Effect of humic acid contact time on the diffusion of Re(VII) in GMZ bentonite

Effect of humic acid contact time on the diffusion of Re(VII) in GMZ bentonite 1 2 first-author 2 3 1 4 1 1 1 corresp Corresponding author, zhaoyaolin@mail.xjtu.edu.cn Corresponding author, zhaoyaolin@mail.xjtu.edu.cn zhaoyaolin@mail.xjtu.edu.cn 1 School of Nuclear Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China School of Nuclear Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China Department of Chemistry, Huzhou Teachers College, Huzhou 313000, China Department of Chemistry, Huzhou Teachers College, Huzhou 313000, China Huzhou Environmental Protection Monitoring Center, Huzhou 313000, China Huzhou Environmental Protection Monitoring Center, Huzhou 313000, China Northwest Institute of Nuclear Technology, Xi’an 710024, China Northwest Institute of Nuclear Technology, Xi’an 710024, China The effect of humic acid (HA) on the diffusion of Re(VII) was investigated in compacted Gaomiaozi (GMZ) bentonite by the through- and out- diffusion method. The effective diffusion coefficient, D e , and accessible porosity, acc , were measured in order to evaluate the impact of humic acid on Re(VII) diffusion. The D e value was in the range of (5.2–8.3)×10 -12 m 2 /s. The diffusion of Re(VII) was enhanced in the presence of HA, and the D e value increased with the increase of ageing time, indicating that the formation of HA-Re complexes was slow. Moreover, the acc was in the range of (0.06– 0.16), which is less than the total porosity. It implies that the HA-Re complexes are anions. Diffusion coefficient Humic acid Re(VII) Complex Bentonite I. INTRODUCTION 1 Humic acids (HA) possess a strong ability for metal complexation. They can alter the migration of radionuclides in clay environments. Several studies have shown that some radionuclides are strongly retained in humics-rich surface soils [ 1 1 , 2 2 ]. For example, the migration of Eu(III) in bentonite is hindered by HA because of the formation of the Eu-HA complexes at the surface of compacted bentonite [ 1 1 ]. In addition, the diffusion of Cu(II) is prevented by humic gels and the diffusion coefficient in the presence of humic gels is two orders of magnitude lower than that in water [ 2 2 ]. However, humics also facilitate radionuclide transport by the formation of radionuclides-colloid complexes. Wold et al . [ 3 3 ] found that humics could make the apparent diffusion coefficient D a increase and the distribution coefficient, K d , values decrease for both Co(II) and Eu(III) in compacted bentonite. 99 Tc is a significant environmental contaminant with a long half-life of 2.1×10 5 years. 99 Tc presents a challenge for clean up because it is difficult to understand the key processes that affect its mobility in the environment. Tc(VII) is the most stable in natural oxic environments and it has high solubility in water and essentially no absorptive properties toward minerals. As contrasted with Tc(VII), Tc(IV) is less soluble and can be immobile in underground water and soils [ 4 4 - 6 6 4-6 ]. Several researchers have focused on the effect of humic substances (HS) on Tc(IV) mobility [ 7 7 - 9 9 7-9 ]. For instance, Artinger et al . [ 7 7 ] show that humic substances (HS) enhanced the mobility of Tc(IV) to form the soluble Tc(IV)-HS species. Boggs et al . [ 9 9 ] found that TcO(OH)-HA was the predominant complex under acidic conditions, whereas TcO(OH) 2 0 and TcO(OH) 2 -HA were the major species under neutral and alkaline conditions, respectively. XANES and EXAFS measurements show that the soluble Tc(IV)-HS species are formed in terms of ligand-metal