The influence of citrate on surface dissolution and alteration of the micro- and nano-structure of biotite

Abstract

Unraveling the kinetics and mechanisms of K-bearing mineral dissolution in the presence of organic acids on the nanometer scale is important for understanding the effectiveness of organic acids present in most soil environments. Herein, batch experiments were used coupled with atomic force microscopy (AFM) to image the surface dissolution and alteration of the micro- and nano-structure of biotite in pH 4.0 aqueous (H₂O), citric acid (CA), and citric acid with sodium chloride (CA + NaCl) solutions. We directly measured the release of potassium (K), aluminum (Al), and silicon (Si) from biotite into solution and observed the alteration of biotite at room temperature (25 °C) during long reaction times (0–168 h). In the acidic solution, biotite dissolved slowly, but the K, Si and Al release rates could be increased by adding citric acid and salt. The AFM observation indicated that the alteration of biotite was a coupled dissolution–precipitation process. An amorphous Si-bearing coating was deposited on the (001) surface or edge sites in the acidic environment after long time dissolution. Bumps or bulges on the (001) surface accelerated the stripping of the biotite segment from the surface after 96 h of reaction in citrate solutions. Na⁺–K⁺ ion exchange occurred in the biotite interlayer resulting in swellings and cracks on the biotite (001) surface and eventually forming Na-bearing hydrated mica. These observations may contribute to resolving the previously unrecognized interaction of organic acids and K-bearing minerals in a more complex soil system.

---

1. Introduction

Reactions occurring at mineral–water interfaces are central to most geochemical processes,^1,2 and affect the composition of natural waters, contaminant fate, weathering and soil formation, especially the formation of clay minerals and iron/aluminum oxides, which affect soil structure and fertility.³ Mineral elements can be released in the process of mineral weathering and are essential for plant growth and living microorganisms.⁴ Potassium (K) absorbed by plants mainly comes from phyllosilicate (mainly mica), feldspar and their weathering products (e.g., illite, vermiculite, chlorite and montmorillonite);⁵ thus, where K deficiency is common, such as in China, India, and regions of Southeast Asia, the breakdown of K-bearing minerals is important for improving the available K content in soils.^6,7

Low molecular weight organic acids (LMWOA) can be released by plants, microorganisms and ectomycorrhizal-forming fungi in soil, especially by plants living in forests without fertilization to acidify the rhizosphere and influence the availability of essential plant mineral nutrients (e.g., N, K, P, and Fe) or directly complex nutrients.^8–11 Due to its supposed importance, the dissolution of K minerals in the presence of organic acids has been studied for decades.^12–15 However, the organic acids (e.g., citric, oxalic, and malic) usually exist as organic salts in the soil solution, and the rates of the processes are ultimately controlled by atomic-scale reactions occurring at mineral surfaces.¹ Hence, quantifying the kinetics of dynamic interfacial processes such as dissolution, deposition or adsorption occurring at mineral interfaces will give valuable evidence for understanding K balance or natural weathering (dissolution) processes in the soil environment.

Within the scope of microscopic observation etch pits originated at the chlorite (001) surface are only expressed a layer by layer dissolution compared to non-stoichiometric dissolution at macro steps or the chlorite edge in pH 2–5.¹⁶ Oxalic acid could accelerate the dissolution rate on the biotite (001) surface due to complexation of aqueous cations and surface sites.¹⁴ Also, Cappelli et al. reported swelling and contraction of biotite edge layers within 5 h (70 °C) at pH 1.0. At a high temperature (200 °C), nano-coating and secondary minerals (e.g., vermiculite and montmorillonite) were noticeable on the mica surface after 168 h of interaction with an acidic (pH 5.7) aqueous solution.^17,18 Because the sheet edges ((hk0) surface) behave very differently from the (001) surface, the complexity of the study object will increase. Additionally, background electrolytes (IS from 0.01 to 1) or solution compositions such as Na⁺, K⁺, Mg²⁺, Cl⁻, NO₃⁻, and SO₄²⁻ have a significant impact on the formation of etch pits and the changes of mineral microstructures.^19–22 Results reported by Qin et al.²³ showed that the dissolution of calcium hydrogen phosphate (CaHPO₄·2H₂O) was promoted in citrate solution at varying pH (4.0–8.0) conditions by adding NaCl. Under geologic CO₂ sequestration conditions (e.g., 328–423 K and 102 atm CO₂), the brine cations could alter the biotite morphology and induce the deposition of fibrous illite on the biotite basal plane.²⁴ Most studies were performed under extreme geological environment (e.g., strong acidity, high solution concentration, or hydrothermal and high pressure). However, for an agricultural environment, the release of elements from clay minerals and mineral weathering always occur at ambient pressure and temperature.

