Nanocellulose from Spinifex as an Effective Adsorbent to Remove

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Nanocellulose from Spinifex as an Effective Adsorbent to Remove Cadmium(II) from Water Priyanka R. Sharma, Aurnov Chattopadhyay, Sunil K Sharma, LiHong Geng, Nasim Amiralian, Darren J. Martin, and Benjamin S. Hsiao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03473 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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ACS Sustainable Chemistry & Engineering

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Nanocellulose from Spinifex as an Effective Adsorbent to Remove

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Cadmium(II) from Water

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Priyanka R. Sharma1, Aurnov Chattopadhyay2, Sunil K. Sharma1, Lihong Geng1,

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Nasim Amiralian3, Darren Martin3, Benjamin S. Hsiao1*

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Department of Chemistry, Stony Brook University, 100 Nicolls Road, Stony Brook, New York 11794-3400, United States

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University High School, 4771 Campus Dr., Irvine, CA 92612 United States

Australian Institute for Bioengineering and Nanotechnology, Corner College and Cooper Rds, The University of Queensland, QLD 4072, Australia

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

23 1 ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

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Abstract

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Nanocelluloses, in the form of carboxycellulose nanofibers, with low crystallinity (CI ~

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50 %), high surface charge (-68 mV) and hydrophilicity (static contact angle 38°), were prepared

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from an untreated (raw) Australian spinifex grass using the nitro-oxidation method employing

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nitric acid and sodium nitrite. The resulting nanofibers (NOCNF) were found to be an effective

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medium to remove Cd2+ ions (cadmium(II)) from water. For example, a low concentration of

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NOCNF suspension (0.20 wt%) could remove Cd2+ ions over a large concentration range (50-

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5,000 ppm) in a relatively short time period (≤ 5 minutes). The results showed that at low Cd2+

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concentrations (below 500 ppm), the remediation mechanism was dominated by interactions

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between carboxylate groups on the NOCNF surface and Cd2+ ions, which also acted as a

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crosslinking agent to gel the NOCNF suspension. At high Cd2+ concentrations (above 1,000

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ppm), the remediation mechanism was dominated by the mineralization process of forming

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Cd(OH)2 nanocrystals, which was verified by TEM and WAXD. Based on the Langmuir

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isotherm model, the maximum Cd2+ removal capacity of NOCNF was around 2,550 mg/g,

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significantly higher than those of any adsorbents reported in the literature. NOCNF exhibited the

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highest removal efficiency of 84%, when the Cd2+ concentration was 250 ppm. This study

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demonstrated a simple pathway to convert underutilized biomass into valuable absorbent

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nanomaterials that can effectively remove cadmium(II) ions from water.

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Keywords Spinifex, carboxycellulose, nanofibers, cadmium(II) removal

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2 ACS Paragon Plus Environment

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Introduction

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Cadmium is one of the top ten chemicals posing a major public health problem to the

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society mainly through water contamination.1 In specific, cadmium is a confirmed carcinogen,

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mutagen and teratogen.2,3 As cadmium is a common component in many electronic devices and

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other products, such as batteries, solar cells, paints and pigments,4 it can enter the water sources

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through industrial waste and run-offs. When human is exposed to cadmium by consumption of

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contaminated foods or water,5 cadmium can be adsorbed by the lungs or gastrointestinal tract and

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transported in the bloodstream to other parts of body and accumulated in the liver and kidneys.

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The biochemical pathways indicate that cadmium can bind to proteins, non-protein sulfhydryl

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groups and various other macromolecules, such as metallothionein, in kidneys.6 According to a

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report from the Agency for Toxic Substances and Disease Registry (ATSDR),5 the acute oral

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toxicity dose of cadmium is in between 1,500 and 8,900 mg (i.e., 20 and 30 mg/kg) that would

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lead to human fatalities. Although fatalities due to the cadmium exposure are rare, even a short-

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term exposure is known to cause severe gastrointestinal irritation, resulting in vomiting,

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abdominal pain and diarrhea.5 There have been several recent studies reporting the increasing

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level of cadmium contaminant in some regions of Africa, Asia and South America.7-9 The

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problem is not unique only in these regions, whereas similar challenges also exist in other parts

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of the world. It is thus imperative to identify sustainable, cost-effective and environmental-

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friendly solutions to deal with this challenge.

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The goal of this study is to demonstrate that carboxycellulose nanofibers (CNF)

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extracted from a raw biomass by the newly developed nitro-oxidation method can be used as an



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effective adsorbent to remove cadmium(II) ions from water. This approach represents a

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sustainable and cost-effective solution to tackle the heavy metal contamination problems (such as

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cadmium) in drinking water. In previous studies, nanocelluloses prepared by acid hydrolysis and

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oxidation methods (e.g. carboxymethylation, TEMPO) have been utilized as means to take out

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heavy metal ions from water. For example, nanocrystalline cellulose (CNC) produced by acid

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hydrolysis, incorporated with succinic acid, was used to remove lead(II) and cadmium(II) ions

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from water;10 CNF containing carboxylate groups prepared by oxidation methods was also used

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for chromium(III), nickle(II) and uranyl(II) ions removal.11,12 In addition, thiol modified CNF

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was found to be effective for chromium(VI) and lead(II) removal,13 and amine-modified CNC

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was capable of remove negatively charged chromium(VI) metal ions from water.14

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Recently, Carpenter et al. reported the properties and life cycle assessment of different

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nanocelluloses and compared them with carbon nanotubes (CNT)15. Thakur et al. also described

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the applications of cellulosic derivatives for water purification and compared their effectiveness

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with that of CNT.16 Both studies showed that CNF appeared to be a superior adsorption medium

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over CNT for water purification, due to CNF’s higher functionality, lower cost, greater

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sustainability and better safety. In addition, CNF has been shown to be a new and improved

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barrier material for membrane applications, due to its good chemical resistance, hydrophilicity

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(leading to low fouling tendency) and high porosity (leading to high flux). For example, in our

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laboratory, thin-film nanofibrous composite (TFNC) membranes, containing multi-layered

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fibrous scaffolds including a barrier layer made of TEMPO mediated CNF (diameter ~ 5 nm), an

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electrospun polyacrylonitrile (PAN) nanofibrous mid-layer (diameter ~ 150 nm), and a non-

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woven polyethylene terephthalate (PET) substrate (diameter~ 20 µm), were demonstrated. These


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membranes exhibited a significantly higher flux (i.e., 2-10 X) for separation of oil/water

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emulsions than commercial ultrafiltration (UF) membranes with similar rejection capability.17-20

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There are many other naturally occurring sorbent materials that have also been

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demonstrated to be able to remove small contaminants, such as heavy meal ions and dye

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molecules, from water.21-27 For example, materials such as ZnO, AgO, TiO2 and alumina,

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especially in the nanoscale form, have all been shown to be effective adsorption media for water

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remediation.28-43 From the sustainability perspective, we are particularly interested in the use of

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underutilized biomasses, such as agriculture waste, grass and weed, as resources to extract

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valuable nanomaterial (CNF) for water purification because of their abundance and up-cycling

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potential.

