Parth Raval

Neethu Thomas

Lama Hamdouna

Laurent Delevoye

Olivier Lafon

G N Manjunatha Reddy
email: gnm.reddy@univ-lille.fr

Boron adsorption kinetics of microcrystalline cellulose and polymer resin

Keywords: boron capture, chelate complexes, biomass valorization, solid-state NMR spectroscopy TOC

Tailoring boron-polysaccharide interactions is an important strategy for developing functional soft materials such as hydrogels, fire retardants and sorbents for environmental remediation, for example, using lignocellulosic biomass. For such applications to be realized, understanding the adsorption kinetics of borate anions on cellulose and their local structures is paramount. Here, the kinetics of boron adsorption by microcrystalline cellulose, lignin and polymeric resin are investigated and compared. Borate anions interact with the vicinal diols in the glucopyranoside moieties of cellulose to yield chemisorbed boron chelate complexes. In contrast to cellulose, technical lignin contains fewer cis-vicinal diols, and does not tend to form such chelate complexes upon treatment with the aqueous boric acid solution. The formation kinetics and stability of these chelate complexes strongly depend on nanoscale structures as well as reaction conditions such as pH and concentration of the sorbate and sorbent. Specifically, insights into the distinct boron adsorption sites were obtained by solid-state 1D 11 B magic-angle spinning NMR and the local structures and intermolecular interactions in the vicinities of boron chelate complexes are elucidated by analyzing 2D 13 C-1 H and 11 B-1 H heteronuclear correlation NMR spectra. The total boron adsorption capacity of cellulose is estimated to be in the 1.3-3.0 mg range per gram of sorbent, which is lower than the boron adsorption capacity of a poly-styrene based resin, ~17.2 mg of boron per gram of Amberlite ® IRA 743. Our study demonstrates that the local backbone and sidechain flexibility as well as the structures of polyol groups play a significant role in determining the kinetic and thermodynamic stability of chelate complexes, yielding different boron adsorption capabilities of lignocellulosic polymers.

Introduction

The valorization of various biomass feedstocks and biomass-derived polymers has emerged as one of the most viable routes to develop materials for energy, sustainability, health and environment. [1][2][3][4][5] Plant-derived lignocellulose consists of (hemi)cellulose carbohydrate polymers and lignin composites, the compositions and structures of which strongly depend on the type of biomass feedstock. Chemical conversion through pre-treatment and catalytic conversion pathways are sought to obtain valuable chemicals such as biofuels, [START_REF] Appels | Biomass Valorization to Energy and Value Added Chemicals: The Future of Chemical Industry[END_REF] platform chemicals, and biogas (or syngas). 7 Other biomass valorization routes include the development of bioplastics, pharmaceutical and healthcare materials, functional hydrogels, fabrics, fire retardants, and environmental remediation materials. [8][START_REF] Ates | Structures, and Advanced Applications of Nanocomposites from Biorenewable Resources[END_REF][10] For example, biomass-derived materials have been designed for the removal of pollutants from aqueous solutions, membranes for water filtration and oil cleanup operations. [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF][START_REF]Line shape analysis of 11 B NMR spectrum of boron-treated Amberlite IRA743[END_REF][13] Specifically, cellulose derivatives have been used as borate and phosphate adsorbents. [START_REF]Comparison of boron adsorption kinetics of Amberlite IRA743 and cellulose[END_REF][START_REF]Estimation of boron adsorption capacities of lignin, cellulose, and IRA743[END_REF][START_REF]Solid-state 1D 1 H and 13 C{ 1 H} NMR spectra of boron adsorbed resin[END_REF] Boric acid chemistry plays an important role in developing functional materials derived from lignocellulose polymers. [START_REF][END_REF][18][19][START_REF] Uddin | Hybrid Films of Chitosan, Cellulose Nanofibrils and Boric Acid: Flame Retardancy, Optical and Thermo-Mechanical Properties[END_REF][START_REF] Mohamed | Flame Retardants: Edition: 1[END_REF][START_REF] Yang | An Efficient Flame Retardant to Improve Simultaneously the Interfacial Interaction and the Flame Retardancy of EVA[END_REF][START_REF] Zhang | Improving Lignocellulose Thermal Stability by Chemical Modification with Boric Acid for Incorporating into Polyamide[END_REF] In particular, cellulose-borate interactions have been tailored to engineer wood composites, fabrics, textiles, and fire retardants by forming an impenetrable glass coating on the fire-exposed surface. [START_REF][END_REF][18][19][START_REF] Uddin | Hybrid Films of Chitosan, Cellulose Nanofibrils and Boric Acid: Flame Retardancy, Optical and Thermo-Mechanical Properties[END_REF][START_REF] Mohamed | Flame Retardants: Edition: 1[END_REF][START_REF] Yang | An Efficient Flame Retardant to Improve Simultaneously the Interfacial Interaction and the Flame Retardancy of EVA[END_REF][START_REF] Zhang | Improving Lignocellulose Thermal Stability by Chemical Modification with Boric Acid for Incorporating into Polyamide[END_REF] The molecular origins of such properties are related to the degree of boron crosslinking in cellulose polymers and the kinetics of boron adsorption, which enhance the mechanical strength of cellulose and facilitate self-healing capacity. As a result, boron-based functionalization of cellulose has been utilized in a wide range of research areas; nanocomposite hydrogels, [START_REF] Esquivel-Pena | Hybrids Based on Borate-Functionalized Cellulose Nanofibers and Noble-Metal Nanoparticles as Sustainable Catalysts for Environmental Applications[END_REF][START_REF] Zhang | Crosslinked Carboxymethyl Cellulose-Sodium Borate Hybrid Binder for Advanced Silicon Anodes in Lithium-Ion Batteries[END_REF] environmental remediation, [START_REF]Line shape analysis of 11 B NMR spectrum of boron-treated Amberlite IRA743[END_REF][START_REF] Lim | Molecular Gel Sorbent Materials for Environmental Remediation and Wastewater Treatment[END_REF][START_REF] Notley | Extraordinary Adhesion of Phenylboronic Acid Derivatives of Polyvinylamine to Wet Cellulose: A Colloidal Probe Microscopy Investigation[END_REF] and actuators/sensors. [START_REF] Nawaz | Cellulose-Based Fluorescent Sensor for Visual and Versatile Detection of Amines and Anions[END_REF][START_REF] Kim | Paper Actuators Made with Cellulose and Hybrid Materials[END_REF] Owing to its low solubility in water and tendency to interact with borate anions, cellulose can be used as a low-cost sorbent for scalable boron removal technology [START_REF] Wang | Removal Technology of Boron Dissolved in Aqueous Solutions -A Review[END_REF] . Among the boron removal technologies such as reverse osmosis, [START_REF] Rodríguez Pastor | Influence of PH in the Elimination of Boron by Means of Reverse Osmosis[END_REF] electrodialysis [START_REF] Bandura-Zalska | Desalination of Boron-Containing Wastewater at No Boron Transport[END_REF] and electrocoagulation, [START_REF] Yilmaz | Boron Removal from Geothermal Waters by Electrocoagulation[END_REF] boron capture by ion exchange [START_REF] Liao | Mesoporous Polymer Nanosponges Immobilized with Functional Polyols for Rapid Removal of Boric Acid and Organic Micropollutants[END_REF][START_REF] Hilal | Boron Removal from Saline Water: A Comprehensive Review[END_REF][START_REF] Kluczka | Boron Adsorption Removal by Commercial and Modified Activated Carbons[END_REF][START_REF] Tang | Development of Novel Diol-Functionalized Silica Particles toward Fast and Efficient Boron Removal[END_REF] is the most efficient approach due to its high adsorption capacity, economic viability and recyclability. Examples of anion exchange resins include Diaion CRB02 ® and Dowex ® (Figure S1), which are capable of achieving the total boron adsorption capacity of tens of milligrams of boron per gram of resin. [START_REF] Ezechi | Boron in Produced Water: Challenges and Improvements: A Comprehensive Review[END_REF][START_REF] Nasef | Polymer-Based Chelating Adsorbents for the Selective Removal of Boron from Water and Wastewater: A Review[END_REF] Within the recent past, polysaccharides [START_REF] Peters | Interactions between Boric Acid Derivatives and Saccharides in Aqueous Media: Structures and Stabilities of Resulting Esters[END_REF] and biopolymers [START_REF] Liao | Synthesis of Glycopolymer Nanosponges with Enhanced Adsorption Performances for Boron Removal and Water Treatment[END_REF][START_REF] Miyazaki | Interaction of Borate and Phenylboronate with the Cross-Linked Dextran Gel Matrix[END_REF] such as chemically modified cellulose, [START_REF] Wei | Functionalization of Regenerated Cellulose Membrane via Surface Initiated Atom Transfer Radical Polymerization for Boron Removal from Aqueous Solution[END_REF] chitosan, [START_REF] Bursalı | Synthesis of Chitosan Beads as Boron Sorbents[END_REF] and lactose-modified chitosan have been employed as a low-cost alternative for boron removal from aqueous solutions. [START_REF] Sacco | Boric Acid Induced Transient Cross-Links in Lactose-Modified Chitosan (Chitlac)[END_REF][START_REF] Li | Boronate Affinity Materials for Separation and Molecular Recognition: Structure, Properties and Applications[END_REF] For such applications to be realized, a more elaborative understanding of the surface boron adsorption kinetics as well as the borate-carbohydrate intermolecular interactions is required to be systematically analyzed and compared.

