Subsurface Pollution Research Articles

Geoelectrical and seismoelectric mapping of subsurface pollution in a closed landfill near the Tongo Bassa and Ngongue river, Douala Cameroon

Located in Douala V (Cameroon), Makepé Missoké is a lowland area that is influenced by the Tongo Bassa and Ngongué rivers. The site has a closed dumpsite, that operated from the 1975’s to 2003 for the disposal and storage of domestic waste produced in Douala. Geoelectric profiling (ERT) coupled with seismo-electric imaging was used to characterize the shallow aquifer of the Douala subbasin and map subsurface pollution. Fourteen geoelectrical profiles were deployed between June and August 2018 and March 2019. Along these lines, 49 sets of seismo-electric point data were collected in December 2021. A total of 118 wells drilled (2018) were used to obtain water level and topographic data. The aquifer geometry, hydraulic characteristics, permeability, lithology and leachate plume extent were determined. This survey enables us to visualize waste infiltration and migration within Makepé. The drilled wells indicate that the main lithologies observed are clay, sandy clay, sand, clayey sand and gravel. The leachate plume observed after processing the electric profiles had a resistivity signature of ≤ 10 Ωm, and high electrical conductivity are observed in some wells. The leachate migrates within the subsurface along a northwest‒southwest trend, where ground water pollution is observed due to leachate infiltration. This infiltration resulted in poor water quality indices in some collected samples. Such pollution is common in unconfined aquifers (< 50 m) due to the absence of a confining layer at the landfill. An increase in resistivity values with depth toward the northeast direction indicates progressive vertical dilution during leachate mineralization. This study integrated geoelectric and seismo-electric tomography with basic water chemistry analysis to effectively characterize the groundwater within the phreatic Quaternary/Mio-Pliocene aquifers of the Douala basin.

Estimation of Subsurface Pollutant Load and Its Seasonal Variation in an Irrigation Channel

Concentrations of heavy metals in irrigation water and soil were examined to determine the level of contamination in a particular irrigation channel in Gazipur, Bangladesh. In this conduit, effluent from households, industries, and municipalities was combined with irrigation water. The study was carried out in order to estimate pollutant loads along a wastewater irrigation channel and to conceptualize seasonal changes in composite wastewaters. A total of six wastewater and nine contaminated soil samples were collected directly from the study area and analyzed for this project. From February to March, before the monsoon, the pH of the water is too low for this channel to be used for irrigation. At the end of the monsoon season, from April through May, all wastewater tests were within these parameters, making them suitable for irrigation. Considering the standard limits, Pb, Cr, and Cu concentrations in all wastewater samples were found unsuitable for irrigation and thus problematic for soils and crops. The concentration of Cd in the wastewater samples ranged from 27.82 to 88.36 mg/L, whereas the tolerance limit for Cd in irrigation water is 0.01 mg/L. In soil samples from the three places chosen, the levels of Pb were below the safe limit and the levels of Cr were above the safe limit. The heavy metal pollution index of all the sites crossed the safe limit. On the basis of the geo-accumulation index and the pollution index, Pb indicates no pollution and Cd indicates very high levels of pollution in all locations. The maximum enrichment factor was found for Cd, Pb, and Cu, which have minor enrichment, Ni, which has no enrichment, and Cr, which has moderate enrichment. The study also tried to link heavy metals in wastewater to their deposition in soil and seasonal and temporal variations. The study revealed that industrial effluent utilized for irrigation was closely related to the accumulation of heavy metals in soils. Since irrigated agricultural soils are likely to be exposed to harmful wastes, it is crucial that the effluents from the industrial region of Gazipur are appropriately controlled.

