DIVISION S-3—SOIL MICROBIOLOGYAND BIOCHEMISTRY

Peroxy Radical Interactions with Soil Constituents1

A. A. POHLMAN AND T. MILL2

ABSTRACTInteraction of peroxy (RO2«) radicals with soil constituents in di-

lute aqueous solutions and suspensions at 50 °C was examined using/7-isopropylphenol (PIP) as a probe. Dissolved and suspended humicsubstances as well as suspended silicate surfaces were found to trapROj« radicals. Competitive trapping of RO2« radicals occurs with 10mg • L~' of solid suspended humic acid polymers and by 1.0% silicatesuspensions containing 3-74 mg-kg~' of total organic C. Electronand H-atom transfer between RO2- radicals and solid surfaces isattributed to the availability of electron-donating or phenolic func-tional groups and not to total organic C content.

Additional Index Words: silicate surfaces, humic acids, phenol,oxidation, H-atom transfer, clay, metal-ions.

Pohlman, A.A., and T. Mill. 1983. Peroxy radical interactions withsoil constituents. Soil Sci. Soc. Am. J. 47:922-927.

THERE is CONSIDERABLE EVIDENCE to indicate thatelectron transfer processes play a major role in

the transformation of many natural and synthetic or-ganic compounds distributed in the aquatic and soilenvironments (10, 11, 17, 23). Oxidants such as al-kylperoxy radical (RO2») and singlet oxygen ('02) gen-erated by photooxidation have been identified as im-portant oxidants in natural waters (3, 11, 23). Theseoxidants exhibit high reactivity towards electron-richstructures such as hydroquinones, eneamines, aro-matic amines, and phenols (4, 7).

In soils RO2» radicals so far have not been identifiedas major oxidants, although there is evidence to in-dicate that abiotic electron-transfer processes can playimportant roles in transformation of some chemicals(5). The aim of this study is to investigate RO2» radicalinteraction with constituents of soils such as humicsubstances, sand, silt, clay, and dissolved ionic speciesto identify possible sinks for RO2« radicals.

We have used 2,2'-azobisisobutyronitrile (AIBN) asa source of RO2» radicals. This compound is slightlysoluble in water (~ 10~3M) and undergoes thermo-lysis at a convenient and constant rate at 50°C (12,19, 20). p-Isopropylphenol (PIP) served as a radicaltrap to measure quantitatively the number of RO2«radicals produced from AIBN and to estimate the rel-ative reactivities of soil constituents towards RO2»radicals in competition with it. In a related study, wehave examined the kinetics of the oxidation of PIP by

1 Contribution from the Physical Organic Chemistry Dep., SRIInternational, Menlo Park, CA 94025. The work was supported byNSF Grant PFR 78-2740. Received 3 Aug. 1982. Approved 25 Mar.1983.2 Former Research Associate and Department Director, respec-tively. Senior Author is now Postgraduate Research Soil Scientist,Dep. of Plant and Soil Biology, 108 Hilgard Hall, Univ. of Califor-nia, Berkeley, CA 94720.

RO2» radicals in waters following both the loss ofphenol and formation of oxidation products (14).Those experiments provide a quantitative basis forkinetic analysis of the RO2» radicals' reactions withsoil constituents discussed here.

MATERIALS AND METHODSReactant Materials and Oxidation Products

p-Isopropylphenol (PIP) was recrystallized from «-hexane(melting point 60-62°C). Commercial 2,2'-azobisisobutyl-ronitrile (AIBN) was used without further purification. Max-imum solubility of AIBN in 5% acetonitrile and 95% H2Ois 2.2 X 10-3M

Montmorillonite (Osage, Wyo.), kaolinite no. 9 (Mesa Alta,N. Mex.), and halloysite no. 29 (Wagon Wheel Gap, Colo.)were obtained from Wards Natural Science Establishment,Rochester, N.Y. Montmorillonite clay was a fine powder,and kaolinite no. 9 and halloysite no. 29 were clay lumpsand had to be ground in a mortar and pestle. Pink and whiteOtay montmorillonite clay samples were obtained from Dr.Isaac Barshad, Univ. of California, Berkeley. Commercialsand, washed and ignited, was ground in a ball grinder andmechanically screened. The model humic acid polymers wereobtained from Dr. J.P. Martin, Univ. of California, River-side.

