Adsorption-desorption behavior of dissolved organic carbon by soil clay fractions of varying mineralogy

Geoderma 280 (2016) 47–56

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Adsorption-desorption behavior of dissolved organic carbon by soil clay fractions of varying mineralogy Mandeep Singh a, Binoy Sarkar a,b, Bhabananda Biswas a, Jock Churchman a,c,⁎, Nanthi S. Bolan d a

Future Industries Institute (FII), University of South Australia, Mawson Lakes, SA 5095, Australia Department of Geological Sciences, Indiana University, Bloomington, Indiana 47405, USA School of Agriculture, Food and Wine, University of Adelaide, Urrbrae, SA 5064, Australia d Global Centre for Environmental Remediation, University of Newcastle, Callaghan, NSW 2308, Australia b c

a r t i c l e

i n f o

Article history: Received 16 December 2015 Received in revised form 22 March 2016 Accepted 4 June 2016 Available online 16 June 2016 Keywords: Organic carbon stabilization Soil clay fractions Sesquioxides Adsorption-desorption Specific surface area Background electrolyte

a b s t r a c t Soil clay minerals significantly influence the accumulation and stabilization of organic carbon (OC). However, the effect of interactions among phyllosilicate clay minerals, native OC and sesquioxides (Fe/Al oxides) on the adsorption-desorption of dissolved organic carbon (DOC) under different background electrolyte types and concentration is poorly understood. A set of batch adsorption-desorption experiments were conducted using pedogenic clays extracted from soils dominated by kaolinite-illite (Kaol-Ill), smectite (Smec) and allophane (Allo). The clay samples were sequentially treated to remove native OC and sesquioxides, and tested for adsorption-desorption of DOC under various solution conditions. All the experiments were conducted at pH 7 using water extractable fraction of OC from wheat residues. DOC adsorption increased with increasing background electrolyte concentration, and the presence of Ca2+ significantly enhanced the uptake in comparison to Na+ due to a possible cationic bridging effect. Under all electrolyte conditions, the maximum DOC adsorption capacity (Qmax) (mg g−1) of the soil clay fractions (SCF) maintained the order: Allo N Smec N Kaol-Ill. A similar order was also observed when the adsorption capacities were normalized to the specific surface area (SSA) of the SCFs (mg m−2). DOC adsorption showed a positive relationship with SSA, and sesquioxides and allophanic minerals provided the largest contributions to the SSA in the SCF. Removal of sesquioxides from the SCF resulted in a decrease in SSA and thus DOC adsorption, whereas removal of native OC increased the SSA and subsequent DOC adsorption. Because this study used pedogenic SCFs which represented soils formed in different environments instead of processed clays from geological deposits, it provided realistic information about the interaction of DOC with SCF in relation to their native OC and sesquioxide contents. It also revealed the importance of Ca2+ in enhancing the carbon adsorption capacities of these SCFs. © 2016 Published by Elsevier B.V.

1. Introduction In the global carbon (C) cycle, adsorption of organic carbon (OC) on clay mineral surfaces is an important process for stabilizing soil OC under the natural environment (Feng et al., 2005). Adsorption of dissolved organic carbon (DOC) by phyllosilicate clays and Fe/Al oxides controls its mobility, retention and degradation through mineralization, thus leading to stabilization of OC in soils (Mikutta et al., 2007; Kothawala et al., 2008; Bolan et al., 2011; Saidy et al., 2015). DOC, along with other plant nutrients can also be leached into the groundwater or lost from the soil surface through runoff (Baldock and Skjemstad, 2000), which ultimately degrades the drinking water quality (EPA-South-Australia, 1998). Also, leaching of DOC from the soil surface can impact the nutrient ⁎ Corresponding author at: University of Adelaide, Australia. E-mail addresses: [email protected] (B. Sarkar), [email protected] (J. Churchman).

http://dx.doi.org/10.1016/j.geoderma.2016.06.005 0016-7061/© 2016 Published by Elsevier B.V.

availability to plants and degradation of soil structure (Kalbitz et al., 2000; Mavi et al., 2012). The free movement of DOC is mainly controlled by its adsorption to soil clay surfaces (Ussiri and Johnson, 2004). The ability of a soil to sequester C may depend on the nature of dominant clay minerals and also the clay content (Shen, 1999; Kahle et al., 2003). In general, soil clays with higher specific surface area (SSA), cation exchange capacity (CEC) and Fe/Al oxides lose less C through mineralization (Ransom et al., 1998; Kahle et al., 2003). Despite having similar clay contents (texture), allophanic and smectitic soils retained more C than kaolinitic or vermiculitic soils (Saggar et al., 1996, 1999). Kaolinitic clay minerals (low activity clays) have a 1:1 layer structure, a low CEC, low SSA and some pH-variable charge. Smectitic clay minerals have a 2:1 structure, a higher CEC and SSA, and predominantly permanent negative charge. Illitic clay minerals, which are non-expanding, have a SSA and CEC that are higher than kaolinitic clay minerals but lower than smectitic types. Allophanes are poorly crystalline, have a very high SSA, but a pH-variable charge that only gives a moderate CEC

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M. Singh et al. / Geoderma 280 (2016) 47–56

