DSpace at VNU: Release of potassium accompanying the dissolution of rice straw phytolith

Chemosphere 119 (2015) 371–376 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere Release of potassium accompanying the dissolution of rice straw phytolith Minh Ngoc Nguyen a,⇑, Stefan Dultz b, Flynn Picardal c, Anh Thi Kim Bui d, Quang Van Pham a, Juergen Schieber e a Department of Pedology and Soil Environment, Faculty of Environmental Science, VNU University of Science, Vietnam National University, 334-Nguyen Trai, Thanh Xuan, Hanoi, Vietnam b Institute of Mineralogy, Leibniz Universität Hannover, Herrenhäuser Straße 2, 30419 Hannover, Germany c School of Public and Environmental Affairs, Indiana University, MSBII, Room 418 N Walnut Grove Ave, Bloomington, IN 47405-2204, USA d Institute of Environmental Technology, Vietnam Academy of Science and Technology, 18-Hoang Quoc Viet, Hanoi, Vietnam e Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, IN 47405-1405, USA h i g h l i g h t s  Potassium is present in rice stems and leaves  Potassium can be immobilized in the mineralized silica (phytolith)  We found that potassium co-exists with organic matter in phytolith structure  Desilification of the phytolith is a main factor regulating potassium release  Pretreatment of the rice straw at 600 °C is optimal in providing available potassium a r t i c l e i n f o Article history: Received 31 May 2014 Accepted 21 June 2014 Handling Editor: X Cao Keywords: Rice straw Phytolith Silicon Potassium Release a b s t r a c t In rice, Si assimilated from the soil solution is deposited in inter- and intracellular spaces throughout the leaf and stems to form silicified structures (so-called phytoliths) Because K is also present in significant concentrations in rice stems and leaves, the question arises if K is immobilized in the mineralized silica during the precipitation of Si This work determined whether desilification of the phytolith is a factor regulating K release by implementing batch experiments Solubility of Si and K of the rice straw heated at different temperatures were examined to identify effect of pretreatment Analyses of phytoliths using SEM–EDX and X-ray tomographic microscopy in conjunction with the results from batch experiments revealed that K might co-exist with occluded organic matter inside the phytolith structure In the kinetic experiments, corresponding increases of K and Si concentrations in the supernatants were observed which suggested that desilification of the phytolith is a main factor regulating K release The extent of heat pretreatment of the rice straw is of significant importance with respect to dissolution of the phytolith by affecting organic removal and surface modification At temperatures lower than 600 °C, corresponding increases of the soluble Si and K with heating temperature have been obviously observed In contrast, the solubility of Si and K gradually decreased at temperatures above 600 °C This work provides insights into factors that control release of K and Si from phytolith and a practical recommendation for practices of burning rice straw that may maximize subsequent release of Si and K for crops Ó 2014 Elsevier Ltd All rights reserved Introduction Silicon (Si) and potassium (K) are two of the most abundant nutrient elements in rice (Tiwari et al., 1992; Epstein, 1999) These ⇑ Corresponding author E-mail address: minhnn@hus.edu.vn (M.N Nguyen) http://dx.doi.org/10.1016/j.chemosphere.2014.06.059 0045-6535/Ó 2014 Elsevier Ltd All rights reserved two elements are taken up from the soil solution and transported to leaf, stem and other parts of rice While K provides the appropriate ionic environment for metabolic processes in the cytosol and, as such, functions as a regulator of various processes including growth regulation (Leigh and Wyn Jones, 1984), Si is known to accumulate as silicaceous phytolith by deposition in inter- and intracellular spaces throughout the leaf and stem of rice 372 M.N Nguyen et al / Chemosphere 119 (2015) 371–376 (Parr and Sullivan, 2005) Several studies have reported that, during the precipitation of Si to form phytolith, organic molecules can be trapped within the silica (Kelly et al., 1991; Elbaum et al., 2003; Piperno and Stothert, 2003) This suggests that inorganic compounds, e.g., K in the rice xylem/phloem sap, can also be occluded in the silica body (so-called PhytOK) It is hypothesized that the PhytOK can be released and contribute to the K-pool of the soil once desilification of the phytolith occurs It is generally accepted that the desilification of silica in aqueous solutions occurs via hydrolysis of Si–O–Si bonds of the SiO2 crystal structure Water itself is a strong promoter of hydrolysis via orientation of the electronegative water oxygens towards the Si atom, leading to a transfer of electron density to the Si–O–Si bond, thereby increasing its length and eventually breaking