The monomeric D-glucose units of cellulose are covalently linked by β(1→4)-glycosidic bonds, whereby all hydrogen atoms are in an axial position, and hydroxyl groups are in an equatorial position. The hydroxyl groups provide the surface and interfacial reactivity, for example, via hydrogen bonding interactions, leading to different degrees of crystallinity, structural and mechanical properties. Due to these inter-and intramolecular interactions and conformational flexibility, [START_REF] Tongye | Conformational Flexibility of Soluble Cellulose Oligomers: Chain Length and Temperature Dependence[END_REF][START_REF] Anggara | Identifying the Origin of Local Flexibility in a Carbohydrate Polymer[END_REF] a substantial amount of cellulose and the cellulose-derived materials exist as amorphous materialsthough crystallites of nanocellulose with dimensions of ~5 nm have been reported [START_REF] Park | Cellulose Crystallinity Index: Measurement Techniques and Their Impact on Interpreting Cellulase Performance[END_REF] making it difficult to characterize these materials by long-range techniques.

Nevertheless, X-ray diffraction (XRD), and Fourier transform (FT) Raman spectroscopy have been employed to gain insight into the degree of order and crystallinity in cellulose. [START_REF] Schenzel | New Method for Determining the Degree of Cellulose I Crystallinity by Means of FT Raman Spectroscopy[END_REF] As a local probe, solid-state (ss)NMR spectroscopy has previously been applied to gain insight into the local structures of cellulose and cellulose-lignin interactions in neat cellulose and biomass-derived lignocellulose compounds. [START_REF] Earl | High Resolution, Magic Angle Sample Spinning 13C NMR of Solid Cellulose I[END_REF][START_REF] Kolodziejski | Carbon-13 Nuclear Magnetic Resonance Spectrometry with Cross Polarization and Magic-Angle Spinning for Analysis of Lodgepole Pine Wood[END_REF][START_REF] Lesage | Through-Bond Carbon-Carbon Connectivities in Disordered Solids by NMR[END_REF][START_REF] Bardet | 13C High-Resolution Solid-State NMR for Structural Elucidation of Archaeological Woods[END_REF][START_REF] Foston | Advances in Solid-State NMR of Cellulose[END_REF][START_REF] Gutmann | Dynamic Nuclear Polarization Signal Amplification as a Sensitive Probe for Specific Functionalization of Complex Paper Substrates[END_REF][START_REF] Terrett | Molecular Architecture of Softwood Revealed by Solid-State NMR[END_REF][START_REF] Kumar | The Surface Chemistry of a Nanocellulose Drug Carrier Unravelled by MAS-DNP[END_REF][START_REF] Cresswell | Importance of Water in Maintaining Softwood Secondary Cell Wall Nanostructure[END_REF][START_REF] Berruyer | Spatial Distribution of Functional Groups in Cellulose Ethers by DNP-Enhanced Solid-State NMR Spectroscopy[END_REF] Within the recent past, surface-enhanced dynamic nuclear polarization (DNP) NMR spectroscopy has been used to characterize functional groups on the surface of cellulose fibers in paper materials, [START_REF] Gutmann | Dynamic Nuclear Polarization Signal Amplification as a Sensitive Probe for Specific Functionalization of Complex Paper Substrates[END_REF] drug-functionalized TEMPO-oxidized cellulose nanofibrils, [START_REF] Kumar | The Surface Chemistry of a Nanocellulose Drug Carrier Unravelled by MAS-DNP[END_REF] and cellulose ethers. [START_REF] Berruyer | Spatial Distribution of Functional Groups in Cellulose Ethers by DNP-Enhanced Solid-State NMR Spectroscopy[END_REF] Here, we investigate the kinetics of boron adsorption in cellulose, lignin and polymeric resin material (Amberlite ® IRA-743) are studied and compared. The local structures of different boron adsorption sites and the intermolecular interactions between borate anions and cellulose polymers are characterized by 1D 11 B MAS and 2D 1 H-11 B correlation NMR spectroscopy. 1D 11 B MAS NMR of boron-adsorbed cellulose provides information on the surface boron adsorption kinetics, chelate complex formation and provides an estimate of the boron adsorption capacity of cellulose. The roles of external factors such as sorbate and sorbent concentration, and pH of the solution on the boron adsorption capacity are examined. SSNMR results are correlated and complemented with FTIR spectroscopy. The impact of boron uptake on the local structures and conformations of the cellulose is investigated by 2D 13 C-1 H and 11 B-1 H heteronuclear correlation NMR experiments. These results indicate that the cellulose exhibits a greater tendency towards boron adsorption at basic (pH = 11 or greater) conditions, due to the formation of kinetically stable bis-chelate complexes than the labile mono-chelate complexes. [START_REF] Peters | Interactions between Boric Acid Derivatives and Saccharides in Aqueous Media: Structures and Stabilities of Resulting Esters[END_REF][START_REF] Peters | G4-Quartet•M+ Borate Hydrogels[END_REF][START_REF] Peters | A G4•K+ Hydrogel Stabilized by an Anion[END_REF][START_REF] Wicklein | Tuning the Nanocellulose-Borate Interaction to Achieve Highly Flame Retardant Hybrid Materials[END_REF] The cellulose fibers adsorb in the range of 1.3 to 3.0 mg of boron per gram of sorbate depending on the concentration of the sorbent, but the polymeric resin Amberlite ® IRA-743 can uptake up to 17.2 mg of boron per gram of resin. A detailed ssNMR study of these materials provides a molecular level understanding of borate-sorbate interactions and the boron adsorption kinetics functionalized by polyol moieties, while unraveling the vital role of the backbone structures, molecular confirmation, and the densities of polyol groups in the total boron adsorption properties of cellulose fibers, thus demonstrating the potential for developing inexpensive and environmentally friendly alternatives for boron capture.

Experimental Methods

Materials and sample preparation.

Cellulose (20 m), Amberlite ® IRA743, lignin, NaOH, and boric acid were obtained from Sigma Aldrich (analytical grade) and used as received. To understand the role of external stimuli on the boron adsorption properties of lignin and cellulose, boron uptake reactions were carried out under different conditions (i) by varying the boric acid concentration in aqueous solution and (ii) by modifying the pH of the solution. In the first case, vials containing 80 mg of cellulose were treated with 1 ml of aqueous solutions containing 5.8, 11.6, 22.7, 33.8, 67.2, and 134.2 g/L of boric acid. This concentration range (5.8 -134.2 g/L) was chosen based on the theoretical maximum boron chelation obtained when all hydroxyl groups in cellulose structure are involved. The pH of these aqueous solutions was maintained constant at ~11 by adding 1 M NaOH solution and progressively monitoring the basicity using a pH meter.

In the second case, vials containing 80 mg of cellulose were treated with 1 ml of 33.5 g/L boric acid solutions, whereby the pH values were adjusted to 5, 7, 9, 11 and 13 by adding 0, 4, 120, 460, and 650 µL of 5 M NaOH, respectively. All vials were subjected to vortex agitation for 5 minutes, and the resulting milky-white solutions were allowed to be set at room temperature overnight.

Followed by this, centrifugation (12000 rpm) was carried out for 10 min, and the supernatants were pipetted out. The boron-adsorbed cellulose pellets (solid residue) were extracted, placed in a petri dish, and dried in the hot air oven (72 °C) overnight. The dried samples were collected and stored for characterization by various solid-state NMR spectroscopy methods. Amberlite ® IRA743 was used as a control material for the quantification of adsorbed boron by cellulose. For the vials containing 80 mg of Amberlite ® IRA-743 resins each, boric acid solution (1 mL) with different concentrations of 5.8, 11.6, 22.8, 33.8, 67.3, and 134.8 g/L was added. The pH of the solutions was maintained at 11, consistently with the boron adsorption conditions used for the cellulose. The resultant precipitate was soaked overnight, then centrifuged (12000 rpm), and the precipitate was collected and dried at 72 °C.