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In this study, the chosen biomass is a raw (untreated) grass from Australia, named

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spinifex. There are 69 species of spinifex grasses (genus Triodia) in the arid/semi-arid regions of

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Australia, which covers over one quarter of the continent.44 These grasses are abundant but

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underutilized. There are plenty of other similar arid grasses exist in Africa, Asia and South

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America, where many of these regions are also experiencing the cadmium contamination

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problems 7-9 due to the rapid development of electronic industry. Hence, the goal of this study is

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to demonstrate that spinifex grass, as a model system for other arid grasses, is a good biomass

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source to extract functional carboxycellulose nanofibers, whereby the extracted nanofibers could

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be used as effective adsorbent media to remove cadmium ions from water.

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It has been reported that typical spinifex grass has a high content of hemicellulose (~44

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%), where this characteristics might lead to an easy nanofiber fibrillation tendency when treated

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with mild alkali conditions followed by a mechanical process (e.g. high pressure

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homogenization).45,46 The resulting cellulose nanofibers have been successfully demonstrated as

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an ingredient for nanopaper with high toughness, and as an efficient reinforcement for latex

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natural rubber and thermoplastic polyurethane nanocomposites.45,47,48 However, spinifex

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nanofibers produced by using the above process (combined alkali and homogenization

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treatments) were found to possess no or small surface charge, where their utility as an adsorbent

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material for water purification is limited.

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To extract CNF from untreated spinifex grasses, we employed the nitro-oxidation

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method, involving the mixtures of nitric acid and sodium nitrite. This method has been

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successfully used to prepare charged CNF (abbreviated as NOCNF hereafter) directly from

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untreated (or raw) jute fibers,49. Nitro-oxidation is a simple approach that can oxidize biomass

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and produce carboxylated nanocellulose with significant reduction in chemicals, water, and

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energy consumption. In addition, the effluent from the nitro-oxidation treatment can be recycled

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to nitrogen-rich fertilizer, thus offering great potential to advance the nexus of food, energy and

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water. Recently, carboxylated cellulose nanocrystals with a maximum zeta potential of -45 mV,

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have also been extracted using the hydrothermal treatment of MCC (microcrystalline cellulose)

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in the presence of nitric acid and hydrochloric acid.50 We envision that the use of nitro-oxidation

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method to extract functional nanocelluloses will become more popular due to its simplicity.

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In the present work, the effectiveness of NOCNF extracting from spinifex to remove

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heavy metal ions from water was demonstrated in the study of Cd2+ removal. The extracted

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NOCNF was found to have medium crystallinity (CI = 53 %), high surface charge (-68 mV),

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carboxylate content (0.86 mmol/g) and hydrophilicity (static contact angle = 38°). The

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mechanism of Cd2+ removal by NOCNF was explored by using the combined Fourier transform

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infra-red (FTIR) spectroscopy, scanning electron microscopy/ energy dispersive X-ray

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spectroscopy (SEM/EDS), confocal microscopy and transmission electron microscopy (TEM)

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techniques to characterize the floc coagulant formed upon the mixing of metal ion solutions at

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different concentrations and a NOCNF suspension (0.2 wt%). The results indicated that at low

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Cd2+ concentrations (below 500 ppm), the remediation process was dominated by the

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interactions between Cd2+ ions and COO- groups on the NOCNF surface, where the adsorption

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pathway prevails. In a way, Cd2+ ions also behaved as an effective crosslinking agent to gel

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NOCNF in suspension. At high Cd2+ concentrations (above 1,000 ppm), the remediation

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mechanism was dominated by the mineralization of cadmium hydroxide crystals within the

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gelled NOCNF scaffold. The combination of these two processes greatly enhanced the

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adsorption capacity of NOCNF.

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The quantitative determination of the adsorption capacity of NOCNF against the Cd2+

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ions was accomplished by the static adsorption study using the inductively coupled plasma mass

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spectroscopy (ICP-MS) technique with different cadmium concentrations and under varying pH

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levels. It was found that NOCNF possessed a maximum removal capacity (Qm) of 2550 mg/g,

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which, to the best of our knowledge, is about 40% higher than the most effective adsorbent

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reported in the literature.51 Finally, we demonstrated that the NOCNF-cadmium floc containing



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mineralized cadmium hydroxide could be easily filtered using the low-cost gravity-driven

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microfiltration or decantation methods. We truly believe that the simple nitro-oxidation approach

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to extract functional carboxylated nanocelluloses from underutilized raw biomass, such as

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spinifex grass, can provide a practical pathway to tackle the toxic metal ions (such as

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cadmium(II)) contamination problems for drinking water purification.

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Experimental

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Materials

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Spinifex grass was obtained from The University of Queensland, Australia. Nitric acid

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(ACS reagent, 70 %), sodium nitrite (ACS reagent ≥ 97 %), sodium bicarbonate, cadmium

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nitrate heptahydrate, sodium hydroxide, hydrochloric acid (36% assay) were purchased from

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Fisher Scientific. The above chemicals were used without further purification. Sodium nitrite,

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nitric acid, sodium bicarbonate and spinifex grass were used for the preparation of NOCNF,

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where cadmium nitrate heptahydrate, sodium hydroxide and hydrochloric acid were used for the

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cadmium remediation studies.