complexation under slightly basic pH and reducing conditions [ 10 10 ]. Moreover, Maes et al . [ 11 11 ] explained the transport of Tc(IV) with organic matter as an organic matter complex/colloid that slowly dissociates. Studies also show that organic ligand like oxalate and acetate increase Tc(IV) solubility [ 4 4 , 12 12 ]. Even in the case of Tc(VII), Maes et al . [ 13 13 ] found that Tc(VII) was reduced to Tc(IV) by the solid Boom clay phase, then the competition effect for Tc(IV) was described between the solid phase and dissolved HS. In the safety assessment of high level radioactive waste repository, the radionuclide species with high mobility, such as 99 TcO 4 - , 125 I - , 36 Cl - , and 79 SeO 3 2- , are given the most concerned in the pre-safety assessment [ 14 14 - 17 17 14-17 ]. The bentonite in Gaomiaozi county China (GMZ bentonite) has been selected as the backfill material for the high level radioactive waste repository. The effective/apparent diffusion coefficient, D e / D a , porosity, tot and distribution coefficient, K d , are the fundamental parameters in modeling the radionuclide transport behaviors for a safety assessment of the repository. Our recent work showed that EDTA can enhance TcO 4 - transport, but not alter the mobility of ReO 4 - (as analogue to 99 TcO 4 - ) due to less reactivity of ReO 4 - [ 18 18 ]. In addition, Liu et al . [ 19 19 ] found that HA has insignificant effect on the diffusion of Tc(VII) in granite. However, the investigations were performed without consideration of the effect of aging time of HA. The difference might be overlapped by the uncertainty or other unknown factors. In this work, the effect of HA on the diffusion of ReO 4 - is under different aging time conditions. The D e and accessible porosity ( acc ) were obtained by fitting the experimental data of both the transient and steady states. Out-diffusion method was employed to clarify if the interaction between ReO 4 - and HA reached equilibrium. II. MATERIALS AND METHODS 1 A. Material and experimental conditions 2 The experiments were carried out by through- and out-diffusion methods. Table 1 Table 1 shows the experimental conditions for the diffusion experiments. 1 1 Item 1 1 Condition/method 1 1 Method 1 1 Through- and out-diffusion methods 1 1 Bentonite dry density (kg/m 3 ) 1 1 GMZ (75.4 wt.% montmorillonite) 1800 1 1 Temperature (℃) 1 1 20±5 1 1 Atmosphere 1 1 ambient condition 1 1 Saturated solution 1 1 artificial pore water of Gansu Beishan (pH 7.6) 1 1 Humic acid (mg/L) 1 1 10 1 1 pH of tracer solution 1 1 2.5 - 3 The GMZ bentonite was obtained from the Beijing Research Institute of Uranium Geology without further processing. ReO 4 - was prepared in distilled water using NH 4 ReO 4 (Alfa Aesar, 99.997%). HA (Sigma-Aldrich) was dissolved by adding a small amount of 0.01 mmol/L NaOH. B. Data processing 2 For the through-diffusion, the diffusion parameters D e and α were obtained by fitting the following equations <math display="block" id="M1"><mrow><mtable columnalign="left"><mtr columnalign="left"><mtd columnalign="left"><mrow><msub><mi>A</mi><mrow><mtext>cum</mtext></mrow></msub><mo>=</mo></mrow></mtd><mtd columnalign="left"><mrow><mi>S</mi><mi>L</mi><msub><mi>C</mi><mn>0</mn></msub><mrow><mo>(</mo><mrow><mfrac><mrow><msub><mi>D</mi><mtext>e</mtext></msub><mi>t</mi></mrow><mrow><msup><mi>L</mi><mn>2</mn></msup></mrow></mfrac><mo>−</mo><mfrac><mi>α</mi><mn>6</mn></mfrac></mrow></mrow></mrow></mtd></mtr><mtr columnalign="left"><mtd columnalign="left"><mrow/></mtd><mtd columnalign="left"><mrow><mrow><mrow><mo>−</mo><mfrac><mrow><mn>2</mn><mi>α</mi></mrow><mrow><msup><mrow/><mrow><msup><mi>π</mi><mn>2</mn></msup></mrow></msup></mrow></mfrac><mstyle