Soil Chemistry Effect of pH and Weathering Indices on the ... · Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou 510640, PR China and Graduate School of

Soil Science Society of America Journal

Soil Sci. Soc. Am. J. 76:1579–1591doi:10.2136/sssaj2011.0225Received 15 June 2011.*Corresponding author ([email protected]). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USAAll rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Effect of pH and Weathering Indices on the Reductive Transformation of 2-Nitrophenol in South China

Soil Chemistry

Iron is one of the most important chemical elements found in soil (Cornell and Schwertmann, 1996). Iron in minerals and soil solution infl uences soil genesis and properties, as well as the transformation of contaminants

(Schwertmann, 1988; Lovley et al., 2004). Iron cycling in soil is important in the transformation of contaminants in soil environments, including chlorinated aliphatic and nitroaromatic compounds, certain heavy metals, and radionuclides (Pecher et al., 2002; Li et al., 2008b).

A number of studies have reported that mineral-bound Fe(II) species sub-stantially promote the reductive transformation of nitroaromatic compounds to the corresponding anilines under abiotic conditions (Hatter, 1985; Klausen et al., 1995; Stumm and Sulzberger, 1992; Rügge et al., 1998; Hofstetter et al., 1999; Klupinski et al., 2004; Colón et al., 2006; Naka et al., 2006). In addition,

Liang TaoGuangdong Key Lab. of Agricultural Environment Pollution Integrated ControlGuangdong Institute of Eco- Environmental and Soil SciencesGuangzhou, 510650, PR China

Wei ZhangGuangdong Key Lab. of Agricultural Environment Pollution Integrated ControlGuangdong Institute of Eco- Environmental and Soil SciencesGuangzhou, 510650, PR China

andGuangzhou Institute of GeochemistryChinese Academy of SciencesGuangzhou 510640, PR China

andGraduate School of The Chinese Academy of SciencesBeijing 100039, PR China

Hui LiSchool of Computer ScienceSouth China Normal Univ.Guangzhou 510631, PR China

FangBai Li*WeiMin Yu

Guangdong Key Lab. of Agricultural Environment Pollution Integrated ControlGuangdong Institute of Eco- Environmental and Soil SciencesGuangzhou, 510650, PR China

ManJia ChenGuangdong Key Lab. of Agricultural Environment Pollution Integrated ControlGuangdong Institute of Eco- Environmental and Soil SciencesGuangzhou, 510650, PR China

andGuangzhou Institute of GeochemistryChinese Academy of SciencesGuangzhou 510640, PR China

andGraduate School of The Chinese Academy of SciencesBeijing 100039, PR China

Iron is the fi rst abundant transition metal in the Earth’s crust; soils in subtropical and tropical zones contain a large amount of free Fe oxide and have thus been paid increasing attention. In this study, 22 soil samples were collected from the A (0–20-cm) horizon in South China and divided into three groups (i.e., vegetable soils in Pearl River Delta, orchard soils in Pearl River Delta, and forest soils in tropical zone). Two types of experiments, one without buffer 2-(N-morpholino)ethanesulfonic acid (MES) and another with MES, were conducted to investigate the effect of pH and weathering indices on the reductive transformation of 2-nitrophenol (2-NP) on soils. Kinetic measurements showed an increase in pH resulted in an enhanced reaction rate (k) of 2-NP reduction. From cyclic voltammogram tests, the enhanced activity of Fe(II) species was attributed to the negative shift of peak oxidation potential of the Fe(III)–Fe(II) couple. The results were subjected to statistical analysis, including the evaluation of variance and correlation, and the application of stepwise regression. Signifi cant differences in k values were obtained in the different soil types and parent materials. The reaction pH proved to be an essential factor in 2-NP reductive transformation, and the weathering indices of soils had a critical effect on the 2-NP reductive transformation processes in soils. With an increase in the level of soil desilicifi cation and allitization from Groups I to III, the k value increased consistently in the order of the decrease in the weathering indices. These fi ndings improve our general understanding of the effect of pH and weathering indices on soil Fe redox chemistry and provide valuable information on the implications for research on soil pollution control.

Abbreviations: 2-AP, 2-aminophenol; 2-NP, 2-nitrophenol; CV, cyclic voltammetry; DDW, deaerated, deionized water; Fed, citrate–bicarbonate–dithionite-extractable iron; Feo, ammonium-oxalate-extractable iron; Fep, pyrophosphate-extractable iron; GC, glassy carbon; HPLC, high-performance liquid chromatography; MES, 2-(N-morpholino)ethanesulfonic acid; OM. organic matter; SCE, saturated calomel electrode.

1580 Soil Science Society of America Journal

mechanistic studies on this heterogeneous reaction have dem-onstrated that the formation of surface complexes is responsible for an enhanced reaction rate of contaminants (Strathmann and Stone, 2003; Li et al., 2009; Tao et al., 2009, 2010). Other stud-ies have found that the reductive reactivity of surface-complex Fe(II) correlates with reaction pH, adsorbed Fe(II) density, re-dox potential, reaction temperature, and organic compounds, among others (Krishnaswamy and Richter, 2002; Hakala et al., 2007; Colón et al., 2008; Li et al., 2008a, 2008b; Kothawala et al., 2009; Tao et al., 2009). Using cyclic voltammetry (CV) and electrochemical impedance spectrometry to characterize the Fe(III)–Fe(II) couple directly, Li et al. (2009) found that Fe(II) complexes, whether dissolved or surfi cial, dramatically increase the reaction rate and extent of 2-NP reductive transformation by relieving thermodynamic or kinetic restraints that hinder elec-tron transfer from uncomplexed aqueous Fe(II). Th e dependence of adsorbed Fe(II) reactivity on pH has been proven in previous experiments through evaluation of the mutual linear dependence between natural logarithm of the reaction rate (lnk), pH, and the measured peak oxidation potential (Ep) in mineral suspensions (Li et al., 2009; Tao et al., 2009, 2010). Nonetheless, research has been minimal because of the complexity of the soil solution and the diffi culty in analyzing the existing forms of adsorbed Fe(II) species, as well as their concentrations on soil surfaces.

Various soil minerals are capable of providing functional sites to stabilize Fe(II) ions (Strathmann and Stone, 2003). Iron(II) adsorption onto soil surfaces is an important environmental pro-cess because surface-complex Fe(II) enables the reductive trans-formation of organic and inorganic pollutants in contaminated soils (Rügge et al., 1998; Li et al., 2008a, 2008b). Th e soils in South China developed under a subtropical and tropical mon-soon climate (Tao and Feng, 2000). Desilicifi cation and allitiza-tion decrease the Si content while increasing Fe and Al contents (Huang et al., 2008; Tan, 2008, p. 93–128). Soils in subtropical and tropical zones contain a large amount of free Fe oxide with a special biogeochemistry (Osei and Singh, 1999; Li et al., 2008b). Th e Fe cycle is an important energy source for soil geochemical processes in this area (Liu, 1993). Th erefore, the transformation of reducible pollutants may strongly depend on the biogeochem-istry of the Fe species. Weathering is the key process in soil for-mation and the development of soil physicochemical properties (Vaughan, 1988). Th e weathering process aff ects the chemical composition of soils by governing the mineral components in tropical and subtropical areas (Guan et al., 2001; Rasmussen et al., 2007). Weathering indices are closely associated with the physicochemical properties of soils, which are important for af-fecting the concentration and activity of adsorbed Fe(II) (Li et al., 2008b). Th e objective of the present study was to determine the eff ect of pH and weathering indices on the reductive transforma-tion of reducible pollutants in soils in South China.

Nitroaromatic compounds are a series of widespread organic compounds that are naturally stable and diffi cult to biodegrade (Hatter, 1985; Karim and Gupta, 2003). In the present study, 2-NP was selected as the probe reducible contaminant. A total of

22 soil samples (including Paleaquults, Acrorthoxs, Rhodudults, Ochraquoxs, and Haplorthoxs) were collected from South China and used as an adsorption interface to enable the adsorption of Fe(II). Th e soils were divided into three groups based on their as-sociated land use: Group I, vegetable soils in the Pearl River Del-ta; Group II, orchard soils in the Pearl River Delta; and Group III, forest soils in the tropical zone. Laboratory experiments were performed to investigate the reductive transformation of 2-NP on these soils to determine the eff ect of pH and weathering indices based on the analyses of soil physicochemical properties. Two types of experiments, one without MES and another with MES, with equal added concentration of Fe(II) (typical experiment sus-pensions prepared for the redox reactor contained 3.0 mmol L−1 FeSO4) were conducted to focus on the dependence of the 2-NP reductive transformation rate on pH and the soil weathering in-dex. Th e variance and correlation were evaluated, and stepwise re-gression analysis was applied. Th e results of the current study are valuable for understanding the correlation between the soil Fe(II) content and the kinetics of reductive transformation. Moreover, the results have implications for research on soil Fe redox chemis-try and soil pollution control.

