Long-Term Uptake of Phenol-Water Vapor Follows Similar Sigmoid

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Long-Term Uptake of Phenol-Water Vapor Follows Similar Sigmoid Kinetics on Pre-Hydrated Organic Matter- and Clay-rich Soil Sorbents Mikhail Borisover, Nadezhda Bukhanovsky, and Marcos Lado Lado Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01558 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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Environmental Science & Technology

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Long-Term Uptake of Phenol-Water Vapor Follows Similar Sigmoid

4

Kinetics on Pre-Hydrated Organic Matter- and Clay-rich Soil Sorbents

5 6 7 8 9

Mikhail Borisover1*, Nadezhda Bukhanovsky1, Marcos Lado2

10 11

1

Agricultural Research Organization, Institute of Soil, Water and Environmental Sciences, The Volcani Center, Rishon LeZion, POB 15159, 7505101, Israel

2

Faculty of Sciences, University of A Coruna. A Zapateira s/n 15071 A Coruna, Spain.

12 13 14 15 16

Mikhail Borisover: [email protected]

17

Nadezhda Bukhanovsky: [email protected]

18

Marcos Lado: [email protected]

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*Corresponding author. The e-mail address: [email protected]; the telephone

21

number: 972-3-9683314; the fax number 972-3-9604017.

22

1

ACS Paragon Plus Environment


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Abstract

23

Typical experimental time frames allowed for equilibrating water-organic vapors with soil

24

sorbents might lead to overlooking slow chemical reactions finally controlling a

25

thermodynamically stable state. In this work, long-term gravimetric examination of kinetics

26

covering about 4000 hours was performed for phenol-water vapor interacting with four

27

materials pre-equilibrated at three levels of air relative humidity (RHs 52, 73 and 92%). The

28

four contrasting sorbents included an organic matter (OM)-rich peat soil, an OM-poor clay

29

soil, a hydrophilic Aldrich humic acid salt and water-insoluble leonardite. Monitoring

30

phenol-water vapor interactions with the pre-hydrated sorbents, as compared with the sorbent

31

samples in phenol-free atmosphere at the same RH, showed, for the first time, a sigmoid

32

kinetics of phenol-induced mass uptake typical for second-order autocatalytic reactions. The

33

apparent rate constants were similar for all the sorbents, RHs and phenol activities studied. A

34

significant part of sorbed phenol resisted extraction, which was attributed to its abiotic

35

oxidative coupling. Phenol uptake by peat and clay soils was also associated with a

36

significant enhancement of water retention. The delayed development of the sigmoidal

37

kinetics in phenol-water uptake demonstrates that long-run abiotic interactions of water-

38

organic vapor with soil may be overlooked, based on short-term examination.

39 40

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1.

41

Introduction

Interactions of water-organic vapors with soils are of long-standing interest due to their

42

significance in environmental, engineering and agricultural scenarios. These include the

43

transport of volatile organic compounds in the vadose zone and their interactions with

44

particles migrating in the atmosphere,1,2 remediation of contaminated soils by heating and

45

vapor extraction,3 application of volatile pesticides in the field,4,5 use of fumigant vapors in

46

laboratory protocols6 and field treatments,2,7 and soil interactions with multiple volatile

47

organic compounds produced by plants and soil bacteria8,9 or released by industrial and

48

transportation activities (e.g., hydrocarbons, nitro-substituted organic compounds, and many

49

others10).

50

Multiple studies have examined the interactions of organic vapors with mineral

51

phases, inorganic surfaces, soils, and soil (natural) organic matter (OM) at varied air relative

52

humidity (RH).11-22 Different techniques were used in such studies, like dynamic

53

measurements with extracting organic compounds from a sorbing material,12 chromatography

54

with the material of interest as a stationary phase,17 mass balance-based headspace

55

analysis,19,22 monitoring sorbed mass changes gravimetrically23 or by means of a quartz

56

crystal microbalance.21 Typically, these works were mainly focused on sorption interactions

57

without considering chemical transformations of the sorbed organic compounds. Depending

58

on the time needed to attain an apparent equilibrium, which may also be influenced by the

59

use of a specific methodology, the duration of the experiments varied from minutes, when

60

monitoring IR spectra of a sorbed compound under different humidity levels,24 to few or tens

61

of hours, when using other type of techniques. 12,21-23

62

Organic compounds may participate in multiple (variously catalyzed) chemical

63

reactions on active surfaces of soils, clay minerals, metal oxides and soil OM; e.g., clay-

64

catalyzed

hydrolysis,25,26

oxidative

transformations

phenols,

65

hydrocarbons and other organic molecules,27-29 formation of covalent bonds with humic

66

3

of


aromatic

amines,


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substances,30-32 or reductive dehalogenation.33,34 The typical timeframes in most of the

67

experimental setups used for equilibrating water-organic vapors with soil sorbents might lead

68

to overlooking slow chemical reactions which in the end can control a thermodynamically

69

stable state. Therefore, investigations involving long-term water-organic vapor interactions

70

with soils are of crucial importance.

71

In this work, such an examination was performed monitoring, for 4000 hours, kinetics

72

of phenol-water vapor interacting with a series of pre-hydrated materials. The materials

73

included two contrasting soils, i.e., one OM-rich peat soil and another OM-poor clay-rich

74

soil, and two sorbents containing very different types of natural OM, i.e., a hydrophilic

75

Aldrich humic acid salt, and water-insoluble leonardite. Phenols are capable of strong

76

specific interactions with soil OM and clay surfaces.35,36 Phenolic compounds sorbed on soils

77

and clays may be coordinated with metals and oxidized through direct electron transfer to an

78

electron-accepting center in the sorbent or, possibly, by means of a soil OM conduit, which

79

may lead under ambient conditions to a continuous formation of persistent free radicals.37-39

80

Such radicals may be further stabilized by interactions with soil OM,37 but also, without the

81

stabilizing effect of OM, the formed free radicals can persist for a long time in the clay in

82

relatively dry enviroments.40 Phenolic compounds are well-known to undergo polymerization

83

on clay surfaces27,41 and to bind covalently to natural OM.42-44 Oxidative polymerization of

84

phenolic compounds and reactions with soil components occur in the presence of transition

85

metals like iron,29,45 enzymes,32,46,47 or adsorbed oxygen.30,41 Examining for as long as 140

86

hrs the uptake kinetics of dry saturated vapors of three phenols on dry smectite demonstrated

87

that phenolic oligomers may be formed on bentonite clays saturated with aluminum, sodium

88

or calcium.41,45 However, to the best of our knowledge, there are no studies examining and

89

comparing long-term kinetics of such complex interactions occurring between phenolic

90

compounds vapors and OM and soil matrices at varied RHs.

