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Cr(vi) represents a worldwide issue due to its carcinogenicity and toxicity, and its removal from water/waste-water is of great relevance for the protection of human health and the environment. We here present an investigation of the adsorption and reduction properties towards Cr(vi) of two different kinds of industrial lignins: a kraft softwood lignin (HMW) and a hardwood lignin (EH) obtained by an enzymatic process. Moreover, we prepared and characterized an acetylated and a phosphorylated lignin starting from HMW, along with a lignin@magnetite hybrid material. The influence of various experimental parameters on the adsorption process, such as pH, contact time, quantity of lignin, Cr(vi) concentration and ionic strength, was evaluated. The best performances can be obtained at acidic pH (pH = 2). With an initial Cr(vi) concentration of 20 mg L−1 and a contact time of 24 hours, a quantitative Cr(vi) reduction was observed, accompanied by a removal of total chromium of up to 35% when HMW was used as the biosorbent. A comparison of the adsorption profiles of the different biosorbents highlighted the higher performance of EH, endowed with a higher surface area and characterised by a maximum adsorption capacity of about 208 mg g−1, while acetylation of the hydroxyl group led to a drop in the adsorption profile. The best fitting of the adsorption isotherm by using the Langmuir model suggests that a monolayer coverage of metal ions onto the homogeneous active sites of the lignin's surface better describes the interactions occurring at the biosorbent interface. Overall, this study demonstrates that technical lignins, and particularly EH hardwood lignin, are effective and economic materials that could be successfully employed in the Cr(vi) water remediation process.

Solvothermal reactions between three bis-pyridine-bis-amide ligands containing a fluorene scaffold bearing different substituents on the C9 position (CH2, 1; CMe2, 2; C = O, 3) with 2,6-naphthalenedicarboxylic acid (H2ndca) and Zn(NO3)2·6H2O led to the isolation of three new microporous mixed-ligand MOFs (PUM310, containing 1; PUM310Me2, containing 2; PUM310CO, containing 3). The structural characterization conducted with X-ray quality single crystals revealed parallel polycatenated frameworks of thick layers in all cases, corresponding to the topological type 5,6L18. The entangled nets contain complete and truncated Zn-paddle wheels. In the truncated paddle wheel, a pyridine of the bis-amide linker has been replaced by a DMF molecule. Solvent removal leads to framework rearrangement, as evidenced by XRPD analysis performed on desolvated PUM310, with consequent framework shrinkage as also evidenced by volumetric adsorption analyses. The three MOFs are fluorescent in the solid state. The optical spectra are indicative of a Förster resonance energy transfer (FRET) involving ndca2− as an excitation energy donor and the bis-pyridine linkers as acceptors, with a FRET efficiency close to 100% in the case of PUM310CO.

Two novel pillared Zn(II)-based Metal-Organic Frameworks were de novo synthesized exploiting N,N′-(1,1′-biphenyl)-4,4′-diylbis-4-pyridinecarboxamide (bpba) and its fluorinated analogous N,N′-(perfluoro-1,1′-biphenyl-4,4′-diyl)diisonicotinamide (F-bpba) as suitable pillar linkers and 2,6-naphthalene dicarboxylic acid as carboxylic ligand. The resulting heteroleptic MOFs, namely, PUM210, [Zn4(bpba)1.5·(ndc)4·(H2O)]n and PUM210F, [Zn3(F-bpba)1·(ndc)3·(DMF)]n, feature an uncommon truncation of the Zn(II) paddle-wheel nodes along the pillaring direction. PUM210 and PUM210F exhibit a polycatenated architecture, resulting in microporous channels decorated by amide moieties. The activated forms show a permanent porosity and a selective adsorption of CO2 over N2. Moreover, the partially transmetalated Cu-PUM210 and Cu-PUM210F were obtained by convenient transmetalation protocol and their adsorption propriety toward CO2 were subsequently investigated.

Two novel pillared Zn(II)-based Metal-Organic Frameworks were de novo synthesized exploiting N,N′-(1,1′-biphenyl)-4,4′-diylbis-4-pyridinecarboxamide (bpba) and its fluorinated analogous N,N′-(perfluoro-1,1′-biphenyl-4,4′-diyl)diisonicotinamide (F-bpba) as suitable pillar linkers and 2,6-naphthalene dicarboxylic acid as carboxylic ligand. The resulting heteroleptic MOFs, namely, PUM210, [Zn4(bpba)1.5·(ndc)4·(H2O)]n and PUM210F, [Zn3(F-bpba)1·(ndc)3·(DMF)]n, feature an uncommon truncation of the Zn(II) paddle-wheel nodes along the pillaring direction. PUM210 and PUM210F exhibit a polycatenated architecture, resulting in microporous channels decorated by amide moieties. The activated forms show a permanent porosity and a selective adsorption of CO2 over N2. Moreover, the partially transmetalated Cu-PUM210 and Cu-PUM210F were obtained by convenient transmetalation protocol and their adsorption propriety toward CO2 were subsequently investigated.