Thus, the objective of this study was to investigate the effects of organic salts on mica alteration (i.e., dissolution, morphological evolution, and secondary mineral precipitation) at ambient temperature. Biotite was used as a model of mica. Our previous experiments found that it was difficult to observe dissolution on biotite surfaces using AFM in a short time (0–100 min) and with weakly acidic or basic solutions (pH 6.0–8.0).²⁵ Therefore, batches of dissolution experiments (0–168 h) at pH 4.0 were conducted with biotite flakes to provide unique information on organic acid–salt–mica interactions in the emulated soil environment.

2. Materials and methods

2.1. Materials

Natural biotite samples were supplied by China University of Geosciences (Wuhan Fig. S1†). One single layer of biotite is approximately ∼6.8 Å in height.^5,26 On the basis of electron probe microanalysis (JXA-8230 Wavelength Dispersive EPMA Japan), the elemental composition of the biotite sample was K(Mg_2.46Fe_0.45Ti_0.09)[AlSi₃O₁₀](OH)₂.²⁵ Thin flakes of biotite (∼0.1 mm thick) were prepared by cleaving along the (001) basal planes and cutting into 3.1 cm × 1.3 cm rectangles, and were then cleaned with deionized water and anhydrous ethanol.²⁷ Biotite powder was prepared by grinding cleaned flakes and passing through a 75 μm sieve. Powder samples were used only for secondary mineral identification.

The selected inorganic salt was NaCl (ACS reagent, Sigma-Aldrich) and the low molecular weight organic acid (LMWOA) was citric acid (CA; ACS reagent, Sigma-Aldrich), ligand form H₃L, dissociation constants (aqueous solution, 25 °C): 3.13, 4.76, and 6.40.²⁸ Prior to the experiment, citric acid and sodium chloride were oven-dried for 3 h at 40 °C to eliminate moisture.

2.2. Experimental

Batch extraction experiments were conducted to measure the effect of citrate on the dissolution process of biotite, as: (1) pH 4.0 aqueous solution, (H₂O); (2) pH 4.0 10 mM citric acid solution, (CA), and (3) pH 4.0 citrate solution, (CA + NaCl), in which the concentrations of citric acid and NaCl were 10 mM and 100 mM, respectively. The pH values were adjusted by adding HCl or NaOH (ACS, Sigma-Aldrich). The small volumes of acid and base solution were not expected to affect the concentration of citric acid. Additionally, the citric acid solution at pH 4.0 was a mixed solution, and fractions of species included H₃L 10.3%, H₂L⁻ 76.4%, and HL²⁻ 13.3%, estimated by means of Geochem software. A biotite flake and 4 mL of H₂O, CA and CA + NaCl solutions were placed in several test tubes (the liquid/solid ratio was 34 : 1 by weight, Table 1). Based on our pre-experiment, separate batches were made for each of the desired elapsed times (24, 48, 72, 96, 120, 144, and 168 h). After the static reaction, the dissolved ion concentrations (K, Si and Al for example) in solution were measured by inductively coupled plasma optical emission spectrometry (ICP-OES, Perkin Elmer Optima 4300DV). The reacted biotite basal surfaces were observed with Atomic Force Microscopy (Multimode 8 Bruker). Both tapping (probe MESP, 3 Hz, phase and amplitude images) and contact mode (probe NP-S10, nominal spring constant 0.24 Nm⁻¹ 3 Hz, deflection and height images) AFM were used.

