4.2 Continuous Metal Removal Using DFCR With Immobilized SRB Biomass
4.2.1 Single component system
4.2.1.1 Heavy metal and COD removal and sulfate reduction
Figure 4.19a represents EDX spectrum of the whole cell immobilized bead (control bead);
insert shows FESEM image of the bead. Similarly, Fig. 4.19(b-g) illustrates EDX spectra of metal laden immobilized beads; insert show FESEM images of the corresponding beads.
The EDX spectrum and FESEM image of immobilized SRB bead cut at its cross-section and metal laden bead are shown in Fig. 4.20. A comparison of FESEM-EDX profiles of these immobilized SRB containing beads (Figs. 4.19 and 4.20) confirms metal precipitation by SRB together with the presence of other elements that constituted the modified Postgate medium, similar to the findings obtained in the previous study (Figs. 4.9 and 4.14).
Moreover, metal removal by the beads is attributed to their precipitation outside the beads (Figs. 4.19 and 4.20). Based on the results obtained, a schematic of metal removal mechanism by immobilized SRB beads is shown in Fig. 4.21.
Figure 4. 21 Schematic of metal removal mechanism by immobilized SRB beads.
was obtained at 48 h HRT for all the metals, except for Fe(III), which showed nearly 75%
(Fig. 4.22a). At 24 h HRT, more than 86% removal is obtained for Pb(II), Cd(II), Zn(II) and Cu(II), whereas in case of Fe(III) and Ni(II), more than 75% removal could be achieved at the respective inlet metal concentrations (Fig. 4.22b). Figures 4.23-4.24 present sulfate and COD reduction profiles obtained using the DFCR operated at different conditions. A low inlet metal loading (48 h HRT) yielded sulfate removal efficiency in the range 54-71% for all the metals (Fig. 4.23a).
Figure 4. 22 Continuous removal of heavy metals using the DFCR with immobilized SRB beads: (a) 48 h HRT and (b) 24 h HRT (●: Metal outlet, ○: Metal inlet, ▲: % Metal removal).
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% Removal
Metal concentration (mg/L)
Time (Days)
M Outlet M Inlet % Removal
Cd Cu Ni Fe Pb Zn
(b)
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% Removal
Metal concentration (mg/L)
Time (Days)
M Outlet M Inlet % Removal
Cd Cu Ni Fe Pb Zn
(a)
However, these values were reduced (46-62%) at 24 h HRT (high inlet metal loading) (Fig.
4.23b). Figure 4.24a reveals that COD removal ranged from 57-73% for all the metals at a low inlet metal loading (48 h HRT). At a high metal loading (24 h HRT), COD removal was slightly low (Fig. 4.24b).
Figure 4. 23 Sulfate removal in the presence of different heavy metals using the DFCR: (a) 48 h HRT and (b) 24 h HRT (●: Sulfate outlet, ○: Sulfate inlet, ▲: % Sulfate removal).
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% Removal
Sulfate concentration (mg/L)
Time (days)
Out let In let % Removal
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% Removal
Sulfate concentration (mg/L)
Time (days)
Out let In let % Removal
(b)
(a)
Figure 4. 24 COD removal in the presence of different heavy metals using the DFCR: (a) 48 h HRT and (b) 24 h HRT (●: COD outlet, ○: COD inlet, ▲: % COD removal).
A slight reduction in the metal removal at a high inlet metal concentration (Fig. 4.22) indicated inhibitory effect of the metals on SRB activity and growth (Hsu et al., 2010), which is also evident from the corresponding low sulfate and COD reduction values presented in Figs. 4.23-4.24. This small reduction in the continuous removal of heavy metals, sulfate and COD reduction at a high inlet metal concentration can be attributed to diffusion limitation of the substrates (sulfate and COD), as noted previously in the case of batch heavy metal removal using immobilized SRB (Figs. 4.16-4.17). Similar results of
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% Removal
COD concentration (mg/L)
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Out let In let % Removal
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COD concentration (mg/L)
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Out let In let % Removal
(b)
(a)
sulfate and COD reduction were reported by Tucker et al. (1998) in their study, but with formate and lactate as the electron donors. Results of continuous heavy metal removal using the DFCR packed with immobilized SRB beads (Fig. 4.22) reveal that the metal removal was maximum for both 24 h and 48 h HRT at their respective low inlet metal concentrations, which correlated well with SRB activity in terms of sulfate reduction, COD removal and sulfide formation (Figs. 4.23-4.25). Values of sulfide generated during the experiments are shown in Fig. 4.25 for 48 h and 24 h HRT, which correlated well with the sulfate reduction values observed earlier.
