A.B. Volikov

Nature-like solution for removal of direct brown 1 azo dye from aqueous phase using humics-modified silica gel

The objective was to estimate suitability of humics-modified silica gels for adsorptive removal of the Direct Brown 1 trisazo dye from aqueous phase. The major advantage of the proposed adsorbents is that of an ecologically sound procedure of immobilizing silanized humic derivatives onto silica gel in aquatic solutions. The silanized humic derivatives, in turn, are obtained without a use of organic solvent by reacting natural humic materials from peat and coal with 3-aminopropyltriethoxyorganosilane in water. These silanized derivatives are surface active and are capable of self assembling into humic adlayers at the water solid interface. A use of this approach allows for immobilization of up to 220 mg of humic materials per 1 g of SiO2. The adsorption capacity of humics-modified silica gels with respect to the Direct Brown 1 trisazo dye varied from 3.5 up to 8.8 mg per 1 g of SiO2. The maximum sorption obtained for the silanized derivative with 50% modification degree was comparable to adsorption capacity of activated coal to this dye (7.5 mg g(-1)). The results of this adsorption study, warrant further studies of azo dye removal from aqueous environments.

Extraction of Humic Substances from Fresh Waters on Solid-Phase Cartridges and Their Study by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Humic substances (HS) have been extracted from fresh waters on macroporous adsorbents of different types, Amberlite XAD-8 resin and Bond Elute PPL solid-phase cartridge. Comparative analysis of the extracted samples by ultra-high resolution mass spectrometry demonstrated selectivity of the selected adsorbents to the components of the molecular assembly of HS: the XAD-8 resin has affinity to the higher oxidized aromatic compounds and PPL has affinity to substances of stronger aliphatic character and nitrogen-containing compounds. Because of this fact, a comparison of the molecular composition of HS isolated from various sources must be performed taking into account selective adsorption; therefore, it is necessary to compare the products extracted by the same adsorbent for correct interpretation.

Targeted design of water-based humic substances-silsesquioxane soft materials for nature-inspired remedial applications

Water-based humic substances-silsesquioxane (HS-SQ) soft materials are synthesized by hydrolysis of (3-aminopropyl-triethoxy)-silane in the HS solution. The aggregation dynamics of this system was studied using in situ small-angle X-ray scattering (SAXS) technique, which revealed three consecutive stages in the evolution of the HS-SQ system based on its fractal dimension (D): HS-SQ oligomeric polyelectrolyte complexes with D 3 (surface fractals). It was suggested that the reaction time needed for the HS-SQ system to transit from mass to surface fractal stage can be used to control its self-assembly onto a solid support. The corresponding studies have confirmed that the HS-SQ networks could be successfully immobilized onto sand columns only at the aggregation stage with fractal dimensions of 2.5 < Dm < 3. This enabled the targeted design of HS-SQ systems capable of guided self-assembly onto the solid support. The corresponding lab column studies have demonstrated successful passive installation of a humic permeable reactive barrier on sand which was capable of intercepting azo dyes from contaminated water. The prospects of using HS-SQ soft materials in nature-inspired remedial technologies and soil restoration are discussed.

Non-classical growth of water-redispersible spheroidal gold nanoparticles assisted by leonardite humate

The growth of gold nanoparticles (AuNPs) assisted by humate – a natural hyperbranched polyelectrolyte – was studied using in situ and ex situ techniques. The conditions for formation of almost monodisperse gold nanospheres of 13 ± 3 nm diameter with pronounced plasmonic properties were defined. A striking similarity was found between humate- and citrate-mediated growth of AuNPs: the formation of gold nanospheres involved rapid nucleation of gold seeds, slow growth within the intermediate agglomerates (visible as nanoworms after drying on a TEM grid), and rapid peptization into the final gold nanospheres. Both humate and citrate syntheses produced ultrastable gold sols with pronounced plasmonic properties. The substantial difference was slower kinetics of humate synthesis, 240 min versus 15 min for citrate, and the water redispersible properties of the humate-capped gold nanospheres after freeze drying, which was not seen with the citrate AuNPs. Theoretical calculations revealed a leading role of steric factors in the formation of intermediate aggregates of capped AuNPs at the stage of their slow growth in the case of both citrate and humate. We suggested that it was the polyelectrolyte nature of humate which enabled the water-redispersibility of humate- versus citrate-capped gold nanoparticles.

Sorption of Humic Substances on a Weakly Basic Anion-Exchange Resin: Relationship with the Adsorbate Structure

Adsorption of a broad range of humic substances (HS) of different origins and fractional compositions on a macroporous weakly basic anion-exchange resin is studied. It is found that the nature of the HS has a substantial effect on both the efficiency of sorption and the mechanism of interaction with the adsorbent. The dependence of the determined thermodynamic parameters of sorption on the HS origin, composition, and structure is shown for a broad range of HS. It is concluded that the results can be used to predict the sorption properties of weakly basic anion-exchange resins with respect to HS of known origin and structural group composition.

Morphology and Hydrophobicity of Humic Coatings on Glass as Studied by Atomic Force Microscopy (AFM) and Contact Angle Measurements

In nature, many functional nanoarchitectures are self-assembled due to supramolecular interactions of natural organic matter and surfaces of mineral particles. However, direct evidence for molecular mechanisms responsible for these interactions is still missing. Here, we developed silanol derivatives of humic substances capable of self-assembling onto hydroxylated surfaces. By using humic substances from peat and coal and DNA array glass slides, we show that humic adlayers were homogeneous on macroscale, but separated into domains of nanosize in coal HS and submicro size in peat HS. The peat adlayers were characterized with larger roughness of 11 nm versus 5 nm for coal. Measurement of the contact angle showed that application of humic coatings leads to an increase in hydrophobicity of glass. It was shown that treatment of soil aggregates with silanol derivatives of humic acids increases water stability of the aggregates.

Nitrogen release from natural and aminoorganosilane-modified humic substances

A study of nitrogen release from native and aminoorganosilane derivatives of humic substances (HS) has been conducted. It was demonstrated that dissolution of humics in water resulted in nitrogen release mainly in the form of ammonium ions, whereas under soil conditions, nitrogen detected in soil extract was presented mainly in nitrate form due to nitrification processes. The amount of nitrogen transformed into nitrate form from aminoorganosilane-modified HS varied from 36 to 58% of introduced and was significantly less than that for (NH4)2SO4 (98%), demonstrating that further study of aminoorganosilane-modified HS as slow-nitrogen-release additional fertilizing or supplements to main nitrogen fertilizers is very promising.