Table 1 Summary of experimental parameters^a

Reaction systems	Objectives	Solution volume (mL)	Mass of biotite (g)	Water/rock ratio	Flake area (cm^2)
a All the experiments were conducted under room temperature (25 °C) and normal pressure.b The dimensions of a biotite flake was 3.1 cm × 1.3 cm × 0.1 mm.c The biotite fragment was passed over 2 mm sieve and cleaned with pure water, dried at 40 °C.d The uncertainties are the standard deviation of the average mass of 10 biotite flake samples.
H2O–biotite flake^b	To study the dissolution of biotite	4	0.1171 ± 0.0002^d	34 : 1	3.95 ± 0.01
CA solution–biotite flake		4	0.1179 ± 0.0002	34 : 1	3.93 ± 0.01
CA + NaCl solution–biotite flake		4	0.1174 ± 0.0002	34 : 1	3.96 ± 0.01
H2O–biotite fragment^c	To identify secondary mineral phases	6	0.30	20 : 1	Fragment
CA solution–biotite fragment		6	0.30	20 : 1	Fragment
CA + NaCl solution–biotite fragment		6	0.30	20 : 1	Fragment

---

Secondary minerals were difficult to detect on biotite flakes except edge sites because of their small quantity and low reaction rates under normal pressure and temperature.^25,29 To facilitate the observation of secondary minerals that formed on biotite edge surfaces, additional batch experiments were conducted with biotite fragment (2 mm in diameters). Biotite fragments (0.30 g) were added to test tubes containing 6 mL solution (the liquid/solid ratio was 20 : 1 by weight, Table 1). After reaction at 25 °C for 288 h, the solid separated by centrifuging (for 5 min, at 5000 rpm) was washed with deionized water to remove excess salt and organic acid prior to drying and analysis with Fourier transform infrared spectroscopy (FT-IR, Nexus Thermo Nicolet). The supernatant was analyzed with FT-IR, transmission electron microscopy (TEM, JEM-2100F), and field scanning electron microscope plus energy dispersive spectroscope (FSEM-EDAX, Sirion 200 FEI). One caveat of this experimental approach, however, is that the production from biotite fragments might be different from those observed in AFM images, and also would be disturbed by edge sites.

The deionized water used in the experiments was ultra-high purity water (18 MΩ cm, pH 5.8) from a two-step purification treatment including double distillation (YaR, SZ-93) and deionization (Milli-Q). The deionized water was passed through two 0.2 μm filters before use in all experiments.

2.3. Data analysis

Element release rates were conducted on three replicates and statistical analyses were performed using OrignPro8.0 software. Captured images were processed using Bruker NanoScope Analysis version 1.40 and processed using Adobe Photoshop 7.0.

3. Results

3.1. Effects of citrate solution on the dissolution of biotite (001) surface

The preferential release of biotite (001) surface interlayer K over the framework ions (Al and Si) was observed in all solutions (Fig. 1). Throughout the 168 h experiments, released K concentrations in CA + NaCl solution were 1.7 and 2.0 times higher than those in CA and H₂O, respectively. In the initial stage (0–96 h), the K release increased linearly as a function of incubation time in all solutions where the linearly dependent coefficients were r = 0.99**, r = 0.99**, and r = 0.99** for H₂O, CA, and CA + NaCl treatments, respectively, and was followed by a slow release rate in the late stage (96–168 h).

Fig. 1 Dissolved ion (a) K, (b) Al, and (c) Si concentrations from biotite (001) surface into solution after reaction in pH 4.0 H₂O, CA, and CA + NaCl solutions for different incubation time at 25 °C.

During the reaction for 0–96 h, a linear relationship between dissolved ion (Al and Si) concentrations with time was observed (Fig. 1). The Al and Si concentrations in the CA and CA + NaCl solutions were significantly higher than those in the H₂O solution. As the reactions continued, the releases of Al and Si from the biotite in H₂O solution were nearly stationary where the slopes of the linear equation between concentration and time in 96–168 h were 0.0003 and 0.0002, with a weaker increase in CA and CA + NaCl solutions.

