A nano-scale study of the mechanisms of non-exchangeable potassium release from micas

Applied Clay Science 118 (2015) 131–137

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A nano-scale study of the mechanisms of non-exchangeable potassium release from micas Ting Li a,b,c, Huoyan Wang a,⁎, Zijun Zhou a,c, Xiaoqin Chen a, Jianmin Zhou a a b c

State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 21008, China College of Resources, Sichuan Agricultural University, Chengdu 611130, China University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e

i n f o

Article history: Received 9 March 2015 Received in revised form 15 September 2015 Accepted 16 September 2015 Available online xxxx Keywords: Non-exchangeable potassium Release mechanisms Chemical extraction method Atomic force microscopy X-ray diffraction

a b s t r a c t Non-exchangeable potassium (NEK) is released from minerals by chemical extraction methods through dissolution and cation-exchange reactions. Assessing the contribution of these two processes to release budgets is important to predicting plant/mineral interactions and soil sustainability. Three NEK extraction methods, 1 M boiling nitric acid (HNO3), 2 M hot hydrochloric acid (HCl) and 0.2 M sodium tetraphenylboron (NaTPB), were applied to investigate the mechanisms of NEK release from trioctahedral biotite and dioctahedral muscovite. Atomic force microscopy and X-ray diffraction were used to investigate the surface morphology and transformation of mica, respectively. NEK released from biotite and muscovite through cation-exchange reaction accounted for 87.88%, 85.93% and 83.23%, 78.31% of the total extracted K when extracted by boiling HNO3 and hot HCl, respectively, with channel and elliptical dissolution pits on mica surface. NEK extracted by NaTPB was an almost complete cation-exchange reaction, and the extracted micas showed an obvious vermiculitic character and no dissolution features. Results showed the release of NEK from micas is essentially a cation-exchange reactions in both acid and salt solutions, indicating that NEK released through cation-exchange reaction but not via dissolution may make up the main soil K pool available to plant. © 2015 Published by Elsevier B.V.

1. Introduction Potassium (K) is an essential nutrient in plant nutrition that has recently become the target of scientific discussion owing to the consequences of nutrient management on farms in terms of deficiency, surpluses and related animal health issues (Öborn et al., 2005; Romheld and Kirkby, 2010; Zörb et al., 2014). Soil K is traditionally subdivided into soluble, exchangeable, non-exchangeable, and structural forms that are in dynamic equilibrium with each other (Öborn et al., 2005; Sparks, 1987). The most common method used to assess the K status of a soil for the likelihood of obtaining a response in crop yield to fertilizer addition is measurement of the exchangeable K (EK). This method has been shown to be quite successful for soils not containing 2:1 K-bearing minerals when adequate calibration has been carried out (Barbagelata and Mallarino, 2012). However, when the contribution of non-exchangeable K (NEK) in soils is increased by the presence of 2:1 K-bearing minerals that have the ability to retain K, the power of prediction using EK soil extraction methods is lost (Cox et al., 1999; Mengel et al., 1998; Prasad, 2009). Therefore, quantification of NEK might facilitate prediction of long-term K release capacity in field balance calculations of soils containing 2:1 K-bearing minerals. ⁎ Corresponding author at: East Beijing Road 71#, Nanjing 210008, China. E-mail address: [email protected] (H. Wang).

http://dx.doi.org/10.1016/j.clay.2015.09.013 0169-1317/© 2015 Published by Elsevier B.V.

