Removal of sulphate from aqueous solution using modified rice straw: Preparation, characterization and adsorption performance

Abstract

Rice straw as a lignocellulosic agricultural residue was chemically converted into a strong basic anion exchanger (RS-AE). Epoxy and amino groups were introduced into raw rice straw by reaction with epichlorohydrin and trimethylamine after it was treated in sodium hydroxide solution. The exchangers were characterized by element analysis, Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM). The results showed that total exchanger capacity of rice straw was increased by 1.32 mEq/g, and quaternary amino groups were formed on its new fibrous surface after modification. Batch adsorption experiments suggested RS-AE exhibits a much higher sulphate maximum adsorption capacity (74.76 mg/g) in contrast to that of raw straw (11.68 mg/g). The equilibrium data were described by Langmuir and Freundlich isotherm models, respectively, and were found to agree very well with the former. RS-AE also showed a good performance in regeneration cycles and a high selectivity for sulphate ions.

Introduction

Sulphate is a major anion occurring in natural water and industrial effluent, such as inorganic chemical industry wastewater and acid mine drainage. Naturally, it mainly originates from the processes of chemical weathering of sulphur containing minerals and the oxidation of sulphides and sulphur. Sulphate is nontoxic, and sulphur is a necessary nutrient element for many kinds of living systems, however, high sulphate concentrations can cause unbalance of the natural sulphur cycle (Pol, Lens, Stams, & Lettinga, 1998), and also endanger human health when excessive ingestion.

Established methods for removal of sulphate from industrial effluents include chemical precipitation, biological treatment and adsorption technologies. Chemical precipitation, for example, to add barium or calcium salts, is rapid and effective, but it may produce another kind of pollution and secondary treatment for solid phase is necessary (Silva, Varesche, Foresti, & Zaiat, 2002). Removal of sulphate by sulphate-reducing bacteria is another alternative; however, the efficiency of biological treatment is susceptible to environmental conditions because the growth requirements of this microbial are relatively rigid (Muyzer & Stams, 2008). Adsorption method may be preferred for their rapid and high selectivity, and sulphur can be recovered. The commonly used zirconium loaded adsorbent is an efficient material to adsorb sulphate (Mulinari & Silva, 2008), but it is too expensive for consumption of the rare metal zirconium. So there is an interest to develop a low cost and effective anion adsorbent to remove sulphate from aqueous solution.

Recently, lignocellulosic agricultural residues such as banana stem and coconut coir pith (Anirudhan et al., 2006, Anirudhan and Unnithan, 2007), wheat straw (Wang et al., 2007a, Wang et al., 2007b), sugarcane bagasse and rice hull (Orlando et al., 2002a, Orlando et al., 2002b), corn stover (Wartelle & Marshall, 2006), soybean hull (Marshall & Wartelle, 2004) used as raw material to prepare anion exchangers have been intensively reported, which seems a potential way to obtain novel and low cost anion adsorbent. China has abundant lignocellulose agricultural resources, especially rice straw. World rice production in 2010/2011 is forecast to be 459.7 million tons, and 137.5 million tons will be harvested in China (USDA, 2010) thus the annual production of rice straw is very high.

Generally, to prepare anion exchanger, cationic groups such as amino groups need to be introduced into a starting material. In the earlier studies (Hwang and Chen, 1992, Laszlo, 1996, Orlando et al., 2002a, Orlando et al., 2002b, Simkovic, 1999), two kinds of chemical conversion ideas to produce anion adsorbent from agricultural by-products had been proposed based on the reactivity of the primary alcoholic –OH mainly from cellulose contained in the materials. The first idea is to synthesize a complex cationic monomer in advance, commonly N-(3-chloro-2-hydroxypropyl) trimethylammonium chloride or 3-bis (3-chloro-2-hydroxypropyl) imidazolium hydrogen sulphate, and then react with agricultural residues. The second is that agricultural residues are reacted with epichlorohydrin in the presence of pyridine and N,N-dimethylformamide, respectively, as catalyst and organic solvent, and then combined with amines to obtain lignocelluloses-based anion exchangers. The latter was widely adopted nowadays. For example, Anirudhan, Jalajamonya, and Suchithraa (2009), Anirudhan et al. (2006) and Anirudhan & Unnithan (2007) converted banana stem, coconut coir pith, and pure cellulose into anion exchangers to remove phosphate, arsenate, and chromate, respectively, from wastewater. Wang et al., 2007a, Wang et al., 2007b prepared an anion exchanger from wheat straw to adsorb nitrate from aqueous solution. Gao, Xu, Wang, Yue, and Xu (2009) also obtained a quaternary amino anion exchanger from wheat straw through pyridine and N,N-dimethylformamide reaction system. However, the use of the catalyst and the organic solvent will inevitably impair the product's cost advantage promised by its abundant and renewable raw material.

In the present work, we focused our attention on using rice straw as a starting material to produce an anion exchanger for removal of sulphate. Although rice straw is a huge lignocellulosic resource both in China and the world, there has been few reports about rice straw based anion adsorbent. We also wanted to develop a low cost and effective procedure to prepare anion exchangers from lignocellulosic materials. Thus, rice straw was first treated with NaOH solution, followed by partially removal of water, and then reacted with epichlorohydrin and trimethylamine to prepare rice straw based anion exchanger (RS-AE). Element analysis, Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM) were employed to characterize the exchangers, and its adsorption performance for removal of sulphate, including adsorption isotherms, competitive adsorption and regeneration of spent adsorbent, were evaluated by batch adsorption experiments.