91

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Investigating the behavior of phenol-water vapor at soil interfaces will elucidate the

92

physico-chemical reactivity of a wide class of oxidized hydroxylated aromatic compounds,

93

both including anthropogenic substances (e.g., pesticides and industrial chemicals) and

94

natural phenols. It will contribute to a better understanding of the natural and engineered

95

transformations of organic compounds that result in their incorporation into the soil

96

matrices.43,48 Therefore, the specific objectives of this study were (1) to examine the kinetics

97

of the long-term interactions of phenol-water vapor, at two phenol activities, with soil and

98

OM sorbents pre-hydrated at three levels of air RH, and (2) to get an insight on the process of

99

phenol sorption and immobilization in a long-term run.

100 101

2. Materials and Methods

102

2.1 Materials. Phenol (C6H5OH, >99%, EP/BP, UN1671, Bio-Lab Ltd., Israel) was used

103

without additional purification as a sorbing compound. Water (ULC/MS-GC/SFC) and

104

acetonitrile (HPLC supra-gradient), used in the HPLC mobile phase and for extractions

105

purposes, were obtained from Bio-Lab Ltd. (Israel). Sodium chloride (NaCl; analysis grade,

106

>99.5%, Merck KGaA, Darmstadt, Germany), potassium nitrate (KNO3; ReagentPlus,

107

>99.5%, Sigma-Aldrich, Co., St. Louis, MO) and magnesium nitrate hexahydrate

108

(Mg(NO3)2×6H2O; for analysis, >99%, Merck KGaA, Darmstadt, Germany) were used

109

without additional purification for preparing aqueous salt-saturated solutions, in order to

110

maintain an experimental atmosphere with constant RH.

111

Sorbent materials included the OM-rich Pahokee peat soil, the clay-rich Revadim soil

112

(Israel), the OM-rich leonardite (a material formed by the natural oxidation of a low-grade

113

coal, lignite), and the Na salt of humic acid (HA-Na). The bulk Pahokee peat and leonardite

114

materials (85 and 87% ww-1 of OM, respectively) were obtained from the International

115

Humic Substances Society (IHSS; some selected characteristics including ash, C, H, N

116

contents of both materials and 13C NMR spectra of Pahokee peat can be found there49). The

117

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agricultural clay soil from Revadim was sampled and characterized earlier,50 and contained

118

42% of clay, 21% of silt, 37% of sand, 0.6% of organic C, 0.06% of total N, and 13.4% of

119

CaCO3. The HA-Na was purchased from Sigma-Aldrich (Steinheim, Germany). The Pahokee

120

peat and Revadim soil were selected as two natural materials differing fundamentally in OM

121

and clay contents. Leonardite has high OM content, similar to that of Pahokee peat, but with

122

much lower N content,49 thus providing a different natural OM system. The HA-Na serves as

123

a model of a hydrophilic (and water-soluble) OM based on a humic substance, an important

124

constituent of natural OM. Initial moisture, organic C and nitrogen adsorption-based specific

125

surface area (SSA) were determined in all sorbents. The relevant experimental procedures are

126

detailed in Supporting Information S1.

127 128

2.2 Vapor sorption experiment. Interactions of phenol-water vapor with pre-hydrated

129

materials, at a certain air RH, were examined gravimetrically using a two-phase experimental

130

setup:

131 (I) First phase: Initial hydration of samples. One g of sorbing material was weighed

132

and placed into an open 10-mL glass vial (with section diameter and height of approximately

133

2 and 3 cm, respectively), in three replications. The height of the solid material layer inside

134

the vial was about 5 mm. Then, the 12 vials (4 different sorbents × 3 replications) plus 2

135

additional empty vials used as blanks were placed into a ~ 1.3 L glass container with a height

136

of 13 cm. A beaker containing about 100 mL of Mg(NO3)2×6H2O, NaCl or KNO3 saturated

137

aqueous solution (with an excess of the solid salt phase in its bottom) was placed also into the

138

glass container in order to hydrate the sorbents in atmospheres with different RH. Three

139

identical glass containers were prepared, for each salt solution. The 9 glass vessels (3 RH

140

levels ×3 containers) were closed hermetically and placed into a Lab Companion SI-300R

141

thermostat, protected from the sunlight, to keep the temperature constant at 25.0oC. At this

142

temperature, the saturated aqueous solutions of Mg(NO3)2×6H2O, NaCl and KNO3 are

143

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characterized by water activities of 0.529, 0.753 and 0.936, respectively (or with nominal

144

RHs of 52.9, 75.3 and 93.6%, respectively51).

145

Within the first 1100 hours of the experiment, at time intervals that varied between

146

120 and 190 hours, the glass containers were opened 7 times; and all the vials were

147

hermetically closed immediately and weighed. The time since the glass container was opened

148

until the 14 vials were closed did not exceed 2 min. After weighing, the vials were mixed

149

manually, with caution, and opened again before returning them to the glass containers for

150

further equilibration in the closed atmosphere of selected humidity. In order to minimize the

151

development of biological activity at the highest atmospheric RH (93.6%), samples after 930

152

hours of equilibration under the KNO3 solution atmosphere were autoclaved at 121 oC for 15

153

min in a Tuttnauer 2540M, weighed, and returned back to the glass container for further

154

equilibration. HA-Na samples experimented significant changes in texture and shape during

155

autoclaving, which were visually detected, most probably caused by water absorption by this

156

hydrophilic material and its further solidification upon cooling. Therefore, these samples

157

were discarded and not used for further analysis.

158

Mass changes were obtained gravimetrically for each sorbent sample and humidity

159

environment (except HA-Na at the highest air RH, as explained above). Regular weighing of

160

blank vials showed no systematic mass accumulation: at each weighing time, mass

161

differences with the initial mass of a dry empty vial did not exceed 1 mg. Nevertheless these

162

differences were considered when calculating mass changes of sorbents during equilibration.