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Preparation and Characterization of NOCNF

178 179

NOCNF were prepared from spinifex grass using the nitro-oxidation method, which has

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been reported earlier by us.49 In brief, 2 g of water washed spinifex grass was placed in a three-

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neck round bottom flask, where 10 mL of nitric acid (65-70 wt%, 0.239 mol) was slowly added


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until the grass was completely soaked. Subsequently, 0.028 mol of sodium nitrite was added to

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the mixture under a continuous stirring (using a magnet stirrer). Upon the addition of sodium

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nitrite, red fumes were generated. To prevent the escape of these red fumes, the mouths of the

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round bottom flask were immediately closed. This reaction was allowed to continue at 50°C for

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12 h, and then the reaction was quenched by adding 250 mL of water. After that, the reaction

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mixture was transferred into a beaker, and further quenched by adding 250 mL of distilled water.

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The suspended product was allowed to settle down at the bottom of the beaker, where the upper

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portion was decanted off to remove the excess acid. The above decantation procedure was

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repeated 2-3 times until fibers started to suspend in water. In the next stage, the fiber suspension

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was centrifuged at 3,000 xg for 10 min, until the pH of the supernatant reached above 2.5.

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Subsequently, the bottom suspension was transferred to a dialysis bag (6-8 kDa) in deionized

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water for 4-5 days until the conductivity of water reached below 5 µS. After dialysis, the

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suspension was again centrifuged at 3,000 xg for 5 min, to separate microsized and nanosized

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fibers. The pH level of the resulting NOCNF suspension was found to be 5.30, which might be

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due to the presence of remnant (unconverted) carboxyl acid groups in the NOCNF. The yield of

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carboxycellulose nanofibers obtained by the above procedure was 10 wt% and that of

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carboxycellulose microfibers was 20 wt%. The carboxyl groups (COOH) on NOCNF were

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converted to carboxylate groups (COONa) by treating the fiber suspensions with 8% sodium

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bicarbonate solution at room temperature for 30 min.

201 202

The functional groups on the NOCNF surface were determined using Fourier transform

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infrared (FTIR) spectroscopy (PerkinElmer Spectrum One instrument-ATR mode), 13C CPMAS

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NMR (Bruker Utrashield 500WB plus), conductometric titration and Zeta probe analyzer



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(Colloidal Dynamics) instruments. The morphology, polydispersity and crystal structure of

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NOCNF were characterized by transmission electron microscopy (TEM, FEI Tecnai G2 Spirit

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BioTWIN instrument), scanning electron microscopy (SEM, Zeiss LEO 1550 SFEG-SEM) with

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energy dispersive X-ray spectroscopy (EDS) capability, and wide-angle X-ray diffraction

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(WAXD, Benchtop Rigaku MiniFlex 600) techniques. The specific surface area of NOCNF was

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measured by Novatouch LX2 (Quantachrome Instruments). The contact angle of NOCNF film

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was measured using the Future Digital Scientific Corp. (FD) contact angle instrument (Model no.

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OCA 15 EC). Descriptions of the above instruments and corresponding sample preparation

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schemes for characterization are listed in Supporting Information.

214 215

Determination of Cadmium(II) Remediation Mechanism and Maximum Removal Capacity

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The cadmium(II) remediation mechanism by NOCNF was determined by the following

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experiments. Cd2+ solutions with concentrations ranging from 50 to 5,000 ppm were prepared,

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which were subsequently mixed with a NOCNF suspension of fixed concentration. In specific, a

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2 mL of 0.2 wt% NOCNF suspension was mixed with a 2 mL of Cd2+ solution at different

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concentration. Upon mixing, a floc appeared and precipitated to the bottom of the vessel by

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manual shaking. The non-flocculated portion was diluted by a factor of 1,000 to reach the

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dilution level below 100 ppb to characterize the removal efficiency, where the flocculated

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portion was separated by using a 0.1µ size microfiltration filter. The extracted floc sample,

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containing aggregates of cadmium hydroxide and NOCNF, was characterized by TEM,

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SEM/EDS, WAXD, and confocal microscopy (Leica TCS SP8X, also described in Supporting

227

Information).


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The adsorption efficiency and adsorption capacity of NOCNF against cadmium(II) were

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determined as follows. The non-floc portion, after dilution with a 2 wt% nitric acid solution to

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the level below 100 ppb, while maintaining the same pH, was characterized by the inductively

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coupled plasma mass spectroscopy (ICP-MS) technique (Supporting Information). The

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adsorption efficiency for NOCNF was calculated based on the difference in Cd2+ concentration

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before and after the mixing with the NOCNF suspension divided by the original Cd2+

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concentration (Ce). This efficiency was a function of the Cd2+ concentration. To determine the

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adsorption capacity of NOCNF, an ideal adsorption capacity was first calculated based on the

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available mass (grams) of NOCNF in suspension and the available Cd2+ ions. The experimental

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adsorption capacity (Qe) was the product of the adsorption efficiency and the ideal adsorption

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capacity. As a result, the experimental adsorption capacity of NOCNF was also a function of the

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Cd2+ concentration. The effect of pH value on the cadmium(II) adsorption efficiency by NOCNF

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was further evaluated. In this study, a constant 10,000 ppm solution of cadmium(II) at different

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pH values (3, 5, 7, 9, and 11) were prepared for the mixing with the 0.2 wt% NOCNF

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suspension.

244 245

The maximum adsorption or removal capacity (Qm) of NOCNF against cadmium(II) was

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determined using the Langmuir Isotherm model, which is based on a monolayer adsorption on

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the active site of adsorbent and has the following expression:

248

஼௘

஼௘

ଵ

= ொ௠ + ொ௠௕ ொ௘

Eq. 1



249

where Ce is the equilibrium (or original) concentration of cadmium(II) and Qe is the adsorption

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capacity of cadmium(II) by NOCNF at equilibrium. The value of Qm (and b – the Langmuir

251

constant) was calculated from the slope of the linear plot based on Ce/Qe versus Ce.52

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Results and Discussion

254 255

Characterization of NOCNF Extracted from Spinifex Using the Nitro-Oxidation Method

256 257

Figure 1(i) illustrates the FTIR spectra of untreated but water-washed spinifex grass and

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extracted NOCNF using the nitro-oxidation method. In these spectra, the characteristic peaks of

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cellulose at 3340 cm-1 due to the O-H stretching and 2900 cm-1 due to the CH and CH2 stretching

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were seen. The prominent peak (highlighted in yellow) in NOCNF at 1581 cm-1 was indicative of

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the carboxylate groups, which confirmed the conversion of -CH2OH at the C6 position in the

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anhydroglucose unit to -COO-. Other peaks at 1372, 1150, 1100, and 1030 cm-1 were due to the

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stretching and bending vibrations in the glycosidic bonds of cellulose. In the FTIR spectrum of

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untreated spinifex grass, the peaks at 1739, 1515, 1460, 1240 and 810 cm-1 could be attributed to

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

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of hemicellulose.