(Churchman and Lowe, 2012). The presence of sesquioxides in the soil clay fractions (SCF) greatly influences the DOC adsorption capacity due to their higher SSA (Wiseman and Püttmann, 2006; Spielvogel et al., 2008; Saidy et al., 2013) and specific chemical interactions (Kleber et al., 2015). A number of mechanisms for the adsorption of DOC on clay mineral surfaces have been proposed including inner sphere complexation (e.g., ligand exchange and cationic bridging) or outer sphere complexation also known as physical adsorption (e.g., hydrogen bonding), van der Waals forces and pi–donor–acceptor interactions (Gu et al., 1994; Guggenberger and Kaiser, 2003; Mikutta et al., 2007; Rashad et al., 2010; Keiluweit and Kleber, 2009). Earlier studies also showed strong effects of background electrolyte, ionic strength, pH, native OC, sesquioxides and soil mineral contents on adsorption-desorption of DOC (Kahle et al., 2004; Lützow et al., 2006; Kothawala et al., 2009; Rashad et al., 2010). An increased DOC adsorption to clay surfaces could be attributed to a higher background electrolyte concentration in the soil solution (Mavi et al., 2012). Divalent cations (e.g., Ca2 +) would form stronger bonds between negatively charged clay minerals and DOC through cation bridging than monovalent cations (e.g., Na+) (Marchuk and Rengasamy, 2011; Roychand and Marschner, 2014; Setia et al., 2013). The ratio between Ca2 + to Na+ in soil- solution would also affect the DOC adsorption-desorption behavior on clay mineral surfaces (Chorom and Rengasamy, 1995). Despite many studies reporting on the adsorption of DOC by clay minerals under various conditions, a lack of understanding of adsorption mechanisms remains. Most of the previous studies were conducted using refined aluminosilicate minerals obtained from geological claydeposits (Guggenberger and Kaiser, 2003; Feng et al., 2005; Rashad et al., 2010; Saidy et al., 2013) while pedogenic soil clays with or without native OC or sesquioxides could behave differently than clay minerals from deposits (Churchman and Lowe, 2012). Soil clay minerals are very different from processed clay minerals from clay-deposits because of their formation in highly heterogeneous and dynamic soil environments (Churchman, 2010; Churchman et al., 2012; Churchman and Lowe, 2012). Coating with OM or sesquioxides, or both, on their reactive surfaces typify many soil clays, and the properties of the clays may reflect the characteristics of the coated material rather than the aluminosilicate component of clay itself (Kahle et al., 2004; Churchman and Lowe, 2012). Soil clay minerals might undergo a greater degree of weathering, represent a smaller particle size, contain a greater proportion of amorphous materials and have different SSA and charge characteristics in comparison to processed clay minerals (Wilson, 1999; Churchman, 2010; Churchman and Lowe, 2012). Also, the dominant exchangeable cations in soil clay minerals could be completely different from that of a processed clay mineral (Churchman and Lowe, 2012). As a result, the interactions of DOC with soil clays could be different from that of clay minerals from deposits. The interactions could also vary for soil clays originating in different environments (Churchman, 2010; Churchman and Lowe, 2012). Therefore, batch DOC adsorptiondesorption experiments were conducted in this study with three pedogenic SCFs (kaolinitic-illitic, smectitic and allophanic) with and without native OC and sesquioxides under different solution conditions. Instead of using aluminosilicates from deposits, soil-isolated clays were used in

order to get a better understanding of the behavior of the C sequestration capacity of clay minerals in soils. It was hypothesized that: (1) high electrolyte concentration would favor DOC uptake in contrast to low electrolyte concentration; (2) higher adsorption would occur in the presence of a divalent cation (Ca2 +) than a monovalent cation (Na+) on the exchange complex; and (3) DOC uptake would vary with varying SSA of the SCF as influenced by their native OC and sesquioxide contents. 2. Materials and methods 2.1. Clay isolation, removal of OC and physico-chemical characterization This study included clays which were extracted from clay-rich (5– 20 cm depth) soils, collected from 3 different sites in South Australia Hoyleton, Waite campus and Mt. Schank. The Hoyleton soil (from the location of the virgin soil studied by Churchman et al. (2010)) was rich in kaolinite and illite with small amounts of inter-stratified illitesmectite, smectite, quartz, feldspar and iron oxide (goethite). Smectite was the dominant clay mineral in the Waite campus soil (from the same location as soil A1024 in Stace et al. (1968)) but with a small amount of inter-stratified illite-smectite, smectite, illite, quartz, feldspar and iron oxides (goethite and hematite). Mt. Schank soil (from the Laslett Road location in Lowe and Palmer (2005)) was dominant in allophane with small amounts of inter-stratified kaolinite- smectite, quartz, feldspar and Fe oxides (ferrihydrite and goethite). The mineralogical composition and pH of the experimental soils and the CEC values of the isolated SCF are summarized in Table 1. The soils were passed through a 2 mm sieve and stored air-dry prior to clay extraction. Clay extraction was carried out using a prolonged shaking method for the disintegration of soil particles (Churchman and Tate, 1986; Roychand and Marschner, 2013). The prolonged shaking method was employed in order to retain the natural physico-chemical properties of the isolated clays. The air-dried soil (50 g) was placed in a 2 L Schott bottle with 500 mL Milli-Q water (18.2 Ω) (1 soil: 10 water ratio). After end-over-end shaking (40 rev min−1) for 24 h, the suspension was transferred to a 1 L measuring cylinder. The 30 cm height of the soil-water solution in the measuring cylinder was achieved by topping up with Milli-Q water, stirring, and then left to settle for 16 h at 22 °C. According to Stoke's Law, the b 2 μm clay fraction was collected by siphoning off the top 22 cm of the soil suspension using a pump to avoid disturbance in the suspension. The clay suspension was centrifuged for 30 min at 3500 rpm, and the clear supernatant was discarded. The precipitated clay was collected, freeze dried, and stored in a desiccator for further analysis. Total OC concentration in the SCF was analyzed by combustion using a Leco C/N analyzer (Leco TruMac® CNS/NS, USA) following complete elimination of inorganic carbon using HCl. The combustion temperature and oxygen flow time were 1300 °C and 5 s, respectively. The CEC of SCF was measured using a method described by Rayment (2011). In brief, a 0.5 g sample was mixed with 30 mL solution of 1 M sodium acetate (pH 7) and shaken for 15 min using an end-over-end shaker. This process was repeated 4 times. After complete saturation with sodium acetate, the sediment was washed with ethanol (N 99% purity) to achieve