Solid-state NMR spectroscopy.

To distinguish between the different boron adsorption sites and to elucidate the local structures of chemisorbed chelate complexes, multinuclear ( 1 H, 13 C, and 11 B) 1D and 2D MAS NMR experiments were performed. All spectra were acquired at room temperature on a 9.4 T Bruker Avance Neo NMR spectrometer (Larmor frequencies were 1 H = 400 MHz, [START_REF] Zhang | Improving Lignocellulose Thermal Stability by Chemical Modification with Boric Acid for Incorporating into Polyamide[END_REF] Na = 159 MHz, 11 B = 128 MHz, and 13 C = 100 MHz, respectively) using a doubleresonance 4 mm H-X MAS probehead. In each case, 50 mg of the solid material was packed into a 4 mm (outer diameter) zirconia rotor and fitted with Kel-F ® caps. 1D 11 B NMR spectra were acquired at 8 kHz MAS using high power decoupling (SPINAL-64) pulse sequence for proton decoupling. [START_REF] Fung | An Improved Broadband Decoupling Sequence for Liquid Crystals and Solids[END_REF] All 1D [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B MAS NMR spectra were acquired with a dedicated 4 mm H-X probehead to eliminate the strong boron background signal (i.e., boron-free). The nutation frequency was 100 kHz corresponding to a 90-degree pulse length of 2.5 µs. The 1D spectra result from averaging 512 transients with a recycle delay of 5 s, corresponding to a total experimental time of 43 min.

The 11 B shifts were calibrated using sodium borohydride (NaBH4) as an external reference ( 11 B shift at -42.06 ppm). [START_REF] Łdziana | NMR Chemical Shifts of 11B in Metal Borohydrides from First-Principle Calculations[END_REF] 1D [START_REF] Zhang | Improving Lignocellulose Thermal Stability by Chemical Modification with Boric Acid for Incorporating into Polyamide[END_REF] Na MAS NMR spectrum of boric acid-treated cellulose was acquired using 160 co-added transients using single-pulse experiment with 8 kHz MAS frequency, whereby 90-degree pulse duration was 3 µs and the recycle delay was set to 3s. To elucidate the local structures of boron chelate complexes, cross-polarization (CP) based 2D [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B-1 H heteronuclear correlation (HETCOR) NMR spectra of the cellulose-borate sample were acquired and analyzed.

The nutation frequencies of 1 H and 11 B were 100 kHz and 84.5 kHz. The 2D 11 B-1 H HETCOR spectrum was acquired with a CP contact time of 1 ms, and a high-power decoupling (DUMBO) pulse sequence was applied in the indirect 1 H dimension. [START_REF] Lesage | Experimental Aspects of Proton NMR Spectroscopy in Solids Using Phase-Modulated Homonuclear Dipolar Decoupling[END_REF] The spectra were acquired with 128 t1 increments in the indirect dimension, and 16 transients were co-added with a recycle delay of 3 s.

To further corroborate these experiments, solid-state 1 H-detected cross-polarization (CP) based 1 H-13 C heteronuclear correlation (HETCOR) NMR spectra of cellulose-borate and neat cellulose were acquired and compared. [START_REF] Wiench | Chemical Shift Correlation NMR Spectroscopy with Indirect Detection in Fast Rotating Solids: Studies of Organically Functionalized Mesoporous Silicas[END_REF] These spectra were acquired at 18.8 T Bruker Avance Neo NMR spectrometer at a spinning frequency of 50 kHz using a double-resonance H-X 1.3 mm MAS probehead. 2D 1 H-13 C HETCOR spectra were recorded with 128 t1 increments in the indirect dimension, each by co-adding 128 and 96 scans for cellulose-borate and neat cellulose, respectively, with a recycling delay of 5 s.

For boron adsorbed polymeric resin (Amberlite ® IRA743), all 1D and 2D MAS NMR spectra were acquired at 18.8 T Bruker Avance Neo NMR spectrometer (Larmor frequencies of 1 H = 800.1 MHz, 11 B = 257 MHz and 13 C = 201 MHz) with 50 kHz MAS using a double-resonance H-X 1.3 mm MAS probehead. 1D 1 H MAS spectra were acquired with 100 kHz r.f. pulses by co-adding 16 transients with an interscan delay of 4 s. 1D 11 B MAS spectra were acquired with 320 co-added transients with an interscan delay of 3s. Similar acquisition was carried out with an empty rotor under identical experimental conditions, in order to suppress the background peak by subtracting this signal with the 11 B MAS spectrum of the boron adsorbed resin. For 1D 13 C{ 1 H} CP-MAS spectra, 2048 co-added transients were acquired with 0.2 ms and 4 ms of CP contact times. Solidstate 1 H-detected cross-polarization based 1 H-13 C heteronuclear correlation (HETCOR) NMR spectra of boron adsorbed Amberlite ® IRA743 spectra were recorded with 106.4 kHz and 162.3 kHz nutation frequencies of 1 H and 13 C, respectively. The 2D 1 H-13 C CP-HETCOR NMR spectrum was acquired with a CP contact time of 4 ms, using 180 t1 increments in the indirect dimension, each with 64 transients using a recycle delay of 3 s. For 2D [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B-1 H CP-HETCOR NMR spectra were acquired with 1 ms and 4 ms of CP contact times. The indirect dimension was acquired by adding 56 t1 increments, each with 64 transients using an interscan delay of 3 s. The 2D 1 H-1 H double-quantum (DQ) single-quantum (SQ) spectra were acquired using the Back-to-Back (BaBa) sequence with one rotor period. For the indirect DQ dimension, 115 t1 increments were acquired, each by co-adding 16 transients, using States method to achieve sign discrimination.

Since boron-treated lignin does not precipitate, we carried out [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B liquid state NMR experiments on lignin suspended in a pH-controlled boric acid solution. Approximately 40 L of the liquid sample was packed into a 4mm zirconia rotor and closed with a plastic lid and Kel-F ® drive cap.

The 11 B NMR spectra were acquired with the same experimental parameters used for cellulose but under static conditions.

Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Perkin

Elmer FTIR Spectrum two UATR spectrometer was used to acquire IR spectra of neat and boricacid treated cellulose powders. The spectra were acquired with 10 scans with a spectral resolution of 0.5 cm -1 .

Results and discussion

Upon addition of boric acid solution to lignin, no precipitate was observed for a wide range of boric acid concentrations (5.8-33.8 g/L) and pH values (7.0-11.0). Therefore, the lignin suspension in boric acid solution was investigated by liquid-state 11 B NMR experiments. These results are presented in supporting information (Figure S2). In comparison, cellulose is insoluble in water, and only sparingly soluble in a buffer solution, leading to the precipitation of borate-sorbent complexes, which were filtered, dried, and characterized using solid-state FT-IR and NMR spectroscopy techniques (vide infra). The boron adsorption kinetics of cellulose and polymer resin Amberlite ® IRA743 were separately studied and compared, as discussed below.

Distinct boron binding sites in cellulose.

In order to understand the boron adsorption kinetics of cellulose, it is essential to distinguish between different boron adsorption sites and estimate their relative populations. Studies have shown that the borate anion interacts with polyhydroxy groups by forming chelate complexes. [START_REF] Reddy | Nanoscale Surface Compositions and Structures Influence Boron Adsorption Properties of Anion Exchange Resins[END_REF][START_REF] Yoshimura | Complexation of Boric Acid with the N-Methyl-D-Glucamine Group in Solution and in Crosslinked Polymer[END_REF][START_REF] Miyazaki | 11 B NMR Investigation of the Complexation Behavior of Borate with Polysaccharides in Aqueous Solution[END_REF][START_REF] Peters | G4-Quartet•M+ Borate Hydrogels[END_REF][START_REF] Peters | A G4•K+ Hydrogel Stabilized by an Anion[END_REF][START_REF] Wicklein | Tuning the Nanocellulose-Borate Interaction to Achieve Highly Flame Retardant Hybrid Materials[END_REF] The equatorial hydroxyl groups in the glucopyranose ring are open to cross-linking, i.e. interchain dimerization, reactions between the adjacent cellulose or directly coordinate to borate anions to form chelate complexes as shown in the inset of Figure 1.