displaystyle="true"><munderover><mo>∑</mo><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow><mi>∞</mi></munderover></mstyle><mfrac><mrow><msup><mrow><mo stretchy="false">(</mo><mo>−</mo><mn>1</mn><mo stretchy="false">)</mo></mrow><mi>n</mi></msup></mrow><mrow><msup><mi>n</mi><mn>2</mn></msup></mrow></mfrac><mi>exp</mi><mrow><mo>{</mo><mrow><mo>−</mo><mfrac><mrow><msub><mi>D</mi><mtext>e</mtext></msub><msup><mi>n</mi><mn>2</mn></msup><msup><mi>π</mi><mn>2</mn></msup><mi>t</mi></mrow><mrow><msup><mi>L</mi><mn>2</mn></msup><mi>α</mi></mrow></mfrac></mrow><mo>}</mo></mrow></mrow><mo>)</mo></mrow><mn>,</mn></mrow></mtd></mtr></mtable></mrow></math> based on the following initial and boundary conditions: <math display="block" id="M2"><mrow><mtable columnalign="left"><mtr columnalign="left"><mtd columnalign="left"><mrow/></mtd><mtd columnalign="left"><mrow><mi>c</mi><mo stretchy="false">(</mo><mi>x</mi><mn>,</mn><mi>t</mi><mo stretchy="false">)</mo><mo>=</mo><mn>0,</mn><mtext> </mtext><mi>t</mi><mo>=</mo><mn>0</mn><mo>;</mo><mtext> </mtext><mo>∀</mo><mi>x</mi><mn>,</mn></mrow></mtd></mtr><mtr columnalign="left"><mtd columnalign="left"><mrow/></mtd><mtd columnalign="left"><mrow><mi>c</mi><mo stretchy="false">(</mo><mn>0,</mn><mi>t</mi><mo stretchy="false">)</mo><mo>=</mo><msub><mi>C</mi><mn>0</mn></msub><mn>,</mn><mtext> </mtext><mi>x</mi><mo>=</mo><mn>0</mn><mo>;</mo><mtext> </mtext><mi>t</mi><mo>></mo><mn>0,</mn></mrow></mtd></mtr><mtr columnalign="left"><mtd columnalign="left"><mrow/></mtd><mtd columnalign="left"><mrow><mi>c</mi><mo stretchy="false">(</mo><mi>L</mi><mn>,</mn><mi>t</mi><mo stretchy="false">)</mo><mo>=</mo><mn>0,</mn><mtext> </mtext><mi>x</mi><mo>=</mo><mi>L</mi><mo>;</mo><mtext> </mtext><mi>t</mi><mo>></mo><mn>0,</mn></mrow></mtd></mtr><mtr columnalign="left"><mtd columnalign="left"><mrow/></mtd><mtd columnalign="left"><mrow/></mtd></mtr></mtable></mrow></math> where L (m) is the sample thickness; C 0 (μg/m 3 ) is the initial concentration of the diffusing species; A cum (μg) is the total amount of diffused species through a boundary at x = L and at diffusion time t ; S (m 2 ) is the cross section area of the clay. The equations of out-diffusion can be described as follows: <math display="block" id="M3"><mrow><mi>J</mi><mo stretchy="false">(</mo><mn>0,</mn><mi>t</mi><mo stretchy="false">)</mo><mo>=</mo><mn>2</mn><msub><mi>J</mi><mi>L</mi></msub><mstyle displaystyle="true"><munderover><mo>∑</mo><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow><mi>∞</mi></munderover></mstyle><msup><mi>e</mi><mrow><mo>−</mo><mrow><mo>(</mo><mrow><mfrac><mrow><mi>n</mi><mi>ρ</mi></mrow><mi>L</mi></mfrac><mfrac><mrow><msub><mrow><msub><mrow/><mi>D</mi></msub></mrow><mtext>e</mtext></msub></mrow><mi>α</mi></mfrac><mi>t</mi></mrow><mo>)</mo></mrow></mrow></msup><mtext> </mtext><mtext>at</mtext><mtext> </mtext><mi>x</mi><mo>=</mo><mn>0,</mn></mrow></math> <math display="block" id="M4"><mrow><mi>J</mi><mo stretchy="false">(</mo><mi>L</mi><mn>,</mn><mi>t</mi><mo stretchy="false">)</mo><mo>=</mo><mn>2</mn><msub><mi>J</mi><mi>L</mi></msub><mstyle displaystyle="true"><munderover><mo>∑</mo><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow><mi>∞</mi></munderover></mstyle><msup><mrow><mo stretchy="false">(</mo><mo>−</mo><mn>1</mn><mo stretchy="false">)</mo></mrow><mi>n</mi></msup><msup><mi>e</mi><mrow><mo>−</mo><mrow><mo>(</mo><mrow><mfrac><mrow><mi>n</mi><mi>π</mi></mrow><mi>L</mi></mfrac><mfrac><mrow><msub><mrow><msub><mrow/><mi>D</mi></msub></mrow><mtext>e</mtext></msub></mrow><mi>α</mi></mfrac><mi>t</mi></mrow><mo>)</mo></mrow></mrow></msup><mtext> </mtext><mtext>at</mtext><mtext> </mtext><mi>x</mi><mo>=</mo><mi>L</mi><mn>,</mn></mrow></math> where