The discharge of untreated water into aquatic natural resources has resulted in widespread contamination by various pollutants, including organic dyes, endocrine-disrupting chemicals, agrochemicals, halogenated aliphatic and aromatic compounds, and sulfur and nitrogen containing organic compounds [1-6]. The increasing accumulation of these toxins requires efficient, practical and cost effective technologies for treatment of wastewater. Traditional treatment systems such asphysico-chemical (flocculation, chlorination and ozonation), adsorption or thermal treatment generally require post-treatments to achieve complete de-contamination [7-8]. However, these treatments are not efficient, especially in the case of highly polluted water, andsome of these methods result in the production of even more toxic compounds. Alternatively, advanced oxidation processes (AOPs) are suitable for efficient removal of pollutants in wastewater. With versatile oxidants, these AOPs can generate different reactive oxygen species (ROS) such as hydroxyl radicals (•OH), singlet oxygen (1O2) and superoxide anion (•O2-) for complete removal of toxic pollutants [9-11]. It is well known that use of ozone or molecular oxygen as an oxidant is subject to gas/liquid solubility problems, which result inhigh energy consumption, while treatment with H2O2 appears to be a feasible protocol among AOPs [12]. The lack of a gas-liquid boundary makes use of H2O2 as an oxidant greatly attractive, and the degradation can be conducted under mild conditions, demonstrating its advantages when compared with wet air oxidation (WAOs). In previous studies, Fenton’s reagent (Fe2+) or other transition metals such as Fe3+, Cu2+, Mn2+ and Co2+ have been widely applied as homogeneous catalysts during the degradation of pollutants [13-15]. However, the limited range of pH (2-4) and the recovery of homogeneous catalyst are serious issues in their practical application. The strategies of immobilization of transition metals over different supports have been extensively explored to minimize the catalyst loss and heavy metal pollution [16-21]. However, during advanced oxidation treatment of wastewater, the solution pH may gradually drop, encouraging catalyst leaching and deactivation. As shown in Table 1, the solution pHs of different systems clearly demonstrate their influence on catalyst leaching [22-29]. Therefore, a constant pH isimportant for process stability and reduced environmental damage during wastewater treatment.
Bicarbonate is highly soluble in water, relatively non-toxic and widely distributed in nature (50-200 ppm in natural water), as well as in biological systems (14.7-25 mmol/L), and exists in different forms constituting the main biological buffer Eq. (1) [30]. Normally, bicarbonate is more soluble at pH 7.0-9.0, while at above or below this range it changes to less soluble Na2CO3 or carbonic acid, respectively.
Notably, carbonate/bicarbonate ions are believed to change the redox properties of metals ions in ocean water, providing an opportunity to organisms to selectively take up metals [31]. Moreover, bicarbonate is reportedly involved in the structure and function of water oxidizing complex (a part of the photosystem Ⅱ protein complex that oxidizes water to dioxygen), and increases the rate of electron transfer from Mn2+ to apo-PS Ⅱ by forming ternary complex [31-33]. Apart from numerous functions in biological systems, the role of bicarbonate activation has also been studied extensively in organic reactions [34-39]. However, the application of this reagent in combination with certain AOPs for wastewater treatment is a newly emerging topic. In fact, the roles of bicarbonate in AOPs during wastewater treatment are controversial among researchers because enough data are available to support as the view of its promoting or scavenging AOPs during wastewater degradation. Nevertheless, bicarbonate still possesses some encouraging properties for its application in wastewater. For example, its buffer capacity (pH 8.0-9.0), low cost, high solubility and particularly its capability to activate H2O2 are attractive properties for developing a sustainable treatment system [30, 34-39]. As in AOPs, various reactive oxygen species (ROS) such as •OH,•O2- and 1O2 mainly controlled the degradation of pollutants. The generation of even more reactive species (HCO4-,•CO3-, HCO3•) along with the aforementioned ROS was proposed in bicarbonate activated hydrogen peroxide (BAP) systems in different studies [40-45]. Although the •CO3- radical produced through (Eq. 2) is less reactive (Eo +1.78 V, pH 7) than •OH radical (Eo +2.3 V, pH 7), its occurrence in high concentration (10-13-10-15 mol/L) and long lifetime in reaction medium may provide it with sufficient opportunity for pollutant degradation [46]. Moreover,•CO3- radical is a very strong single electron oxidant that can react by both electron transfer and hydrogen abstraction. For example, it was reported that •CO3- radical can react rapidly with organic pesticides, sulfur containing compounds and aromatic compounds [39, 46]. Similarly, peroxymonocarbonate (HCO4-) produced through Eq. (3) is a well-known two-electron oxidant that has been investigated since its discovery in 1984 [47]. HCO4- anion is considered to be more reactive than H2O2 (100-500 times) in different reactions depending on the substrate [47]. In view of various active oxidizing reagents generated from BAP systems and their important roles in other fields, the addition of the BAP system to wastewater treatment is attractive. Although this is a newly emerging method and many issues still need to be addressed, summarizing its progress will help researchers to explore new technologies for the future development of a sustainable and green solution for wastewater treatment.
