Compositional changes in the humin fraction resulting from the long-term cultivation of an Irish grassland soil: Evidence from FTIR and multi-NMR spectroscopies.

Soil humin (HN), a major long-term sink for carbon in the pedosphere, plays a key role in the global carbon cycle, and has been less extensively studied than the humic and fulvic acids components. There are increasing concerns about the depletions of soil organic matter (SOM) arising from modern soil cultivation practices but there has been little focus on how HN can be altered as the result. This study has compared the HN components in a soil under cultivation for wheat for >30 years with those from an adjacent contiguous soil that had been under long-term grass for all that time. A urea-fortified basic solution isolated additional humic fractions from soils that had been exhaustively extracted in basic media. Then further exhaustive extractions of the residual soil material with dimethyl sulfoxide, amended with sulphuric acid isolated what may be called the "true" HN fraction. The long-term cultivation resulted in a loss of 53 % soil organic carbon in the surface soil. Infrared and multi-NMR spectroscopies showed the "true" HN to be dominated by aliphatic hydrocarbons and carboxylated structures, but with clear evidence for lesser amounts of carbohydrate and peptide materials, and with weaker evidence for lignin-derived substances. These lesser-amount structures can be sorbed on the soil mineral colloid surfaces and/or covered by the hydrophobic HN component or entrained within these which have strong affinities for the mineral colloids. HN from the cultivated site contained less carbohydrate and more carboxyl groups suggesting slow transformations took place resulting from the cultivation, but these were much slower than for the other components of SOM. It is recommended that a study be made of the HN in a soil under long-term cultivation for which the SOM content has reached a steady state and where HN will be expected to dominate the components of SOM.

A two-year incubation study of transformations of crop residues into soil organic matter (SOM) and a procedure for the sequential isolation and the fractionation of components of SOM.

Maize (Zea mays) stover, with its natural 13C abundance, was incubated for two years in a gravelly brown earth sandy loam soil that had been under long term cultivation to wheat (Triticum aestivum) for more than 30 years. The relative abundances of 13C in the maize amendment allowed the contributions of the stover to be traced in the components of soil organic matter (SOM) isolated and fractionated using a sequential exhaustive extraction (SEE) process that gave 16 distinct fractions. These were caracterised using elemental, δ13C, FTIR, and 13C NMR analyses. Emphasis is placed on results for two years of incubation but to some extent data are compared with those for similar fractions taken after one year of incubation. Amounts of maize-derived organic carbon in the humic (HA) and fulvic (FA) isolates were more than twice those in the fractions after one year of incubation. The NMR results highlighted compositional differences between the fractions and showed increased contributions of lignin to the HAs and FAs (and especially in the cases of the HAs) as pH increased, and it was evident that humification was taking place after two years of incubation. The most recalcitrant humin fraction, isolated in the final solvent in the sequence, dimethylsulphoxide (DMSO) and sulfuric acid, is composed predominantly of methylene moieties, is compositionally and structurally very different from the humic and hydrophilic isolates, but identical to that which did not dissolve in the solvent. That suggests that exhaustively pre-extracting soil with the NaOH/urea solvent system used will allow a truly representative humin to be obtained using the DMSO/acid solvent system.

Characterization of transformations of maize residues into soil organic matter.

An awareness of the transformation of plant residues returned to cultivated soils is vital for a better understanding of carbon cycles, the maintenance of soil fertility and the practice of a sustainable agriculture. The transformation of maize (Zea mays L) straw residues into soil organic matter (SOM) in a one year incubation experiment was studied in a soil that had been under long term cultivation with wheat (Triticum aestivum L) for >30years. A novel sequential exhaustive extraction and fractionation procedure isolated a series of fractions of SOM. The samples were characterized by elemental and δ13C analyses, by amino acids and neutral sugars analyses, by Fourier transformed infrared (FTIR) spectrometry, and by solid state 13C nuclear magnetic resonance (NMR) spectroscopy and with chemical shift anisotropy (CSA) -filter and dipolar dephasing (DD) spectral editing NMR techniques. The δ13C data indicated that 59% and 38% of the newly transformed organic carbon was in the humic and fulvic acid fractions, respectively, and in general a greater proportion of the transformed carbon was in the fractions isolated at the higher pH values. Results for SOM fractions from the amended soil indicate dominant contributions from carbohydrate and lignin-like material, and that can be clearly identified by FTIR, CP/TOSS, and spectral editing of CSA-filter and DD. The compositions of the fractions from the amended and non-amended soils fractions can be clearly differentiated using principal component analysis (PCA) for the data collected. The sequential extraction procedure showed that the hydrophilicity of humic fractions increased as the result of the maize amendment, and the aromaticity of the fraction decreased. The data may give some indications of transformations that take place during humification processes.

