In Bisulfite/Permanganate for Organic Compounds Oxidation
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Water Research 148 (2019) 198e207 Contents lists available at ScienceDirect Water Research journal homepage: www.elsevier.com/locate/watres New insight into the reactivity of Mn(III) in bisulfite/permanganate for organic compounds oxidation: The catalytic role of bisulfite and oxygen * Shifa Zhong, Huichun Zhang Department of Civil Engineering, Case Western Reserve University, 2104 Adelbert Road, Cleveland, OH, 44106-7201, USA article info abstract À À Article history: A recently discovered bisulfite(HSO3 )/permanganate(MnO4 ) system was reported to produce highly Received 24 June 2018 reactive free Mn(III) that can oxidize organic compounds in milliseconds. However, this characteristic Received in revised form reactivity was not found in all other known reaction systems that can also produce free Mn(III). Why can 18 September 2018 Mn(III) in NaHSO /KMnO be so active? Here, we found NaHSO and O acted as catalysts for the reaction Accepted 12 October 2018 3 4 3 2 between Mn(III) and organic compounds. Without O , 0% of organic compounds were oxidized in Available online 23 October 2018 2 NaHSO3/KMnO4, indicating the absence of O2 inactivated Mn(III) reactivity. When the reaction between NaHSO and KMnO was monitored in air, Mn(III) catalyzed rapid oxidation of NaHSO by O . Then, the Keywords: 3 4 3 2 Bisulfite and permanganate Mn(III) that could oxidize organic compounds was found to be the ones involved in the catalytic reaction Bisulfite/oxygen reaction between NaHSO3 and O2, thus the link between O2 and Mn(III) reactivity was established. Finally, Disproportionation NaHSO3/O2 can be viewed as catalysts for the reaction between Mn(III) and organic compounds because Mn(III) catalyst 1) when Mn(III) was involved in oxidizing organic compounds, it stopped being the catalyst for the Mn(III) oxidant reaction between NaHSO3 and O2 so that they were consumed to a much smaller extent; and 2) without Water treatment NaHSO3 and O2, Mn(III) lost its oxidation ability. To the best of our knowledge, this is the first report on “catalytic role exchange” where Mn(III) is the catalyst for NaHSO3/O2 reaction while NaHSO3/O2 are the catalysts for Mn(III)/organic compounds reaction. Understanding the critical role of oxygen in NaHSO3/ KMnO4 will enable us to apply this technology more efficiently toward contaminant removal. © 2018 Elsevier Ltd. All rights reserved. 1. Introduction Nowack and Stone, 2002). The preparation of Mn(III)-ligands is routinely through adding ligands prior to starting the reaction that Manganese(III) (Mn(III)) is viewed as a potentially strong can generate free Mn(III) (Davies, 1969; Jiang et al., 2010; Klewicki oxidant due to its high reduction potential (E0 ¼ 1.51V) (Kostka and Morgan, 1998). Examples of such reactions can be easily found 2þ 2- 2þ et al., 1995), but it is very unstable in aqueous solution because of in literature, including (1) oxidation of Mn , such as S2O8 /Mn , 2þ 2- 2þ À 2þ its fast disproportionation to Mn and MnO2 (Anderson and Kochi, Cr2O7 /Mn , and MnO4 /Mn (Davies, 1969; Gupta and Ghosh, À 1970; Ladbury and Cullis, 1958). However, this disproportionation 1958; Jiang et al., 2010); and (2) reduction of MnO4 or MnO2,e.g. 2þ À À reaction can be effectively prevented by some ligands, e.g. pyro- oxalic acid/MnO2,Mn /MnO4 and H2O2/MnO4 (Chen et al., 2013; phosphate (PP), oxalate, citrate, and siderophores, through forming Davies, 1969; Jiang et al., 2010; Taube, 1947). À À stable Mn(III)-ligand complexes (Anderson and Kochi, 1970; Chen Recently, a newly discovered system HSO3 /MnO4 was also et al., 2013; Duckworth and Sposito, 2005; Gao et al., 2018; Jiang found to produce free Mn(III) (eq. (1))(Gao et al., 2017; Hu et al., et al., 2009, 2010; Klewicki and Morgan, 1998; Taube, 1947). 2017; Sun et al. 2015, 2016), but it can oxidize organic com- These Mn(III)-ligand complexes are often directly used as oxidants pounds in milliseconds (eq. (2)). This millisecond reaction has for oxidation purposes, such as oxidative degradation of contami- triggered substantial interest in the advanced oxidation community nants in wastewater (Gao et al., 2018; Jiang et al., 2009, 2010; because the system is very simple, practical, and fast. In addition to oxidizing organic compounds, some Mn(III) undergoes dispropor- 2þ tionation to form Mn and MnO2 (eq. (3)). Moreover, NaHSO3 can compete with organic compounds for Mn(III) (eq. (4)). * Corresponding author. E-mail address: [email protected] (H. Zhang). https://doi.org/10.1016/j.watres.2018.10.053 0043-1354/© 2018 Elsevier Ltd. All rights reserved. S. Zhong, H. Zhang / Water Research 