9] had been based solely on ICP, which measures total chromium. The authors had assumed that all Cr(VI) is retained after passing through the column, and had not considered the possibility of reduction to Cr(III). Reduction may indeed not
Figure 4. Amount of chromium removed in mg/g (q) by trimmed original wool as function of Ce, the equilibrium concentration of Cr(VI). Adsorbent dosage = 8.0 g/L, T = 25.0˚C, shaking speed = 150 rpm.
Figure 5. Langmuir isotherm for Cr(VI) and total chromium using original wool as adsorbent in long term mode (5 days). Adsorbent dosage = 8.0 g/L, pH = 1.5, T = 25.0˚C, shaking speed = 150 rpm.
Figure 6. Langmuir isotherm for Cr(VI) and total chromium using original wool as adsorbent in short term mode (contact time 3 h). Adsorbent dosage = 8.0 g/L, pH =1.5, T = 25.0˚C, shaking speed =150 rpm.
Table 2. Langmuir parameters (b, Q and R2) for total chromium and Cr(VI) for short term and long term equilibration periods, with original wool as adsorbent. Adsorbent dosage = 8.0 g/L, T = 25.0˚C, shaking speed =150 rpm.
Table 3. Freundlich isotherm parameters (Kf, n and R2) for total chromium and Cr(VI) for short and long term modes using trimmed wool as adsorbent. Adsorbent dosage = 8.0 g/L, T = 25.0˚C, shaking speed =150 rpm.
occur when the contact time between wool and the eluent is very short. Under such conditions, all forms of chromium are essentially removed. However, when solutions containing Cr(VI) were left in contact with wool in the column for longer periods, the first fraction contained higher concentrations of total chromium, and this can now be attributed to the presence of Cr(III). To verify this conclusion, 100 ppm Cr(VI) solution was applied to a column packed with original wool and eluted at a constant rate of 2.0 ml/min. 100 ml fractions of eluents, representing short term equilibration were collected for four successive overnight periods, with the elution stopped at the end of each day, thereby allowing long term equilibration of Cr(VI) on wool. The data for total chromium and Cr(VI) are presented in Figure 7. Virtually, the only form of chromium eluted overnight is Cr(III). This corroborates the finding that upon prolonged exposure on wool, Cr(VI) undergoes reduction to Cr(III).
3.5. FTIR Spectroscopy
Figure 8 displays FTIR spectra of wool before and after adsorption of Cr(VI). The very weak peak at 939 cm−1 in free wool becomes much more pronounced in wool loaded with chromium for long term contact, due to the oxidation of wool by Cr(VI). This peak can be attributed to S=O bonds that form when cystine in wool is oxidized. A similar assignment has been reported for wool oxidized by UV irradiation  . This finding supports the proposed mechanism for removal of Cr(VI) by wool from aqueous solution (Equation (1)).
3.6. Electron Dispersive X-Ray Spectroscopy (EDS)
Scanning electron microscopy (SEM) was employed to study the surface of wool before and after loading with chromium. In order to obtain the elemental analysis of the surface, EDS spectra were recorded for both samples (Figure 9). Inspection of this figure reveals that chromium is retained on wool at the end of the long term contact time, with the appearance of a chloride peak from HCl.
Figure 7. Mass of chromium species in eluted fractions. Fraction volume = 100 ml, total applied volume = 2.5 L, pH of applied solution = 1.5, wool mass 26.0 g, wool depth = 29.0 cm. T = 25.0˚C.
Figure 8. FTIR spectra of free wool (bottom) and wool loaded with ionic chromium species (top).
Figure 9. EDS results for original free wool (top) and wool loaded with ionic chromium species (bottom).
The affinity of wool for Cr(VI) varies with contact time and pH. For short contact times, the Langmuir adsorption isotherm was obeyed with no detectable change in the oxidation state. However, removal percentages did not exceed 90%. Long contact times resulted in more than 99% removal of Cr(VI). A 2-step mechanism for this removal is proposed. The first involves fast adsorption of Cr(VI) on wool and the second a slow catalytic reduction of Cr(VI) to Cr(III), followed by desorption of Cr(III) into solution. The surface of wool before and after adsorption was characterized by FTIR and EDS and the results with the suggested mechanism. The optimum parameters for this significant improvement in Cr(VI) removal and hence its environmental remediation can now be identified as pH 1.5, contact time of at least 5 days, and a minimum adsorbent dosage of 8.0 g/L. For the short term study, pH 2.0 was selected so as to provide comparison with previous results. Cr(VI) adsorption follows the Langmuir adsorption isotherm with Q = 64.5. This high capacity of wool for Cr(VI) provides a practical solution for the removal of Cr(VI) from industrial wastewater. A plant can be constructed consisting of a batch adsorption reactor with a 5-day retention time in which Cr(VI) is mostly removed. The effluent could then subjected to pH 10.0 at which Cr(III) precipitates as hydroxide which can be recovered by sand filtration for further reuse.
This research was supported by the American University of Sharjah, Grant FRG12-2-10.
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