Solid-state 13C NMR Spectra Research Articles

β-(1 → 4)-Polyglucuronic acids with C2/C3-ketones prepared from regenerated cellulose by catalytic oxidation using solid NaOCl·5H2O

Three commercial regenerated cellulose samples were subjected to TEMPO-catalyzed oxidation using solid NaOCl·5H2O as the primary oxidant for structural analyses of the oxidized products (TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxyl). The regenerated cellulose/water slurries became transparent solutions after oxidation for 60 min. The yields of the oxidized products were almost 100 % when they were isolated as precipitates in ethanol/water mixtures. The solution-state NMR spectra revealed that the oxidized products were almost pure water-soluble β-(1 → 4)-polyglucuronic acids; the reaction conditions described herein ensured the complete oxidation of the C6–OH groups in the regenerated cellulose samples to C6-carboxy groups. However, the solid-state 13C NMR spectra of the oxidized products indicated that C2/C3-ketones (<20 % of the total units) were formed during side reactions, which is characteristic for oxidized products prepared from regenerated cellulose with the C2/C3-glycol structure. These ketones were likely to form intermolecular hemiacetal linkages in the oxidized products. During conductivity titration of the oxidized products, it is necessary to control the sample masses to accurately determine the carboxy contents. The mass-average degree of polymerization decreased from 330 to 890 for the original regenerated cellulose samples to 65–79 for the oxidized products; substantial depolymerization is inevitable during TEMPO-catalyzed oxidation of the regenerated cellulose samples.

Structural analyses of supernatant fractions in TEMPO-oxidized pulp/water reaction mixtures separated by centrifugation and dialysis

Side reactions occurring on cellulose during 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TMEPO)-catalyzed oxidation have not been considered to be significant. Then, TEMPO-oxidized hardwood and softwood bleached kraft pulps (HBKP and SBKP) were prepared with an excess NaOCl·5H2O. Supernatant fractions (SFs) were obtained in the aqueous reaction mixtures of TEMPO-oxidized pulps by centrifugation and dialysis. The SFs with carboxyl contents of 5.0 and 4.2 mmol/g were obtained in the yields of 19 % and 30 % from HBKP and SBKP, respectively. These carboxy contents are much higher than those (2.6–2.7 mmol/g) of the precipitate fractions in the TEMPO-oxidized pulps. Solid-state 13C NMR spectra and other analyses revealed that the water-soluble β-(1 → 4)-polyglucuronic acids were predominantly present in the SFs. In addition, water-insoluble TEMPO-oxidized cellulose nanocrystals were present in the SFs, but they constituted less than ~10 % of the SFs. The mass-average degrees of polymerization (DPw) of the SFs obtained from HBKP and SBKP were 166 and 155, respectively, whereas the original HBKP and SBKP had DPw values of 1990 and 2140, respectively. These substantial depolymerization and formation of the water-soluble β-(1 → 4)-polyglucuronic acids occur on cellulose and oxidized cellulose molecules as side reactions during TEMPO-catalyzed oxidation, which should be considered for structural analyses of TEMPO-oxidized products.

Near-infrared-emitting silsesquioxane-based porous polymer containing thiophene for highly efficient adsorption and detection of iodine vapor and solution phase