Occurrence and removal of titanium at full scale wastewater treatment plants: implications for TiO2 nanomaterials.

Titanium dioxide nanoparticles increasingly will be used in commercial products and have a high likelihood of entering municipal sewage that flows to centralized wastewater treatment plants (WWTPs). Treated water (effluent) from WWTPs flows into rivers and lakes where nanoparticles may pose an ecological risk. To provide exposure data for risk assessment, titanium concentrations in raw sewage and treated effluent were determined for 10 representative WWTPs that use a range of unit processes. Raw sewage titanium concentrations ranged from 181 to 1233 µg L(-1) (median of 26 samples was 321 µg L(-1)). The WWTPs removed more than 96% of the influent titanium, and all WWTPs had effluent titanium concentrations of less than 25 µg L(-1). To characterize the morphology and presence of titanium oxide nanoparticles in the effluent, colloidal materials were isolated via rota-evaporation, dialysis and lyophilization. High resolution transmission electron microscopy and energy dispersive X-ray analysis indicated the presence of spherical titanium oxide nanoparticles (crystalline and amorphous) on the order of 4 to 30 nm in diameter in WWTP effluents. This research provides clear evidence that some nanoscale particles will pass through WWTPs and enter aquatic systems and offers a methodological framework for collecting and analyzing titanium-based nanomaterials in complex wastewater matrices.

Characterization and liquid chromatography-MS/MS based quantification of hydroxylated fullerenes.

Highly water-soluble hydroxylated fullerene derivatives are being investigated for a wide range of commercial products as well as for potential cytotoxicity. However, no analytical methods are currently available for their quantification at sub-ppm concentrations in environmental matrixes. Here, we report on the development and comparison of liquid chromatography-ultraviolet/visible spectroscopy (LC-UV/vis) and liquid chromatography-mass spectrometry (LC-MS) based detection and quantification methods for commercial fullerols. We achieved good separation efficiency using an amide-type hydrophilic interaction liquid chromatography (HILIC) column (plate number >2000) under isocratic conditions with 90% acetonitrile as the mobile phase. The method detection limits (MDLs) ranged from 42.8 ng/mL (UV detection) to 0.19 pg/mL (using MS with multiple reaction monitoring, MRM). Other MS measurement modes achieved MDLs of 125 pg/mL (single quad scan, Q1) and 1.5 pg/mL (multiple ion monitoring, MI). Each detection method exhibited a good linear response over several orders of magnitude. Moreover, we tested the robustness of these methods in the presence of Suvanee River fulvic acids (SRFA) as an example of organic matter commonly found in environmental water samples. While SRFA significantly interfered with UV- and Q1-based quantifications, the interference was relatively low using MI or MRM (relative error in presence of SRFA: 8.6% and 2.5%, respectively). This first report of a robust MS-based quantification method for modified fullerenes dissolved in water suggests the feasibility of implementing MS techniques more broadly for identification and quantification of fullerols and other water-soluble fullerene derivatives in environmental samples.

Isolation and fractionation of soil humin using alkaline urea and dimethylsulphoxide plus sulphuric acid.

