Hydraulic and Volume Change Behaviour
The temporal variations of hydraulic infiltrations for kaolin and XGK under 20 kPa mechanical load with distilled water as pore-fluid was presented in Fig. 2(a). The hydraulic infiltration rate of the kaolin sample was found to be higher at the beginning of the test, which reduces from 4×10− 6 to 1.5×10− 8 m/s within 10 minutes from the start of the experiment. The hydraulic infiltration rate did not change significantly further with time and could not attain the limiting value of 1×10− 9 m/s. The initial sudden drop in the hydraulic infiltration rate is due to the breaking of the card-house structure upon inundation. The hydraulic infiltration of kaolin amended with 10% XG (XGK) was also plotted in the same figure. The hydraulic infiltration rate was found to be 2.55×10− 6 m/s at the start of the experiment, which reduced with time. The hydraulic infiltration rate of the XGK decreased gradually and achieved the limiting hydraulic infiltration rate in 510 minutes from the start of the experiment as shown in Fig. 2(a). The XGK sample attained an equilibrium hydraulic infiltration value of 3.5 × 10− 11 m/s after 8000 minutes from the start of the experiment. A reduction of three orders of magnitude in hydraulic infiltration rate was observed in the values before and after its amendment with XG.
The dry XG is generally found in the coiled structure form, the presence of the negative charges on the XG chain attracts the water and forms a layer of water around it upon hydration as shown in Fig. 3(a). The FESEM micrographs of kaolin and XGK samples permeated with different pore-fluids were presented in Fig. 4. The FESEMs of kaolin and XGK were performed after permeation with distilled water as Fig. 4(a) and 4(b), respectively. The saturated samples were collected for FESEM from the middle of the compacted clay samples. The kaolin sample shows various water paths in the sample, which contributes to the higher hydraulic infiltration of pore-fluid. The XGK sample shows the coating of the xanthan gum layer over the kaolin particles and the clogging of pores of kaolin is also visible in Fig. 4(b). The reason behind the reduction in hydraulic infiltration of XGK is the clogging of pores with xanthan gum gel as confirmed from Fig. 4(b).
The volume change behavior of kaolin and XGK in terms of normalized thickness with distilled water as pore fluid was plotted in Fig. 2(b) under mechanical stress. The normalized thickness (h/h0) is the ratio of the present thickness of sample with the initial thickness. A collapse of the kaolin sample was observed due to the application of hydro-mechanical loading conditions. The normalized thickness of the kaolin sample did not increase after the initial collapse, which represents the insignificant development in diffused double layers (DDLs) of the kaolin sample. For XGK, a slight collapse of approximately 1% was observed initially due to the application of hydro-mechanical load. The sample achieved its original volume within 2 minutes of the start of the experiment and it started further swelling with time due to the complete formation of xanthan gum gel. An equilibrium normalized thickness, which represents the overall swelling of 1.41 was achieved with XGK. The swelling of XGK is taking place due to higher osmotic pressure between XG gel than the applied stress. The swelling of the sample upon inundation with water is attributed to the formation of xanthan gum gel, which can absorb water molecules with electrostatic interaction between a water molecule and carboxyl groups present in XG. The mechanical force applied is countered by the developed repulsive osmotic forces between the XG-coated kaolin as shown in Fig. 3(b).