267 268

It was interesting to note that the intensity of the peak assosiated to the C-H stretching

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groups at 2900 cm-1 in NOCNF was found to be decreased, while the intensity of COONa peak

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at 1581 cm-1 was notably increased, when compared to those in untreated spinifex grass. This

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further confirmed the oxidation of anhydroglucose units at the C6 position by the nitro-oxidation


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method. The significant decrease (or disappearance) in intensity at 1739, 1515, 1460, 1240, and

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810 cm-1, also verified that the nitro-oxidation method effectively reduced or even removed the

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hemicellulsoe and lignin content from the cell walls of spinifex grass. Table 1 summarizes some

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physical and chemical properties of NOCNF extracted from spinifex using the nitro-oxidation

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method. It was seen that the carboxylate content of spnifex-based NOCNF measured by the

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conductometric tititration method was 0.86 mmol/g; the surface charge of NOCNF in suspension

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measured by zetaprobe analyzer was -68 mV; and the specific surface area obtained for the

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freeze dried NOCNF sample was 5.7 m2/g (this was comparatively lower than that of NOCNF

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obtained from wood, algae and bacetrial cellulose using other extraction methods,52 which was

281

probably due to a lower degree of polymerization by using the nitro-oxidation method). In

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addition, the static contact angle of the extracted NOCNF film was 38° (Figure S1 in Supporting

283

Information), indicating the hydrophillic nature of NOCNF.

284 285

Solid state

13

C CPMAS NMR spectra of untreated spinifex grass and the extracted

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NOCNF are shown in Figure 1(ii). The spectrum of spinifex grass (Figure 1(ii)A) clearly

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indicated the presence of cellulose, hemicellulose, and lignin components. For example, the

288

small peaks at 21.8 and 173 ppm could be attributed to the methyl and carboxyl carbons in the

289

acetyl groups of glucoroxylans in hemicellulose, whereas the small peak at 56.6 ppm was due to

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the methoxyl (-OCH3) carbons in lignin.54 In this spectra (spinifex grass), the presence of

291

cellulose was also evident: the peak at 104.60 ppm was due to the C1 carbons, the peaks at 89.4

292

and 84.1 ppm (doublet) were due to crystalline and amorphous regions of the C4 carbons, and

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the sharp doublet peaks with highest intensity in between 72.4-75.2 ppm, could be contributed to

294

the C2, C3 and C5 carbons. The doublet peaks at 65.2 and 63 ppm were due to the crystalline



295

and amorphous regions of the C6 carbons in the cellulose component of spinifex. However, in

296

extracted NOCNF, the

297

difference. For example, the disappearance of peaks at 153, 56.6 and 21.4 ppm, corresponding

298

to the C3, C4 carbons of syringyl units of lignin and methyl groups of glucoroxylans in

299

hemicellulose, confirmed the significant reduction/removal of lignin and hemicellulose content

300

in spinifex grass by the nitro-oxidation method. The intensity increases and broadening of the

301

carbonyl peak at 173 ppm, along with the reduction in the amorphous region of the C6 peak at

302

about 63 ppm, provided the evidence of oxidation of the cellulose moiety at the C6 position.

303

Interestingly, the crystalline peaks of the C6 carbons (65.2 ppm) and C4 carbons (89 ppm) all

304

became sharper in NOCNF, indicating that the oxidation mainly took place in the amorphous

305

region of cellulose. These analyses were consistent with the analysis of FTIR spectral results.

13

C CPMAS NMR spectrum (Figure 1(ii)B) exhibited some notable

306 307

WAXD measurements of spinifex grass and extracted NOCNF were also carried out and

308

the results are illustrated in Figure 2(i). These patterns indicate that both samples exhibited the

309

characteristic signature of a cellulose I structure, with three distinct diffraction peaks at 2θ angles

310

of 16.64° (spinifex)/16.86°(NOCNF), 22.45°(spinifex)/22.77°(NOCNF), and 34.80°, which

311

could be indexed as the (110), (200), and (004) refelctions.55,56 The crystallinity index (CI)

312

calculated using the “Segal equation” (details given in Supplementary Information) was about

313

53% for NOCNF, and about 50% for untreated spinifex grass. The CI of spinifex-based NOCNF

314

was found to be higher than that of jute-based NOCNF49 extracted by the nitro-oxidation

315

method, which was probably due to the different nature of the biomass.

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A typical TEM image of NOCNF is shown in Figure 2(ii), which exhibited long filament-

318

like fibers entangled in a random fashion. Using a modified ImageJ software, it was possible to

319

estimate the average fiber length and fiber width. Based on the measurements from 20 individual

320

filaments, the average NOCNF fiber length was 190 ± 90 nm, the average fiber width was 4.0 ±

321

1.5 nm and the polydispersity index (PDI) was 0.35 ± 0.01 (the results also are listed in Table 1)

322

for the spinifex-based nanocellulose sample extracted by the nitro-oxidation method. The

323

average fiber width was in excellent agreement with that of NOCNF extracted from untreated

324

jute fibers, but the average length was shorter than that from jute.49

325 326

Explore the Cadmium(II) Removal Mechanism by NOCNF

327 328

The remediation mechanism of cadmium(II) by NOCNF was determined by

329

characterization of floc formed the mixing of cadmium(II) solutions at different concentration

330

with a 0.2 wt% NOCNF suspension. The visual examination that elucidates the interactions

331

between NOCNF and Cd2+ ions is shown Figure 3(i), where the left photo was a cadmium(II)

332

nitrate solution (Cd2+~5,000 ppm) that was clear and transparent. However, upon the addition of

333

5 mL of NOCNF suspension, a white precipitate or floc appeared at the bottom of the bottle in a

334

very short period of time (< 2 min) (the right photo in Figure 3(i)). The floc sample contained

335

aggregates of NOCNF and cadmium contaminant, which was analyzed carefully to determine the

336

remediation mechanism at different cadmium(II) concentration.

