BioMOF@PAN Mixed Matrix Membranes as Fast and Efficient Adsorbing Materials for Multiple Heavy Metals’ Removal

Heavy metal ions are a common source of water pollution.
In this
study, two novel membranes with biobased metal–organic frameworks
(BioMOFs) embedded in a polyacrylonitrile matrix with tailored porosity
were prepared via nonsolvent induced phase separation methods and
designed to efficiently adsorb heavy metal ions from oligomineral
water. Under optimized preparation conditions, stable membranes with
high MOF loading up to 50 wt % and a cocontinuous sponge-like morphology
and a high water permeability of 50–60 L m–2 h–1 bar–1 were obtained. The
tortuous flow path in combination with a low water flow rate guarantees
maximum contact time between the fluid and the MOFs, and thus a high
heavy metal capture efficiency in a single pass. The performances
of these BioMOF@PAN membranes were investigated in the dynamic regime
for the simultaneous removal of Pb2+, Cd2+,
and Hg2+ heavy metals from aqueous environments in the
presence of common interfering ions. The new composite adsorbing membranes
are capable of reducing the concentration of heavy metal pollutants
in a single pass and at much higher efficiency than previously reported
membranes. The enhanced performance of the mixed matrix membranes
is attributed to the presence of multiple recognition sites which
densely decorate the BioMOF channels: (i) the thioether groups, deriving
from the S-methyl-l-cysteine and (S)-methionine amino acid residues, able to recognize and
capture Pb2+ and Hg2+ ions and (ii) the oxygen
atoms of the oxamate moieties, which preferentially interact with
Cd2+ ions, as revealed by single crystal X-ray diffraction.
The flexibility of the pore environments allows these sites to work
synergically for the simultaneous capture of different metal ions.
The stability of the membranes for a potential regeneration process,
a key-factor for the effective feasibility of the process in real
life applications, was also evaluated and confirmed less than 1% capacity
loss in each cycle.