Different attempts have been made to solve the serious issues occurring during pollutant degradation when treating wastewater [48-57]. However, the challenges associated with these available technologies still require the development of new efficient, cost effective and sustainable treatment systems. In the recently developed BAP system, the operation conditions are generally mild, with a pH around neutral or weakly alkaline, unlike the strongly alkaline conditions commonly used during peroxide bleaching [58]. Similarly, the components of the BAP system are economical, environmentally friendly and have the potential for treatment ofchemical warfare agents such as HD [bis(2-chloroethyl) sulfide], GB (pinacolyl methylphosphono- fluoridate), and VX [O-ethyl S-(2-diisopropylamino)ethyl methylphosphonothioate], indicating its potential for use as a broad-spectrum decontamination solution [59]. In particular, these merits of the BAP system are common challenges associated with other AOPs for wastewater [60-62]. To date, the BAP system has been investigated for use in treatment of a wide range of organic contaminants. These techniques have used either direct BAP systems or those that are activated with homogeneous or heterogeneous catalysts. These investigations are reviewed here, as well as studies introduced from other fields.
Generally, AOPs are considered one of the most efficient systems for treatment of wastewater, and the development of sustainable AOPs requires inexpensive oxidants that are easily soluble under neutral conditions. Among different oxidants, H2O2 is most commonly applied in AOPs because of its eco-friendly nature. However, the poor leaving tendency of the hydroxide ion from H2O2 makes it less reactive and thus always requires activation. The activation of H2O2 can be achieved by different methods such as transition metal salts or complexes [34], carboxylic acid (peroxyacid) [63], acetonitrile (peroxycarboxymidic acid) [64], urea (peroxyo urea) [65], or sodium perborate and sodium percarbonate in strong basic medium [66]. These systems all offer one or more broad disadvantages, such as toxicity associated with metals, organic byproducts, severe pH conditions and other problems created by the oxidative decomposition of organic ligands. The BAP system was first introduced by Drago et al. [67] and Yao et al. [34], and has since been used extensively in many organic reactions.
In view of the naturally abundant availability, high solubility, less toxic nature and its activation ability for H2O2, a metal free system (HCO3-/H2O2) was introduced by Xu et al. [68] for degradation of various organic compounds, including methyl blue (MB), methyl orange (MO), rohdamine B (RhB) and chlorophenols (CPs). Indeed, this system is a completely green method of wastewater treatment, and the findings are highly encouraging. The authors found that this system is much better than using H2O2 alone for the degradation of various organic pollutants such as MB, MO, RhB and CPs as shown in Fig. 1. The enhanced activity was initially attributed to the highly active HCO4‾ anion, but •O2‾ was later confirmed as the sole reactive speciesinvolved in degradation. This method is also safe for the environment, given that NaHCO3 is abundant and naturally present in surface and ground water. However, the long reaction time may be considered a limitation for its practical application.
To date, different metal ions such as Fe, Co, Cu, Ce, Mn, Cr and Mo were reported in catalytic wet peroxide oxidation (CWPO) during wastewater treatment [7]. Among versatile AOPs, Fenton or Fenton like, photo-H2O2 or electron-H2O2 are considered more efficient technologies for a wide range of organic pollutants in wastewater treatment [69-73]. However, the ferric ion (Fe3+) produced through Eq. (4) during degradation is converted to insoluble Fe(OH)3 (sludge) at neutral or nearly neutral pH that slows down the efficiency of the system, which is a serious drawback in Fenton’s system; accordingly, its activity is strictly limited to a low pH range (2.5-3.5).
Different efforts have been made to overcome this challenge, and in this context, the introduction of BAP using various redox metal ions is currently a new emerging alternative. In the BAP system, the solution pH remains neutral or slightly alkaline. Recent studies of wastewater treatment using BAP systems with different redox metal ions are summarized in Table 2. These studies provided some encouraging results, such as enhanced de-colorization of organic dyes, in situ formation and stabilization of reactive intermediates, kinetic and mechanism studies. The studies of BAP systems conducted to date have mainly focused on degradation of dyes with different redox metal ions such as Co, Mn and Cu. For example, the Mn2+/HCO3-/H2O2 system was applied for a variety of dyes and landfill leachate [74-76]. Xu et al. [74] found that the BAP system was very efficient for degradation of MB, as complete de-colorization was achieved in 180 min (Fig. 2). Similarly, other organic pollutants, such as MO, RhB and landfill leachate, were also degraded at different time intervals. Based on kinetics and UV-visible studies, the MnⅣ=O species were proposed as possible intermediates involved in degradation. In another study, Ember et al. [75] claimed a clean and rapid degradation of orange Ⅱ (AOⅡ) with relatively non-toxic Mn2+ salt at 25 ℃, and they conducted extensive spectroscopic studies to identify the key features of the degradation mechanism in which MnⅣ=O was proposed as the primary intermediate.
