Prediction of arsenic retention in vadose zone based on empirical relationship between soil properties and segmented retardation factors.

Arsenic (As) is a widespread environmental contaminant that poses a significant threat to ecosystems and human health. Although previous studies have qualitatively revealed the effects of individual soil properties on the transport and fate of As in the vadose zone, their integrated impacts remain obscure. Moreover, studies investigating the retardation factor therein, which is a key parameter for comprehending As transport in the vadose zone, are extremely limited. In this study, we investigated the interplay of soil properties with As transport and retention within the vadose zone, while focusing on the retardation factor of As. We employed steady-state unsaturated water-flow soil column experiments coupled with a mobile-immobile model and multiple linear regression analysis to elucidate the dependence of As retardation factors on the soil properties. In the mobile water zone, iron and organic matter contents emerged as the two most influential properties that impedes As mobility. Whereas, in the immobile water zone, the coefficient of uniformity and bulk density were the most influential factors that enhanced As retention. Finally, we derived an empirical equation for calculating the As retardation factors in each zone, offering a valuable tool for describing and predicting As behavior to protect the groundwater resources underneath.

Influence of iron (hydr)oxide mineralogy and contents in aquifer sediments on dissolved organic carbon attenuations during aquifer storage and recovery.

Aquifer storage and recovery (ASR) is a promising approach for managing water resources that enhances water quality through biogeochemical reactions occurring within aquifers. Iron (hydr)oxides, which are the predominant metallic oxides in soil, play a crucial role in degrading dissolved organic carbon (DOC), primarily through a process known as dissimilatory iron reduction (DIR). However, the efficiency of this reaction varies depending on the mineralogy and composition of the aquifer, and this understanding is essential for adequate water quality in ASR. The objective of this study is to investigate the impact of iron (hydr)oxide on acetate, as an organic carbon source, attenuation during the ASR. To achieve this, three sets of laboratory sediment columns were prepared, each containing a different type of iron (hydr)oxide minerals: ferrihydrite, goethite, and hematite. Following an acclimation period of 28 days to simulate the microcosm within an aquifer, the columns were continuously supplied with the simulated river water spiked with acetate (DOC 40-60 mg L-1), and the acetate concentration in the effluent was monitored. The result revealed that the column containing ferrihydrite achieved 97% acetate attenuation through DIR with anoxic conditions (DO < 0.1 mg L-1), while the goethite and hematite columns exhibited limited attenuation rates of 40 and 50%, respectively. Furthermore, the efficiency of acetate attenuation in the ferrihydrite columns increased with the content of ferrihydrite but experienced a rapidly declined at higher contents (3-4%), possibly due to the partial conversion of ferrihydrite to goethite as a result of the interaction between ferrihydrite and the Fe(II) produced during DIR. Additionally, an analysis of the microbial community demonstrated that microorganisms known to possess the ability to reduce iron (hydr)oxides under anaerobic conditions were abundant in the ferrihydrite columns.

Synergetic effect of nitrate on dissolved organic carbon attenuation through dissimilatory iron reduction during aquifer storage and recovery.

Aquifer storage and recovery (ASR) is a promising water management technique in terms of quantity and quality. During ASR, iron (Fe) (hydr)oxides contained in the aquifer play a crucial role as electron acceptors in attenuating dissolved organic carbon (DOC) in recharging water through dissimilatory iron reduction (DIR). Considering the preference of electron acceptors, nitrate (NO3⁻), possibly coexisting with DOC as the prior electron acceptor to Fe (hydr)oxides, might influence DIR by interrupting electron transfer. However, this phenomenon is yet to be clarified. In this study, we systematically investigated the potential effect of NO3⁻ on DOC attenuation during ASR using a series of sediment columns representing typical aquifer conditions. The results suggest that DOC attenuation could be enhanced by the presence of NO3⁻. Specifically, total DOC attenuation was notably higher than that from the stoichiometric calculation simply employing NO3⁻ as the additional electron acceptor to Fe (hydr)oxides, implying a synergetic effect of NO3⁻ in the overall reactions. X-ray photoelectron spectroscopy analyzes revealed that the Fe(II) ions released from DIR transformed the Fe (hydr)oxides into a less bioavailable form, inhibiting further DIR. In the presence of NO3⁻, however, no aqueous Fe(II) was detected, and another form of Fe (hydr)oxide appeared on the sediment surface. This may be attributed to nitrate-dependent Fe(II) oxidation (NDFO), in which Fe(II) is (re)oxidized into Fe (hydr)oxide, which is available for the subsequent DOC attenuation. These mechanisms were supported by the dominance of DIR-relevant bacteria and the growth of NDFO-related bacteria in the presence of NO3⁻.

