1324
Ind. Eng. Chem. Res. 1998, 37, 1324-1328
Sunflower Stalks as Adsorbents for the Removal of Metal Ions from Wastewater Gang Sun* and Weixing Shi Division of Textiles and Clothing, University of CaliforniasDavis, Davis, California 95616
Sunflower stalks as adsorbents for the removal of metal ions such as copper, cadmium, zinc, and chromium ions in aqueous solutions were studied with equilibrium isotherms and kinetic adsorptions. The maximum adsorptions of four heavy metals are 29.3 mg/g (Cu2+), 30.73 mg/g (Zn2+), 42.18 mg/g (Cd2+), and 25.07 mg/g (Cr3+), respectively. Particle sizes of sunflower stalks affected the adsorption of metal ions; the finer size of particles showed better adsorption to the ions. Temperature also plays an interesting role in the adsorption of different metal ions. Copper, zinc, and cadmium exhibited lower adsorption on sunflower stalks at higher temperature, while chromium showed the opposite phenomenon. The adsorption rates of copper, cadmium, and chromium are quite rapid. Within 60 min of operation about 60-80% of these ions were removed from the solutions. Sunflower stalks are excellent adsorbents for the removal of organic dyes and colorants from textile effluents (Sun and Xu, 1997). In our laboratories, a series of applications for the utilization of sunflower stalks and other biomasses are undergoing. Of them, utilizing the agricultural residues in the removal of heavy-metal ions have revealed good potential in wastewater treatment in textile and agricultural industry. Plant residues, which are mainly ligno-cellulosic materials, can inherently adsorb waste chemicals such as dyes and cations in water due to the Coulombic interaction between the two substrates and physical absorption. They are renewable agricultural wastes available abundantly at no or low cost. Disposal of the agricultural biomasses in California and many states is the major obstacle to the sustainable agriculture and environment. Utilization of the biomasses has been studied extensively and some alternatives have been developed. Among many new technologies, utilizing plant residues as adsorbents for the removal of toxic chemicals in wastewater is a prominent technology (McKay et al., 1987; El-Geundi, 1991). The adsorption capacities of plant residues such as maize cob, sugarcane bagasse, and sunflower stalks to cationic dyes were significantly higher compared to the anionic compounds (Sun and Xu, 1997; Laszlo, 1994, 1996). Therefore, it is necessary to explore the adsorption capacities of sunflower stalks to inorganic cations such as these toxic heavy-metal ions as well. Adsorptive removal of heavy metals in wastewater is usually achieved by using activated carbon or activated alumina (Faust and Aly, 1987). Activated carbon is a porous material with an extremely large surface area and intrinsic adsorption to many chemicals. However, active carbon is only able to remove around 30-40 mg/g of cadmium, zinc, and chromium in water and is nonregenerable, which is quite costly to wastewater treatment. Polymer resins that can form complexes with the heavy metal ions are the best adsorbents (Lu * To whom correspondence should be addressed: 129 Everson Hall, University of CaliforniasDavis, One Shields Avenue, Davis, CA 95616. Phone: 530-752-0840. Fax: 530-752-7584. E-mail:
[email protected].
et al., 1994); however, they are more expensive and more selective to special ions. Regeneration of the exhausted adsorbents on site is not very easy to perform for customers. Thus, the need for an economic and effective adsorbent for the removal of toxic ions from wastewater is existing highly in textile and agricultural wastewater treatment. The present article is to report the feasibility of utilizing sunflower stalks as low-cost adsorbent materials for the removal of toxic heavy-metal ions from wastewater. Sunflower stalks are composed of polyol structures which have proven relatively strong Coulombic adsorption to cations such as organic bases as well as intrinsic adsorption to other materials such as acidic and anionic compounds (Sun and Xu, 1997). Sunflower stalks have relatively large surface areas that can provide intrinsic adsorptive sites to many substrates. On the basis of the structural analysis of the sunflower stalks, it was expected that the adsorbents should be able to remove cationic metal ions as well. Thus, four heavy metal ions, cadmium(II), chromium(III), copper(II), and zinc(II), were selected and used in adsorption isotherm and in kinetic adsorption studies of sunflowers stalks. The effects of temperature and size of sunflower stalks were also evaluated. Experimental Methods Materials. Sunflower stalks were kindly provided by Pioneer Hi-Bred International, Inc., Woodland, CA. Samples were ground to pass through 25-45- and 60mesh sieves and then washed with deionized water until the effluent was colorless. Copper sulfate (CuSO4‚5H2O) and cadmium nitrate (CdNO3‚4H2O) were obtained from Mallinckrodt, Inc. (Paris, KY). Zinc sulfate (ZnSO4‚7H2O) was supplied by J. T. Baker Chemical Co. (Phillipsburg, NJ). Chromium chloride (CrCl3‚6H2O) was purchased from Aldrich Chemical Co. (Milwaukee, WI). All chemicals were used without further purification. The concentrations and pH values of the stock solutions are listed in Table 1. The standard reference solutions of the four ions were purchased from Fisher Scientific. Methods. The concentrations of metal ions were measured with an inductively coupled plasma-atomic
S0888-5885(97)00468-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/13/1998
Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1325 Table 1. Initial Concentration and pH Values of Ion Solution and Their Analytical Wavelengths Used in ICP-AES metal ion cadmium(II) chromium(III) copper(II) zinc(II)
initial conc.
