Article pubs.acs.org/JAFC
Cite This: J. Agric. Food Chem. 2019, 67, 7183−7189
Extraction and Analysis of Available Boron Isotopes in Soil Using Multicollector Inductively Coupled Plasma Mass Spectrometry Aide Sun,*,†,‡ Dianda Gou,† Yuliang Dong,† Qingcai Xu,*,† and Guangjie Cao† †
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Shandong Provincial Key Laboratory of Water and Soil Conservation and Environmental Protection, College of Resource and Environment, Linyi University, Linyi, Shandong 276005, People’s Republic of China ‡ State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, Guangdong 510640, People’s Republic of China S Supporting Information *
ABSTRACT: As a result of the important roles of boron (B) in the growth of plants, the uptake of B by plants is dependent upon the existing form and content of available B in soil, which can bring about the local cycle of B isotope equilibrium. A method using water-heating extraction combined with three-step ion-exchange chromatography was developed for the extraction and isotopic analysis of available B in soil. The extraction efficiency and fractionation of B isotopic composition in the procedure were investigated. The results showed that, in the upper layers of soils, the change of δ11B values was opposite that of the mass concentration and a similar variation between δ11B and content occurred in the lower layers. The isotope of available B in soil can create a featured isotopic signature to further understand the geochemical details related to the soil properties and molecular mechanism of B uptake in plants. KEYWORDS: available boron, brown soil, cinnamon soil, isotopic composition, multicollector inductively coupled plasma mass spectrometry
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there can be toxicity to plants.18 The uptake effectiveness of available B by the plant varies with plant species,8 which may enable larger differences of available B in the contents, isotopic composition, and fractionation in the absorption and utilization by different plants. Many methods have been reported to determine available B in soil using different extractants,19,20 including boiling water, hot calcium chloride, potassium dihydrogen phosphate, tartaric acid, and mannitol− CaCl2. In these methods, experimental conditions affecting the extraction efficiency of available B, such as extraction time, temperature, pH, and volumes and types of extractants have been investigated. In these methods, as much available B was extracted as possible; however, in these procedures of available B extraction when available B is extracted from soil, the other fractions of B in soil can be transformed to available B. It is difficult to distinguish whether other fractions of B in the soil are transformed when available B is extracted. These reports on the extraction of available B laid a good foundation to investigate the variation and fractionation characteristics of isotope compositions of available B in soil taken up by plants. The mass difference between B stable isotopes 10B and 11B results in featured isotopic signals of δ11B values with a measured range from −70 to 75‰ in various geological environments.21−23 The major forms of B are as H3BO3 in acid or B(OH)4− in alkaline solution. The isotopic equilibrium between B(OH)3 and B(OH)4− in aqueous solution is expressed as the following in eq 1:
INTRODUCTION Boron (B) is an essential micronutrient required for the growth and development of higher plants, marine algae, and cyanobacteria and has a low concentration in environmental samples.1−4 However, excessive B in soil is toxic to plants,5−7 but B deficiency results in abnormality or death in root and shoot growth, barrier function of the leaf surface, and vessel formation.8,9 Given the narrow range between deficiency and toxicity of B,10 a slight fluctuation beyond the optimum level can be toxic to plants.11,12 In nature, B exists in the soil solution as hydrosoluble B in the forms of trihydroxyboric acid (H3BO3) and tetrahydroxyborate [B(OH)4−].13 Total B in soil is divided into five fractions: readily soluble, specifically adsorbed, oxide bound, organically bound, and residual B.14 These soil fractions can have mutual transformations