Comparison of Biosorption Behaviors of Various Metals by Aerobic Granules

Figures 11.19 and 11.20 compare the biosorption of cadmium, copper, and zinc by aerobic granules (Xu, 2006). The biosorption rate constant is in the order of Cd2+<Cu2+<Zn2+, indicating that zinc biosorption by aerobic

Fig. 11.19. Comparison of various metals on biosorption capacity at Cq = 10mg/l and Xq = 100mg/l (Xu, 2006).
Fig. 11.20. Comparison of various metals on biosorption rate constant at Cq = 10mg/l and Xq = 100mg/l (Xu, 2006).

granules was the fastest, while cadmium biosorption by aerobic granules was the slowest. These results could be attributed to the different active sites used for adsorption, which reflect the strength of the metal sorptive bond and the rate of adsorption onto the active sites (Kaewsarn and Yu, 2001). The factors that affect the biosorption preference of biosorbent for different kinds of adsorbates may be related to the characteristics of the binding sites (e.g. functional groups, structure, surface properties, etc.), the properties of the adsorbates (e.g. concentration, ionic size, ionic weight, ionic charge, molecular structure, ionic nature or standard reduction potential, etc.), and the solution chemistry (e.g. pH, ionic strength, etc.) (Aksu and Gulen, 2002).Under comparable test conditions, the amount of metal adsorbed by aerobic granules was subjected to the following order of Cd2+ >Cu2+ >Zn2+ (Fig. 11.19). These results are consistent with the observations by Xu (2002) that the complex stability and binding affinity of metal ions to Laminaria japonica is in the order of Zn < Ni < Cu < Cd simply because cadmium usually has higher affinity and stability to bond with carboxyl groups.

A proportional relationship between Qe and C0 (Figs 11.7-11.9) indicates that the cadmium, copper, and zinc biosorption on aerobic granule surface could be driven by the concentration gradient of metal at a constant granule concentration. This implies that the driving force for metal biosorption would result from a soluble metal concentration that is higher than the concentration that would be in equilibrium with the amount of metal adsorbed on the aerobic granules. For a constant initial metal concentration, Figs 11.13-11.15 show that Qe declined as the initial granule concentration increased. Similar phenomena had been reported in studies on lead and zinc uptake by S. cinnamoneum, P. chrysogenum, and Citrobacter sp. (Puranik et al., 1999) and iron (III) and iron (III)-cyanide complex ion uptake by Rhizopus arrhizus (Aksu and Gulen, 2002). In the environmental engineering literature, the effect of metal concentration on Qeq was mainly presumed to be due to dilution of the metal with the increased biomass concentration (Taniguchi et al., 2000). It is a reasonable consideration that the number of binding sites to metal on aerobic granules is proportional to the amount of aerobic granules added to the batch tests, i.e. high granule concentration could result in a lower relative metal concentration on the basis of unit mass of aerobic granules.

The biosorption capacity of cadmium, copper, and zinc by aerobic granules was inversely related to their initial aerobic granules concentration (Figs 11.13-11.15), i.e. the metal uptake decreased with the increase of the initial aerobic granules concentration. Othman and Amin (2003) also found that Zn2+, Cu2+, and Mn2+ biosorption capacities by a conventional biosorbent decreased from 24.2 to 10.5 mg/g for Zn2+,

5 to 2.8mg/g for Cu2+, and 37.5 to 8.2mg/g for Mn2+ when the biosor-bent concentration was increased from 0.5 to 3 g/l. Similar trends were also observed in the biosorption of heavy metals by Oscillatoria anguistissima, marine algae, and fungal biomass (Ahuja et al., 1999; Khoo and Ting, 2001).

Figures 11.7-11.9 and 11.13-11.15 indicate that both initial metal and granule concentrations can influence the biosorption capacity of metal at equilibrium, i.e. the biosorption process of metal by aerobic granules cannot be described by C0 or X0 alone. When biosorption tests are carried out at a given metal concentration, higher biomass concentration could lower real metal concentration on the basis of unit biomass added. In this case, a concept of relative metal concentration is proposed and defined as the ratio of initial metal concentration to initial granule concentration, i.e. C0/X0. This ratio indeed quantifies dilution of metal concentration with the added biomass. The observed relationship between Qe and C0/X0 ratio obtained at various C0 or X0 for cadmium, copper, and zinc are presented in Figs 11.21 and 11.22, respectively. It appears that Qe increases with the increase of C0/X0 ratio. These results imply that the individual effects of C0 and X0 on the metal biosorption on the surfaces of aerobic granules can be unified by the C0/X0 ratio for batch tests initiated at different C0 and X0. An important implication of Figs 11.21 and 11.22 is that if C0 or X0 is not strictly controlled in batch experiments, the

Fig. 11.21. Effect of Co/Xo ratio on the heavy metal biosorption capacities at equilibrium (Qe) at various initial metal concentrations (Xu, 2006).

Fig. 11.21. Effect of Co/Xo ratio on the heavy metal biosorption capacities at equilibrium (Qe) at various initial metal concentrations (Xu, 2006).

m 300

m 300

Cq/Xq (mg metal/mg granules)

Fig. 11.22. Effect of C0IX0 ratio on the heavy metal biosorption capacities at equilibrium (Qe) at various initial aerobic granules concentrations (Xu, 2006).

Cq/Xq (mg metal/mg granules)

Fig. 11.22. Effect of C0IX0 ratio on the heavy metal biosorption capacities at equilibrium (Qe) at various initial aerobic granules concentrations (Xu, 2006).

Cq/Xq ratio could better reflect the real driving force for metal biosorption by microorganisms, and provides a unified basis for interpretation of the biosorption data obtained at different initial metal and biomass concentrations.

It should be realized that biosorbents currently used are microbial flocs or dispersed bacteria. One serious operation problem associated with those biosorbents is separation of used biosorbents from the treated effluent. For achieving an efficient solid-liquid separation, an additional settling facility is required. As compared to conventional floc-form biosorbents, aerobic granules have the advantages of compact microbial structure, and excellent settling ability. The settling velocity of the aerobic granules used was 71 m/h, which is 5-8 times higher than that of microbial flocs. In this study, the aerobic granules can be completely separated out from the treated effluent by gravity in one minute. When selecting appropriate biosorbents for the removal of heavy metals from industrial wastewa-ter, three criteria should be seriously taken into account, i.e. effectiveness, robustness, and reliability of biosorbents. It appears that the characteristics of aerobic granules could satisfy these requirements for biosorbents. It can be expected that aerobic granule-based biosorption process is an efficient and cost-effective technology for the removal of heavy metals from industrial wastewater streams.

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