Complex behavior of petroleum hydrocarbons in vadose zone: A holistic analysis using unsaturated soil columns.

The migration of petroleum hydrocarbons in vadose zone involves complex coupled processes such as downward displacement and natural attenuation. Despite its significance in determining groundwater vulnerability to petroleum contamination and optimizing the remedial strategy, it has not been comprehensively studied in terms of overall processes under field-relevant conditions. In this study, a series of unsaturated soil column experiments were conducted by simulating subsurface diesel contamination within a vadose zone using different soil textures at different soil bulk densities and initial diesel concentrations, while partly exposing them to simulated precipitation. The results showed that the soil column with less fine fraction was favorable for the downward migration of diesel but unfavorable for its natural degradation. However, precipitation complicated the relative conductivities of multiple fluids (water, air, and diesel) through the pore network, therby decreasing diesel migration and degradation. For example, the downward migration of diesel in the SL column decreased by 8.4% under precipitation, while the overall attenuation rate dropped to almost 0.24% of its original state. Lowering bulk density (from 1.5 to 1.23 g/cm3), however, could enhance the attenuation rate presumably due to the secured void space for the incoming fluids. A high initial concentration of diesel (2%; w/w) inhibited its natural attenuation, while its influence on its vertical propagation after the precipitation was not significant. The present findings provide a mechanistic basis for approximating the behavior of petroleum hydrocarbons in a random vadose zone.

Variable effects of soil organic matter on arsenic behavior in the vadose zone under different bulk densities.

The nonstationary nature of water and oxygen content in the vadose zone determines various biogeochemical reactions regarding arsenic (As) therein, which affects the groundwater vulnerability to As contamination at a site. In the present study, we evaluated the effect of soil organic matter (OM) on the behavior of As using specifically designed soil columns that simulated the vadose zone. Three wet-dry cycles were applied to each of the four columns with different OM contents and bulk densities. OM was found to exhibit variable effects, either inhibiting or accelerating the mobilization of As, depending on bulk density. At a moderate bulk density (< 1.27 g/cm3), OM slightly lowered the pH of pore water, which enhanced the sorption of As onto the iron (Fe) oxides, promoting the retention of As in soil. In the soil column with a relatively higher bulk density (1.36 g/cm3), however, the dissimilatory reduction of iron oxides was triggered by rich OM under oxygen-limited conditions. X-ray absorption spectroscopy analysis revealed that alternate wetting and drying transformed the Fe oxides in the soil by reductive dissolution and subsequent re-precipitation. Consequently, As was not stably retained in the soil, and its mobilization downwards was further accelerated.

Denitrification dynamics in unsaturated soils with different porous structures and water saturation degrees: A focus on the shift in microbial community structures.

Despite its environmental significance, little is known about denitrification in vadose zones owing to the complexity of such environments. Here, we investigated denitrification in unsaturated soils with different pore distributions. To this end, we performed batch-type denitrification experiments and analyzed microbial community shifts before and after possible reactions with nitrates to clarify the relevant denitrifying mechanism in the microcosms. For quantitative comparison, pore distribution in the test soil samples was characterized based on the uniformity coefficient (Cu) and water saturation degree (SD). Micro-CT analysis of the soil pore distribution confirmed that the proportion of bigger-sized pores increased with decreasing Cu. However, oxygen diffusion into the system was controlled by SD rather than Cu. Within a certain SD range (51-67%), the pore condition changed abruptly from an oxic to an anoxic state. Consequently, denitrification occurred even under unsaturated soil conditions when the SD increased beyond 51-67%. High throughput sequencing revealed that the same microbial species were potentially responsible for denitrification under both partially (SD 67%), and fully saturated (SD of 100%) conditions, implying that the mechanism of denitrification in a vadose zone, if it exists, might be possibly similar under varying conditions.