Drone-towed controlled-source electromagnetic (CSEM) system for near-surface geophysical prospecting: on instrument noise, temperature drift, transmission frequency, and survey set-up

Abstract. Drone-borne controlled-source electromagnetic (CSEM) systems combine the mobility of airborne systems with the high subsurface resolution in ground systems. As such, drone-borne systems are beneficial at sites with poor accessibility and in areas where high resolution is needed, e.g. for archaeological or subsurface pollution investigations. However, drone-borne CSEM systems are associated with challenges, which are not observed to the same degree in airborne or ground surveys. In this paper, we explore some of these challenges based on an example of a new drone-towed CSEM system. The system deploys a multi-frequency broadband electromagnetic sensor (GEM-2 uncrewed aerial vehicle, UAV), which is towed 6 m below a drone in a towing-bird configuration together with a NovAtel GNSS–IMU (global navigation satellite system–inertial measurement unit) unit, enabling centimetre-level position precision and orientation. The results of a number of controlled tests of the system are presented together with data from an initial survey at Falster (Denmark), including temperature drift, altitude vs. signal, survey mode signal dependency, and the effect of frequency choice on noise. The test results reveal the most critical issues for our system and issues that are likely encountered in similar drone-towed CSEM set-ups. We find that small altitude variations (± 0.5 m) along our flight paths drastically change the signal, and a local height vs. signal correlation is needed to correct near-surface drone-towed CSEM data. The highest measured impact was −46.2 ppm cm−1 for a transmission frequency of 91 kHz. We also observe a significant increase in the standard deviation of the noise level up to 500 % when going from one transmission frequency to five. We recommend not to use more than three transmission frequencies, and the lowest transmission frequencies should be as high as the application allows it. Finally, we find a strong temperature dependency (up to 32.2 ppm∘C-1), which is not accounted for in the instrumentation.

Reductive dehalogenation in groundwater by Si-Fe(II) co-precipitates enhanced by internal electric field and low vacancy concentrations

Fe(II) and silicate can form Si-Fe(II) co-precipitates in anoxic groundwater and sediments, but their phase composition and reactivity towards subsurface pollutants are largely unknown. Three types of Si-Fe(II) co-precipitations with the same chemical composition, namely Si-Fe(II)-I, Si-Fe(II)-II, and Si-Fe(II)-III, have been synthesized by different hydroxylation sequences in this work. It was found that Si-Fe(II)-III reduce carbon tetrachloride (CT) much faster (k1=0.04419 min−1) than Si-Fe(II)-I (0 min−1) and Si-Fe(II)-II (7.860 × 10−4 min−1). XRD results show that the main component of Si-Fe(II)-III is ferrous silicate (FeSiO3), which is quite different from that of Si-Fe(II)-I and Si-Fe(II)-II. The unique arrangement of hydroxyl coordination, the less distorted octahedral structure, the polyhedral morphology and the absence of Si-A center vacancies in Si-Fe(II)-III are responsible for its high reductive dehalogenation reactivity. The highest redox activity of Si-Fe(II)-III was shown by electrochemical characterization. The [FeII-O-Si]+ in Si-Fe(II)-III may stabilize the dichlorocarbene anion (˸CCl2−), which favors the transformation of CT to methane (9.2%). The Si-Fe(II) co-precipitates consist of countless internal electric fields, and the transformation of hydroxyl and CT both consumed electrons. The coexistence of hydroxyl and CT increases the electron density in the electron-rich region due to their electronegativity, enhancing their electron-accepting capabilities. This study deepens our understanding of the phase composition and electronic structure of Si-Fe(II) co-precipitates, which fills the gap in the reductive dehalogenation of halides by Si-Fe(II) co-precipitates.

Enhanced remediation of surface-bound hexavalent chromium in soils using the acidic and alkaline fronts of electrokinetic technology

In the subsurface environment, highly toxic hexavalent chromium (Cr(VI)) control and remediation are essential to avoid further ecological impacts and reduce environmental risks. This paper investigated the enhanced Cr(VI) electrokinetic removal in the soil through the approaching cathode method. Besides, a novel four-step sequential fractionation method was used to reflect the strength of Cr(VI) binding to the soil. The approaching cathode enhanced the electrokinetic delivery of surface-bound Cr(VI) by advancing the alkaline front for Cr(VI) desorption and improving the electric potential flattening of the soil layers. Desorption of Cr(VI) by the alkaline front involved converting the inner-sphere complexes form of Cr(VI) to a weakly adsorbed form susceptible to ionic strength. In addition, the acidic front provided a favorable environment for the photochemical reduction of Cr(VI) by soil species or the added citrate as the electron donors. Improving the potential distribution could regulate the energy consumption of individual soil layers and efficiently operate the electrokinetic transfer of pollutants. The work results have significant scientific and practical significance for applying the in-situ electrokinetic technique in subsurface pollution control.

