Permeable Reactive Barrier Research Articles

Enhanced Biological Nitrate Removal from Groundwater in Humid Tropical Regions Using Corn Cob-Based Permeable Reactive Barriers: A Case Study from Panama

Nitrate contamination in groundwater is a global concern due to its widespread presence and consequential social, environmental, and economic ramifications. This study investigates the efficacy of biological denitrification in a humid tropical setting, utilizing corn cob in batch and column tests to assess nitrate removal under varying conditions. Batch tests demonstrated the nitrate removal efficiencies of 93.14%, 91.58%, 90.77%, and 98.74% for initial concentrations of 22.18 ± 2.82 mg/L, 27.3 mg/L, 69.1 ± 1.2 mg/L and 115.08 ± 1.88 mg/L, respectively. In the column test, the removal efficiency was 99.86%, 87.13%, and 74%, and the denitrification rate was 32.82, 53.43, and 83.53 mg NO3−-N/L d, for a hydraulic retention time (HRT) of 24 h, 16 h, and 7 h, respectively. Predominantly, nitrate removal occurred via biological denitrification, particularly favoring a 24 h HRT. The corn cob effectively removed high nitrate concentrations of up to 115 mg NO3−-N/L. Scanning electron microscopy and Fourier transform infrared spectroscopy revealed surface characteristic changes of the carbon source pre- and post-denitrification. This research sheds light on the potential of biological denitrification using corn cob in humid tropical environments, offering a promising avenue for addressing nitrate contamination challenges in groundwater systems.

A Two-dimensional Analytical Model for Volatile Organic Compound Diffusion Through the Unsaturated Soil and Horizontal Permeable Reactive Barriers

A Two-dimensional Analytical Model for Volatile Organic Compound Diffusion Through the Unsaturated Soil and Horizontal Permeable Reactive Barriers

Permeable Reactive Barrier Remediation Technique Using Carbonized Food Waste in Ground Contaminated with Combined Cu and Pb

In recent years, with the escalation of food waste generation, stringent legal constraints on landfill usage and incineration have necessitated the exploration of alternative disposal methods, augmenting interest in diverse recycling strategies. Notably, carbonized food waste (CFW), a byproduct of food waste carbonization, has emerged as an efficacious adsorbent for pollutant removal. This study focuses on the application of in situ remediation techniques, specifically electrokinetic (EK) remediation combined with enhancers, to decontaminate soil afflicted with single or multiple heavy metals. The utilization of a permeable reactive barrier (PRB) infused with CFW aims to mitigate secondary environmental repercussions, including the propagation of contaminants in soil and groundwater. Experiments were conducted on clay samples contaminated with copper, lead, or a combination thereof. Observations revealed that the current density peaked during the initial 1–2 days of the experiment, experienced a resurgence post-electrode exchange, and subsequently diminished. The efficacy of metal removal was predominantly pronounced for copper, with remediation rates ranging from 85% to 92% in singly contaminated samples and 75% to 89% in dually contaminated samples after a 10-day treatment period, incorporating an electrode exchange on the eighth day. Conversely, the efficacy of lead removal was markedly lower, with rates of 0.6% to 33% in singly contaminated samples and 14% to 25% in combined contamination scenarios, suggesting the necessity for extended treatment durations. The post-experimental moisture content indicated successful enhancer injection. Additionally, pH measurements affirmed that heavy metals migrated effectively within the sample matrix, facilitated by the EK phenomenon after the electrode exchange. This study highlights the potential of CFW within PRBs for the remediation of heavy metal-contaminated soils, although the removal efficiencies between different metals is variable, emphasizing the need for tailored approaches in the treatment of lead-contaminated environments.