MATERIALS AND METHODSSoil Sampling

In total, 22 soil samples were collected from A (0–20 cm, the leached layer) horizons in South China and divided into Groups I, II, and III. Th eir classifi cation and sampling locations are shown in Fig. 1 and listed in Table 1. Th e soil samples were air dried, sieved (2 mm), and stored at 4°C before the analyses and experiments.

ChemicalsTh e chemicals, including MES (>99.5%), were purchased

from Sigma-Aldrich, and 2-NP (99.5%), 2-aminophenol (2-AP, 99.5%), FeSO4⋅7H2O (>99.5%), and methanol for high-performance liquid chromatography (HPLC) were purchased from Acros. All chemicals were of analytical grade or higher and were used without further purifi cation.

Deaerated, deionized water (DDW) with resistivity of 18 MΩ cm (Easy Pure II RF/UV) was used to prepare all solutions. Before use, the DDW was sparged with ultra-high-purity N2 for 2 h and immediately stored in a Bactron Anaerobic/Environment Chamber II (95% N2, 5% H2; Shellab, Shedon Manufacturing Inc.) for 12 h. Stock solutions of NaCl (0.43 mol L−1) and MES (0.60 mol L−1) were prepared with DDW. Stock solution of 2-NP (1.1 mmol L−1) was dissolved in methanol in an O2–free glove box and preserved in a dark-brown container. All organic stock solutions were fi ltered through 0.22-μm (Millipore Millex-GS) fi lters before use. To minimize Fe(III) contamination, Fe(II) stock solutions were prepared by acidifying fi ltered FeSO4 solution through a 0.02-μm fi lter (Anotop 25-Plus inorganic membrane, Whatman Scientifi c) with 0.1 mol L−1 HCl. Th e fi nal concentration of Fe(II) stock solutions was kept approximately at 1.0 mol L−1, which was

www.soils.org/publications/sssaj 1581

determined by atomic absorption spectrophotometry (Aanalyst 100, PerkinElmer) (Strathmann and Stone, 2001).

Experimental ProcedureTwo groups of experiments, one with MES and another

without MES, were conducted to investigate the eff ects of pH

Fig. 1. Distribution of soil samples: Group I, vegetable soils in Pearl River Delta; Group II, orchard soils in Pearl River Delta; and Group III, forest soils in the tropical zone.

Table 1. Description of the soil classifi cation and sampling locations.

Group SampleSoil taxonomy (Great Group) Parent materials Location

I: Vegetable soils in Pearl River Delta

I-1 Paleaquult alluvial deposit Xiazhou Village, Doumen Town, Doumen District, Zhuhai CityI-2 Paleaquult alluvial deposit Dahaihuan Village, Pingsha Town, Jinwan District, Zhuhai City

I-3 Paleaquult sandstone Liantang Village, Gaoyao County, Zhaoqing City

I-4 Paleaquult granite Tangbei Village, Qiuchang Town, Huiyang District, Huizhou City

I-5 Paleaquult granite Zhendong Village, Yonghan Town, Longmen District, Huizhou City

I-6 Paleaquult sandstone Fantou Village, Chenjiang Town, Huichen District, Huizhou City

I-7 Paleaquult basalt Nianmian Village, Xiegang Town, Dongguan City

I-8 Paleaquult alluvial deposit Huangxi Village, Shiwan Town, Bolo County, Huizhou City

I-9 Paleaquult alluvial deposit Tielu Mountain in Shenxi Village, Shenwan Town, Zhongshan City

I-10 Paleaquult alluvial deposit Tanzhou Town, Zhongshan City

II: Orchard soils in Pearl River Delta

II-1 Acrorthox granite Southern slope of Luofu Mountain in Bolo County, Huizhou City

II-2 Acrorthox granite Baiyun Mountain, Guangzhou City

II-3 Acrorthox granite Lanxi Village, Longgang District, Shenzhen City

II-4 Acrorthox granite Gudou Mountain, Xinhui City

II-5 Rhodudult granite Dongshan Forest, Taishan City, Jiangmen City

II-6 Rhodudult sandstone Jinwuwu Village, Guanjiao Town, Foshan City

II-7 Rhodudult quaternary period red clay Bengqiang Village, Xishan Town, Luxi City

II-8 Haplorthox alluvial deposit Nanlong Village, Sanxiang Town, Zhongshan City

III: Forest soils in tropical zone

III-1 Acrorthox basalt Tangjia Town, Leizhou City

III-2 Acrorthox basalt Qujie Town, Xuwen Country

III-3 Ochraquox sedimentary rocks Tiaoshu Village, Jijia Town, Leizhou CityIII-4 Ochraquox sedimentary rocks Beipo Town, Suixi County, Zhanjiang City


on the reductive transformation of 2-NP. Typical experiment suspensions prepared for the redox reactor without buff er contained 3.0 mmol L−1 FeSO4, 22.0 μmol L−1 2-NP, 200 mmol L−1 NaCl, and 68.0 mg of each soil sample. Th e experiments were conducted at the original pH of the soil suspension. Another set of experiments with MES was conducted under conditions identical to those of the typical suspensions without MES, except that 28 mmol L−1 MES was added to the reactor to keep the pH at approximately 6.5. Th ree series of control experiments were conducted: (i) 2-NP in homogeneous suspension without Fe(II), (ii) 2-NP in homogeneous suspension with 3 mmol L−1 Fe(II), and (iii) 2-NP in soil suspensions without 3 mmol L−1 Fe(II). In addition, the sorption control studies of organic chemicals were conducted under the various conditions except adding 3 mmol L−1 Fe(II). All experiments were performed in triplicate and were replicated thrice simultaneously.

Reductive Transformation ProceduresAbiotic reduction experiments were conducted on aqueous

suspensions in 20-mL borosilicate glass serum bottles with aluminum crimps and Tefl on-lined butyl rubber septa using the methods described previously (Li et al., 2009; Tao et al., 2009, 2010; Tao and Li, 2011). To prevent any Fe(II) oxidation, all experiments were conducted inside a Bactron Anaerobic/Environment Chamber II at 25°C. A typical reaction medium was prepared by combining 8 mL of MES, 8 mL of NaCl, and 51 μL of FeSO4 from the stock solutions, with the fi nal concentrations kept at 28, 200, and 3 mmol L−1, respectively. Th ese mixtures were added to the serum bottles (containing 68.0 mg of each soil sample). Aft er adding 0.34 mL of 2-NP (fi nal concentration = 22 μmol L−1) to the reactors, the reduction reactions were begun by transferring the serum bottles to an orbital shaker at 200 rpm and 25 ± 1°C in darkness. Th e total fi nal volume of the aqueous phase in the soil–water mixture was kept at 17 mL, the number of serum bottles of each kinetic experiment was 10, and the total experiment time was set to 240 min. For the 2-NP kinetic studies, one of the 20-mL serum bottles was removed from the shaker and transferred to the O2–free glove box before the routine analysis. Aft er the serum bottle was mixed vigorously, it was opened and spiked with 2 mol L−1 HCl (<60 μL) to adjust the pH to below 3 and prevent further degradation of the 2-NP (Klupinski et al., 2004). Th e suspension was immediately centrifuged at 10,000 rpm for 10 min (Sigma-3K 15) to remove the particles. Th e remaining supernatants were further analyzed.

Iron(II) SorptionIron(II) adsorption onto diff erent soil samples was

determined under conditions identical to those for the kinetic experiments, except that 22.0 μmol L−1 2-NP was not added to the reactor. Adsorption measurements were conducted under pH conditions similar to those for the kinetic experiments. Owing to the possibility that Fe(II) might be oxidized at circumneutral

pH, the adsorption experiments at pH ≥6.5 were conducted with continuously bubbled N2 rather than on a rotator. Th e fl ow rate of N2 was 90 mL min−1 to enable suffi cient stirring of the suspension (Li et al., 2009). When equilibrium was reached, the fi nal pH of each suspension was recorded before fi ltering (0.2-μm membrane fi lter, MCE) (Nano and Strathmann, 2006). Th e acidifi ed fi ltrate was collected for analysis of the Fe(II) content.

