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Environmental Processes

Fast Photomineralization of Dissolved Organic Matter in Acid Mine Drainage Impacted Waters Chenyi Yuan, Rachel L Sleighter, Linda K. Weavers, Patrick G. Hatcher, and Yu-Ping Chin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b00202 • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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Environmental Science & Technology

1

Fast Photomineralization of Dissolved Organic Matter in Acid Mine Drainage

2

Impacted Waters

3 4

Chenyi Yuana,e, Rachel L. Sleighterb, Linda K. Weaversc, Patrick G. Hatcherb, Yu-Ping

5

Chind,f

6 7

aEnvironmental

8

43210.

9

bDepartment

Science Graduate Program, The Ohio State University, Columbus, Ohio,

of Chemistry and Biochemistry, Old Dominion University, Norfolk,

10

Virginia, 23529.

11

cDepartment

12

University, Columbus, Ohio, 43210.

13

d

14

epresent

15

National Exposure Research Laboratory, U.S. Environmental Protection Agency, Athens,

16

GA, 30605.

17

fpresent

18

Delaware, Newark, DE, 19716.

of Civil, Environmental and Geodetic Engineering, The Ohio State

School of Earth Sciences, The Ohio State University, Columbus, Ohio, 43210. address: Oak Ridge Institute for Science and Education (ORISE), hosted at

address: Department of Civil and Environmental Engineering, University of

19 20

1 ACS Paragon Plus Environment


21

ABSTRACT

22

Acid mine drainage (AMD) formed from pyrite (iron disulfide) weathering contributes to

23

ecosystem degradation in impacted waters. Solar irradiation has been shown to be an

24

important factor in the biogeochemical cycling of iron in AMD impacted waters, but its

25

impact on dissolved organic matter (DOM) is unknown. With a typical AMD impacted

26

water (pH 2.7-3) collected from the Perry State Forest watershed in Ohio, we observed

27

highly efficient (> 80%) photochemical mineralization of DOM within hours in a solar

28

simulator resembling twice summer sunlight at 40°N. We confirmed that the

29

mineralization was induced by ●OH formed from FeOH2+ photodissociation and was

30

inhibited 2-fold by dissolved oxygen removal, suggesting the importance of both the

31

photochemical reaction and oxygen involvement. Size exclusion chromatography and

32

Fourier transform ion cyclotron resonance mass spectrometry elucidated that any

33

remaining organic matter was comprised of smaller and highly aliphatic compounds. The

34

quantitative and qualitative changes in DOM are likely to constitute an important

35

component in regional carbon cycling and nutrient release and to influence downstream

36

aquatic ecosystems in AMD affected watersheds.

37 38


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39

TOC/ABSTRACT ART

40 41

42 43



44

INTRODUCTION

45

Acid mine drainage (AMD) impairs more than 10,000 miles of streams (Table S1) in

46

over half of the states in the US, especially in the Rocky Mountains and Appalachian

47

regions.1–3 AMD is produced from the chemical and biological weathering of sulfide

48

minerals (mainly pyrite, FeS2) in abandoned coal/mineral mining areas and is rich in

49

heavy metals (such as Fe and Mn) and sulfuric acid.4 With continuing worldwide

50

exploitation of natural resources and ineffective mitigation operations,3 AMD remains a

51

persistent concern to the local and regional aquatic ecosystems.

52

Dissolved organic matter (DOM), ubiquitous in aquatic systems, is a heterogeneous mix

53

of organic molecules derived from biological precursors. It serves important ecological

54

roles as a part of the carbon cycle,5,6 as a substrate for heterotrophic microorganisms,7–9

55

and as a photo-reactive component that can screen cell-damaging UV light and generate

56

reactive intermediates that participate in a number of biogeochemical and environmental

57

processes.10–12 DOM is also present in AMD impacted waters at low concentrations (< a

58

few mg-C L-1). These low levels have mainly been attributed to DOM adsorption by Fe

59

and Al oxides that are formed in abundance in these systems.13,14 Previous research on

60

the photochemistry of AMD impacted waters has mainly focused on diel Fe redox

61

cycling15–17 and hydroxyl radical (●OH) production18,19 from the photolysis of ferric

62

complexes. Although AMD impacted waters are highly photoreactive, the

63

phototransformation of DOM in these waters, to our knowledge, has not been previously

64

studied, and we hypothesize that the extent of ●OH generation in these special aquatic

65

environments plays an oversized role with respect to fate of DOM.


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66

In this paper we quantified DOM photomineralization (i.e., complete conversion to

67

carbon dioxide or monoxide) kinetics, evaluated primary reactive species, and

68

characterized its molecular transformation products in AMD impacted waters sampled

69

from the Perry State Forest watershed in southeastern Ohio. Besides irradiation inside a

70

solar simulator, we also investigated similar DOM changes between naturally shaded

71

water samples and naturally sunlit water samples.

72 73

EXPERIMENTAL SECTION

74

Reagents. Suwannee River fulvic acid (SRFA) and Pony Lake fulvic acid (PLFA) were

75

obtained from the International Humic Substances Society. All chemical reagents were

76

purchased from commercial sources without further purification. See the Supporting

77

Information (SI, Text S1 on page S4) for details.