163

This correction was between 1.5-10% of the sample mass gain after 360 hrs of equilibration,

164

and became negligible at longer equilibration times.

165

Actual atmospheric RH in the glass containers was determined by means of a Lutron

166

HT-3015 humidity meter with resolution 0.01%. Along this first phase, air RH values were in

167

the ranges 50.9-52.6%, 71.8-73.6%, and 90-92.9% over the saturated solutions of

168

Mg(NO3)2×6H2O, NaCl and KNO3, respectively, with no significant time trends observed.

169

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(II) Second phase: Interactions of sorbent materials with phenol-water vapor. After

170

the apparent (partial) equilibration of the sorbents at the selected RHs for 1100 hrs during the

171

first phase of the experiment, the samples were brought into contact with phenol vapor in an

172

environment that maintained the same water activity (or RH) as the initial hydration phase.

173

For this, solutions containing 20,000 (A) and 60,000 (B) mg L-1 of phenol were

174

prepared by dissolving solid phenol in each of the salt-saturated aqueous solutions. These

175

phenol solutions were again re-equilibrated with excess amounts of Mg(NO3)2×6H2O, NaCl

176

or KNO3. Phenol concentrations were tested by diluting an aliquot of the resulting solution

177

and measuring absorbance at 268 nm with a Genesys 10UV spectrophotometer (Thermo

178

Scientific). Phenol standards for these spectrophotometric determinations were also prepared

179

in appropriate salt-containing solutions. If phenol concentrations were found different from

180

those initially prepared, they were corrected by adding a phenol-containing salt-saturated

181

aqueous solution.

182

In this second phase, the three glass containers from phase (I) with the same RH were

183

treated differently. One continued to be monitored under the same atmosphere as a control

184

treatment, whereas in the other two, the saturated salt solutions were replaced with solutions

185

A and B containing 20,000 and 60,000 mg L-1 of phenol, respectively. Temporal mass

186

changes of each sample continued to be gravimetrically monitored as in phase (I). The overall

187

duration of the vapor sorption experiment, including the initial hydration phase (I), was 5094

188

hrs and involved 14 additional gravimetric determinations for each sample during this phase

189

(II).

190 Alongside the gravimetric measurements, small portions of the sorbents were sampled

191

at 5 times for determining extractable/soluble phenol contents (see below). The mass of

192

sorbent sampled was accurately accounted for when calculating the mass remaining in the

193

vials.

194

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Phenol concentration in the salt-saturated solutions A and B was also tested at

195

different times during sample equilibration, by means of a spectrophotometer as described

196

above. Although phenol was mobilized from the aqueous solutions to the gas phase, changes

197

in concentration did not exceed 10% and 7% in the solutions A and B, respectively. In order

198

to prevent a further decrease of phenol concentration in these solutions, these were replaced

199

by fresh ones after certain periods of time. Since absorbance-based phenol concentration

200

measurements might not be selective, they were also verified by independent HPLC

201

determinations at one sampling time. For this purpose, aliquots of the phenol containing

202

solutions were diluted by a factor of 10,000 in double-distilled water and examined against

203

similarly prepared standards (the details of HPLC measurements are provided below). This

204

showed that spectrophotometric measurements may overestimate phenol concentrations, as

205

compared with those determined by HPLC by, at max, 17% in solutions A and 4% in

206

solutions B.

207

The effect of dissolved phenol on the water activities in the salt-saturated solutions

208

and, therefore, on the respective atmospheric RHs was expected to be minimal. This is due to

209

the following consideration: at 25oC, the actual partial pressure of water vapor over the

210

saturated phenol solution was reported to be only 2% lower than that calculated by means of

211

Raoul’s law from the respective mole fraction of water in the aqueous phase and the vapor

212

pressure of pure water.52 Hence, based on the reported composition of the aqueous phase in

213

the phenol-water binary system, it can be computed that the presence of about 8% of phenol

214

in the saturated aqueous solution reduces the partial pressure of water vapor, as compared

215

with that of pure water, by, at max, 4%. Therefore, the influence of 20,000 and 60,000 mg L-1

216

phenol on water activities (and on the respective atmospheric RH values) in the salt-saturated

217

solutions used in this work should be even lower than this value. In fact, along the second

218

equilibration phase, the atmospheric RH values were in the ranges 51.7-52.6%, 72.6-73.9%,

219

91.7-93.1% over the phenol-containing saturated solutions of Mg(NO3)2×6H2O, NaCl and

220

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KNO3, respectively. These RH ranges were similar to those determined in the parallel

221

controls without phenol (50.7-52.5, 72.9-73.9, and 91.7-92.2%, for the salt solutions of

222

Mg(NO3)2×6H2O, NaCl and KNO3, respectively), and also to the RH values found in the first

223

hydration phase. No temporal trends were noted for the RH values during this second phase

224

of sample equilibration. Therefore, the averaged RH values of 52, 73 and 92% are used in

225

this paper to describe the atmosphere humidity over the saturated solutions of

226

Mg(NO3)2×6H2O, NaCl and KNO3, respectively.

227 228 gravimetric

229

measurements, extractable/water-soluble contents of sorbed phenol were determined. For

230

this, in each sampling event, three 5-mg portions of sorbing material were taken from each

231

vial, including the control ones. Since each combination of sorbing material, atmosphere RH

232

and phenol concentration was triplicated, 9 sorbent portions were obtained for these

233

measurements.

234

2.3

Sample

extraction/dissolution.

During

second

phase,

alongside

Samples were first mixed with 6.0 mL of water for 20 min, after which 14.0 mL of

235

acetonitrile were added. The overall mixing continued for 2 hrs, in order to allow the

236

extraction of phenol from soil, peat and leonardite and the dissolution of HA-Na together

237

with phenol. Then, samples were centrifuged for 20 min at 1200 relative centrifugal force and

238

analyzed by HPLC (see below). Preliminary tests showed that the repeated extraction of

239

phenol-loaded peat and leonardite samples added only 2 and 5%, respectively, to the phenol

240

concentrations recovered during the first extraction. Therefore, one extraction step was

241

considered sufficient. No meaningful phenol concentration was found in clay soil extracts.