Th e original suspension sample was split into two separate samples for measurement of diff erent Fe(II) concentrations, and the Fe(II) concentration was determined colorimetrically by the 1,10-phenanthroline method at 510 nm using an ultraviolet-visible (UV-vis) spectrophotometer (TU-1800PC) (Fadrus and Malý, 1975; Jeon et al., 2005). Solutions containing dissolved Fe(II) were fi ltered through 0.22-μm fi lters to remove the particles in the samples before analysis. Th e total Fe(II) in the samples was extracted using 0.5 mmol L−1 HCl for 1.5 h (Fadrus and Malý, 1975; Jeon et al., 2005) and then analyzed by applying the same procedures as those for dissolved Fe(II). Adsorbed Fe(II) was calculated as the diff erence between total Fe(II) and dissolved Fe(II).

Electrochemical TestsCyclic voltammetry was conducted using the methods

described by Li et al. (2009). Up to 22 soil-modifi ed glassy carbon (GC) electrodes were prepared using bare GC electrodes with a diameter of 3 mm, in which a GC electrode was fi rst polished with emery paper, followed by Al2O3 powders (particle sizes of 0.06 and 1 μm). Between polishing, the electrode was rinsed thoroughly with DDW. Th e polished electrode was successively cleaned with acetone and then water in an ultrasonic bath for 10 min. Th e mineral slurry containing 5 mg of soil sample was then prepared in a dilute Nafi on solution (0.5% w/w, 250 μL) ultrasonically for 15 min. Using a microsyringe, aliquots (2 μL) of the above slurry were coated on the clean GC electrode, and the electrode was air dried for 30 min before use. Th e mineral loading for the electrochemical tests was estimated to be 2 × 10−3 g L−1. Th e 22 soil-modifi ed GC electrodes are referred to here as “soil/GC,” whereas the bare GC electrode is referred to as “GC.”

Electrochemical measurements were performed in a conventional three-electrode cell equipped with the soil/GC electrode as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a platinum spiral wire as the counter electrode. Cyclic voltammetry was recorded using an Autolab Potentiostat (PGSTAT 30, Eco Chemie) at a scan rate of 50 mV s−1. Th e electrochemical cell was fi lled with a solution containing 3.0 mmol L−1 FeSO4 and 0.20 mol L−1 NaCl. Th e solution pH was adjusted by adding diluted HCl or NaOH solution. High-purity N2 gas was bubbled through the electrolyte to remove the dissolved O2.

Analysis of Physicochemical PropertiesCitrate–bicarbonate–dithionite-extractable Fe (Fed) was

determined according to Mehra and Jackson (1958). Ammonium-


oxalate-extractable Fe (Feo) was extracted by acidifi ed ammonium oxalate buff er solution at pH 3.0 (Schwertmann, 1964). Pyrophosphate-extractable Fe (Fep) was determined using alkaline sodium pyrophosphate at pH 8.5 (Alexandrova, 1960). Th e concentration of Fe in the extracting solution was determined using the 1,10-phenanthroline method (Fadrus and Malý, 1975). Th e total Fe and Mn contents were determined by digestion with HClO4 and HF, respectively, followed by measurement using atomic absorption spectrophotometry (Pansu and Gautheyrou, 2006). Total Al was measured by an inductively coupled plasma atomic emission spectrometer (PS 1000AT, Leeman Labs Inc.) aft er digestion with HClO4 and HF (Pansu and Gautheyrou, 2006). Th e measured total Fe, Mn, and Al contents in the soils were subsequently recalculated and are reported as the equivalent oxide contents: Fe2O3, MnO, and Al2O3, respectively. Th e organic matter (OM) in the soils was determined using the potassium dichromate oxidation–volumetric method (Pansu and Gautheyrou, 2006). Th e cation exchange capacity (CEC) of the soils was determined using the NH4OAc method at pH 7.0 (Pansu and Gautheyrou, 2006). Th e SiO2 contents of the soils were determined by the Na2CO3

melting-mass method (Lu, 1999, p. 48–50). Th e weathering indices of the soils are presented as the molecular ratios SiO2/Al2O3 (silica/Al ratio), Al2O3/Fe2O3 (Al/Fe ratio), and SiO2/(Al2O3 + Fe2O3) (sesquioxide ratio), which were based on the total Si, Al, and Fe contents obtained (Duzgoren-Aydin et al., 2002; Rasmussen et al., 2010). Th e physicochemical properties of the soil samples are listed in Table 2.

Analytical MethodsTh e concentrations of 2-NP and 2-AP as a function of

reaction time were monitored by Waters HPLC 2487 (5 μm Symmetry-C18, 4.6 mm, 250 mm), which consists of a Waters 1525 binary pump, an analytical reversed-phase column, and a Waters dual-λ absorbance UV-vis detector. Th e isocratic mobile phase contained 80/20 (v/v) methanol/water and 3 mmol L−1 HCl (maintaining the mobile-phase pH at approximately 2.8) at a fl ow rate of 1.0 mL min−1 under isocratic conditions at 30 ± 1°C; the wavelength was set to 213 nm. Th e 2-NP and 2-AP concentrations were calculated against standard solutions (Klupinski et al., 2004). Th e mass recovery fraction of 2-NP was

Table 2. Physicochemical properties of the soil samples.

Sample pHs† CEC‡ OM§ Fed¶ Feo# Fep†† Fe2O3 MnO SiO2 Al2O3SiO2/

Al2O3‡‡Al2O3/

Fe2O3‡‡

SiO2/(Al2O3 + Fe2O3)‡‡

cmol(+) kg−1 ——————————————————— g kg−1 ———————————————————3.60 14.58 30.66 15.52 10.29 1.09 48.46 0.52 600.8 176.8 5.78 5.72 4.92

7.92 16.27 21.13 13.75 13.82 0.06 58.67 1.51 522.0 184.2 4.82 4.92 4.00

4.90 12.14 38.01 8.41 5.73 0.64 41.88 0.26 562.3 206.0 4.64 7.72 4.11

4.42 7.37 16.84 5.88 3.52 0.33 18.18 0.32 753.6 109.1 11.74 9.41 10.61

I-5 5.88 7.02 20.56 3.91 2.88 0.86 24.20 0.15 750.0 110.8 11.51 7.18 10.10

I-6 6.88 6.75 12.74 4.59 2.12 0.72 10.78 0.19 835.4 67.2 21.13 9.78 19.17

I-7 5.50 6.92 22.91 2.90 1.86 0.69 10.57 0.27 681.5 109.6 10.57 16.27 9.96

I-8 5.25 3.38 10.37 1.48 0.74 0.37 4.99 0.06 923.5 32.4 48.46 10.19 44.12

I-9 4.40 7.57 19.01 5.30 1.06 0.13 27.33 0.16 699.6 151.9 7.83 8.72 7.02

I-10 6.09 17.32 42.77 6.34 5.62 0.16 58.86 0.09 548.6 176.7 5.28 4.71 4.35

II-1 5.08 7.79 29.47 5.31 2.07 0.15 32.57 0.64 652.9 156.9 7.07 7.56 6.25

II-2 4.26 7.00 23.85 2.76 0.66 0.27 12.62 0.11 683.1 159.4 7.29 19.81 6.94

II-3 5.19 4.40 23.17 1.89 0.68 0.09 8.74 0.54 743.9 128.6 9.83 23.08 9.43

II-4 4.51 7.16 25.97 2.19 0.80 0.15 14.53 0.50 722.8 142.3 8.63 15.36 8.11

II-5 4.56 4.84 17.40 8.38 1.59 0.76 25.44 0.13 819.5 82.7 16.85 5.10 14.08

II-6 5.70 11.80 2.35 2.97 0.57 0.01 48.46 0.65 626.8 168.1 6.34 5.44 5.35

II-7 6.73 12.76 12.27 7.43 2.64 0.03 71.21 0.20 502.6 221.6 3.86 4.88 3.20

II-8 6.65 17.12 34.53 7.52 5.09 0.30 53.68 1.02 556.8 179.2 5.28 5.24 4.44

III-1 4.79 8.33 25.06 4.92 3.15 0.04 106.51 0.50 410.0 257.8 2.7 3.80 2.14

III-2 4.90 10.59 38.72 7.60 3.32 0.03 115.09 1.36 400.3 304.0 2.24 4.14 1.80

III-3 4.84 2.68 5.88 3.95 0.63 0.05 20.05 0.65 826.8 94.3 14.91 7.38 13.13III-4 4.72 3.07 10.04 4.83 0.73 0.04 16.24 0.88 561.3 72.6 13.14 7.01 11.50

† Original pH of soil suspension.‡ Cation exchange capacity.§ Organic matter.¶ Dithionite-extractable Fe.# Oxalate-extractable Fe.†† Pyrophosphate-extractable Fe.‡‡ Molecular ratio.