78

Sample Collection and Preparation. Surface water samples from Essington Lake (EL)

79

and a shaded upstream pond (SP) were collected in Perry State Forest watershed in Ohio

80

(Figure S1) and filtered through 0.45 μm Pall AquaPrep groundwater filters (Port

81

Washington, NY). For Fourier transform ion cyclotron resonance mass spectrometry

82

(FTICR-MS) and size exclusion chromatography (SEC) measurement, DOM samples

83

(ELDOM and SPDOM) was extracted from 20-50 L of raw water by adsorption onto

84

Agilent PPL cartridges (bed mass: 5 g) as described by Dittmar et. al.20 and freeze-dried

85

before analysis. Extraction efficiencies for EL and SP were about 50%, estimated based

86

on the influent and effluent DOC concentrations.

87

Whole Water Irradiation Experiments. Quartz tubes (path length of 0.9 cm, sealed

88

with Teflon lined O-rings and glass caps) filled with water samples were irradiated in an



89

Atlas Suntest CPS+ solar simulation system equipped with a xenon lamp and

90

a solar standard filter at 25 ± 2 °C with an intensity of 500 W m-2 (Mount

91

Prospect, IL) from 290 nm to 800 nm (Figure S2). At the designated times, sample tubes

92

were sacrificed for total organic carbon (TOC), iron, dissolved oxygen, and/or UV-Vis

93

analysis. Dark control samples in foil-wrapped phototubes or amber vials were conducted

94

for all photolysis experiments and no dark reaction was observed. Water samples include

95

EL, SP, and EL spiked with 2-6 mg-C L-1 SRFA, 6 mg-C L-1 PLFA, or 6 mg-C L-1

96

SPDOM. Among them, the FA-containing EL water samples were made by spiking EL

97

water with 60 mg-C L-1 stock FA solutions and the 10-15% dilution factor was

98

considered in data analysis. Irradiance was recorded with a Solar Light PMA2100 data-

99

logging radiometer with a PMA2107 UVA+UVB detector (Glenside, PA) at an interval

100

of one second or one hour, depending on irradiation time, and any fluctuations during

101

light ignition (< 10 s) were corrected accordingly. For the anoxic experiments, 80 mL SP

102

water was purged with argon gas for 80 minutes and transferred to quartz tubes (sealed

103

with layers of Parafilm and Teflon to avoid possible gas exchange) in a glove box. The

104

photolysis and sample analysis were conducted in the same manner as the oxic

105

experiments. Natural sunlight irradiation for EL waters was also performed in the same

106

quartz tubes laid horizontally above ground near Essington Lake (39°45’30” N,

107

82°12’15” W) from 10 am to 2 pm on Oct 15, 2015 (a clear sunny day). SP waters for

108

FTICR-MS characterization were photolyzed in UV-transparent bags inside the solar

109

simulator and detailed in Text S1 on page S8.

110

●OH

111

caffeine (CAF, 6 concentrations) or 0.5 - 50 μM terephthalic acid (TPA, 8

Determination and Model Predication. EL waters spiked with 0.5 - 100 μM


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112

concentrations) were filled in quartz tubes and irradiated as described above. The probes

113

were selected because of their negligible light absorption by wavelengths present in the

114

solar range and their high reactivity towards ●OH.21,22 The product of TPA reaction with

115

●OH,

116

system. At the designated time points, phototubes were withdrawn for high performance

117

liquid chromatography (HPLC) measurements. ●OH production rates (R●OH) and

118

background scavenging (S●OH) was calculated by plotting

119

on the following equation:

120

hydroxy-TPA, was not monitored due to its susceptibility towards ●OH in our water

1 [●

OH]𝑠𝑠 ― 𝑝𝑟𝑜𝑏𝑒

=

𝑘𝑝𝑟𝑜𝑏𝑒,●OH 𝑅●OH

1

1 [●OH]ss - probe

versus [probe]0 based

(1)

[𝑝𝑟𝑜𝑏𝑒]0 + 𝑅● 𝑆●OH OH

121

where kprobe,●OH is the second order rate constant of reaction between ●OH and the

122

respective probe (CAF: 5.9 × 109 M-1s-1;23 TPA: 4.4 × 109 M-1s-1).21 [●OH]ss - probe

123

represents the ●OH steady state concentration in the presence of the selected probe

124

compound and was calculated by fitting the following equation:

125

Ln

126

R●OH in AMD waters under different conditions was also estimated based on the integral

127

of the product of light absorption flux and ●OH quantum yield (Φ𝜆) from FeOH2+, which

128

is the most important species responsible for the photochemically generation of ●OH in

129

our system:24

[𝑝 𝑟𝑜𝑏𝑒 ]𝑡 [𝑝𝑟𝑜𝑏𝑒]0

=―k

probe,●OH

𝜆 𝐼𝜆𝐹𝑠𝜆𝐹𝑐 𝜆𝛷𝜆 𝑑𝜆 ∫𝜆𝑚𝑎𝑥 𝑙 𝑚𝑖𝑛

× [●OH]ss - probe × 𝑡

―2.3𝛼

(2)

𝑙

𝑠𝜆 )𝜖𝜆𝑐𝛷𝜆 𝜆𝑚𝑎𝑥𝐼𝜆(1 ― 𝑒 ∫𝜆 𝑑𝜆 𝛼 𝑙 𝑠𝜆 𝑚𝑖𝑛

130

𝑅●OH =

131

where Iλ (einstein m-2 s-1 nm-1) is the photo flux at a particular wavelength λ (nm) and was