Table 1 Properties of experimental soils and clay fractions. Soil types

Clay % in soil

Clay fractions present

pH

CEC of SCF (cmol (P+) kg−1)

Kaolinitic-illitic

18

7.83

9.4

Smectitic

44

8.25

70

Allophanic

28

Co-dominant: kaolinite, illite sub-dominant:illite- smectite, smectite, quartz Co-dominant: smectite, kaolinite sub-dominant: illite-smectite, kaolinite, quartz, feldspar Dominant: allophane sub-dominant: kaolinite-smectite, quartz, feldspar

7.7

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49

Table 2 Carbon contents in soil clay fractions before and after NaOCl treatment and the removal efficiency. Clay type

Kaol-Ill Smec Allo

Untreated

NaOCl-treated

Removal efficiency

TOC (g kg−1)

N (g kg−1)

C/N

OC (g kg−1)

N (g kg−1)

C/N

TOC (%)

N (%)

15 22.5 129.5

3.3 3.1 14

4.5 7.3 9.3

3.9 3 31.7

2.3 2.3 3

1.7 1.3 10.6

74 86.7 75.5

32.4 25.8 78.6

Table 3 Composition of DOC solution extracted from wheat straw. Parameter

Value

pH (1:2) C (mg L−1) N (mg L−1) Ca (mg L−1) Mg (mg L−1) Na (mg L−1) K (mg L−1) Fe (mg L−1) Al (mg L−1)

6.12 1981 92 17.1 14.4 23.4 198.9 0.14 0.18

the EC b 40 μS cm−1. The sediment was then equilibrated with 30 mL of 1 M ammonium acetate at pH 7 for 15 min. This process was repeated 3 times and the final volume of supernatant was made up to 100 mL using Milli-Q water. Sodium in the supernatant was measured using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) (Perkin Elmer 5300 V, USA). The SSA of SCF was analyzed by performing N2 adsorption experiments at liquid nitrogen temperature (− 196 °C) on a Micromeritics Gemini 2380 surface area analyzer. Samples were outgassed at 70 °C for 12 h under high vacuum (2 Torr). Removal of OC from SCF was conducted using a method proposed by Mikutta et al. (2005). In brief, 5 g of each SCF was mixed with 40 mL of 6% NaOCl (pH 8) and shaken for 4 h at 22 °C using a rotary shaker. The process was repeated several times until all effervescence ceased and clear supernatant started appearing. The C-removed samples were washed several times with Milli-Q water to remove residual NaOCl. After centrifugation for 30 min at 3500 rpm, all samples were dialyzed for 4–5 weeks to achieve EC b 40 μS cm−1 and then freeze dried. Following removal of OC, a small part of the sample was used to estimate the OC removal efficiency of the employed method (Table 2). A portion of OC-removed sample was further treated to remove the sesquioxides. Citrate-dithionite-bicarbonate (CDB) treatment was used for removing the free (non-structural) Fe (Fed) and Al (Ald) (Blakemore et al., 1987). This procedure was repeated several times to achieve a homogenous light bluish/greyish color which was an indication of complete removal of sesquioxides. To confirm the presence of allophane mineral in Mt. Schank SCF, acid oxalate-extractable Fe (Feo), Al (Alo), and Si (Sio), and pyrophosphate–extractable Fe (Fep) and Al (Alp) contents were also measured by using ICP-OES (Perkin Elmer 5300 V) (Blakemore et al., 1987). The natural SCF samples, OC-removed samples, and OC and sesquioxides-removed samples were denoted as Kaol-Ill-1, Smec-1, and Allo-1; Kaol-Ill-2, Smec-2, and Allo-2; and Kaol-Ill-3, Smec-3, and Allo-3, respectively. 2.2. Preparation of DOC solution DOC was extracted from oven-dried and finely ground wheat straw. Wheat straw was selected for DOC extraction in order to obtain a concentrated stock solution of the adsorbate with consistent composition throughout the study. In brief, 200 g straw was mixed with 900 mL of