Reduction of nitrogen loss in runoff from sloping farmland by a ridged biochar permeable reactive barrier with vegetated filter strips

IntroductionEutrophication due to nitrogen (N) loss from sloping farmland has a high risk in the Three Gorges Reservoir. Biochar and vegetated filter strips (VFS) are used to control nutrient runoff and increase soil water-holding capacity, soil nutrient retention, and crop yield. However, surface biochar application has limited ability to control N loss, especially from sloping farmland.MethodsIn this study, different widths of ridged biochar permeable reactive barrier (RB-PRB) with VFS were employed to intercept N loss in runoff from sloping farmland. Adsorption characteristics of biochar for nitrate and ammonium N were evaluated using isothermal and kinetic adsorption models before field experiments. N index values for ammonium (NH4+), nitrate (NO3−), dissolved N (DTN), particulate N (PN), and total N (TN) lost through runoff were monitored from April 2019 to January 2020.ResultsNO3− and NH4+ sorption on biochar was predominantly physical adsorption with a maximum capacity of 4.51 and 4.12 mg g-1, respectively. During the research period, the dominant transportation pathway of N loss involved dissolved total N movement through subsurface flow, which accounted for 65.55% of the total loss. TN loss for CK was 1954 g·hm-2, while RB-PRB and VFS decreased N loss from sloping farmland by 36.7%. The interception efficiency of RB-PRB was highest at 0.3 m width. VFS successfully intercepted particulate N and reduced it by 32.75%. In terms of soil nutrients, the RB-PRB and VFS interventions led to a substantial 41.69% increase in the TN content of the soil at a 0.4 m width.DiscussionThe findings suggest that biochar has a favorable adsorption effect on NH4+ and NO3−, an appropriate width of RB-PRB with VFS could effectively reduce nitrogen loss from sloping farmland. Simultaneously, it enhances the water and fertilizer retention capacity of sloping cropland soil; however, the long-term implications necessitate further validation.

PRB technology incorporating acidic ground conditions and bio-geochemical clogging – A critical review

Groundwater acidity resulting from pyrite oxidation in acid sulphate soil terrain presents a severe threat to the environment. The exposure of low-lying acidic coastal belts to the atmospheric oxygen, exacerbated by phreatic surface lowering in dry seasons and activities like infrastructure development and agriculture, leads to pyrite oxidation and sulfuric acid production in soil. This paper reviews the challenges posed by acid sulphate soils by emphasising the environmental and infrastructure damage caused by acidic soil leaching into water bodies. Permeable reactive barriers (PRBs) have emerged as a promising method of passive treatment for mitigating groundwater acidity in pyritic terrain. This review mainly focuses on the effectiveness of PRBs in low-lying floodplains by addressing the bio-geochemical clogging that diminishes the reactivity and porosity of PRBs over time. This paper also summarises the numerical methods needed to design PRBs in acidic terrains by identifying gaps in current research that could enhance the accuracy of future PRB designs. This comprehensive review contains valuable insights into the ongoing efforts of addressing the challenges associated with groundwater contamination in regions containing acid sulphate soil.

A Comparative Analysis of Brick Powder and Limestone as Reactive Media in Permeable Reactive Barriers for Remediation of Fluoride Contamination: Insights from Column Study

A Comparative Analysis of Brick Powder and Limestone as Reactive Media in Permeable Reactive Barriers for Remediation of Fluoride Contamination: Insights from Column Study

Quantitative assessment of the impact factors upon remediation performance of the PRB technique in conjunction with pumping/recharge

Installation of pumping/recharge wells adjacent to contaminant plume and permeable reactive barrier (PRB) can effectively improve PRB's performance by artificially altering groundwater flow field attracting more contaminants to pass through PRB within a shorten time. However, the impact factors affecting the performance of PRB in conjunction with pumping/recharge (abbreviated as PRB-PR technique) have not been quantitatively assessed. In this study, numerical approach was adopted and multiple scenarios were implemented based on the developed MODFLOW and MT3MDS models to quantitatively evaluate the influences of impact factors on PRB-PR's remediation performance over a hypothetical contaminated site. Simulation results indicated that: (1) PRB should be installed nearby the downstream boundary of contaminant plume; (2) PRB's contaminant removal efficiency (η), pumping/recharge rate (Q), and aquifer hydraulic conductivity (K) affect PRB-PR's performance significantly, and a linear relationship exists between contaminant removal rate (R) and the product of K and Q no matter η changes (a series of fitting equations representing the linear relationship between R and KQ under various η were yielded based on data fitting); and (3) PRB's hydraulic conductivity, pumping/recharge wells' locations, and pumping/recharge wells' quantity do not affect PRB-PR's performance seriously. The equations indicating PRB-PR's remediation performance and key impact factors can be utilized to calculate the minimum values of Q and η that can meet the requirements of remediation objectives of real contaminated sites, contributing to supervising the installation and operation of the PRB-PR technique accordingly.