it (Dove and Crerar, 1990) Recently, it has been reported by Nguyen et al (2013) that dissolution of Si is controlled by aqueous solution chemistry and pretreatment of the rice straw However, additional data on co-releases of Si and other occluded elements such as K is currently lacking Our current studies of the release of K accompanying phytolith dissolution in aqueous solutions helps address this knowledge gap On-site burning after harvesting is the primary method of handling rice straw to return nutrients to the soils In recent decades, burning of rice straw has been predominant because it is a costeffective method of straw disposal, doesn’t interfere with soil preparation, and helps to reduce pest and populations of pathogens resident in the straw biomass (Dobermann and Witt, 2000) The mode of pretreatment of rice straw and burning at different temperatures might result in various degrees of dehydroxylation of biogenic silica and organic matter contents In this study, we identify co-release of Si and K using batch experiments with rice straw phytolith ash samples obtained from different ashing temperatures A further treatment by H2O2 to remove organic matter, which is often mentioned as a method for phytolith preparation (Parr et al., 2001), will be used to relate losses of K and Si that accompany the removal of organic matter SEM–EDX spectra reflecting the presence of various elements, and X-ray tomographic microscopy providing 3D-segmentation and visualization of the solid phase were also used in this study to evaluate distribution of K in the rice straw phytoliths Improved knowledge of the mechanisms and release processes of K from phytolith will open different management options and is of importance for the continued efficient cultivation of rice Materials and methods 2.1 Sample production Rice straw was collected from a paddy field in the rice-growing area of the central part of the Red River Delta (105°440 1700 E, 20°590 5700 N) directly after harvesting The rice straw was air-dried, ground in a blade grinder, and passed through a 1.0 mm sieve The rice straw had 73.6 g kgÀ1 Si examined by an X-ray fluorescence analyzer (XRF-1800 Shimadzu, Japan), 387 g kgÀ1 C, and 13.2 g kgÀ1 N as determined by an Elementar Vario EL (Hanau, Germany) elemental analyzer with a respective C:N ratio of 29 Dry ashing of rice straw was performed by heating finely-ground, air-dried, rice straw in an oven at a target temperature (between 300 °C and 1000 °C) for h To avoid strong exothermic reactions during dry-ashing the weight of sample was limited to g For kinetic experiments we selected two samples treated at 400 °C and 800 °C 2.2 Methods For determination of Si and K distribution and dissolution from phytoliths, soluble salts from the ashes were removed by washing with deionized water for five minutes followed by centrifugation and decantation The procedure was repeated five times, and samples were finally freeze-dried To identify distribution of elements in phytolith using SEM–EDX, the samples were examined using a FEI Quanta 400 FEG ESEM (FEI Company, Hillsboro, OR, USA) at 20 kV under low vacuum conditions using a gaseous secondary electron detector The stage was at room temperature with a chamber pressure of 80 Pa Surface point elemental analysis was performed while the samples were being viewed in the SEM using a Princeton Gamma-Tech energy dispersive spectrometer The visualization was performed by X-ray tomographic microscopy using the 3D segmentation and visualization software YaDiV (Friese et al., 2013) to provide a three-dimensional image of the principal arrangement of silicified structures and organic matter for a vascular bundle in a dried stem of a rice plant The samples were analyzed at the synchrotron light source (SLS) of the Paul– Scherrer-Institute in Villigen, Switzerland The TOMCAT (TOmographic Microscopy and Coherent rAdiology experimenTs) beamline receives photons from a 2.9 T superbending magnet with a critical energy of 11.1 keV, producing a monochromatic beam A sample is fixed on a centering and rotation stage in front of a microscope, detecting the monochromatic X-ray beam In all experiments, 200 mg of sample was mixed with 200 mL of solution in 250 mL polypropylene tubes Suspensions were gently shaken by hand directly after mixing and allowed to stand for 24 h at room temperature Some of the batch experiments were extended up to d with sampling at 24 h intervals The experiments were terminated by filtration of the suspension through a 0.45 lm pore-size cellulose acetate filter Si and K in solution was determined in triplicate using the molybdate blue method (Mortlock and Froelich, 1989) with a Spectrophotometer UV–Vis (L-VIS-400, Labnics Company, Fremont, CA, USA) and flame photometer (PFP7-Jenway, OSA, UK), respectively In more detail, we performed the following experiments: Experiment 1: We analyzed the dissolution kinetics