97 ± 2% (n = 5), while the mass recovery fraction of 2-AP was 98 ± 1% (n = 5).

Statistical AnalysisEach treatment was replicated three times and included

blanks. Analytical determination was performed in duplicate. All measurements of the soil physicochemical properties were expressed based on the air-dried soil. A multiple comparison of the means of the soil CEC and the measured contents of OM, Fed, Feo, Fep, Fe2O3, MnO, SiO2, and Al2O3 was conducted using Tukey’s honestly signifi cant diff erence test. Th e variance and correlation were evaluated. Stepwise regression analyses were applied to investigate the eff ects of pH and weathering indices on the variation in 2-NP reduction rates in the 22 soil samples. Regression models were developed through stepwise multiple

regression analyses with forward selection using SPSS 18.0 (IBM SPSS Inc.).

RESULTSEffects of pH on the Reductive Transformation of 2-Nitrophenol

Two types of experiments, one with MES and another without MES, were conducted to investigate the eff ects of pH on the reductive transformation of 2-NP. Figure 2A shows a comparison of the reaction kinetics obtained from Sample III-3 soil interfaces under various conditions. No loss of 2-NP was observed in homogeneous solution without Fe(II), whereas nearly 45% of the 2-NP disappeared in the homogeneous solution containing 3 mmol L−1 of Fe(II) at pH 6.7 (MES), indicating that Fe(II) species were the active species in 2-NP reductive transformation. Th e mass recovery fraction of 2-NP in

Fig. 2. (A) Reductive transformation of 2-nitrophenol (2-NP) in Sample III-3 soil interfaces under various conditions; (B) complete time course showing 2-NP disappearance and formation of 2-aminophenol (2-AP) in Sample III-3 suspension with 3 mmol L−1 Fe(II) with 2-(N-morpholino)ethanesulfonic acid (MES) at pH 6.5; and (C) concentration of adsorbed Fe(II) and peak oxidation potential (Ep) referenced to electrochemical measurement using a saturated calomel electrode (SCE) under different pH conditions. Reaction conditions: 3 mmol L−1 FeSO4, 0.022 mmol L−1 2-NP, 28 mmol L−1 MES, 0.2 mol L−1 NaCl, 4.0 g L−1 soil, and 298 K.


the sorption control studies for the various soils were consistently above 95% (data not shown); for instance, the adsorption of 2-NP in Sample III-3 under diff erent pH conditions (pH 4.7 with MES, pH 4.7 without MES, and pH 6.5 with MES) was approximately <3% (Fig. 2A). Th e results of the sorption control studies indicate that the level of adsorption under all conditions was considerably low, and the adsorption behavior showed a negligible diff erence. In addition, the homogeneous reaction of Fe(II) with 2-NP at a reaction pH of 6.7 with MES yielded a considerably lower rate than that resulting from the heterogeneous reaction, which involved the adsorbed Fe(II) species. Th ese observations demonstrate that Fe(II) adsorbed to the soil interface is a reactive electron donor that promotes the reductive transformation of 2-NP.

Figure 2A indicates that the rate of 2-NP reductive transformation signifi cantly increased with an increase in pH. Th e transformation of 2-NP generally followed the pseudo-fi rst-order kinetics rate law. Th e constant rate (k) values were obtained from the pseudo-fi rst-order model fi tting. Th e results (Fig. 2A) demonstrate the dependence of 2-NP degradation on the solution pH. Th e k value for 2-NP transformation in Sample III-3 suspensions was 6.9 ± 0.4 × 10−4 min−1 at pH

4.71 (the original pH of the soil solution without MES). Th is value is approximately fi ve times smaller than that obtained at pH 6.5 (with 28 mmol L−1 of MES). Th e k value of 2-NP transformation increased to 8.4 ± 0.3 × 10−3 min−1 when the reaction was conducted at pH 6.70 (with 28 mmol L−1 MES buff er solution). Th is value is approximately 12 times higher than that obtained at the original pH of 4.71 (Table 3). A similar behavior was also observed in the other soil samples. Th e k values of 2-NP transformation under various conditions are listed in Table 3. In comparison, the reaction conducted under various pH conditions shows that the smallest k value for 2-NP transformation was 1.2 ± 0.1 × 10−4 min−1, found in Sample II-1 at pH 4.74 (without MES solution). Moreover, the largest k value for 2-NP transformation was 9.0 ± 0.3 × 10−2 min−1 found in Sample II-6 at pH 6.70 with 28 mmol L−1 MES solution (Table 3).

Figure 2B shows a complete time course of 2-NP disappearance and 2-AP formation. Th e mass balance indicates that 2-NP reduction proceeded through some intermediates to 2-AP. A similar behavior was also observed in the other soil samples (the ratio of conversion for all reactions was consistently

Table 3. Results of experiments with and without 2-(N-morpholino)ethanesulfonic acid (MES), including the reaction pH (pHr), peak oxidation potential (Ep) referenced to electrochemical measurement using a saturated calomel electrode (SCE), the amount of Fe(II) sorbed, and the reaction rate (k).

SampleExperiments without MES Experiments with 28 mmol/L MES

pHr Ep vs. SCE Fe(II) sorbed k† pHr Ep vs. SCE Fe(II)sorbed k†

mV mg L−1 min−1 mV mg L−1 min−1

I-1 4.20 352 2.2 ± 0.7‡ 4.2 (±0.4) × 10−4‡ 6.50 −20 21.2 ± 1.4‡ 2.5 (±0.2) × 10−3‡I-2 6.05 115 16.0 ± 0.7 2.0 (±0.2) × 10−4 6.50 −17 25.8 ± 1.7 4.6 (±0.2) × 10−3

I-3 3.84 410 14.3 ± 0.7 3.6 (±0.3) × 10−4 6.50 −16 21.6 ± 1.5 9.8 (±0.6) × 10−3

I-4 3.82 389 1.3 ± 0.5 3.5 (±0.2) × 10−4 6.50 −27 4.0 ± 0.2 3.6 (±0.2) × 10−3

I-5 4.03 338 14.8 ± 0.6 3.1 (±0.4) × 10−4 6.50 21 18.2 ± 0.6 2.7 (±0.2) × 10−3

I-6 4.24 303 6.1 ± 0.1 3.6 ( 0.2) × 10−4 6.50 1 6.8 ± 0.9 7.0 (±0.3) × 10−4

I-7 5.03 368 13.3 ± 0.2 1.6 (±0.1) × 10−4 6.50 31 19.3 ± 1.7 1.3 (±0.2) × 10−3

I-8 3.82 359 2.3 ± 0.2 2.7 (±0.2) × 10−4 6.50 −2 6.2 ± 0.1 1.7 (±0.4) × 10−3

I-9 4.62 300 14.0 ± 0.7 3.8 (±0.2) × 10−4 6.50 4 19.9 ± 0.8 1.3 (±0.1) × 10−3

I-10 5.03 302 1.9 ± 0.1 4.9 (±0.6) × 10−4 6.50 15 22.4 ± 1.3 5.3 (±0.3) × 10−3

II-1 4.74 300 5.1 ± 0.4 1.2 (±0.1) × 10−4 6.50 11 17.6 ± 0.3 6.8 (±0.3) × 10−3

II-2 3.81 359 28.6 ± 0.6 2.9 (±0.3) × 10−4 6.50 −4 29.1 ± 1.2 4.2 (±0.2) × 10−3

II-3 5.04 410 7.2 ± 0.1 1.3 (±0.2) × 10−3 6.50 84 66.8 ± 0.4 4.3 (±0.2) × 10−3

II-4 4.73 430 3.1 ± 0.1 8.9 (±0.6) × 10−4 6.50 45 60.4 ± 0.3 2.5 (±0.3) × 10−3

II-5 3.82 359 12.6 ± 0.5 4.8 (±0.4) × 10−4 6.50 −20 37.0 ± 0.9 7.1 (±0.6) × 10−3

II-6 5.04 400 10.2 ± 0.8 2.5 (±0.2) × 10−4 6.50 −9 24.4 ± 1.0 8.0 (±0.4) × 10−2

6.70 −16 71.6 ± 3.4 9.0 (±0.3) × 10−2

II-7 5.31 354 8.1 ± 0.1 1.7 (±0.2) × 10−3 6.50 93 35.1 ± 1.2 2.4 (±0.3) × 10−2

II-8 5.01 364 16.4 ± 0.8 1.8 (±0.2) × 10−4 6.50 67 23.5 ± 0.6 3.0 (±0.3) × 10−3

III-1 4.74 366 9.4 ± 0.1 8.8 (±0.4) × 10−4 6.50 27 47.0 ± 0.1 5.7 (±0.4) × 10−2

III-2 4.62 380 8.8 ± 0.1 1.2 (±0.1) × 10−3 6.50 32 62.7 ± 0.6 1.7 (±0.3) × 10−2

III-3 4.71 302 1.6 ± 0.1 6.9 (±0.4) × 10−4 6.50 5 11.5 ± 0.9 3.0 (±0.2) × 10−3

6.60 −25 12.5 ± 0.5 4.4 (±0.3) × 10−3

6.70 −65 22.5 ± 0.8 8.4 (±0.3) × 10−3

III-4 5.02 398 2.6 ± 0.1 1.2 (±0.2) × 10−3 6.50 94 39.6 ± 0.2 1.9 (±0.1) × 10−2

† k was determined from the pseudo-fi rst-order model fi tting. Each kinetic experiment consisted of 10 data points, with a total experiment time of 240 min.