Opportunities and challenges of machine learning in bioprocesses: Categorization from different perspectives and future direction.

Recent advances in machine learning (ML) have revolutionized an extensive range of research and industry fields by successfully addressing intricate problems that cannot be resolved with conventional approaches. However, low interpretability and incompatibility make it challenging to apply ML to complicated bioprocesses, which rely on the delicate metabolic interplay among living cells. This overview attempts to delineate ML applications to bioprocess from different perspectives, and their inherent limitations (i.e., uncertainties in prediction) were then discussed with unique attempts to supplement the ML models. A clear classification can be made depending on the purpose of the ML (supervised vs unsupervised) per application, as well as on their system boundaries (engineered vs natural). Although a limited number of hybrid approaches with meaningful outcomes (e.g., improved accuracy) are available, there is still a need to further enhance the interpretability, compatibility, and user-friendliness of ML models.

A multi-scale framework for modeling transport of microplastics during sand filtration: Bridging from pore to continuum.

The fate and transport of microplastics (MPs) during deep bed filtration were investigated using combined laboratory experiments and numerical modeling. A series of column experiments were conducted within the designated ranges of six operating parameters (i.e., size of the MP and collector, seepage velocity, porosity, temperature, and ionic strength). A variance-based sensitivity analysis, the Fourier amplitude sensitivity test, was conducted to determine the priority in affecting both the attachment coefficient at the pore scale, and the subsequent stabilized height of the breakthrough curve at the continuum scale, which follows non-monotonic trends with singularity in the size of MP (i.e., 1 µm). Finally, Damkohler numbers were introduced to analyze the dominant mechanisms (e.g., attachment, detachment, or straining) in the coupled hydro-chemical process. The robustness of conceptual frameworks bridges the gap between pore-scale interactions and the explicit MPs removal in the continuum scale, which could support decision-making in determining the priority of parameters to retain MPs during deep bed filtration.

Thermodynamic investigation of nanoplastic aggregation in aquatic environments.

In this study, the aggregation behavior of polystyrene nanoplastics (PS NPs) in the absence or presence of oppositely charged particulate matters is systematically investigated for a wide range of electrolyte conditions. Herein, we used isothermal titration calorimetry combined with time-resolved dynamic light scattering to provide kinetic and thermodynamic insights into the NP aggregation. The thermodynamic profiles of homoaggregation and heteroaggregation were fit using an independent site and two independent sites models, respectively, demonstrating different interaction modes of both aggregation processes. We found that the contribution of solvation entropy was significant and variable in most cases, and this thermodynamic parameter was a large determinant of the thermodynamics of NP aggregation. Furthermore, the stability of PS NPs in natural water matrices was found to be correlated with ionic strength and the content of natural colloids (e.g., metal oxides and clay particles). These results point to the importance of considering the role of thermodynamic variables when studying the fate of NPs within various environmental conditions.

Coupled effect of porous network and water content on the natural attenuation of diesel in unsaturated soils.

The natural attenuation potential of a vadose zone against diesel is critical for optimizing remedial actions and determining groundwater vulnerability to contamination. Here, diesel attenuation in unsaturated soils was systematically examined to develop a qualitative relationship between physical soil properties and the natural attenuation capacity of a vadose zone against diesel. The uniformity coefficient (Cu) and water saturation (Sw, %) were considered as the proxies reflecting the degree of effects by porous network and water content in different soils, respectively. These, in turn, are related to the primary diesel attenuation mechanisms of volatilization and biodegradation. The volatilization of diesel was inversely proportional to Cu and Sw, which could be attributed to effective pore channels facilitating gas transport. Conversely, biodegradation was highly proportional to Cu under unsaturated conditions (Sw = 35-71%), owing to nutrients typically associated with fine soil particles. The microbial community in unsaturated soils was affected by Sw rather than Cu. The overall diesel attenuation including volatilization and biodegradation was optimized at Sw = 35% for all tested soils.