3.2. Effects of citrate solution on the morphological evolutions of biotite (001) surface

Tapping mode AFM analysis (Fig. 2) demonstrates that the morphological changes of biotite (001) surfaces are greatly influenced by citric acid and salt. In the initial stage (0–96 h), the formation of etch pits was the predominant feature on the biotite (001) surface. After reaction in water (H₂O) for 24 h, there was a widespread “roughening” of the biotite surface with small indentations (<1 nm in depth). The (001) surface layer slowly dissolved in water and the total area of the pits only occupied 4.3% of the surface (Fig. 2d). The depth changes of the biotite basal plane (Fig. 2e-i and ii) indicated that the dissolution started from defect/kink sites on the surface layer. Phase images also showed that secondary minerals did not form except in the etch pits and as debris on the biotite (001) surface (Fig. S2b†).

Fig. 2 The morphological evolution of biotite (001) surface in pH 4.0 H₂O, CA, and CA + NaCl solution at 25 °C in contact mode for 24, 48, 72, and 96 h, of which (a)–(d), (f)–(i), and (k)–(n) are height images and the scale bar is 5 μm × 5 μm, (e), (j), and (o) are linear sections from the corresponding height images.

The second series of images, Fig. 2f-i, show biotite in a 10 mM CA solution. In this case, the process of dissolution was visibly distinct and the retreat rate of etch pits on the (001) surface was much faster than that in pH 4.0 H₂O solution at the same reaction time. In total, 91.8% of the first layer was dissolved and released into solution within 72 h; afterwards, a new cycle of morphological evolutions (second layer) began at 96 h, as shown in Fig. 2i. In addition, several circular precipitates with heights of 0.600–1.351 nm were observed on the biotite (001) surface (Fig. 2i). Details are provided in Fig. S2e† which indicated that the circular precipitates had the same structure and composition as the bulk sample.

The morphological evolution rate of the (001) surface in the CA + NaCl solution was accelerated compared to that in the CA solution. After reaction for only 24 h, 51.8% of the first layer (∼1 nm in depth) dissolved (Fig. 2k). A newly exposed layer (second layer) (shown in Fig. 2m) dissolved gradually in 72 h. Similarly, the circled precipitates, as shown in Fig. 2i, also generated on the biotite (001) surface (Fig. 2n). The heights of the bulges on the biotite basal surface in CA + NaCl solution were significantly higher than those in the CA solution (Fig. S2†). The evolution of the circled structure shown in Fig. S3† suggests that the particular structures may be induced by internal pressure (i.e., ion exchange action Na⁺–K⁺). Thus, the circled structure and particles scattered on the (001) surface (Fig. 2i and n) were the residues of biotite segments caused by the rupture of bulges and resulted in the occurrence of incongruent dissolution in the late reaction stage. Furthermore, the phase images (Fig. S2 and S3†) indicated some secondary coatings were precipitated on biotite (001) surface in CA and CA + NaCl solutions.

The morphological changes of the biotite (001) surface after a longer reaction time (120–144 h) are shown in Fig. 3. A coating of unstable secondary minerals was formed in water and resulted in a blurred image due to forcing by the probe tip. Accumulated traces were also produced at the height of 1.2–1.4 nm on the sides of the scanned zone (Fig. 3a). At the end of the experiment, SEM images showed only several etch pits (dark area in the panel) formed on the biotite (001) surface after reacting with weakly acidic water for 168 h (Fig. S4a†).

Fig. 3 The morphological changes of the biotite (001) surface in pH 4.0 H₂O, CA, and CA + NaCl solution for 144 h of incubation, of which (a) is contact/deflection mode image, the iconography in (a) is height image, (b) is tapping/amplitude mode image and the scale bars of (a) and (b) are 5 μm × 5 μm. (c) is phase image and the scale bar is 15 μm × 15 μm, and the iconography is amplitude mode image after 120 h of incubation. (i)–(iv) are linear sections from the corresponding images.

Bulges, segments, and fractured ring structures could be observed on the biotite (001) surface after reacting with CA and CA + NaCl solutions for 144 h. Fibrous swellings, at a height of 11 nm, appeared on the (001) surfaces, while 1–2 nm depth cracks appeared after 120 h of incubation in the CA + NaCl solution (Fig. 3iii). More fibrous swellings were apparent on the biotite (001) surface, as the reactions continued (144 h), and the phase image indicated that the properties of the swellings were different from the bulk property of the biotite (Fig. 3c). SEM images show that the biotite swelling layers could crack into micro segments and detach from the surface (Fig. S4d†).

