Copper Removal Using Electrosterically Stabilized Nanocrystalline Cellulose

Removal of heavy metal ions such
as copper using an efficient and low-cost method with low ecological
footprint is a critical process in wastewater treatment, which can
be achieved in a liquid phase using nanoadsorbents such as inorganic
nanoparticles. Recently, attention has turned toward developing sustainable
and environmentally friendly nanoadsorbents to remove heavy metal
ions from aqueous media. Electrosterically stabilized nanocrystalline
cellulose (ENCC), which can be prepared from wood fibers through periodate/chlorite
oxidation, has been shown to have a high charge content and colloidal
stability. Here, we show that ENCC scavenges copper ions by different
mechanisms depending on the ion concentration. When the Cu­(II) concentration
is low (C0 ≲ 200 ppm), agglomerates
of starlike ENCC particles appear, which are broken into individual
starlike entities by shear and Brownian motion, as evidenced by photometric
dispersion analysis, dynamic light scattering, and transmission electron
microscopy. On the other hand, at higher copper concentrations, the
aggregate morphology changes from starlike to raftlike, which is probably
due to the collapse of protruding dicarboxylic cellulose (DCC) chains
and ENCC charge neutralization by copper adsorption. Such raftlike
structures result from head-to-head and lateral aggregation of neutralized
ENCCs as confirmed by transmission electron microscopy. As opposed
to starlike aggregates, the raftlike structures grow gradually and
are prone to sedimentation at copper concentrations C0 ≳ 500 ppm, which eliminates a costly separation
step in wastewater treatment processes. Moreover, a copper removal
capacity of ∼185 mg g–1 was achieved thanks
to the highly charged DCC polyanions protruding from ENCC. These properties
along with the biorenewability make ENCC a promising candidate for
wastewater treatment, in which fast, facile, and low-cost removal
of heavy metal ions is desired most.