for the 400 °C-, 800 °C-, 400 °C/H2O2-, and 800 °C/H2O2-treated rice straw samples by monitoring Si and K releases into solution The dissolution experiments lasted d and sampling was carried out every 24 h Experiment 2: To evaluate the effect of heat-pretreatment of rice straw, the rice straw samples treated at 300, 400, 500, 600, 700, 800, 900 and 1000 °C were mixed with deionized water and kept at room temperature for 24 h Experiment 3: To examine whether K sorbs on siloxane surfaces of the phytolith, we used 400 °C- and 800 °C-treated rice straw samples Ca2+ and NH+4 prepared in solutions with a concentration range of 10–100 mmolc LÀ1 from pure analyzed chloride salts were used as exchangers Suspensions were terminated after 24 h Results 3.1 Sample properties The different ashing temperatures of straw changed organic-C content of the straw to different extents (Table 1) The organic-C was most completely removed by heating at 800 °C, whereas only less than 12% of total organic-C was removed at 400 °C With a subsequent treatment with H2O2, remaining organic-C contents were 2.85% and 0.48% for the ashes treated at 400 °C and 800 °C, respectively The total Si and K contents were 20.7% and 9.4%, respectively, after treatment at 400 °C, and 32.0% and 10.3% following 800 °C treatment The ashes had an alkaline reaction (pH 10–11) Soluble K of the 400 °C-and 800 °C-treated samples in a 1:10 373 M.N Nguyen et al / Chemosphere 119 (2015) 371–376 Table Specific surface area (SSA), chemical composition of the original rice-straw sample (1), dry-ashed samples treated at 400 °C and 800 °C (2, 3) Soluble K was analyzed for the heattreated samples alone SSA (m2 gÀ1) Treatment Original sample (1) 400 °C (2) 800 °C (3) – 68.6 1.0 Soluble K (g kgÀ1) Total content (%) C Si K Ca Mg K+ 38.7 34.2 0.53 7.36 20.7 32.0 2.37 9.4 10.3 1.13 3.0 4.9 0.53 1.3 2.3 – 52 15 extract with deionized water were 52 and 15 g kgÀ1 which are equal to 55% and 15% of the total contents, respectively A marked decrease of the specific surface area (SSA) determined by the N2-adsorption method (Quantachrome, NOVA 4000e, Boynton Beach, FL, USA) was obtained with increasing heating temperature The SSAs of the sample treated at 400 °C and 800 °C were found to be 68.6 and 1.0 m2 gÀ1, respectively, indicating a strong condensation of silica structures (Fig 1a and c) Temperatures >800 °C are known to cause formation of crystalline SiO2 phases such as cristobalite or tridymite (Fig 2) Formation of these minerals by pyrolysis of biogenic silica was also reported by Kordatos et al (2008) Subsequent treatments by H2O2 resulted in scabrous surfaces as shown in Fig 1b and d For the original sample, SSA determination by the N2-adsorption method failed because N2 did not enter the micropores of OM * Cristobalite * + Trydimite Treatment temperature + + + * * 1000oC 900oC 800oC 700oC 550oC 10 20 30 40 Theta 3.2 EDX characterization The elemental composition of phytolith is shown by the EDX spectra in Fig It can be seen that Si, C, O, K and Ca are major elements in the phytoliths The relative abundance of these elements varied between samples based on the chosen pretreatment An obviously high signal of C in the 400 °C-treated sample in Fig XRD patterns of rice straw ashes treated at different temperatures comparison with those in the 800 °C-, 400 °C/H2O2-oxidized-, and 800 °C/H2O2-treated samples denoted that oxidation reactions by elevated heating or H2O2 treatment removed large amount of organic matter In contrast, Si signals in the 800 °C-, 400 °C/H2O2-, Fig SEM images of a leaf fragment in treated rice-straw samples: 400 °C (a), 400 °C/H2O2 (b), 800 °C (c) and 800 °C/H2O2 (d) 374 M.N Nguyen et al / Chemosphere 119 (2015) 371–376 (a) Soluble ions / heat treatments: 40 o Si / 400 C Si / 800oC o K / 400 C o K / 800 C 30 (a) -1 Soluble ions (mg L ) 20 10 (b) 40 30 Fig EDX spectra of the treated-rice straw samples: 400 °C (a), 400 °C/H2O2 (b), 800 °C (c) and 800 °C/H2O2 (d) 20 and 800 °C/H2O2-treated samples were found to be higher than that of the 400 °C-treated sample By comparing the EDX signals of K, it can be seen that K contents of the 800 °C-treated samples are higher than those of 400 °C-treated samples While a subsequent wet-ashing with H2O2 result in almost no change in K abundance of the 800 °C-treated sample, a significant amount of K has been removed from 400 °C-treated sample by the same H2O2 treatment 3.4 Effect of heat treatment on solubility of Si and K Solubility of Si and K of the rice straw phytolith showed a high dependence on heating temperature (Fig 5) With a change in Time (day) Fig Si and K release from (a) dry-ashed rice straw samples treated at 400 °C and 800 °C, and (b) those with a subsequent treatment with H2O2, in a time sequence up to d 25 K Si 20 -1 Batch experiments carried out with the heat-treated ash samples and those with a subsequent treatment with H2O2 showed