During the kinetic and mechanistic studies, a similar trend of enhanced degradation was observed for a series of dyes with increasing bicarbonate concentration. Ember et al. [76] reported that the rate constants for all dyes depend on the second order of HCO3- concentration, indicating two equivalents of HCO3- contribute to degradation in which one equivalent of bicarbonate was proposed to participate in the formation of more reactive [Mn(CO3)(H2O)4] intermediate, and the other equivalent produces peroxycarbonate. Another important feature of this study is the stability and re-usability of the catalyst system. The authors found that Mn2+ salts are able to form a highly reactive aquatic intermediate that changes to inactive Mn(Ⅱ) complex in the absence of any other stabilizing ligands. They also proposed different stoichiometric Mn2+···HCO3- complexes with increasing bicarbonate concentration. These complexes were found to be relatively stable and their deactivation occurred in less than 20 min. However, the stability increased (1-4 days) in the presence of some organic dyes at high concentrations of bicarbonate (0.4 mol/L), for which the authors provided extensive evidence from different dyes in monodentate manure, including some with low binding affinity or binding in monodentate manure. In all cases, the catalytic cycle was found to repeat several times without significant loss in catalytic activity, which was attributed to in situ stabilization of catalyst with dyes.
Similar studies not involving wastewater treatment also investigated the decomposition of H2O2 and oxidation of amino acids using the Mn2+/HCO3-/H2O2 system [77-79]. In these studies, the reaction rate of H2O2 decomposition was found to increase in a manner directly proportional to [Mn2+] and [H2O2] and to the third power of HCO3- concentration [77]. For example, Stadtman et al. [77] proposed the use of three equivalentsof HCO3- combined with one equivalent of Mn2+ to form a stable complex allowing facile reaction between MnⅡ and H2O2. In another study, the disproportionation of H2O2 was proportional to [HCO3-]3, suggesting the formation of an MnⅡ(HCO3-)3 complex [78]. However, in the presence of amino acid (AA), the decomposition of H2O2 is proportional to the square of the HCO3- concentration. The higher efficiency of the suggested complex was a result of the favorable redox potential of Mn(Ⅱ) in the (Mn2+(HCO3-)2AA) complex. The proposed mechanism in this study initially involved the generation of a cage type (Me)-AA-HCO3- complex that reacts with H2O2 to produce •OH radicals very close to the oxidized moiety (oxidized amino acid). To examine the structural identity of free radicals during the oxidation of amino acids and the catalytic decomposition of H2O2, electron paramagnetic resonance (EPR) with spin trapping using DMPO was conducted by Yim et al. [79]. In this study, the EPR results confirmed the presence of both •OH and •O2- as intermediates in the dismutation reaction, and •O2- radical was considered a major product trapped by DMPO. They suggested the formation of •O2- likely occurred in the inner coordination sphere of metal ions, where the •OH radicals were also produced. According to the proposed mechanism shown in Scheme 1, transferring the electron from Mn2+ to H2O2 at the axial position in structure 1 resulted in homolytic cleavage of H2O2 and the formation of cage •OH radicals in structure 2. The majority of •OH radicals in the cage structure 2 then abstract the hydrogen atom from another H2O2 at the equatorial position to form HO2•. These studies clearly confirmed that the oxidative activity of Mn2+ primarily depends on the concentration of bicarbonate.
Bicarbonate in biological systems has been reported to exist in equilibrium with CO2. In the literature, the oxidative activity of Cu, Zn dismutase was initially attributed to bicarbonate for a variety of reactions at physiological conditions [80-86]; however, the actual activity of Cu, Zn dismutase was later confirmed to be caused by CO2 rather than bicarbonate [87]. To clarify the actual dependence of Mn2+ reactivity, Liochev et al. [88] conducted a study of the oxidation of NADH. They found an apparent synergism between [HCO3-] and [CO2] during oxidation, and thisoxidation was inhibited using Mn-SOD, reflecting the generation of superoxide as an important reactant. Based on the consistent properties of this system, the following Scheme 2, from Eqs (7)-(18), was proposed for the oxidation of NADH.