3.3. Observation of the secondary mineral phases

Infrared spectra show significant changes for biotite solid powder and supernatants after reaction with citrate (Fig. 4a). Solid samples displayed peak areas in the ranges of 900–1200 cm⁻¹ and 400–700 cm⁻¹ (attributed to Si–O bands)²⁷ which increased significantly in CA and CA + NaCl solutions compared to those in aqueous solution. This suggests that the Si–O bands are more abundant in the secondary mineral phases. In supernatants, the peaks at 815, 730, and 655 cm⁻¹, belonging to Al–O bands,²⁷ in CA and CA + NaCl solutions were considerably higher than those in aqueous solution.

Fig. 4 Observation of the secondary mineral phase. (a) FT-IR analysis of the biotite powders (solid) and the supernatants (liquid) after reacting with different solutions for 288 h at 25 °C. (b), (c) and (d) are TEM images of the biotite powders in solution after reacting with pH 4.0 H₂O and CA solutions for 288 h at 25 °C, respectively. (e) and (f) are FSEM images of the Si-bearing particles extracted from the supernatant of biotite powders after reacting with CA + NaCl solution. (g) is EDAX analysis on site (i).

In TEM analysis of the supernatants, biotite powders were found in the H₂O treatment (Fig. 4b) and had no obvious dissolution on its surface and edge. In contrast, for biotite powders in the CA solution, distinct dissolution signs on (hk0) surface and edges could be seen (shown in Fig. 4c). These same dissolved phenomena were observed in the CA + NaCl treatment (see Fig. S5†). After 45 s of irradiation by the electron beam (∼150 °C), a molten phenomenon occurred on the biotite segments edges (Fig. 4d) but did not occur in pH 4.0 aqueous solution. Compared with NaCl (melting point 801 °C), in citrate solution, it indicated that only citric acid (melting point 156 °C) could melt.²⁸ Thus, citric acid could be easily adsorbed onto the phyllosilicate mineral defect sites or edges.³⁰ The supernatant of the CA + NaCl treatment was purified with deionized water and alcohol, and then examined using FSEM. Biotite debris was mixed with flat precipitates (shown in Fig. 4e). The EDAX analysis showed that aggregates contained bulk minerals comprised of Si, O, Al, Fe, Mg, and K, with a small quantity of Na (0.72% wt).

4. Discussion

4.1. Dissolution of biotite and release of ions in acidic citrate solution

The present study confirms that citric acid and sodium have significant effects on biotite dissolution and elemental release. Throughout the 168 h experiments, the elemental release rates went through different stages in the solutions. Released K concentration increased from 1.2- to 2.0-fold when the treatment changed from CA to CA + NaCl compared to that in aqueous solution (pH 4.0 H₂O treatment), while the releases of Al and Si increased from 2.9- and 2.6- to 3.8- and 4.1-fold, respectively.

Previous reports indicated that the K release rate was influenced by the ion exchange rate (e.g., H⁺–K⁺, Na⁺–K⁺, and Mg²⁺–K⁺) and hydrolysis rate (H⁺ reacts with Al–O–Al and Si–O–Si to break the lattice charge-balancing and release K⁺) on mineral surface.³¹ Therefore, in slightly acidic citrate solutions, several mechanisms may control the surface K release rate. For example, (1) higher ion strength in the bulk solution can accelerate the ion change reaction.³² Calculated with the formula, the IS (ionic strength) in H₂O, CA and CA + NaCl solutions were 0.0001, 0.005 and 0.05, respectively. Therefore the Na⁺–K⁺ and H⁺–K⁺ ion exchange rate would be subjected to cationic concentration. Secondly, (2) H⁺ and Na⁺ can facilitate cation exchange with phyllosilicate interlayer K⁺.³³ Thus, the formation of defects or etch pits is closely linked to the release of interlayer K⁺ that will be affected by organic ligands complexation (i.e., –COO⁻) with frame ions (Al, Si, Mg, etc.).¹² Thirdly, (3) cation exchange reactions, that contained sodium (higher ionic strength in NaCl), result in a relatively faster element release rate.³⁴ However, the release of Al and Si were subjected to the combination affinity between citrate and framework ions instead of the salt ions under same pH conditions.^18,25

