Remediation of arsenic-contaminated groundwater using media-injected permeable reactive barriers with a modified montmorillonite: sand tank studies

Recent advances in clay minerals for groundwater pollution control and remediation

Clay minerals are abundant on Earth and have been crucial to the advancement of human civilization. The ability of clay minerals to absorb chemicals is frequently utilized to remove hazardous compounds from aquatic environments. Moreover, clay-based adsorbent products are both environmentally acceptable and affordable. This study provides an overview of advances in clay minerals in the field of groundwater remediation and related predictions. The existing literature was examined using data and information aggregation approaches. Keyword clustering analysis of the relevant literature revealed that clay minerals are associated with groundwater utilization and soil pollution remediation. Principal component analysis was used to assess the relationships among clay mineral modification methods, pollutant properties, and the Langmuir adsorption capacity (Qmax). The results demonstrated that pollutant properties affect the Qmax of pollutants adsorbed by clay minerals. Systematic cluster analysis was utilized to classify the collected data and investigate the relationships. The pollution adsorption mechanism of the unique structure of clay minerals was investigated based on the characterization results. Modified clay minerals exhibited changes in surface functional groups, internal structure, and pHpzc. This review provides a summary of recent clay-based materials and their applications in groundwater remediation, as well as discussions of their challenges and future prospects.

Groundwater arsenic contamination: impacts on human health and agriculture, ex situ treatment techniques and alleviation

Groundwater is consumed by a large number of people as their primary source of drinking water globally. Among all the countries worldwide, nations in South Asia, particularly India and Bangladesh, have severe problem of groundwater arsenic (As) contamination so are on our primary focus in this study. The objective of this review study is to provide a viewpoint about the source of As, the effect of As on human health and agriculture, and available treatment technologies for the removal of As from water. The source of As can be either natural or anthropogenic and exposure mediums can either be air, drinking water, or food. As-polluted groundwater may lead to a reduction in crop yield and quality as As enters the food chain and disrupts it. Chronic As exposure through drinking water is highly associated with the disruption of many internal systems and organs in the human body including cardiovascular, respiratory, nervous, and endocrine systems, soft organs, and skin. We have critically reviewed a complete spectrum of the available ex situ technologies for As removal including oxidation, coagulation-flocculation, adsorption, ion exchange, and membrane process. Along with that, pros and cons of different techniques have also been scrutinized on the basis of past literatures reported. Among all the conventional techniques, coagulation is the most efficient technique, and considering the advanced and emerging techniques, electrocoagulation is the most prominent option to be adopted. At last, we have proposed some mitigation strategies to be followed with few long and short-term ideas which can be adopted to overcome this epidemic.

A comprehensive review on permeable reactive barrier for the remediation of groundwater contamination

Groundwater quality is deteriorating due to contamination from both natural and anthropogenic sources. Traditional "Pump and Treat" techniques of treating the groundwater suffer from the disadvantages of a small-scale and energy-intensive approach. Permeable reactive barriers (PRBs), owing to their passive operation, offer a more sustainable strategy for remediation. This promising technique focuses on eliminating heavy metal pollutants and hazardous aromatic compounds by physisorption, chemisorption, precipitation, denitrification, and/or biodegradation. Researchers have utilized ZVI, activated carbon, natural and manufactured zeolites, and other by-products as reactive media barriers. Environmental parameters, i.e., pH, initial pollutant concentration, organic substance, dissolved oxygen, and reactive media by-products, all influence a PRB's performance. Although their long-term impact and performance are uncertain, PRBs are still evolving as viable alternatives to pump-and-treat techniques. The use of PRBs to remove anionic contaminants (e.g., Fluoride, Nitrate, etc.) has received less attention since precipitates can clog the reactive barrier and hinder groundwater flow. In this paper, we present an insight into this approach and the tremendous implications for future scientific study that integrates this strategy using sustainability and explores the viability of PRBs for anionic pollutants.