Organic matter and free iron oxides were removed fromclay and soil samples according to a procedure described byKunze (8). Synthetic alumino-silicate gels were prepared bythe procedure of Chu and Johnson (2). Suspensions wereprepared by adding a known dry weight of solid to a knownvolume of solution. Solids were removed from solution bycentrifugation at 20 000 rpm for 30 min. Fulvic and humicacids were extracted from Kentwood sandy loam soil by aprocedure described by Stevenson (18). The pH of suspen-sions and filtrates was adjusted by addition of either diluteHC1 or NaOH.

The water used to prepare the suspensions, reagents, etc.,was obtained from a Millipore Corporation Milli-Q system(reverse osmosis, ion exchange, carbon, and filtration through0.45-Atm filter). Chelex-100 resin (200-400 mesh) was ob-tained from Bio-Rad Laboratories. Chelex-100 treated waterwas prepared by adding 1 volume of Chelex to 5 volumesof Milli-Q water, stirring for about 2 h, filtering to removethe resin, and neutralizing with dilute HC1. Products of re-action, discussed below, were characterized by methods de-scribed elsewhere (14).

Kinetic MethodsOxidation reactions were run in 100-mL volumetric flasks

containing the desired amount of phenol, initiator, and sus-pended matter in 5% acetonitrile (AN) and 95% water. Allsalt and suspended matter additions and final pH adjust-ments were made before adding measured amounts of PIPand AIBN to the reaction mixture. Before oxidation wasinitiated, enough sample was withdrawn from the reactionflask to obtain a [PIP]0 value and to provide sufficient ox-ygen for peroxy radical formation. All flasks were shaken in

922

POHLMAN & MILL: PEROXY RADICAL INTERACTIONS WITH SOIL CONSTITUENTS 923

a water bath at 50 ± 0.01°C for 20 to 45 h. Samples weretaken at convenient intervals, cooled to 20°C, and the sus-pended matter removed by centrifugation at 20 000 rpm for30 min. The supernatant was stored in a refrigerator untilanalysis. Control measurements (no AIBN initiator added)were made in the presence of sand, alumino-silicate gel, ka-olinite, oxidized and reduced montmorillonite clay samplesusing identical conditions to the oxidation experiments todemonstrate PIP was not lost by a nonoxidative process.

Analytical MethodThe disappearance of PIP and AIBN from the reaction

mixture was analyzed by reverse phase HPLC with gradientelution. The gradient used was 5 to 70% AN in Milli-Q waterthrough a 4- by 300-mm M-Bondapak (Waters Associate) Qgcolumn with linear programming at 2 mL/min for 30 min.p-Isopropylphenol and its oxidation products were detectedat 254 nm and AIBN at 330 nm using a Water AssociatesM440 photometer. Quantisation of PIP, AIBN, and prod-ucts was made by direct calibration of known concentrationsusing a Spectral Physics Minigrator to measure peak areas.Each analysis was run in duplicate.

Atomic absorption analyses for metals in water were per-formed by Accurex, Mountain View, Calif. Analyses for or-ganic matter in clays was performed on HF-HCl-digestedsamples (Broadbent, 1965) and total organic C (TOC) byDohrman DC-54 system.

RESULTS AND DISCUSSIONThe production of free C radicals (R«) by thermol-

ysis of AIBN is not very efficient in water because anappreciable fraction of initially formed R» radicals reactwith each other before they diffuse apart (6). The frac-tion of R« radicals that escape from the water-solventcage at 50°C is 0.20 ± 0.03 in metal-free water (14)(designated by the efficiency factor a). These R» rad-icals rapidly combine with O2 to form peroxy radicalRO2«. The thermal initiation process is first order (19,20), and follows the sequence

Table 1—Effect of clays and sand on decomposition of AIBNin 95% water, 5% acetonitrile at 50°C.

R2N2

(2R«)cage

(2R»)cage

(2R«)cage + N2 , [1]

[2]

[3]R.+ 02——R02», [4]

where R» is (CH3)2 CCN.We measured the loss of AIBN in water at 50 °C,

using HPLC. Data on the rate constant, kd, for thedecomposition of AIBN at 50°C in water alone (allexperiments were performed in 95:5, H2O:AN to fa-cilitate solubilization of PIP) and water suspension ofseveral clays and sand are shown in Table 1. Reactionsfollowed first-order kinetics with kd = (1.56 ± 0.06)X 10~6 s~', and the loss of AIBN was not affected byclays or sand in the reaction mixture.

