective space for intercalated MoS₄²⁻ anions against oxidation, and facilitate Mn-MoS₄ with the advantage of easy storage and application over other adsorbents.