Red mud (RM) is an industrial waste produced in large amounts during alumina extraction from bauxite. Its disposal generates serious environmental pollution due to high alkalinity. Therefore, a strategy for the effective utilization of RM must be developed. For instance, RM may be transformed into useful products, such as adsorbents. Given its high concentrations of aluminum oxides, iron oxides, titanium oxides, silica oxides, and hydroxides, RM may be developed as a cheap adsorbent for the removal of various ions from aqueous solution and soils (e.g., metal and non-metal ions, phenolic compounds, and dyes) and waste gas purification (sulfide and carbide). This review summarizes the background, properties, and applications of RM as an adsorbent. Proper approaches of removing metal and non-metal elements from wastewater are also systematically reviewed and compared. Emphasis is placed on the surface modification of RM to obtain high adsorption. Finally, the scope for future research in this area for RM is discussed in depth.
Understanding the effect of the microstructure of a sodium
silicate
solution on the growth behavior of silica nanoparticles is necessary
for the preparation of functional silica. The structural evolution
of silica aggregates in sodium silicate solutions was studied by small-angle
X-ray scattering (SAXS) and transmission electron microscopy (TEM).
The sodium silicate solution mainly contained three types of particles:
monomers with a radius of gyration (R
g) of <0.6 nm, SiO2 clusters formed by monomer polymerization,
and large colloidal particles. Notably, primary particles with different
structures in sodium silicate solutions exhibited a structure-directing
effect for silica nanoparticles formation. Assembly growth occurs
through the continuous addition of primary particles to the surface.
For SiO2/Na2O < 4.2, the primary particles
are ellipsoidal, and there are more hydroxyl groups grafted on both
ends of the ellipsoid, so the condensation reaction is more likely
to occur at both ends, eventually the ellipsoidal aggregates are formed.
For SiO2/Na2O > 4.2, condensation reactions
occur at equal rates in all directions, resulting in the formation
of spheroid aggregates. Additionally, for SiO2/Na2O > 4.2, the primary particles maintain the fractal structure
and
are not easily destroyed during the carbonization reaction, so the
aggregates formed by primary particles have relatively denser fractal
structure than SiO2/Na2O < 4.2. Moreover,
an understanding of the sodium silicate structure and different structural
regulation mechanisms for silica nanoparticles synthesis provided
an important theoretical foundation for fabricating high-performance
silica.
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