Unraveling the mystery of subsurface microorganisms in bioremediation

Microorganisms are the key players in biogeochemical processes in the shallow subsurface for bioremediation. Estimation of genetic diversity, microbial activity, and their metabolic functions are the most applicable methods to explore the biodegradation processes in the subsurface. Several techniques such as community-level physiological profiling, sequencing, metagenomics, enzyme assay, and culture-based methods are used to investigate the metabolic potential, functional diversity, and genetic diversity of inherent microbial communities. These studies help to understand the metabolic pathways and biodegradation patterns in the subsurface, however, the interspecies microbial interactions and their transport mechanism in the porous media toward bioremediation of subsurface pollutants are still not well understood. Despite the advanced characterization methods for microbes, multiple crucial knowledge gaps of the dynamic subsurface microbial community remain unraveled. A detailed understanding of subsurface microbial interactions, mineral-metal-microbial correlation, transport mechanism, and degradation pathways will help in the optimization and design of in-situ bioremediation in the future. There is a need for detail-oriented exploration of these fundamental microbial processes to gain a greater understanding of microbial dynamics to facilitate the advanced bioremediation processes sustainably.

IMPACT OF BIOLOGICALWASTEWATER TREATMENT PLANT AND PAHARRANG DRAIN ON GROUNDWATER CONTAMINATION AND ITS HEALTH RISK ASSESSMENT

In developing nations like Pakistan, the dumping of untreated industrial effluents into drains is a major source of subsurface pollution. This research was carried out at the area of Chokera, Faisalabad and focused on the Paharrang drain to examine its impact on groundwater quality since companies in the area discharge untreated sewage into the drain at various points along its length. It was primarily concerned with determining the impact of the outflow on groundwater quality and observing discharge rate fluctuations. The goal of this study was to assess the toxicity of the drain’s effluent and treatment plant on groundwater quality. Physio-chemical parameters such as pH, EC, TDS, TSS, DO, Carbonates, Bicarbonates, Cl, Arsenic, Lead, Chromium, Copper, Cadmium, and Zinc were measured in the collected samples. The findings of these samples were compared to WHO recommendations. The values of these data were represented using ArcView GIS v10.2 and a mapping of quality parameters. None of the values in the wastewater samples were determined to be within the WHO acceptable limit. Similarly, groundwater investigation revealed that all samples were unsuitable for human consumption. Drinking groundwater directly might be hazardous to one’s health.

Sampling Frequency Fluctuations of the Sensors and Software SPIDAR Ground Penetrating Radar: Impact on Probing Passive Surface Acoustic Wave Delay Lines for Pollution Sensing

In the context of the subsurface pollutant detection using cooperative targets probed by ground penetrating RADAR (GPR), we assess the sampling frequency stability of the instrument as needed to make sure any time delay variation between echoes is associated with subsurface chemical detection and not with instrument drift. Thanks to surface acoustic wave (SAW) transducers designed as GPR cooperative targets, the sampling frequency of the sensors and Software SPIDAR GPR control unit is characterized for short- and long-term sampling rate stabilities. The long-term stability of the instrument is below the phase measurement noise of the SAW cooperative target. However, short-term (within each trace) phase fluctuations are observed, hinting at short-term stroboscopic delay synthesis artifacts. Having demonstrated the stability of the baseline, we demonstrate gas-phase pollutant detection in the subsurface environment using GPR interrogation of a SAW sensor functionalized with a dedicated coating for reacting with hydrogen sulfide. Finally, for onsite long-term monitoring, an embedded vector network analyzer is shown to exhibit sufficient stability and the targeted performance for this measurement by recovering the time-domain response through the inverse Fourier transform of the frequency swept characterization of the sensor.