pH
wavelength (nm)
1000 ppm 1000 ppm 1000 ppm 1000 ppm
5.16 2.40 4.25 5.14
228.8 267.7 324.7 206.2
emission spectrophotometer (ICP-AES) (Thermo Jarrell Ash Atomscan 25). The analytical wavelengths of each metal ion were selected and are listed in Table 1. Before each test, calibration curves corresponding to different ions were prepared and referenced. The equilibrium isotherms were determined by mixing 0.20 g of sunflower stalks, which has been conditioned at 21 °C and a relative humidity (RH) of 65% for at least 24 h, with 50.00 mL of ion solution in a 125mL Erlenmeyer flask at 25 and 50 °C. Each isotherm consisted of 10 different concentrations varied from 50 to 1000 ppm for the ions. The flasks containing ion solution and sunflower stalk particles were placed in a shaker (Precision Instrument) and agitated for 5 days at constant temperature. The equilibrium concentrations of different combinations were measured by the ICP-AES and referenced with the calibration curves. The kinetic measurement of adsorption properties of sunflower stalks were carried out with the same equipment and similar conditions. The sample mass was 1.00 g and the volume of the dye solution was 200 mL in this series of tests. Kinetic tests usually ran about 3 h, with samples taken periodically. All of the above tests were at least duplicated or even triplicated to ensure accuracy. Results and Discussion Adsorption Capacity. Adsorption of metal ions on sunflower stalks can be attributed to two terms, intrinsic adsorption and Coulombic interaction. Coulombic term results from the electrostatic energy of interactions between the adsorbents and adsorbates. The charges on both substrates as well as softness or hardness of the charges on both sides are mostly responsible for the intensity of the interaction, or amount of adsorption. Coulombic interaction can be reflected from adsorption of cationic species versus anionic species on adsorbents. The intrinsic adsorption of the materials is determined by their surface areas, which can be observed by the effect of different sizes of adsorbents on the adsorption. It is the major driving force in adsorption processes of activated carbon. Both factors may interact with each other during the course of adsorption of chemicals in aqueous solution as well. The adsorption capacities of sunflower stalks for the four metal ions were determined by measuring equilibrium isotherms. Of the four ions, copper(II), chromium(III), and zinc(II) are in fourth period and cadmium is in fifth, which is softer compared to the other three. Also, chromium contains three positive charges, which has more charges than the other ions. The sunflower stalks were used in different particle size ranges (2560 and >60 mesh) and under two different temperatures (25 and 50 °C). Adsorption isotherms of sunflower stalks (25-45 mesh) for the four metal ions in water are shown in Figure 1. The adsorption capacity of sunflower stalks to the cadmium ion is much higher than that of the copper and zinc ions, though they all
Figure 1. Adsorption isotherms of four metal ions on sunflower stalks (25-45 mesh) at 25 °C.
Figure 2. Langmuir adsorption plots of four metal ions on sunflower stalks (25-45 mesh) at 25 °C.
have two positive charges. The chromium ion, having three positive charges, should be more strongly attracted to the cellulose surface due to the stronger Coulombic interaction; instead, it has the lowest adsorption on sunflower stalks. It seems to us that the affinities of ions to sunflower stalks is not purely determined by the greater or fewer charges they contain. The adsorption tendency of the ions is also possibly dependent on the softness of ion species as well. A similar trend can be found in the higher adsorption of cationic organic dyes on sunflower stalks, which are even softer (Sun and Xu, 1997). The Coulombic forces between ion species and negatively charged cellulose in water may be the major interaction which significantly affects the adsorption of the dye substrates with opposite charges on the materials (Sun and Xu, 1997). However, the negatively charged cellulose surface is extremely soft; it may not have very strong interaction with the core ions which are fully charged and hard species. Thus, the equilibrium adsorptions of metal ions were much smaller than that of cationic dyes. Langmuir Isotherm. Utilizing the Langmuir isotherm (eq 1) to analyze the equilibrium isotherms of the four ions gave the linear plots over a broad concentration range, which are shown in Figure 2. The values of constant KL/aL represent the maximum adsorption capacity of the adsorbents to the particular substrate. By using the analytical method (Sun and Xu, 1997), the maximum adsorption capacities of all adsorbents for different ions were obtained from the Langmuir plots:
Ce/Qe ) 1/KL + (aL/KL)Ce
(1)
1326 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 Table 2. Langmuir Adsorption Parameters of Sunflower Stalks metal ions Cu2+
size (mesh) 25-45
