S1

Supporting Information 1

Mutual interactions between reduced Fe-bearing clay minerals and humic 2

acids under dark, oxygenated condition: hydroxyl radical generation and humic 3

acid transformation 4

Qiang Zeng1,2, Xi Wang1,2, Xiaolei Liu1,3, Liuqin Huang4,5, Jinglong Hu1,2, Rosalie 5

Chu5, Nikola Tolic5 and Hailiang Dong*1,2 6

1. Center for Geomicrobiology and Biogeochemistry Research, State Key Laboratory 7 of Biogeology and Environmental Geology, China University of Geosciences, Beijing 8

100083, China. 9

2. School of Earth Sciences and Resources, China University of Geosciences, Beijing 10 100083, China 11

3. School of Water Resources and Environment, China University of Geosciences, 12 Beijing 100083, China 13

4. State Key Laboratory of Biogeology and Environmental Geology, China University 14 of Geosciences, Wuhan 430074, China 15

5. Environmental Molecular Sciences Laboratory, Pacific Northwest National 16 Laboratory, Richland, Washington 99352, UnitedStates 17

* Corresponding author at: State Key Laboratory of Biogeology and Environmental 18 Geology, China University of Geosciences, Beijing 100083, China. Tel.: 19

+86-10-82320969; Email: dongh@cugb.edu.cn 20

21 22 23

Total number of pages: 28 24 Total number of tables: 1 25

Total number of figures: 17 26 27

28

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Section S1: RIC preparations 29

Freeze-dried NAu-2 and SWy-2 solid samples were dissolved in 30

citrate-bicarbonate buffer and were reduced with sodium dithionite.1 Specifically, 31

the prepared clay slurries (10 g/L) were heated in a water bath (90 ℃) and purged 32

with high purity nitrogen for 30 min. Afterwards, sodium dithionite was added as a 33

reducing agent at a weight ratio of 4:1 (dithionite vs. clay). The serum bottles were 34

sealed with rubber stoppers and placed in a 70 ℃ incubator overnight to complete the 35

reduction. The reduced clay minerals were washed with deoxygenated Milli-Q water 36

(18.2 MΩ cm) four times through repeated centrifugation, decanting, and 37

resuspension to remove the residue reductant and buffer. Washed clay minerals were 38

resuspended in deoxygenated H2O to achieve a final concentration 10 g/L and sealed 39

in serum bottles for further use. All washing procedures were conducted inside a 40

glove box (95% N2 and 5% H2, Coy Laboratory Product, Grass Lake, Michigan, USA) 41

to avoid oxidation. 42

Section S2: Measurements of total Fe(II), Fe(II)(aq), and total Fe(aq) 43

The concentration of total Fe(II) in RIC systems was measured with the 44

1,10-phenanthroline method as previously described.2 Specifically, clay slurries (0.2 45

mL in volume) were sampled at selected time points with a 1 mL syringe and injected 46

into black centrifuge tubes. Hydrofluoric acid and sulfuric acid were added followed 47

by boiling for 30 min to thoroughly dissolve the clay minerals. 1,10-phenanthroline 48

(10% in ethanol) was added as a color-developing agent. After color development, 49

S3

absorbance at 510 nm was recorded by a UV-vis spectrophotometer (UV-2550, 50

Shimadzu). 51

Fe(II)(aq) and total Fe(aq) concentrations in the filtrates of RIC experiments 52

(obtained after removing the solids by centrifugation and filtering through 0.22 μm 53

nylon filters) and in Fe(II)(aq) experiments were determined with Ferrozine method.3 54

For Fe(II)(aq) measurement, the samples were acidified with 1 N HCl solution. Color 55

development was achieved by Ferrozine agent (1 g/L) and absorbance at 562 nm was 56

recorded. For total Fe (aq) measurements, Fe(III) was pre-reduced by 10% 57

hydroxylamine hydrochloride in the 1 N HCl solution to convert all Fe (aq) into 58

Fe(II)(aq) followed by the same measurement. 59

Section S3: Measurements of H2O2 60

H2O2 in RIC system was measured with a modified 61

N,N-diethylp-phenylenediamine (DPD) method.4 Due to possible interference of Fe(II) 62

and Fe(III), the filtrates were sampled and immediately mixed with 2,2-bipyridine and 63

