Multivariate Metal–Organic Framework/Single-Walled Carbon Nanotube Buckypaper for Selective Lead Decontamination

Abstract

The search for efficient technologies empowering the selective capture of environmentally harmful heavy metals from wastewater treatment plants, at affordable prices, attracts wide interest but constitutes an important technological challenge. We report here an eco-friendly single-walled carbon nanotube buckypaper (SWCNT-BP) enriched with a multivariate amino acid-based metal–organic framework (MTV-MOF) for the efficient and selective removal of Pb 2+ in multicomponent water systems. Pristine MTV-MOF was easily immobilized within the porous network of entangled SWCNTs, thus obtaining a stable self-standing adsorbing membrane filter ( MTV-MOF/SWCNT-BP ). SWCNT-BP alone shows a moderately good removal performance with a maximum adsorption capacity of 180 mg·g –1 and a considerable selectivity for Pb(II) ions in highly concentrated multi-ion solutions over a wide range of lead concentration (from 200 to 10000 ppb). Remarkably, these features were outperformed with the hybrid membrane filter MTV-MOF/SWCNT-BP , exhibiting enhanced selectivity and adsorption capacity (310 mg·g –1 , which is up to 42% higher than that of the neat SWCNT-BP) and consequently enabling a more efficient and selective removal of Pb 2+ from aqueous media. MTV-MOF/SWCNT-BP was able to reduce [Pb 2+ ] from the dangerous 1000 ppb level to acceptable limits for drinking water, below 10 ppb, as established by the current EPA and WHO limits. Thus, the eco-friendly composite MTV-MOF/SWCNT-BP shows the potential to be effectively used several times as a reliable adsorbent for Pb 2+ removal for household drinking water or in industrial treatment plants for water and wastewater lead decontamination.

GO-SWCNT Buckypapers as an Enhanced Technology for Water Decontamination from Lead

“…Such results gave a lead adsorption capacity per unit of adsorbent mass of 337 ± 13, 377 ± 9, 424 ± 13, and 479 ± 25 mg g −1 , and confirmed again the best adsorption capacity shown by 75% GO-SWCNT BPs. The obtained adsorption capacity per unit of adsorbent mass is larger than the values recently found for the lead removal by MOF powders (108 mg g −1 ) [54] and BP membranes hosting MOF (310 mg g −1 ) [36] but lower than the lead adsorption capacity by GO flakes (555 mg g −1 ) [54]. It is worth noting that, despite the slight reduction in the adsorption capacity (−15%) due to the presence of SWCNTs, 75% of GO-SWCNT BPs are self-standing films, which can be more easily used, recovered, and regenerated than any powder-like adsorbent (see infra).…”

“…The beneficial substitution of SWCNT with GO in BPs was further confirmed through kinetic experiments. From the literature [36,38], it is known that the rate equation for Pb 2+ capture in solutions follows the Lagergren first-order Equation (1):…”

“…Several adsorbents have been proposed for the removal of heavy metals from waste-water including activated carbon [19], clays and zeolites [20,21], nanoparticles [22,23], organic resins and polymers [24], metal oxides [25], agricultural waste products [26,27], zero-valent ions [28]. More recently, other materials have been investigated as efficient adsorbents for heavy metals ions, including single and multiwalled carbon nanotubes (SWCNTs and MWCNTs) [29][30][31], graphene oxides (GOs) [32][33][34], and organic metal frameworks (MOF) [35,36] as either single components or in mixtures. Despite their enhanced adsorption properties, due to their large surface area, porosity, and the possibility of tuning their interactions with the target pollutants by easy functionalization [37], these materials lack the possibility of easy recovery and regeneration after use due to their powdery consistency.…”

“…In addition, BPs are characterized by a porous structure, low density, and interesting thermal, rheological, and electrical properties [43]. In order to enhance their properties, BPs are used to host other materials such as MOF [36,44] and GO [42,45], leading to the formation of composite materials with superior performance in different fields such as flexible sensors [45], batteries [46], smart packaging [47], artificial muscles [48], fire retardant [49], and membrane-based processes, namely desalinization and catalysis [40,50]. In the literature, few works deal with the application of composite BPs as adsorbent membranes.…”

“…In the literature, few works deal with the application of composite BPs as adsorbent membranes. Recently, SWCNT BPs incorporating MOFs have been proposed as boosted adsorbents for the recovery of rare-earth elements [44] and the selective capture of lead [36] from wastewater. The presence of MOF in SWCNT BPs was found to be beneficial for the adsorption of all tested lanthanides, with the Ce 3+ recovery reaching the value of 263.30 mg of cerium adsorbed per gram of MOF-SWCNT BP.…”