Removal of arsenic from semiarid area groundwater using a biosorbent from watermelon peel waste

Groundwater is one of the most important reservoirs in semi-arid and arid zones of the world, particularly in Mexico. The aims of this work were to produce a biosorbent from watermelon peel waste and a biosorbent with citric acid treatment and to evaluate both biosorbents with different concentrations of arsenic in groundwater. The biosorbents were produced with watermelon peel residues, which were observed by SEM microscopy to evaluate their physical morphology. Its removal potential was tested at concentrations of 0, 1, 13, 22, and 65 μg/L of arsenic, and both adsorption capacity and removal percentage were analyzed by final measurement obtained by atomic absorption spectrometry. The pH was measured throughout the experimentation maintaining ranges between 5.5 and 7.5. The biosorbent without treatment presented clearer and more compact flakes. At the microscopic level, the biosorbent without treatment presented pores with a more circular shape, and the biosorbent with treatment was more polygonal, similar to a honeycomb. The highest removal percentage was 99.99%, for both treatments at 4 h. The biosorbent without treatment at 4 h with arsenic concentrations of 65 μg/L presented the highest adsorption capacity (2.42 μg/g). It is concluded that watermelon peel biosorbent is a material that has the potential to remove arsenic from groundwater. This type of biosorbent is effective to remove arsenic and could be used in the field, however, it still needs to be optimized to convert it into a material completely suitable for large-scale use.

A hybrid data-driven framework for diagnosing contributing factors for soil heavy metal contaminations using machine learning and spatial clustering analysis

The efficacy of source apportionment is often limited by a lack of information on natural and anthropogenic contributing factors influencing soil heavy metal (HM) contaminations. To overcome this limitation and develop the data mining methods, a novel hybrid data-driven framework was proposed to diagnose the contributing factors in an industrialized region in Guangdong Province, China, mainly using a combination of naive Bayes (NB), random forest (RF), and bivariate local Moran's I (BLMI) on the basis of the multi-source big data. The medium industry types of enterprises from the freely available Baidu point of interest data were successfully classified, and then the 250 contaminating enterprises as a contributing factor were identified by the optimized NB classifier. The quantitative contributions of the nine contributing factors for the As, Cd, and Hg concentrations were determined by the optimized RF. The twelve spatial clustering maps between the three HM concentrations and the four key contributing factors were generated by BLMI, explicitly revealing their mutual interactions and internal effects and also intuitively showing the "high-high" areas and their distributions. This framework can obtain rich information on contributing factors such as medium industry types, contribution rates, spatial clusters, and spatial distributions.

Evaluation of Zeolite as a Potential Reactive Medium in a Permeable Reactive Barrier (PRB): Batch and Column Studies

The purpose of this work is to evaluate the capacity of a natural zeolite to be used as a reactive material in a permeable reactive barrier (PRB) to remove inorganic contaminants from groundwater. To this aim, zeolite samples were subjected to characterization tests, column experiments, batch tests and a flushing process to evaluate the adsorption and desorption capacities of the zeolite. In the column experiments, the samples were subjected to eight successive cycles involving the percolation of a potassium aqueous solution (1500 mg/L) and a subsequent flushing process with water. Batch tests were conducted by mixing 20 g of zeolite with 100 mL of single-element aqueous solutions of K and Zn with concentrations of 200 mg/L. The results indicate that the zeolite rock is composed predominantly of clinoptilolite species and has a Si/Al ratio of 6.8, a high cationic exchange capacity (CEC) of 180 cmolc/kg and a high K+ adsorption rate with a removal efficiency of 78%. The adsorption isotherms of the zeolite follow the Langmuir model and are well fit by a pseudo-second-order kinetic model showing a high correlation coefficient (r2 > 0.999) for both K+ and Zn2+ cations. Additionally, the contaminant transport parameters for K+ ions (Rd = 24.9; Dh = 1.32 × 10–2 cm2/s and α = 1.42) reveal that the zeolite is resistant to the dispersion of ions in the barrier, indicating that the material has advantageous characteristics for use in a PRB. However, the flushing process of the material is not efficient, indicating that the appropriate use of the zeolite is in clean-up systems in which the adsorbent material can be exchanged after losing its efficiency as a reactive barrier.

