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Membrane Bioreactor for Perchlorate Treatment

Introduction

Perchlorate (C104) is a highly persistent and toxic oxyanion that is continuously being released in our waters by natural and anthropogenic means (e.g., poor industrial practices, military' testing, unregulated disposal, and natural deposition). Salts of perchlorate are widely used as the primary ingredient in the manufacture of explosives and solid-state rocket fuel due to their high chemical stability and powerful oxidizing capacity. Since perchlorate is highly soluble in water (i.e., water solubility of NaC104 at 25°C equals 2,100 mg/1) and has poor retention in soils and minerals, it becomes extremely mobile and strongly persists in the aquatic environment (Urbansky and Brown 2003, Cao et al. 2019).

Numerous cases of perchlorate detection in water resources have been reported globally and some even mentioned СЮ4~ concentrations as high as 811 pg/L, 120 mg/L and 3,700 mg/L in drinking water, surface water and groundwater, respectively (Guan et al. 2015). Such concentrations can pose potential health risks to aquatic and terrestrial organisms. Schmidt et al. (2012) investigated the effects of the different perchlorate concentrations on zebrafish and results showed conspicuous alterations on the vertebrate’s thyroidal tissue and pituitary gland at concentrations greater than 250 pg/L. Chronic exposure of perchlorate (10 mg/L) to subadult threespine stickleback (Gasterosteus aculeatus) was found by Gardell et al. (2017) to cause obesogenic (adipose-inducing) effects. Moreover, perchlorate can also be accumulated in plants in two ways: (1) when water contaminated with perchlorate is used for irrigation, or (2) when perchlorate-containing fertilizer is added to soil (Calderon et al. 2017). The rate of perchlorate uptake by plants mainly depends on several factors, namely, plant species, C104 concentration and co-existing ions (Yu et al. 2004).

Ingestion of perchlorate-contaminated drinking water or edible plants, such as fruits and vegetables, may affect the thyr oid function in humans. Perchlorate is a known competitive inhibitor of sodium/iodide syrnporter (NIS) that is responsible for the uptake of iodine needed for the production of metabolic and developmental hormones (Leung et al. 2010). Thyroid hormone deficiency during pregnancy or after birth has been linked to problems in neuropsychological functions such as visual, motor, language and memory skills on the fetus or infant (Zoeller and Rovet 2004). Higher doses (6 mg/kg of body wt d) even result in fatal bone marrow disorders (Coates et al. 2001). The official reference dose (RfD) for perchlorate as set by the United States Environmental Protection Agency

(U.S. ЕРА) is 0.7 pg/kg of body wtd, corresponding to a Drinking Water Equivalent Level (DWEL) of 24.5 ng/L in solution (Yao et al. 2017).

Physicochemical techniques, namely, anion exchange (Gu et al. 2007), membrane filtration (Yoon et al. 2009) and adsorption (Mahmudov and Huang 2010), were found effective for small- scale treatment of perchlorate-laden aqueous solution. However, these methods only separate perchlorate from the bulk liquid and undesirable by-products are usually generated requiring subsequent treatment, thereby increasing the total treatment costs (Wan et al. 2019). In particular, contaminated resins or brine solutions from ion exchange processes comprise 4-6% of the total disposal costs (Boles et al. 2012). Membrane filtration utilizing porous membranes, such as reverse osmosis (RO), nanofiltration (NF) and ultrafiltration (UF) membranes, was also investigated (Yoon et al. 2005a, b, 2009).

Rejection mechanisms of perchlorate by the membranes were found to be governed by size exclusion and electrostatic exclusion, and that the removal was enhanced at increasing solution pH and conductivity (Yoon et al. 2005a, b). RO membrane can be used for direct pollutant treatment since it can retain low-molecular mass compounds and ions due to the very high dense properties of its separating layer (Velizarov et al. 2005). With RO filtration, total retention of ions can be achieved, albeit undesired because non-toxic minerals such as hardness (Ca2+, Mg2+) may drop to very low levels causing corrosion of some metals (e.g., lead, copper, iron, zinc, etc.) (Velizarov et al. 2005). NF membrane is another alternative technology as it selectively separates ions according to their valent forms (i.e., multi-valent or mono-valent ions) via diffusion through the membranes (at low pressure), convection (at high pressure) or repulsion (Donnan exclusion) (Velizarov et al. 2005). However, pH and ionic strength of the source water significantly affect NF membrane and its ability to reject target anions, thereby requiring a proper selection of operating conditions (Yoon et al. 2009). UF membranes, on the other hand, need larger aggregates for effective filtration, especially in the presence of co-existing anions; hence they are seldom used in the direct removal of perchlorate from solution. Nevertheless, they can be combined with other processes (i.e., biodegradation, coagulation, etc.) for solid exclusion and better effluent quality (Ensano et al. 2019).

Biodegradation is an economically sustainable treatment method because of its capability to completely convert perchlorate into harmless end products at low operating and capital costs. Under anaerobic or anoxic condition, ClOy is used as an electron acceptor, while organic (e.g., methane, ethanol, glucose, acetate, etc.) and inorganic (e.g., elemental iron (Fe°) hydrogen gas (H,), and elemental sulfur (S°), etc.) compounds are used as electron donors by heterotrophic and autotrophic perchlorate-reducing bacteria (PRB), respectively (Han et al. 2011, Zhang et al. 2016, Song et al. 2019) (Figure 1). Heterotrophic PRB use organic compounds as the electron donor and carbon source, while autotrophic PRB use CO, as the carbon source. Perchlorate is reduced to chlorate (C103-) and chlorite (CIO,-) by perchlorate reductase and CIO,- is further degraded to nontoxic Cl- and 02 by chlorite dismutase (1).

The addition of electron donor to the contaminated water must be carefully controlled as insufficient amount may lead to incomplete perchlorate removal, while over dozing, particularly for organic compounds, can stimulate substantial biomass accumulation in water distribution systems as well as the possible formation of toxic disinfection by-products (Gao et al. 2016). This constitutes one of the major drawbacks of the biological method. In lieu of this, scientists around the world have been searching for new and promising approach to improve the biodegradation process of perchlorate for both in situ and ex situ applications, the latter being more prevalent in practice. One emerging teclmology is the membrane bioreactor (MBR), which is the combination of membrane technology and biodegradation process. This paper puts an emphasis on the applicability of perchlorate treatment by MBR, its advantages over other treatment methods, limitation for practical implementations and some future challenges.

 
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