337 338

FTIR spectra of pure NOCNF and the floc obtained by mixing 500 ppm of cadmium(II)

339

nitrate solution and NOCNF suspension (0.2 wt%) are illustrated in Figure 3(ii). In Figure 3(ii)A,



340

the NOCNF spectrum depicted the main characteristic peaks at 3400 cm-1 from the hydroxyl (-

341

OH) stretching, 2883 cm-1 from the C-H symmetrical stretching, 1232 cm-1 from the C-OH

342

bending at the C6 position, 1204 cm-1 from the C-O-C symmetric stretching, and 1581 cm-1 from

343

the COO- stretching. However, the floc spectrum (Figure 3(ii)B) exhibited some notable

344

difference. For example, the COO- stretching peak was shifted to 1630 cm-1 versus the initial

345

1581 cm-1 in NOCNF, which is in agreement with the occurrence of interactions between COO-

346

and Cd2+ ions. As one Cd2+ ion can effectively interact with two COO- groups, cadmium(II) can

347

be considered as a good crosslinking agent for the charged NOCNF particles in water. The peak

348

at 3400 cm-1 in the floc spectrum also displayed a significant broadening and intensification

349

when compared with that of pure NOCNF. As this peak was due to the hydroxyl (-OH)

350

stretching, its broadening and intensification could no longer be attributed only to the presence of

351

water in NOCNF, but due to the formation of cadmium hydroxide (Cd(OH)2), which is discussed

352

later.

353 354

The steady state shear viscosity measurement of the cadmium(II) nitrate aqueous solution

355

(Cd2+=500 ppm), the NOCNF suspension at concentration 0.1 wt%, and the floc portion of the

356

mixture containing Cd2+ ions (500 ppm) and NOCNF (0.1 wt%) in suspension was carried out to

357

understand the interactions between Cd2+ ions and NOCNF. The viscosity results are illustrated

358

in Figure S2 (Supporting Information). It was seen that the cadmium nitrate solution possessed

359

very low viscosity, in which instability was observed at low shear rates. In contrast, the NOCNF

360

suspension (0.1 wt%) behaved as a Newtonian fluid with slight shear-thinning tendency at high

361

shear rates. However, the floc of Cd2+ and NOCNF aggregates displayed a drastic increase in

362

viscosity and pronounced shear-thinning behavior. In fact, the floc portion of the mixture


Page 16 of 50

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363

behaved exactly like a gel. This observation supported the notion that Cd2+ ions functioned as an

364

effective crosslinking agent to bind negatively charged NOCNF particles together, similar to the

365

hydrogel formation induced by ionic interactions between the oppositely charged molecules.57

366

Furthermore, the enhanced shear-thinning behavior in the floc implied that the total attraction

367

forces between Cd2+ and NOCNF, even through strong ionic interactions, were not sufficiently

368

high to resist the deformation forces at high shear rates. The above viscosity results are

369

complementary to the FTIR results (Figure 3(ii)), also indicating that Cd2+ is an effective

370

crosslinking agent to gel the NOCNF suspension.

371 372

It was interesting to note that the actual Cd2+ adsorption capacity of NOCNF in

373

suspension was much higher than the expected value based on the available carboxylate groups

374

on NOCNF. The very high adsorption capacity was due to the combined effects of cadmium(II)

375

adsorption by the charged NOCNF scaffold (as a polyelectrolyte) and the mineralization of

376

cadmium hydroxide crystals through nucleation and growth pathways. The mineralization of

377

metal ions in polyelectrolyte has been well documented. For example, Hirasawa et al. reported

378

that the mixing of lead sulfate solution with polyethyleimine (PEI) polyelectrolyte could lead to

379

nanosized lead crystals.58 In this system, the PEI component clearly acted as a nucleating agent

380

in the aqueous environment. Initially, no visible changes were observed upon the addition of lead

381

sulfate to the PEI polyelectrolyte. However, with the increasing lead sulfate content, the lead

382

concentration suddenly displayed a sharp decrease, which was termed as the ‘bend point’ by the

383

authors. Subsequently, the tranparent solution became to a milky suspension, containing a large

384

amount of nanocrystals. In the present study, a similar behavior was also observed after the

385

addition of Cd+2 ions into the NOCNF suspension.



386 387

The cadmium(II) remediation mechanism by NOCNF was revealed by the SEM/EDS

388

measurements. Figure 4 shows SEM images of two floc samples formed upon the addition of

389

Cd2+ solutions having two different concentrations: (i) 500 ppm and (ii) 1,000 ppm, to the 0.2

390

wt% NOCNF suspension. In Figure 4(i), the image of the floc with lower Cd2+ concentration

391

(500 ppm) exhibited the clear appearance of agglomerated nanofibers, partially covered with

392

white clouds of cadmium compound (as seen in the EDS inset), indicating the adsorption of Cd2+

393

ions onto the NOCNF surface. In fact, no evidence (by X-ray diffraction) of cadmium

394

mineralization was observed at this concentration. The quantitative evidence of the presence of

395

cadmium (Cd) compound was supported by the EDS spectrum, where the carbon (C), oxygen

396

(O) and sodium (Na) peaks were due to NOCNF, and the very large silicon (Si) peak was due to

397

the use of silicon wafer as the support substrate. The similar intensity of the peaks associated

398

with NOCNF and Cd indicated that mass ratio between NOCNF and the adsorbed cadmium was

399

similar. This suggests that at cadmium concentrations ≤500 ppm, the remediation mechanism

400

was mainly due to the adsorption of Cd+2 ions onto the NOCNF surface.

401 402

However, SEM image of the floc sample obtained upon the mixing of a cadmium(II)

403

solution with a higher concentration (1,000 ppm) and the NOCNF suspension showed very

404

different morphology (Figure 4(ii)). In this image, the fiber appearance was less visible, while

405

thick cloudy structures associated with cadmium hydroxide crystals became dominant in some

406

regions. The corresponding EDS spectrum (inset) showed the Cd peak with much higher

407

intensity, while other peaks (C, O, Na and Si) were maintained at the same level. This indicated

408

the mineralization of cadmium hydroxide crystals occurred. It also indicates that the adsorbed


Page 18 of 50

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409

cadmium on the NOCNF surface acted as a nucleus, which initiated the growth of cadmium

410

hydroxide crystals in the presence of high Cd+2 concentration. In this stage, the remediation

411

mechanism is mainly dominated by the crystal growth of cadmium hydroxide.

