Mercury Removal from WastewaterUsing Cysteamine FunctionalizedMembranes
2020
This
study demonstrates a three-step process consisting of primary
pre-filtration followed by ultrafiltration (UF) and adsorption with
thiol-functionalized microfiltration membranes (thiol membranes) to
effectively remove mercury sulfide nanoparticles (HgS NPs) and dissolved
mercury (Hg2+) from wastewater. Thiol membranes were synthesized
by incorporating either cysteine (Cys) or cysteamine (CysM) precursors
onto polyacrylic acid (PAA)-functionalized polyvinylidene fluoride
membranes. Carbodiimide chemistry was used to cross-link thiol (−SH)
groups on membranes for metal adsorption. The thiol membranes and
intermediates of the synthesis were tested for permeability and long-term
mercury removal using synthetic waters and industrial wastewater spiked
with HgS NPs and a Hg2+ salt. Results show that treatment
of the spiked wastewater with a UF membrane removed HgS NPs to below
the method detection level (<2 ppb) for up to 12.5 h of operation.
Flux reductions that occurred during the experiment were reversible
by washing with water, suggesting negligible permanent fouling. Dissolved
Hg2+ species were removed to non-detection levels by passing
the UF-treated wastewater through a CysM thiol membrane. The adsorption
efficiency in this long-term study (>20 h) was approximately 97%.
Addition of Ca2+ cations reduced the adsorption efficiencies
to 82% for the CysM membrane and to 40% for the Cys membrane. The
inferior performance of Cys membranes may be explained by the presence
of a carboxyl (−COOH) functional group in Cys, which may interfere
in the adsorption process in the presence of multiple cations because
of multication absorption. CysM membranes may therefore be more effective
for treatment of wastewater than Cys membranes. Focused ion beam characterization
of a CysM membrane cross section demonstrates that the adsorption
of heavy metals is not limited to the membrane surface but takes place
across the entire pore length. Experimental results for adsorptions
of selected heavy metals on thiol membranes over a wide range of operating
conditions could be predicted with modeling. These results show promising
potential industrial applications of thiol-functionalized membranes.
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