Supplementary Materials s33

1 Supplementary Materials

2 Authors: Sofia Semitsoglou-Tsiapou, Astrid Mous, Michael R. Templeton, Nigel J.D. Graham,

3 Lucía Hernández Leal, Joop C. Kruithof

5 Title: The role of natural organic matter in nitrite formation by LP-UV/H2O2 treatment of nitrate-

6 rich water

7 Theory

1 8 1. Photochemistry of nitrate photolysis

9 1.1 UV photolysis of nitrate

10 The photochemistry of the nitrite formation from nitrate consists of a complex set of reactions.

11 The photolysis of nitrate is initiated by three reactions, shown below (Lu et al. 2009).

12 φ < 0.001 (1)

13 φ = 0.09 (2)

14 φ = 0.1 (3)

15 The quantum yields shown are for the wavelength of 254nm (Mark et al. 1996). Reaction (1) is

16 dominant at very low pH, but with increasing pH the reaction loses its significance to the nitrite

17 production. Reaction (2) is mainly significant for wavelengths > 280 nm (Goldstein and Rabani,

18 2007). Plumb and Edwards (1992) stated that at a wavelength of 254 nm direct photolysis of

19 nitrate will account for about 9% of the nitrite formed. The rest is produced via the intermediate

20 production of the peroxynitrite ion (ONOO-), see reaction (3).

21 Peroxynitrite is a strong, relatively long-lived intermediate (Alvarez et al. 1995). Aime et al

22 (2004) stated that peroxynitrite formation is essential for the nitrite formation by MP photolysis.

23 At pH ≥ 8, it is the dominant species compared to its conjugate base (ONOOH) (Aime et al.

24 2004). Various reactions originate from peroxynitrite, leading to formation of intermediates with

25 nitrite as end product.

26 (4)

27 k = 0.017 s-1 (5)

28 pKa = 6.6 (6)

29 k = 1.4 s-1 (7)

2 30 (8)

31 Reaction (4) shows the photolysis of peroxynitrite. Mark et al (1996) argued that this reaction, at

32 best, only plays a minor role in the nitrite formation at 254 nm. At high pH the peroxynitrite is

33 predominantly present because of the pKa value of 6.8 (reaction 6), therefore hardly any

34 ONOOH is available and thus at high pH the reactions (7) and (8) are not expected to account for

35 a big part of the nitrite formation or the decomposition to nitrate (Mack et al., 1999). Coddington

36 et al (1999) showed that nitrite formation through reaction (8) occurred for a pH value up to 8.5,

37 at which it reached a plateau. Taking this into account and the fact that reaction (5) and (6) will

38 not result into the formation of nitrite on their own, low formation of nitrite is expected under LP

39 photolysis of nitrate.

40 1.2 Hydrogen peroxide effect

41 Nitrite formation is enhanced when hydrogen peroxide is added to the nitrate solution. The

42 photolysis of hydrogen peroxide results into the production of hydroxyl radicals, as seen in

43 reaction (9).

44 (9)

45 These hydroxyl radicals, along with the peroxynitrite, take part in a series of reactions producing

46 a variety of radicals as intermediates subsequently leading to increased formation of nitrite.

47 k = 5 109 M-1s-1 (10)

48 (11)

49 (12)

50 (13)

51 k = 1 1010 M-1s-1 (14)

52 k = 4.5 108 M-1s-1 (15)

3 53 k = 1.1 109 M-1s-1 (16)

54 (17)

55 (18)

56 k = 1.3 109 M-1s-1 (19)

57 (20)

58 k = 5.3 102 s-1 (21)

59 k = 1 103 s-1 (22)

60 k = 3 108 M-1s-1 (23)

61 The reactions show that the addition of hydrogen peroxide will increase the formation of nitrite

62 due to an increase in the number of pathways leading to this formation (reactions 11, 13, 17, 18,

63 21-23).

64 Hydrogen peroxide itself can also react with the peroxynitrite (reactions 24-26) (Alvarez et al.

- ∙ 65 1995). The reaction of the superoxide ion (O2∙ ) with the NO2 radical (reaction 26) is faster by

∙ 66 three orders of magnitude than the reaction with the HO2 radical (reaction 25), resulting in nitrite

67 formation.

