3. Results and discussion
Fig. 1 shows a typical SEM image of the poly(PPA)-Cys nanofibers.
The average diameter of the poly- (PPA)-Cys nanofibers was about 200 nm. The lengths of the nanofibers depended on the amount of charge passed. SEM images indicate that the nanofibers were sufficiently dispersed and aggregation was not a major issue. Before exposure to nanofibers, sample from tannery waste water was tested for several parameters such as chromium concentration, chemical oxygen demand (COD), biological oxygen demand, (BOD), suspended solids (SS), phosphate and pH. The resulting data is presented in Table 1. Various factors that are known to affect chromium ions sorption/complexation processes (Scheme 1d) were investigated. These factors include i) time ii) pH and iii) concentration of the nanofibers. The removal of chromium by the nanofibers (1 mg of nanofibers/mL of waste water) was investigated at various times ranging from 5 to 60 min at pH of 3.28. The results of these experiments are summarized in Fig. 2. It was evident that over 80% of the Cr(III)was removed within 10 min. Maximum Cr(III) removal was achieved in about 45 min when almost 90% Cr(III) was removed. 45 min was therefore selected as the optimum time for Cr(III) removal. The next parameter to be investigatedwas the pH of the tannery waste water from the initial pH of 3.2e9.0 by addition of 1 N NaOH. The pH of the medium is an important factor in determining the rate of surface reactions. The effect of pH on the adsorption of metal ions on the adsorbent is presented in Fig. 3. For Cr(III), the amount adsorbed increased from 91% to 93.5% as the medium pH was adjusted from 3.2 (initial pH of wastewater) to 5.5 respectively. Increase in pH resulted in further adsorption/ complexation of the Cr(III) as was reflected in removal rates of 99.8 and 99.9% at pH 7.0 and 9.0 respectively. The variation in adsorption capacity is largely due to the influence of pH on the surface adsorption characteristics of nanofibers. The increase in metal ion removal as pH increased can be explained on the basis of a decrease in competition between proton (Hþ) and positively charged metal ion at the surface sites, and by decrease in positive charge which results in a lower repulsion of the adsorbing Cr(III) ions. This indicated that the adsorption capacity of the nanofibers is clearly pH dependent (Cimino et al., 2000; Selomulya et al., 1999). However, a pH of 5.5 was selected for further studies to ensure that the removal of Cr(III) in the water was due to the sorption/complexation of the ion with the nanofibers and not due to any precipitation of hydroxides or oxides of chromium at higher pHs and/or basic pHs.