‡ 95% confi dence interval in parentheses.


>0.95). With 2-NP as a probe chemical, the intermediates of 2-NP reductive transformation to 2-AP were not detected.

Effects of pH on the Amount of Adsorbed Iron(II)Th e present study proves that the amount of adsorbed Fe(II)

on the mineral surface is a crucial factor aff ecting the reduction rate of organic pollutants, as asserted in the literature (Stumm and Sulzberger, 1992; Strathmann and Stone, 2003; Hiemstra and van Riemsdijk, 2007; Li et al., 2009; Tao et al., 2009, 2010; Tao and Li, 2011). Th e eff ect of the sorbed Fe(II) concentration on the reductive transformation of 2-NP was investigated in the 22 soil suspensions with the same initial Fe(II) concentration [3.0 mmol L−1 Fe(II)] but under diff erent pH conditions, as described above. Th e results (Table 3) show that the amount of adsorbed Fe(II) signifi cantly increased with the increase in pH. Moreover, the k values for 2-NP reductive transformation increased with increasing sorbed Fe(II) concentration in all cases. Th is observation indicates that the amount of adsorbed Fe(II) is a key parameter that must be considered to improve the reductive transformation rate. For example, in the Fe(II) suspension in Sample III-3, the sorbed Fe(II) concentration increased from 1.6 ± 0.1 to 22.5 ± 0.8 mg L−1 when the pH increased from 4.7 to 6.7, and the k value for 2-NP transformation increased from 6.9 ± 0.4 × 10−4 to 8.4 ± 0.3 × 10−3 min−1 (Fig. 2C). Similar trends were observed in the other soil suspensions.

Electrochemical Behavior of Sorbed Iron(II) on the Soil-Modifi ed Electrodes

Using homemade, soil-modifi ed electrodes, CV was performed to evaluate the Fe(II) to Fe(III) electron transfer

behavior of adsorbed Fe(II), which provides direct evidence of the change in redox potentials (Li et al., 2009; Tao et al., 2009, 2010; Tao and Li, 2011). Figure 3 illustrates the redox behavior of the Fe(III)–Fe(II) redox couple on bare GC and Sample I-2/GC electrodes. Based on the CV results, no signifi cant peaks were observed in the absence of aqueous Fe(II) (from FeSO4), indicating that the contribution of native Fe fraction to the voltammetric signal can be neglected. All the other voltammograms with aqueous Fe(II) exhibited a pair of peaks: an anodic oxidation peak for Fe(II) at the top of the CV cycles, with potentials ranging from −50 to 200 mV (vs. SCE), and a cathodic reduction peak for Fe(III) at the bottom of the CV cycles, with potentials ranging from −450 to −200 mV (vs. SCE).

Th e results of the present study are consistent with theoretical (Nano and Strathmann, 2006) and previous experimental results for simple interfaces (Li et al., 2009; Tao et al., 2009, 2010). Both peaks shift ed toward a more negative direction because of the adsorption interfaces (Sample I-2). For instance, at pH 6.05, the peak oxidation potential (Ep) of Fe(II) adsorbed onto Sample I-2 signifi cantly decreased from 158 mV on bare GC electrodes to 115 mV (vs. SCE). Moreover, the peak redox potential of Fe(III) adsorbed onto Sample I-2 signifi cantly decreased from −267 mV on bare GC electrodes to −278 mV (vs. SCE). Conversely, both peaks shift ed toward a more negative direction with the increase in pH. When the pH was modulated from 4.7 to 6.7, the peak oxidation potential of Fe(II) adsorbed onto Sample III-3 signifi cantly decreased from 302 to −65 mV (vs. SCE). Th e decrease in the peak oxidation potential of adsorbed Fe(II) corresponded to the increase in the sorbed Fe(II) concentration and the k value for 2-NP transformation (Fig. 2B). A similar behavior was observed on the other soil minerals. Th e results are listed in Table 3.

Reductive Transformation of 2-Nitrophenol in Different Soil Types and Parent Materials

Figure 4 presents the average k values for 2-NP reductive transformation obtained in soils of diff erent types (i.e., Paleaquult, Acrorthox, Rhodudult, and Ochraquox) and parent materials (i.e., alluvial deposit, sandstone, granite, basalt, and sedimentary rocks) using variance analysis. No signifi cant diff erence was found in the average k value without MES among the Acrorthox, Rhodudult, and Ochraquox, but a signifi cant diff erence was observed between the Paleaquult and Ochraquox (Fig. 4A). Th e average k value with MES in the Rhodudult was notably higher than that in the other three soil types. A signifi cant diff erence in the average k value was observed between the Rhodudult and the others, whereas no signifi cant diff erence in the average k value was found among the Paleaquult, Acrorthox, and Ochraquox (Fig. 4C). Th e results of variance analysis show no signifi cant diff erence in the average k value among alluvial deposits, sandstone, granite, and basalt, whereas signifi cant diff erences were observed among sedimentary rocks, alluvial deposit, and sandstone. As shown in Fig. 4D, the average k values with buff er in sandstone and basalt were notably higher than those in the

Fig. 3. Cyclic voltammetry results of adsorbed Fe(II) species under various conditions: 1, Sample I-2/glassy carbon (GC) electrode without Fe(II) at pH 6.50 [2-(N-morpholino)ethanesulfonic acid, MES]; 2, bare GC electrode with Fe(II) at pH 6.05; 3, Sample I-2/GC electrode with Fe(II) at pH 6.05; 4, Sample I-2/GC electrode with Fe(II) at pH 6.50 (MES). Electrochemical measurements were conducted in the cell (25 mL) containing 3.0 mmol L−1 FeSO4, 0.2 mol L−1 NaCl solution, and 28 mmol L−1 buffer at 25°C. Mineral loading was estimated to be 2 × 10−3 g L−1. Solution pH was 6.5, and the scan rate was 50 mV s−1.


other three types. Signifi cant diff erences in the average k values were observed among sandstone, alluvial deposit, granite, and sedimentary rocks.

DISCUSSIONEssential Effect of pH on 2-Nitrophenol Reductive Transformation

To investigate the eff ects of pH and weathering indices on 2-NP reductive transformation, statistical analyses interpreting the results were performed. Without the eff ect of the buff er solution, the correlation analysis (Table 4) shows that sorbed Fe(II), Ep, and lnk values have weak correlations with a single soil physicochemical property, as indicated by the low Pearson’s correlation coeffi cient. Nevertheless, the correlations between Ep and both solution pH and Feo are signifi cant at the 0.01 level, whereas the correlations between Ep and both the reaction pH and Fed are signifi cant at the 0.05 level. Th e Pearson correlations between Ep and solution pH, reaction pH, Fed, and Feo are −0.571, −0.432, −0.461, and −0.600, respectively, indicating that Ep has a negative correlation with solution pH, reaction pH, Fed, and Feo. Using electrochemical methods (CV and electrochemical impedance spectrometry), Li et al. (2009) confi rmed that the Ep of complex Fe(II) is signifi cantly aff ected by solution pH, and the enhanced reductive transformation of 2-NP is related to the reduced Ep of surface-complex Fe(II) as

well as the reduced charge transfer resistance of the Fe(III)–Fe(II) couple. Th e correlation analysis illustrates that the reductive reactivity of surface-complex Fe(II) species in soils is not dependent on a single soil physicochemical property, but it has a signifi cant relationship with the pH conditions (i.e., solution and reaction pH conditions) and Fe components (i.e., Fed, and Feo) (Table 4). Th is trend illustrates the decisive eff ect of pH on the reductive transformation reactions.