Humin, the most recalcitrant and abundant organic fraction of soils and of sediments, is a significant contributor to the stable carbon pool in soils and is important for the global carbon budget. It has significant resistance to transformations by microorganisms. Based on the classical operational definition, humin can include any humic-type substance that is not soluble in water at any pH. We demonstrate in this study how sequential exhaustive extractions with 0.1 M sodium hydroxide (NaOH) + 6 M urea, followed by dimethylsulphoxide (DMSO) + 6% (v/v) sulphuric acid (H(2)SO(4)) solvent systems, can extract 70-80% of the residual materials remaining after prior exhaustive extractions in neutral and aqueous basic media. Solid-state (13)C NMR spectra have shown that the components isolated in the base + urea system were compositionally similar to the humic and fulvic acid fractions isolated at pH 12.6 in the aqueous media. The NMR spectra indicated that the major components isolated in the DMSO + H(2)SO(4) medium had aliphatic hydrocarbon associated with carboxyl functionalities and with lesser amounts of carbohydrate and peptide and minor amounts of lignin-derived components. The major components will have significant contributions from long-chain fatty acids, waxes, to cuticular materials. The isolates in the DMSO + H(2)SO(4) medium were compositionally similar to the organic components that resisted solvation and remained associated with the soil clays. It is concluded that the base + urea system released humic and fulvic acids held by hydrogen bonding or by entrapment within the humin matrix. The recalcitrant humin materials extracted in DMSO + H(2)SO(4) are largely biological molecules (from plants and the soil microbial population) that are likely to be protected from degradation by their hydrophobic moieties and by sorption on the soil clays. Thus, the major components of humin do not satisfy the classical definitions for humic substances which emphasise that these arise from microbial or chemical transformations in soils of the components of organic debris.

Biogenic nanoscale colloids in wastewater effluents.

The size, surface area, metal complexation capacity, organic pollutant sorption potential, reactivity with disinfectants, and elevated nitrogen content of biogenic organic nanoscale material (BONM) can potentially affect aquatic environments. BONM in effluents from 11 full-scale wastewater treatment plants (WWTPs), which use a range of biological processes, were characterized in two ways. First, BONM was measured by hydrodynamic size-exclusion chromatography coupled with an online organic carbon and UV detector. Second, BONM was isolated from the wastewater using rotary evaporation and dialysis and then characterized by elemental analysis, transmission electron microscopy, and Fourier transform infrared spectroscopy. The wastewaters contained 6-10 mg/L of dissolved organic carbon (DOC). BONM accounted for 5%-50% of the DOC in wastewater effluent organic matter, and the largest size fraction (>10 kDa) of organic carbon correlated with the organic carbon content determined after rotary evaporation and dialysis. Membrane bioreactor WWTPs had the lowest fraction of BONM (<10% of the DOC), followed by conventional activated sludge (10% to 30% of the DOC), with other processes (e.g., trickling filters, aerated lagoons) containing larger BONM percentages. BONM had a lower carbon to nitrogen ratio (6.2 ± 1.7) compared with the literature values for humic or fulvic acids, exhibited chemical bonds that were indicative of amides and polysaccharides, and contained fibril entangled networks. This work has important implications for operations efficiency of WWTPs, including controlling membrane fouling and release of organic nitrogen into sensitive environments.

Unraveling the structural components of soil humin by use of solution-state nuclear magnetic resonance spectroscopy.

Humin is the most recalcitrant and least understood fraction of soil organic matter. By definition, humin is that fraction not extracted by traditional aqueous alkaline soil extractants. Here we show that > or = 70% of the traditional humin fraction is solubilized when 0.1 M NaOH + 6 M urea and dimethyl sulfoxide (DMSO) + 6% H2SO4 are used in series after conventional extraction. Multidimensional solution-state NMR is applied in this study to gain an understanding of the major constituents present in these "solubilized humin fractions". The spectra indicated strong contributions from five main categories of components, namely, peptides, aliphatic species, carbohydrates, peptidoglycan, and lignin. Diffusion edited spectroscopy indicated that all species are present as macromolecules (or stable aggregate species). Although the distribution of the components is generally similar, peptidoglycan is present at significant levels supporting a higher microbial contribution to humin than to humic and fulvic fractions. The abundance of plant- and microbial-derived materials found does not exclude "humic" materials (e.g., oxidized lignin) or the presence of novel compounds at lower concentrations but suggests that a large proportion of humin is formed from classes of known compounds and parent biopolymers.