In Situ Treatment of Arsenic-Contaminated Groundwater via Extraction Well–Integrated Permeable Reactive Barriers

In Situ Treatment of Arsenic-Contaminated Groundwater via Extraction Well–Integrated Permeable Reactive Barriers

A field study of a novel permeable-reactive-biobarrier to remediate chlorinated hydrocarbons contaminated groundwater

Chlorinated hydrocarbons (CHs) pose significant health risks due to their suspected carcinogenicity, necessitating urgent remediation efforts. While the combination of zero-valent iron (Fe0) and microbial action shows promise in mitigating CH contamination, field studies on this approach are scarce. We devised a novel three-layer permeable reactive barrier (PRB) material incorporating Fe0 and coconut shell biochar, effectively implemented at a typical CH-contaminated site. Field monitoring data revealed conducive conditions for reductive dechlorination of CHs, characterized by low oxygen levels and a relatively neutral pH in the groundwater. The engineered PRB material consistently released organic carbon and iron, fostering the proliferation of CH-dechlorinating bacteria. Over a 250-day operational period, the pilot-scale PRB demonstrated remarkable efficacy in CH removal, achieving removal efficiencies ranging from 21.9% to 99.6% for various CH compounds. Initially, CHs were predominantly eliminated through adsorption and iron-mediated reductive dechlorination. However, microbial reductive dechlorination emerged as the predominant mechanism for sustained and long-term CHs removal. These findings underscore the economic viability and effectiveness of our approach in treating CH-contaminated groundwater, offering promising prospects for broader application in environmental remediation efforts.

The Performance of Iron Nanocomposites as Persulfate Activators in Permeable Reactive Barrier Technology for Treatment of Landfill Leachate

Landfill leachate contains a large organic load and a wide spectrum of pollutants, and it needs appropriate treatment to reduce the severity of these contaminants. Therefore, the present study aimed to remove chemical oxygen demand (COD) and ammonia (NH3 ) as target contaminants from the landfill leachate. To this end, the zero-valent iron nanofibers/reduced ultra-large graphene oxide (ZVINFs/rULGO) and converter sludge (CS) were used as persulfate (PS) activators in permeable reactive barrier (PRB) technology for removing COD and NH3 . The Box–Behnken design (BBD) was employed to determine the possibility of using each activator and optimize three parameters: activator, pore volume (PV), and flow (mL/min) using target contaminants. In the Batch process, the effects of pH, activator concentration, and PS/COD mass ratio on COD removal were examined. The BBD experiments provided a satisfactory predictive model, indicating that ZVINFs/rULGO is superior to CS when used as activators of PS in PRB technology. The biochemical oxygen demand (BOD5 )/COD ratio increased from 0.24 in raw leachate to 0.67 in treated leachate, and leachate toxicity was reduced by more than 85%. Additionally, the ZVINFs/rULGO and CS contributed to the activation of PS.

Enhancing Pb2+ removal efficiency in flow-through systems using flue gas desulfurization gypsum-based porous materials: Experimental and numerical studies

This study addresses the poor permeability of flue gas desulfurization (FGD) gypsum in water remediation and further explores its potential for resource utilization. FGD gypsum-based porous materials (FGPMs) were prepared using different foaming behavior as permeable reactive barrier (PRB) media materials. The dynamic removal performance of Pb2+ was investigated, and a numerical model was developed to validate the results. The findings reveal that the adsorption mechanisms of FGPMs primarily involve ion exchange and surface precipitation. Pb2+ does not alter the structure or functional groups of the materials. By optimizing the foaming behavior to enhance pore connectivity, the adsorption capacity increased from 8.78 mg/g to 13.42 mg/g, thereby extending the service life of the PRB. The presence of closed and capillary pores hinders the dynamic removal performance of Pb2+. Furthermore, the FGPMs exhibit superior removal performance at low flow rates, low concentrations, and high column heights, as confirmed by the numerical model. These results offer valuable insights for designing and optimizing FGD gypsum-based adsorption processes to remove Pb2+ from wastewater efficiently.

