CFD-DEM study on transport and retention behaviors of nZVI-clay colloids in porous media.

Transportation process of nano scale zero valent iron (nZVI) in clay-rich soils is complicated and crucial for in-situ remediation of contaminated sites. A coupled computational fluid dynamic and discrete element method (CFD-DEM) was used to investigate the interplays of repulsive and attractive forces and the injection velocity of this process. The screened Coulomb's law was used to represent the electrostatic interaction, and surface energy density was introduced to represent the effects of the van der Waals interaction. A phase diagram was constructed to describe the interplay between injection velocity and repulsive force (in terms of charge of colloids). Under the boundary and initial conditions in this study, clogging formed at low repulsive force (colloidal charge = -1 ×10-15 C), where increment of injection velocity (from 0.002 m/s to 0.02 m/s) cannot prevent clogging, as in the case of bare nZVI transportation with limited mobility; On the other hand, excessive repulsive force (charge = -4 ×10-14 C) is detrimental to nZVI-clay transportation due to repulsion from the concentrated colloids in pore throats, a phenomenon as in the overuse of stabilizers and was defined as the "membrane repulsion effect" in this study. At moderate charge (-1 ×10-14 C), injection velocity increment induced clogging due to aggregates formed at the windward of cylinder and accumulated at the pore throats.

Monitoring the remediation of groundwater polluted by MSW landfill leachates by activated carbon and zeolite with spectral induced polarization technique.

The municipal solid waste (MSW) landfill in Hangzhou, China utilized zeolite and activated carbon (AC) as permeable reactive barrier (PRB) fill materials to remediate groundwater contaminated with MSW leachates containing ammonium, chemical oxygen demand (COD), and heavy metals. The spectral induced polarization (SIP) technique was chosen for monitoring the PRB because of its sensitivity to pore fluid chemistry and mineral-fluid interface composition. During the experiment, authentic groundwater collected from the landfill site was used to permeate two columns filled with zeolite and AC, and the SIP responses were measured at the inlet and outlet over a frequency range of 0.01-1000 Hz. The results showed that zeolite had a higher adsorption capacity for COD (7.08 mg/g) and ammonium (9.15 mg/g) compared to AC (COD: 2.75 mg/g, ammonium: 1.68 mg/g). Cation exchange was found to be the mechanism of ammonium adsorption for both zeolite and AC, while FTIR results indicated that π-complexation, π-π interaction, and electrostatic attraction were the main mechanisms of COD adsorption. The Cole-Cole model was used to fit the SIP responses and determine the relaxation time (τ) and normalized chargeability (mn). The calculated characteristic diameters of zeolite and AC based on the Schwarz equation and relaxation time (τ) matched the pore sizes observed from SEM and MIP, providing valuable information on contaminant distribution. The mn of zeolite was positively linear with adsorbed ammonium (R2 = 0.9074) and COD (R2 = 0.8877), while the mn of AC was negatively linear with adsorbed ammonium (R2 = 0.8192) and COD (R2 = 0.7916), suggesting that mn could serve as a surrogate for contaminant saturation. The laboratory-based real-time non-invasive SIP results showed good performance in monitoring saturation and provide a strong foundation for future field PRB monitoring.

A passive sink-zeolite permeable reactive barrier to control NH4+-N pollution plume within groundwater: Conceptual design and numerical modeling.

Ammonium nitrogen (NH4+-N) is a typical inorganic pollutant in the groundwater at landfill sites, and high-concentration NH4+-N is toxic to humans and organisms. Zeolite can effectively remove NH4+-N in water by adsorption, and it is suitable to be used as a type of reactive materials for permeable reactive barriers (PRBs). A passive sink-zeolite PRB (PS-zPRB) with higher capture efficiency than a continuous permeable reactive barrier (C-PRB) was proposed. And a passive sink configuration was integrated with PRB in the PS-zPRB, this configuration enabled the high hydraulic gradient of groundwater at the treated sites to be fully utilized. In order to explore treatment efficiency for groundwater NH4+-N using the PS-zPRB, numerical modeling on decontamination of NH4+-N plumes at a landfill site was performed. The results indicated that the NH4+-N concentrations of PRB effluent gradually decreased from 21.0 mg/L to 0.5 mg/L within 5 y, and met the drinking water standards after treatment for 900 d. The decontamination efficiency index of PS-zPRB was consistently higher than 95% within 5 y, and the service life of PS-zPRB appeared over 5 y. The capture width of PS-zPRB effectively exceeded the PRB length by around 47%. Compared with C-PRB, the capture efficiency of PS-zPRB was increased by around 28%, and the reactive material of PS-zPRB was saved by approximately 23% in volume.

