Efficiency of chitosan–algal biomass composite microbeads at heavy metal removal

Abstract

A new chitosan/algal (Cladophora sp.) composite microbead was produced and used in removal of heavy metal ions. Bleached algal biomass was incorporated into the chitosan matrix through cross-linking with glutaraldehyde. Fourier transform infrared spectroscopy analysis demonstrated that bleached algal biomass consisted of mainly cellulosic residues. Scanning electron microscopy images exhibited that algal particles were immobilised in the polymeric matrix. Sorption capacity of the microbeads was determined; Cd(II): 0.240, Cr(III): 1.128, Cu(II): 1.059, Ni(II): 0.239 and Zn(II): 0.310 mmol g^− 1. The microbeads with bleached algal biomass exhibited higher sorption capacity for Cd(II) and Zn(II) ions than the plain glutaraldehyde cross-linked chitosan microbeads, demonstrating that the contribution of the algal biomass to the sorption. Equilibrium, kinetic and thermodynamic evaluation of the experimental data was performed. The findings revealed that chitosan–algal composite microbeads can be used in heavy metal removal.

Introduction

Composite material production from renewable sources is a promising area for heavy metal removal thanks to eco-friendly nature of biological waste materials [1]. Chitosan, the by-product of food processing industry, is a functional biopolymer which is desired in heavy metal removal due to its metal ion interacting groups; –OH and –NH₂. Also through these functional groups, chitosan polymer can be easily cross-linked, which makes it an effective matrix for composite material production [2], [3], [4].

Cladophora is genus of filamentous green algae usually found in shallow fresh and saline waters around the world [5]. In case of excessive input of nutrients into an aquatic system, which is called eutrophication, these organisms thrive in large numbers. In a eutrophic aquatic ecosystem, excessive growth and reproduction of algae (macro algal bloom) are usually observed. When left in the water, the algal biomass is consumed by the bacteria. This bacterial decomposition causes further eutrophication in the system by disrupting the mainly dissolved oxygen level. Accumulation of floating algal mats on the surface prevents sun rays from reaching the deeper, which further deteriorates the conditions for photosynthesis organisms [6], [7]. All these concerns stipulate that macroalgal biomass should be removed from the medium following a bloom of macroalgae, and the utility of algal biomass should be addressed for a wide range of applications. Cladophora biomass can be preferred in heavy metal removal studies for a couple of reasons: (1) It is available in huge amounts worldwide, (2) it is low-cost, (3) earlier studies have established that cell wall of green algae consists of mainly cellulose, and their cell wall structure also has proteins bound to polysaccharides [8]. Therefore, cell wall composition of green algae provides binding sites such as carboxyl hydroxyl amino and sulphate for metal ions [9], [10], [11], and (4) its collection from coastal regions can solve possible eutrophication problem.

Many studies have been conducted for heavy metal removal by using chitosan as a main composite material. In some studies, chitosan composites were produced by blending chitosan solution with inorganic materials such as clay [12], sand [13] and magnetite [14] etc. Chitosan composite materials with biological origin such as alginate [15], cellulose [16] and cotton [17] have also been synthesised and tested for heavy metal removal. On the other hand, algal-based biosorbents are known to be effective at treatment of wastewater with heavy metal contaminants [1]. However, few studies have been conducted on heavy metal removal by chitosan–algal biomass (Sargassum sp.) composite [18], [19].

Production of adsorbents by exploiting biological materials is advantageous. Bio-based adsorbents are superior to the conventional synthetic adsorbents in certain aspects; they are biodegradable and nontoxic. They are also considered as low-cost thanks to their abundance in the biosphere. The functional moieties on their surfaces often eliminate chemical modification treatments for functionalisation [1], [20]. Biomass from many organisms including bacteria, fungi and algae has been extensively studied as an alternative adsorbent in removal of heavy metal ions [21], [22], [23]. Compared to other support materials used in adsorption studies, chitosan has excellent properties; it can be cross-linked with agents such as glutaraldehyde, epichlorohydrin and β-cyclodextrin polyaldehyde, and it exhibits a high affinity for heavy metal ions [2], [24]. In the present study, macroalgal bloom biomass from a cosmopolitan algal genus (Cladophora sp.) was used to produce chitosan composite microbeads for heavy metal removal; Cd(II), Cr(III), Cu(II), Ni(II) and Zn(II). Powdered algal biomass was easily incorporated into the chitosan matrix via cross-linking of chitosan with glutaraldehyde. The composite microbeads were characterised by Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and X-ray diffraction (XRD).