Unusual dye adsorption behavior of κ-carrageenan coated superparamagnetic nanoparticles

Highlights

• Synthesis of superparamagnetic κ-carrageenan coated Fe₃O₄ nanoparticles.

• κ-Carrageenan coated Fe₃O₄ nanoparticles for removal of methylene blue from water.

• The nanoparticles display fast MB removal (5 min) and high MB adsorption capacity.

• The MB adsorption follows a Z-type adsorption isotherm.

Abstract

κ-Carrageenan coated magnetic iron oxide nanoparticles (NPs) were synthesized and tested as adsorbents for the magnetically assisted removal of methylene blue (MB) from aqueous solutions, a cationic dye commonly present in wastes from the textile industry. The resulting composite NPs were superparamagnetic and contained ca. 12 wt% carrageenan. The MB uptake ability was found to vary with the pH solution and was larger in alkaline conditions. Both pseudo-first-order and pseudo-second-order equations predicted well the kinetics with the maximum adsorption achieved very fast, within 5 min. The MB adsorption has shown an unusual Z-type isotherm which suggests the generation of new adsorbing sites with increasing MB initial concentration. Under the experimental conditions used (23 °C, pH 9) the materials presented here displayed MB adsorption capacity (185.3 mg/g) higher than other magnetic sorbents previously reported. Thus κ-carrageenan coated magnetic NPs are very promising eco-friendly materials for removing MB from wastewater using magnetic separation.

Introduction

Dyes are present in the effluents of a number of industries, including textile, paper, plastic, cosmetic, leather and rubber. The discharge of dyes in water supplies is a matter of concern due their harmful impact on the environment. Most of these compounds cause depletion of water’s oxygen that may result in death of the aquatic life. In addition several dyes and their degradation products exhibit toxicity and potential mutagenic and carcinogenic effects [1]. Methylene blue (Fig. 1a) is commonly used for dying textiles and wood [2] and although MB is less hazardous as compared to other dyes, it has also various harmful effects namely because acute exposure causes nausea and chest pain [3].

The methods used for the removal of methylene blue and other cationic dyes from industrial effluents include chemical precipitation [4], photochemical degradation [5] and adsorption [2], [6]. Among these purification techniques, adsorption is by far the most commonly used due to its simplicity, low cost and effectiveness. A number of materials have been explored as dye adsorbents, such as inorganic materials, activated carbon and inexpensive waste materials [2], [7]. The research for environmentally friendly and low-cost dye adsorbents has raised the attention to biopolymers obtained from renewable resources, such as polysaccharides. Polysaccharides have specific functional groups in their structure which explains their affinity towards a wide diversity of molecules and ions. The list of low cost polysaccharides tested as dye adsorbents is extensive [8] and include alginate [9], chitosan [10], [11], [12], cyclodextrins [13] and starch [14].

Carrageenan comprises a family of linear water-soluble sulfated polysaccharides extracted from red seaweeds. Due to their biocompatibility and ability to form hydrogels, carrageenan has been extensively used as gelling agent in food and pharmaceutical industries [15]. κ-Carrageenan (Fig. 1b) bears one sulfonate group per disaccharide unit, that corresponds to ca. 20 wt% sulfate content. It forms hydrogels stabilized by alkali-metal cations, such as potassium ions and presents the best gelation properties within the carrageenan family, thus being selected as adsorbent for this study. The sulfonate anions of carrageenan are the groups primarily responsible for the interaction with cationic dyes [16]. Taken together, the ability of carrageenan to form complexes with cationic dyes [16] and their gelling properties, make these biopolymers interesting candidates for dye adsorbents in wastewater treatment.

Limitations associated to the separation of dispersed polymer based adsorbents from the treated effluents have restrained their use mostly in fixed-bed systems or packed in sorption columns [8]. Hence, magnetic adsorbents appear as advantageous alternatives that allows the easy magnetically assisted separation of the adsorbents from the treated effluents [17], [18]. Magnetic nanoparticles (MNPs) such as iron oxide nanoparticles, besides bearing a large surface area favorable for adsorption processes, possess magnetic features which enables them to be separated within a short time by employing an external magnetic field [18], [19], [20]. Moreover iron oxide nanoparticles (e.g. magnetite – Fe₃O₄ and maghemite-γ-Fe₂O₃) show no severe toxicity hence these materials are attractive for the development of water purification systems. Polymer coated MNPs have received much attention since the polymer shell may prevent the aggregation of the magnetic cores and improve the dispersion stability of the nanoparticles in aqueous medium. The combination of MNPs with polysaccharides for dye adsorption from wastewater was investigated in the past for cyclodextrins [13], alginate [9] and chitosan [12], [21], [22]. Magnetic κ-carrageenan hydrogels have been explored for biomedical purposes [23], [24], [25] but to the best of our knowledge, the use of this polysaccharide for coating magnetic nanoparticles for the removal of cationic dyes from aqueous solutions is an unexplored strategy, despite its affinity to cationic dyes being known for several years [16].

In this work we report the preparation of κ-carrageenan coated magnetite (Fe₃O₄) nanoparticles and their applications for the removal of methylene blue from aqueous solutions. The removal capability was evaluated in a batch mode process and the kinetics and equilibrium of the adsorption were investigated, as well as the reusability of the sorbents. The system reported here combines the possibility of easy and fast magnetic separation with the advantages of low cost biodegradable materials, resulting in efficient eco-friendly materials for wastewater treatment.