Investigation of Cu Adsorption and Migration with Spectral Induced Polarization in Activated Carbon.

In this paper, the adsorption process of copper ions on activated carbon (AC) was simulated in a column test. It was deduced that it is consistent with the pseudo-second-order model. Cation exchange was observed to be the major mechanism of Cu-AC interactions through scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) measurements. Adsorption isotherms were fitted well using the Freundlich model. Adsorption thermodynamics at 298, 308, 318 K demonstrated that the adsorption process is spontaneous and endothermic. Spectral induced polarization (SIP) technique was used to monitor the adsorption process, and the double Cole-Cole model was used to analyze the SIP results. The normalized chargeability was proportional to the adsorbed copper content. Two measured relaxation times from the SIP testing were converted into the average pore sizes of 2, 0.8, 0.6, 100-110, 80-90, and 53-60 µm by the Schwartz equation, which are consistent with the measured pore sizes from both mercury intrusion porosimetry and scanning electron microscopy (SEM). The reduction in the pore sizes by SIP during the flow-through tests suggested that the adsorbed Cu2+ gradually migrated into small pores as with continued permeation of the influent. These results showcased the feasibility of using SIP technique in engineering practice involving the monitoring of copper contamination in land near a mine waste dump or in adjacent permeable reactive barriers.

Investigation of aqueous Fe(III) and Mn(II) removal using dolomite as a permeable reactive barrier material.

Iron (Fe) and manganese (Mn) are the most frequently detected heavy metals in the soil and groundwater near municipal landfill sites. Natural calcium-carbonate-based materials, such as dolomite, effectively remove metal ions and are suitable as reactive materials for permeable reactive barriers (PRBs). However, multiple heavy metals usually coexist in contaminated groundwater, the effectiveness and competitive precipitation mechanisms in the removal of Fe(III) and Mn(II) are unclear. In this study, we investigated the efficiency and influencing factors of the removal of single and coexisting Fe(III) and Mn(II) by dolomite through experimental batch and column tests, property characterization, and PHREEQC simulations. Dolomite with 1.18-2.36 mm particle size showed the best removal efficiency for Fe(III) and Mn(II) through precipitation. Fe(III) was preferentially precipitated by dolomite with higher removal efficiency, attributed to the lower solubility product (Ksp) of iron precipitates. Compared with Fe(III), Mn(II) was precipitated conditionally, and the removal efficiency was restricted by the concentration of Fe(III) in the system. Considering the application of PRB in the field, dolomite would be effective for the remediation of coexisting heavy metals with lower precipitate Ksp. The half-time of Mn(II) removal could serve as a reference for PRB thickness designs if the target metal contaminants were in a similar concentration range as Fe(III) and Mn(II). Additionally, the PRB performance could be affected by the reduction of hydraulic permeability induced by precipitation, and the fine precipitates migrating from PRB might affect downstream groundwater quality.

Evaluation of aqueous Cd2+ and Pb2+ removal by natural loess using spectral induced polarization and microscopic characterization.

Mining and landfill activities can cause serious soil and groundwater contamination with lead (Pb) and cadmium (Cd). Loess soils are common and have been reported as effective for the removal of heavy metals. The spectral induced polarization (SIP) technique has been approved for its nondestructive ability to characterize the contaminant transport process and surface geochemical properties in porous media. In the present study, SIP was applied to monitor Pb2+ and Cd2+ removal processes using loess through column flow-through experiments. The outflow aqueous geochemical analyses indicated a better retention capability of loess for Pb2+, which was through precipitation induced by calcite dissolution and aqueous pH increment, as confirmed by SEM-EDS and XRD results. Cd retention took place mainly through ion exchange with Ca2+ and Mg2+ on the loess surface. The SIP signals showed a continuous decrement on the magnitude of imaginary conductivity during both Pb2+ and Cd2+ flow-through, which was attributed to the total surface area and decrement of polarizable surface charges. The SIP signals differentiated the interactions between loess and Pb2+/Cd2+ by displaying a peak shift to a higher frequency on the imaginary conductivity spectra during Pb2+ flow-through, which was attributed to calcite dissolution and proved by the high correlation (R2 = 0.9366) between the estimated dissolved calcite mass and the peak of imaginary conductivity. The above results suggest that loess has a great potential for field heavy metal remediation applications, and the SIP technique displays a promising capability of monitoring the remediation performance.