132

adjusted using p-nitroanisole/pyridine actinometry for indoor experiments or calculated

133

using the National Renewable Energy Laboratory’s (NREL) Simple Model of the

=


(3)


134

Atmospheric Radiative Transfer of Sunshine (SMARTS)25 for outdoor experiments (see

135

parameters in Table S2). Fsλ is the fraction of light absorbed by the system with an

136

attenuation coefficient of αsλ (cm-1), and Fcλ is the fraction of light absorbed by FeOH2+. c

137

(M) is the concentration of FeOH2+ calculated with Visual MINTEQ 3.1 using the

138

experimentally determined ion concentrations and pH value, ϵλ is its molar absorptivity

139

(M-1 cm-1) (Table S9), and Φλ is its wavelength dependent ●OH production quantum

140

yield.26 l (cm) is the photo path length of the studied water (0.9 cm for our photo tubes).

141

The equation was integrated over the wavelengths where the solar spectrum and the

142

absorbance of FeOH2+overlap. See Figure S3 for representative spectra at noon in mid-

143

summer.

144

The Roles of SO42− in AMD Photochemistry. The production of SO4●− in EL was

145

determined using two probes of different selectivity towards ●OH and SO4●−: Benzoic

146

acid (BZA) has high reactivity towards both ●OH (bimolecular rate constant of 4.3 × 109

147

M-1s-1)27 and SO4●− (1.2 × 109 M-1s-1),28 while t-butyl alcohol (TBA) is a strong ●OH

148

scavenger (4.2 - 7.6 × 108 M-1s-1)27 but a weak SO4●− quencher (4 - 9.1 × 105 M-1s-1).29

149

Of the two probes used for ●OH, only the rate constant between SO4●− and TPA was

150

known at pH 7 (1.7 ×108 M-1s-1, which is one order of magnitude lower than its reaction

151

rate constant with ●OH),28 so we used the better-studied probes BZA and TBA to

152

determine the importance of SO4●−. Both BZA and TBA have negligible light absorption

153

and ability to complex Fe in our system, thus do not interfere with the photochemistry.

154

The degradation of 30 μM BZA in EL in the presence of 0, 10, or 100 mM TBA was

155

monitored in the solar simulator described above. The contribution of SO4●− production (

156

RSO●4 ― ) relative to ●OH was calculated based on competitive kinetics. To assess the


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157

impact of SO42− on ●OH production, the degradation of CAF (a ●OH probe) was

158

monitored in the presence of varying [SO42−] (1 mM, 11 mM, and 21 mM) in synthetic

159

AMD waters with controlled pH (2.7), [Fe(III)] (660 μM), and ionic strength (0.06 M from

160

ClO4−, SO42−, and Na+).

161

Analytical techniques. pH was measured using a Beckman 240 pH / Temp Meter (Brea,

162

CA) with daily calibration. pH levels remained unchanged for irradiated water samples

163

(2.7-3). Total organic carbon (TOC) was measured using a Shimadzu TOC-VCPN analyzer

164

(Kyoto, Japan) in the non-purgeable organic carbon mode calibrated with potassium

165

hydrogen phthalate standards (detection limit: 0.05 mg-C L-1, precision between replicate

166

injections: 1.5% as coefficient of variation). Water sample absorbance was determined in

167

a 1-cm quartz cuvette with a Shimadzu double-beam UV-1800 spectrophotometer

168

(Kyoto, Japan). Deionized water was used as the blank solution in the reference side of

169

the spectrophotometer. TPA, CAF, and BZA were measured using a Waters HPLC with a

170

Restek C18 column (Text S1 on page S5). Dissolved oxygen was measured with a Lazer

171

Research Laboratories micro-DO probe (Los Angeles, CA). Ferrous and ferric iron were

172

determined using FerroZine colorimetry (Text S1 on pages S4 and S5). Elements were

173

determined using a Teledyne-Leeman Labs Prodigy Dual View Inductively Coupled

174

Plasma Optical Emission Spectrometer (ICP-OES, Hudson, NM) by Service Testing And

175

Research Laboratory at Ohio State University.30 Major anions were measured using a

176

ThermoDionex Ion Chromatograph (ICS-2100).31,32 Extracted DOM samples were

177

analyzed on a Bruker Daltonics 12 Tesla Apex Qe FTICR-MS instrument with an Apollo

178

II ESI ion source in negative ionization mode (Text S1 on pages S6 to S8). SEC of



179

extracted DOM was determined using a waters HPLC with a waters Protein-Pak column

180

(Text S1 on page S5).

181 182

RESULTS AND DISCUSSION

183

Photomineralization Rates of DOM in AMD Impacted Waters. The 5.5 km2 Perry

184

State Forest watershed (Figure S1) has typical AMD impacted low-order streams in

185

southeast Ohio.33,34 Two sites in this watershed were investigated: Essington Lake (EL),

186

which is largely un-shaded and represents a natural photolytic reactor, and a highly

187

shaded upstream pond (SP), which is covered by vegetation and represents a less

188

irradiated surface water body. While both waters had similar low pH values (2.7-3) and

189

high total Fe concentrations (300-700 μM) (Table S3), SP (5.4 mg-C L-1) had 5 times

190

more dissolved organic carbon (DOC) compared to EL (0.9 mg-C L-1), which strongly

191

suggests the possible impact of natural sunlight irradiation on DOM biogeochemistry.

