Adsorption and desorption of cationic malachite green dye on cellulose nanofibril aerogels.

TL;DR: Ultra-light aerogels have been assembled from cellulose nanofibrils into hierarchically macroporous (several hundred μm) honeycomb cellular structure surrounded with mesoporous (8-60nm) thin walls, demonstrating facile recovery of both dye and aerogel as well as the robust capability of thisaerogel for repetitive applications.

About: This article is published in Carbohydrate Polymers.The article was published on 2017-10-01 and is currently open access. It has received 208 citations till now. The article focuses on the topics: Adsorption & Langmuir adsorption model.

Summary

1. Introduction

• To further increase the specific surface, inter-CNF hydrogen bondings were reduced by solvent exchange the hydrogels with tert-butanol prior to freeze-drying into aerogels.
• The adsorption mechanism, kinetics and capacity of cationic malachite green dyes on these cellulose nanofibril aerogels as well as their desorption were investigated.

2.1. Materials

• Pure cellulose was isolated from rice straw (Calrose variety) following a previously reported process to a 36% yield (Lu & Hsieh, 2012) .
• Cellulose nanofibrils (CNFs) were derived from pure rice straw cellulose employing 5 mmol NaClO per gram of cellulose and mechanical blending at 37,000 rpm for 30 min (Jiang et al., 2013) .
• Malachite green (Fisher Scientific), sodium chloride (NaCl, >99%, Fisher Scientific), and tert-butanol (certified, Fisher Scientific) were used as received without further purification.
• All water used was purified and deionized by Milli-Q plus water purification system (Millipore Corporate, Billerica, MA).

2.3. Cellulose aerogel characterization

• CNF aerogel was cut in the transverse direction, coated with gold and visualized by a field emission scanning electron microscope (FE-SEM) (XL 30-SFEG, FEI/Philips, USA) at a 5-mm working distance and 5-kV accelerating voltage.
• The specific surface was determined by the Brunauer-Emmett-Teller (BET) method from linear region of the isotherms in the 0.06-0.20 relative P/P 0 pressure range.
• Pore size distributions were derived from desorption branch of the isotherms by the Barrett-Joyner-Halenda (BJH) method.
• The total pore volumes were estimated from the amount adsorbed at a relative pressure of P/P 0 = 0.98.
• CNF aerogels before and after dye adsorption (MG concentration of 100 mg/L, dried in air) were pressed into KBr pellets (1:100, w/w).

2.4. Adsorption of malachite green

• For MG removal from dilute solution, the adsorption was carried out at 10 mg/L MG concentration and 1:5 aerogel-to-MG v/w ratio for 2 h.
• After adsorption, a piece of fresh aerogel was put into the residual solution for three repeated times until the solution becomes colorless.

3.2. Adsorption kinetics

• While both kinetic models of the adsorption data yielded correlation coefficient R 2 values over 0.97 (Table 1 ), pseudo-second order kinetic model was clearly a better fit with all R 2 values above 0.9943, showing linearity in the t/q t versus t plots (Fig. 2d ).
• This indicates that the overall rate of the MG adsorption onto CNF aerogels is controlled by chemisorption.

3.3. Adsorption mechanism

• The intra-particle diffusion rate constant k i and boundary layer thickness or resistance constant C values were calculated from the second intra-particle diffusion region using Eq. ( 8) to yield R 2 ranging from 0.9835 to 0.9961, very high at all concentrations (Fig. 3b ).
• The boundary layer thickness constant C also increased with increasing MG concentrations, indicating greater adsorption via the initial electrostatic interaction at higher MG concentrations.

3.5. Effectiveness of CNF aerogel in complete removal of malachite green

• The effectiveness of CNF aerogel to remove MG dye was first evaluated using varying aerogel quantities at a constant 100 mg/L MG concentration.
• Therefore, repetitive adsorption using fresh aerogels is effective in completely removing MG at low concentrations.

