Structure and stability of natural organic matter/soil complexes and related synthetic and mixed analogues

Organic ligands activate the dark formation of hydroxyl radicals (HO•) in surface soil/sediment: Yields, mechanisms, and applications

Soil is an important sink for various pollutants. Recent findings suggest that soil and sediment would spontaneously form HO^• through Fenton or Fenton-like reactions under natural conditions. In this study, the effects and mechanisms of organic ligands (OLs) on the occurrence of HO^• in surface soil/sediment were experimentally and computationally examined. Results confirmed that HO^• generation was ND-12.92 nmol/g in surface soil/sediment, and the addition of EDTA-2Na would significantly enhance the yields of HO^• by 1.4-352 times. Moisture was the decisive factor of soil HO^• generation. The release of Fe(II) from solid into the aqueous phase was essential for the stimulation of HO^• in EDTA-2Na suspensions. Furthermore, complexation reactions between Fe(II) and OLs would enhance single electron transfer (SET) reactions and the formation of O₂^•-. Interestingly, for specific OLs, their stimulations on SET and formation of O₂^•- would depress HO^• generation. Provoking HO^• generation by OLs could be efficiently used to degrade sulfamethoxazole in rice field sediment. The study provided new knowledge on how commonly synthetic OLs affect the HO^• generation in surface soil/sediment, and it additionally shed light on the engineered stimulation of in-situ Fenton reactions in natural soil/sediment.

Molecular composition of raw peat and humic substances from permafrost peat soils of European Northeast Russia as climate change markers

Humic substances (HSs) from the mire peat soils of the forest-tundra zone of the European northeast part of Russia have been characterized in terms of molecular composition. This was accomplished using solid-state ¹³C nuclear magnetic resonance (¹³C NMR) techniques and electron spin resonance (ESR) spectroscopy. The composition depended on the intensity of cryogenic processes in the active layer, the quality of the humification precursors (the degree of peat material transformation), and the biochemical selection of aromatic fragments during humification. Humic acids (HAs) and fulvic acids (FAs) of the peat soils showed the presence of compounds with a low extent of condensation and a low portion of aromatic fragments, which increased with depth. A higher proportion of aliphatic carbon species was found in the HAs, indicating a low degree of organic matter stabilization. Based on the data from the two types of peat soils, we suggest that particular changes in the proportion of aromatic and unoxidized aliphatic fragments on the border of the bottom of the active layer and permafrost layers can be used as markers of current climatic change.

Adsorption of Polyelectrolytes at the Manganese Oxide(IV)–Polymer Solution Interface: Structure of Adsorbed Layers

The influence of the molecular weight of polyelectrolytes on their adsorption and on the structure of the adsorbed layers at the manganese oxide(IV)–polymer solution interface was determined. Polyacrylic acid (PAA) and polyacrylamide (PAM) were applied as ionic polymers.

An explanation was proposed for the observed changes in surface charge and zeta potential of the solid in the presence of these polymers. The thickness of the adsorption layer of PAA and PAM was determined and the free energies of adsorption of these polymers on the MnO2 surface were calculated from the zeta potential measurements.

Study of the Adsorption Mechanism and the Structure of Adsorbed Layers of Polyelectrolytes at the Metal Oxide/Solution Interface

The influence of the molecular weight of polyacrylic acid (PAA) and polyacrylamide (PAM) as well as of inorganic contaminants on the ZrO2 surface on the adsorption and electrokinetic properties of the metal oxide/polyelectrolyte solution interface were studied for ZrO2 and Fe2O3 solid particles.

The calculated concentrations of the various surface groups on ZrO2 and Fe2O3 enabled an investigation of the possible mechanism for the bonding of the studied polyelectrolytes with the surfaces of both oxides. PAA and PAM macromolecules bond with the solid surface mainly via the –OH groups of the oxides, which may interact with the carboxy groups of polyelectrolytes through hydrogen bridging.

