Efficient Removal of UO22+ from Water Using Carboxycellulose

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Efficient Removal of UO from Water using Carboxycellulose Nanofibers Prepared by the Nitro-Oxidation Method Priyanka R. Sharma, Aurnov Chattopadhyay, Sunil K Sharma, and Benjamin S. Hsiao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03659 • Publication Date (Web): 20 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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Industrial & Engineering Chemistry Research

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Efficient Removal of UO22+ from Water Using Carboxycellulose Nanofibers

2

Prepared by The Nitro-Oxidation Method

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Priyanka R. Sharma1, Aurnov Chattopadhyay2, Sunil K. Sharma1, Benjamin S. Hsiao1* 1

Department of Chemistry, Stony Brook University, Stony Brook, NY11794-3400, United States

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2

University High School, Irvine, CA 92612, United States

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* Corresponding author

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E-mail: [email protected]; Tel: +1(631)632-7793

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1 ACS Paragon Plus Environment


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ABSTRACT

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Carboxycellulose nanofibers (NOCNF) were extracted from untreated jute fibers using a

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simple nitro-oxidation method, employing nitric acid and sodium nitrite. The resulting NOCNF

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possessed high surface charge (-70 mV) and large carboxylate content (1.15 mmol/g), allowing

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them to be used an effective medium to remove UO22+ ions from water. The UO22+ (or U(VI))

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removal mechanism was found to include two stages: the initial stage of ionic adsorption on the

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NOCNF surface following by the later stage of uranyl hydroxide mineralization, as evidenced by

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the FTIR, SEM/EDS, TEM and WAXD results. Using the Langmuir isotherm model, the

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extracted NOCNF exhibited a very high maximum adsorption capacity (1,470 mg/g), about

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several times higher than the most efficient adsorbent reported (polyacrylic acid hydrogel). It

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was also found that the remediation of UO22+ ions by NOCNF was pH dependent and possessed

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the maximum adsorption at pH = 7. The removal efficiency of NOCNF was between 80-87%

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when the UO22+ concentration was below 1,000 ppm, while it decreased in to 60% when the

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UO22+ concentration was around 1,250 ppm.

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KEYWORDS

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Carboxycellulose, nanofiber, jute, nitro-oxidation, uranium removal

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INTRODUCTION

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Uranium is a naturally occurring radioactive heavy metal that can cause many adverse

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effects on animal and human health,1,2 such as nephrotoxicity, genotoxicity and developmental

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defects. 3,4,5 In certain regions of New Mexico, Australia, Austria, Kazakhstan, Canada, India and

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Czech Republic, where uranium abundantly exists in the bedrocks and groundwater,6 local

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civilians can suffer high elevation of uranium concentration in blood through contaminated water

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source.7-8 In addition, the potential of radioactive discharge from nuclear plants during meltdown

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can also pose a long-term water hazard.7-9 According to the US EPA (Environment Protection

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Agency) guideline, the maximum contamination limit of uranium is 30 µg/L,10 based on the

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long-term intake of the material in every day water intake of 2 L for 70 years. The major reasons

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for its presence in the environment, besides the leaching from bedrocks and emission from

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nuclear plants, also include coal combustion and fertilizer use containing uranium traces,12 which

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can all cause ground water contamination. Upon ingestion, uranium can rapidly appear in the

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blood stream and bind with red blood cells, forming uranyl-albumin complex that would

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accumulate in kidney and skeleton.11,12

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Several methods have been demonstrated to tackle the problem of uranium contamination

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in water. These methods include anion exchange, lime softening, enhanced coagulation, reverse

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osmosis, activated alumina adsorption and electrodialysis.13 Among these methods, the simplest

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and most effective method is through the usage of coagulant/flocculant, followed by

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microfiltration to remove the uranium contaminants from water.14 In this method, alum, ferric

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sulfate and ferrous sulfate are often used as the coagulant or flocculant agents, whereby their



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efficiencies lie between 50-90% and are more effective at pH 6 or 10. Alternatively, lime

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softening can be used as the primary treatment, having an average efficiency of 80-90% with the

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maximum effectiveness at higher pH values (e.g. pH ~ 10).15 Subsequently, anion exchange and

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reverse osmosis are used as the secondary treatment, where reverse osmosis is far more effective

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than anion exchange, and is capable of removing more than 99% of uranium impurities.13 The

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use of inorganic coagulant can cause environmental hazard due to their non-biodegradable

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nature, whereas the multi-staged treatment process is often time consuming, expensive and

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requires special set-up.