With sufficient PIP, RO2- radicals [RO2- =(CH3)2C(CN)O2«] are rapidly scavenged to form mix-tures of several products. At concentrations between4 and 15 X \0~5M PIP and 4 X 10~4M AIBN theprincipal reactions areRO2- + p-iPrC6H4OH _

ROOM + p-iPrC6H4O •, [5]

Reaction addition[AIBN]0,

10'M

Milli-Q water:]: 15.0 1.53 ± 0.01 1261.0%Kaoliniteno. 9

(< 2-^m particle size) 15.0 1.58 ± 0.01 1225.0% Kaolinite no. 9

(< 2-/im particle size) 15.0 1.52 ± 0.03 126Milli-Q waterj 4.00 1.54 ± 0.05 1250.1% Montmorillonite (Wyom)

(< 10-^m particle size, Mg-saturated) 4.00 1.62 ± 0.04 1181.0% Sand

417 to 147-f.m particle size) 4.00 1.51 ± 0.06 128

t Half life for decomposition.t Produced by Milli-Q system.

RO2« + /?-iPrC6H4O

OOR

i-Pr OOR i -P r

= RO2ArO, [6]

and the stoichiometry of the overall oxidation processis 2:1, RO2«:ArOH. Under these conditions the effi-ciency factor a can be estimated from the relationship

a = A[ArOH]/A[AIBN]. [7]Several complications may occur in the presence of

clays, sands, and humic substances that will changethe 2:1 stoichiometric relationship. These include (i)trapping of RO2« radicals by clay surfaces or by humicsubstances, (ii) reactions of RO2» radicals with dis-solved low valent metal ions that reduce the radicalsto inactive species, and (iii) reactions of metal ionswith intermediate peroxides, ROOM or RO2ArO, toform additional radicals

Under these conditions several other important re-actions can compete:2ArO _

i-Pr i -Pri-Pr

= (ArO)2

2RO2« —> inactive productsRO2» + surface or humic substances —»

ROOH + surface or humic radicals

[8]

[9]

[10]if reactions [8], [9], or [10] effectively compete withreactions [5] and [6], then the 2:1 stoichiometry willshift to 1:1 (n < 2) if reaction [8] is dominant, andto n > 2 if reactions [9] or [10] are dominant.

924 SOIL SCI. SOC. AM. J., VOL. 47, 1983

To explore these possible reactions we examined theaffect on PIP oxidation (by AIBN at 50°C) of a num-ber of suspended and dissolved species consisting ofor derived from montmorillonite (Wyoming) clay andfine sand, all of which were substantially free of or-ganic matter. We also examined the effect of humicand fulvic acids on PIP oxidation. In all cases thereactions were followed by the loss of PIP and theformation of para-substituted cyclohexadienone (p-RO2ArO) and dimeric [(ArO)2] products. Rates of ox-idation of PIP are calculated from A[PIP],/A?. Resultsare summarized in Table 2.

Deviation of the stoichiometric values from 2 forphenol oxidations in the presence of suspended andsoluble clay or sand constituents and humic sub-stances suggests that Eq. [5] and Eq. [6] do not holdexactly.

We could account for from 2 to 74% of oxidizedPIP as p-RO2ArO or dimer. In addition we detectedmany other products by HPLC during oxidations ofPIP but only p-quinone was identified. Probably PIPoxidizes to give both p- and osubstituted cyclohex-adienones (RO2ArO), but we have identified only the^-substituted RO2ArO. In general 0-substitution givesunstable intermediates that cleave to o-quinones andfurther degradation products. The formation of hy-droperoxide (reaction [5]) could not be detected bythe HPLC uv detector because of its low extinctioncoefficient (t = 6.2) at 254 nm.