Critical Effect of Weathering Indices on 2-Nitrophenol Reductive Transformation

Table 4 also presents the results of the correlation analyses with MES (excluding pH as a variable) to determine other soil aspects that have important eff ects on the 2-NP reductive transformation reactions. Th e results show signifi cant diff erences compared with the results without buff er. As shown in Table 4, the correlations among sorbed Fe(II), Ep, the single physicochemical property (i.e., SiO2 and Al2O3), and weathering indices [i.e., SiO2/Al2O3 and SiO2/(Al2O3 + Fe2O3)] are signifi cant at the 0.05 level. Moreover, the amount of sorbed Fe(II), the single physicochemical property (Fe2O3, Fep, SiO2, and Al2O3), and weathering indices [i.e., SiO2/Al2O3, Al2O3/Fe2O3, and SiO2/(Al2O3 + Fe2O3)] also have signifi cant correlations with the lnk value. Th e amounts of Al2O3 and Fe2O3 have positive correlations with the amounts of sorbed Fe(II) and lnk, but the

Fig. 4. Results of variance analysis of average 2-nitrophenol (2-NP) reduction rates (k) (A) without 2-(N-morpholino)ethanesulfonic acid (MES) and (C) with MES in different soil types, and (B) without MES and (D) with MES in different parent materials. Average value within the same treatment that has the different lowercase letters (i.e., a or b) are signifi cantly different at P < 0.05, whereas average value within the same treatment that has the same lowercase letters are not signifi cantly different at P = 0.05.


amounts of SiO2, SiO2/Al2O3, and SiO2/(Al2O3 + Fe2O3) have negative correlations with the amounts of sorbed Fe(II) and the lnk value. Th e larger the molecular ratio of SiO2/Al2O3 and SiO2/(Al2O3 + Fe2O3), the smaller are the amounts of the sorbed Fe(II) and lnk values (Tables 2 and 3).

Previous studies illustrating the eff ect of pH on reductive transformations (Li et al., 2009; Tao et al., 2009, 2010) have mainly focused on the reductive reactivity of adsorbed Fe(II) species. Soil weathering processes strongly infl uence the amount of adsorbed Fe(II) based on soil physicochemical properties

(Li et al., 2008a, 2008b). Th e potential correlations among the results listed in Tables 2 and 3 were distinguished using stepwise regression analysis. Th e observed results without MES solution were excluded in the analysis, however, to avoid the variable infl uence of pH. Using stepwise regression analysis, two equations were obtained when the sorbed Fe(II) and lnk values were chosen as dependent variables. When sorbed Fe(II) was chosen as the dependent variable, a linear correlation resulted for sorbed Fe(II) and Al2O3:

Table 4. Results of correlation analysis for sorbed Fe(II), peak oxidation potential (Ep) and the logarithm of the reaction rate (k) for experiments with and without 2-(N-morpholino)ethanesulfonic acid (MES).

Parameter Statistical test

Without MES (n = 22) With MES (n = 25)

Fe(II)sorbed Ep lnk Fe(IIsorbed Ep lnk

pHs† Pearson correlation 0.159 −0.571** −0.210 −0.079 0.215 0.012Signifi cance (two-tailed) 0.480 0.006 0.348 0.707 0.301 0.953

pHr‡ Pearson correlation −0.050 −0.432* 0.106 0.209 −0.452* 0.330

Signifi cance (two-tailed) 0.824 0.045 0.639 0.317 0.023 0.107

Organic matter Pearson correlation 0.151 0.026 −0.077 0.119 0.218 −0.199


CEC§ Pearson correlation 0.177 −0.318 −0.217 0.093 0.086 0.201


Fed¶ Pearson correlation 0.025 −0.461* −0.077 −0.115 −0.152 −0.026


Feo# Pearson correlation 0.073 −0.600** −0.243 −0.147 −0.136 −0.111


Fep†† Pearson correlation 0.089 0.079 −0.346 −0.364 −0.212 −0.502*


Fe2O3 Pearson correlation 0.046 −0.116 0.280 0.377 0.144 0.580**


MnO Pearson correlation 0.001 −0.312 0.043 0.321 0.102 0.296


SiO2 Pearson correlation −0.163 0.055 −0.246 −0.434* −0.392 −0.546**


Al2O3 Pearson correlation 0.240 −0.010 0.237 0.464* 0.195 0.501*


SiO2/Al2O3‡‡ Pearson correlation −0.292 0.036 −0.142 −0.406* −0.192 −0.406*


Al2O3/Fe2O3‡‡ Pearson correlation 0.212 0.310 0.007 0.149 0.210 −0.422*


SiO2/(Al2O3+Fe2O3)‡‡ Pearson correlation −0.288 0.046 −0.144 −0.401* −0.180 −0.420*


Sorbed Fe(II) Pearson correlation 1 −0.155 −0.348 1 0.411* 0.532**

Signifi cance (two-tailed) 0.492 0.112 0.041 0.006

Ep Pearson correlation −0.155 1 0.373 0.411* 1 0.071

Signifi cance (two-tailed) 0.492 0.087 0.041 0.734

lnk Pearson correlation −0.348 0.373 1 0.532** 0.071 1Signifi cance (two-tailed) 0.112 0.087 0.006 0.734

* Correlation is signifi cant at the 0.05 level (two-tailed t-test).** Correlation is signifi cant at the 0.01 level (two-tailed t-test). † Original pH conditions of soil suspension.‡ Reaction pH.§ Cation exchange capacity.¶ Dithionite-extractable Fe.# Oxalate=extractable Fe.†† Pyrophosphate-extractable Fe.‡‡ Molecular ratio.


( ) 2 3

2

sorbed Fe II 8.27 0.14Al O

0.22 0.001R P

= +

= ≤ [1]

When the lnk value was chosen as the dependent variable, an equation with a high coeffi cient of determination (R2 = 0.65, P ≤ 0.001) was obtained:

2 3 2

2

ln 0.43 0.02Fe O 0.07OM 0.01SiO

0.65 0.001

kR P

=− + − −

= ≤ [2]

A linear correlation was shown for the lnk value, Fe2O3, SiO2, and OM. Equation [1] shows that an increased amount of adsorbed Fe(II) species can be attributed to the increased amount of Al2O3. Equation [2] indicates that decreased amounts of Si and OM, as well as an increased amount of Fe2O3, have a signifi cant correlation with the 2-NP reductive transformation rate. Given that the desilicifi cation and allitization of soils lead to Fe and Al accumulation, the Fe2O3 and Al2O3 contents increase with weathering intensity because of clay mineral oxidation (Guan et al., 2001; Huang et al., 2008; Tan, 2008, p. 93–128). Th e stepwise analysis showed that the desilicifi cation and allitization of soils aff ect the Fe(II) adsorption degree and 2-NP reductive transformation rate. Moreover, it reveals the critical eff ect of weathering indices on the 2-NP reductive transformation processes in soils.

Promotion of 2-Nitrophenol Reductive Transformation by Soil Desilicifi cation and Allitization

Weathering is a signifi cant process in the formation of soil and its physicochemical properties (Vaughan, 1988). Th e weathering process signifi cantly aff ects the chemical composition of soils by governing the mineral components in tropical and subtropical areas. Figure 5 illustrates the trends in the average amount of Fe2O3, sorbed Fe(II) with MES, and average k value for soils classifi ed as Groups I (vegetable soils in the Pearl River Delta), II (orchard soils in the Pearl River Delta), and III (forest soils in the tropical zone). In these groups, the average amounts of OM, SiO2, and Fep and the molecular ratio of SiO2/(Al2O3 + Fe2O3) decreased, whereas the average amounts of Fe2O3, Al2O3, and sorbed Fe(II) with MES increased (Tables 2 and 3). For example, the average amounts of Fe2O3 and Al2O3 increased from 0.2 ± 0.1 (Group I) to 0.4 ± 0.3 mol kg−1 (Group III) and from 1.3 ± 0.5 (Group I) to 1.8 ± 1.1 mol kg−1 (Group III), respectively, whereas the molecular ratio of SiO2 decreased from 11.5 ± 2.2 (Group I) to 9.2 ± 3.3 mol kg−1 (Group III). Th is result indicates that the level of soil desilicifi cation and allitization in Group III is stronger than that in Groups I and II (Fig. 5).

Moreover, in Groups I to III (Fig. 5), the average amount of sorbed Fe(II) with MES (excluding the infl uence of variable pH) increased from 0.3 ± 0.1 (Group I) to 0.7 ± 0.3 mmol L−1 (Group III). Th e average k value of 2-NP reductive transformation, either without MES or with MES, also increased in the order Group I < Group II < Group III (Fig. 5). Th e trend in the average amount

of sorbed Fe(II) with MES, as well as the order of the increase in the average k value of 2-NP reductive transformation, are consistent with the increased levels of soil desilicifi cation and allitization. Previous reports have illustrated the dependence of adsorbed Fe(II) reactivity on pH values and have revealed the positive correlation between the 2-NP reductive transformation rate and sorbed Fe(II) (Li et al., 2009; Tao et al., 2009, 2010). Th e results of the correlation analyses and stepwise analyses in the current study prove the signifi cant correlation between both soil desilicifi cation and allitization and the amount of sorbed Fe(II) and the lnk value.

