(NaOCl) AnRemoval From Soils Using Hydrogen Peroxide,d Disodium Peroxodisulfate

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Review: Organic Matter Removal from Soils using Hydrogen Peroxide,Sodium Hypochlorite, and Disodium Peroxodisulfate

R. Mikutta,* M. Kleber, K. Kaiser, and R. Jahn

ABSTRACT termination of soil mineral properties requires completeremoval of organic matter without modification of theWe compare the performance of three most accepted reagents formineral phase. An incomplete removal of organic mat-organic matter removal: hydrogen peroxide (H2O2), sodium hypochlo-

rite (NaOCl) and disodium peroxodisulfate (Na2S2O8). Removal of ter hampers the evaluation of phyllosilicate clay miner-organic matter from soil is mostly incomplete with the efficiency of alogy (Tributh and Lagaly, 1991), SSA determinationremoval depending on reaction conditions and sample properties. (de Jonge et al., 2000) and may also affect infrared (IR)Generally, NaOCl and Na2S2O8 are more effective in organic C re- and differential thermal analysis (Mitchell and Farmer,moval than H2O2. Alkaline conditions and additives favoring disper- 1962).sion and/or desorption of organic matter, such as sodium pyrophos-

Hydrogen peroxide was introduced by Robinsonphate, seem to be crucial for C removal. Pyrophosphate and additives(1922) for soil texture analysis and became the mostfor pH control (bicarbonate) may irreversibly adsorb to mineral sur-widely used chemical reagent for organic matter de-faces. In soils with a large proportion of organic matter bound to thestruction. Some alternative reactants have been pro-mineral matrix, for example subsoils, or rich in clay-sized minerals

(Fe oxides, poorly crystalline Fe and Al phases, expandable phyllo- posed to increase C removal efficiency and to reducesilicates), C removal can be little irrespective of the reagents used. possible effects of H2O2 on minerals. Sodium hypochlo-Residual organic C seems to seems to represent largely refractory rite (Anderson, 1963) and Na2S2O8 (Meier and Mene-organic matter, and comprises mainly pyrogenic materials and ali- gatti, 1997) have emerged as reagents with the greatestphatic compounds. If protected by close association with minerals, potential to replace H2O2. These reagents have beenother organic constituents such as low-molecular weight carboxylic

recently applied to soils (Kaiser et al., 2002; Kiem andacids, lignin-derived and N-containing compounds may escape chemi-Kogel-Knabner, 2002; Eusterhues et al., 2003) and sedi-cal destruction. For determination of mineral phase properties, treat-ments (Mayer, 1999).ment with H2O2 should be avoided since it may promote organic-

assisted dissolution of poorly crystalline minerals at low pH, disinte- To date, there is a vast number of oxidation protocolsgration of expandable clay minerals, and transformation of vermiculite and much uncertainty regarding the effects of oxidativeinto mica-like products due to NH4

�fixation. Sodium hypochlorite reagents on soil constituents. A thorough understanding

and Na2S2O8 are less harmful for minerals than H2O2. While the NaOCl of treatments for soil organic matter removal with oxi-procedure (pH 9.5) may dissolve Al hydroxides, alkaline conditions dants is the key to a correct interpretation of experimen-favor the precipitation of metals released upon destruction of organic

tal results. Thus, our objective is to review and comparematter. Prolonged heating to �40�C during any treatment may trans-the suitability of H2O2, NaOCl, and Na2S2O8 for organicform poorly crystalline minerals into more crystalline ones. Sodiummatter removal from soils, sediments, and minerals.hypochlorite can be used at 25�C, thus preventing heat-induced min-

eral alteration. Special emphasis was put on (i) the evaluation of indi-vidual treatment procedures including reaction condi-tions required, (ii) mechanisms of interaction betweenthe reagents and organic matter, (iii) the organic C re-Removal of organic matter by chemical reagents ismoval efficiency and reasons for organic matter resis-a common pretreatment for analyses of the soiltance, (iv) the effects of reactants on soil minerals andmineral phase such as particle-size distribution (Gee(v) metal precipitation, and (vi) implications for the useand Bauder, 1986), mineral composition (Tributh and

Lagaly, 1991), cation exchange capacity (CEC), and spe- of chemical treatments for soil analysis.cific surface area (SSA; Kahle et al., 2003). Chemicaldestruction of organic matter is also used to uncover

PROCEDURES, REACTION CONDITIONS,mineral surfaces for subsequent sorption experimentsAND APPLICATION(Jardine et al., 1989; Kaiser and Zech, 2000; Pagel-Wieder

et al., 2004) and to assess organically bound metals (e.g., Removal of organic materials from soil by aqueousShuman, 1983). Other fields for the use of chemical degra- reactants is controlled by multiple factors, including thedation of soil organic matter include the isolation of reaction conditions (pH, temperature, contact time,refractory organic fractions (Balesdent, 1996; Righi et chemical additives) and soil properties (mineralogy, or-al., 1995; Eusterhues et al., 2003) and the exploration ganic matter content, and quality). Attempts have beenof organic matter properties (Cuypers et al., 2002). De- made to optimize reaction conditions to increase the

efficiency of organic C removal from soils. This resultedInstitut fur Bodenkunde und Pflanzenernahrung, Martin-Luther-Uni- in a variety of treatment protocols compiled in Table 1,versitat Halle-Wittenberg, Weidenplan 14, D-06108 Halle, Germany.

which will be briefly discussed here.Received 18 May 2004. *Corresponding author ([email protected]).

Abbreviations: Ac, acetate; CEC, cation exchange capacity; IR, infra-Published in Soil Sci. Soc. Am. J. 69:120–135 (2005).© Soil Science Society of America red; PAH, polynuclear aromatic hydrocarbons; SSA, specific surface

area; XRD, x-ray diffraction.677 S. Segoe Rd., Madison, WI 53711 USA

120

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MIKUTTA ET AL.: REMOVAL OF ORGANIC MATTER FROM SOILS 121

Table 1. Methods for organic matter removal from soils, sediments, and minerals compiled from the literature.

OC§removal

Material Volume Concentration Reaction conditions efficiency Determination Citation

mL % %Hydrogen peroxide

2 g soil �15 30 heating – organic matter content Jackson (1958)I) 5 g soil 1 30 10 mL H2O, 60–70�C, 5 � 11–77 effect on minerals Lavkulich and Wiens (1970)

15 minII) 10 g soil �5 30 20 mL H2O, initially 28–81 effect on minerals Lavkulich and Wiens (1970)

room temperature, thenheating to 60–70�C

10 g soil 20 30 80 mL H2O, initially 56–94 surface area and charge Sequi and Aringhieri (1977)room temperature, thenheating to 70–80�C

10 g soil 20 30 80 mL Na4P2O7, initially 88–100 surface area and charge Sequi and Aringhieri (1977)room temperature, thenheating to 70–80�C

10 g clay to 60 g sand �5 �30 initially room temperature, – particle size distribution Gee and Bauder (1986)then heating to 90�C

�10 g soil �5 30 then 10 10–20 mL H2O, initially – clay mineralogy Kunze and Dixon (1986)room temperature, thenheating to 65–70�C

N.A. vermiculite N.A. 30 80�C, 3 min and 20�C, 24 h – effect on clay properties Muromtsev et al. (1990)15–20 g clay �100 10 initially room temperature, – clay mineralogy Tributh and Lagaly (1991)

then heating to 60–70�C3 g size fraction 50 50 (vol. %) initially room temperature, 85 surface area Feller et al. (1992)

then heating to 50�C10 g soil �10 30 or 50 initially room temperature, – particle size distribution Sheldrick and Wang (1993)

then heating to 90�C10 g clay 200 5 60�C, 5 d N.A.# investigation of clay- Righi et al. (1995)

humus complexes0.9 g clay† 10 30 NaOAc buffer pH 5, 53 Cs adsorption Dumat et al. (1997)

1 h at room temperature,then 70�C for 24 h¶

1 g soil 50 6 80�C, five repetitions 66–97 surface area Theng et al. (1999)10 g decarbonated sediment 20 30 HNO3, 50–90�C, 100 optimization of Schultz et al. (1999)

three additions of H2O2, sequential metalNH4OAc to prevent extractionmetal precipitation

20 g soil – 10 initially room temperature, 2–34 (wt.) particle size Schmidt et al. (1999)then boiling for 24 or 72 h fractionation

30 g soil, 500 15 then 30 initially room temperature, 83–94 analysis of residual Leifeld and Kogel-Knabnerdiam. � 20 �m then heating to 70�C, 1–3 d organic matter (2001)

Sodium hypochlorite

10 g soil 20 6 boiling, 3 � 15 min, pH 9.5 32–96 clay mineralogy Anderson, (1963)5 g soil 10 6 boiling, 3 � 15 min, pH 9.5 69–83 comparison H2O2 vs. Omueti (1980)

NaOCl for texturalanalysis

10 g soil 20 5.3 boiling, 3 � 15 or 30 min, N.A. sequential metal Shuman (1983)DTPA‡, pH 8.5 extraction