3.6. MG-CNF aerogel interactions

• As sodium carboxylate dissociates, the negatively charged carboxylate anions on CNF surfaces can electrostatically attract the MG iminium cations (Fig. 6c ).
• The FTIR spectra of MG adsorbed CNF aerogel showed several peaks in the 800-400 cm −1 fingerprint region, consistent of the mono-substituted and para-disubstituted benzene rings in MG and confirming its presence in the aerogel.
• The sodium carboxylate carbonyl peak at 1618 cm −1 became smaller which could be due to the interaction between the carboxylate and iminium groups, whereas the carboxylic acid peak at 1720 cm −1 remained essentially unchanged, suggesting that the carboxylic acid did not participate in the adsorption.
• Therefore, FTIR spectra further confirmed electrostatic interactions to be predominantly responsible for MG dye adsorption onto CNF aerogel.
• The maximum adsorption of 212.7 mg g −1 MG/aerogel corresponds to 0.59 mmol/g MG/aerogel or approximately 46% of the CNF surface carboxyls were involved in binding MG.

3.7. Desorption of malachite green from CNF aerogel

• MG dye desorption from aerogels at varied NaCl concentrations also fitted to pseudo-second order kinetic well, with all R 2 above Fig. 7 .
• Desorption of malachite green from CNF aerogel at varied salt concentrations, insets are pictures of aerogels after desorption (a); fitting of pseudo-second-order desorption kinetics (b); Pseudo second order kinetics fitting parameters for dye desorption from CNF aerogels (c). 0.9958 (Fig. 7b & c ).
• The remaining less than 10% of residual dyes could be removed using a fresh NaCl solution.
• The substantial and fast desorption process indicates that the aerogel could be facilely regenerated for repeated dye removal applications.

4. Conclusions

• Ultra-lightweight aerogels have been fabricated from TEMPO oxidized cellulose nanofibrils (CNFs) via a three-step freezingthawing induced hydrogel formation, tert-butanol exchange and freeze-drying to a honeycomb cellular structure consisting of a few hundred m wide irregularly shaped open cells surrounded by mesoporous (8-60 nm) thin walls of self-assembled CNFs.
• This unique hierarchical macro-and meso-porous structure coupled with high specific surface (193 m 2 /g) and surface carboxyl (1.29 mmol/g) has shown to be highly effective in removing cationic dye via electrostatic interactions between the CNF surface anionic carboxylate groups and MG iminium groups.
• With increasing initial dye concentrations from 10 to 200 mg/L, the adsorption kinetics of MG on CNF aerogels showed a pseudosecond-order adsorption model with the initial adsorption rate increasing from 2.3 to 59.8 mg g −1 min −1 and the equilibrium adsorption capacity increasing from 53.0 to 203.7 mg g −1.
• The excellent wet resiliency and fast release of the adsorbed dye could be achieved by increasing ionic strength to recover the aerogel for repetitive use.

Figures

Table 1 Pseudo-first and pseudo-second order kinetic model parameters for MG adsorption onto CNF aerogels.

Fig. 4. Langmuir adsorption isotherm fitting (a), and experimental data with both Langmuir and Freundlich isotherm fitting (b), adsorption isotherm parameter for the adsorption of malachite green by CNF aerogel at 1:5 mg/mL aerogel/dye w/v ratios with 10–400 mg/L dye concentration (c).

Fig. 5. Effectiveness of MG removal by CNF aerogels: (a) aerogel dosage (1:5–10:5 mg/mL aerogel/dye w/v rations) on MG (100 mg/L) adsorption and removal at 2 h, inset are pictures of the original dye solutions; (b) complete removal of malachite green (10 mg/L) with 1:5 mg/mL aerogel/MG w/v ratio by cyclic adsorption, insets are pictures o

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Adsorption and desorption of cationic malachite green dye on cellulose nanofibril aerogels" ?

The rapid MG adsorption was driven by electrostatic interactions and followed a pseudosecond-order adsorption kinetic and monolayer Langmuir adsorption isotherm.