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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 12: 1901 - 1914

DOI: https://doi.org/10.7536/PC240324

Review

Reductive Transformation of Perchlorate: Fundamentals and Applications

  • Junhua Fang 1 ,
  • Ruofan Li 1 ,
  • Wenjun Zhang 2 ,
  • Weixian Zhang , 1, *
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  • 1 State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China
  • 2 Shangli fireworks development research center, Pingxiang 337000, China
* e-mail:

Received date: 2024-03-26

  Revised date: 2024-08-18

  Online published: 2024-09-06

Supported by

National Key Research and Development Program of China(2022YFC3702102)

National Natural Science Foundation of China(51978488)

Key-Area Research and Development Program of Guangdong Province(2020B0202080001)

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Abstract

Perchlorate, a persistent inorganic pollutant in water, poses a global environmental challenge due to its high solubility, mobility, and stability, making it difficult to degrade in the environment. Contamination by perchlorate has become a worldwide environmental issue, as residues of perchlorate in surface water and groundwater enter food and drinking water through various pathways, posing potential health risks. Chemical and biological methods have been extensively studied for perchlorate removal, each with its unique advantages and challenges. This paper systematically summarizes the recent research progress in chemical and biological treatment technologies for removing perchlorate from water, elaborating on the mechanisms, influencing factors, and advantages and disadvantages of these technologies. Chemical degradation, catalytic reduction, and electrochemical reduction are effective methods for treating perchlorate pollution. Organic electron donors such as acetate, glycerol, ethanol, and methane, as well as inorganic electron donors such as hydrogen and elemental sulfur, are widely used in the biological degradation process of perchlorate. Chemical methods provide rapid reduction rates and convenient implementation, while biological methods offer environmentally friendly solutions and long-term sustainable potential. However, both methods have limitations. In recent years, researchers have begun to explore combined removal techniques that integrate chemical and biological methods to enhance the remediation efficiency of perchlorate pollution. This paper reviews the research progress of three combined removal techniques: adsorption-biological method, bio-electrochemical method, and chemical reduction-biological method. In addition, future research directions are discussed, including engineering implementation studies, materials and microbiology research, practical application studies, and in-depth exploration of perchlorate degradation mechanisms.

Contents

1 Introduction

2 Chemical degradation of perchlorate

2.1 Chemical reduction

2.2 Catalytic reduction

2.3 Electrochemical reduction

3 Biodegradation of perchlorate

3.1 Organic electron donor

3.2 Inorganic electron donor

4 Combined methods for perchlorate degradation

4.1 Adsorption-biological method

4.2 Bio-electrochemical method

4.3 Chemical reduction-biological method

5 Conclusion and Outlook

Key words： perchlorate; chemical degradation; biodegradation; combined removal technology; bio- electrochemical method

Cite this article

Junhua Fang , Ruofan Li , Wenjun Zhang , Weixian Zhang . Reductive Transformation of Perchlorate: Fundamentals and Applications[J]. Progress in Chemistry, 2024 , 36(12) : 1901 -1914 . DOI: 10.7536/PC240324

1 Introduction

Perchlorate (ClO4-) is a strong oxidizer widely used in fireworks, rocket propellants, and military blasting. Due to its stable chemical properties and high solubility, once perchlorate is released into the environment, it easily migrates with water and may transfer to soil and plants, leading to its long-term persistence in the environment[1⇓-3]. Especially during the production and use of fireworks, perchlorate residues can enter surface water and groundwater systems through infiltration and runoff, increasing the concentration of perchlorate in water bodies[4]. This not only brings pollution pressure to the environment but also poses potential threats to human health.
Perchlorate primarily harms the human body by inhibiting the absorption of iodide ions (I-) by the thyroid, leading to a decrease in the synthesis of thyroid hormones and triiodothyronine, which in turn disrupts the normal function, metabolism, and development of the thyroid. In severe cases, it can cause lesions in bone marrow and muscle tissue, and even induce thyroid cancer, thus posing a threat to human health[5-6]. As early as 2005, the U.S. Environmental Protection Agency stipulated that the reference dose for ClO4- should not exceed 15 μg/L[7]. Wu et al. collected 300 water samples from 15 locations across 13 provinces and cities in China. The analysis results showed that perchlorate was detected in most (86%) of the water samples, with concentrations ranging from <0.02 to 54.4 μg/L[8]. Among these, the highest average concentration of perchlorate was found in the water samples from Hengyang City, Hunan Province ((36.5±18.2) μg/L), followed by Nanchang City, Jiangxi Province ((4.70±4.62) μg/L). This is closely related to the thriving fireworks industry in these two provinces. Therefore, seeking effective methods to remove perchlorate from water bodies is crucial for ensuring the safety of drinking water and promoting economic development.
At present, the treatment processes for perchlorate-containing wastewater mainly focus on physical, biological, and chemical methods[9]. Physical methods primarily include adsorption, ion exchange, and membrane separation[10⇓-12], among which adsorption and ion exchange have been applied for a longer time, with relatively mature and stable processes. Membrane separation mainly includes electrodialysis and reverse osmosis. However, physical methods can only concentrate or enrich pollutants without achieving the transformation of perchlorate forms. Chemical methods involve using reductants or reactive substances to convert perchlorate ions into harmless by-products[13]. These methods include chemical reduction, catalytic reduction, and electrochemical reduction. Although chemical methods possess high reactivity and versatility, they also face challenges such as electrode scaling, energy consumption, and the need for continuous supply of reagents[14]. Biological methods utilize the metabolic capabilities of microorganisms to enzymatically reduce perchlorates into harmless chloride ions[15]. Anaerobic microorganisms play a crucial role in the bioremediation of perchlorates, using perchlorate as the terminal electron acceptor in their metabolic pathways. Bioremediation technologies, such as fixed-bed reactors, bioreactors, and bioaugmentation strategies, have shown promise in enhancing the rate of microbial perchlorate reduction[16-17]. However, biological methods may be limited by slow reaction rates, sensitivity to environmental conditions, and the requirement for specific microbial communities.
In this review, the mechanisms, advantages, limitations, and recent advances in the degradation of perchlorate via chemical and biological methods are comprehensively discussed. The synergistic effects of combined approaches are also discussed, and emerging strategies for enhancing the efficiency of perchlorate remediation are highlighted.