Waste-derived permeable reactive barrier to treat heavy metals and organic matter in groundwater affected by landfill leachate

Waste-derived permeable reactive barrier to treat heavy metals and organic matter in groundwater affected by landfill leachate

Nano Zero-Valent Iron (nZVI) Encapsulated with ABS (nZVI/(ABS + EC)) for Sustainable Denitrification Performance and Anti-Aggregation

Aggregation and sharp reactivity decrease are the key problems of using nano zero-valent iron (nZVI) as a potential reaction medium for a permeable reactive barrier (PRB). In this study, nZVI particles encapsulated within an acrylonitrile–butadiene–styrene (ABS) matrix (nZVI/(ABS + EC)) was fabricated, which for the first time successfully simultaneously solved the above problems via accurately regulating the distribution of nZVI particles in the ABS matrix and regulating the contact between nZVI particles and the contaminated aqueous environment. In addition, the size and number of the pores throughout the ABS matrix were first regulated by ethyl cellulose (EC) for the purpose of controlling the contact between nZVI particles and the nitrate contaminant, affording apparent rate constants (kobs) for denitrification performance in the range of 0.0423 to 0.0820 min−1. The remediation of simulated nitrate-contaminated solution by nZVI/(ABS + EC) was suitably described by the first-order kinetics model, with kobs ranging from 0.0423 to 0.2036 min−1, and functional relationship models of kobs with the dosages of EC (dEC) and nZVI (dFe) during encapsulation were developed for the quantitative regulation of a sustainable denitrification performance. Results revealed that encapsulation prevents the aggregation of nZVI, rendering a sustainable denitrification performance of the material; the denitrification performance was demonstrated to be affected and quantitatively regulated by the encapsulation and application conditions. Using nZVI/(ABS + EC) as the reaction medium for PRB, the pore blocking of PRB can be avoided, and the sustainable remediation performance can be quantitatively regulated and predicted.

Activation of persulfate by g-C3N4/nZVI@SBC for degradation of total petroleum hydrocarbon in groundwater

In this study, we synthesized a high removal efficiency catalyst using biochar-supported nanoscale zero-valent iron and g-C3N4, denoted as g-C3N4/nZVI@SBC, to activate persulfate (PS) for the degradation of total petroleum hydrocarbon (TPH) in groundwater. We characterized the morphology and physiochemical properties of g-C3N4/nZVI@SBC with scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), BET surface area analysis, and X-ray photoelectron spectroscopy (XPS). To assess the performance of the g-C3N4/nZVI@SBC catalyst, we investigated various reaction parameters, such as the mass ratio of g-C3N4 to nZVI@SBC, PS concentration, initial pH, initial TPH concentration, and the presence of coexisting ions in the system. The results from batch experiments and repeated use trials indicate that g-C3N4/nZVI@SBC exhibited both excellent catalytic activation capability and impressive durability, making it a promising choice for TPH degradation. Specifically, when the PS concentration reached 1 mM, the catalyst dosage was 0.3 g/L, and the g-C3N4 to nZVI@SBC mass ratio was 2, we achieved a remarkable TPH removal efficiency of 93.8%. Through electron paramagnetic resonance (EPR) testing and quenching experiments, we identified sulfate radicals, hydroxyl radicals, and superoxide radicals as the primary active substance involved in the TPH degradation process. Moreover, the g-C3N4/nZVI@SBC composite proved highly effective for in-situ TPH removal from groundwater and displayed an 86% removal rate, making it a valuable candidate for applications in permeable reactive barriers (PRB) aimed at enhancing environmental remediation. In summary, by skillfully utilizing g-C3N4/nZVI@SBC, this study has made notable advancements in synthesis and characterization, presenting a feasible and innovative approach to addressing TPH pollution in groundwater.

Assessment of rapid initiators and long-lasting nutrients for developing biological permeable reactive barriers to treat mine-contaminated groundwater