A comparison of the change in values of the diffuse layer charge with the surface charge enabled the principal factors responsible for the changes in the zeta potential of the oxides, e.g. pH, polymer molecular weight and concentration of the polymer solutions, to be evaluated. From such zeta potential values, it was possible to determine the free energies of adsorption of PAA and PAM on the surfaces of both oxides.

The thicknesses of adsorbed polymer layers on Fe2O3 and ZrO2 were calculated on the basis of measurements of their suspension viscosities in the absence and presence of adsorbed polymer. It was shown that the thickness of the adsorption layer increased with increasing polymer molecular weight, pH and concentration of the polymer solution.

Because the experimental determination of the number and the length of trains, loops and tails in such polyelectrolytes was not possible, the participation of such segments of polymer structure at the interface was computed using the Scheutjens–Fleer model of polymer adsorption. The polymer adsorption expressed as the number of equivalent monolayers was calculated and compared with the experimental data.

Low-temperature thermal desorption of diesel polluted soil: influence of temperature and soil texture on contaminant removal kinetics

Five soil size aggregate fractions, corresponding to coarse (500-840 μm), medium (200-350 μm), fine (75-200 μm) sand, silt (10-75 μm) and clay (<4 μm), were artificially contaminated with diesel, and thermally treated using a laboratory scale apparatus to investigate the effect of soil texture on contaminant adsorption and removal. Ex situ thermal process was simulated using helium as the carrier gas at a flow rate of 1.5 L min(-1), different temperatures (100-300 °C) and different treatment times (5-30 min). The amount of contaminant adsorbed on the soil and the residual amount after thermal treatment was determined by gas chromatography. Results showed that adsorption phenomena and desorption efficiency were affected by the soil texture and that temperature and time of treatment were key factors in remedial process. A temperature of 175 °C is sufficient to remedy diesel polluted sandy and silty soils, whereas a higher temperature (250 °C) is needed for clays. Thermal desorption of diesel polluted soil was shown to be governed by first-order kinetics. Results are of practical interest and may be used in scaling-up and designing desorption systems for preliminary cost and optimal condition assessment.

Molecular rigidity and diffusivity of Al3+ and Ca2+ humates as revealed by NMR spectroscopy

NMR techniques were applied to follow changes in molecular rigidity and diffusion of complexes formed between a humic acid (HA) and either aluminum or calcium ions, added in amounts ranging from 0.05% to 1% of HA carboxylic acidity. Spin-lattice relaxation time in the rotating frame (T1rho,(H)) and diffusion coefficients (D) of humic-metal complexes were obtained from 13C cross-polarization magic angle spinning (CP-MAS), and 1H-diffusion order spectroscopy (DOSY) spectra, respectively. Molecular rigidity of humic complexes increased significantly with metal addition throughout the full carbon spectral region, being more pronounced for triple-charged Al than for double-charged Ca. However, T1rho(H) values of spectral intervals suggested that molecular rigidity increase was generally in the following order: aliphatic C > aromatic/double bonds C > carboxyl C. Concomitantly, DOSY spectra showed that addition of both Al and Ca decreased substantially the diffusivity of humic alkyl components and increased that of aromatic and hydroxyalkyl components, thereby indicating that complexation induced a molecular-size increase in the former and a decrease in the latter. These results suggest that saturated and unsaturated long-chain alkanoic acids in HA were preferentially involved in metal complexation with Al and Ca, with consequentincrease of conformational rigidity and molecular size of humic hydrophobic domains. Conversely, more hydrophilic or mobile humic components appeared relatively less affected by the molecular and intermolecular rearrangements induced in HA by complexation with metals. Such NMR approach appears thus liable to evaluate the response to metal complexation of specific chemical entities present in the bulk HA and provides a further insight in the molecular architecture of humic-metal complexes.