5 g clay 50 5.25 boiling, 5 � 15 min, pH 9.5 N.A. charge and exchange Cavallaro and McBride (1984)properties

20 g soil 100 7 80�C, 5 � 1 h 88–94 B adsorption and Marzadori et al. (1991)desorption

1 g soil 2 5.3 96�C, 2 � 0.5 h, pH 8.5 N.A. sequential metal Qiang et al. (1994)extraction

0.9 g clay‡ 2 6.6 boiling, 2 � 15 min, pH 8.5 76 Cs adsorption Dumat et al. (1997)1–1.5 g sediment – 13 60–80�C, 6 h, pH 9–9.5 N.A. surface area Mayer (1999)1 g sediment 20 N.A. 95 � 5�C, 3 � 1 h, pH 9.5 N.A. sequential metal La Force and Fendorf (2000)

extraction10 g soil 30 6 room temperature, 3 � 16 h, 26–80 P adsorption and McDowell and Condron (2001)

pH 8.5 desorption2 g soil 100 6 room temperature, 5 � 6 h, �87 investigation of Kaiser et al. (2002)

pH 8 clay-humus complexes4.5 g clay 250 6 room temperature, 6 � 6 h, 49–81 isolation of stable Mikutta et al. (2004)

pH 8 organic matterDisodium peroxodisulfate

0.2 g clay – 8 g NaHCO3 buffer, 98�C, 1 h, 78–97 clay mineralogy Meier and Menegatti (1997)pH 7.8–8.5

0.5 g clay minerals – 20 g 22 g NaHCO3 buffer, 80�C, – effect on clay minerals Menegatti et al. (1999)1 h, pH 7.8–8.5

0.2 g of size fractions – 8 g NaHCO3 buffer, 80�C, 16 h 93 surface area Kiem et al. (2002)0.5 g clay – 20 g 22 g NaHCO3 buffer, 80�C, 16–99 isolation of stable Eusterhues et al. (2003)

2 d organic matter

† Pretreated with Na4P2O7 � NaOH.‡ Diethylenetriaminepentaacetic acid � chelator.§ OC, organic C.¶ Ac, acetate.# Not available.

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122 SOIL SCI. SOC. AM. J., VOL. 69, JANUARY–FEBRUARY 2005

Hydrogen Peroxide Lack of visible frothing and bleached soil color indicatea complete reaction, but frothing may continue due toMost protocols for sample preparation before texturalthe decomposition of excess H2O2 at mineral surfacesand mineralogical analysis propose the use of 30% (wt/wt)(REACTION OF OXIDANTS WITH INORGANICH2O2 but 10 and 50% (wt/wt) H2O2 have been suggestedAND ORGANIC MATTER section).as well (Table 1). McLean (1931b) found that doubl-

Sometimes H2O2 is used in combination with acetateing the H2O2 concentration from 6 to 12% resulted inbuffer (pH 5) to prevent acidic conditions resulting fromlittle increase in C removal from soils (Fig. 1). Thisformation of acid oxidation products (Douglas and Fies-suggests that the concentration of H2O2 is not decisivesinger, 1971; Pennell et al., 1995). The pH of unbufferedfor C removal efficiency, probably because of the de-soil–H2O2 suspensions may drop by up to three unitscomposition of H2O2 (REACTION OF OXIDANTSand final pH values between 2 and 4 have been reportedWITH INORGANIC AND ORGANIC MATTER sec-(Douglas and Fiessinger, 1971; Lavkulich and Wiens,tion), and the presence of chemically stable and mineral-1971; Griffith and Schnitzer, 1977). Using acetate buffer,protected organic compounds (EFFICIENCY OF OR-H2O2 is additionally consumed due to the oxidation ofGANIC CARBON REMOVAL DEPENDS ON SAM-acetate. Acetate (Ac) may also adsorb to minerals (e.g.,PLE PROPERTIES section).van Hees et al., 2003) and thus contribute to residualTypically, the treatments start at room temperatureC (17–41% C; Pennell et al., 1995).because of a strong initial reaction with easily decom-

There is little consistency in H2O2 use before SSAposable organic matter. If frothing subsides, the sampledetermination of the mineral phase. Hydrogen peroxideis commonly heated to 60 to 90�C (Table 1). Increasingconcentrations vary from 6 to 30% (Sequi and Arin-temperature accelerates the decomposition of H2O2 butghieri, 1977; Theng et al., 1999) and reaction tempera-shortens the reaction time necessary to oxidize organictures from 50 to 80�C (Feller et al., 1992; Theng et al.,matter (Schultz et al., 1999). At temperatures �70�C,1999) (Table 1). Similarly, H2O2 concentration and con-H2O2 is rapidly consumed and additional H2O2 is neededtact time proposed for isolation of stable organic matterto perpetuate organic matter destruction. At lower tem-vary strongly (Righi et al., 1995; Theng et al., 1992).peratures, the contact time needs to be extended (SchultzThese examples demonstrate little agreement on proto-et al., 1999). In practice, removal of organic C from soilscols to achieve given research objectives.using H2O2 often requires several days and no reliable

indicator exists showing completion of the reaction.Sodium Hypochlorite

The use of NaOCl for organic matter removal wasfirst proposed by Anderson (1963) for mineralogicalanalysis of clays (Table 1). The method utilizes 6%(wt/wt) NaOCl at pH 9.5 and three consecutive cyclesincluding boiling for each 15 min. The restriction ofreaction time to 15 min is due to the fast decompositionof NaOCl at high temperatures. Heat-induced mineralchanges (TREATMENTS INDUCE MODIFICATIONSOF MINERAL CONSTITUENTS section) can be mini-mized at room temperature, which requires extendedcontact times (Table 1; Kaiser et al., 2002). If appliedat pH 9.5, NaOCl may partly dissolve Al secondaryphases (TREATMENTS INDUCE MODIFICATIONSOF MINERAL CONSTITUENTS section). To avoidthis, NaOCl can be used at lower pH (Table 1). Treatingsoils with NaOCl avoids vigorous frothing and boilingover as often experienced when using H2O2. ModifiedNaOCl protocols have been used for multiple purposessuch as for metal extraction from soil organic matter(Shuman, 1983), before metal sorption studies (McDow-ell and Condron, 2001), and to investigate stabilizationof organic C by mineral phases (Kaiser et al., 2002)(Table 1).

Disodium PeroxodisulfateFig. 1. (a) Effect of added H2O2 volume on the efficiency of organic

Meier and Menegatti (1997) proposed the use ofC removal from Welsh carbonate-free Madryn soil (�, 5.1% C)and Bodrwyn soil (�, 4.3% C) using 6%(wt/wt) H2O2. (b) Organic Na2S2O8 to remove organic matter from mineral phasesC removal efficiency from bulk soils with increasing H2O2 concen- in one single step (Table 1). Thermal decomposition oftration. Soils contained between 1.3 and 6.5% organic C. Sixty Na2S2O8 did not decrease C removal at temperaturesmilliliters H2O2 were added equivalent to 0.08 g C. The samples

�80�C. By using a NaHCO3 buffer, the pH of reactionwere heated gently then boiled (data adopted from McLean,1931a, 1931b). suspensions with clay minerals and soils can be kept

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between 7 and 8.5 (Menegatti et al., 1999; Kiem and ally decomposed into O2 and H2O (Eq. [1]) via produc-Kogel-Knabner, 2002), thus preventing acid-mediated tion of odd-electron (radical) intermediates (Evans andmineral dissolution. After treatment, Meier and Mene- Upton, 1985). Fenton-like chemistry has been appliedgatti (1997) applied a hot wash (98�C, 1 min) with formic in remediation of soils (Watts et al., 2002; Kanel et al.,acid to remove any traces of salts. However, SO4

2� and 2003) and in wastewater treatment (Waite, 2002).HCO3

� have a high affinity for hydroxyl-bearing mineral�M2� � H2O2 → �M3� � •OH � OH�

surfaces (Ali and Dzombak, 1996; Su and Suarez, 1997)and are likely to become attached to minerals such as (Fenton reaction) [2]Fe and Al oxides or short range order minerals when

�M3� � H2O2 → �M2� � HO2• � H� [3]applied in high amounts (Table 1). Peroxodisulfate oxi-dation was employed for predicting the bioavailability �M2� � •OH → �M3� � OH� [4]of polynuclear aromatic hydrocarbons (PAH) in soils

•OH � H2O2 → H2O � HO2• [5]and sediments since both, oxidation and biodegradation,removed similar portions of PAH (Cuypers et al., 2000). �M3� � O2�• → �M2� � O2 [6]Recently, the Na2S2O8 procedure has been adopted to