4.2. Morphological changes and alteration of the micro- and nano-structure of biotite

The AFM analysis showed slow formation of shallow etch pits on the (001) surface in pH 4.0 aqueous solutions and rapid growth of etch pits in the citrate solution in the initial stage (24–96 h). In an acidic environment, the K⁺ ions on the (001) surface could be totally released. Only Si–O bonds (400–550 cm⁻¹ and 900–1200 cm⁻¹) were found, and their disordered arrangement may cause a rough surface.³⁵ Haward et al. found similar results with a hydrochloric acid solution and oxalic acid of 0.1 mol L⁻¹ on the biotite basal plane within 60 min.¹⁴ However, there were a few surface coatings precipitated on the biotite basal plane in the CA + NaCl solution at 96 h and in H₂O at 144 h, which also made the surface appear rough. A similar gel-like coating was found on the phlogopite surface.^27,36 The reaction of aluminosilicate minerals with aqueous solutions mainly involves breaking of Al–O, Si–O and Al–Si bonds by H⁺, generating Al³⁺ and orthosilicic acid, which can be expressed as Al₂Si₂O₅(OH)₄ + 6 H⁺ = 2 Al³⁺ + 2 H₄SiO⁴ + H₂O.³⁷

Usually, the dissolution of a mineral by an aqueous solution results in an interfacial fluid that may become supersaturated with respect to a new mineral phase, and that this phase may nucleate within this interfacial region,^38,39 especially in a pH 4.0 solution, where Si-bearing minerals (e.g., quartz and kaolinite) are easily deposited on the biotite (001) surface.³⁵ In contrast to the flake observation, the powder experiment confirmed more secondary mineral phases containing Si–O bands in CA and CA + NaCl solutions developed and were identified via FT-IR, due to the dissolution rate of edge surface being around 250 times higher than at the basal plane.^29,40 Therefore, the coating was speculated to be amorphous silicon mineral in a closed, steady space, and by its formation on the (001) surface further impeded dissolution because of the ions release rate declined in late stage (Fig. 1).

During the late stage (96–168 h), the biotite basal layer expanded vertically in the citrate solution. In particular, as the Na⁺ concentration increased, bumps and bulges on the biotite (001) surfaces were more evident (Fig. 3). One possible reason was that the ion-exchange of aqueous Na⁺ (hydrated radius r = 0.358 nm) with interlayer K⁺ (ion radium r = 0.133 nm and hydrated radius r = 0.331 nm) in biotite resulted in a swelling on the mica surface layer.^17,29,41 Another possibility is the hydrolysis of the TOT-layer and preferential leaching of the cations from the interlayer and octahedral positions. However, the formation of the swelling layer probably requires an extended exposure of the surface to the solution and may in addition require specific compositional or structural conditions or defects.⁴² When aqueous Na⁺ continuously enters into the interlayer, the action of internal pressure and expansion in the surface layers will continue until it bursts and produces mineral fragments.⁴³

The coupled dissolution–precipitation process in citrate solution is shown in Fig. 5, including several typical characteristics as secondary coating, bulge, swelling, cracking, and detachment of the biotite surface layer. Defects on the biotite basal or edge sites, where complexation and hydration effects and bulges were reported to occur faster.⁴² Leaching of octahedral cations and internal stress could build up within the bent surface, and result in cracking and detachment of the surface layer.^24,42 Fig. S2† and 3c show that the bulge rupture and fibrous swellings commonly occurred in the surface layer of 2–20 nm. Meanwhile, cracks around the swellings were induced, but the cracks were only about 1–3 nm in depth. Chemical reactions that result in bulges, bumps, or cracks, are relatively weak in the ambient environment compared to hydrothermal solution conditions.^24,42 This replacement reaction via coupled dissolution–precipitation makes the biotite surface layers transform into Na-bearing hydrated mica, which also could verify that a small quantity of Na (0.72% wt) was present in the mineral after reaction (Table S1† and Fig. 4g). Under natural conditions, the hydrated mica is not stable, and may weather further to form clay minerals such as illite, chlorite, montmorillonite, and kaolinite,³ after which the secondary coating on the mineral surface would restrain the weathering rate in the real environment.