Milli-Q water and stirred for 1 h using an end-over-end shaker and then left to settle for 4 days in a 4 °C constant-temperature room. The supernatant was separated by centrifugation (4000 rpm for 20 min), decanted off, and filtered through a 0.45-μm membrane filter (Millipore Corporation, USA) using a vacuum suction unit. The C concentration (1981 mg C L− 1) in the filtrate was measured using a TOC analyzer (Shimadzu: TOC-LCSH), and the cationic components were measured using ICP-OES (Perkin Elmer 5300V, USA) (Table 3). 2.3. Batch adsorption experiment Freeze dried SCF sample (30 mg) was mixed with the background electrolyte (0.1 M/0.01 M Ca(NO3)2/NaNO3) and topped up with a known amount of DOC stock solution to obtain a final volume of 30 mL and desired DOC concentrations: 0, 25, 50, 75, 100, 150 and 200 mg C L− 1, which supplied 0, 250, 500, 750, 1000, 1500 and 2000 mg C kg−1 clay. A wide adsorbate to solution ratio (1:1000) was used in order to achieve the maximum adsorption capacity of SCF. Batch adsorption experiments were conducted in triplicate at pH 7.0 in 50-mL polypropylene centrifuge tubes. The suspensions were equilibrated for 24 h (a preliminary experiment confirmed achievement of adsorption equilibrium in about 18 h) at 4 °C in the dark to minimize any C loss by mineralization. Blank experiments, with only DOC and only SCF, were included to measure any other possible DOC loss and likely desorption of native C, respectively. The suspension was then centrifuged (4000 rpm for 20 min) and subsequently filtered through a 0.45-μm membrane filter (Millipore Corporation, USA). The DOC concentration in the supernatant was measured using a TOC analyzer (Shimadzu: TOC-LCSH, Japan). Adsorption of DOC, Cs (mg C kg−1), was calculated using (Eq. (1)): Cs ¼ ðCo −Ce ÞV=W

ð1Þ

where Co is the initial DOC concentration (mg L−1), Ce is the equilibrium DOC concentration (mg L−1), V is the suspension volume (L), and W is the weight of sample (kg). The adsorption data was fitted to the Langmuir isotherm model which is expressed as (Eq. (2)): qm ¼ ðXm KL Ce Þ=ð1 þ KL Ce Þ

ð2Þ

where qm is the amount of adsorbate adsorbed (mg kg−1), Ce is the adsorbate concentration in the solution phase (mg L−1), KL is the Langmuir binding constant, and Xm is an estimate of the maximum adsorption capacity (mg kg−1). 2.4. Desorption experiment The reversibility of adsorption was examined by conducting desorption experiments. The SCF (after adsorption) were first washed with Milli-Q water to remove the DOC entrained in the equilibrium solution. In brief, 20 mL Milli-Q water was added in centrifuge tubes and shaken vigorously for a few seconds, and the supernatant was discarded following centrifugation (at 4000 rpm for 20 min). Then 30 mL Milli-Q water was added to the samples, the mixture was equilibrated under similar conditions as those for the adsorption experiment, and the supernatant

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M. Singh et al. / Geoderma 280 (2016) 47–56

Fig. 1. Isotherms of DOC adsorption on sequentially treated soil clay fraction in 0.1 M Ca(NO3)2 (a, b and c) and 0.01 M Ca(NO3)2 (d, e and f). Clay types 1, 2 and 3 represent natural SCF , Cremoved SCF, and C and sesquioxides-removed SCF, respectively. Symbols denote experimental points while the curves represent Langmuir isotherm fitted to the experimental data. Error bars indicate standard error of the mean (n = 3; p b 0.05).

was analyzed for DOC following filtering through a 0.45-μm filter. All desorption data was normalized using the weight of dry clay sample. The percentage of desorption was calculated by using Eq. (3): Desorption% ¼ ðQ d  100Þ=Cs

ð3Þ

where Cs is the amount of DOC adsorbed (mg C kg−1) and Qd is the amount of DOC desorbed in Milli-Q water (mg C kg−1).

3. Results and discussion The DOC adsorption data for all the SCF with and without native C and sesquioxides best fitted the Langmuir isothermal model with R2 values ranging from 0.90 to 0.97 at 95% confidence level (Figs. 1 and 2; Table 4). The DOC adsorption capacities calculated as the Qmax values using the Langmuir model differed according to the soil clay mineral composition. DOC uptake was positively correlated with the presence and/or contents of sesquioxides and SSA, but negatively correlated with the native C contents.

2.5. Statistical analysis

3.1. Effect of soil mineral composition on DOC uptake

Two-way analysis of variance (ANOVA) was performed to determine the effect of treatments (SCF) and electrolyte types on the adsorption of DOC. Duncan's Multiple Range Test (DMRT) at p b 0.05 was used to determine whether means differed significantly. Microsoft Excel (Microsoft Corporation, USA) and/or SPSS windows version 20.0 (SPSS Inc., Chicago, USA) were used for data analyses.

