Packed-bed sorption of copper using spent animal bones: factorial experimental design, desorption and column regeneration
Introduction
Toxic materials are substances that have adverse effects on health. Heavy metals belong to this category of materials due to the many diseases that result from them. Industrial waste containing heavy metals is one of the major sources of water pollution. Copper is among the most toxic metals that affect the environment. Many studies have shown that these metals are toxic even at very low concentrations. Bearing in mind the adverse effects of heavy metals, environmental agencies set permissible limits for their levels in drinking water and other types of water. According to WHO (World Health Organization) guidelines for drinking water, the permissible level of Cu2+ is 1.0 ppm (Van der Leeden et al., 1990).
As such, it is necessary to decrease the concentration of heavy metals in wastewaters to their permissible limits before discharge to the environment. The use of biological, microbial and plant materials, to remove heavy metals from aqueous solutions is well known (Volesky, 1987, Avery and Tobin, 1993, Crist et al., 1990, Lee and Low, 1989, Al-Asheh and Duvnjak, 1996, Lee et al., 1997, Lee et al., 1999, Matheickal and Qiming, 1999, Baily et al., 1999, Al-Asheh et al., 1999). These materials are known as biosorbents. The uptake of metals by these materials has been attributed to their constituents of proteins, carbohydrates and lignin that contain functional groups, such as carboxyl, hydroxyl and amine groups that are responsible for metal sorption (Kuyucak and Volesky, 1988). Therefore, biosorption is a physio-chemical binding of a substance or sorbate, to a biological material.
Sorption of metals by biological materials is often carried out in batch or continuous mode. The practical application of the adsorption of heavy metals is most economically carried out in a packed-column operation (Volesky, 1987, Wagner and Jula, 1981). Packed column adsorption using plant and microbial sorbents has been studied by several researchers. Volesky and Prasetyo (1994) used columns of 4-mm ID and varying bed depth (70, 90 and 120 cm) packed with the marine Alga Ascophyllum nodosum to remove Cd2+. Macchi et al. (1986) used a bed volume of 12-ml with an ID of 1.34-cm, packed with 2 g of exhausted coffee grounds to remove mercury ions from a water solution. Sphagnum-moss peat was packed in two columns, 1-m and 5-cm ID, for the removal of Cr6+ from aqueous solutions (Sharma and Forster, 1995).
In this work, the effect of different operating parameters, such as bed-depth, influent solution flow rate and influent copper concentration has been investigated during the recovery of copper using packed-bed adsorption. The spent animal bones and copper ions as sorbent and sorbate, respectively, were selected as examples for this process. The effect of other operating parameters characterizing the sorption process, such as pH, has been investigated previously using the same adsorption system (Al-Asheh et al., 1999). The study was performed using factorial experimental design. The interaction among the variables is also considered. Some desorption tests have also been investigated in this work.
Section snippets
Adsorbent
Animal bones were collected from nearby butcher shops. All of the attached meat and fat were removed and cleaned from the bones. The bones were then washed several times with tap water and left in open air for several days to get rid of odors. Later, they were transferred to an oven at 80°C for 24 h. Dry bones were crushed and milled into different particle sizes in the range 0.71–2.0 mm and were later used as a sorbent in the sorption tests. Such particle size distribution did not result in
Results and discussion
A 23 complete factorial design can be performed with the values of the operating variables shown in Table 1. This results in 8 tests with all possible combinations of x1, x2 and x3. The beak-through time, y was measured for each of these tests as shown in Table 2. Table 2 also shows replicates for some runs for the purpose of analysis. The tests were performed randomly in order to avoid any time trend or other types of influences on the experiment.
The complete factorial model that can be used
Conclusions
Experiments on the adsorption of Cu2+ from aqueous solutions by spent animal bones were conducted in a packed-bed column. The main effects and interactions among the studied parameters such as bed-depth, influent flow rate and influent metal concentration on the break-through time were analyzed using a two-level factorial design model. The effect of the parameters was assessed at 95% confidence intervals. It was found that although the effects of influent flow rate and influent metal
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