Na2EDTA to complex with Fe(II) and Fe(III), respectively. Phosphate was added as a 64

buffering agent. Afterwards, DPD was added as a color developer and peroxidase 65

(Type VI, from Horseradish, Sigma-Aldrich) was added as a catalyst. After color 66

development, absorbance at 551 nm was recorded by a UV-vis spectrophotometer. 67

Section S4: Quantification of hydroxybenzoic acid by HPLC 68

The oxidation products of benzoic acid, including three monohydroxybenzoic 69

acids (4-hydroxybenzoic acid, 4-HBA; 3-hydroxybenzoic acid, 3-HBA; salicylic acid, 70

SA) and one dihydroxybenzoic acid (2,5-DHBA) were measured with a HPLC 71

S4

method as previously described.5 Aqueous concentrations of these products in the 72

filtrates were analyzed with a Shimadzu HPLC (LC-20AT) equipped with a diode 73

array detector (SPD-M20A) and a fluorescence detector (RF-20A). A C18 column 74

(Agilent ZORBAX Eclipse Plus, 4.6×150 mm, 5 μm) and a gradient elution 75

procedure were used for separation of the four possible products. Specifically, mobile 76

phase A consisted of 0.1% trifluoroacetic acid and mobile phase B was acetonitrile 77

(HPLC-grade). The gradient elution procedures were as follows: 0-13 min, 10% B; 78

13-15 min, 10-30% B; 15-23 min, 30% B; 23-25 min, 30-60% B; 25-30 min, 60% B; 79

30-31 min, 60-100% B; 31-35 min, 100% B; 35-36 min, 100-10% B; 36-40 min, 10% 80

B. 4-HBA, 2,5-DHBA, 3-HBA and SA were eluted at 8.2, 11.5, 13.4 and 24.0 min, 81

respectively. The detection wavelengths were set at 276 nm for 4-HBA, 330 nm for 82

2,5-DHBA, and 300 nm for 3-HBA. SA was measured by a fluorescence detector 83

(λex=314 nm, λem=400 nm). The standard curves were established with four pure 84

hydroxybenzoic acids in the range of 0.2-50 μmol/L concentrations using a linear 85

regression (R2≥0.999). 86

Section S5: XRD analysis 87

XRD was used to examine the possible effect of HA and oxygenation on 88

mineralogical transformation of RIC. Clay suspensions were smeared onto 89

petrographic glass slides and dried overnight inside a glove box.6 Analysis was 90

performed with a Rigaku Smart lab X-ray powder diffractometer, using CuKa 91

radiation and a rotating-anode generator, with power of 9000 W (200 kV 45 mA). The 92

samples were scanned from 2 to 15 2θ-degree in a step scan mode. The 2θ step size 93

S5

was 0.02° and a counting time of 1 s per second. 94

Section S6: Gel permeation chromatography (GPC) measurement 95

GPC was performed to investigate the change of molecular weight of HA after 96

oxidation by •OH. The filtrate samples were analyzed with a Shimadzu HPLC system 97

equipped with a refractive index detector (RID-20) and a TSKgel GMPWXL column. 98

The mobile phase consisted of 0.1 N NaNO3 and 0.06% NaN3 in dd H2O, with a 0.6 99

mL/min flow rate. The detection range was 200-500,000 calibrated with polyethylene 100

glycol as a standard. 101

Section S7: 3D-EEM measurement and data analysis 102

EEM spectroscopy was used to detect change of chromophoric groups in HA 103

after oxidation. EEM spectra were obtained using a Hitachi Instruments F-7000 104

fluorescence spectrometer. A 150W Xenon arc lamp was used as a radiation source. 105

An excitation wavelength range of 250-500 nm and an emission wavelength range of 106

250-600 nm were recorded, with a fixed sampling interval of 5 nm. The slit width was 107