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We aimed to improve the use of coagulant/flocculant approach to remove the uranium

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containments from water in a sustainable, eco-friendly and cost-efficient manner. Our approach

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was based on the use of nanomaterials extracted from biomass, the most abundant polymer

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resource on earth.16 This is because these functional materials in nanoscale that can be extracted

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from varying cellulose resources have been demonstrated as very effective sportive media to

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remove heavy metal ions from water.17,18 For example, carbon aerogel prepared from

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microcrystalline cellulose were found to be very capable of removing Cr(VI) and Pb(II) ions;19

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the oxidized form of nanocelluloses, such as carboxycellulose nanofibers (CNF), were also found

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to possess excellent capability to remove uranyl ions (UO22+) or uranium (IV) ions (U(VI)), and

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arsenic ions, As (III) and As(V).20-23

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Oxidized CNF can possess very high carboxylate content, and be considered as a

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polyelectrolyte in water due to the presence of high surface charge (i.e., through carboxylate

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group, COO-). The negatively charged fiber surface can interact with positively charged heavy


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metal ions, such as chromium, lead,22 and uranium23. These oxidized CNF can be prepared from

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any biomass resources using different chemical pathways, including TEMPO mediated

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oxidation,24-28 carboxymethylation,29 phosphorylation,30 acetylation,31 and silylation.32 However,

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most of these pathways only work well for the cellulose component. Therefore, additional steps

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are required to pretreat the biomass and remove the hemicellulose and lignin components.

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Recently, we have demonstrated a simple method to extract carboxylated CNF directly from

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untreated (or raw) biomass using nitric acid (HNO3) or nitric acid-sodium nitrite (NaNO2)

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mixtures,33 which is termed as the “nitro-oxidation” method (hereafter, nitro-oxidized cellulose

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nanofibers are abbreviated as NOCNF). The presence of nitric acid can initiate the fibrillation

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process of untreated biomass by removing the components of lignin and hemicellulose, whereas

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the reaction of HNO3 (an oxidant) and NaNO2 would generate HNO2 and release nitroxonium

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ions (NO+) in the presence of excess acid. The produced nitroxonium ion is a very effective

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oxidizing agent that can attack the primary hydroxyl group (-CH2OH) of cellulose at the C6

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position and produce carboxylate groups. This method thus greatly reduces the need for multi-

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chemicals, and offers substantial benefits in reducing the consumption of water and electric

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energy.

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In the current study, NOCNF extracted from untreated jute fibers using the nitro-

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oxidation method, was used to test as an absorbent to remove UO22+ (uranyl) ions from water.

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The maximum adsorption capacity of NOCNF for removal of UO22+ ions was compared with the

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most effective adsorbent reported in the literature, i.e., polyacrylic acid based hydrogel

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adsorbent34, as well as with other existing adsorbents. In addition, the mechanism of removing

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UO22+ ions by NOCNF was explained thorough the use of Fourier transform infrared



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(FTIR),

scanning

electron

microscopy

(SEM)/energy

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spectroscopy

dispersive

X-ray

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spectroscopy (EDS), transmission electron microscopy (TEM) and wide-angle X-ray diffraction

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(WAXD) results. Evidently, the extracted NOCNF from jute fibers was found to be a very

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effective medium to purify the uranium contaminated water, whereby the floc, containing large

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aggregates of UO2(OH)2 crystals and NOCNF, could be easily removed using a simple and

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inexpensive gravity-driven microfiltration method.

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EXPERIMENTAL

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Materials

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Jute fibers were provided by Toptrans Bangladesh Ltd. (Bangladesh). Nitric acid (ACS

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reagent, 70 wt%), sodium nitrite (ACS reagent ≥ 97 wt%), sodium hydroxide and hydrochloric

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acid (36% assay) were obtained from Fisher Scientific and used without further purification.

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Uranyl acetate-2 wt% solution (Depleted Uranium) was purchased from Electron Microscopy

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Sciences, where the uranyl acetate solution was diluted using different amounts of distilled water

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to obtain the desired concentrations for the purification analysis.

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NOCNF Preparation Scheme

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NOCNF samples were prepared from untreated jute fibers by using the nitro-oxidation

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method with the mixture of nitric acid-sodium nitrite.33 Briefly, 14 mL (22.2 mmol) of nitric acid

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was added to 1 g of jute fibers in a three-neck flask. After 10 min of mixing, when all the fibers


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were soaked in the acid, 0.96 g (14 mmoL) of sodium nitrite was added. Immediately after the

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sodium nitrite addition, red fumes were formed. To avoid the escape of the fumes, the flask was

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closed with stoppers. The reaction was allowed to run for 12 h under magnetic stirring. The

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reaction was subsequently quenched by adding 250 mL of distilled water into the flask. Then, the

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reacted mixture was transferred into a 500 mL beaker allowing the white suspension to settle

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down. The upper layer in the beaker was removed by decantation and the sediment nanofibers

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were re-dispersed in 250 mL of ethanol/water (20:80 ratio) mixture. The procedures of

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decantation and nanofiber re-dispersion in the ethanol/water mixture were repeated 5-6 times,

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until the pH of the filtrate from suspended fibers reached 2.5. The nanofibers were then dialyzed

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using the dialysis bag (Spectral/Por, MWCO: 6-8 kD), until the conductivity of water reached 5

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µS. The carboxyl groups (COOH) on the NOCNF surface were converted to carboxylate groups

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(COONa) by the post-treatment using 8 wt% sodium bicarbonate for 30 min.