Methods commonly used to determine the NEK often involve chemical extraction with dilute or concentrated acids, typically nitric acid (HNO3) or hydrochloric acid (HCl) (Andrist-Rangel et al., 2013; Ghosh and Singh, 2001; Øgaard and Krogstad, 2005; Reitemeier et al., 1948), or a large excess of displacing cations such as sodium tetraphenylboron (NaTPB) (Cox and Joern, 1997; Cox et al., 1999; Fabián et al., 2008; Jonathan et al., 2014; Schulte and Corey, 1965; Vetterlein et al., 2013; Wang et al., 2010; Wentworth Sally and Rossi, 1972). Previous studies have shown that release of NEK from micas via the chemical extraction method proceed by exchanging the adjacent interlayer K+ with hydrated cations and the dissolution of micas, resulting in the formation of weathering products (Andrist-Rangel et al., 2013; Darunsontaya et al., 2010; Jalali, 2005; Mortland and Ellis, 1959; Reed and Scott, 1962; Srinivasa Rao et al., 2006). However, there is still a great deal of uncertainty and variation among these batch experiments with respect to the respective contribution of the exchange and dissolution to release budgets, which has important implications for predicting plant/mineral interactions and soil sustainability. To date, investigation of the relationships between NEK extracted by traditional chemical methods and soil mineralogy has mainly been performed by elemental analysis and X-ray diffraction (XRD) (Andrist-Rangel et al., 2013; Ghosh and Singh, 2001; Hosseinpur and Zarenia, 2012; Øgaard and Krogstad, 2005). Indeed, previous studies have obtained a full NEK release characterization from 2:1 K-bearing

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minerals, and minerals extracted by the chemical method of interlayer anhydrous K+ ions expanded the basal spacing of these layers, which led to significant changes in XRD patterns (Andrist-Rangel et al., 2013; Cox and Joern, 1997; Darunsontaya et al., 2010). However, there have been few nano-scale studies of the crystallo-chemical changes of 2:1 K-bearing minerals caused by the traditional NEK extraction chemical method. The dynamics of NEK release from 2:1 K-bearing minerals is related to their crystal structure and atomic bonding (Nagy, 1995; Sparks, 1987). Micas are layer phyllosilicates consisting of two tetrahedral (T) sheets with an octahedral (O) sheet in between (TOT structure). The octahedral unit is linked to the other units via shared octahedral edges. In biotite, all three octahedral sites are occupied by chemical elements (trioctahedral), whereas only two octahedral sites are occupied in muscovite (dioctahedral). Since the 2:1 structure is negatively charged, an interlayer cation neutralizes the bulk electrical structure. Biotite has K+ as an interlayer cation, Mg2 + and Fe2 + are in the trioctahedral layer and Si4 + and Al3+ make up the tetrahedral layer. Muscovite has a similar structure, but contains Al3+ in the dioctahedral layer. There are two main types of mica surfaces, the basal parallel to the layers (001) and the lateral (hk0) surface (Nagy, 1995; Schoonheydt and Johnston, 2006). It is generally agreed that NEK release progresses via breakage of bridge oxygen bonds, Si-O-Al, at the crystal edges, although in some cases basal planes may contribute to the dissolution reaction (Nagy, 1995; Vetterlein et al., 2013; Wieland and Stumm, 1992). The present study was conducted to quantify the relative contribution of NEK released through exchange with hydrated cations and the dissolution of micas. Our approach was to analyze the 2:1 K-bearing minerals before and after extraction by combining mineralogical data obtained by XRD and surfaces change data at the sub-nanometer level measured by atomic force microscopy (AFM). 2. Materials and methods 2.1. Materials and characterization The materials used in this study include trioctahedral biotite and dioctahedral muscovite. Biotite and muscovite were obtained from the Huayuan Mica Factory in Hebei Province, China. The chemical composition of the mica samples was analyzed by X-ray fluorescence (XRF) (Table 1). For both micas the sum of oxides in Table 1 lies below 100 wt.%. Most likely, these variations in results are explained by presence of structural water in the ignited material at the time of weighing before fusion. This interpretation is in accord with the analyses by Kalinowsk and Schweda (1996) and Newman (1967), which showed the deficit of 3.5 wt.% to 4.3 wt.% in the micas analyses may be explained by presence of structural water. Also contribution to the oxide sum deficiency of the micas analyses is that we did not analyze TiO2 and F. But the chemical composition given here and the total analysis are close to the data of Kalinowsk and Schweda (1996), who found TiO2 and F represent 3.32 wt.% and 0.41 wt.% in chemical analyses of biotite and muscovite, respectively. The discrepancies between the data of Kalinowsk and Schweda (1996) and our possible that the mica samples coming from different places are of different composition. Under these circumstances it does not affect the calculated mica structural formulas. Based on the XRF and XRD results, the formulas of micas used in this work study were calculated to be: (Ca0.62Na0.11K1.22) (Al0.33Fe1.63Mn0.03Mg2.56) (Si5.93Al2.21)O20(OH)3.97F0.03 (biotite) and (Ca0.01Na0.12K1.76)(Al3.11Fe0.58Mn0.002Mg0.25) (Si6.37Al2.10)O20(OH) Table 1 Chemical composition of the micas determined by XRF analysis (wt.%).