Seasonal Variation on the Hydrochemistry and Height of Brackish-Water from Commodore Channel, Lagos, Nigeria

The current study deals with the evaluation of physicochemical characteristics, nutrient status, and variation in the height of surface water from the commodore channel, and its comparison with the hydrochemical characteristics of the monitoring borehole from the NIOMR Jetty, Lagos, Nigeria. We aim to further improve our understanding of the interrelationship of climate change and hydrological cycle, and its impacts in the Lagos coastal environments. The observed ranges of physicochemical characteristics of the surface water are dissolved oxygen (DO) 3.07-6.05mg/L; pH: 7.99-8.48; temperature 26.97-30.34°C, electrical conductivity (EC) 9680-47800mS/cm; salinity (Sal) 6.51-28.05‰ and water level 1.24-1.79m. The concentration of nutrient range from nitrite 10-36mg/l, nitrate 28-44mg/l, sulphate 38-90mg/l and phosphate 2-78mg/l. The analyzed hydrochemical characteristics from all the stations are within the desired values for healthy marine ecosystems when compared with Federal Ministry of Environment (FMENV) Nigeria permissible limits which are: DO >5 mg/l, pH 6.5-9, temperature <40°C, nitrite 1mg/l, nitrate 20mg/l, phosphate 5mg/l and sulphate 500mg/l; except low DO (<4.5mg/l) in January, November and December, and elevated nitrite, nitrate and phosphate values across the season. The low DO values may be attributed to enhanced transportation of non-biodegradable organic loads that utilize the available DO for chemical oxidation and microbial decomposition. While the enriched nitrite, nitrate and phosphate concentrations suggest enhanced productivity and influx of nutrients rich flood water from the anthropogenic source. Our study showed a coincidence in the highest brackish water height of the surface water (1.24m) from the commodore channel with the highest groundwater level (1.58m) in October. The lowest height of water levels was recorded in February and March at 2.23m and 1.79m for groundwater and surface water respectively. The similarities in the height of water levels were recorded with contrasting hydrochemical variables (e.g., high Ec vs. low Ec; and high Sal vs. low Sal), which confirmed the effect of rainfall/freshwater incursion on the surface water and simultaneous occurrence of subsurface pollution arising from seawater intrusion on the groundwater samples in October. This study had shown that a long term monitoring of rise in water levels and hydrochemical variables of surface and groundwater can be used to understand climate change and early warning predictions of flooding in coastal environments.

Numerical simulation of groundwater contaminant transport in porous media

Groundwater has been considered as a major pathway for contamination migration which results in both subsurface pollution and soil contamination. This investigation makes an attempt to set up a two-dimensional numerical model to simulate groundwater flow and pollutant transport through porous media. The numerical model is used to simulate both groundwater flow and contaminant migration in porous media; besides, it is capable of considering the effects of sorption, retardation, and volatilization on the contaminant. Jajrud rural district located near Tehran City in Iran is a potentially contaminated site; furthermore, it is regarded as a potential peril for that zone. Groundwater flow and contaminant transport in Jajrud rural been investigated. This model is based on the finite-difference method which has enough ability to predict the pollution migration, and it is validated by several analytical and experimental test cases. The result revealed that the advection could possibly have been regarded as the main process on the contaminant transport in both reactive and non-reactive form of pollutant, as a result of high velocity in this region; furthermore, the reactive pollutant concentration mitigates 1.33 times more that non-reactive one as a result of high dispersion coefficient and contaminant decay term.

Bacteria transport and deposition in an unsaturated aggregated porous medium with dual porosity.