that the concentration of soluble Si and K increased with time at different rates depending on the treatment (Fig 4a and b) After d, Si and K releases were 32.7 and 12.8 mg kgÀ1 for the 400 °Ctreated sample, 14.2 and 8.0 mg kgÀ1 for the 800 °C-treated sample, 3.8 and 0.3 mg kgÀ1 for the 400 °C/H2O2-treated sample, and 42.7 and 9.2 mg kgÀ1 for the 400 °C/H2O2-treated sample, respectively For the 400 °C- and 800 °C-treated samples without H2O2 treatment, Si and K concentration in the supernatant increased over the initial d and stayed almost constant after day For the samples with a subsequent treatment with H2O2, Si and K concentrations continuously increased with time and no saturation was reached after d Different changes in Si and K concentration in the supernatant of the 400 °C/H2O2- and 800 °C/H2O2-treated samples were also observed (Fig 4b) Lower Si and K were released for the 400 °Ctreated sample when it was subsequently treated with H2O2, whereas, in the case of Si release, an opposite trend was observed for the 800 °C-treated sample Nguyen et al (2013) found that a subsequent treatment with H2O2 decreased Si dissolution from rice straw ash phytolith The varying response of the samples to H2O2 treatment could be a change in the efficiency of the H2O2 oxidation reaction between the 400 °C- and 800 °C-treated samples We found that the 400 °C-treated sample became finer and whiter after treatment with H2O2, while no obvious change was observed for the 800 °C-treated sample Soluble ions (mg L ) 3.3 Si and K dissolution kinetics 10 15 10 300 400 500 600 700 800 900 1000 o Treatment temperature ( C) Fig Dependence of the solubility of Si and K on treatment temperature ashing temperature from 300 °C to 600 °C, increases of the soluble Si and K from to 15 mg LÀ1 and to 22 mg LÀ1, respectively, were observed At a higher temperature range, from 700 °C to 1000 °C, the solubility of Si and K were obviously decreased Lowest values of soluble Si and K were and mg LÀ1 for the sample treated at 1000 °C Over the entire temperature range from 300 °C to 1000 °C, K showed a lower solubility in comparison with Si It can be seen that soluble Si and K have similar trends (similar ‘‘peak shape’’ as shown in Fig 5), and the maximum values were at 600 °C This strongly suggests that a similar mechanism controls the dissolution of Si and K from the samples 375 M.N Nguyen et al / Chemosphere 119 (2015) 371–376 Discussion 3.6 x + 05 85 0.9 0.2 = y= R 14 12 -1 Soluble K (mg L ) K is taken up from soil and transferred to different parts of the rice plant, e.g., stems and leaves, to provide the appropriate ionic environment for metabolic processes in the cytosol, and, as such, functions as a regulator of various processes including growth (Leigh and Wyn Jones, 1984) During the growth of rice, precipitation of Si to build up the phytolith might result in an occlusion of plant transport sap which contains organic molecules and dissolved substances within the silica cells The visualization performed with X-ray tomographic microscopy showed a number of holes inside the phytolith (Fig 6a) and these holes were contained organic substances (Fig 6b) K is one of the most dominant ions in the transport sap, suggesting that it might also be trapped inside these ‘‘closed holes’’ These holes might also exist as micropores (Mohamad Remli et al., 2014) that were not visible in Fig 6, and which also some occluded organic compounds and K The EDX spectra showed that Si, O, C and K are the most dominant elements present in the phytolith (Fig 2) A low EDX signal for K of the 400 °C/H2O2-treated sample suggests that both K and organic matter were removed when the sample was treated with H2O2 This indicates that K can be a component of the occluded material inside the phytolith structure that we described above as ‘‘PhytOK’’ However, a detailed analysis of the chemistry of this K-pool and its relationship to the organic matter coexisting in the holes of the phytolith structure was not included in this study In the kinetic experiments, increases of soluble Si with time were observed as a result of phytolith decomposition A corresponding increase of the K concentration in the supernatants was observed and suggested a relationship between K and Si releases In order to clarify this relationship, the kinetic data was used to create linear trend lines (as shown in Fig 7) A strong correlation between the soluble Si and K for all the 400 °C-, 800 °C-, 400 °C/ H2O2-, and 800 °C/H2O2-treated samples was evident We can therefore conclude that the K release is controlled by the dissolution of the phytolith The temperature of the sample treatment may also be a factor controlling phytolith dissolution and K release It can be clearly seen in Fig that line (a), representing the 400 °C-, 400 °C/H2O2-treated samples is clearly separated from line (b) for the 800 °C-, 16 91 (b) 10 (a) 215 - x 84 06 0.2 