Copper is known to catalyze the oxygenation of hydrocarbon and demonstrate Fenton’s like chemistry during the degradation of organic compounds [89-92]. Cheng et al. [93] reported that, in BAP systems, Cu2+ degrades different dyes including AOⅡ, MO, methyl red, and toluidine blue. The activityof the BAP system with Cu2+ was found to be high enough for complete de-colorization of AOⅡ to occur within 10 min (Fig. 3). In particular, this BAP system was found to be even more efficient than the copper based Fenton’s like system, as a very small amount of Cu2+ (0.01 mmol/L) is sufficientfor complete de-colorization, while in the Cu2+/H2O2 system,0.1 mmol/L Cu2+ was required for de-colorization of reactive black 5 under neutral or alkaline conditions [94].
Cobalt cation was also reported to be useful for degradation of a wide range of dyes using the BAP system [95-99]. These studies mostly focused on degradation of dyes with other aspects such as degradation mechanism and kinetic studies. In the BAP system using (Co2+/HCO3-/H2O2), there was no direct evidence of the participation of •OH radicals in the degradation process. For example, in the degradation of MB, Xu at al. [95] proposed the term transient caged •OH radical as suggested by Yim et al. [79] for dismutation of H2O2 in the Mn2+/HCO3-/H2O2 system. Here, the authors found that the fluorescence intensity, which was used to quantify •OH radicals, increased sharply with the addition of bicarbonate. They initially assumed that this reflected the formation of an in situ Co2+-HCO3- complex between Co2+ ions and HCO3- ions because of the significant role of bicarbonate in the production of •OH radicals. This intermediate complex was assumed to act as an in situ catalyst in the activation of H2O2. In the CV studies, they later confirmed that this in situ complex (Co2+-HCO3-) shifts the potential peak of the Co2+ ion in a more negative direction. Based on the bicarbonate concentration, they proposed different stoichiometric complexes. For example, a more stable and reactive [Co2+(HCO3-)]+ complex was suggested at modest concentration (5-10 mmol/L), while another [Co2+(HCO3-)2] complex was assumed to exist at high concentration (50-100 mmol/L). The coordination numbers of these complexes were suggested to play important roles in the generation of •OH radical. For example, bicarbonate at low concentration (0-5 mmol/L) was assumed to produce dimers or oligomers in aquatic Co2+ ions. In such cases, maximum coordination positions are unavailable for H2O2 to generate efficient •OH radicals, resulting in lower MB degradation. Similarly,[Co2+(HCO3-)2] complex with no binding equatorial position was recommended at high bicarbonate concentration (50-100 mmol/L), which caused lower degradation. However, the [Co2+(HCO3-)]+ complex formed at 5-10 mmol/L of bicarbonate was believed to be more stable, which can produce rapid and active transient cage type •OH radicals.
Similarly, Lou et al. [96] studied the changes in redox properties of Co2+ ions after the addition of H2O2 during degradation in the BAP system. As shown in Fig. 4, the authors found significant increases in UV-visible absorbance at 260 and 400 nm, which they attributed to oxidation of Co2+ to Co3+. They also observed that theabsorbance at 260 nm initially rapidly increased, then decreased linearly with time (Fig. 4). This behavior was attributed to the initial rapid conversion of Co2+ into high valent state Co3+, and the subsequent slower reduction of Co3+ to Co2+. Based on this behavior, the authors assumed that the production of •OH radicals occurred during oxidation of Co2+ and •O2- during the Co3+reduction process. The existence of •OH and •O2- radicals was confirmed by EPR and NBT experiments. However, the term tightly associated •OH radical with cobalt complex was used, as no scavenging of •OH radical occurred, even with high concentrations of 2-propanol. The authors attributed the increase in de-colorization of dye with 2-propanol to the increased stability of free •O2- radicals in organic solvent, which was also confirmed by SOD scavenging studies.
In continuation of the Co2+/HCO3-/H2O2 system, Li et al. [97] conducted CV and UV-visible studies of the coordination behavior of MB and Co2+ to find clear information describing the reaction mechanism. For example, CV studies revealed that MB alone produces a clear oxidation peak (Ea = -0.159 V) and reduction peak (Ec = -0.178 V) analogous to the methylene blue/leucomethylene blue redox couple reaction that corresponds to two electron transfer processes. However, in the presence of Co2+ cations, the oxidation peak of MB was found to decrease with changes in peak current. Similarly, when both Co2+ ions and HCO3- were present in the system, the oxidation peak of MB shifted to a much lower potential (Ea = -0.095 V) with increasing peak intensity, which they attributed to the formation of Co2+-MB complexes. They also suggested different stoichiometric [Co2+(MB)]+ complexes for different Co2+ concentrations in bicarbonate solution. These different complexes appeared to change the UV-visible spectra of MB at 664 nm. In the radical scavenger and fluorescence studies, the production of •OH radicals was detected. However, in the fluorescence studies, the peak intensity was much higher in the presence of MB than in the Co2+/HCO3-/H2O2 system. This increase in fluorescence intensity was attributed to the role of MB in the generation of •OH radicals, which was supported by the findings in EPR studies. These studies [95-97] relied on the generation of transient cage •OH radicals during the oxidative degradation of pollutants. Based on the consistent experimental results, the following general degradation pathway Eqs. (19)-(24) was suggested during pollutants degradation [97].