3.91F0.09 (muscovite). Mica specimens were cleaved to thin samples (~20 μm, (001) cleavage surfaces), successively cleaned with ultrapure water, acetone, ethanol, and isopropanol to remove organic matter, then air dried at room temperature, ground and passed through a 90 μm sieve. All salts and acids used in this study were of guarantee reagent, salts were obtained from Aladdin Industrial CO. (Shanghai, China), and acids were obtained from Nanjing Chemical Reagent CO. (Nanjing, China).

2.2. Non-exchangeable K extraction experiments 2.2.1. Boiling HNO3 Mineral samples were determined by 1 M boiling HNO3 according to the method described by Srinivasa Rao et al. (2000) and Wood and De Turk (1941). Briefly, 2.5 g of mineral samples were placed into 100 mL digestion tubes (in quadruplicate) and 25 mL of 1 M HNO3 was added. The samples were then heated to 100 °C until they began bubbling, after which boiling was continued for 10 min. After boiling, the contents were cooled and filtered into a 100 mL volumetric flask. The micas were further washed with four 15 mL portions of 0.1 M HNO3, after which the residues were collected into centrifuge tubes and washed at least five times with deionized water to reduce the acidity. Finally, the samples were air dried at room temperature. 2.2.2. Hot HCl Mineral samples were extracted in 2 M HCl at 100 °C according to the procedure described by Andrist-Rangel et al. (2013). Briefly, 1.0 g of solids (in quadruplicate) and 25 mL of 2 M HCl was placed in 50 mL centrifuge tubes in a boiling water-bath for 2 h. The tubes were swirled after 30, 60, and 90 min, then transferred to a cool water-bath after 120 min. Next, samples were centrifuged (2000 rpm, 5 min), after which the extracts were vacuum filtered through membrane filters. To reduce the acidity and salinity of the HCl-treated residues, they were washed at least four times with 0.5 M MgCl2, twice with deionized water, and then air dried at room temperature. 2.2.3. NaTPB Mineral samples were extracted by 0.2 M NaTPB using a method similar to that described by Cox et al. (1999) and Wang et al. (2007). Briefly, samples of 0.5 g of minerals (in quadruplicate) were weighed into 50 mL centrifuge tubes, after which 3 mL of extractant (1.7 M NaCl, 0.01 M EDTA, 0.2 M NaTPB) was added. After shaking at 200 rpm for 80 days, 25 mL of quenching solution (0.5 M NH4Cl, 0.14 M CuCl2) was added to the tubes to stop the extraction of soil K. The tubes were then heated in boiling water for 60 min to dissolve the KTPB precipitate, after which the suspension was vacuum filtered through membrane filters and stabilized by the addition of three drops of 6 M HCl. To reduce the salinity of the NaTPB-treated residues, samples were washed at least four times with ethanol, twice with deionized water, and then air dried at room temperature. 2.3. Solution analysis Solutions were analyzed for K, Si, Al, Mg and Fe concentrations using inductively coupled plasma optical emission spectrometry (ICP-AES, IRIS-Advantage, Thermo Elemental, MA, USA). The element release rate formula is: A(X) = CX × V/m; where A(X) is the extracted amount of element X (mmol kg−1), CX is the concentration of element X in extraction solution (mmol L−1), V is the solution volume (mL) and m is the mineral mass (g). 2.4. Surface morphological analysis