2 Chemical Degradation of Perchlorates

Perchlorate contamination has caused serious environmental and health risks worldwide. To address this issue, various chemical degradation methods have been developed to remediate perchlorate-contaminated sites. This paper explores the reduction mechanisms employed by different chemical degradation methods, including chemical reduction, catalytic reduction, and electrochemical reduction processes.

2.1 Chemical Reduction

Chemical reduction technology can completely convert perchlorate into chloride, which is an environmentally friendly method for perchlorate removal in terms of environmental impact. The reaction equation for the removal of ClO4- from water is shown in Equation 1[13]. Some researchers use oxidized titanium ions to reduce perchlorate. Park et al.[18] used soluble Ti(II) generated from the dissolution of zero-valent titanium as a perchlorate reductant (Equation 2). The results showed that a lower pH value is required for the generation of Ti(II) from Ti(0), and the amount of acid increases with the increase in Ti(0) concentration. Kinetic data indicated that under lower pH conditions, HCl is more effective than H2SO4 in promoting perchlorate degradation (Figure 1a, b). In addition, they studied the chemical degradation of perchlorate by Ti(II) and Ti(III)[18], finding that the degradation rate of perchlorate was fastest at an F/Ti(0) ratio of 0.5 (25 mM KF). It was also concluded that high ionic strength does not enhance the perchlorate-Ti(III) reaction, while higher acid concentrations do enhance the reaction[18]. When using 5 mol/L HCl, the degradation rate constant of perchlorate in Ti(III) is twice that in Ti(II).
ClO4- + 8H+ + 8e- → Cl- + 4H2O E0=1.287 V
2Ti + 6H+ + 6F-→Ti2+ + TiF62- + 4H2
2Fe0 + 4H+ + O2 → 2Fe2+ + 2H2O
Fe0 + 2H2O → Fe2+ + H2 + 2OH-
ClO4- + 4Fe0 + 8H+ → Cl- + 4Fe2+ + 4H2O
in recent years, nanoscale zero-valent iron (nZVI) particles have attracted considerable attention due to their large specific surface area, high reactivity, and strong reduction capability, and have been successfully used for the removal of various wastewater pollutants (such as nitrates, selenates, organic dyes, aromatic halides, and heavy metal contaminants)[19-20]. Studies have shown that nZVI, especially modified nZVI, exhibits excellent perchlorate degradation rates. Cao et al.[21] investigated the removal of perchlorate by nZVI and found that nZVI can effectively reduce ClO4- in water at relatively high reaction temperatures. Their results indicated that at 75 ℃, with a nanoparticle dosage of 10 g/L, the removal efficiency of ClO4- (initial concentration of 200 mg/L) could reach nearly 90% within 24 h. Related studies have shown that when nZVI is added to a perchlorate solution under acidic conditions, the nZVI particles undergo the following reactions (Equations 3-5)[22].
However, due to the easy aggregation and oxidation of nZVI, its stability and reactivity are significantly reduced, which limits its practical application in pollutant removal. Coated nZVI involves encapsulating nZVI in polymer materials or surfactants, maintaining the high reactivity of nZVI while enhancing dispersion and antioxidant properties. Xiong et al.[23] tested two different stabilized nZVI materials (including nZVI coated with starch and carboxymethyl cellulose (CMC)) for the feasibility of removing perchlorate from water or brine. The study showed that at a moderate to high temperature (90~95 ℃) and an iron dose of 1.8 g/L, about 90% of perchlorate in fresh water and simulated IX brine (NaCl = 6% (w/w)) was destroyed within 7 h. The addition of metal catalysts (aluminum, copper, cobalt, nickel, palladium, or rare earth) did not show significant improvement in perchlorate degradation. Xie et al.[22] prepared chitosan-stabilized nZVI (CS-nZVI) to test the degradation of perchlorate in water (Figure 1c). The results showed that CS-nZVI exhibited superior removal rates compared to ZVI and nZVI, especially in water contaminated with high concentrations (200 mg/L) of perchlorate. Most of the perchlorate could be reduced to chloride through initial adsorption followed by reduction. This study provides an effective method for the degradation of perchlorate in highly concentrated or saline polluted water. Due to the high dispersibility and high reactivity of coated nZVI, its ability to degrade perchlorate far exceeds that of conventional nZVI.
图1 (a) HCl和(b) H2SO4中产生的Ti(II)溶液还原高氯酸盐性能[18]，(c) 壳聚糖负载nZVI降解水溶液中的高氯酸盐示意图[22]，(d) PHW去除高氯酸盐的结果示意图[24]

Fig. 1 Schematic diagram of perchlorate reduction in Ti(II) solution generated in (a) HCl and (b) H2SO4[18]; (c) schematic diagram of perchlorate degradation in aqueous solution of chitosan-loaded nZVI[22] and (d) schematic diagram of perchlorate removal results by PHW[24].

Increasing the temperature can enhance the decomposition efficiency of ClO4-[25]. Hori et al. [24] investigated the effect of pressurized hot water (PHW) on perchlorate decomposition and found that ClO4- showed almost no reactivity in pure PHW up to 300 ℃. Meanwhile, adding zero-valent metals to the reaction system can promote the reduction of ClO4- to Cl, with the activity order being Al < Cu < Zn < Ni ≪ Fe. Notably, this method has been successfully applied in the United States for the degradation of ClO4--contaminated water samples. The initial concentration of ClO4- at 5.22 μmol/L was significantly reduced to (0.03±0.01) μmol/L, meaning that 99% of the initial concentration was effectively removed from the water (Figure 1d). Vellanki and Batchelor [26] used a sulfite/UV advanced reduction process to reduce perchlorate, and their results indicated that the degradation rate of perchlorate by the sulfite/UV system increased with the increase in pH, temperature, and sulfite concentration.