Several patterns in product formation are worthnoting. Product balances improved (to 60-70%) whentrace transition metal ions were removed by Chelexor EDTA (ethylenediaminetetraacetic acid) or whenreactions were run in the presence of humic but notfulvic acids. Filtrates from clay and sand preparationstended to catalyze the rate of PIP oxidation and lowerproduct yields. The accelerated rates probably are dueto small quantities of metal ions (probably Cu2+) thatdissociated from clay and sand samples during filtra-tion. Atomic absorption analysis of the filtrate derivedfrom 1% sand suspension detected 1.47 X 10~5, 3.1X 10~6, and 5 X iq-8Mof Cu, Fe, and Mn, respec-tively. These transition metal ions can undergo one-electron transfer reactions with hydroperoxides (21)

RO2H + Mn _ RO + Mn+ + OH- [11]or

RO2H + Mn+1 Mn + H+ [12]with production of additional free radicals. The gen-eration of additional free radicals from the decom-position of hydroperoxide [Me2C(CN)OOH = ROOH]was confirmed by mixing ROOH and PIP in the pres-ence of 1% sand at 50 °C. The reaction generated boththe dimer (ArO)2 and p-quinone, but not RO2ArO. Ina similar experiment we observed that RO2ArO de-composes in the presence of sand suspensions aloneto ^7-quinone and another unidentified product. Cop-per ions are particularly effective in decomposingROOH. About 35% of initial PIP (8 X 10~5M) wasoxidized by radicals generated by the decompositionof 4 X 10-5M ROOH in the presence of 1.4 X 10~5MCu2+ at 50°C. Both p-quinone and the dimer (ArO)2were detected as major products of oxidation. Neither

1.4 X 10~5M Fe2+ nor Fe3+ was effective in decom-posing ROOH and the combined action of 1.4 X10-5A/Cu2+ and 3.0 X 10"6A/Fe3+ had no differenteffect on ROOH than Cu2H" alone. In the absence ofROOH, the concentration of PIP was not affected byCu2+ ions.

We have noted elsewhere (14) that oxidation by RO2»radicals in Mill-Q filtered water gives faster rates ofoxidation and poorer material balances than oxida-tion in metal-free water, presumably because of tracequantities of metal ions (see Experimental Methodsfor description of Milli-Q filter process). The com-bined action of 2.9 X 10~7 and 1.1 X 10~7Mof Cuand Fe, respectively, found in Milli-Q water, reducedthe initial phenol concentration by about 8% in thepresence of ROOH. In Chelex resin-treated water,thermal decomposition of ROOH was slow and ac-counted for only 2% of phenol lost under the sameconditions.

Unlike filtrates from clay and sand preparations andsand suspensions, which accelerate the oxidation ofPIP, clay suspensions and humic substances inhibitthe oxidation of PIP. The free radical scavenging ac-tion of humic substances is due to the presence ofhydroquinoid-OH, phenolic-OH, NH2, and SH groups(16). These groups can act as electron donor groupsand successfully compete for with PIP for RO2« rad-

Table 2—Effect of soil constituents on the rate of PIP oxidation_______and product formation by RO2« at 50°C.t_______

Products of oxidationneacuon a[fif\t

additives* (10'M)

None* 0.92

iva ue ———————10"M. A[PIP]j

a") in)

12 2.0

(ROjArO)(10'Af)

0.36

l(ArO),](10W)

0.10

LIFKUJJHA[PIP](

0.610.08% Montmorillonite clayttSuspension 0.37Filtrate 1.33

517

5.01.4

0.090.06

0.050.38

0.500.62

0.08% Montmorillonite clayttSuspension 0.53Filtrate 2.22

1.0% Sand§§Suspension 2.42Filtrate 2.38

Suspension+ 8.0 x10-!MEDTA 0.88

Humic acidllSrngL'1 0.82SOmgL'1 0.57

Fulvic acidll5mgL" 0.8020mgL-' 0.27

627

2930

10

107

113

3.60.9

0.80.8

2.3

2.53.5

2.27.1

0.000.01

0.000.02

0.28

0.330.16

0.240.11

0.140.54

0.030.25

0.11

0.120.13

0.040.01

0.530.50

0.020.23

0.57

0.690.74

0.400.41

t Oxidations in 95% H,O, 5% AN; 20-24 h; [AIBN]0 = 4.00 x 1Q-M,[PIP]0 4.00 to 8.00 x 10-'M,pH6-7.

