Cadmium ion sorption onto lignocellulosic biosorbent modified by sulfonation: the origin of sorption capacity improvement

Abstract

Juniper (Juniperus monosperma), a small-diameter underutilized material, has been studied as a lignocellulosic biosorbent for removing heavy metals from water. In this study, juniper wood was modified by sulfonation to enhance sorption capacity for cadmium in water. The origin of the enhancement was investigated by observing the sorption behaviors and the change in surface functional group concentrations. Cadmium sorption by all juniper wood biosorbents studied was fast and the sorption capacity decreased with decreasing pH, similar to results found for other biosorbents. Sulfonated juniper was found to have at least twice the sorption capacity for cadmium removal from water compared to that of untreated juniper, though the sorption capacity increased with increasing pH. A slight increase in carboxylate content after sulfonation was likely responsible for a small portion of the enhancement. Elemental analysis showed an increase in sulfur content after sulfonation. Diffuse reflectance infrared Fourier transform (DRIFT) spectra showed a decrease in the band at 1660 cm⁻¹ in the range of carbonyl groups as a result of sulfonation. This indicates that coniferaldehyde groups in the lignin of juniper wood corresponding to this band were substituted into sulfonic acid groups after sulfonation. This interpretation was supported by both the color forming reaction with phloroglucinol–hydrochloric acid and the reaction mechanisms from the acid sulfite pulping process. Consequently, the enhancement of cadmium sorption capacity of juniper wood by sulfonation mainly originated from the production of sulfonic acid groups, which are binding sites for heavy metals.

Introduction

Water contaminated by heavy metals remains a serious environmental and public problem. Cadmium is a toxic heavy metal that not only causes choking, abdominal pain, anemia, renal dysfunction, and diarrhea, but also has been listed as a carcinogen by the Environmental Protection Agency (Gaballah and Kibertus, 1998). Diverse technologies have been used to reduce the contents of heavy metals in water, including chemical precipitation, ion exchange, activated carbon adsorption, separation by membrane, electrolytic process, and biological treatment (Gaballah and Kibertus, 1998). Recently, adsorption methods using biosorbents have been widely noticed because of their low cost. Bark, a by-product of the timber industry, has been introduced as a low-cost and effective biosorbent (Gaballah and Kibertus, 1998). Research has suggested that the active species that chelate heavy metals in water are the chemical structures of polysaccharide and polyphenolic compounds in bark (Vazquez et al., 1994; Gaballah and Kibertus, 1998; Bailey et al., 1999). Seaweed and algae were also applied to heavy metal biosorption (Fourest and Volesky, 1996; Romero-Gonzalez et al., 2001; Yun et al., 2001; Davis, Llanes, et al., 2003). In those biosorbents, uronic acids of alginate present in the outer cell wall of the brown algae are the main binding sites for heavy metals (Davis, Mucci, Volesky, 2003). In addition, peat moss (Crist et al., 1992, Crist et al., 1996; Ho and McKay, 2000), alfafa (Tiemann et al., 1999), husk (Saeed and Iqbal, 2003), and sugar beet pulp (Reddad et al., 2002) have been identified as potential biosorbents for heavy metal removal.

Juniper (Juniperus monosperma) and modified juniper have been used as biosorbents for removal of heavy metals in lab-scale tests (Han, 1999; Min et al., 2004). Juniper is a small-diameter and underutilized forest material. More than 19 million hectares of land in the southwestern United States is covered with pinyon juniper woodland. Over the years, these trees have overgrown and dominated large areas of rangeland (Buckman and Wolters, 1986). In addition, abundance of these trees has led to a buildup of biomass that contributes to wildfire fuel loading (LeVan-Green and Livingston, 2001). Accordingly, it is desirable to produce value-added products such as biosorbents from juniper to find uses for this underutilized raw material resource and to reduce fuel loading.

Raw biosorbents have generally been modified with chemical treatments to increase their sorption capacity. Metal ion binding to lignocellulosic biosorbents is thought to occur through chemical functional groups, such as carboxyl, amino, or phenolic groups (Tiemann et al., 1999). Sodium hydroxide treatment has long been used in saponification to produce carboxylate sites that serve as the binding sites for heavy metals (Gardea-Torresdey et al., 1990; Tiemann et al., 1999; Min et al., 2004). Morita et al. (1987) used carbon disulfide and amidoximes to modify wood surfaces. Fourest and Volesky (1996) found that sulfonate groups in various types of biomass contribute to heavy metal binding. Even in polymer membranes used for removing heavy metals, sulfonation has been employed to produce active sites for heavy metals (Choi et al., 2003). According to Sjöstrom (1993), sulfonation in acidic sulfite pulping can generate hydrophilic sulfonic groups as well as free phenolic groups. Therefore, when juniper wood is treated under a mild acidic sulfite pulping condition, its sorption capacity is expected to be improved as a result of the production of sulfonic sites that serve as new binding sites for cadmium ions.

In this study, we demonstrated, through sorption tests, that sulfonation enhances the cadmium ion sorption capacity of a lignocellulosic biosorbent. The origin of the enhancement of sorption capacity can be explained by the change in surface functional groups on the biosorbent caused by sulfonation.