Clay and oxide destabilization induced by mixed alum/macromolecular flocculation aids

The review points out typical differences and analogies of the bulk characteristics of aluminum ion complexed polyelectrolytes and of their adsorption behaviors when such systems were supplied to inorganic colloids such as oxides and clays. It reports some particular investigations that were carried out in aqueous media to determine (i) the nature of the interactions existing between clay or oxides, aluminum ions and polyelectrolytes and (ii) the effects on the interfacial characteristics and the colloid stability related to the relative concentrations of these different constituents. The investigations concerned the synthetic alumina/polyacrylic acid systems and the natural kaolinite/humic acid systems, as well as partly the mixed alumina/humic acid systems. Different adsorption features and destabilization kinetics were determined to develop within these systems. One of the main constraints of the investigation arose from the presence of three interacting components which developed amphoteric and amphipatic interactions, the latter being generated by the hydrophobic moieties induced by the aluminum ions/carboxylic acid groups ion-pairing. The investigations concerned the extent and the rate of transfer of hydrogen, aluminum ions and polyelectrolytes from the bulk solution to the solid surface. Electrical surface charge characteristics were expressed in terms of the zeta potential of the colloid/polymer complexes. The colloid stability of the systems was determined as a function of time at short and long terms. The variation as a function of time of the number and weight average masses was correlated with the variation with time of the zeta potential. All these systems were determined to reach the kinetic and thermodynamic equilibrium only slowly. Despite the fact that the supply of mixed coagulants provoked the initial aggregation and the subsequent fragmentation processes for both systems, the mechanisms responsible for the two processes were found to be different as revealed by comparatively investigating the synthetic and the natural systems. The fragmentation originated from the slow segregation process of positively and negatively charged groups for the natural kaolinite/humic acid systems, while the segregation process affected hydrophobic and hydrophilic moieties for the synthetic alumina/polyacrylic acid systems.

Soil remediation: humic acids as natural surfactants in the washings of highly contaminated soils

The remediation of the highly contaminated site around the former chemical plant of ACNA (near Savona) in Northern Italy is a top priority in Italy. The aim of the present work was to contribute in finding innovative and environmental-friendly technology to remediate soils from the ACNA contaminated site. Two soils sampled from the ACNA site (A and B), differing in texture and amount and type of organic contaminants, were subjected to soil washings by comparing the removal efficiency of water, two synthetic surfactants, sodium dodecylsulphate (SDS) and Triton X-100 (TX100), and a solution of a natural surfactant, a humic acid (HA) at its critical micelle concentration (CMC). The extraction of pollutants by sonication and soxhlet was conducted before and after the soil washings. Soil A was richer in polycyclic aromatic hydrocarbons, whereas soil B had a larger content of thiophenes. Sonication resulted more analytically efficient in the fine-textured soil B. The coarse-textured soil A was extracted with a general equal efficiency also by soxhlet. Clean-up by water was unable to exhaustively remove contaminants from the two soils, whereas all the organic surfactants revealed very similar efficiencies (up to 90%) in the removal of the contaminants from the soils. Hence, the use of solutions of natural HAs appears as a better choice for soil washings of highly polluted soils due to their additional capacity to promote microbial activity, in contrast to synthetic surfactants, for a further natural attenuation in washed soils.

Application of the SAXS method and viscometry for determination of the thickness of adsorbed polymer layers at the ZrO2–polymer solution interface

The authors studied the influence of the molecular weight of polyacrylic acid (PAA) and polyacrylamide (PAM), solution pH and ionic strength, and the background electrolyte type on adsorption and the thickness of polyelectrolyte adsorption layers formed on ZrO(2) surface. Carboxyl groups distributed along PAA and PAM chains were shown to be responsible for their interface conformation, which directly influences the thickness of the adsorbed polyelectrolyte layers. Bonding of macromolecules with solid surface occurs through the hydrogen bridges of these groups. Two methods were applied to determine the PAA and PAM adsorption layer thickness on ZrO(2), i.e., SAXS (small angle X-ray scattering) and viscometry. Despite some limitations of the SAXS method resulting from the relationship between the size of solid pores, polymer molecular weight, and conformation of the adsorbed macromolecule, all obtained SAXS results were very close to those calculated from viscometry data.