Despite the high de-colorization rate of dyes in the Co2+/HCO3-/H2O2 system, the degree of mineralization (COD and TOC removal) in the reviewed studies is very limited[95, 97-98]. One possible protocol for obtaining good efficiency is the combination of different systems. Normally, AOPs are recommended to improve the biodegradability, while subsequent biological treatment is recommended for final degradation [98]. Yang et al. [98] applied the BAP system to improve the biodegradability of X-3B before the biodegradation system [98]. The ratio of BOD5/COD is usually expressed as biodegradability, and a value higher than 0.30 indicates that the wastewater can be effectively degraded with biological treatment [98]. The authors observed that, as shown in Fig. 5, the biodegradability factor for the BAP system improved from 0.1 to 0.46 or higher when there were high concentrations of H2O2, indicating that the BAP system may be suitable for pretreatment before application of biological treatment.
The efficiency of the system is the most important factor when evaluating its practical application. In general, these studies have shown excellent efficiency in the de-colorization of different dyes, even when small amounts of transition metals are utilized (Table 2). Importantly, in the studied metal ions, Co2+ showed the best efficiency, followed by Mn2+ and Cu2+ [95]. However, the remaining metals, including Fe2+ and Ag+, showed minor efficiency. Comparative studies of BAP systems with ordinary Fenton’s systems revealed that Co2+/HCO3-/H2O2 is more efficient than the Fenton’s system [99]. For example,5 μmol/L of Co2+ is sufficient to de-colorize AOⅡ dye within 10 min, while Fe2+ at this concentration (pH 3.0) provided only 25% de-colorization. The selective removal of more toxic compounds in the presence of less toxic ones is extremely difficult. However, this system is able to degrade AOⅡ in the presence of numerous pollutants including MB, sulforhodamine B, as well as inorganic anions including Cl−, CO32−, SO42−, NO3−, and HPO42− [99].
The application of direct metal ions as catalysts will never be considered safe during wastewater treatment using the BAP system. Although some author's have claimed only very small amounts of metals ions (e.g.,5-20 μmol/L of Co) were discharged during treatment [95-99], theseconcentrations will rise owing to the lack of a feasible recovery system, resulting in heavy metal pollution. To date, only three metal ions, Mn2+, Cu2+ and Co2+, have shown excellent activity in BAP systems, while other metals, including Fe2+ and Ag+, were found to be less reactive [74, 93, 95-99]. Among the tested metal ions, Co2+ is able to d egrade a variety of dyes including MB, RhB, MO, X-3B, AOⅡ, andsulforhodamine B under mild reaction conditions [95-99]. However, dyes represent only one class of pollutants as wastewater is composed of mixtures of versatile organic pollutants. Therefore, it is important to evaluate the efficiency of the Co2+/HCO3-/H2O2 system when applied to otherclasses of pollutants such as phenolic compounds, which are difficult to degrade and pose significant threats to the environment. Xiao et al. [97] reported that the Co2+/HCO3-/H2O2 system is very efficient for de-colorizing different azo dyes (reactive brilliant red X-3B and methyl orange) and triphenylemethane dyes (sulforhodamine B and RhB). However, as shown in Fig. 6, its activity toward phenolic compounds (e.g., p-nitrophenol (PNP),2,4-dinitrophenol (DNP)) is negligible, and only 7% PNP was removed, while DNP was not degraded. Among transition metal ions that catalyzed oxidation of wastewater, manganese is an efficient, economical and environmentally friendly catalyst. In the BAP system, Mn2+ salt was applied by Xu et al. [74] and Ember et al. [75-76] for degradation of series of dyes (aromatic azo dyes, aromatic azo dyes with adjacent functional groups, flavonoid dyes and phenolic derivatives) under mild reaction conditions. The results of these studies demonstrated that Mn2+ produces different reactive intermediates that, in the absence of any further coordinating ligands, rapidly decomposed into insoluble Mn(OH)2, MnCO3 or MnO2. They also found that these intermediates can be stabilized with electron rich organic molecules such as dyes. For example, Ember et al. [75] investigated the stability of catalyst formed in situ by exposing the catalyst todye in bicarbonate solution for 20 h. The catalyst formed in situ showedexcellent stability and demonstrated the same reactivity as fresh catalyst with the addition of H2O2. Moreover, the stability and efficiency of this in situ formed catalyst increased with substitution of functional groups in aromatic azo dyes adjacent to azo linkages. These studies also revealed that the degradation of dyes with aquatic Mn2+ is only possible in the presence of bicarbonate buffer and H2O2. Indeed, no other buffer containing aqueous Mn2+ ions is able to de-colorize the dyes, which was attributed to amplification of HCO4- as a key reactive species. Ember et al. [76] found that the Mn2+/HCO3-/H2O2 system is highly sensitive to solution pH. The activity of this system was found to be highest at pH 8.2-8.6. At high pH (≥ 9.0), the decrease in the degradation rate was attributed to deprotonation of HCO4- to less electrophiylic CO42- oxidant or decomposition of H2O2, respectively.