Micas

SiO2

Al2O3

Fe2O3

MgO

MnO

CaO

Na2O

K2O

Total

Biotite Muscovite

40.70 47.06

14.80 30.01

15.08 5.80

11.80 1.25

0.22 0.014

4.00 0.096

0.40 0.44

6.54 10.22

93.54 94.89

Atomic force microscopy (AFM) is a high resolution technique that enables the investigation of surfaces on a sub-nanometer level, yielding three-dimensional data sets that may be used for quantitative

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measurement of process rates and energies alongside information on changes in surface topography at the atomic scale (Aldushin et al., 2006; Bosbach et al., 2000; Gratz et al., 1991; Li et al., 2014; Simon et al., 2011). All height and phase contrast images of boiling HNO3, hot HCl and NaTPB extracted mineral samples were collected under ambient laboratory conditions in tapping mode with AFM (Dimension Edge, Bruker). Probes were 125 μm long and contained phosphorus (n) doped silicon tips (a nominal tip radius of 10 nm, MPP-11100-10, Veeco probes). The images were collected with drive frequencies of 300 kHz, typical spring constants of 40 N/m, and scan rates of 1 Hz. The AFM images were analyzed using the NanoScope software (Version 5.12b48). 2.5. Characterization of extracted minerals X-ray diffraction was performed on the original and extracted minerals to identify whether any new crystalline phases had formed. Analysis was conducted using a Rigaku Ultima IV high resolution XRD with CuKα radiation at λ = 1.5418 Å. Data were obtained on random powder mounts in stepscan mode with a step size of 0.02°2θ, fixed slits, and an 8 s counting time. The composition of the minerals was determined by semi-quantitative analysis using the intensity and ‘full width at half maximum’ of the (00l) reflections of the phyllosilicates normalized to that of the (101) quartz peak, and the mineral intensity factors for calculation of the peak area (Kahle et al., 2002). 2.6. Data analysis Element release rates were calculated in quadruplicate and statistical analyses were performed using the SPSS 19.0 software. The normality and homogeneity of variances were checked before ANOVA was conducted. Differences among groups were analyzed by the least significant difference (LSD) test and a P b 0.05 was considered statistically significant. 3. Results 3.1. Element release from micas The amount of major cations released from micas extracted using the three different methods is presented in Table 2. The results showed that the extraction methods of boiling HNO3 and hot HCl showed similar element release trends. Fe and Mg were extracted in the highest levels, followed by Al and interlayer K, while the framework element Si showed lower values in both biotite and muscovite. Both extraction methods recovered 23.69% and 34.46% of the total K from biotite, but only 1.02% and 1.10% of that from muscovite, respectively (Fig. 1). However, the release of the interlayer K in NaTPB solutions appeared to be much greater than that in Mg, Al and Si from both biotite and muscovite, with 97.54% and 45.13% of the total K being recovered, respectively. 3.2. Surface morphological changes in micas in response to extraction Atomic force microscopy deflection images (Fig. 2) revealed the mica (001) surfaces before and after extraction using different chemical

Fig. 1. Efficiency of the extraction methods to released potassium from micas. Values with different small and capital letters indicate significant differences among extraction methods in biotite and muscovite, respectively, by LSD (P b 0.05).