DOC uptake by all the SCF increased with increasing adsorbate concentrations (Figs. 1 and 2). Among the three clay types studied, the highest DOC adsorption (mass basis) was observed in Allo-2 followed by Smec-2 and Kaol-Ill-2 (Figs. 1 and 2) in the presence of 0.1 M Ca(NO3)2 background electrolyte. Kaolinite and smectite are the most abundant phyllosilicate minerals in soils and they have very different

M. Singh et al. / Geoderma 280 (2016) 47–56

51

Fig. 2. Isotherms of DOC adsorption on sequentially treated soil clay fraction in 0.1 M NaNO3 (a, b and c) and 0.01 M NaNO3 (d, e and f). Clay types 1, 2 and 3 represent natural SCF , Cremoved SCF, and C and sesquioxides-removed SCF, respectively. Symbols denote experimental points while the curves represent Langmuir isotherm fitted to the experimental data. Error bars indicate standard error of the mean (n = 3; p b 0.05).

physico-chemical characteristics (see Introduction). While smectite and kaolinite are both layered crystalline silicate minerals with moderate or low SSA, allophane is a poorly crystalline mineral consisting of smaller

particles with high SSA (Parfitt, 1990; Wattel-Koekkoek et al., 2001). The structural characteristics of the soil clay minerals greatly influence their DOC adsorption capacities. The trend of DOC uptake

Table 4 Langmuir parameters and affinity coefficients (K) of SCF -organic associations in Ca(NO3)2 and NaNO3 (0.1 and 0.01 M) at pH 7. Clay typea

Background electrolyte 0.1 M Ca(NO3)2 Qmax (mg g

Kaol-Ill-1 Kaol-Ill-2 Kaol-Ill-3 Smec-1 Smec-2 Smec-3 Allo-1 Allo-2 Allo-3 a

23.14 33.11 19.19 52.08 81.96 42.55 108.69 135.13 72.99

−1

)

0.01 M Ca(NO3)2 K (L mg 0.013 0.010 0.011 0.006 0.014 0.006 0.020 0.035 0.014

−1

)

2

r

Qmax (mg g

0.95 0.96 0.92 0.93 0.94 0.93 0.95 0.92 0.92

21.97 29.24 16.92 50.25 77.51 36.90 101.10 123.45 70.92

−1

)

0.1 M NaNO3 K (L mg 0.011 0.011 0.011 0.006 0.014 0.007 0.016 0.035 0.014

−1

)

r

2

0.90 0.97 0.95 0.92 0.93 0.90 0.92 0.96 0.92

Qmax (mg g 16.83 22.57 13.03 43.29 61.72 33.00 97.08 120.48 60.60

0.01 M NaNO3

−1

)

K (L mg 0.013 0.017 0.010 0.007 0.020 0.008 0.018 0.036 0.018

−1

)

r

2

0.97 0.94 0.95 0.90 0.95 0.93 0.94 0.96 0.93

Qmax (mg g−1)

K (L mg−1)

r2

15.36 19.37 10.19 36.23 59.17 21.73 93.45 113.63 60.24

0.012 0.026 0.014 0.009 0.019 0.018 0.019 0.037 0.017

0.96 0.96 0.93 0.94 0.97 0.92 0.93 0.97 0.94

Kaol-Ill-1, Smec-1 and Allo-1 = Natural SCFs; Kaol-Ill-2, Smec-2 and Allo-2 = C-removed SCFs; Kaol-Ill-3, Smec-3 and Allo-3 = C & sesquioixdes-removed SCFs.

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M. Singh et al. / Geoderma 280 (2016) 47–56

3.2. Effect of native C and oxides on DOC uptake

Table 5 SSA and calculated Qmax with respect to SSA for different SCF. Clay typea

SSA (m2

Kaol-Ill-1 Kaol-Ill-2 Kaol-Ill-3 Smec-1 Smec-2 Smec-3 Allo-1 Allo-2 Allo-3

55 69 39 74 93 60 119 140 90

g−1)

Qmax with respect to SSA (mg m−2)b 0.1 M Ca(NO3)2

0.01 M Ca(NO3)2

0.1 M NaNO3

0.01 M NaNO3

0.42 0.48 0.49 0.70 0.88 0.70 0.91 0.96 0.81

0.40 0.42 0.43 0.68 0.84 0.61 0.84 0.88 0.79

0.31 0.33 0.33 0.58 0.66 0.55 0.81 0.86 0.67

0.28 0.28 0.26 0.49 0.64 0.36 0.78 0.81 0.67

a Kaol-Ill-1, Smec-1 and Allo-1 = Natural SCFs; Kaol-Ill-2, Smec-2 and Allo-2 = C-removed SCFs; Kaol-Ill-3, Smec-3 and Allo-3 = C & sesquioixdes-removed SCFs. b Qmax values, which were obtained from the Langmuir model, were divided by the respective SSA.

Fig. 3. Relationship between SSA and Qmax values obtained under various experimental conditions (values are mean; n = 3).

(Allo N Smec N Kaol-Ill) in this study is in agreement with several previous reports (Saggar et al., 1996; Dontsova and Bigham, 2005; Wang and Xing, 2005; Saidy et al., 2013). The difference in SSA (Table 5) of SCF was one of the major factors governing the uptake of DOC (Lützow et al., 2006; Saidy et al., 2013). SSA was assessed by N2 adsorption on the adsorbents, and gave an estimation of the external surface area of the minerals potentially available for DOC uptake. The results showed a high correlation (R2 = 0.97; at 95% confidence level) between DOC adsorption capacities and SSA of the clay fractions (Fig. 3). In some cases, DOC adsorption could be related more to the pedogenic oxide contents of soils than the SSA or CEC (Kahle et al., 2004). In this study, DOC adsorption capacities correlated moderately with the CEC values (from Table 1) of the clay fractions with dominant crystalline physllosilicates (Kaol-Ill-1 and Smec-1), but not with those dominated by the poorly crystalline Allo-1.