5 nm for both excitation and emission windows. The intensity of all EEM spectra was 108

corrected by subtracting the intensity of Milli-Q water from the measured intensity of 109

samples. To evaluate the humification and condensed (aromatic) nature of HA, 110

humification index (HIX) was calculated as follows 7: 111

HIX =(∑I435→438)/ (∑I300→345) 112

where I represents the emission fluorescence intensity at the excitation wavelength of 113

255 nm. 114

Section S8: XPS and FTIR measurement and data analysis 115

S6

XPS was used to characterize the change of major functional groups of HA after 116

oxidation by •OH. Specifically, aqueous HA samples (i.e., the filtrates) were 117

freeze-dried for XPS and FTIR analyses. XPS was performed with a Thermo 118

ESCALAB250Xi X-ray photoelectron spectrometer equipped with a monochromatic 119

Al Kα radiation source (hv = 1486.6 eV). High resolution spectra of C1s were 120

collected and fitted using the XPSPEAK 4.1 software after subtracting background 121

(Shirley type). The C1s level at 284.6 eV was used for calibration. FTIR was 122

performed with a Perkin-Elmer Frontier infrared spectrometer in transmittance mode. 123

Approximately 2 mg solid sample was ground in an agate mortar, homogeneously 124

mixed with 200 mg KBr powder, and then pressed into a disk. For each spectrum, 60 125

scans over the range 400-4000 cm-1 were accumulated with a spectrum resolution of 4 126

cm-1. 127

Section S9: ESI-FT-ICR-MS measurement and data analysis 128

FT-ICR-MS was applied to determine molecular-level change of HS after 129

oxidation by •OH. Aqueous filtrate samples were extracted with Bond Elut PPL 130

cartridges (©Agilent Technologies) to remove salts and eluted in HPLC grade 131

methanol. Extracted samples were analyzed with a 12 T Bruker SolariX FTICR mass 132

spectrometer equipped with a standard Bruker ESI source to generate 133

negatively-charged primary molecular ions. Samples were introduced into the ESI 134

source through a syringe pump at a flow rate of 3.0 µL/min. The experimental 135

parameters were set as follows: needle voltage +4.4 kV, Q1 set to 100 m/z and glass 136

capillary temperature at 180°C, which were optimal experimental parameters for 137

S7

characterizing natural organic matter.8 A mass range 100-900 m/z was collected with a 138

mass detection limit of < 1 ppm. Raw spectra were converted to a list of m/z values 139

using Bruker Daltonik version 4.2. Chemical formula assignment was performed with 140

an in-house software 9, with a S/N > 7, mass measurement error < 0.5 ppm, and 141

inclusions of C, H, O, N, S, and P elements. On average, more than 80% of observed 142

peaks in all measured samples were assigned to a formula. 143

For zoning of van Krevelen diagram, the diagram was divided into several 144

regions according to previous studies,10–12 including lignin, tannin, aliphatic, and 145

condensed aromatic. The specific boundaries for each compound are established as 146

follows: 147

Lignin, H/C = 0.7-1.5, O/C = 0.1-0.67; 148

Tannin, H/C = 0.6-1.5, O/C = 0.67-1.0. 149

aliphatic, H/C>1.5 150

The condensed aromatic compound is defined as compounds with the aromatic index 151

AI≥0.67,11 where AI=1+𝐶𝐶−0.5𝑂𝑂−𝑆𝑆−0.5𝐻𝐻𝐶𝐶−0.5𝑂𝑂−𝑆𝑆−𝑁𝑁−𝑃𝑃

152

For KMD (Kendrick Mass Defects) calculation, the KMD(COO) and KMD(H2) were 153

calculated as follows: 154

Kendrick Mass (COO) = exact m/z of peak × (nominal mass of COO/exact mass of 155

COO) 156

KMD (COO) = Observed nominal mass - Kendrick Mass (COO) 157

where nominal mass of COO = 44.00000, exact mass of COO = 43.98983 158

Kendrick Mass (H2) = exact m/z of peak × (nominal mass of H2/ exact mass of H2) 159

S8

KMD (H2) = Observed nominal mass - Kendrick Mass (H2) 160

where nominal mass of H2 = 2.00000, exact mass of H2= 2.01565 161

Section S10: DOC measurement 162

DOC was used to monitor possible mineralization of HA during oxygenation. At 163

selected time points, the filtrate samples were analyzed with an Analytik-jena multi 164