methods. The results indicated that fine materials of less than a micrometer remained attached to the larger grains in the original biotite, and the roughness of the original biotite (001) surface was 9.73 nm in height (Fig. 2A). However, the morphology of the biotite (001) surface underwent several changes after extraction by boiling HNO3 and hot HCl and NaTPB (Fig. 2B, C and D). In the presence of boiling HNO3, a small number of channel-like structures (hereafter referred to as dissolution channels) were randomly distributed on the biotite (001) surface (Fig. 2B-b), with a 5 × 5 μm surface roughness of 3.88 nm in height. After extraction by hot HCl, the biotite surface not only showed channel dissolution (Fig. 2C-b), but also elliptical dissolution pits (Fig. 2C-c) and a surface roughness of 3.71 nm in height. In addition, the fine mineral sample attached to the larger grains was dissolved in the hot HCl solution. For NaTPB, there was no dissolution feature formation, but the surface of the biotite became blurry and the interlayer space was expanded, with a roughness of 25.7 nm in height (Fig. 2D). This finding was consistent with our observation of little to no dissolution of framework elements in the presence of NaTPB. Atomic force microscopy observations of muscovite (001) surface dissolution using different extraction solutions are shown in Fig. 2F, G and H. Some fine materials of less than a micrometer remained attached to the larger grains in the original muscovite, with a 5 × 5 μm surface roughness of 10.50 nm in height (Fig. 2E). For boiling HNO3 and hot HCl, the depth of dissolution pits ranged from 8 to 20 nm and 6 to 10 nm (Fig. 2F-b and G-b). As the acidity increased, dissolution channels grew in size, vertically and horizontally (Fig. 2G-c), and terraces on the surface of muscovite appeared with heights of 2.13 nm (base layer) and 9.17 nm (top layer) (Fig. 2G-d). Similar to biotite, the surface of muscovite extracted with NaTPB had no dissolution features except expansion of the interlayer space (Fig. 2E).

Table 2 Mean amount of elements in micas released using three different extraction methods (mmol kg−1). Data are the means (sd) of four replicate extractions. Micas

Extraction method

K

Mg

Fe

Al

Si

Biotite

Boiling HNO3 Hot HCl NaTPB Boiling HNO3 Hot HCl NaTPB

329.0 (16.8) c⁎ 478.5 (18.2) b 1354.4 (22.1) a 22.2 (0.7) b 23.8 (1.6) b 979.3 (72.0) a

1246.4 (74.2) b 2160.5 (65.1) a 49.9 (4.9) c 31.6 (1.2) b 38.3 (2.6) a 1.9 (0.5) c

774.7 (53.8) b 1528.3 (53.0) a bd⁎⁎ 42.5 (1.4) b 51.6 (1.5) a bd

551.7 (37.5) b 980.0 (27.9) a 4.3 (1.3) c 48.8 (1.4) b 63.6 (2.1) a 2.6 (0.2) c

53.5 (0.7) b 107.8 (2.1) a 9.2 (0.5) c 11.3 (0.4) b 18.6 (0.8) a 3.3 (0.4) c

Muscovite

⁎ Values followed by different letters differ significantly based on the LSD (P b 0.05). ⁎⁎ bd indicates that the concentration of elements was below that of the detection limit.

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Fig. 2. AFM images show the evolution of morphology of biotite and muscovite (001) surfaces before and after extraction by three different extraction methods. A, B, C and D are the original biotite, 1 M boiling HNO3 extracted biotite, 2 M hot HCl extracted biotite and 0.2 M NaTPB extracted biotite, respectively. E, F, G and H are the original muscovite, 1 M boiling HNO3 extracted muscovite, 2 M hot HCl extracted muscovite and 0.2 M NaTPB extracted muscovite, respectively. The linear sections indicate changes of sites on the biotite and muscovite (001) surface.

Fig. 3. X-ray diffraction traces (Cu K-radiation) are shown for untreated and treated micas; (A) biotite; (B) muscovite.