Transient behavior of arsenic in vadose zone under alternating wet and dry conditions: A comparative soil column study.

The water and oxygen contents of the vadose zone change cyclically depending upon the meteorological condition (e.g., intermittent rainfall), which can affect the biogeochemical reactions that govern the fate of arsenic (As). To simulate and evaluate the transient behavior of As in this zone when subjected to repeated wet and dry conditions, soil column experiments with different soil properties were conducted. Three wetting-drying cycles resulted in the fluctuation of water and dissolved oxygen contents, and consequently, the reduction-oxidation potential in the soil columns. Under these circumstances, the biotic reduction of As(V) to As(III) was observed, especially in the column filled with soils enriched in organic matter. Most of the As was found to be associated with soil particles rather than to be dissolved in the pore water in all of the columns tested. Retention of As was more preferable in the soil column with a higher Fe content and bulk density, which provided more sorption sites and reaction time, respectively. However, a considerable amount of soil-bound As could be remobilized and released back to the pore water with the repetition of wetting and drying due to the transformation of As(V) to As(III).

What determines the efficacy of landfarming for petroleum-contaminated soils: Significance of contaminant characteristics.

Identifying the cause of inconsistent landfarming efficacy is critical to designing optimal remedial strategies for petroleum-contaminated sites. We assessed contaminated soils collected from two former military bases in South Korea to better understand the role and influence of different factors. Landfarming remediation was simulated in the laboratory by applying comparable practices (such as tillage and bioaugmentation) and the relevant mechanism was examined. We then systematically examined potential factors affecting petroleum-removal efficacy, including the content of fine soil particles, the initial concentration and composition of petroleum contaminants, and the degree of soil-contaminant interaction. The distribution range of total petroleum hydrocarbons (TPHs) and the size of unresolved complex mixture (UCM) found in gas chromatography data showed that petroleum composed of TPHs with lower carbon numbers and having smaller size of UCM could be treated more effectively by landfarming. Incorporating the evaluation of the distribution range and UCM properties of petroleum, rather than simply considering its total concentration, is a more accurate and efficient method for determining the site-specific suitability of landfarming as a remedial option, as well as for assessing the necessity of supplementary processes.

Prediction of biogas production rate from dry anaerobic digestion of food waste: Process-based approach vs. recurrent neural network black-box model.

The stability of dry anaerobic digestion (AD) of food waste (FW) as well as the resulting methane gas generation was investigated from the perspective of system dynamics. Various organic loading rates were applied to the system by modifying the water content in the FW feed and solid retention time (SRT). The excessive organic loading due to the accumulation of volatile fatty acids (VFAs) from the feed with 80% water content during the short SRT (15 and 20 d) caused system failure. In contrast, more intermediate materials, such as VFAs, was easily converted into methane at higher water contents. In addition, the biogas production rate of dry AD was effectively predicted based on SRT, soluble chemical oxygen demand, total VFA, total ammonia, and free ammonia using a recurrent neural network-the so-called "black-box" model. This implies the feasibility of applying this data-based black-box model for controlling and optimizing complex biological processes.

Microplastic removal in conventional drinking water treatment processes: Performance, mechanism, and potential risk.

The effectiveness of traditional drinking water treatment plants for the removal of Microplastics (MPs) in the size range of tens of micrometers is currently uncertain. This study investigated the behavior and removal efficiency of four different sized polystyrene MPs (10-90 µm in diameter) in a simulated cascade of coagulation/sedimentation, sand filtration, and UV-based oxidation over technically relevant time frames. In the coagulation and sand filtration steps, the larger the MP size, the better it was removed. The coagulant type and water characteristics (i.e., pH and the presence of natural organic matter) influenced the coagulation efficiency for MPs. X-ray microcomputed tomography technique and two-site kinetic modeling were used to identify the mechanisms involved in sand filtration. The MPs > 20 µm could be completely retained in sand by straining, while the attachment to the sand surface was likely responsible for the retention of MPs < 20 µm. However, approximately 16% of 10 µm MPs injected passed through the sand, which were further fragmented by UV oxidation. UV/H2O2 treatment promoted the MP fragmentation and chemical leaching more significantly than UV treatment, resulting in a higher toxicity for UV/H2O2-treated water. Our findings pave the way for deeper understanding of how MPs behave and transform in a sequential drinking water treatment process.