Fragmentation of Kaolinite Aggregates Induced by Ion-Exchange Reactions within Adsorbed Humic Acid Layers

Mixing of humic acids and kaolinite clay suspended in aqueous solution containing aluminum ions promotes aggregation of the clay instantaneously. To this fast clay destabilization succeed reversible fragmentation processes which display varying kinetic ranges for which the changes in the average masses of fragments are determined. We show the rate of formation of fragments to decay with the mass i as i(-2) and the maximal rate of aggregate breakup to increase with the fragment mass as i(lambda), where lambda lies between 0.35 and 0.40. At short terms the rate of variation of the average masses of the fragments is lower than expected from the theory. Aggregate fragmentation is attributed to slow progressive modifications in the positive and negative charge distributions within the amphoteric humic acid adsorbed layer, which are induced by the high density of positive charge existing on the clay surface after aluminum ion adsorption. Kaolinite-aluminum ion-humic acid complexes aggregate by forming initially linear small portions of stacked platelets. These portions are connected together to form larger agglomerates of fractal dimension close to 2.5. Copyright 2000 Academic Press.

Polyacrylamide at Solid/Liquid Interfaces

In the literature, the term polyacrylamide refers to neutral, hydrolyzed, and chemically modified polyacrylamide and in this sense polyacrylic acid may likewise be considered to consist of hydrolyzed acrylamide groups. These hydrosoluble polymers are employed in wide range of applications where the polymer characteristics at solid-liquid interfaces play an important role. The interaction with solid adsorbents is very complex since electrostatic, hydration, van der Waals, and other forces operate simultaneously. As in most applications the polyacrylamide is brought into contact with oxides, clays or soils, synthetic alumino-silicates, and aluminum oxide may be used as model adsorbents. Thus, with reference to the two systems polyacrylamide/alumino-silicate and polyacrylamide/aluminum oxide, the author attempts to define some of the interfacial processes affecting the adsorbed macromolecules and to describe certain interfacial characteristics of these complex systems. The main results are as follows. -Neutral polyacrylamide. Hydrogen bonding between acrylamide and neutral aluminol groups is responsible for adsorption and the amount of polymer adsorbed parallels the surface density of aluminol sites. At ambient temperature, the kinetics of layer formation is governed initially by the random deposition of solution macromolecules and finally by tunneling. Thermodynamic and kinetic considerations lead to a model where the majority of the adsorbed macromolecule share identical dynamic characteristics under equilibrium conditions. Departure from equilibrium induced by changing the polymer concentration in the supernatant phase greatly modifies the dynamic features of the adsorbed macromolecules. -Hydrolyzed polyacrylamide. Charge-charge interactions superimpose on the persistent effects of hydrogen bonding to alter the amount of polymer adsorbed and the adsorption kinetics. The strong affinity of hydrolyzed polyacrylamide for positively charged surfaces induces interfacial spreading of the adsorbed macromolecules and in some instances desorption of part of this population (overshoots). -Complexed polyacrylamide. The presence of positively charged, negatively charged, and neutral chain segments confers amphoteric character on the polymer chain. Strongly complexed polymers display fast adsorption, while initially weakly complexed polymers show delayed adsorption. In addition, under certain circumstances, the presence of hydrophobic microdomains confers an amphiphilic character on the macromolecules, which profoundly modifies the nature of the interaction forces. The specific interaction typical of neutral polyacrylamide progressively disappears with increasing hydrolysis until for polyacrylic acid the surface coverage corresponds to nonselective interfacial deposition. Extremely slow displacement of hydrophilic and hydrophobic groups are found to occur within the adsorbed layer. Despite the promising use of macromolecules in environmental strategies, a consideration of the complexity of the underlying interfacial phenomena points to the difficulties which may arise at short or long terms in such applications. Copyright 1999 Academic Press.