t Additive wt % based on g/100 mL of solution.§ [ROj.] was estimated from 0.4 IAIBN]0 [1 -exp ( -k d t|).1 (|RO,ArO] + 2[ArO]!)/4PIP.# Chelex-100 treated water.

tt Montmorillonite (Wyoming) sample was oxidized with 30% H,Oj, Na-exchanged, < 10 /on. Suspension and filtrate in Chelex-100 treatedwater.

tt Montmorillonite (Wyoming) sample was oxidized with 30% H,O,, thenreduced with NasSjO4 and Fe extracted with Na-citrate; Mg-exchanged,< 10 /*m suspension and filtrate in Chelex-100 treated H,0.

§§ Washed and ignited, 38- to 60-jim fraction, suspensions in Milli-Q H2O,filtrate in Chelex-100 treated water.

11 Extracted from Kentwood sandy loam soil; FA concentration was esti-mated based on FA contains 50% TOC.

POHLMAN & MILL: PEROXY RADICAL INTERACTIONS WITH SOIL CONSTITUENTS 925

icals. Semiquinone free radicals known to be presentin humic substances (16) might also interact with RO2»radicals by radical combination. Our studies indicatethat 30 mg-L-1 of HA and 20 mg-L-1 of FA can de-crease the rates of PIPP oxidation from 12 X 10~n

to 7 and 3 X 10~nAf s~' corresponding to about 42and 75% inhibition, respectively, compared with re-actions run in metal-free water. The greater inhibitoryproperties of FA are presumably due to its higher sol-ubility in the reaction mixture.

To gain greater insight into the inhibitory propertiesof soil constituents, we examined the effect on PIPoxidation of a wide range of materials including 1:1and 2:1 clays and clay, silt, and sand fractions isolatedfrom two soils. The results are summarized in Table3. Montmorillonite (Wyoming) clay samples and clayfractions obtained from the two soils exhibited highreactivities towards RO2« radicals. In contrast, syn-thetic aluminosilicate, kaolinite, halloysite, and pinkand white Otay montmorillonite clays do not inhibitthe oxidation of phenol within the range of concen-trations tested. The difference in behavior betweenWyoming and Otay montmorillonite clays is quite un-usual because both samples form very finely dispersedsuspensions with low TOC contents of 1 to 3 mg • kg~'

Table 3—Effect of suspended soil constituents on the rate of_________PIP oxidation by RO2. at 50°C.t_________

Rate, 1RO''H TOC#ofTreat- A[PIP]t, 10"M- A[PIP]t solid

Reaction additivest ment§ 10'M 3-' mg-kg-'

13

0

0

14

22

1.8

178

50

1.7

1.1

<0.5

6.9

3.0

1.0

1.9

5.0% 7:3 Aluminosilicate 1 0.84 10 2.3 TT1.0% Kaolinite no. 9, < 2 /an 2 0.81 10 2.3 <0.81.0% Halloysite no. 29,

< 10/mi 3 0.971.0% Montmorillonite

(Wyoming), < 2 /im 3 0.011.0% Montmorillonite

(Wyoming), < 2^m 4 0.041.0% Montmorillonite

(Pink Otay), < 1 (im 4 1.181.0% Montmorillonite

(White Otay), < 10 ^m 4 1.861.0% Kentwood sandy<oam

clay, Typic Haploxerolls,coarse-loamy, mixedmesic family, < 2 /tm 5 0.35 4 5.5 74

1.0% Linne clay loam clay,< 2 (im 5 0.00 0 - 56

1.0% Kentwood sandy loamsilt, 38-2 urn 5 0.28 3 6.9 34

1.0% Linne clay loam silt,38-2 urn 5 0.43 5 4.5 36

1.0% Kentwood sandy loamsand, 147-38 /im 5 0.19 2 10.2 34

1.0% Linne clay loam sand,Calcic Pachic Haploxerolls,fine-loamy mixed thermicfamily, 147-38 pm 5 0.53 6 3.6 42

1.0% Commercial sand,147-60 urn 6 2.77 33 0.9 TT

2.0% Commercial sand,417-147 ion__________6 3.42 33____0.6 TT

T Oxidation conditions as listed in Table 2.} Wt % additive based on g/100 mL solution basis.§ Treatments: 1) Synthesis according to Chu and Johnson (2); 2) Oxidized

with 30% H£>i, reduced with Na2S»O,, Mg-exchanged; 3) Mg-ex-changed. 4) Oxidized with 30% H,O,, Na-exchanged; 5) Like 4 and auto-claved; 6) Washed and ignited.