ABSTRACT The formation of mine-contaminated groundwater as a result of acidic mine drainage from the oxidation of sulfur-containing minerals entering the groundwater. Biological permeable reactive barrier (Bio-PRB) technology is excellent for the remediation of mine-contaminated groundwater. Usually, the organic substrates utilized in Bio-PRB are a combination of rapid initiators, which are readily bioavailable, and long-lasting nutrients, which are more difficult to degrade. Herein, we investigated the effectiveness of three rapid initiators and three long-lasting nutrients to remove sulfate from simulated mine-contaminated groundwater via simulated column experiments. The rapid initiators comprised crude glycerol, sodium acetate, and industrial syrup (IS), and the long-lasting nutrients included biodiesel emulsified oil, soybean oil emulsified oil, and high-carbon alcohol emulsified oil (HO). Microorganisms were stimulated using IS to create a sulfate reduction system owing to its high total organic carbon content (24.30 g L−1), achieving optimal sulfate removal rate (1.69 mmol dm−3 d−1). The fastest (2.93 mmol dm−3 d−1) and highest (88%) sulfate removal rates were achieved using HO, which is probably associated with the ability of HO to provide the most suitable C/N ratio (111.75) and induce the growth of sulfate-reducing bacteria (SRB) for substrate degradation. Conversely, a high concentration of sulfate reduction products inhibited SRB growth in the HO column. The addition of organic materials promoted SRB growth and various organic substrate–degrading bacteria. Furthermore, the competitive growth of methanogens (86.6%) may be responsible for the decrease in the relative abundance of SRB during the later stages of the experiment in the HO column.

Mechanical properties of reaction mediums in permeable reactive barriers

The mechanical properties of reaction media in permeable reactive barriers (PRB) is vital in geoenvironmental applications. Bentonite, activated carbon and zeolite, recognized for their excellent adsorption capabilities, are employed as the main reaction media in PRB for the treatment of contaminated underground water. The compaction test and the undrained and unconsolidated triaxial test were carried out to investigate the compression and shear strength of the activated carbon-zeolite mixture and activated carbon-bentonite mixture at various composition ratios. The impact of compaction degree on samples' shear strength was analyzed. The influence of different composition ratios on the mechanical properties and the permeability of each reaction medium were also evaluated. The results show that the mechanical performance of most activated carbon-zeolite (AZs) is not satisfactory compared to natural soil and activated carbon-bentonite mixtures. Activated carbon‑sodium bentonite (ANBs) and activated carbon‑calcium bentonite (ACBs) present similar compaction characteristics and shear properties. In ANBs and ACBs, the cohesion of mixes with a mass ratio of 1:2 (ANB2 and ACB2) was found lower than that of mixes with mass ratios of 1:1 (ANB1 and ACB1) and 1:3 (ANB3 and ACB3). And in most ANB and ACB mixes, 100 % compaction produced higher moisture content and higher friction angle, but lower cohesion, compared to 92 % compaction degree. And the shear strength behavior of ANBs is dominated by both bentonite and activated carbon. The permeability of ACB1, AZ3 and ACB1-sand are at 1.31 × 10−6 m/s, 1.37 × 10−6 m/s and 7.72 × 10−7 m/s, respectively. These results provide valuable insights into the selection and optimization of reaction media for PRBs in geoenvironmental engineering applications.

Preparation of iron composite filler for PRB technology and its application in the removal of toxic metals(loids) from groundwater

Iron-based materials have garnered significant research interest due to their high surface reactivity, exhibiting potential for remediating metal(loid)s in groundwater. However, agglomeration and blockage limit its long-term and stability performance in application in permeable reactive barrier (PRB) technology. In this study, three innovative and low-cost iron composite fillers were prepared using a mass-producible impregnation method and applied as fillers for PRB. Static sorption and dynamic column experiments were conducted to assess the long-term performance of these composites to remediate multiple toxic metals(loids) contaminated groundwater. The results showed that the iron composite fillers had the excellent adsorption capacities for Co(II), Cu(II), Cr(VI) and As(III) from simulated contaminated groundwater even under the interference of Fe(III) and Mn(II). The pH of the influent favored the adsorption of toxic metals(loids) by the iron composite fillers in the order of 3.00＞5.00＞8.00＞7.00＞9.00＞6.00. The overall adsorption process was more consistent with the quasi-secondary kinetic equation. The corresponding constant values of Cr, Co, Cu, and As on CZ were 0.022, 0.093, 0.756, and 0.342 h−1, respectively. And that of CP-I and CP-II were 0.004, 0.022, 0.001, 0.126, 0.008, 0.041, 0.001, and 0.154 h−1, respectively. The specific surface area of the composite zeolite (Na92Al92Si100O384) and composite pumice was 48.185 m2/g and 0.63 m2/g, respectively. The dynamic column experiment was run steadily for 40 days, during which the average removal rates of toxic metals(loids) were above 95% and approximate 20 times its own volume of contaminated water was purified. FeO(OH) on composite zeolite and Fe3O4 on composite pumice played a dominant role in the removal of toxic metals(loids). The toxic metals(loids) adsorbed in the PRB were mainly in a stable phase of residual fraction and Fe-Mn oxide fraction, making them less likely to cause secondary pollution problems.