148 (2019) 198e207 199 þ À À 2- 2H þ 2HSO3 þ MnO4 / Mn(III) þ 2H2O þ 2SO4 (1) monohydrate (MnSO4$H2O) (99%þ, Acros Organics), oxalic acid dehydrate (99.5%þ, Acros Organics), hydroxylamine hydrochloride 2þ Mn(III) þ organic compound/ products þMn (2) (99%þ, Acros Organics), sulfuric acid (H2SO4, 95.7%þ, Fisher Chemical) and caffeine (Sigmal-Aldrich) were purchased at the 2þ þ 2Mn(III) þ 2H2O / Mn þ MnO2 þ 4H (3) highest available purity and used as received. Deionized water (DI water, 18.2 MU cm) was obtained from a Millipore Milli-Q water À 2þ 2- þ HSO3 þ 2Mn(III) þ H2O / 2Mn þ SO4 þ 3H (4) purification system. These findings are inspiring in that there is no need to complex 2.2. Oxidation experiments for organic compounds free Mn(III) with ligands as stable oxidants anymore because free Mn(III) itself can now oxidize organic compounds. However, if free The batch reactors were used rather than stop-flow reactors Mn(III) itself is so active, (1) why has this high reactivity not been because we did not intend to monitor the reaction kinetics of discovered for many decades, especially when reactions that can organic compounds. The reaction kinetics of this system as well as readily produce free Mn(III) are widely available, as mentioned the Mn(III) concentration measurements have been carefully above? (2) Why does Mn(III) have to be complexed with ligands investigated already (Sun et al., 2015). Hence, we focused on the rather than directly used as oxidants in previous systems? (3) Is this oxidation efficiency of this system, that is, the overall conversion of high reactivity of Mn(III) unique for NaHSO3/KMnO4? To address organic compounds at the reaction time of up to ~10 seconds these questions, preliminary experiments were conducted to pro- (because the reaction was reported to complete in milliseconds duce free Mn(III) based on two known reactions without adding (Sun et al., 2015), and Fig. S1 shows that 10 seconds were enough ligands but in the presence of organic compounds. Table S1 in the for the reaction to finish). The batch reactors were open to air supporting information (SI) indicated that, unlike the high reac- (referred to as “in air” hereafter) unless otherwise specified. For tivity of Mn(III) in NaHSO3/KMnO4, Mn(III) in oxalic acid/MnO2 or typical NaHSO3/KMnO4 systems, 20 mM phenol was first mixed 2þ KMnO4/Mn could not oxidize organic compounds (phenol as a with 250 mM NaHSO3 solution using a magnetic stirrer, then 50 mM probe) in milliseconds. In fact, Chen et al. were only able to use the KMnO4 was added into the above mixture (the total volume of Mn(III) in oxalic acid/MnO2 to catalyze carbadox reduction by oxalic solution was 50 ml). After reaction in 5e10 seconds, 500 ml samples acid (Mn(III) was also reported to catalyze atrazine de-alkylation) were taken and immediately quenched by 10 ml of 1 M hydroxyl- (Chen et al., 2013; Hu et al., 2017), while Jiang et al. had to com- amine hydrochloride to be analyzed by an Agilent 1260 High Per- 2þ plex the Mn(III) in KMnO4/Mn with ligands to oxidize bisphenol formance Liquid Chromatography (HPLC) equipped with a diode A(Jiang et al., 2009, 2010). These results indicate that: (1) the high array detector. Most experiments were conducted with phenol as reactivity of Mn(III) in NaHSO3/KMnO4 is unique and not found in the probe compound because it is known to be easily oxidizable. To other systems, which makes it understandable why Mn(III) has test if the observed oxygen effect is not unique to phenol, caffeine, been complexed with ligands as a routine way for further appli- an organic compound that is known to be hardily oxidizable (Sun cations; and (2) the high reactivity of Mn(III) in NaHSO3/KMnO4 et al., 2015), was also examined in some experiments. cannot be seen as a common property for Mn(III), and Mn(III) itself For the oxalic acid/MnO2 system (Chen et al., 2013), 200 mMor cannot be directly used as an oxidant due to its facile dispropor- 10 mM MnO2 colloids were prepared by reacting MnSO4$H2O with tionation. Meanwhile, these results raised a new question: why can KMnO4 at the 2.5:1 molar ratio for 2 h in solution. 40 mM phenol Mn(III) in NaHSO3/KMnO4 be so active? This question is exactly was first mixed with 10 mM or 250 mM oxalic acid and then the what this study aims to address. mixture was added into the above MnO2 colloids. After all the MnO2 Here, we found the high reactivity of Mn(III) was due to the was completely dissolved, 500 ml samples were taken and imme- catalytic effect of NaHSO3 and O2. The importance of O2 on the diately quenched by 10 ml of 1 M hydroxylamine hydrochloride for fi 2þ reactivity of Mn(III) was rst investigated in an oxygen-free glove HPLC analysis.