192

Under simulated sunlight, which is twice the midday irradiance in June (after lens effect

193

correction for our photo tubes) at the location of the Perry State Forest watershed

194

(39°45’30” N, 82°12’15” W, Figure S2), the photomineralization of DOM in both EL

195

and SP waters was extremely fast, with initial (within the first hour) pseudo-first order

196

half-lives of 0.7 ± 0.2 and 0.7 ± 0.1 h, respectively, and deviated from first-order kinetics

197

as it approached nearly complete mineralization (Figure 1a, Figure S9). No dark reaction

198

or iron precipitation was observed in any experiment. The initial photochemical

199

mineralization rates (RDOC) ranged from 0.5 to 2.9 mg-C L-1 h-1 in the photic zone, which

200

are one-to-four orders of magnitude higher than most natural surface waters at near

201

neutral or slightly acidic pH values.35–37 The corresponding apparent quantum yields


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(Φapp, defined as the ratio of moles of DOC mineralized over moles of photons absorbed

203

by the whole sample at 290-400 nm where irradiation was strongly absorbed) ranged

204

from 1.4 × 10-3 to 3.7 × 10-3. It is an indirect apparent quantum yield since light is mainly

205

absorbed by inorganic Fe complexes rather than DOM, which will be discussed in the

206

next section. Because of light attenuation, photomineralization was elevated in the top

207

several centimeters of the water column (Figure S4). When integrated through the water

208

column, mineralization rates under noon summer sunlight were estimated to be 11 and 29

209

mg-C h-1 m-2 for in-situ EL and SP waters, respectively, based on the apparent quantum

210

yields. We estimated a maximum of about 6 kg-C could be mineralized for EL (an area of

211

6.6 × 104 m2) during a single day in middle summer. This daily mineralization translates

212

into about 1% all DOC in EL assuming a depth of 10 m and a hydraulic retention time of

213

more than one day. The actual mineralized carbon amount per surface area will greatly

214

depend on light intensity, hydrologic conditions, and the water chemical constituents of

215

the impacted system.

216

In an effort to assess the impact of DOM type and concentration on mineralization, we

217

added Suwannee River fulvic acid (SRFA, which represents terrestrially derived DOM),

218

Pony Lake fulvic acid (PLFA, which represents microbially derived DOM), and solid-

219

phase-extracted (SPE) shaded pond DOM (SPDOM, from our field site) to EL water and

220

assessed their mineralization kinetics. The initial pseudo-first order mineralization rate

221

constant (kDOC) decreased with increasing DOC concentration (Figure 1b), as predicted

222

from our derived rate law equations describing the effect of varying DOM concentrations

223

(Text S2). PLFA and the native DOM in EL were shown to be below the kinetic line to



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224

which SRFA data was fitted, implying that they contained moieties that were more photo-

225

resistant relative to SRFA and SPDOM.

226

Initial Light Absorption and Reactive Species Production. Solar energy is absorbed

227

by various chromophoric species in our water samples, mainly DOM and Fe complexes.

228

At a pH of 2.7-3 in AMD waters, the most abundant Fe species is FeSO4+ (76-81%,

229

calculated using Visual MINTEQ 3.1, Table S4), followed by FeOH2+ (8-14%) and

230

Fe(SO4)2− (5%). Complexation between Fe and DOM is limited (< 5%), due to the

231

protonation of Fe complexing DOM ligands under these acidic conditions and was

232

neglected in our interpretation (Text S3). Figure 2 shows the contribution of light

233

absorbance from Fe complexes and DOM in a EL water sample spiked with 6 mg-C L-1

234

SRFA: FeSO4+ > SRFA > FeOH2+ ≈ Fe(SO4)2−. We will discuss the role of different light

235

absorption constituents on initial DOM transformation as follows.

236

Light absorption by FeOH2+ and ●OH production

237

𝐹𝑒𝑂𝐻2 + 𝐹𝑒2 + + ●𝑂𝐻

238

The photoreduction of FeOH2+ (Equation 4) is the main photochemical reaction in a

239

variety of acidic Fe-rich waters, such as AMD impacted water, clouds, fog, rain, and

240

certain wastewater treatment systems.15,18,38,39 We observed consistent Fe(II) increase

241

during DOM mineralization (see Figure S10b as an example). As a result, the dynamic

242

production and scavenging of ●OH will change with irradiation time, but our following

243

analysis only focused on the initial radical generation stage.