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The extracted NOCNF were characterized using FTIR (PerkinElmer Spectrum One

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instrument), conductometric titration and Zeta probe analyzer (Colloidal Dynamics), TEM (FEI

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Technai G2 Spirit BioTwin). The specific surface area of NOCNF was measured by

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NovatouchLX2 (Quantachrome Instruments) under the N2 atmosphere. The descriptions of the

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instruments used and corresponding techniques are given in Supporting Information.

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Preparation of UO22+ Solutions

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Uranyl acetate solutions with varying UO22+ concentrations from 25 to 2,120 ppm were prepared through the serial dilution of well-stirred 2,500 ppm of UO22+ stock solution.



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Remediation Measurements

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For the remediation measurement, 5 mL of uranyl acetate solution with different UO22+

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concentrations were mixed with 5 mL of NOCNF suspension having a concentration 0.23 wt%.

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Upon mixing, a floc was formed and settled down at the bottom of the holder. Floc and

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supernatant samples were collected and analyzed using the following procedures.

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Preparation of Supernatant Samples for Inductively Coupled Plasma Mass Spectroscopy

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(ICP-MS) Analysis

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The supernatant (non-flocculated) portion was diluted by 100 times and was then filtered

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through a 0.1-micron filter to remove NOCNF. The resulting sample was first analyzed using the

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UV-visible spectroscopy (Thermo Scientific GENESYS 30 Visible Light Spectrophotometer).

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Later, the samples were further diluted by 10 times for the ICP-MS (SQ-ICP-MS, Thermo Fisher

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Scientific) analysis (Supporting Information).

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Preparation of Supernatant Samples at Different pH Values

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The effect of pH on the adsorption of uranyl ions by the NOCNF suspension (0.23 wt%)

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was also investigated. In this study, UO22+ solutions at a fixed concentration (2,222 ppm) and

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different pH values (3, 5, 7, 9, 10) were prepared and were subsequently added with the NOCNF

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suspension. As per above procedures, the non-flocculated portion was taken out and diluted


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below 100 ppb, followed by the addition of 2 wt% nitric acid and filtered through 0.1-micron

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filter and then submitted for the ICP-MS analysis.

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Calculation of Adsorption Efficiency of NOCNF

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The removal efficiency of NOCNF was calculated based on the initial and final UO22+

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concentrations divided by the final UO22+ concentration (measured by ICP-MS). The ideal

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adsorption capacity of NOCNF was calculated by the amount of UO22+ (mg) in solution to the

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amount of NOCNF (g) used in the experiment, assuming that NOCNF could remove all UO22+

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ions from the solution. The experimental adsorption capacity of NOCNF (abbreviated as Qe)

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was the product of the percent efficiency of NOCNF and the ideal adsorption capacity of

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NOCNF.

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Characterization of UO22+ Adsorption Mechanism by NOCNF

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The mechanism of the UO22+ removal from water using NOCNF was revealed by FTIR

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(PerkinElmer Spectrum One), UV-visible spectroscopy (Thermo Scientific GENESYS 30), SEM

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(Zeiss LEO 1550 SFEG-SEM) with EDS capability, TEM (FEI Technai G2 Spirit BioTwin) and

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WAXD. The descriptions of these instruments and the techniques used are also given in

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Supporting Information.

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Static Adsorption Study

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In order to determine the maximum UO22+ adsorption capacity by NOCNF, the

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adsorption equilibrium was estimated. In this study, the Qe value (i.e., the adsorption capacity

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adsorbed at equilibrium) was calculated based on the data obtained from the ICP-MS data using

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the Langmuir adsorption model. This model is based on a monolayer adsorption on the active

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site of adsorbent, having the following expression:

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஼௘ ொ௘

஼௘

ଵ

= ொ௠ + ொ௠௕

(1)

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where Ce is the original concentration of UO22+, Qe is the experimental adsorption capacity of

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UO22+ ions at equilibrium; Qm and b are the constants which can be calculated from the slope

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and the intercept of the linear plot based on Ce/Qe versus Ce (using the Langmuir model).35

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RESULTS AND DISCUSSION

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Characterization of NOCNF Prepared by the Nitro-Oxidation Method

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Figure 1(i) illustrates the FTIR spectra of jute fibers and extracted NOCNF using the

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nitro-oxidation method. There were significant differences between the two spectra, but also

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some similarity. To start, both jute fibers and extracted NOCNF exhibited 1372, 1150, 1100, and

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1030 cm-1 peaks, corresponding to the stretching and bending vibrations of glycosidic bonds in

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cellulose. The peaks in FTIR of jute fibers at 1515, 1739, 1460, 1240 and 810 cm-1 were due to

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the C=C aromatic symmetrical streching in lignin, xylan and glucomannan of hemicellulose,

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respectively. Notably, the intensities of these peak due to the hemicellulsoe and lignin moieties

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were all reduced or completely disappeared in NOCNF, indicating that the treatment of nitro-

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oxidation was effective in removing hemicellulose and lignin polymers in the cell walls of jute


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fibers. Two distinctive cellulose peaks: 3340 cm-1 due to the O-H stretching and 2900 cm-1 due

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to the CH and CH2 stretching, were present in both jute fibers and extracted NOCNF, but the

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intensity assosiated with the C-H stretching peak at 2900 cm-1 was found to decrease and that of

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the COONa peak at 1594 cm-1 was found to increase notably in NOCNF. This observation

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indicated that the nitro-oxdation treatment effectively coverted hydroxyl groups into carboxylate

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groups at the C6 position of the anhydroglucose unts.