J (0, t ) and J ( L , t ) are fluxes at the high and low concentration boundaries, respectively. The experimental procedure was consistent with the literature [ 14 14 , 18 18 , 20 20 ]. The equilibrium condition is that the artificial pore water containing 10 mg L -1 humic acid is in contact with bentonite for 5 weeks. Aging conditions were that 10 mg/L humic acid mixed with ReO 4 - in artificial pore water was aged for 0 and 60 days, respectively. The source reservoir was spiked with (0.6–1.6) g/L ReO 4 - . The solutions in the target reservoirs were replaced to keep the concentration of ReO 4 - in this reservoir as low as possible. After finishing the through- diffusion experiment, a 10 mL solution without ReO 4 - and HA in both reservoirs was replaced and the out-diffusion experiment was started. The sample was determined by ICP-OES (PerkinElmer Optima 2100DV). The experimental and theoretical data processing of the through- and out- diffusion methods have been described previously [ 14 14 , 21 21 , 22 22 ]. The composition of the GMZ bentonite samples were analyzed by the powder X-ray diffraction (XRD) with a Model XD-2 (Beijing Purkinje General Instrument Co., Ltd., China) using CuK (radiation at 35 kV/20 mA). The morphology and structure of bentonite were observed using a scanning electron microscope (Hitachi S-3400N). III. RESULTS AND DISCUSSION 1 Constituent mineral composition of bentonite could be detected through X-ray diffraction. Fig. 1(a) Fig. 1(a) shows scanning electron microscopy (SEM) images of GMZ bentonite. The most composition was montmorillonite with a characteristic peak at 2 θ = 6.14 °, 19.9 °, 36.7 ° and 55.4 °. Other minerals, such as feldspars, quartz, and Cal - Fe(Ca)CO 3 , were also present in GMZ bentonite. The layer structure of GMZ bentonite consisted of one alumina octahedral sheet located between the two silica tetrahedral sheets. The same result can be found in other GMZ [ 23 23 , 24 24 ] and MX-80 [ 25 25 ] bentonite research. The results of the analysis by scanning electron microscopy show ( Fig. 1(b) Fig. 1(b) ) that GMZ bentonite has a multi-layer structure and smoothed shapes. Figure 2 Figure 2 shows the A cum (2(a)) and Flux (2(b)) vs. time for ReO 4 - in HA equilibrium, HA aging 0 days and aging 60 days 3 kinds of condition at 1 800 kg/m 3 compacted GMZ bentonite by through-diffusion method. Through- diffusion experiments of ReO 4 - in the presence of different HA conditions (HA equilibrium, HA aging 0 days and aging 60 days) were studied by compacting GMZ bentonite at 1 800 kg/m 3 , respectively. Fig. 2 Fig. 2 shows the A cum and flux as a function of diffusion time. Diffusion parameters are listed in Table 2 Table 2 . It shows a typical diffusion curve. The flux increased dramatically at the transient phase, then remains unchanged at the steady phase. The D e and rock capacity factor α values were obtained from the steady and transient phases, respectively. α is equal to the accessible porosity acc for anions [ 20 20 ]. The time to reach the steady phase was 5, 8 and 12 days under equilibrium, aging 0 and 60-day conditions. The breakthrough curves of flux were almost identical under equilibrium and aging 0 conditions. Since humic substances can absorb on silica sheets of montmorillonite in lower pH [ 26 26 ] and clogged bentonite pore space [ 27 27 ], it implies that the interaction between HA and Re(VII) is stronger than that of between HA and bentonite. Therefore, Re(VII)-HA complexes can be formed. Moreover, the flux under aging 60-day condition was higher than the others, indicating that the formation of Re(VII)-HA complexes was slow. Fig. 1(b) Fig. 1(b) verified the breakthrough curves of flux, that is, the A cum was almost identical under equilibrium and aging 0 conditions and it increased fastest under aging 60-day conditions. 