Hydrogen peroxide can also be consumed by catalase-soils to isolate stable organic matter and to study thepositive bacteria, which are common members of theinfluence of mineral surfaces on organic matter storagemicrobial community in soil environments (Pardieck et(Kiem and Kogel-Knabner, 2002; Eusterhues et al.,al., 1992). Catalase decomposes H2O2 into O2 and H2O2003).without formation of OH radicals (k � 4 � 107 M s�1;Pardieck et al., 1992). Manganese oxides can also induceREACTION OF OXIDANTS WITH the catalase-type reaction resulting in reduced efficiencyINORGANIC AND ORGANIC MATTER of organic C removal in Mn-containing soils. When the

In theory, CO2 and H2O are the end products of oxida- soil reaction is acid, protonation of H2O2 may yield elec-tive organic matter destruction but a variety of by-prod- trophilic OH cations (Eq. [7]) while at alkaline condi-ucts form during the chemical treatments. Intermediate tions nucleophilic perhydroxyl ions (HOO–) formdegradation products adsorb to mineral surfaces, form (Eq. [8]).precipitates (Martin, 1954), complex trace metals, and

H2O2 � H� ↔ H2�OOH ↔ H2O � OH� [7]thus hinder their redistribution to solid phases (Hoff-

man and Fletcher, 1981) and have the potential to pro- H2O2 � HO� → H2O � HOO� [8]mote mineral dissolution at low pH (Cornell and Schwert-

The possible decomposition pathways of H2O2 in soilsmann, 1996; Zhang and Bloom, 1999). The interactionimply that several reactive H2O2 species can interactof oxidants with organic matter involves complex reac-with organic matter. In the simplest case, H2O2 directlytions, depending on either one of the following factors:oxidizes organic compounds in a peroxidic-type reaction(i) reaction conditions (pH, temperature), (ii) the pres-by a two-electron process without O2 formation (Schumbence of inorganic catalysts that are capable to transformet al., 1955). Reaction of organic matter with OH radi-reagents into more reactive forms, and, in case of H2O2,cals are far more complex. Under laboratory conditions,(iii) the presence of enzymes that reduce the oxidant’s re-OH radicals react with alkenes and aromatics at diffu-activity.sion-controlled rates (Watts et al., 2002). Hydroxyl radi-cals can also initiate radical reactions among organicHydrogen Peroxideradicals produced by hydrogen abstraction from C–HHydrogen peroxide is thermodynamically unstablebonds. There is evidence that OH radicals preferentiallyand decomposes into O2 and H2O according to Eq. [1]attack aromatic compounds: Westerhoff et al. (1999)(Pardieck et al., 1992). Decomposition of H2O2 increasesnoted that aliphatic structures of natural organic matterwith pH, being maximal close to the reagent’s pKa atreact slower with OH radicals than aromatic moieties.11.6 (Xiang and Lee, 2000).Xie and Barcelona (2003) reported a higher chemical

H2O2 � H2O2 → 2H2O � O2 (log k � 36.12) [1] resistance of low-molecular weight aliphatic hydrocar-bons (C5–C8) compared with aromatic compounds inWhen present in soils, reduced metal species can cata-jet-fuel contaminated sediments treated with H2O2.lyze the decomposition of H2O2 and thereby produceThese results are consistent with the general order ofOH radicals that are much more powerful oxidants thanreactivity: aromatic � –CH2– � –CO– � –COOH (Pey-H2O2 (Strukul, 1992). The Fe2�–catalyzed decomposi-ton, 1993) and may account for the enrichment of ali-tion of H2O2 into OH species according to Eq. [2] resem-phatic compounds in H2O2–treated soils (Schulten et al.,bles the classical Fenton reaction. Surface sites of oxides1996; Leifeld and Kogel-Knabner, 2001; COMPOSI-and montmorillonite (Fe2� or Fe3�) may similarly pro-TION OF ORGANIC MATTER section).duce OH radicals by Fenton-like reactions (Huang et

In addition, certain aromatic compounds may selec-al., 2001; Gournis et al., 2002; Kwan and Voelker, 2002;tively be removed by H2O2. Andreozzi et al. (2002)Petigara et al., 2002). Equations [2] through [6] repre-observed that phenols adsorbed to goethite containingsent the metal-catalyzed Haber-Weiss mechanism, wheretwo adjacent OH groups (or one OH and one NH2)the metal (M) is cycled between a lower [M(II)] and

higher [M(III)] oxidation state and where H2O2 is gradu- were oxidized by H2O2 while those containing two

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COOH groups or one COOH plus one OH group were phatic compounds in natural organic matter. Chakrabar-tty et al. (1974) used 12% (wt/wt) NaOCl (pH 12) tonot reactive toward H2O2.

Degradation of lignin strongly depends on the reac- investigate the structural composition of humic acidsextracted from soil and coal. They reported that reactiontion milieu. At alkaline pH, degradation of lignin is

more complete than at acid pH, possibly because of results in aliphatic carboxylic acids (malonic, succinic,glutaric, and adipic acid) and aromatic polycarboxylicmore reactive H2O2 species present and larger lignin

solubility (Sun et al., 2000; Xiang and Lee, 2000). This acids (�50% CO2, up to 32% non-volatile acids). Hypo-chlorite is presumed to cleave all methine, methylene,suggests that under acid conditions lignin components

may contribute to residual organic C in H2O2–treated and methyl goups activated by electron-withdrawingheteroatoms without simultaneously disrupting aro-soil. Chemical degradation of humic substances, lignin,

and simple carbohydrates by H2O2 yields a variety of matic systems.water-soluble compounds such as mono- and dicarbox-ylic low-molecular-weight organic acids (e.g., formic, Disodium Peroxodisulfateacetic, oxalic, and malonic acid), phenols and benzene-

In alkaline to slightly acid solutions, S2O82� reacts withcarboxylic acids (Craik, 1924; Kuchlein, 1932; Griffith

H2O according to Eq. [11] (Kolthoff and Miller, 1955)and Schnitzer, 1977; Xiang and Lee, 2000; Goldstone etto finally give SO4

2� by dissociation of HSO4�.al., 2002). Martin (1954) estimated that in two soil clays

reacted with H2O2, 30 to 40% of the initial organic C 2S2O2�8 � 2H2O → 4HSO4

� � O2 [11]was transformed into oxalate. Similarly, Harada and When heated, S2O2�

8 undergoes thermal homolysis orInoko (1977) using redox titration have calculated that reacts with reduced metals, for example, Fe2� ions, to100 g of soil yielded between 0.01 and 0.2 moles of yield SO4� radicals that can react with organic matteroxalate during H2O2 treatment. (Edwards and Curci, 1992). Hydroxyl radicals can alsoLess is known about the interaction of H2O2 with form when SO4

� radicals react with H2O and with OH�

N-containing compounds. Herriott (1947) reported that ions at alkaline conditions (Eq. [12] and [13]). Reactionat room temperature 5% (wt/wt) H2O2 caused a loss of [12] becomes negligible at dissolved organic C concen-60% of amino-N from virus protein. Harada and Inoko trations �1 mg L�1, where organic compounds are the(1977) showed that after treating soils with H2O2, resid- major sink for SO4� radicals (Peyton, 1993).ual N was mainly water-soluble (60–100%) and domi-

nated by inorganic N species (�70%). Ammonia is a SO�4 • � H2O → H� � SO2�

4 � OH• [12]product of the oxidative deamination of amino acids SO�

4 • � OH� → SO2�4 � OH• [13](Schnitzer and Hindl, 1980) and has been found after

Like OH radicals, SO4� radicals have a similar reactiv-oxidation of humic acid (14 mg NH3 out of 46 mg total

ity toward organic structures (Peyton, 1993). Larger,N) (Miles et al., 1985).more resistant phenolic compounds may be synthesizedby coupling of phenols during radical reactions inducedSodium Hypochloriteby SO�

4 radicals.In aqueous solution, NaOCl principally exists in the

following equilibria (Eq. [9] and [10]). EFFICIENCY OF ORGANICMATTER REMOVALOCl� � H2O ↔ HOCl � OH� [9]

Organic matter cannot completely be removed fromandsoils by wet oxidative treatments (Fig. 1). Early work

HOCl ↔ OCl� � H� [10] took advantage of this fact by using H2O2 for determina-tion of the degree of humification in soils (Robinson

When natural organic matter is treated with hypo- and Jones, 1925) and to gain information about thechlorite species (HOCl, OCl�), high-molecular-weight composition of organic matter (McLean, 1931a, 1931b).chlorinated compounds form in a first step that is fol- Progressing research revealed a number of factors re-lowed by the cleavage of benzene rings into trihalometh- sponsible for incomplete C removal such as soil reaction,anes and haloacetic acids (Jimenez et al., 1993; Li et al., presence of carbonates, chemically resistant organic2000; Pomes et al., 2000). Sodium hypochlorite-treated compounds, and protection of organic matter by min-humic acid yielded more chloroform than treated fulvic eral surfaces.acid (Peters et al., 1980). Trihalomethane and haloaceticacids likely derive from 3,5-dihydroxybenzene structures. Hydrogen PeroxideIn accordance, Norwood et al. (1987) noted that chlorina-tion of aquatic fulvic acid produced chlorinated aliphatics The extent of organic C removal by H2O2 varies with

soils and particle-size separates, ranging from �20%and resulted in preferential removal of lignin phenols.Chlorinated phenols were also found after reaction of (Bartlett et al., 1937) to �93% (Kahle et al., 2003).