Th e 22 soil samples belong to vegetable soils (Group I), orchard soils (Group II), and forest soils (Group III). Forest soils have the characteristics of undisturbed soils, whereas vegetable soils and orchard soils have been strongly disturbed by human activities. Soils in Groups I and II are distributed across the Pearl River Delta with the characteristics of a subtropical marine monsoon climate (the annual rainfall is more evenly distributed and the wet and dry seasons are not very distinct). Th e soils in Group III are distributed across a tropical zone (tropical rainforest or monsoon forest area) with the characteristics of a monsoon climate, i.e., dry and foggy winter with wet and rainy summer. Th e degree of soil development and weathering in the tropical zone is stronger than that in the Pearl River Delta. Hence, the variance presents a signifi cant diff erence between Groups I and III but none between Groups I and II or between Groups II and III (Fig. 5).

In real subsurface environments, the active Fe(II) species mainly come from dissimilatory Fe reduction (Lovley et al., 1996, 2004; Li et al., 2010). Th ese species promote the reductive transformation of nitroaromatic compounds. Various microbes in soils have diff erent Fe-reduction abilities (Li et al., 2010; Wu

Fig. 5. Trends of average amount of Fe2O3, sorbed Fe(II) with 2-(N-morpholino)ethanesulfonic acid (MES), and average 2-nitrophenol (2-NP) reduction rates (k) classifi ed into groups: Group I, vegetable soils in Pearl River Delta; Group II, orchard soils in Pearl River Delta; and Group III, forest soils in the tropical zone. Average value within the same parameter that has the different lowercase letters (i.e. a or b) are signifi cantly different at P < 0.05, whereas average value within the same parameter that has the same lowercase letters are not signifi cantly different at P = 0.05.


et al., 2010). In addition, diff erent inorganic or organic acids in soils combine with Fe(II) to form active Fe(II)–L complexes, where L represents inorganic or organic ligands (Li et al., 2008a; Rakshit et al., 2009). Nevertheless, certain aspects of their correlations, such as Fe(II) species activities during dissimilatory Fe reduction and the reduction mechanism, are not well understood. Based on the results and discussion in the present study, soil-weathering indices are inferred to have an important infl uence on the nitroaromatic compound reduction capacity in soils. Th e rate of dissimilatory Fe reduction in soil may have a tight correlation with soil-weathering indices, and further studies are being conducted by our group to verify this hypothesis.

CONCLUSIONSTh e present study investigated the eff ects of pH and weathering

indices on the reductive transformation of 2-NP in 22 soil samples from South China. Th e soil samples were collected from the A (0–20 cm, the leached layer) horizon and divided into three groups: Group I, vegetable soils in the Pearl River Delta; Group II, orchard soils in the Pearl River Delta; and Group III, forest soils in the tropical zone. Experiments were designed and conducted under various conditions, including with and without MES (excluding pH interference). Th e kinetics of 2-NP reductive transformation show that an increase in pH yields an increase in adsorbed Fe(II) on mineral surfaces and a negative shift of the Ep of the Fe(III)–Fe(II) couple. Th is eff ect further results in an enhanced reaction rate of 2-NP reduction. Th rough statistical analyses, signifi cant diff erences in k were proved in diff erent soil types and parent materials. Th e reaction pH proved to be an essential factor in 2-NP reductive transformation, and the weathering indices of soils have a critical eff ect on the 2-NP reductive transformation. In Groups I to III, the increase in the increased level of soil desilicifi cation and allitization (i.e., decrease in Si content and increase in Fe and Al contents), and the decrease in the molecular ratio of SiO2/(Al2O3 + Fe2O3) are consistent with the order of the increase in the k value. Th ese fi ndings suggest that the level of soil weathering aff ects Fe reduction processes in soils. Our group is conducting a further investigation of the weathering indices for indexing Fe(III) reduction rates in subtropical soils to verify this hypothesis.

ACKNOWLEDGMENTSTh is work was fi nancially supported by the National Natural Science Foundation of PR China (no. 41025003, 41001136, and 40971149), the Natural Science Foundation of Guangdong Province for Doctoral Scientifi c Research Program (no. 10451065003005011), the Youth Science Foundation of Guangdong Province Academy of Sciences (no. qnjj201002), and the “973” Program (no. 2010CB134508).

REFERENCESAlexandrova, L.N. 1960. On the composition of humus substances and the

nature of organo-mineral colloids in soils. In: F.A. Van Beren et al. (ed.) Transactions of the 7th International Congress of Soil Science, Madison, WI. 14–21 Aug. 1960. Vol. 2. Elsevier, Amsterdam. p. 74–81.

Colón, D., E.J. Weber, and J.L. Anderson. 2006. QSAR study of the reduction of nitroaromatics by Fe(II) species. Environ. Sci. Technol. 40:4976–4982. doi:10.1021/es052425x

Colón, D., E.J. Weber, and J.L. Anderson. 2008. Eff ect of natural organic matter on the reduction of nitroaromatics by Fe(II) species. Environ. Sci. Technol. 42:6538–6543. doi:10.1021/es8004249

Cornell, R.M., and U. Schwertmann. 1996. Th e iron oxides: Structure, properties, reactions, occurrence and uses. Wiley-VCH Verlag, Weinheim, Germany.

Duzgoren-Aydin, N.S., A. Aydin, and J. Malpas. 2002. Re-assessment of chemical weathering indices: Case study on pyroclastic rocks of Hong Kong. Eng. Geol. 63:99–119. doi:10.1016/S0013-7952(01)00073-4

Fadrus, H., and J. Malý. 1975. Rapid extraction–photometric determination of traces of iron(II) and iron(III) in water with 1,10-phenanthroline. Anal. Chim. Acta 77:315–316. doi:10.1016/S0003-2670(01)95189-X

Guan, P., C.W.W. Ng, M. Sun, and W. Tang. 2001. Weathering indices for rhyolitic and granite in Hong Kong. Eng. Geol. 59:147–159. doi:10.1016/S0013-7952(00)00071-5

Hakala, J.A., Y.P. Chin, and E.J. Weber. 2007. Infl uence of dissolved organic matter and Fe(II) on the abiotic reduction of pentachloronitrobenzene. Environ. Sci. Technol. 41:7337–7342. doi:10.1021/es070648c

Hatter, D.R. 1985. Th e use and importance of nitroaromatic chemicals in the chemical industry. In: D.E. Rickert, editor, Toxicity of nitroaromatic compounds. Hemisphere, Washington, DC. p. 1–13.

Hiemstra, T., and W.H. van Riemsdijk. 2007. Adsorption and surface oxidation of Fe(II) on metal (hydr)oxides. Geochim. Cosmochim. Acta 71:5913–5933. doi:10.1016/j.gca.2007.09.030

Hofstetter, T.B., C.G. Heijman, S.B. Haderlein, C. Holliger, and R.P. Schwarzenbach. 1999. Complete reduction of TNT and other (poly)nitroaromatic compounds under iron-reducing subsurface conditions. Environ. Sci. Technol. 33:1479–1487. doi:10.1021/es9809760

Huang, L., J. Hong, W.F. Tan, H.Q. Hu, F. Liu, and M.K. Wang. 2008. Characteristics of micromorphology and element distribution of iron–manganese cutans in typical soils of subtropical China. Geoderma 146:40–47. doi:10.1016/j.geoderma.2008.05.007

Jeon, B.H., B.A. Dempsey, W.D. Burgos, M.O. Barnett, and E.E. Roden. 2005. Chemical reduction of U(VI) by Fe(II) at the solid–water interface using natural and synthetic Fe(III) oxides. Environ. Sci. Technol. 39:5642–5649. doi:10.1021/es0487527

Karim, K., and S.K. Gupta. 2003. Continuous biotransformation and removal of nitrophenols under denitrifying conditions. Water Res. 37:2953–2959. doi:10.1016/S0043-1354(03)00073-3

Klausen, J., S.P. Trober, S.B. Haderlein, and R.P. Schwarzenbach. 1995. Reduction of substituted nitrobenzenes by Fe(II) in aqueous mineral suspensions. Environ. Sci. Technol. 29:2396–2404. doi:10.1021/es00009a036