The temporal variations of hydraulic infiltrations for kaolin and XGK under 20 kPa mechanical load with 0.5M NaCl as pore-fluid was presented in Fig. 2(c). The hydraulic infiltration rate of the kaolin sample was found to be higher at the beginning of the experiment up to 200 minutes, which reduces from 3×10− 6 to 3×10− 8 m/s within 600 minutes from the start of the experiment. The equilibrium value of hydraulic infiltration rate was found between 10− 8 to 10− 9 m/s. The hydraulic infiltration rate value did not change significantly further with time and could not attain the limiting value of 1×10− 9 m/s. The sudden drop in the hydraulic infiltration rate is due to the breaking of the card-house structure upon inundation. The hydraulic infiltration of the XGK sample was also plotted in the same figure. The hydraulic infiltration rate was found to be 2.4×10− 6 m/s initially, which reduced with time. The hydraulic infiltration rate of the XGK decreased gradually and achieved the limiting hydraulic infiltration rate 370 minutes from the start of the experiment as depicted in Fig. 2(c). The XGK sample attained an equilibrium hydraulic infiltration value of 5×10− 10 – 3.3×10− 11 m/s order after 6000 minutes from the start of the experiment. A reduction of two orders of magnitude in hydraulic infiltration was observed in the values before and after its amendment with XG. The FESEMs of kaolin and XGK samples were performed after permeation with 0.5M NaCl as Fig. 4(c) and 4(d), respectively. The kaolin sample shows various water paths in the sample, which contributes to the higher hydraulic infiltration of pore-fluid. The XGK sample shows the coating of the xanthan gum layer over the kaolin particles and the clogging of pores of kaolin is also visible in Fig. 4(d). The reason behind the reduction in hydraulic infiltration of XGK is the clogging of pores with xanthan gum gel as depicted in Fig. 4(d). The volume change behavior of kaolin and XGK in terms of normalized thickness with 0.5M NaCl as pore fluid was plotted in Fig. 2(d) for 20 kPa mechanical stress. A collapse of 12% for the kaolin sample was observed due to the application of hydro-mechanical loading conditions. The normalized thickness of the kaolin sample did not increase after the collapse, which represents the insignificant development in DDLs of the kaolin sample. For XGK, a slight collapse of approximately 1% was observed initially due to the application of hydro-mechanical load. The sample achieved its original volume within 100 minutes from the start of the experiment and it started further swelling with time due to the formation of xanthan gum gel. An equilibrium normalized thickness of 1.06 was achieved with XGK, which represents the overall swelling of the sample. The swelling of XGK is taking place due to higher osmotic repulsive pressure between XG gel than the applied stress as shown in Fig. 3(b). The swelling of the sample upon inundation with 0.5M NaCl is attributed to the formation of XG gel. The XG gel formed with water molecules interacts with sodium ions due to electrostatic attraction from the carboxyl group in XG.
The temporal variations of hydraulic infiltrations for kaolin and XGK mechanical load with 0.5M KCl as pore-fluid were presented in Fig. 2(e). The hydraulic infiltration rate of the kaolin sample was found to be higher initially up to 4 minutes, which reduced from 2×10− 6 to 4.7×10− 9 m/s within 10 minutes from the start of the experiment. The hydraulic infiltration rate did not change significantly further with time and could not attain the limiting value of 1×10− 9 m/s. The initial sudden drop in the hydraulic infiltration rate is due to the breaking of the card-house structure upon inundation. The hydraulic infiltration rate of XGK was also plotted in the same figure. The hydraulic infiltration rate was found to be 2.53×10− 6 m/s initially, which reduced with time. The hydraulic infiltration rate of the XGK decreased gradually and achieved the limiting value of hydraulic infiltration rate in 380 minutes from the start of the experiment as shown in Fig. 2(e). The XGK sample attained an equilibrium hydraulic infiltration value of 10− 10 − 10− 11 m/s order after 3000 minutes from the start of the experiment. A reduction of two-three orders of magnitude in hydraulic infiltration rate was observed for kaolin before and after its amendment with XG. The reason behind the reduction in hydraulic infiltration of XGK is the clogging of pores with xanthan gum gel. The rate of the reduction of hydraulic infiltration is quick in kaolin as compared to the XGK, because of breaking of card house structure does not take much time as compared to the formation of XG gel. The volume change behavior of kaolin and XGK in terms of normalized thickness with 0.5M KCl as pore-fluid was plotted in Fig. 2(f) under 20 kPa mechanical stress. A collapse of the kaolin sample was observed due to the application of hydro-mechanical loading conditions. The normalized thickness of the kaolin sample did not increase after the initial collapse which represents the insignificant development in DDLs of the kaolin sample. For XGK, a slight collapse of approximately 1% was observed initially due to the application of hydro-mechanical load. The sample achieved its original volume within 1 minute of the start of the experiment and it started further swelling with time due to the complete formation of XG gel. An equilibrium normalized thickness of 1.27 was achieved with XGK, which represents the overall swelling of the sample. The swelling of XGK is taking place due to higher swelling pressure than the applied stress. The swelling of the sample upon inundation with 0.5M KCl solution is attributed to the formation of XG gel. The XG gel interacts with potassium ions due to electrostatic interaction between carboxyl groups of XG and cations. The elemental analysis of samples was performed using energy-dispersive X-ray (EDX) spectroscopy and presented in Fig. 5. EDX analysis Adsorption potential of XGK layer for potassium ion was evaluated using EDX. Spectra of pure kaolin and pure XG were presented as Fig. 5 (a) and (b). Both kaolin and XG sample spectra shows the absence of potassium. Whereas, the potassium is present as one of the major elements in XGK sample permeated with KCl as shown in Fig. 5(c). Therefore, the EDX spectra results confirms the adsorption potential of XGK sample for the potassium cation upon permeation and its ability to arrest the potassium ions.