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The quantitative determination of the carboxylate group (COO-) in NOCNF was carried

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out using the conductometric tititration method, which content was found to be 1.15 mmol/g (the

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conductometric titration graph is shown in Figure 1S in Supporting Information). The surface

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charge on the extracted NOCNF measured by the zetaprobe analyzer was found to be -70 mV,

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indicating the polyelectrolyte behavior of NOCNF in suspension (the graph is shown in Figure

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2S in Supporting Information). The specific surface area obtained from the freeze dried NOCNF

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sample was 6.31 m2/g, which is comparatively lower than the NOCNF samples obtained from

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wood, algae and bacetrial celluloses,36 probably due to a lower degree of polymerization of in

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jute-based NOCNF.

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Information.

The BET adsorption graph is shown in Figure 3S of Supporting

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Figure 1(ii) shows the TEM image of extracted NOCNF from untreated jute using the

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nitro-oxidation method. The image revealed the long fiber morphology of NOCNF. As these

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nanofibers were randomly entangled in the in-plane view, it was difficult to determine the exact

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length of individual fibers. However, by using the ImageJ software of multiple fibers (20

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filaments), the average length of the fiber was found to be 290 ± 40 nm and the average width



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was 4.47 ± 0.5 nm. Table 1 illustrates some characteristic properties of NOCNF extracted by the

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nitro-oxidation treatment.

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Characterizations of Floc Containing Aggregates of UO2(OH)2 and NOCNF

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Photographs of two samples: the UO22+ solution (2120 ppm) and mixture of 5 mL UO22+

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solution and 5 mL NOCNF suspension (0.23 wt%) are illustrated in Figure 2. It was seen that the

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UO22+ solution appeared slightly yellow but was completely transparent. However, the mixture

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exhibited a yellowish precipitate, settled at the bottom of the bottle. The precipitation occurred

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in a relatively short time period (< 2 min) upon the mixing. This simple experiment suggested

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the NOCNF in suspension might be an effective medium for removal of UO22+ impurities from

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water.

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The precipitate was due to the combined effects of NOCNF aggregation, where UO22+

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behaved as a crosslinking agent, and the mineralization of uranyl ions forming uranyl hydroxide

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crystals, which was evidenced by the WAXD measurements. The WAXD profiles of NOCNF

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and floc (obtained by mixing of 0.0046 g NOCNF in suspension and 1,250 ppm of UO22+

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solution) are shown in Figure 3. It was seen that the pattern of NOCNF showed the characteristic

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peaks of (110) (not labeled), (200), and (004) reflections at 2θ angles of 16.5, 22.7 and 35.1°,

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respectively, based on the cellulose I structure. In contrast, the floc sample exhibited prominent

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diffraction peaks, which could be labeled as (020), (002), (022), (151), (062), (171) and (131)

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reflections from the existence of the uranyl hydroxide (UO2(OH)2) crystal structure.37 The


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diffraction profile of NOCNF was found to be buried underneath that of uranyl hydroxide

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crystals.

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The FTIR spectrum of NOCNF was also found to overlap with that of the floc

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(containing aggregates of NOCNF and UO2(OH)2), where both spectra are illustrated in Figure

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4(i). The NOCNF spectra (Figure 4(i)A) exhibited several characteristic peaks of cellulose: 3400

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cm-1 for hydroxyl (-OH) stretching, 2,883 cm-1 for CH symmetrical stretching, 1,232 cm-1 for

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COH bending at the C6 position, and 1,204 cm-1 for COC symmetric stretching. In contrast, the

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spectrum of the floc (obtained by mixing 1250 ppm UO22+ solution and 0.23 wt.% NOCNF

281

suspension) showed a shift in the COO- stretching peak (1594 cm-1 for NOCNF versus 1630 cm-1

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for the floc). This shift was probably due to the crosslinking effect between the two COO- groups

283

and one UO22+ group in the floc. This verified the existence of chemical interactions between the

284

COO- group on NOCNF and the UO22+ ions. It was noted that the peak at 3340 cm-1 (-OH

285

stretching) of the floc was more pronounced than that of NOCNF, probably due to the large

286

number water moiety in the floc.

287 288

It was interesting to find that the actual UO22+ adsorption capacity of NOCNF in

289

suspension was much higher than the expected value based on the available COO- on NOCNF.

