Interaction between Heavy Metals and Aerobic Granules

Abstract

The interaction of heavy metals and aerobic granular sludge occurs on the surface and the inner space of sludge. These physical or biochemical processes are mainly based on sludge and are also influenced by environmental conditions and metal characteristics. Many studies have been conducted to uncover metals bioremediation by aerobic granular sludge and the effects of metals on sludge. These results are helpful for understanding the metal transformation and performance of aerobic granular sludge process. In bioremediation of heavy metals by aerobic granular sludge, adsorption accounts for most metal uptake. The high biomass retention, compact and porous structure, and excellent settling ability of aerobic granules enable them good performance in the biosorption of heavy metals. Aerobic granules show high adsorption capacities when compared with other biosorbents and some commercial adsorbents. Heavy metals can be adsorbed to aerobic granules by replacing the sites of light metal ions (such as Ca2+, Mg2+, K+, and Na+) which already exist in the high content EPS, or binding to the functional groups on EPS and cell wall. Chemical precipitation on the metal–sludge interface also contributes to metal sorption to granules. Further experiments and characterizations of aerobic granule are needed to interpret space distribution of adsorbed metals and the adsorption mechanisms. Besides, good stability and unique structure of aerobic granules provide opportunity to enhance desired surface functional groups by chemical modification, which leads an important field in biosorption. Aerobic granules possess superior settling ability, so the removal of heavy metals in a continuous reactor similar as UASB or SBR maybe more hopeful. Effects of heavy metals on the alive microbe are complicated, especially in the aerobic granular sludge reactor. The inhibition of heavy metal on aerobic granules depends on the metal species and concentrations. Organic pollutants degradation and nitrification are both negatively affected by the prolonged addition of high concentration heavy metals. However, higher toxicity-resistance than flocculent sludge has been proved in aerobic granules. This could be attributed to the unique compact structure and diverse microcosm. Diffusion resistance developed by the layer structure functions as barrier and buffer. The responses to heavy metal exposure are also of interest. Microbes in aerobic granules can regulate EPS synthesis and alternate their metabolic pathways to acclimate to meal toxicity. High concentration heavy metals should weaken the stability of whole microorganisms in the aerobic granules. Microbial test is a useful method for exploring sensitivity of different microorganisms to metals. Better understanding of metal biotransformation and responses of aerobic granules is essential for optimizing the aerobic granule reactor treating or exposed to heavy metals.