The powerful gathering and potent detection of radioactive iodine, which may be created during the production of nuclear energy or the management of nuclear waste, are essential. Herein, near-infrared-emitting (NIR) porous polymeric materials with thiophene (PCS-DPBS) were employed for the first time to capture iodine vapor effectively and reversibly. The fluorophore monomer (CN-DPBS) with a donor–acceptor-donor (D-A-D) typed electronic configuration exhibiting near-infrared fluorescence emission was successfully produced by the Knoevenagel reaction. Then, CN-DPBS reacted with octavinylsilsesquioxane (OVS) via the Friedel-Crafts reaction, resulting in a series of porous polymers based on silsesquioxane (PCS-DPBS). The polymer was effectively synthesized, as evidenced by elemental analysis, FT-IR, solid-state 29Si NMR, and solid-state 13C NMR spectra. The morphology of PCS-DPBS was also demonstrated by XRD and SEM results. Interestingly, the resulting polymer has distinct properties and characteristics such as NIR emission (663 nm), adjustable porosity (specific surface area of 245 m2 g−1 and total pore volumes of 0.150 cm3 g−1), and thermal stability with Td5% and Td10% of 202 and 414 °C, respectively, in N2 atmosphere. With a large micropore volume and a high thiophene content in its polymer structure, this results in high iodine vapor adsorption capacities, with uptake capacities up to 2.81 g g−1 at 70 °C. The recycling ability is at least five times and the iodine uptake capacity is still >50%. The CN-DPBSunit provides PCS-DPBS with unique luminescence, allowing it to function as a probe to detect iodine solution with great sensitivity with KSV values of 1622.4 M−1. This study on a thiophene-containing adsorbent brings up new possibilities for the effective capture and detection of iodine in environment.

Green chemistry fabrication of durable antimicrobial peptide-immobilized silk fibroin films for accelerated full-thickness wound healing

Silk fibroin (SF) has been intensively explored for wound dressings due to its excellent biological and mechanical properties. However, forming SF films with a combination of durable and excellent antibacterial properties as well as biocompatibility is still a challenge. Herein, a marine-derived antimicrobial peptide, actinomycin X2 (Ac.X2), was immobilized onto SF fibers and Ac. X2-immobilized SF (ASF) films was further prepared using all green chemical approaches. The immobilized amount of Ac. X2 can be precisely quantified by its characteristic UV–visible absorption peak at 443 nm. Both Fourier transform infrared spectroscopy (FTIR) and solid-state 13C NMR spectra revealed that the lactone bond on the α/β rings of Ac. X2 was covalently grafted with the amino groups of SF, which greatly improved the degree of immobilization. The ASF film exhibited excellent antimicrobial activity against Staphylococcus aureus and Methicillin-resistant S.aureus (MRSA), with no biofilm formation. Furthermore, the ASF film showed a suitable degradation rate, good hemocompatibility, and reduced cytotoxicity in vitro biocompatibility assay. In vivo rat full-thickness wound healing experiment, the ASF film exhibited effectively promote skin regeneration, wound vascularization, and restore skin function compared with the control and SF wound dressings. These results suggest that the versatile ASF film would be a potential wound dressing for clinical application.

Solid-state NMR study of spin dynamics and local disorder in smart polymers: PDMAEMA

The solid-state 1H and 13C NMR spectra as well as the 1H–13C cross-polarization upon magic angle spinning (CP MAS) kinetics were studied for poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA), i.e. a smart pH- and thermo-responsive polymer. The stereochemical content of PDMAEMA was determined from the complex shaped 13C MAS signal of CH3 group. The kinetic data were processed using the Hirschinger and Raya spin dynamics model that includes the complete scheme of rotating frame spin-lattice relaxation pathways. The general solution was adapted for the spin cluster treatment. The earlier studied experimental CP MAS kinetics data of poly [2-(methacryloyloxy)ethyl trimethylammonium chloride] (PMETAC), i.e. one of its quaternized form, were revisited and newly processed applying this model. The spin-lattice relaxation of protons in the rotating frame in PDMAEMA and PMETAC occurs in the same scale from one to tens of milliseconds. Very high anisotropy of spin-diffusion was found for both polymers. However, the local disorder of various spin sites in PDMAEMA is significantly higher than in PMETAC. It is characterized by the order parameters 0.71–0.77 and 0.87–0.91, respectively. The main chain in PDMAEMA is also more disordered and more flexible than in PMETAC.