242 243

2.4 Analytical determination of phenol. Measurements of phenol in solutions after

244

extraction/dissolution of sorbing materials were performed at 30oC using a Jasco HPLC with

245

PDA detector (MD-2018, Jasco), and a Phenomenex 250×4.6 mm LC column (Kinetex 5 µm

246

10


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C18). The mobile phase was a 70:30 (v v-1) acetonitrile: water mixture with a flow rate of 1

247

and 0.8 mL min-1 in the extraction and dissolution tests, respectively. Phenol standards were

248

prepared in the mobile phase solution. Separate tests performed with phenol dissolved in the

249

mobile phase solution containing 200 mg L-1 of HA-Na showed no effect of dissolved humic

250

substances on quantification of phenol. No phenol was found in the extracts (solutions)

251

obtained with the control sorbent samples having no exposure to phenol vapors.

252 253

2.5 Modeling uptake kinetics. Temporal sample mass changes were modelled using the

254

quasi-Newton method within the Non-Linear Estimation module included in Statistica 7.0

255

(Statsoft Inc.). The minimized function was the sum of squares of deviations of experimental

256

values vs. those calculated by means of the model equation. The convergence of the solution

257

was verified by varying the initial fitted parameters.

258 259

3. Results and Discussion

260

3.1 Phenol-water vapor interactions with pre-hydrated sorbents: mass changes caused

261

by introducing phenol into a system

262

Mass changes measured during the exposure of samples to various atmospheres were

263

converted to total sorbed masses (in %, w w-1 of dry sorbent). In order to characterize the

264

interactions of phenol-water vapor with pre-hydrated samples, at each time interval the total

265

sorbed masses of the control samples at given RH were subtracted from the total sorbed

266

masses measured on the samples exposed to hydration followed by interactions with phenol-

267

containing vapor under the same RH. With this procedure, additional sample hydration (if

268

any) was accounted for, and the effect of phenol introduced into the atmosphere on mass

269

changes was delineated. The temporal dynamics of these differential masses m is shown in

270

Fig. 1 for all sorbents, air RHs and phenol concentrations in salt solutions. The temporal

271

dynamics of sorbed masses in the control samples and in those exposed to hydration and

272

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phenol-containing vapor are provided in Figs. S1, S2 (Supporting Information) and

273

commented there in section SI2.

274

The data shown in Fig. 1 provides clear evidences of the increasing effect of phenol

275

on the total sorbed masses as time progresses and suggests a sigmoidal change of the

276

differential masses as a function of exposure time. When examining the data obtained in the

277

first 1100 hrs (first phase of the experiment), virtually all the changes in mass tend to be close

278

to zero (except those obtained on leonardite in the 92/A series), which was expected since

279

during this initial period the three glass containers with the same RH were under the same

280

hydrating treatment. The non-zero mass differences observed in the 92/A leonardite series

281

prior to the exposure to the phenol-containing vapor suggest a systematic difference between

282

the masses of two groups of samples; the first group (control) was always exposed to 92%

283

RH, and the second group was further additionally exposed to phenol vapor (solution A). The

284

reason for the non-zero mass difference between these two groups of samples, which were

285

supposed initially to be treated identically, is unknown. It is important to note that for each

286

sorbent, the sigmoidal curves do not show any effect of a specific RH or phenol

287

concentration. In order to better elucidate the role of the different factors, the data shown in

288

Fig. 1 were approximated by a kinetics model.

289 290

3.2 Differential mass uptake by sorbing materials follows sigmoidal kinetics: modeling

291

It may be assumed that the mass uptakes occurring in phenol-containing vapor atmospheres

292

(after the initial 1100 hrs hydration) are the result of two independent contributions. The first

293

one is associated with a continuous water uptake, if any, and follows an independent kinetics

294

matching the one observed on the control samples (exposed to water vapor only). The second

295

contribution represents any interactions of phenol in a sorbed state with a possible (positive

296

or negative) contribution of water and, therefore, controls the temporal dynamics of the

297

differential masses m shown in Fig. 1. Modeling sigmoidal kinetics of differential mass

298

uptakes m was carried out by means of the empirical Eqn. (1):

299

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Page 13 of 34


a 1 + P × e − kt

(1)

300

where a is a maximal differential mass taken up in the course of a sample equilibration in an

301

atmosphere containing phenol and water vapors, k is an effective rate constant, P is an

302

empirical parameter; t is the time allowed for sample incubation in the phenol-containing

303

atmosphere. Equation (1) is mathematically equivalent to the model proposed to describe

304

water uptake by seeds.53 In that model, P was related to the time needed to reach an inflection

305

point. This model was used also to describe hydration of beans.54

306

m=

Thus, the m values (in Fig. 1) obtained during the exposure of sorbents with

307

atmospheres containing phenol were fitted by means of Eqn. (1) involving three adjustable

308

parameters, a, P and k. In the modeling, t was defined as a time interval counted from

309

introducing phenol into a system, i.e., after the 1100 hrs given to samples for equilibration at

310

selected RHs. Table 1 summarizes model parameters a, P and k, their standard errors, the

311

root-mean-square deviation (RMSD) and the proportion of variance accounted for (r2) for

312

each sorbent, RH and phenol concentration in solution. In general, Eqn. (1) fitted the data

313

quite well, accounting for a major portion of the variance and resulting in low RMSD values

314

(≤0.91%, Table 1) that did not exceed the experimental error bars (Fig. 1). This suggests that

315

deviations of experimental data from the simulated ones are caused by experimental

316

scattering rather than by an inadequacy of the model. Although the P parameter in Eqn. (1) is

317

necessarily positive, yet, it was determined with relatively large standard errors (Table 1). To

318

the best of our knowledge, it is the first observation of sigmoid kinetics for organic sorbate-

319

induced mass uptake of mixed vapor by soil and OM matrices.

320

The examination of k values in Table 1 suggests that the level of phenol concentration

321

in solutions A and B and the atmospheric RH do not have a meaningful effect on the effective

322

rate constants. In fact, k values determined on peat and clay soil are very similar, and only

323

those associated with HA-Na might be considered slightly different from k values determined

324

on the other materials. The similarity of the rate constants k among different sorbing

325

13



Page 14 of 34

materials suggests also that the mechanisms responsible for the mass uptake in the

326

atmospheres containing phenol and water vapors are similar among peat, clay soil, leonardite

327

and HA-Na. It is clear that mass uptakes are affected by changes in both phenol and water

328

sorbed concentrations. However, the whole dynamics of differential masses m shown in Fig.