and interactions under certain environmental conditions,14,15 which leads to a large difference in the mass concentration, properties, and textures among different fractions of soils.16 Most B in soil is coated on mineral lattices or adsorbed on the surface of soil particles and is difficult for plants to directly absorb.5,10 Nevertheless, available B can only be absorbed by plant roots in the form of H3BO3 or B(OH)4−, which represents 1−3% of total B in soil.1,3,11 The portion of B adsorbed on the surface of clay mineral and metal oxide particles in soil is considered the reservoir of available B, which can dissolve slowly to prevent the exchange equilibrium of available B as a result of mutation and uptake. In the field of plant B nutrition, hydrosoluble B in soil is mostly used to represent available B.15 In general, the threshold of available B in soil is in the range of 0.5−2.0 mg/kg. If the amount of available B is below the threshold, then B deficiency occurs;12,16,17 if the amount of available B in soil is higher, © 2019 American Chemical Society
Received: Revised: Accepted: Published: 7183
March 5, 2019 May 28, 2019 May 31, 2019 May 31, 2019 DOI: 10.1021/acs.jafc.9b01455 J. Agric. Food Chem. 2019, 67, 7183−7189
Article
Journal of Agricultural and Food Chemistry Table 1. Information of Climate and Geography in the Sample Sites soil type
location
altitude (m)
longitude
latitude
temperature (°C)
precipitation (mm)
vegetation type
brown soil cinnamon soil
Pingyi, Shandong Pingyi, Shandong
496 534
117° 45′ 0.6228″ E 117° 45′ 1.7532″ E
35° 18′ 10.4472″ N 35° 18′ 11.322″ N
from −2 to 26 from −3 to 27
830 800
angiosperm, herb angiosperm, herb
10
B(OH)3 + 11 B(OH)4 − ↔ 10 B(OH)4 − + 11 B(OH)3
of the two soils with a depth down to 40 cm in the brown soil profile and 105 cm in the cinnamon soil profile were collected in May 2017 from Pingyi County of Shandong Province, China (Figure S1 of the Supporting Information). The soil profile was sampled from the bedrock to uppermost layer in the interval of 5 cm. According to the sampling sequence, the sample IDs were name from ZR01 to ZR07 for the brown soil profile and from HT01 to HT20 for the cinnamon soil profile. Details of climate and geography in the soil sample site are listed in Table 1. The information on pH, organic matter, and soil texture in the two soil profile samples is shown in Table S1 of the Supporting Information. The main compositions of the clay mineral in the two soils are shown in Table S2 of the Supporting Information. Reagents. Hydrochloric acid (HCl, guaranteed reagent) was redistilled to remove exogenous B and stored in a sealed polypropylene vessel. The isotopic reference standard material used in this study was NIST SRM 951 boric acid [National Institute of Standards and Technology (NIST), Gaithersburg, MD, U.S.A.]. Boric acid, borax, and ammonium hydroxide were of guaranteed reagent quality. The B specific resin Amberlite IRA 743 (20−50 mesh), strong cation-exchange resin Dowex 50W X8 (200−400 mesh), and weak anion-exchange resin Amberlite IRA 67 (20−50 mesh) were purchased from Sigma-Aldrich LLC (Shanghai, China). High-purity water with a B blank of less than 8 ng was redistilled by sub-boiling distillation, passed through a column filled with Amberlite IRA 743 resin, and then used to prepare the standard and working solutions. Apparatus. Inductively coupled plasma optical emission spectrometry (ICP−OES, Vista MPX, Varian, Palo Alto, CA, U.S.A.) equipped with a 40 MHz radio frequency (RF) generator and a charge-coupled device detector was used for B determination.29 The spectral line for the B determination was set at 249.68 nm. Neptune Plus MC−ICP−MS (Thermo Fisher Scientific, Waltham, MA, U.S.A.) was employed for B isotope measurement, coupled with a double-focusing magnetic sector field mass spectrometer equipped with nine Faraday cups and four ion counters, enabling static measurements of m/z 10 and 11. The typical operating conditions for B measurement by MC−ICP−MS are summarized in Table 2.34 The
(1) 11