412 413

The formation of cadmium hydroxide nanocrystals could be confirmed by Figure 5,

414

which illustrates (A) TEM image and (B) corresponding WAXD profile of the floc sample

415

formed by mixing of 1,250 ppm Cd2+ solution and 0.2 wt% NOCNF suspension at pH = 7. In

416

TEM measurement, the floc sample was not stained with uranyl acetate to avoid the overlap of

417

contrast. In Figure 5A, it was seen that the NOCNF scaffold acted as a nucleating network in the

418

formation of cadmium hydroxide nanocrystals (black dots), where the average size of these

419

nanocrystals was in the range of 3-5 nm. The results are in full agreement with the data reported

420

in the previous literature, where the presence of cellulose nanofibers could induce the formation

421

of metal nanocrystals (ferrite, Ag, Au, ZnO, TiO2).59-61 Figure 5B shows the corresponding

422

WAXD profile of the floc sample in Figure 5A. Distinct diffraction peaks were seen, which

423

could be indexed by the unit cells of Cellulose I structure and cadmium hydroxide. In specific,

424

the peaks at the 2θ angle of 16.86° and 22.77° could be indexed by (110), (200) diffractions from

425

the Cellulose I structure, whereas the peaks at the 2θ angle of 29.2°, 31.9°, 35.4°, 38.9°, 42.8°,

426

48.09°, 55.4°, and 56.4° could be indexed by (001), (200), (220), (131ത), (111), (310), (201) and

427

(421ത) diffractions of cadmium hydroxide, respectively.62

428 429

Confocal images of NOCNF cast from the 0.2 wt% suspension, cadmium nitrate deposit

430

from a 1,000 ppm solution, and a floc sample prepared by mixing the NOCNF suspension and

431

1,000 ppm solution, are shown in Figure 6. Figure 6(i) represents the NOCNF aggregates with



432

NOCNF being stained by the acid blue dye, while Figure 6(ii) represents the cadmium nitrate

433

deposit with Cd2+ ions being stained by the LeadmiumTM AM dye (this dye has been commonly

434

used to detect lead or cadmium ions in living cells because of its good binding tendency with

435

these metal ions63). The two dyes have different excitation wavelengths: the acid blue dye has an

436

excitation at λ = 580 nm, and the LeadmiumTM AM dye has an excitation at λ = 405-490 nm. As

437

a result, their imaging by different lasers could be used to differentiate the regions of NOCNF

438

and Cd2+ ions at a macroscopic level. In specific, the laser with a wavelength λ = 580 nm was

439

used to take the image in Figure 6(i), which exhibited the agglomeration of NOCNF rather than

440

individual nanofibers; the laser with wavelength λ = 405-490 nm was used to take the image in

441

Figure 6(ii), exhibiting light shadow of cadmium nitrate deposit. Figure 6(iii) represents the floc

442

sample, prepared by mixing stained NOCNF with the acid blue and stained Cd2+ ions with the

443

LeadmiumTM AM dye, taking at λ = 580 nm. In this image, the same type of red textures (due to

444

the agglomeration of NOCNF) was seen as that in Figure 6(i). However, when imaging the same

445

floc sample at λ = 470 nm (Figure 6(iv)), a very different texture was seen when compared that

446

in Figure 6(ii). In specific, the floc images in Figure 6(iv) exhibited a granular texture (marked

447

by red circles) with the granules having much stronger intensity than the diffuse texture in the

448

image of cadmium nitrate deposit (Figure 6(ii)). This granular texture was also not seen in Figure

449

6(iii), in which the imaging was taken to highlight the stained NOCNF. The granular regions in

450

Figure 6(iv) were due to the clusters of collapsed NOCNF scaffold bonded by cadmium

451

hydroxide nanocrystals. These results further confirmed the occurrence of cadmium hydroxide

452

mineralization within the NOCNF scaffold during the remediation process.

453 454

Determine the Maximum Cadmium(II) Removal Capacity of NOCNF


Page 20 of 50

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455 456

The maximum removal capacity of NOCNF against cadmium(II) was determined as

457

follows. Based on the procedures outlined in the Experimental section, the ideal adsorption

458

capacity, adsorption efficiency and experimental adsorption capacity (Qe) of NOCNF for the

459

removal of cadmium(II) in the concentration range of 50 – 5,000 ppm were determined, and the

460

results are shown in Table 2. It was found that the adsorption (or removal) efficiency of NOCNF

461

against cadmium(II) in the 50-1,250 ppm region (except 100 ppm) was between 74 and 84 %.

462

However, the efficiency of NOCNF in the 2,500 and 5,000 ppm region was between 14 and

463

17%. Based on Table 2, the values of the original Cd2+ concentration (Ce), experimental

464

adsorption capacity (Qe) and Ce/Qe are summarized in Table 3. It was found that the plot of Qe

465

against Ce almost exhibited a linear relationship (Figure 7(i)). These results were rearranged

466

according to Eq. 1, where the plot of Ce/Qe versus Ce could be fitted with the Langmuir

467

Isotherm model, as shown in Figure 7(ii). The slope of the fit is proportional to the reciprocal of

468

the maximum adsorption capacity (Qm), which was about 2,550 mg/g (R2= 0.986).

469 470

The maximum adsorption capacity (Qm) of the spinifex-based NOCNF was compared

471

with other commercially available and experimental adsorbents, which are listed in Table 4. It

472

was found that the Qm of NOCNF in this study was about 40% higher than the best commercial

473

adsorbent based on magnesium oxide;51 it was also several times higher than those of porous

474

chitosan beads64 and carboxymethyl functionalized cellulose,65 all effective cadmium(II)

475

adsorbents, probably due to the much larger available surface area in NOCNF. The combined

476

advantage of the very large surface to volume ratio (nanofibers) and abundant charged groups

477

(COO-) in NOCNF was even more pronounced when compared to raw wood,66 coffee grounds,67



Page 22 of 50

478

agriculture waste (e.g. corn stalk, maize cob and bagasse) and biowaste (e.g. oil cake and shell

479

dust),66-71

480

nanocomposites,82 where the Qm value of spinifex-based NOCNF was significantly higher. In

481

fact, extracted NOCNF also exhibited higher efficiency than the other carboxylated biopolymers,

482

such as carboxylated alginic acid,83 esterified saw dust bearing carboxyl group,84 carboxylated

483

chitosan,85 carboxylated cellulose nanocrystals10 and cellulose nanocrystal modified using

484

sodium succinate.10

synthetic

polymer,72

biochar,73

activated

charcoal,74-80

styofoam,81

and

485 486

The adsorption efficiency of NOCNF for removal of Cd2+ ions from water was found to

487

be pH dependent.