Solid-state ATR-FTIR spectra of neat and boric acid-treated cellulose show identical vibrational frequencies (Figure S3). Next, we applied ssNMR spectroscopy to analyze distinct boron adsorption sites on cellulose polymers. Owing to its high natural abundance (80.1%) and moderateto-low quadrupolar coupling constants (0.2 MHz ≤ CQ ≤ 2.5 MHz), [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B NMR is a sensible probe to detect the chelate complexes of cellulose. Specifically, the isotropic chemical shifts (iso) and CQ values of the 11 B sites provide insight into the coordination geometry around boron atoms. [START_REF] Hušák | Determining the Crystal Structures of Peptide Analogs of Boronic Acid in the Absence of Single Crystals: Intricate Motifs of Ixazomib Citrate Revealed by XRPD Guided by Ss-NMR[END_REF] It has been shown that tetra-coordinated boron atoms exhibit narrow peaks due to low quadrupolar coupling interactions and highly symmetric chemical bonding environments, as compared to tricoordinated boron atoms. S4). This material exhibits well-resolved [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B signals at 1.9 and 6.0 ppm, which are attributed to distinct boron adsorption sites. Based on the 11 B signal assignments of tetrahedrally coordinated borate anions in chelate complexes presented in the literature, 66,71 the 11 B intensities at 1.9 ppm and 6 ppm are attributed to the different boron chelate (most likely mono-and bis-chelate) complexes, which will be further corroborated by analyzing 2D correlation NMR spectra (vide infra). The 11 B peak at ~18 ppm is due to tri-coordinated boron atoms in the unreacted boric acid. [START_REF] Kobera | Structure and Distribution of Cross-Links in Boron-Modified Phenol-Formaldehyde Resins Designed for Soft Magnetic Composites: A Multiple-Quantum 11 B-11 B MAS NMR Correlation Spectroscopy Study[END_REF] Specifically, the high-resolution [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B MAS spectrum obtained at 18.8 T and 50 kHz MAS enabled the different boron signals to be distinguished and identified.

For the 11 B MAS spectra acquired at 18.8 T and 9.4 T, the lineshape fitting analysis is carried out in order to deconvolute the peaks corresponding to the distinct tri-and tetra-coordinated boron atoms in boron chelate complexes and boric acid components. [START_REF] Hušák | Determining the Crystal Structures of Peptide Analogs of Boronic Acid in the Absence of Single Crystals: Intricate Motifs of Ixazomib Citrate Revealed by XRPD Guided by Ss-NMR[END_REF] Using this analysis, the δiso, CQ and asymmetry (ηQ) parameters were obtained and compared (Table 1). These results suggest that the borate anions are chemisorbed on cellulose in the form of chelate complexes, which are expected to be stabilized by the counter cations such as Na + ions that originate from the NaOH used for the regulation of pH. It is further confirmed by acquiring 1D solid-state [START_REF] Zhang | Improving Lignocellulose Thermal Stability by Chemical Modification with Boric Acid for Incorporating into Polyamide[END_REF] Na NMR spectra of boric-acid-treated cellulose (Figure S5), which exhibit a signal centered at ~5 ppm, indicating the presence of Na + species. Next, we analyzed the boron adsorption kinetics by means of chelate complexes on carbohydrate polymer surfaces/interfaces as a function of boric acid concentration between 5.8 and 134.2 g.L -1 . This concentration range is chosen based on the theoretical maximum that postulates all the hydroxyl groups in the cellulose are available and involved in the formation of boron chelate complexes. Figure 2a compares the 11 B{ 1 H} MAS NMR spectra of boric acid treated cellulose acquired at 9.4 T using a dedicated 4 mm H-X probehead to eliminate the background [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B signal, whereby an increase in the peak intensities at 1.9 ppm and 5.9 ppm as a function of increasing boric acid concentration is evidenced. It is noteworthy that the kinetically stable mono-chelate complexes are formed at much faster rate than the thermodynamically stable bis-chelate complexes. To test this, we carried out 11 B{ 1 H} cross-polarization (CP) MAS NMR spectra of boron adsorbed cellulose powder with different CP contact times in the range of 0.2 ms to 8 ms, and the CP intensity build as a function of contact time is plotted and compared for the different chelate complexes (SI, Figure S6). Both mono-and bis-chelate complexes exhibited maximum CP intensity at 1-2 ms of contact time. Due to the presence of labile hydroxyl groups in mono-chelate complexes, they are open to exchange reactions when the boron adsorbed cellulose is dissolved in water and buffer solutions with other metal cations (Na + ions used for adjusting the pH), as will be further confirmed by subsequently washing the boron adsorbed cellulose material with Millipore water (vide infra). The bis-chelate complexes are formed by covalently crosslinking two cellulose fibrils, thus expected to be thermodynamically more stable. The relative populations of these two complexes (Figure 2b) strongly depends on the pH dependent speciation of borate anions and boric acid moieties, as discussed below (vide infra). However, the signal corresponding to boric acid (5-15 ppm, due to large quadrupolar interaction) was observed at 18.6 ppm, the intensity of which increased as the boric acid concentration increased. It indicates that there exists some degree of unreacted boric acid when the material is treated with excess of boric acid (20 g/L or higher) due to the saturation of boron adsorption sites on the cellulose sorbent. To further confirm that the mono-chelate complexes are open to exchange reactions and boric acid is phase separated and does not involve in the formation of chelate complexes, we subsequently washed the resin samples. For the dried powders of boron adsorbed cellulose obtained before and after washing with water, the 11 B MAS NMR spectra were acquired and compared (SI, Figure S7). For the rinsed, washed and dried boron adsorbed cellulose, [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B peak intensities associated with the mono-chelate and boric acid are substantially reduced, however, the [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B peak of the bischelate complex is largely retained. These results confirm that the unreacted boric acid and the labile mono-chelate complexes are open to exchange reactions, which are washed away during this process, allowing the more stable bis-chelate complexes to be retained on the cellulose. The δiso, CQ, and ηQ values of the [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B signals associated with the boron chelate and boric acid sites did not change substantially (Table 1 and SI Table S1), as function of increased boric acid concentration. Subtle changes in the CQ values between 2.1 and 2.4 MHz for the boric acid signal is within the error bars (± 0.1 MHz). These values are consistent with tri and tetra-coordinating boron sites reported in the previous studies. [START_REF] Hušák | Determining the Crystal Structures of Peptide Analogs of Boronic Acid in the Absence of Single Crystals: Intricate Motifs of Ixazomib Citrate Revealed by XRPD Guided by Ss-NMR[END_REF][START_REF] Bishop | Determination of the Mode and Efficacy of the Cross-Linking of Guar by Borate Using MAS 11 B NMR of Borate Cross-Linked Guar in Combination with Solution 11 B NMR of Model Systems[END_REF][START_REF] Reddy | Magic-Angle Spinning NMR Spectroscopy Provides Insight into the Impact of Small Molecule Uptake by G-Quartet Hydrogels[END_REF] However, the tendency of forming bis-chelate complexes is reduced with the increasing boric acid concentration, and the formation of monochelate complexes is more favored (Figure 2). This is because the formation of bis-chelate complexes requires two cellulose units along the polymeric cellulose chain (or between the adjacent polymer chains) to be cross-linked, which is sterically more demanding than the monochelate complexes that can adsorb on the same cellulose monomer. At dilute concentrations of borate anions, ≤ 15 g/L, both of these sites are accessible for the chemisorption of borate anions.

When the concentration of boric acid is > 15 g/L, mono-chelate complexes are sterically more accessible than the bis-chelate complexes. In contrast to this, synthetic polymer resins such as Amberlite ® IRA743 designed for boron adsorption take into account these structural factors to enhance the boron uptake, whereby the flexible surface-functionalized polyhydroxy groups (cis diols) attached to the polymer backbone favor the formation of bis-chelate complexes, which are thermodynamically more stable than the mono-chelate complexes. [START_REF] Reddy | Nanoscale Surface Compositions and Structures Influence Boron Adsorption Properties of Anion Exchange Resins[END_REF] As such, the natural cellulose structure lacks cis-diols, which limits its tendency to form a bis-chelate complex. In addition, the role played by the concentration of borate anions (B(OH) -) over the boric acid (H3BO3) in the formation of these complexes cannot be merely ignored.

Impact of pH-dependent boric acid speciation on boron adsorption kinetics.

