Interfacial Interaction Of Heavy Metal Ions And Aerobic Microbial Granules

Biological treatment is one of the most widely used wastewater treatment processes. Aerobic microbial granules play an important role in the field of biological wastewater treatment due to their advantages over the conventional sludge floes, such as a denser and stronger aggregate structure, better settleability and ensured solid-effluent separation, higher biomass concentration, and greaterability towithstand shock loadings. Heavy metals can be stimulatory, inhibitory, or even toxic in biochemical reactions depending on the metal concentration and speciation, the state of microbial growth, and the biomass concentration. Therefore, study was conducted to investigate the interaction machnisms of aerobic microbial granules and metal ions. The work could provide useful information for the the design and operation of biological systems. In this paper, the binding capacities and mechanisms of aerobic granule with metal ions was investigated. The effects of long-term addition of metal ions on the biochemical properties of aerobic granules were examined. The mechanism and binding sites involved in the interaction of metal ions with modified aerobic granules were also evaluated. Main contents and results are as follows:1. Batch experiments were conducted to study the binding characteristics of a eationic dye, Malachite Green (MG), onto aerobic granules. The Langmuir isotherm was found to provide the best theoretical correlation of the experimental data for the biosorption of MG. The monolayer biosorption (saturation) capacities were determined to be 56.8 mg/g. The aerobic granule had a specific surface of 72.32 m2/g SS. Thermodynamic analysis show that biosorption follows an endothermic path of the positive value of?H0 and spontaneous with negative value of?G0.2. The interaction process of cobalt(?) and zinc(?) and aerobic granules was characterized. Single component and binary equimolar systems were studied at different pH values. The equilibrium was well described by Redlich-Peterson adsorption isotherm. The maximal binding capacity of the granules, in single systems (55.25 mg/g Co; 62.50 mg/g Zn) compared with binary systems (54.05 mg/g Co; 56.50 mg/g Zn) showed reduction in the accumulation of these metals onto aerobic granules. The kinetic modelling of metal sorption by granules has been carried out using Lagergren equations. The regression analysis of pseudo second-order equation gave a higher R2 value, indicating that chemisorption involving valent forces through the sharing or exchange of electrons between sorbent and sorbate may be the rate limiting step. The initial biosorption rate indicated that aerobic granules can adsorb Co(?) more rapidly than Zn(?) from aqueous solutions. Meanwhile, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses revealed that chemical functional groups (e.g., alcoholic and carboxylate) on aerobic granules would be the active binding sites for biosorption of Co(?) and Zn(II).3. The interacting mechanisms of metallic cations (Zn2- and Co2+) to active chemical groups on the extracellular polymeric substances (EPS) of the aerobic granules, including loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS), were examined by XPS and FTIR spectroscopy. For Zn2+ and Co2+, LB-EPS showed stronger binding properties than TB-EPS and the process of them was described well by the Langmuir isotherm. Compared to the single-metal system, binary-metal addition induced competitive binding between the Zn2+ and Co2+ with reduction of the maximal binding capacity for both EPS. The main chemical groups involved in the interactions between contaminants were apparently alcohol, carboxyl and amino. These groups were part of the EPS structural polymers, namely, polysaccharides, proteins, and hydrocarbon-like products. When biosorption and flocculation occurred at the same time, the LB-EPS were used not only as chelate sorbents but also as flocculants to further enhance their sorption capacity.4. This part investigated the individual toxic effects of long-term addition of Cu(?) and Ni(?) on the biochemical properties of aerobic granules in sequencing batch reactors (SBRs). The biochemical properties of aerobic granules were characterized by EPS content, dehydrogenase activity and microbial community biodiversity. One SBR was used as a control system, while another two received respective concentration of Cu (?) and Ni(?) equal to 5 mg/L initially and increased to 15 mg/L on day 27. Results showed that the addition of Cu (?) drastically reduced the biomass concentration, bioactivity, and biodiversity of aerobic granules. The toxic effect of Ni(?) on the biodiversity of aerobic granules was milder and the aerobic granular system elevated the level of Ni(?) toxicity tolerance. Even at a concentration of 15 mg/L, Ni (?) still stimulated the biomass yield and bioactivity of aerobic granules to some extent. The elevated tolerance seemed to be owed to the concentration gradient developed within granules, increased biomass concentration, and promoted EPS production in aerobic granular systems.5. According to quantum chemistry calculation, it is investigated that copper ions and hydrated copper ions are preferred to interact with amine groups. Porous aerobic granules were grafted with polyethylenimine (PEI), due to the presence of a large number of amine groups in the PEI molecule. The biosorption characteristics of cations and anions from aqueous solution using modified aerobic granules were investigated. FTIR and XPS analysis exhibited the presence of PEI on the granule surface. Compared with the raw granule, the modified aerobic granules with PEI showed a significant increase in sorption capacity for both metal ions. The monolayer biosorption capacity of granules for Cu(?) and Cr(?) ions was found to be 71.239 and 348.125 mg/g. The optimum solution pH for adsorption of Cu(?) and Cr(?) from aqueous solutions was found to be 6 and 5.2, respectively. The biosorption data fitted better with the Redlich-Peterson isotherm model. FTIR showed chemical interactions occurred between the metal ions and the amide groups of PEI on the biomass surface. XPS results verified the presence of Cr(?) on the biomass surface, suggesting that some Cr(?) anions were reduced to Cr(?) during the sorption.