2.2 Catalytic Reduction

Catalytic reduction technology can effectively reduce the activation energy for perchlorate reduction (Figure 2a). For example, under normal temperature and pressure, Re-Pd/C can use hydrogen as an electron donor to effectively convert perchlorate aqueous solutions through chemical reduction[27]. Studies have shown that the activity and stability of the catalyst largely depend on the solution composition and the rare earth content in the catalyst. In similar experiments, researchers linked these parameters with the formation and changes in molecular structure of rare earth species immobilized on the catalyst and investigated the mechanism by which rare earth palladium/carbon catalysts remove ClO4- (Figure 2b)[28]. Using X-ray spectroscopy, it was found that rare earths are immobilized as ReVII under aerobic solution conditions, but transform into a mixture of reduced O--coordinated rare earth substances under H2-injection-induced reducing solution conditions. Under aerobic solution conditions, extended X-ray absorption fine structure analysis showed that the immobilized ReVII species were indistinguishable from dissolved tetrahedral perrhenate (ReO4-) anions, indicating the presence of outer-sphere adsorption on the catalyst surface. Under reducing solution conditions, two types of rare earth elements were identified. At low Re loadings (≤1% by weight), monomeric ReI species form in direct contact with Pd nanoclusters[28]. As the Re loading increases, the speciation gradually shifts towards oxidized ReV clusters. The determined Re structures support a revised mechanism for catalytic ClO4- reduction involving oxygen atom transfer reactions between odd-valent oxo-rhenium species and oxyanions (Re oxidation step) and atomic hydrogen species (Re reduction step) formed by H2-palladium catalyzed dissociation. Liu et al.[29] studied the applicability of rare earth palladium/carbon catalysts in the perchlorate reduction process for treating waste brine. Experiments conducted in synthetic brine containing only NaCl (6%~12% by weight) showed that the activity of the Re-Pd/C catalyst was higher than in equivalent freshwater solutions. However, the ClO4- reduction rate constant measured in actual waste brine was 65 times lower than in synthetic NaCl brine due to the presence of NO3- in the waste brine, which inhibits the reduction of ClO4-.
图2 (a) 催化剂催化高氯酸盐的活化过程图[32]、(b)Re-Pd/C催化剂上还原ClO4-的示意图[28]、(c) hoz配位Re物种的固定、反应和分解的示意图[30]

Fig. 2 (a) Activation process diagram of perchlorate catalyzed by catalyst[32], (b) schematic diagram of reduction of ClO4- on Re-Pd/C catalyst[28], (c) schematic diagram of immobilization reaction, and decomposition of hoz-coordinated Re species[30].

Liu et al.[30]prepared a highly active catalyst for the reduction of the inert water pollutant perchlorate to Cl-at 25 ℃ and 1 atm H2by non-covalently immobilizing rhenium complex ReV(O)(hoz)2Cl (where hoz = 2-(4,5-dihydrooxazole-2-yl)phenol) and Pd0 nanoparticles on a porous carbon support. Similar to the molybdenum complex center in biologic oxygen anion reductases, the immobilized rhenium complex serves as a single site for oxygen atom transfer from ClO4 -and ClOx -intermediates, while Pd0nanoparticles provide atomic hydrogen reducing equivalents to sustain the redox cycling of the immobilized rhenium site, replacing the more complex chain of electron transfer steps that maintain the molybdenum center in oxygen anion reductases[31]. The results indicate that the redox cycle between hoz-coordinated ReVand ReVIIspecies is the main catalytic cycle for ClO4 -reduction. Under reducing conditions, approximately half of the immobilized hoz-coordinated ReVis further reduced to ReIII, which does not directly react with ClO4 -. A small portion of hoz-coordinated ReVIIspecies can dissociate into ReO4 -and free hoz, then be re-reduced and reactivated into a mixture of less reactive ReV, ReIII, and ReIspecies (Figure 2c).
Photocatalysis has been successfully applied to the degradation of perchlorate[33]. For example, Ye et al.[34] evaluated the photocatalytic reduction of perchlorate using a copper-titania/silica catalyst under UV conditions. The results showed that the catalyst exhibited optimal catalytic activity when the mass ratio of Cu2+ to TiO2 was 1200. In the presence of a hole scavenger (citric acid, Cit), Cl- was the final product in the reduction process of ClO4-, while ClO3- was the main intermediate. However, after the depletion of Cit, the concentration of ClO4- increased. Then, Jia et al.[35] investigated the photoelectrocatalytic reduction of perchlorate aqueous solutions using silver-loaded titanium dioxide nanotube arrays (Ag-TNTs) under the same environmental conditions. The effects of silver doping amount, photogenerated electrons, photogenerated holes, hydroxyl radicals, initial pH, and applied voltage on the photoelectrocatalytic reduction of perchlorate were studied, and the optimal reaction conditions were determined. The results indicated that Ag-TNTs had the best photoelectrocatalytic activity when the mass ratio of Ag to TiO2 was 0.84%. When the initial concentration of perchlorate was 0.001 mM, the reduction efficiency of perchlorate reached 62% after 6 h of reaction at (368±0.5) K with an applied voltage of 1.5 V in the presence of Cit (0.15 mol/L).

2.3 Electrochemical Reduction

Hydrogen reduction has been proven to be an efficient and clean method for removing perchlorates[29,36]. However, the hydrogen reduction of perchlorates requires an external supply of hydrogen, which may pose risks during transportation and use. To avoid using externally supplied hydrogen, electrochemical hydrogen production has been found to be a suitable method[37]. Electro-catalytic hydrogenation occurs in an electrochemical reactor, where the working electrode acts as a heterogeneous catalyst to enhance the rate of perchlorate reduction[38]. The electrode not only serves as the site of reaction but also as an electron sink, performing dual functions of chemical reactions and electron migration. Due to the low reduction efficiency of direct electron transfer, H* atoms generated through catalysis usually play a dominant role in the perchlorate reduction process. Transition metals, which are predominant as electrocatalysts, have been used for electro-assisted perchlorate reduction[39]. It has been confirmed by voltammetry, chronoamperometry, and impedance spectroscopy that perchlorate ions can be electrochemically reduced on rhodium electrodes[40]. Research found that the reduction process begins with the adsorption of perchlorate ions at free active sites on the rhodium surface. In the same system, the reduction rate of chlorate ions is much higher than that of perchlorate ions. Therefore, the ClO4− → ClO3− conversion is the determining step in this system. The electrochemical reduction process of ClO4− ions can also occur in acidic media on platinum-coated electrodes[41]. Xu et al.[36] used a novel electrode coupling rhenium (Re) and palladium (Pd) with benzalkonium chloride (BC). Under conditions of pH 3.0, anaerobic environment, and current density of 20 mA/cm2, ClO4− can be effectively reduced and completely converted into chlorine, with the reduction rate constant of Re-Pd/BC being 0.9451 L-1·gcat-1 (Figure 3a).
图3 (a) Re-Pd/BC电还原高氯酸盐的机理示意图[36]、(b) 高氯酸盐在N 掺杂的活性炭纤维(Pd/Pt-NACF)电极上还原的示意图[48]