T. Li et al. / Applied Clay Science 118 (2015) 131–137

3.3. Characterization of secondary mineral Some significant differences between the two micas were found upon X-ray analysis of random powder mounts (Fig. 3). The XRD traces from biotite extracted by boiling HNO3 were unchanged except for reduced 00l intensities (Fig. 3A), while those of the biotite extracted with hot HCl showed expanded interlayers, with a broad peak at 14.03 Å, in addition to the 9.99 Å original biotite peak. NaTPB showed the most extensive interlayer expansion and alteration among the three methods used in this investigation. All higher angle peaks from the NaTPB extracted biotite sample, including peaks with hk indices, were broadened. The weak broad peaks from the biotite extracted by hot HCl and NaTPB indicate that the crystallite size of the extracted materials has been reduced. For muscovite samples, extraction with hot HCl and boiling HNO3 did not induce significant changes except for the specific weakening of the 00l intensities (Fig. 3B). The intensities of the 00l reflections found at 8.8, 17.6, 26.8, 36.0, and 45.4° 2θ decreased by 30 to 60% relative to fresh muscovite. Expansion of interlayer space in muscovite was shown by peaks forming at 10.44 Å in the sample extracted by NaTPB in addition to a sharp 10 Å reflection of the unexpanded muscovite (Fig. 3B). 4. Discussion The higher amount of elements released from biotite than muscovite under acid conditions (Table 2 and Fig. 1) can be interpreted in terms of chemical composition and crystal structure of the two micas. The octahedral cations are mainly Fe and Mg in biotite and Al in muscovite (Table 1). The release of structural Fe and Mg from biotite through the attack of hydronium ions was more extensive than that of structural Al from muscovite (Song and Huang, 1988). These reactions apparently led to deterioration of biotite and release of NEK from the mineral structure. In the case of muscovite, the octahedral cation, Al, was more stable than Fe and Mg of biotite in the acid solutions (Table 2 and Fig. 1). In addition to the chemical composition, the atomic bonding and crystal structure may play a role in the release of K from these minerals, even in acid solutions (Kalinowsk and Schweda, 1996). Potassium is held more tightly in muscovite than in biotite (Pachana et al., 2012). The hydroxyl orientation in muscovite is oblique to silicate sheets, the distance between the proton and K is longer and the K is thus less repelled than in biotite. Moreover, the degree of tetrahedral twisting and tilting to fit the octahedral sheet is greater in muscovite than in biotite. Taken together, these factors make the K ions more stable in muscovite than in biotite (Kalinowsk and Schweda, 1996; Pachana et al., 2012; Song and Huang, 1988), indicating that biotite makes a greater contribution to plant-available reserves of K in soils than muscovite. The values of extracted NEK in NaTPB solution were higher than under acid conditions, but the octahedral cations, Mg, Fe and Al, showed an opposite trend (Table 2 and Fig. 1). This was because that the mechanisms by which NEK are extracted by acids differ from those of the NaTPB solution (Andrist-Rangel et al., 2013; Nagy, 1995; Song and Huang, 1988). The K in the mineral structure is replaced by Na in the NaTPB solution through a cation exchange reaction, while the TPB combines with released K and forms precipitates to ensure continuation of the exchange reaction (Carey and Metherell, 2003; Cox et al., 1999; Reed and Scott, 1962; Wang et al., 2010). Therefore, after extraction of NEK by NaTPB from biotite and muscovite, there was no dissolution feature formation, but the surface of the fragment became blurry, the interlayer space of micas was expanded, and the vermiculite-like products were found to contain less interlayer cations than the original micas. These findings were supported by the solution analysis (Table 2 and Fig. 1) and preliminary XRD analyses (Fig. 3). The occurrence of peaks of intensity at 14.03 Å and 10.44 Å demonstrated that an expanded clay was mixed with the original biotite. Evidently, the use of NaTPB solution to extract K did not cause irreversible collapse of micas. These