Bacterial transport and deposition play an important role in the assessment and prediction of subsurface pollution risks. Bacteria transport experiments were performed under unsaturated flow conditions in an aggregated porous medium at the laboratory column scale, to investigate how the inter- and intra-aggregated pore space of this medium could affect transport and deposition under unsaturated flow conditions, where inter- and intra-pore spaces are not fully activated. The results obtained through experimental observations and numerical simulations showed that some intra- and inter-pore space of this medium was excluded from bacteria transport and retention, as confirmed by the non-uniform transport of bacteria pathways in the aggregated porous media under unsaturated flow conditions. Capillary energy was higher the than other forces acting at bacteria air-water-solid interfaces. If this energy should contribute in increasing bacteria deposition under unsaturated conditions, similar to what has been reported for sandy media, similar overall retention of E. coli and R. rhodochrous was obtained under unsaturated flow conditions, suggesting that capillary energy was not the driving force for bacteria deposition.

Characterizing landfill leachate migration potential of a semi-arid duplex soil

Leachate migration from open landfills is an environmental concern of developing cities. This study investigated the base soil-profile pedo-physical and chemical properties of the South African Sepane soil form or referred to as Cutanic Luvisol at the Bloemfontein southern landfill under the Mangaung municipality in the Free State Province. Six soil-profiles pedo-physical, exchangeable-cations and heavy metals concentrations were characterized from in-situ, core and loose soil-samples. The DTPA Test from a 5g air-dried soil extracted heavy metals. The soil profile was characterized by a layered Orthic-A, pedocutanic B- and C-horizons with lower horizons containing mean-total clay of 72%, bulk-density (≥1.5 gcm−3) and saturated hydraulic-conductivity (Ks < 6mmhr-1). Mean soil pH increased with depth from 6.4 to 6.8 along-side exchangeable-cations ranging from 19 to 2573 mgkg-1 in the order Ca > Mg > K > Na > S > P and Ca > Mg > Na > K > S > P for the respective A- and B-horizons. The Mg/K and (Ca + Mg)/K exceeded norm ratios. Soil-profile horizons had respective 44%, 34% and 22% heavy-metal distribution with mean content range of 0.001–37.3 mgkg-1 in the order Mn > Fe > Cr > Zn > Cu > As > Pb > Ni > Cd and Fe > Mn > Cr > Cu > As > Pb > Zn > Ni > Cd for the surface and subsurface horizons, respectively. Heavy-metal mean concentrations were below the norm except for Cr that was higher than 150% from upper horizons and posed serious risk to the near-surface environment. Soil profiles heavy-metal content and pollution-index was unpolluted (0.3–0.4), decreased with depth and reflected no subsurface pollution concerns. This study findings highlighted low internal-migration potential of clay soils and the need for understanding the sources and mode of migration of Cr at the landfill alongside continued monitoring.

One-at-a-time sensitivity analysis of pollutant loadings to subsurface properties for the assessment of soil and groundwater pollution potential.

Chemical leak was numerically simulated for four chemical substances: benzene (light non-aqueous phase liquid (NAPL)), tetrachloroethylene (dense NAPL), phenol (soluble in water), and pentachlorophenol (white crystalline solid) in a hypothetical subsurface leak situation using a multiphase compositional transport model. One metric ton of chemical substances was assumed to leak at a point 3.51m above the water table in a homogeneous unconfined aquifer which had the depth to water table of 7.135m, the hydraulic gradient of 0.00097, the recharge rate of 0.7mm/day, and the permeability of 2.92 × 10-10m2. For comparison, surface spill scenarios, which had a long pathway from source to the water table, were simulated. Using the model results, point-source pollutant loadings to soil and groundwater were calculated by multiplying mass, impact area, and duration above and below the water table respectively. Their sensitivity to subsurface properties (depth to water table, recharge rate, porosity, organic carbon content, decay rate, hydraulic gradient, capillary pressure, relative permeability, permeability) was analyzed, with changing each parameter within acceptable ranges. The study result showed that the pollutant loading to groundwater was more sensitive to the subsurface properties than the pollutant loading to soil. Decay rate, groundwater depth, hydraulic gradient and porosity were influential to pollutant loadings. The impact of influential parameters on pollutant loadings was nonlinear. The dominant subsurface properties of pollution loadings (e.g., decay rate, groundwater depth, hydraulic gradient, and porosity for groundwater) also affect the vulnerability, and the subsurface pollutant loadings defined in this study are dependent on chemical properties as well, which indicates that the influential hydrogeological and physicochemical parameters to pollutant loadings can be used for pollution potential assessment. The contribution of this work is the suggestion that the sensitivity of pollutant loadings can be used for pollution potential assessment. Soil and groundwater pollution potential of chemicals are discussed altogether for leak scenarios. A physics-based model is used to understand the impact of subsurface properties on the fate and transport of chemicals above and below the water table, and consequently their impact on the pollutant loading to soil and groundwater.