Having analyzed the role of boric acid concentration, we then examined the impact of pH on the total boron adsorption properties of cellulose. In an aqueous solution, boric acid act as a Lewis acid , forming a Lewis acid base adduct [B(OH)4] -with hydroxyl groups as depicted in Equation 1.

H 3 BO 3 + H 2 O ⇄ B(OH) 4 -+ H + 1
Due to the weak Lewis acidity of boric acid, the boron speciation in the form of boric acid and borate anions in an aqueous medium depends strongly on the pH. [START_REF] Nasef | Polymer-Based Chelating Adsorbents for the Selective Removal of Boron from Water and Wastewater: A Review[END_REF] The acid dissociation constant (pKa) of boric acid was found to be ~9.14, i.e., at pH = 9.14, the boric acid and borate anion are in equilibrium. At a pH greater than 9.14, the tetrahedral borate anion [B(OH)4] -dominates, while at a pH lower than 9.14, trigonal boric acid B(OH)3 predominates. However, it has been shown that borate anions are preferred over boric acid in order to interact with diols. [START_REF] Bishop | Determination of the Mode and Efficacy of the Cross-Linking of Guar by Borate Using MAS 11 B NMR of Borate Cross-Linked Guar in Combination with Solution 11 B NMR of Model Systems[END_REF] It prompted us to study the boron adsorption by cellulose as a function of pH. to the chelate complexes are barely detected. At pH = 7, the [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B signal (1.9 ppm) associated with the mono-chelate complex becomes evident, and its intensity increases as the pH value increase up to 13. A similar trend is observed for the [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B signal of the bis-chelate (5.9 ppm) complex. The 11 B peaks of mono-chelate and the bis-chelate complexes have reached a maximum intensity at pH = 13. This could be due to deprotonation of hydroxyl groups in aqueous alkali [START_REF] Bialik | Ionization of Cellobiose in Aqueous Alkali and the Mechanism of Cellulose Dissolution[END_REF] and the highly abundant borate anion at alkaline pH, allowing it to interact with diols of cellulose. In contrast to the boron atoms in chemisorbed chelate complexes, the [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B signal intensity of the free boric acid decreases at a relatively higher pH, i.e., a more alkaline medium, because the equilibrium shifts towards borate anion, which facilitates the formation of chelate complexes. These results are presented in SI (Table S3). C, and [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B peaks based on the through-space dipolar-coupled1 H-11 B and 1 H-13 C sites. The linecut row or column 1 H-spectra at specific 13 C and 11 B chemical shifts are presented in SI (Figure S10). In the 2D 1 H-13 C HETCOR spectrum (Figure 4a), the 13 C peaks in the vertical axis at 104. 4b). Specifically, the 11 B signal at 1.9 ppm correlated to H6 at 3.7 ppm and was assigned to the mono-chelate complexes, and the 11 B signal at 5.9 ppm correlated to H6 at 3.

Structure elucidation of boron chelate complexes. Although 1D

Boron adsorption capacities of lignin, cellulose, and Amberlite ® IRA743.

To understand the role of backbone structures and polyol groups on the total boron adsorption capacity, we studied and compared the boron adsorption properties of structurally different polymers such as cellulose, lignin, and synthetic resin. A key limitation associated with technical lignin as a sorbent for boron capture is due to the structural constraints, including the aromatic rings and the presence of phenolic hydroxyl, aliphatic hydroxyl, benzyl alcohol, noncyclic benzyl ether, carbonyl groups, and methoxyl groups in lignin, which makes it readily soluble in water. [START_REF] Katahira | A Brief Introduction to Lignin Structure[END_REF] In synthetic polymer resin, the high density of hydroxyl groups in NMDG moieties and low solubility of polystyrene backbone are responsible for the enhanced boron uptake. In addition, the amine functional group in Amberlite ® IRA743 plays an essential role in the efficient chelating of boric acid by neutralizing the protons released by the formation of chelate complexes, leading to NH3 + groups that may also charge balance the borate anions. For Amberlite ® IRA743, the 11 B signal at 10.2 ppm and 6.0 ppm corresponds to bis-chelate, and mono-chelate complexes, [START_REF] Sasaki | Contribution of Boron-Specific Resins Containing N-Methylglucamine Groups to Immobilization of Borate/Boric Acid in a Permeable Reactive Barrier Comprising Agglomerated MgO[END_REF] and the signal at 19.0 ppm is due to the free boric acid. Detailed lineshape analysis and their corresponding parameters are presented in Figure S11 and Table S4.

In addition, the peak at 2.0 ppm is expected to originate from tetrahedrally coordinated free borate anions with counter cations in the sample. [START_REF] Sasaki | Contribution of Boron-Specific Resins Containing N-Methylglucamine Groups to Immobilization of Borate/Boric Acid in a Permeable Reactive Barrier Comprising Agglomerated MgO[END_REF] At a controlled pH ~11, the boron uptake of Amberlite ® IRA743 increases as the boric acid concentration increases from 5.8 to 134.2 g.L - (SI, Figure S12), similar to that observed for cellulose material.

Figure S13 (SI) presents kinetics of boron adsorption plots by means of normalized

intensities of the mono-chelate and bis-chelate complexes as a function of boric acid concentration (g/L), indicating the different trends in the formation of distinct boron chelating complexes in cellulose and resin. For example, the bis-chelate complex is predominantly observed in Amberlite IRA-743 at low boric acid concentrations, whereas the fractions of the mono-chelate complex are favored in cellulose. This is due to the different polymeric backbone structures as well as the densities of polyol groups combined. Therefore, the Amberlite and cellulose materials exhibit different boron-adsorption properties (Figure 5b) due to different affinity interactions. As indicated in Figure 5b, at low boric acid concentrations (5.0 -33.8 g/L), cellulose exhibits relatively low boron adsorption than the Amberlite ® IRA743, which is intuitive as the structure of Amberlite ® IRA743 is particularly engineered to have a strong affinity towards boron species.

Upon further increase in the boric acid concentration (greater than 33.8 g/L), the sum of the normalized intensity of the mono-chelate complex and the bis-chelate complex in cellulose increases. Because of this, cellulose and Amberlite ® IRA743 exhibit different boron adsorption kinetics. However, both Amberlite ® IRA743 and cellulose showed a saturation in the boron adsorption when the boron concentration reached ~65 g/L. Cellulose captures boron by forming less stable mono-chelate complexes as major species, whereas Amberlite ® IRA743 captures boron by forming stable bis-chelate complexes as major species (Figure S14). This indicates that the surface-functionalized NMDG hydroxyl groups in Amberlite ® IRA743 are key to the enhanced boron adsorption properties. However, understanding the molecular level origins of enhanced boron adsorption by Amberlite ® IRA743 necessitates a detailed analysis of local structures of distinct boron adsorption sites. To gain insight into the local chemical environments of the different boron adsorption sites in Amberlite ® IRA743, we next carried a detailed 1D 1 H, 13 C, [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B MAS and 2D 1 H-1 H, [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B-1 H, 1 H-13 C correlation NMR experiments. In the 1 H one pulse NMR spectrum (SI, Figure S15a) of boron-adsorbed resin, distribution of peaks in the 1-6 ppm range correspond to the overlapping contribution from the -CH and CH2, -NCH3 and -NCH2 groups of pendant NMDG sidechains. In addition, the broad distribution of peaks centered at 4.5 ppm is attributed to the hydroxyl groups of the surface functionalized NMDG moieties. The broad peak in the 6-7 ppm is due to the aromatic protons associated with the polystyrene divinylbenzene backbone moieties. By comparison, the 13 C{ 1 H} cross-polarization NMR spectra acquired with the different CP contact times (SI, Figure S15b) allowed the directly bonded -CH and the quaternary carbon atoms to be distinguished. In addition, a detailed analysis of 2D 1 H-1 H double-quantum-single-quantum (DQ-SQ) correlation spectrum is presented in SI (Figure S16), which provides information on the through-space and dipolar coupled proton pairs in the NMDG groups and the polystyrene divinylbenzene backbones.

2D 1 H-13 C correlation experiments provide enhanced resolution, enabling the 1 H peaks corresponding to the different moieties to be identified and analyzed. In the 2D 13 C-1 H HETCOR spectrum of boron-adsorbed resin (Figure 6a), correlation between 13 C signals at 30-50 ppm and of cellulose materials, owing to the structural/conformational flexibility associated with both pyranose rings and pendent NMDG sidechains. [START_REF]Estimation of boron adsorption capacities of lignin, cellulose, and IRA743[END_REF][START_REF]Solid-state 1D 1 H and 13 C{ 1 H} NMR spectra of boron adsorbed resin[END_REF] To compare the relative boron-adsorption capacities of cellulose and Amberlite ® IRA743, the peak integrals of mono-and bis-chelate complexes obtained from the deconvolution of 1D 11 B NMR spectra of Amberlite ® IRA743 were analyzed and compared. A detailed explanation of the estimation of the boron-adsorption capacity is given in SI (Section 15, Figure S14, Table S5).