To demonstrated this effect, the adsorption efficiency of NOCNF for

488

cadmium(II) removal was measured at different pH level in a very high Cd+2 concentration

489

(10,000 ppm), where the result is shown in Figure 8. It was found that the highest adsorption

490

efficiency was achieved at pH = 7, which would enable practical applications. This behavior

491

could be explained as follows. At the very low pH values, the effective charge density became

492

lower as some carboxylate groups got turned into neutral carboxylic acid groups, while at very

493

high pH values, some NOCNF could be denatured. The observation of the high adsorption

494

capacity of NOCNF occurred at neutral pH is also consistent with the results obtained from

495

biosorption of olive stones.86

496 497

To further evaluate the usage of NOCNF as an adsorbent material to remove Cd2+ ions

498

from water, a freeze-dried NOCNF sample (in powder form) was used in the following

499

experiment. In specific, 0.056 g of NOCNF was re-dispersed in 150 mL of water, where the pH

500

value of the milky NOCNF suspension (Figure 9(i)) was found to be 5.30 and the corresponding


Page 23 of 50 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


501

conductivity was 30.40 µs. The milky appearance indicated that the re-dispersion of freeze-dried

502

NOCNF powder would take time and was in the microscopic level. However, when 10 mL of

503

cadmium nitrate solution (1,500 ppm) was added to this NOCNF suspension, a precipitation

504

formed rather quickly. For example, after 5 minutes of decanting, the coagulant of NOCNF and

505

cadmium hydroxide crystals completely precipitated to the bottom of the flask (Figure 9(ii)). The

506

removal of cadmium(II) by these freeze-dried NOCNF sample was complete, but the pH value of

507

the suspension in Figure 9(ii) was found to decrease to 4.50 and the conductivity increase to

508

220.4 µs. This might be due to the following reason. Upon the addition of cadmium nitrate

509

solution in the NOCNF suspension, the formation of nitric acid (HNO3) could occur, resulting

510

from the release of nitrate (NO3-) ions from cadmium nitrate and H+ ions from the COOH group

511

of NOCNF (in lower pH value). This reaction would lead to the decrease in pH and increase in

512

conductivity. This observation was quite different from a study of cadmium(II) removal from

513

cadmium acetate solution using olive stones.86 In that study, the precipitation of cadmium

514

hydroxide (Cd(OH)2) crystals took place at pH between 10 and 11, in which the maximum

515

removal efficiency of cadmium(II) was seen. The precipitate in Figure 9(ii) (due to the coagulant

516

of NOCNF and cadmium hydroxide crystals) could be easily removed by gravity driven

517

microfiltration process, where Figure 9(iii) illustrates the transparent solution after filtration

518

using a course filter paper (average pore size around 40 µm).

519 520

Conclusions

521 522

In this study, carboxycellulose nanofibers (NOCNF) extracted from an arid grass spinifex

523

using the nitro-oxidation method were demonstrated to be an effective flocculating or



524

coagulating agent for removal of cadmium(II) from water. The maximum adsorption (removal)

525

capacity of the studied NOCNF was found to be 2,550 mg/g, determined by the Langmuir

526

isotherm model using the data from a static adsorption study. This capacity was significantly

527

higher than those reported in the literature for cadmium(II) removal. The removal pathway was

528

found to contain two mechanisms: at low cadmium(II) concentrations (< 500 ppm), the removal

529

was dominated by the charge interactions between the COO- groups and Cd2+ ions, whereas at

530

high cadmium(II) concentrations (> 1,000 ppm), the removal was dominated by the formation of

531

Cd(OH)2 nanocrystals. As Cd2+ ions behaved as an effective crosslinking agent to gel the

532

NOCNF suspension, the precipitate of NOCNF/Cd(OH)2 aggregates could be easily removed by

533

gravity-driven (or very low energy) microfiltration using filters of large pore size (e.g. 40 µm),

534

or by decantation. The nitro-oxidation method was found to be a simple and effective means to

535

extract CNF from spinifex, which could be viewed as a model for many underutilized arid

536

grasses (e.g. agave) or even agriculture waste.

537

adsorbent to purify the cadmium(II) contaminated water because of its very large surface to

538

volume ratio and abundant carboxylate groups. The spinifex-based NOCNF system can be

539

viewed as a mode system to tackle the emerging cadmium pollution problems in many part of the

540

world, where NOCNF can be extracted from a wide range of underutilized biomasses.

The resulting NOCNF is an outstanding

541 542

Supporting Information

543 544

Detailed descriptions on instruments used, such as Fourier transform infra-red 13

545

spectroscopy (FTIR),

C nuclear magnetic resonance (NMR), conductometric titration method,

546

zeta potential measurement, transmission electron microscopy (TEM), scanning electron


Page 24 of 50

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547

microscopy (SEM), wide-angle X-ray diffraction (WAXD), surface area measurement by the

548

Brunauer-Emmett-Teller (BET) method, confocal microscopy, inductively coupled plasma mass

549

spectroscopy (ICP-MS), contact angle measurement (including a figure illustrating the contact

550

angle measurement of NOCNF extracted from spinifex fibers) and rheological measurements

551

with figure showing the steady state shear viscosity data of cadmium(II) nitrate solution,

552

NOCNF suspension, and the floc portion of the cadmium(II)/NOCNF mixture.

553 554

Acknowledgments

555 556

The authors would like to thank the SusChEM Program of the National Science

557

Foundation (DMR-1409507) for the financial support. The authors would also like to thank

558

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

559

Chuang (ThINC facility at AERTC, Stony Brook University, USA) for conducting the TEM

560

measurements, Dr. Jim Quinn (Materials Science and Engineering- Stony Brook University) for

561

the SEM measurements and Dr. David Hirschberg (School of Marine and Atmospheric Science,

562

Stony Brook University) for conducting the ICP-MS analysis.