Currently, it is difficult to compare different homogeneous BAP systems employing Co, Mn and Cu metal ions when applied to the treatment of wastewater. To date, these studies have focused on the degradation of dyes with special interest on reaction kinetics and mechanism. In general, these BAP systems show excellent efficiency during degradation of dyes, with slight differences in the nature of reactive species responsible for degradation. For example, in the Co2+/HCO3-/ H2O2 system, the reactivity was mainly attributed to the formation of in situ active catalyst ([Co2+(HCO3-)]+), while for the Mn2+ based BAP (Mn2+/HCO3-/H2O2) system, they proposed the in situ active catalyst ([Mn2+(HCO3-)]+), high valent MnⅣ=O species and HCO4─ as key reactive intermediates for degradation of dyes. However, a completely different mechanism was proposed in the Cu2+/HCO3 -/H2O2 system. For example, Long et al. [93] found that the contribution of HCO4─ or Cu2+-HCO4─ for degradation of dyes was not so important in the Cu2+/HCO3-/H2O2 system. Similarly, the negligible role of free radicals suggested byscavenger studies strengthensthe argument that higher oxidation state copper (Cu3+) acts as the sole reactive species involved in degradation of dyes.
In view of heavy metal toxicity, the use of metal ions as homogeneous catalysts is not safe for the treatment of wastewater. Obviously, the strategy of incorporating these active metals on certain supports is the best choice to minimize the threat associated with heavy metals. However, the heterogeneous catalysts prepared when adopting such a strategy are also subject to various problems, including low efficiency and poor stability. For example, various acidic products can be produced during degradation, which encourages catalyst leaching because of low pH and causes poor catalyst stability [29, 100, 101]. As in homogeneous systems, the recovery of dissolved metal ions is nearly impossible or need expensive treatment. Accordingly, the application of heterogeneous catalyst in BAP systems will be a good choice for wastewater treatment [29, 100-102]. This method was first applied in a BAP system by Zhou et al. [29], who used Co supported-diatomite clay as a heterogeneous catalyst for degradation of dyes (Fig. 7). The efficiency of heterogeneous catalyst was much better when compared with different control experiments, including homogeneous cobalt ions under the same conditions, and the supported catalyst was found to present excellent stability (180 h) in a fixed bed reactor. This prolonged catalyst lifetime was attributed to bicarbonate, which provides a weak alkaline buffer throughout the degradation, resulting in improved catalyst stability (Co leaching,0.24 ppm or smaller).
The nature of the support is very important in heterogeneous catalysts, as it can encourage effective interactions between metal ions and support, improve the distribution of active components, and retain the properties of impregnated metals [103]. Hydrotalcites based materials present unique properties as catalyst support, including modulating its properties by varying metal proportions or replacing different metal cations within the layered structure [104]. Cobalt supported-LDH catalysts (Co-MgAl-LDH) were recently applied for degradation of phenolic compounds and dyes, which highlighted the importance of bicarbonate on process efficiency, catalyst stability and leaching [100, 101]. During degradation of 4-CP, Jawad et al. [100] found that the activity of a BAP system in the presence of bicarbonate was consistently higher than in various control experiments, even when a small amount of bicarbonate was used (Fig. 8). Similarly, they observed less catalyst leaching (0.14 mg/L) than in previous studies [29]. The authors attributed this to bicarbonate addition as well strong linkages in the LDH structure. The supported catalyst in the fixed bed test was found to present complete stability with excellent activity (Fig. 9). Again, this stability was attributed to bicarbonate addition, which provides constant pH (8.84-8.98) throughout the 300 h reaction, thereby controlling leaching (0.20-0.24 ppm).
In continuation of the BAP system with supported catalysts, another study targeted both the degradation of cationic and anionic dyes (MB and MO) at the bench level and in a fixed bed reactor [101]. In this study, Jawad et al. [101] found that 1.7% Co loading in Co-MgAl-LDH catalyst was sufficient for de-colorization of both cationic and anionic dyes (Fig. 10). The CoMgAl catalyst was found to be more reactive than CoAl, CoMg, MgAl and CuMgAl catalysts, and this behavior was considered to be a function of geometric structure and arrangements of Co2+ in the crystal lattice of LDH. As shown in Fig. 11, the addition of bicarbonate resulted in enhanced catalyst activity relative to control experiments. As shown in Fig. 12, the catalyst stability in the fixed bed was even better than the last two reported studies [29, 100]. This excellent stability of the supported LDH based catalyst was attributed to the absence of Co leaching throughout the 300 h reaction.