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results are consistent with those reported by Darunsontaya et al. (2010) and Singh and Goulding (1997), who observed that Kdepletion resulted in a decrease in the intensity of reflections for illite and an increase for interstratified clay minerals, vermiculite and smectite. However, a small amount of the framework element Si was also released from micas in NaTPB solution. This may have occurred because the protons were sorbed at some hydroxyl sites and new hydroxyls were formed from oxygen ions in the tetrahedral layer, which expelled the framework element Si from the micas structure (Newman, 1967). In acidic solutions, the release of elements from micas proceeded via surface reactions in which octahedral and interlayer cations were displaced by the hydronium ion, H3O+. The substitution of the octahedral and interlayer cations by H3O+ weakened the structure of the micas and was accompanied by a slight expansion of the interlayer space in the mica layers (Fig. 2 and Fig. 3). In addition, the adsorbed H3O+ ions might breakdown the Al-O bonds. Therefore, the element Al was changed and expelled from the micas structure (Bibi et al., 2014; Köhler et al., 2003). As a result, a Si-rich residual phase was likely to form at the surface of the micas. This feature was in agreement with the solution analysis illustrated in Table 2 and Fig. 1, which revealed that the major elements of the micas were released faster than Si. These findings are consistent with the results reported by Li et al. (2014), Shao et al. (2011) and Turpault and Trotignon (1994). The higher values of octahedral and interlayer cations extracted from micas indicated that the release of cations in boiling HNO3 and HCl solution were dominated by a diffusion controlled process (Andrist-Rangel et al., 2013; Mortland and Ellis, 1959; Srinivasa Rao et al., 2006). Although the boiling HNO3 and hot HCl extracted micas had channel or elliptical dissolution pits, and the micas were fissured and partially curled and peeled off (Fig. 2), previous laboratory studies of single mineral phyllosilicate dissolution in acidic solutions showed that only the curled sheets have been transformed into a nearly pure silica phase, and analyses done around or inside the corroded zone do not show such depletion of Mg, Fe, Al and K (Turpault and Trotignon, 1994). Thus, the dissolution of micas was limited by Si release (Bibi et al., 2014; Rozalen et al., 2014; Turpault and Trotignon, 1994). Based on the amount of the framework element Si released and the ratio of K to Si in original micas, we calculated the amount of extracted NEK released through exchange of the elements with hydrated cations and via the dissolution of micas (Table 3). These results indicated that NEK released through the exchange with hydrated cations by the two acid extraction methods, boiling HNO3 and hot HCl, accounted for 87.88% and 83.23% of the total K extracted from biotite, and 85.93% and 78.31% of the total K extracted from muscovite, respectively. Voinot et al. (2013) also found that, in the presence of HCl, the transformation rate of biotite was significantly greater than the dissolution rate of biotite based on boron (B) isotopes in biotite. However, K extracted by NaTPB almost totally a cation-exchange reaction. Previous studies have suggested that mica dissolution rates vary as a function of pH and temperature. Dissolution rates increase in decreasing pH and increasing temperature under acid conditions, with the lowest values being observed near neutral pH, and increasing values being observed with increasing pH (Bibi et al., 2014; Li et al., 2014; Nagy, 1995). The pH of farm soil generally ranges from 4 to 8 may result in lower dissolution rate of micas in soil horizon than in 1 M boiling HNO3 and 2 M hot HCl used for extraction. Meanwhile, plant roots produce more protons, low molecular weight organic acids, and salts to activate K-bearing minerals that release K when they suffer from K deficiencies (Hinsinger et al., 2003). However, Shao et al. (2011) found the release of K from mica was a diffusion-controlled process with the presence of organic ligands. Thus, we assumed that plants may actually satisfy most of their K requirements through exchange with hydrated cations and extremely weak destructive and reversible reactions. Because of K uptake by plants, the values of the K reserve pool of soils may be increased in horizons in which plant roots are active and insufficient K fertilizer is applied.