N/C 2100s series analyzer equipped with nondispersive infrared sensor (NDIR).13 165

Section S11: Electron transfer between RIC and Fe(aq)-HA species 166

Our recent study demonstrated that electron transfer occurs from structural Fe(II) 167

in RIC to Fe(III)(aq)-ligand complex and this electron transfer changes •OH generation 168

pathway and yield.6 To find out if a similar mechanism exists in the presence of HA, 169

aqueous Fe(III)-HA complex was added into a rNAu-2 suspension with different 170

pre-exposure times (to achieve different Fe(II)/total Fe ratio). Aqueous Fe(II) 171

generation was monitored as an indicator of electron transfer from structural Fe(II) in 172

rNAu-2 to Fe(III)aq-HA complex. Experiment was performed under anoxic condition 173

to accurately measure Fe(II)(aq). Specifically, rNAu-2 was pre-equilibrated with HA 174

(100 μmol/L) inside a glove box for 1 h to allow for possible HA-induced dissolution 175

of rNAu-2. Afterwards, Fe(III)aq-HA solution (100 μmol/L, HA to Fe(III) ratio was 1) 176

was added to rNAu-2 (1 g/L), and the filtrate was measured for Fe(II)(aq) generation 177

over time. The amount of Fe(II)(aq) generation was calculated by the difference 178

between the final and the initial Fe(II)(aq) concentrations. All procedures were 179

performed inside a glove box and the reaction solutions were purged with high purity 180

nitrogen for 30 min before use. 181

182

S9

Table S1. Summary of the average molecular weight of HA calculated from Figure 183 S12 184

P1 peak

Mn Mw

PPHA-before oxidation 2862 4021

PPHA-after oxidation 2428 3172

LHA-before oxidation 2096 2398

LHA-after oxidation 1985 2255

P2 peak

PPHA-before oxidation 531 559

PPHA-after oxidation 540 568

LHA-before oxidation 536 564

LHA-after oxidation 543 571

Note: Mn=number-averaged molecular weight, Mw=weight-averaged molecular 185

weight 186

187

S10

188 Figure S1. Correlation between dosage of sodium benzoate and the cumulative •OH 189 production from rNAu-2 oxygenation in the presence of 500 ppm PPHA. The error 190 bars represent standard deviation from duplicate experiments. 191

192

193

S11

194 Figure S2. Sorption of hydroxybenzoic acids to rNAu-2 and rSWy-2 surfaces, where 195 ce represents aqueous concentration of hydroxybenzoic acid and qe represents the 196 sorbed concentration after equilibrium for 2 hours. Experiments were conducted 197 under anaerobic conditions to avoid •OH production. 198

199

200

201

S12

202

Figure S3. Oxidation kinetics of rSWy-2 [2.5 g/L, total 0.7 mM Fe(II)] in the absence 203

and presence of different concentrations of PPHA (A) and LHA (B), corresponding 204

time-course accumulation of •OH (C, D), and linear correlations between •OH yield 205

and the amount of oxidized Fe(II) (E, F). The slope k represents •OH yield 206

(micromole) per millimole of oxidized Fe(II). The correlation coefficients (R2) were 207

greater than 0.98 for all groups. The error bars represent standard deviation from 208

duplicate experiments. 209

210

S13

Figure S4. Modeling of Fe(II) oxidation kinetics of rNAu-2 in the absence and 211 presence of different concentrations of HA. The kinetic data were fitted with a second 212

order rate equation, 1𝐶𝐶− 1

𝐶𝐶0= 𝑘𝑘𝑘𝑘, where C represents the total Fe(II) concentration at 213

selected sampling points, and C0 represents the initial Fe(II) concentration. The 214 correlation coefficients (R2) were greater than 0.92 for all groups. The error bars 215 represent standard deviation from duplicate experiments. 216

217

S14

Figure S5. Production of three monohydroxybenzoic acids during oxygenation of 218 rNAu-2 in the absence and presence of HA (A-G), and their relative percentages after 219 24 h oxygenation (H). The numbers 100, 250 and 500 denotes the HA concentration 220 (ppm). 221 222