Heavy isotope ( B) is preferentially enriched in H3BO3, but light isotope (10B) is preferentially enriched in B(OH)4−. Because of the isotopic equilibrium between H3BO3 and B(OH)4−,24,25 there are different δ11B values in different fractions and types of soils. The B concentration and isotopic compositions in the acid-soluble fraction of the loess−paleosol profile in Luochuan were in the range from 0.8 to 2.7 mg/kg and from −1.8 to +18.6‰, respectively.26 The B content and δ11B values in paleosol were higher than those in the loess layer as a result of the adsorption effect on B of different phases.26,27 The δ11B values of soils in the Orinoco River Basin changed little (from −1.4 to 1.6‰) as a result of the effects of chemical weathering. The sorption of available B to soil is a key process that controls its transport and fate and contributes to the isotopic fractionation in various environmental media, especially the water−soil and soil−rhizosphere interfaces.28 These δ11B values in soils mentioned above were not lower than those in root compartments in three plant species with the range from −6.80 to +0.29‰ reported by Xu et al.29 and that in the root of bell pepper (−11.0‰) by Geilert et al.30 The amounts of B and δ11B values in the acid-soluble fraction and of total B in soil were used to investigate the geochemical characteristics of B isotopes in the loess−paleosol profile and the origin of the chemical weathering; however, these forms of B could not be directly absorbed by plants and have little relationship with that of available B.31,32 The content of available B changed with the depth of soil profile; thus, the absorption and isotopic composition of B in the growth of root in different depths are dependent upon the conditions of available B in soil solution; however, the variation of isotopic compositions and fractionation of available B in different agrotypes have been poorly studied. On the basis of this, it was desirable to develop a method for the extraction and analysis of isotopic compositions of available B in soil samples using multicollector inductively coupled plasma mass spectrometry (MC−ICP−MS). In this study, samples of brown and cinnamon soils were selected to develop a method for the extraction of available B using the B isotopic composition other than the mass concentration of B in the procedure for investigation. The factors influencing the extraction efficiency of available B and fractionation of B isotopic compositions in the method were investigated. The variation and fractionation characteristics of the mass concentrations and isotopic compositions of available B in the two soil samples were examined and discussed preliminarily.
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Table 2. Typical Operating Conditions for B Measurement on Neptune parameter
value
RF forward power (W)
1310
Ar auxiliary gas flow (L/min)
1.0
extraction voltage (V) integration time (s)
−2000 4.194
parameter
value
Ar cooling gas flow (L/min) Ar sample gas flow (L/min) acceleration voltage (kV) idle time (s)
16 1.0 10 3
samples were measured using the standard bracketing technique, with NIST SRM 951 H3BO3 as the isotopic standard reference material.34 The determination of the B isotopic ratio in soil samples were completed in the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, China. Extraction of Available B in Soil Samples. About 10.0 g of soil sample was weighed accurately and placed in a polypropylene tube. Then, 20 mL of high-purity water was added to the tube to make a ratio of water to soil of 2:1 (w/w). The tube with the mixed solution was shaken gently until all of the soil sample was suspended in aqueous solution, which was placed in a water bath at 100 °C. After heating for 20 min, the temperature of the soil solution exceeded 95
MATERIALS AND METHODS
Soil Profile Information. Brown and cinnamon soils are typical soil types in Shandong Province, China, and account for 14.09 and 14.66% of the soil area of Shandong, respectively. “Shandong brown earth” is a typical brown soil in the Chinese soil classification system.33 The pH in brown soil ranges from slightly acidic to neutral, and cinnamon soils can be pH 6−8.2 or more.33 The natural samples 7184
DOI: 10.1021/acs.jafc.9b01455 J. Agric. Food Chem. 2019, 67, 7183−7189
Article
Journal of Agricultural and Food Chemistry
Figure 1. Workflow of the extraction of available B in soil samples.
