Novel chitosan goethite bionanocomposite beads for arsenic remediation

Highlights

• Green synthesis without using toxic reagent; low health risk by releasing no nanoparticles.

• CGB can efficiently remove arsenate as well as arsenite, without a pre-oxidation process.

• CGB can be easily handled (no clogging issue) and has high mechanical property.

• The reactive adsorption and diffusion of arsenic onto CGB was monitored and mapped.

Abstract

We report on the synthesis and As adsorption properties of a novel chitosan – iron (oxyhydr)oxide composite material for the remediation of arsenic-contaminated water supplies. FE-SEM, Mössbauer spectroscopy, ICP-OES and synchrotron (Bulk XAS, μXRF) techniques were applied to determine the composition of the new material and investigate the As uptake efficiency and mechanism. The iron (oxyhydr)oxide phase has been identified as a nano-sized goethite, well dispersed in the chitosan matrix, leading to the name ‘chitosan goethite bionanocomposite’ (CGB). The CGB material is prepared in the form of beads of high density and excellent compression strength; the embedding of the goethite nanoparticles in the chitosan matrix allows for the high adsorption capacity of nanoparticles to be realized. CGB beads remove both As(III) and As(V) efficiently from water, over the pH range 5–9, negating the need for pre-oxidation of As(III). Kinetic studies and μXRF analysis of CGB bead sections show that diffusion-adsorption of As(V) into CGB beads is faster than for As(III). Using CGB beads, synthetic high-arsenic water (0.5 mg-As/L) could be purified to world drinking standard level (<0.01 mg-As/L) using only 1.4 g/L CGB. When considered in combination with the advantages of the low-cost of raw materials required, and facile (green) synthesis route, CGB is a promising material for arsenic remediation, particularly in developing countries, which suffer a diversity of socio-economical-traditional constraints for water purification and sanitation.

Introduction

Wherever surface water resource is limited, there is a greater activity of groundwater extraction (Chapman et al., 1996). In many regions an unintended consequence of utilizing groundwater reserves for drinking water supply has been elevated levels of dissolved arsenic (Nordstrom, 2002), mobilized through the oxidation of aquifer As-containing minerals. Inorganic arsenate (As(V)) and arsenite (As(III)) pose the greatest threat to human health, since they are the main species occurring in natural waters and are the most toxic forms (Bissen and Frimmel, 2003, Bodek et al., 1988, Mohan and Pittman, 2007). Arsenite is considered more toxic than arsenate due to a higher mobility in the environment and more rapid cellular uptake (Jain and Ali, 2000, Rossman, 1998). Some of the best documented and most severe cases of arsenic contaminated aquifers are those in Asia (e.g. parts of Bangladesh, China, India, Nepal) and South America (e.g. Argentina, Mexico) (Aureli, 2006, He and Charlet, 2013, Ravenscroft et al., 2009). Numerous treatment methods have been developed over recent years to remove As from the affected water supplies to address this significant public health problem, in a cost effective way (Charlet and Polya, 2006, Mohan and Pittman, 2007).

Iron (oxyhydr)oxides have a high sorption affinity toward both As(V) and As(III) over a wide pH range (Dixit and Hering, 2003) and commonly feature in As removal technologies (Mohan and Pittman, 2007). Significant advantages are gained by using finely sized iron (oxyhydr)oxide particles, however such materials are difficult to separate from treated water. Hybrid materials composed of iron (oxyhydr)oxide-polymer mixtures potentially overcome this limitation (Rorrer et al., 1993, Wang et al., 2009, Zou et al., 2012), having macroscopically larger size, making it easier to separate them by filtration in water treatment. Polymeric coatings can also stabilize the iron oxide/hydroxide with respect to aggregation, maximizing the adsorption capacity (Wu et al., 2008). Chitosan (poly-d-glucosamine) has been employed in iron (oxyhydr)oxide-polymer composite materials and has distinct practical advantages over other polymer alternatives, being low-cost, biodegradable and non-toxic (Ngah et al., 2011). Due to the low porosity of directly polymerized chitosan, most studies have used modified chitosan prepared by the gel-bead method which allows an expansion of the polymer network, improving access to the internal sorption sites (Guibal et al., 1998, Jin and Bai, 2002). A number of approaches have been used to synthesize iron (oxyhydr)oxide-chitosan composite materials, including: (i) treating chitosan beads with aqueous Fe³⁺ (FeCl₃·6H₂O) solutions or hydrated ferric oxides suspensions, or (ii) dispersing iron oxides into chitosan solution prior to the bead formation step (Dias et al., 2011, Guo and Chen, 2005, Qiu et al., 2012, Rorrer et al., 1993). Chemical cross-linking of chitosan with glutaraldehyde is usually employed to enhance the mechanical properties of the polymer beads (Nedelko et al., 2006).

In this study we report a novel route for the synthesis of a chitosan-iron (oxyhydr)oxide, that does not use glutaraldehyde in the synthesis, and yields a composite bead with superior mechanical properties to that reported previously. The iron (oxyhydr)oxide particles produced are goethite (α-FeOOH) of nanometer size (about 250 nm × 50 nm × 10 nm), homogenously distributed in the chitosan polymer and providing a high adsorption capacity. Micro X-ray fluorescence (μXRF) of arsenic impregnated beads shows that both As(III) and As(V) are adsorbed by the composite material. We call this composite material ‘chitosan goethite bionanocomposite’ (CGB). When compared with previously reported composite materials CGB is simpler (and likely cheaper) to produce, has no nanoparticle-related toxicity and has excellent As removal properties.