290

This suggested that the remediation process of NOCNF, which was a polyelectrolyte in water,

291

involved more than the adsorption mechanism due to the interactions between UO22+ ions and

292

NOCNF. The adsorbed UO22+ ions on the NOCNF surface clearly provided nucleating sites for

293

the mineralization of uranyl hydroxide crystals. In the literature, the mineralization of metal ions

294

in the presence of polyelectrolytes, such as polyethyleimine, has been well documented. For



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example, nanoscale lead crystals could be formed by adding lead sulfate solution to the solution

296

of polyethyleimine (PEI).38 At low lead concentrations, even though the PEI component acted as

297

a nucleating agent for lead crystallization, no visible changes in the mixture upon the addition of

298

lead sulfate to the PEI polyelectrolyte. However, upon the increase in lead sulfate content, the

299

lead concentration exhibited a sudden decrease, where the fall point was refrerred as a ‘bend

300

point’. At this point, some visible changes occurred in the mixture, including the change in

301

solution color from milky white to transparent due to the growth of a large number of

302

nanocrystals. A similar physicochemical change (i.e., pale yellow transparent solution to beige

303

yellow as shown in Figure 2) was also observed upon the addition of UO22+ ions into a NOCNF

304

suspension in the current study.

305 306

The upper layer (non-flocculated portion) of the mixtures prepared by addition of UO22+

307

solutions with various concentrations into a 0.23 wt% NOCNF suspension was taken out (diluted

308

100x using distilled water) for UV spectroscopy measurements and the results are shown in

309

Figure 4(ii). It was interesting to see that solutions with higher UO22+ concentrations (i.e., 2120

310

and 1560 ppm) showed the absorbance above 1. However, solutions with lower UO22+

311

concentrations (880 and 640 ppm) exhibited almost no absorbance. These results indicated that at

312

low UO22+ concentration, NOCNF had a very good capability to significantly remove the UO22+

313

impurities. A more detailed adsorption capability study will be discussed later.

314 315

To understand the mechanism of UO22+ ions (at different concentrations) and NOCNF

316

interactions, the floc samples were further characterized using the SEM/EDS technique, where

317

the results are shown in Figure 5. In this figure, the SEM image of the floc obtained by mixing


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318

low concentration of UO22+ (500 ppm) and NOCNF suspension revealed a relatively uniform

319

NOCNF film (some regions showed patchy aggregates of UO2(OH)2 and NOCNF). The

320

corresponding EDS spectrum in the inset of Figure 5(i) provided quantitative information about

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the different elements in the floc. In this spectrum, the carbon (C), oxygen (O), sodium (Na)

322

peaks were seen, which was consistent with the presence of the carboxylate group (-COONa) in

323

NOCNF. The silicon (Si) peak was also seen due to the use of silicon wafer support. The very

324

intense uranium (U) peak indicated the crosslinking interactions between the UO22+ ions and

325

NOCNF at low UO22+ concentrations (e.g. 500 ppm). Notably, no evidence of uranyl oxide

326

hydroxide mineralization was observed at this concentration. Moreover, the above results were in

327

consistency to the FTIR results.

328 329

However, the SEM image of the floc obtained in the mixture of UO22+ solutions at high

330

concentrations (e.g., 1250 ppm) with the NOCNF suspension showed a very different

331

morphology (Figure 5(ii)). In addition to the uniform layer of UO2(OH)2 and NOCNF

332

aggregates, large flower like (or spherulite like) uranyl hydroxide crystals were also evident.

333

The spherulitic morphology clearly indicated that the mineralization follows the conventional

334

nucleation and growth process. In other words, the adsorbed UO22+ ions onto the NOCNF

335

surface were acting as the nucleating sites for the growth of uranyl hydroxide crystals at high

336

UO22+ concentrations. This process was further confirmed by the corresponding EDS spectra.

337

The appearance of the dominant uranium (U) peak, as compared to much weaker carbon (C),

338

oxygen (O), sodium (Na) peaks from NOCNF, indicated that the mass of uranium present was

339

significantly higher than the mass of the NOCNF present, confirming the mineralization of

340

uranium onto the NOCNF surface. Similar results have been observed in the mixing study of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 40

341

lead sulfate and polyethyleneimine polyelectrolyte, where a large amount of lead nanocrystals

342

could be generated.37 Hence, the above results indicated that the removal of UO22+ ions by

343

NOCNF was mainly due to the adsorption process at low UO22+concentrations (≤ 500 ppm),

344

where the removal was further assisted by the mineralization process at high UO22+

345

concentrations (≥ 1,250 ppm), resulting very high removal efficiency.

346 347

The presence of uranyl hydroxide crystals in the NOCNF scaffold could also be

348

identified by TEM, where a typical TEM image of the floc (prepared by mixing NOCNF with

349

UO22+ ions at 1,250 ppm concentration) taken outside the spherulitic region is shown Figure 6.