Characterization and monitoring of subsurface contamination from Olive Oil Mills' waste waters using Electrical Resistivity Tomography

This work describes the efficiency and ability of Electrical Resistivity Tomography (ERT) to map and monitor the subsurface contamination caused by the wastes created during the production of olive oil. The spatial distribution and temporal variation of these wastes are investigated through an integrated methodological flowchart composed of numerical modeling tests and field data collected from an active waste disposal site. An Olive Oil Mills' Wastes (OOMW) real site was chosen to monitor the subsurface flow of the wastes that are disposed of in an artificial pond for 1.5 years. Synthetic modeling was used to simulate and reconstruct the movement of the OOMW as a conductive target within a layered resistive medium. The results of the ERT data show a high degree of correlation between published ERT, geochemical, and IP geophysical results. This indicates that ERT can be a powerful tool for mapping and monitoring the byproducts of the olive oil industry, in the form of subsurface contamination, as demonstrated by the synthetic modeling. The electrical signature of the OOMW was also verified through the identification of in situ wastes within an excavation trench along the monitoring ERT line. The results show that ERT can be used as a stand-alone tool to characterize the subsurface pollution in OOMW sites.

Design charts for contaminant transport throughslurry trench cutoff walls

Slurry trench cutoff walls with low-permeability backfill material, such as soil-bentonite and slag-cement-bentonite, are used widely for containment of subsurface pollution. In the design of slurry walls the potential service life for a given thickness or the wall thickness for a target service life are typically determined via analyses of one-dimensional contaminant transport. The difficulty of selecting appropriate inlet and outlet boundary conditions and the mathematical complexity of analytical solutions hinder engineers in undertaking a contaminant transport analysis–based design. Design charts for nondimensionalized effluent flux are presented by developing and using an analytical solution. The methodologies of using these charts in design are demonstrated.

Simulating the Fate and Transport of Coal Seam Gas Chemicals in Variably-Saturated Soils Using HYDRUS

The HYDRUS-1D and HYDRUS (2D/3D) computer software packages are widely used finite element models for simulating the one-, and two- or three-dimensional movement of water, heat, and multiple solutes in variably-saturated media, respectively. While the standard HYDRUS models consider only the fate and transport of individual solutes or solutes subject to first-order degradation reactions, several specialized HYDRUS add-on modules can simulate far more complex biogeochemical processes. The objective of this paper is to provide a brief overview of the HYDRUS models and their add-on modules, and to demonstrate possible applications of the software to the subsurface fate and transport of chemicals involved in coal seam gas extraction and water management operations. One application uses the standard HYDRUS model to evaluate the natural soil attenuation potential of hydraulic fracturing chemicals and their transformation products in case of an accidental release. By coupling the processes of retardation, first-order degradation and convective-dispersive transport of the biocide bronopol and its degradation products, we demonstrated how natural attenuation reduces initial concentrations by more than a factor of hundred in the top 5 cm of the soil. A second application uses the UnsatChem module to explore the possible use of coal seam gas produced water for sustainable irrigation. Simulations with different irrigation waters (untreated, amended with surface water, and reverse osmosis treated) provided detailed results regarding chemical indicators of soil and plant health, notably SAR, EC and sodium concentrations. A third application uses the HP1 module to analyze trace metal transport involving cation exchange and surface complexation sorption reactions in a soil leached with coal seam gas produced water following some accidental water release scenario. Results show that the main process responsible for trace metal migration in soil is complexation of naturally present trace metals with inorganic ligands such as (bi)carbonate that enter the soil upon infiltration with alkaline produced water. The examples were selected to show how users can tailor the required model complexity to specific needs, such as for rapid screening or risk assessments of various chemicals nder generic soil conditions, or for more detailed site-specific analyses of actual subsurface pollution problems.