The temporal variations of hydraulic infiltration rates for kaolin and XGK under mechanical load with 0.5M CaCl2 as pore-fluid were presented in Fig. 2 (g). The hydraulic infiltration rate of the kaolin sample was found to be higher at the beginning of the test, which reduces by orders of magnitude within 20 minutes from the start of the experiment. The hydraulic infiltration rate did not change significantly further with time and could not attain the limiting value of 1×10− 9 m/s. The initial drop in hydraulic infiltration rate is due to the breaking of the card-house structure upon inundation. The hydraulic infiltration rate of XGK was also plotted in Fig. 2(g). The hydraulic infiltration rate was in the order of 1.5×10− 6 m/s initially, which reduced with time. The hydraulic infiltration rate of the XGK decreased gradually and achieved the limiting value of hydraulic infiltration rate in 220 minutes from the start of the experiment as shown in Fig. 2(g). The XGK sample attained an equilibrium hydraulic infiltration rate of 3×10− 11 m/s after 2000 minutes from the start of the experiment. A reduction of two-three orders of magnitude in equilibrium hydraulic infiltration rates of kaolin was observed before and after its amendment with XG. The FESEMs of kaolin and XGK were performed after permeation with 0.5M CaCl2 as pore-fluid in Fig. 4(e) and 4(f), respectively. The kaolin sample shows various water paths in the sample, which contributes to the higher hydraulic infiltration of pore-fluid. The XGK sample shows the coating of the xanthan gum layer over the kaolin particles and the clogging of pores of kaolin is also visible in Fig. 4(f). The reason behind the reduction in hydraulic infiltration of XGK is the clogging of pores with XG gel. The rate of the reduction of hydraulic infiltration is quick in alone kaolin as compared to the XGK, because of breaking of card house structure does not take much time as compared to the formation of XG gel. This is also a reason for higher hydraulic infiltration of the XGK sample around after 20 minutes start of the experiment. The volume change behavior of kaolin and XGK in terms of normalized thickness with 0.5M CaCl2 as pore-fluid was plotted in Fig. 2 (h) under the mechanical load. A collapse of 12–13% for the kaolin sample was observed due to the application of hydro-mechanical loading conditions. The normalized thickness of the kaolin sample does not increase after initial collapse which represents the insignificant development in DDLs with a CaCl2 salt environment. For XGK, a slight collapse of approximately 1% was observed initially due to the application of hydro-mechanical load. The sample achieved its original volume within 30 minutes of the start of the experiment and it started further swelling with time due to the complete formation of XG gel. An equilibrium normalized thickness of 1.08 was achieved with XGK, which represents the overall swelling of the sample. The swelling of XGK is taking place due to higher osmotic repulsion pressure between the XG gel. The swelling of the sample upon inundation with 0.5 M CaCl2 is attributed to the formation of XG gel. The XG gel interacts with calcium ions due to electrostatic interaction between carboxyl groups in XG and cations.
The kaolin clay compacted in a dry state comprises of open card house type structure due to the edge–edge interactions. The hydration of kaolin with water alters the charges on its edges from positive to negative [37], which further leads to the breaking of the card-house type structure as illustrated in Fig. 3(c) and (d). The observation of the significant reduction in the volume of the kaolin along with hydraulic infiltration is attributed to the same reason. But the alone kaolin at lower density does not have significant development of diffused double layers to achieve the limiting value of hydraulic conductivity (< 10− 9 m/s). The saturated hydraulic conductivity of the bare kaolin is also governed by the hydrated radius of the cation in pore fluid. The saturated hydraulic infiltration was found to be lowest for the Na+ ions solution due to the highest hydrated radius of the sodium as compared to potassium. The XGK before and after hydration is shown in Fig. 3(e) and 3(f) respectively. The macro-voids present in kaolin samples are reduced due to breaking of card-house structure as well as clogging of pores with xanthan gum gel. The coating of the XG over kaolin restricts the movement of pore fluid across it. Due to the presence of many carboxyl groups, the XG gel has the potential for arresting the cations. Adsorption of cations on the XG surface and clog formations are the main mechanisms for lower hydraulic infiltration of different pore fluids across the tailing barriers.