A Three-Dimensional Silicon-Diacetylene Porous Organic Radical Polymer

A Three-Dimensional Silicon-Diacetylene Porous Organic Radical Polymer

The distribution of carbonate in apatite: The environment model

Abstract The environment model is used to describe the location of carbonate in nine carbonated apatites containing varied percentages of carbonate and Na+, K+, or NH4+ ions. Unlike the traditional model for carbonate substitution, which identifies different locations and orientations of the carbonate ion in the apatite structure, the environment model utilizes the different structural surroundings to describe the different types of carbonate. The A-type carbonate environment is assigned to channels lined only with calcium ions (A-channel configuration = Ca6) or to channels containing one Na+ or a vacancy (A′-channel configuration = Ca5Na or Ca5☐), and the B-type carbonate environment is the surroundings of the replaced phosphate ion. The assignments are made by peak-fitting the carbonate asymmetric stretch region (ν3) of the IR spectrum, following previously published criteria. These assignments lead to the conclusion that the percentage of channel carbonate (A- and A′-environments) is greater than that of B-type for each of these carbonated apatites. In general, the use of triammonium phosphate as the phosphate source in the synthesis produces apatites with larger amounts of channel carbonate (A- and A′-environments), while the use of sodium-containing phosphate reagents produces smaller amounts of channel carbonate. The environment model provides explanations for the differences within IR and NMR spectra obtained for apatites containing a range of total carbonate content. The B-type appearance of the carbonate ν3 region of the IR spectrum is found primarily in apatites containing sodium, which allows increased amounts of carbonation via co-substitution of Na+ with carbonate and creation of A′-environments with populations equal to that of B-type carbonate. The presence of ammonium or alkali metal salts with cations larger than Na+ results in the utilization of a charge-balance mechanism that produces vacancies rather than cation substitution in the channel. The carbonated apatites formed with primary utilization of the vacancy mechanism generally contain greater percentages of carbonate in the A-environment and carbonate IR spectra that contain an obvious high-frequency peak at about 1550 cm–1. The multiple peaks in the solid state 13C NMR spectra previously observed for carbonated apatite are attributed to substitution in the A-, A′-, and B-environments rather than different stereo-chemical orientations of the carbonate ion.

Characterization of pH-responsive high molecular-weight chitosan/poly (vinyl alcohol) hydrogel prepared by gamma irradiation for localizing drug release

pH-sensitive hydrogels prepared by gamma irradiation find promising biological applications, partially, in the field of localized drug liberation. Herein, optimal conditions for fabricating high-molecular-weight chitosan/polyvinyl alcohol hybrid hydrogels using gamma irradiation at 10, 25, and 30 kGy were investigated by studying the water uptake behavior, the pore size on the surface, and thermal stability. Furthermore, the crosslinking mechanism of irradiated hydrogels was examined via solid-state 13C NMR spectrum. The swelling ratio of the gamma-irradiated CS/PVA hydrogel was pH-dependent; particularly, the hybrid hydrogel exhibited high swelling ratios under acidic conditions than those under basic conditions due to the protonation of amino groups on CS-backbone in acidic environments. In addition, amoxicillin was used as a model drug in the in vitro drug release investigations in pH-simulated gastric fluid and deionized water at 37 °C. To identify the drug release mechanism, several kinetic models composing zero-order, first-order, Higuchi, Hixson–Crowell, and Korsmeyer–Peppas models were used. The findings suggested that drug release is mediated by a non-Fickian transport mechanism.Graphical

Gelation Elimination and Crystallization Inhibition by Co-Amorphous Strategy for Amorphous Curcumin