Fig. 3 (a) Schematic diagram of the mechanism of electro-reduction of perchlorate by Re-Pd/BC[36], (b) schematic diagram of perchlorate reduction on N-doped activated carbon fiber (Pd/Pt-NACF) electrode[48]

In addition to precious metals, ClO4reduction can also occur on other metal electrodes, such as technetium (Tc), titanium (Ti), cobalt (Co), iron (Fe), and zinc (Zn). During the deposition of Tc substances in HClO4 electrolyte, the cathodic current increases with the increase of high-intensity radiation or deposition, a recognized explanation being the reduction of perchlorate[42]. Wang et al.[43]investigated the indirect electrochemical reduction process for removing perchlorate and nitrate at the titanium-water interface using either single perchlorate or nitrate solutions and coexisting solutions. The results showed that perchlorate and nitrate could be reduced simultaneously on the surface with titanium serving as the anode. The main final products of this indirect electrochemical reduction are chloride or nitrite, and when these two anions coexist, the concentration of nitrite can be negligible. In contrast to the stable nature of noble metal electrodes, perchlorate catalytic reduction occurs on non-precious metal electrodes, mediated by anodic corrosion at high potentials[44-45]. In 1 mol/L HClO4solution at 25 ℃, the average conversion rate of ClO4 -to Cl-on various anodic metals follows the order: Co > Fe > Zn > Al > Cu[46-47].
In addition to metal electrodes, carbon materials and their composites are chosen as electrode materials due to their excellent chemical inertness and adsorption properties. Yao et al.[48] developed a bifunctional electrode by loading palladium/platinum (Pd/Pt) on nitrogen-doped activated carbon fibers (Pd/Pt-NACF) for perchlorate degradation. The nitrogen functional groups on the surface of Pd/Pt-NACF carry a positive charge, enhancing their adsorption capacity for perchlorate. Subsequently, under constant current, the adsorbed perchlorate is effectively reduced to non-toxic chloride by H* atoms generated through catalysis by Pd/Pt clusters. Additionally, Re-Pd/C has been successful in the heterogeneous catalytic reduction of ClO4- and has been used as a particulate electrode for electrocatalytic reduction of perchlorate in three-dimensional electrochemical reactors[36]. It was found that benzalkonium chloride-modified rare earth and palladium co-doped activated carbon can effectively adsorb perchlorate through ion exchange. The increase in the number of H* atoms and current density promotes the electroreduction of perchlorate; however, the excessive oxygen produced in the solution leads to the deactivation of Re-Pd metals (Figure 3b).

3 Biodegradation of Perchlorates

In the methods for treating water contaminated with perchlorate, biological reduction is widely studied due to its low cost, high efficiency, and lack of secondary pollution[15,49 -50]. Functional microorganisms use perchlorate as an electron acceptor in the respiratory chain, and under anaerobic conditions with suitable electron donors, they reduce it to chloride and oxygen. These functional microorganisms are known as perchlorate-reducing bacteria (PRB)[51-52]. Under the action of (per)chlorate reductase (pcr), perchlorate is sequentially reduced by PRB to chlorate (ClO3-) and chlorite (ClO2-), and then chlorite is further reduced to chloride (Cl-) and molecular oxygen (O2) under the catalysis of chlorite dismutase (Cld)[53]. PRBs are classified into autotrophic and heterotrophic perchlorate-reducing bacteria, depending on whether they use inorganic or organic electron donors. Common organic electron donors include acetate, methane, ethanol, glycerol, acetic acid, and glucose[54-55], while inorganic electron donors such as hydrogen, zero-valent iron, elemental sulfur, and thiosulfate are also used[56-57].

3.1 Organic Electron Donors

Acetic acid is one of the most commonly used substrates for PRB bioreduction of perchlorate, serving not only as a carbon source but also as an electron donor in the process of perchlorate reduction[58-59]. PRB utilizes acetate as an electron donor to gradually reduce perchlorate to Cl- (Equation 6). Patel et al. successfully achieved complete reduction of perchlorate from synthetic ion exchange brine using acetic acid in a granular activated carbon (GAC) based fluidized bed reactor. Some studies have shown that acetic acid exhibits better performance in perchlorate reduction[55,60]. Lian et al.[60] compared the effects of acetate, methanol, glucose, sucrose, and succinate on the biological reduction of perchlorate. The reduction rates of perchlorate by different electron donors within 10 hours varied, specifically, acetate > succinate > sucrose > methanol > glucose. Compared to other organics, due to the simpler molecular structure of acetate, microorganisms can more easily utilize it, making it a more suitable electron donor (Figure 4a, b). Azospira sp. KJ, a heterotrophic PRB, achieved the maximum rate of perchlorate reduction when using acetic acid as the electron donor[55]. The reduction of perchlorate follows first-order degradation kinetics, and the ratio of acetic acid to perchlorate is a key factor affecting the reduction rate and efficiency[61].
图4 (a) 电子供体对高氯酸盐生物还原的影响[60]，(b) 电子供体对输出电压的影响[60]，(c) 以CH4为电子供体的高氯酸盐降解机理[66]，(d) HMBR的去除作用机理[67]

Fig. 4 (a) Effect of the electron donors on perchlorate bioreduction[60], (b) Effect of the electron donors on output voltage[60], (c) Mechanism of perchlorate degradation with CH4 as electron donor[66], (d) Mechanism of removal of HMBR[67]