Setting Times of Early-Age Mortars Determined from Evolution Curves of Poisson's Ratio.

Setting times, as the early-age properties of cement-based materials, are important properties to ensure the quality and long-term performance of engineering structures. To determine the initial and final setting times of cementitious materials, the compressive wave velocity and shear wave velocity of six early-age mortar samples were monitored. Their time evolution curves of Young's modulus, shear modulus, bulk modulus, and Poisson's ratio were then calculated and analyzed. The signature times of the derivatives of the Poisson's ratio evolution curves correlate well with the initial and final setting times, and the remarkably high coefficient of determination values relative to the data from this study are higher than those presented in the current literature. The proposed derivative method on the Poisson's ratio evolution curve is as good as the derivative methods from vs. evolution curves used by prior studies for the estimation of both the initial and final setting times of the early-age properties of cement-based materials. The formation and subsequent disappearance of ettringite of low Poisson's ratio were postulated to cause the initial dip in the Poisson's ratio evolution curves.

The Migration and Deposition Behaviors of Montmorillonite and Kaolinite Particles in a Two-Dimensional Micromodel.

The pick-up, migration, deposition, and clogging behaviors of fine particles are ubiquitous in many engineering applications, including contaminant remediation. Deposition and clogging are detrimental to the efficiency of environmental remediation, and their mechanisms are yet to be elucidated. Two-dimensional microfluidic models were developed to simulate the pore structure of porous media with unified particle sizes in this study. Kaolin and bentonite suspensions were introduced to microfluidic chips to observe their particle deposition and clogging behaviors. Interactions between interparticle forces and particle velocity profiles were investigated via computational fluid dynamics and discrete element method simulations. The results showed that (1) only the velocity vector toward the micropillars and drag forces in the reverse direction were prone to deposition; (2) due to the negligible weight of particles, the Stokes number implied that inertia was not the controlling factor causing deposition; and (3) the salinity of the carrying fluid increased the bentonite deposition because of the shrinkage of the diffused electrical double layer and an increase in aggregation force, whereas it had little effect on kaolin deposition.

Soil heavy metal pollution of industrial legacies in China and health risk assessment.

Rapid urbanization in China has brought about large-scale factory relocation. Severe environmental ecological and human health risks are caused by a large number of contaminated legacies left in the city. To comprehensively review the pollution and assess the health risk of industrial legacies in China, a total of 625 polluted industrial legacies were compiled by document retrieval. Legacies are mainly located in the southwest of China, the North China Plain, Yangtze River Basin, Yangtze River Delta, and Pearl River Delta with a mean operation time of 35 years, and legacies of chemical manufacturing take the biggest proportion of all sites. Health risk assessments considering the uncertainty of exposure and toxic factors reveal that the soil heavy metal pollution in China is serious, with Pb, Cd, Zn, Ni, and As as dominant pollutants. Legacies of chemical manufacturing, ferrous metal processing, non-ferrous metal processing, and mines should be priority controlled for their large number and serious risks. Children are the most vulnerable people with more serious non-carcinogenic and carcinogenic risks, while males are slightly surpassed by females. Insights for better risk management of legacies are provided based on the comprehensive assessment of pollution and human health risk in this study.

Evaluating iron remediation with limestone using spectral induced polarization and microscopic techniques.