The degradation of acid orange 7 was recently reported by Duan et al. [102] using a BAP system with CoxOy-N/GAC catalyst as shown in Fig. 13. The authors described this high activity for the formation of active complexes between cobalt and HCO3-, which produced various reactive oxygen species upon reaction with H2O2. The CoxOy-N/GAC catalyst also showed excellent stability, even after successive runs (Fig. 14). The catalyst activity gradually decreased and finally dropped to 32.8% after the fifth run. This deactivation was attributed to the deposition of intermediate product over the catalyst surface. After drying at 350 ℃ under N2, the catalyst recovered its activity as shown in the sixth run. The good stability of the catalyst at this point was assumed to reflect the interaction of cobalt and nitrogen in CoxOy-N/GAC catalyst, which led to lower catalyst leaching (0.02 ppm). This system appeared to be more promising than the homogeneous BAP system in terms of activity and minimum risk to the environment.
In H2O2 based advanced oxidations, different mechanisms like formation of free radicals, high valent metal oxo species or both mechanisms were generally proposed during degradation of organic pollutants [96]. Most BAP systems using heterogeneous catalysts have relied on the generation of various reactive oxygen species as confirmed by scavengers, fluorescence and EPR studies [29, 100-102]. For example, popular scavengers such as ascorbic acid, t-butanol (TBA), sodium azide (NaN3) and benzoquinone (BQ) were applied to detect •OH,1O2 and •O2- radicals, respectively [29, 100-102]. The decrease in degradation rates with the addition of a particular scavenger was attributed to participation of the respective ROS. For example, Zhou et al. [29] suggested 1O2 was the sole reactive species in the BAP system owing to the apparent inhibition of degradation of MB by sodium azide. They suggested that •OH radicals indirectly promoted the generation of other ROS such as •CO3- and 1O2 through a series of reactions Eqs. (2)-(3),(25)-(29). In another study, Jawad et al. [100, 101] suggested the involvement of •OH and •O2- radicals based on the scavenging effect of TBA, ascorbic acid and BQ in the degradation of 4-CP and dyes. Apart from scavengers, some other techniques such as fluorescence and EPR studies were considered to be more sensitive for detecting different ROS in reaction medium [97]. In a fluorescence study, fluorescent reagents such as sodium terepthalic acid (NaTA) were reacted with •OH to give 2-hydroxyterepthalic acid (HTA), which provides sharp, bright and stable fluorescence [97]. This reaction is considered to be unaffected by other reactive species such as H2O2, HO2• and •O2− [97]. The peak intensity of the fluorescence is considered proportional to the amount of •OH radicals produced in the reaction [97]. Similarly, nitrone based cyclic spin traps that generate characteristic signals after trapping different free radicals in reactions such as (5,5-dimethyl-1-pyrroline N-oxide) (DMPO) and acyclic N-tert-butyl-α-phenylnitrone (PBN) are recommended in EPR studies [105]. Jawad et al. [100] applied both fluorescence and EPR techniques to confirm the radicals as suggested in scavenger studies. Their fluorescence studies revealed sharp fluorescence with intensity that decreased with the addition of TBA, which the authors attributed to •OH radicals. The occurrence of •OH radicals were further confirmed through EPR, where DMPO gives quartet signals of DMOP-OH (1:2:2:1 in intensity), which are characteristic signals of •OH radicals. Moreover, the presence of •O2- was confirmed using PBN as a spin trap, which produces specific triplet BPN-O2- signals in EPR studies. However, Jawad et al. [100, 101] did not confirm the presence of 1O2 oxygen as suggested in the BAP system [29] containing sodium azide. The increase in degradation rate with azide was considered for production of two other reactive species (•N3 and •O2-) through Eqs. (30) and (31) because of their close rate constants for both •OH and 1O2 (1 × 1010 vs. 2 × 109 mol-1 L s−1) [106]. The existence of both •OH and •O2- was also confirmed by Li et al. [41] in a chemiluminescence study of the H2O2-Co2+-NaHCO3 system using ascorbic acid and SOD as scavengers. They also observed an increase in chemiluminescence intensity in response to the addition of NaN3. The authors attributed this increase to the formation of NOx species that resulted from the reaction of NaN3 and •CO3- radicals. The carbonate radicals produced through scavenging of •OH Eq. (2) were also considered key reactive species in the BAP system during degradation of various pollutants [29, 100, 101]. The authors considered the importance of •CO3- radicals as ROS based on its activity in studies of different compounds such as pesticides, dyes, and even methyl t-butyl ether [107-109].
