| Perchlorate is an anion that originates from the dissolution of the salts of perchloric acid. It is widely present in the environment occurring both naturally and through manufacturing. Perchlorate is excessively soluble and mobile in aqueous media and can persist for many decades due to its chemical stability. It adversely affects human health by interfering with iodine uptake into the thyroid gland and consequently damaging thyroid gland which controls growth, development and metabolism. Perchlorate, a serious threat to human health, is being seriously concerned in advanced water treatment.Many different technologies such as ion exchange, biological treatment, and plant uptakeare currently under R&D for efficient perchlorate removal in various water resources. Among them, autotrophic biodegradation by perchlorate reducing microbes (PRMs) emerges as a promising cost-effective treatment process attracting increasing attention worldwide. During this process, perchlorate as the terminal electron acceptor is utilized by PRMs, and can be eventually completely transformed into chloride and O2via the sequence:ClO3-, ClO2-. To accomplish the autotrophic biodegradation of perchlorate, PRMs need electron donors, preferably H2in many cases. Though H2is more cost-effective than organic electron donors and has minor biomass clogging problem, the explosive properties make its broad-scale application risky and constrained to some extent.One alternative safer way to provide H2for PRMs is the addition of zero-valent iron (ZVI), which corrodes in water and produces H2as follows. The effective use of H2released from ZVI corrosion by PRMs (both pure and mixed cultures) as electron donor has been demonstrated by many research groups. Besides H2production, ZVI corrosion increases solution pH and produces iron (hydro)oxides precipitates, which makes perchlorate removal by a PRMs associated with ZVI (PRMs-ZVI) system more complex.Besides, many factors were reported having significant effect on perchlorate biodegradation performance by PRMs such as the initial pH, electron donor, redox potential, competing ions and ionic strength. However, no special efforts have yet been devoted to understanding the complex role of Ca2+, an important divalent cation widely present in aqueous media, in perchlorate removal by PRMS-ZVI system.The amount of Ca2+may have direct/indirect effect on the structure or metabolic activities of bacteria. Besides, the presence of Ca2+may induce physicochemical perchlorate removal in PRMs-ZVI system. For instance, Ca2+may attach to iron (hydro)oxides leading to higher positive surface charge, and subsequently boost the adsorption of anions such as perchlorate for surface charge neutralization. Also, similar to iron (hydro)oxides, CaCO3produced at high solution pH may act as nucleus forming larger particles with iron (hydro)oxides by nucleation. The increased particle size may enhance the removal of anions (i.e. ClO4-) through co-precipitation.This study attempts to prove the feasibility of perchlorate removal by autotrophic PRMs associated with ZVI. Besides, we research the influence of the correlative factors, such as adding amount of ZVI, initial pH, et al. on perchlorate removal in this system. The important part of this study was about the complex effect of Ca2+on perchlorate removal performance in PRMs-ZVI system. According to the increase of chloride concentration, the respective contribution of biodegradation and physicochemical perchlorate removal was differentiated and evaluated. Batch beaker test results showed that perchlorate removal initiated earlier and completed faster with the increasing amount of Ca2+. In the absence of Ca2+, the final perchlorate removal efficiency was around40%fully contributed by biodegradation. In contrast,100%perchlorate removal efficiencies were achieved with corresponding biodegradation efficiencies accounting for around61,69, and83%at Ca2+concentration of60,120, and240mg/L, respectively. It appears that Ca2+can delay pH increase by combining with OH-and consequently extend biodegradation processes. When ZVI and Ca2+co-existed, physiochemical perchlorate removal occurred earlier than biodegradation mainly by surface charge neutralization and co-precipitation. The positive effect of Ca2+on perchlorate removal performance was further corroborated in subsequent continuous column tests. As empty bed residence time decreased from140to27min, more reliable and effective perchlorate removal performance was observed with Ca2+concentration of240mg/L. Nevertheless, Decachlorosoma and Dechloromonas were identified to be the main PRMs species accounting for over90%of the entire bacterial population, regardless of Ca2+... |
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