Acceleration and centralization of a back-diffusion process: Effects of EDTA-2Na on cadmium migration in high- and low-permeability systems

Facilitated transport of toluene and naphthalene with humic acid in high- and low-permeability systems: Role of ionic strength and cationic type

The occurrence of colloids on pollutants transport in groundwater has attracted more attention. However, the research on the regulation mechanism of colloids on combined pollutants transport in heterogeneous aquifers is limited. In this study, a series of tank experiments were conducted to systematically investigate the effects of ionic strength, and cation type on humic acid (HA) facilitated transport of toluene (TOL), and naphthalene (NAP) in high- and low-permeability systems. The results showed that HA facilitated pollutants transport in low Na+ solution. In Ca2+ solution, the presence of HA hindered pollutants transport, and the inhibition increased with the increase of ionic strength. Both in Na+ solution and low Ca2+ solution, the influence of heterogeneous structure on pollutant transport played a dominant role, and TOL and NAP had a greater transport potential in the high permeability zone (HPZ) due to the preferential flow. Whereas, deposition of HA aggregates, and electrostatic attractive interaction had negative effects on transport than groundwater flow in high Ca2+ solution. Pollutants were prone to accumulate at the bottom of the HPZ, and the top of the low permeability zone (LPZ). These new findings provide insights into the mechanism of colloids influence on the pollutants transport in heterogenous aquifer.

Control mechanism of trichloroethylene back diffusion by microstructure in a low permeability zone

The trailing effect caused by the back diffusion (BD) of contaminants in low-permeability zones (LPZs), which prolongs remediation time and increases remediation costs, has caused widespread concern. In this study, the BD of trichloroethylene (TCE) from the LPZ to the high-permeability zone (HPZ) was determined using flow cell experiments. The anomalous variance in the BD flux of the TCE-spanning 2-4 times the deviation under identical experimental conditions, attracted our attention. To determine the cause of this aberrant behavior, a micro computed tomography (micro-CT) characterization of the flow cell was conducted, which revealed significant microstructural disparities in the LPZ. The study found that the pore connectivity of LPZs determines the efficiency of BD and that LPZs with different porosities have different sensitivities to connectivity. The pore shape complexity indicates the possibility of BD retardation, and remediation is more difficult for these types of LPZs. Changing the structure of LPZs to improve their remediation efficiency may be a new research topic. Notably, correcting the model parameters through microstructural characterization significantly refined the prediction accuracy.

Contaminant back-diffusion from layered aquitards subjected to barrier-controlled source zones

Low-permeability aquitards may serve as secondary sources of slow-releasing contaminants into the adjacent aquifer system, creating considerable obstacles to groundwater cleanup. Accurately capturing the exchange of contaminant mass between aquitards and aquifers can facilitate site management and remediation. Previous simulation studies were mainly limited to one-dimensional (1D) back diffusion from aquitards during the remediation of the source zone. In this study, a novel two-dimensional (2D) back-diffusion model is developed to investigate the storage and release of contaminants in aquitards after source isolation. This model coupled the dynamical decay of isolated sources and the diffusion-sorption process of contaminants in the layered aquitards. Exact analytical solutions for the present 2D multilayer model were derived using the finite cosine transform, Duhamel Theorem, separation of variables, and transfer matrix method. Results indicated that the previous 1D model would overestimate the contaminant concentration in the aquitard and the back-diffusion risk when the source zone was isolated. The proposed 2D back-diffusion model enables quantitative prediction of how source zone width, source concentration, and aquitard heterogeneity impact plume trailing time, thus aiding in understanding the mechanisms of aquifer contamination beyond barrier-controlled source zones.

Choline-based deep eutectic solvent combined with EDTA-2Na as novel soil washing agent for lead removal in contaminated soil

Lead-contaminated soil was cleaned through ethylene-diamine-teraacetic acid disodium salt (EDTA-2Na) combined with diluted deep eutectic solvent (DES) which was prepared by mixing choline chloride with ethylene glycol. The influences of leaching temperature, leaching time, liquid-solid (L/S) ratio, concentration of EDTA-2Na, water-DES ratio, and the molar ratio of choline chloride-ethylene glycol (Ch-E) on the leaching rate of lead were investigated. The mineral phases of the soil and DES before and after washing were analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR). The changes to the DESs before and after dissolving lead nitrate (Pb(NO₃)₂) were analyzed by high resolution mass spectrometry (HRMS) and nuclear magnetic resonance (NMR). Hydrogen bonds and EDTA-2Na in the Ch-M system resulted in the conversion of Pb(NO₃)₂ to other complex ions such as [Pb·Ch-E]^- and [Pb·EDTA-2Na]^- and other complex ions due to the dissolution of the washing agent. The results showed that the soil mineral phase did not change significantly and up to 95.79% of Pb could be washed under temperature, time, L/S ratio, EDTA-2Na concentration, DES/water ratio, Ch-E molar ratio, and stirring speed conditions of 40 °C, 2 h, 6, 0.02 M, 2, 0.75 and 300 rpm, respectively. The hydrogen bonds and EDTA-2Na may play a key role in the remediation of lead-contaminated soil by a washing agent. This research describes a rapid, efficient, and environmentally friendly method for remediation of lead-contaminated soil.