244

●OH

245

mg-C-1 s-1,21,40–43 leading to mineralization, bleaching, and other transformations of

246

DOM.8,11 We determined the initial ●OH production rates (R●OH) and the steady state

ℎ𝑣

(4)

is a potent oxidant and reacts with DOM at rate constants of 1 × 104 to 7 × 104 L


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●OH

concentrations ([●OH]ss) in EL water using the ●OH probes: terephthalic acid (TPA)

248

and caffeine (CAF) (Table 1). Reaction of the two probes in our irradiated AMD samples

249

resulted in a consistent R●OH of 1.7 ± 0.2 × 10-7 M s-1, [●OH]ss of 4.3 ± 0.8 × 10-12 M,

250

and natural ●OH scavenging S●OH of 4.0 ± 0.2 × 104 s-1. Our model estimate of ●OH

251

production based on light absorption and the quantum yield of ●OH from FeOH2+

252

corroborates our experimental data for both EL and a synthetic AMD solution within a

253

factor of two (Table 1) and suggested that FeOH2+ is the principal ●OH photosensitizer in

254

AMD waters. Experiments conducted under natural sunlight with EL waters collected

255

and irradiated hourly from 10 am to 2 pm on a sunny mid-October day (39o 45’ N)

256

revealed similar [●OH]ss values (2 × 10-12 - 4 × 10-12 M, Table S5). Our [●OH]ss is on

257

the high end relative to the only other reported measurements of [●OH]ss in AMD

258

impacted waters (7 × 10-15 - 4 × 10-12 M)18 and is many orders of magnitude higher

259

than values reported for sunlit natural waters not affected by AMD (10-15 - 10-18 M).42,44,45

260

Effect of AMD chemical constituents on ●OH production. Like most AMD impacted

261

waters, EL and SP had low pH and high salt concentrations (Table S3) compared to

262

common freshwater systems. The fluctuation of pH and the presence of ligands such as

263

sulfate (SO42−), chloride (Cl−), and fluoride (F−) in AMD impacted waters may

264

potentially impact the speciation of FeOH2+. The FeOH2+ concentration increases with

265

[OH−] when pH increases from 2 to 4.38 Using the SP sample as an example, our modeled

266

R●OH values decreased by approximately 40% when pH decreased from its native pH

267

(3.0) to the pH of EL (2.74) (Table 1).

268

The presence of sulfate in AMD impacted waters also greatly impacted Fe(III) speciation.

269

Because Fe sulfate complexes have similar molar extinction coefficients as FeOH2+



270

(Table S9), it also results in strong light absorption without the production of ●OH. We

271

corroborated this phenomenon by conducting photolysis experiments using synthetic

272

AMD solutions (pH 2.7, Fe(III): 600 µM, ionic strength: 0.06 M from ClO4−, SO42−, and

273

Na+). No degradation was observed in the matrix control in the absence of Fe2(SO4)3 at

274

the same pH and ionic strength. Figure 3a revealed a roughly 5-fold decrease in ●OH

275

production with increasing [SO42−] (from 1 mM to 21 mM). Modeling based on FeOH2+

276

speciation agreed well with experimental data at high SO42− levels but overestimated

277

R●OH at 1 mM [SO42−] by 66% (Figure 3a).

278

The presence of F− could also influence iron photochemistry because of its strong

279

complexation to Fe3+ (two orders of magnitude stronger than SO42−) to form non-

280

absorbing FeF2+. However, the large amount of total Al(III) in EL out-competed Fe3+ for

281

nearly all of the fluoride present, making this ligand unimportant in Fe(III) speciation.

282

Unlike F−, sub mM levels of Cl− mainly existed as the free anion in EL and SP and was

283

an unimportant ligand in our system, minimizing its impact on ●OH production.

284

Estimated contribution of AMD chemical constituents on ●OH scavenging. Besides

285

DOM, ions in the AMD water samples such as Cl−, Fe2+, and HSO4− can also scavenge

286

●OH

287

on their concentrations and second-order rate constants (Table S10). DOM was the major

288

●OH

289

rates. Cl− (kCl,●OH= 4.4×109 M-1 s-1)46 reacts with ●OH to produce ClOH●−, which then

290

forms species like Cl● and Cl2●−.47,48 However, the back reaction to reform ●OH is also

291

very fast,49 leading to 99.4% and 99.7% ClOH●− to reform ●OH in EL and SP water

292

samples, respectively. As a result, the apparent scavenging by Cl− is was estimated to be

(Equations S3-S8, Table S8). We estimated the contribution of each species based

scavenger in both EL and SP systems, in agreement with its high mineralization


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293

15% of our measured scavenging in unaltered EL water and its percent contribution to

294

●OH

295

competition from DOM. About two thirds of the total Fe(II) existed as Fe2+, with the rest

296

being FeSO4(aq) (Table S4). Because of the low initial concentration of Fe2+, it was

297

estimated to only react with approximately 6% ●OH in EL. The scavenging by Fe2+ will

298

become more important as photoreduction of FeOH2+ proceeds. HSO4− reacts with ●OH

299

to form SO4●−. The low concentration of HSO4− resulted in only 3% contribution to ●OH

300

scavenging in EL. While these estimates can provide an idea of ●OH scavenging

301

composition in AMD impacted waters, we recognize its limitation and future research is

302

needed to confirm the relative importance of different scavengers.