Removal of native C resulted in an increase in SSA and a higher uptake of DOC by all the clay fractions (Tables 4 and 5; Figs. 1 and 2). It led to an increase of SSA to the extent of 25.5, 26 and 17.6% in the Kaol-Ill-2, Smec-2 and Allo-2 fractions, respectively. These results are in agreement with previously reported findings (Mikutta et al., 2005; Anda et al., 2008; Herbrich et al., 2015). Native C removal increased the SSA by facilitating N2 penetration into the pores which otherwise were clogged by the C compounds (Sarkar et al., 2011). Native OC might adsorb a very small amount of N2 gas due to its own very low surface area (Theng et al., 1999). Additionally, the C removal treatment using NaOCl at high pH (pH = 8) might lead to a partial dispersion of the micro-aggregates, which subsequently provided finer sized clay particles and improved SSA (Yukselen-Aksoy and Kaya, 2010). Under the 0.1 M Ca(NO3)2 electrolyte condition, the increase in Qmax values for Kaol-Ill-2, Smec-2 and Allo-2 were 43, 57.4 and 24.3%, respectively, in comparison to the untreated adsorbents (with native C) (Table 4). Similarly, in the 0.01 M NaNO3 electrolyte condition, the increase was to the extent of 26.1, 63.3 and 55.1%, respectively (Table 4). High contents of native C might also reduce the amount of adsorbed DOC because organic matter would already occupy most of the available binding sites on the clay surfaces (Kaiser and Zech, 2000; Nguyen and Marschner, 2014; Vogel et al., 2014). However, DOC uptake might not be related to the native C contents in some instances where pedogenic oxide contents of soils appeared to be the more important factors controlling SSA and solute adsorption (Kahle et al., 2004). Contrarily, removal of sesquioxides from the samples resulted in a significant decline in SSA (Table 5), which subsequently reduced the capacities of the adsorbents to uptake DOC (Figs. 1 and 2). In Kaol-Ill-3, Smec-3 and Allo-3 fractions, removal of sesquioxides accounted for 41, 23.3 and 32.2% reduction in SSA, respectively (Table 5). As a consequence, the DOC adsorption capacities of Kaol-Ill-3, Smec-3 and Allo-3 fractions decreased by 72.5, 92.6 and 85.1%, respectively, in 0.1 M Ca(NO3)2 electrolyte (Table 4). The given mineralogical composition of Fe/Al oxides was based on the CDB, acid-oxalate and pyrophosphate extractable fractions. The mineralogical composition of the hydrous iron oxide in the SCF might also have influenced the DOC uptake. The Kaol-Ill SCF was dominant in goethite and hematite, whereas the Allo SCF was rich in ferrihydrite; the Smec SCF may have contained both (goethite and hematite – measured together) and also ferrihydrite, in similar proportions (Table 6). Ferrihydrite showed the strongest effect on SSA and thus DOC adsorption. Previous reports (Kahle et al., 2003, 2004) had also indicated that DOC adsorption was directly related to the content of sesquioxides. On the other hand, many previous studies had also found adsorption was reduced due to saturation of the reaction sites with native C compounds (Kothawala et al., 2009; Kindler et al., 2011; Pengerud et al., 2014; Zhang et al., 2014). In addition, the Allo clay fraction contained about 21% allophane (Table 6). There is a strong relationship between the contents of OC and allophane in soils (Percival et al., 2000; Basile-Doelsch et al., 2005; Rasmussen et al., 2007). Parfitt (2009) also reported that allophanic soils protect more OC by forming a strong complex and by reducing bacterial activity due to the poor

Table 6 Fe, Al and Si contents (%) in SCF after different treatments and calculated contents of allophane, ferrihydrite and (goethite and hematite). SCF type

Feda

Kaol-Ill Smec Allo

1.49 0.12 0.14 0.75 0.35 0.18 0.22 – 0.9 0.28 0.08 0.19 0.08 0.07 0.51 – 5.72 5.68 0.21 0.66 6.8 0.9 0.97 3

a b c

Feob

Fepc

Ald

Alo

Alp

Sid

Fed, Ald: dithionite–citrate extractable Fe and Al. Feo, Alo, Sio: oxalate-extractable Fe, Al and Si. Fep, Alp, Sip: pyrophosphate extractable Fe, Al and Si.

Sio Sip

Al / Si ratio (Alo − Alp / Sio) Allophane (Sio ∗ 7) Ferrihydrite (Feo ∗

0.12 – 0.18 – 0.21 1.97

– – 21

Goethite and hematite (Fed −

1.7)

Feo)

0.20 0.48 9.66

1.37 0.62 0.04

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availability of phosphorus, low pH and presence of free iron and aluminium. 3.3. Effect of background electrolyte type and concentration on DOC uptake DOC adsorption increased with increasing concentration of background electrolyte, and the effect was more pronounced in the presence of Ca(NO3)2 than NaNO3. At a higher electrolyte concentration (0.1 M), Ca(NO3)2 in comparison to NaNO3, led to an increase in DOC adsorption by Kaol-Ill-1, Smec-1 and Allo-1 to the extent of 37.5, 20.3 and 12%, respectively. Such a trend was maintained at lower electrolyte concentrations (0.01 M) as well (43, 38.7 and 8.2% increase, respectively by KaolIll-1, Smec-1 and Allo-1), with the strongest effect in the case of Kaol-Ill SCF. Comparing the effect of electrolyte concentrations, a 10-fold increase in Ca(NO3)2 concentration increased the Qmax values by 5.3, 3.6 and 7.5% for Kaol-Ill-1, Smec-1 and Allo-1, respectively. An equal rise of NaNO3 concentration resulted in an increase of Qmax values by 9.6, 19.5, and 3.9%, respectively, for the three clay fractions. Therefore, the maximum DOC uptake in the presence of different background electrolytes was in the order: 0.01 M NaNO3 b 0.1 M NaNO3 b 0.01 M Ca(NO3)2 b 0.1 M Ca(NO3)2 (Table 4). These results corroborated previous reports that a divalent cation (Ca2+) improved DOC adsorption over a monovalent cation (Na+) and a higher electrolyte concentration increased the adsorption capacity (Mikutta et al., 2007; Rashad et al., 2010; Mavi et al., 2012; Setia et al., 2013). Di- or multivalent cations