To achieve complete reduction of perchlorate, it is usually necessary to provide an excess of acetic acid, as it can alleviate the inhibitory effect of co-contaminant nitrate in water on perchlorate reduction[62-63]. Contrary to previous studies, Guan et al.[64] argued that acetic acid may inhibit the growth of PRB, but they also pointed out that nitrate can mitigate this inhibition and promote the growth of PRB, further accelerating the degradation rate of perchlorate. However, excessive supply of acetic acid may lead to incomplete oxidation of acetic acid during the bioreduction process of perchlorate, causing residual acetic acid and fermentation metabolites to cause severe secondary organic pollution in the effluent, which is actually a major drawback in its practical application[65].
CH3COOH + ClO4- → Cl- + 2CO2 + 2H2O
glycerol is an increasingly abundant byproduct of the biodiesel production industry, making it inexpensive and easily accessible[68]. In ion exchange membrane bioreactors (IEMB), when glycerol is used as an electron donor, the biological reduction efficiency of perchlorate exceeds 99%, with perchlorate concentrations added up to 250 mg·L−1 (Equation 7)[69]. The advantage of using glycerol as an electron donor lies in its much lower diffusion rate compared to ethanol, at 6.9×10-9 cm2·s-1, which is 60% lower than that of ethanol[70]. As a result, the organic carbon concentration in the effluent remains at a low level, averaging 1.82 mg-C·L-1, always below 3 mg-C·L-1[69]. To further avoid cross-contamination caused by organic carbon, the carbon source can be supplied based on the stoichiometric requirements for complete perchlorate degradation. Glycerol is a complex organic compound that is difficult to utilize directly by PRB. Therefore, the biological reduction of perchlorate with glycerol as an electron donor occurs in two stages: glycerol is first fermented in the IEMB suspension, and then PRB attached to the membrane use these fermentation products to achieve the biological reduction of perchlorate[69]. Population analysis shows that the glycerol-fermenting bacterium Klebsiella oxytoca is the dominant species in the suspension phase, while typical PRB (Azospirillum sp.) are the main strains on the biofilm.
4C3H8O4 + 7ClO4- → 7Cl- + 12CO2 + 16H2O
ethanol is used as an effective organic electron donor and is widely applied for the removal of perchlorate in IEMB. Studies[71] have found that PRB Dechloromonas can proliferate in large quantities in IEMB, utilizing ethanol to remove perchlorate (Equation 8). The ethanol concentration in biological treatment systems must be controlled. Low concentrations of ethanol may limit the reduction of perchlorate[72]. Under conditions of limited ethanol supply, the reduction of perchlorate decreases from (33.8 ± 3.0) mg·L-1 to (8.3 ± 0.8) mg·L-1. Although some ethanol may also be consumed by heterotrophic microorganisms, excess ethanol can lead to residual COD in the effluent[70]. IEMB technology addresses the issue of organic residue caused by the low diffusion coefficient of ethanol in the membrane (1.8×10−8 cm2·s−1). The biofilm on the membrane surface can reduce the ethanol concentration at the membrane surface, thereby ensuring that the organisms can fully utilize the ethanol[73].
C2H5OH + ClO4- → Cl- + 2CO2 + 2H2 + H2O
in recent years, methane (CH4) has received considerable attention as a carbon source and electron donor for biological perchlorate reduction[74]. Firstly, the cost of CH4 is relatively low, and it can be obtained from anaerobic digesters[75]. Moreover, unlike common organic carbon sources such as acetate and ethanol, which have high water solubility, CH4 has poor solubility in water, thus avoiding secondary organic pollution[76]. Although the availability of CH4 for PRB use is limited by its low solubility, this issue can be addressed by membrane biofilm reactors (MBfRs), which achieve high gas transfer rates; this technology has been successfully applied to biological wastewater treatment with CH4 as an electron donor[77]. When perchlorate acts as the electron acceptor, the reaction is thermodynamically feasible (Equation 9)[66]. There are two possible mechanisms for CH4-mediated perchlorate degradation: (1) anaerobic methane oxidation coupled with perchlorate reduction (ANMO-PR) and (2) microaerobic methane oxidation coupled with perchlorate reduction (mAMO-PR)[66]. ANMO-PR, the combination of anaerobic methane oxidation and perchlorate reduction, is performed by a bacterium that dismutates chlorine dioxide intracellularly to form chlorine and oxygen, with oxygen serving as a co-substrate for methane monooxygenation (aerobic type). mAMO-PR, the combination of microaerobic methane oxidation and perchlorate reduction, involves two microorganisms: one bacterium reduces ClO4- to Cl- and produces O2 extracellularly, while the O2 is used by methanotrophs for methane oxidation (extracellular microaerobic type) (Figure 4c). As mentioned above, the pathways of biological perchlorate reduction involve steps that generate O2, which is necessary for the oxidation of CH4[78].
CH4 + ClO4- → Cl- + 2CO2 + 2H2O