Typical efficiencies of C removal are listed in Table 1.hypochlorite with humic and fulvic acid (Quimby et al.,1980), substituted benzoic acids (Larson and Rockwell, Hosking (1932) noted for a number of calcareous Aus-

tralian Black soils that 20% (wt/wt) H2O2 removed only1979), and after chlorination of lignin (Rajan et al., 1996).Westerhoff et al. (2004) observed a higher reactivity of 5 to 20% of organic matter when the soil reaction was

alkaline (pH 9–10) while 50 to 90% were removed whenaqueous chlorine for aromatic compounds than for ali-

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the soil pH was between 6 and 7.5. In soils containing could be removed by this procedure. Reduction of thenumber of repetitive treatments may decrease the re-Mn oxides, only �30% of organic matter was removed

because of rapid H2O2 decomposition. Hosking (1932) moval of organic matter in soils rich in poorly crystallineFe and Al phases (Siregar et al., 2004).and Anderson (1963) increased the C removal by re-

moving carbonates first, using HCl (pH 2–3) or a Naacetate buffer (pH 5). In calcareous soils, carbonate Disodium Peroxodisulfatecoatings favor occlusion and thus physical protection of

The efficiency of Na2S2O8 to remove organic matterorganic matter against destruction. Oxalate, which is ahas been proposed to be superior to H2O2 and NaOClcommon product of organic matter destruction (HY-(Meier and Menegatti, 1997; Table 1). When the authorsDROGEN PEROXIDE under section REACTION OFtested NaH2PO4 as buffer, C removal was less than whenOXIDANTS WITH INORGANIC AND ORGANICusing NaHCO3 buffer. This can be attributed to theMATTER), can form insoluble complexes with Ca2�,lower pH (drop from approximately 8 to 5) in case ofthus leaving a residual organic C fraction behind afterthe Na2S2O8–NaH2PO4 mixture, which disfavors desorp-H2O2 treatment. Moreover, HCO�

3 and CO2�3 ions are

tion of mineral-bound organic matter. Kiem and Kogel-known to inhibit organic matter degradation during wa-Knabner (2002) applied Na2S2O8 to particle-size frac-ter purification by scavenging OH radicals (Wang et al.,tions of loamy and sandy surface soils and found a2001). The same may hold true for H2O2 treatment ofcontact time of 16 h sufficient to reduce organic C con-calcareous soils where carbonates likely dissolve due tocentrations by about 93% (Table 1). Oxidative destruc-the production of organic acids.tion of humic acid (Martin et al., 1981) and organicUsing Na4P2O7 as a dispersing agent in the H2O2 treat-matter from aquifer sediments (Powell et al., 1989),ment, organic C removal is more complete than byhowever, not exceeded 50%. Thus, desorption of or-applying H2O2 alone (Simon et al., 1992). Sequi andganic matter by HCO�

3 or SO2�4 is likely the key stepAringhieri (1977) increased the average organic matter

in removal of organic C from soils. In contrast, Eust-removal by using H2O2–Na4P2O7 from 79 to 96%, likelyerhues et al. (2003) found that in some cases even 2 dby disruption of aggregates, thereby releasing occludedof reaction time were insufficient to destroy organic Corganic matter, and by displacing sorbed organic com-in acid soils rich in secondary minerals (Table 1). Thispounds from mineral surfaces. Pyrophosphate and alsoindicates that either chemically stable compounds repre-PO3�

4 sorb strongly and hysteretically to soil mineralssented a major organic fraction (not tested) or strong(Varadachari et al., 1995; Celi et al., 2000). Hence, when interactions with mineral surfaces limited desorption ofcarrying out a sorption experiment with soil treated with organic matter (PROTECTION OF ORGANIC MAT-a mixture of H2O2 and Na4P2O7, occupation of reactive TER BY SOIL MINERALS section under EFFI-sorption sites by pyrophosphate needs to be considered. CIENCY OF ORGANIC CARBON REMOVAL DE-Furthermore, organo–mineral complexes and chemi- PENDS ON SAMPLE PROPERTIES).cally stable compounds contribute to the resistance of

organic matter (EFFICIENCY OF ORGANIC CAR-EFFICIENCY OF ORGANIC CARBONBON REMOVAL DEPENDS ON SAMPLE PROP-

REMOVAL DEPENDS ONERTIES section).SAMPLE PROPERTIES

Sodium Hypochlorite Sample-specific properties like the presence of chemi-cally stable organic compounds and protective mineralAccording to Anderson (1963), the C removal effi-phases are also crucial for organic C removal efficiency.ciency of 6% (wt/wt) NaOCl at pH 9.5 was less affected

by the presence of carbonates (up to 9% CaCO3) thanComposition of Organic Matterwhen using 30% (wt/wt) H2O2. Several studies revealed

that 6% (wt/wt) NaOCl at pH 9.5 removed more C from Pyrogenic materials (black carbon) represent a groupbulk soils and clays than 30% (wt/wt) H2O2 (Anderson, of chemically resistant compounds in soils. They com-1963; Omueti, 1980; Cheshire et al., 2000). The NaOCl prise randomly oriented condensed aromatic rings lo-treatment does not require additional dispersing re- cated in graphite-like structures (Schmidt and Noack,agents because of dispersion by NaOCl itself. In spite 2000). Such materials can make up a significant portionof a higher electric potential of NaOCl with decreasing of organic matter in soils, especially in soils adjacent topH, C removal efficiency is maximal at pH 9.5 (Lavkul- coal-processing industries (Schmidt et al., 1999) or inich and Wiens, 1970). This indicates that desorption soils from technogenic materials (Zikeli et al., 2004).from mineral surfaces under alkaline conditions is deci- Hydrogen peroxide is ineffective to degrade graphite,sive for the removal of organic compounds. Desorption anthracite, lignite, charcoal, and ash (Robinson, 1927;may be of similar importance for organic matter removal Schmidt et al., 1999).as the oxidative breakdown of organic compounds, sug- Low-molecular-weight organic acids produced duringgesting the term “wet oxidation” to be misleading. Kai- oxidative treatments are relatively resistant against fur-ser and Guggenberger (2003) tested a modified NaOCl ther degradation (Luft and Stoffler, 1998) and thus be-treatment (pH 8, room temperature, five repetitions) come enriched during oxidative treatments. Analyzingon 196 heavy soil fractions (density �1.6 g cm�3) and H2O2–treated clay and bulk soil fractions by differential

thermal and infrared analysis, Farmer and Mitchellfound that between 77 to 95% of the initial organic C

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(1963) and Harada and Inoko (1977) found no evidence soil minerals. Contemporary work distinguishes be-tween the mechanisms of intercalation and sorptive pro-for resistant organic compounds other than oxalate. For-

mation of insoluble calcium oxalates is favored under tection.Intercalation includes replacement of hydrated in-alkaline conditions, while in acidic soils adsorption of

oxalate to variable charge minerals is more likely. organic interlayer cations of expandable 2:1 phyllosili-cate clay minerals by organic molecules. This requiresFarmer and Mitchell (1963) noted that in some cases

complexed oxalate couldn’t be extracted by H2O but by either cationic or neutral organics and a strongly acidicsoil reaction. Theng et al. (1992) and Righi et al. (1995)5% ethylenediaminetetraacetate. In contrast, Escudey

et al. (1999) suggested extensive washing with H2O after assumed that organic matter intercalated in expandableclay minerals is hardly decomposable by H2O2 due tothe H2O2 treatment to remove oxalate from soils com-

pletely but did not quantify their results. a limited accessibility toward the reagent. In contrast,Kodama and Schnitzer (1971) found no evidence thatSeveral studies confirmed that organic compounds

other than black carbon and oxalate could survive oxida- organic matter sorbed into the interlayer regions of mica-vermiculite-montmorillonite-interstratified clay resistedtive treatments (Righi et al., 1995; Schulten et al., 1996;