Microbial reduction of nitrate in the presence of zero-valent iron

Microbial reduction of nitrate in the presence of zero-valent iron (ZVI) was evaluated in anoxic shake flasks to assess the feasibility of ZVI-facilitated biological nitrate removal. Nitrate was completely reduced within 3days in the presence of both ZVI and microorganisms (ZVI-M). In contrast, only 75% of the nitrate was reduced in the presence of ZVI but without microbial inoculum. Nitrate removal was affected by ZVI-M flasks initial pH, nitrate concentration and ZVI dosage. Nitrate removal in the inoculated ZVI flasks system could be divided into two phases: adaptation phase and log phase which could be described by first-order kinetic equations. The analysis of bacterial communities in the inoculated flasks in the absence and presence of ZVI, indicated that the addition of ZVI increased the relative abundance of Methylotenera spp., Alcaligenes eutrophus, Pseudomonas spp. which might play an important role in nitrogen removal. The presence of ZVI could enhance biological denitrification through four mechanisms: the biological reduction of nitrate with 1) electrons derived directly from ZVI; 2) with hydrogen released from ZVI; 3) with Fe²⁺ released from ZVI; and 4) with acetate generated by homoacetogens which utilize H₂ released from ZVI.

Isolation and characterization of aerobic, culturable, arsenic-tolerant bacteria from lead-zinc mine tailing in southern China

Bioremediation of arsenic (As) pollution is an important environmental issue. The present investigation was carried out to isolate As-resistant novel bacteria and characterize their As transformation and tolerance ability. A total of 170 As-resistant bacteria were isolated from As-contaminated soils at the Kangjiawan lead-zinc tailing mine, located in Hunan Province, southern China. Thirteen As-resistant isolates were screened by exposure to 260 mM Na₂HAsO₄·7H₂O, most of which showed a very high level of resistance to As⁵⁺ (MIC ≥ 600 mM) and As³⁺ (MIC ≥ 10 mM). Sequence analysis of 16S rRNA genes indicated that the 13 isolates tested belong to the phyla Firmicutes, Proteobacteria and Actinobacteria, and these isolates were assigned to eight genera, Bacillus, Williamsia, Citricoccus, Rhodococcus, Arthrobacter, Ochrobactrum, Pseudomonas and Sphingomonas. Genes involved in As resistance were present in 11 of the isolates. All 13 strains transformed As (1 mM); the oxidation and reduction rates were 5-30% and 10-51.2% within 72 h, respectively. The rates of oxidation by Bacillus sp. Tw1 and Pseudomonas spp. Tw224 peaked at 42.48 and 34.94% at 120 h, respectively. For Pseudomonas spp. Tw224 and Bacillus sp. Tw133, the highest reduction rates were 52.01% at 48 h and 48.66% at 144 h, respectively. Our findings will facilitate further research into As metabolism and bioremediation of As pollution by genome sequencing and genes modification.

Discontinuous permeable adsorptive barrier design and cost analysis: a methodological approach to optimisation

The following paper presents a method to optimise a discontinuous permeable adsorptive barrier (PAB-D). This method is based on the comparison of different PAB-D configurations obtained by changing some of the main PAB-D design parameters. In particular, the well diameters, the distance between two consecutive passive wells and the distance between two consecutive well lines were varied, and a cost analysis for each configuration was carried out in order to define the best performing and most cost-effective PAB-D configuration. As a case study, a benzene-contaminated aquifer located in an urban area in the north of Naples (Italy) was considered. The PAB-D configuration with a well diameter of 0.8 m resulted the best optimised layout in terms of performance and cost-effectiveness. Moreover, in order to identify the best configuration for the remediation of the aquifer studied, a comparison with a continuous permeable adsorptive barrier (PAB-C) was added. In particular, this showed a 40% reduction of the total remediation costs by using the optimised PAB-D.