329

1 is induced by phenol brought to a system and undergoing interactions with the sorbing

330

materials. Therefore, the k values may be considered as describing the kinetics of phenol-

331

sorbent interactions, at a given RH level (but in a potentially variably hydrated sorbed state).

332 333

3.3 Contribution of extractable phenol to the differential mass uptake

334

Extractable phenol (mPhOH, in %, w w-1 of dry sorbent) in peat, HA-Na and leonardite samples

335

is plotted vs. time of equilibration with phenol-containing vapors in Figs. 2, and Figs. S3 and

336

S4 of Supporting Information, respectively. Results obtained in clay soil samples are not

337

presented since no meaningful contents of extractable phenol were detected. In addition, the

338

ratios (m-mPhOH)/mPhOH are also shown in these figures. The time trends of the ratios (m-

339

mPhOH)/mPhOH are illustrative of the temporal changes of any contributions to the masses m

340

other than extractable phenol, relatively to mPhOH.

341

By examining the mPhOH and (m-mPhOH)/mPhOH values in Figs. 2, S3, S4, it is clear that,

342

initially after introducing phenol into the atmosphere, the contents of extractable phenol were

343

a significant, and even dominant, portion of the m values, evidenced by the (m-mPhOH)/mPhOH

344

ratios not exceeding 1. In some cases, (e.g., at the beginning of the exposure of peat samples

345

to phenol-containing vapors, Fig. 2), these ratios were even negative, most probably

346

evidencing the removal of water from the samples due to phenol-water competition. In the

347

course of the incubation, the contents of extractable phenol in peat and HA-Na samples

348

increased, reaching maximal values after approximately 2400 hrs (Figs. 2, S3). However, the

349

content of extractable phenol in leonardite samples did not show any substantial changes with

350

time (Fig. S4). The (m-mPhOH)/mPhOH ratios increased with time in all the samples, showing

351

14


Page 15 of 34


also a tendency to saturation although, in samples of peat and leonardite, this was reached

352

more slowly than for extractable phenol contents (Figs 2, S4). Hence, it is clear that: (1) the

353

(m-mPhOH) differences may show changes even when the uptake kinetics of extractable

354

phenol is completed (i.e., on peat and leonardite samples, Fig. 2,S4); (2) in the course of the

355

incubation, the extractable phenol becomes a minor fraction of the whole attained mass m,

356

which is evidenced by the (m-mPhOH)/mPhOH values significantly exceeding 1. Thus, no simple

357

relation between the gained differential masses m in Fig. 1 and the contents of extractable

358

phenol (Figs. 2, S3, S4) may be sought. This conclusion is also supported by the above

359

observation that no substantial extractable phenol contents were detected in clay soil samples,

360

demonstrating yet the kinetics of differential mass uptakes similar to those on three other

361

sorbents (see the k values in Table 1). Although for the purposes of kinetics modeling with

362

Eqn. (1) the accumulation of extractable phenol in the peat and HA-Na samples (Figs. 2, S3)

363

might need to be subtracted from the overall dynamics of the differential masses m (Fig. 1),

364

the role of this correction was considered minor. This was because in peat and HA-Na

365

samples the overall change of the extractable phenol content was only about 5% compared to

366

the changes in differential masses m. Hardly any correction would be needed in clay soil and

367

leonardite samples, where extractable phenol content was not determinable or did not show

368

any dynamics, respectively.

369

The amounts of extractable phenol observed at the end of peat incubation (after 4000

370

hrs) seem to increase with RH of the environment, and this increase is more pronounced

371

when the samples were exposed to an atmosphere over solution B, containing the highest

372

phenol concentration, as compared to solution A (Fig. 2). The RH-induced trend could be

373

well understood by considering that hydration (solvation) of natural OM may disrupt non-

374

covalent intra-OM interactions thus opening new sorption sites and enhancing sorption of

375

organic compounds, capable of specific interactions with the sorbent.55,56 In particular,

376

elevating water activity has been demonstrated to enhance interactions of phenol and

377

15



Page 16 of 34

carbamazepine with the peat used in the current work.57,58 Larger sorptive phenol-peat

378

interactions caused by OM hydration were observed at higher phenol concentrations

379

(activities) and quantitatively explained using a model accounting for water-induced opening

380

of new sorption sites in OM and the successful competition of phenol molecules with water

381

for these sites.57

382

The increase in extractable phenol content mPhOH with increasing RH was also

383

detected in HA-Na exposed to the atmosphere over solution A but not over solution B (Fig,

384

S3). On the contrary, no effect of RH on extractable phenol content was detected in

385

leonardite samples (Fig. S4). The differences among the three sorbents may be related to the

386

different nature of OM in all of them, probably involving differences in their swelling

387

capability and in the tradeoff between opening new sorption sites and further sorbate/water

388

competition. Such interplays were suggested to explain, for example, the strong hydration-

389

enhanced sorption of some organic compounds on peat-extracted humin, the modest effects

390

for the same compounds on peat OM, and the negligible hydration effect when the sorbent

391

was peat-extracted humic acid.55 In addition, it must be kept in mind that the relations

392

between extractable phenol contents, RH of air atmosphere and phenol activity (concentration

393

in solutions and in gas phase) may become complicated by the occurrence of chemical

394

reactions with phenol participation. Due to phenol transformations, it is hardly possible to

395

further interpret the influences of RH and phenol concentration in solutions on the changes of

396

the (m-mPhOH)/mPhOH quantities shown in Figs. 2, S3 and S4.

397 398

3.4 Contribution of non-extractable OM to the surplus of organic C

399

Oxidative coupling of phenols occurring on clays, soils and soil OM components was

400

reported in multiple studies.29,32,33-36,41-45 In this work, oxygen participating in coupling was

401

always present in great excess in all studied systems. For example, oxygen contained in one

402

volume of the glass container used for vapor sorption experiments was about 50% more than

403

16


Page 17 of 34


the total needed to polymerize or bind with the sorbent OM all phenol dissolved in the salt

404

solution A (more detailed considerations are provided in Supporting Information, S3).