53

could form ion-bridges between functional groups of DOC and clay surfaces, and thus would increase its adsorption (Mikutta et al., 2007; Setia et al., 2013; Roychand and Marschner, 2015). As found in the current study, DOC adsorption was more influenced by the electrolyte type than by their concentrations (Setia et al., 2013). The greater DOC uptake at a higher electrolyte concentration, although only small, was the result of a compressed double layer which resulted in a smaller distance between the solute and surface, and consequently a lower zeta potential value (Naidu et al., 1994; Münch et al., 2002). In general, irrespective of the background electrolyte type and concentration, SCF which had a higher SSA adsorbed more DOC. Similarly, when Qmax values were expressed on a SSA basis (mg m−2) after normalizing DOC adsorption to the SSA, the adsorption capacity followed the same order: Allo N Smec N Kaol-Ill under all background electrolyte types and concentrations (Table 5). There was a slight but insignificant increase in DOC adsorption by the Kaol-Ill clay fraction after native C and/or sesquioxide removal under 0.01 M NaNO3, 0.1 M NaNO3, 0.01 M Ca(NO3)2 solution conditions (Table 5). However, there was no such trend with Smec and Allo clay fractions (Table 5). 3.4. Desorption of DOC The all adsorbed DOC could not be removed in a single extraction step using Milli-Q water; only 6.41 to 55.28% could be removed (Figs. 4 and 5). The extent of desorption (%) in the current study was higher

Fig. 4. Desorption of DOC in Milli-Q water by sequentially treated soil clay fractions following adsorption in either 0.1 M or 0.01 M Ca(NO3)2. Kaol-Ill-1, Smec-1 and Allo-1 = Natural SCFs; Kaol-Ill-2, Smec-2 and Allo-2 = C-removed SCFs; Kaol-Ill-3, Smec-3 and Allo-3 = C & sesquioxides-removed SCFs. Error bars indicate standard error of the mean (n = 3; p b 0.05).

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M. Singh et al. / Geoderma 280 (2016) 47–56

Fig. 5. Desorption of DOC in Milli-Q water by sequentially treated soil clay fractions following adsorption in either 0.1 M or 0.01 M NaNO3. Kaol-Ill-1, Smec-1 and Allo-1 = Natural SCFs; Kaol-Ill-2, Smec-2 and Allo-2 = C-removed SCFs; Kaol-Ill-3, Smec-3 and Allo-3 = C & sesquioxides-removed SCFs. Error bars indicate standard error of the mean (n = 3; p b 0.05).

than that reported by (Saidy et al., 2013) who observed about 6–12% desorption by purified kaolinite, illite and smectite. However, our results were in line with the findings of Kahle et al. (2004), who observed approximately 13–50% desorption of adsorbed DOC either from purified kaolinite, montmorillonite and illite, or similar clay minerals extracted from natural soils. Mikutta et al. (2007) also observed 2–35% desorption of adsorbed DOC by pyrophyllite, goethite and vermiculite in a single extraction step. Similarly, Setia et al. (2013) also observed 5 to 60% desorption of DOC from their soils in various background solutions. Desorption of DOC was the lowest from the SCF where adsorption was conducted in Ca(NO3)2 and at a higher electrolyte concentration. As expected, the highest desorption occurred where the background electrolyte during adsorption was NaNO3 at a lower concentration. These results again confirmed a stronger ion-bridge type of bond formation by Ca2+ aiding adsorption of DOC on clay surfaces (Setia et al., 2013; Roychand and Marschner, 2014). Fettig and Sontheimer (1984) also proposed that the presence of divalent cations could make the DOC molecules more hydrophobic leading to the formation of hydrophobic bonds and consequently resulting in a reduced desorption of DOC in Milli-Q water. Marchuk and Rengasamy (2011) reported that Ca2+bonds were more covalent and less ionic in nature, while the monovalent Na+bonds in contrast were highly ionic. Water molecules cannot easily break the covalent Ca2 + bonds; therefore less DOC is

desorbed in water. Thus, Ca2+ not only led to a higher adsorption, but also led to less desorption of DOC compared to Na+. The effect of sesquioxide removal on DOC desorption was also evident in all the SCF. Sesquioxides, phyllosilicates, and allophane are different in terms of their surface properties. Removal of the sesquioxides from SCF led to an increased extent of DOC desorption (Figs. 4 and 5). Organic matter appears to form very strong and stable ligand-exchange interaction with sesquioxides at low pH (Kahle et al., 2004; Saidy et al., 2013, 2015). As a result of this chemisorption type of interaction, DOC desorption would become extremely difficult and slow at low pH (Kaiser and Guggenberger, 2000; Mikutta et al., 2007). The experimental conditions of the present study which included clay minerals with or without sesquioxides at pH values around 7 meant that an inner sphere complexation was unlikely but cationic bridging could play a key role when Ca2+ was present in the background electrolyte. 4. Conclusions Under the present experimental conditions, DOC adsorption-desorption varied with the types of soil clay fractions; the allophanic soil clay fraction, which had a higher SSA and sesquioxides content, adsorbed more DOC than the smectitic or kaolinitic-illitic soil clay