In the previous study, the development of amorphous curcumin (CUR) aimed to enhance the solubility/dissolution of CUR by disrupting its crystal lattice, but it unexpectedly showed a decreased dissolution than its crystalline counterpart on account of gel formation in its dissolution process. Whether such gelation could be eliminated by co-amorphous strategy was answered in this study. Herein, CUR by co-amorphization with chlorogenic acid (CHA) was successfully prepared using quench cooling. The formed co-amorphous material (namely CUR-CHA CM) eliminated the gelation and hence performed superior dissolution performance than crystalline/amorphous CUR. Meanwhile, it exhibited higher physical stability than amorphous CUR during dissolution as well as under long-term/accelerated conditions. To further study the such enhancement mechanism, the internal molecular interactions were investigated for CUR-CHA CM in the solid state as well as in aqueous solution. FTIR and solid-state 13C NMR spectra confirmed that intermolecular hydrogen bonds formed between CUR and CHA after co-amorphization. Furthermore, the nucleation of CUR was significantly inhibited by CHA in an aqueous solution, thus maintaining the supersaturated dissolution for a long time. The present study offers a feasible strategy to eliminate gelation and enhance stability of amorphous solids by co-amorphization and crystallization inhibition.

Metal/metal oxide-decorated covalent organic frameworks as electrocatalysts for electrocarboxylation and water splitting

A o-tolidine based triazine (TTP) covalent organic framework was prepared, and successful formation of the porous polymer network was confirmed by solid state 13C and 15N CP-MAS NMR spectra, XRD, and other morphological techniques. The synthesized TTP was decorated with transition metal/metal oxide nanoparticles (Cu, Ni, and Cu–Ni). The presence of CuO and NiO nanoparticles on the TTP matrix were confirmed by Raman spectroscopy and the PXRD analysis and different morphologies with modest changes in particle size and porosity were confirmed by FE-SEM and HR-TEM studies. As evidenced by onset potential and current density, the Cu-TTP sample exhibited excellent electrocatalytic activity for the electrochemical carboxylation of propylene oxide in the presence of CO2 and HER, whereas the Ni-TTP sample displayed excellent OER activity. The presence of pyridinic, pyrrolic, and quaternary nitrogen was attributed to the excellent catalytic activity of Cu-TTP for electrocarboxylation and HER activity, whereas secondary amine bonds and a combination of mechanisms were responsible for the increased OER activity of Ni-TTP the sample, as evidenced by XPS.

Chlorine-free extraction and structural characterization of cellulose nanofibers from waste husk of millet (Pennisetum glaucum)

This study aims to extract cellulose nanofibers (CNFs) from a sustainable source, i.e. millet husk, which is an agro-waste worthy of consideration. Pre-treatments such as mercerisation, steam explosion, and peroxide bleaching (chlorine-free) were applied for the removal of non-cellulosic components. The bleached millet husk pulp was subjected to acid hydrolysis (5% oxalic acid) followed by homogenization to extract CNFs. The extracted CNFs were characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Dynamic Light Scattering (DLS), Energy Dispersive X-ray Spectroscopy (EDX), Thermogravimetry (TG and DTG), Differential scanning calorimetry (DSC), and Solid state 13C nuclear magnetic resonance spectroscopy (solid state 13C NMR). The isolated CNFs show a typical cellulose type-I structure with a diameter of 10-12 nm and a crystallinity index of 58.5%. The appearance of the specific peak at 89.31 ppm in the solid state 13C NMR spectra validates the existence of the type-I cellulose phase in the prepared CNFs. The prepared CNFs had a maximum degradation temperature (Tmax) of 341 °C, that was 31 °C greater than raw millet husk (RMH). The outcome of the study implies that the nanofibers are prominent alternatives for synthetic fibers for assorted potential applications, especially in manufacturing green composites.

