| Currently, studies on antimony-containing wastewater purification through microbial adsorption process mainly focus on flocculate adsorbents, such as bacteria, fungi, and algae. Due to the weak settling ability of these adsorbents, post-separation operation can be time-consuming and costly. Moreover, microorganisms can not adsorbe oxyanions efficiently owning to the limit imposed by their natural negative surface charge. Therefore this study utilized a combination of microbial granulation technology and surface modification technology to transform the characteristics and form of activated sludge, which could enhance the removal capacity of Sb(V) and improve the efficiency of post-separation. Study on the modification process and sorption mechanism can provide fundamental knowledge to improve the adsorption capacity of AGS (aerobic granular sludge), and facilitate its further engineering application. This study therefore can promote the effective recycling of resources, and construction of a resource efficient and environment friendly society.Firstly, the methods for biomass surface modification were explored using flocs produced during the granulation process. Five low cost and low pollution modifiers, namely, NaCl, CaC12, FeC13, C2H5OH and HC1, were tested with granule flocs (GF) for screening based on their individual adsorption capacity. Results indicated that Fe-treatment resulted in the highest removal capacity of 99%for Sb(V) at pH 4.3, therefore the modification method of FeC13 would be applied in the subsequent studies. Furthermore, two types of aerobic granular sludge (AGS) were cultivated in this study through different conditions, namely, AGS-B (Bacteria) and AGS-F (Fungi). The morphology, physical structure and microbial community composition in these two kinds of AGS were investigated and found to show significant difference. They were further modified by FeC13 for adsorption studies.Fe-treated AGS exhibited significant difference from the original AGS in both color and texture. The surface of Fe-treated AGS-B seemed less viscous, but the structure was more compact. Chemical modification also changed the contents of metal elements in AGS drastically. Before modification, the major metal element in AGS-B was calcium (Ca), with the concentration of 8.8%(w/w), while the content of Fe was only 0.11%. In the case of AGS-F, potassium (K) was enriched with concentration of 2.5%, and the level of Fe was 0.06%. After treatment by FeCl3, both types of granules were attached with significant amount of Fe, the concentrations of which were 5.9% and 1.6%, respectively, while other metal elements such as K, Na, Ca and Mg could hardly be found. In terms of surface of morphology, untreated AGS-B mainly contained coccus and bacillus, and untreated AGS-F was composed of filamentous fungi with abundant mycelia twisting around each other. After treatment by FeC13, small particles were observed to distribute on the surface of microorganisms in the AGS. The element deposition on the granule surface was further analyzed by energy dispersive spectrometer (EDS), and results confirmed that the relative abundance of Fe increased dramatically. Changes of the surface functional groups were analyzed by Fourier transform infrared spectroscopy (FTIR). For AGS-B, significant shift of the adsorption peak of hydroxyl group was found to occur after modification, and inference was made that precipitate of Fe(OH)3 resulted from hydrolysis of FeC13, then the hydroxyl groups in the extracelluler polymer substance (EPS) of AGS interacted with Fe(OH)3 by surface complexation, which attached Fe(OH)3 onto the AGS.This study proceeded to investigate the effect of various conditions on the sorption of Sb(V) onto Fe-treated AGS, including pH, biomass concentration, temperature, mixing condition, ioinic strength as well as in interfering ions. The pH value of the bulk solution affected adsorption capacity of modified AGS significantly. Strong acidic environment (pH 2.0 and pH 2.8) could lead to dissolution of Fe(OH)3 formed by modification, and adsorption capacity was reduced accordingly. Strong base environment (pH 11.3) could promote the dissolution of EPS on the AGS, which resulted in the disintegration of Fe-treated AGS-B. In addition, a general alkaline condition could cause the Fe-treated AGS to be charged negatively on the surface, which would reduce its capacity to adsorb Sb (V) in the form of Sb(OH)6-.3.4 was found to be a suitable pH value for the removal of Sb(V) by Fe-treated AGS. High biomass concentration could improve the removal rate of Sb(V), which reached 95% when concentration was higher than 18g/L (wet basis) in the system. The influence of temperature was found to be insignificant on the sorption effect, but higher temperature could enhance adsorption to some degree. Increase in the shaking rate could improve mixing strength in the bulk solution, and promote diffusion of Sb (V) into inside of adsorbent. When shaking rate was increased to 175rpm, adsorption rate of the entire process increased and time to reach adsorption equilibrium was shortened. Addition of NaCl to increase ion strength could lead to significant inhibitory effect and lower the sorption capacity. Fe-treated AGS-B showed no affinity with possible interfering cations like Ni2+, Cu2+ and Zn2+, but the corresponding anions of these salts could reduce the sorption capacity through competitive adsorption. Based on the experimental results and a relevant surface complexation model, the mechanism of Sb (V) adsorption onto Fe-treated AGS was inferred. Due to protonation at pH 3.4, the surface of AGS was positively charged. Sb (V) in the form of Sb(OH)6-then went near the micro-interface of Fe(OH)3 by electrostatic interaction. Subsequently, outer-sphere complex could be formed through ion exchange, and inner-sphere complex could be formed further.Various kinetic, isotherm, and thermodynamic models were then applied to study the sorption behavior of Fe-treated AGS. Among the three kinetic models, pseudo-second-order model was found to best describe the adsorption process, and the correlation coefficient was higher than 0.998. This indicated that adsorption process could be classified as chemisorption. The velocity of Fe-treated AGS-F was found to be 2.8 times,4.4 times and 3.7 times higher than that of Fe-treated AGS-B under the initial Sb (V) concentrations of 20,60, and 100 mg/L by calculation of kinetic model, respectively. Fitting results of intraparticle diffusion models indicated that process of Sb (V) adsorption included a faster film diffusion and a slower intraparticle diffusion. Langmuir model showed a better correlation than Freundlich model for Fe-treated AGS, which suggested that monolayer chemisorption could be the main sorption mechanism. Results of thermodynamics illustrated that the process of sorption was endothermal and the reaction was spontaneous.Finally, some other sorption processes were explored with AGS. Direct adsorption of cations onto unmodified AGS in antimony-containing wastewater was investigated in last part of this study. Adsorption of Cu (II), Zn (II) and Ni (II) onto AGS-B and AGS-F was studied at different pH. Strong acidic environment (pH 1.9 and pH 2.6) was found to be unfavorable for the adsorption. AGS-B had a strong stronger adsorption ability of cations than AGS-F, and Ni (II) had weaker affinity with AGS than Zn (II) and Cu (II). Adsorption of Sr (â…¡) onto AGS-B was also studied systematically in this part, including the various influencing factors. Study of a continuous adsorption-desorption process of Sr (II) indicated that NaCl and KC1 were ideal agents for desorption, and the re-adsorption capacity of Sr (II) was not affected, but CaCl2 and H2O were not suitable. Combining the results from the ion exchange experiment and adsorption-desorption experiment, it was inferred that adsorption mechanism of Sr (â…¡) mainly included ion exchange and physical adsorption, while the contribution of other mechanisms such as complexation and precipitation was insignificant. Both pseudo-second-order model and Langmuir model could describe the sorption process well for AGS, and results of thermodynamics study indicated that adsorption of Sr (â…¡) was also endothermal.In conclusion, aerobic granular sludge proved to be a very promising biosorbent, and had shown significant potential in the practice of metal cations and oxyanions removal from wastewater via adsorption processes. |
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