303

Light absorption by FeSO4+ and sulfate radical (SO4●−) production. As the dominant Fe

304

species, FeSO4+ also undergoes photolysis and produces the more selective SO4●− as

305

shown in Equation (5).26

306

𝐹𝑒𝑆𝑂4+ 𝐹𝑒2 + + 𝑆𝑂●4 ―

307

The production of SO4●− in EL waters was determined by monitoring benzoic acid (BZA)

308

degradation in the presence and absence of t-butyl alcohol (TBA). Unlike BZA, which

309

reacts similarly with both ●OH and SO4●−, TBA scavenges ●OH 1000-times faster relative

310

to SO4●−, allowing us to assess the importance of SO4●− in the photomineralization of

311

DOM. The more-than-one-order-of-magnitude decrease in the BZA disappearance rate

312

constant in the presence of TBA in Figure 3b shows that the SO4●− production rate (RSO●4 ―

313

) was only about (3 ± 1) % of R●OH. This production rate is smaller than what is predicted

314

based upon FeSO4+ light absorption and its quantum yield for the production of SO4●−

315

(14% of R●OH),26 and is possibly due to matrix effects in the actual AMD waters. Further,

scavenging was likely much less in high DOM samples such as SP water due to the

ℎ𝑣

(5)



316

the possibility of SO4●− production from the reaction between ●OH and HSO4− (1 mM)

317

was evaluated to be quite small (< 3% R●OH) based on its kinetics (Table S10). In

318

addition, the reaction between SO4●− and DOM is approximately 2 times slower than with

319

●OH.50

320

waters, it’s photoproduct, SO4●−, is likely a minor contributor to DOM transformation.

321

Light absorption by DOM and its direct photolysis. Absorption of sunlight by DOM itself

322

can, directly or through generated reactive species, transform DOM51,52 to smaller and/or

323

labile organic compounds53,54 and mineralize it to inorganic carbon.55–57 Although DOM

324

is one of the most important light attenuators in natural waters, the fraction of light

325

absorbed by DOM in AMD impacted waters is relatively insignificant due to its low

326

abundance relative to light-absorbing Fe species (Figure 2, Figure S5). Reported quantum

327

yields of DOM photomineralization typically decrease exponentially with increasing

328

wavelengths, and the averaged quantum yields in the UV portion of sunlight are on the

329

order of 10-4-10-5 for fresh water35,58 and seawater.55 Through integration of the product

330

of absorbed light and quantum yield over 290-400 nm, the rate of direct

331

photomineralization of DOM would be in the range of 4×10-4 – 8 ×10-4 mg-C L-1 h-1 in

332

EL solution spiked with 6 mg-C L-1 SRFA under our experimental conditions. This rate is

333

at least 3 orders of magnitude lower than our observed mineralization value (2.4 mg-C L-

334

1 h-1,

335

would be of minimum importance in AMD waters and the observed DOM

336

photomineralization was initiated by indirect photolysis

337

Secondary Reactions Initiated by Photochemical Processes. The aforementioned

338

major light absorption process that leads to reaction with DOM, i.e., light absorption by

Therefore, although FeSO4+ is the most abundant chromophore in AMD impacted

Figure 1a) in the photic zone, and we surmise that the direct photolysis of DOM


Page 16 of 38

Page 17 of 38


339

FeOH2+, requires Fe(III) to be an electron acceptor for the mineralization/oxidation of

340

DOM. If we assume that the oxidation states of carbon in DOM is approximately zero8

341

and Fe(III) is the only electron acceptor, approximately 0.25 mole of CO2 is produced for

342

every mole of Fe(III) reduced based on redox stoichiometry (Equation S20, Table S8).

343

Thus, we anticipate that for the SRFA spiked EL sample and native SP waters, a

344

maximum of 0.9 and 1.9 mg-C L-1 of DOM would be mineralized, respectively, based

345

upon the initial Fe(III) levels present in each sample. In contrast, we observed

346

photomineralization of 5 mg-C L-1 in EL water (spiked with an initial concentration of 6

347

mg-C L-1 SRFA) and 4 mg-C L-1 in SP (initial concentration of 5.4 mg-C L-1 in SP) water

348

(Figure 1a). Therefore, electron acceptors other than Fe(III) must also participate in the

349

secondary mineralization of DOM following the photomineralization of DOM mediated

350

by ●OH.

351

Dissolved oxygen may be the most important electron acceptor besides Fe(III). In surface

352

waters at equilibrium with the atmosphere (~300 μM aq O2), we estimate that O2 can

353

mineralize 300 μM or 3.6 mg-C L-1 of DOM based upon redox stoichiometry (Equation

354

S19, Table S8). In a study that examined the photodegradation of 2,4-dichlorophenoxy

355

acetic acid (pH 2.8, Fe = 1.0 mM), the presence of O2 enhanced its mineralization by a

356

factor of four.59 We observed an enhancement by about a factor of two for mineralized

357

DOC in air-saturated SP water relative to argon-saturated SP water, and the enhancement

358

was quantitatively attributed to the change in Fe and oxygen levels (Figure 1c, Figure

359

S10). We observed that even in the oxic experiments, the oxygen concentration in the

360

sealed phototubes dropped dramatically (as much as 80%) as a result of photolysis in SP

361

water (Figure S10a). While in the field, where oxygen is continuously replenished by



362

exchange with the atmosphere, a higher enhancement in DOM mineralization is possible

363

compared to what was determined in sealed phototubes. Reaction involving O2, ●OH, and

364

DOM could produce organoperoxyl radicals (Equations S9 and S13, Table S8) , which

365

may be the first step in the oxygen-dependent secondary transformation of DOM.59 These

366

organoperoxyl radicals may be involved in the reoxidation of Fe(II) back to Fe(III) or in the

367

generation of H2O2 to produce more •OH via reaction with Fe(II) (Equations S15-S18,

368

Table S8).59

369

Characterizing DOM Phototransformation Products. While most of the target DOM

370

used in this study was mineralized to inorganic carbon, a recalcitrant fraction remained

371

after photolysis and is likely similar in composition to DOM in EL. We determined

372

compositional differences between DOM samples extracted from the sunlit EL and

373

shaded SP waters by Fourier transform ion cyclotron resonance mass spectrometry

374

(FTICR-MS) and size exclusion chromatography (SEC). DOM was isolated by SPE to

375

remove the undesired high salt content60 and to increase its concentration for the assays.

