Selective Sorbent Design: CaS Aerogel for Rapid Remediation of Aqueous Pb (II)

Heavy metals are a persistent environmental problem due to their high toxicity, even at very low concentrations (parts per billion, ppb). The removal of such diluted heavy metals is challenging because of the competition the counterions (Ca²⁺, Na⁺, Mg^2+, etc.) present in natural water bodies. The design of sorbents capable of removing ions below the action limit (15 ppb for Pb²⁺) requires a strong driving force for selective uptake and rapid removal. In this work, we report the synthesis of porous CaS aerogels (surface area = 143.6 m²/g) by oxidative assembly of CaS nanoparticles and describe their use in selective Pb²⁺ ion remediation from water. Despite the presence of amorphous CaCO₃ (up to 50 wt %) in the gel network, the gels demonstrated a capacity of 17.1 mmol Pb/g aerogel (3543 mg/g), and this could be augmented to 22.5 mmol Pb/g aerogel (4593 mg/g) by modifying the synthesis to reduce CaCO₃ content to ca. 15 wt %. Moreover, the selectivity of CaS aerogels toward Pb²⁺ ions is high, as evidenced by little-to-no change in the distribution constant (K _d ∼ 10⁴) in the presence of competing ions (1 M) such as Na⁺, Mg²⁺, and Ca²⁺. During remediation with low concentrations (100 ppb) of Pb²⁺ with CaS aerogels, the level of Pb²⁺ dropped to 5.4 ppb (below the 15 ppb EPA limit) within 1 h with a 95.4% removal efficiency. In contrast to the CO₂ supercritically dried aerogels, lower surface area ambient dried gels (xerogels) only remove 40% of the lead ions from a 100 ppb solution, saturating within 1 h. The efficiency and rapidity of selective Pb²⁺ uptake using CdS aerogels arise from a combination of a strong thermodynamic driving force for cation exchange (K _eq = 2.5 × 10²⁷) and chemisorption along with favorable kinetics associated with the high surface area porous architecture. These results show that formation of high surface area metal chalcogenide aerogels by oxidative assembly to form nanocrystalline architectures, as previously demonstrated for II-VI and IV-VI semiconductors, can be extended to the more highly ionic alkaline earth sulfides.