Cuypers et al., 2002). Aliphatic compounds are a signifi- 15% (wt/wt) H2O2.Eusterhues et al. (2003) observed that the concentra-cant portion of chemically resistant organic matter. Grif-

fith and Schnitzer (1977) estimated that n-alkanes and tion of Na2S2O8–resistant organic matter related posi-tively to the clay content (r � 0.93) in twelve horizonsn-fatty acids accounted for up to 40% of the H2O2–resis-

tant organic matter, while a substantial fraction of resid- from two acid soil profiles (Typic Haplorthod and TypicDystrochrept). In the Spodosol, a significant relationshipual C could not be extracted by organic solvents. Study-

ing the H2O2–resistant organic matter in mica–beidellite of residual organic C to dithionite–citrate–bicarbonate-extractable Fe (r � 0.90) suggests that the resistantinterstratified clay by Py-FIMS, Schulten et al. (1996)organic matter was likely associated with Fe (hydr)-found it to be enriched in N-containing compounds,oxides (sorptive protection). Similar results were re-n-C22–C26 carboxylic acids, n-alkanes, n-diols and alkyl-ported by Mikutta et al. (2004) for NaOCl-resistantsubstituted aromatic esters. Leifeld and Kogel-Knabnerorganic matter in clay subfractions from subsurface hori-(2001) showed by CPMAS 13C-NMR analysis that H2O2

zons of smectitic, vermiculitic, illitic, kaolinitic, and chlo-preferentially removed sugars (O-alkyl C) and ligninritic soils (Table 1). Despite the heterogeneous samplecompounds (mainly aromatic C) from �20-m particle-set, the concentration of residual organic C correlatedsize separates of agricultural soils leaving an aliphaticwell with oxalate-extractable Fe (Feo) and Al (Alo) (Fig.residue behind. In accordance, persulfate treatment evi-2a). Acid oxalate primarily extracts Fe and Al fromdenced the presence of aliphatic biopolymers in sedi-poorly crystalline aluminosilicates, ferrihydrite and Fementary and terrestrial humic acids (Saiz-Jimenez, 1992).and Al humus complexes (Wada, 1989).Martin et al. (1981) found that n-C16–C18 fatty acids were

The efficiency of poorly crystalline minerals to protectthe most abundant compounds after reaction of humicorganic matter against oxidative destruction was con-acid with acid K2S2O8. However, a variety of benzenecar-firmed in batch experiments. Singer and Huang (1993)boxylic and phenolic acids were also detected. Cuypersand Huang (1995) showed that humic, tannic, and citricet al. (2002) reported that persulfate preferentially re-acid associated with non-crystalline Al hydroxide weremoved labile and more amorphous organic matter whileonly partly destructible by H2O2 and NaOCl, while tan-the residuum was enriched with long-chain aliphatics.nic and citric acid were completely oxidized in the ab-The resistant organic matter was proposed to have asence of Al hydroxide. The higher resistance of mineral-more condensed structure, a higher affinity for hydro-associated organic compounds was explained by incor-phobic compounds and being more thermostable thanporation of organic compounds by coprecipitation oramorphous organic matter.by formation of strong surface complexes.Lower C/N ratios of H2O2–resistant organic matter

Mikutta et al. (2004) showed that the capability ofindicate a higher chemical stability of some N com-minerals in clay subfractions to protect organic matterpounds. Cheshire et al. (2000) found that some aminodecreases as more organic C is associated with mineralacids were protected against degradation possibly withinsurfaces. The C removal efficiency related closely tomicroaggregates or by interaction with mineral surfaces.the initial organic C concentration normalized to theInterpretation of C/N ratios after organic matter re-mineral SSA (C loading; Fig. 2b). This result is consis-moval is, however, questionable since mineral-boundtent with the finding that at small C loadings, organicNH�

4 or NH�4 produced during organic matter degrada-

matter occupies a larger surface area with more func-tion and subsequently fixed to minerals can also affecttional groups involved in multiple-site attachments withC/N ratios (Miles et al., 1985; Leifeld and Kogel-Knab-mineral surfaces (Kaiser and Guggenberger, 2003). Or-ner, 2001).ganic matter in direct contact to the mineral surfacethus appears less susceptible to chemical destruction.Protection of Organic Matter by Soil Minerals Eusterhues et al. (2003) provided further evidence thatthe C removal efficiency depends on the protective ca-Hosking (1932) recognized that more organic matter

resisted the H2O2 treatment in soils with higher clay pability of mineral surfaces. They observed that C re-moval by Na2S2O8 decreased with soil depth. Nearly allcontent. This suggests that organic matter can be pro-

tected against oxidative treatments by interaction with organic C was removed from A and EA horizons (�98%)

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while in the deeper B and C horizons 9 to 84% of theinitial organic C resisted Na2S2O8. This effect can beexplained by larger amounts of mineral-bound organicmatter and by stronger association of organic matterwith mineral surfaces with increasing soil depth. In sur-face horizons, there is much unprotected, particulateorganic matter and binding sites at mineral surfacesmay already be occupied by organic matter resulting inweaker bindings. In deeper horizons where less of themineral surface area is covered by organic compounds(small C loading), organic matter may sorb in a morespread-out, uncoiled conformation with more ligandsinvolved in direct contact with the mineral surface (Kai-ser and Guggenberger, 2003). As a result, desorptionof organic matter from mineral surfaces during oxidativetreatments is more difficult in subsoils than in topsoils.

TREATMENTS INDUCE MODIFICATIONSOF MINERAL CONSTITUENTS

The use of chemical destruction of organic matter isbased on the supposition that minerals are unaffected bythe treatments. Here we summarize reports on mineralchanges likely induced by the treatments.

Fig. 2. (a) Relation between the concentrations of residual organicThermal Effect on MineralsC and oxalate-extractable Fe plus Al in fine (�, �0.2 �m) andcoarse clay (�, 0.2–2 �m) fractions of 12 acid subsoil samples withMost treatments are conducted under elevated tem-different mineralogy after treatment with 6% (wt/wt) NaOCl. (b)peratures (60–100�C) (Table 1). Kaiser and Guggen-Relationship between the organic C loading of mineral surfaces andberger (2003) showed that temperatures �40�C lasting the amount of removable organic C (data adopted from Mikutta et

several hours can transform moist amorphous Al hy- al., 2004).droxide into gibbsite while temperatures �80�C con-verted ferrihydrite into hematite. The total SSA of both the interlayer spaces of phlogopite (mica with low Femineral phases decreased on heating by about 90%, content) and vermiculite via exchange with H2O andpartly due to the entire loss of microporosity (�2 nm). cations and decomposes into O2 and H2O (Ucgul andSo even moderate heating can alter oxide surface prop- Girgin, 2002; Obut and Girgin, 2002). The gas evolvederties and thus gives rise to artifacts, at least in soils rich can disrupt individual silicate layers. Increasing thick-in poorly crystalline components. The only procedure ness of the minerals (80–120 fold) was noted with in-reported to be efficient under ambient temperature is creasing H2O2 concentration (1–50%), temperature (40–the modified NaOCl treatment proposed by Kaiser et 60�C), and contact time (1–30 h). Above 60�C, phlogopiteal. (2002) (Table 1). However, the method is time-con- started to exfoliate as described by Drosdoff and Milessuming and laborious. (1938), possibly because of accelerated decomposition

of H2O2 with increasing temperature. However, treat-ment with 30% (wt/wt) H2O2 (60�C, 70 min) caused noHydrogen Peroxidephase change of phlogopite according to x-ray diffrac-

Phyllosilicates tion (XRD) (Ucgul and Girgin, 2002).Douglas and Fiessinger (1971) showed by XRD thatMany studies on the effects of H2O2 on individual

minerals were conducted outside the field of soil re- the (001) signals of smectite and vermiculite decreasedafter reaction with H2O2 in presence of large amountssearch (Hayashi and Oinuma, 1964; Muromtsev et al.,

1990; Ucgul and Girgin, 2002). The use of unweathered of sucrose (168 g C kg�1 clay) while little effect wasobserved without sucrose or in presence of sodium ace-minerals and the variety of procedures applied, compli-

cates the transferability to soil systems. Despite this, tate buffer (pH 5). They inferred that both minerals,especially vermiculite, were partly destroyed due to thesome general remarks can be made.

Drosdoff and Miles (1938) first noted destruction of low pH (pH 1.8–3) induced by incomplete oxidation ofsucrose. However, decreasing (001) basal peak reflec-mica and some vermiculite samples when treated with

6 and 30% (wt/wt) H2O2. Mineral exfoliation resulted tions following H2O2 treatment may also result fromresidual organic matter producing less than perfect ori-from catalytic decomposition of H2O2 by Mn oxides

located in mineral interlayers. Amonette et al. (1985) entation of clay specimen (Dohrmann, 2003).Miles et al. (1985) observed NH4

� produced duringshowed that H2O2 penetrates biotite interlayer spacesexpanded by tetraphenylborate and oxidizes 85% of the organic matter degradation to sorb to exchange sites of

vermiculite, inducing collapse of the 1.48-nm spacings tostructural Fe2� ions within 24 h. Also, H2O2 diffuses into

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Fig. 4. X-ray diffractograms of Mg-saturated, glycerated, and orientedFithian illite (I) (�2 �m) before and after treatment with 10%(wt/wt) H2O2 (Fithian illite: 85% illite, 15% mixed layers, kaolinite,quartz). The treatment was conducted in the presence of straw andmanure (S, M, diam. 0.1 mm). Before treatment, organo–mineralassociations (5% organic matter) were prepared by three dryingand wetting cycles. Treatments: (a) no, (b) I � H2O2, (c) I � H2O2 �S, and (d) I � H2O2 � M.