Based on this analysis, the boron-adsorption limit of the cellulose was estimated to be about 1.3-3.0 mg per gram of cellulose, as compared to ~17.2 mg/g in Amberlite ® IRA743 resin, when treated with a boric acid solution of concentration of 5.8 g/L at pH = 11. The reason behind employing a 5.8 g/L boric acid concentration is to ensure the minimal presence of free boric acid, and basic pH favors the formation of borate anions. For the materials used in this study, boron adsorption properties of lignin, cellulose, and Amberlite IRA743 are presented in Tables 3 andS5. Specifically, cellulose exhibits higher boron uptake than lignin owing to a flexible backbone structure. [START_REF] Tongye | Conformational Flexibility of Soluble Cellulose Oligomers: Chain Length and Temperature Dependence[END_REF] Besides that, the higher concentration of polyhydroxyl groups is the key factor causing the better boron adsorption in Amberlite ® IRA743. Nonetheless, a detailed understanding of cellulose-borate interactions and the related boric acid chemistry are expected to offer far-reaching insights for developing other functional materials, such as cellulose-based hydrogels, remediation, and fire retardants.

FT-IR spectra of neat cellulose and boron-treated cellulose

Figure S2 presents the FT-IR spectra of untreated and boric acid-treated cellulose. The vibrational band at (1) ~3300 cm -1 is attributed to the free O-H stretching frequency of hydroxyl groups, and those at (2) 2920 -1 and (4) 1420 cm -1 correspond to C-H bending and stretching frequencies in cellulose, respectively. 1 Trigonal BO and tetragonal BO are known to have bending frequencies at approximately 1470 cm -1 (3) and 970 cm -1 (6), respectively. 2 The peaks around 1040 cm -1 ( 5)

and 870 cm -1 (7) corresponded to the C-O-C stretching vibration of the pyranose ring. As an alternative method using powdered NaBH4 (wt.% of boron is 28.54%) as an external standard, the boron adsorption capacities of cellulose, and Amberlite were further estimated by acquiring and comparing the single-pulse 11 B NMR spectra of known concentration of NaBH4 and boron adsorbed cellulose and Amberlite IRA743 materials obtained under identical acquisition conditions. Dried powders of boron adsorbed cellulose and Amberlite IRA743 materials were used to acquire the 11 B MAS NMR spectra at 18.8 T and 50 kHz MAS. The 11 B peak integral of NaBH4 was calibrated to unity (2.9 mg of NaBH4) and the integral corresponding to chelate boron sites in the spectra of both cellulose and Amberlite shown in Figure S14 are compared with respect to this standard in order to estimate the boron adsorbed by cellulose and polymeric resin at the same concertation of boric acid used (33.8 mg/mL) for the reaction.

Figure 1 .

 1 Figure 1. Solid-state 1D (a) 11 B MAS NMR spectra of boron adsorbed cellulose acquired at (a) 18.8 T (50 kHz MAS using a Hahn echo sequence) indicating the distinct boron adsorption sites

Figure 1

 1 Figure 1 presents 1D 11 B MAS NMR spectra of boron-adsorbed and dried cellulose powder acquired at (a) 18.8 T (50 kHz MAS) and (b) 9.4 T (8 kHz MAS with 1 H-decoupling). The protocol used for the boron adsorption is presented in the experimental details. In addition, the 11 B MAS

Figure 2 .

 2 Figure 2. Boron adsorption kinetics of borate mono-and bis-chelate complexes on microcrystalline cellulose at pH = 11. (a) solid-state 1D 11 B{ 1 H} MAS NMR spectra plotted as a function of boric acid concentration. Peaks corresponding to mono-and bis-chelate complexes and boric acid are depicted. (c) 11 B peak intensity of mono-and bis-chelate complexes to the plotted as a function of boric acid concentration.

Figure 3 .

 3 Figure 3. (a) Solid-state 1D 11 B{ 1 H} MAS NMR spectra of cellulose treated with boric acid at different pH values, as indicated in the inset. (b) A plot of normalized intensities of boron signals

Figure 3a compares the 1D 11 B

 11 Figure 3a compares the 1D 11 B MAS NMR spectra of 33.5 g.L -1 boric acid-treated cellulose as a

  Figure 3b presents the normalized intensities of boron signals (obtained by comparing the intensity of each peak with respect to the most intense peak for the particular site) associated with boric acid and mono-and bis-chelate complexes plotted as a function of pH. It shows a monotonic increase in the adsorbed boron on the cellulose surface by means of mono-and bis-chelate complexes, and simultaneously, the amount of free boric acid decreases as a function of increasing pH, consistently with the dissociation of boric acid (Equation 1). For the different 11 B signals shown in Figure 3a, the quadrupolar fitting parameters are listed in Table S2. The quadrupolar coupling constants (CQ) for mono-chelate and bis-chelate signals vary from 0.8 MHz (pH 5) to 0.3 MHz (pH 13) but remain within 1 MHz. As shown in the 11 B spectra plotted as a function of boric acid concentration, the free boric acid signal with iso = 18.6 ppm exhibits a quadrupolar coupling constant (CQ) ~2.4 MHz for different pH values. This analysis suggests that the total boron adsorption capacity of cellulose depends on the pH and boric acid concentration in an aqueous solution. During the centrifugation process, the mass of boron adsorbed on cellulose and the mass of boron leached into the supernatant solution were estimated and compared for the different boric acid concentrations.

  7 ppm was assigned to the bis-chelate complexes, as shown in Figure 4b. Together, the observation of 2D correlation signals in Figures 4a and 4b indicates through-space proximities between boron atoms in the chelate complexes and the proton atoms in the cellulose polymers.

Figure 4 .

 4 Figure 4. (a) Solid-state 2D 1 H-13 C HETCOR NMR spectrum (18.8 T and 50 kHz MAS) of boricacid treated cellulose using a CP contact time of 2 ms. (b) Solid-state 2D 11 B-1 H heteronuclear correlation NMR spectrum (9.4 T and 8 kHz MAS) of boric-acid treated cellulose were acquired using a CP contact time of 1 ms. The 2D 1 H-13 C spectrum is tilted by 90 degrees to compare the 1 H chemical shifts on the vertical axis. Dashed lines depict the assignments of 2D 1 H-11 B and 1 H-13 C correlation peaks to the corresponding 1 H and 13 C sites indicated in the schematic structure (top).

Figure 5 .

 5 Figure 5. (a) Solid-state 1D 11 B NMR spectrum of boron-treated polymeric resin Amberlite ® IRA743 (top) and cellulose (bottom) acquired at 9.4 T (8 kHz MAS) and 18.8 T (50 kHz MAS). Resolution enhancement in the 11 B MAS spectra acquired at 18.8 T is noteworthy. (b) A plot of normalized 11 B intensity of mono-and bis-chelate complexes combined as a function of boric acid concentration for Amberlite ® IRA743 (red dots) and cellulose (black dots). The boric acid concentration used for the boron treatment was 33.8 mg/mL.

Figure 5a compares 11

 11 B MAS spectra of boron-treated (pH = 11) Amberlite ® IRA743 and cellulose materials acquired at different magnetic fields. While the spectra acquired at 9.4 T yield to partially overlapped 11 B peaks, the well resolved 11 B signals obtained in the spectra acquired at 18.8 T enabled the 11 B signals corresponding to three distinct boron sites in cellulose and four distinct boron adsorption sites in the polymeric resin to be identified. The enhanced resolution associated with the 11 B MAS spectra further enabled the estimation of boron adsorption capacity of these materials, as discussed below.

Figure 6 .

 6 Figure 6. Solid-state (a) 2D 1 H-13 C (b) 2D 11 B-1 H heteronuclear correlation NMR spectra of boricacid treated polymeric resin (amberlite IRA743) acquired with a CP contact time of 4 ms. The 2D 11 B-1 H spectrum is tilted by 90 degrees to compare the 1 H chemical shifts on the vertical axis. Dashed lines depict the assignments of 2D 11 B-1 H and 1 H-13 C correlation peaks to the corresponding 1 H and 13 C sites indicated in the schematic structure (top). All spectra were acquired at 18.8 T with 50 kHz MAS.