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Table 3 The amount and ratio of extracted non-exchangeable potassium released through cation-exchange reaction and the dissolution of micas. Mica

Biotite

Muscovite

Method

Boiling HNO3 Hot HCl NaTPB Boiling HNO3 Hot HCl NaTPB

NEK released through cation-exchange reaction

NEK released through dissolution

Amount (mmol kg−1)

Ratio of total extracted NEK (%)

Amount (mmol kg−1)

Ratio of total extracted NEK (%)

289.15 398.32 1347.58 19.06 18.64 978.37

87.88 83.23 99.50 85.93 78.31 99.91

39.80 80.20 6.83 3.12 5.15 0.92

12.12 16.77 0.50 14.07 21.69 0.09

5. Conclusion Our results clearly demonstrated that the values of NEK extracted by boiling HNO3 and hot HCl were larger for biotite than muscovite, but that both showed an expansion of the interlayer space in layers and channel or elliptical dissolution pits on the mica surface. A significant proportion of the extracted K was released through the interlayer K exchanged with hydrated cations. In contrast, for micas saturated in NaTPB, almost all K released through the interlayer K+ was exchanged with Na. Additionally, these samples showed an obvious vermiculitic nature and no dissolution features on the mica surface. Overall, K extracted from micas by traditional K extraction methods were dominantly released through a diffusion controlled process, and NEK released through cation-exchange reaction but not via dissolution may make up the main soil K pool available to plant. This finding has important implications for predicting plant/mineral interactions and soil sustainability. Acknowledgments This study was financially supported by the National Department Public Benefit Research Foundation of China (grant No. 201203013), the National Basic Research Program of China (grant No. 2013CB127401) and the National Natural Science Foundation of China (grant Nos. 40971176 and 41271309). The authors wish to thank Gong H. for the ICP analyses, Dr. Gao B. H. for the AFM observation, Chen J. for the XRD measurement, and especially to Prof. Zhang S. R. for his writing assistance. References Aldushin, K., Jordan, G., Schmahl, W.W., 2006. Basal plane reactivity of phyllosilicates studied in situ by hydrothermal atomic force microscopy (HAFM). Geochim. Cosmochim. Acta 70, 4380–4391. Andrist-Rangel, Y., Simonsson, M., Öborn, I., Hillier, S., 2013. Acid-extractable potassium in agricultural soils: source minerals assessed by differential and quantitative X-ray diffraction. J. Plant Nutr. Soil Sc. 176, 407–419. Barbagelata, P.A., Mallarino, A.P., 2012. Field correlation of potassium soil test methods based on dried and field-moist soil samples for corn and soybean. Soil Sci. Soc. Am. J. 77, 318–327. Bibi, I., Singh, B., Silvester, E., 2014. Dissolution kinetics of soil clays in sulfuric acid solutions: ionic strength and temperature effects. Appl. Geochem. 51, 170–183. Bosbach, D., Charlet, L., Bickmore, B., Hochella, M.F., 2000. The dissolution of hectorite: in situ, real-time observations using atomic force microscopy. Am. Mineral. 85, 1209–1216. Carey, P.L., Metherell, A.K., 2003. Rates of release of nonexchangeable potassium in New Zealand soils measured by a modified sodium tetraphenyl-boron method. New Zeal. J. Agr. Res. 46, 185–197. Cox, A.E., Joern, B.C., 1997. Release kinetics of nonexchangeable potassium in soils using sodium tetraphenylboron. Soil Sci. 162, 588–598. Cox, A.E., Joern, B.C., Brouder, S.M., Gao, D., 1999. Plant-available potassium assessment with a modified sodium tetraphenyboron method. Soil Sci. Soc. Am. J. 63, 902–911. Darunsontaya, T., Suddhiprakarn, A., Kheoruenromne, I., Gilkes, R.J., 2010. The kinetics of potassium release to sodium tetraphenylboron solution from the clay fraction of highly weathered soils. Appl. Clay Sci. 50, 376–385. Fabián, G.F., Sylvie, M.B., Craig, A.B., Jeffrey, J., Raymond, H., 2008. Assessment of plantavailable potassium for no-till, rainfed soybean. Soil Sci. Soc. Am. J. 72, 1085–1095. Ghosh, B.N., Singh, R., 2001. Potassium release characteristics of some soils of Uttar Pradesh hills varying in altitude and their relationship with forms of soil K and clay mineralogy. Geoderma 104, 135–144.

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