405

The significance of chemical transformations involving phenol may be demonstrated

406

by comparing the C content of extractable phenol to the surplus of organic C determined for

407

some selected samples at the end of sample equilibration (i.e., after 5094 hrs). Exposing all

408

four sorbents to phenol atmospheres at 73% RH resulted in surpluses of organic C contents

409

(in w w-1 of dry sorbent), when compared to the controls exposed to a phenol-free atmosphere

410

(Table 2). However, phenol-associated C (calculated from the extractable phenol mPhOH)

411

made only a minor fraction of this surplus, i.e., 5, 12 and 33% in leonardite, HA-Na and peat

412

samples, respectively (Table 2), and this contribution was negligible in clay soil samples.

413

Therefore, the major part of the organic C added after exposing the samples to phenol-water

414

vapor was not an extractable phenol but, most probably, the products of phenol coupling.

415

Interestingly, a sigmoid accumulation of the reaction products (as dictated by Eq. 1) is

416

expected for a second order autocatalytic reaction59 when the product is initially present in

417

small amounts, as a seeding, or formed through a non-catalytic pathway.60 Indeed, the model

418

formally expressed by Eqn. (1) represents the assumption that hydration of a material follows

419

a "second-order autocatalytic" kinetics (Eq. 2),61

420

dm = k × m × (1 − m / a ) dt

(2)

421

Intuitively, applicability of Eqn. (2) to the kinetics of differential mass uptake shown in Fig. 1

422

may be understood as indicating that the rate of mass accumulation increases with the uptake,

423

providing more opportunities for phenol coupling. But it tends to pass a maximum, due to the

424

uptake boundary a that may be caused by steric or any other limitations. No actual

425

mechanism can be hypothesized at this moment.

426

The surplus in organic C content and its fraction considered to represent the products

427

of phenol coupling are hardly associated with SSA of the sorbents. For example, clay soil

428

was characterized by 10 times greater SSA as compared with peat (3.14 vs. 0.28 m2 g-1).

429

17



Page 18 of 34

However, the increase in organic C content was greater more than by an order of magnitude

430

in peat as compared to clay soil (i.e., 5.1 vs. 0.24 % w w-1, respectively, in the atmosphere

431

over solution A; Table 2). The non-extractable fractions of this surplus, obtained by

432

subtracting the fraction of extractable phenolic C (Table 2), were 67 and 100% in peat and

433

clay soil thus corresponding to 3.4 and 0.24 % w w-1, respectively.

434

Leonardite and HA-Na are characterized by a similar surplus in organic C content (6.7

435

and 5.4% w w-1, respectively; Table 2). Also, the fractions of phenol coupling components in

436

this surplus are comparable for leonardite and HA-Na, i.e., 95 and 88% (based on subtracting

437

the fraction of extractable phenolic C, Table 2). This correspond to the similar non-

438

extractable organic C contents in leonardite and HA-Na, i.e., 6.4 and 4.5% w w-1, although

439

the SSA values of these two materials are different by more than 6 times (1.52 and 0.23 m2 g-

440

1

, respectively). It seems that the lowest initial organic C content in clay soil is responsible for

441

the lowest increase in organic C and the minimal formation (by absolute value) of products of

442

phenol coupling (among the four sorbing materials). In turn, larger organic C contents in

443

peat, HA-Na and leonardite provide greater opportunities for phenol coupling with the

444

organic matrix but also for larger contents of extractable phenol. It appears that the presence

445

of OM in sorbing materials may enlarge opportunities for phenol transformations but also, in

446

part, by sorbing phenol molecules, protect it from further immobilization.

447

Formation of non-extractable OM due to the exposure of sorbents to phenol-water

448

vapors was not expected to be associated with microbial activities, because neither

449

autoclaving the samples previously exposed to RH of 92% nor varying the levels of air RH

450

from 52 to 92% had impacts on the effective rate constants of mass uptakes, k (Table 1).

451

Also, there were no significant effects of autoclaving or atmosphere RH on the parameters a

452

and P of Eqn. (1) (except leonardite at RH of 92%, due to the above-explained systematic

453

mass differences prior any exposure to phenol, see 3.1). By the same reasons, a participation

454

of enzymes in phenol immobilization is also questionable. The observed delay in developing

455

18


Page 19 of 34


a sigmoid kinetics (Fig. 1) suggests that long-run abiotic interactions of water-organic vapor

456

with soil may be overlooked based on short-term examinations.

457

The increase of non-extractable organic C can hardly be explained by means of

458

phenol interactions with coordinating and electron-accepting metal centers in sorbent

459

matrices, similarly to those demonstrated on montmorillonite loaded with copper(II) and

460

poly-p-phenylene39 or exchanged with iron(III).40 In that case, the coordination of phenol

461

with active metal centers in the clay matrix was strongly air RH-dependent, which was

462

related to sorption of water molecules on active sites thus desorbing the organic molecules

463

and inhibiting further reaction.40 In contrast, no effect of air RH on the rate constant k and

464

other kinetic parameters was detected in the clay soil (Table 1). In addition, the overall

465

patterns of mass uptake kinetics (Fig. 1) and the effective rate constants k (Table 1) were very

466

similar in all four sorbents, which points to the same processes controlling phenol interactions

467

in all the studied matrices. Considering various nature, composition and sources of the

468

sorbent materials, as well as the significant fractions of added non-extractable organic C

469

determined in OM-rich leonardite, HA-Na and peat (i.e., 6.4, 4.5 and 3.4% w w-1 of dry

470

sorbent, respectively; see above) in contrast with its small value in clay soil (0.24% w w-1),

471

the role of metal centers stoichiometrically coordinating phenol and demonstrating similar

472

kinetics in materials that are so different seems to be unlikely.

473

It is worth recalling that the experimental setup involved simultaneous exposure of all

474

types of sorbing materials to the same atmosphere, containing phenol and/or water vapors.