A framework for estimating soil water characteristic curve and hydraulic conductivity function of permeable reactive media

The unsaturated behavior of permeable reactive barriers (PRB) is a critical component in predicting the removal efficiency through the adsorption of contaminants. This study investigates the framework to estimate the soil water characteristic curve (SWCC) and hydraulic conductivity function (HCF) for iron oxide-coated sand (IOCS) and zeolite, which are common materials used in PRBs. A multistep outflow (MSO) experiment was performed and the results of the MSO experiment were used to optimize associated parameters in Kosugi's SWCC and HCF. In addition, three scenarios of optimization analysis were investigated to evaluate the best-fitting model for estimating SWCC and HCF. The low root mean square error (RMSE) of fitted parameters indicates the Kosugi model well described the observed suction profiles in MSO experiments. In addition, the lowest RMSE and coefficient of variation suggested the inclusion of the additional parameter β provided the best estimation of the three materials (clean sand, IOCS, and zeolite). The physically reasonable estimation of SWCC and HCF of the three materials from the optimized parameters suggests the proposed framework is a reasonable model for the unsaturated behavior of PRBs.

Groundwater Bioremediation through Reductive Dechlorination in a Permeable Bioelectrochemical Reactor

A new membrane-less bioelectrochemical reactor configuration was developed for contaminated groundwater remediation. The new bioelectrochemical reactor configuration was inspired by the utilisation of a permeable reactive barrier (PBR) configuration with no separation membrane. The corresponding reactive zones were created by using graphite granules and mixed metal oxide (MMO) electrodes to stimulate the reductive and oxidative biological degradation of chlorinated aliphatic hydrocarbons. In the present study, the PBR-like bioelectrochemical reactor has been preliminarily operated with synthetic contaminated groundwater, testing the reductive dechlorination activity on cis-dichloroethylene (cisDCE). Moreover, to assess the effects of competing anions presence for the electron donor (i.e., the cathode), the synthetic wastewater contained sulphate and nitrate anions. In the PBR-like reactor operation, nearly all cisDCE was removed in the initial sampling port, with only VC detected as the observable RD product. During the same biotic test of the PRB reactor, the presence of both the reductive dechlorination and anions reduction was confirmed by the complete nitrate reduction in the cathodic chamber of the PRB reactor. On the contrary, sulphate reduction showed a lower activity; indeed, only 25% of the influent sulphate was removed by the PRB reactor.

Laboratory Experiments on Remediation of Landfill Leachate Contamination with Permeable Reactive Barriers (PRBs) of Reactive Media Derived from Waste

Groundwater at landfills is often contaminated with heavy metals. Permeable reactive barrier (PRB), a potential in-situ treatment system, was investigated in this study, using 'composite reactive media' (COM), primarily composed of waste materials, to remediate heavy metals in groundwater. The COM consisted of 40% Brick and Mortar Waste (BMW), 40% Biochar (BC), 10% Granular Activated Carbon (GAC), and 10% Zeolite, based on volume. In an up-flow column, the average removal efficiencies of Fe, Pb, Cu, As, Mn, Zn, and Cd were 70.9±2.7, 80.9±3.2, 72.2±1 .6, 76.3±2.5, 81.6±6.3, 88.0±2.0, and 67.3±4.6%, respectively. In a bench-scale COM-filled PRB (0.85 x 0.25 x 0.3 m), the average removal efficiencies of the same species were 71.4±1.3, 75.4±4.9, 67.4±4.3, 72.9±1.3, 83.6±7.6, 89.8±7.4, and 80.9±7.1%, respectively. Adsorption associated with fixation, and ion exchange could be the main treatment mechanisms. COM also has potential as a filtration medium in other treatment unit processes and to treat some other types of wastewater than landfill-leachate. Further, this use of COM could alleviate the burden of disposing of BMW waste, thereby addressing a significant issue in waste management. As the identification of potential failure points in reactive media is crucial, field-scale experiments are suggested to be conducted for further investigation.