Design Charts for Contaminant Transport through Slurry Trench Cutoff Walls

AbstractSlurry trench cutoff walls with low-permeability backfill material, such as soil-bentonite and slag-cement-bentonite, are used widely for containment of subsurface pollution. In the design ...

HYDROGEOCHEMICAL CHARACTERISTICS OF WATER INTAKES OF FRESH SUBSURFACE WATERS OF THE TAZOVSKY DRAINAGE BASIN

In this article the object of researches covers the subsurface interpermafrost waters of Palaeogene deposits considered within the largest water intakes of the Tazovsk underground waters drainage basin. The authors present a comparative description of the hydrogeochemical setting of the water-bearing complexes, the analysis of the technogenic pollution «from above» which is directly related to the development and operation of fields of hydrocarbon raw materials, oil products storage, transportation and processing. Additionally, the problem of subsurface waters pollution «from below» during leakage of fluids through injectors and producers casing strings resulted from not always proper quality cementing. The correlation dependences of some components are presented in the paper.

Kinetic Data Analysis by MLR and ANN Models for Phenol Attenuation in Peat Soil

AbstractThe efficacy of phenol transport from the aqueous phase to peat soil for assessment of the attenuation capacity for migratory phenol in subsurface water pollution was investigated by the application of multiple linear regression (MLR) and artificial neural network (ANN) models. The batch kinetics study was performed, which revealed that the Freundlich isotherm model fits reasonably well with experimental results. A maximum value of 43% phenol removal efficiency was achieved in 200 g/L of soil, an initial concentration of phenol of 10 mg/L, and an equilibration time of 6 h. A sum total of 270 laboratory batch adsorption tests were conducted, and the results were applied in MLR and ANN models. Some of the influencing factors, such as pH, initial concentration, mass of soil, contact time, and so forth, on removal of sorbate by peat were also investigated in the present research. The experimental results exhibit a reasonable goodness of fit [higher coefficient of determination, R2, and lower root-mean...

Coupled effect of extended DLVO and capillary interactions on the retention and transport of colloids through unsaturated porous media

Colloids are potential vectors of contaminants in the subsurface environment. The knowledge of transport and retention behaviors of colloids is of primary importance for assessment and prediction of subsurface pollution risks. In this study, sand column experiments were conducted to investigate the coupled effects of various interfacial forces on the retention and transport of a hydrophilic silica colloid and a relatively hydrophobic latex colloid. Water column experiments were performed to observe the movement of colloids with air bubbles. Extended DLVO interaction energies and capillary potential energy were calculated to analyze colloid retention at air-water interface (AWI), solid-water interface (SWI), and air-water-solid interface (AWS). Results show that colloid retention decreases due to increase in electrostatic repulsion and Born repulsion as well as decrease in Lewis acid-base attraction and hydrophobic interactions. Water content effect and hydrophobic effect on colloid retention become more predominant in the solution of higher ionic strength. Colloid retention at AWI is minimal (i.e., due to nonexistence of primary and secondary minima) at the ionic strengths <75mM. Capillary potential energy (107–108 KBT) of colloids is 4–5 orders of magnitude greater than the extended DLVO interaction energy (~103 KBT), suggesting that capillary retention at AWS is the primary mechanism controlling colloid retention in unsaturated porous media. Results from this study show that immobile solid phase (e.g., soil) could be much more important than air phase in determining colloid retention in unsaturated porous media under unfavorable conditions, especially in the solutions of high ionic strengths.