350

This TEM image showed highly entangled NOCNF morphology (in some regions the edges of

351

fibers can be observable), whereby the black dots of uranyl hydroxide crystals in nanoscale were

352

observed throughout the region in a relatively uniform manner. The size of these nanocrystals

353

measured using the ImageJ software was in the range of 4-10 nm. These results are consistent

354

with several previous works, where the presence of cellulose nanofibers facilitated the formation

355

of different inorganic nanocrystals (e.g. ferrite, Ag, Au, ZnO, TiO2).39-41

356 357

Adsorption Capacity of NOCNF

358 359

The adsorption capacity of NOCNF in suspension was determined by the following

360

method. The ICP-MS results were used to calculate the Qe value (i.e., the experimental

361

adsorption capacity of NOCNF) and Ce/Qe ratio (i.e., the original UO22+ concentration of

362

NOCNF divided by the experimental adsorption capacity of UO22+ ions at equilibrium per gram

363

of NOCNF in suspension), where the plot of Ce/Qe versus Ce was then fitted with the Langmuir


Page 17 of 40

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364

adsorption model. The results are illustrated in Figure 7(i). The value of Qe was calculated by

365

multiplying the adsorption efficiency of NOCNF by the ideal adsorption capacity of NOCNF,

366

based on the available carboxylate content (1.15 mmol/g). The results of the ideal adsorption

367

capacity and the experimental capacity of NOCNF are shown in Table 2. It was found that the

368

adsorption efficiency of NOCNF at the UO22+ concentration from 25 to 900 ppm was in the

369

range of 80-87%. The relationship between the adsorption efficiency (%) and the concentration

370

of uranyl ions (UO22+) is illustrated in Figure 4S (Supporting Information). It was seen that the

371

adsorption efficiency of NOCNF decreased to 66-67%, when the concentration of uranyl ions

372

was above 1,000 ppm. However, these adsorption efficiency values are comparatively lower than

373

those of ion exchange resins, such as layered sulfide materials, silica and MnO2 based resins.42

374

Based on the Langmuir adsorption model, the coefficient of LSRL (least-squares regression

375

line), or the slope, in the Ce/Qe versus Ce plot was 6.8149x10-4 (this slope was the reciprocal of

376

the adsorption capacity). This correlation had a R2 (adjusted R squared) value of 0.977,

377

indicating the relationship had excellent conformity to the Langmuir isotherm model (Figure

378

7(i)). Thus, the maximum adsorption capacity (Qm) of NOCNF was 1,467 mg/g, based on Eq. 1.

379 380

Floc Removal by Microfiltration

381 382

A simple gravity-driven microfiltration experiment was carried out to demonstrate the

383

possibility of removing the floc by a low energy filtration means. In this demonstration, the floc

384

was formed by mixing 5 mL of NOCNF suspension (0.23 wt%) and 5 mL of uranyl acetate

385

solution (the UO22+ concentration was 500 ppm). Photographs of the floc in suspension and the

386

used filter paper containing the gel like floc containing aggregates of UO2(OH)2 and NOCNF are



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Page 18 of 40

387

shown in Figure 8. It was interesting to note that the use of a relatively porous filter paper having

388

an mean pore size of 40 µm was sufficient to remove the floc aggregates from water by gravity.

389

This can be explained as follows. Although the uranyl ions (UO22+) have a very small size

390

(aabout 3 Å43), they are effective crosslinking agents binding the NOCNF together and forming

391

large aggregates. Within these aggregates, the mineralization process of UO2(OH)2 further

392

increased the aggregate density resulting in precipitation of the floc. The average size of the floc

393

was very large, thus could be easily separated by microfiltration driven even by gravity. This

394

indicates that the extracted NOCNF in suspension has great potential to be used as the primary

395

medium to remove UO22+ ions from water with the removal efficiency in the range of 80-87%,

396

where the floc can be subsequently separated by any low energy filtration means.

397 398

Effect of pH on the Adsorption Efficiency

399 400

The pH effect on the adsorption efficiency for the removal of UO22+ ions from water was

401

also investigated, where the results are shown in Figure 7 (ii). It was found that the highest

402

removal efficiency of the NOCNF suspension was achieved at the neutral condition (i.e., pH =

403

7), where the maximum removal efficiency was in the range of 90%. The ICP-MS data obtained

404

from the analysis of the upper layer removed from the mixture of UO22+/ NOCNF at different pH

405

values are shown in Table 3. The efficiency of NOCNF was calculated as the original UO22+

406

concentration divided by the final UO22+ concentration after the floc formation. It was found that

407

the efficiency of NOCNF for removal of UO22+ ions was quite high (> 89%) at pH > 5.

408

However, at lower pH values (e.g. pH =2), the adsorption efficiency of NOCNF became lower (~

409

86%). This could be due the denaturing of NOCNF at very acidic conditions.