Application of iron-biochar composite in topsoil for simultaneous remediation of chromium-contaminated soil and groundwater: Immobilization mechanism and long-term stability

This study was aimed to prove the effectiveness and practicability of an integrated technology for simultaneous remediation of Cr-contaminated soil and groundwater. The remediation system was built by pumping Cr-contaminated groundwater into top contaminated soil round after round, enabling the pre-applied iron-biochar composite (Fe-BC) in topsoil to stabilize Cr from both groundwater and soil. Immobilization ability and mechanism of Cr in soil were explored by toxicity characteristic leaching procedure test, scanning electron microscopy-elemental mapping, and X-ray photo spectroscopy. Hydrus-1D software was used to examine the long-term stability of immobilized Cr in soil. Results showed that Fe-BC-amended soil could remove about 71% Cr from contaminated groundwater. Meanwhile, Cr from both groundwater and soil was simultaneously immobilized in topsoil, leachability of Cr in which was reduced by over 81%. The immobilization of Cr in soil was attributed to the reduction of Cr(VI) into Cr(III) to form stable Cr_xFe_(1-x)(OH)₃. After remediation, the average transport rate of Cr in the soil profile was only 0.420 cm y^-1 along with the local rainfall. Our study demonstrated that integrated technology could effectively remove Cr from groundwater and stabilize Cr in soil and the simultaneous remediation target for both soil and groundwater reached.

Contaminant occurrence and migration between high- and low-permeability zones in groundwater systems: A review

In recent decades, water quality problems that impact human health, especially groundwater pollution, have been intensely studied, and this has contributed to new ideas and policies around the world such as Low Impact Development (LID) and Superfund legislation. The fundamental to many of these problems is pollutant occurrence and migration in saturated porous media, especially in groundwater. Such environments often contain contrasting zones of high and low permeability with significant differences in hydraulic conductivity (~10^-4 and 10^-8 m/s, respectively). High-permeability zones (HPZs) represent the primary pathways for pollutant transport in groundwater, while low-permeability zones (LPZs) are often diffusion dominated and serve as both sinks and sources (i.e., via back-diffusion) of pollutants over many decades. In this review, concepts and mechanisms of solute source depletion, contaminant accumulation, and back-diffusion in high- and low-permeability systems are presented, and new insights gained from both experimental and numerical studies are analyzed and summarized. We find that effluent monitoring and novel image analysis techniques have been adroitly used to investigate temporal and spatial evolutions of contaminant concentration; simultaneously, mathematical models are constantly upscaled to verify, optimize and extend the experimental data. However, the spatial concentration data during back-diffusion lacks diversity due to the limitations of pollutant species in studies, the microscopic mechanisms controlling pollutant transformation are poorly understood, and the impacts of these reactions on contaminant back-diffusion are rarely considered. Hence, most simulation models have not been adequately validated and are not capable of accurately predicting pollutant fate and cleanup in realistic heterogeneous aquifers. Based on these, some hypotheses and perspectives are mentioned to promote the investigation of contaminant migration in high- and low-permeability systems in groundwater.

Effects of EDTA on adsorption of Cd(II) and Pb(II) by soil minerals in low-permeability layers: batch experiments and microscopic characterization

Ethylenediaminetetraacetic acid (EDTA) can serve as a washing agent in the remediation of low-permeability layers contaminated by heavy metals (HMs). Therefore, batch adsorption experiments, where pure quartz (SM1) and mineral mixtures (SM2) were used as typical soil minerals (SMs) in low-permeability layers, were implemented to explore the effects of different EDTA concentrations, pH, and exogenous chemicals on the HM-SM-EDTA adsorption system. As the EDTA concentration increased, it gradually cut down the maximum Cd adsorption capacities of SM1 and SM2 from approximately 135 to 55 mg/kg and 2660 to 1453 mg/kg; and the maximum Pb adsorption capacities of SM1 and SM2 were reduced from 660 to 306 mg/kg and 19,677 to 19,262 mg/kg, respectively. When the initial mole ratio (MR = moles of HM ions/sum of moles of HM ions and EDTA) was closer to 0.5, the effect of EDTA was more effective. Additionally, EDTA worked well at pH below 7.0 and 4.0 for Cd and Pb, respectively. Low-molecular-weight organic acids (LMWOAs) affected the system mainly by bridging, complexation, adsorption site competition, and reductive dissolution. Cu²⁺, Fe²⁺ ions could significantly increase the Cd and Pb adsorption onto SM2. Notably, there were characteristic changes in mineral particles, including attachment of EDTA and microparticles, agglomeration, connection, and smoother surfaces, making the specific surface area (SSA) decrease from 16.73 to 12.59 m²/g. All findings indicated that EDTA could effectively and economically reduce the HM adsorption capacity of SMs at the reasonable MR value, contact time, and pH; EDTA reduced the HM adsorption capacity of SMs not only by complexation with HM ions but also by decreasing SSA and blocking active sites. Hence, the acquired insight from the presented study can help to promote the remediation of contaminated low-permeability layers in groundwater.