Fig. 3. (a) Scanning electron microscope (SEM) image of oxide-free(dithionite–citrate-treated) Prassa-Kimolos bentonite (Greece),

(1997) showed that deposit illites were not attacked by(b) oxide-free bentonite treated with 30% (wt/wt) H2O2 (6 h, 80�C)in the presence of 230 mg L–1 dissolved organic C. Bars represent H2O2. We studied illite treated with 10% (wt/wt) H2O2100 �m. (80�C, 120 h) in the presence and absence of organic

matter. Before treatment, clay specimens were sub-the 1.03-nm position characteristic for mica-like phases. jected to drying–wetting cycles in the presence of ma-Infrared spectroscopy confirmed NH4

� fixation by the nure and straw (0.1 mm) to facilitate the formation oftreated vermiculite. Therefore, the H2O2 treatment is clay–organic associations. Using XRD, we observed nonot suitable for XRD-based identification of vermiculite structural change of illite induced by H2O2 treatmentin soils. (Fig. 4).

Van Langeveld et al. (1978) inferred from reduced The results compiled above indicate that the alter-XRD reflections that montmorillonite (6% organic C) ation of phyllosilicates by H2O2 depends on (i) the pres-was structurally altered during H2O2 treatment. Figure ence of catalyzing materials, (ii) the existence of accessi-3a and 3b display the transformation of bentonite on ble interlayer mineral surfaces, and (iii) the abundancetreatment with 30% (wt/wt) H2O2 at 80�C for 6 h in pres- of organic matter. The results imply that pure depositoryence of dissolved organic matter. Compared with un- minerals not weathered in the soil environment and thustreated particles, the treated ones show distinctively lacking structural modifications such as cracks, microfis-frayed structures suggesting corrosion likely caused by sures, and expansion zones as well as sorbed organicdecomposing H2O2 and acidic organic matter degrada- matter are little affected by treatment with H2O2.tion products.

In contrast, Hayashi and Oinuma (1964) found noCarbonates, Oxides, and Sulfidesindications for major alterations of repository clays (ver-

miculite, montmorillonite, illite, chlorite) treated with In calcareous soils, H2O2 treatment may cause dissolu-tion and corrosion of carbonate minerals because of the30% (wt/wt) H2O2 at 80 to 90�C over several days. Mu-

romtsev et al. (1990) recognized no structural alteration low pH of H2O2 (Pingitore et al., 1993). Typically, Mnoxides are destroyed during the H2O2 treatment (Shu-of pure vermiculite treated with 30% (wt/wt) H2O2 by

applying XRD analysis, nevertheless the CEC doubled man, 1983; Papp et al., 1991; Table 2). Manganese (III,IV) oxides are affected by reduction to Mn2�, which isbecause of layer expansion. Rosenberg and Hooper

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Table 2. Destruction of Mn oxide and sulfide minerals by oxidative treatments (data adopted from Papp et al., 1991).

Mn and Fe extracted, % Cu and Zn extracted, %

Pyrite Pyrrhotite Chalcopyrite Chalcocite Chalcopyrite SphaleriteTreatment MnO2 FeS2 FeS CuFeS2 Cu2S CuFeS2 ZnS

H2O2† 97 92 46 39 99 88 80H2O2 and Na4P2O7‡ 98 41 21 75 99 97 93NaOCl¶ 1.7 0.8 0.9 8.7 4.2 2.3 1.9

† 1 g sample � 8 mL of 3:2 mixture of 30% (wt/wt) H2O2 and 0.025 M HNO3.‡ Conducted at pH 6.5 according to Sequi and Aringhieri (1977); Table 1.¶ Modified from Lavkulich and Wiens (1970); followed by acid wash.

subsequently oxidized to yield MnOOH (Pardieck et tain mineral constituents like amorphous Al(OH)3 (Par-al., 1992), Mn3O4 (Jackson, 1958), or a mixture of differ- fitt and Childs, 1988; Kaiser and Zech, 1996). In addition,ent Mn oxides (Moon et al., 1999). Sulfide minerals are pyrophosphate may induce precipitation of Ca phos-readily dissolved by reaction with H2O2 (Mukherjee et phates (Hawke et al., 1989; Celi et al., 2001) and Alal., 2001). Papp et al. (1991) showed that H2O2 dissolved phosphates (e.g., poorly crystalline Al phosphate, tara-21 to 99% of the total metals present in sulfides nakite, wavellite) (Kim and Kirkpatrick, 2004).(Table 2). Consequently, H2O2 should not be used toestimate organically bound metals when sulfide miner- Sodium Hypochloriteals are present, for example in acid sulfate soils fromcoastal lowlands (Shamshuddin et al., 2004). In general, Mn oxides and sulfide minerals are dis-

Iron and Al (hydr)oxides may be affected during the solved to a lesser extent by NaOCl compared with H2O2H2O2 treatment by heat-induced transformation into (Table 2). Lavkulich and Wiens (1970) found that, atmore crystalline forms and by organic-assisted disso- pH 9.5, NaOCl dissolved significantly less oxalate-extract-lution at low pH. At alkaline pH, for example, in cal- able Fe and Al than H2O2 (Table 3). This can be ex-careous soils or when using H2O2 in combination with plained by the alkaline pH of NaOCl, which preventsNa4P2O7, oxides remain unaltered (Marzadori et al., 1991). acid-induced mineral dissolution and may probably sup-At low pH (pH 2–4), large concentration of oxalate, port hydrolysis and precipitation of hydroxides (METALand high temperature, the H2O2 treatment is comparable PRECIPITATION section). However, some Al can beor even more aggressive than the acid-oxalate method dissolved at pH 9.5, for example in the Alouette Ap(pH 3, room temperature, darkness, 4 h) designed for horizon (Table 3). Using the method of Lavkulich andextraction of poorly crystalline minerals and Fe and Al Wiens (1970), Osei and Singh (1999) reported that nobound to organic matter (Blakemore et al., 1987). For Fe was released from tropical surface soils by NaOClacid soils treated with 30% (wt/wt) H2O2, Lavkulich while extracted Al and Si accounted for up to 0.3 g kg�1.and Wiens (1970) reported that the amount of oxalate- Negligible amounts of Fe and Al were dissolved duringextractable Fe and Al decreased by 34 to 80% and 1 to treatment of a Mollisol surface soil with 5.3% (wt/wt)90% (II–H2O2 treatment, Table 3). They assumed that in- NaOCl (Qiang et al., 1994; Table 1). In soils rich inorganic Fe and Al phases dissolved during the H2O2 poorly crystalline minerals, 6% (wt/wt) NaOCl at pH 8treatment. However, mass balance calculations utiliz- (room temperature, three repetitions) dissolved no Feing selective extractions prior and after H2O2 treatment and �3% of dithionite-citrate extractable Al and Sisuggest that dissolution of poorly crystalline Fe and Al (Siregar et al., 2004). These findings support the viewcomponents alone does not entirely explain the smaller that crystalline oxides and silicates are not affected byFeo and Alo concentrations after H2O2 treatment (Table 3).

NaOCl.More likely, they result from heat-induced recrystalliza-However, NaOCl treatments at higher temperaturestion of poorly crystalline oxides. However, after mild

may induce changes in extractable pedogenic Al andH2O2 treatment (I-H2O2 treatment; Table 3), the massFe (e.g., Langley Ap; Table 3) due to transformationbalance was positive indicating higher extraction effi-into more crystalline forms. In contrast, Marzadori etciency of acid oxalate after the H2O2 treatment due toal. (1991) studying calcareous soils found an increase indisaggregation and removal of organic matter coatingsFeo and Alo after destruction of organic matter by 7%from oxide surfaces (Marzadori et al., 1991). When test-(wt/wt) NaOCl despite heating to 80�C (Table 1). In thating the influence of 10% (wt/wt) H2O2 on six acid subsoilcase, the thermal transformation of a poorly crystallinesamples, we found an average decrease of Feo and Alooxide fraction was probably overcompensated by anconcentrations by approximately 90% (Table 3). Alumi-increased extractability of poorly crystalline phases duenum and Fe in reacted H2O2 solutions were little (Ta-to the removal of organic coatings. Mayer (1999) foundble 3), suggesting the decrease in oxalate-extractablean increased enthalpy for the adsorption of N2 on marinecomponents to be related to the transformation of min-sediments treated with 13% (wt/wt) NaOCl at pH 9 toerals at high temperature (80�C).9.5. The change in adsorption enthalpy was attributedPyrophosphate, when used in conjunction with H2O2

to microtopographic or chemical changes of mineralas a dispersing reagent to enhance C removal, can causesurfaces due to the highly alkaline reaction conditions.loss of peptized fine mineral particles, especially in pres-

ence of adsorbed organic matter, or dissolution of cer- This effect warrants further research.

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Table 3. Effect of H2O2 and NaOCl treatment on oxalate-extractable Fe and Al fractions in acid bulk soils. Negative values in the ‘massbalance’ column indicate that poorly crystalline components were transformed during heating into more crystalline forms. Data forthe H2O2 (30%) and the NaOCl treatment adapted from Lavkulich and Wiens (1970).

