Environmental Engineering Reference
In-Depth Information
The AFm results show that all humic adlayers were separated into multiple domains of nano- and submicrosize. Again sub-
stantial differences can be seen in the morphology of coatings produced by the humic materials of different origins. The rough-
est surfaces were observed for PHS-APTS and AHS-APTS at 5.9 and 3.8 nm, respectively, which is in line with indications of
fluorescent images showing the largest portion of humic matter attached to the surfaces in the cases of PHS and AHS. For
coal-based HS, a much smoother surface was observed with an rms roughness of 1.5 nm. The coal HS adlayer consisted of flat
circular particles (2-15 nm in diameter) homogeneously distributed over the glass surface. The particles did not exhibit intimate
contact with one another. This might be indicative of repulsive interactions between particles produced by the negatively
charged humic polyanions, which preclude coalescence and induce the appearance of the diffuse structure in the adlayer. The
domains observed in the adlayer assembled by peat humic acids are very different from those of coal and present much larger
particles of 10-50 nm in diameter, aggregates of these particles (20-100 nm), and ridge-like assemblies (up to 200 nm in length
and up to 30 nm in height). Aquatic humic materials produced an intermediate picture consisting of domains ranging from 5 to
30 nm and chain-like assemblies (up to 100 nm). The particles had “geometric” shapes dissimilar to those of both peat and coal
materials. The aquatic adlayer was characterized with the highest heterogeneity provided by the irregular location of large
particle chains reaching 100 nm in length and up to 40 nm in height.
The nanostructures described earlier for the adlayers of silanized humic materials are in agreement with the AFm images
observed for the natural humic colloids and for the humic adlayers immobilized onto different mineral surrogates (glass, oxi-
dized silicon wafer, mica, carbonaceous or goethite surfaces) [48-52]. The coatings composed of separated islands of aquatic
humic materials were observed for immobilized SRFA and SRHA (IHSS standard humic materials from the Suwannee River)
[49]. close estimates of particle shapes and sizes are reported for these in situ studied humic colloids: (i) flat particles (8-13 nm
in diameter); (ii) aggregates of particles (20-100 nm); and (iii) chain-like assemblies, networks, and torus-like structures [53].
The results provide an insight into the molecular features of the surface morphology of humic coatings, which can be immobi-
lized onto the hydroxyl-carrying solid supports using the guided self-assembly of alkoxysilanized humic materials.
Of particular interest was the comparative assessment of the approach with other immobilization techniques. For this
purpose, the trimethoxysilyl derivatives of coal and peat humic acids were also immobilized onto silica gels and the corresponding
quantities of HS were determined [46]. The amounts found were 210 and 232 mg of HS/g SiO
2
, respectively. A comparison of
these results with those reported elsewhere [54] for immobilized humic materials using alternative techniques showed that the
new immobilization method generates 2-10 times more humic material on the silica gel. Immobilization of HS onto the APTS-
modified silica gel in the aqueous phase by the adsorption mechanism without further heating yielded 65 mg HS/g SiO
2
. The
immobilization method based on HS binding to SiO
2
modified with APTS in the presence of N-(3-dimethylaminopropyl)-N'-
ethylcarbodiimide hydrochloride (eDS) had a higher yield (107 mg HS/g SiO
2
), and the binding of the HS to APTS-modified
silica gel at high temperature in anhydrous solvents (e.g., dimethylformamide (DmF)) yielded 124 mg HS/g SiO
2
. When binding
HS to the APTS-modified silica via glutaric aldehyde, a value of 60 mg HS/g SiO
2
was obtained. An additional advantage of the
proposed approach is that the binding of HS was practically irreversible, while alternative techniques released up to 50% of the
immobilized HS into the solution upon washing with 0.1 m Nacl at pH 10 [54].
The conclusion reached is that the unique properties of these developed silanized humic derivatives include (i) their solu-
bility in water; (ii) the capability to adhere to different minerals; and (iii) their environmental compatibility. A progressive
combination of these properties allows us to consider these derivatives as important reagents for the treatment of both actinide-
and metal-contaminated aquatic environments. They can serve either as liquid scavengers—phase-switchers—or as reactive
agents for in situ installation of permeable reactive barriers (PRBs) aimed at sequestering mobile higher-valence actinides from
contaminated groundwaters.
29.3.2
sequestration of mobile actinides in their Higher-valence states onto Humic nanocoatings
To further develop the concept of a remedial technology based on the use of “mineral-adhesive” humic agents, consideration
was given to the in situ installation of PRBs, which are capable of sequestering actinides in the higher-valence states, and the
execution of experiments for demonstrating the immobilization of waterborne Np(V) and Pu(V) [55]. Humic coatings were
produced by self-assembly of the alkoxysilanized humic derivatives onto silica gel as described in the previous paragraph.
leonardite humic acid (HA) and its hydroquinone derivatives were used as parent humic materials for the incorporation of
alkoxysilyl groups. The amount of organic carbon immobilized onto silica gel in case of the leonardite HA was 9.2% (240 mg
of HA per 1 g of silica gel), whereas for the hydroquinone derivative it was much less accounting for 3.3% (100 mg HA per 1 g
of SiO
2
). concentrations of Np(V) and Pu(V) in the working solutions were 4.68 10
−6
m and 4.90·10
−9
m, respectively.
experiments with Np(V) were executed at pH 4.5. experiments with Pu(V) were conducted at three pH values: 3.5, 4.6, and
7.7. All experiments were executed in the absence of oxygen under batch conditions. The sequestration of Np(V) and Pu(V) by
the humic-coated silica gels is shown in Figure 29.14 and Figure 29.15, respectively.