376

While SPE is a necessary sample preparation step, we recognize that it only partially

377

recovers the DOM in our samples (~ 50%) and, as such, limits our ability to completely

378

characterize the refractory material.20 SEC of extracted DOM (Figure 4a) clearly shows

379

the shift in the size of the major light-absorbing (λ = 224 nm) molecules from about 1000

380

Da in SP to the lower size limit of our column (about 100 Da) in EL. Consistently,

381

FTICR-MS showed a difference in both the number of peaks (a 60% decrease between

382

SP and EL DOM respectively) and number-averaged m/z for the DOM in EL (467 Da)

383

relative to SP (523 Da) (Table S6 and S7). Although both SEC and FTICR-MS may

384

detect different DOM pools within their own limitations, the significant and consistent


Page 18 of 38

Page 19 of 38


385

results from the two independent analytical techniques corroborate our observations

386

regarding DOM size. Thus, the resident DOM in sunlit EL consist of smaller compounds

387

and undetectable moieties, likely as a result of extensive photolysis in the lake compared

388

to the shaded SP sample. Compared to SP, EL had a higher H/C ratio and lower double

389

bond equivalents (DBE) (Table S7) and possesses an unusually highly aliphatic content

390

as qualitatively visualized in van Krevelen diagrams (Figure 4b, Figure S6). Aromatic

391

and other light absorbing components were largely transformed to aliphatic moieties

392

upon irradiation. The photo-lability of aromatic compounds (including its susceptibility

393

by ●OH) has been widely recognized by either MS-based techniques or more simple

394

optical techniques (e.g., specific UV absorbance, spectral slope, etc.) in both whole

395

waters and solutions of DOM isolates upon solar irradiation.8,61–64 Low molecular weight

396

acids have also been shown to be formed from reactions between DOM and ●OH.8,11 The

397

O/C values decreased from SP to EL (Figure 4b, Table S7), indicating oxygen-based

398

functional groups were degraded upon solar irradiation.

399

In order to assess whether compositional differences between SPDOM and ELDOM is

400

caused by photolysis, we irradiated SP whole waters for 3 or 6 hours and found that

401

remaining SPDOM has a similar MS spectrum as ELDOM (Table S6 and S7, Figure S7

402

and S8), i.e., aromatic, oxygen-rich compounds underwent degradation and aliphatic

403

compounds remained. Unlike ELDOM, the irradiated SP sample also revealed a high

404

abundance of newly formed high-intensity S-containing peaks (Table S6, Figure S7 and

405

S8), suggesting the incorporation of S in DOM molecules. Possible mechanisms include

406

esterification by sulfuric acid65 and SO4●− addition to double bonds66 to form

407

organosulfates.



408

Combining the product molecular signature and the photochemically induced reactive

409

species, we proposed that in sunlit AMD impacted waters, aromatic organic matters react

410

with photogenerated ●OH and subsequently other reactive oxygen species, resulting in

411

OH addition, oxidative radical reaction, ring opening, bond cleavage, and

412

decarboxylation as depicted by previous studies from other aquatic systems.51,61 In this

413

process, large amount of inorganic carbon and nutrients such as N and P, could be

414

released, which might impact the ecosystem.

415

Implications for Future Work

416

We discovered near complete mineralization of DOM on a time scale of hours under

417

sunlight in the photic zone of the studied AMD impacted waters in our phototubes. Our

418

estimates indicate a maximum of 1% and 0.5% DOM could be mineralized in a sunny

419

day in mid-summer for a lake with a depth of 10 m and with water properties resembling

420

EL and SP, respectively. This process was initiated by the formation of exceedingly high

421

levels of ●OH, accelerated by dissolved oxygen, and was highly dependent on FeOH2+

422

speciation and other water substituents. Season, weather, water depth, hydrology of the

423

impacted lake/stream will also greatly impact the actual mineralization rate. Beyond the

424

AMD impacted waters used in this study, similar DOM phototransformation might be

425

expected in other highly acidic Fe-rich waters, such as those impacted by natural airborne

426

acidic fumigation in extreme environments67 or the general acid rock drainage and waters

427

released from FeS2-containing sediments.68 We also demonstrated that any un-

428

mineralized DOM was transformed to smaller and highly aliphatic compounds, whose

429

effect on AMD impacted watersheds, on especially downstream ecosystems, necessitates

430

further research.


Page 20 of 38

Page 21 of 38


431

ACKNOWLEDGEMENTS

432

Funding was provided by the John C. Geupel endowment in the Department of Civil,

433

Environmental and Geodetic Engineering at the Ohio State University. We thank Perry

434

State Forest for providing access to AMD waters. We thank Cody Chandler, Carissa

435

Hipsher, and Jeff Hudson for helping with AMD water sampling, Sue Welch for IC

436

measurement, and Franklin (Sandy) Jones for freeze-drying DOM samples. We also

437

thank Kimberly Parker for discussion and the four anonymous reviewers for their

438

suggestions to improve the manuscript.