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fractions. Background electrolytes, concentrations and soil clay fractions's composition influenced the adsorption-desorption phenomenon. DOC adsorption increased with increasing background electrolyte concentration, and the presence of Ca2+ enhanced the uptake in comparison to Na+ due to a possible cationic bridging effect. Both on unit mass and unit surface area bases the DOC adsorption capacity followed the order: allophanic N smectitic N kaolinitic-illitic under all electrolyte conditions. Hence on a unit mass basis the allophanic soil clay fraction always adsorbed more DOC than the smectitic clay fraction and both adsorbed more DOC than the kaolinitic-illitic soil clay fraction. Furthermore, on a unit surface area basis the allophanic soil clay fraction adsorbed more DOC than the smectitic clay fraction and both adsorbed more DOC than the kaolinitic-illitic soil clay fraction. Adsorption and desorption of DOC were also influenced by the presence of sesquioxides. The extent of adsorption of DOC was reduced when sesquioxides were removed and DOC adsorbed in their absence was desorbed more readily than when they were present. This indicated their importance for DOC uptake and association with soil clay fractions. At the experimental pH (≈ 7) inner sphere complexation by sesquioxides was unlikely, but cation bridging became prevalent in the presence of Ca2+. Removal of native C increased the SSA and DOC adsorption, presumably by providing new adsorption sites on surfaces and in pores. This study of pedogenic soil clay fractions, rather than clay minerals from geological deposits provided realistic information about the interaction of DOC with soil clay fractions in relation to their native C and sesquioxide contents. The results indicated that the studied soil clay fractions would stabilize C in the order: allophanic N smectitic N kaolinitic-illitic. It would be worthwhile though to further validate these conclusions by studying other soil clay types from different agro-ecological conditions. Acknowledgements Mandeep Singh is thankful to the University of South Australia and Department of Education and Training, Government of Australia, for awarding him a US-APA PhD Scholarship. This research was partly supported by the Co-operative Research Centre for Contamination Assessment and Remediation of the Environment (CRC CARE) and Australian Research Council Discovery-Project (DP140100323). References Anda, M., Shamshuddin, J., Fauziah, I.C., Syed Omar, S.R., 2008. Pore space and specific surface area of heavy clay oxisols as affected by their mineralogy and organic matter. Soil Sci. 173 (8), 560–574. Baldock, J.A., Skjemstad, J., 2000. Role of the soil matrix and minerals in protecting natural organic materials against biological attack. Org. Geochem. 31 (7), 697–710. Basile-Doelsch, I., Amundson, R., Stone, W.E.E., Masiello, C.A., Bottero, J.Y., Colin, F., Masin, F., Borschneck, D., Meunier, J.D., 2005. Mineralogical control of organic carbon dynamics in a volcanic ash soil on La Réunion. Eur. J. Soil Sci. 56 (6), 689–703. Blakemore, L.C., Searle, P.L., Daly, B.K., 1987. Methods for Chemical Analysis of Soils. NewZealand Soil Bureau Scientific Report, 80. Department of Scientific and Industrial Research, Lower Hutt, New Zealand. Bolan, N.S., Adriano, D., Kunhikrishnan, A., James, T., McDowell, R., Senesi, N., 2011. Dissolved organic carbon: biogeochemistry, dynamics and environmental significance in soils. Adv. Agron. 110, 1–75. Chorom, M., Rengasamy, P., 1995. Dispersion and zeta potential of pure clays as related to net particle charge under varying pH, electrolyte concentration and cation type. Eur. J. Soil Sci. 46 (4), 657–665. Churchman, G.J., 2010. Is the geological concept of clay minerals appropriate for soil science? A literature-based and philosophical analysis. Phys. Chem. Earth 35, 927–940. Churchman, G.J., Lowe, D.J., 2012. Alteration, formation, and occurence of Minerals in Soils. In: Huang, P., Li, Y., Sumner, M.E. (Eds.), Handbook of Soil Sciences: Properties and Processes, second ed. CRC Press, USA. Churchman, G.J., Tate, K., 1986. Aggregation of clay in six New Zealand soil types as measured by disaggregation procedures. Geoderma 37 (3), 207–220. Churchman, G.J., Foster, R.C., D'Acqui, L.P., Janik, L.J., Skjemstad, J.O., Merry, R.H., Weissmann, D.A., 2010. Effect of land-use history on the potential for carbon sequestration in an Alfisol. Soil Tillage Res. 109 (1), 23–35. Churchman, J., Hesterberg, D., Singh, B., 2012. Soil clays (editorial). Appl. Clay Sci. 64, 1–3. Dontsova, K.M., Bigham, J.M., 2005. Anionic polysaccharide sorption by clay minerals. Soil Sci. Soc. Am. J. 69 (4), 1026–1035.

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