1 H signals at 1 - 3

 13 ppm are due to the dipole-dipole interactions between protons and carbon atoms the CH2 moieties of the NMDG groups (pale green dot) and the polymer backbone (gray dots). 2D peaks correlating 54-78 ppm ( 13 C) and 2-4.5 ppm ( 1 H) indicates the close proximity between carbon and proton sites in the NMDG side chains (green, blue and magenta dots). Specifically, high intensity 2D correlation between 1 H signal around 6-8 ppm and 13 C signal in the range of 122-138 ppm is attributed to the 1 H-13 C dipole-dipole interactions in the phenyl group of styrene moieties (red dots). Further, the intermediate-range dipole-dipole interactions between carbon atoms of polymer backbone and proton sites of the NMDG sidechains could also be identified by the low intensity correlated intensities of 13 C signal centered around 130-138 ppm with1 H signals at 1-2 ppm and 4-5 ppm range. These results are further corroborated by analyzing the 2D 11 B-1 H CP HETCOR spectrum, whereby the through-space and dipolar coupled 1 H and 11 B moieties produce 2D peaks. The most notable of all are the 1 H signals associated with the CH and OH sites in the vicinity of mono-and bis-chelate complexes are resolved and identified. A strong intensity 2D peak correlating 1 H (1.7, 3.8 and 7.4 ppm) and 11 B (10.2 ppm) shifts is attributed to the formation of bis-chelate complexes, which further correlate with the 1 H and the 13 C chemical shifts in the range of 35-50 ppm and 60-78 ppm that are associated with the -NCH3, -NCH2 and CH sites of NMDG groups. This suggests that the hydroxyl groups of NMDG is mainly involved in formation of such complexes. In addition, low intensity correlation peak between 1 H (3 and ~5 ppm corresponds to -CH2 and -NCH2 and -OH sites of NMDG moieties) and 11 B (6 ppm) chemical shifts are due to the mono-chelate complexes. However, there are no 2D correlation peaks associated the boron peaks at 1.5 and ~17.6 ppm in the HETCOR spectra acquired with different CP contact times, further conforming that the free boric acid (~17.6 ppm) and agglomerates of the borate anions (~1.5) ppm are phase separated and boron atoms of which do not exhibit throughspace dipolar interactions with the proton sites of the polymeric resin (SI, Figure S17). Although the attribution of 11 B shift at 10.2 ppm to the formation of amine coordinated borate complexes cannot be completely ignored, the 11 B peak at 1.5 ppm does not correspond to the chelate complexes. In addition, comparison of 11 B MAS spectrum of the boron adsorbed polymeric resin acquired before and after subsequently washing with Millipore water indicates that the unreacted boric acid species are leached into the solution during the subsequent rinsing and drying process (SI, Figure S18). The different 11 B shifts associated with the chemisorbed boron sites in cellulose and amberlite are due to the different local bonding environments of boron chelate complexes. These results further confirm that the local chemical environments of the mono-and bis-chelate complexes in the resin are different from the chelate complexes formed by the borate-cellulose interactions. Additionally, this analysis highlights the crucial role of NMDG groups in facilitating boron adsorption by means of mono-and bis-chelate complexes and leading to enhanced boron adsorption of polymeric resin. The surface modification and hyperbranching of cellulose by functional groups such as NMDG groups are expected to improve the boron adsorption properties

Figure S3 . 8 .

 S38 Figure S3. Solid-state ATR FT-IR transmittance plots of pristine cellulose and boric acid-treated cellulose acquired at room temperature.

13. Boron adsorption kinetics of Amberlite IRA743 Figure S12 .Figure S13 .

 IRA743S12S13 Figure S12. Comparison of 1D 11 B{ 1 H} MAS NMR spectra of Amberlite IRA743 resin treated with different concentrations of boric acid. All spectra were acquired at 9.4 T with 8 kHz MAS frequency at room temperature.

B background signal acquired by an empty rotor under identical experimental condition. 56 16.

 56 Figure S15. Solid-state 1D (a) 1 H MAS and (b) 13 C{ 1 H} CP-MAS NMR spectra of boric acid treated Amberlite IRA743 acquired at 18.8 T and 50 kHz MAS frequency. Spectral analysis is indicated by the color dots in the structure of polymeric resin Amberlite IRA743 (inset in the top).

Figure S15a displays a 1 H

 1 Figure S15adisplays a 1 H MAS spectrum of boron adsorbed resin with the color dots representing proton sites as depicted in the schematic structure. In the 1D 13 C{ 1 H} CP-MAS NMR spectrum of the same materials acquired with a short CP contact time of 0.2 ms (FigureS15b), signals at 30-50 ppm are due to methylene carbon sites in the polymer backbone and methylene groups bonded to amine group of NMDG moieties, and the broad distribution of peaks in the 55-80 ppm range are due to -NCH2 and alkyl groups of NMDG sidechain. The 13 C signals in the 124-135 ppm range are due to aryl carbons in polymer backbone of resin. Additionally, the quaternary carbon sites (135-150 ppm) are observed in a 13 C{ 1 H} CP MAS spectrum acquired with 4 ms of contact time.

Table 1 . 11 B best-fit NMR parameters, δiso, CQ, and ηQ of distinct boron sites in cellulose treated with 33.8 gL -1 boric acid at pH = 11.

 1 Errors on the iso, CQ, and Q are estimated as ±0.1 ppm, 0.1 MHz, and 0.01, respectively.

		Boron species	δiso (ppm)	CQ (MHz)	Q
	18.8 T	Mono-chelate	1.8	0.3	0.01
		Bis-chelate	6.1	0.4	0.02
		Unreacted boric acid	19.1	2.5	0.10
	9.4 T	Mono-chelate	1.9	0.4	0.01
		Bis-chelate	6.0	0.4	0.01
		Unreacted boric acid	18.6	2.4	0.13
	a

Table 2 . 2D peaks correlating 1 H, 13 C, and 11 B shifts, originating from 1 H-13 C proximities and 1 H-11 B proximities in the boron-treated cellulose

 2

	1 H-13 C sites	1 H-11 B sites	1 H chemical shift (ppm)	13 C chemical shift (ppm)	11 B shift (ppm)
	C1-H1	-	4.6	104.8	-
	C4-H4	-	3.4 3.5	81.7 88.3	-
	C6-OH	-	4.7	64.4	-
	C6-H6	B-H6	4.2 3.7	64.4 64.4	5.9 1.9

  As a result, lignin treated with an aqueous solution of borate anions yields a dark brown solution, and no precipitation occurred. Although the hydroxyl groups present in lignin are expected to interact with borate anions from the aqueous solution, they are open to exchange reactions with counter cations in solution, as depicted in solution-state[START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B NMR spectrum of boric acid-treated lignin that did not show the peaks corresponding to the mono-and bis-chelate complexes. This spectrum is identical to the liquid-state[START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B NMR spectrum of neat boric acid at pH =11, further confirming that the lignin exhibits low affinity towards borate anions. It is noteworthy that the signals from bound boron cations may not be visible in this solution-state 11 B NMR spectrum due to the fast relaxation properties of quadrupolar nuclei such as 11 B. By comparison, cellulose is a potential biomass-

derived compound for boron capture from aqueous solutions due to its low solubility in water and ability to form chemisorbed boron chelate complexes. Therefore, the total boron adsorption capacity of cellulose is compared with a synthetic boron-adsorbing resin Amberlite ® IRA743, which consists of a polystyrene matrix with surface functionalized N-methyl D-glucamine (NMDG) groups.

Table 3 . Boron adsorption capacities of lignin, cellulose, and Amberlite ® IRA743.

 3 The roles of external stimuli such as boric acid concentration and pH on the boron adsorption kinetics of cellulose are explored by measuring and analyzing[START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B NMR spectra, whereby an alkaline condition with a pH greater than 11 is beneficial for boron adsorption.Analysis of 2D 1 H-13 C and 1 H-11 B HETCOR spectra of boron-adsorbed cellulose suggest that chemisorbed chelate complexes are in cellulose proximities to the pyranose rings of the cellulose moieties. The boron adsorption capability of cellulose is estimated to be 1.2 mg of boron per gram of sorbent, while Amberlite ® IRA743 resin exhibited higher performance due to the presence of Applications of Lignocellulosic Biomass. Waste and Biomass Valorization 2021,12,

		Adsorbed boron (mg) / g of
	Polymer
		sorbent
	Lignin	-
	Cellulose	1.3-3.0
	Amberlite ® IRA-743	17.2
	Conclusions
	The molecular-level origins of different boron adsorption kinetics in lignocellulose

biopolymers, notably, microcrystalline cellulose and lignin are examined and compared with a polymeric resin. Lignin is readily soluble in water and exhibits poor affinity towards boron species.