1 Estimated as in Table 2.# TOC measured on HF/HC1 digested samples by Dohrman DC-54 TOC

system.tt Not determined.

The coarser particle-size fractions from the two soilsalso exhibited inhibitory properties. But all of thesefractions also contained associated organic matter withTOC contents ranging from 34 to 42 mg-kg"1. Theinhibitory effect of the associated organic matter canbe clearly demonstrated by comparing oxidation ratesfor reactions including sand fractions obtained fromsoils to reactions including commercial sand (Table3) which contains no organic matter. Sand fractionscontaining organic matter inhibited oxidation; sandfree of organic matter catalyzed oxidation.

The high inhibition by montmorillonite (Wyoming)clay is quite surprising because the clay contains onlyvery small quantities of associated organic matter (6.9mg-kg-1) making it unlikely that simple humic acid-like inhibition was responsible. Nor is it likely thatinhibition is due to electron transfer from low-valenttrace metal ions present in octahedral coordination orpresent as impurities, because oxidized clay samplesalso inhibit the reaction, and structural Fe2+ can bereadily oxidized by even milder conditions (15) thanused in our treatment. We have, however, shown andreported elsewhere (14) that Fe2+ ions in concentra-tions 10~5 to 10~4Mcan completely inhibit oxidationof PIP in water by electron transfer to RO2» radicals.Transfer of electrons from RO2» radicals to metals inhigher vacancy states such as Fe+3 is very unlikelybecause of the high ionization potential for oxygen(13.64V) (13).

There appears to be little relationship between par-ticle size and inhibitory properties of the clays: a 1%suspension of < 1 pm pink Otay montmorillonite,which does not inhibit is close in total surface area tothat of ̂ 2 jum Wyoming montmorillonite, which doesinhibit. One main difference between the two clays isthat in general, there is a lower A13+ substitution forSi4+ in the tetrahedral sheet in the Wyoming-typemontmorillonites than in the Otay (22). Thus thecharge density on the Wyoming is more localized inspots on the surface, whereas the charge density in theOtay is more diffused. This difference could conceiv-ably be a factor for the lack of inhibition by Otay.

The most likely explanation for the inhibition is thepresence of trace quantities of an exceptionally effec-tive organic inhibitor. This assumption is supportedby our observations that the activity of the inhibitingclay can be significantly decreased by drying it at 130°Cfor 24 h. Under these conditions, volatilization as wellas structural modifications of organic constituents canoccur (i.e., decarboxylation). However, oxidation ofFe2+ to Fe3+ is possible but structural modificationsto the alumino-silicate are highly unlikely at such lowtemperatures.

To learn whether inhibition of PIP oxidation canbe caused by RO2« radical interaction with active solidsurface, in addition to dissolved species, we studiedPIP oxidations in the presence of 8 to 10 mg-L-1 ofsolid synthetic humic acid polymers (HAP). Samplesof HAP were prepared by Martin and Haider (9) usingperoxidase-H2O2 to catalyze the polymerization of dif-ferent hydroxytoluenes, phenols, and hydroxy benzoicacid. Inhibition of PIP oxidation under conditionslisted in Table 4 was complete for every HAP testedexcept that derived only from orcinal where inhibition

926 SOIL SCI. SOC. AM. J., VOL. 47, 1983

Table 4—Oxidation of PIP by ROi* in the presence of synthetichumic acid polymers at 50°C.t

AdditiveA[PIP](,

10'MRate,

10"M-s-'

Humid acid polymers:):Constituent units

10 mg-L-' Orcinol 0.53 610 mg-L-' Orcinol + catechol 0.00 010 mg«L~' Hydroxy toluene + phenols§ 0.00 09 mg-L-'Hydroxy acidsl 0.00 0

Insoluble fraction of humic acid polymers^Constituent units

10 mg-L- Orcinol 0.78 911 mg-L"' Orcinol + catechol 0.00 08 mg«L-' Hydroxy toluene + phenols§ 0.00 0

t Oxidation conditions as in Table 2.} Synthesized by Martin and Haider (9).§ Constituent units consist of: 2,3- 2,6-, 3,4-dihydroxytoluenes, catechol,

orcinol, phloroglucinol, pyrogallol, and resorcinol.1 Constituent units consist of: 2,4- 3,5-dihydroxybenzoic, 2,3,4-, 2,4,6-tri-

hydroxybenzoic, caffeic, ferulic, gallic, protocatechuic, vanillic, and p-hydroxycinnamic acids.