563 564

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636

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791 35 ACS Paragon Plus Environment


Page 36 of 50

792

Table 1. General properties of NOCNF extracted from spinifex using the nitro-oxidation

793

method. Charge

- 68 mV

COO-

Size

content

(L/D) nm

0.86

190±90/

mmol/g

4.0±1.5

PDI

0.35±0.01

794 795

PDI: polydispersity index; CI: crystallinity index; SA: surface area

796 797


CI

SA

(%)

(m2/g)

53

5.7

Page 37 of 50 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


798

Table 2. Calculated ideal adsorption capacity and experimental adsorption capacity of NOCNF

799

for the cadmium(II) removal in the Cd+2 concentration range of 50-5,000 ppm. Final Cd+2

Original Cd+2

Adsorption

Original

Original NOCNF

Ideal

Experimental

conc. by

conc. by

efficiency

quantity

(0.2 wt%)/2mL

adsorption

adsorption

ICPMS

ICPMS

Cd2+(mg)

(g)

capacity

capacity

(ppb)

(ppb)

(mg/g)

Qe (mg/g)

4,300

5,000

0.14

20

0.004

5,000

736

2,075

2,500

0.17

10

0.004

2,500

428

325

1,250

0.74

5

0.004

1,250

233

110

500

0.78

2

0.004

500

99

40

250

0.84

1

0.004

250

53

100

125

0.20

0.5

0.004

125

26

9.5

50

0.81

0.2

0.004

50

10

800 801

Total amount of NOCNF used in 2 mL of 0.2 wt% suspension = 0.004 g

802

Adsorption efficiency = (original Cd2+ conc. - final Cd2+ conc.) / original Cd2+ conc.

803

Ideal adsorption capacity = milligrams of Cd2+in solution / grams of NOCNF in suspension

804

Experimental adsorption capacity = adsorption efficiency x ideal adsorption capacity

805

Qe: experimental adsorption capacity; Ce: original concentration of Cd2+ in ppm

806



807

Table 3. The relationship between the values of Ce/Qe versus Ce; the results could be fitted by the

808

Langmuir adsorption model to determine the maximum adsorption capacity (Qm) of NOCNF. 809

Ce (ppm)

Qe (mg/g)

Ce/Qe

810

5,000

736

6.80

811

2,500

428

5.84

812

1,250

233

5.36

813

500

99

5.08

814

250

53

4.76

815

125

26

4.9

816

50

10

5

817


Page 38 of 50

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818

Table 4. Comparison of the Qm value obtained for NOCNF with those of different adsorbents

819

reported in the literature. Type of adsorbent

Maximum adsorption

Reference

capacity (mg /g) NOCNF

2,550

This study

flower – like magnesium oxide

1,500

51

chitosan beads

518-188

64

epichlorohydrin cross-linked carboxymethyl

150

65

meranti wood

150 – 175

66

untreated coffee grounds

15 – 17

67

corn stalk

3.39

68

maize cob

105

69

jatropha oil cake

86

70

sugarcane bagasse

69

70

waste shell dust of water mussel

18.18

71

thiol functionalized polyacrylonitrile fibers

350.6

72

MnO2-biochar

45.8

73

activated charcoal

8-40

74-80

styrofoam

50-70

81

attapulgite/ carbon nanocomposites

46

82

carboxylated alginic acid

50-100

83

esterified saw dust bearing carboxyl group

198

84

cellulose



Page 40 of 50

carboxylated chitosan

720-800

85

carboxylated cellulose nanocrystals

1.9

10

cellulose nanocrystals modified using

345

10

sodium succinate 820 821


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Figure 1. (i) FTIR spectra and (ii) 13C CPMAS NMR spectra of spinifex fibers and NOCNF.

ACS Paragon Plus Environment


Figure 2. (i) WAXD profiles of (A) spinifex and (B) NOCNF; (ii) TEM image of NOCNF extracted from spinifex using the nitro-oxidation method.


Page 42 of 50

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Figure 3. (i) Comparative images of two samples: (left) 5 mL of solution based on 5,000 ppm of cadmium nitrate and distilled water, (right) 5 mL of a NOCNF suspension (0.20 wt%) mixed the cadmium nitrate solution (5,000 ppm) at pH 7, (ii) FTIR spectra of (A) NOCNF and (B) floc obtained from the mixture of cadmium nitrate solution (500 ppm of Cd2+) and NOCNF suspension (0.20 wt%).



Figure 4. (i) SEM of the floc obtained from the mixture of NOCNF suspension and 500 ppm of cadmium nitrate solution (inset: EDS spectra); (ii) SEM of the floc obtained from the mixture of NOCNF suspension and 1,000 ppm of cadmium nitrate solution (inset: EDS spectra).


Page 44 of 50

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Figure 5. (i) TEM image of the floc containing NOCNF and Cd(OH)2 nanocrystals (black dots); (ii) corresponding WAXD profile (reflection peaks were indexed by the unit cells of cellulose I and Cd(OH)2) of the floc obtained by mixing a 1,250 ppm of Cd2+ solution and NOCNF suspension (0.20 wt%) at pH = 7.



Figure 6. Confocal images of (i) NOCNF, stained with the acid blue (using the laser with = 580 nm), (ii) cadmium nitrate deposited with a 1,000 ppm solution, where Cd2+ ions were stained by LeadmiumTM AM dye (using the laser with = 405-490 nm); (iii) the floc obtained by mixing stained NOCNF (with acid blue) and stained Cd2+ ions (with LeadmiumTM AM dye) taken at =580 nm; (iv) the same floc taken at =470 nm (the red circles indicated the aggregates of NOCNF and cadmium hydroxide nanocrystals).


Page 46 of 50

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(i)

(ii) Figure 7. (i) Adsorption of Cd2+ ions by the NOCNF suspension at the Cd+2 concentration between 500 and 5,000 ppm, (ii) the fitting of the adsorption data using the Langmuir isotherm model (Qe is the adsorption capacity measured in milligrams of Cd2+ per gram of NOCNF, Ce is the original Cd2+ concentration measured in mg of cadmium(II) per liter of solution).



Page 48 of 50

Figure 8. The effect of pH value on the Cd2+ adsorption efficiency of NOCNF at the cadmium concentration of 10,000 ppm.


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Figure 9. Photographs of (i) freeze dried NOCNF (0.056 g) dispersed in 150 mL of water, (ii) 5 minutes after the addition of 10 mL cadmium nitrate solution (1,500 ppm) in the (i) NOCNF suspension, and (iii) filtration of (ii) through a filter paper (average pore size = 40 µm) using the gravity filtration method.



Table of Content Graphic

Carboxylated nanofibers (NOCNF) extracted from spinifex using nitro-oxidation approach was efficient in removing cadmium(II) from water via adsorption and mineralization.

1


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Nanocellulose from Spinifex as an Effective Adsorbent to Remove

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