3.2 Inorganic Electron Donor

Hydrogen (H2) has been widely studied and applied in the biotreatment of perchlorate[57,79]. Research indicates that in H2-based MBfR, when the influent perchlorate concentration is 10 mg·L−1, the removal rate of perchlorate can reach 98%[80]. MBfR is one of the most commonly used reactors, capable of achieving nearly 100% gas utilization efficiency and improving the removal rate of perchlorate[77,81]. Using H2 as an electron donor in heterotrophic perchlorate reduction has many advantages, including (1) low solubility of H2, making it easy to remove from water; (2) H2 is non-toxic; (3) hydrogen-oxidizing bacteria using inorganic carbon sources (CO2, HCO3-) grow slowly, with the main metabolic product being H2O, resulting in less metabolic sludge (Equation 10); (4) H2 as an electron donor does not require additional organic matter, avoiding secondary pollution[82-83]. The removal of perchlorate using H2 as an electron donor conforms to a zero-order reaction kinetics model[57]. When H2 supply is insufficient, facultative denitrifying bacteria are superior to strict autotrophs because organic products can serve as electron donors[84]. At the same time, high nitrate content inhibits perchlorate reduction. When the surface loading of nitrate exceeds 0.65 g-N·m-2·day-1, the perchlorate removal rate in H2-based MBfR is only 30%. As the surface loading decreases to 0.34~0.53 g-N·m-2·day-1, the perchlorate in the effluent is below the detection limit[83]. Simultaneously, the reduction of nitrate to N2 gas produces alkalinity, leading to an increase in pH, which exacerbates the precipitation of magnesium and calcium carbonates on the MBfR fibers. Precipitates affect the flux of H2 to the biofilm and lead to a decline in denitrification and perchlorate reduction performance, but this inhibition can be alleviated by mild citric acid washing[85]. Bosu et al.[67] developed a hydrogen-based membrane reactor for the removal of anions, toxic metal ions, organic compounds, and antibiotics (Figure 4d). This reactor achieved a perchlorate removal rate of 30%~100% through a hydrogen-mediated microbial reduction process. However, the high cost of H2 limits its widespread use. Additionally, the risk of explosion during the transportation, storage, and utilization of H2 is also a concern.
4H2 + ClO4- → Cl- + 4H2O
elemental sulfur (S0) is widely used as an inorganic electron donor in biological wastewater treatment due to its ease of handling and low cost[86-87]. It has been confirmed that Sulfuricella, Sulfuritalea, Thiobacillus, and Sulfurimonas are effective PRBs using S0 as an electron donor[20]. The maximum observed yield coefficient for microbial consortia using S0 is 0.19 mg-dry weight (DW) mg-perchlorate-1, which is lower than the yield coefficients (0.34~0.36 mg-DW mg-perchlorate-1) of heterotrophic perchlorate-reducing bacteria[88]. This indicates that, in autotrophic perchlorate reduction, less additional biomass is produced due to the lower microbial growth rate, which is more beneficial in practical applications. Filling insoluble S0 particles into bioreactors provides a good approach for long-term supply of the electron donor. Ucar et al.[89] compared the performance of sulfur-based autotrophic and methanol-based heterotrophic perchlorate removal, with perchlorate reduction rates of 12 and 24 mg·L-1·day-1, respectively. However, the effluent from the heterotrophic reactor contained up to about 20 mg·L-1 of methanol. Therefore, although methanol-based perchlorate reduction is fast and efficient, sulfur-based perchlorate reduction may be preferred because it eliminates organic substrate pollution in the effluent by using S0 as an electron donor. The removal of perchlorate using S0-based reactors follows a half-order kinetic model, indicating that the reduction rate is mainly limited by diffusion in the biofilm. Ju et al.[9] found that the effect of increasing S0 concentration on perchlorate reduction kinetics is related to the particle size of S0. The perchlorate reduction rate of 4 mmol/L powdered S0 with a particle size of 10-130 μm was comparable to that of 200 mM granular S0 with major and minor ellipsoid axes of 1.75 and 1 mm, respectively. The main disadvantage of sulfur-based autotrophic perchlorate reduction is the production of sulfate and acid (Equation 11)[90].
4S0 + 4H2O → 3H2S + SO42- + 2H+
anthraquinone-2,6-disulfonate sodium (AH2DS) as an effective electron donor can selectively stimulate the bioremediation of perchlorate[91-92]. Studies have shown that media containing Azospira suillum, Geobacter, Dechloromonas agitata, and Dechloromonas aromaticacan reduce perchlorate while oxidizing AH2DS to anthraquinone-2,6-disulfonate (AQDS)[91⇓-93]. Thrash et al.[93]inoculated three representative PRBs (Azospira suillum, D. aromatica, and Dechloromonas agitata) in a bioelectrochemical reactor and found that the total concentration of anthraquinones remained constant during the experiment, indicating equal conversion rates between AH2DS and AQDS. Meanwhile, no perchlorate reduction was observed with the addition of only AQDS. In contrast, perchlorate was easily reduced by PRB dechloromonads if AH2DS was provided. The same phenomenon has been observed in other studies[91]. This suggests that AH2DS is oxidized to AQDS while reducing perchlorate. There are many advantages to using AH2DS as an electron donor. First, the oxidation product AQDS can act as an electron shuttle, accelerating electron transfer efficiency and enhancing perchlorate degradation in BER[93]. Second, AQDS can be regarded as a quinone-based redox mediator due to its high redox buffering capacity, which can mitigate the inhibitory effect of long-term exposure to oxygen and high concentrations of nitrate on perchlorate reduction[94]. Of course, the presence of AQDS also has a downside, as it increases the cost and requirements for further treatment of residual AQDS in wastewater[93].

4 Combined Methods for Perchlorate Degradation

After discussing the chemical and biological methods for removing perchlorate, this paper finds that each method has its unique advantages and challenges. While the chemical method offers a fast reduction rate and simple operation, the biological method provides an environmentally friendly solution with the potential for long-term sustainability. However, both methods have limitations. A large number of studies indicate that the combination of multiple technologies may be a more efficient strategy for the remediation of perchlorate contamination.

4.1 Adsorption-Biological Method

Biological activated carbon (BAC) has a good removal effect on perchlorate[95-96]. BAC is a variant of granular activated carbon (GAC) that supports microbial growth. In addition to serving as a growth matrix, the carbon also promotes the adsorption of perchlorate, enhancing microbial removal efficiency[97-98]. Choi et al.[95] used fixed-bed biofilm reactors with GAC or glass beads as support media to evaluate the impact of increased influent dissolved oxygen (DO) concentration on biological perchlorate removal over short-term (12 h) and long-term (23 d) periods. In the biological activated carbon reactor, when the influent DO concentration was as high as 8 mg/L, both oxygen and perchlorate were fully removed within a short time (12 h). When using non-adsorptive support media, complete breakthrough of perchlorate was observed with an increase in influent DO concentration. During the long-term exposure period (23 d), the adsorption and removal rates of DO and perchlorate gradually decreased. Chemical adsorption of oxygen occurred on BAC, which reduced the DO concentration, thereby promoting microbial reduction of perchlorate[99].
Brown et al.[96] studied the removal of perchlorate by GAC and BAC through batch and column experiments. GAC primarily achieves removal through ion exchange rather than chemical reduction, but the perchlorate adsorption capacity of GAC is limited[100]. When evaluating perchlorate reduction on BAC, they observed that perchlorate was reduced below the detection limit, indicating a sustained reduction potential over time, suggesting that BAC could be a viable option for perchlorate treatment in drinking water. Brown et al.[101] conducted a pilot study on groundwater to understand perchlorate reduction in fixed-bed bioreactors. They demonstrated that perchlorate was reduced below the detection limit using a microbial reactor supported by GAC (using indigenous biomass). The system ensured no breakthrough through microbial reduction and activated carbon adsorption. Performance was not affected even during shutdown, backwashing, and in the presence of other anions.
Lehman et al.[102]investigated the feasibility of using an ion exchange process followed by biological treatment of concentrated brine solutions to simultaneously remove perchlorate and nitrate. The pilot study utilized a commercially available resin column, with the biological treatment being conducted in a microbial sequencing batch reactor (SBR). Perchlorate concentrations in the concentrated brine were as high as 3 mg/L, and the SBR was able to reduce this to below 500 μg/L. After more than 20 regeneration cycles, perchlorate was reduced to below the detection limit, with no accumulation of perchlorate in the system. A cost assessment revealed that this method would be 20% cheaper than conventional ion exchange followed by brine treatment. However, the process is limited by the number of cycles, and eventually, perchlorate will accumulate on the resin and have to be disposed of. In a related study, it was found that the presence of divalent cations (such as Ca2+and Mg2+) in the brine significantly increased the microbial reduction rate of perchlorate[103]. This is because appropriate concentrations of metals are important for the cellular growth of perchlorate-reducing bacteria[104]. Chung et al.[105]studied the microbial treatment of perchlorate in ion-exchanged brine using hydrogen-based MBfR, finding that the reduction rate of perchlorate was significantly affected by the salt concentration in the brine, with higher salt concentrations inhibiting microbial growth. An IEMB for the simultaneous removal of perchlorate and nitrate was evaluated[72]. IEMB is based on driving the target anions through an anion exchange membrane into a bioreactor, where perchlorate is microbially reduced to chloride. This approach was capable of reducing perchlorate from 100 μg/L to less than 4 μg/L in simulated contaminated water, and the reactor could operate for over 2 months with the same membrane without any fouling issues. Even under substrate-limited conditions, IEMB was able to perform at the same level.