Figure 4. 25 Results of sulfide generated in the DFCR with immobilized SRB beads at (a) 48 h HRT and (b) 24 h HRT (■ Sulfide generated (mg/L)).
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Sulfide (mg/L)
Time (days)
Cd Cu Ni Fe Pb Zn
(a)
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Sulfide (mg/L)
Time (days) Cu
Cd Ni Fe Pb Zn
(b)
Tucker et al. (1998) investigated the potential of SRB immobilized polyacrylamide gel in column reactors for the removal of uranium and molybdenum, and achieved almost 99%
metal removal without any inhibitory effect on SRB activity at a low inlet concentration in the range 5-10 mg/L. Zhang et al. (2016) achieved more than 99% metal removal at very high initial concentrations of Cd, Cu, Fe and Zn using upflow anaerobic packed bed reactor with immobilized SRB beads; the beads were prepared based on a mixture of PVA, sodium alginate, silicon sand, maize straw powder, zero valance iron and SRB sludge.
Compared with 24 h HRT, 48 h HRT yielded a maximum removal of metals, sulfate and COD reduction with the DFCR (Figs. 4.22a-4.24a). Effect of HRT on reactor performance is well reported in the literature; in general, metal precipitation by SRB occurs in the range of a few days to 5 days (Kuyucak and St-Germain, 2006). Moreover, it is usually observed that at short HRT values, sufficient time is unavailable for the SRB to reduce sulfate, thereby hindering metal precipitation as sulfide. Whereas, a long HRT offers a greater extent of sulfate reduction by using the carbon source more efficiently, thereby precipitating the metals as their respective sulfide salts (Dvorak et al., 1992).
Another reason for a low heavy metal removal efficiency at 24 h HRT may be due to substrate diffusion limitation and intermittent biomass washout from the reactor, which could be overcome by improving the mechanical stability and porosity of the bio-support material. Among the different metals tested in this study, Cu(II) removal was maximum at all inlet concentration levels owing to its low solubility product value with sulfide.
Consequently, the metal removal order matched well with their solubility product values reported in the literature (Hill et al., 2005). The metal removal thus followed the order: Cu >
Zn > Pb > Cd > Ni > Fe, which matches well with the metal removal obtained in the batch study (Fig. 4.16).
Performance of the DFCR in terms of metal removal rate with respect to inlet metal loading rate (ILR) is shown in Fig. 4.26, which reveals that the metal removal efficiency is maximum at a low ILR, indicating that SRB activity is unaffected due to the metals at a low inlet concentration (Fig. 4.26). However, with an increase in the ILR, the removal rate and removal efficiency decreased, indicating an inhibitory effect of the metals at a high inlet concentration (175 mg/L in case of Cu(II) and 90 mg/L for the other metals) on SRB activity (Fig. 4.26).
From Fig. 4.26, the line passing through the origin indicates a stable performance of the system at different input conditions; the removal rate values that are offset from the line passing through the origin indicate that ILR values beyond this point are unfavorable for achieving a high metal removal using the DFCR. Hence, metal loading rates greater than 4.29 mg/L∙h in case of Cu(II) and 2.20 mg/L∙h, in case of Ni(II), Zn(II), Pb(II), Fe(III) and Cd(II) are considered toxic and inhibitory to SRB activity and, therefore, detrimental to the reactor performance.
Figure 4. 26 Heavy metal removal rate as a function of inlet metal loading rate obtained using the continuously operated DFCR with immobilized SRB beads: (a) Cd(II), (b) Cu(II), (c) Ni(II), (d) Fe(III), (e) Pb(II) and (f) Zn(II) (▲: Metal removal rate from single component system; ○: Metal removal rate from multi-metal solution).