68 (24)

69 k = 1 105 M-1s-1 (25)

70 k = 1 108 M-1s-1 (26)

71 Hydrogen peroxide can also scavenge the hydroxyl radicals that are produced (reaction 27).

72 k = 2.7 107 M-1s-1 (27)

4 73 1.3 Natural organic matter effect

74 The presence of a background water matrix is expected to lead partly to consumption of a part of

75 the UV light and the hydroxyl radicals, reducing the contribution of pathways for nitrite

76 formation via nitrate photolysis. The UV light consumption and OH-radical scavenging by the

77 NOM are well-known processes (Crittenden et al. 2005), but the reactions involved in the UV

78 photolysis of NOM itself affect the pathways leading to nitrite formation. UV photolysis of

1 ∙- ∙ 79 NOM leads to species, such as O2, O2 , OH, other peroxyl radicals and solvated electrons

80 (Cooper et al. 1988, Frimmel et al. 1994, Bems et al. 1999). The production of those species is

81 summarized by the reactions (28)-(36) given below.

82 The excited states of NOM (HS in the reactions stands for Humic Substances) lead to the

1 3 83 production of singlet oxygen ( O2) (which is quenched by the producing molecular oxygen, O2)

84 via reactions (28), (30) and (31). Superoxide radicals are produced via reactions (29), (33) and

∙ ∙- 85 (34) and solvated electrons via reaction (32). Hydroperoxyl (HO2 ) and superoxyl radicals (O2 )

86 form hydrogen peroxide which in turn is photolyzed into hydroxyl radicals (reactions 35-36).

87 (28)

88 (29)

89 (30)

90 ] (31)

91 (32)

92 k= 1.9-2.2 1010 M-1s-1 (33)

93 (34)

94 (35)

5 95 (36)

96 The solvated electrons generated from the excitation of NOM can act as reducing agents and lead

97 to the formation of nitrite via reactions (37)-(39).

98 (37)

99 (38)

100 (39)

101 2. Photochemistry of nitrite photolysis

− ∙ ∙− 102 The photolysis of NO2 in the region of 200–400 nm leads to the formation of NO and O

∙ 103 radicals (reactions 40-41). At pH<12 the O − radical gets protonated and via reaction 42 forms

∙ 104 OH radicals (Mack and Bolton, 1999).

105 (40)

106 (41)

107 (42)

6 108 Figures

110 Figure S.1. Absorption spectra of the Suwannee River, Nordic Lake and Pony Lake NOM

111 (concentration of 4 mg/L for all three NOMs, UV wavelength range 190-350 nm).

7 112 References

113 Aime A., 2004. Effect of hydrogen peroxide and water quality on nitrite formation during nitrate

114 photolysis. KWR report for KIWA N.V.

115 Alvarez B., Denicola A., Radi R., 1995. Reaction between peroxynitrite and hydrogen peroxide:

116 Formation of oxygen and slowing of peroxynitrite decomposition. Chemical Research in

117 Toxicology, 8 (6), 859-864.

118 Bems B., Jentoft F.C., Schlögl R., 1999. Photoinduced Decomposition of Nitrate in Drinking

119 Water in the Presence of Titania and Humic Acids. Applied Catalysis B: Environmental 20 (2),

120 155-163.

121 Coddington J.W., Hurst J.K., Lymar S.V., 1999. Hydroxyl radical formation during

122 peroxynitrous acid decomposition. Journal American Chemical Society, 121 (11), 2438-2443.

123 Cooper W.J., Zika R.G., Petasne R.G., Fischer A.M., 1988. Sunlight-Induced Photochemistry of

124 Humic Substances in Natural Waters: Major Reactive Species. Aquatic Humic Substances.

125 Influence on Fate and Treatment of Pollutants, chapter 22, 333–362.

126 Crittenden J.C., Trussell R., Hand D.W., Howe K.J., Tchobanoglous G., 2012. MWH's Water

127 Treatment: Principles and Design, Third Edition. John Wiley & Sons, Inc.

128 Frimmel F.H., 1994. Photochemical aspects related to humic substances. Environment

129 International 20 (3), 373-385.

130 Goldstein S., Rabani J., 2007. Mechanisms of Nitrite formation by nitrate photolysis in aqueous

131 solutions: the role of peroxynitrite, nitrogen dioxide, and hydroxyl radical. Journal American

132 Chemical Society 129 (34), 10597-10601.

8 133 Lu N., Gao N.-Y., Deng Y., Li Q.-S., 2009. Nitrite formation during low pressure UV lamp

134 irradiation of nitrate. Water Science Technology, 60 (6), 1393-1400.

135 Mack J., Bolton J.R., 1999. Photochemistry of nitrite and nitrate in aqueous solution: a review.

136 Journal of Photochemistry and Photobiology 128 (1-3), 1-13.

137 Mark G., Korth H.-G., Schuchmann H.-P., von Sonntag C., 1996. The photochemistry of

138 aqueous nitrate ion revisited. Journal of Photochemistry and Photobiology 101 (2-3), 89-103.

139 Plumb R.C., Edwards J.O., 1992. Color Centers in UV irradiated Nitrates. Journal of Physical

140 Chemistry 96 (8), 3245-3247.