4.2 Bio-electrochemical Method

In recent years, bioelectrochemical systems (BES) have become a promising technology for perchlorate degradation. BES are defined as electrochemical processes in which electrochemically active microorganisms act as catalysts in the cathode and/or anode regions[106-107]. Perchlorate is the most favorable electron acceptor in microbial reduction processes, with the highest reduction potential (E° = 0.87 V), making it an ideal electron acceptor. The use of biocathodes in BES systems has advantages over the use of abiotic cathodes (Figure 5a). Both indirect and direct electron transfer mechanisms exist in the microbial degradation processes of perchlorate and nitrate.
图5 (a) BES的示意图显示了高氯酸盐和硝酸盐在阴极处的还原[117]、(b)自养反硝化生物阴极微生物燃料电池同时去除高氯酸盐和硝酸盐示意图[109]、(c)新型电渗析离子膜生物反应器高效去除水溶液中的硝酸盐和高氯酸盐示意图[114]

Fig. 5 (a) Schematic diagram of BES showing the reduction of perchlorate and nitrate at the cathode[117], (b) schematic diagram of simultaneous removal of perchlorate and nitrate from an autotrophic denitrifying biological cathode microbial fuel cell[109], and (c) schematic diagram of the efficient removal of nitrate and perchlorate from aqueous solution by a novel electrodialysis ion membrane bioreactor[114]

Shea et al.[108]reported the perchlorate remediation in microbial battery cathodes, demonstrating the effectiveness of perchlorate removal without electron shuttles. Mieseler et al.[65]documented the microbial remediation of perchlorate in contaminants using acetate as an electron donor. Aerobic cultures from bioreactor sludge were inoculated into the biocathode. The enrichment of perchlorate-degrading bacteria led to the highest reduction rate of perchlorate (∼0.01 mmol/L/d). Jiang et al.[109]investigated the role of denitrifying microbial cathodes in the remediation of perchlorate and nitrate (Figure 5b). They showed that in dual MFCs, nitrate reduction was unaffected, but perchlorate reduction decreased by 20%. Open-circuit and closed-circuit experiments indicated that the removal of nitrate and perchlorate should be attributed to the biological reduction by autotrophic denitrifying biocathodes.
Butler et al.[110] studied microbial fuel cells with denitrifying biocathodes for perchlorate reduction, finding that the removal rate of perchlorate was 24 mg/L-d and the cathode conversion rate was 84% without the addition of any exogenous electron shuttles. The experiments showed that a pH value of 8.5 increased the reduction of perchlorate by the biocathode, while neutral pH values were considered optimal conditions for denitrification in bioelectrochemical cathodes[111]. Perchlorate was removed through the autotrophic reduction process by the microbial cathode[112]. The current applied to the cathode hindered the degradation of perchlorate, achieving a significant degradation of 98.99% at 60 mA. Compared to the same unmodified cathode, the biofilm on the modified graphite cathode of the microbial fuel cell reduced perchlorate reduction by 12%, demonstrating that the biofilm had a positive effect on the transfer of electrons from the cathode to the electrolyte[113]. Chen et al.[114] demonstrated the functionality of a novel modified dialysis membrane bioreactor (EDIMB) for the combined removal of nitrate and perchlorate (Figure 5c). This reactor combined electrodialysis and biological reduction technologies to remediate pollutants while avoiding organic and microbial sources. Sequencing revealed that the primary organisms involved in nitrate and perchlorate reduction were Thauera and Lactobacillus paracasei.
Yang et al.[115]constructed and operated two-chamber MESs using four types of cathode materials, namely iron-carbon granules (Fe/C), zero-valent iron particles (ZVI), activated carbon (AC), and carbon felt (CF). They demonstrated that perchlorate was degraded at the cathode with varying efficiencies without external energy supply or perchlorate-degrading microorganisms. The highest ClO4- removal rates in these reactors were 18.96 (Fe/C, 100 Ω, 2 d), 15.84 (ZVI, 100 Ω, 2 d), 14.37 (CF, 100 Ω, 3 d), and 19.78 mg/L/d (AC, 100 Ω, 2 d), respectively. Ucar et al.[116]demonstrated the simultaneous reduction of perchlorate and nitrate in a polyethersulfone (PES) ultrafiltration membrane bioreactor. The influent nitrate concentration varied between 25 to 100 mg NO3−-N/L. Complete denitrification of 100 mg NO3--N/L could be achieved with a hydraulic retention time as low as 6 h. Perchlorate could also be completely removed when the influent perchlorate concentration was 3000 μg/L and the hydraulic retention time was 12 h.