Characterization of cellulose and TEMPO-oxidized celluloses prepared from Eucalyptus globulus

Abstract Eucalyptus (Eucalyptus globulus) cellulose was isolated from wood powder by dewaxing, delignification, and subsequent 4% NaOH extraction. 2,2,6,6-Tetramethyl-piperidine-1-oxyl (TEMPO)-oxidized eucalyptus celluloses were prepared from never-dried eucalyptus cellulose (EC) in yields of 96% and 72% (based on the dry weight of EC) when oxidized with NaOCl of 5 and 10 mmol/g-EC, respectively. Their carboxy contents were 1.4 and 1.8 mmol/g, respectively, when determined by conductivity titration. The crystallinity of cellulose I for EC decreased by TEMPO-mediated oxidation, showing that the originally crystalline region in EC was partly converted to disordered regions by TEMPO-mediated oxidation. Correspondingly, the relative signal area of C6‒OH/C1 with the trans-gauche (tg) conformation attributed to crystalline cellulose I in the solid-state 13C NMR spectrum of EC decreased from 0.42 to 0.34 by TEMPO-mediated oxidation with NaOCl of 10 mmol/g-EC. TEMPO-oxidized EC prepared with NaOCl of 10 mmol/g-EC was almost completely converted into individual TEMPO-oxidized EC nanofibrils (TEMPO-ECNFs) of homogeneous widths of ∼3 nm widths and lengths of &gt;1 μm by mechanical disintegration in water. However, the TEMPO-ECNFs contained many kinks and had uneven surfaces, probably owing to significant damage occurring on the surface cellulose molecules of crystalline cellulose microfibrils during TEMPO-mediated oxidation.

Isomeric sp2-C-conjugated porous organic polymer-mediated photo- and sono-catalytic detoxification of sulfur mustard simulant under ambient conditions

Isomeric sp2-C-conjugated porous organic polymer-mediated photo- and sono-catalytic detoxification of sulfur mustard simulant under ambient conditions

Surface functionalization of mesoporous silica with maltodextrin for efficient adsorption of selective heavy metal ions from aqueous solution

A simple non-hydrothermal method for the synthesis of maltodextrin functionalized SBA-16 (SBA-16@MD) is reported for selective adsorption of heavy metal ions (HMIs) including Cd2+, Zn2+, and Cu2+ from aqueous solution. The as-synthesized SBA-16@MD is fully characterized by FESEM, EDX, HRTEM, PXRD, solid-state 13C NMR spectrum, TGA, and BET surface area measurement. The HMIs adsorption was investigated for several parameters, for example, HMIs concentration, adsorbent dose, temperature, pH, contact time, and stirring speed. The SBA-16@MD shows high values of adsorption capacity for Cd2+ (575.53 mg/g), Zn2+ (564.67 mg/g), and Cu2+ (509.73 mg/g) respectively, at pH 6. The calculated adsorption data are best fitted with the Langmuir isotherm inferring homogeneous HMIs adsorption on the surface of SBA-16@MD. The kinetic data are consistent with the pseudo-second-order and intraparticle diffusion models. The calculated thermodynamic data reveals that the nature of adsorption is endothermic and spontaneous. The high values of adsorption capacity, SBA-16@MD for Cd2+, Zn2+, and Cu2+ can be explained based on electrostatic interactions and ion exchange taking place between the SBA-16@MD surface and HMIs.

Structure and Performance of Cuticles Isolated from Wool Fibers Using Different Approaches

ABSTRACT Wool cuticles have a complex hierarchical structure and inherent resistance to general chemicals and enzymes. However, data on the structure and properties of the cuticle, which informs its processing and utilization remains scant. In this paper, we successfully isolated flaky cuticles using the ultrasonic technique from wool residues after keratin extraction with combined L-cysteine and urea (Cuticle-L + U). We then characterized and analyzed the chemical structure and thermal stability of the cuticle-L + U and compared it with the raw wool and wool cuticle isolated with the formic acid method (cuticle-FA). Our data show that the cuticle-L + U has a lower disulfide bond content compared to either raw wool or cuticle-FA. The solid-state 13C NMR spectra demonstrated that the raw wool had a higher content of α-helix structures (61.57%), while cuticle-FA (80.08%) and cuticle-L + U (52.32%) had more β-sheet or disorder structures. Differential scanning calorimetry (DSC) analysis indicated that both the cuticle-L + U and cuticle-FA have lower α-helical crystallinity compared to the raw wool, which further proved that the cuticle mainly consists of β-sheet and random coil structure. In addition, TG analysis and SDS-PAGE showed higher thermal stability and insolubility with the cuticle-L + U compared with cuticle-FA.