Page 19 of 40

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410 411

Performance Comparison

412 413

The comparison of the maximum adsorption capacity (Qm) from varying adsorbents is

414

shown in Table 4. It was interesting to note that NOCNF extracted from jute fibers using the

415

nitro-oxidation method was found to be surprisingly effective in removing UO22+ ions from

416

water. In specific, the Qm value of our NOCNF was about three times more efficient than

417

polyacrylic acid hydrogels, which has been considered as the most efficient adsorbent to date.34

418

NOCNF was significantly better than carbonaceous adsorbent and calcium alginates beads,44,45

419

as well as mesoporous silica (SBA-15).46

420

underutilized biomasses such as grass, shrubs, weeds and agriculture waste, the potential use of

421

NOCNF for UO22+ removal or remediation of other metal ion contaminants from water can be

422

very promising. In Table 4, it was very interesting that the UO22+ removal efficiency using

423

TEMPO oxidized cellulose nanofibers,22 reported by our group, was around eight times lower

424

than NOCNF in this study. The major difference is because our earlier study did not include

425

measurements at high UO22+ concentrations, where the mineralization of uranyl oxide hydroxide

426

crystals took place.

427

contaminant concentration.

Considering the sources of NOCNF can be

In typical remediation study, the test should include a wide range of

428 429

CONCLUSIONS

430 431

NOCNF extracted from jute fibers using the nitro-oxidation method has exhibited

432

excellent removal efficiency of UO22+ (or U(IV)) ions from water. Being a highly charged



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Page 20 of 40

433

colloidal particle (surface charge = - 70 mV, carboxylate content = 1.15 mmol/g) and having the

434

shape of nanofiber, NOCNF behaves as a unique polyelectrolyte in water and possesses great

435

potential for water purification applications. The mechanism of UO22+ removal includes the

436

combined effect of adsorption and mineralization, leading to a very high maximum adsorption

437

capacity of 1,470 mg/g. In specific, The adsorption efficiency for NOCNF at UO22+

438

concentrations below 1,000 ppm was in the range of 80-87%, however, the adsorption efficiency

439

decreased to 66% at concentration of 1,250 ppm. The adsorption efficiency was found to be pH

440

dependent and the maximum removal efficiency lied at the neutral condition (pH = 7). This

441

study clearly demonstrated that NOCNF extracted form biomass, such as untreated jute, using

442

the simple nitro-oxidation method, is an excellent medium to aggregate U(IV) ions in

443

contaminated water, where the resulting floc can be easily removed by low energy (e.g. gravity)

444

filtration methods. The simplicity of the NOCNF approach (from extraction to deployment) can

445

provide an attractive alternative, which is sustainable, eco-friendly and cost-efficient, to replace

446

the existing U(VI) removal methods using either inorganic flocculating/coagulating agents, such

447

as alumina, ferrous oxide, or sophisticated reverse osmosis and ion exchange techniques.

448 449

SUPPORTING INFORMATION

450 451

Instrumental and experimental descriptions of Fourier transform infrared spectroscopy

452

(FTIR), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS)

453

capability, inductively coupled plasma mass spectroscopy (ICP-MS), transmission electron

454

microscopy (TEM), wide-angle X-ray diffraction (WAXD), conductometric titration

455

measurements. Experimental results showing the relationship between the consumed volume of


Page 21 of 40

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456

sodium hydroxide (NaOH, 0.4 M) and the conductivity during the conductometric titration study;

457

Zeta potential of NOCNF extracted from jute using the nitro-oxidation method; BET adsorption

458

curve for NOCNF extracted from jute using the nitro-oxidation method; The relationship

459

between the adsorption efficiency (%) of NOCNF and the UO22+ concentration used.

460 461

ACKNOWLEDGMENT

462 463

The authors would like to thank the financial support by the SusChEM Program of the

464

National Science Foundation (DMR-1409507). In additions, the authors would like to thank

465

Susan von Horn (iLab, Stony Brook University), Dr. Chung-Chueh Chang and Ya-Chen Chuang

466

(ThINC facility at AERTC, Stony Brook University) for conducting the TEM measurement, Dr.

467

Jim Quinn (Materials Science and Engineering, Stony Brook University) for the SEM analysis,

468

as well as Dr. David Hirschberg (School of Marine and Atmospheric Science, Stony Brook

469

University) for conducting the ICP-MS measurement.

470 471

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472 473

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474

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476

2. WHO, World Health Organization. Guidelines for Drinking-Water Quality 2008, 198-200.



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22. Yang, R.; Aubrecht, K. B.; Ma, H. Y.; Wang, R.; Grubbs, R. B.; Hsiao, B. S.; Chu, B. Thiol-

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Biomacromolecules 2006, 7, 1687-1691.

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26. Saito, T.; Okita,Y.; Nge, T. T.; Sugiyama, J.; Isogai, A. TEMPO-mediated oxidation of

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586

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593 594



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Page 28 of 40

595

Table 1. Properties of NOCNF extracted from untreated jute fibers using the nitro-oxidation

596

method.

597

Charges

NOCNF

-70 mV

COO-

Size

Surface

content

(L/D)

Area

1.15

290±40/

6.31 m2/g

mmol/g

4.4±0.5 nm

598 599


Page 29 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


600

Table 2. Calculated ideal adsorption capacity and experimental adsorption capacity against

601

UO22+ ions by NOCNF in the concentration range of 25-1250 ppm. Original

Mass of

Original

Final

UO22+ conc.