4.3 Chemical Reduction-Biological Method

ZVI, as a strong reductant, can easily reduce perchlorate from a thermodynamic perspective (ΔG° = -2495.9 kJ·mol-1). However, the activation energy barrier between perchlorate and ZVI is too high, making the chemical process too slow for in situ remediation[118⇓-120]. It has been found that ZVI, by supplying H2 generated during the iron corrosion process in situ, becomes the ultimate electron donor promoting perchlorate reduction[119,121]. The batch addition of ZVI achieves continuous H2 supply, which is significantly superior to the continuous supply of other electron donors (such as organics, H2 gas, etc.). In addition, iron has advantages such as low cost, safe operation, and no organic residues in wastewater[119]. Typically, the reduction of perchlorate by ZVI-PRB is divided into three steps: (1) ZVI reduces H+ to H2 on its surface. (2) The diffusion pathway of H2 is first to the bacteria attached to the iron surface, then to the solution, and finally to the suspended bacteria. (3) H2 is used as an electron donor to reduce perchlorate[120]. Perchlorate is mainly reduced through PRB attached to ZVI because the microbial mass attached to ZVI is much larger than that in the liquid[122]. Moreover, when energetic compounds coexist with perchlorate, ZVI pretreatment can degrade these energetic compounds, eliminating their toxic inhibitory effect on PRB, while the small molecular degradation products can also serve as electron donors for the bioremediation of perchlorate[123-124].
Yu et al.[120] studied the reduction of perchlorate by microorganisms in the presence of ZVI. They used an autotrophic Dechloromonas strain, which utilizes H2 gas as an electron source and carbon dioxide as a carbon source. Through batch experiments using only ZVI and ZVI with bacteria, they found that the degradation rate of perchlorate increased from 0.0033 to 0.0758 h-1. In the presence of higher concentrations of nitrate, they observed a significant decrease in the reduction rate of perchlorate. The enhancement in the perchlorate reduction rate with ZVI as the electron donor should be attributed to: (1) reactivity on the ZVI surface, (2) H2 mass transfer, and (3) microbial cell density. By altering these factors through conductive experiments, the process was limited by bacterial kinetics, and increasing cell density had a direct impact on the increase in the perchlorate reduction rate. Son et al.[119] also demonstrated similar results for microbial reduction of perchlorate in the presence of ZVI in both batch and column experiments. They found that H2 gas produced by anaerobic iron corrosion supported complete removal of 65 mg/L of perchlorate by a microbial mixture within 8 d. This study claimed that the reduction kinetics were comparable to those of acetate or hydrogen feed systems, which may have higher operational costs and pose explosion hazards. Compared to other systems, the ZVI-microbial combined process is more favored for the remediation of perchlorate. In stark contrast to this study, Shrout et al.[125] found that adding ZVI to mixed bacterial cultures significantly inhibited the reduction of perchlorate. Through characterization studies, they inferred that the addition of Fe0 would increase the pH and lead to the formation of iron precipitates on the cell surface, thereby inhibiting microbial perchlorate reduction by encapsulating the bacteria.
A summary and comparison of chemical methods, biological methods, and combined methods for the removal of perchlorates are provided (Table 1). Chemical methods convert perchlorates into harmless substances by using reducing agents, which have a fast processing speed and significant effects, but may produce by-products and have higher costs. Biological methods use microorganisms to reduce perchlorates to chloride ions, offering advantages such as environmental friendliness and low cost, but the process is slower and more susceptible to environmental conditions. Combined methods, through synergistic effects, improve the efficiency and stability of removal, gradually becoming an integrated solution in the environmental management of perchlorates.
表1 化学法、生物法及联合法去除高氯酸盐的总结和比较

Table 1 Summary and comparison of chemical, biological, and combined perchlorate removal

Method Principle Advantages Disadvantages Efficiency Applications Ref.
Chemical Method Oxidation or reduction of perchlorate using chemicals Rapid removal rates
Simple process
Easy to control		 High chemical cost
Potential secondary pollution
Toxic by-products		 Moderate to High Industrial wastewater treatment, emergency remediation 18-48
Biological Method Microbial reduction of perchlorate by specific bacteria (e.g., perchlorate-reducing bacteria) Environmentally friendly
Low operational cost
No harmful by-products		 Slower reaction rates
Requires specific conditions		 High Groundwater and drinking water treatment 51-93
Combined Method Integration of chemical and biological processes to enhance perchlorate removal Synergistic effects
Higher efficiency
Can address complex contamination		 More complex operation
Higher initial costs
Requires careful management		 Very High Complex contamination scenarios, large-scale remediation 95-125

5 Conclusions and Future Prospects

Perchlorate pollution control is one of the important topics in the current environmental field, and the research on its degradation methods appears particularly urgent. This paper discusses the degradation of perchlorates, focusing on chemical methods, biological methods, as well as combined approaches such as chemical reduction-biological methods and bio-electrochemical methods. Through a comprehensive analysis of these methods, it is found that each method has its own advantages and challenges.
Chemical methods are favored for their rapid reaction rates and simple operation, but the by-products they generate may have negative impacts on the environment, and there are challenges in dealing with these by-products. Biological methods, on the other hand, have advantages such as being environmentally friendly and sustainable, but they have higher operational complexity and are more significantly influenced by external environmental factors.
Therefore, the combination of chemical and biological methods has become a highly regarded solution. By integrating the strengths of both, it is possible to overcome their respective limitations and achieve more efficient perchlorate degradation. For example, the adsorption-biological method utilizes the adsorption capacity of bio-activated carbon for perchlorate and the reduction ability of microorganisms, achieving efficient removal. The bio-electrochemical method, on the other hand, leverages the reduction capability of microorganisms in electrochemical systems for perchlorate, overcoming some of the constraints of traditional biological methods.
Looking ahead, in-depth research can be conducted in the following areas:
(1) Engineering Implementation Research: Further optimize the engineering implementation of the combined method, including reactor design, operation optimization, and byproduct treatment, etc. By improving the reactor design, optimizing operating parameters, and effectively handling byproducts, enhance its effectiveness and sustainability in practical applications.
(2) Materials and microbial research: In-depth exploration of novel materials and microbial strains to enhance the efficiency and stability of perchlorate degradation. By researching and screening materials and microorganisms with high degradation capabilities, more options and possibilities are provided for the practical application of combined methods.
(3) Practical application research: Conduct application research in real-world scenarios to comprehensively evaluate the actual effects and environmental impacts of the combined method in water bodies and soil. Through field trials and monitoring, obtain more accurate data and information to provide a scientific basis and guidance for its wide application in environmental governance.
(4) Mechanism study: Deeply explore the degradation mechanism of perchlorate, revealing its reaction process and influencing factors. By combining theoretical models and experimental validation, a deeper understanding of the perchlorate degradation mechanism will be achieved, providing more in-depth theoretical support and guidance for the optimization and improvement of combined methods.
Overall, the combined methods for perchlorate degradation have broad application prospects and are of great significance in addressing water environmental pollution and soil remediation. With further research and continuous optimization of these methods, it is believed that they can make greater contributions to achieving clean environments and sustainable development goals.

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