The sealing time for XGK with distilled water was found to be higher (510 min.) than other pore fluids and it was found to be lowest (270 min.) for calcium ions (Ca2+). The sealing time for monovalent cations (i.e., Na+, and K+) was found to be almost the same, i.e. (360–380 min.). Calcium ions act as cross-linker between polymeric chains of XG, which reduced the sealing time, presence of cross-linked chains of XG is also visible from the FESEM of the XGK sample permeated with calcium salt in Fig. 4(g). Monovalent cations show slightly higher sealing time than divalent cations due to a reduction in cross-linking. The highest sealing time of water is attributed to the absence of crosslinking of XG.
Diffusion rates of salts through Kaolin and XG amended Kaolin
Through-diffusion experiments were conducted for three different salts having a 0.5 M initial concentration in the source reservoir. The saturated kaolin and XGK samples were placed in different diffusion cells and reservoirs were attached to either end of the diffusion cell. The test was started by placing 0.5 M salt solution on the source side and distilled water on the collector side. The samples were collected and analyzed for the presence of inorganic ions at different time intervals up to 60 days. The temporal variations of relative concentrations of measured source and collector reservoirs for the NaCl diffusion experiment with kaolin were presented in Fig. 6(a). The temporal variations in relative concentrations of ions in both the reservoirs for XGK with 0.5M NaCl solution as an initial concentration in the source reservoir were presented in Fig. 6(b). The concentrations in the source reservoir decreased with time and increase in the collector reservoir for both kaolin and XGK. A relative concentration of 0.073 and 0.089 were observed after 60 days in the collector reservoir for kaolin and XGK, respectively, with NaCl solution. The diffusion coefficient and retardation factors were evaluated by minimizing the error between measured and theoretical concentration profiles obtained by solving equations (2–6), and optimization [44, 45]. The theoretical profiles for source and collector reservoirs were also presented in Fig. 6(a) and Fig. 6(b) for kaolin and XGK, respectively. The diffusion coefficient and retardation factor of kaolin with NaCl solution were found to be 1.41×10− 10 m2/sec and 18.69, respectively. These values were reduced to 1.09×10− 10 m2/sec and 14.05, respectively, for NaCl solution upon amendment of kaolin with XG. The theoretical profiles for kaolin and XGK with NaCl salt solution showed a very good theoretical estimation with a root mean square error (RMSE) value of 0.0199 and 0.0132, respectively.
The temporal variations of relative concentrations of measured source and collector reservoirs for the KCl diffusion experiment with kaolin and XGK were presented in Fig. 6(c) and 6(d), respectively. The relative concentrations in the source reservoir decreased with time and increased in the collector reservoir for both kaolin and XGK. A relative concentration of 0.048 and 0.14 was observed after 60 days in the collector reservoir for kaolin and XGK respectively with KCl salt solution. The diffusion coefficient and retardation factors were evaluated by minimizing the error between measured and theoretical concentration profiles obtained by solving the Eq. (2–6), using optimization. The theoretical profiles for source and collector reservoirs were also presented in Fig. 6(c) and Fig. 6(d) for kaolin and XGK, respectively. The diffusion coefficient and retardation factor of kaolin with KCl solution were found to be 1.61×10− 10 m2/sec and 9.36, respectively. These values were changed to 7.44×10− 11 m2/sec and 12.82, respectively, for KCl solution upon amendment of kaolin with XG. The theoretical profiles for kaolin and XGK with KCl salt solution showed a very good theoretical estimation with an RMSE value of 0.0195 and 0.0151, respectively.
The temporal variations of relative concentrations of measured source and collector reservoirs for the CaCl2 diffusion experiment with kaolin and XGK were presented in Fig. 6(e) and 6(f), respectively. The concentrations of CaCl2 in the source reservoir decreased with time and increased in the collector reservoir for both kaolin and XGK. A relative concentration of 0.09 and 0.18 were observed after 60 days in the collector reservoir for kaolin and XGK, respectively, with CaCl2 salt solution. The diffusion coefficient and retardation factors were evaluated by minimizing the error between measured and theoretical concentration profiles obtained by solving equations (2–6) and using optimization [44, 45]. The theoretical profiles for source and collector reservoirs were also presented in Fig. 6(e) and Fig. 6(f) for kaolin and XGK, respectively. The theoretical profiles for kaolin and XGK with CaCl2 salt solution showed a very good fitting with RMSE values of 0.0239 and 0.0163, respectively. The diffusion coefficient and retardation factor of kaolin with CaCl2 solution were found to be 1.92×10− 10 m2/sec and 11.62, respectively. These values were reduced to 8.36×10− 11 m2/sec and 7.69, respectively, for CaCl2 solution upon amendment of kaolin with XG. The values of the diffusion coefficient and retardation factor for three different salts through kaolin and XGK were also tabulated in Table 1.