Klupinski, T.P., Y.P. Chin, and S.J. Traina. 2004. Abiotic degradation of pentachloronitrobenzene by Fe(II): Reactions on goethite and iron oxide nanoparticles. Environ. Sci. Technol. 38:4353–4360. doi:10.1021/es035434j

Kothawala, D.N., T.R. Moore, and W.H. Hendershot. 2009. Soil properties controlling the adsorption of dissolved organic carbon to mineral soils. Soil Sci. Soc. Am. J. 73:1831–1842. doi:10.2136/sssaj2008.0254

Krishnaswamy, J., and D.D. Richter. 2002. Properties of advanced weathering-stage soils in tropical forests and pastures. Soil Sci. Soc. Am. J. 66:244–253. doi:10.2136/sssaj2002.0244

Li, F.B., X.M. Li, S.G. Zhou, L. Zhuang, F. Cao, D.Y. Huang, et al. 2010. Enhanced reductive dechlorination of DDT in an anaerobic system of dissimilatory iron-reducing bacteria and iron oxide. Environ. Pollut. 158:1733–1740. doi:10.1016/j.envpol.2009.11.020

Li, F.B., L. Tao, C.H. Feng, X.Z. Li, and K.W. Sun. 2009. Electrochemical evidences for promoted interfacial reactions: Th e role of adsorbed Fe(II) onto γ-Al2O3 and TiO2 in reductive transformation of 2-nitrophenol. Environ. Sci. Technol. 43:3656–3661. doi:10.1021/es8033445

Li, F.B., X.G. Wang, Y.T. Li, C.S. Liu, F. Zeng, L.J. Zhang, et al. 2008a. Enhancement of the reductive transformation of pentachlorophenol by polycarboxylic acids at the iron oxide–water interface. J. Colloid Interface Sci. 321:332–341. doi:10.1016/j.jcis.2008.02.033

Li, F.B., X.G. Wang, C.S. Liu, Y.T. Li, F. Zeng, and L. Liu. 2008b. Reductive transformation of pentachlorophenol on the interface of subtropical soil colloids and water. Geoderma 148:70–78. doi:10.1016/j.geoderma.2008.09.003

Liu, A.S. 1993. Soils in Guangdong Province. (In Chinese.) Sci. Publ. of China, Beijing.


Lovley, D.R., J.D. Coates, E.L. Blunt-Harris, E.J.P. Phillips, and J.C. Woodward. 1996. Humic substances as electron acceptors for microbial respiration. Nature 382:445–448. doi:10.1038/382445a0

Lovley, D.R., D.E. Holmes, and K.P. Nevin. 2004. Dissimilatory Fe(III) and Mn(IV) reduction. Adv. Microb. Physiol. 49:219–286. doi:10.1016/S0065-2911(04)49005-5

Lu, R.S. 1999. Th e analysis method of soil agricultural chemistry. (In Chinese.) Publ. Co. of Agric. Sci. Technol. of China, Beijing.

Mehra, O.P., and M.L. Jackson. 1958. Iron oxide removed from soils and clays by a dithionite–citrate system buff ered with sodium bicarbonate. Clays Clay Miner. 7:317–329. doi:10.1346/CCMN.1958.0070122

Naka, D., D. Kim, and T.J. Strathmann. 2006. Abiotic reduction of nitroaromatic compounds by aqueous iron(II)–catechol complexes. Environ. Sci. Technol. 40:3006–3012. doi:10.1021/es060044t

Nano, G.V., and T.J. Strathmann. 2006. Ferrous iron sorption by hydrous metal oxides. J. Colloid Interface Sci. 297:443–454. doi:10.1016/j.jcis.2005.11.030

Osei, B.A., and B. Singh. 1999. Electrophoretic mobility of some tropical soil clays: Eff ect of iron oxides and organic matter. Geoderma 93:325–334. doi:10.1016/S0016-7061(99)00078-6

Pansu, M., and J. Gautheyrou. 2006. Handbook of soil analysis: Mineralogical, organic and inorganic methods. Springer-Verlag, Dordrecht, the Netherlands.

Pecher, K., S.B. Haderlein, and R.P. Schwarzenbach. 2002. Reduction of polyhalogenated methanes by surface-bound Fe(II) in aqueous suspensions of iron oxides. Environ. Sci. Technol. 36:1734–1741. doi:10.1021/es011191o

Rakshit, S., M. Uchimiya, and G. Sposito. 2009. Iron(III) bioreduction in soil in the presence of added humic substances. Soil Sci. Soc. Am. J. 73:65–71. doi:10.2136/sssaj2007.0418

Rasmussen, C., R.A. Dahlgren, and R.J. Southard. 2010. Basalt weathering and pedogenesis across an environmental gradient in the southern Cascade Range, California, USA. Geoderma 154:473–485. doi:10.1016/j.geoderma.2009.05.019

Rasmussen, C., N. Matsuyama, R.A. Dahlgren, R.J. Southard, and N. Brauer. 2007. Soil genesis and mineral transformation across an environmental gradient on Andestic lahar. Soil Sci. Soc. Am. J. 71:225–237. doi:10.2136/sssaj2006.0100

Rügge, K., T.B. Hofstetter, S.B. Haderlein, P.L. Bjerg, S. Knudsen, C. Zraunig, et al. 1998. Characterization of predominant reductants in an anaerobic leachate-contaminated aquifer by nitroaromatic probe compounds. Environ. Sci. Technol. 32:23–31. doi:10.1021/es970249p

Schwertmann, U. 1964. Diff erenzierung der eisenoxide des bodens durch photochemische extraktion mit sauer ammoniumoxalat-losung. Z. Pfl anzenernaehr. Bodenkd. 105:194–202. doi:10.1002/jpln.3591050303

Schwertmann, U. 1988. Occurrence and formation of iron oxides in various pedoenvironments. In: J.W. Stucki et al., editors, Iron in soil and clay minerals. NATO Ser. D. Reidel Publ. Co., Dordrecht, the Netherlands. p. 267–308.

Strathmann, T.J., and A.T. Stone. 2001. Reduction of the carbamate pesticides oxamyl and methomyl by dissolved FeII and CuI. Environ. Sci. Technol. 35:2461–2469. doi:10.1021/es001824j

Strathmann, T.J., and A.T. Stone. 2003. Mineral surface catalysis of reactions between FeII and oxime carbamate pesticides. Geochim. Cosmochim. Acta 67:2775–2791. doi:10.1016/S0016-7037(03)00088-7

Stumm, W., and B. Sulzberger. 1992. Th e cycling of iron in natural environments: Considerations based on laboratory studies of heterogeneous redox processes. Geochim. Cosmochim. Acta 56:3233–3257. doi:10.1016/0016-7037(92)90301-X

Tan, K.H. 2008. Soil in the humid tropics and monsoon region of Indonesia. CRC Press, Boca Raton, FL.

Tao, F.L., and Z.W. Feng. 2000. Terrestrial ecosystem sensitivity to acid deposition in South China. Water Air Soil Pollut. 118:231–243. doi:10.1023/A:1005139719658

Tao, L., and F.B. Li. 2011. Electrochemical evidence of Fe(II)/Cu(II) interaction on titanium oxide for 2-nitrophenol reductive transformation. Appl. Clay Sci. (in press). doi:10.1016/j.clay.2011.05.020

Tao, L., F.B. Li, C.H. Feng, and K.W. Sun. 2009. Reductive transformation of 2-nitrophenol by Fe(II) species in γ-aluminum oxide suspension. Appl. Clay Sci. 46:95–101. doi:10.1016/j.clay.2009.07.005

Tao, L., F.B. Li, Y.K. Wang, and K.W. Sun. 2010. Reductive activity of adsorbed Fe(II) on iron (oxyhdydr)oxides for 2-nitrophenol transformation. Clays Clay Miner. 58:682–690. doi:10.1346/CCMN.2010.0580507

Vaughan, P.R. 1988. Characterizing the mechanical properties of in-situ residual soil. In: Proceedings of the 2nd International Conference on Geomechanics in Tropical Soils, Singapore. 12–14 Dec. 1988. A.A. Balkema, Rotterdam, the Netherlands. p. 469–487.

Wu, C.Y., L. Zhuang, S.G. Zhou, F.B. Li, and X.M. Li. 2010. Fe(III)-enhanced anaerobic transformation of 2,4-dichlorophenoxyacetic acid by an iron-reducing bacterium Comamonas koreensis CY01. FEMS Microbiol. Ecol. 71:106–113. doi:10.1111/j.1574-6941.2009.00796.x

Documents

Soil Chemistry Effect of pH and Weathering Indices on the ... · Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou 510640, PR China and Graduate School of