After treatment in After treatment inInitial residues extracts Mass balance†

Soil Horizon Feo Alo Feo Alo Fe Al Fe Al

g kg�1

30% (wt/wt) H2O2 (Treatment I; see ‘Material’ column in Table 1)

Abbotsfort Ap 7.6 10.0 6.3 8.6 0.06 1.9 �1.2 0.5Abbotsfort Bir 3.5 7.8 3.1 7.0 N.D.‡ 0.2 �0.4 –0.6Alouette Ap 7.8 10.2 7.7 3.6 1.2 3.1 1.1 –3.5Cloverdale Ap 8.6 4.6 11.8 3.0 0.9 3.5 4.1 1.9Cloverdale C 4.2 2.6 3.9 2.6 0.1 0.3 �0.2 0.3Langley Ap 15.0 12.0 17.9 6.6 0.7 3.6 3.6 –1.8Whatcom Ap 9.9 6.7 11.6 4.5 0.5 2.8 2.2 0.6Whatcom Bir 12.0 16.8 11.2 18.0 0.01 0.3 �0.8 1.5

30% (wt/wt) H2O2 (Treatment II; see ‘Material’ column in Table 1)

Abbotsfort Ap 7.6 10.0 3.8 8.3 0.1 1.8 �3.7 0.1Abbotsfort Bir 3.5 7.8 2.3 7.7 0.02 0.6 �1.2 0.5Alouette Ap 7.8 10.2 1.6 1.1 7.2 11.5 1.0 2.4Cloverdale Ap 8.6 4.6 5.3 2.7 1.7 4.0 �1.6 2.1Cloverdale C 4.2 2.6 1.9 2.3 0.04 0.1 �2.3 –0.2Langley Ap 15.0 12.0 8.9 6.2 1.9 8.0 �4.2 2.2Whatcom Ap 9.9 6.7 5.8 3.4 2.0 5.8 �2.1 2.5Whatcom Bir 12.0 16.8 6.0 14.5 0.2 1.2 �5.8 –1.1

6% (wt/wt) NaOCl (according to Anderson, 1963; Table 1)

Abbotsfort Ap 7.6 10.0 7.3 9.7 0.03 0.1 –0.3 –0.2Abbotsfort Bir 3.5 7.8 3.3 6.0 N.D. 0.1 –0.2 –1.7Alouette Ap 7.8 10.2 4.9 5.7 0.7 1.7 –2.2 –2.8Cloverdale Ap 8.6 4.6 8.7 4.4 0.1 0.2 0.2 0Cloverdale C 4.2 2.6 4.7 3.0 N.D. 0.0 0.5 0.4Langley Ap 15.0 12.0 11.1 7.0 0.2 0.5 –3.7 –4.5Whatcom Ap 9.9 6.7 11.3 7.2 0.2 0.6 1.6 1.1Whatcom Bir 12.0 16.8 12.2 16.0 0.01 0.02 0.2 –0.8

10% H2O2 (wt/wt) (according to Tributh and Lagaly, 1991; Table 1)

Rottleberode Bw 2.3 1.0 0.2 0.1 0.01 0.1 �2.1 –0.8Kyffhauser E 1.1 1.5 0.1 0.1 0.3 0.7 �0.7 –0.7Kyffhauser Bt 1.1 1.9 0.2 0.2 0.4 0.8 �0.5 –0.9Bahrental Bw 2.9 2.5 0.3 0.2 0.4 1.2 �2.2 –1.1Hewenegg AB 8.9 3.4 0.9 0.4 0.3 0.2 �7.7 –2.8Kohlerwald Bt 13.4 13.3 1.4 1.2 N.D. 1.8 �12.0 �10.3

† Mass balance (Fe, Al) (g kg�1) � (Feo, Alo)residuum � (Fe, Al)extract � (Feo, Alo)initial.‡ N.D. � not detected.

Disodium Peroxodisulfate phase properties and organically bound metal fractions.Sequi and Aringhieri (1977) noted that Fe and Al associ-Using XRD, IR spectroscopy and N2 adsorption,ated with organic matter are released during H2O2 oxi-Menegatti et al. (1999) found no structural alterationdation and subsequently precipitate as hydroxides onof Na2S2O8–treated illite, kaolinite, and montmorilloniteinorganic and organic components. These precipitatesreference minerals (�0.2 and �2 m). Minor losses ofare assumed to alter SSA and charge of the mineralCa, K, and Mn were explained by partial removal ofphase and to protect organic matter against further deg-fine-grained accessory minerals such as feldspars andradation. The formation of metal precipitates during thecarbonates. The SSA of most treated samples remainedH2O2 treatment was inferred from the increase in SSAunaltered while the SSA of montmorillonite increased(N2–BET) and positive surface charge, and the concomi-by �20 m2 g�1. This was attributed to the dispersiontant decrease of negative charge. However, it is moreof large aggregates. The CEC of the treated mineralslikely that the alteration of surface charge derived fromremained unchanged. However, HCO�

3 as used for pHthe removal of organic matter carrying much negativecontrol may partly be converted into CO2�

3 on heating,charges located in carboxylic groups and from exposurewhich at 80�C, can extract Al from allophanes and hy-of positively charges on oxide surfaces. Despite that,droxides (Wada, 1989). Thus, if used for organic matterprecipitation of metals formerly complexed with organicremoval from soils where these phases are abundant, formatter cannot be ruled out. Schultz et al. (1999) pre-example volcanic ash soils and Spodosols, this potentialferred H2O2 instead of alkaline NaOCl to assess organi-defect requires further attention.cally bound metals in carbonate-free sediments to avoidmetal precipitation (Table 1). Addition of NH4 acetateMETAL PRECIPITATION during the H2O2 treatment was intended to prevent re-adsorption of metals to the solids. Hoffman and FletcherThe question whether metal ions complexed with or-(1981) showed that NaOCl (6.5%, pH 9.5) induced littleganic matter are released and precipitate during oxida-

tion is relevant for the reliable assessment of mineral precipitation of trace metals (Cu, Zn, Fe, Mn, Mo) when

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inorganic precipitates from soils and sediments were surface-controlled processes (e.g., adsorption, diffusion).Hydrogen peroxide has been shown to disintegrate mi-redissolved by acidified H2O. In spite of the alkaline

conditions, soluble organic compounds can effectively cas, vermiculites, and smectites. Phyllosilicate disinte-gration may become more significant with increasingscavenge metals released and thus hinder their precipi-

tation. In accordance, Shuman (1983) showed for six H2O2 concentration, temperature and in the presenceof decomposable organic matter. In soils, destructionbulk soils that 5% (wt/wt) NaOCl (pH 8.5) dissolved

similar amounts of metals (Cu, Fe, Zn) from the organic of phyllosilicates might change the surface propertiesbut little experimental evidence has been given. Forfraction as 30% (wt/wt) H2O2 while H2O2 extracted sig-

nificantly more Mn. In summary, there is no consistent example, Theng et al. (1999) hypothesized the SSA in-crease of a smectitic soil following H2O2 treatment toperception on the degree of precipitation of organically

bound metals released during organic matter destruc- result from destruction of smectite. Treatment withNaOCl produced only a minor increase in SSA althoughtion and on the potential effects on mineral properties.both reagents removed similar amounts of organic mat-ter. Compared with H2O2, the effects of NaOCl or

SYNTHESIS Na2S2O8 on phyllosilicates seem negligible, but morestudies on soils are needed to confirm that.This review shows that a large number of protocols

Poorly crystalline constituents are most susceptiblefor removal of organic matter by H2O2, NaOCl, andto alteration during treatments for organic matter re-Na2S2O8 is available. Removal of soil organic mattermoval. In organic matter-rich soils treated with H2O2,is never complete and largely relies on the reactionorganic oxidation products like low-molecular-weightconditions chosen and the sample properties. Sodiumorganic acids may assist mineral dissolution at low pH.hypochlorite and Na2S2O8 are more effective in organicDuring the alkaline NaOCl procedure (pH 9.5), Al fromC removal than H2O2, especially in calcareous soilshydrous oxides can dissolve. This effect can be avoidedwhere oxidation-resistant Ca oxalates may form. Car-by using NaOCl at lower pH. Moreover, temperaturesbon removal efficiency can be increased by desorbing�40�C applied during organic matter removal involvesreagents such as NaHCO3 and Na4P2O7, but those com-the risk of recrystallization of poorly crystalline Alpounds strongly sorb to minerals, which needs to bephases, while temperatures �80�C may convert poorlyconsidered in sorption experiments with treated sam-crystalline into more crystalline Fe oxides. Since poorlyples. In soils with large portions of mineral-bound or-crystalline phases significantly contribute to the physicalganic matter, Fe and Al (hydr)oxides and other poorlyand chemical properties of soils (Percival et al., 2000;crystalline constituents, even NaOCl and Na2S2O8 failKiem and Kogel-Knabner, 2002), more effort shouldto reduce the organic C concentrations effectively. Clay-be put on that question. At present, only the NaOClsized Fe and Al phases provide large SSA and reactivetreatment has been shown to remain efficient at roomhydroxyls favoring ligand exchange reactions with or-temperature and thus avoids heat-induced transforma-ganic matter (Gu et al., 1995). The resulting mineral–tion of sensitive mineral phases.organic attachments are difficult to break, especially