Table 1
The diffusion coefficient and retardation factor for kaolin and XG amended kaolin with different salts
Material | Pore Fluid | Effective Diffusion Coefficient, De | Retardation Factor, Rd | Root mean square error RMSE |
Kaolin | 0.5M NaCl | 1.41×10− 10 m2/sec | 18.69 | 0.02 |
0.5M KCl | 1.61×10− 10 m2/sec | 9.36 | 0.0195 |
0.5M CaCl2 | 1.92×10− 10 m2/sec | 11.62 | 0.024 |
XG amended Kaolin | 0.5M NaCl | 1.09×10− 10 m2/sec | 14.05 | 0.013 |
0.5M KCl | 7.44×10− 11 m2/sec | 12.82 | 0.015 |
0.5M CaCl2 | 8.36×10− 11 m2/sec | 7.69 | 0.016 |
Numerical Simulation of Contaminant through Barriers for Evaluating Design Period
The attenuation ability of the cations is based on the evaluated model parameters (Ks, De, and Rd) on the engineered barrier facilities. The contaminant migration through barriers is mainly governed by the advection-diffusion equation. After evaluating independent advection (saturated hydraulic conductivity) and diffusion (effective diffusion coefficient and retardation factor) characteristics, the transport of contaminants was simulated in 500 mm-thick kaolin clay layer as well as the XGK layer. The normalized concentration distribution for the studied contaminants was given at different periods in Fig. 7. The migration of contaminants for kaolin alone was mainly governed by the advection, whereas diffusion is the dominant phenomenon in XGK. A kaolin layer of 500 mm thickness attenuates the migration of sodium cation for up to a period of 10 years. Moreover, the XGK layer of the same thickness can attenuate sodium ions for more than 50 years as shown in Fig. 7 (a) and (b). The migration of potassium through the kaolin layer attenuates the contaminant for a period less than 5 years, further, the XGK improved the migration rates by decreasing the saturated hydraulic conductivity lower than 10− 9 m/s and effective diffusion coefficient to a smaller value. Thus, the XGK attenuates potassium for more than 50 years as shown in Fig. 7 (c) and (d). Similarly, the 500 mm thick layer of kaolin attenuates the migration of calcium for up to 5–10 years, which improves significantly after kaolin is amended with XG, and the 500 mm thick XGK layer attenuate the calcium ion migration for more than 50 years as shown in Fig. 7 (e) and (f).
The hydraulic head of leachate also influences the migration of the contaminants through barrier material. The concentration profile of 0.5M KCl for 500 mm thick kaolin layer for different hydraulic heads were plotted in Fig. 8(a). The plotted profiles showed that hydraulic head significantly influenced the contaminant profiles. The lowest hydraulic head (i.e., 1 meter) showed a migration 200 mm in 1 year, while 10 meters’ hydraulic head showed the failure of barrier after 1 year. The concentration profiles of 0.5M CaCl2 for 500 mm thick kaolin layer for three different hydraulic heads were plotted in Fig. 8(b). The calcium migration also showed similar results as of potassium, the barrier system showed very less migration of calcium at lower head. The barrier system started failing after 1 year and showed rapid migration of calcium at a hydraulic head of 10 meters. The contaminant profiles of three different contaminants for 500 mm thick XGK layer with different hydraulic head for a period of 50 years were plotted in Fig. 8(c). The migration profiles after 50 years didn’t change significantly with hydraulic head. Moreover, hydraulic head of 10 meters for all contaminant showed higher migration as compared to lower heads. The calcium ion showed higher migration when compared with other two cations. For all three contaminants, a 500 mm layer of XGK is sufficient to act as a barrier for more than 50 years. Based on the concentration profiles with different cations for the XGK layer evaluated design time was found to be 50 years.