Speaker
Prof.
Baoshan Xing
(University of Massachusetts, USA)
Description
Rapid development and expansion of nanotechnology and
growing use of nano-products have raised numerous safety
concerns among the public and scientific community.
Potential release of nanoparticles (NPs) into the
environment is predictable through point and/or non-point
sources. Environmental parameters such as pH, ionic
strength, and natural organic material (NOM) profoundly
affect their environmental behavior in aquatic systems. This
in turn will influence the biological interaction and
potential toxicity of NPs in the environment. In this study
we examined how the physico-chemical characteristics of NOM
affect the colloidal behavior of C60 fullerene and
α-aluminum oxide NPs as two model NPs. Three structurally
different humic acids and tannic acid were used as model
NOMs. Aggregation behavior of NPs and structural
characterizations of the adsorbed NOM on Al2O3 NPs were
studied using DLS, AFM, TEM and DRIFT techniques. Fullerene
suspension had a negatively charged surface over a wide
range of pH, while Al2O3 had pH dependent charge with ZPC of
7.9. Surface charge of pure fullerene suspension decreased
to more negative values after addition of any type of NOM,
leading to a more stable colloidal system. Surface charge of
Al2O3 NPs also decreased with the addition of NOM, which
enhanced the colloidal stability at pH near or above its
ZPC. However, below ZPC the presence of free NOM decreased
stability of the colloidal Al2O3 system due to charge
neutralization as well as bridging flocculation. Early stage
aggregation kinetics studies were conducted by addition of
varying concentrations of Ca2+ to the NPs suspensions.
Addition of Ca2+ to the fullerene + NOM and the NOM-coated
Al2O3 NPs systems increased zeta potential almost uniformly
for all types of NOM. Critical coagulation concentration
(CCC) was calculated for each NP and NOM pair combination.
The CCCs increased with increasing molecular weight and
decreasing polarity of NOM. Our data clearly showed that
high molecular weight NOMs promoted the colloidal stability
of fullerene + NOM system and the NOM-coated Al2O3 NPs
through steric stabilization. This is in agreement with the
DRIFT spectra of NOM-coated Al2O3 NPs showing higher
aliphatic content in the complexes prepared with high
molecular weight NOMs. However, low molecular weight NOMs
also enhanced colloidal stability of fullerene and
NOM-coated Al2O3 NP suspensions but mainly through
electrostatic repulsion. In contrast to NOM-coated Al2O3 and
fullerene systems, we observed that the presence of free
long chain polymeric materials in NOM destabilized the
colloidal suspension of pure Al2O3 NPs in acidic pHs. The
plausible mechanism could be the coiling of long molecular
chains of NOM followed by NPs entrapment. NPs aggregate size
would be a determining factor in their fate, mobility and
sedimentation in aqueous systems, which will consequently
affect their biological interactions. This study highlights
the effect of physico-chemical characteristics of NOM on the
aggregation of NPs through modification of their surface
properties.
Primary author
Prof.
Baoshan Xing
(University of Massachusetts, USA)