Groundwater contamination with iron caused by mining and landfill activities has fueled the development of remediation strategies. Permeable reactive barriers (PRBs) are commonly applied in subsurface remediation because of their high removal effect and low costs. Spectral induced polarization (SIP) technique has been approved for its nondestructive ability to monitor the geochemical processes in porous media. In this study, SIP technique was applied for monitoring iron remediation by limestone at column scale. The chemical analysis showed the pH of the porous fluid increased - attributed to the dissolution of limestone, which promoted the precipitation of iron. The precipitate phases included both γ-FeOOH and Fe2O3 based on X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) results. The micro computed tomography (CT) technique investigated the uneven distribution of the precipitates in the column, which indicated the existence of preferential flow. SIP signals revealed the quantity of the accumulated iron precipitates, which was proved by the chemical measurement and calculation. SIP signals also derived the time evolution of both the average precipitate size and size distribution, which elucidated the processes of precipitate crystal growth and aggregation during Fe flow-through. Above results suggest that SIP holds the promise of monitoring the engineering barrier performance.

Content and morphology of lead remediated by activated carbon and biochar: A spectral induced polarization study.

Soil and groundwater contamination with lead (Pb) poses serious challenges for the environment. Activated carbon (AC) and biochar have huge potential application in the in-situ remediation processes through permeable reactive barriers (PRB). Spectral induced polarization (SIP) technique recently showed promises in nondestructively monitoring the spatio-temporal characteristics of physical, chemical and biological processes in porous media. In this study SIP technique was used for monitoring Pb remediation by AC and biochar in column scale. The calculated characteristic grain/pore size evolutions from SIP signals on AC, agreed well with the size of precipitates measured by SEM and mercury intrusion porosimetry (MIP) methods. The content increment process of the retained Pb on AC was also recorded via the magnitude increment of the imaginary conductivity. The mechanisms of Pb-AC and Pb-biochar interactions were investigated using SEM-EDS, TEM, FTIR, XRD, and XPS measurements. It showed that AC immobilizes through physical adsorption and precipitation, whereas complexation with functional groups is the remediation mechanism for biochar. Furthermore, the observed SIP responses of both AC and biochar are two orders of magnitude higher than those of typical natural soils or silica materials. This distinct difference is an additional advantage for the field application of SIP technique in PRB scenarios.

Emerging sustainable technologies for remediation of soils and groundwater in a municipal solid waste landfill site -- A review.

Remediation of soils and groundwater in a municipal solid wastes (MSW) landfill site emerges as a global challenge to the living environment on earth with significant market potential. Unlike contaminants in an industry or agricultural site, contaminants from MSW landfills are diverse, primarily consisting of chemical oxygen demand (COD), inorganic matter (ammonia-nitrogen, nitrate-nitrogen, total phosphorus) and heavy metals. This renders new challenges to remediation contaminants of different characters altogether. A status quo of existing technologies, including permeable reactive barriers, electrokinetic remediation, microbial remediation, and injection of either solubilizing agents or micro or nanobubbles were thoroughly reviewed, with an emphasis on removal efficiency based on existing projects at lab, pilot or field scales. A design chart tailored for the remediation of a landfill contaminated site was developed, verified by a few case studies, which supplement the chart. Future trends of technical innovation (such as multi-layer permeable reactive barriers (PRBs)) and challenges (such as flow pattern) were identified.

Complex dielectric permittivity of metal and polymer modified montmorillonite.

Owing to its high surface area and high surface charge density, clay, either contaminated with heavy metal ions or modified with organic additives as barrier materials, is difficult to assess and monitor. Complex dielectric permittivity (κ*) showed promising potential in tackling the above issues. In this study, the complex dielectric permittivity (κ*) of clays modified with a surfactant, four polymers, and four metal ions was measured at frequencies from 0.2-20 GHz. With the addition of polymer and metal ions, increasing frequency caused a slight decrement in real permittivity but a significant decrement in effective permittivity. A modified linearity polynomial equation, which considered the particle conductivity, was developed to fit the relationship between effective conductivity (σeff) and porosity ranging from 0.7 to 1.0. A three-dimensional Cole-Cole plot (κ'-κeff″-w) shows Cole-Cole circle expansion at higher water content. The resonance strength of modified clays was observed to increase with water content, which suggests that the number of water molecules in the diffuse layer of polymer or metal ions saturated clay increased. However, sorbed polymer and metal ions have an insignificant influence on the resonance time τs and stretching exponent 1-α. κ* can provide nondestructive characterization of metal or polymer modified clays.