1 1 Sample 1 1 Condition 1 1 C 0 (mg/L) 1 1 D e (× 10 -11 m 2 /s) 1 1 D a (× 10 -11 m 2 /s) 1 1 ε acc 1 1 Re(VII) 1 1 HA equilibrium 1 1 1600 ± 50 1 1 0.52 ± 0.05 1 1 8.67±2.28 1 1 0.06 ± 0.01 1 1 1 1 HA aging 0 day 1 1 1600 ± 50 1 1 0.59 ± 0.03 1 1 4.92±3.12 1 1 0.12 ± 0.07 1 1 1 1 HA aging 60 days 1 1 1600 ± 50 1 1 0.83 ± 0.09 1 1 5.19±1.12 1 1 0.16 ± 0.02 Table 2 Table 2 shows the measured diffusion parameters for ReO 4 - in GMZ bentonite. The apparent diffusion coefficient ( D a ) is calculated as follows: <math display="block" id="M5"><mrow><msub><mi>D</mi><mtext>a</mtext></msub><mo>=</mo><mfrac><mrow><msub><mi>D</mi><mtext>e</mtext></msub></mrow><mi>α</mi></mfrac><mn>.</mn></mrow></math> The D e value under aging 60-day condition was higher than that of under equilibrium and aging 0 day conditions. It demonstrates that HA can enhance the mobility of Re(VII). The acc values were less than the total porosity ε values, which is measured by the HTO through-diffusion method in GMZ bentonite [ 22 22 ]. It indicates that the Re(VII)-HA complexes are anionic species. In order to verify the slow reaction kinetics of Re(VII) and HA, Fig. 3 Fig. 3 shows the out-diffusion results of Re(VII) diffusion under aging 0 and 60-day conditions. J ( L , t ) was the flux in target reservoir boundary, and J (0, t ) was the flux in source reservoir boundary. The curves were calculated by using the D e and acc values obtained from the through-diffusion experiments ( Table 2 Table 2 ). The data of Re under aging 60-day condition at both boundaries (original target and source reservoir) were in good agreement with the predictions from the through-diffusion experiment, except the first three data points. The dead volume was probably the major factor to enhance the mass of Re, especially in a short time interval (from 30 min to a few hours), to change the reservoirs. In contrast, the J ( L , t ) of predicted and measured data is in agreement with each other, however the J (0, t ) were systematically underestimated by the model predictions for the diffusion of Re-HA under aging 0 day conditions. This analogue discrepancy at J (0, t ) was in agreement with the Cl - out-diffusion experiment [ 17 17 , 28 28 , 29 29 ]. J (0, t ) showed the exponent decreased with time, it can be explained that heterogeneous porosity distributions are of much less importance for through- and out-diffusion at a low concentration boundary. However, they significantly influence out-diffusion results at a high concentration boundary. Fig. 3 Fig. 3 also shows that the experimental data was identical under 0 (solid dots) and 60-day (empty dots) conditions. Moreover, the expected curves obtained from aging 60 days provided a better fit than that from aging 0 day. It can be explained that the reaction equilibrium between Re(VII) and HA was achieved when the out-diffusion was started under 0 day conditions. IV. CONCLUSION 1 Diffusion experiments for ReO 4 - in the presence of humic acid at different humic acid condition (humic acid equilibrium, aged 0 and aged 60 days) were investigated by using through- and out- diffusion method. Both D e and acc values were obtained by fitting experimental data using a computer code FDP. 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