439 440

ASSOCIATED CONTENT

441

The Supporting Information is available free of charge on the

442

ACS Publications website: Text S1-S3, Figures S1−S10, and Tables S1−S10 addressing

443

materials, experimental procedures, FTICR-MS spectra, and other supplementary data.

444



445

FIGURES AND TABLES

446 447

Figure 1. The photomineralization of DOM. a. DOM photomineralization (DOC

448

decrease) in EL, EL spiked with 6 mg-C L-1 SRFA and SP waters. EL (0.9 mg-C L-1) was

449

collected from an open lake while SP (5.4 mg-C L-1) was from a shaded pond. Data

450

points in the same color of different transparency indicate replicate irradiation

451

experiments. b. pseudo 1st order rate constants (kDOC, calculated from initial 1-hour

452

irradiation) of DOM photomineralization for different water samples (EL, EL spiked with

453

2, 4, or 6 mg-C L-1 SRFA, EL spiked with 6 mg-C L-1 PLFA or SPDOM, and SP). The

454

regression line (with 95% confidence intervals) is fitted from the experiment using EL

455

solutions spiked with SRFA (Text S2). c. comparison of different electron acceptors in

456

argon (Ar) (Fe(III) only) and air (Fe(III) and O2) equilibrated SP solutions. “DOC

457

equivalent” represents either measured DOC decrease or oxidizable DOC calculated from

458

electron acceptor concentrations based on redox stoichiometry. Horizontal lines labelled

459

as “Armax” and “airmax” were calculated based on initial Fe(III) and O2 levels while vertical

460

bars labeled as “DOC oxidized by Fe(III) or O2” were calculated based on measured


Page 22 of 38

Page 23 of 38


461

changes in Fe(III) and O2 (Figure S10). pH in all solutions: 2.7-3. [Fe(III)]0 in EL based

462

solutions: 250-300 μM; [Fe(III)0] in SP: 610-650 μM.



463 464

Figure 2. Light absorbance composition for a EL solution containing 6 mg-C L-1 SRFA.

465

Path length: 1 cm. Absorbance for the whole solution (total) and 6 mg-C L-1 SRFA

466

solution was experimentally determined (exp). Absorbance for Fe complexes was

467

calculated (model) based on the modeled concentration (FeOH2+: 29 µM; FeSO4+: 220

468

µM; Fe(SO4)2−: 14 µM) and molar absorptivity26,69 (Table S9) when information was

469

available. Dilution of EL (10-15%) by 60 mg-C L SRFA stock solution was considered in

470

speciation calculation. Reproduced with permission from reference 26. Copyright 1995

471

American Chemical Society Reproduced with permission from reference 69. Copyright

472

1953 American Chemical Society


Page 24 of 38

Page 25 of 38


473

Table 1. Water chemistry and measured/modeled ●OH kinetics in irradiated AMD

474

impacted waters under simulated sunlight. system

pH

EL

2.72 2.74 2.74 2.99a 2.74b 2.76 2.76

SP synthetic AMD

Fe(III)0 μM 330 330 330 612 612 660 660

SO42− mM 17.2 17.2 17.2 15.2 15.2 21.0 21.0

probe/model R●OH 10-7 M s-1 CAF 1.8 ± 0.1 TPA 1.5 ± 0.1 model 1.0 model 2.2 model 1.4 CAF 1.1 ± 0.1 model 1.1

S●OH 104 s-1 3.8 ± 1.0 4.1 ± 0.6 NAd NA NA --c NA

[●OH]sse 10-12 M 4.9 ± 1.4 3.7 ± 0.6 NA NA NA NA NA

475 476

Note: Data are adjusted using p-nitroanisole/pyridine actinometry for lens effect for

477

modeled values. anative pH of SP water. ban alternative pH calculated for SP to compare

478

with EL data. climited data points resulted in a negative value and large uncertainty in

479

fitting S●OH, thus omitted. dNA: not applicable. ecalculated from S● . Error bars represent

480

95% confidence intervals.

𝑅●𝑂𝐻 OH



481

482 483

Figure 3. a. The effect of sulfate concentration on ●OH production (R●OH) in synthetic

484

AMD solutions measured by caffeine (CAF) or modeled based on FeOH2+ speciation.

485

[CAF]0 = 5, 50, and 300 μM, [Fe(III)]0 = 660 μM, I = 0.06 M, pH = 2.8. b. The

486

contribution of SO4●− production (RSO●4 ― ) relative to ●OH (R●OH) in the EL water probed

487

by benzoic acid (BZA, reacting similarly with both radicals) with different t-butyl alcohol

488

(TBA, a ●OH quencher) concentrations. [BZA]0 = 30 μM, [Fe(III)]0 = 330 μM, I = 0.04 M,

489

pH = 2.9, [SO42-] = 17.2 mM. Error bars represent 95% confidence intervals.


Page 26 of 38

Page 27 of 38


490 491

Figure 4. Characterization of SPE-extracted DOM from EL and SP. a. size exclusion

492

chromatograph (detected by absorbance at 224 nm). b. van Krevelen diagram of formulas

493

unique or common to both samples. Aliphatic (0 < AImod < 0.5), aromatic (0.5 < AImod
0.67) areas are marked, and AImod is the modified

495

aromaticity index (Text S1).



496

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