As in previous studies, Duan et al. [102] assumed the presence of •OH radical and •O2- during degradation of AO7. However, because of the important role of bicarbonate in the degradation of AO, the authors assumed the schematic activation of H2O2, releasing both •OH and •O2- anion radicals on the catalyst surface as shown in Scheme 4 Eqs. (32)-(34).
AOPs appear to be an important technology for wastewater treatment because of their high efficiency at ambient conditions. However, all AOPs require application of certain oxidants such as H2O2, PMS, PS, or O3 to generate different ROS for degradation. The generation rate of these ROS can be improved by different methods including use of catalysts, light, etc., along withbicarbonate activation. All AOPs containing bicarbonates/carbonates were reported as scavengers of •OH radicals [110]. However, the radical-radical recombination of bicarbonate radical (1.2 × 107 mol-1 L s-1) is considered much lower than that of •OH radical (5.5 × 109 mol-1 L s-1) [29, 110]. Moreover, the carbonate radical generatedfrom (eq. (2)) exists at much higher concentrations (10-13-10-15 mol/L), about two order of magnitude higher than •OH radical (10-14-10-18 mol/L) in natural water, and was reported to be an active oxidant in many reactions [39, 46, 106]. Similarly, since its discovery in 1983, many studies have documented the role of bicarbonate as an activator of H2O2 [40-45]. More recently, the ability of bicarbonate to activate oxidants such as PS, O2 or O3 also attracted attention owing to its potential applications in wastewater [110-113]. For example, the Mn2+/HCO3-/O3 system was reported to degrade atrazine, and influencedboth degradation of atrazine and the residual O3 concentration as shown in Fig. 15 [110]. The slight increase in degradation of atrazine in this system was attributed to bicarbonate, which produces •CO3- as a reactive species during degradation.
Dioxygen is the cleanest and most abundant oxidizing agent in nature, and its activation by different methods has been reported in wastewater treatment [114, 115]. Li et al. reported a Co2+/HCO3-/O2 system for degradation of O-aminophenol (OAP) as shown in Fig. 16 [112]. They found that 64.7% of OAP could be converted to the less harmful compound,2-aminophenoxazine-3-one (APZ) in 90 min, and this conversion increased to 92% after 240 min. However, other transition metals such as Mn2+, Fe2+ and Ni2+ showed nearly no reactivity, while Cu2+ ions reacted four time slower than Co2+ ions.
Advanced oxidations based on peroxymonosulfate (PMS, HSO5-) or persulfate (PS, S2O82-) recently emerged as efficient technologies for degrading different kinds of organic pollutants [113, 116]. For example, Lei et al. [113] reported the activation of PS with bicarbonate during phenol degradation, in which bicarbonate can increase both the efficiency and stability of the CuO-Fe3O4/PS system. As shown in Fig. 17, the degradation efficiency increased by nearly 18% with the addition of 10 mmol/L bicarbonate. They also found that bicarbonate plays a role in minimizing catalyst leaching because of its buffer capacity (pH 8.0 ± 0.5). For example, the leaching of Cu2+ decreased from 5.6 mg/L to less than 1.0 mg/L with the addition of 1 mmol/L bi carbonate. The leaching of Cu2+ decreased further as the bicarbonate concentration increased, and became negligible at ≥ 5 mmol/L. The authors assumed that PS has a structure similar to H2O2, and could therefore react in a similar fashion as bicarbonate Eq. (35) to produce HCO4- anion, which may further accelerate the oxidation of Cu2+ to Cu3+.
This review summarized recent progress in utilizing bicarbonate to activate H2O2 for pollutant degradation. The merit of the BAP system is its buffer pH (around neutral or weakly alkaline) throughout the wastewater treatment process, which makes the heterogeneous catalysts more resistant to leaching and thus more stable with time. The formation of HCO4- and •CO3- oxidants are additional merits of this system. The data reviewed here also highlighted the promotion effect of bicarbonate in H2O2 based pollutant degradation, whereas it was previously regarded as a drawback to wastewater treatment. Although many studies have been conducted to address the use of ROS for degradation, it seems that ROS are catalyst dependent in different systems. Accordingly, more detailed investigations are still needed to address the mechanisms for degradation. It should also be noted that bicarbonate has been gradually applied in H2O2 based AOPs, as well as in other advanced oxidation systems such as those using PS, O2 and O3 as oxidants, which will offer more attractive applications of bicarbonate for pollutant degradation.
2016,
Vol. 37 Issue (6): 810-825 
PDF (1116 KB)
2016,
Vol. 37 Issue (6): 810-825 PDF (1116 KB)