475

Therefore, if volatile radicals could be generated in a system by any of the materials, they

476

might be able to initiate oxidative phenol coupling/polymerization on each of the four

477

sorbents. Thus, OOH radicals may be formed by oxygen adsorbed on clay surfaces,62 a

478

mechanism that was proposed41 as a possible reason for the formation of oligomers during

479

sorption of saturated dry alkyl- and chlorophenol vapors on dried smectites exchanged with

480

different cations (including sodium and calcium). Humic substances of any origin and nature

481

19



Page 20 of 34

are considered to contain free radicals, being an integral part of humic matter structure.63-65

482

The radicals moieties may involve semiquinone conjugated with aromatic rings,

483

methoxybenzene or N-associated radicals65 and may be separated into permanent ones, with

484

long life spans, and transient ones.66 When examining soil-extracted commercial and

485

synthesized humic materials,65 enhanced electronic spin resonance signals associated with the

486

concentration of radicals were reported in the presence of oxygen, and oxygen sorption and

487

penetration into coals and carbonaceous polymers were suggested. Thus, formation of radical

488

species in an air environment, due to the presence of humic matter (in all the types of

489

materials) or clay surfaces (in Revadim soil), might be hypothesized to lead to slow oxidative

490

coupling of phenol. In addition, interactions of phenol with electron-accepting metals present

491

even as impurities or in minor amounts in any of the studied sorbents might lead to the

492

formation of phenol radicals and the production of reactive oxygen species,38,39 thus inducing

493

further phenol transformations. The understanding of long-term uptake of phenol-water vapor

494

by natural materials developed in this work may be applicable to other phenolic substances.

495

However, these results cannot be extrapolated to other organic pollutants belonging to

496

different chemical classes.

497 498

3.5 Enhanced water retention due to phenol uptake

499

It is instructive, finally, to compare the surplus in OM mass derived from phenol uptake and

500

transformations to the gravimetric mass increases m. The mass C fraction of the phenol

501

molecule is 76.6%. If phenol is dimerized via oxidative coupling or chemically linked to a

502

sorbent matrix (i.e., soil OM), the C fraction of the organic mass added is 77.4%. In case of

503

extended phenol polymerization, the C fraction of the organic mass added would be 78.2%.

504

Therefore, considering 77% as the mass C fraction of the phenol moiety associated with a

505

sorbent by some of the mechanisms, the surplus of organic C content due to sample

506

exposures to phenol vapor may be converted to the surplus of OM mass content (in w w-1 of

507

20


Page 21 of 34


dry sorbent). The calculated values and their contribution to the differential mass increase m

508

under an atmospheric RH of 73% are shown also in Table 2.

509

It can be seen that OM content increase in the clay soil samples as a result of their

510

interactions with phenol-water vapors accounted only for 4-8% of the differential mass

511

increases m observed. Very similar results were obtained when the clay soil samples were

512

exposed to phenol vapors at RH of 52% over the solutions A and B, with increases of OM

513

mass explaining only 6 and 10% of the total increase of mass m, respectively. Therefore, the

514

major part of gravimetric increases m in masses of clay soil samples should be related to

515

moisture retention induced by phenol immobilized in the clay soil matrix. In leonardite and

516

HA-Na samples, the OM-associated mass increases made dominant contributions to the

517

gravimetrically determined mass increases m, i.e., 85 and 84%, respectively (Table 2). In peat

518

samples, 63% of the mass increases m may be assigned to sorbed phenol and its oligomerized

519

or immobilized forms, but the reminder (37%) reflects most probably the water bound and

520

co-sorbed along the phenol uptake and transformation. Organic compounds capable of

521

specific interactions, including phenol, when sorbed by OM-rich natural sorbents may bring

522

water to a sorbed state.

67

Specifically, based on an earlier experimental study of peat

523

hydration effect on phenol sorption57 and the concept67, the enhanced water retention by peat,

524

due to its interactions with phenol molecules, is expected. The underlying mechanism is the

525

solvation of OM by organic molecules, thus exposing new sorption sites for water to

526

occupy.58,67

527 528

Acknowledgements

529

The authors wish to thank Mrs. Anna Berezkin (ARO, The Volcani Center, Israel) and Dr.

530

Efrat Sheffer (The Robert H. Smith Faculty of Agriculture, Food and Environment, Hebrew

531

University of Jerusalem, Rehovot) for their help in determining the OC&TN contents of the

532

21



Page 22 of 34

materials studied. The authors declare no competing financial interest. The constructive

533

comments of the three anonymous reviewers were greatly appreciated.

534 535

Supporting Information. The procedures used for the determination of moisture, organic C

536

and SSA; the data (Figs. S1,S2) and description of mass changes of the samples exposed to

537

water and phenol-water vapors; the examination of extractable phenol content in HA-Na and

538

leonardite samples (Figs. S3,S4); details on oxygen excess in vapor uptake experiments.

539

22


Page 23 of 34


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Water—4% Succinic Acid. J. Am. Chem. Soc. 1937, 59, 2481–2488.

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(Vignaunguiculata) and Bambara Groundnuts (Voandzeia Subterranea) Seeds. J. Food

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of Organic Compounds by Humin and Humic Acid Fractions vs Original Peat Material.

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Environ Sci. Technol. 2004, 38, 4120-4129.

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713

Environmentally Important Sorbents: Estimating and the LFER Analysis. Adsorption

714

2013, 19, 241-250.

715 716 717

28


Page 29 of 34


718

Figure Captions

719 Fig. 1. Differences m (in %, w w-1 of dry sorbent) between the masses of the samples

720

sequentially contacting water vapor and water/phenol atmospheres (shown in Figs. S1c-f,

721

S2c-f) and of those exposed to the control hydration experiments (Figs. S1ab, S2ab) plotted

722

against incubation time (in hours). The sorbent materials (peat, HA-Na, leonardite and clay

723

soil) are indicated on the plots. The mass differences are associated with specific RH values

724

(i.e., 52, 73 and 92%) and phenol concentrations, 20,000 and 60,000 mg L-1, denoted as A

725

and B, respectively, in a salt solution used to maintain a required air humidity. The error bars

726

represent one standard deviation.

727 728

Fig. 2. Extractable phenol (mPhOH, in %, w w-1 of dry sorbent; black squares referred to the

729

left Y-axis) and ratios (m-mPhOH)/mPhOH (open squares referred to the right Y axis) in peat

730

incubated in different phenol-containing atmospheres plotted against time of incubation (hrs).