Fig. 5 Schematic diagram of the process of biotite surface layer dissolution and transformation in pH H₂O, CA, and CA + NaCl solutions.

Collectively, we have presented AFM observations on the dissolution of biotite by citrate, and the results indicate that citrate has a bimodal effect (promoter/inhibitor) on the dissolution of biotite (001) surfaces, which mainly depends on the reaction rate. Therefore, our results approximate the contribution of (001) surface dissolution on the release of K, and it may suggest possible implications for analyzing the role of low-carbon–organic salt in a more complex soil–rhizosphere environment.

5. Conclusions

We have used batches of experiments combined with AFM observation to provide visual information of citrate on surface dissolution and alteration of the micro- and nano-structure of biotite. The experimental results that in the early stage of CA and CA + NaCl solutions at pH 4.0, elemental release and etch pit coverage as a function of time allows straight forward assessment of the dissolution rate; while in the late stage, we observed a relatively slow release of elements and expansion of etch pits over the surface because of the formation of burst and amorphous Si-bearing coatings on the mineral surface.

In the absence of citrate (i.e., in HCl solution at pH 4.0), release of Al⁺ and Si⁺ decreases significantly compared with that in a citrate solution and the formation of discrete etch pits occupies 4.8% of the surface dissolved after 96 h reaction. The biotite (001) surface in citric acid may dissolve by carboxyl complexate tetrahedral (Al³⁺) and octahedral (Mg²⁺, Fe²⁺) cation processes that expose the biotite (hk0) surface to attack and result in depletion of interlayer K⁺ by exchange with H⁺ and Na⁺. Another process may involve an increase of Na⁺ ions in the citrate solution, whereby bumps or bulges on the (001) surface could accelerate the dissolution rate. In addition, large quantities of Na⁺ ions via pits or defect sites can enter into the deeper domain of biotite structures, leading to the appearance of swellings on the surface, and eventually forming Na-bearing hydrated mica or clay minerals.

Acknowledgements

The authors acknowledge support from the Open-end Fund of Engineering Research Center of Ecology and Agricultural Use of Wetland, Ministry of Education (KF201613), the Special Fund for Agro-scientific Research in the Public Interest (201203013) and a Specialized Research Fund for the Doctoral Program of Higher Education (801180010140). We thank Prof. Phillip M. Chalk, the University of Melbourne, Australia, for critical reading and commenting on the manuscript.