0.9 y= R = 0 10 20 30 40 50 -1 Soluble Si (mg L ) Fig Correlation between soluble Si and K in the supernatant of 400 °C- and 400 °C/H2O2-treated samples (a), and 800 °C- and 800 °C/H2O2-treated samples (b) in the kinetic experiments 800 °C/H2O2-treated samples Ratios of soluble Si and K varied for the samples treated at different temperatures as shown in Fig We believe that differences in organic removal rate and transformation of the silica-containing surface groups during the heat treatment are two of the primary factors limiting Si and K release from rice straw ash phytolith High contents of organic matter in the samples treated at low temperature prohibited Si and K releases This is in accordance with findings from other studies (Van Cappellen et al., 2002; Parr and Sullivan, 2005) in which it was reported that organic matter strengthens the phytolith surface and its resistance to dissolution With increasing pretreatment temperature, dehydroxylation of silanol groups will form siloxane bonds and the surface becomes hydrophobic (Zhuravlev, 2000) This reaction reduces adsorption of water molecules on the surface and prevents the breakage of the surface siloxane bonds It can therefore explain why little Si and K were released from the phytolith samples in this case In addition, the formation of stable silica at high temperature resulted in a product with low specific surface area and less activity (Kordatos et al., 2008; Nguyen et al., 2013), likely also resulting in a decrease of Si and K release Fig Three-dimensional image of the principal arrangement of silicified structures (a) and it is fulfilled by occluded substances (b) Example of a silica cell with occluded substances (c) and destructed cell (d) Phytolith appear gray and occluded substances dark gray The pixel width is 0.37 lm 376 M.N Nguyen et al / Chemosphere 119 (2015) 371–376 Another hypothesis is that K might be located in the siloxane hexagonal cavities and it is released as a result of the breaking of the surface siloxane bonds As reported by Zhuravlev (2000), siloxane bonds can only be formed by dehydroxylation of silanol groups of the amorphous silica during high temperatures in the pretreatment process This implies that phytolith in rice straw has no siloxane surface, i.e., hexagonal cavities not occur, before burning During the ashing process, some internal K in the xylem/phloem sap might be occluded in these newly-formed hexagonal cavities However, extractions using Ca2+ and NH+4 as exchange ions showed that an increase of Ca2+ or NH+4 in the supernatant resulted in lower amounts of both Si and K released (data not shown) This suggests that K was released by dissolution of the phytolith rather than exchanged from the siloxane surface Conclusion In rice straw phytolith samples, K content was up to 55%, indicating that this K-pool cannot be ignored when considering crop and soil fertility management By combining the results from batch experiments, analysis of SEM–EDX, and X-ray tomographic microscopy, we believe that K might co-exist with organic matter in the ‘‘closed-holes’’ of the phytolith structure This implies that this amount of PhytOK is locked and unavailable for plants prior to dissolution of the phytolith structure Co-release of Si and K was observed and it allowed us to conclude that dissolution of the phytolith is a main factor regulating K release The ashing temperature of the rice straw can affect Si and K releases by enhancing the removal of occluded carbon or stabilizing the silica surface of the phytolith The highest values of soluble Si and K observed at 600 °C suggests that pretreatment of the rice straw around this ashing temperature is optimal in providing available Si and K for soils and crops Acknowledgements This research was funded by the Vietnam National Foundation for Science & Technology Development (Project 105.08-2013.01) X-ray-tomographic microscopy was performed with skilful help by Julie Fife at the TOMCAT beamline of the synchrotron facility of the Paul Scherrer Institute, Villigen, Switzerland Great help of Sarah B Cichy and Karl-Ingo Friese for morphological characteriza- tion of phytoliths from the tomographic dataset is acknowledged We would 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Zhuravlev model Colloids Surf., A 173, 1–38  ... solution The dissolution experiments lasted d and sampling was carried out every 24 h Experiment 2: To evaluate the effect of heat-pretreatment of rice straw, the rice straw samples treated at 300,... and 800 °C/H2O2-treated samples was evident We can therefore conclude that the K release is controlled by the dissolution of the phytolith The temperature of the sample treatment may also be... groups during the heat treatment are two of the primary factors limiting Si and K release from rice straw ash phytolith High contents of organic matter in the samples treated at low temperature prohibited