731

The RH levels and the types of solutions (A and B) used for generating phenol vapor are

732

indicated on each plot. The error bars represent one standard deviation.

733

29



Page 30 of 34

Table 1. Parameters of the model (Eqn. 1) fitted to the temporal changes of differential mass uptakes (Fig. 1) Sorbent

RH, %

Clay soil

52 52 73 73 92 92 52 52 73 73 92 92 52 52 73 73 52 52 73 73 92 92

Leonardite

HA-Na

Peat

a

Initial phenol concentration in salt a, w w-1, % solutions controlling air RH, mg L-1 20,000 6.9±0.4b 60,000 5.0±0.3 20,000 7.0±0.5 60,000 6.5±0.4 20,000 7.5±0.4 60,000 6.9±0.4 20,000 7.9±0.5 60,000 7.6±0.5 20,000 9.8±0.5 60,000 9.7±0.5 20,000 16.4±0.4 60,000 11.8±0.5 20,000 6.5±0.2 60,000 6.1±0.2 20,000 8.2±0.3 60,000 8.2±0.3 20,000 9.5±0.6 60,000 9.4±0.4 20,000 10.3±0.5 60,000 12.1±0.5 20,000 9.8±0.7 60,000 10.3±0.4

734 735

k×103, hrs-1

P

RMSDa, w w-1, %

r2

2.6±0.5 2.5±0.5 2.6±0.6 2.6±0.5 2.6±0.5 2.8±0.6 3.2±0.9 3.5±0.9 3.3±0.7 3.1±0.6 2.3±0.3 3.5±0.7 6.4±1.1 7.2±1.1 4.7±0.8 4.7±0.9 3.0±0.7 2.8±0.4 2.2±0.3 2.2±0.3 3.0±1.0 2.8±0.5

39±21 39±21 52±35 42±24 28±13 38±22 31±22 43±35 37±23 32±18 6.9±1.3 32±19 278±256 203±171 57±38 34±23 41±27 22±8 13±4 10±2 19±15 18±7

0.52 0.37 0.64 0.53 0.53 0.55 0.77 0.75 0.77 0.69 0.70 0.91 0.46 0.38 0.53 0.58 0.80 0.52 0.60 0.61 0.89 0.64

0.96 0.96 0.95 0.96 0.97 0.96 0.94 0.95 0.96 0.97 0.98 0.96 0.97 0.98 0.97 0.96 0.96 0.98 0.97 0.98 0.92 0.97 736 737 738

b

root-mean-square deviation; "±" indicates standard error.

30 ACS Paragon Plus Environment

Page 31 of 34


Table 2. Organic C contents, and some parameters explaining extractable phenol contribution to organic C surplus, and OM contribution to the

739

total differential mass m, due to exposure of samples of the four sorbents to phenol vapor at RH of 73%.

740 741

Sorbent

Clay soil

Leonardite

HA-Na

Peat

a

Type

Control Exposed to slnd A Exposed to sln B Control Exposed to sln A Control Exposed to sln A Control Exposed to sln A

Organic C content

Surplus in organic C content

Extractable Fraction of Surplus in Differential phenol content, phenolic C in the OM content mass, m mPhOH organic C surplus

%, w w-1 of dry sorbent 0.73±0.08 a nab 0.97±0.15 0.24±0.17

ndc nd

% na 0

%, w w-1 of dry sorbent na na 0.31±0.22 7.2±1.0

Fraction of the differential mass increase m due to OM surplus % na 4

1.15±0.29

0.42±0.30

nd

0

0.54±0.39

6.6±1.2

8

53.2±0.2 59.9±0.6

nab 6.7±0.7

nd 0.39±0.01

na 5

na 8.7±0.9

na 10.3±0.6

na 85

44.2±0.5 49.7±0.3

na 5.4±0.6

nd 0.85±0.01

na 12

na 7.0±0.8

na 8.3±1.0

na 84

44.8±0.1 49.9±0.9

na 5.1±0.9

nd 2.21±0.01

na 33

na 6.6±1.2

na 10.5±0.9

na 63

b

c

d

± indicates standard error. "na" means non-applicable. "nd" means not determinable. "sln" means solution.


742 743 744 745


Page 32 of 34

746

Relative humidity (%)/phenol feeding sln: 73/A 92/A 52/B 73/B

52/A

Peat

Mass difference (sample minus control) -1 (%, w w of dry sorbent)

12

8

4

4

0

0 0

1000

HA-Na

12

8

2000

3000

4000

5000

20

92/B

0

1000

2000

3000

4000

5000

2000

3000

4000

5000

10

Leonardite

16

Clay soil

8 6

12

4

8

2

4

0 0 -2 0

1000

2000

3000

4000

5000

0

1000

Time (hours)

Time (hours)

747 748 749 Figure 1


750


-1

Content of extractable phenol (%, w w )

Extractable phenol, mPhOH 2.8

6

RH 52%

2.8 4

2.4

2

2.0

0

2.8

2

2.0

3.2

RH 52%

4 2.8

2.4

0

Phenol sln A

1200 2400 3600

RH 73%

2.4 2 2.0

2.0 1.6

0

Phenol sln B 1200 2400 3600

-2 0

3.2 4 2.8

1200 2400 3600

RH 92%

4

2.4 2 2.0

1.6

0

2

2.0

-2 1.2 0

3.2

1.2

4

2.4

0 1.6

-2 1.2

1200 2400 3600

6

RH 92%

Phenol sln A

Phenol sln A

1.2

6 4

2.4

0 1.6

1.6

2.8

RH 73%

(m-mPhOH)/mPhOH

2

1.6

Phenol sln B

1.2 0

0

Phenol sln B

1.2

1200 2400 3600

0

1200 2400 3600

Time of exposure to phenol-water vapor (hrs)

Figure 2


0

(m-mPhOH)/mPhOH

Page 33 of 34


Page 34 of 34

-1

Phenol-induced mixed water vapor uptake (% w w )

TOC Graphics

14

m= 12

a 1 + P × e− kt

10

peat organic matter (OM)

8 6

soil (clay) 4 2

kOM ≈ kclay 0

0

1000

2000

3000

Time (hours)

34


4000

Long-Term Uptake of Phenol-Water Vapor Follows Similar Sigmoid

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