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°C. The extraction of available B was performed for 30 min. Then, after quickly cooling to room temperature, the solution was filtered by negative-pressure filtration. The quantified eluate was used to determine the mass concentration of available B using ICP−OES and to extract B prepared for the measurement of B isotope composition in the soil sample. Three-step ion-exchange chromatography was used for the extraction of B in obtained aqueous solution.35−37 First, 10 mL of the aqueous solution of available B was transferred to a mixing ionexchange resin column, consisting of Dowex 50W X8 and Amberlite IRA 67 exchange resins. After the filtered solution was collected, the resin was rinsed with three batches of 0.5 mL of deionized water. The collected solutions were mixed together. Then, after pH in the aqueous solution was adjusted to 9 with ammonium hydroxide solution, the solution was transferred to a resin column loaded with Amberlite IRA 743 B-selective resin. The solution was leached through the resin at a rate of 0.5 mL/min. Then, three batches of 0.5 mL of deionized water were used to rinse the B-selective resin, which was performed to selectively extract B and to remove remaining impurities meanwhile. After that, 5 mL of 0.1 mol/L HCl was used to elute B in the resin 3 times. All of the acid solutions were collected into a polyfluoroalkoxy (PFA) tube, which was evaporated at 60 °C with a clear air flow until nearly dry. Finally, the concentrated solution in the tube with 0.5 mL of high-purity water added was transferred to another mixing ion-exchange resin column equipped with strong cation- and weak anion-exchange resins. Five batches of 3 mL of deionized water were used to rinse the mixing resins. All eluates were collected into a 15 mL PFA beaker. With the addition of mannitol solution, the solution was evaporated to about 0.2 mL remaining. Then, the solution was diverted to a 0.5 mL centrifuge tube, and the evaporation continued until the approximate concentration of the solution was 1 mg of B/mL, which was then stored at 4 °C for mass spectrometric analysis.35,38,39 An outline of the general workflow of available B extraction in soil samples is shown in Figure 1. Isotopic Measurement of B. The B isotopic composition of the sample measured by MC−ICP−MS is expressed as δ11B (‰) relative to that of the NIST SRM 951 standard in eq 2
δ11 B (‰) = [(11 B/10 B)Sam /(11 B/10 B)Std − 1] × 1000 11
10
11
RESULTS AND DISCUSSION
Effect of the Extraction Time on the Extraction Efficiency of Available B in Soil. The mass fraction distribution and isotopic compositions of available B were dependent upon the interactions and transformation in the five fractions of B in soil. When available B is continually extracted from a soil sample, the other types can convert to available B. The real amount of available B extracted from soil is affected by the extraction time. If the other forms of B in soil convert to available B and are extracted simultaneously, this may alert the isotopic compositions of available B. Thus, authentic information about the geochemical characteristics in isotopic signals of available B in soil would not be obtained. In the experiments, when the soil solution was boiling, the time required for extraction was accurately recorded. The extraction time was set at 5, 10, 20, 30, 40, and 60 min to investigate extraction efficiency of available B. The results are shown in Figure 2.
Figure 2. Change of the mass concentration of available B in brown and cinnamon soils with extraction time.
(2)
10
where ( B/ B)Sam and ( B/ B)Std are the B isotopic ratios of the sample and NIST SRM 951 H3BO3, respectively. Statistical Analysis. The Microsoft Office Excel software (Chinese version) was used for statistical analyses. Single element analysis was performed to screen out the optimum conditions between the amount of available B and the effect of factors. The mass concentration of B and δ11B values in soil samples was replicated thrice, and a relative standard error was obtained.
With an increasing extraction time, the amount of available B extracted in soil also increased (Figure 2). After extraction for 30 min, the content of available B became stable at a certain range. For the effect of the gradient concentration, available B in the soil sample was gradually dissolved into the aqueous solution. When extraction time exceeded 30 min, available B had been extracted completely. When the extraction time 7185
DOI: 10.1021/acs.jafc.9b01455 J. Agric. Food Chem. 2019, 67, 7183−7189
Article
Journal of Agricultural and Food Chemistry
affect the recovery in the procedure and the isotopic composition of available B in soil. The recovery of B in the extraction of three-step ion-exchange chromatography in previous reports35−37 showed no isotopic fractionation of B during the procedure. In the tests, the recoveries in waterheating extraction, three-step ion-exchange chromatography, and the whole procedure were analyzed using standard spiking with H3BO3 as the reference standard material. The recoveries for available B are shown in Figure 4. It was clear in Figure 4 that the recoveries in water-heating extraction, three-step ion-exchange chromatography, and the whole procedure were within the range of 98−105% and the amounts of B in the respective parts were determined using ICP−OES to be within the allowable error. This indicated that no available B was lost and isotopic fractionation of available B did not occur in the extraction. The blank of available B with the amount of less than 8 ng was determined 10 times, showing that the introduction of exogenous B of