Chemical composition of soil organic carbon and aggregate stability along an elevation gradient in Helan Mountains, northwest China

Exploring the composition of soil organic carbon (SOC) and aggregate stability over a range of elevations is essential to evaluate the stability of SOC and the conversion of carbon source or sink of mountain ecosystems. However, information of those changes in mountain ecosystems in arid and semi-arid regions is still scarce to date. In this study, we estimated the response of SOC stability along elevation gradient (1,380–2,438 m) by combining physical protection of aggregates with chemical protection of SOC using a redundancy analysis method in the Helan Mountains, northwest China. Results showed that the dominated aggregates of Helan Mountains changed from microaggregates to macroaggregates with increasing elevation, and the mean weight diameter (MWD) in mid and high elevations (2,139–2,248 m) was significantly higher than that in low elevations (1,380–1,650 m). Solid-state 13C NMR spectra showed that O-alkyl C was the main chemical composition of SOC in the different elevations. Regression analysis indicated that with the increase of elevation, the proportion of aromatic C and aromaticity gradually increased, peaked at the mid elevation and subsequently decreased. These findings indicated that the soil structure and chemical composition of SOC are more stable in the mid elevation. Macroaggregates were positively correlated with O-alkyl C and aromatic C, while silt and clay were positively correlated with alkyl C, indicative of a lower decomposition rate of macroaggregates. The RDA analysis confirmed that aggregate stability were significant associated with SOC content and chemical composition such as O-alkyl C, indicating that the soil labile carbon pool was most related to affect aggregate stability. Our findings suggest that the Helan Mountains had a better soil structure and a more stable SOC chemical composition at mid elevation ranges. The soil structure at low elevations is sensitive to environment change; therefore, a reduction in anthropogenic disturbance is suggested for improved sustainability of the ecosystem.

Ribitol in Solution Is an Equilibrium of Asymmetric Conformations.

Ribitol (C5H12O5), an acyclic sugar alcohol, is present on mammalian α-dystroglycan as a component of O-mannose glycan. In this study, we examine the conformation and dynamics of ribitol by database analysis, experiments, and computational methods. Database analysis reveals that the anti-conformation (180°) is populated at the C3–C4 dihedral angle, while the gauche conformation (±60°) is seen at the C2–C3 dihedral angle. Such conformational asymmetry was born out in a solid-state 13C-NMR spectrum of crystalline ribitol, where C1 and C5 signals are unequal. On the other hand, solution 13C-NMR has identical chemical shifts for C1 and C5. NMR 3J coupling constants and OH exchange rates suggest that ribitol is an equilibrium of conformations, under the influence of hydrogen bonds and/or steric hinderance. Molecular dynamics (MD) simulations allowed us to discuss such a chemically symmetric molecule, pinpointing the presence of asymmetric conformations evidenced by the presence of correlations between C2–C3 and C3–C4 dihedral angles. These findings provide a basis for understanding the dynamic structure of ribitol and the function of ribitol-binding enzymes.