UO22+

UO22+ conc.,

UO22+ conc.,

(ppm), Ce

used

ICP-MS

(mg) 1,250

a

b

Adsorption

c

Experimental

Ce/Qe

adsorption

adsorption

(g/L)

ICP-MS

capacity

capacity

(ppb)

(ppb)

(mg/g)

(mg/g), Qe

2.5

72.5

24.4

0.66

543.47

360.57

3.47

1,000

2

100

32.7

0.67

434.78

292.61

3.42

875

1.75

87.5

12.1

0.86

380.43

327.83

2.67

750

1.5

75

10.7

0.86

326.08

279.57

2.65

500

1

50

7.3

0.85

217.39

185.65

2.69

250

0.5

25

5

0.80

108.69

86.96

2.88

50

0.1

50

7.6

0.85

21.73

18.43

2.71

25

0.05

25

3.3

0.87

10.86

9.43

2.65

efficiency

Ideal

602

total amount of NOCNF used in 2 mL of 0.23 wt% suspension = 0.0046 g

603

a

604

b

605

c

606

Qe = experimental adsorption capacity; Ce = original concentration of UO22+ in ppm

adsorption efficiency = (original UO22+ conc. - final UO22+ conc.) / original UO22+conc. ideal adsorption capacity = milligrams of UO22+ in solution / grams of NOCNF in suspension

experimental adsorption capacity = adsorption efficiency x ideal adsorption capacity

607



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608

Table 3. Effect of the pH value on the adsorption efficiency of NOCNF pH

Original UO22+

Final UO22+

efficiency

value

concentration

concentration

%

ICP-MS (ppb)

ICP-MS (ppb)

2

22.22

3.50

86.2

5

22.22

2.55

89.9

7

22.22

2.14

90.5

9

22.22

2.71

89.3

10

22.22

2.66

89.0

609 610


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611


Table 4. Comparison of the maximum adsorption capacity (Qm) from different adsorbent. Adsorbent

Qm

Reference

NOCNF

1,470 mg/g

This study

Polyacrylic acid hydrogels 445 mg/g

34

Carbonaceous adsorbent

206 mg / g

44

Calcium alginate beads

237 mg/g

45

SBA – 15

170 mg/g

46

TEMPO oxidized CNF

167 mg/g

22

612



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Figure 1. (i) FTIR spectra of jute fibers and NOCNF (ii) TEM of NOCNF extracted from jute fibers (taken at scale bar of 100 nm and magnification of 395,000x).

ACS Paragon Plus Environment

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Figure 2. (i) Comparative image depicts two samples; left: 5 mL of distilled water with 5 mL of 0.02 wt% uranyl acetate solution (2,120 ppm), right: 5 mL NOCNF suspension (0.23 wt%) with 1g of 0.02 wt% uranyl acetate solution (2,120 ppm); at pH 7.



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Figure 3. WAXD of NOCNF and the floc obtained by mixing of 1,250 ppm of UO22+ and NOCNF (0.0046 g) in suspension. NOCNF was indexed by the cellulose I crystal structure, and the foc was indexed by the uranyl hydroxide (UO2(OH)2) crystal structure.


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Figure 4. (i) FTIR spectra of (A) NOCNF (B) the floc obtained by mixing of uranyl acetate (1,250 ppm of UO22+) and NOCNF suspension, (ii) Ultraviolet visible spectrum of nonflocculated portion obtained on addition of various concentrations of UO22+ ions into 5 mL of 0.23 wt% of NOCNF. Samples were diluted by 100x before analysis.



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Figure 5. (i) SEM image of the floc obtained on interaction of 500 ppm of UO22+ with 5 mL of NOCNF (0.23 wt%), inset left: EDS spectra; (ii) SEM image of the floc obtained on interaction of 1,250 ppm of UO22+ with 5 mL of NOCNF (0.23 wt%), inset left: EDS spectra (nucleating sites are marked by red circles).


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Figure 6. TEM image of floc consist of NOCNF with UO22+ ions (concentration used was 1,250 ppm). The black dots on the edge of NOCNF represent the UO2(OH)2 nanocrystals.



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Figure 7. (i) The experimental results of Ce/Qe versus Ce and the fitting by the Langmuir isotherm model; (ii) the pH effect of on the UO22+adsorption by NOCNF.


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Figure 8. Pictures of (A) NOCNF + UO22+ ions (500 ppm); (B) the floc obtained by gravity driven microfiltration using a filter paper (pore size: 40 µm).



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Table of Content Graphic

Efficient Removal of UO22+ from Water Using Carboxycellulose Nanofibers Prepared by The Nitro-Oxidation Method

Priyanka R. Sharma1, Aurnov Chattopadhyay2, Sunil K. Sharma1, Benjamin S. Hsiao1* 1

Department of Chemistry, Stony Brook University, Stony Brook, NY11794-3400, United States 2

University High School, Irvine, CA 92612, United States


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Efficient Removal of UO22+ from Water Using Carboxycellulose

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