Cellulose-based sorbents are capable of adsorbing borate anions from aqueous solutions. Specifically, 1D

[START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF]

B MAS NMR spectra acquired at different magnetic fields, 9.4 T and 18.8 T, have been analyzed to identify distinct adsorption sites, i.e., cellulose mono-chelate and bis-chelate complexes. Quantitative lineshape analysis of the

[START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF]

B MAS spectra suggests that the boron atoms preferentially form kinetically labile mono-chelate complexes over the thermodynamically stable bis-chelate complexes. (14) Anirudhan, T. S.; Senan, P. Adsorption of Phosphate Ions from Water Using a Novel Cellulose-Based Adsorbent. Chem. Ecol. 2011, 27, 147-164. (15) Hong, M.; Li, D.; Wang, B.; Zhang, J.; Peng, B.; Xu, X.; Wang, Y.; Bao, C.; Chen, J.; Zhang, Q. Cellulose-Derived Polyols as High-Capacity Adsorbents for Rapid Boron and Organic Pollutants Removal from Water. J. Hazard. Mater. 2021, 419, 126503. (16) Ee, L. Y.; Chia, S. Y. R.; Xue, K.; Chin, S. Y.; Cho, C. A. H.; Tan, X. Y.; Li, S. F. Y. (19) Yang, H.; Yu, B.; Xu, X.; Bourbigot, S.; Wang, H.; Song, P. Lignin-Derived Bio-Based Flame Retardants toward High-Performance Sustainable Polymeric Materials. Green

  3,4

Table S1 . Best-fit 11 B MAS NMR parameters, δiso, CQ, and ηQ a , for distinct boron sites in boric acid-treated cellulose as a function of boric acid concentration at pH = 11.

 S1 Errors on the iso, CQ, and Q are estimated as ±0.1 ppm, 0.1 MHz, and 0.01, respectively.

	Conc. of	Mono-chelate complex	Bis-chelate complex	Unreacted boric acid
	boric acid (g/L)	δiso (ppm)	CQ (MHz)	ηQ	δiso (ppm)	CQ (MHz)	ηQ	δiso (ppm)	CQ (MHz)	ηQ
	5.8	1.9	0.2	0.01	5.9	0.2	0.01	18.6	2.2	0.34
	11.6	1.9	0.3	0.01	5.9	0.3	0.02	18.6	2.3	0.18
	22.7	1.9	0.4	0.01	5.9	0.4	0.02	18.6	2.4	0.11
	33.8	1.9	0.4	0.01	5.9	0.4	0.02	18.6	2.4	0.14
	67.2	1.9	0.4	0.01	5.9	0.4	0.02	18.6	2.4	0.18
	134.2	1.9	0.5	0.02	5.9	0.4	0.02	18.6	2.4	0.19

a

Table S2 . Best-fit 11 B MAS NMR parameters, δiso, CQ, and ηQ a , of distinct boron sites in boric acid, treated cellulose as a function of pH at a boric acid concentration of 33.5 g.L 1 .

 S2 Errors on the iso, CQ, and Q are estimated as ±0.1 ppm, 0.1 MHz, and 0.01, respectively.

		Mono-chelate complex	Bis-chelate complex	Unreacted boric acid
	pH	δiso (ppm)	CQ (MHz)	ηQ	δiso (ppm)	CQ (MHz)	ηQ	δiso (ppm)	CQ (MHz)	ηQ
	13	1.9	0.3	0.01	5.9	0.3	0.02	18.2	2.4	0.15
	11	1.9	0.4	0.02	5.9	0.4	0.02	18.6	2.4	0.14
	9	1.9	0.4	0.02	5.9	0.7	0.02	18.6	2.4	0.01
	7	1.9	0.9	0.02	5.9	0.5	0.02	18.6	2.5	0.25
	5	1.9	0.8	0.02	5.9	0.8	0.02	18.6	2.3	0.15

a

Table S5 . The boron adsorption capacity of Amberlite IRA 743 and cellulose treated with 33.8 gL 1 boric acid at pH = 11.

 S5 Integrals of[START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B peaks associated with mono-and bis-chelate complexes are considered as boron adsorption sites. Contribution form the background signal for the[START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B MAS NMR spectra of sorbents acquired at 18.8 T with 1.3 mm HX probe is suppressed by subtracting the[START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF]

	Material	Mass of boron	Sample mass	11 B peak	Mass of boron	Boron adsorbed
		used to treat 80	in rotor taken	integral #	in the rotor	(mg) / gram of
		mg sorbate in 1	for analysis		(mg)	sorbent
		mL (mg)	(mg)
	NaBH4	---	2.9	1	0.83	---
	Amberlite	33.8	2.6	0.06	0.11	17.2
	IRA743
	4C cellulose	33.8	2.9	0.01	0.008	3.0

#

H-13 C proximities in the cellulose framework are listed in Table

2, whereby the C1 at 104.8 ppm

hydroxyl groups in the NMDG groups combined with the backbone and sidechain flexibility of which favors the accessibility towards borate anions to penetrate into the resin surface. Insights into the local structures of different boron chelate complexes in heterogeneous polymeric sorbates are obtained by analyzing 2D correlation NMR spectra. These results reveal that cellulose polymers could seemingly be an option for the removal of boron from aqueous solutions, owing to its low solubility in water, conformational backbone flexibility, and hydroxyl groups. However, surface functionalization of cellulose is required to further enhance the boron adsorption properties. A detailed structural characterization of cellulose-boron interactions further helps to tailor the cellulose-based materials for environmental remediation applications. Supporting Information. Chemical structures, solution-state [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B NMR spectrum of lignin, FTIR spectra of cellulose before and after boron adsorption, solid-state 1D 1 H, 11 B, 13 C and [START_REF] Zhang | Improving Lignocellulose Thermal Stability by Chemical Modification with Boric Acid for Incorporating into Polyamide[END_REF] Na MAS NMR spectra of boron adsorbed cellulose, 1D 1 H, 13 C, [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B and 2D 1 H-1 H, [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] B-1 H correlation spectra of boron adsorbed resin, and 1D 11 B MAS NMR spectra of boron adsorbed sorbates after rinsing and washing, and quantitative analysis of boron adsorption capacities of cellulose and resin.

Conflicts of Interest.

Authors declare no conflicts of interest.

Electronic Supplementary Information for:

Boron adsorption kinetics of microcrystalline cellulose and polymer resin

Parth Raval, Neethu Thomas, Lama Hamdouna, Laurent Delevoye, Olivier Lafon, and G. N. Manjunatha Reddy * University of Lille, CNRS, Centrale Lille Institut, Univ. Artois, UMR 8181-UCCS-Unité de Catalyse et Chimie du Solide, F-59000, Lille, France *corresponding author: gnm.reddy@univ-lille.fr Although mono-chelate complexes show a faster CP build up owing to stronger dipolar couplings due to its proximity to hydroxyl protons, it can be seen that the optimal contact time for monochelate signal is 1 ms due to the presence of labile -OH groups, but 2 ms for bis-chelate peaks.

Table of Contents

Estimation of boron present in the supernatant solution of cellulose

Concentration of boron species that are not adsorbed on the cellulose was estimated by analyzing the supernatant of boron treated cellulose powder using solution state [START_REF]Projections of 2D 13 C-1 H and 11 B-1 H HETCOR spectra of boron adsorbed cellulose[END_REF] S3, however, it excludes the unreacted boric acid precipitated in the cellulose sorbate. When boric acid solution greater than 33.8 mg/mL, unreacted boron acid precipitates in the network of cellulose (Figure 2, main manuscript).

Table S3. Mass of boron leached into the solution during the centrifugation process.

Mass

Figure S16

presents the 1 H DQ-SQ correlation spectrum of the boron adsorbed polymeric resin (Amberlite IRA743), together with the analysis of 1D 1 H MAS NMR spectrum with peaks corresponding to the aromatic protons and the different. DQ-SQ spectrum was acquired with Backto-Back (BaBa) sequence with a DQ excitation time corresponding to 1 rotor period ( r = 20 s).

In the 2D 1 H-1 H DQ-SQ correlation NMR spectrum of boron-adsorbed resin, autocorrelation (i.e., on-diagonal) peaks are observed for -CH moieties of methylene groups in polymer backbone at