Metals released during degradation of organic matterwhen a single molecule is attached to multiple mineralmay precipitate, and thereby possibly reduce the C re-surface sites, thus hampering desorption of organic mat-moval efficiency and the amount of organically boundter, which is an important prerequisite for organic mat-trace metals in sequential extraction studies, and mayter removal. However, sorptive protection of organicalso affect surface properties of minerals. However, lit-matter by minerals is hardly discernable from protectiontle awareness exists of that problem. Hydrogen peroxideof organic matter due to occlusion in microaggregatesand NaOCl seem to induce similar metal precipitation,or coprecipitation with hydrous oxides. Since intercala-with the degree of precipitation depending on the pH,tion is confined to expandable clay minerals and stronglymetal content and the amount of organic matter. In-acid conditions, the relevance of protected organic com-organic precipitates may probably become relevant forpounds in interlayer spaces is presumably small. Organicsoils and particle-size separates rich in organically com-matter resistant to oxidative treatment likely representsplexed metals such as Andisols and Spodosols.a refractory C pool since it seems to be older than bulk

organic matter before treatment (Theng et al., 1992).Similar composition of residual organic matter after IMPLICATIONSchemical and biological degradation suggests the treat-ment with oxidative reagents to mimic biodegradation At present, the knowledge on oxidative removal of

soil organic matter is incomplete with respect to the ef-(Rihani et al., 1995; Cuypers et al., 2002). However,selective degradation of certain structures of organic fects of oxidants on soil minerals. Therefore, systematic

studies on changes of mineral properties on oxidativematter by the three reagents (aromatics versus aliphat-ics) and, in case of H2O2, the likely adsorption of organic removal of soil organic matter are required. These stud-

ies should involve pure minerals (poorly crystalline ox-intermediates deriving from labile organic matter (e.g.,oxalate) to minerals needs to be considered. ides, expandable clay minerals), defined organo–mineral

complexes and soils with a wide range of properties (min-Based on the evidence shown, changes of mineralphase properties when treated with oxidative reagents eral composition, pH, organic matter content). Conclu-

sions on the feasibility of a certain oxidant/protocol toseem inevitable. This attributes some ambiguity to stud-ies concerned with soil mineralogical composition or remove organic C are possible only when several oxidants

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Table 4. Oxidative treatments in soil analysis. Advantages and drawbacks of reactants: (�) � suitable, (�) � not recommended,(�/�) � ambiguous.

Purpose Hydrogen peroxide Sodium hypochlorite Disodium peroxodisulfate

Textural analysis �/� uncertain for soils with high contents �/� little effect on silicates, �/� large reactant-soil andof vermiculite, mica and smectite formation of haloorganics reactant-buffer ratio

Mineralogical analysis � dissolution of carbonates � little effect on silicates and � no effect on reference phyllosilicateand determination of � destruction of smectitic, vermiculitic oxides (pH 8, 25�C) claysmineral properties and micaceous minerals � dissolution of Al hydroxides �/� bicarbonate buffer may extract

� transformation of vermiculites into (pH 9.5) allophan and hydroxidesmica-like minerals by NH4

� fixation � heating may alter poorly � heating may alter poorly crystalline� dissolution of poorly crystalline crystalline oxides (100�C) oxides

oxides in acid soils (especially whenNa4P2O7 is used as dispersant)

� heating may alter poorly crystallineoxides

Extraction of organically- � dissolution of Mn oxides, sulfide � little effect on Mn oxides, not used for this purposebound metals minerals, and poorly crystalline sulfides and poorly

oxides in acid soils crystalline minerals�/� alkaline pH may favor

metal precipitation (exceptin soils rich in OM)

Amonette, J., F.T. Ismail, and A.D. Scott. 1985. Oxidation of iron inare comparatively applied to the same sample collective.biotite by different oxidizing solutions at room temperature. SoilWhen modifying oxidation protocols, mineral alterationsSci. Soc. Am. J. 49:772–777.

should be elucidated by using combined methods like Anderson, J.U. 1963. An improved pretreatment for mineralogicalXRD, IR spectroscopy, gas adsorption (e.g., N2, CO2), analysis of samples containing organic matter. Clays Clay Miner.

10:380–388.and selective dissolution techniques.Andreozzi, R., A. D’Apuzzo, and R. Marotta. 2002. Oxidation ofTable 4 may serve as a guide for the use of reagents

aromatic substrates in water/goethite slurry by means of hydrogenfor organic matter removal. For textural analysis, H2O2 peroxide. Water Res. 36:4691–4698.is adequate when the soil mineral phase is not domi- Balesdent, J. 1996. The significance of organic separates to carbon

dynamics and its modelling in some cultivated soils. Eur. J. Soilnated by mica, vermiculite or smectites. This recommen-Sci. 47:485–493.dation must be used with caution since the impact of

Bartlett, J.B., R.W. Rubble, and R.P. Thoma. 1937. The influence ofmineral disintegration on particle-size distribution hashydrogen peroxide treatment on the exchange capacity of Maryland

not yet been tested. Sodium hypochlorite is impractical soils. Soil Sci. 44:123–128.for this purpose because of the formation of haloorgan- Blakemore, L.C., P.L. Searle, and B.K. Daly. 1987. Methods for chemi-

cal analysis of soils. New Zealand Soil Bureau. Scientific Rep. 80.ics that have to be disposed separately. Similarly, theDep. of Scientific and Industrial Research, Lower Hutt, Newlarge amounts of oxidant needed render the Na2S2O8 Zealand.procedure unsuitable for organic matter removal before Cavallaro, N., and M.B. McBride. 1984. Effect of selective dissolution

textural analysis. For organic C removal before sorption on charge and surface properties of an acid soil clay. Clays ClayMiner. 32:283–290.experiments and when surface properties of soils and

Celi, L., M. Presta, F. Ajmore-Marsan, and E. Barberis. 2001. Effectsminerals (SSA, CEC) are assessed, we recommend theof pH and electrolytes on inositol hexaphosphate interaction withuse of NaOCl (pH 8, 25�C) instead of 30% (wt/wt) H2O2 goethite. Soil Sci. Soc. Am. J. 65:753–760.

since phyllosilicate disintegration and heat-induced trans- Celi, L., E. Barberis, and F.A. Marsan. 2000. Sorption of phosphateformations of minerals are kept to a minimum. For quan- on goethite at high concentrations. Soil Sci. 165:657–664.

Chakrabartty, S.K., H.O. Kretschmer, and S. Cherwonka. 1974. Hypo-tification of clay minerals, H2O2 should be avoidedhalite oxidation of humic acids. Soil Sci. 117:318–322.because of the transformation of vermiculites into mica-

Cheshire, M.V., C. Dumat, A.R. Fraser, S. Hiller, and S. Staunton.like minerals by NH�4 fixation. The use of Na2S2O8 is 2000. The interaction between soil organic matter and soil clay

appropriate for clay preparation before XRD analysis minerals by selective removal and controlled addition of organicmatter. Eur. J. Soil Sci. 51:497–509.since it seems not to alter the properties of reference

Cornell, R.M., and U. Schwertmann. 1996. The iron oxides. Structure,clays. The application of NaOCl (pH 9.5) should beproperties, reactions, occurrences and uses. VCH, Weinheim.avoided for soils containing interlayered Al and Al hy-

Craik, J. 1924. The mechanisms of the oxidation of typical carbohy-droxides. Before further analysis, a test for the residues drates. J. Soc. Chem. Ind. 43:171–177.of organic C and the reagents used is recommended. Cuypers, C., T. Grotenhuis, J. Joziasse, and W. Rulkens. 2000. Rapid

persulfate oxidation predicts PAH bioavailability in soils and sedi-ments. Environ. Sci. Technol. 34:2057–2063.ACKNOWLEDGMENTS

Cuypers, C., T. Grotenhuis, K.G.J. Nierop, E.M. Franco, A. de Jager,We are grateful to Christian Mikutta (Berlin University of and W. Rulkens. 2002. Amorphous and condensed organic matter

Technology) and Adelina Siregar for comments and discus- domains: The effect of persulfate oxidation on the composition ofsion. Adelina Siregar provided the XRD diffractograms of soil/sediment organic matter. Chemosphere 48:919–931.

De Jonge, H., L.W. de Jonge, and M.C. Mittelmeijer-Hazeleger. 2000.H2O2–treated illite. This study was funded by the DeutscheThe microporous structure of organic and mineral soil materials.Forschungsgemeinschaft priority program SPP. 1090 “Soils asSoil Sci. 165:99–108.sources and sinks for atmospheric CO2”.

Dohrmann, R. 2003. The stability of clay minerals in acidic watercontaining organic substances and XRD detection of clay modifiedREFERENCES by organic substances. Z. Angew. Geol. 2:18–21.

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