S15

Figure S6. Time course changes of Fe(II)(aq) concentration (A, B), total Fe(aq) 223 concentration (C, D), and Fe(II)aq/total Fe ratio (E, F) during oxidation of rNAu-2 in 224 the presence of different concentrations of HA. The error bars represent standard 225 deviation from duplicate experiments. 226 227

S16

228

Figure S7. XRD pattern showing d(001) peak of rNAu-2 after oxygenation for 24 h in 229 the absence and presence of PPHA and LHA. 230

231

S17

232

Figure S8. Modeling of Fe(II)aq oxidation kinetics in the absence and presence of 500 233 ppm HA. An external supply of H2O2 (50 μmol/L) had no effect on Fe(II)aq oxidation. 234 The kinetic data were fitted with a second order rate equation. The correlation 235 coefficients were all higher than 0.98. 236 237

S18

238

Figure. S9. Time course production of H2O2 in aqueous phase during oxygenation of 239 rNAu-2. Unaltered NAu-2 produced a negligible amount of H2O2. 240

241

S19

Figure S10. Time course increase of Fe(II)(aq) after addition of either HA (100 ppm) or 242 Fe(III)(aq)-HA complex (100 μmol/L Fe(III) and 100 ppm HA) into a rNAu-2 243 suspension (1 g/L). In a separate experiment, rNAu-2 was pre-exposed in air for 12 h 244 to achieve a different Fe(II)/Fe(III) ratio. The first sampling point was measured 245 immediately after Fe(III)(aq)-HA addition. Different amounts of Fe(II) generation upon 246 addition of Fe(III)(aq)-PPHA and Fe(III)(aq)-LHA are likely due to different reduction 247 potential of these complexes. 248 249

S20

250 Figure S11. Plots of relative percentages of three monohydroxybenzoic acids as the 251 oxidation products of SB in rNAu-2-HA and Fe(II)(aq) -HA systems. For Fe(II)(aq) -HA 252 system, the Fe(II)(aq) concentration was 150 μmol/L with 500 ppm HA, and 90 μmol/L 253 with 250 ppm HA, respectively, simulating the Fe(II)(aq)-HA concentration in 254 rNAu-2-HA system (Figure S6A-B). Both PPHA and LHA were used. 255 256

S21

Figure S12. GPC elution profiles of HA before (A, B) and after (C, D) oxidation as 257 well as change in cumulative molecular mass distribution of the P1 peak after 258 oxidation (E, F). 259 260 261 262

S22

263

264 Figure S13. Time course changes of the HIX index of PPHA and LHA caused during 265 oxidation of rNAu-2. 266 267 268

S23

269 Figure S14. High resolution C1s XPS profiles of PPHA and LHA before (A, B) and 270 after (C, D) oxidation by •OH. The inset tables show the relative percentages of 271 carbon-related functional groups calculated from the corresponding peak areas. 272 273 274 275 276 277 278

S24

Figure S15. Progressive increase of the intensity of COOH group (1403 cm-1) in 279 PPHA and LHA samples with increasing oxygenation time. The sharp peak at 1384 280 cm-1 was caused by the presence of NO3- in natural OM.13,14 281 282

S25

283 Figure S16. Plots of KMD(COO) value against Kendrick nominal mass in CHO 284 formulas. The removed compounds were only present before oxygenation. The 285 produced compounds were only present after oxygenation. The conserved compounds 286 were present in both un-oxygenated and oxygenated samples. (A, C) showed plots of 287 conserved and produced compounds for PPHA and LHA samples, respectively. (B, D) 288 showed plots of removed and produced compounds for PPHA and LHA samples, 289 respectively. 290 291

S26

Figure S17. Plots of KMD(H2) value against Kendrick nominal mass in CHO 292 formulas. The removed compounds were only present before oxygenation. The 293 produced compounds were only present after oxygenation. The conserved compounds 294 were present in both un-oxygenated and oxygenated samples. (A, C) showed plots of 295 conserved and produced compounds for PPHA and LHA samples, respectively. (B, D) 296 showed plots of removed and produced compounds for PPHA and LHA samples, 297 respectively. 298 299 300 301

S27

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