Continuous flow adsorption of methylene blue by cellulose nanocrystal-alginate hydrogel beads in fixed bed columns

Highlights

• An unusual fixed bed column breakthrough profile was observed.

• Anomalous bed expansion and shrinkage was elucidated.

• Hydrogel beads Q_m was found to be 255.5 mg/g.

Abstract

The adsorption behavior of methylene blue by cellulose nanocrystal-alginate (CNC-ALG) hydrogel beads in a fixed bed column was studied by varying the initial dye concentrations, bed depths and flow rates. An unusual phenomenon was observed in the early phase of the adsorption, and the phenomenon was elucidated by varying other critical design parameters, such as the flow direction, diameter of column and composition of adsorbent. The swelling and shrinkage of hydrogel beads during the adsorption was responsible for the anomalous concentration versus time profile of the adsorption process. The maximum adsorption capacity of the column was 255.5 mg/g, which is in agreement with the batch study determined from the Langmuir adsorption isotherm. A comprehensive understanding on the adsorption mechanism of CNC-ALG hydrogel beads during the early stages of adsorption was derived from this study.

Introduction

Dyes used across textile, pulp and paper, dye and dye intermediate, pharmaceutical, tannery and kraft bleaching industries are considered one of the important organic contaminants introduced to the natural water resources or wastewater streams (Carmen & Daniel, 2012). This is alarming both from toxicological and esthetical perspectives and hence the treatment of the dye effluents prior to discharging them to water bodies is crucial (Métivier-Pignon, Faur-Brasquet, & Le Cloirec, 2003). Several researchers have considered methylene blue (MB) as a model cationic organic dye for adsorption studies (Auta and Hameed, 2013, Batmaz et al., 2014, Tan et al., 2008a). This is because of the high solubility of MB in aqueous solution and the harmful side effects of MB that necessitates its removal from wastewater (Higashijima and Fuchigami, 1992, Imasaka et al., 1989, Rafatullah et al., 2010). MB is used extensively for dying cotton, wood and silk and exposure to MB molecules can cause eye burns, breathing complications, nausea, vomiting, profuse sweating, mental confusion and methemoglobinemia (Rafatullah et al., 2010).

Among the various treatment methods employed for dye removal, adsorption is considered superior because of its simple design and operation, low initial investment, effectiveness and insensitivity to toxic substances (Crini, 2006, Ho and McKay, 2003, Jain et al., 2003). Moreover, this technique when performed using a well-designed system offers the best results that produce high quality treated water (Sharma et al., 2011, Unuabonah et al., 2008). Activated carbon produced from various sources is the most widely used adsorbent for wastewater treatment (Cazetta et al., 2011, Hameed et al., 2007, Liu et al., 2013). However, the high cost and energy involved in its production and regeneration have resulted in an increasing interest to develop alternative low cost adsorbents, such as rice husk, fly ash, hazelnut shell, banana peel, orange peel, kaolin etc. (Crini, 2006).

Recently, our group has demonstrated that adsorbents based on the sustainable nanomaterial, i.e. cellulose nanocrystal (CNC) displayed adsorption capacity that was significantly higher that the non-conventional low cost adsorbents (Batmaz et al., 2014). CNCs are rod-like nanoparticles having a diameter of 10–20 nm and lengths of several hundred nanometers obtained by the sulfuric acid hydrolysis of pulp fibers (Beck, Bouchard, & Berry, 2012). These CNCs possess high specific surface area, good mechanical strength, biodegradability and high functionality (Peng, Dhar, Liu, & Tam, 2011). Moreover, the use of CNCs provides a more sustainable alternative compared to other conventional nanostructures derived from petrochemical and activated carbon sources due to the negligible carbon footprint, biodegradability and non-toxicity. CNCs are now being produced by CelluForce Inc. at 1000 kg/day and the synthesis process employed is eco-friendly and sustainability as the sulphuric acid used for the sulphuric acid hydrolysis is regenerated (Li et al., 2013, Roman, 2015, Walker, 2012). Hence, adsorbents based on these nanomaterials should have an enormous capability to remove a wide variety of pollutants from wastewater. However, the practical application of these nanomaterials in large scale water treatment processes is limited due to the difficulty of separating them from aqueous solutions (Batmaz et al., 2014). To address this difficulty, it is desirable to embed the nanomaterials into a three dimensional network that can then be separated using sieves after batch adsorption or they can be used in a continuous fixed bed process.

Hydrogels are crosslinked three dimensional polymer network structures that have received increasing interest in wastewater treatment due to their high adsorption capacities, regeneration capacities and reusability for continuous processes (Kurecic & Sfiligoj, 2012). Alginate obtained from cell walls of brown seaweed is the most widely used biopolymer for preparing hydrogel beads for wastewater treatment due to their biocompatibility, biodegradability, large scale availability, lower cost and non-toxicity (Charumathi and Das, 2012, Lee and Mooney, 2012). Our group has recently developed CNC incorporated alginate (CNC-ALG) hydrogel beads that possess excellent adsorption capability, which can be used in fixed bed columns for large scale water treatment processes. CNC-ALG hydrogel beads possess an overall anionic charge and batch adsorption studies have shown that they have very good adsorption capacity for a model cationic dye, methylene blue (Mohammed, Grishkewich, Berry, & Tam, 2015).

Application of adsorption techniques for large scale wastewater treatment usually employ continuous operations, such as fixed bed units, as this allows for large volumes of contaminated water to be treated within a shorter time period. These units may be easily scaled up from laboratory to pilot unit, and the process is easy to monitor and operate (Auta and Hameed, 2013, Tan et al., 2008a). Continuous adsorption processes using fixed bed columns are effective processes for cyclic sorption/desorption as they depend on concentration gradient as a driving force for adsorption. This in turn allows for efficient utilization of the sorbent capacity and results in better quality of the effluent (Charumathi & Das, 2012).

Several studies have reported on the use of hydrogel beads prepared using biopolymers, such as alginate and chitosan in fixed bed columns for the adsorption of contaminants from water (Charumathi and Das, 2012, Fiol et al., 2006, Lazaridis and Keenan, 2010, Lezehari et al., 2012, Xu et al., 2008). However, none of the studies have concentrated on the adsorption behavior of these hydrogel beads when used in fixed bed column studies. Moreover, until now there is no reported study where CNCs based hydrogel beads have been used for fixed bed column studies. Hence this study reports on the adsorption behavior of CNC-ALG hydrogel beads in a fixed bed column for the removal of MB.