References

1. D. H. McNear Jr, Nature Education Knowledge, 2013, 4, 1.
2. S. L. Brantley, J. D. Kubicki and A. F. White, Kinetics of water–rock interaction, Springer, 2008.
3. M. J. Wilson, Clay Miner., 1999, 34, 7–25.
4. G. M. Gadd, Microbiology, 2010, 156, 609–643.
5. T. J. McMaster, Curr. Opin. Biotechnol., 2012, 23, 562–569.
6. P. Barre, B. Velde, C. Fontaine, N. Catel and L. Abbadie, Geoderma, 2008, 146, 216–223.
7. D. Sparks, Adv. Soil Sci., 1987, 6, 1–63.
8. J. F. Banfield, W. W. Barker, S. A. Welch and A. Taunton, Proc. Natl. Acad. Sci. U. S. A., 1999, 96, 3404–3411.
9. F. D. Dakora and D. A. Phillips, Plant Soil, 2002, 245, 35–47.
10. D. L. Jones, P. G. Dennis, A. G. Owen and P. A. W. van Hees, Plant Soil, 2003, 248, 31–41.
11. Z. Balogh-Brunstad, C. Kent Keller, J. Thomas Dickinson, F. Stevens, C. Y. Li and B. T. Bormann, Geochim. Cosmochim. Acta, 2008, 72, 2601–2618.
12. J. Cama and J. Ganor, Geochim. Cosmochim. Acta, 2006, 70, 2191–2209.
13. J. I. Drever and L. L. Stillings, Colloids Surf., A, 1997, 120, 167–181.
14. S. J. Haward, M. M. Smits, K. V. Ragnarsdóttir, J. R. Leake, S. A. Banwart and T. J. McMaster, Geochim. Cosmochim. Acta, 2011, 75, 6870–6881.
15. P. Hinsinger, C. Plassard, C. X. Tang and B. Jaillard, Plant Soil, 2003, 248, 43–59.
16. F. Brandt, D. Bosbach, E. Krawczyk-Bärsch, T. Arnold and G. Bernhard, Geochim. Cosmochim. Acta, 2003, 67, 1451–1461.
17. C. Cappelli, A. E. S. Van Driessche, J. Cama and F. J. Huertas, Cryst. Growth Des., 2013, 13, 2880–2886.
18. K. Pachana, P. Zuddas and P. Censi, Appl. Geochem., 2012, 27, 1738–1744.
19. H. E. King and C. V. Putnis, Geochim. Cosmochim. Acta, 2013, 109, 113–126.
20. H. S. Lee, K. Lee, S. S. Kim and S. H. Ko, J. Ind. Eng. Chem., 2003, 9, 360–365.
21. E. Ruiz-Agudo, M. Kowacz, C. V. Putnis and A. Putnis, Geochim. Cosmochim. Acta, 2010, 74, 1256–1267.
22. E. Ruiz-Agudo, M. Urosevic, C. V. Putnis, C. Rodriguez-Navarro, C. Cardell and A. Putnis, Chem. Geol., 2011, 281, 364–371.
23. L. H. Qin, W. J. Zhang, J. W. Lu, A. G. Stack and L. J. Wang, Environ. Sci. Technol., 2013, 47, 13365–13374.
24. Y. D. Hu, J. R. Ray and Y. S. Jun, Environ. Sci. Technol., 2013, 47, 191–197.
25. J. Li, W. Zhang, S. Li, X. Li and J. Lu, Appl. Geochem., 2014, 51, 101–108.
26. M. E. Fleet, Rock-forming Minerals, Geological Society of London, London, 2003.
27. H. B. Shao, J. R. Ray and Y. S. Jun, Environ. Sci. Technol., 2010, 44, 5999–6005.
28. D. R. Lide, CRC Handbook of Chemistry and Physics, CRC Press Inc., Boca Raton, FL, USA, 82th edn, 2001.
29. Y. Hu, J. R. Ray and Y. S. Jun, Environ. Sci. Technol., 2011, 45, 6175–6180.
30. M. Elena Ramos and F. Javier Huertas, Appl. Clay Sci., 2014, 90, 27–34.
31. K. G. Dunkel and A. Putnis, Eur. J. Mineral., 2014, 26, 61–69.
32. L. M. McDonald, V. P. Evangelou and M. A. Chappell, Cation Exchange, in Encyclopedia of Soils in the Environment, Elsevier Academic Press, Oxford, 2005.
33. A. C. D. Newman, Clay Miner., 1970, 8, 361–373.
34. G. Sposito, The Chemistry of Soils, Oxford University Press, New York, 2008.
35. A. W. Bray, L. G. Benning, S. Bonneville and E. H. Oelkers, Geochim. Cosmochim. Acta, 2014, 128, 58–70.
36. H. H. Teng, P. Fenter, L. K. Cheng and N. C. Sturchio, Geochim. Cosmochim. Acta, 2001, 65, 3459–3474.
37. B. E. Kalinowski and P. Schweda, Geochim. Cosmochim. Acta, 1996, 60, 367–385.
38. R. K. Tang, C. A. Orme and G. H. Nancollas, ChemPhysChem, 2004, 5, 688–696.
39. E. Ruiz-Agudo, C. V. Putnis and A. Putnis, Chem. Geol., 2014, 383, 132–146.
40. M. P. T. Turpault and L. Trotignon, Geochim. Cosmochim. Acta, 1994, 58, 2761–2775.
41. B. Tansel, J. Sager, T. Rector, J. Garland, R. F. Strayer, L. Levine, M. Roberts, M. Hummerick and J. Bauer, Sep. Purif. Technol., 2006, 51, 40–47.
42. K. Aldushin, G. Jordan and W. W. Schmahl, Geochim. Cosmochim. Acta, 2006, 70, 4380–4391.
43. E. B. A. Bisdom, G. Stoops, J. Delvigne, P. Curmi and H. J. Altemuller, Pedologie, 1982, 2, 225–252.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra24068b

---