Overview of global status and challenges for end-of-life crystalline silicon photovoltaic panels: A focus on environmental impacts.

Recent developments in photovoltaic (PV) technology have enabled a reduction of fossil fuel usage and subsequent carbon dioxide (CO2) release from energy production. However, end-of-life (EoL) crystalline silicon (c-Si) PV panels have become an emerging waste issue. This overview attempts to update and forecast the global status of renewable energy capacity and c-Si PV waste generation under different scenarios and to present a summary of the recent literature on recycling technologies and life cycle assessment (LCA) of EoL c-Si PV panels with a focus on reclaimable resources. For most LCA studies in the 1980s and the 2000s, the EoL phase of PV systems has often neglected or oversimplified (e.g., disposal after low-rate recovery) the fact that various recycling procedures and reclaimable resources from each stage cannot be appropriately considered. A limited number of studies have been available since the 2010s that highlight the high-rate recovery from EoL PV panels. However, the differences in functional unit, system boundary and impact analysis methodology make it difficult to compare the results directly, and spatio-temporal uncertainties are yet to be thoroughly quantified due to the lack of workable localized data. More efforts are needed to identify complementary environmental impacts (i.e., burden and credit) from the individual recycling processes. Correspondingly impacts from transport need to be fully incorporated for the optimization of the recycling process which has been neglected in most of the previous studies.

Microfluidic pore model study of precipitates induced by the pore-scale mixing of an iron sulfate solution with simulated groundwater.

Precipitates induced by the pore-scale mixing of iron sulfate solutions with simulated groundwater were investigated using a microfluidic pore model to assess the environmental impacts of the infiltration of acid mine drainage into a shallow aquifer. This model was employed to visualize the formation of precipitates in a porous network and to evaluate their physicochemical influences on pore flow. Four types of groundwater (Na-HCO3, Na-SO4, Na-Cl, and Ca-Cl) were evaluated, and precipitation rates were calculated by processing images of precipitates in the pores captured via microscopy. The results showed that all groundwater types formed a yellow-brownish precipitate at the interface of the iron solution and simulated groundwater flow. Microscopic X-ray analyses demonstrated that precipitate morphology varied with groundwater type. Faster precipitation was observed in the following order by groundwater type: Na-HCO3 > Na-Cl > Na-SO4 > Ca-Cl, which was attributed to the different stability constants of the major anions in each simulated groundwater with Fe ions. Chemical equilibrium models suggested that precipitates were Fe minerals, with FeOOH as the predominant form consistent with the results of X-ray photoelectron spectrometry. The presence of FeOOH implies that precipitates may serve as an effective sorption barrier against some nutrients and heavy metals for the underlying groundwater. However, dye-flow experiments suggested that the precipitates may clog aquifer pores, thereby altering hydrogeological properties in the aquifer.

Role of ranitidine in N-nitrosodimethylamine formation during chloramination of competing micropollutants.

Ranitidine (RNT) is a widely known precursor of N-nitrosodimethylamine (NDMA) as evinced by the self-catalytic formation of NDMA during chloramination. In the present study, the NDMA formation potentials (NDMA-FP) of 26 micropollutants were assessed, particularly when mixed with RNT. 11 compounds were identified as individual precursors, including trimebutine and cimetidine, which exhibited substantial NDMA-FP, with up to 10% molar yield. In addition, nitrosamines, other than NDMA, namely N-nitrosodiethylamine and N-nitrosomethylamine, were observed from diethylamine-containing precursors, such as metoclopramide. In a 1:1 mixture of RNT and a competitor, the change in NDMA-FP was mostly comparable (within 20% deviation), while antagonistic interactions were observed for competitors, such as diethylhydroxylamine. The scattered overall NDMA-FP should be considered as a product of competition among the precursors for core substrates and intermediates for NDMA formation. The co-existence of either trimebutine or metoclopramide with RNT led to an exceptionally synergetic NDMA generation. Degradation kinetics and chlorination/nitrosation experiments combined with mass spectroscopy analyses indicated that RNT would accelerate both the initial chlorination and nitrosation of trimebutine and metoclopramide, leading to N-nitroso complexes, which have well-understood NDMA formation pathways, i.e., amination with subsequent aminyl radical generation. This work demonstrates a wide array of precursors with NDMA-FP, suggesting that nitrosamine formation is potentially underestimated in field environments.