Tunable amine-functionalized silsesquioxane-based hybrid networks for efficient removal of heavy metal ions and selective adsorption of anionic dyes

By changing the acid scavenger from NEt3 to NaHCO3, two novel kinds of amine-functionalized silsesquioxane (SQ)-based hybrid networks (PCS-N1, PCS-N2) with different molar ratios of –NH2/NH– and NH3+ were successfully prepared via nucleophilic substitution of octa(3-chloropropyl) silsesquioxane (SQ-Cl8) with triaminoethylamine, respectively. Both networks were fully characterized by FTIR, SEM, TEM, XPS, BET, TGA, WCA and solid-state 13C and 29Si NMR spectra. No porosity was observed for these two hybrid networks; however, they exhibited good thermal stability and hydrophilicity. To our surprise, they strongly adsorb heavy metal ions due to ion exchange and coordination. The adsorption capacities for Ag+ and Hg2+ are as high as 358 ± 6 and 721 ± 5 mg g−1, respectively. More interestingly, PCS-N1 can selectively adsorb the anionic dye methyl orange with a capacity as high as 731 ± 13 mg g−1. Furthermore, they regenerate easily and offer potential for water purification. This work provides a new approach to prepare and tune amine-functionalized SQ-based networks for multipurpose.

TEMPO/NaBr/NaClO and NaBr/NaClO oxidations of cotton linters and ramie cellulose samples

Dried cotton linters and ramie cellulose samples were oxidized with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)/NaBr/NaClO and NaBr/NaClO (i.e., TEMPO-free) in water at pH 10. The carboxy contents, degrees of polymerization (DPs), X-ray diffraction patterns, and solid-state 13C NMR spectra were measured or obtained for the oxidized products with and without subsequent NaBH4 reduction. Cellulose nanofibrils were prepared from the oxidized cellulose samples by sonication in water and observed by atomic force microscopy and transmission electron microcopy. Because the cellulose molecules were depolymerized with NaBr/NaClO, the depolymerization behavior of the cellulose samples with TEMPO/NaBr/NaClO can be mainly explained by depolymerization with NaBr/NaClO (i.e., not TEMPO-related compounds or reactions). However, because C6-aldehydes formed in the disordered regions periodically present along the longitudinal direction of each cellulose microfibril, the viscosity-average DP values of the TEMPO/NaBr/NaClO-oxidized cellulose samples decreased to 200–300, while those with subsequent NaBH4 reduction exhibited much higher DP values. The nanofibrils prepared from the TEMPO/NaBr/NaClO-oxidized cellulose samples had smallest fibril heights or widths of 5–6 nm. However, significant amounts of unfibrillated bundles with heights of 10–40 mm were present in the nanofibril/water dispersions. The high carboxy contents of the TEMPO/NaBr/NaClO-oxidized cellulose samples (1.62–1.63 mmol/g) indicated that significant amounts of carboxy groups were likely present in the disordered regions, probably forming tail-like polyglucuronate chains. Solid-state 13C NMR analysis revealed that some of the glucosyl units originally with the tg C6–OH conformation were transformed to other conformations by TEMPO/NaBr/NaClO oxidation, while the crystalline C4 signal areas remained constant.

Assessment of the biochemical degradability of crop derived biochars in trace elements polluted soils

This study assesses the biochemical degradability of biochars produced from five contrasting crop residues (rice husk, olive pit, almond shell, maize straw and wood chips) in acidic soils contaminated with trace elements. For this purpose, highly and moderately acid soils were amended with each of the proposed biochars and subjected to a microbial degradation experiment under controlled conditions for 155 days. CO2 production of pure biochars fitted with a single exponential model, indicating that they only had one C pool. Among the biochars studied, those derived from olive pit and almond shell showed the greatest stability, with mean residence times (MRT) of 293 and 214 years, respectively, whereas, maize straw biochar revealed MRT of 82 years. Despite that the average MRT of all tested biochars under controlled conditions were shorter than usually reported, biochar addition increased the MRT of the recalcitrant C pool of the studied soils by a factor from 2x to 13 × . Solid-state 13C NMR spectra confirmed a partial cracking of the biochar aromatic network during the experiment due to microbial degradation or physical alteration processes. It was further noted that biochar degradability depended on its aromaticity but we have also observed that in highly acidic and metal-contaminated soils biochar is less degraded, probably due to the difficulties for microbial development at these soils.