# Humic acid polymers were extracted with water for 20 h at 50°C and theinsoluble portion was separated.

was about 50%. Because these HAP contained varyingamounts of FA (about 8, 19, 9, and 84% for orcinol,orcinol + catechol, hydroxytoluenes + phenols, andhydroxyacid-types, respectively) (9) which presum-ably could dissolve and act as homogeneous inhibi-tors, we extracted HAP with water for 20 h at 50°Cto remove the FA. Water extraction did not reducethe inhibitory properties. The results seem to suggestthat inhibition of PIP oxidation by RO2» radicals canoccur with both dissolved and suspended humic sub-stances. That about 10 mg-L"1 of solid suspendedHAP can successfully compete with 8.00 X 10~5Afdissolved phenol is significant, because RO2» radicalsmust first diffuse to the site of activity on the solidsurface before inactivation can occur.

The differences in activities between orcinol— andorcinol + catechol-type-HAP suggests that some humicsubstances may be more active than others dependingon the number and type and arrangement of electron-donating functional groups. These differences in reac-tivities may also account for our observations thatsome clays containing organic impurities are more ac-tive than others, and that reactivities of clays and othersoil minerals may not be proportional to the amountof organic impurities measured as total organic C.

CONCLUSIONSThis study provides a basis for semiquantitatively

assessing the fate of free radical oxidants such as RO2«radicals in soils or sediments. Although the detailedfate in a specific soil system will depend on the relativeproportion of clays, humic substances, minerals, andtransition metal ions, some generalizations are pos-sible. The clay and mineral components of soils havelittle effect on radical reactivity but some trace tran-sition metals (at least Cu2+) associated with these frac-tions may have significant effects on peroxide inter-mediates. Only Fe2+ ion appears capable of scavengingRO2» and inhibiting oxidation but its presence in aero-bic systems with RO2» seems unlikely. The dominanteffects in many soils will be trapping of RO2» by humic

and fulvic acids probably with formation of hydro-peroxides and some peroxycyclohexadienones. In highorganic soils, peroxides also may oxidize phenolicstructures but in sandy soils the peroxides probablywill decompose via metal-catalyzed processes.

Our results with the dissolved and suspended HAand FA are surprising in that natural HA and FA dis-solved in water failed to inhibit PIP oxidations at con-centrations (30 mg-L"1) of HA where insoluble syn-thetic HAP completely suppressed oxidation. Theconclusion is clear that, in most natural waters, RO2»reactions with reactive synthetic organic chemicals willcompete favorably with reactions of RO2» with HA orFA. This is consistent with what we observed in anearlier investigation of photolytic production of RO2»in natural water (11).

The differences in reactivity of synthetic and naturalHAs may be related to the fact that soil HA but notsynthetic HA has polysaccharades, peptides, and ali-phatic material linked into the polymer. These ma-terials generally are less reactive toward RO2» and thusreduce the reactivity of the HA. The unusual reactiv-ity of Wyoming montmorillonite toward RO2« appar-ently is due to traces of very reactive organic material,possibly quinones, formed in H2O2 treatment of theclay. In general, total organic C appears to be a poorguide to predicting reactivity for free radical oxida-tions.

ACKNOWLEDGMENTSWe thank Walter Farmer, Univ. of California, Riverside,

for many helpful discussions on soil chemistry and physics,and J.P. Martin, Univ. of California, Riverside, and IsaacBarshad, Univ. of California, Berkeley, for generous gifts ofmaterials. The referees made several helpful suggestions. Ms.Maria Buyco prepared the several versions of this manu-script.

CLARK & GILMOUR: DECOMPOSITION AT OPTIMUM & SATURATED SOIL WATER CONTENTS 927