Comparative analysis of I2-KI and HNO3 leaching in a life cycle perspective: Towards sustainable recycling of end-of-life c-Si PV panel.

An iodine-iodide system was investigated as an alternative lixiviant for HNO3 for leaching precious metals from the end-of-life c-Si photovoltaic (PV) cell. A series of batch experiments were conducted for the optimization of leaching kinetics and thermodynamic equilibrium followed by a life cycle assessment (LCA) using data from the experiments. The results showed that more than 95% of Ag and Al leached out within the first 5 min. The optimum conditions for equilibrium leaching were as follows: solid to liquid ratio of 1:10 for Ag (1:9 ml for Al), and I2 concentration of 0.35 M for Ag (0.3 M for Al), with I- concentration of 0.7 M. In addition, selective leaching of Ag could also be accomplished by adjusting the reaction pH to 9.6%, and 93% of reproducibility was achieved via the rejuvenation of the exhausted leaching solution, which can benefit the subsequent recovery process. The leaching efficiency of iodine-iodide system was nearly comparable to that of HNO3, and the environmental impacts of the two cycle of continuous process with rejuvenation of the iodine leaching solution can be effectively reduced especially in the acidification & eutrophication, respiratory effect, and mineral extraction categories with subsequent exclusion of the additional neutralization process.

Material- and Site-Specific Partition Coefficients for Beneficial Use Assessments.

Partition coefficient (Kd) values available in the literature are often used in fate and transport modeling conducted as part of beneficial use risk assessments for industrial byproducts. Because element partitioning depends on soil properties as well as characteristics of the byproduct leachate, site-specific Kd values may lead to more accurate risk assessment. In this study, contamination risk to groundwater of beneficially reused byproducts was assessed using batch leaching tests on waste to energy bottom ash and coal combustion fly ash. Leachates were equilibrated with eight different soils to obtain the waste-soil-specific Kd,exp values for the metals of interest. The Kd,exp values were used as inputs in the Industrial Waste Management Evaluation Model to demonstrate the degree to which Kd estimates affect risk assessment outcomes. Measured Kd,exp values for the most part fell within the large range of Kd values reported in the literature, but IWEM results using default Kd values for some types of soils resulted in overestimated risk compared to those derived from Kd,exp values. Modeled concentration at the receptor location was much lower for some elements for those soils with high concentrations of iron and aluminum.

Experimental determination of nonequilibrium transport parameters reflecting the competitive sorption between Cu and Pb in slag-sand column.

Competitive sorption and resulting nonequilibrium transport of Cu and Pb were investigated using slag as a primary sorbent. A series of estimation models were applied based on the equilibrium, and nonequilibrium sorption respectively, and finally calibrated by incorporating the experimentally determined batch kinetic data. When applied individually, the behavior of metals in slag-sand column were well predicted by both equilibrium and nonequilibrium models in CXTFIT code. However, coexisting Cu and Pb exhibited competition for sorption sites, generating an irregular breakthrough curves such as overshoot (higher concentration in effluent than the feed concentration) of Cu and corresponding earlier peak of Pb followed by gradual re-rising. Although two-site nonequilibrium model further considers coupled hydrochemical process, desorption of the Cu from competition made the model prediction inaccurate. However, the parameter estimation could be improved by incorporating the experimentally determined mass transfer rate, ωexp from batch kinetics. Based on the calibrated model, the fraction of instantaneous retardation, ßexp of Pb decreased from 0.41 in the single system to 0.30 in the binary system, indicating the shift from equilibrium to nonequilibrium state, where which of Cu increased from 0.39 to 0.94, representing the shift towards equilibrium. The modified results were also compared with five-step sequential extraction data, confirming that the shift of particular metal fractions from the competition triggered the nonequilibrium transport.